Improvement in the Performance of Perovskite Solar Cells by Reducing Defect Density with Antisolvent Treatment

Abstract

In this work, we have investigated the influence of chlorobenzene antisolvent treatment on perovskite film formation in inverted planar perovskite solar cells. Herein, we report the formation of smooth, pin-hole free flat homogeneous perovskite film by one-step antisolvent induced crystallisation method for the device configuration of ITO/PEDOT:PSS/ CH3NH3PbI3 xClx/PC71BM/Al. The process involves spin-coating of perovskite precursor followed by addition of antisolvent liquid (chlorobenzene) in the mid of the spin-coating process. This antisolvent treatment process has shown an effective way to improve the morphology of the perovskite film as well as photovoltaic performance of the PSCs. The significant superior photovoltaic performance has been obtained with antisolvent treated PSCs having average PCE ~ 11.5% as compared to conventional spin-coated planar PSCs with an average PCE ~ 8% as shown in Fig 1.
In order to investigate this enhanced photovoltaic performance in antisolvent assisted PSCs, systematic investigations using optical absorption, IPCE, electroluminescence (EL), electrochemical impedance spectroscopy (EIS), capacitance-voltage (C-V), XRD, and scanning electron microscopy (SEM) have been carried out.
An enhanced absorption in antisolvent treated films in 400-650 nm wavelength range was observed which was also supported by IPCE measurements. We studied the electroluminescence (EL) emission spectrum for distinguishing the recombination losses. Conventional PSCs showed an increased loss due to non-radiative recombination as compared to antisolvent treated PSCs. This increased recombination loss was confirmed with electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) analysis. Capacitance-voltage (C-V) measurement using MS plot has been used for distinguishing the effects taking place in bulk of the active layer and those occurring at the interface of perovskite/ETL or HTL [1,2]. We found lower built-in potential (Vbi) of 0.77 V and higher defect density (N) of 24.6 x 1016 cm-3 for conventional planar PSCs as compared to antisolvent treated PSCs with Vbi ~ 1.135 V and N ~ 11.1 x 1016 cm-3 [3]. The higher defects exert extra potential barrier and hinder the charge carrier’s extraction. It also induces trap-assisted recombination and reflects as lower Voc and PCE as in case of conventional PSCs. The lower trap density and higher Vbi results in lower non-radiative recombination which is beneficial to raise both the Voc and PCE for antisolvent PSCs. In EIS studies, we have found two signals in the form of arcs in Nyquist plot and in the form of peak in bode plot at 104-105 Hz and 1 Hz respectively. We found a good agreement between ideality factor (ηid) determined by two different methods viz. Sun-Voc (extracted from J-V curves) and Sun-RHF (extracted from EIS). Ideality factor of 1.64 and 2.28 has found for antisolvent treated and conventional PSCs respectively [4]. The higher ηid confirms the increased recombination loss in conventional PSC.

Fig. 1. Current density−voltage curves of devices fabricated with conventional and antisolvent treated PSCs. Inset shows the schematic diagram of the planar inverted perovskite device.

The XRD measurements on the films revealed that antisolvent treatment causes improved crystallinity of perovskite structure and preferred crystal growth in (110) direction. SEM studies also confirmed the formation of smooth and pin-hole free perovskite film formation with antisolvent treated perovskite film. It is therefore concluded that the enhanced photovoltaic performance in antisolvent-assisted PSCs is attributed to smooth pin-hole free perovskite film morphology having lower defect density and recombination losses.