Effects of Metal Oxyhydroxide Coatings on Photoanode in Quantum

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Effects of Metal Oxyhydroxide Coatings on Photoanode in Quantum Dot Sensitized Solar Cells Zhenwei Ren, Zhiqiang Wang, Rong Wang, Zhenxiao Pan, Xueqing Gong, and Xinhua Zhong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00434 • Publication Date (Web): 12 Mar 2016 Downloaded from http://pubs.acs.org on March 17, 2016

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Effects of Metal Oxyhydroxide Coatings on Photoanode in Quantum Dot Sensitized Solar Cells Zhenwei Ren, Zhiqiang Wang, Rong Wang, Zhenxiao Pan,* Xueqing Gong,* Xinhua Zhong* Key Laboratory for Advanced Materials, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China Email: [email protected] (Z.P.); [email protected] (X.G.); [email protected] (X.Z.) Tel/Fax: (+86) 21 6425 0281

ABSTRACT: Exploring facile modifications on photoanode to suppress charge recombination at photoanode/electrolyte interfaces is an efficient way to improve the performance of quantum dot sensitized solar cells (QDSCs). Herein, a series of metal oxyhydroxides gel have been overcoated on CdSeTe QD sensitized photoanodes via a hydrolysis and condensation process from the corresponding metal chloride (NbCl5, ZrOCl2, SnCl4, FeCl3, AlCl3, CoCl2, CuCl2, MgCl2 and ZnCl2) aqueous solutions, and their effects on the photovoltaic performance are systematically investigated. Photovoltaic measurement results indicate that the NbCl5 and ZrOCl2 modifications offer a remarkable enhancement in photovoltaic performance, especially in photovoltage, while the SnCl4 AlCl3, MgCl2 and ZnCl2 treatments give a negligible influence, and the FeCl3, CuCl2 and CoCl2 treatments present a negative effect on the performance. DFT calculations suggest that different metal oxyhydroxide coatings bring forward distinct densities of empty states at the surface of TiO2, which correspond to different charge recombination kinetics and therefore different photovoltaic performance. Electrochemical impedance spectroscopy (EIS) and open circuit voltage decay (OCVD) measurements confirm further the suppressed charge recombination process after coated with the amorphous Zr or Nb oxyhydroxide layer. In all, an impressive PCE of 9.73% (Jsc = 21.04 mA/cm2, Voc = 0.720 V, FF = 0.642) was obtained for CdSeTe based QDSC with ZrOCl2 modification on photoanode.

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INTRODUCTION Being light harvesters, colloidal quantum dots (QDs) offer excellent features with high absorption coefficient, solution processability, facile band gap tunability and multiple exciton generation possibility.1-8 These properties have realized a promising progress for QD based solar cells (including both QD sensitized solar cells QDSCs, and depleted heterojunction quantum dot solar cells) with certified power conversion efficiency (PCE) over 9%.9-13 Benefiting from the optimization of QDs sensitizers, and the valid interface engineering in suppressing charge recombination, QDSCs experience a rapid evolution in recent years with PCE increase from less than 5% to beyond 9%.9,14-24 However, further improvement of the PCE is still an urgent task to render QDSCs competitive with other kinds of emerging solar cells. The rather moderate PCE of QDSCs should be partially attributed to the unfavorable charge recombination losses. The intrinsic trap state defects in QD sensitizers as well as the multiple interfaces formed in the sensitization configuration increase the opportunity for charge recombination, compromising the charge collection efficiency and the resultant PCE. Therefore, further improvement of the PCE still relies on the inhibition of charge recombination processes effectively.25-29 Previous works have consistently demonstrated that the recombination process occur mainly at the TiO2/QD/electrolyte triple-junction interfaces.29 Although the application of high quality QDs can partially reduce the charge recombination, it is powerless to control the recombination occurring at photoanode/electrolyte interface. Furthermore, the surface coverage of QDs on TiO2 is relative low (6%) Cd1−x MnxSe Quantum Dot Sensitized Solar Cell. J. Mater. Chem. A 2014, 2, 19653−19659. (25) Roelofs, K. E.; Herron, S. M.; Bent, S. F. Increased Quantum Dot Loading by pH Control Reduces Interfacial Recombination in Quantum-Dot-Sensitized Solar Cells. ACS Nano 2015, 9, 8321−8334. (26) Tachan, Z.; Hod, I.; Shalom, M.; Grinis, L.; Zaban, A. The Importance of the TiO2/Quantum Dots Interface in the Recombination Processes of Quantum Dot Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 3841−3845. (27) Mora-Seró, I.; Gimenez, S.; Fabregat-Santiago, F.; Gomez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1848−1857. (28) Hodes, G. Comparison of Dye- and Semiconductor-Sensitized Porous Nanocrystalline Liquid Junction Solar Cells. J. Phys. Chem. C 2008, 112, 17778−17787.

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Effects of Metal Oxyhydroxide Coatings on Photoanode in Quantum Dot Sensitized Solar Cells Zhenwei Ren, Zhiqiang Wang, Rong Wang, Zhenxiao Pan,* Xueqing Gong,* Xinhua Zhong*

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