Effect of Crystal Orientation and Conduction Band Grading of Absorber on Efficiency of Cu(In,Ga)Se₂ Solar Cells Grown on Flexible Polyimide Foil at Low Temperature

Abstract

Flexible Cu(In,Ga)Se2 (CIGS) solar cells are developed on polyimide (PI) foil by using a three‐stage co‐evaporation method. To deposit CIGS on a PI substrate, low temperature deposition process (below 440 °C) is investigated. By optimizing the three‐stage process, power conversion efficiency (PCE) values of 12.1% and 13.6% are obtained at maximum process temperatures of 400 °C and 440 °C, respectively (without anti‐reflection coating). The CIGS absorber deposited at 400 °C is grown with (220)/(204) rather than (112) as its dominant growth orientation, accompanied by highly homogeneous crystal structure and phases. Admittance spectroscopy reveals that the defect levels were lower in the CIGS absorber with (220)/(204) dominant growth orientation, which could have contributed to the PCE of 12.1%. The CIGS solar cell deposited at a temperature of 440 °C exhibits worse defect characteristics compared to that fabricated at 400 °C. However, the CIGS solar cell fabricated below 440 °C exhibited marginal double‐grading of Ga/(Ga + In) in the absorber layer, which resulted in an improved PCE of 13.6%. This result implies that at a temperature below 440 °C, the double‐grading of Ga/(Ga + In) is the more dominant factor causing the PCE improvement above 13%.

Controllable Formation of Ordered Vacancy Compound for High Efficiency Solution Processed Cu(In,Ga)Se2 Solar Cells

“…Figure shows capacitive frequency (C–f) spectra of the CIGS devices measured under dark condition with the temperature range of 120–300 K. The capacitance at high frequency mainly represents the response caused by free carrier density, while the capacitance at low frequency represents the response caused by free carrier and deep trap. [ 37,43 ] As shown in Figure 8, the capacitance without OVC device showed larger variation as a function of frequency (Figure 8a), while the capacitance with OVC device displayed smaller variation (Figure 8b). The significant frequency dependence of the without OVC device capacitance demonstrated the higher trap densities within the absorber layer.…”

“…This result demonstrated that the interface recombination was significantly passivated by the formation of OVC phase. [ 37 ]…”

“…[ 44 ] The capacitance data converging at high frequency and low temperature may be resulting from the carrier‐freezing induced geometric capacitance of the device. [ 37,44 ]…”

Morphological–Electrical Property Relation in Cu(In,Ga)(S,Se)2 Solar Cells: Significance of Crystal Grain Growth and Band Grading by Potassium Treatment

“…[39][40][41][42] Bandgap grading structure is commonly exhibited by all device types, which takes advantage of the minimum bandgap to increase light absorption and the bandgap grading toward the back interface to facilitate charge collections. [3,4,26] In the BK-CIGS absorber layer, the minimum E c and maximum E v regions are extended from the decreasing tendency of S/(S+Se) and Ga/(Ga+In) ratio. The notch region, which refers to the minimum E g region, extends and the notch depth is reduced throughout the depth of the BK-CIGS film (Figure 1e).…”

“…In general, a bandgap grading structure is suggested in bulk CIGS films with a deep notch point near the surface, as well as increased bandgap at both the top and bottom surfaces to enhance device open-circuit voltage (V oc ) and short-circuit current density (J sc ), respectively. [3,4,26] To further optimize the bandgap grading structure, a wide notch region and a moderately steep gradient are necessarily required to maximize the optical absorption without compromising V oc . [16,25] When an incident light propagates from the pn-junction to the CIGS light absorber, minority carrier electrons generated within the space charge region (SCR) of the pn-junction are subsequently drifted to the electrode by the device electrical field.…”