Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat Enhanced efciency and thermal stability of mesoscopic perovskite solar cells by adding PC 70 BM acceptor Rahul Ranjan a , Belal Usmani b , Sudhir Ranjan a , Hasitha C. Weerasinghe c , Anand Singh d,∗∗ , Ashish Garg b,∗∗∗ , Raju Kumar Gupta a,e,∗ a Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India b Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, India c Flexible Electronics Lab, CSIRO Manufacturing, Clayton South, Victoria, 3168, Australia d Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, 208016, India e Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, Uttar Pradesh, India ARTICLEINFO Keywords: Mesoscopic perovskite solar cells PC 70 BM Additives Reduced grain boundaries Thermal stability ABSTRACT Perovskite-based solar cells (PSCs) have emerged as a very promising solar photovoltaic technology with fvefold increase observed in the device efciency within less than 10 years of its inception. The efciency of these solar cells is strongly afected by the quality of perovskite layer surface coverage and its grain size. Although use of antisolvents such as chlorobenzene has been used to treat methylammonium lead iodide (CH 3 NH 3 PbI 3 ) resulting in grain size of ca. 200–300 nm, one still needs to minimize the grain boundaries to reduce their impact on recombination. In this study, we have successfully incorporated [6, 6]-phenyl-C70-butyric acid methyl ester (PC 70 BM) acceptor as an additive in the perovskite layer of the mesoscopic solar cell devices, which not only leads to the formation of larger grains with fewer grain boundaries resulting in reduced charge carriers re- combination, the band alignment between CH 3 NH 3 PbI 3 and TiO 2 was also found to improve. The device having the optimum PC 70 BM concentration in perovskite showed enhanced power conversion efciency (PCE) due to improvement in current density (J SC ) and fll-factor (FF), correlated to the decrease in charge transfer resistance (R CT ). The devices using PC 70 BM additive exhibit over 15% improvement in the power conversion efciency or PCE (15.5% for PC 70 BM containing device and 13.3% PCE for the reference device with no acceptor additive). More importantly, the device containing PC 70 BM acceptor additive, retained ca. 75% of the original PCE for 250 h when kept at 85 °C without encapsulation in an ambient at 20% RH. 1. Introduction Solution-processed Organic-Inorganic hybrid perovskites (MAPbX 3 ) have emerged very strongly as next generation photovoltaic materials, owing to fast ambipolar charge transport, lower exciton binding energy, tunable band gap, strong light absorption in visible region (400–800 nm), long difusion length (up to 100 nm for halides and > 1000nm for mixed halides perovskites), and long carrier lifetime [1–6]. These excellent properties of perovskites along with the inter- facial engineering, additive incorporation, and improved flm deposi- tion methods have led to tremendous improvement in PCEs increasing from ~3% in 2009 to over 22% recently. These numbers are compar- able to silicon-based solar cells, thus making perovskite photovoltaic extremely attractive, easily processable low temperature technology to replace silicon solar cells in the near future whose processing is far more tedious starting from making semiconductor grade silicon [7,8]. Conventionally, PSCs can be fabricated either on mesoscopic or planar heterojunction structures. Planar confguration often consists of an active layer (perovskite) sandwiched between an electron transport layer (TiO 2 , SnO 2 )[9,10] and a hole transport layer (Spiro-OMeTAD (N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-9,9′-spirobi [9H-fuorene]-2,2′,7,7′-tetramine), P3HT (poly-3-hexylthiophene), PTAA (poly(triaryl amine), PEDOT/PSS (poly(3,4-ethylenediox- ythiophene) doped with poly(styrene sulfonate) anions) [11–13], whereas mesoscopic confguration employs a mesoporous scafold (TiO 2 ,Al 2 O 3 ) as electron transport layer into which perovskite material https://doi.org/10.1016/j.solmat.2019.110130 Received 2 June 2019; Received in revised form 18 July 2019; Accepted 15 August 2019 ∗ Corresponding author. Department of Chemical Engineering, Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, 208016, Uttar Pradesh, India. ∗∗ Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: anands@iitk.ac.in (A. Singh), ashishg@iitk.ac.in (A. Garg), guptark@iitk.ac.in (R.K. Gupta). Solar Energy Materials and Solar Cells 202 (2019) 110130 0927-0248/ © 2019 Elsevier B.V. All rights reserved. T