Optimisation of TCO Bi-Layer Front Contact for Multijunction Thin-Film Silicon Solar Cells

Abstract

Thin film solar cells are devices that make use of thin layers to generate electricity from solar energy. Among various layers present in a thin film solar cell architecture, transparent conductive oxide (TCO) is crucial. TCOs are a set of materials with a unique set of properties. As a front contact, TCOs are required to offer high transparency to allow photons to reach the absorber layer to generate charge carriers. At the same time, TCOs need to be conductive as well to transport charge carriers from absorber to the metal electrode. However, there exists a trade-off where improvement to one property often comprises the other.
The concept of a bi-layer TCO configuration is explored in this project which makes use of two single layer TCOs: one with superior electrical properties and the other with superior optical properties. Hydrogenated indium oxide (IOH) is investigated as the conductive layer while the intrinsic zinc oxide (i-ZnO) as the transparent layer. Various other conductive layers are also tried in combination with i-ZnO, those being - Cerium doped indium oxide (ICO) and Cerium and hydrogen co-doped indium oxide (ICOH).
The first step to produce highly conductive bi-layer films is optimise post-deposition annealing parameters. Emphasis is placed on optimising annealing time, which tracing out the behavior of change in opto-electrical properties of the bi-layer after every 10 min increment in annealing time. 140 minutes is found as the optimum annealing time for IOH/i-ZnO bi-layers on both flat glass and textured glass substrates.
Novel highly conductive IOH/i-ZnO bi-layer were deposited demonstrating mobility of 143 𝑐𝑚^2/𝑉 𝑠 and carrier concentration of 1.05𝑥10^19 𝑐𝑚^−3 on flat glass. The bi-layer has the ability to not only retain individual constituent TCO properties and also to surpass the limitations of their individual TCOs. This attributes to the fact that the free charge carriers transport takes place a t the interface between TCO layer in the bi-layer stack. The ICO/i-ZnO and ICOH/i-ZnO structures were deposited, both bi-layers demonstrated mobility below 100 𝑐𝑚^2/𝑉 𝑠, however they outperformed their layers both electrically and optically.
IOH/i-ZnO bi-layer construction was also deposited on textured glass substrates. Best sample demonstrates mobility as high as 110 𝑐𝑚^2/𝑉 𝑠 with a carrier concentration of 3.63𝑥10^19 𝑐𝑚^−3. The reason for slightly lower mobilities is attributed to the inconsistency in shape and size of the textures present on the substrate, which leads to non-uniform deposition of IOH across the entire surface. Further investigation will lead to understanding hydrogen’s role in the IOH/i-ZnO bi-layer and the extent to which bi-layer performance on textured substrates can be improved.