|Long-term efficiency gain of Cu(In,Ga)Se2 solar cell|
|Deok-In Kim, Sang-Wook Park, Soon-Rok Park, Ju-Young Baek, Tae-Young Yun, Hye-Jin Han, Chan-Wook Jeon
Yeungnam University, Gyeongsan, Korea
Long-term efficiency gain of Cu(In,Ga)Se2 solar cell Deok-In Kim, Sang-Wook Park, Soon-Rok Park, Ju-Young Baek, Tae-young Yun, Hye-Jin Han, Chan-Wook Jeon* Yeungnam University, Gyongsan, Gyoengbuk, 712-749, Korea The electrical properties of (Al-doped ZnO)/ZnO/CdS/CIGS/Mo/glass solar cells were measured repeatedly to trace the long-term variation of device characteristics over more than six months from the date of the first measurement. The solar cell efficiency was found to increase from 11.5% to 14.3% (or from 14.9% to 16.7%) over the time period. While the short-circuit current density and fill factor was saturated within 20 days, the open circuit voltage increased continuously onward. The long-term aging effect was found to be irreversible by keeping the device in the dark unlike the reversible short-term transient behavior. Also, the aging effect was found to completely disappear by thermal annealing as mild as 100 oC for 10 min. The persistent improvement of the devices was accompanied with substantial increase of doping density in the space charge region. The observation of such a long-term performance gain cannot be interpreted by the same mechanism as used in the short-term transient, where most of variations take place within a day at most and they are reversible. An actual mass transport should be considered to account for the variation observed over several months period. It seems plausible that Cu migration, driven by the electric field in p-n junction, increased acceptor density, most probably VCu, in the space charge region, which neutralize the compensating donors. By annealing, formation of InCu is suggested to become more favorable and the doping density would decrease again resulting in extinction of the aging effect.