Optical study of high index substrate effect in multilayer InAs/GaAs quantum dot solar cells

Abstract

We present a study of the optical properties of InAs quantum dot (QD) solar cells (SC) grown by molecular beam epitaxy on GaAs(11N)A orientation substrates (N = 4, 5). Optical properties of InAs/GaAs (11N)A QD SCs were characterized by spectroscopic ellipsometry (SE), photoluminescence (PL) and photocurrent (PC) measurements. From SE data, optical constants are calculated and the different transition energies for the two SCs are calculated and identified in the energy range 1–6 eV. The surface and volume energy loss functions are calculated and show different dependences on the incident photon energy for the two InAs/GaAs QD SCs in the energy interval 1–6 eV. PL measurements revealed good optical properties of the InAs QD SC grown on GaAs(115)A compared to that grown on GaAs(114)A orientation substrate. Two additional peaks are observed at 1.17 and 1.34 eV in the PL spectrum of the SC grown on the (115)A direction and are attributed to the transitions between excited states in the InAs QDs. Photocurrent measurements show that above GaAs bandgap, higher photocurrent is measured for the sample grown on the (114)A orientation. This was attributed to the multiple exciton generation in InAs QDs since in this sample the built-in field is strong enough to accelerate photocarriers generated in GaAs.