High Performance Bulk Photovoltaics in Narrow-bandgap Centrosymmetric Ultrathin films

In conventional bulk photovoltaics (BPVs), it is difficult to acquire both intensive photocurrent and large photovoltage output, which greatly limits the practical application. Here, we report a new strategy that can significantly increase photocurrent by five orders of magnitude whilst retaining the nature of high photovoltage of BPVs by introducing the local-chemistry-induced polar nanoregions (PNRs) into narrow-bandgap centrosymmetric ultrathin films. It is believed that these PNRs distribute randomly in non-ferroelectric insulator matrix and contribute to ferroelectric-like polarization when the sample thickness is comparable to the PNRs sizes. This allows to establish an internal field to effectively separate photogenerated electron-hole pairs even in centrosymmetric materials with narrow bandgap, and thus the power conversion efficiency (PCE) will increase significantly. BiVO4 (BVO) hereof is exemplified. Through intergrowing polar Bi4V2O11 nanoregions into the ultra-thin narrow bandgap BVO film, the PCE increases 1000 times compared with that of the BVO ceramic without taking advantage of the PNRs. This new strategy of producing high PV output in narrow-bandgap semiconductor thin films therefore will extend the candidate pool of bulk PV materials beyond traditional ferroelectricity, providing a pathway for practical application of bulk PV effect. Additionally, the combination of ferroelectric-like feature with a narrow bandgap in one material may create new optoelectronic functions.