Journal Article Research Support, Non-U.S. Gov't Research Support, U.S. Gov't, Non-P.H.S. Review bioelectronics bulk heterojunctions charge transport dopant-free small molecules electronic skin flexible electronics formamidinium hole conductors light-emitting diodes low bandgap polymers oxide semiconductor oxide semiconductors perovskite perovskites phototransistors printable electronics sensors solar cells solution processes solution-process structural properties sub-bandgap emissions thin-film transistor