Takahito Nishimura

Control of valence band offset at CdS/Cu(In,Ga)Se2 interface by inserting wide-bandgap materials for suppression of interfacial recombination in Cu(In,Ga)Se2 solar cells

We inserted Cu(In,Ga)3Se5 into the CdS/Cu(In,Ga)Se2 interface of Cu(In,Ga)Se2 solar cells with a flat band profile and energy bandgaps (Eg) of 1.2 and 1.4 eV in order to investigate the repelling of holes by the effect of valence band offset (ΔEv). We found that open circuit voltage (VOC) was clearly improved from 0.66 to 0.75 V with Eg of 1.4 eV, although VOC was only increased from 0.63 to 0.64 V with Eg of 1.2 eV. For high efficiency, we fabricated Cu(In,Ga)Se2 solar cells with a single-graded band profile and an average Eg of 1.4 eV. Eventually, a conversion efficiency of 14.4% was obtained when Cu(In,Ga)3Se5 with a thickness of 30 nm was inserted, although the conversion efficiency was 10.5% without Cu(In,Ga)3Se5. These results suggest the importance of ΔEv in the suppression of interfacial recombination by repelling holes and possibility that the highest efficiency of Cu(In,Ga)Se2 solar cells with an average Eg of 1.4 eV could be achieved.

Interfacial quality improvement of Cu(In,Ga)Se2 thin film solar cells by Cu-depletion layer formation

Se irradiation with time, t Se, was introduced after the second stage of a three-stage process to control the Cu2Se layer during Cu(In,Ga)Se2 (CIGS) deposition. Open circuit voltage and fill factor of CIGS solar cells could be improved by introducing Se irradiation. We concluded that the control of the Cu2Se layer led to the formation of a Cu-depletion CIGS layer (CDL), which improved conversion efficiency owing to suppression of interfacial recombination by a valence band offset formed between CIGS and the CDL. Finally, highest efficiency of 19.8% was achieved with t Se of 5 min. This very simple and new technique is promising for the improvement of photovoltaic performance.

Improvement of Cu(In, Ga)Se 2 photovoltaic performance by adding Cu-poor compounds Cu(In, Ga) 3 Se 5 at Cu(In, Ga)Se 2 /CdS interface

We report on the improvement of Cu(In, Ga)Se₂ (CIGS) photovoltaic performance by inserting Cu-poor compounds Cu(In, Ga)₃Se₅ at CIGS/CdS interface. It was experimentally found that formation of the Cu(In, Ga)₃Se₅ layer at the CdS/CIGS interface was quite important to boost the photovoltaic performance. Cd diffusion was promoted by introducing the layer, and conductivity type turned to n-type near the surface. As a result, cell performance was improved owing to suppression of recombination at the interface by stronger band-bending. Additionally, it was shown that the interval time before the growth of the Cu(In, Ga)₃Se₅ layer is important to eliminate the Cu intermixing in CIGS.