High efficient ITO free inverted organic solar cells based on ultrathin Ca modified AZO cathode and their light soaking issue

Abstract

Aluminum doped zinc oxide (AZO) was used to be the cathode instead of indium-tin-oxide (ITO) in the poly (3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) based bulk heterojunction inverted organic solar cells (IOSCs). For the AZO only IOSC, the device shows a poor power conversion efficiency (PCE) of 1.34% and a light soaking issue related to the energy barrier at the AZO/P3HT:PCBM interface. When a 5nm Ca modifying layer is inserted between AZO and P3HT:PCBM, the obtained AZO/Ca (5nm) IOSC shows an increased PCE from 1.74% to 2.69% after 15min illumination. It is thought that the increased photoconductivity of AZO/Ca (5nm) film upon illumination and the enhanced electron transport across the AZO/Ca interface may be responsible for the light soaking issue. When an ultrathin Ca modifying layer of 1nm is employed, a further improved PCE of 3.17% is obtained, and remarkably, no light soaking issue is observed in this case. However, this unexpected issue appears after the un-encapsulated AZO/Ca (1nm) IOSC has been stored in air for several days, which may be due to the energy loss in the electron transport across the interface between partly oxidized Ca and AZO layers induced by the oxidization of Ca. Furthermore, the AZO/Ca (1nm) IOSC has a comparable PCE to the referenced ITO/Ca (1nm) IOSC and presents a better air-stability. It is thus concluded that the AZO cathode is a promising alternative of ITO to fabricate the high efficient and long-lifetime IOSCs.