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

Abstract

The effect of absorber stoichiometry in (Ag,Cu)(In,Ga)Se 2 (ACIGS) solar cells with bandgaps (E g ) > 1.40 eV is studied on a large sample set. It is confirmed that moving away in composition from ternary AgGaSe 2 by simultaneous reduction in Ga and Ag content widens the chalcopyrite single-phase region and thereby reduces the amount of ordered vacancy compounds (OVCs). As a consequence, a distortion in currentÀvoltage characteristics, ascribed to OVCs at the back contact, can be successfully avoided. A clear anticorrelation between open-circuit voltage (V OC ) and short-circuit current density (J SC ) is detected with varying absorber stoichiometry, showing decreasing V OC and increasing J SC values for [I]/[III] > 0.9. Capacitance profiling reveals that the absorber doping gradually decreases toward stoichiometric composition, eventually leading to complete depletion. It is observed that only such fully depleted samples exhibit perfect carrier collection, evidencing a very low diffusion length in wide-gap ACIGS films. The results indicate that OVCs at the surface play a minor or passive role for device performance. Finally, a solar cell with V OC ¼ 0.916 V at E g ¼ 1.46 eV is measured, which is, to the best of our knowledge, the highest value reported for this bandgap to date.

Rubidium Fluoride Absorber Treatment for Wide‐Gap (Ag,Cu)(In,Ga)Se2Solar Cells

“…For more information about the lateral spread in composition within each ACIGS deposition, we refer to. [ 11 ]…”

“…Recently, we revealed a clear anticorrelation between V OC and the short‐circuit current density ( J SC ) with varying group‐I stoichiometry for wide‐gap ACIGS solar cells. [ 11 ] Perfect carrier collection (high J SC ) is only achieved for very close‐stoichiometric compositions, due to a complete depletion of the absorber (i.e., low doping). By contrast, absorbers with [I]/[III] < 0.9 show “common” doping densities and space charge region widths (<550 nm), resulting in higher V OC and significantly lower J SC values.…”

“…While some of the large V OC loss of wide-gap chalcopyrite solar cells can be mitigated by using alternative (i.e., not CdS) buffer layers with tunable electron affinity (χ), [6][7][8][9][10] a major part of the deficit seems to arise from a poorer bulk quality. [11,12] The lifetime deterioration with increasing GGI was suggested to originate from higher Shockley-Read-Hall (SRH) recombination via 1) energetically deeper Ga Cu antisite donor defects [13][14][15][16] or (V Se -V Cu ) complexes in acceptor state [17] ; 2) an increasing density of deep acceptors [18,19] ; 3) an increasing tetragonal lattice distortion [20] ; and/or 4) an increased fraction of Cu-enriched (supposedly detrimental [21] ) grain boundaries (GBs). [22] Instead of utilizing low-χ alternative buffers, interface recombination can also be reduced (or canceled out) for CdS-buffered wide-gap CIGS solar cells when the absorber is alloyed with silver, i.e., forming (Ag,Cu)(In,Ga)Se 2 (ACIGS).…”

“…[7,26,27] Consequently, high V OC values close to 900 mV and beyond were achieved for wide-gap ACIGS solar cells with CdS buffer layers by several groups. [7,11,[28][29][30][31][32] However, it was reported that the chalcopyrite single-phase region of the ACIGS system is narrowing toward GGI and AAC ¼ 1 (i.e., AgGaSe 2 ). [33] As a result, a high volume share of ordered vacancy compound (OVC) patches is observed, which significantly increases for AAC and GGI > 0.5.…”

“…This composition is chosen because a positive CBO at the ACIGS/CdS interface is expected and the substantial formation of OVCs can be curbed. [7,11,31] The I-V parameters of corresponding solar cells (all using CdS as a buffer) will be compared with those from a large number of devices with very similar composition but without PDT, which were already presented in an earlier publication. [11] In the second part, the chemical and morphological absorber modifications introduced by the RbF-PDT are illustrated and possible impacts on device characteristics discussed.…”