Current Progress of Efficient Active Layers for Organic, Chalcogenide and Perovskite-Based Solar Cells: A Perspective

Abstract

Photovoltaics has become one of the emerging alternatives to progressively supply/replace conventional energy sources, considering the potential exploitation of solar energy. Depending on the nature of the light harvester to influence on its light-absorption capability and the facility to produce electricity, different generations of solar devices have been fabricated. Early studies of organic molecules (dye sensitizers) with good absorption coefficients, going through metal chalcogenides and, lastly, the timely emergence of halide perovskites, have promoted the development of novel and low-cost solar cells with promising photoconversion efficiency (PCE), close to the well-established Si-based devices. However, main drawbacks such as the degradation/photocorrosion of the active layer, the existence of intrinsic defect sites, and the inherent toxicity of the material due to the presence of some harmful elements have blocked the future commercialization of the above kind of solar cells. In this review, we highlight the current progress in achieving efficient photomaterials for organic, chalcogenides and halide perovskites-based solar cells with the purpose of achieving high PCE values, some of which are breakthroughs in this research topic, and the diverse approaches used to extend the stability of the active layer and improve the performance of the solar devices.

1. Introduction

Photovoltaics (PVs) is one of the most important topics in the development of prominent strategies for finding sustainable energies, with sunlight being an unlimited energy source [1]. This factor has been pivotal considering that both the high energy demand around the world and the subsequent replacement of traditional energy sources such as fossil fuels, coke, and natural gas, among others, have been promoted to mitigate the growing environmental contamination and global warming [2,3]. To guarantee a maximum capability of light absorption, the main component of solar devices is based on semiconductor materials, which collect the sunlight to produce free electrons (sunlight-to-current transformation), which are transported through the fabricated cell [4,5]. In order to optimize the absorption of sunlight, various light trapping techniques, including texturing [6], anti-reflective coatings [7,8] and plasmonic effects [9], can be employed. These methodologies are implemented to enhance the capture of light within the active layer of a solar cell. Furthermore, it is relevant to mention that to certify the performance of the fabricated devices, some operational parameters are established, such as the simulated sunlight intensity, which should contain the AM 1.5 G filter, and the direction of the voltage scanning (forward/reverse) during the photovoltaic measurements. For this contribution, we show the maximum photoconversion efficiency developed by the different solar cells, independent of the direction of voltage scanning, considering that the reported performance is established for optimized devices (ideally similar efficiency in forward and reverse measurements).

For decades, silicon (Si)-based solar cells have dominated the photovoltaics market because these devices can reach photoconversion efficiencies (PCEs) of more than 20%, the photomaterial is non-toxic, providing good photoconductivity properties, resistance to corrosion, and high light intensities and temperatures [4,5,10]. However, even though Si-based solar cells can be obtained by using different Si active layers (monocrystalline, polycrystalline, nanostructured and amorphous Si films), the material preparation and processing are high energy demanding processes to melt the Si source, which increases the fabrication cost for Si-based solar cells [11]. Hence, the final costs of the device production and installation increases, which is a huge challenge in solar technologies. With the purpose of offering a step forward in the fabrication of lower-cost solar cells with promising PCE, another kind of semiconductor has been introduced, as in the case of II-VI type CdTe thin films [12,13]. This type of chalcogenide can be directly deposited as a polycrystalline film through low-cost techniques including close vapor transport [14], laser ablation [15], sputtering [16] and electrodeposition [17]. CdTe thin films have also been prepared by using chalcogenide nanoparticles dispersed in organic or aqueous solutions and deposited by dip-coating/spin-coating techniques, followed by an annealing process [18,19,20].

On the other hand, CdTe shows an optimal band gap ~1.42 eV (indicating a suitable light harvesting feature in a wide range of the UV-vis-NIR spectrum), direct band transitions, and a high absorption coefficient ~10⁴ cm^–1 [21]. This feature is adequate for absorbing ~90% of incident illumination for 1 µm thickness CdTe film. Additionally, this chalcogenide can be produced with n- or p-type semiconductivity, making it ideal for establishing heterojunctions for carrier transfer. Nevertheless, some disadvantages, such as a high resistance to carrier transport and fast surface recombination, hinder electron mobility through the solar devices, decreasing their operational performance [22]. In this way, reasonable approaches such as the preparation of chalcogenide-based heterostructures, for instance, CdS/CdTe ones, are known to enhance carrier injection and transfer into the solar cell, which increases the net PCE [23,24]. Unfortunately, the recent PCE is far from the established record of solid-state solar cells, some of them based on tandem perovskite-Si devices (whose main properties are highlighted below), and it is deductible that a wide understanding of the required components of the solar cells and the quality of the active layer should be pivotal in order to increase the PV efficiency.

The first step to maximizing the sunlight harvesting in low-cost PV devices was introduced by Grätzel in 1991 [25], to fabricate dye-sensitized solar cells (DSSCs) by using natural/synthetic organic materials. These molecules favored carrier generation and transport, obtaining an initial PCE of 10%. As shown in Figure 1 [26], DSSCs are based on the deposition of ruthenium (II) polypyridyl complex monolayers on TiO₂ films acting as working electrodes (photoanodes). Meanwhile, platinum-transparent glass is used as a counter electrode (also called a cathode), being coupled with the active layer to achieve the expected device. The operational performance of the DSSCs to produce electricity depends on the efficiency of exciton formation. In this scenario, the photogenerated electrons coming from the highest occupied molecular orbital (HOMO) of the organic complex (hereafter named sensitizer) are accumulated in the lowest unoccupied molecular orbital (LUMO), and then transferred to the conduction band (CB) of the TiO₂ layer. Then, these carriers are transported to the external circuit of the PV cell to reach the cathode, generating photocurrent. The electron depletion in the HOMO of the dye is filled by electrons from the electrolyte, which is composed of a redox system based on the I⁻/I₃⁻ pair. Lastly, iodide is regenerated by reducing the triiodide species through electrons collected by the cathode, closing the circuit where the current is produced. This contribution opened the door to new organic photomaterials based on dyes, polymers, and small molecules with suitable electronic properties, giving the possibility to fabricate organic solar cells [27,28,29]. Although the selective contacts (electron and hole transporter layers, ETL and HTL, respectively) impact carrier generation and transport into the PV device, organic molecules exhibit limited light harvesting in a wide range of the energy spectrum and restrained exciton diffusion [30]. These features favor a decrease in carrier density to be transported into the solar cells, hampering their operational performance.

Attending to the above device configuration from the DSSCs, organic molecules have been replaced by inorganic hybrid sensitizers, as in the case of Cd-free chalcogenides (in form of quantum dots, QDs); see Figure 2a [31,32,33,34] and then halide perovskite (HPs) type active layers (in the form of bulk); see Figure 2b [35,36], with fascinating features such as high absorption coefficient, long carrier diffusion length, and modulable optical properties depending on the photomaterial size and composition. These advantages allow for the extension of the the light harvesting even to the IR range of the energy spectrum, improving the capability of the sensitizer to generate the photocarriers required to favor a high PCE [37]. In this context, the band gap and relative band positions of the QDs and bulk perovskites can be modified to match the ETL and HTL and thereby facilitate the carrier injection to the selective contacts to generate the expected photoresponse [38,39]. Chalcogenide QDs have reached a maximum PCE of 14.4% [40], while HP bulk films have achieved PCE close to 26% [41,42], overpassing the actual performance of the Si- and CdTe-based technologies. Interestingly, most of the abovementioned features of both kind of active layers are also useful for another kind of research application, such as the fabrication of photodetectors with high responsivity and specific detectivity [43,44,45,46,47,48,49,50,51].

The combination of HPs-based solar cells (PSCs) and Si-solar devices has promoted the development of tandem solar cells, with PCE values of 33.2% [52]. The transparency and thickness control of the active layers and the band gap tuning have been considered to be promising devices for future commercialization. Despite the light-to-current transformation being eventually improved in current prototypes of solar cells, the use of inorganic materials is found to be restrained due to their ionic nature, which produces photocorrosion in chalcogenides [53] and the formation of non-photoactive crystalline phases in HPs, as in the case of iodide-HPs.

As shown above, reports have widely discussed the fundamentals of the photovoltaic effect in solar cells, which have been adopted to describe the way that the active layers, or sensitizers, capture the sunlight to produce electricity through the formation of photocarriers, which are separated and transported through the devices. This fact has led some researchers to believe that the emergence of the following generation of solar cells offers more advantages than the previous one, but it has not been deeply explained from the point of view of individual components how the fabrication of innovative solar devices can be more efficient and exhibit enhanced long-term stability. More specifically, few reports have consolidated the advantages/disadvantages of using different types of materials as active layers, where the photovoltaic effect starts, and some explanation about how the integration of these sensitizers into a coupled solar system can magnify the net performance of a solar cell to be competitive with the actual commercialized Si-based ones has not been widely highlighted.

Overall, in this review, we highlight the current progress in the preparation of novel active layer materials to fabricate more efficient organic, chalcogenide, and perovskite solar cells, obtaining valuable information about their operational stability, and including the use of less toxic species to produce more profitable solar technologies in the near future. Unlike actual reports where a general description of the characteristics of each component of the solar device (electron/hole transporting and active layers) is provided, we mainly focused on the light absorption and carrier separation/transport capabilities of suitable active layers to fabricate efficient solar cells from diverse device generations. Some critical disadvantages, such as band structure mismatch, low absorption coefficient, presence of defect sites for carrier recombination, and degradation of the photoactive crystalline phase, are mentioned throughout the manuscript. Nevertheless, some benefits, such as a tunable band gap depending on the nature of the photomaterial and a versatile surface chemistry, are analyzed to understand the actual challenges of using several types of active layers, offering the opportunity to establish an adequate one for future commercialization purposes. In this context, a perspective on prominent and suitable photomaterials acting as active layers in solar devices is also addressed.

The review is divided into three main topics: organic solar cells, chalcogenide solar cells, and halide perovskite-based solar cells, and the application of novel photomaterials as active layers in solar devices is thoroughly addressed. This order of showing the main features of each active layer was chosen, considering the level of actual interest in the development of this type of photomaterial. Although the current interest in some of them has decreased, we consider these materials have opened the door to understanding the photovoltaic process in a solar cell configuration and what the most relevant properties of the material itself are to make it suitable for photovoltaics.

2. Progress in the Development of Active Layers for Efficient PV Devices

2.1. Organic Solar Cells

Organic solar cells (OSCs) have recently attracted attention worldwide since they are one of the most promising energy sources due to their distinctive advantages, such as light-weight mechanical flexibility, being colorful, translucent, nontoxic and being solution-processable for huge area fabrication [54,55,56,57,58]. Over the last three decades, the area of OSCs has made impressive progress in efficiency, which probably makes them a competitive alternative to inorganic solar cells in the near future. In this review, we are going to classify OSCs into three different groups based on their structure: binary, ternary, or tandem OSCs and dye-sensitized solar cells (DSCs).

2.1.1. Binary, Ternary, and Tandem Organic Solar Cells

The first manufacture of organic photovoltaic solar cells was based on single organic films inserted between two metal electrodes of different work functions [59,60]. Initially, organic materials with p- and n-type conductivities are deposited on a transparent electrode, and then the metal electrode is placed over them. The operation of OSCs consists of the fact that sunlight is absorbed by organic layers, and produced excitons are dissociated to electrons and holes at the interface amongst p- and n-type organic layers. Finally, holes and electrons are diffused at the electrodes, generating electricity [59,60,61]. The PCEs reported by these devices were really poor (around 0.01%), although they reached a significant 0.7% with the incorporation of merocyanine dyes [62,63]. The next evolution in this area was performed by introducing the heterojunction concept, in which two organic films with special electron- or hole-transporting properties were sandwiched between the electrodes. Tang and coworkers [64] manufactured the first organic photovoltaic device based on planar heterojunction (PHJ) of two organic materials (phthalocyanine and perylene derivatives), one carrier donor, and one carrier acceptor, which were used to distribute the excitons. These organic layers were sandwiched between a transparent conducting oxide and a semitransparent metal electrode, affording effective charge extraction. Although there were very few donor/acceptor interfaces, this device was quite limited. This solar cell afforded around 1% of PEC, which was the best result obtained for almost 10 years.

On the other hand, the development of conjugated polymers grew enormously in the early 1990s, although their incorporation as single-layer devices was not initially successful since the values of PCEs obtained by this type of material were less than 0.1% [65,66]. Nevertheless, the discovery of photoinduced electronic transference between optically excited conjugated polymers and the fullerene molecule (C₆₀) [67,68], along with the elevated photoconductivities shown by the addition of C₆₀ to the conjugated polymers [69], allowed the fabrication of devices based on polymer-fullerene bilayer heterojunction [70,71] or bulk heterojunction (BHJ) [72,73,74]. This advance significantly increased the values of PCEs of the corresponding devices for almost two decades (Table 1) [75,76,77,78,79,80,81,82]. The photoinduced electron mobility favorably happens in the polymer-fullerene system when the electron in the excited state of the polymer is transferred to the fullerene, which is quite more electronegative since the electron is injected from a p-type hole belonging to the conducting polymer, donor (D), to the n-type electron from the conducting C₆₀ molecule, acceptor (A) [83]. This occurs because fullerene derivatives present a fully conjugated structure, which gives them powerful electron-accepting and isotropic electron-transport abilities and favors electron delocalization at the D:A interfaces [84,85]. Some time ago, it was believed that fullerene materials such as PC₆₁BM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) [86], F1 ([6,6]-phenyl-C₆₁-propionic acid methyl ester) [87], and F [88] (Figure 3) were a basic component for efficient operation of OSCs. However, this scenario has recently changed with the appearance of non-fullerene acceptors (NFA) such as ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-ditheno [2,3-d:2′,3′-d′]-s-indaceno [1,2-b:5,6-b′]-dithiophene) [89], ITIC-Th1 (3,9-bis(2-methylene-(5&6-fluoro-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylthienyl)-dithieno [2,3-d:2′,3′-d′]-s-indaceno [1,2-b:5,6-b′]-dithiophene) [90], FOIC [91], and Y6 ((2,20-((2Z,20Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo [3,4-e]thieno [2,”30′:4′,50]thieno [20,30:4,5]pyrolo [3,2-g]thieno [20,30:4,5]thieno [3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile)) [92] (Figure 3), which have increased notably the values of PCEs obtained [84,93]. The performance of the NFA-based OSCs is closely related to the morphology of the functional layers and the material processing, although in some cases the change in the stratification of the donor and acceptor in the BHJ [94,95,96] and the chemical stability at the BHJ/contact interfaces [97] have also played a decisive role in increasing the PCE values in these devices. The fast development of OSCs based on NFA has caused these devices have crossed the PCE barrier of 19% over the last years; see Table 1 [98,99,100,101,102,103,104,105]. Nevertheless, the broad range of donor molecules previously used for fullerene OSCs provided a splendid library for direct use in NFA OSCs. From the device perspective, excitons in NFA OSCs can be effectively separated by negligible driving energies, which contributes to increasing their PCEs. Consequently, this type of devices regularly shows elevated photocurrent and low voltage losses simultaneously; nonetheless, fullerene-based OSCs present problems under low driving energies frequently [99].

The PCE is calculated in OSCs by multiplying the values of open-circuit voltage (V_oc), short-circuit current density (J_sc), and a fill factor (FF). The value of V_oc essentially depends on the level of the donor’s HOMO and acceptor’s LUMO; the J_sc value is correlated to the absorption and surface of the active layer; and the value of FF is related to the charge carrier mobility. Table 1 includes the highest PCE values recently reported for binary OSCs [106].

Table 1. Photovoltaic performances of the recently reported binary OSCs.

Binary OSCs	Jsc (mA/cm2)	Voc (V)	FF	PCE (%)	Stability	Ref.
PF2:PC71BM	15.00	0.75	0.744	8.40	72% of initial performance, 3.33 h, air	[75]
DCPY2	22.24	0.90	0.650	13.02	90% of initial performance, 700 h, 85 °C, air	[77]
PM6:L8-BO	26.68	0.88	0.805	19.02	70% of initial performance, 130 h, air	[107]
BTI-2T-CNA:IDIC	12.90	0.89	0.581	6.70	-	[108]
BTP-eC9:BTP-ICBCF3	27.40	0.85	0.778	18.20	-	[109]
PBDB-TF:HF-PCIC	17.24	0.89	0.710	10.90	90% of initial performance, 700 h, 130 °C, air	[110]
BTP-O-S	24.40	0.90	0.782	17.10	-	[111]
PM6:L8-BO	25.81	0.90	0.801	18.69	86% of initial performance, 500 h, air	[112]
PM6:BTP-eC9	27.88	0.86	0.804	19.31	70% of initial performance, 1000 h, air	[98]
D18:L8-BO	26.70	0.89	0.800	19.00	74% of initial performance, 200 h, 80 °C, air	[101]
PEDOT:PSS	25.70	0.87	0.791	18.03	88% of initial performance, 1200 h, air	[113]

Table 1. Photovoltaic performances of the recently reported binary OSCs.

Binary OSCs	Jsc (mA/cm2)	Voc (V)	FF	PCE (%)	Stability	Ref.
PF2:PC71BM	15.00	0.75	0.744	8.40	72% of initial performance, 3.33 h, air	[75]
DCPY2	22.24	0.90	0.650	13.02	90% of initial performance, 700 h, 85 °C, air	[77]
PM6:L8-BO	26.68	0.88	0.805	19.02	70% of initial performance, 130 h, air	[107]
BTI-2T-CNA:IDIC	12.90	0.89	0.581	6.70	-	[108]
BTP-eC9:BTP-ICBCF3	27.40	0.85	0.778	18.20	-	[109]
PBDB-TF:HF-PCIC	17.24	0.89	0.710	10.90	90% of initial performance, 700 h, 130 °C, air	[110]
BTP-O-S	24.40	0.90	0.782	17.10	-	[111]
PM6:L8-BO	25.81	0.90	0.801	18.69	86% of initial performance, 500 h, air	[112]
PM6:BTP-eC9	27.88	0.86	0.804	19.31	70% of initial performance, 1000 h, air	[98]
D18:L8-BO	26.70	0.89	0.800	19.00	74% of initial performance, 200 h, 80 °C, air	[101]
PEDOT:PSS	25.70	0.87	0.791	18.03	88% of initial performance, 1200 h, air	[113]

In general, the working mechanism in bulky heterojunction OSC devices occurs as follows: (a) photons are absorbed and excitons are generated; (b) excitons are diffused to D/A interfaces; (c) excitons are dissociated in holes and electrons at D/A interfaces, which are transported to the electrodes; see Figure 4 [114,115]. The BHJ system was first introduced by combining two polymers with acceptor and donor properties in solution [116], which solved the problems shown by PHJ systems. One of the best OSCs reported to date is based on this type of structure due to its numerous advantages, such as semi-transparency, light weight, and high mechanical flexibility [117,118,119,120]. However, BHJ structure has some drawbacks, such as their morphology; it is difficult to control them since an inadequate morphology could promote a fast charge recombination, affecting electron diffusion at the interface between donor and acceptor moieties, which also influences the recombination between holes and electrons [120]. These disadvantages could influence the industrial development of OSCs based on BHJ because controlling the nanostructure of the donor and acceptor is a complicated task. Accordingly, the performance of OSCs has been significantly improved by efforts to design new donor and acceptor materials.

The main limitation of OSCs is that organic materials show an absorption width of only about 0.5 eV, so the binary single-junction can only take advantage of a small part of the solar radiation, hindering the improvement in device efficiency [84,121]. For these reasons, the introduction of a ternary blend based on a dye, nanoparticle, polymer, or small molecule into the device could solve the aforementioned problems [100], and ternary OSCs have recently focused the attention of the scientific community because these devices could afford great PCE values (Table 2). Basically, ternary OSCs are formed by three units, two acceptor moieties, and one donor (or two donor moieties and one acceptor) (Figure 5) [122].

Firstly, the third component has been used as a sensitizer, assisting in extending the maximum absorption of the acceptor and the donor based on the 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6] methano-fullerene (PCBM) matrix. In addition, considering the first reports about ternary OSCs, scientists have employed a huge variety of organic entities as third components, which act as an efficient charge transport pathway, boosting charge transport and exciton dissociation for OSC devices [100]. These ternary devices, which are mainly based on polymer/small molecule/PCBM, polymer/PCBM/non-fullerene, and polymer/ICBA/PCBM (ICBA, indene-C₆₀ bis-adduct), reported PCE values above 10% (Figure 6) [123,124,125]. Ternary OSCs can be classified in four categories according to their function: charge transfer, energy transfer, parallel like, and alloy models. It is important to mention that the most efficient ternary OSCs operate under an alloy-like model since the gradual changes in V_oc depend on the third element.

Table 2. Photovoltaic performances of the recently reported ternary OSCs.

Ternary OSCs	Jsc (mA/cm2)	Voc (V)	FF (%)	PCE (%)	Stability	Ref.
TO:PEDOT:PSS (S11)	17.62	0.95	0.701	11.78	70% of initial performance, 1008 h, 85 °C, air	[126]
PM6:PY-V-γ:PFBO-C12	25.80	0.91	0.770	18.00	80% of initial performance 1000 h, air	[127]
PEDOT:PSS:LiF	25.60	0.86	0.742	16.70	55% of initial performance, 360 h, air	[128]
PM6:Y6:7.5%FPDI-2PDI	27.47	0.85	0.772	18.00	55% of initial performance, 30 h, air	[129]
PM6:M-Cl:O-Cl	27.40	0.87	0.762	18.10	-	[130]
D18:Y6:SN-O	26.80	0.88	0.781	18.30	-	[131]
PBQx-TF:eC9-2Cl:F-BTA3	26.70	0.88	0.809	19.00	-	[105]
PBDB-T-2F:Y6:SF(BR)4	29.31	0.89	0.800	20.87	-	[132]
PBQx-TCl:PBDB-TF:eC9-2Cl	27.15	0.89	0.811	19.51	-	[133]

Table 2. Photovoltaic performances of the recently reported ternary OSCs.

Ternary OSCs	Jsc (mA/cm2)	Voc (V)	FF (%)	PCE (%)	Stability	Ref.
TO:PEDOT:PSS (S11)	17.62	0.95	0.701	11.78	70% of initial performance, 1008 h, 85 °C, air	[126]
PM6:PY-V-γ:PFBO-C12	25.80	0.91	0.770	18.00	80% of initial performance 1000 h, air	[127]
PEDOT:PSS:LiF	25.60	0.86	0.742	16.70	55% of initial performance, 360 h, air	[128]
PM6:Y6:7.5%FPDI-2PDI	27.47	0.85	0.772	18.00	55% of initial performance, 30 h, air	[129]
PM6:M-Cl:O-Cl	27.40	0.87	0.762	18.10	-	[130]
D18:Y6:SN-O	26.80	0.88	0.781	18.30	-	[131]
PBQx-TF:eC9-2Cl:F-BTA3	26.70	0.88	0.809	19.00	-	[105]
PBDB-T-2F:Y6:SF(BR)4	29.31	0.89	0.800	20.87	-	[132]
PBQx-TCl:PBDB-TF:eC9-2Cl	27.15	0.89	0.811	19.51	-	[133]

Since single organic materials have the ability to absorb at different wavelength regions between the blue and the infrared regions, “tandem” architectures have been designed to accomplish higher performances in OSCs with better use of near-infrared (NIR) solar energy [134]. Tandem OSCs are formed by two sub-cells, a front and back sides, both made of organic materials. For certain types of OSCs with a specific bandgap and absorption region, the balance between the loss of photon energy by thermalization and spectrum unitization is limited [135]. However, in tandem OSCs, the absorption spectrum can be extended by using a wide bandgap sub-cell to employ high-energy photons and another narrow bandgap sub-cell for employing low energy photons [136]. In this context, the V_oc value presented by tandem OSCs is normally higher than that of binary or ternary OSCs because it is the sum of those of the two sub-cells, whereas the J_sc value usually derives from the sub-cell with relatively lower J_sc; hence, the PCE in tandem OSCs depends directly to the balance between V_oc and J_sc for each sub-cell, which is decisive for the high efficiency of the device [134,137]. At the current stage, the PCE values of tandem OSCs are very similar to those presented by binary or ternary OSCs, as seen in Table 3. The main difficulty for the development of tandem OSCs is related to their design since the current of each sub-cell has to be matched, so the total current of the device is limited by the lowest current generating sub-cell [85]. Another limitation of this device is related to the low efficiency presented by organic materials for narrow band gap sub-cells, which would allow the use of the NIR region of the solar spectrum.

Table 3. Photovoltaic performances of the recently reported tandem OSCs.

Tandem OSCs (Front/Back Cell)	Jsc (mA/cm2)	Voc (V)	FF	PCE (%)	Stability	Ref.
PTB7-Th:BTPV-4F/PTB7-Th:BTPV-4F:PC71BM	14.50	1.65	0.690	16.40	-	[120]
PBDT [2F]T:PC71BM/PCE10:PC61BM	7.10	1.61	0.705	8.30	-	[138]
PBDB-T:F-M/PTB7-Th:O6T-4F:PC71BM	14.35	1.64	0.737	17.36	96% of initial performance, 3984 h, air	[139]
PM7:TfIF-4Cl/PTB7-Th:COi8DFIC:PC71BM	14.59	1.64	0.780	18.71	95% of initial performance 500 h, air	[140]
PM6:GS-ISO/PM6:BTP-eC9	13.14	2.01	0.768	20.27	87% of initial performance, 450 h, air	[141]

Table 3. Photovoltaic performances of the recently reported tandem OSCs.

Tandem OSCs (Front/Back Cell)	Jsc (mA/cm2)	Voc (V)	FF	PCE (%)	Stability	Ref.
PTB7-Th:BTPV-4F/PTB7-Th:BTPV-4F:PC71BM	14.50	1.65	0.690	16.40	-	[120]
PBDT [2F]T:PC71BM/PCE10:PC61BM	7.10	1.61	0.705	8.30	-	[138]
PBDB-T:F-M/PTB7-Th:O6T-4F:PC71BM	14.35	1.64	0.737	17.36	96% of initial performance, 3984 h, air	[139]
PM7:TfIF-4Cl/PTB7-Th:COi8DFIC:PC71BM	14.59	1.64	0.780	18.71	95% of initial performance 500 h, air	[140]
PM6:GS-ISO/PM6:BTP-eC9	13.14	2.01	0.768	20.27	87% of initial performance, 450 h, air	[141]

2.1.2. Dye-Sensitized Solar Cells

Conceptually similar to the BHJ, there is another broad research field based on DSSCs. The early development of these devices was performed by the Grätzel research group [25,142,143,144]. As discussed previously, DSSCs have greatly focused the attention of the scientific community over the last few years due to their promising performance and low fabrication cost [145,146]. It is important to highlight that water-based electrolytes DSSCs are being developed owing to their interesting advantages such as reduced volatility, non-flammability, and environmental compatibility, affording a PCE value over 7% [147]. Additionally, DSSCs are able to be produced on a large scale and present a commercial application for self-powering systems and portable electronics [148,149]. On the other hand, specifically DSSCs prepared with copper-complex-based redox mediator have shown great progress in obtaining PCE values close to 13%, since their V_oc can achieve values over 1.0 V [148,149], lower compared to the values observed by high performance tandem OSCs; see Table 3. However, these devices exhibit low J_sc values, considerably reducing their PCE performance. Probably, the principal disadvantage of DSSCs compared with other OSCs is related to the relatively low PCE values reported for these devices; it is difficult to find DSSCs with PCE values over 14%. On the contrary, the most important advantage of DSSCs is their ability to exhibit outstanding stability, maintaining high percentages of their initial PCE after long periods of time.

2.1.3. Comparison of J–V Curves and Normalized PCE vs. Time for Different Types of Organic Solar Cells

Binary, ternary, and tandem OSCs or DSSCs are devices with different characteristics that are determined by their values of V_oc, fill factor (FF) and J_sc. These variables are defined by the current density and voltage (J–V curves) properties of the solar cells [99]. These parameters are closely related to the photoactive materials (acceptor and donor materials). Thus, the number of free carriers collected at the electrodes at zero applied potential (V_oc = 0) is what determines the value of J_sc. The energy difference amongst the HOMO and LUMO orbitals of the acceptor and donor materials influences the highest voltage that a solar cell could extract for an external circuit, ascribed to the value of V_oc. For example, in PM1:BTP-eC9 systems, device efficiency improved from 17.86% to 19.10% with TCB processing. It is important to comment that the TCB-ISM device exhibits the highest PCE value (19.31%) reported to date for a binary OSCs; see Figure 6a [98]. Additionally, the ternary blend PBQx-TCl:PBDB-TF:eC9-2Cl displays a maximum PCE of 19.51%, a V_oc of 0.886 V, a J_sc of 27.15 mA cm⁻², and an FF of 81.14%; see Figure 6b [133]. The tandem OSC based on ICLs of e-TiOx/PEDOT:PSS system achieved a V_oc value over 2.0 V, which is considerably higher than the values of V_oc showed by binary or ternary OSCs, affording a PCE value of 20.27%; see Figure 6c [141]. It is important to highlight that DSSCs with ZS4 performed a J_sc of 16.3 mA cm⁻², as well as a great V_oc value of 1.05 V, accomplishing an impressive PCE of 13.2% under AM1.5G sunlight. This efficiency is the highest one reported for copper-electrolyte-based DSSCs with a single sensitizer; see Figure 6d [145].

Figure 6. (a) J–V curves for the DIO processed and TCB processed OSCs based on PM6:BTP-eC9. Reproduced under the terms of the CC-BY license [98]. Copyright 2023. The Authors. Published by Springer Nature. (b) J–V curves of the OPV cells based on the PBQx-TCl:eC9-2Cl, PBDB-TF:eC9-2Cl, and PBQx-TCl:PBDB-TF:eC9-2Cl (0.5:0.5:1.2) systems. Reproduced with permission [133]. Copyright 2023. John Wiley and Sons. (c) The J–V curves of tandem OSCs based on ICLs of e-TiOx/PEDOT:PSS under AM 1.5G, 100 mW/cm². Reproduced with permission [141]. Copyright 2021. Elsevier Inc. (d) J–V curves of the DSCs with ZS4 and ZS5 measured under AM1.5G illumination. Reproduced with permission [145]. Copyright 2023. John Wiley and Sons.

Figure 6. (a) J–V curves for the DIO processed and TCB processed OSCs based on PM6:BTP-eC9. Reproduced under the terms of the CC-BY license [98]. Copyright 2023. The Authors. Published by Springer Nature. (b) J–V curves of the OPV cells based on the PBQx-TCl:eC9-2Cl, PBDB-TF:eC9-2Cl, and PBQx-TCl:PBDB-TF:eC9-2Cl (0.5:0.5:1.2) systems. Reproduced with permission [133]. Copyright 2023. John Wiley and Sons. (c) The J–V curves of tandem OSCs based on ICLs of e-TiOx/PEDOT:PSS under AM 1.5G, 100 mW/cm². Reproduced with permission [141]. Copyright 2021. Elsevier Inc. (d) J–V curves of the DSCs with ZS4 and ZS5 measured under AM1.5G illumination. Reproduced with permission [145]. Copyright 2023. John Wiley and Sons.

Having discussed the typical performance of the binary, ternary, tandem OSCs and DSSCs, it is pivotal to focus on studying their stability over time. To address this point, we highlight the stability presented by some of the previously mentioned devices; see Figure 7. The operational stability of binary PM6:BTP-eC9-based OSCs showed a stronger initial decay in PCE, dropping a 17% efficiency during the first 75 h when DIO (1,8-diiodooctane) was used as an additive, while the efficiency of the TCB (1,3,5-trichlorobenzene) treated device only decreased by 7% at the same time. After 1000-h simulated 1-sun illumination stress test at maximum power point (MPP), the TCB-treated device maintained a 78% of initial efficiency; see Figure 7a [98]. The ternary PBDB-TF/HC-PCIC/PC₇₁BM experienced an intense drop in efficiency (15%) after only 1 h. However, its efficiency remained constant during the following 10 h; see Figure 7b [110]. Tandem cells dropped sharply in the first 10 h and became stable within 456 h under continuous 1 sun illumination; see Figure 7c [141]. Finally, DSSC fabricated with ZS4 exhibited excellent stability, keeping 95% of their initial PCE under continuous light soaking at 45 °C for 1000 h; see Figure 7d [145]. In general, binary and ternary OSCs reported high PCE values (~20%); however, most of these devices are not very stable over time since their efficiency decreases significantly after a few hours. In contrast, although the PCE values shown by DSCs are moderate (~14%) compared to binary or ternary OSCs, the most important benefit of these types of devices is possibly their capability to display outstanding stability, maintaining elevated percentages of their initial PCE after long periods of time.

2.2. Chalcogenide Solar Cells

Chalcogenide solar cells are a class of thin-film solar cells that utilize semiconductor materials from the chalcogenide group (e.g., sulfur, selenium, tellurium) to absorb the solar radiation, produce photocarriers, and thereby generate electricity. These solar cells have the advantage of being cost-effective, with good conversion efficiencies. They typically consist of a substrate, transparent conductive oxide layer, p-type and n-type chalcogenide layers, and an intrinsic layer. Here, we will explain some characteristics of binary, ternary, and quaternary chalcogenides usually employed for the fabrication of solar devices.

2.2.1. Binary, Ternary, and Quaternary Chalcogenides

Binary chalcogenide solar cells use semiconductor materials composed of two elements, one from the chalcogenide group, such as sulfur (S), selenium (Se), or tellurium (Te) and a metal. These materials are abundant and relatively low-cost, making them attractive for solar cell applications. The structure of binary chalcogenide solar cells typically consists of a substrate, a transparent conductive oxide layer (TCO), a p-type chalcogenide layer, an intrinsic layer, and an n-type chalcogenide layer. The TCO layer allows light to pass through while providing electrical conductivity. The p-type and n-type chalcogenide layers help in creating an electric field and facilitating the movement of charge carriers. When sunlight is absorbed by the chalcogenide layers, electron-hole pairs are created, which are then collected to generate an electric current [150]. Among the main types of binary solar cells are as follows:

Cadmium Telluride (CdTe)

A thin-film of CdTe semiconductor absorbs radiation in the visible part of the spectrum, creating electron-hole pairs. The generated electrons and holes are separated at the junction, resulting in a flow of electric current [150]. One of the examples of efficient CdTe solar devices has been described by Rashid and coworkers [151] where a silicon carbide (3C–SiC)-based buffer layer was introduced into the preparation of CdTe films and subsequently device fabrication. By improving the crystallinity of the active layer and providing an adequate heterojunction of the chalcogenide and the 3C-SiC, a PCE value up to 17.29%; see Figure 8a. Clearly, diverse device optimizations have been incorporated during the fabrication of CdTe solar cells, allowing PCE values up to 23% [106]. Among its advantages are cost-effectiveness due to abundant and low-cost materials and compatibility with large-scale manufacturing. However, its main problem is the use of toxic materials [152]. To improve the stability of these devices and avoid the possible lixiviation of Cd, the incorporation of small amounts of cupper during chalcogenide etching process (chemical etch), can be carried out [153]. In this context, optimized CdTe solar devices can preserve 80% of initial performance for close to 1200 h in air; see Figure 8b.

Cadmium Selenide (CdSe)

These thin-film solar cells operate in the same way as CdTe ones, changing the layer that absorbs the sun’s photons for one of CdSe [154].

Lead Sulfide (PbS)

These solar cells are known for their ability to efficiently absorb infrared light. PbS solar cells can be fabricated using low-cost solution-based processes [155]. They have shown promise for applications in low-light conditions. For PbS solar cells, PCEs have reached values up to 13.8% [156].

Meanwhile, ternary and quaternary chalcogenides solar cells are a type of thin-film solar cells that are composed of three or four different elements from groups 11–16 of the periodic table, typically including a combination of metal, chalcogen, and sometimes halide elements [157]. These materials have unique electronic properties, such as their tunable bandgap and high absorption coefficient, that make them promising candidates for use in solar cells. Some of the principal ternary and quaternary chalcogenides used in solar cell applications are as follows:

Copper-Indium-Gallium Selenide (CIGS)

The CIGS solar cell structure consists of several layers, including a back contact, a p-type CIGS absorber layer, an n-type buffer layer, and a transparent front contact [158]. CIGS solar cells have advantages including high efficiency, low temperature coefficient, and the ability to be manufactured using a variety of techniques [158]. The high efficiency of CIGS solar cells is due to the material’s ability to absorb a large fraction of the solar spectrum, including both visible and infrared light [159,160]. Additionally, CIGS solar cells have a low temperature coefficient [161], meaning their efficiency is less sensitive to changes in temperature compared to other types of solar cells [162]. CIGS solar cells have achieved record PCE of up to 23.35% [163]. The main drawbacks that CIGS cells have are the use of toxic materials and the high temperatures that are needed for their manufacture [164]. On the other hand, by taking advantage of the band gap modulation, CIGS can reach PCEs higher than 21.8% [165], which becomes useful for the fabrication of tandem devices; see Figure 8c. Some optimized devices for this type of active layer can keep ~88% of initial efficiency in harsh conditions of H₂O/O₂ for close to 350 h [166]; see Figure 8d.

Figure 8. Operational performance and stability of some chalcogenide solar cells. (a) J–V curve of CdTe solar cell. Reproduced with permission [151] Copyright 2022, Elsevier. (b) Stability of CdTe solar cell in function of etching cycles. Reproduced with permission [153]. Copyright 2017. Elsevier. (c) J–V curve of CIGS solar cell in function of modulated band gap. Reproduced with permission [165] Copyright 2020. Elsevier. (d) Stability of CIGS solar cells under short-circuit condition (SCC) and open-circuit condition (OCC). Reproduced with permission [166]. Copyright 2015. Elsevier.

Figure 8. Operational performance and stability of some chalcogenide solar cells. (a) J–V curve of CdTe solar cell. Reproduced with permission [151] Copyright 2022, Elsevier. (b) Stability of CdTe solar cell in function of etching cycles. Reproduced with permission [153]. Copyright 2017. Elsevier. (c) J–V curve of CIGS solar cell in function of modulated band gap. Reproduced with permission [165] Copyright 2020. Elsevier. (d) Stability of CIGS solar cells under short-circuit condition (SCC) and open-circuit condition (OCC). Reproduced with permission [166]. Copyright 2015. Elsevier.

Copper-Zinc-Tin Sulfide (CZTS)

CZTS has a similar structure to CIGS and offers the advantage of using abundant and non-toxic elements [167]. The CZTS solar cell structure typically consists of a molybdenum back contact, a CZTS absorber layer, a buffer layer, and a transparent conductive oxide front contact [168]. CZTS solar cells have lower PCE than CIGS solar cells, reaching values of up to 11.0% [169].

Copper-Zinc-Tin Selenide (CZTSe)

CZTSe solar cells operate in a similar manner to other thin-film solar cells. They consist of a substrate, a back contact, a CZTSe absorber layer, a buffer layer, and a transparent conductive oxide front contact [168]. CZTSe solar cells are like copper indium gallium selenide (CIGS) solar cells in terms of their crystal structure, which is a kesterite structure [170]. CZTSe has a higher bandgap than CIGS, which means that it can absorb a larger portion of the solar spectrum and potentially achieve higher efficiencies. CZTSe also has the advantage of being composed of earth-abundant and non-toxic materials. CZTSe solar cells face several challenges, such as low open-circuit voltage and fill factor due to the presence of defects and recombination centers in the absorber layer [171]. PCE currently reached values ~12.5% [172].

Cadmium-Free Copper Zinc Tin Sulfide (CZTSSe)

In cadmium-free CZTS solar cells, alternative cadmium sulfide. This allows for the elimination of toxic cadmium, resulting in a more environmentally friendly and sustainable solar cell technology [173]. CZTSSe solar cells have achieved PCE values of up to 12.6% [174].

Due to the fact that most of the fabricated chalcogenide devices do not report stability measurements,

Table 4. Photovoltaic parameters of main chalcogenide-based solar cells.

Active Layer	JSC (mA/cm2)	VOC (V)	FF	PCE	Ref.
CdTe	26.30	0.81	0.808	17.3	[151]
CdTe	30.25	0.87	0.794	21.0	[175]
CdTe	31.69	0.88	0.785	22.1	[106]
CIGS	39.80	0.72	0.765	21.8	[165]
CIGS	39.68	0.73	0.804	23.4	[163]
CIGS	39.33	0.73	0.805	23.3	[176]
CZTS	21.77	0.71	0.826	10.0	[177]
CZTS	21.74	0.73	0.693	11.0	[169]
CZTSe	38.30	0.46	0.663	11.8	[178]
CZTSe	37.40	0.49	0.682	12.5	[172]
CZTSSe	35.60	0.51	0.697	12.6	[174]
CZTSSe	35.20	0.51	0.698	12.6	[179]

shows a summary of the conversion efficiencies of the main solar cells of this generation and their corresponding parameters.

2.2.2. Chalcogenide Tandem Solar Cells

Chalcogenide tandem solar cells have emerged as a promising candidate for achieving high-efficiency photovoltaic devices. In this type of solar cells, two or more different chalcogenide-based semiconductors with diverse bandgaps are assembled to absorb a broader range of the solar spectrum; see Figure 9a [180]. The upper cell absorbs the higher-energy photons, while the lower cell absorbs the lower-energy photons that pass through the upper cell [181]. This allows for a more efficient utilization of the solar spectrum and can lead to a higher overall efficiency. Chalcogenide-based materials, such as copper indium gallium selenide (CIGS), cadmium telluride (CdTe) [182], lead sulfide (PbS) [183,184] and earth-alternative materials such as kesterite-based chalcogenides [185,186] have been extensively studied for use in tandem solar cells. CIGS is particularly attractive due to its high absorption coefficient and tunable bandgap, making it suitable for use in both the top and bottom cells of the tandem device. Besides, CIGS tandem solar cells have high carrier mobility, scalability, potential for low-cost, and compatibility with alternative materials.

In recent years, perovskite-chalcogenide tandem solar cells have gained significant attention due to their high-power conversion efficiencies and potential for low-cost, scalable production [187,188,189,190]. Perovskites are inorganic materials that have unique optical properties. For more details, see Section 2.3. This kind of tandem solar cell typically combines a perovskite material and other material as a different perovskite, CIGS and CdTe [191,192]. The perovskite material is used to absorb high-energy photons, while the other materials are used to absorb low-energy photons. High-energy photons absorbed by the perovskite generate electron-hole pairs. The electrons and holes then move to the interface between the perovskite and other material layers, where they are separated by a built-in electric field. The electrons flow through the bottom layer, while the holes flow through the perovskite layer, generating a photocurrent. The two currents are then combined at the electrode, producing a higher overall current than a single junction solar cell. These kinds of cells have a maximum efficiency of around 31% [106]. For instance, Al-Ashouri and workers [193] have fabricated a monolithic CIGSe/perovskite tandem solar cells, introducing the concept of self-assembled monolayers to choose suitable hole selective contacts and favor an efficient carrier separation/transport. For the optimized device, a maximum PCE of 23.26% was reached (Figure 9b) keeping the 97% of initial performance after 2400 h under operation; see Figure 9c.

Table 5. Photovoltaic parameters of main chalcogenide tandem solar cells.

Active Layer	JSC (mA/cm2)	VOC (V)	FF	PCE	Stability	Ref.
Perovskite/CIGS	19.2	1.68	0.719	23.3	97% of initial performance, 2400 h, air	[193]
Perovskite/CIGS	19.24	1.77	0.729	24.2	-	[194]
DSSC/CIGS	14.6	1.17	0.77	13.0	67% of initial performance, 12 h	[180]
Perovskite/PbS	-	-	-	20.3	90% of initial performance, 12.5 h, without encapsulation	[195]
Perovskite/Perovskite	16.0	2.01	0.800	25.6	88% of initial performance, 500 h, air	[196]
CuInSe2/Perovskite	12.9	1.34	0.635	11.0	~70% of initial performance, 2400 h, in a desiccator	[197]
Perovskite/PbS	16.67	1.79	0.783	23.4	-	[198]
GaInP/AlGaAs/CIGS	11.72	2.95	0.818	29.1	90% of initial performance, after 50 cycles of operation	[199]
GaAs0.75P0.25/Si	20.47	1.91	0.798	31.3	-	[200]

shows different examples of tandem chalcogenide solar cell, their efficiencies, and parameters such as V_OC, J_SC, and FF.

Figure 9. (a) Representation of a tandem solar cell based on a DSSC/CIGS assembly. Reproduced under the terms of the CC-BY license [180]. Copyright 2015. The authors. Springer Nature. (b) J–V curve and (c) stability measurements of a tandem perovskite-CIGS solar cell. Reproduced under the terms of the CC-BY license [193]. Copyright 2019, The authors, Royal Society of Chemistry.

Figure 9. (a) Representation of a tandem solar cell based on a DSSC/CIGS assembly. Reproduced under the terms of the CC-BY license [180]. Copyright 2015. The authors. Springer Nature. (b) J–V curve and (c) stability measurements of a tandem perovskite-CIGS solar cell. Reproduced under the terms of the CC-BY license [193]. Copyright 2019, The authors, Royal Society of Chemistry.

2.2.3. New Chalcogenide Materials

II-IV-N₂ materials such as MgSnN₂ (MTN) and ZnSnN₂ (ZTN) are being investigated for their potential as absorber layers in solar cells [201,202]. Both materials are nitrides that belong to the wurtzite crystal structure and are composed of earth-abundant elements [203]. MTN and ZTN have bandgap energies in the visible range, making them well-suited for solar absorption [203]. They also have high carrier mobility, which is important for efficient charge transport in solar cells [203]. However, there are still challenges to the manufacture of these materials. For instance, to create high-quality thin films of MTN and ZTN high-temperature growth techniques such as molecular beam epitaxy or metalorganic chemical vapor deposition are used, which can be expensive and difficult to scale up for large-scale production. Moreover, the properties of the films can be sensitive to the growth conditions, leading to variations in their electronic and optical properties. Another challenge is the development of suitable heterojunctions and contacts for efficient charge collection in solar cells based on MTN and ZTN. Several studies have explored the use of different materials, such as graphene or molybdenum disulfide, as contacts for MTN and ZTN solar cells, but further research is needed to optimize their performance. Efficiencies of ZTN solar cells reach 1.5% [204], which is still low compared to conventional solar cells.

Antimony chalcogenides (Sb₂X₃, where X is a chalcogen element such as sulfur, selenium, or tellurium) have recently gained attention as potential materials for solar cell applications due to their advantageous optoelectronic properties such as high absorption coefficients and suitable bandgap energies for solar energy conversion [205]. One advantage of using antimony chalcogenides is that they are relatively abundant and less toxic compared to other chalcogenide materials such as cadmium telluride (CdTe) [206]. Additionally, antimony chalcogenides have shown promising photovoltaic efficiencies, with reported values ranging from 2% to 10.7% for various device structures [207,208]. However, challenges remain for antimony chalcogenide-based solar cells, including issues related to material stability and durability. For example, Sb₂Se₃ is prone to degradation when exposed to moisture and air, which can limit its practical application. Furthermore, antimony chalcogenides have relatively low carrier mobilities, which can affect device performance [209,210].

2.3. Halide Perovskite Based Solar Cells

2.3.1. Pb-Based Perovskites Active Layers

Since Miyasaka and coworkers integrated HPs as promising light harvesters to provide a step forward in a new generation of solar cells in 2009 [211], these materials have been consolidated as the most prominent semiconductors for photovoltaics. Typically, HPs show a 3D structure based on ABX₃ formula (A = Cs⁺, cesium; MA⁺, methylammonium; FA⁺, formamidinium; B = Pb²⁺, Sn²⁺; X = Cl⁻, Br⁻, I⁻, or combinations), where the composition and particle size engineering can promote the preparation of HPs with diverse photophysical, chemical, and electronic properties [212,213,214]. Due to the fact that a solar device should collect as much incident illumination as possible in a broad range of the energy spectrum to reach an efficient light-to-current transformation, low-band gap HPs are ideal for this process. Therefore, HPs with high iodide content are in the focus of many research groups, obtaining different strategies to reach a suitable active layer, compatible and stable with the selective contacts of the solar cell, to facilitate the carrier mobility through the device [215,216]. Although an acceleration in the number of publications have been reported to show current alternatives to prepare adequate HPs active layers (open the door to an extensive number of reviews), here, we will highlight some of these contributions where the modification of the perovskite layer has been vital to increasing the performance of the solar devices.

Similar to the generations of solar devices shown above, PSCs are composed by an electron transporter layer (ETL) and hole transporter layer (HTL), which favor the carrier transport through the device [217]. Moreover, a transparent conducting glass as fluorine-doped tin oxide (FTO) is used to promote the carrier mobility and allow the direct illumination of the active layer, while on top of the PSC architecture, metals such as silver or gold are deposited [218]. As we previously mentioned, iodide HPs are the most relevant photomaterials to achieve a high photoconversion efficiency (PCE) in PSCs, especially perovskites based on FAPbI₃. This material presents a band gap around 1.4 eV, indicating a broad absorption capability of solar energy in the UV-vis-NIR spectrum [219,220,221]. This band gap value also corresponds to a theoretical PCE ~32.3%, near to the Shockley–Queisser limit of efficiency [222]. Unfortunately, this feature is achieved for the photoactive phase of the FAPbI₃, denominated as black α-phase, which is stable at high temperatures, but is eventually transformed into the yellow δ-phase at room conditions; see Figure 10a [223]. To suppress the fast α-to-δ phase conversion into this kind of perovskite, Meng and coworkers [224] have prepared bulk films based on FA_0.94MA_0.06Pb(I_0.94,Br_0.06)₃, which have been coupled with carbon electrodes on top, in a n-i-p device configuration, to accelerate the carrier separation and transport into the fabricated solar cell. Here, bromide anions were incorporated into the perovskite to provide better stability to the material since iodide species are more labile, being rapidly diffused from the perovskite lattice to create structural defects. However, the presence of Br also offers the enlargement of the band gap, hampering the sunlight absorption capability of the perovskite. In this context, a high FA content ~94% was introduced into the material, to guarantee a narrow band gap. Attending to the final device, the PCE was estimated to be 20.04% at 1 sun illumination. At this point, the 94% of the initial performance of the solar cell was maintained for 1000 h.

On the other hand, Kim and coworkers [225] have deeply investigated the impact of the addition of MACl as an additive for the stabilization of FAPbI₃ to obtain highly-performance solar cells. Here, they demonstrated that the α-to-δ phase transition is promoted by the weak interaction between FA⁺ cations and the iodide species from the [PbI₆]⁴⁻ octahedra building blocks, which is one of the key factors to trigger the octahedra tilting [226,227,228]. Thus, the presence of Cl provides a better p orbital localization from I, improving the FA-I interactions beyond compensating halide vacancies into the perovskite structure. Then, MA⁺ cations also facilitate the perovskite stabilization, causing a contraction of the cubo-octahedra sites in the material. MA⁺ shows a higher dipolar moment than that of FA cations (~10 times), strengthening the volume shrinkage, making closer a direct interaction between FA and I. Therefore, authors have estimated a PCE ~24% (under 1 sun) in the devices containing the mixed cation FA/MA-perovskites, keeping 90% of their initial performance after 1200 h of operation. In this context, it is well known that the lattice distortion of the perovskites negatively impacts the net performance of the PSCs, favoring the emergence of non-radiative recombination traps, hindering the carrier transport into the fabricated device [227,229]. Thus, it is suitable to decrease the level of local strain into the perovskite structure to increase the PCE and thereby, extend the device stability. In these studies, Seok and coworkers [230] have proposed the combination of A-site cations to replace some FA⁺ species from the FAPbI₃ and delay the phase conversion of its photoactive black α-phase. Here, a dual substitution of FA⁺ by larger methylenediammonium (MDA²⁺) and small Cs⁺ cation in a same molar ratio (0.03 mol fraction) relaxes the lattice strain into the perovskite (denominated as (FAPbI₃)_1−x(MC)_x) also contributing to suppress the carrier traps and improving the radiative recombination pathway in the photomaterial. At this point, a maximum PCE ~25.2% was achieved in fabricated PSCs under 1 sun illumination; see Figure 10b, keeping 80% of their initial efficiency for 1300 h, at 85 °C; see Figure 10c. In addition, Seok’s research group have introduced some alkylammonium chlorides (RACl) as the case of propylammonium and butylammonium chloride to improve the crystallinity and compensate for structural defects contained into the FAPbI₃ based HPs [42]. Here, a maximum PCE of 26.08%, retaining over 88% of the initial efficiency for 600 h under operation.

To date, the octahedral tilting is induced by the loss of halide species such as iodide anions during the perovskite film preparation [223,231,232], generating a high density of halide vacancies (thereby, high density of carrier traps is produced), leading to the degradation of the photophysical and electronic features of the material. More specifically, the I⁻ anions released from the perovskite structure are easily oxidized to obtain I⁰, which is the main species to trigger chemical chain reactions and accelerate the quenching of the intrinsic properties of the active layer [232]. For this purpose, different organic agents have been introduced to avoid the formation of halide vacancies and strengthen the Pb-I bonds in the octahedra building blocks, as in the case of caffeine [233], theobromine [234], alkylammonium halides [234], phosphonopropionic acid [235] and some amino-based organic ligands [236]. Although some of these organic agents show a high binding capability to the [PbI₆]⁴⁻ octahedra units, avoiding the halide deficiency, they also exhibit fast decomposition under heating or can acts as barrier layers (due to their long carbon chains). This fact hinders the carrier extraction from the perovskite, hampering the net performance of the solar device. Therefore, with the purpose of reducing the iodide migration and thereby inhibit the formation of halide defect sites in the perovskite layer, Zhao and coworkers [237] have added 3-amidinopyridine (3AP) during the material preparation, inducing the coordination between amidino moieties and the Pb-I frameworks. In this scenario, a maximum PCE of 25.3% was obtained in optimized PSCs under 1 sun irradiation, preserving ~92% of the initial efficiency for 5000 h in ambient air.

Other alternatives to stabilize the perovskite active layer have been focused on the direct incorporation of large alkylammonium species to react with the 3D HPs to form multidimensional 2D/3D systems with longer durability [236,238,239]. Considering that big cations do not match with the Goldschmidt tolerance factor, breaking the 3D preferential crystalline structure of the perovskite, the [PbX₆]⁴⁻ octahedra in form of stacking layers are separated by these of organic agents [240,241]; see Figure 10d. The 2D perovskites are environmentally stable and promote Vander Waals interactions with the Pb-X frameworks. However, these materials exhibit a large band gap and poor carrier transport capability, decreasing the PCE in modified PSCs. In this way, it is favorable the mixture of 2D/3D systems to facilitate the separation/mobility of electrons through the device. Grancini and coworkers [242] have proposed the incorporation of 2-thiophenemethylammonium halides, 2-TMAX (X = Cl, Br and I) during the preparation of triple-cation [(FAPbI₃)_0.87(MAPbBr₃)_0.13]_0.92(CsPbI₃)_0.08 perovskite, where the thiophene organic species react with the photomaterial to create the 2D layer. By adding 2-TMABr and 2-TMAI agents, a suitable band structure (valence band shift) in between the 2D-3D perovskites is mediated, favoring the hole extraction into the heterojunction, contrary to 2-TMACl, where the carrier diffusion was hampered. Accordingly, PSCs with a maximum PCE of 20.8% (under 1 sun) were fabricated, keeping 74% of their initial performance for 1000 h of operation without any encapsulation. Another example is mentioned by Zhou and coworkers [241], where the 2-thiophenemethylammonium (ThMA) spacer was also added during the preparation of FA/MA based 3D perovskite. Here perovskite crystals were orientally grown, inducing the passivation of a high density of carrier traps to avoid the non-radiative recombination losses. Here, devices provided high PCE up to 21.5%; see Figure 10e, preserving 99% of its initial performance after 1680 h ambient air; see Figure 10f.

Figure 10. (a) Typical phase transformation of the FAPbI₃ crystalline structure. The δ–to–α phase transition is promoted at high temperature. Reproduced with permission [223]. Copyright 2020. American Chemical Society. (b) Performance and (c) long-term stability measurements of the (FAPbI₃)_1−x(MC)_x (x = 0.03 mol%) and control–based PSCs (uncapsulated devices). Reproduced with permission [230]. Copyright 2020. Science AAAS. (d) Schematic of the proposed 2D/3D FA/MA perovskite film where ThMA spacer is incorporated between the [PbI₆]⁴⁻ octahedra layers, which is perpendicular to the substrate. (e) Performance and (f) stability test of the control and 2D/3D FA/MA–based PSC. Reproduced with permission [241]. Copyright 2019. John Wiley & Sons, Inc.

Figure 10. (a) Typical phase transformation of the FAPbI₃ crystalline structure. The δ–to–α phase transition is promoted at high temperature. Reproduced with permission [223]. Copyright 2020. American Chemical Society. (b) Performance and (c) long-term stability measurements of the (FAPbI₃)_1−x(MC)_x (x = 0.03 mol%) and control–based PSCs (uncapsulated devices). Reproduced with permission [230]. Copyright 2020. Science AAAS. (d) Schematic of the proposed 2D/3D FA/MA perovskite film where ThMA spacer is incorporated between the [PbI₆]⁴⁻ octahedra layers, which is perpendicular to the substrate. (e) Performance and (f) stability test of the control and 2D/3D FA/MA–based PSC. Reproduced with permission [241]. Copyright 2019. John Wiley & Sons, Inc.

Then, a mixture of organic spacers such as iso-butylammonium iodide (i-BAI) and FA-iodide (FAI) reported by Cho and coworkers [243] suppressed the interfacial carrier traps and thereby reduced the interfacial energy barrier to avoid carrier diffusion. A PCE as high as 21.7% was obtained for the optimized solar device, keeping 87% of the original efficiency after 912 h with a relative humidity of 75%.

To maximize the absorption ability and electronic features of the bulk HPs, these materials have also been integrated with other types of solar devices such as Si, copper indium gallium selenide, different PSCs, DSSCs and QDs-based solar cells to fabricate tandem devices, as shown above [192]. Some works have reached near to 30%, as the case of the contribution reported by Albrecht and coworkers [244], where a monolithic perovskite tandem solar cell (PTSC) was fabricated. Here, a wide-band gap Cs_0.05(FA_0.77MA_0.23)_0.95Pb(I_0.77Br_0.23)₃ bulk film was integrated with a Si-based solar device, also incorporating diverse hole transporter layers such as C₆₀, 2PACz and Me-4PACz, to accelerate the carrier extraction. A maximum of PCE ~29% was achieved for the optimized PTSC, retaining the 95% of the initial performance after 300 h under operation. Interestingly, De wolf and coworkers [245] have incorporated MgF₂ as an interlayer between the perovskite based on triple cation Cs_0.05FA_0.8MA_0.15Pb(I_0.755Br_0.255)₃ and C₆₀ to improve the electron extraction (better adjustment of the relative energy positions in the perovskite) and separate the C₆₀ from the active layer surface to restrain the non-radiative recombination dynamics. Accordingly, a stabilized PCE of 29.3% was achieved, where the optimized PTSC device retained ~95% of its initial performance after 1000 h of operation, even carrying out damp-heat testing at 85 °C with 85% relative humidity. Recently, this research group provided a new record of the PCE ~33.2% using a similar perovskite layer on top of the cell, while a textured Si solar device is in the bottom of the configuration.

Even though the surface passivation with interesting alkylammonium ligands and the incorporation of 2D/3D heterostructures enhance the operational performance of the solar devices, it is reasonable that some differences in the PCE values are obtained. So, the highest PCE value is reached by filling/replacing structural defects in the active layer, improving its optical and electronic properties, suppressing carrier traps, and facilitating carrier transport. Although 2D/3D assembly can also achieve defect passivation in the perovskite, the appearance of a heterostructure can promote the emergence of interfacial energy levels where the carriers can be trapped, hampering the development of a higher device efficiency.

2.3.2. Partially Pb-Substituted/Pb-Free Perovskite Active Layers

As we have highlighted above, a series of approaches are prominent to reach long-term stability and suitable electronic properties in perovskites to favor carrier separation/extraction and mobility through the fabricated device. However, it is also known that one of the main drawbacks of the PSCs is the inherent toxicity of Pb, which hampers the future commercialization of this technology [246,247]. Therefore, the fact of substituting Pb partially or completely with less toxic metals, preserve/increase the final properties of the active layer, and thereby, reaching high performance in the solar device, is still a huge challenge. One of the most attractive metals to replace Pb is Sn, which also generate [SnX₆]⁴⁻ octahedra units to generate multidimensional HPs structures as the case of Pb and provide HPs lower band gap than that of Pb-analogous [248]. Nevertheless, Sn-HPs exhibit a poor stability consequence of the fast Sn²⁺-to-Sn⁴⁺ oxidation under ambient conditions, degrading the net efficiency in solar devices [249,250]. Hence, we will focus on the fabrication of PSCs based on a mixture of Sn/Pb- or Sn-HPs introducing novel strategies to avoid Sn oxidation in the active layer and thereby increase the performance of Sn-PSCs.

Firstly, for Sn/Pb-HPs systems, Wang and coworkers [251] have studied the addition of antioxidants such as tea polyphenol (TP) during the preparation of optimized CsPb_0.5Sn_0.5I₂Br bulk film to avoid the rapid Sn oxidation. Through the coordination bonds between high Lewis acidity Sn sites and hydroxybenzene groups, the Sn⁴⁺/Sn²⁺ ratio into the HP was decreased from 68.1% to 5.85%, deducing that TP is a good reducing agent. In addition, this antioxidant mediated an improved HP crystallization, making that at 0.5 wt% TP, a homogeneous film with a low content of pinholes is obtained. Therefore, low density of grain boundaries appears to hinder the carrier extraction. A maximum PCE of 8.1% was reached under 1 sun illumination, retaining 95% of original performance after ~1440 h into a N₂-filled glove box. Meanwhile, Guo and coworkers [252] have used the mixture of 2D/3D HPs in order to prepare suitable heterostructures for PSCs. Here, authors have introduced bulky alkylammonium based on 2-(4-fluorophenyl)ethylammonium iodide (FPEAI) during the preparation of (MAPbI₃)_0.75(FASnI₃)_0.25 films. In presence of the organic cation, highly oriented crystals are grown, and phase segregation occurring as the result of the Sn oxidation (emerging two distinguished signals from MAPbI₃ and FASnI₃ HPs), is suppressed. This fact makes that the recombination lifetime is longer than that of the absence of the organic molecules, associated with the reduction of high density of trap states. In context, a high PCE of 17.51% was reached (Figure 11a), keeping 90% of the initial performance after 1200 h into a glove box (Figure 11b), and 70% of initial efficiency close to 400 h under ambient air; see Figure 11c.

Then, Xu and coworkers [253] employed the surface passivation concept to prolong the stability of the FA_0.7MA_0.3Pb_0.7Sn_0.3I₃ HPs and remove structural defects from the material using guanidinium bromide (GABr). Considering that GA⁺ cations provide more charge density than FA⁺ and MA⁺ cations, this could be one of the main explanations to restrain the fast Sn²⁺-to-Sn⁴⁺ transformation. In addition, iodide vacancies are formed by the depletion of Sn²⁺, creating high content of halide vacancies into the perovskite. Thus, Br⁻ domains fill/replace these empty states to enhance the final properties of the prepared active layer. The optimized device has reached a maximum PCE of 20.63% without any encapsulation; see Figure 11d, conserving over 85% of the original efficiency after 1000 h under ambient air; see Figure 11e, and 80% of the original efficiency after 24 h of operation at 80 °C; see Figure 11f. Lastly, Huang and coworkers [245] have demonstrated that the addition of an alkylammonium pseudo-halide additive based on octylammonium tetrafluoroborate (OA⁺BF₄⁻) for the preparation of MA-free Sn/Pb HPs. While OA⁺ cations can fill some A-site cations into the HPs, BF₄⁻ species compensate the iodide vacancies generated by the Sn oxidation, also reducing the likelihood of emerging interstitial iodide and iodine. The suppression of defect density, main factor to promote carrier trapping, facilitates the fabrication of PSCs with PCE up to 23.7% (a record for Sn/Pb-PSCs), keeping over 88% of the initial performance after 1000 h of continuous operation at 50 °C in air and tracking the device under MPP.

For the case of the stabilization of Sn-based PSCs, some examples can be addressed. Mora-Seró and coworkers [254] have made use of the chemical engineering strategy, where Dipropylammonium iodide (DipI) and sodium borohydride (NaBH₄) (known as a good reducing agent) have been added to the preparation of FASnI₃ films and prevent the Sn oxidation. While DipI can block the loss of iodine in form of I₂, NaBH₄ reduce the released I₂ to produce iodide into the HPs, avoiding the halide migration. Therefore, the Sn²⁺/Sn⁴⁺ and I⁻/I₂ ratios are increased, inhibiting the formation of interstitial iodine/iodide vacancies, which are the main energy states where the hole/electrons can be trapped. In this alternative for stabilizing Sn, optimized devices boost a high PCE of 10.61%; see Figure 12a, providing up to 96% of the initial performance after 1300 h of continuous operation at MPP in N₂ condition; see Figure 12b,c. Another approach to stabilizing the black phase of the FASnI₃, Mi and coworkers [255] have introduced trimethylthiourea (3T) during the spin-coating process for HPs preparation. The presence of this bifunctional ligand induces coordination bonds with Sn²⁺ cations through the formation of Sn-S species. Simultaneously, N-H moiety from the organic agent produces hydrogen bond iodide species from the octahedra building blocks to avoid their diffusion out from the HPs lattice. Due to these effects, the carrier recombination is prolonged in comparison to the pristine HPs, hindering the emergence of carrier traps. In this context, PSCs were able to develop a maximum PCE up to 14%, maintaining 100% of the original efficiency for more than 700 h, under N₂ atmosphere. On the other hand, He and coworkers [256] have included 4-fluoro-phenethylammonium bromide (FPEABr) into the precursor solution to prepare 2D/3D Sn-HPs with low density of structural defects. FPEA cations are incorporated into the perovskite surface, while F- and Br-anions fill/replace halide vacancies. In terms of the device architecture, the 2D Sn-perovskite acts as an interface layer between the active layer and PEDOT:PSS hole transporter layer, restraining the iodide migration into the device. In presence of 10 wt% FPEABr, encapsulated 2D/3D PSCs generate a PCE up to 14.81%; see Figure 12d, exhibiting over 80% initial performance after 432 h under operation at room temperature (Figure 12e), and 60% of initial efficiency after 1 h at 80 °C (Figure 12f), both kinds of tests under ambient air.

Table 6. Summary of photovoltaic parameters of the optimized devices based on Pb-, Sn/Pb-, Sn-perovskite films.

Pb-Based Perovskite Films
Active Layer	Jsc (mAcm−2)	Voc (V)	FF	PCE (%)	Stability	Ref.
FA0.94MA0.06Pb(I0.94,Br0.06)3	23.60	1.11	0.765	20.04	94% of initial performance, 1000 h, air	[224]
FAPbI3–40 wt% MACl	25.92	1.13	0.820	24.02	90% of initial performance, 1200 h, air	[225]
(FAPbI3)1−x(MC)x, x = 0.03 mol%	26.23	1.17	0.822	25.20	80% of initial performance, 1300 h, 85 °C, air	[230]
FAPbI3–10 mol% RACl	25.69	1.18	0.862	26.08	88% of initial performance, 600 h, air	[42]
FAPbI3–4 mol% 3AP	26.04	1.18	0.822	25.30	92% of initial efficiency, 5000 h, air	[237]
2-TMABr–modified (FAPbI3)0.87(MAPbBr3)0.13]0.92(CsPbI3)0.08	23.20	1.15	0.780	20.80	74% of initial efficiency, 1000 h, air	[242]
ThMA–modified 2D/3D FA/MA perovskite	22.88	1.16	0.810	21.49	99% of its initial performance, 1680 h, air	[241]
i-BAI/FAI–modified (FAPbI3)0.85(MAPbBr3)0.15	~23	~1.18	~0.825	21.70	87% of the original efficiency, 912 h, air	[243]
Cs0.05(FA0.77MA0.23)0.95Pb(I0.77Br0.23)3/Si PTSC	19.26	1.90	0.795	29.05	95% of the initial performance, 300 h, air	[244]
MgFx–modified Cs0.05FA0.8MA0.15Pb(I0.755Br0.255)3	20.58	1.92	0.807	29.30	95% of the initial performance, 1000 h, 85 °C, air	[245]
Partially Pb-Substituted/Pb-Free Perovskite Films
Perovskite Layer	Jsc (mAcm−2)	Voc (V)	FF	PCE (%)	Stability	Ref.
TP–modified CsPb0.5Sn0.5I2Br	20.10	0.62	0.650	8.10	95% of original performance, ~1440 h, N2–atmosphere	[251]
(MAPbI3)0.75(FASnI3)0.25–3.75 wt% FPEAI	28.42	0.79	0.780	17.51	90% of the initial performance, 1200 h N2–atmosphere, 70% of initial efficiency, 400 h, air.	[252]
FA0.7MA0.3Pb0.7Sn0.3I3–12 wt% GABr	26.61	1.02	0.760	20.63	85% of original efficiency, 1000 h, air 80% of original efficiency, 24 h, 80 °C, air	[253]
OABF4–modified Cs0.2FA0.8Pb0.5Sn0.5I3	33.80	0.88	0.800	23.70	88% of initial performance, 1000 h, continuous operation at 50 °C, air, tracking under MPP	[245]
DipI/NaBH4–modified FASnI3	22.13	0.65	0.731	10.61	96% of initial performance, 1300 h, continuous operation at MPP, N2-atmosphere	[254]
3T–modified FASnI3	20.50	0.91	0.757	14.06	100% of original efficiency, >700 h, N2–atmosphere	[255]
FPEABr–modified 2D/3D FASnI3 (10 mol%)	24.91	0.84	0.708	14.81	80% of initial performance, 432 h, continuous operation, air 60% of initial efficiency, 1 h at 80 °C, air	[256]

shows the comparison in photovoltaic performances of the above perovskite-based solar devices, providing main parameters, such as J_sc, V_oc, and FF, to estimate the corresponding PCE. It is evident that PSCs fabricated with Pb-based HP films are more stable and still offer a high performance, mainly associated to hampered oxidation condition of Pb itself. However, even though the net efficiency offered by Sn/Pb- or Sn-PSCs are lower compared to their Pb-analogous, diverse alternatives are presented to improve their long-stability in room conditions (inhibiting the fast Sn oxidation), which is pivotal to finding the suitable conditions to increase the PCE in future Pb-free PSC technologies.

After describing the main characteristics of each kind of active layer used to fabricate the corresponding solar devices, some advantages and disadvantages of the related systems can be compared. Even aromatic moieties can promote carrier mobility in the organic sensitizers; carrier extraction is facilitated by the preparation of bulk heterojunctions, where an organic carrier acceptor and donor are assembled. Nevertheless, the formation of these heterostructures produce inhomogeneous morphology of the active layer, being of the critical reasons to favor the carrier recombination and the decrease in the PCE of the OSCs. In this line, chalcogenides were then integrated for the fabrication of more innovative solar devices, where the carrier injection depends on the relative energy positions of these kind of sensitizers, which can be modulated through material composition or particle size. Unfortunately, chalcogenides are prone to photocorrosion, damaging their optical and electronic properties. Under this view, it seems that halide perovskites are the unique and most prominent option to be incorporated as active layer for solar cells, taking into account their high absorption coefficient, better electronic properties and versatile surface chemistry. However, some reports and review articles disagree with the former qualification, attending to the fact that the ionic structure of perovskites is highly labile, generating high density of defect sides, which decreases the stability of the photomaterial. Therefore, one interesting question emerges at this point: what is the suitable active layer to produce a solar device with high light-harvesting capability, efficient PCE with long-term stability? According to the results exposed through the different operational parameters of each generation of solar cells (Table 1, Table 2, Table 3, Table 4, Table 5 and Table 6), the tandem systems are the most promising devices to have an important impact in photovoltaics, covering the absorption of the light in a broad range of the energy spectrum (UV-vis-NIR) by integrating wide- and narrow-band gap active layers (from diverse generation of solar cells), improving carrier generation/separation and diffusion. At this scenario, these types of devices offer one step forward the commercialization of solar technologies and are competitive against the well-established Si-based solar cells.

3. Concluding Remarks

This review shows the different generations of solar cells, considering the most important advances in the preparation of efficient active layers (organic, chalcogenide and halide perovskites) with improved capability to harvest most of the solar energy spectrum and promote the photocarrier transport into the solar device. As highlighted first, dye sensitizers were the first low-cost active layers to be incorporated with the purpose of replacing conventional and expensive silicon-based solar cells, taking advantage of their good absorption coefficient and facile carrier extraction. However, most of the organic materials show optical properties at specific wavelengths, hindering the photocarriers generation and lowering the PCE values. Therefore, organic molecules or species based on phthalocyanine and perylene derivatives have gained growing interest and are being integrated into novel OSCs. This fact provides the opportunity to achieve a better light harvesting performance and increase the density of carriers flowing into the device. Then, highly conductive materials such as fullerenes, and the concept of “bulk heterojunction” have been taken advantage to enhance the electron diffusion into the solar cell component interfaces, facilitating the photocarrier separation. Unfortunately, the inhomogeneity in the morphology of the organic active layers and their relatively fast deterioration make the density of carrier traps increase, hindering the electrons reach the external circuit of the solar cell. Here, the best OSCs can achieve PCE of more than 20% recently, but low stability in the range of hours can be achieved.

To overcome the above issues, chalcogenide-based solar cells have emerged to expand the light absorption capability of the active layer in a broad range of the energy spectrum (with a higher absorption coefficient than organic sensitizers), also considering the multiple pathways to prepare suitable films for efficient device prototypes. In this way, by attending to the chemical composition of the metal chalcogenides such as CdTe, PbS or some of them with metal combinations to obtain CZTSe/CZTSSe derivatives, light harvesting in the IR range is unlocked to provide more photocarriers, and thereby, more photocurrent is developed. Moreover, p-type and n-type chalcogenides can be assembled to offer an adequate band structure with electron and hole transporting layers, promoting effective carrier injection and mobility through the solar cells. Accordingly, PCE values close to 25% can be reached, also enhancing the stability of the device for long time of operation, even under harsh conditions such as H₂O, O₂, and high temperatures. Nevertheless, some disadvantages, such as high resistance for carrier mobility, the appearance of high density of energy traps favoring the surface recombination, and the use of toxic elements such as Cd, S, Te among others, decrease the possibility of using chalcogenides for further improvements in device performance.

Attending to the latter, HPs active layers have emerged as the most prominent semiconductors, gaining attention in photovoltaics due to their exponential enhancement in light-to-electricity transformation capability is some couple of years. Because of their versatile surface chemistry and variable composition, intrinsic properties of perovskites such as band gap can be easily modifiable, in order to improve the material stability without modifying their photoactivity. In this context, diverse strategies to delay the α-to-δ phase transformation in iodide-HPs have been applied in order to decrease the density of structural defects (main reason to promote the octahedra titling and thereby favoring the phase transition), preserve the optical performance and restrain the carrier recombination dynamics. Mainly, bulky organic cations are introduced to prepare 2D/3D perovskite systems, where a suitable heterojunction is established to facilitate the carrier extraction. In addition, the introduction of alkylammonium halides is pivotal to suppress halide deficiency (carrier traps), contributing to the carrier diffusion to the external circuit of the cell, increasing the net efficiency of the solar cell. Therefore, PCE values higher than 25% have been reached, with long-term stability for hours in air ambient, being competitive with recent Si-based devices. However, like chalcogenides, inherent toxicity of HPs is found due to the presence of Pb, one of the main explanations to hinder their future commercialization. Therefore, partially Pb-substituted/Pb-free HPs have been prepared replacing Pb instead of Sn, including promising additives to prevent the progressive oxidation of the metal. Even the net performance of Sn-based HPs layers is not so high compared to their Pb-analogous, long-term stability under ambient air has been analyzed, being the first step to find the device parameters with the purpose to increase the operational efficiency. Attending to the fact that innovative strategies are being studied to produce a more stable photomaterial with good optical performance and stability, the use HPs have reached an exponential interest during recent years, establishing them as the most prominent active layers to fabricate solar cells. Unlike organic sensitizers and metal chalcogenides, the facile control of the surface chemistry of HPs, mainly surface passivation, make them versatile materials with improved intrinsic properties, even to be applied in other interesting research topics such as optoelectronics, solar-driven chemical reactions and so on.

From the perspective of developing solar devices with maximum photovoltaic performance, the combination of different generations of solar cells to fabricate tandem devices will be the most rentable/profitable option to maximize sunlight absorption, carrier photogeneration, and transport, due to the use of wide-band gap and narrow-band gap active layers, increasing the density of electrons to flow through the cell. At this point, matching the relative energy positions of the components into the device provides the suitable pathway for the carrier injection and mobility, avoiding the carrier trapping. On the other hand, the partial or total replacement of toxic metals, such as Cd of Pb from chalcogenides and perovskites, respectively, are recently one of the top research challenges to provide a step forward to future commercialization and being competitive with Si-solar cells. In this way, some ultrathin or flexible perovskite devices could be competitive with the recent state-of-the art. Without a doubt, more prominent works will be focused on this topic, preserving of increasing the optical performance, operational efficiency, and stability of innovative devices. This contribution gives an insight about the different active layers useful to continue future studies in photovoltaics and fabricate more efficient, less toxic, and stable devices with promising potential to compensate for energy consumption in the coming years.