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Review

Classification, Functions, Development and Outlook of Photoanode Block Layer for Dye-Sensitized Solar Cells

by
Youqing Wang
*,
Wenxuan Wu
and
Peiling Ren
Research Center for Semiconductor Materials and Devices, Shaanxi University of Science and Technology, Xi’an 710021, China
*
Author to whom correspondence should be addressed.
Inorganics 2025, 13(4), 103; https://doi.org/10.3390/inorganics13040103
Submission received: 25 February 2025 / Revised: 21 March 2025 / Accepted: 24 March 2025 / Published: 27 March 2025
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Abstract

:
The block layer situated between the active material and electrode in photoelectrochemical devices serves as a critical component for performance enhancement. Using dye-sensitized solar cells as a representative model, this review systematically examines the strategic positioning and material selection criteria of block layers following a concise discussion of their fundamental mechanisms. We categorize block layer architectures into three distinct configurations: single layer, doped layer, and multilayer structures. The electron generation and transport mechanisms to photoelectrodes are analyzed through structural design variations across these configurations. Through representative literature examples, we demonstrate the correlation between material properties and photoconversion efficiency, accompanied by comprehensive performance comparisons. In the single-layer section, we comparatively evaluate the merits and limitations of TiO2- and ZnO-based block layers. The doped layer discussion traces the evolutionary trajectory from single-dopant systems to co-doping strategies. For multilayer architectures, we elaborate on the flexibility of its functional regulation. Finally, we present a forward-looking perspective on the hot issues that need to be urgently addressed in photoelectrochemical device block layers.

1. Introduction

At present, the large amount of pollution and damage to the environment caused by the use of fossil energy has become a serious problem that needs to be addressed globally. The rational and effective use of existing energy sources, as well as the development of clean new energy sources, play a crucial role in industrial production and daily life [1]. Solar energy is an emerging renewable energy source that has the characteristics of wide distribution and large amount of resources. Rational use of solar energy is a key way to effectively solve the energy problem. Therefore, various ways of utilizing solar energy have been extensively explored [2,3], such as photoelectrochemical devices [4,5,6]. Common photoelectrochemical devices include dye-sensitized solar cells (DSSCs), photoelectrochemical detectors, and photocatalytic devices. The working mechanism of such devices is usually to use the barrier at the solid–liquid interface to separate the electron–hole pairs, so as to achieve the conversion of light energy to electric energy. How to improve the photoelectric conversion efficiency of photoelectrochemical devices is a long-term concern of researchers.
As a typical example, DSSCs are devices with a sandwich structure consisting of a photoanode, electrolyte, dye molecules, and a counter electrode [7]. It works as follows [8]: (1) After the sunlight irradiation on the device, the dye molecules transition from the ground state to the excited state; (2) the dye molecules in the excited state inject the electrons into the conduction band of the semiconductor; (3) the electrons diffuse from the conduction band of the semiconductor material to the conductive substrate, and then migrate through the external circuit to the counter electrode; (4) the dyes in the oxidized state are regenerated through redox mediation by the electrolyte; (5) the electrolyte in the oxidized state is reduced after accepting electrons to the electrode, thus completing a cycle. How to restrain these negative reactions and improve the collection efficiency of photogenerated electrons is of great importance to the device’s performance.
In DSSCs, the active materials need to be in sufficient contact with the electrolyte for more efficient charge exchange, while the porous structure, due to its large specific surface area, ensures this. However, while this reaction occurs, the electrolyte may pass through the porous active layer to reach the conductive electrode and exchange charge with it. At the same time, the photogenerated electrons that reach the conductive electrode may return to the electrolyte and recombine with the electrolyte [9]. This non-ideal electron reverse motion significantly reduces the conversion efficiency. The key to solving this problem is to add a dense block layer (as shown in Figure 1, also known as a compact layer or barrier layer) to inhibit the recombination process of electrons [10]. Ideally, a dense block layer should completely cover the conductive substrate surface, isolating the electrolyte from the electrode to suppress the easily occurring recombination reactions between electrons in the electrode and the electrolyte, while facilitating electron conduction from the active material to the electrode. In the current research on block layers, researchers have prepared block layers with many materials, for example, TiO2 [10,11,12], ZnO [13,14], SnO2 [15,16], WO3 [17], and perovskite [18]. The thickness of the block layer ranges from a few nanometers to hundreds of nanometers [19,20]. The methods for preparing the block layer include atomic layer deposition [21], sol-gel [22], magnetron sputtering [23], and so on. These semiconductor materials, when prepared as dense films, can effectively prevent electrons in the conductive electrode from reacting with the electrolyte through the active layer.
As shown in Figure 2, a block layer of different structures prepared using a range of materials play a variety of positive roles in photoelectrochemical devices. In recent years, researchers have conducted in-depth research on the application of a block layer from different aspects. The purpose of this review is to classify and summarize the current research on block layers of DSSCs’ photoanode, which mainly includes: (1) The block layer is divided into three types in terms of structure: single layer, doped layer, and multilayer, and the performance of the block layer is comprehensively evaluated, which is convenient to understand the performance parameters of the block layer prepared by different research methods and different materials reported in the literature; (2) The working principle of the different structure block layer used in DSSCs is summarized, and the role of the block layer in DSSCs is further analyzed; (3) The future research direction of the block layer is discussed, which has reference significance for the research of DSSCs and other novel photoelectrochemical devices. Further improving the photoelectric conversion efficiency is crucial for the application of DSSCs as third-generation solar cells, and we believe the design and optimization of the block layer will play a critical role in this process.

2. Role of the Block Layer

In order to improve the power conversion efficiency (PCE), researchers have explored and applied the principles of light scattering [29], semiconductor energy band modulation [23], surface plasma excitations [30], and so on to the research work of block layers. The block layer also evolved from the initial single layer of dense TiO2 [10] or ZnO [23] to the dense film doped with elements such as Nb [25], Eu [31], Cr [19], and the multilayer structural block layer by preparing a heterojunction structure such as TiO2/ZnO [26] and TiO2/Al2O3 [32] (as illustrated in Figure 3). With the continuous optimization of the structure, the functions of the block layer have gradually become diversified.

2.1. Single Block Layers

The single block layer can play the following roles [33,34]: (1) As described above, by adding a barrier layer between the electrode and the active substance can isolate the contact between the electrolyte and the electrode, and thus effectively inhibit the electron recombination between the two. (2) Acting as a transition layer to improve the adhesion of the active substance to the electrode. (3) Providing a light scattering process in the light propagation path, thereby enhancing the light absorption ability of the photoanode. (4) Acting as a protective layer in some special electrodes, such as metal nanowire electrodes, to enhance the mechanical, chemical, and thermal stability of the whole electrode. The choice of semiconductor materials as the block layer is the outcome of comprehensive considerations of factors such as conductivity, light transmission, band structure, light scattering, and the binding force between the block layer and the electrode. The following will present a comprehensive analysis in terms of conductivity, light transmission, and electrode protection.
(1) Regarding conductivity [13,35,36,37]. The first requirement to be realized as a block layer is to separate the electrolyte and the electrode, and to ensure that the photogenerated electrons can be successfully injected into the electrode through the block layer. Therefore, the block layer needs to have certain conductivity when the material is selected. When the block layer is thin enough, electrons can easily pass through the film due to quantum tunneling effects, and thus insulator materials like Al2O3 can also be used as block layers.
(2) Regarding optical properties [38,39,40]. The device requires a high transmittance of the block layer to ensure that as much light as possible reaches the active material, which requires the material to have a high transmittance and be thin and dense enough, and most of the materials with good light transmittance are semiconductors or insulators. On the other hand, researchers have also used thick block layers containing larger particles to achieve light scattering, which in turn increases the distance that light travels through the active material [41].
The analysis from the band structure requires the semiconductor material to have a suitable band gap width. Taking the most widely used single-layer semiconductor material TiO2 as an example, we note that its bandgap is about 3.2 eV, so there is a strong absorption of ultraviolet light below 400 nm, resulting in the TiO2 block layer only being used for visible light devices, but not for ultraviolet devices [42]. The energy band structure also affects the electron transport. The conduction band position of the material used for the block layer should be lower than the active material; in addition, the formation of a composite block layer with a type-II energy band structure is very effective for inhibiting electron recombination, which will be discussed in the multilayer block layer [43].
(3) Regarding electrode protection [44,45]. In order to achieve better conductivity, flexibility, or to realize a wider range of light transmission, electrodes based on metal nanowires or carbon materials are used to replace conventional FTO or ITO. In this case, the function of the block layer is no longer just to inhibit the electron recombination, but also to act as a protection for the electrodes, e.g., through enhanced mechanical stability, chemical corrosion inhibition, or improved thermal stability.
As shown in Figure 3a′ above, in the absence of a block layer, the charge generated by the DSSCs subjected to light appears as a recombination reaction between the conducting electrodes and the porous active layer, and the electrons that should have flowed to the outer circuit are re-associated with the electrolyte. After adding a block layer as shown in Figure 3b′, the recombination reaction appears to be reduced.

2.2. Doped Block Layers

In order to further improve the PCE through the block layer, researchers have tried to obtain block layer materials with better optical and conductive properties through doping. By doping in semiconductor materials, impurity energy levels (mostly N-type doping) appear inside the material, and the electrons at this impurity energy level will more easily transition from the impurity level to the bottom of the conduction band, improving the electron mobility of semiconductor materials. As shown in Figure 3c′, the mobility of photogenerated electrons in the block layer is improved while the photogenerated electron recombination is inhibited. This technique not only improves the performance of the original semiconductor block layer, but also opens up the possibility of using more materials as block layers. Some materials with excellent light transmission but insufficient electrical conductivity, which had been excluded, may be used for the block layer of DSSCs. Various doping techniques are well established and are not limited to a single element, but researchers also try to co-dope two different elements on the block layer to modify the performance parameters such as transmittance and light scattering of the block layer in addition to conductivity.

2.3. Multilayer Block Layers

Although the single layer and doped block layer isolate the electrolyte liquid from the electrode and effectively inhibit the recombination reaction between the electrode and the electrolyte, there are still a large number of electrons transported to the block layer that undergo backflow and recombine with the electrolyte. The researchers redesigned the structure of the block layer and built a multilayer block layer as shown in Figure 3d′. Its main working principle is to construct a type-II band alignment between two materials with different energy bands to realize unidirectional conduction of electrons, so that electrons can only flow from the electrolyte to the photoelectrode, which greatly inhibits the recombination reaction.

3. Recent Developments

3.1. Single Block Layers

Researchers have made many attempts to find the ideal materials for the block layer, including TiO2 [21,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60], ZnO [13,14,23,24,61], SnO2 [15], BaTiO3 [62], CuO [63], Eu [64], Ga2O3 [65], HfO2 [66], Al2O3 [67], Nb2O5 [68,69,70,71], NiO [72], WO3 [17], and Zn2SnO4 [73]. TiO2 and ZnO are mostly used as dye carriers in DSSCs due to their simple preparation, easy access to raw materials and high stability. In order to match the energy bands and to increase the bonding of the block layer films to the porous carriers, TiO2 and ZnO were chosen as the block layer materials at the beginning of the research. After repeated verification, TiO2 and ZnO were also proved to be the most effective barrier materials. In this review, we enumerate the classic works while describing their working mechanisms and comparing their advantages and disadvantages.
In 2008, Anthony et al. [46] found that adding a compact TiO2 block layer between the transparent conductive film and electrolytes effectively reduces charge recombination between the electrode and the electrolyte, which can greatly improve the PCE of the DSSCs (Figure 4a). After that, researchers have systematically investigated the electron transport mechanism in TiO2 block layers. Subsequently, Yang Yanbo et al. [74] confirmed that when TiO2 is used as the block layer, it has no effect on the open-circuit voltage (VOC) of the DSSCs, but has a significant effect on the short-circuit current density (JSC) and fill factor (FF), and this enhancement increased firstly and then decreased with an increase in film thickness (Figure 4b–d).
After adding a layer of the TiO2 block layer on the FTO photoelectrode of the DSSCs, no energy barrier will be formed between the FTO and the mesoporous TiO2, and the Fermi energy level of the mesoporous layer will not be changed, which is why the VOC will not change. The block layer works because the dense film isolates the electrolyte from diffusion and blocks the highly reactive electrons in the conducting electrode from exchanging charges with the oxidation state ions. Therefore, densification is the most important requirement for a block layer. How to ensure that the block layer is homogeneous, dense, and free of cracks on a macroscopic scale is a key concern of researchers.
With the maturation of TiO2 block layer technology, researchers now routinely implement this architecture as a performance-enhancing standard in DSSC fabrication. Due to the simplicity of the preparation, the easy availability of raw materials, and the low cost, TiO2 has become one of the most common choices for preparing a block layer.
As ZnO has similar energy band positions to TiO2, as well as higher electron mobility, stronger photocorrosion resistance, and more varied micro-nanostructures, researchers have also turned their attention to ZnO to examine the potential of its application in DSSCs. Although the PCE of ZnO-based DSSCs so far is generally lower than that of TiO2-based DSSCs under the same condition, due to the instability of ZnO in the acidic dye environment and the slow electron injection kinetics from dye to ZnO [24], it has some advantages in regulating the device voltage, as well as being better adapted in devices with ZnO as the active material.
In 2012, Guan et al. [24] referred to the method of preparing a dense ZnO block layer between the FTO and mesoporous TiO2 of TiO2-based DSSCs; a dense ZnO block layer was prepared on the ZnO-based DSSCs photoelectrode between the FTO and mesoporous ZnO by sol-gel method. The electrochemical impedance spectroscopy (EIS) confirms that the block layer has the effect of lowering the interfacial resistance of the device, inhibiting charge recombination, and improving VOC and JSC. It is also demonstrated by experimental tests that the ZnO block layer improves the PCE of the DSSCs by about 20% (Figure 5a,b). In 2020, Kouhestanian et al. [23] prepared a dense ZnO block layer on the FTO of TiO2-based DSSCs by magnetron sputtering. The deposition method was improved by changing the time of the magnetron sputtering to accurately control the film thickness of the final block layer. At the early stage of film formation, the block layer presents a slight enhancement to VOC and JSC, but soon with the increase in the thickness of the ZnO block layer, the VOC of the device remains unchanged, but the JSC exhibits a very fast decreasing trend. The reason for this phenomenon is explained by the fact that the gradual thickening of the ZnO layer with increasing sputtering time creates additional trap states, which impede the photogenerated electron transfer path from the electrolyte to the FTO (Figure 5c,d). Consistent with the previous conclusions, the thickness of the block layer is a very critical factor.
Whether it is TiO2-based DSSCs or ZnO-based DSSCs, the JSC of the DSSCs with the ZnO block layer always shows a tendency to increase firstly and then decrease with increasing film thickness, but the efficiency enhancement at the highest point is mostly inferior to that of the TiO2 block layer. Table 1 list the representative literature of a single-layer block layer in recent years.

3.2. Doped Block Layers

Single block layers based on materials such as TiO2 play a great role in improving the performance of DSSCs. Then, researchers tried to further enhance the PCE of the cells by doping other elements. The main objectives of doping the block layer include [36,79]: (1) Enhancing electron transport efficiency. Doping can improve the conductivity of block layers, reduce electron recombination, and thereby enhance electron transport efficiency. (2) Optimizing band structure. Doping adjusts the band structure of semiconductors to better align its energy levels (work function) with those of the active materials and electrodes, facilitating electron injection and transport. (3) Improving light absorption. Certain dopants can broaden the light absorption range of the semiconductor, partially enhancing the utilization of solar energy. (4) Enhancing material stability. Doping also improves the chemical and thermal stability of the block layer, extending the lifespan of the device.
Researchers have conducted a lot of exploration in the types of doped materials. The Al [80], F [35], Mn [31], La [31], Eu [31], Nb [25,81,82,83], N [27], I [27], Cr [19], Ni [84], Zn [84], and Sm3+/Y3+ [85] were attempted to dope in TiO2 block layers, the Eu3+/Tb3+ [86] and In [87] were attempted to dope in ZnO, or doped in Nb2O5 [88] with Ag as both block and plasmon-enhance layers, and doped SnO2 layer with K [89]. The following are some representative doped schemes for the block layer.
Koo et al. [25] have tried to prepare an Nb-doped TiO2 block layer by horizontal ultrasonic spray pyrolysis deposition (HUSPD) to improve the electrical conductivity of the TiO2 block layer. Through a series of controlled experiments, it was determined that the best performance of the block layer was obtained when the molar ratio of Nb-Ti in the sprayed solution was 6. The PCE was 7.5% when applied to DSSCs, which was a 22.3% enhancement compared to that of the non-blocked layer device (as shown in Figure 6a,b). In 2020, Lv et al. [27] attempted to improve the performance of a TiO2 block layer by N-I co-doping by sol-gel method. Compared with the no block layer (5.08%), the TiO2 block layer (5.77%), N-doped TiO2 block layer (6.05%), and I-doped TiO2 block layer (6.11%), it was found that the N-I co-doped TiO2 block layer has the highest efficiency (6.79%) and the PCE improvement was as high as 17.7% (as shown in Figure 6c,d).
Initially, researchers doped block layers to address certain limitations of TiO2 and ZnO, such as their poor conductivity, which resulted in hindered electron transport through thicker block layers. To date, research objectives have diversified, encompassing goals such as band structure modulation, light absorption regulation, and enhancing overall stability. As a result, there is a proliferation of doping and co-doping schemes in the block layer. Table 2 shows the representative literature on doped block layers in recent years.

3.3. Multilayer Block Layers

Multilayer block layers, particularly those based on type-II band-aligned heterojunction structures, have emerged as a research hotspot in recent years due to their superior performance [92]. These multilayer block layers typically consist of 2–3 thin films, forming heterojunctions [82,93], homojunctions [94], or metal-semiconductor junctions [95], and exhibit more versatile functionalities compared to single-layer counterparts.
(1) Enhancing charge separation efficiency and suppressed electron recombination. Heterojunctions composed of two distinct semiconductor materials (e.g., TiO2/SnO2 [96,97], TiO2/ZnO [26]) generate a built-in electric field at the interface due to band offset. This field accelerates directional transport of photogenerated electrons while inhibiting electron–hole recombination. The stepwise band structure of heterojunctions creates an “electron funneling effect”, driving rapid electron migration toward the electrode and reducing interfacial residence time [98].
(2) Optimizing energy-level alignment. By tuning the conduction band and valence band positions of heterojunction materials, the energy-level structure can be optimized to facilitate electron transport from active materials to the block layer and ultimately to the electrode.
(3) Improving light absorption. Multilayer films accommodate more scattering structures, altering the light propagation path to enhance light absorption. Incorporating noble metal nanoparticles (e.g., Au/TiO2 [95]) into heterojunctions amplifies interfacial light fields via surface plasmon resonance (SPR) effects, further boosting light absorption and charge generation efficiency.
(4) Enhancing electron transport kinetics. Integrating high electron mobility materials (e.g., SnO2 [99,100]) with TiO2 reduces the series resistance of the block layer, improving overall electron transport rates. Heterojunction interfaces passivate surface defect states in TiO2 (e.g., oxygen vacancies [80]), minimizing scattering losses during electron transport.
(5) Improving material stability. Introducing chemically robust materials (e.g., Al2O3 [101]) into heterojunctions protects the TiO2 block layer from electrolyte corrosion, extending the device’s lifespan. Differences in thermal expansion coefficients between heterojunction materials alleviate mechanical stress under high-temperature conditions.
Research on multilayer block layers predominantly employs TiO2 or ZnO as core materials, which are combined with other materials to form multilayer thin-film structures, such as TiO2/ZnO [26], ZnO/TiO2 [26,28,74,102], and TiO2/Al2O3 [32]. Below are some seminal studies on multilayer block layers in DSSCs.
In 2017, Marimuthu et al. [28] prepared a ZnO-TiO2 seed layer using spin-coating and sintering techniques, which also acts as a block layer. The precursor solution consisted of dehydrated zinc acetate and TiO2 powder as solutes, isopropyl alcohol as the solvent, and monoethanolamine as the stabilizer. ZnO nanowire arrays (NWAs) and ZnO nanoneedle arrays (NNAs) were subsequently grown on this seed layer by controlling the growth duration to leverage the diverse micro-nanostructures of ZnO. Experimental results confirmed that ZnO-NNAs exhibited superior UV light absorption compared to ZnO-NWAs. This enhanced performance was attributed to the more complex micro-nanostructures of ZnO-NNAs and the composite ZnO/TiO2/ZnO-NNAs block layer, which provided better suppression of recombination and light scattering properties. Consequently, dye-sensitized solar cells (DSSCs) incorporating ZnO-NNAs achieved a 61.5% improvement of PCE (as shown in Figure 7a,b).
Recent advances in multilayer block layers highlight their potential for overcoming conductivity limitations in DSSCs. In 2020, Zatirostami et al. [26] demonstrated that a 50 nm TiO2/ZnO double block layer fabricated by RF sputtering achieved a PCE of 7.1%, representing a 24.6% improvement over single-layer ZnO configuration (as shown in Figure 7c,d). This work underscores the importance of interfacial engineering and thickness control in optimizing charge transport and device efficiency.
In recent years, as research on DSSCs has advanced, the rate of PCE enhancement has gradually slowed. Consequently, researchers have shifted focus from traditional optimization domains—such as device architecture and dye molecular engineering, which historically drove significant PCE improvements—to less-explored components like the block layer, despite its relatively minor direct impact on efficiency. With advancements in material science and nanofabrication technologies, challenges associated with block layer thickness, which traditionally limited charge transport kinetics, are expected to be mitigated through precise control of multilayer structures.
Notably, multilayer block layers can achieve type-II band alignment, a configuration that outperforms single-layer and doped block layers in suppressing electron back-transfer from the photoelectrode to the electrolyte. This is attributed to the built-in interfacial electric field and stepwise energy gradients, which synergistically enhance charge separation and directional transport. Given these advantages, we propose that multilayer block layers represent an underexplored yet promising avenue for advancing DSSC performance, offering substantial potential to elevate both scientific understanding and technological relevance in photovoltaics research.

4. Summary and Outlook

4.1. Summary

This review systematically investigates the critical role of block layers in photoelectrochemical energy conversion devices, using DSSCs as a representative model. We present a comprehensive classification framework for block layer architectures, categorizing them into three distinct structural types: single layer, doped layer, and multilayer configurations. Each structural variant is analyzed through its unique operational mechanisms, including charge transport modulation, recombination suppression, and interfacial energy alignment. The paper further evaluates recent advancements in block layer engineering, highlighting material innovations and performance optimization strategies across different architectural paradigms. Through comparative analysis of these layered systems, we identify structure–property relationships that inform rational design principles for next-generation photoelectrochemical devices.
Single block layers are characterized by their simplicity in fabrication and low cost. They effectively suppress recombination between the electrode and electrolyte, serve as an interfacial transition layer to improve contact, and provide basic light scattering effects. Among the extensively studied single-layer materials, TiO2 demonstrates optimal performance, and its related research has laid the foundation for subsequent in-depth investigations.
To enhance the electrical conductivity of block layers, doping techniques have been introduced into the thin-film fabrication process, enabling researchers to develop doped block layers with superior properties. Beyond conductivity improvements, numerous studies have explored additional benefits, including band structure optimization, enhanced light transmittance, and improved stability.
Finally, multilayer block layers exhibit remarkable versatility in functional modulation. By integrating two to three materials with distinct stability, distinct band structures, distinct conductivity, and distinct optical properties, researchers have achieved exceptional results in efficient charge separation/transport and overall stability—performance unattainable with single-layer films. However, the fabrication of multilayer block layers involves relatively complex processes, particularly in precise thickness control. Additionally, as evidenced by numerous studies, the sol-gel method remains the most cost-effective approach for block layer preparation.

4.2. Outlook

Research on block layers in photoelectrochemical devices has been extensively explored, with well-established mechanisms for suppressing electron recombination and other functionalities. However, as a specialized component in these devices, we argue that significant opportunities remain for advancing block layer technologies.
(1)
Multilayer block layers with tailored band structures
Multilayer block layers offer complex structural design possibilities. By integrating doping techniques, ideal band structures with electron funneling effects can be achieved, simultaneously enhancing stability and light absorption. We propose that multilayer architectures will emerge as a focal point for future research.
(2)
Expanding applications to ultraviolet (UV) and infrared (IR) devices
Current block layer studies predominantly focus on visible-light devices like DSSCs. For UV or IR applications, existing designs may prove inadequate. For instance, TiO2 block layers exhibit strong UV absorption, which detrimentally impacts UV photodetectors. Alternative strategies, such as employing ultra-wide bandgap materials or bandgap tuning via doping, could address these limitations.
(3)
Adapting block layers for emerging electrode architectures
Conventional block layers are primarily designed for TCO-based (transparent conductive oxides) electrodes (e.g., FTO, ITO) on rigid glass substrates. For novel electrodes like metal nanowire or graphene-based systems, traditional block layers may be incompatible. Adjustments to fabrication processes, thickness, and band alignment are essential. Additionally, enhancing the chemical, mechanical, and thermal stability of block layers for these electrodes should be prioritized, as systematic studies in this area remain scarce.
(4)
Block layers for flexible photoelectrochemical devices
Flexibility has become a critical focus in DSSC research. For flexible electrodes, block layers must exhibit bend resistance and maintain robust interfacial adhesion with active materials. These requirements necessitate innovations in material selection and deposition techniques tailored to deformable substrates.

Funding

This work was financially supported by the National Natural Science Foundation of China (No. 61804092) and the Innovation Capability Support Program of Shaanxi (Program No. 2024CX-GXPT-21).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the basic operating principle of the DSSCs and the position of the block layer.
Figure 1. Schematic diagram of the basic operating principle of the DSSCs and the position of the block layer.
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Figure 2. Overview of the classification of block layers [23,24,25,26,27,28].
Figure 2. Overview of the classification of block layers [23,24,25,26,27,28].
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Figure 3. (a,a′) Schematic and cross-sectional view of the photoanode without block layer, (b,b′) with a single block layer, (c,c′) with doped block layer, and (d,d′) with multilayer block layer. The red arrows represent the conduction of electrons.
Figure 3. (a,a′) Schematic and cross-sectional view of the photoanode without block layer, (b,b′) with a single block layer, (c,c′) with doped block layer, and (d,d′) with multilayer block layer. The red arrows represent the conduction of electrons.
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Figure 4. (a) J–V curves of the DSSC with TiO2 block layer [46]; (b–d) SEM images, dark current curves and J–V curves of DSSC with TiO2 block layer [74]. All J–V tests were conducted under AM1.5 spectral conditions with an illumination intensity of 100 mW/cm2.
Figure 4. (a) J–V curves of the DSSC with TiO2 block layer [46]; (b–d) SEM images, dark current curves and J–V curves of DSSC with TiO2 block layer [74]. All J–V tests were conducted under AM1.5 spectral conditions with an illumination intensity of 100 mW/cm2.
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Figure 5. (a) Schematic structure and (b) J–V curves of DSSCs prepared by Jie Guan et al. [24]. (c) Band diagram and (d) J–V curves of DSSCs prepared by Elham Kouhestanian et al. [23]. All J–V tests were conducted under AM1.5 spectral conditions with an illumination intensity of 100 mW/cm2.
Figure 5. (a) Schematic structure and (b) J–V curves of DSSCs prepared by Jie Guan et al. [24]. (c) Band diagram and (d) J–V curves of DSSCs prepared by Elham Kouhestanian et al. [23]. All J–V tests were conducted under AM1.5 spectral conditions with an illumination intensity of 100 mW/cm2.
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Figure 6. (a,b) The structure diagram and photovoltaic performance of DSSCs based on Nb-doped TiO2 block layers [25]; (c,d) The morphology and photovoltaic performance of DSSCs based on N–I co-doped TiO2 block layers [27]. All J–V tests were conducted under AM1.5 spectral conditions with an illumination intensity of 100 mW/cm2.
Figure 6. (a,b) The structure diagram and photovoltaic performance of DSSCs based on Nb-doped TiO2 block layers [25]; (c,d) The morphology and photovoltaic performance of DSSCs based on N–I co-doped TiO2 block layers [27]. All J–V tests were conducted under AM1.5 spectral conditions with an illumination intensity of 100 mW/cm2.
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Figure 7. (a,b) Structure diagram and performance curve of DSSCs using the seed layer as block layer [28]; (c,d) The SEM image and performance of DSSCs based on the TiO2/ZnO double block layer [26]. All J–V tests were conducted under AM1.5 spectral conditions with an illumination intensity of 100 mW/cm2.
Figure 7. (a,b) Structure diagram and performance curve of DSSCs using the seed layer as block layer [28]; (c,d) The SEM image and performance of DSSCs based on the TiO2/ZnO double block layer [26]. All J–V tests were conducted under AM1.5 spectral conditions with an illumination intensity of 100 mW/cm2.
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Table 1. Performance comparison of DSSCs with and without a single block layer.
Table 1. Performance comparison of DSSCs with and without a single block layer.
MaterialPreparation MethodBlock Layer/
No Block Layer		Voc (V)Jsc(mA/cm−2)FFη (%)Ref.
TiO2RF-magnetron sputteredNBL0.6710.880.644.67[75]
BL0.6613.140.76.07
TiO2ALDNBL0.6984.910.625.15[20]
BL0.7175.870.586.18
TiO2Heat treatment-assisted electrospinningNBL0.525.950.561.73[76]
BL0.589.790.543.06
TiO2Spray pyrolysis techniqueNBL0.6764.870.581.9[77]
BL0.7198.160.583.39
TiO2RF-magnetron sputtered (TiO2)NBL0.6710.70.574.06[78]
BL0.6811.210.614.59
ZnODC-magnetron sputteredNBL0.669.720.664.25[23]
BL0.6810.830.75.12
ZnOVUV irradiationNBL0.72130.645.97[61]
BL0.8116.90.679.14
EuSol-gelNBL0.555.610.471.45[64]
BL0.547.150.461.77
NiOElectrodepositionNBL0.102-1.770.310.055[72]
BL0.151-3.080.460.166
TiO2/ZnOSol-gelNBL0.6958.480.663.86[22]
BL/ZnO0.7288.180.734.34
BL/TiO20.6979.320.674.36
TiO2/ZnOSol-gelNBL0.7889.130.684.93[74]
BL/ZnO0.7998.070.694.44
BL/TiO20.78610.660.736.16
ZnOSol-gelNBL0.5367.550.682.76[24]
BL/ZnO0.5638.550.703.34
Table 2. Performance comparison of DSSCs with doped block layers.
Table 2. Performance comparison of DSSCs with doped block layers.
MaterialPreparation MethodBlock Layer/
No Block Layer		Voc (V)Jsc(mA/cm−2)FFη (%)Ref.
YbF3-Eu3+-doped SnO2Spin-coating and sinteredNBL0.7516.080.67.24[90]
BL YbF3-Eu3+ co-doped SnO20.7817.150.618.16
Ag-doped Nb2O5RF-magnetron sputteredNBL0.7612.180.666.19[88]
BL Ag-doped Nb2O50.7817.330.699.24
Al-doped TiO2Sol-gelBL TiO20.71314.760.677.02[80]
BL Al-doped TiO20.70216.50.667.66
Mg-La-dop-
ed TiO2		Sol-gelBL TiO20.7912.50.667.17[31]
BL Mg-La co-doped TiO20.7813.10.657.42
Nb-doped TiO2HUSPDNBL0.7114.020.626.13[25]
BL TiO20.7615.110.617.16
BL Nb-doped TiO20.7416.90.67.5
N-I-doped TiO2Sol-gelNBL0.68711.480.645.08[27]
BL TiO20.6912.650.665.77
BL N-doped TiO20.71413.410.636.05
BL I-doped TiO20.73812.740.656.11
BL N-I co-doped TiO20.73414.230.656.79
Cr-doped TiO2Sol-gelNBL0.653.790.651.8[19]
BL0.688.520.673.92
Ni-Zn-doped TiO2Sol-gelBL TiO20.6451.0390.4250.47[84]
BL Ni-Zn co-doped TiO20.6941.4360.4590.76
Sm3+-Y3+-
doped TiO2		Sol-gelNBL0.7099.10.543.48[85]
BL TiO20.7179.670.513.52
BL Sm3+-Y3+ co-doped TiO20.72311.120.514.09
rGO-doped TiO2Sol-gelNBL0.6096.7920.62.49[91]
BL TiO20.626.9340.652.82
BL rGO-
doped TiO2		0.6512.130.645.08
Eu3+-Tb3+-
doped ZnO		Sol-gelBL TiO20.76.380.673.01[86]
BL ZnO/TiO20.748.130.563.34
BL Eu3+-Tb3+ co-doped 0.7610.130.675.13
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Wang, Y.; Wu, W.; Ren, P. Classification, Functions, Development and Outlook of Photoanode Block Layer for Dye-Sensitized Solar Cells. Inorganics 2025, 13, 103. https://doi.org/10.3390/inorganics13040103

AMA Style

Wang Y, Wu W, Ren P. Classification, Functions, Development and Outlook of Photoanode Block Layer for Dye-Sensitized Solar Cells. Inorganics. 2025; 13(4):103. https://doi.org/10.3390/inorganics13040103

Chicago/Turabian Style

Wang, Youqing, Wenxuan Wu, and Peiling Ren. 2025. "Classification, Functions, Development and Outlook of Photoanode Block Layer for Dye-Sensitized Solar Cells" Inorganics 13, no. 4: 103. https://doi.org/10.3390/inorganics13040103

APA Style

Wang, Y., Wu, W., & Ren, P. (2025). Classification, Functions, Development and Outlook of Photoanode Block Layer for Dye-Sensitized Solar Cells. Inorganics, 13(4), 103. https://doi.org/10.3390/inorganics13040103

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