| AVS 62nd International Symposium & Exhibition | |
| Energy Frontiers Focus Topic | Tuesday Sessions |
| Session EN-TuP |
| Session: | Energy Frontiers Poster Session |
| Presenter: | Kenji Harafuji, Ritsumeikan University, Japan |
| Authors: | K. Harafuji, Ritsumeikan University, Japan H. Sato, Ritsumeikan University |
| Correspondent: | Click to Email |
Organic solar cells (OSCs) offer many desirable properties such as flexibility, low cost, and easy fabrication. The lifetime is, however, still short. The degradation may be driven by light illumination, exposure to external air, and high temperature.
The degradation phenomenon under repetitive light illumination and high-vacuum is experimentally investigated in small molecular OSCs. The OSC has a structure of an indium tin oxide (anode)/copper phthalocyanine (donor, 20 nm)/fullerene (acceptor, 40 nm)/bathocuproine (buffer, 10 nm)/Ag (cathode, 100 nm). Xenon lamp illumination with an intensity of 100 mW/cm2 is performed, and an air mass 1.5 global spectrum is obtained using a solar simulator. The repetitive illumination with the period of 30 s is composed of 100 repetitions of 3 s illumination followed by 27 s in the dark. The effect of light wavelength on the degradation is studied with the use of a long-pass filter of cut-wavelength λc. The filter allows the transmission of light into the OSC only with its wavelength longer than λc. Six kinds of filter with λc=0, 320, 370, 400, 550 and 665 nm are used.
It is found that the OSC degradation is dominated by the UV (ultra-violet) light with wavelength less than 400 nm. The short-circuit current density Jsc, open-circuit voltage Voc, and fill factor FF in the case of λc=0 nm (without the filter) are decreased approximately 20%, 40%, and 60% compared with the case of λc greater than 400 nm at the 100th illumination, respectively. Resultantly, power conversion efficiency ηp is decreased 80%. The ηp value is decreased 60% with the filter of λc =320 nm, whereas initial ηp is almost kept even at 100th illumination with the filter of λc =400 nm. An S-shaped kink appears near the Voc region in the current-voltage (J-V) characteristics at 100th illumination in the case of λc shorter than 400 nm. This shows the increase of series resistance in the OSC. The origin may be due to the disorder of crystal structure at the anode/donor interface. There are no appreciable changes of J-V characteristics between at 1st and 100th illuminations in the case of λc longer than 400 nm.
On the other hand, the initial ηp value at the first illumination is much smaller as λc is increased in the range of λc longer than 400 nm, accompanied by the large decrease of Jsc. This is because the total amount of light energy transmitted into active organic layers is decreased as λc is longer.
The origin of the degradation by illumination is revealed to be the UV light component with its wavelength shorter than 400 nm. The optimum λc value is 400 nm from the compromise between initial efficiency ηp and OSC degradation.