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

© 2018, Springer-Verlag GmbH Germany, part of Springer Nature. 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 (PR) 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 (θ), 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 θ ≤ 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 F pn of QDSCs with θ ≥ 2.5 ML approached a constant value with a relatively high I ex , which suggests that the decrease in photo-generated carriers in the QDSC was caused by the re-capturing and trapping process

Understanding intermediate-band solar cells

The intermediate-band solar cell is designed to provide a large photogenerated current while maintaining a high output voltage. To make this possible, these cells incorporate an energy band that is partially filled with electrons within the forbidden bandgap of a semiconductor. Photons with insufficient energy to pump electrons from the valence band to the conduction band can use this intermediate band as a stepping stone to generate an electron–hole pair. Nanostructured materials and certain alloys have been employed in the practical implementation of intermediate-band solar cells, although challenges still remain for realizing practical devices. Here we offer our present understanding of intermediate-band solar cells, as well as a review of the different approaches pursed for their practical implementation. We also discuss how best to resolve the remaining technical issues