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 750nm wavelength fabricated in bottom absorption geometry with the semiconducting polymer blend PTB7:PC71BM. A maximal responsivity of ~100mA/W at 630nm with 54mW/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 μs regime, with an on/off-ratio of four orders of magnitude at −2V 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. [Display omitted] •Organic photodiodes using PTB7:PC71BM as absorption layer have been structured and electro-optically analyzed.•Improvements in the on/off-ratio are realized by changing the thickness of the absorption layer.•We discuss the absorption spectra in dependence to the achieved photocurrent for the photodiodes.•The μs transient photo current response time is measured with a responsivity of 98mA/W.