Multilayer broadband anti-reflective coatings for bulk heterojunction polymer solar cells

The photocurrent produced by solar cells is limited by reflection losses for all types of photovoltaic devices. The first reflection loss occurs at the glass/air interface of the photovoltaic device. A solar cell without a light trapping mechanism in place loses around 4 % of the solar energy at this interface. To minimise the losses, a broadband multilayer thin film anti-reflection (MAR) coating has been designed and deposited onto the glass surface of a solar cell. The coating consisted of four dielectric layers of alternating thin films of ZrO2 and SiO2. The layers were deposited by using high rate pulsed DC magnetron sputtering using time only for nanometre thickness control. Spectrophotometer measurements confirm that the transmission increased over the spectrum utilized by the bulk heterojunction (BHJ) solar cell (350nm- 700nm). The weighted average reflection reduced from 4.22% to 0.99%. BHJ solar cells with a PCDTBT:PCBM blend serving as the active layer were prepared on a MAR coated soda lime glass slides to verify the effectiveness of the coating. The efficiency increased by 0.18% at STC from 4.98% to 5.17% (a 3.7% relative increase). The gain was achieved by increasing the photocurrent from 11.96 mA/cm2 to 12.36 mA/cm2.