Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Abstract

Poor crystallinity, high degree of porosity and rough surfaces are the main drawbacks of solution‐processed CIGS absorbers resulting in lower power conversion efficiencies when compared to vacuum‐based CIGS solar cells. Therefore, promoting absorber grain growth is key to further improve solution‐based solar cell performance. The effect of alkali elements such as Na in CIGS absorbers is generally recognised to have beneficial effects not only on the absorber opto‐electronic properties but also on the grain growth. In this work, thermal evaporation of a thin layer of NaCl prior to selenisation resulted in absorbers with significantly larger CIGS grains than previously seen with Na diffusing directly from the from soda‐lime glass substrate. NaCl is non‐toxic, abundant and readily available compound that has not been typically used as an evaporation source, but rather as an additive into CIGS precursor solution. The effect of Na on these solution‐processed CIGS devices was primarily observed in the spectacular morphological changes leading to improved carrier collection and minority carrier lifetimes, but less on the absorber doping. Transmission electron microscopy (TEM) revealed voids forming around large CIGS grains upon NaCl addition and these had a negative effect on inter‐grain carrier transport. Nonetheless, the resulting device performance doubled from 5% to 10% with addition of Na using this doping approach; however, a compromise between the optimum grain growth and optimum electronic properties had to be made. This study demonstrates a novel, simple and effective Na‐doping strategy for CIGS absorbers and reveals the current limitations of the Na‐doping in solution‐processed atmospherically deposited cells.

Extrinsic Doping of Ink‐Based Cu(In,Ga)(S,Se)2‐Absorbers for Photovoltaic Applications

“…[ 116,118,206 ] These results cemented the importance of adding sodium to the absorber layer and spurred research on developing new incorporation strategies. [ 207 ] Over the years, several contradictory results relating to the effect of Na addition to the CIGSSe absorber appeared including i) grain size (increased, [ 204,208,209 ] decreased, [ 197,210–213 ] no relation, [ 28,214 ] ) ii) preferentially oriented growth (promoted (220/204) growth, [ 215 ] promoted (112) growth [ 216,217 ] ), iii) decreased concentration of traps or deep levels, [ 196,218 ] iv) GB passivation [ 28,104,214,219–221 ] ), and v) p‐type conductivity (increased, [ 28,115,222 ] decreased, [ 51,223,224 ] ). The differences in findings can be related to the vast differences in absorber composition and growth conditions.…”

“…[ 84,244 ] A wide range of compounds have been studied in ink‐based routes (e.g., Na 2 S, [ 208 ] KF, [ 245 ] sodium citrate, [ 33 ] KCl, [ 138 ] and Sb 2 S 3 [ 246 ] ) as opposed to the predominantly used alkali halide salts (e.g., Na 2 S, NaCl, Na 2 Se, NaF, KF, RbF, and CsF) in vacuum processing. [ 51,223,224 ] The following sections further expand on the PV applications of the most commonly used dopants (e.g., Na, K, Sb, Bi) in MI‐based CIGSSe/CISSe PV, and the advancements achieved by these approaches. Notably, unless specifically mentioned, all devices mentioned in the subsequent sections utilize an SLG substrate, without a barrier layer.…”

“…Reproduced with permission. [ 224 ] Copyright 2015, Wiley. b) Influence of different Na sources (Na from SLG, and/or NaCl added to precursor molecular solutions of metal formates in MeOH and TMG) on device PCEs, highlighting the effect of different doping strategies (top).…”

“…NaCl was thermally evaporated on to as‐coated films in varying thicknesses (0, 15, 30, 50, 100, and 150 nm). [ 224 ] Furthermore, Mo‐coated alkali‐free substrates (Corning Eagle XG glass) were used in place of standard SLG substrates to better control Na contents. The as‐coated films were selenized at 550 °C for 70 min (RTP furnace) and air annealed at 180 °C for 5 min.…”

“…While sodium addition was found to greatly enhance the grain growth of the films, a trade‐off was required between grain morphology and device performance. [ 224 ] The best performing devices were those with modest grain growth at 15 nm NaCl addition, which demonstrated PCEs double that of the Na‐free devices (5.0% vs 10.3%). Excess sodium (>30 nm NaCl) enhanced grain growth (see Figure 7a), however pinhole and void formation were reported, hindering device performance in these samples.…”