The use of HfO2 in a point contact concept for front interface passivation of Cu(In,Ga)Se2 solar cells

Johannes Löckinger,Shiro Nishiwaki,Benjamin Bissig,Giedrius Degutis,Yaroslav E Romanyuk,Stephan Buecheler,Ayodhya N Tiwari

doi:10.1016/j.solmat.2019.03.009

Abstract

We report on the use of a high bandgap metal-oxide at the front interface of Cu(In,Ga)Se2 (CIGS) solar cells in a point contact concept for reduced interface recombination. Highly resistive HfO2 is applied on the CIGS surface by atomic layer deposition (ALD). Aspects of the surface passivating effect of HfO2 on CIGS were investigated by time-resolved photoluminescence (TRPL), electron beam induced current (EBIC) and capacitance-voltage (C-V) measurements. Two structuring methods for point contact formation are compared, a lithographic top-down and a simple bottom-up approach using NaCl as template. The former method employed a plasma etch step which was found to degrade the performance of solar cells when applied on the CIGS surface. The template method omitted sputtering and allowed patterning of HfO2 up to 10 nm thickness without adversely impacting the open-circuit voltage (VOC). EBIC revealed an improved carrier collection due to the HfO2 coating and a long term stable PL decay was observed. Yet, the point contact concept with HfO2 was not significantly influencing the performance of a CIGS solar cell for the investigated parameter range.

Highlights

Photovoltaic (PV) devices based on chalcogenide Cu(In,Ga)Se2 (CIGS) absorber layers are among the most promising thin-film PV technologies reaching power conversion efficiencies (PCE) of 20.4% and 23.35% on flexible and rigid substrates [1,2]
We report on the use of a high bandgap metal-oxide at the front interface of Cu(In,Ga)Se2 (CIGS) solar cells in a point contact concept for reduced interface recombination
Aspects of the surface passivating effect of HfO2 on CIGS were investigated by time-resolved photoluminescence (TRPL), electron beam induced current (EBIC) and capacitance-voltage (C-V) measurements

Summary

Introduction

Photovoltaic (PV) devices based on chalcogenide Cu(In,Ga)Se2 (CIGS) absorber layers are among the most promising thin-film PV technologies reaching power conversion efficiencies (PCE) of 20.4% and 23.35% on flexible and rigid substrates [1,2]. The strong variations in device performance depending on which and how the buffer layer – CdS, Zn (S,O), ZnxMgyO, InxSy, ZnxSnyO – is applied suggests that the front CIGS/buffer interface is crucial for achieving a high PCE [4]. The concept of using high band gap dielectrics to passivate the front surface with point contacts was successfully developed in Si solar cells with the PERC (passivated emitter and rear cell) structure [5]. Both Al2O3 and HfO2 have shown both chemical and field-effect passivation qualities in Si solar cells [6,7]. While for HfO2 both a negative or positive Qeff was reported, depending on the magnitude of χCIGS [14]

Methods

Results

Conclusion