Major growth in the image sensor market is largely as a result of the expansion of digital imaging into cameras, whether stand-alone or integrated within smart cellular phones or automotive vehicles. Applications in biomedicine, education, environmental monitoring, optical communications, pharmaceutics and machine vision are also driving the development of imaging technologies. Organic photodiodes (OPDs) are now being investigated for existing imaging technologies, as their properties make them interesting candidates for these applications. OPDs offer cheaper processing methods, devices that are light, flexible and compatible with large (or small) areas, and the ability to tune the photophysical and optoelectronic properties - both at a material and device level. Although the concept of OPDs has been around for some time, it is only relatively recently that significant progress has been made, with their performance now reaching the point that they are beginning to rival their inorganic counterparts in a number of performance criteria including the linear dynamic range, detectivity, and color selectivity. This review covers the progress made in the OPD field, describing their development as well as the challenges and opportunities.

No abstract

Tremendous interest in organic semiconductors for the development of light-weight and flexible electronic devices has been generated, owing to their ease of fabrication and suitability to large-area applications. [1][2][3][4][5] However, there are still many unexplored possibilities offered by these conjugated materials. [6,7] We demonstrate here a new structure that permits efficient integration of both light-and current-generation functions from a rubrene/fullerene heterostructure into an efficient organic dual device (ODD). [8][9][10] The device behaves like a compound semiconductor device: The electroluminescence (EL) turn-on voltage is < 1 V with the characteristic color of rubrene. The solar power conversion efficiency reaches 3 % with a 5.3 mA cm -2 short-circuit current density and almost 1 V open-circuit voltage under AM 1.5 illumination. Surprisingly, the EL turn-on voltage is about half the value of the rubrene bandgap (2.2 eV), a fact that cannot be explained using current models of charge injection into organic semiconductors. A physical interpretation is proposed in terms of the so-called Auger fountain mechanism, [11] which we could implement into our molecular heterojunction. During the EL process in heterojunction devices, holes and electrons are injected into the hole and electron conducting layers, respectively. They then recombine near the interface to raise a photon that has a color characteristic of the recombination region. Whereas, for the photovoltaic (PV) effect in organic materials, electron-hole pairs created by a broadband light source form excitons that dissociate at the donor-acceptor (D-A) junction. It then generates a net bipolar current flow across the device. Interest in organic-based solar cells (OSCs) has grown because of experience and understanding gained from organic light-emitting diode (OLED) developments. The general understanding treats light and current generation as competing phenomena. Indeed, the working principle of organic solar cells is that photoluminescence (PL) is quenched by ultrafast charge transfer from the donor to the acceptor.[4] However, we have successfully integrated both efficient light-and current-generating functions in a discrete ODD device. 5,6,11,12-Tetraphenylnaphthacene, commonly known as rubrene, is used as a hole-transporting material, while fullerene (C 60 ) is used as an electron-transporting material. Both rubrene and fullerene are widely studied semiconductors, with among the highest field-effect mobility for holes and electrons, respectively. [12,13] Moreover, rubrene is also currently used as a yellow dopant for achieving OLEDs with white-light emission suitable for display and lighting applications, and fullerene is omnipresent as an acceptor in efficient OSCs. [14,15] We have made heterojunction devices that combine 35 nm thick rubrene and 25 nm thick C 60 layers sandwiched between transparent indium tin oxide (ITO) and 60 nm thick metal electrodes. A 40 nm thick layer of poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDO...