No abstract

In this study, the effects of light-soaking (LS), heat-soaking (HS), and combined LS and HS, that is, heat-light soaking (HLS) on potassium fluoride (KF)-treated and KF-free copper indium gallium selenide (CIGS) solar cells with CBD-CdS buffer layer were investigated. LS and HS did not change the basic solar cell parameters of CIGS solar cells when they were performed separately.In contrast, HLS improved cell efficiency with increased open-circuit-voltage for KF-treated CIGS solar cells, whereas it reduced cell performance for KF-free CIGS cells. Capacitance-voltage measurements confirmed a significantly increased carrier concentration in KF-treated CIGS solar cells, as compared to KF-free cells by HLS. X-ray photoelectron spectroscopy measurement revealed that the HLS did not change the atomic concentration of Cd, S, and O in CBD-CdS buffer layer.However, the concentration of Na atoms slightly increased at the CIGS surface region, as confirmed from SIMS measurement. It implies a possible reason for increased carrier concentration in KF-treated CIGS solar cells after HLS. Temperature-dependent current-voltage measurements suggests that HLS modify a K-containing new layer and affects cell performance.

The combined effect of heating and forward bias voltage, that is, heat‐bias soaking (HBS) on cesium fluoride (CsF)‐free and CsF‐treated Cu(In1‐xGax)Se2 (CIGS) solar cells using chemical bath deposition‐cadmium sulfide (CBD‐CdS) buffer layer was investigated with varying the heating temperature, bias voltage, and biasing time. Heat bias soaking followed by heat soaking (HS) (ie, HBS/HS) treatments improved the open‐circuit voltage (VOC) and conversion efficiency for the CsF‐treated CIGS solar cell, whereas such a beneficial effect was not observed for CsF‐free CIGS solar cells. Capacitance‐voltage measurement confirmed the significantly increased carrier concentration (NCV) after HBS for the CsF‐CIGS solar cell, which is one of the possible reasons for the increased VOC. Because of the extremely high NCV, the short‐circuit current density (JSC) decreased owing to the narrower depletion width. However, the high NCV could be tuned to an appropriate value via a subsequent moderate heating procedure without biasing. As a result, the JSC loss was reduced, thereby improving the cell efficiency. These results open a new route to improve the efficiency of alkali‐treated CIGS solar cells.

The combined effect of the heat–light soaking (HLS) and subsequent heat‐soaking (HS) processes is investigated on NaF‐free and NaF‐treated CIGS solar cells with CdS buffer layer. The HLS treatment improves cell efficiency slightly with increased open‐circuit voltage for NaF‐treated CIGS solar cells, whereas the cell efficiency deteriorates for NaF‐free CIGS solar cells. The different behaviors between both types of solar cells may be due to a Na‐containing new layer, which is formed at the CIGS surface region by NaF‐treatment. On the other hand, the short‐circuit current density (JSC) decreases for both types of solar cells after HLS treatment, which is attributed to the extremely high carrier concentration (NCV), leading to a narrower space charge region. However, it is found that the large NCV can be tuned to an appropriate value using the subsequent HS technique. As a result, the JSC loss is successfully reduced, therefore improving cell efficiency. The details of this improved efficiency are discussed with respect to the change of NCV in CIGS solar cells by the HLS and HS processes.

The effects of heat-light soaking (HLS) and heat-bias soaking (HBS) were investigated on NaF-treated Cu (In,Ga)Se 2 (CIGS) solar cells. The HLS improved the cell efficiency with increased open-circuit voltage (V OC ) for NaF-treated CIGS solar cells, whereas the short-circuit current density (J SC ) slightly decreased. The reduced J SC was caused by the significantly high carrier concentration due to the HLS, leading to a fairly narrow space charge region. However, the reduced J SC could be successfully recovered by the subsequent moderate heat soaking (HS) in the dark, thereby improving cell efficiency. On the other hand, no beneficial effect was observed for NaF-free solar cells. Secondary ion mass spectrometry (SIMS) measurements revealed that the Na and K ions increased especially near the CIGS surface region after the HLS process. This result suggests that these ions were shifted by the photo-induced electric field. The thermally and electrically activated alkali ions may eliminate the donorlike defects in CIGS absorber, resulting in an increased acceptor concentration. Furthermore, we found that an external forward bias without light irradiation (HBS) resulted in the same effect as HLS. Based on these results, a possible mechanism of HLS and HBS effects was proposed on NaF-treated CIGS solar cells.

In this work, potassium fluoride (KF)-treated Cu(In,Ga)Se2 (CIGS) thin films were rinsed in ammonia and water solutions before buffer layer (CdS) deposition and the effects of rinsing on photovoltaic properties were investigated. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) measurements revealed that sodium atoms out-diffused at the surface region during KF deposition. Water and ammonia rinsing processes of KF-treated CIGS thin films reduced alkali metals from the surface. However, sodium at the Cu-depleted surface layer remained at a high concentration, suggesting the occupation of Cu vacancies with sodium atoms. On the other hand, ammonia rinsing removed the Cu-poor region from the surfaces of KF-treated CIGS thin films affecting the growth (or nucleation) of the CdS layer. The surface coverage of the CdS layer deposited on the ammonia-rinsed KF-treated CIGS thin film was inferior to than that of water-rinsed samples, resulting in the poor cell performance due to an increased interface recombination.