Highly Efficient Flexible Perovskite Solar Cells Using Solution-Derived NiO x Hole Contacts Xingtian Yin,* ,† Peng Chen, † Meidan Que, † Yonglei Xing, † Wenxiu Que,* ,† Chunming Niu, ‡ and Jinyou Shao § † Electronic Materials Research Laboratory, International Center for Dielectric Research, Key Laboratory of the Ministry of Education, School of Electronic & Information Engineering, ‡ Center of Nanomaterials for Renewable Energy (CNRE), State Key Lab of Electrical Insulation and Power Equipment, School of Electrical Engineering, and § State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi People’s Republic of China *S Supporting Information ABSTRACT: A solution-derived NiO x film was employed as the hole contact of a flexible organic−inorganic hybrid perovskite solar cell. The NiO x film, which was spin coated from presynthesized NiO x nanoparticles solution, can extract holes and block electrons efficiently, without any other post-treatments. An optimal power conversion efficiency (PCE) of 16.47% was demonstrated in the NiO x -based perovskite solar cell on an ITO-glass substrate, which is much higher than that of the perovskite solar cells using high temperature-derived NiO x film contacts. The low-temperature deposition process made the NiO x films suitable for flexible devices. NiO x -based flexible perovskite solar cells were fabricated on ITO-PEN substrates, and a preliminary PCE of 13.43% was achieved. KEYWORDS: NiO x , hole contact, flexible, perovskite solar cell O rganic−inorganic hybrid perovskites have been demonstrated to be efficient light absorbers for solar cells with a power conversion efficiency (PCE) exceeding 20%. 1−4 Perovskite solar cells with different structures have been explored and investigated in detail during the past several years, including perovskite sensitized solar cells, 5−7 mesoscopic perovskite solar cells, 8−10 and planar heterojunction perovskite solar cells. 11−13 The planar hetero- junction perovskite solar cells have the simplest structures among these devices due to the absence of high-temperature- derived mesoporous layers. 14 As one important class of perovskite solar cells, inverted planar heterojunction perovskite solar cells with a p-i-n structure attracted considerable attentions. They not only have less serious hysteresis than the normal n-i-p structured planar heterojunction devices, 15 but also can be fabricated through a low-temperature solution route using poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT:PSS) films as the hole contact layers, making them very suitable for flexible solar cells. 16−19 For example, hysteresis-less inverted planar heterojunction perovskite solar cells with a PCE of 18.1% have been demonstrated with PEDOT:PSS hole contact films. 20 However, unfortunately, PEDOT:PSS is not good for device long-term stability due to its high acidity and hygroscopicity, which has been already demonstrated in organic photovoltaics and light emitting diodes (LED). 21 Therefore, different materials, especially the inorganic hole transport materials, have been employed to replace PEDOT:PSS layers in inverted perovskite devices, such as PbS quantum dots, 22 CuSCN films, 23, 24 and NiO x films. 23,25−29 Particularly, research on NiO x -based perovskite solar cells have achieved a great progress. 25 However, the employed NiO x films were deposited by using an expensive pulse laser deposition method, which is not suitable for large scale fabrication. Furthermore, a postannealing treatment must be conducted at 200 °C to improve the quality of the NiO x films, making them incompatible with flexible substrates. Besides, efficient NiO x hole contacts can also be prepared by using low cost solution methods. Unfortunately, the indis- pensable annealing process at 300−500 °C 23,26−30 prevents their application in flexible solar cells. Recently, Jen et. al employed a combustion method to prepare Cu-doped NiO x hole contact for perovskite solar cell, and improved the PCE to 17.74%. 31 Although low-temperature sputtered NiO x films may be compatible with flexible devices, their low PCE even on rigid substrates (below 10%) makes them unattractive to the research society. 32 Actually, most reported inverted flexible perovskite solar cells are based on organic hole transport materials, especially PEDOT:PSS. 18,33 Thus, it is very mean- Received: December 24, 2015 Accepted: March 9, 2016 Article www.acsnano.org © XXXX American Chemical Society A DOI: 10.1021/acsnano.5b08135 ACS Nano XXXX, XXX, XXX−XXX