On/off-ratio dependence of bulk hetero junction photodiodes and its impact on electro-optical properties

Optical sensing from the near ultra-violet to the visible range of light is very important for industrial process monitoring, health care, and life science. Here, we present organic photodiodes (OPD), operating between 300 and 750 nm wavelength fabricated in bottom absorption geometry with the semiconducting polymer blend PTB7:PC71BM. A maximal responsivity of ~ 100 mA/W at 630 nm with 54 mW/cm2 light intensity has been achieved at room temperature. An approach to improve the on/off-ratio (light to dark) by modifying the absorption layer thickness has been investigated. The dark current decreases with the thickness of polymer blend PTB7:PC71BM. The transient response for OPD rise- and fall-time is in the ms regime, with an on/off-ratio of four orders of magnitude at - 2 V bias. The rise of responsivity is tunable by increasing the absorption layer thickness and shows a dependency in relation to dark current and transient response.