Influence of Cu-atomic ratio in the 3-stage deposition technique on the efficiency of CuIn_1-xGa_xSe₂ solar cells

Abstract The 3-stage co-evaporation technique is one of the deposition processes used to fabricate photon absorber layer in high efficiency CuIn1-xGaxSe2 (CIGS) solar cells. For this technique, the [Cu]/[III] ratio (y), where [III] refers to group-III elements, evolves from Cu-poor (y &lt; 1) to Cu-rich (y &gt; 1) in the 2nd stage and finally ends with slightly Cu-poor (y ~ 0.9) in the 3rd stage of the 3-stage process. Here, the highest values of [Cu]/[III] in the 2nd stage are intentionally varied from 1.0 to 1.5 by setting the deposition time of the pre-calibrated Cu and Se fluxes in the 2nd stage. The [Ga]/[III] ratio (x) is set at 0.37 during the 1st and 3rd stages in all devices. The influences of the Cu-atomic ratio are examined for the crystal grain growth, elemental depth profiles of the CIGS absorbers as well as the photovoltaic parameters and external quantum efficiency (EQE) of the CIGS solar cells. The optimal value of y = 1.3 is found to provide the highest efficiency CIGS device. The double-grading depth profile in the [Ga]/[III] ratio has also been observed despite the constant fluxes of group-III elements set during the whole process. The performances of the CIGS solar cells are investigated under AM1.5 condition and found to have open-circuit voltage (V OC) of 670 mV, short-circuit current density (J SC) of 33.2 mA/cm2, fill factor (FF) of 75.5% and the power conversion efficiency of 16.8% for the best CIGS device. The J SC of the device with y = 1.3 is relatively higher than other devices due to the increase of photo-generated currents in the short wavelength region as seen in the EQE spectrum.</jats:p

Fabrication of SnO₂ by RF magnetron sputtering for electron transport layer of planar perovskite solar cells

Abstract The requirements of electron transport layer (ETL) for high efficiency Perovskite solar cells (PSCs) are, for example, appropriate band energy alignment, high electron mobility, high optical transmittance, high stability, and easy processing. SnO2 has attracted more attention as ETL for PSCs because it has diverse advantages, e.g., wide bandgap energy, excellent optical and chemical stability, high transparency, high electron mobility, and easy preparation. The SnO2 ETL was fabricated by RF magnetron sputtering technique to ensure the chemical composition and uniform layer thickness when compared to the use of chemical solution via spin-coating method. The RF power was varied from 60 - 150 W. The Ar sputtering gas pressure was varied from 1 × 10−3 - 6 × 10−3 mbar while keeping O2 partial pressure at 1 × 10−4 mbar. The thickness of SnO2 layer decreases as the Ar gas pressure increases resulting in the increase of sheet resistance. The surface morphology and optical transmission of the SnO2 ETL were investigated. It was found that the optimum thickness of SnO2 layer was approximately 35 - 40 nm. The best device shows Jsc = 27.4 mA/cm2, Voc = 1.03 V, fill factor = 0.63, and efficiency = 17.7%.</jats:p