Ultrathin Cu(In,Ga)Se2 transparent photovoltaics: an alternative to conventional solar energy-harvesting windows

Abstract

Transparent photovoltaic (TPV) devices using very thin absorbers, which are sandwiched by transparent conducting front and rear contacts, provide efficient solar energy yields and partially visible transparency. These characteristics allow such devices to be used in applications including building-integrated photovoltaics (BIPV), solar vehicles, and wearable devices. In this study, we developed the TPVs with tunable power conversion efficiencies (PCEs) and average visible transmittance (AVT) values using ultrathin Cu(In,Ga)Se2 (CIGS) absorber layers with thicknesses ranging from 30 to 300 nm, prepared using a single-stage coevaporation process. The PCE and AVT values of these devices could be easily tuned by controlling the absorber thickness; the TPV devices exhibited respective PCEs ranging from 11.0% to 2.1% (front illumination) and 6.6–1.7% (rear illumination), as well as AVTs ranging from 9.1% to 47.8% as the absorber thickness was varied from 300 to 30 nm. The TPV device prepared with a 100 nm-thick fully depleted CIGS absorber layer exhibited an enhanced bifaciality factor close to unity owing to the field-driven charge transport and reduced recombination at the rear contact and/or in the bulk of this device. Similar PCEs of ~5% under front and the rear illumination at an AVT of ~35% were achieved. This TPV device demonstrated constant PCEs under low-light irradiance (10–100 mW cm−2) irrespective of the illumination direction, as well as an enhanced bifacial performance as JSC linearly increased from 11.4 to 19.3 mA cm−2 as the rear illumination intensity was increased from 0 to 60 mW cm−2. It also exhibited high light-soaking stability.