Atomic-layer-deposited Al-doped zinc oxide as a passivating conductive contacting layer for n+-doped surfaces in silicon solar cells

Bart Macco,Wilhelmus M.M Kessels,Bart B Van Pelt,Marc Dielen,Jimmy Melskens,Nga Phung,Bas W.H Van De Loo,Marcel A Verheijen,Dennis G.J.A Loeffen

doi:10.1016/j.solmat.2021.111386

Abstract

Stacks consisting of an ultrathin SiO 2 coated with atomic-layer deposited (ALD) zinc oxide (ZnO) and aluminum oxide (Al 2 O 3 ) have been shown to yield state-of-the-art passivation of n -type crystalline silicon surfaces. The distinguishing aspect of this novel passivation stack is the very conductive nature of the passivating ZnO layer. In this work, it is demonstrated that such a stack can provide additional functionalities relevant for silicon solar cells. Specifically, it is shown that the conductive and transparent stacks can passivate textured and n + -diffused silicon surfaces and that they can form an Ohmic contact to n + -diffused surfaces with a low contact resistivity, provided the ZnO is Al-doped. The Al 2 O 3 capping layer has previously been shown to be crucial in the passivation mechanism by preventing the effusion of hydrogen during annealing. Here, it is demonstrated to enable a significant improvement in both the transparency and lateral conductivity of the ZnO upon annealing as well, up to a level typically only attainable by In-based transparent conductive oxides. It is furthermore shown that the passivation of the stacks is thermally stable up to 500–600 o C, depending on the preparation method for the interfacial SiO 2 . Together, these properties make the presented stack an interesting building block for crystalline silicon solar cells, with possibilities for integration as passivating front contact in Passivated Emitter and Rear Cell (PERC)-like solar cells, e.g. as bottom cell top contact in silicon-perovskite tandem cells, as well as a conductive hydrogenation source for poly-Si passivating contacts. • SiO 2 /ALD ZnO:Al/Al 2 O 3 stacks yield excellent passivation of n -type, textured c -Si. • Contact resistivities as low as 15 mΩcm 2 on n + -doped silicon surfaces were obtained. • Doping of the c-Si and ZnO:Al is crucial for obtaining a low contact resistivity. • ALD ZnO:Al can be made highly conductive and transparent by capping and annealing. • The stack has potential in PERC, poly-Si contact and c -Si/perovskite tandem cell.

Highlights

In the field of crystalline silicon (c-Si) solar cells, engineering and tailoring of the contacts is becoming progressively important as the conversion efficiencies creep more and more towards the efficiency limit, which is set at 29.43% for single-junction c-Si solar cells [1]
It is good to note that in a similar study on sputtered zinc oxide (ZnO):Al on poly-Si(n) passivating contacts, it was found that the contact resistivity increased strongly at an annealing temperature of 400 oC, which was attributed to the formation of 2–3 nm of SiOx at the transparent conductive oxide (TCO)/poly-Si interface [73]
In this work it is shown that stacks consisting of atomic-layer deposited (ALD) ZnO(:Al)/Al2O3 on a thin SiO2 layer can provide a combination of functionalities rele vant for solar cell operation

Summary

Introduction

In the field of crystalline silicon (c-Si) solar cells, engineering and tailoring of the contacts is becoming progressively important as the conversion efficiencies creep more and more towards the efficiency limit, which is set at 29.43% for single-junction c-Si solar cells [1]. Optical transparency is a challenge for the TCO layers, as there is an inevitable trade-off between conductivity and op tical transparency: In order to achieve a sufficiently low sheet resistance (typically less than 100 Ω/sq.), the TCO is degenerately n-type doped to a carrier density Ne of a few times 1020 cm− 3 This high doping brings the plasma frequency ωp of the TCO into the N(IR) region, resulting in optical free-carrier (or Drude) losses through absorption and reflection [34]. It is shown that the stacks can passivate both textured and n+-diffused silicon and that ALD ZnO on a thin SiO2 layer can form an Ohmic contact to such n+-diffused surfaces with a low contact resistivity. Opportunities and po tential benefits for the application of the stack as front contact in PERCtype solar cells and poly-Si(n) passivating contacts are outlined

Experimental section

Results and discussion

Lateral conductivity and transparency of the ZnO:Al TCO

Conclusions and outlook