A research team led by Professor Kung-Hwa Wei, in the Materials Science & Engineering Department of NYCU had used a novel approach involving controlling molecule diffusion at the interface of sequentially deposited (SD) p-type and n-type materials layers to achieving a pseudo p-i-n, as opposed to conventional bulk heterojunction (BHJ), active layer for semitransparent organic photovoltaics (St-OPVs); the objectives are (i) providing St-OPVs with both high power conversion efficiency (PCE) and high visible light transmission (VLT) for simultaneous power generation and photosynthesis that are difficult to achieve with devices having conventional BHJ active layers because of severe trade-off occurring between PCE and VLT and (ii) flexible and light St-OPVs that can be incorporated into green houses and building without requiring large land uses, particularly favorable for high population-densities countries.

For example, semitransparent organic photovoltaics were fabricated having the structure of glass/ITO/PEDOT:PSS/(D:A for BHJ; D/A for SD)/ZnO nanoparticles (NP)/Au/Ag. For the SD architecture, a solution of the PBDB-T-2F was spin-coated on the PEDOT:PSS layer to form the front layer, and then a solution of the Y6 was spin-coated onto the surface of the donor layer, followed by thermal deposition of a 1- and a 15-nm thick Au and Ag layer as the anode.

Semitransparent organic photovoltaics of polymer (PBDB-T-2F)/small molecule (Y6) with active layer thickness of 115, 100 and 85nm were prepared. The results show the SD device and BHJ device provide (i) the champion power conversion efficiency (PCE) of 12.91% (visible light transmission (VLT) of 14.5%) and 12.77% (VLT of 13.4%), respectively, at 115 nm; (ii) the champion PCE of 12.73% (VLT of 18.3%) and 12.17% (VLT of 15.2%), respectively, at 100 nm; (iii) the champion PCE of 12.22% (VLT of 22.2%) and 11.23% (VLT of 16.6%), respectively, at 85 nm. The slopes for the visible light transmissions (VLTs) vs. active layer thickness (T) curves for the cases of bulk heterojunction (BHJ) and sequential deposition (SD) devices are 0.11 and 0.26, respectively, indicating the increase in VLTs of the SD devices are more sensitive to the reduction in the active layer thickness. The ratios of dPCEBHJ/dVLTBHJ and dPCESD/dVLTSD are 0.45 and 0.08, respectively, implying a larger trade-off between the power conversion efficiencies and the visible light transmissions for the devices with bulk heterojunction active layer than that for the devices with pseudo p-i-n active layer structure. (Adv. Energy Mater., 2021, 11, (13), 2003576). The direct evidence of p-i-n active layers be found in the (a) Energy-dispersive X-ray (EDX) spectra and (b) X-ray Photoelectron spectra (XPS) that show a gradient concentration distribution of a n-type material across the layer, while the BHJ structure having an almost constant concentration of a n-type material. The detailed structure of the active layer was determined with small angle X-ray scattering as shown in the Front cover inside figure (Adv. Energy Mater., 2021, 11, (13), 2003576).

The impacts of this study in the field are (i) a first psuedo p-i-n active layer structure with tunable light absorption for semitransparent organic photovoltaics having both high power conversion efficiency and high visible light transmission and (ii) low-weight and flexible devices being developed; these contributions are important for the green houses and buildings applications that require visible light transmission for photosynthesis and infrared light absorption for suitable power generation. As opposed to conventional silicon solar cells that require large land areas for installations and wirings for electric transport, flexible and light semitransparent organic photovoltaics not only can be fitted into green houses or existing buildings that allows power generation and enough visible light transmission for photosynthesis for agriculture purposes that will also reduce CO2 emission.