Elsevier

Optik

Volume 179, February 2019, Pages 837-847
Optik

Original research article
A path-finding toward high-efficiency penternary Cu(In,Ga)(Se,S)2 thin film solar module

https://doi.org/10.1016/j.ijleo.2018.10.154Get rights and content

Abstract

The optimal p-n junction structure in a state-of-the-art Cu(In,Ga)(Se,S)2 thin-film solar module technology is investigated. For co-optimization design and path-finding, a TCAD model is developed with experimental samples. The engineerable parameters, i.e., FGa, GGIavg, and CdS thickness, are demonstrated to play a critical role in determining the p-n junction properties such as dark current characteristics Jdark(V), voltage-dependent photocurrent, localized carrier collection efficiency, and interface carrier transportation. We show the optimal Ga-grading is determined by a trade-off between the recombination loss in space charge region and the photo-carrier collection in quasi-neutral region. The optimal CdS thickness is determined by a trade-off between carrier collection efficiency, short-circuit current (JSC) loss, and Jdark(V), which depends on varied Ga-profiles. Overall, thin CdS (≦10 nm) is preferred to reduce the JSC loss in accumulated Ga-profiles, while thicker CdS is preferred to enhance the carrier collection efficiency in flatter Ga-profiles. The band alignment effect on varied Cu(In,Ga)(Se,S)2/CdS junctions is also investigated. It is found sulfur-incorporation can suppress the VOC saturation behavior at wide bandgap. For CIGSeS absorber with SS = 20% and DP =15%, the maximum VOC of 780 mV can be achieved by co-optimized Ga-profile. Furthermore, varied Ga-profiles and CdS buffer layers are explored for pathfinding. An optimal p-n junction structure shows a relative +40% efficiency improvement from 15.5% to 21.9%. This work shows the efficiency headroom of reported CIGSeS thin-film solar module technology through co-optimized CIGSeS composition gradient and buffer layer.

Introduction

Penternary Cu(In,Ga)(Se,S)2 (CIGSeS) solar cell has shown the greatest potential to replace Si-based solar cells due to the 22.9% record efficiency in thin-film photovoltaics [1]. In typical “sulfuration after selenization” (SAS) process, Ga-accumulation near the back contact limits the bandgap engineering in absorber layer [[2], [3], [4]]. The accumulated Ga-profile has been considered as an efficiency limiting factor in CIGSeS solar cell. For further efficiency breakthrough, several methods were proposed to modulate Ga-profile [[5], [6], [7]]. In reference [7], a controllable Ga-engineering in CIGSeS thin-film solar module technology has been reported. Quasi-linear gallium and sulfur profiles can be modulated by process steps. Accordingly, the optimization of composition gradients and device structure becomes more critical for further efficiency enhancement.

Due to the complicated Cu(In,Ga)(Se,S)2 material system, the design of composition gradients and device structure is more difficult as compared to co-evaporated CIGS solar cells. Very few studies for the optimal CIGSeS solar cell are reported. In this work, an advanced TCAD model is developed for co-optimization design and path-finding. The corresponding physical model is well-calibrated by experimental samples produced in the reported solar module manufacturing platform [7]. Based on the TCAD model, the optimal p-n junction structure in CIGSeS thin-film solar module is investigated. To provide a clear physical picture, detailed analysis including dark current characteristics Jdark(V), voltage-dependent photocurrent Jph(V), collection efficiency profile ηc(x,V), and band-alignment effect are performed. The engineerable parameters in manufacturing, i.e., FGa, GGIavg, and CdS thickness, are demonstrated to play a critical role in determining the p-n junction characteristics. Furthermore, varied Ga-profiles and CdS buffer layers are co-optimized, and an optimal p-n junction structure shows a relative +40% efficiency improvement as compared to the typical CIGSeS solar module. This work shows the co-optimization of CIGSeS composition gradient and buffer layer is critical for further efficiency breakthrough.

Section snippets

Device structure and simulation model

In this work, Technology Computer Aided Design (TCAD) simulation [8] is performed to solve coupled equations of optical, electrostatic, and carrier continuity in 3D solar module structure. To consider the resistance losses in conventional thin-film solar-module, Simulation Program with Integrated Circuit Emphasis (SPICE) [9] is combined to perform mixed-mode simulation. By this method, unit solar cell can be simulated by varied physical and boundary conditions, and each electrical node in

The role of Ga-grading and its performance impacts

Fig. 3 shows device performance parameters with varied FGa (GGIavg = 0.26). A typical 10 nm CdS buffer is used for fair comparison. Simulation shows increased FGa improves the bandgap near p-n junction, and it results in lower recombination loss in space charge region (SCR). On the other hand, increased FGa causes more recombination losses in quasi-neutral region (QNR) due to reduced back electric field and narrowed bandgap near the back contact. Consequently, an increased FGa gives rise to a

Conclusions

In this work, co-optimization of penternary Cu(In,Ga)(Se,S)2 composition gradients and CdS buffer in a reported CIGSeS thin-film solar module technology is investigated. With experimental samples, a TCAD model is developed for performance impact analysis, co-optimization design, and path-finding. The engineerable parameters, i.e., FGa, GGIavg, and CdS thickness, are demonstrated to play a critical role in determining the p-n junction properties such as dark current characteristics Jdark(V),

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