Novel Doping Free Metal-oxide Carrier Selective Passivating Contacts for Solar Cells

Master thesis (2019)

Authors

Y. Zhou Electrical Engineering, Mathematics and Computer Science

Contributors

O. Isabella Photovoltaic Materials and Devices - EEMCS (supervisor 1)
G. Yang Photovoltaic Materials and Devices - EEMCS (supervisor 2)
Faculty
Electrical Engineering, Mathematics and Computer Science
Copyright: © 2019 YiLong Zhou
Solar cells MoOx Passivation Carrier selective passivating contacts Transition metal oxides TiOx
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Published Date

27-05-2019

Language

English

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Faculty

Electrical Engineering, Mathematics and Computer Science

Abstract

Carrier selective passivating contact (CSC) is considered to be a promising direction for surface passivation research because it performs passivation for both non-contacted and contacted areas. Conventional heavy doping CSCs are subject to a few drawbacks such as significant Auger recombination, parasitic absorption, complicated processing and relatively high fabrication cost. An alternative approach is by using transition metal oxides (TMO) where nm-thick metal oxides are introduced with easy and cost effective processes to realize the asymmetric conductivity for charge carriers. In this project, we investigated the passivation property and carrier selectivity of two types of TMOs, namely MoOx as hole transport layer (HTL) and TiOx as electron transport layer (ETL).

The performance of MoOx based HTL is characterized at device level. By optimizing the metallization process, interfacial (i)a-Si:H layer thickness, textured surface pre-treatment and annealing conditions, an ultimate PCE of 17.60% is achieved, with Voc being 655 mV, Jsc being 38.36 mA/cm2 and FF being 70.40%.

The passivation quality of TiOx based ETL is characterized both by symmetric test for the passivation properties and at device level. First, the passivation property of single TiOx layer and NAOS-SiO2/TiOx stacks are investigated. With the optimization of layer thickness and FGA conditions, a Voc of ~680 mV is obtained in both cases. After which, we investigated the passivation degradation caused by changing a new TiO2 source material. We conclude that the passivation degradation is mainly attributed to the non-uniform TiOx coating resulted from the reduced free mean path of evaporated TiOx due to TiO2 outgassing during deposition. Meanwhile, we also studied the influence of different interfacial layer between c-Si and TiOx such as (i)a-Si:H thin layer where a Voc of ~660mV is achieved by using new TiO2 source. Eventually, all optimized ETL structures are tested in FBC solar cells with p+ poly-SiOx at the front side as HTL. The results demonstrate that all cells are showing similar passivation quality (Voc=~575 mV) and the champion cell of 14.21% efficiency is achieved in c- Si/TiOx stack, with Jsc being 30.53 mA/cm2 and FF being 74.97%.