Abstract—Mesoscopic perovskite solar cells (mp-PSCs) with mesoporous bilayer were fabricated under ambient conditions. The bilayer was formed by capping the mesoporous TiO 2 layer with a layer of In 2 O 3 . CH 3 NH 3 I 3-x Cl x mixed halide perovskite was prepared through the one-step method and was used as the light absorber. The mp-PSCs with the composite TiO 2 /In 2 O 3 mesoporous layer exhibited optimized electrical parameters, compared with the PSCs that employed only a TiO 2 mesoporous layer, with a current density of 23.86 mA/cm 2 , open circuit voltage of 0.863 V, fill factor of 0.6 and a power conversion efficiency of 11.2%. These results indicate that the formation of a proper semiconductor capping layer over the basic TiO 2 mesoporous layer can facilitate the electron transfer, suppress the recombination and subsequently lead to higher charge collection efficiency. Keywords—Ambient conditions, high efficiency solar cells, mesoscopic perovskite solar cells, TiO 2 /In 2 O 3 bilayer. I. INTRODUCTION N the past years a new class of third generation photovoltaics has emerged based on organometal halide perovskites. While results of the first efficient perovskite solar cells (PSCs) were published in 2012 [1], their efficiencies were rapidly lifted to 20% the very next year. Moreover, in early 2016 researchers managed to manufacture PSCs with improved stability and a power output of 21.1% [2]. Perovskite materials are direct bandgap semiconductors described by the general formula ABX 3 , where X is an anion and A and B represent cations. The cation A is organic, typically methylammonium or formamidinium while cation B is lead (Pb) or tin (Sn) and the anion X is a halogen ion usually iodine, chlorine, bromine or a mixture of them [3]-[5]. For the fabrication of PSCs there are basically two different device architectures [6]. Planar or thin-film PSCs (Fig. 1 (a)) consist of a flat perovskite layer between n-type and p-type semiconductor. In this device structure, once the incident light is absorbed, the charge generation and the charge extraction are both occurring in the perovskite layer [7]-[9]. In the Andigoni Apostolopoulou and Dimitra Sygkridou are PhD candidates with the Physics Department, University of Patras, 26500, Greece (e-mail: andapostolop@upatras.gr, dsigridou@upatras.gr). Alexandros N. Kalarakis is with the Mechanical Engineering Department, Technological-Educational Institute of Western Greece, 26334 Patras, Greece; (e-mail: a_kalarakis@teiwest.gr). Elias Stathatos is with the Electrical Engineering Department, Technological-Educational Institute of Western Greece, 26334 Patras, Greece (corresponding author to provide phone: 0030-2610-369242; e-mail: estathatos@teiwest.gr). mesoscopic or sensitized PSCs (Fig. 1 (b)) the active layer consists of a mesoporous semiconductor which is sensitized with the perovskite. The perovskite absorber infiltrates the semiconductor’s layers creating a semiconductor-perovskite interface. Once the light is absorbed from the perovskite, the generated electrons are injected to the n-type semiconductor from where they are extracted and the holes are transferred to the p-type semiconductor [10], [11]. Fig. 1 Schematic figure of (a) planar (thin-film) and (b) mesoscopic (sensitized) PSCs [12] In this work, we demonstrate the results of CH 3 NH 3 PbI 3- x Cl x mixed halide organic-inorganic mesoscopic PSCs where the mesoporous layer is a combination of semiconductors, particularly TiO 2 and In 2 O 3 . The combination of two (or more) different semiconductors can facilitate the electron transfer from the second semiconductor (In 2 O 3 ) to the first one (TiO 2 ), provided that the conduction band edge of the second semiconductor is higher than the conduction band edge of the first semiconductor (Fig. 2) [13], [14]. Composite semiconductors have also been used in dye-sensitized solar cell (DSSC), which is another type of third generation photovoltaics, as they suppress the recombination processes and accelerate the electron transport, leading to higher charge collection efficiency [15], [16]. The structure that was used for the reference PSCs, without the additional step of the surface treatment of the mesoporous TiO 2 layer, was FTO/compact- TiO 2 /mesoporous-TiO 2 /CH 3 NH 3 PbI 3-x Cl x /P 3 HT/Au. For the PSCs with the capping layer of In 2 O 3 the structure that was employed was FTO/compact-TiO 2 /mesoporous-TiO 2 /In 2 O 3 / CH 3 NH 3 PbI 3-x Cl x /P3HT/Au. These cells are compared against each other morphologically and electrically and the results are presented and discussed in the following paragraphs. High Efficiency Perovskite Solar Cells Fabricated under Ambient Conditions with Mesoporous TiO 2 /In 2 O 3 Scaffold A. Apostolopoulou, D. Sygkridou, A. N. Kalarakis, E. Stathatos I World Academy of Science, Engineering and Technology International Journal of Electronics and Communication Engineering Vol:11, No:1, 2017 113 International Scholarly and Scientific Research & Innovation 11(1) 2017 ISNI:0000000091950263 Open Science Index, Electronics and Communication Engineering Vol:11, No:1, 2017 publications.waset.org/10006933/pdf