Investigation of solar cell carrier lifetime using photoconductance lifetime measurements

• Main Author: Sng, Joseph Ding Wen
• Other Authors: Rusli
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/141015
• Institution: Nanyang Technological University

Transition metal oxides (TMO) are commonly used as hole selective contacts on n-type silicon (Si) to form Si/TMO heterojunction solar cell. The hole-selective contact is achieved by inducing a strong band bending in Si, arising from the high work function (≳ 5.5 eV) of the TMO, to result in a strong upward bending at the n-type Si, leading to carrier inversion at the Si surface. Besides serving as hole-selective contact, the TMOs also play as an important role in passivating the Si surface. The surface of Si is defective due to the termination and disruption of its crystal lattice. This results in the presence of many impurities and defects that enhances surface recombination and contributes to a high surface recombination rate. The TMOs are able to passivate the Si surface by forming bonds with Si, resulting in a reduced number of dangling bonds and lower surface recombination rate. The final year project studies three TMOs, namely MoOx, V2Ox and WOx, formed on n-type Si. The focus is to compare and evaluate how effective they are in serving as a passivation layer. The TMOs are fabricated using solution-based process, which is advantageous as it is low cost and can be carried out at room temperature and over a large area. Consequently, such solution based TMOs are attractive for the fabrication of Si/TMO solar cells. For comparison, we have also studied electron beam deposited MoOx as a passivation layer. The evaluation of the quality of the passivation is done by measuring the effective lifetime of Si via the Sinton WCT-120 Photoconductance Lifetime tester. By flashing short pulses of light on the Si wafers and monitoring the decay of the induced photoconductance, the effective lifetime of the carriers and the open circuit voltage can be determined. Nowadays Si nanostructures are commonly introduced into Si solar cells to minimize light reflection, enhance light trapping and increase optical absorption. In this work, besides planar Si, we have deposited the TMOs on nanostructured Si to assess the effectiveness of the TMO serving as a passivation layer on the defective nanostructured Si surface. Both Si nanowires and Si nanoholes sample have been prepared using the solution-based metal assisted chemical etching technique and investigated in this work. The results obtained show that among the three transition metal oxides, the Si wafer passivated with MoOx has the highest lifetime of 69.6 μs and lowest surface recombination velocity of 464.95 cm/s. The long lifetime achieved through the passivation is important for realizing good efficiency for the Si/TMO heterojunction solar cell. Nanostructured sample which are passivated with MoOx had the highest improvement of 89.7 mv for its Voc as compared to their non-passivated and non -nanostructured counterparts. This improvement of the performance of the Si sample is essential for producing more efficient solar cells and show the feasibility of using MoOx hole -selective contact to reduce the surface recombination velocity of Si sample.