Metastable Behavior on Cesium Fluoride‐Treated Cu(In_1–x,Ga_x)Se₂ Solar Cells

Abstract

Metastable behavior of cesium fluoride (CsF)‐treated Cu(In1–x,Gax)Se2 (CIGS) solar cells is investigated under heat‐light soaking (HLS) and heat‐soaking (HS) treatments. HLS increases open‐circuit voltage, fill factor, efficiency, and net carrier concentration and decreases short‐circuit current density, whereas heat‐soaking treatment acts oppositely. The performance of a CsF postdeposition treatment to CIGS thin film in selenium vapor, and closer to stoichiometry copper content, did not mitigate the open‐circuit voltage improvement after HLS. These results argue the traditional concept of the VSe–VCu divacancy complex for the total beneficial effect of HLS in alkali‐treated CIGS solar cells. The metastable behavior observed in the CsF‐treated devices due to the HLS and HS treatments is explained by the specific behavior of alkali‐containing new compounds at the surface and/or the migration of alkali metals at the surface and bulk regions.

Proton Irradiation on Cesium‐Fluoride‐Free and Cesium‐Fluoride‐Treated Cu(In,Ga)Se2 Solar Cells and Annealing Effects under Illumination

“…Recently, chalcopyrite Cu(In,Ga)S,Se 2 (CIGSSe) thin‐film solar cells with air mass (AM) 1.5 conversion efficiency exceeded 23%, the highest value among the thin‐film solar cell technologies . These highest power conversion efficiency and high radiation tolerance show good potential for space application . Flight data demonstrate no degradation of Cu(In,Ga)Se 2 (CIGS) solar cell performance by proton irradiation and thermal annealing .…”

Front passivation of Cu(In,Ga)Se2 solar cells using Al2O3: Culprits and benefits

“…chemical passivation [6,15], and 2) repelling minority carriers by a built-in electrical field due to fixed charges at the dielectric surface [16][17][18]. While this approach has been successfully implemented in the rear contact, the front contact has been mostly improved using alkali post deposition treatments [9,19,20], However, it is still heavily debated its full effects and if benefits are due to bulk recombination changes [21][22][23][24]. In PDT processes, after the CIGS deposition, a fluorine-alkali compound is evaporated on the CIGS, leading to several optoelectronic changes that improve device performance [23,25,26].…”

Behavior of Na and RbF‐Treated CdS/Cu(In,Ga)Se2 Solar Cells with Stress Testing under Heat, Light, and Junction Bias

“…Cu(In,Ga)(S,Se) 2 or CIGS photovoltaic devices are a promising thin‐film photovoltaics (PV) technology with a record 23.4% cell and 19.2% module efficiency, respectively. [ 1 ] Alkali treatments (Na, [ 2–6 ] K, [ 6–10 ] Rb, [ 10–12 ] and Cs [ 11,13 ] ) are a crucial step toward attaining high efficiency devices with improved p‐type conductivity, [ 4,7,12–15 ] and grain boundary and interface passivation, [ 12,16–22 ] resulting in improved open‐circuit (OC) voltage ( V OC ). The increased p‐type doping in Na‐treated CIGS devices has been speculated to originate from many possible mechanisms such as the introduction of Na‐related acceptor defects, introduction of O Se deep acceptor defects, elimination of In Cu compensating donors or elimination of defects at grain boundaries, and surface passivation.…”

“…[ 27–35 ] A number of groups have reported an increase in V OC under light soaking which has been attributed to increase in apparent doping density. [ 4,7,13–15,34,35 ] Erslev et al [ 35 ] observed an increase in carrier density in Na‐containing devices after light exposure, but no increase in carrier density in Na‐free CIGS devices. Heise et al [ 36 ] recently reported a metastable decrease in current collection efficiency of CIGS solar cells in addition to a metastable increase in V OC after light soaking.…”

“…[ 7,38 ] Similarly, CsF‐PDT devices when exposed to heat‐ and light‐soaking have also shown an increase in V OC and doping density. [ 13 ] Other mechanisms that have been used to explain metastability in CIGS solar cells include the following: a) lattice relaxation around the ( V Se – V Cu ) divacancy defect complex, which has also been associated with persistent photoconductivity (PPC) of bulk CIGS material; [ 36–40 ] b) fast diffusion of Cu as interstitial Cu i + ions leads to increase in V oc upon forward bias soaking of the CIGS devices; [ 41,42 ] c) formation of In Cu DX‐center defects [ 43,44 ] which has been reported to have metastable shallow and deep donor configurations; d) photodoping of CdS or ZnS buffers; and [ 45,46 ] e) large lattice relaxation in the bulk CIGS driven by charge injection, similar to the ( V Se – V Cu ) mechanism, but with much longer characteristic times. [ 47 ] Although these studies have explored the effect of light soaking on Na‐PDT, RbF‐PDT, and CsF‐PDT devices, the combined effect of junction bias and light soaking on the metastability in CIGS solar cells remains relatively unexplored.…”