Effect of heat‐bias soaking on cesium fluoride‐treated CIGS thin film solar cells

Abstract

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.

Proton Irradiation on Cesium‐Fluoride‐Free and Cesium‐Fluoride‐Treated Cu(In,Ga)Se2 Solar Cells and Annealing Effects under Illumination

“…The performance was even higher than that initially with efficiency rising from 16.0% to 16.6%, after HLS, owing to the improvement in open‐circuit voltage. This result is consistent with previous reports of HLS or heat‐bias soaking (HBS) on alkali‐treated CIGS solar cells, where an electric field generated in the CIGS absorber by light irradiation (or forward bias) was suggested as a driving force for the effect.…”

“…The completed solar cell structure was Al/Ni/ZnO:Al (300 nm)/ZnO (100 nm)/CdS (60 nm)/CIGS (2.5–3.0 μm)/Mo (600 nm)/SLG. CsF‐treated CIGS solar cells were prepared as mentioned in the previous report …”

CuGaSe2 Thin Film Solar Cells: Challenges for Developing Highly Efficient Wide‐Gap Chalcopyrite Photovoltaics

“…Alkali‐elements in CIGSe films are also likely correlated with metastable properties, for example, light‐soaking or bias‐induced effects . CGSe cells showed no significant improvements in V oc and FF but showed an enhancement in J sc values as mentioned above (Figure ).…”

“…A buried p‐n junction model has been suggested as the operating principle of CIGSe cells . Group II elements such as Cd or Zn are believed to diffuse into the CIGSe film surface during the deposition process of an n ‐type buffer‐layer such as CdS or ZnS(O,OH) on a CIGSe film.…”

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

“…This indicates that CsF‐treated CIGS solar cells are applicable to terrestrial and space applications. Furthermore, our previous studies revealed the beneficial effect of heat‐light soaking (HLS) or heat‐bias soaking (HBS) on alkali metal‐treated CIGS solar cells. These post‐treatments increased the efficiency ( η ) of solar cells by enhancing the open‐circuit voltage ( V OC ).…”