Exploring reverse-bias characteristics of CIGS solar cells: impact of alkali-post-deposition treatment and CdS buffer layer

Abstract

The characteristics of solar cells in the reverse voltage direction are essential for the resilience of a photovoltaic module against partial-shading induced damage. Therefore, it is important to establish a thorough understanding of the mechanisms that lead to reverse breakdown in solar cells. This work studies thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) absorber layers. Systematic material variations are investigated in order to learn more about the mechanisms governing reverse breakdown in these devices. To this end, devices with different thicknesses of the CdS buffer layer and with and without a RbF-post-deposition treatment (PDT) of the absorber layer were fabricated. The resulting current-voltage characteristics at negative voltage biases reveal that devices break down at much more negative voltages if they underwent a PDT, if the buffer layer thickness is increased, or if the buffer layer is not photoexcited. This implies that possibly a PDT may be disadvantageous for the shading tolerance of a module. The further analysis indicates that several mechanisms are involved in the reverse breakdown. Whereas tunneling currents in the buffer layer seem to play a major role for the actual breakdown, the strong effect of the PDT is probably caused by a reduction of shunt leakage currents along grain boundaries which lowers material heating.

Reverse-bias behaviour of thin-film solar cells: effects of measurement-induced heating

“…Both models predict an increase of tunneling currents with increasing temperature, which agrees with the observed decrease in breakdown voltage. Further, it has been shown that the voltage drop in the CdS buffer layer plays a crucial role for the breakdown [5,16,19,20]. So far, these models have only been tested on IV characteristics measured with continuous voltage sweeps, so that heating effects may have led to misinterpretations.…”

“…Here, the common assumption was made, that the applied voltage V app drops entirely over the buffer layer for all T, so that the electric field may be calculated as E = V app /d CdS (with d CdS the thickness of the buffer layer) [6,16,20]. The data clearly show a linear behaviour above a certain voltage (for 220 K this voltage apparently exceeded the measured range).…”