Refractive index extraction and thickness optimization of Cu₂ZnSnSe₄thin film solar cells

Abstract

Cu2ZnSnSe4 (CZTSe) thin film solar cells are promising emergent photovoltaic technologies based on low‐bandgap absorber layer with high absorption coefficient. To reduce optical losses in such devices and thus improve their efficiency, numerical simulations of CZTSe solar cells optical characteristics can be performed based on individual optical properties of each layer present in the cell structure. In this contribution, we have first determined the optical coefficients of individual thin films (i.e., (n, k) of the absorber, buffer, and window layers) to build a realistic model simulating the optical behavior of the whole cell stack we propose. Optical characterization was performed using two approaches, one based on ellipsometry measurements for characterizing thin flat cadmium sulfide (CdS) and zinc oxide (ZnO) layers and the other relying on reflectance and transmission (R/T) analysis for the rough CZTSe absorber. Then, we performed numerical simulations using as input experimental optical parameters predicting optimal CZTSe cell structure minimizing optical losses. The impact of each layer's thickness on the cell's short‐circuit current has been studied. A set of optimal thicknesses of each of the active layers was proposed. Finally, the proposed optical optimization was experimented practically leading to CZTSe cells with 9.7% and 10.4% efficiencies.

Is the Cu/Zn Disorder the Main Culprit for the Voltage Deficit in Kesterite Solar Cells?

“…A similar optical study has been conducted by ElAnzeery et al, yet in pure selenide CZTSe devices. [ 30 ] In this latter work, the thicknesses of the various layers within the optimized CdS/i-ZnO+AZO/MgF 2 stack were 20/435/130 nm, respectively, what yielded a short circuit current density (J sc ) close to 40 mA cm −2 . The deposition of the "as thin as possible" CdS buffer layer (around 20 nm) without a shunting issue, was achieved by a so-called "cadmium treatment", [ 31 ] which also happened to decrease the series resistance of the device, thereby improving the Filling Factor ( FF ) and the overall photovoltaic performance.…”

Self-Aligned Hierarchical ZnO Nanorod/NiO Nanosheet Arrays for High Photon Extraction Efficiency of GaN-Based Photonic Emitter

“…These structures were simulated on the 200 nm thick ITO layer deposited on the 430 nm photonic emitters. The refractive indices of various materials were calculated using EMA [15][16][17]:…”

“…Especially, one-dimensional (1D) ZnO nanostructures can be widely used in photonic emitters and photodetectors because of their easy refractive index control, transparency in the visible light range, high photoreactivity, and light waveguide properties [12][13][14]. According to effective medium approximation (EMA), the effective refractive index (n eff ) of ZnO (ZnO film: n = 2.1 at visible wavelengths) decreases when it is converted into nanostructures [15][16][17][18].…”

Optimized solar cells based on changes in resonance structure as a function of the refractive index and the thickness

“…5 Another way to enhance efficiency of thin solar cells is to minimize the energy losses due to reflection. 6 A possible way to achieve this is to add one or more thin dielectric layers on the top of the solar cell as anti-reflection coatings (ARC). Single layer ARCs are standard in the industry today.…”

“…It can be further used to investigate, how absorption can be enhanced by tuning the refractive index and the thicknesses of the absorbing and non-absorbing layers involved. Therefore our work is strongly related to optimization of anti-reflection coatings, 6 but it focuses on a different aspect, namely the effect of resonances in layered films on the absorption in the energy converting film.…”