Charge Recombination Control for High Efficiency Quantum Dot

Jan 13, 2016 - quantum dot light absorbers, quantum dot sensitized solar cell (QDSCs) are a promising candidate for the low-cost third-generation sola...
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Charge Recombination Control for High Efficiency Quantum Dot Sensitized Solar Cells Ke Zhao, Zhenxiao Pan, and Xinhua Zhong* Key Laboratory for Advanced Materials, Institute of Applied Chemistry, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: Benefiting from the unique excellent optoelectronic properties of quantum dot light absorbers, quantum dot sensitized solar cell (QDSCs) are a promising candidate for the low-cost third-generation solar cells. Over the past few years, the power conversion efficiency (PCE) of QDSCs presents a rapid evolution from less than 1% to beyond 8%. Charge recombination is regarded as one of the most significant factors in limiting the photovoltaic performance of QDSCs. A significant improvement in the PCE of QDSCs has been obtained by charge recombination control. Some effective routes to suppress charge recombination processes, such as adopting preprepared high-quality QD sensitizers, tailoring the electronic properties of QDs, and interface engineering with the use of organic or inorganic thin layer overcoating the sensitized photoanode have been overviewed in this perspective. Also, the possible accesses to better performance (higher efficiency and stability) of the QDSCs have been proposed on the basis of achievements obtained previously.

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In QDSCs, a series of charge transfer processes have to occur cooperatively so that the electrical output can be harnessed efficiently. As shown by blue arrows in Figure 1a, excited

s a promising candidate for the low cost third-generation solar cells, quantum dot sensitized solar cells (QDSCs) have being achieved a significant progress in photovoltaic performance over the past few years, creating a considerable improvement in power conversion efficiency (PCE) from less than 1% to beyond 8%.1−4 The intrinsic excellent properties of QD sensitizers such as tunable band gap, high absorption coefficient, large dipole moment, and solution processability5−7 make them an outstanding light-absorber candidate. Additionally, the potential of hot electron extraction and multiple exciton generation (MEG) render the theoretic efficiency of QDSCs beyond the Shockley−Queisser limit.8−11 Systematic study on QDSCs has been carried out in the past decade. Researchers have explored several routes, such as chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR), electrophoresis, and linkerassisted self-assembly approaches,12−20 to deposit QDs on mesoporous nanocrystalline metal-oxide (mainly TiO2 and ZnO) film electrodes. A series of QDs (CdS, CdSe, PbS, CdSexTe1−x, and CuInS2) and their derivatives21−32 were employed as light-absorber sensitizers. The metal-oxide film electrode, electrolyte as well as counter electrode were also systematically optimized to improve the performance of QDSCs.33−47 Benefiting from all these efforts, the photovoltaic performance of QDSCs has achieved a steady and significant improvement. Sharing similar principles and configurations, QDSCs still lag behind dye sensitized solar cells (DSCs) in PCE record.48 A critical factor resulting in the poorer performance of QDSCs is charge recombination occurring at photoanode/QD/electrolyte interfaces. Therefore, strategies to minimize charge recombination processes will play a key role in improving the performance of QDSCs. © XXXX American Chemical Society

Figure 1. Schematic drawing of the bare QD sensitized nanocrystalline TiO2 electrode showing (a) four major recombination paths and (b) recombination paths related to the trap states in QDs.

electrons are injected from conduction band (CB) of QDs into CB of TiO2 substrate with lower energetic level, followed by transportation to the collecting electrode surface. Simultaneously, holes remained in QD valence band (VB) are reduced by redox couple in electrolyte, and the oxidized species of redox couple will be regenerated at the counter electrode.10 However, there are several recombination paths counteracting these favorable processes and deteriorating the performance of the device. Charge recombination takes place mainly at the interfaces of TiO2/QD/electrolyte and inside QD, including the following four major paths as shown in Figure 1a with Received: September 27, 2015 Accepted: January 13, 2016

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tether the presynthesized high-quality QDs on the metal oxide film electrodes.16−20 However, the photoanodes sensitized by these deposition approaches are still plagued by the relatively low surface coverage of QDs and the agglomeration of QDs. The low surface coverage of QDs on the metal oxide film electrode on one hand leads to the inefficient absorption of light; on the other hand, results in large portion area of bare TiO2 surface exposed directly to electrolyte media, and this will facilitate the recombination of electrons from TiO2 with the oxidized species in electrolyte at the TiO2/electrolyte interface.57,58 Therefore, the advantage of high-quality QDs is overshadowed by the low surface coverage and QDSCs assembled by the above-mentioned postsynthesis assembly routes show unsatisfactory performance usually.30,59−62 It is clear that an efficient way to improve the PCE of QDSCs with use of presynthesized high-quality QDs is to increase the loading amount of QD on metal oxide film electrode. A capping ligand-induced self-assembly approach was developed by Zhong’s group in which the high-quality colloidal QDs were first synthesized using organometallic high-temperature synthesis methods,25,63−65 then the presynthesized QDs underwent an ex situ ligand exchange process to obtain bifunctional linker molecules (3-mercaptopropionic acid or thioglycolic acid) capped water-soluble QDs, followed by immobilization of the linker molecules capped water-soluble QDs onto the plain TiO2 film electrodes. Experiment results have demonstrated that this approach is a fast and efficient way to deposit QDs onto TiO2 film with QD coverage as high as 34%.25 On the basis of this method, great progress has been achieved, and a series of PCE records of QDSCs were reported.4,25,31,34,63,66,67 As shown in Figure 2a−d, optical spectroscopy characterization demonstrated the high-quality of colloidal QDs with the sharp first excitonic absorption peak and the high photoluminescence (PL) intensity, indicating the low nonradiative recombination rate, high crystallinity, and low trap state density of the obtained QDs. The excellent optical properties were retained after the QDs tethered on the oxide film electrode as demonstrated by the absorption spectra (Figure 2d). Such a distinct feature cannot be achieved by the QD direct growth approaches such as CBD or SILAR route.60,68,69 Wide-field transition electron microscopy (TEM) and cross-sectional scan electron microscopy (SEM) images and elemental mapping of QD-sensitized TiO2 film (Figure 2e,f) proved the high-density and uniform QDs distribution on the TiO2 film electrodes.31,66 A PCE of 5.4% was obtained for CdSe QDSCs under full 1 sun illumination,25 demonstrating the success of capping ligandinduced self-assembly technique integration with the highquality QDs synthesized via the high-temperature route in the construction of high efficiency QDSCs. Tailoring electronic properties of QD sensitizers with the use of core/shell structured QD materials through epitaxial growth of a second semiconductor material around the core QD is also an important strategy for the suppression of charge recombination in QDSCs. Type-II Core/Shell Structure QDs. The core/shell structure offers QDs various photoelectric properties by tailoring the nature or the shell thickness of the shell materials.70 The core/ shell structure provides a method for tailoring the QD band alignment to facilitate the electron−hole separation and electron injection into the TiO2 CB. It is noted that the property of tunable band alignment could be used to inhibit charge recombination process in QDSCs. Among them, type-II structured QDs, in which both the valence and conduction

yellow arrows: (1) internal charge recombination inside QD, (2) excited electron recombination from QD to the oxidized species in electrolyte at QD/electrolyte interface, (3) electrons in the TiO2 back transfer to QD at TiO2/QD interface, and (4) direct electron loss from TiO2 nanoparticles to the electrolyte at TiO2/electrolyte interface.49,50 A significant difference between QDSCs and their analogue DSCs in the recombination paths is the potential contribution of surface trap states within QDs, which act as recombination centers and lead to the degradation of the cell device. The recombination paths that the trap states participate in are illustrated in Figure 1b. Photoexcited electrons in the CB of QD can be trapped by surface trap state defects. When the trap state is energetically located below the CB edge of TiO2, the electron injection from QD into TiO2 CB will be hindered. Trapped electrons have a higher probability of being captured by a hole in QD or the oxidized species in electrolyte even if the trap state is energetically located above the CB edge of TiO2. Meanwhile, trapped holes, on the contrary, can recombine with excited electrons in the QD or pull the injected electrons back from TiO2. Furthermore, the accumulation of holes inside QDs would result in oxidation degradation of QDs and the cell performance.51,52 These recombination paths and their strong negative effects to the device performance have been recognized, and considerable efforts have been made to address this issue. In this Perspective, we present an overview on various methods to control the undesirable charge recombination processes, and classify them according to their inhibition effect on different recombination paths.

The adoption of preprepared high quality colloidal QD sensitizers is an effective route in reducing surface trap state defects-related recombination in QDSCs. The adoption of preprepared high quality colloidal QD sensitizers is an effective route in reducing surface trap state defects-related recombination in QDSCs. Because of the strong negative effects of the trap states in QDs, it is widely recognized that as few as possible trap state density is favorable for the suppression of charge recombination and the improvement of photovoltaic performance of the resultant cell devices. This means that the quality of QD sensitizers plays a decisive role on the performance of cell devices. Although the widely adopted direct growth route like CBD and SILAR can ensure high coverage of QDs on the surface of TiO2 film electrode, and some modified methods have been developed to further increase the QDs loading and suppress the charge recombination,53−55 precise control of particle size, size distribution, and especially the high quality QDs with low trap state density cannot be conveniently achieved due to the uncontrollability of nucleation and growth of QDs on the defined space of the microporous channels in mesoporous TiO2 film at relatively low temperature.20,56 It is well known that the well-developed organometallic high-temperature synthesis methods could provide high-quality QD with much less trap state density. Postsynthesis assembly approaches, including direct adsorption, electrophoretic deposition, and in situ ligand exchange linker-assisted QD deposition methods, were developed to 407

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Figure 2. UV−vis absorption (a) and PL emission (b) spectra of CdSe QDs dispersions in toluene (c) UV−vis absorption spectra of MPA-capped CdSe QDs dispersions in water. (Reprinted from ref 25 with permission of The Royal Society of Chemistry.) (d) Temporal evolution of absorption spectra of CdSe QD sensitized TiO2 film. (e) Wide-field TEM images of CIS-Z QD-sensitized TiO2 film. The scale bar is 20 nm. (Reprinted from ref 31 with permission.) (f) Cross-sectional SEM image of CdTe/CdSe-sensitized TiO2 film electrode (left), and elemental mapping of Ti, Cd, Te, and Se (right, the scale bars are 2 μm). (Reprinted from ref 66 with permission.)

simultaneously suppresses the charge-recombination process because the shell acts as a tunneling barrier for electron recombination with the oxidized species in electrolyte.73 Additionally, type-II QDs show a remarkable red shift of absorption edge benefited from the nature of “spatially indirect” energy gap (also called exciplex state), so the optoelectronic response range in this system is expanded.71,74 Attracted by all these superiorities, type-II core/shell structured QDs were employed as sensitizers for the construction of QDSCs toward lower recombination rate and higher conversion efficiency. Among the few attempts to use type-II core/shell QDs, including ZnSe/CdS and ZnTe/ZnSe, as the light harvester to construct QDSCs, the obtained PCE values are relative low (6%) Cd1−xMnxSe Quantum Dot Sensitized Solar Cell. J. Mater. Chem. A 2014, 2, 19653−19659. (95) Luo, J.; Wei, H.; Huang, Q.; Hu, X.; Zhao, H.; Yu, R.; Li, D.; Luo, Y.; Meng, Q. Highly Efficient Core−Shell CuInS2−Mn Doped CdS Quantum Dot Sensitized Solar Cells. Chem. Commun. 2013, 49, 3881−3883. (96) Huang, Z.; Zou, X.; Zhou, H. A Strategy to Achieve Superior Photocurrent by Cu-Doped Quantum Dot Sensitized Solar Cells. Mater. Lett. 2013, 95, 139−141. (97) Yang, S.; Huang, C.; Zhai, J.; Wang, Z.; Jiang, L. High Photostability and Quantum Yield of Nanoporous TiO2 Thin Film Electrodes Co-Sensitized with Capped Sulfides. J. Mater. Chem. 2002, 12, 1459−1464. (98) Rosenbluth, M. L.; Lewis, N. S. ″Ideal″ Behavior of the Open Circuit Voltage of Semiconductor/Liquid Junctions. J. Phys. Chem. 1989, 93, 3735. (99) Tan, M. X.; Laibinis, P. E.; Nguyen, S. T.; Kesselman, J. M.; Stanton, C. E.; Lewis, N. S. Principles and Applications of Semiconductor Photoelectrochemistry. Prog. Inorg. Chem. 1994, 41, 21−144. (100) Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T. High Efficiency of CdSe Quantum-Dot-Sensitized TiO2 Inverse Opal Solar Cells. Appl. Phys. Lett. 2007, 91, 023116. (101) Shen, Q.; Kobayashi, J.; Diguna, L. J.; Toyoda, T. Effect of ZnS Coating on the Photovoltaic Properties of CdSe Quantum DotSensitized Solar Cells. J. Appl. Phys. 2008, 103, 084304. (102) Guijarro, N.; Campina, J.; Shen, Q.; Toyoda, T.; LanaVillarreal, T.; Gómez, R. Uncovering the Role of the ZnS Treatment in the Performance of Quantum Dot Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2011, 13, 12024−12032. (103) Yang, H.; Guo, C.; Guai, G. H.; Song, Q.; Jiang, S. P.; Li, C. M. Reduction of Charge Recombination by an Amorphous Titanium Oxide Interlayer in Layered Graphene/Quantum Dots Photochemical Cells. ACS Appl. Mater. Interfaces 2011, 3, 1940−1945. 416

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