Impact of front contact layers on performance of Cu(In,Ga)Se₂ solar cells in relaxed and metastable states

Abstract

We studied the impacts of front contact layers composed of transparent conducting oxide (TCO) and transparent oxide semiconductor (TOS) layers on the performance of Cu(In,Ga)Se 2 (CIGS) solar cells with glass/Mo/CIGS/CdS/TOS/TCO in relaxed and metastable states. After annealing under illumination, the CIGS solar cells with KF and NaF postdeposition treatments exhibited metastable increases in open-circuit voltage, fill factor, and the resulting conversion efficiency with a metastable decrease in short-circuit current density. These are partly attributed to an increase in metastable acceptor density in the CIGS layers, being stable for a day even at room temperature, with a reduced space charge width. We found that the net acceptor density evaluated by capacitance-voltage measurements largely varied depending on the TOS/TCO layers, whereas the open-circuit voltage was identical. On the other hand, no significant differences in both the net acceptor density and open-circuit voltage were observed regardless of the TOS/TCO layers in the relaxed state produced by annealing in the dark. The results suggest that the CdS/CIGS heterojunction can no longer be treated as an n + /p junction in the metastable state and that the evaluated acceptor density was apparent values with large difference originating from different potential distribution in the CdS layer, which is related to the TOS/CdS and TCO/CdS junctions. In addition, the TOS/TCO layers largely affected the short-circuit current density owing to their optical constants and film thicknesses. The presented results highlight the importance of the design of CdS/TOS/TCO layers in solar cells operated in metastable states.

Metastable Behavior on Cesium Fluoride‐Treated Cu(In1–x,Gax)Se2 Solar Cells

“…W190113). HLS treatment increased the V OC and carrier concentration ( N CV ) and reduced J SC, whereas the HS treatment yielded opposite results similar to the previous reports on alkali‐treated samples . The change in the width of the space charge region (SCR) was consistent to the change in N CV .…”

“…Such improvement has a turn-on time constant in the order of a few minutes and turn-off time constant in the order of a day or months. [29] A metastable phenomenon on non alkali-treated CIGS solar cells due to a Zncompound buffer layer and CIGS thin films under light illumination and bias has been observed in earlier studies. [30,31] Previous studies revealed that HLS improved the fill factor (FF) and η due to a rearrangement of the S/(S þ O) atomic ratio in the CBD-ZnS(O,OH) buffer layer to adjust the conduction band offset at the ZnS(O,OH)/CIGS interface.…”

“…Both the positive and negative effects due to this metastable behavior have been reported . However, in CdS/alkali metal‐treated CIGS solar cells, HLS guarantees an improvement in V OC and FF under the optimized condition . In this study, we analyzed the contributing factors for the metastable effects in CsF‐treated CIGS solar cells.…”

“…These post‐treatments increased the efficiency ( η ) of solar cells by enhancing the open‐circuit voltage ( V OC ). Such improvement has a turn‐on time constant in the order of a few minutes and turn‐off time constant in the order of a day or months …”

“…[34][35][36][37] However, in CdS/alkali metal-treated CIGS solar cells, HLS guarantees an improvement in V OC and FF under the optimized condition. [5,10,13,[27][28][29] In this study, we analyzed the contributing factors for the metastable effects in CsF-treated CIGS solar cells. Our investigation disclosed that the metastability increased in V OC , and FF could be explained by the specific behavior of alkali metal migration and/or presence of Cs-related compounds at the very near-surface region of the CIGS thin film.…”