Effect of space layer doping on photoelectric conversion efficiency of InAs/GaAs quantum dot solar cells

We report an effect of photoelectric conversion efficiency (PCE) by space layer doping in InAs/GaAs quantum dot solar cells (QDSC) and delta-doped QDSC grown by molecular beam epitaxy. The PCEs of QDSC and delta-doped QDSC without anti-reflection coating were 10.8% and 4.3%, respectively. The QDSC had about four electrons per QD, and its ideality factor was temperature-independent, which implies that recombination of electron-hole pairs is suppressed by strong potential barriers around charged dots. From the deep level transient spectroscopy measurements, four defect levels, including QD with the activation energy ranges from 0.08 eV to 0.50 eV below GaAs conduction band edge, appeared. Especially, the M1 defect (E-c-0.14 eV) was newly formed in delta-doped QDSC and its density was higher than those of M3 (E-c-0.35 eV) and M4 (E-c-0.50 eV) levels in QDSC. These results suggest that the photo-carriers recombining at M1 defect might be responsible for the reduction of PCE in delta-doped QDSC. (c) 2015 AIP Publishing LLC.