Vol.:(0123456789) 1 3 Applied Physics A (2018) 124:824 https://doi.org/10.1007/s00339-018-2251-8 RAPID COMMUNICATION Thin flms of nanostructured ZnO used as an electron-transporting material for improved performance of perovskite solar cells Khalid Mahmood 1  · Arshi Khalid 2  · Madsar Hameed 1  · Faisal Rehman 1  · Muhammad Taqi Mehran 3  · Rak‑Hyun Song 4 Received: 4 July 2018 / Accepted: 13 November 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract A low-temperaure hydrothermal method has been used to grow ZnO nanostructures including two distinct morphologies, nanodisks and nanowells as electron-transporting materials (ETMs) for high-efcieny perovskite solar cells. The perovskite solar cells based on ZnO nanowells demonstrate the maximum power conversion efciency (PCE max ) of 16.65%, compared to ZnO nanodisks and mesoporous flm, possibly thanks to better morphology, faster electron transport and complete infltra- tion of the perovskite absorber into the ZnO nanowells. 1 Introduction In the recent past, perovskite solar cells (PSCs) have attained rapid development, with power conversion efciency (PCE) from 3 to 22.1%, attributed to high carrier mobility, high optical absorption coefficient, longer carrier diffusion lengths and its simple fabrication process at low temperature [1–3]. Until now, the state-of-the-art PSCs mainly employ a TiO 2 nanoparticulate flm as an electron transport material (ETM), both in the mesostructured and planar confgurations [4–6]. However, processing at high annealing temperatures, precise interface modifcations and elemental doping are mostly required to enhance the functionality of TiO 2 -based ETMs, which mainly limits their further usage in commer- cial applications such as stretchable cell devices [6, 7]. On the other hand, ZnO facilitates the growth of various nano- structures at much lower temperatures, with high electron mobility and ease of tailoring its optical and electrical prop- erties by simply altering its composition and morphology [8–14]. Specifcally, in PSCs, ZnO with one-dimensional (1-D) and two-dimensional (2-D) morphologies in the form of nanorods, nanosheets and mesoporous flm have been studied recently with device efciencies of maximum up to 16.1% [9, 10]. However, the use of 2-D ZnO nanowells and nanodisks synthesized by a low-temperature hydrothermal method has not been reported yet and needs to be explored to obtain devices with better efciency and functionality. Considering that the device efciency is predominantly ETMs’ morphology dependent, we herein report the fab- rication of PSCs based on ZnO nanowells and nanodisks prepared by a simple and facile hydrothermal route. ZnO nanowells showed superior performance with a maximum PCE (PCE max ) of 16.65%, compared to both ZnO nanodisks and only mesoporous flms, thanks to the better perovskite infltration and improved electron transfer at the ZnO/per- ovskite interface. Khalid Mahmood and Arshi Khalid contributed equally to this work. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00339-018-2251-8) contains supplementary material, which is available to authorized users. * Khalid Mahmood khalid@kaist.ac.kr 1 Department of Chemical and Polymer Engineering, University of Engineering and Technology Lahore, Faisalabad Campus, 3½ Km. Khurrianwala, Makkuana By-Pass, Faisalabad, Pakistan 2 Department of Humanities and Basic Sciences, University of Engineering and Technology Lahore, Faisalabad Campus, 3½ Km. Khurrianwala, Makkuana By-Pass, Faisalabad, Pakistan 3 University of Science and Technology (UST), 217-Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea 4 New and Renewable Energy Research Division, Korea Institute of Energy Research (KIER), 152-Gajeong ro, Yuseong gu, Daejeon 34129, South Korea