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.

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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...

We report the observation of a spin-dependent dark transport current, exhibiting spin coherence at room temperature, in a -conjugated polymer-fullerene blend using pulsed electrically detected magnetic resonance. The resonance at g ¼ 2:0028ð3Þ is due to polarons in the polymer, and exhibits spin locking at high microwave fields. The presence of an excess of fullerene, and the operating voltage (1 V) used, suppresses negative polaron formation in the polymer. It is concluded that spin-dependent transport is due to the formation of positive bipolarons. DOI: 10.1103/PhysRevLett.105.176601 PACS numbers: 72.80. Le, 71.38.Mx, 72.25.Rb, 76.30.Mi Organic semiconductors provide a range of commercial optoelectronic display devices, and show great promise in the field of photovoltaics (PV) [1][2][3][4] if improved efficiency, combined with low cost and ease of production, can be achieved. In addition, the weak spin-orbit coupling of organic semiconductors is attractive for carrier spin transport and manipulation, and is driving efforts to develop spintronic devices [5]. All these applications depend upon detailed knowledge of the relevant transport processes, in particular, those influenced by spin selection rules. The observation of large magnetoresistance (MR) has, for example, attracted a range of explanations [6][7][8][9], but aspects of the proposed mechanisms remain controversial [10].Conduction in disordered organic semiconductors is dominated by hopping of charge carriers between localized states. Because of strong electron-phonon coupling the carriers are polarons. Oppositely charged polarons can form excitons and eventually recombine; the process normally depends on the spin state of the coupled pair immediately prior to exciton formation. In addition, the strong coupling between carriers and the environment can markedly reduce the energy cost of doubly occupying states. Two like-charge polarons can form a bipolaron, the correlation energy between the pair and the lattice deformation lowering the formation energy [11]. However, the on-site exchange requires that the final state is a spin singlet, and bipolaron formation will be ''spin-blocked'' if two polarons have the same spin component along the common axis of quantization [8].Organic PV devices have advanced dramatically with the development of bulk heterojunction materials [2][3][4], which comprise a -conjugated polymer blended with an electron acceptor such as a fullerene derivative. The two phase-separated components give interpenetrating networks with vastly increased interfacial regions [3]. The PVeffect is due to photoexcitation of the polymer, followed by highly efficient electron transfer to the fullerene phase. Positive polarons (P þ ) are transported through the polymer matrix, negative polarons through the fullerene phase, efficiently suppressing carrier loss by P þ P À recombination. However, unipolar transport to the electrodes may be influenced by bipolaron formation. Gaining insight into this process, which affects charge carrier collection...

Application of non-metal doped titania for inverted polymer solar cells

Flexible thin film solar cells based on a pentacene/C60 heterojunction on indium tin oxide coated polyethylene terephthalate (PET) substrates are described. Devices grown on PET are compared to those on glass substrates. The nature of the organic grains grown on PET and glass substrates is shown to have a significant impact on the device short circuit current. Device performance under mechanical strain and elevated temperature is reported. Devices deliver an 8.8mAcm−2 short circuit current density with a 1.6% power conversion efficiency under an AM1.5 simulated solar intensity of 80mWcm−2. The relatively low 300mV open circuit voltage appears to limit device efficiency.

Solar cells exhibiting efficient photon harvesting are built from molecular blends of pentacene: PTCDI-C13H27. Absorption of the composition spans the whole visible spectrum with an onset at 730nm. External quantum efficiency approaches unity at the peak absorption of pentacene. A 2.0% power conversion efficiency is obtained under 80mWcm−2 AM 1.5 illumination, with 8.6mAcm−2 short-circuit current. The comparison with bilayer devices suggests directions of improvement in conversion efficiency such as controlled growth of the blend layers.

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