Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Abstract

Despite favorable optical properties and band-gap tunability, Cu(In,Ga)S 2 solar cell performance is often limited due to bulk and interface recombination losses. We show that Cu-deficient absorbers have lower bulk recombination, owing to the suppression of the detrimental antisite defects. Zn(O,S) buffer layer further lowers the interface recombination due to appropriate band alignment and suppression of defects at the interface. This leads to a high-quality absorber with lower interface losses, resulting in a high power conversion efficiency of over 15%.

Dominant Processing Factors in Two-Step Fabrication of Pure Sulfide CIGS Absorbers

“…Shukla et al detailed the reduction of interface recombination paths by using a Cd-free buffer layer with a comparably higher conduction band (Zn (O, S)) and the utilization of Cu-poor absorbers. They found the latter, together with gallium inclusion, to positively affect carriers' lifetimes in their work [13]. An observation also substantiated by others in the improvement of open-circuit voltages by the omission of deep level defects [14][15][16].…”

“…Analyzing the EQE plot in Figure 5.b, losses at high energies of above 2.4 eV could be associated with quality and thickness of CdS buffer layer (with a bandgap of ~2.43 eV), and with the emitted electron-hole pairs not getting collected. A concern actually addressed in the work of others and Hiroi's, (partially) by using alternative/thicker buffer/window layers and increasing the relative work function [11,13,27,28]. The samples without a barrier layer generally appeared to show higher J SC and V OC responses than the samples with a barrier layer.…”

Formation of Cu(In,Ga)S2 chalcopyrite thin films following a 3-stage co-evaporation process

“…The investigated Cu(In,Ga)S 2 films were grown following the 3-stage process, known to result in the highest device performances for pure selenide absorbers (Cu(In,Ga)Se 2 ). Amongst the reasons mentioned for the superiority of the 3-stage process, one may highlight (i) the recrystallization of the growing film during Cu-poor/Cu-rich transition which decreases the density of detrimental extended crystalline defects [4,17,18], and (ii) the bandgap variation 8 throughout the layer, evolving along with GGI V-shaped gradient [19][20][21]. This latter GGI gradient results from different diffusion coefficients of indium and gallium within the growing Cu(In,Ga)Se 2 layer.…”

“…While a pure selenide absorber layer demonstrated 22.6 % record efficiency [2], devices based on pure sulfides remains lower with a 15.5 % record efficiency achieved using an absorber with a bandgap below 1.6 eV [3]. Nevertheless, by combining both improved material properties and growth process understanding, several groups have shown improved performance launching pure sulfide based devices towards much higher efficiency [4,5]. Moreover, the bandgap of CuIn 1-x Ga x S 2 can be tuned between 1.5 eV and 2.4 eV by increasing x from 0 through 1 [6].…”

Toward Understanding Chalcopyrite Solar Cells via Advanced Characterization Techniques

“…[87,88] Through PL measurement, evaluating the recombination characteristics by calculating the quantum yield in the high energy spectrum and deriving the quasi-Fermi level splitting (QFLS) value which corresponds to the upper limit of the device V oc has also been reported. [89,90] Photogenerated carriers should not be recombined until they are collected at the electrode. Because their lifetime is inversely proportional to the charge carrier recombination rate, [91] accurately estimating the lifetime is important in understanding device physics and in the design of solar cell devices with reduced charge carrier recombination.…”