The impact of alkali elements on the degradation of CIGS solar cells

Abstract

Unencapsulated CIGS solar cells with high and low contents of sodium (Na) and potassium (K) were simultaneously exposed to damp heat and illumination. The solar cells with a high alkali (Na, K) content exhibited higher initial conversion efficiencies, but degraded severely within 100 h, while the alkali poor samples kept relatively stable performance under damp heat and illumination. The degradation of the samples with a high alkali content resulted in the formation of sodium rich spots on the top ZnO:Al surface of the samples. This is likely caused by light-induced Na + migration via the grain boundaries in the absorber to the depletion region, where the Na + accumulated. This allowed subsequent Na + transport through the depletion region due to the lowering of the internal electric field caused both by the Na + accumulation and illumination. The migration resulted in the formation of shunt paths, which reduced the shunt resistance and open circuit voltage. Furthermore, ingression of water into the ZnO:Al is expected to be responsible for a slow but steady increase in series resistance for both high and low alkali solar cells. Additionally, sodium migration led to a severe increase of the series resistance in case of alkali rich samples.

The impact of atmospheric species on the degradation of CIGS solar cells

“…SIMS measurements also showed an increase in hydroxide content of one to two magnitudes in the zinc oxide layers in the H 2 O/air and H 2 O/CO 2 /N 2 samples. SIMS also indicated that the sodium profiles did not change greatly, while it has been shown to migrate under illumination or bias, leading to degradation of the solar cells [10,11]. Therefore, it is unlikely that similar reactions are occurring under these conditions.…”

Incorporation of alkali metals in chalcogenide solar cells

“…The amount of Na present in a CIGSe film is higher in films with a smaller grain size [37] and Na accumulates at the surface of the CIGSe films [6,21,38,39] and possibly in the grain boundaries [40][41][42][43].…”

The effect of Na on Cu-K-In-Se thin film growth

“…A PDT without KF (just Se) has also been shown to significantly alter CIGS absorbers [21,22], calling into question what 'control' absorber is appropriate for KF PDT comparison. High absorber Na and K composition has also been linked to drastically accelerated degradation in photovoltaic (PV) performance [23], which undermines the industrial relevance of initial performance gains achieved with high Na and K levels. Specifically, Na and K diffused into and degraded the ZnO layer after 100 h in damp heat and light [23], underscoring the importance of understanding alkali metal bonding in CIGS.…”

“…High absorber Na and K composition has also been linked to drastically accelerated degradation in photovoltaic (PV) performance [23], which undermines the industrial relevance of initial performance gains achieved with high Na and K levels. Specifically, Na and K diffused into and degraded the ZnO layer after 100 h in damp heat and light [23], underscoring the importance of understanding alkali metal bonding in CIGS. In a departure from work utilizing KF precursors and PDTs, recent work used Cu, KF, In, and Se co-evaporation to form Cu1-xKxInSe2 (CKIS) alloys with K/(K+Cu) composition, or x, varied from 0 to 1 [24,25].…”