Current Applied Physics, Vol.20, No.1, 219-225, 2020
Effect on the reduction of the barrier height in rear-emitter silicon heterojunction solar cells using Ar plasma-treated ITO film
In this study, we investigated the effect of plasma treatment on an indium fin oxide (ITO) film under an ambient Ar atmosphere. The sheet resistance of the plasma-treated ITO film at 250 W (37.6 Omega/sq) was higher than that of the as-deposited ITO film (34 Omega/sq). Plasma treatment was found to decrease the ITO grain size to 21.81 nm, in comparison with the as-deposited ITO (25.49 nm), which resulted in a decrease in the Hall mobility. The work function of the Ar-plasma-treated ITO (WFITO = 4.17 eV) was lower than that of the as-deposited ITO film (WFITO = 5.13 eV). This lower work function was attributed to vacancies that formed in the indium and oxygen vacancies in the bonding structure. Rear-emitter silicon heterojunction (SHJ) solar cells fabricated using the plasma-treated ITO film exhibited an open circuit voltage (V-OC) of 734 mV, compared to SHJ cells fabricated using the as-deposited ITO film, which showed a V-OC of 704 mV. The increase in V-OC could be explained by the decrease in the work function, which is related to the reduction in the barrier height between the ITO and a-Si:H (n) of the rear-emitter SHJ solar cells. Furthermore, the performance of the plasma-treated ITO film was verified, with the front surface field layers, using an AFORS-HET simulation. The current density (J(SC)) and V-OC increased to 39.44 mA/cm(2) and 736.8 mV, respectively, while maintaining a WFITO of 3.8 eV. Meanwhile, the efficiency was 22.9% at V-OC = 721.5 mV and J(SC) = 38.55 mA/cm(2) for WFITO = 4.4 eV. However, an overall enhancement of 23.75% in the cell efficiency was achieved owing to the low work function value of the ITO film. Ar plasma treatment can be used in transparent conducing oxide applications to improve cell efficiency by controlling the barrier height.