Annealing effect of pentacene and ultrathin metal anode buffer on the performance of small-molecule organic solar cells

Abstract

The annealing effect of thin pentacene and ultrathin gold/silver anode buffer layers on both the initial performance and durability against repetitive illumination stress in small-molecule organic solar cells (OSCs) was experimentally investigated. The OSC had a structure of indium tin oxide (ITO, anode)/anode buffer/copper phthalocyanine (CuPc, donor)/fullerene (acceptor)/bathocuproine (cathode buffer)/Ag (cathode). OSCs with a 3-nm-thick pentacene anode buffer layer provided a 15% increase in the power conversion efficiency compared to OSCs with no pentacene layer. OSCs with an ultrathin 0.3-nm-thick Au anode buffer layer provided an increase of 63% in durability compared to OSCs without the Au layer. OSCs with the Au layer did not show apparent leakage current, while those with the Ag layer did. OSCs with a combinational anode buffer of the pentacene layer/Au layer were mainly examined. The spatial distribution of Au atoms on pentacene was uniform even after annealing, which helped to improve the quality of CuPc crystals. The ultrathin Au layer brought both a large absorption of ultraviolet (UV) light and highly dense nucleation sites for overlaid CuPc film formation. These were possible reasons for durability improvement. The Au layer thickness of 0.3 nm on pentacene was optimal to achieve the condition with both less attenuation of visible light necessary for electric power generation and larger absorption of UV light causing degradation. Annealing treatment before CuPc evaporation brought both preferable initial performance and durability for OSCs with a combinational structure of the ITO/pentacene layer/Au layer. An appropriate region of annealing temperature had, however, a small window around 100°C.