InGaAs/GaAs Quantum Dot Solar Cells by Metal Organic Chemical Vapour Deposition

Abstract

Along with the ongoing research and industry development to reduce the cost of conventional PV devices such as Si-based solar cells, significant research efforts have been focused on exploring new concepts and approaches for high efficiency III-V compound semiconductor solar cells, especially through the fast emerging nanotechnology to exploit the unique properties of nanostructures such as self-assembled quantum dots (QDs). By incorporating self-assembled QDs into the intrinsic region of a standard p-i-n solar cell structure during the epitaxial growth, photons in the solar spectrum with energy lower than the energy gap of the host material can be absorbed by the QD layers, leading to an extended photoresponse to longer wavelengths and hence larger photocurrent. In addition, the size and composition of the QDs can be varied and thereby allowing the bandgap to be tuned for absorption in different regime of the solar spectrum. However, due to the small QD absorption cross section, the increase of photocurrent in QDSCs is not significant and always accompanied with some reduction in other device characteristics such as the open circuit voltage and fill factor. In this thesis, self-assembled In0.5Ga0.5As/GaAs QDSCs have been designed, fabricated, characterized and investigated in comparison with conventional GaAs p-i-n solar cells. The properties and fundamental mechanisms behind their complicated photoelectrical behaviours were analysed and understood. Several approaches were proposed and carried out to improve the device performance of QDSCs, either during the epitaxial growth process or after the growth and fabrication of the solar cells. Stacking more QD layers is supposed to enhance the total volume of QD material and hence the light absorption. We carried out experiments to grow QDSC structures with increased number of QD layers. However, much reduced photocurrent and conversion efficiency for 15 and 20-layer samples were observed, which could be due to low carrier extraction efficiency and strain-induced defects. In order to improve the carrier extraction efficiency and consequently more enhanced photocurrent, modulation doping has been introduced into QDs layers to partially populate the confined states with electrons. The modulation doping has been found to be effective to improve carrier transport and collection efficiency, leading to an enhancement of the external quantum efficiency over the whole solar cell response range and thus the conversion efficiency. We have also taken two different post-growth approaches to improve the QDSC efficiency, namely the rapid thermal annealing and surface plasmonic light trapping. Firstly, QDSCs with different layers were annealed at various temperatures between 700 and 850 °C with the device annealed at the highest temperature of 850 °C displayed the highest efficiency increase of 41.42 % from 10.26 % to 14.51 %, compared to the as-grown sample. Secondly, it was found that a combination of 120 nm diameter hemispherical Ag nanoparticle and a 5 nm thick TiO2 dielectric film pre-deposited on the back of the GaAs substrate was the optimum light trapping configuration for our QDSC. The QDSC spectral response was improved by 35.7% over the 900 nm- 1200 nm wavelength range, leading to enhancements in both Jsc and Voc and an overall efficiency enhancement of 7.6 % compared to the reference QD solar cell. Show more