Long term stability of c-Si surface passivation using corona charged SiO2

Ruy S Bonilla,Christian Reichel,Martin Hermle,Phillip Hamer,Peter R Wilshaw

doi:10.1016/j.apsusc.2017.03.204

Ruy S Bonilla, Christian Reichel + Show 3 more

Open Access

https://doi.org/10.1016/j.apsusc.2017.03.204

Copy DOI

Abstract

Recombination at the semiconductor surface continues to be a major limit to optoelectronic device performance, in particular for solar cells. Passivation films reduce surface recombination by a combination of chemical and electric field effect components. Dielectric films used for this purpose, however, must also accomplish optical functions at the cell surface. In this paper, corona charge is seen as a potential method to enhance the passivation properties of a dielectric film while maintaining its optical characteristics. It is observed that corona charge can produce extreme reductions in surface recombination via field effect, in the best case leading to lifetimes exceeding 5ms at an injection of 1015cm−3. For a 200μm n-type 1Ωcm c-Si wafer, this equates to surface recombination velocities below 0.65cm/s and J0e values of 0.92fA/cm2. The average improvement in passivation after corona charging gave lifetimes of 1–3ms. This was stabilised for a period of 3 years by chemically treating the films to prevent water absorption. Surface recombination was kept below 7cm/s, and J0e<16.28fA/cm2 for 3 years, with a decay time constant of 8.7 years. Simulations of back-contacted n-type cells show that front surface recombination represents less than 2% of the total internally generated power in the cell (the loss in power output) when the passivation is kept better than 16fA/cm2, and as high as 10% if front recombination is worse than 100fA/cm2.

Highlights

Crystalline silicon (c-Si) continues to be the leading material for solar cell production
To the authors’ knowledge, this is among the lowest surface recombination velocity achieved on ∼1 cm n-type crystalline silicon passivated with a thermal oxide
In the present work it has been demonstrated that FEP can be applied extrinsically to thermally grown silicon dioxide to improve its passivation quality, producing lifetimes as high as 5 ms at an injection of 1015 cm−3, which equate to surface recombination velocities below 0.65 cm/s, on n-type 1 cm c-Si, equivalent to J0e values of 2.88 fA/cm2

Summary

Introduction

Crystalline silicon (c-Si) continues to be the leading material for solar cell production. In highly efficient mono c-Si cells, surface recombination of charge carriers is a limiting factor in achieving optimal performance. Known as surface passivation, is of utmost importance. As cell geometries in which all contacts are on the cell’s backside become increasingly popular, front surface passivation becomes even more crucial. The surface in a semiconductor is an abrupt crystal discontinuity. At a bare silicon surface, many atoms may be partially bonded and possess dangling bonds that create intermediate band-gap energy levels, known as surface energy traps or surface states, which promote recombination [1]. In practical solar cells, bare semiconductor surfaces are not present and recombination

Objectives

Methods

Findings

Discussion

Conclusion