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 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.
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.
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