TiO₂ Passivation Layer on ZnO Hollow Microspheres for Quantum Dots Sensitized Solar Cells with Improved Light Harvesting and Electron Collection

Abstract

Light harvesting and electron recombination are essential factors that influence photovoltaic performance of quantum dots sensitized solar cells (QDSSCs). ZnO hollow microspheres (HMS) as architectures in QDSSCs are beneficial in improving light scattering, facilitating the enhancement of light harvesting efficiency. However, this advantage is greatly weakened by defects located at the surface of ZnO HMS. Therefore, we prepared a composite hollow microsphere structure consisting of ZnO HMS coated by TiO₂ layer that is obtained by immersing ZnO HMS architectures in TiCl₄ aqueous solution. This TiO₂-passivated ZnO HMS architecture is designed to yield good light harvesting, reduced charge recombination, and longer electron lifetime. As a result, the power conversion efficiency (PCE) of QDSSC reaches to 3.16% with an optimal thickness of TiO₂ passivation layer, which is much higher when compared to 1.54% for QDSSC based on bare ZnO HMS.

1. Introduction

The collapse of current climate because of global warming and huge demand of energy have forced us to conduct extensive research on solar cells over the past decades. Tremendous progress has been achieved in solar cell technology because of the increasing research focusing on the development of nanotechnology and new materials [1,2,3,4]. During the research development of solar cells, quantum dots sensitized solar cells (QDSSCs) have received wide attention because of their several merits including low temperature fabrication, solution-based processing, and high stability in different surroundings [5,6,7]. QDSSCs comprise three main parts: photoanode, electrolyte, and counterelectrode [8]. The photoanode usually consists of fluorine-doped tin oxide (FTO) glass, mesoporous metal oxide semiconductor (MOS) film, and quantum dots (QDs) [9,10]. Among these components of photoanode, the MOS as supporting architecture plays an important role in light utilization and charge transfer process in QDSSCs. Hence, design and decoration of an appropriate MOS architecture has been considered as an effective approach to improve light harvesting and electron transport in QDSSCs [11], enhancing the photovoltaic performance.

Various MOS materials such as ZnO, TiO₂, and SnO₂ are suitable for application in QDSSCs [12,13,14]. Among them, ZnO is a potential candidate material for the photoanode because of its high electron mobility (200–1000 cm⁻² V⁻¹ S⁻¹) [15] and various fabrication strategies such as atomic layer deposition (ALD) and hydrothermal method, for variety of structures including 1D, 2D, and 3D [16,17,18]. The most common ZnO architecture in QDSSCs is 1D nanostructures such as nanowire, nanotube, and nanorod arrays [12,19,20], because they can provide direct pathway for electron transport to reduce the probability of charge recombination [21,22]. However, weak light scattering of 1D ZnO structure limits the further improvement of photovoltaic performance of QDSSCs. According to the Mie theory and Anderson localization of light [23], resonant scattering of light is expected for spherical particles only when the particle size is comparable to the wavelength of incident light. To tackle this challenge, MOS hollow microspheres (HMS) as photoanode architecture seem to be a good choice for strong light scattering because of size controllability and multiple light reflection inside of HMS [24]. Our group’s previous research found that ~500 nm ZnO HMS can generate strong light scattering, improving the power conversion efficiency (PCE) of Zn_xCd_1−xSe QDSSC [25].

Another factor that limits ZnO application as architecture in QDSSCs is that there are multiple surface defects in ZnO [26], which increase the charge recombination and result in the drop of short-circuit current (J_sc). Covering passivation layer on surface of photoanode is an effective way to lower the surface defects and suppress charge recombination [27,28]. For example, Haifeng Zhao et al. [29] modified ZnO nanorods with TiO₂ nanoparticles, and revealed that TiO₂ passivation layer can facilitate the deposition of CdS/CdSe QDs on TiO₂/ZnO surfaces, and effectively suppress the charge recombination. Lou et al. [30] reported a PCE of 1.97% for QDSSCs based on ZnO nanorods passivated with TiO₂ as a barrier layer. In view of these backgrounds, it can be found that ZnO surface modification with TiO₂ has attracted interest of researchers to enhance photovoltaic performance of QDSSCs, because of the combination of excellent electron mobility of ZnO and high chemical stability of TiO₂ [29].

In this investigation, we synthesized ZnO HMS as architectures for QDSSCs through carbonaceous template method, and further passivated surface of ZnO HMS with TiO₂ by a simple immersion route in TiCl₄ aqueous solution. The photovoltaic performance of CdSe/CdS QDSSCs has been improved by adjusting the thickness of TiO₂ passivation layer through optimization of immersing time in TiCl₄ aqueous solution. The coating of TiO₂ passivation layer not only enlarges the deposition of QDs on TiO₂-passivated ZnO HMS surfaces, but also reduces the charge recombination and prolong the electron lifetime, leading to the enhancement of light harvesting and charge collection efficiency. As a result, the PCE of QDSSCs increased from 1.54% for bare ZnO HMS to 3.16% for TiO₂-passivated ZnO HMS with an optimal thickness of passivation layer.

2. Materials and Methods

2.1. Materials

All the chemicals used in this work including sucrose, zinc nitrate, cadmium nitrate, selenium powder, sodium borohydride, ethylcellulose, terpinol, ethanol, titanium tetrachloride, copper sulphate, thiourea, sodium sulfide, and sulfur powder were obtained from Aladdin Co., Ltd. (Shanghai, China), and were analytical grade reagents without further purification. Other materials such as fluorine-doped tin oxide (FTO) conductive glass were purchased from Opvtech Co., Ltd. (Dalian, China).

2.2. Preparation Carbonaceous Spheres Templates

The carbonaceous spheres template was obtained by a classical hydrothermal route [31]. Generally, 1.5 M sucrose aqueous solution was transferred in to Teflon autoclave filling 80% volume, then sealed the autoclave and heated at 180 °C for 8 h. After cooling down to room temperature, the autoclave was opened and the black powder was collected. The black powder was washed three times with deionized water and dried in electric oven at 80 °C for 12 h, finally getting the carbonaceous spheres templates.

2.3. Synthesis of ZnO Hollow Microspheres (HMS) and Fabrication of ZnO HMS Photoanode

Total of 1.0 g newly prepared carbonaceous microspheres were soaked in 20 mL 1.0 M zinc nitrate aqueous solution, after 30 min ultrasonication, the mixture was aged for 12 h at room temperature. Then the mixture was filtered, washed three times with deionized water, and dried at 80 °C for 12 h. Subsequently, the resultant product was heated to 500 °C in Muffle furnace with rate a of 5 °C/min, and was held at this temperature for 4 h, producing white ZnO hollow microspheres (HMS) powders.

The ZnO HMS photoanode was fabricated according to the method proposed in our previous report [25]. Typically, ZnO HMS (3.0 g), ethylcellulose (0.5 g), terpinol (10 mL), and ethanol (3 mL), were mixed together under magnetic stirring for 1 h, forming a viscous paste. Then the paste was doctor-bladed onto conductive surface of FTO glass (2.0 × 1.5 cm). The thickness of the ZnO HMS film was controlled to be ~15 μm, and the active area of ZnO HMS film was tuned to be 0.25 cm² with aid of same size and thickness spacer. Subsequently, the film was dried in room temperature, and was heated to 500 °C for 1 h to remove any organic residuals.

2.4. Fabrication of TiO₂-Passivated ZnO Hollow Microspheres Photoanode

The TiO₂-passivated ZnO hollow microspheres photoanodes which are designated as TiO₂@ZnO HMS were fabricated via one-step approach. First, the prepared ZnO HMS photoanode was immersed in 1.5 M TiCl₄ aqueous solution for a period at room temperature to guarantee the Ti⁴⁺ precursors can be deposited on the surface of ZnO HMS. After rinsing in deionized water, TiO₂@ZnO HMS photoanodes were obtained by a further calcining process at 450 °C for 1 h. In order to investigate the thickness effect of TiO₂ passivation layer, the immersing time in TiCl₄ aqueous solution was adjusted to 5 min, 10 min, and 15 min (designated 5-min, 10-min, and 15-min).

2.5. Assemble of ZnS/CdSe/CdS Quantum Dots Sensitized Solar Cells

For in situ assembly of CdSe/CdS QDs, the classic successive ionic layer adsorption reaction (SILAR) method was employed. Initially, six SILAR cycles of CdS QDs were deposited on TiO₂@ZnO HMS photoanode by dipping photoanode in 0.1 M cadmium nitrate solution and 0.1 M sodium sulfide solution in sequence according to our previous work [32]. Then seven SILAR cycles of CdSe QDs were sensitized by dipping CdS/TiO₂@ZnO HMS photoanodes in 0.1 M cadmium nitrate solution and 0.1 NaSeH₄ solution in sequence to form CdSe/CdS QDs-sensitized systems. Finally, a ZnS passivation layer was formed with two SILAR cycles by dipping alternatively into 0.1 M zinc nitrate and 0.1 M sodium sulfide solutions for 1 min/dip. The identical QDs systems were applied to TiO₂@ZnO HMS (5-min, 10-min, and 15-min) and bare ZnO HMS photoanodes for comparative investigation.

Cu₂S/FTO was used as the counter electrode for QDSSCs, which was prepared according to previous literature [33]. Polysulfide electrolyte was prepared by 1 M sulfur and 1 M sodium sulfide in water/methanol (1:1 in volume) solution before each test. The QDSSC was assembled by sandwiching the QDs sensitized photoanode and counter electrode in an open way, and the active area of the QDSSCs was 0.25 cm². Moreover, each solar cell have been repeatedly tested three times for the purpose of reproduction.

2.6. Characterization

The morphologies and microstructures of the products were characterized by Tecnai G2 F20 transmission electron microscope (TEM), and Quanta 450 FEG scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) for elemental scanning. The crystalline nature and structure of TiO₂-passivated ZnO HMS was analyzed by X-ray diffraction (XRD, D/MAX—2400, Rigaku, Tokyo, Japan) using a Cu Kα source operated at 40 kV and 30 mA with scanning rate of 2°/min. The optical absorption properties of the photoanodes were recorded by a U-3900H UV-vis spectrophotometer which is equipped with integrating sphere attachment for diffuse reflection measurement. The I-V performances of the QDSSCs were obtained under illumination using a solar simulator to simulate sunlight with intensity of 100 mW cm⁻². The incident photon to charge carrier generation efficiency (IPCE) was measured as a function of wavelength by a 150 W Xe lamp coupled with a computer-controlled monochromator. In order to reveal the influence of TiO₂ passivation layer on charges transfer dynamics, the electrochemical impedance spectroscopy (EIS) tests of the QDSSCs were performed by CHI852C electrochemical workstation under dark at an applied bias of −0.5 V with the frequency range of 10⁻¹ to 10⁻⁵ Hz.

3. Results and Discussion

The formation of carbonaceous spheres templates is the first key step for the synthesis of ZnO HMS. Figure 1a shows the SEM image of prepared carbonaceous spheres, presenting smooth surface and uniform sizes of around 800–900 nm. The spherical structure of template are also confirmed by TEM in Figure 1b, showing that the carbonaceous spheres prepared by hydrothermal process using sucrose aqueous solution are solid spheres, and the size distribution is almost in agreement with the SEM results. Based on carbonaceous spheres, the Zn²⁺ were adsorbed on their surface just by soaking them in Zn²⁺ aqueous solution. Figure 1c shows the carbonaceous spheres after soaking treatment in Zn²⁺ aqueous solution. Obviously, this treatment make the carbonaceous spheres thicker and the size of spheres becomes a little larger than bare carbonaceous spheres shown in Figure 1b, indicating the successful adsorption of Zn²⁺ on carbonaceous spheres templates. A further annealing process at 500 °C with heating rate of 5 °C/min leads to the formation of ZnO HMS. Figure 1d shows the SEM image of ZnO products; spherical structure that is composed of large amount of nanoparticles can be clearly identified. The inset of Figure 1d is the magnified SEM image of one sphere, indicating a hollow inside of ZnO spheres. The ZnO hollow microspheres (HMS) could be identified by TEM images as shown in Figure 1a and c. A number of ZnO HMS can be seen in low magnification TEM image, indicating that massive production of ZnO HMS is achievable by this template method. The TEM of ZnO HMS in high magnification offers typical hollow spherical structure with a single shell. From the SEM and TEM of ZnO HMS, it can be seen that the size distribution of ZnO HMS is around 450–600 nm, which are smaller than carbonaceous spheres templates. When heated, the template gradually changed to CO₂, becoming smaller and smaller. Meanwhile Zn²⁺ precursors on the surface of carbonaceous spheres were oxidized into ZnO nanoparticles and compact together as the shrinkage of template, forming ZnO HMS in smaller than primitive carbonaceous spheres templates.

Figure 2 presents the TEM images of ZnO HMS passivated by TiO₂. As displayed in Figure 2a–c, there is almost no change in hollow microsphere structure after the TiO₂ passivation but an extra layer on surface of ZnO HMS. A bright layer with different thickness coating on shell of ZnO HMS is apparently observed in each of ZnO HMS, as shown in the insets of Figure 2a–c, indicating that TiO₂ passivation layer are formed on surface of ZnO HMS. Figure 2a shows the thickness of passivation layer is ~10 nm for the 5-min TiO₂@ZnO HMS. When immersing time of ZnO HMS photoanode in TiCl₄ aqueous solution prolongs to 10-min, the thickness of TiO₂ passivation layer increases to ~20 nm, as shown in inset of Figure 2b. A ~32 nm thickness of TiO₂ passivation layer is obtained by further prolonging the immersing time to 15 min, as presented in the inset of Figure 2c. These TEM images of TiO₂@ZnO HMS indicate that the thickness of passivation layer is controllable by changing the soaking duration of ZnO HMS in TiCl₄ aqueous solution. Figure 2d presents the HRTEM of TiO₂@ZnO HMS (10-min), showing a lattice fringe of 0.26 nm which can be ascribed to the (002) plane of hexagonal ZnO (PDF # 36-1451). It can be identified that the outmost layer of crystallites has a lattice fringe of 0.24 nm close to the (002) lattice plane of ZnO, which is recognized as the (103) plane of anatase TiO₂ (PDF # 21-1272). Figure S1 provides the XRD of TiO2-passivated ZnO HMS, apart from diffraction peaks that can be ascribed to the hexagonal ZnO, the diffraction peaks around 2θ = 24.6° and 53.1° belongs to the anatase phase of TiO2 (See Supplementary Materials), confirming the anatase TiO2 layer formation on ZnO HMS. The TiO2 has a higher electrical conductivity, which is favorable for electron transport [29].

The surface elemental composition of the TiO₂@ZnO HMS photoanode (10-min) obtained by immersing ZnO HMS photoanode in TiCl₄ aqueous solution for 10 min was characterized by EDS elemental mapping, which is shown in Figure 3. Ti, O, and Zn elements are detected from the selected zone of the photoanode. These elements are uniformly distributed on the surface of photoanode, further proving the formation of TiO₂ passivation layer on ZnO HMS photoanode. Figure 4 illustrates the formation mechanism of TiO₂@ZnO HMS. First, ZnO HMS are doctor bladed onto the surface of FTO glass, forming ZnO HMS photoanode, then Ti⁴⁺ precursors are absorbed by ZnO HMS photoanode when immersed in TiCl₄ aqueous solution. Finally, Ti⁴⁺ precursors are oxidized into TiO₂ nanoparticles during the annealing process, assembling into TiO₂ passivation layer on surface of ZnO HMS photoanode. The thickness of passivation layer are controllable by adjusting immersing duration in TiCl₄ aqueous solution.

Based on TiO₂@ZnO HMS photoanode, CdSe/CdS QDSSCs were fabricated to investigate the influence of passivation layer and layer thickness on photovoltaic performances. In Supplementary Materials, Figure S2a–d presents the three times tested J-V curves CdSe/CdS QDSSCs based on ZnO HMS passivated with TiO₂ by immersing in TiCl₄ aqueous solution for 0 min (blank), 5 min (5-min), 10 min (10-min), and 15 min (15-min). Tables S1–S4 summarize the photovoltaic performance of solar cells, including short-circuit current density (J_sc), open voltage (V_oc), fill factor (FF), power conversion efficiency (PCE), and their corresponding average value and standard deviation (See Supplementary Materials). Furthermore, Figure 5 presents the champion J-V curves of QDSSCs, and

Table 1. Average photovoltaic performance and standard deviation extracted from J-V curves of QDSSCs based on ZnO HMS passivated with TiO₂ by immersing in TiCl₄ aqueous solution for 0 min (blank), 5 min (5-min), 10 min (10-min), and 15 min (15-min) ^a.

Solar cells	Voc (V)	Jsc (mA cm−2)	FF	PCE (%)
Blank	0.39 (0.40)	8.75 (8.79)	0.43 (0.44)	1.49 ± 0.05 (1.55)
5-min	0.40 (0.41)	10.44 (10.49)	0.45 (0.46)	1.88 ± 0.12 (1.98)
10-min	0.45 (0.46)	14.57 (14.64)	0.46 (0.47)	2.99 ± 0.48 (3.16)
15-min	0.44 (0.45)	12.83 (12.89)	0.42 (0.43)	2.41 ± 0.10 (2.49)

^a The numbers in parentheses represent the values obtained from the champion solar cells.

shows the average photovoltaic performance and standard deviation extracted from J-V curves. For the QDSSC based on bare ZnO HMS (blank), the V_oc, J_sc, and FF are 0.40 V, 8.79 mA cm⁻², and 0.44, only producing a PCE of 1.54%. The increment of PCE occurred when TiO₂ passivation layer formed on surface of ZnO HMS photoanode. For instance, the PCE of TiO₂@ZnO HMS (5-min) reaches to 1.98% with V_oc = 0.41 V, J_sc = 10.49 mA cm⁻², and FF = 0.46. Moreover, it is noteworthy that 10-min TiO₂@ZnO HMS solar cell shows better performance with a V_oc of 0.46 V, a J_sc of 14.64 mA cm⁻², and an FF of 0.47, yielding a PCE of 3.16%, which is much higher than the PCE of 1.54% obtained with blank photoanode. Figure 6 summarizes the relative variation of PCE and thickness of TiO₂ passivation layer with different immersing time of ZnO HMS photoanode in TiCl₄ aqueous solution. Obviously, the photovoltaic performance show an increase with rising immersing time, and V_oc, J_sc, FF, and PCE reach their maxima at 10 min. However, with further prolonging of immersing time, J_sc and PCE dramatically decrease at 15 min, with a decrease of J_sc from 14.64 mA cm⁻² to 12.89 mA cm⁻², and a decrease of PCE from 3.16% to 2.49%. In our series solar cells, the only difference is thickness of TiO₂ passivation layer; therefore, it is reasonable to believe that effects related to light harvesting and electron transport resulted from adjusting of TiO₂ passivation layer thickness are responsible for the variation of photovoltaic performance.

To investigate the influence of TiO₂ passivation layer on the optical absorption performance of ZnO HMS photoanode, Figure 7 displays the UV-vis absorption spectra of TiO₂@ZnO HMS photoanodes obtained by immersing in TiCl₄ aqueous solution for different periods. It is well-known that the absorbance and absorption range is related to the loading amount and band gap of QDs, respectively. As shown in Figure 7, the onset of absorption for all sample photoanodes is around 650 nm, indicating the typical band gap characteristic of CdSe QDs. However, the absorbance of the CdSe/CdS co-sensitized TiO₂@ZnO HMS photoanode is higher than that of bare ZnO HMS photoanode, which means that a larger amount of CdSe/CdS QDs can be deposited on TiO₂@ZnO HMS. According to Equation (1) [34], the light harvesting efficiency (LHE) is related to the absorbance.

LHE=1–10^-absorbance,

(1)

Apparently, with prolong treatment time in TiCl₄ aqueous solution, the absorption intensity increases and reaches maximum value at 10 min. The increased absorbance from 0 to 10 min of immersing time can be ascribed to the improved surface stability and specific surface area, which can facilitate adsorption of more QDs [30,35]. Therefore, the LHE of 10-min TiO₂@ZnO HMS photoanode is higher than bare ZnO HMS and other TiO₂@ZnO HMS photoanodes. When immersing time prolonged to 15 min, a decrease of absorbance occurred because longer immersing time likely reduces the average pore size in the TiO₂@ZnO HMS photoanode, which is unfavorable for QDs loading. As Equation (1) indicated, higher LHE obtained with 10-min TiO₂@ZnO HMS photoanode means more photons are captured by the CdSe/CdS co-sensitized TiO₂@ZnO HMS photoanode, contributing to the enhancement of J_sc.

The significant enhancement of J_sc for our champion solar cell is a major key factor to improve photovoltaic performance. Under a given light source, J_sc is expressed by the Equation (2) [36]:

$$J_{sc}=q{\displaystyle {\int }_{{\lambda }_{\min }}^{{\lambda }_{\max }}{\eta }_{IPCE}{\varphi }_{ph}^{source}}(\lambda )d\lambda ,$$

(2)

where λ_max and λ_min are the wavelengths where the IPCE vanishes, Φ_ph is the incident photon flux. Figure 8 compares the IPCE of QDSSCs based on TiO₂-passivated ZnO HMS photoanodes with different treatment duration in TiCl₄ aqueous solution. Two phenomena are displayed in IPCE results, one is that the similar photoresponse range to the profile of UV-vis spectra; another is that the IPCE values follow the order of 10 min > 15 min > 5 min > blank, which is in accordance with the corresponding changing trend of J_sc. It is well-known that IPCE is determined by LHE, charge injection efficiency (Φ_ing), and charge collection efficiency (η_cc), as expressed in Equation (3) [37,38]:

$$IPCE=LHE\times {\varphi }_{ing}\times {\eta }_{cc},$$

(3)

As discussed in Figure 7, higher LHE obtained with TiO₂@ZnO HMS photoanode is one of the reasons responsible for the enhancement of IPCE. Moreover, the influence of TiO₂ passivation layer on charge transfer may be another factor contributing to the enhancement of IPCE. Therefore, EIS was carried out to explore the kinetics of electrochemical and photoelectrochemical processes in the QDSSCs. Figure 9 gives the Nyquist plots of QDSSCs based on bare ZnO HMS and 10-min TiO₂@ZnO HMS photoanodes, and the inset presents the equivalent circuit model for QDSSC. Two semicircles appear in EIS spectra. The first very small semicircle is assigned to the redox reaction at counter electrode/electrolyte interface at high frequencies (R_c); the second larger semicircle reveals the charge transfer across the photoanode/QDs/electrolyte interface at intermediate frequency, and the size of the semicircle represents the charge transport/recombination resistance (R_ct) [29,39].

Table 2. Fitting results of EIS of QDSSCs based on ZnO HMS and TiO₂@ZnO HMS photoanodes.

Photoanodes	Rc (Ω)	Rct (Ω)	Cμ (μF)	τn (ms)
ZnO HMS	5.1	218	948	207
TiO2@ZnO HMS	4.8	309	945	292

also summarizes the fitted results from EIS data by using the “Zview” software. (Zview 2, Solartron, London, UK).

The R_c of QDSSCs based on bare ZnO HMS is 5.1 Ω, which is similar to 4.8 Ω for cell based on TiO₂@ZnO HMS because our QDSSCs employed the same counterelectrode and electrolyte. However, remarkable variation of R_ct is observed between QDSSC based on bare ZnO HMS (218 Ω) and 10-min TiO₂@ZnO HMS (309 Ω). The increased recombination resistance as a result of the TiO₂ passivation layer leads to lower electron recombination, which is due to the energy matching between ZnO and TiO₂ which could hinder the back reaction of photo-generated electrons with oxidized species in polysulfide electrolyte. Moreover, lower recombination resistance caused by TiO₂ passivation layer means a longer electron lifetime (τ_n) can be expected. The τ_n is calculated by Equation (4) [40]:

$${\tau }_{\mathrm{n}}=C_{\mu }\times R_{ct},$$

(4)

As shown in Table 2, τ_n increases from 207 ms to 292 ms for the bare ZnO HMS and 10-min TiO₂@ZnO HMS cells. A longer τ_n indicates a decreased charge recombination, proving that TiO₂ passivation layer on ZnO HMS can greatly suppress the charge recombination in QDSSC. In addition, a reduced recombination will lead to the enhancement of η_cc, not only resulting in the increase of IPCE, but also causing the increase of V_oc which is determined by the following Equation (5) [41]:

$$V_{oc}=\frac{E_F{}_n-E_{redox}}{e}=\frac{k_{B}T}{e}\ln (\frac{n}{n_0}),$$

(5)

where E_fn is the quasi-Fermi level of the electrons in semiconductor under illumination; E_redox is the potential of the redox electrolyte; e is the positive elementary charge; k_BT is the thermal energy; n is the electron concentration in conduction band of the semiconductor under illumination; n₀ is the electron concentration in the dark condition. As EIS results show, the TiO₂ passivation layer caused a longer electron lifetime and improved η_cc, which will lead to more electron accumulation in ZnO HMS photoanode, causing the n to increase, while the E_redox remains the same because of the identical polysulfide. Therefore, higher V_oc is obtained using TiO₂-passivated ZnO HMS photoanode.

From UV-vis, IPCE, and EIS results, it is obvious that enhancement of LHE and decrease of charge recombination play decisive role in TiO₂@ZnO HMS based solar cell for the improvement of J_sc and V_oc. As discussed above, a possible mechanism for the contribution of TiO₂ passivation layer toward improve QDSSC performance is proposed in Figure 10. First, the formation of TiO₂ passivation layer can enhance the adsorption of QDs, improving the light capture ability. Second, the energy level between ZnO and TiO₂ can prevent the back transfer of electrons in ZnO with polysulfide electrolyte, increasing electron lifetime. Therefore, it can be concluded that the increased light harvesting and charge collecting efficiency caused by TiO₂ passivation layer lead to the improvement of J_sc and V_oc, resulting in the superior photovoltaic performance of TiO₂@ZnO HMS-based QDSSC. However, the lower V_oc and FF seem key factors that limit the further improvement of PCE, leading to our champion solar cell is still lower than the best solar cell reported in literature [1,4]. Some factors including the types of QDs, electrolyte, and counter electrode may have a role in enhancing the V_oc and FF. In future, we will pay more attention in developing new types of sensitizers or counter electrodes to realize the full utilization of light and band alignment to boost the V_oc and FF that may further help to enhance PCE of QDSSC based on TiO₂-passivated ZnO HMS architectures.

4. Conclusions

In conclusion, we have synthesized ZnO HMS by a simple carbonaceous sphere template method, and further developed a feasible approach to passivated ZnO HMS with TiO₂ layer for QDSSC application. TEM, SEM confirmed the structure of TiO₂-passivated ZnO HMS and the thickness controllability of TiO₂ layer by changing treatment time in TiCl₄ aqueous solution. When used as photoanode in QDSSCs, TiO₂-passivated ZnO HMS shows a positive influence on photovoltaic performance. UV-vis, IPCE, and EIS analysis results proved that TiO₂ passivation layer could improve light harvesting efficiency, reduce charge recombination, enhance charge collecting efficiency, leading to the enhancement of J_sc and V_oc. As a result, the PCE of QDSSC based on TiO₂-passivated ZnO HMS with optimal thickness layer increased to 3.16%, which is much higher than 1.54% produced by QDSSC based on bare ZnO HMS.