Heavy Alkali Treatment of Post‐Sulfurized Cu(In,Ga)Se₂ Layers: Effect on Absorber Properties and Solar Cell Performance

Abstract

This contribution evaluates a sequential post‐deposition treatment of Cu(In,Ga)Se2 (CIGS) films, consisting of 1) a post‐sulfurization in elemental S‐atmosphere and 2) a subsequent treatment by heavy alkali fluorides (Alk‐PDT). First, the effect of the sulfurization step on the corresponding solar cell performance is investigated and optimum process parameters, leading to an efficiency improvement, are identified. Losses in carrier collection observed after S‐incorporation are attributed to an increased grain boundary (GB) recombination. It is found that the corresponding reduction in short‐circuit current density can be mitigated by a RbF‐ or KF‐PDT, supposedly by depleting GBs in Cu. However, in strong contrast to non‐sulfurized CIGS, the Alk‐PDT results in a lower open‐circuit voltage and distortions in the current–voltage (I–V) characteristics for sulfurized absorbers. Possible explanations are the absence of a wide‐gap surface phase and/or air exposure between the post‐treatment steps. It is further proposed that a back contact barrier may be responsible for the distortions in I–V.

Performance Limitations of Wide‐Gap (Ag,Cu)(In,Ga)Se2 Thin‐Film Solar Cells

“…Its potential absence or mitigation in low-gap absorbers allows for V OC values approaching 0.9 • V OC, SQ . [2,46,75] Nevertheless, other factors like Cu-enriched grain boundaries [15] or a larger density of acceptor defects [12,13] may contribute to the V OC loss at high GGI values as well.…”

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

“…[ 4 ] In the case of low‐gap absorbers with pure CuInSe 2 surface regions, efficiencies ( η ) of η ≥ 18% are reported and further progress is expected. [ 5,6 ] However, to this date, the efficiency of CuGaSe 2 ‐based solar cells is still limited to η < 12%. [ 7 ] The most pronounced loss, as compared with absorbers with [Ga]/[III] < 0.4 ([III] = [Ga]+[In]), is the significantly increased deficit in open‐circuit voltage ( V OC ) with respect to the bandgap energy.…”

Postdeposition Sulfurization of CuInSe2 Solar Absorbers by Employing Sacrificial CuInS2 Precursor Layers

“…More details about the sample preparation with FIB can be found in our earlier studies. [ 5,13–15 ] In addition, compositional depth profiles were obtained with glow discharge optical emission spectroscopy (GDOES) using a Spectruma Analytik GMBH GDA 750 instrument with the sputtering crater of 4 mm in diameter.…”

“…[ 9 ] For the purpose of avoiding toxic H 2 S, several studies performed sulfurization of coevaporated absorbers in elemental sulfur atmosphere with the intention of similar device improvement. [ 5,10–15 ] However, this approach led to the formation of a pure CuInS 2 (CIS) layer on top of Cu‐depleted CIGSe with a Ga pile‐up near the CIGSSe/CIS interface. This approach resulted in a reduced fill factor (FF) and blocking behavior, presumable caused by the wide‐gap Ga‐rich CIGSe near the interface.…”

“…[ 13,16 ] While the CIS layer provides a higher surface bandgap and, thereby, increases open‐circuit voltage ( V OC ), [ 13 ] the conversion efficiency was not increased, possibly due to the formation of an electron barrier at the CIGSe/CIS interface. For the CIGSe absorbers with Ga‐free surfaces, a much higher in V OC , FF, and efficiency was observed after sulfurization [ 5,14 ] although short‐circuit current density ( J SC ) was always lower. The lower J SC after sulfurization has been attributed to: 1) emergence of an electron barrier at the CuInS 2 /CuIn(S,Se) 2 interface, 2) shrinkage of the space‐charge region width due to higher doping in the newly formed CuInS 2 , or 3) enhanced recombination at the grain boundaries (GBs) owing to the higher S concentration therein.…”