(Invited) Lead-Free Perovskite Solar Cells

Abstract

Perovskite solar cells (PSC) have attracted much attention in recent years due to the rapid progress on the performance of PSC to attain PCE over 22 % within 5 years. However, conventional perovskite light absorbers contain the toxic lead element that should be replaced by other environmentally benign elements. Herein we report pure tin-based PSC based on various device configurations. To make a stable pure tin perovskite free of lead, we varied SnCl2/SnBr2 ratios to yield tin perovskites with three halides (I, Br and Cl) co-crystallized inside the tin-perovskite crystals (MASnIBr2-x Cl x ) according to the stoichiometric ratios of the precursors. When the SnCl2 proportion was equal to or greater than 50 % (x ³ 1), phase separation occurred to generate MASnI3-y Br y and MASnCl3-z Br z according to the stoichiometric proportions of their precursors, confirmed with the corresponding XRD analysis. A device with MASnIBr1.8Cl0.2 (SnCl2 = 10 %) showed the best photovoltaic performance: J SC = 14.0 mA cm-2, V OC = 380 mV, FF = 0.573 and PCE = 3.1 %, with great reproducibility and long-term stability. For the inverted planar tin-based PSC, we investigated the doping effect of bulky organic cations with ethylenediammonium diiodide (EDAI2) and butylammonium iodide (BAI) as additives to enhance the performance and stability of the FASnI3 perovskite solar cells. The additive EDAI2 plays a key role to cause slow passivation of the surface and relaxation of crystal strain such that the device performance increases gradually with increasing duration of storage. In the presence of EDAI2 additive (1 %) the FASnI3 device attained the best initial efficiency 7.4 % and the device performance continuously increased as a function of duration of storage; the maximum PCE, 8.9 %, was obtained for a device stored in a glove box for over 1400 h with only slight degradation for storage beyond 2000 h. Moreover, hybrid cationic GA/FA tin-based PSC was optimized at precursor ratio GAI:FAI = 20:80 to attain PCE 8.5 % when prepared freshly; the efficiencies continuously increased to attain a record PCE 9.6 % after storage for 2000 h. The fresh device passed all strict verification steps and was certified to give the efficiency of power conversion 8.3 %. The great performance and stability of the device reported herein have attained a new milestone for lead-free perovskite solar cells on a path toward commercial development.