Characterisation of long wavelength Photovoltaic devices based on GaAs/InAs Quantum Dots

Photovoltaic (PV) devices based on semiconductor materials have been demonstrated to be a good option for electricity generation. Many research efforts have been devoted to semiconductors solar PV in order to improve their efficiencies. One of the ways is to extend the absorption range of the device to the mid infra red. This can be achieved with the insertion of quantum dots (QDs) into a host lattice, which introduce new energy states. The doping of such structures has also proved to enhance the open-circuit voltage which in turn increase the device efficiency. The key challenge though is the growth of QDs based materials with expanded absorption spectrum and high open circuit voltage (Voc). We present the electrical and optical properties of several GaAs/InAs quantum dots (QDs) structures grown on GaAs substrate using Molecular Beam Epitaxy and having different doping profiles (0, 8x1010 and 16x1010 /cm2 in between the dots). In addition to the 1.4 eV (GaAs bandgap) energy state, two more energy states (1.37 and 1.2 eV) were generated by the incorporation of the InAs quantum dots. Very simple structures consisting of circular diodes and without any antireflection coating or attempts at reducing shadow effects from the contacts were tested. The external quantum efficiency (EQE) of the fabricated devices was measured for both undoped and n-doped structures. The doped samples exhibited lower EQE compared to the undoped one, which has a maximum EQE of 40%. The I-V characteristics under light were obtained using a 100mW/cm2 power light. The Voc of the n-doped material was extended up to the value of 0.8 volts but at the expense of the short-circuit current (Isc), which was reduced to 15.6 from ~ 20mA/cm2. The obtained Voc and Isc values are suitable for solar cell efficiency improvement. The QD device efficiencies were calculated. The doped 8x1010 /cm2 samples showed the highest efficiency of ~10 %, an excellent value for these simple structures.