Assessing the roles of Cu- and Ag-deficient layers in chalcopyrite-based solar cells through first principles calculations

Abstract

Chalcopyrites are a demonstrated material platform for realizing efficient thin-film photovoltaics, with the most well known Cu(In,Ga)Se2 (CIGS)-based solar cells exceeding 23%. Several factors, including flexibility in tuning the absorber bandgap, enhanced surface treatments, and the electrically benign nature of common defects are responsible for the existing high performance and future promise in chalcopyrite-based photovoltaic devices. The introduction of Cu-poor phases (also known as ordered-vacancy compounds or OVCs) between the absorber and buffer layers in CIGS solar cells is known to enhance device performance; however, the overall properties and role of OVCs remain poorly understood. Using first principles calculations based on the density functional theory with screened hybrid functionals, we explore the electronic structure and stability of OVCs and their band offsets with defect-free chalcopyrite layers in Cu- and Ag-based compounds (ABX2 where A=Cu, Ag; B=In, Ga, Al; and X=S, Se). Using AB3X5 and AB5X8 stoichiometries as model OVC systems, we report on the variation of the bandgap with the A/B ratio and discuss the trends in other Cu- and Ag-based chalcopyrites beyond CuInSe2. We find that the valence and conduction bands are lower in energy in OVCs with respect to the parent ABX2 chalcopyrite owing to a reduced p–d interaction between X and A atoms. We additionally perform device-level simulations to assess the implications of the results, finding that the valence band offsets of OVCs are favorable, while the conduction band offsets of chalcopyrites beyond CuInSe2-based absorbers may be detrimental in conventional solar cell device designs.

On the Paramount Role of Absorber Stoichiometry in (Ag,Cu)(In,Ga)Se2 Wide‐Gap Solar Cells

“…[4,19,[39][40][41] Indeed, OVCs are generally supposed to exhibit lower conduction band and valence band edges than the corresponding 1:1:2 phase, potentially creating an electron transport barrier for electrons captured in the OVCs. [42] ratio may be explained by the extension/thickness of the TEM lamella and morphology of the OVCs. However, the presence of some minor 1:5:8 OVCs cannot be ruled out (see second OVC peak in Raman; Figure 5).…”

Silver Surface Treatment of Cu(In,Ga)Se2 (CIGS) Thin Film: A New Passivation Process for the CdS/CIGS Heterojunction Interface

“…According to the result of Boyle et al, [ 37 ] Ag can lower both the valence band edge and conduction band edge of the CIGS film, and the former is more obvious than the latter. [ 5,55,56 ] To determine if the lowered valence band edge was the main factor for the increased defect level, we performed UPS, as shown in Figure 9d–f. The VBM value of the samples of 0, 2 and 4 min are 4.21, 4.19, and 4.17 eV, respectively.…”

“…[ 2–4 ] Highly efficient CIGS‐based solar cells possess a multilayer stacked structure, which includes a p‐type CIGS absorber layer, an n‐type buffer layer (CdS or Zn(O,S)), and a transparent conductive oxide (TCO: ZnO and aluminum (Al)‐doped ZnO). [ 5–7 ] In consequence, interface engineering of the CIGS‐based solar cell plays a critical role in the improvement of device performance, particularly for the CdS/CIGS heterojunction interface. [ 8 ] Generally, serious carrier recombination can be observed at the CdS/CIGS interface when deep acceptor defects (such as ) are unoccupied at the Fermi level, [ 9–12 ] which is detrimental to device performance.…”

“…Several surface treatments, post‐deposition treatment (PDT) with an alkali metal, [ 14–21 ] the formation of a copper (Cu)‐deficient layer (CDL) on the CIGS surface, [ 5,13,22–24 ] and the interface passivation of CdS/CIGS with Al 2 O 3 , [ 25–28 ] have been used to improve the device performance of solar cells. PDT, such as potassium fluoride, modifies the CIGS surface (reduced gallium (Ga) concentration and depleted Cu), which allows the reduction of CdS thickness along with the maintenance of high efficiency.…”

“…[ 4,29 ] And the CDL between the CIGS film and the buffer layer (such as CdS) is made up of the ordered vacancy compounds (OVCs) that have a lower valence band maximum (VBM) than that of the CIGS film, forming an interfacial hole barrier and decreasing recombination at the CdS/CIGS interface. [ 5,13,22,24 ] Furthermore, interface passivation with Al 2 O 3 can reduce the interfacial defect concentration significantly. [ 25 ] In summary, these surface treatments all aim at the optimization of the CIGS surface and passivation of the defects at the CdS/CIGS interface.…”