Low-Temperature ZnO Thin Film and Its Application in PbS Quantum Dot Solar Cells

Zinc oxide (ZnO) has been widely deployed as electron conducting layer in emerging photovoltaics including quantum dot, perovskite and organic solar cells. Reducing the curing temperature of ZnO layer to below 200 oC is an essential requirement to reduce the cell fabrication cost enabled by large-scale processes such as ink-jet printing, spin coating or roll-roll printing. Herein, we present a novel water-based ZnO precursor stabilized with labile NH3, which allow us to spin coat crystalline ZnO thin films with temperatures below 200 oC. Thin film transistors (TFTs) and diode-type quantum dot solar cells (QD SCs) were fabricated using ZnO as electron conduction layer. In the QD SCs, a p-type 1,2-ethylenedithiol treated PbS QDs with a bandgap of 1.4 eV was spin-coated on top of ZnO layer by a layer-by-layer solid state ligand exchange process. Electron mobility of ZnO was about 0.1 cm2V-1s-1 as determined from TFT measurements. Power conversion efficiency of solar cells: FTO/ZnO/PbS/Au-Ag was 3.0% under AM1.5 irradiation conditions. The possibility of deposition of ZnO at low temperatures demonstrated herein is of important for solution processed electronic and optoelectronic devices. Keywords ZnO, low-temperature, quantum dots, solar cells, TFTs References [1] A. Janotti, A. Janotti, C.G. Van De Walle-fundamental of ZnO as a semiconductor, Reports on Progress in Physics, 72 (2009) 126501.[2] H. You, Y. Lin-investigation of the sol-gel method on the flexible ZnO device, International Journal of Electrochemical Science, 7 (2012) 9085–9094.[3] Y. Lin, C. Hsu, M. Tseng, J. Shyue, F. Tsai-stable and high-performance flexible ZnO thin-film transistors by atomic layer deposition, Applied Materials &Interfaces, 7(40) (2015) 22610–22617.[4] C. Lin, S. Tsai, M. Chang-Spontaneous growth by sol-gel process of low temperature ZnO as cathode buffer layer in flexible inverted organic solar cells, Organic Electronics, 46 (2017) 218-255.[5] H. Park, I. Ryu, J. Kim, S. Jeong, S. Yim, S. Jang-PbS quantum dot solar cells integrated with sol−gel-derived ZnO as an n‑type charge-selective layer, Journal of Physical Chemistry C, 118(2014) 17374−17382.[6] Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger-inverted polymer solar cells integrated with a low- temperature-annealed sol-gel-derived ZnO film as an electron transport layer Advanced Materials, 23(2011) 1679–1683.[7] V.A. Online, R. Suriano, C. Bianchi, M. Levi, S. Turri, G. Griffini-the role of sol-gel chemistry in low-temperature formation of ZnO buffer layers for polymer solar cells with improved performance, RSC Advances, 6(2016) 46915-46924.[8] X. D. Mai, J. An, H. Song, J. Jang-inverted Schottky quantum dot solar cells with enhanced carrier extraction and air-stability, Journal of Materials Chemistry A, 2 (2014) 20799–20805.[9] H. Choi, J. Lee, X.D. Mai, M.C. Beard, S.S. Yoon, S. Jeong - supersonically spray-coated colloidal quantum dot ink solar cells, Scientific Report, 7(2017) 622.[10] C.R. Newman, C.D. Frisbie, A. Demetrio, S. Filho, J. Bre- introduction to organic thin film transistors and design of n-channel organic semiconductors, Chemistry Materials, 16(2004) 4436-4451.[11] M. Asad, N. Abdul, Chapter 9: Sol-Gel-Derived Doped ZnO Thin Films: Processing, Properties, and Applications, in Recent Applications in Sol-Gel Synthesis, Edt:C. Usha. InTech, Rijeka, Croatia, 2017. [12] D. Guo, K. Sato, S. Hibino, T. Takeuchi, H. Bessho, K. Kato, Low-temperature preparation of (002)-oriented ZnO thin films by sol–gel method, Thin Solid Films, 550 (2014), 250-258. [13] S. T. Meyers, J. T. Anderson, C. M. Hung, J. Thompson, J. F. Wager, D. A. Keszler, Aqueous Inorganic Inks for Low-Temperature Fabrication of ZnO TFTs, J. Am. Chem. Soc, 130 (2008), 17603-17609.