Development of an Optical Library for Coevaporated CdSexTe1−x

The conversion efficiency of CdTe solar cells may be improved by bandgap engineering, i.e., changing the bandgap value through the addition of Se in the absorber. The Se alloying enables a short-circuit current density improvement, as it leads to a bandgap energy value decrease. Furthermore, it has been associated with increased minority carrier lifetimes, assuring high open-circuit voltage values. An Se gradient profile control can further optimize the solar cell performance. Thus, an optical model baseline of the CdSe_xTe_1−x (CST) compound was developed. Spectroscopic ellipsometry measurements were conducted to accurately extract the optical constants of ten CST layers deposited through coevaporation with x varying from 0 to 1. Using the measured dielectric function spectra from the discrete CST layers with varying x, and considering the composition-induced shift in the critical point energies, an energy-shift model was employed to develop the accurate optical library for the CST compound for any x value to provide data for future modeling and optimization. The library accuracy was validated through optical simulations of the quantum efficiency of a graded CST solar cell using the finite-difference time-domain method by replicating the Se profile in the absorber layer measured through secondary ion mass spectrometry.

I. Introduction

Thin-film solar cells, such as CdTe and Cu(In,Ga)Se₂ (CIGS), allow for lower cost manufacturing and reduced material consumption compared with mono-Si while maintaining high performance [1], [2]. CdTe technology currently dominates the photovoltaics (PV) thin-film market, with the annual global PV module production having increased from below 1 GWp in 2008 to over 12 GWp in 2023 [3]. This expansion correlates with a notable increase in module power conversion efficiency to over 19% [4]. Concurrently, research-scale record cells have achieved a light-to-power conversion efficiency value of over 23% [1], [2], [5]. The latest efficiency improvements have been mostly a result of the CdS window layer removal and introduction of a graded CdSe_xTe_1−x (CST) absorber layer [6], [7], [8]. The incorporation of Se enables the formation of a CST compound with a lower bandgap than pure CdTe, leading to an improved long-wavelength spectral response and, thus, a higher short-circuit current density value [8]. Bandgap engineering through Se diffusion to the CdTe absorber has also been associated with decreased interface recombination due to an improved band alignment at the front absorber interface and a passivation of deep defects within the absorber bulk and at grain boundaries [9], [10], [11]. Thus, a high open-circuit voltage value is maintained despite the bandgap energy value decrease, as adding Se allows for increased carrier lifetimes [6], [10]. The solar cell efficiency can be further optimized by a strict control over the Se gradient profile [6], [12], [13], [14]. However, the performance highly depends on slight fluctuations in the compositional profile. Consequently, optimizing the graded profile requires the substantial utilization of device modeling [12], [13], [15]. Therefore, for the CST compound grading optimization, a detailed knowledge of the optical properties of the alloy is essential.