Solution-processable all-small molecular bulk heterojunction films for stable organic photodetectors: near UV and visible light sensing†
We report stable organic photodetectors with all-small molecular bulk heterojunction (BHJ) sensing layers prepared using solutions of electron-donating and electron-accepting small molecules. As an electron-donating molecule, 2,5-bis(2-ethylhexyl)-3,6-bis(4′-methyl-[2,2′-bithiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (EHTPPD-MT) was synthesized via a Stille coupling reaction, whereas [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) was used as an electron-accepting component. The devices with the EHTPPD-MT:PC61BM BHJ layer could detect photons at a wavelength of 400–800 nm and exhibited a stable photoresponse under on/off modulation of near UV (405 nm) and visible (532 nm and 650 nm) light even at bias voltage conditions. The corrected responsivity reached ∼175 mA W−1 for the near UV detection at −1 V. An extremely durable photoresponse was measured for the present devices (including flexible devices) under illumination with high intensity green light (133.4 mW cm−2 at 532 nm) which is much stronger than standard sun light (100 mW cm−2, white). The excellent stability has been attributed to the tiny EHTPPD-MT crystals, which are formed in the EHTPPD-MT:PC61BM layers during the coating processes.