Transparent-conductive-oxide-free front contacts for high-efficiency silicon heterojunction solar cells

Shenghao Li,Kaifu Qiu,Kaining Ding,Friedhelm Finger,Thomas Kirchartz,Martina Luysberg,Paul Steuter,Ruijiang Hong,Weiyuan Duan,Hui Shen,Manuel Pomaska,Depeng Qiu,Andreas Lambertz,Karsten Bittkau,Malte Köhler,Uwe Rau,Zhirong Yao

doi:10.1016/j.joule.2021.04.004

Abstract

<h2>Summary</h2> In order to compensate the insufficient conductance of heterojunction thin films, transparent conductive oxides (TCO) have been used for decades in both sides of contacted crystalline silicon heterojunction (SHJ) solar cells to provide lateral conduction for carrier collection. In this work, we substitute the TCO layers by utilizing the lateral conduction of c-Si absorber, thereby enabling a TCO-free design for SHJ solar cells achieving a low series resistivity of 0.32 Ωcm<sup>2</sup> and a good fill factor of 80.7% with a conventional finger pitch of 1.8 mm. Achieving high efficiencies in TCO-free SHJ solar cells requires suppressing deterioration of the passivation quality induced by the direct metal-to-a-Si:H contacts. We show that an ozone treatment at the a-Si:H/metal interface suppresses the metal diffusion into the a-Si:H layer and improves the passivation without increasing the contact resistivity. SHJ solar cells with TCO-free front contacts and ozone treatment achieve efficiencies of >22%.

Highlights

One of the fundamental challenges in the design of both sides contacted crystalline Si solar cells is the development of a contacting scheme that combines a highly conductive front contact with negligible parasitic absorption losses
In order to compensate the insufficient conductance of heterojunction thin films, transparent conductive oxides (TCO) have been used for decades in both-sides contacted crystalline silicon heterojunction (SHJ) solar cells to provide lateral conduction for efficient carrier collection
A series resistance of 0.32 Ωcm[2] and a fill factor of 80.7% were measured for a TCO-free back-junction SHJ solar cell with a conventional finger pitch of 1.8 mm, thereby proving that relying on lateral conduction in the c-Si bulk is compatible with low series resistances

Summary

Introduction

One of the fundamental challenges in the design of both sides contacted crystalline Si solar cells is the development of a contacting scheme that combines a highly conductive front contact with negligible parasitic absorption losses. We show the feasibility of a TCO-free design at the front contact of a SHJ solar cell showing efficiencies > 22% These high efficiencies are enabled by the excellent majority carrier conductivity in the c-Si absorber under illumination, leading to a low series resistance of 0.32 Ωcm[2] and a fill factor of 80.7%. Potential of substituting TCO layer by lateral c-Si bulk conduction TCO is removed and the lateral conduction between fingers relies only on the c-Si absorber, high-quality carrier collection is achieved with a small series resistance of 0.32 Ωcm[2] and a high fill factor of 80.7% in completely TCO-free SHJ solar cells. Further development that contributes to TCO-free SHJ solar cells could be replacing rear side TCO by proper back reflection design[28,29] and developing more transparent front-side heterojunction materials such as MoOx30, nc-SiOx31 and μc-SiC32,33

Conclusions

Findings

Methods