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Article

Black TiO2-Based Dual Photoanodes Boost the Efficiency of Quantum Dot-Sensitized Solar Cells to 11.7%

by
Danwen Yao
1,†,
Zhenyu Hu
2,†,
Ruifeng Zheng
2,
Jialun Li
2,
Liying Wang
2,
Xijia Yang
2,
Wei Lü
2,* and
Huailiang Xu
1,3,*
1
State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China
2
State Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, China
3
State Key Laboratory of Precision Spectroscopy and Chongqing Institute, East China Normal University, Shanghai 200062, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Nanomaterials 2022, 12(23), 4294; https://0-doi-org.brum.beds.ac.uk/10.3390/nano12234294
Submission received: 15 October 2022 / Revised: 28 November 2022 / Accepted: 29 November 2022 / Published: 2 December 2022
(This article belongs to the Special Issue Laser Fabrication of Functional Micro/Nanomaterials)
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Abstract

:
Quantum dot-sensitized solar cells (QDSSC) have been regarded as one of the most promising candidates for effective utilization of solar energy, but its power conversion efficiency (PCE) is still far from meeting expectations. One of the most important bottlenecks is the limited collection efficiency of photogenerated electrons in the photoanodes. Herein, we design QDSSCs with a dual-photoanode architecture, and assemble the dual photoanodes with black TiO2 nanoparticles (NPs), which were processed by a femtosecond laser in the filamentation regime, and common CdS/CdSe QD sensitizers. A maximum PCE of 11.7% with a short circuit current density of 50.3 mA/cm2 is unambiguously achieved. We reveal both experimentally and theoretically that the enhanced PCE is mainly attributed to the improved light harvesting of black TiO2 due to the black TiO2 shells formed on white TiO2 NPs.

1. Introduction

In recent decades, energy shortages and environmental pollution have emerged as some of the most important concerns that need to be addressed [1]. Solar cells are an effective strategy to utilize solar energy as a kind of clean energy [2]. Among the third-generation solar cells, quantum dot-sensitized solar cells (QDSSCs) have attracted considerable attention due to the stunning properties of quantum dots (QDs) such as a tunable bandgap, a large light absorption coefficient, high stability, and multiple exciton effects [3,4]. Typical QDSSCs are composed of a photoanode, an electrolyte, and a counter electrode (CE). The photoanode is the crucial part and is responsible for the collection and transfer of photogenerated electrons, and generally consists of a mesoporous, wide bandgap oxide film (electron acceptor) coated with a light harvesting material (QD sensitizer). An ideal photoanode should have high specific surface area to absorb QD sensitizers, a suitable crystal structure to obtain extensive light absorption, and excellent ability for electron injection and transfer while having a small charge recombination rate [5]. Although numerous wide bandgap semiconductors such as ZnO, SnO2, and ZnS have been used as photoanodes in QDSSCs, TiO2 is still the most extensively used material due to its excellent chemical stability, non-toxicity, and low cost, etc. [6,7,8]. Limited by the wide bandgap, TiO2 can only absorb ultraviolet light, which accounts for ~5% of sunlight. Although the absorption range of TiO2 can be expanded by combining it with narrow bandgap semiconductors such as CdS, CdSe, PbS, CdSe, etc., the power conversion efficiency (PCE) of QDSSCs is still far from the theoretical value in spite of the great efforts dedicated to optimizing the species of narrow bandgap semiconductors and the interface between TiO2 and narrow bandgap semiconductors. The highest efficiency of QDSSCs reported so far is 15.31% by Zhong et al., which is based on an elegantly designed Zn–Cu–In–S–Se QD sensitizer [9].
Black TiO2 was first discovered and reported by Chen et al. in 2011. Compared with traditional TiO2, black TiO2 has a higher light absorption intensity and a wider light absorption range, which can be extended to the visible and near-infrared region with lower impedance, and thus has attracted attention in the fields of photodegradation and hydrogen production, etc. [10,11]. In the following several years, numerous researchers tried to fabricate black TiO2 by doping S, Se, N, P, and other elements and the final products obtained could be yellow, blue, and black [12,13]. Some researchers inferred that the color change in TiO2 NPs may be related to the increase in the oxygen vacancy concentration, leading to a narrower bandgap and redshift in the absorption spectrum to the visible and even the infrared region [14,15,16]. In contrast, some researchers believe that the increase in oxygen vacancy concentration and the doping of other elements would not narrow the bandgap of TiO2, and the significant redshift in the absorption spectrum is owing to the amorphous surface state of black TiO2 [17,18,19]. In spite of the argument around the mechanism of black TiO2, including the effects of oxygen vacancies, Ti3+, and doped elements [20,21,22,23], it is clear that black TiO2 exhibits enhanced and extended light absorption, which is expected to improve the performance of QDSSCs [24,25,26,27].
Herein, we fabricate black TiO2 by femtosecond laser treatment of commercial TiO2 nanoparticles (NPs). The black TiO2 NPs prepared from anatase, rutile, and P25 (mixed-phase TiO2 crystal with the anatase/rutile ratio of ~80:20) are further used to assemble QDSSCs. A solar cell architecture combining the concentrated photovoltaic cell (CPV) concept with the design of a dual photoanode is proposed, where CuS is used as the CE and sandwiched between two black TiO2 photoanodes. A PCE of 11.7% with a recorded Jsc of 50.3 mA/cm2 is achieved, showing a 290% enhancement when compared with that assembled by traditional P25 photoanodes. According to theoretical and experimental analyses, the PCE improvement is attributed to the black TiO2 shells formed on white TiO2 NPs, which expand the absorption of black TiO2 to the visible and infrared region and induce more excited photoelectrons in the photoanode. In addition, the dual photoanode design and CPV integration enable the augmented light to transmit to the bottom and top photoanodes, providing additional excitation energy for light harvesting.

2. Materials and Methods

2.1. Preparation of Black TiO2

Black P25, rutile TiO2, and anatase TiO2 were prepared by a titanium–sapphire laser system (Spectra-Physics, Spitfire ACE), which generated linearly polarized femtosecond laser pulses with a center wavelength of 800 nm, a repetition rate of 500 Hz, and a pulse width of about 40 fs [27]. A powdered sample of TiO2 was flattened on the bottom of the container with a glass slide, and the container was fixed on a two-dimensional movable platform equipped with electric motors. A rotatable half-wave plate and a polarizer were placed in the laser propagation path to control the laser pulse energy at 1.0 mJ. The laser beam was then reflected by two mirrors with high reflection at 800 nm to make the beam propagate vertically to the surface of the TiO2 samples. The laser beam passed through a f = 1 m focal lens to form a single filament [28,29,30], and then hit the flattened TiO2 sample to produce black TiO2. The container was moved with the electric motors so as to raster it. As an example, the pristine and processed P25 TiO2 samples are shown in Figure 1a with their crystalline structures.

2.2. Preparation of Photoanodes

Typically, 0.8 g TiO2 (Aladdin, Shangai, China), 0.4 g ethyl cellulose (Aladdin), and 3.245 g α-terpineol (Aladdin) were dispersed in 8.5 mL ethanol (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) under stirring. The obtained paste was spin-coated on FTO substrates (Zhuhai Kaiwo Optoelectronics Technology Co., Ltd., Zhuhai, China) and dried at 80 °C for 20 min, where the FTO substrate had been cleaned by immersing in deionized water, absolute ethanol, acetone, absolute ethanol, and deionized water in turn and washing by ultrasonic methods for 30 min, respectively. Then, the films were annealed at 450 °C for 30 min to acquire porous TiO2 films. The prepared TiO2 film was sensitized with CdS QDs by the successive ionic layer adsorption and reaction method using cadmium acetate (0.05 M, Sinopharm Chemical Reagent Co., Ltd.) and anhydrous sodium sulfite (0.05 M, Aladdin). Then, the chemical bath deposition method was used to deposit CdSe QDs using selenium powder (0.08 M, Tianjin Guangfu Science and Technology Seven Development Co., Ltd., Tianjin, China), Na2SO3 (0.05 M, Aladdin), Cd(CH3COO)2 (0.05 M, Aladdin), and trisodium nitrilotriacetic acid monohydrate (0.12 M, TCI Shanghai). Finally, the ZnS passivation layer was coated by immersing the photoanode in 0.1M Zn(AC)2·2H2O solution (Sinopharm Chemical Reagent Co., Ltd.) and 0.1 M Na2S·9H2O (Aladdin) solution repeatedly for two cycles.

2.3. Preparation of CuS and CuS/Brass-Mesh Counter Electrode (CE)

To prepare CuS CEs, a 50 mL solution including Na2S2O3·5H2O (1 M) and CuS·5H2O (1 M, Tianjin Guangfu Science and Technology Seven Development Co., Ltd.) was prepared and the pH was adjusted to 2.0 by acetic acid (Aladdin). Clean FTO substrates were put in the solution and kept at 70 °C for 3 h. Then, the substrates were dried at 130 °C for 30 min to acquire CuS CEs. As for the preparation of the CuS/brass-mesh CE, the copper mesh (Hebei Xingheng Materialtech Co., Ltd., China) was soaked in 70 °C hydrochloric acid (36%, Sinopharm Chemical Reagent Co., Ltd.) for 2 h to remove the zinc on the surface. Then, the copper mesh was washed with deionized water and dried at room temperature. The treated Cu mesh was soaked in a mixed solution of CH4N2S (0.01 M, Sinopharm Chemical Reagent Co., Ltd.) and C2H8N2 (Ethylenediamine, 1.5 M, Tianjin Guangfu Science and Technology Seven Development Co., Ltd.) for 24 h, and then the previous cleaning and drying steps were repeated.

2.4. Assembly of QDSSCs and Dual Photoanode QDSSCs

The polysulfide electrolyte was prepared by dissolving 2.4 g Na2S (Aladdin) and 0.32 g S powder (Sinopharm Chemical Reagent Co., Ltd.) in a 10 mL solution with a methanol/deionized water volume ratio of 7:3. Finally, the CuS CE, polysulfide electrolyte, and different photoanodes were assembled into a sandwich structure device divided by a polymer gasket filled with polysulfide electrolyte. The CPV concept was integrated into QDSSCs with dual-photoanode architecture (D-A), where the prepared CuS/brass mesh CE mentioned in Section 2.3 was used as the counter electrode and sandwiched between two identical photoanodes. Each photoanode and counter electrode were separated by the aforementioned polymer gasket. The fabrication procedure of QDSSCs is shown in Figure 1b.

2.5. Characterization and Electrochemical Measurement

The materials and devices were characterized, respectively, by X-ray diffractometry (XRD, Rigaku X-ray diffractometer, Rigaku, Tokyo, Japan), field emission scanning electron microscopy (FESEM, S4800, Hitachi, Tokyo, Japan), transmission electron microscopy (TEM, FEI Talos F200, FEI, Thermo Scientific, Waltham, MA, USA), ultraviolet/visible-near infrared spectrophotometry (UV-3150), an electrochemical workstation (CHI660C, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China), and by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) (Thermo Scientific, Waltham, MA, USA). The photovoltaic performances (J–V curves) were measured by a Keithley 2400 source meter under illumination of an AM 1.5 G solar simulator (Zolix Instruments Co., Ltd., Beijing, China).

3. Results

To investigate the effect of phase structure on device performance, three different black TiO2 NPs have been prepared based on pristine P25, rutile TiO2, and anatase TiO2, respectively. The X-ray diffraction (XRD) results in Figure 2a–c indicate that the main diffraction peaks do not change after the laser ablation treatment for all three samples. The peak intensities of anatase and rutile TiO2 are weakened slightly, which may be due to the transformation of the surface of TiO2 into amorphous TiO2, resulting in a decrease in crystallinity, while the decrease in P25 TiO2 is almost invisible. Fourier transform infrared spectra (FTIR) are shown in Figure 2d–e. Similar to the XRD results, the infrared peak positions of pristine TiO2 are the same as those of black TiO2. The characteristic peaks of anatase TiO2 and rutile TiO2 can be observed in FTIR spectra of P25 TiO2 and black P25 TiO2 in Figure 2f. In the Raman spectra (Figure S1 in the Supplementary Materials), it can be seen that there are no significant shifts in the Raman peak positions for the black TiO2, while the bands seem slightly smaller than the pristine TiO2 in all three cases. This indicates that there is no significant difference between black TiO2 and white TiO2 in structure and functional groups [17,18,19].
The structural variation was further investigated by X-ray photoelectron spectroscopy (XPS). Figure 3a shows the XPS surveys of P25 TiO2 with and without the laser treatment, indicating that there is no obvious difference in their chemical compositions. High-resolution XPS spectra are used to further clarify the detailed variations. The N1s XPS spectrum of black P25 TiO2 in Figure 3b indicates that laser treatment of P25 TiO2 in air can induce N doping [10,11,13]. Figure 3c shows the O1s XPS spectra of pristine P25 TiO2 and black P25 TiO2. The peaks at 530.3 eV, 532.2 eV, and 533.2 eV correspond to OTi-O, OV, and OO-H bonds, respectively. The oxygen vacancy related peak is located at 532.2 eV, and it can be observed that the OV peak intensity in black P25 TiO2 is higher than that in pristine P25 TiO2, indicating that the oxygen vacancy after the laser treatment is significantly increased. This is an important reason for the color change of P25 TiO2 after the laser treatment. Figure 3d demonstrates the Ti2p XPS spectra of pristine P25 TiO2 and black P25 TiO2. The two characteristic peaks of Ti4+ are located at 459.25 eV and 465.1 eV, and the two characteristic peaks of Ti3+ are located at 458.2 eV and 463.95 eV. The proportion of Ti3+ in black P25 TiO2 is higher than that of pristine P25 TiO2, which is consistent with the change of oxygen vacancy content in Figure 3b, confirming the aforementioned explanation [31,32,33].
Shown in Figure 4 are the UPS spectra of anatase TiO2 and rutile TiO2 before and after laser treatment. Both results indicate that the top of the valence band of black TiO2 is lower than that of pristine TiO2, which is consistent with previous reports [14,15,18]. From the UV–vis absorption spectra (Figure 5a) and diffuse reflectance spectra (Figure S2b in the Supplementary Materials) of the six samples it can be seen that the main absorption of the six samples occurs in the spectral range of less than 400 nm, mainly due to the inherent bandgap of TiO2 (3.02–3.20 eV) [34]. It should be emphasized that the change in the absorption up to 2500 nm can also be observed (see Figure S2a in the Supplementary Materials), which indicates the strong effect of the laser treatment on the optical properties of TiO2.
The absorption of black TiO2 in the visible and infrared region is overall higher than that of pristine TiO2, and the significant enhancement in light absorption will generate more photoexcited electrons, and thus a higher photocurrent Jsc. Shown in Figure 5b,c are the bandgap diagrams of anatase TiO2 and black anatase TiO2, and rutile TiO2 and black rutile TiO2, respectively, from which it can be seen that the bandgap of the black TiO2 has no obvious narrowing. This is in contrast to the black TiO2 calcined at high temperature in the reducing gas, in which the reduction of TiO2 is comparatively complete and there is a more obvious bandgap narrowing. The laser treatment only acts on the surface of TiO2 NPs. The test thickness of UPS is about 1–2 nm; therefore, UPS spectra mainly reflect the surface properties of NPs. Therefore, the black TiO2 we prepared is likely to be a core-shell structure, the surface layer should be amorphous TiO2, and the core is untreated crystalline TiO2. The XPS results of anatase TiO2 and rutile TiO2 before and after the laser treatment are similar to that of P25 (see Figures S3 and S4 in the Supplementary Materials). The detailed analysis of the energy band variation induced by the laser treatment is further investigated by first-principle calculations and will be shown later.
Figure 6a–c show the TEM images of black P25 TiO2, and Figure 6d–f show the TEM images of pristine P25 TiO2. It can be seen in Figure 6 that the size of P25 does not change significantly before and after the laser treatment. Both the high-resolution TEM (HRTEM) images shown in Figure 6c,f show a clear lattice spacing of 0.35 nm, which corresponds to the (101) plane of TiO2. The results are consistent with the aforementioned prediction that the core is untreated crystalline TiO2, and the difference exists on the surface area. For black P25, an amorphous layer with 1.02 nm thickness can be observed. That is to say, the black TiO2 prepared by the laser treatment is a core-shell structure, the surface layer is amorphous TiO2, and the core is untreated crystalline TiO2, which confirms the conclusion drawn from Figure 3, Figure 4 and Figure 5. Moreover, the TEM images of anatase TiO2 and rutile TiO2 before and after the laser treatment (see Figures S5–S8 of the Supplementary Materials) give the same results as those in Figure 6.
Then, we investigated the PCEs of QDSSCs fabricated with a single TiO2 photoanode, and the corresponding device parameters are shown in Table 1. For J–V curves of anatase TiO2 in Figure 7a, the PCE of anatase TiO2 is 3.6% with a Jsc = 18.3 mA/cm2, and for black anatase TiO2 the PCE is 4.7% with a Jsc = 22.9 mA/cm2. The overall PCE is increased by 30%, which is due to the improved absorption of TiO2 after the laser treatment. The schematic diagram of the dual photoanodes is shown in Figure 8. For the PCE of D-A TiO2 QDSSCs, by one sun irradiating from the top photoanode and with concentrated sunshine illumination from the bottom photoanode with the help of a parabolic reflector, the PCE is found to be greatly enhanced. The PCE of dual anatase TiO2 photoanode is 7.2%, and that of black anatase TiO2 is 9.1%. For rutile TiO2 in Figure 7b, the PCE increased from 1.6% to 2.3% after the laser treatment, and the overall PCE increased by 43.75%. The PCE of the dual rutile TiO2 photoanode is 3.9% and that of the black rutile TiO2 is 5.3%. The best performance is achieved by P25 TiO2 and is shown in Figure 7c, which is due to the synergic effect of rutile and anatase TiO2. The Jsc increased from 16.6 mA/cm2 to 25.0 mA/cm2 after the laser treatment, inducing an increasing in PCE from 4.0% to 5.9%. The PCE of the dual P25 TiO2 photoanode is 8.0% and that of the black P25 TiO2 is 11.7% with a Jsc of 50.3 mA/cm2. From the J–V curves, it can be confirmed that the increase in PCE is mainly due to the enhanced Jsc, which suggests the improved collection efficiency of photogenerated electrons.
To further verify the above mechanism, we measured incident photon-to-electron conversion efficiency (IPCE), as shown in Figure 7d–f. For pristine TiO2 without the laser treatment, all three kinds of devices exhibit a photo-response in the visible region, which is consistent with previous reports since CdS/CdSe QD sensitizers absorb visible light. Replacing pristine TiO2 with black TiO2 induces the extension of the IPCE to the near-infrared region as shown by the red dotted curves, which is responsible for the increase in Jsc. It should be noticed that the QD sensitizers used in the present study only absorb visible light, and thus the extended IPCE should be caused by the black TiO2, which is consistent with the absorption properties of black TiO2, shown in Figure S2 in the Supplementary Materials. In addition, it should be mentioned that we adopted a dual photoanode architecture, and two photoanodes are connected in parallel and share a CuS mesh CE. Meanwhile, the CPV concept was integrated into QDSSCs. Light management as an important technology to improve the conversion efficiency in solar cells; aiming to increase the photon flux received by solar cells. By the light trapping effect, the optical path length is increased, thereby improving the PCE of solar cells. The IPCE test on two different measurement devices (Crowntech QTest Station 1000 CE, America and DG-6050 Zolix, China) are different from the PCE test on two different solar simulators (SS150 Solar Simulator Zolix and Sirius-SS150A-D Zolix), where sunlight is illuminated on both sides of cell. This is because the IPCE test only allows illumination from the top side due to the device design. Therefore, the actual IPCE should be higher than the present test value in Figure 7. The reported PCE 11.7% is an average value, and a larger PCE of over 12% could be acquired.
Shown in Figure 7g–i are Nyquist curves of anatase TiO2, rutile TiO2, and P25 TiO2 photoanodes with and without the laser treatment, which were measured in the dark and under illumination, respectively. The Nyquist curves are obtained from the Electrochemical Impedance Spectroscopy (EIS) measurements, which can reveal the interfacial reactions of photoexcited electrons in the QDSSCs. The electrochemical system can be regarded as the equivalent circuit shown in the inset, where Rs represents the series resistance in the high frequency region, and CPE1 and CPE2 are the chemical capacitances of the cathode and photoanode, respectively. R1 and R2 refer to the charge transfer resistance between the counter electrode and electrolyte (R1) and that at the TiO2/QDs/electrolyte interface (R2), which can be determined from the radii of the first and second circles of the Nyquist curve, respectively. As shown in Figure 7g–i, the radii of all of the first circles of the Nyquist curves are unobservable, indicating that the charge transfer resistance R1 is negligible for all the QDSSC devices. Furthermore, the radii of the second circles of the black TiO2 based photoanodes are remarkably smaller than those of the pristine TiO2, corresponding to the smaller charge transfer resistance, which indicates that the charge transport of black TiO2-based devices is faster than that of pristine TiO2-based photoanodes. This is due to oxygen vacancy doping providing a more convenient transport channel for electrons [8,35]. This greatly improves the ability of the TiO2 thin film to separate, collect, and transport photogenerated carriers in the cell, thereby reducing the loss of the photogenerated carriers during the device operation. Among the three kinds of TiO2, the smallest impedance is achieved by black P25 TiO2, which is consistent with the J–V results. Compared with the pristine sample, the decrease in interfacial electron transfer resistance enables facilitation of electron transport, leading to a significant increase in Jsc.
XPS has confirmed that the laser processing can induce oxygen vacancies accompanied by N-doping in TiO2, while TEM results suggest that the laser processing only affects the surface of TiO2 NPs, inducing a core/shell structure; thus, the oxygen vacancies and N-doping should mainly exist in the surface layer. The results in Figure 2 indicate that the bandgap does not change obviously before and after the laser treatment, which is believed to originate from the measurement depth of XPS, and reflects the information on near surface layer. Therefore, the analysis of energy band variation induced by the laser processing is further investigated by first-principle calculation of pristine TiO2 and N-doped TiO2 rich in oxygen vacancies induced by laser processing [28,30,36,37,38,39]. It is found that the oxygen atoms in the anatase phase are equivalent, so any oxygen vacancies will not affect the calculation results. Shown in Figure 9 are the calculated results in the range of −10–10 eV for anatase and rutile TiO2 before and after the laser treatment (see Figure S9 in the Supplementary Materials for a larger energy range of −60–30 eV). For anatase TiO2, it can be observed from Figure 9a,b that the bandgap of TiO2 has been significantly narrowed due to the doping of oxygen vacancies and nitrogen atoms. This is mainly due to the donor behavior of the introduced oxygen vacancies, leading to the formation of a donor level. Thus, the forbidden band width is reduced from 3.171 eV to 1.498 eV. However, the substitutional doping of O atoms by N atoms would introduce holes, which act as acceptors and produce a shallow acceptor energy level above the top of the valence band of pristine anatase TiO2; therefore, there is no sharp narrowing in the bandgap. For rutile TiO2 in Figure 9c,d, similar bandgap narrowing from 3.083 to 0.13 eV can be confirmed. This is significantly larger than that of anatase TiO2. Compared with the anatase TiO2 structure, the rutile unit cell has fewer atoms (see Figure S10 in the Supplementary Materials for unit cells of the pristine TiO2 and black TiO2 with nitrogen-doping and oxygen vacancies). The same number of oxygen vacancies and nitrogen atoms account for a larger atomic proportion, which should be the reason for the smaller calculated bandgap than that of anatase TiO2. The experimental and theoretical investigations in the present work have shown that the laser processing mainly induces N-doping and oxygen vacancies near the surface layer of TiO2, which reduces the bandgap, inducing a broadened absorption [40].
Since black TiO2 itself shows broad absorption, it is possible to use black TiO2 as a photoanode, without loading CdS/CdSe sensitizers, to realize high PCE (the corresponding results are shown in Figure S11 of the Supplementary Materials). It could be confirmed that the pure black TiO2 photoanode shows very poor PCE, suggesting that the pure black TiO2 photoanode cannot realize effective photogenerated electron extraction, which is probably due to a failure to find a suitable electrolyte and CE that matched black TiO2. The polysulfide (S2–/Sn2–) electrolyte is commonly used in QDSSCs since it can stabilize the commonly used chalcogenide QD sensitizers and provide an acceptable photovoltaic performance [41]. However, in the absence of sulfide QDs, the polysulfide electrolyte could not directly combine with TiO2 to achieve redox reaction. Photosensitizer can only be used in solar cells if the right electrolyte is combined with a matching pair of electrodes [26].
While the debate around the photoelectrical and photocatalytic mechanism of black TiO2 still exists, the improvement in PCE of QDSSCs in this work could be attributed to two reasons according to our experimental and theoretical investigations: (i) the expanded absorption of black TiO2 to the near-infrared region induces more excited photoelectrons in the photoanode, and thus increases the Jsc; and (ii) the dual photoanode design and CPV integration, where the two photoanodes are connected in parallel and share a CuS mesh CE. The CuS grown on Cu mesh as a CE allows the transmitted light from the top cell to arrive at the bottom photoanode. Thus, the dual photoanode structure can capture more light and increase current density. In addition, the CPV systems can also enable an augmented light transmission to the top photoanode through the parabolic reflector at the bottom, providing additional excitation energy for light harvesting. To our best knowledge, the present work achieves the highest PCE of CdS/CdSe QD co-sensitized QDSSCs, which is mainly due to the obvious enhancement in Jsc (Table S1 in Supplementary Materials).

4. Conclusions

In the present study, we have developed a strategy to boost performance of QDSSCs, in which black TiO2 produced by laser processing was used as a photoanode material. It has been shown that the laser fabrication induces N-doping and oxygen vacancies near the surface layer of TiO2 and forms the core/shell structure, the synergic effect of which may be the origin for the extension of the absorption of black TiO2 into the visible and infrared region. Combined with the CPV structure design, a PCE of 11.7% with a Jsc of 50.3 mA/cm2 in the QDSSCs were achieved, which is due to the expanded absorption of black TiO2, the dual photoanode design, and CPV integration. While great efforts have been dedicated to synthesizing novel QD sensitizers and to the complicated structure design of photoanodes, the present work has provided an alternative way to boost performance of QDSSCs, which are expected to exhibit higher performance by further optimization of cell design parameters.

Supplementary Materials

The following supporting information can be downloaded at: https://0-www-mdpi-com.brum.beds.ac.uk/article/10.3390/nano12234294/s1. Figure S1: (a) Raman spectra of anatase TiO2 and black anatase TiO2. (b) Raman spectra of rutile TiO2 and black rutile TiO2. (c) Raman spectra of P25 TiO2 and black P25 TiO2, Figure S2: (a) Diffuse reflectance spectra of anatase, rutile, P25, black anatase, black rutile, black P25 nanoparticles. (b) Diffuse absorb spectra of anatase, rutile, P25, black anatase, black rutile, black P25 nanoparticles, Figure S3: (a) O1s XPS spectrum of anatase TiO2. (b) Ti2p XPS spectrum of anatase TiO2. (c) UPS spectrum of anatase TiO2. (d) O1s XPS spectrum of black anatase TiO2. (e) Ti2p XPS spectrum of black anatase TiO2. (f) UPS spectrum of black anatase TiO2. (g) XPS survey of anatase TiO2. (h) XPS survey of black anatase TiO2. (i) N1s XPS spectra of anatase TiO2 and black anatase TiO2. The green line is the baseline of the curve, Figure S4: (a) O1s XPS spectrum of rutile TiO2. (b) Ti2p XPS spectrum of rutile TiO2. (c) UPS spectrum of rutile TiO2. (d) O1s XPS spectrum of black rutile TiO2. (e) Ti2p XPS spectrum of black rutile TiO2. (f) UPS spectrum of black rutile TiO2. (g) XPS survey of rutile TiO2. (h) XPS survey of black rutile TiO2. (i) N1s XPS spectrum of rutile TiO2 and black rutile TiO2. The green line is the baseline of the curve, Figure S5: TEM and HRTEM images of anatase TiO2 with different magnifications (a-f), Figure S6: TEM and HRTEM images of black anatase TiO2 with different magnifications (a-f), Figure S7: TEM and HRTEM images of rutile TiO2 with different magnifications (a-f), Figure S8: TEM and HRTEM images of black rutile TiO2 with different magnifications (a-f), Figure S9: Energy band diagram and density of states spectrum obtained by first-principles calculations for (a) anatase TiO2, (b) black anatase TiO2, (c) rutile TiO2, and (d) black rutile TiO2, Figure S10: Unit cells of anatase TiO2, black anatase TiO2, rutile TiO2, and black rutile TiO2 for first-principles calculations, Figure S11: (a), (b) and (c) are J-V curves of black anatase TiO2, rutile TiO2, and P25 TiO2 samples assembled with a S2−/Sn2− electrolyte and copper sulfide counter electrode without quantum dot sensitization. (d), (e) and (f) are J-V curves of black anatase TiO2, rutile TiO2, and P25 TiO2 samples assembled with a platinum electrode using S2−/Sn2− electrolyte without quantum dot sensitization, Table S1: Performance parameters of CdS/CdSe co-sensitized QDSSCs based on different reports. References [42,43,44,45,46,47,48,49,50,51] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, W.L. and H.X.; methodology, D.Y., Z.H., L.W., W.L. and H.X.; software, J.L., L.W.; validation, D.Y., Z.H, W.L. and H.X.; formal analysis, J.L., D.Y., Z.H., L.W., W.L. and H.X.; visualization, D.Y., Z.H., R.Z., W.L. and H.X.; investigation, D.Y., Z.H., X.Y., L.W., R.Z., W.L. and H.X.; data curation, X.Y., R.Z., D.Y. and Z.H.; writing—original draft preparation, D.Y., Z.H, W.L. and H.X.; writing—review and editing, W.L. and H.X.; supervision, H.X. and W.L.; funding acquisition, H.X. and W.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China (grant number 62004014, 62004015, 61625501, 62027822), and Department of Science and technology of Jilin Province (grant number 20210101077JC).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data supporting the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Schematic illustration of structures of pristine and black TiO2; (b) Fabrication procedure and working principle of QDSSCs.
Figure 1. (a) Schematic illustration of structures of pristine and black TiO2; (b) Fabrication procedure and working principle of QDSSCs.
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Figure 2. (a) XRD patterns of anatase TiO2 and black anatase TiO2; (b) XRD patterns of rutile TiO2 and black rutile TiO2; (c) XRD patterns of P25 and black P25; (d) FTIR spectra of anatase TiO2 and black anatase TiO2; (e) FTIR spectra of rutile TiO2 and black rutile TiO2; (f) FTIR spectra of P25 and black P25. A and R in (c) and (f) represent the peaks from anatase and rutile phases, respectively.
Figure 2. (a) XRD patterns of anatase TiO2 and black anatase TiO2; (b) XRD patterns of rutile TiO2 and black rutile TiO2; (c) XRD patterns of P25 and black P25; (d) FTIR spectra of anatase TiO2 and black anatase TiO2; (e) FTIR spectra of rutile TiO2 and black rutile TiO2; (f) FTIR spectra of P25 and black P25. A and R in (c) and (f) represent the peaks from anatase and rutile phases, respectively.
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Figure 3. (a) XPS survey of P25 TiO2 and black P25 TiO2; (b) N1s XPS spectra of P25 TiO2 and black P25 TiO2; (c) O1s XPS spectra of P25 TiO2 and black P25 TiO2; (d) Ti2p XPS spectra of P25 TiO2 and black P25 TiO2.
Figure 3. (a) XPS survey of P25 TiO2 and black P25 TiO2; (b) N1s XPS spectra of P25 TiO2 and black P25 TiO2; (c) O1s XPS spectra of P25 TiO2 and black P25 TiO2; (d) Ti2p XPS spectra of P25 TiO2 and black P25 TiO2.
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Figure 4. UPS spectra of anatase TiO2 (a) and black anatase TiO2 (b); UPS spectra of rutile TiO2 (c) and black rutile TiO2 (d). The blue and red lines are the fits to be tangent to the regression curves.
Figure 4. UPS spectra of anatase TiO2 (a) and black anatase TiO2 (b); UPS spectra of rutile TiO2 (c) and black rutile TiO2 (d). The blue and red lines are the fits to be tangent to the regression curves.
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Figure 5. (a) The UV–vis absorption spectra of six samples of P25 TiO2, rutile TiO2, anatase TiO2, black P25 TiO2, black rutile TiO2, and black anatase TiO2; (b) Plots of (Ahν)2 vs. photon energy of anatase TiO2 and black anatase TiO2; (c) Plots of (Ahν)2 vs. photon energy of rutile TiO2 and black rutile TiO2. The blue lines are the fits to be tangent to the experimental curves.
Figure 5. (a) The UV–vis absorption spectra of six samples of P25 TiO2, rutile TiO2, anatase TiO2, black P25 TiO2, black rutile TiO2, and black anatase TiO2; (b) Plots of (Ahν)2 vs. photon energy of anatase TiO2 and black anatase TiO2; (c) Plots of (Ahν)2 vs. photon energy of rutile TiO2 and black rutile TiO2. The blue lines are the fits to be tangent to the experimental curves.
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Figure 6. (a–c) are TEM and HRETM images of black P25 TiO2; (d–f) are TEM and HRETM images of P25 TiO2 without laser treatment. Insets in (c,f): Zoomed-in views for the TEM images.
Figure 6. (a–c) are TEM and HRETM images of black P25 TiO2; (d–f) are TEM and HRETM images of P25 TiO2 without laser treatment. Insets in (c,f): Zoomed-in views for the TEM images.
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Figure 7. (a–c) are J–V curves of QDSSCs based on anatase TiO2, rutile TiO2, and P25 TiO2 photoanodes with and without laser treatment, respectively; (d–f) are IPCE curves of QDSSCs based on anatase TiO2, rutile TiO2, and P25 TiO2 photoanodes with and without laser treatment, respectively; (g–i) are Nyquist curves of anatase TiO2, rutile TiO2, and P25 TiO2 photoanodes with and without laser treatment measured in the dark and under illumination, respectively.
Figure 7. (a–c) are J–V curves of QDSSCs based on anatase TiO2, rutile TiO2, and P25 TiO2 photoanodes with and without laser treatment, respectively; (d–f) are IPCE curves of QDSSCs based on anatase TiO2, rutile TiO2, and P25 TiO2 photoanodes with and without laser treatment, respectively; (g–i) are Nyquist curves of anatase TiO2, rutile TiO2, and P25 TiO2 photoanodes with and without laser treatment measured in the dark and under illumination, respectively.
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Figure 8. Schematic diagram of the cell design with dual photoanode used in the present work.
Figure 8. Schematic diagram of the cell design with dual photoanode used in the present work.
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Figure 9. Energy band and density of states diagram based on first-principle calculation for (a) anatase TiO2, (b) black anatase TiO2 rich in oxygen vacancies, (c) rutile TiO2, and (d) black rutile TiO2 rich in oxygen vacancies.
Figure 9. Energy band and density of states diagram based on first-principle calculation for (a) anatase TiO2, (b) black anatase TiO2 rich in oxygen vacancies, (c) rutile TiO2, and (d) black rutile TiO2 rich in oxygen vacancies.
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Table
Table 1. Performance parameters of QDSSCS based on different photoanodes, and D-A means dual photoanodes.
Table 1. Performance parameters of QDSSCS based on different photoanodes, and D-A means dual photoanodes.
SampleJsc (mA/cm2)Voc (V)FFPCE (%)
Rutile TiO29.2 ± 0.20.510 ± 0.0030.267 ± 0.0011.6 ± 0.1
Anatase TiO216.5 ± 0.20.569 ± 0.0010.385 ± 0.0033.6 ± 0.1
P25 TiO216.6 ± 0.30.594 ± 0.0020.403 ± 0.0044.0 ± 0.1
Black Rutile TiO212.1 ± 0.20.542 ± 0.0030.379 ± 0.0032.3 ± 0.1
Black Anatase TiO222.9 ± 0.30.607 ± 0.0020.341 ± 0.0064.7 ± 0.1
Black P25 TiO225.0 ± 0.30.616 ± 0.0040.383 ± 0.0085.9 ± 0.2
D-A Rutile TiO218.2 ± 0.20.634 ± 0.0020.345 ± 0.0043.9 ± 0.2
D-A Anatase TiO234.2 ± 0.30.616 ± 0.0050.342 ± 0.0067.2 ± 0.1
D-A P25 TiO233.1 ± 0.40.602 ± 0.0030.405 ± 0.0058.0 ± 0.2
D-A Black Rutile TiO222.1 ± 0.40.612 ± 0.0060.398 ± 0.0065.3 ± 0.2
D-A Black Anatase TiO247.7 ± 0.40.607 ± 0.0070.326 ± 0.0089.1 ± 0.3
D-A Black P25 TiO250.3 ± 0.40.619 ± 0.0070.399 ± 0.00711.7 ± 0.3
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Yao, D.; Hu, Z.; Zheng, R.; Li, J.; Wang, L.; Yang, X.; Lü, W.; Xu, H. Black TiO2-Based Dual Photoanodes Boost the Efficiency of Quantum Dot-Sensitized Solar Cells to 11.7%. Nanomaterials 2022, 12, 4294. https://0-doi-org.brum.beds.ac.uk/10.3390/nano12234294

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Yao D, Hu Z, Zheng R, Li J, Wang L, Yang X, Lü W, Xu H. Black TiO2-Based Dual Photoanodes Boost the Efficiency of Quantum Dot-Sensitized Solar Cells to 11.7%. Nanomaterials. 2022; 12(23):4294. https://0-doi-org.brum.beds.ac.uk/10.3390/nano12234294

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Yao, Danwen, Zhenyu Hu, Ruifeng Zheng, Jialun Li, Liying Wang, Xijia Yang, Wei Lü, and Huailiang Xu. 2022. "Black TiO2-Based Dual Photoanodes Boost the Efficiency of Quantum Dot-Sensitized Solar Cells to 11.7%" Nanomaterials 12, no. 23: 4294. https://0-doi-org.brum.beds.ac.uk/10.3390/nano12234294

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