Electrical and optical characterizations of InAs/GaAs quantum dot solar cells

The electrical and optical characterizations of InAs/GaAs quantum dot solar cells (QDSCs) were investigated by frequency dependent capacitance-voltage (C-V) measurements and photoreflectance (P-R) spectroscopy. The C-V results confirmed that the frequency dependent junction capacitance (C-j) of QDSC is sensitive to the carrier exhaustion process through trapping and recapturing in the strain-induced defects and QD states caused by the interface strain between InAs and GaAs materials. As a result, at a low frequency (<= 200 kHz), the C-j of the QDSCs decreased with increasing InAs deposition thickness (0), leading to the decrease in carrier concentration (N-d) of the n-GaAs absorber layer due to the carrier losses processes caused by the trapping and re-capturing in the defects and the relatively large QDs. At 0 <= 2.0 ML, the p-n junction electric field strength (F-pn) of the QDSCs which was evaluated by PR spectra decreased with increasing excitation photon intensity (I-ex) due to the typical field screening effect in the SC structure. On the other hand, the Fpn of QDSCs with theta >= 2.5 ML approached a constant value with a relatively high Iex, which suggests that the decrease in photo-generated carriers in the QDSC was caused by the re-capturing and trapping process.