Carrier Selective Contacts for Thin Film and Nanowire InP Solar Cells

Abstract

This thesis investigates, both theoretically and experimentally, the use of electron selective contacts for both planar and nanowire InP solar cells. We start this thesis with the optimization of electron selective contact for thin film InP solar cells. We proposed a new device structure, ITO/ZnO/i-InP/p+ InP (p-i-ZnO-ITO) which according to our simulation results in higher efficiency compared to conventional p-i-n junction solar cells because of improved passivation and carrier selectivity. As a proof-of-concept, we fabricated ITO/ZnO/i-InP solar cell on a p+ InP substrate and achieved an open-circuit voltage and efficiency as high as 819 mV and 18.12 %, respectively, along with ~90% internal quantum efficiency. Following the successful optimization of carrier selective contact for InP thin film solar cells, we optimized the electron selective contact for InP nanowire solar cells, both theoretically and experimentally. Using Finite-difference time-domain (FDTD) simulations, we showed that the use of a metal-oxide coating over InP core could significantly increase its absorption, while our device simulation results showed that even for a core minority carrier lifetime of 50 ps, an efficiency of 23% can be obtained, under optimized conditions. Based on simulation results, we fabricated a radial p-n junction nanowire solar cell by etching a heavily doped p-type InP substrate that has an extremely low minority carrier lifetime (less than 100 ps) to form the nanowire core, which was then coated with an aluminium zinc oxide/ ZnO shell. These radial junction nanowire devices exceeded an efficiency of 17%, the best reported value for radial junction nanowire solar cells. Although CuI is a suitable material as a hole selective contact, it can lose its conductivity and transparency with time, which made it less than desirable in real device application. Therefore, a part of this thesis investigates the optimization of CuI to increase its stability in ambient condition without compromising its transparency and conductivity. We showed that the introduction of TiO2 in CuI made it more stable in ambient condition while also improving its conductivity and transparency. A detailed comparative analysis between CuI and CuI-TiO2 composite thin films were performed to ascertain the reasons for improved performance of CuI-TiO2 composite thin films in comparison to pure CuI thin films. Finally, we used simulation to show that a high-efficiency device could be obtained by utilizing an InP thin film of only 280 nm in thickness, without the requirement of a conventional p-n homojunction or epitaxial window layer. This is achieved by utilizing a wide bandgap carrier selective contacts for charge carrier separation. Under ideal conditions, the proposed solar cell structure can achieve an efficiency as high as 28%. Although, the efficiency reduces in the presence of bulk and interface recombination, for a bulk minority carrier lifetime as low as 2 ns and an interface recombination velocity (IRV) as high as 105 cm/s, an efficiency of ~22% can still be achieved with the same thickness of InP. Overall, this thesis presents a thorough study of carrier selective contact in InP planar and nanowire solar cells using a combination of both theoretical simulation and experimental verification. The proposed thin film device structures are particularly beneficial in cases where the growth of controlled p-n junction and heterostructure window layer can be challenging as in case of low-cost deposition techniques, such as thin-film vapour-liquid-solid and close-spaced vapour transport. The results from CuI is promising in terms of its stability, transparency and conductivity but more work is required to utilize it as a hole selective contact in III-V materials. Finally, the work on carrier selective contact for nanowire solar cell is expected to simplify the complications associated with the growth and doping of nanowires