Optimization of Back Contact Grid Size in Al2O3-Rear-Passivated Ultrathin CIGS PV Cells by 2-D Simulations

Jackson Lontchi,Julie Goffard,Stephane Collin,Viktoria Gusak,Janez Krc,Wei-Chao Chen,Marika Edoff,Milan Kovacic,Sourav Bose,Pedro M P Salome,Denis Flandre,Maria Zhukova,L Gouillart,Andrea Cattoni

doi:10.1109/jphotov.2020.3012631

Abstract

We present a simulation strategy using ATLAS-2D to optimize the back-contact hole grid (i.e., size and pitch of openings) of the Al 2 O 3 -rear-passivation layer in ultrathin Cu(In,Ga)Se 2 photovoltaic cells. We first discuss and compare our simulation model with a series of experimental nonpassivated and passivated cells to decouple the crucial passivation parameters. The simulation results follow the experimental trends, highlighting the beneficial effects of the passivation on the cell performances. Furthermore, it stresses the influence of the passivation quality at the Al 2 O 3 /Cu(In,Ga)Se 2 (CIGS) interface and of the contact resistance at the Mo/CIGS interface within the openings. Further simulations quantify significant improvements in short-circuit current and open-circuit voltage for different sizes of openings in the Al 2 O 3 layer, relative to an excellent passivation quality (i.e., high density of negative charges in the passivation layer). However, a degradation is predicted for a poor passivation (i.e., low density of such charges) or a high contact resistance. Consequently, we point out an optimum in efficiency when varying the opening widths at fixed hole-pitch and fixed contact resistance. At equivalent contact resistance, simulations predict that the sizes of the pitch and openings can be increased without optimal performance losses when maintaining a width to pitch ratio around 0.2. This simulation trends have been confirmed by a series of experiments, indicating that it is crucial to care about the dimensions of the opening grid and the contact resistance of passivated cells. These simulation results provide significant insights for optimal cell design and characterizations of passivated UT-CIGS PV cells.

Highlights

Nowadays, ultra-thin Cu(In, Ga)Se2 (UT-CIGS) technology is widely investigated for low-cost fabrication, low material usage and for flexible photovoltaic (PV) and Building integrated photovoltaic applications [1, 2]
We present in this paper a 2D model of reference and Al2O3-rear-passivated UT-CIGS cells built with ATLAS-2D simulation tool
They consist of a series of reference and Al2O3-rear-passivated UT-CIGS cell with 500 nm CIGS thickness fabricated on soda lime glass (SLG) substrates at the International Iberian Nanotechnology Laboratory (INL)

Summary

Introduction

Ultra-thin Cu(In, Ga)Se2 (UT-CIGS) technology is widely investigated for low-cost fabrication, low material usage and for flexible photovoltaic (PV) and Building integrated photovoltaic applications [1, 2]. The group at Uppsala university has reported on several experiments with grading of the CIGS absorber layer by different Ga/(In+Ga) ratios [5,6,7,8] They reported the beneficial effect of the Ga-grading on a 0.5 μm thick CIGS cell with an improvement in absolute value of efficiency (Eff) around 2.5% due to an increase of the open-circuit voltage (Voc) and fill-factor (FF) values [5] with the possibility to engineer the bandgap by means of evaporation profile variations [6]. The main effect is the reduction of the rear recombination of photogenerated carriers that leads to an increase of the short-circuit current (Jsc) and the Voc especially for ultra-thin cells [18, 19] Another largely investigated approach is the intentional introduction of alkali elements to improve the transport properties and the resulted performance of thin CIGS PV cells [20, 21]. The influence of the fixed charge density at the rear passivation layer along with the dimensions of the rear contact holes (i.e. size and pitch of opening grid) and the effect of the contact resistance have not yet been investigated in UT-CIGS PV cells

Methods

Results

Conclusion