The optoelectronic properties of polymeric semiconductor materials can be utilized for the fabrication of organic electronic and photonic devices. When key structural requirements are met, these materials exhibit unique properties such as solution processability, large charge transporting capabilities, and/or broad optical absorption. In this review recent developments in the area of π-conjugated polymeric semiconductors for organic thin-film (or field-effect) transistors (OTFTs or OFETs) and bulk-heterojunction photovoltaic (or solar) cell (BHJ-OPV or OSC) applications are summarized and analyzed.

π-Conjugated Polymers for Organic Electronics and Photovoltaic Cell Applications†

Electron and ambipolar transport in organic field-effect transistors.

Acenes have long been the subject of intense study because of the unique electronic properties associated with their pi-bond topology. Recent reports of impressive semiconductor properties of larger homologues have reinvigorated research in this field, leading to new methods for their synthesis, functionalization, and purification, as well as for fabricating organic electronic components. Studies performed on high-purity acene single crystals revealed their intrinsic electronic properties and provide useful benchmarks for thin film device research. New approaches to add functionality were developed to improve the processability of these materials in solution. These new functionalization strategies have recently allowed the synthesis of acenes larger than pentacene, which have hitherto been largely unavailable and poorly studied, as well as investigation of their associated structure/property relationships.

The larger acenes: versatile organic semiconductors.

This article is written from an organic chemist's point of view and provides an up-to-date review about organic solar cells based on small molecules or oligomers as absorbers and in detail deals with devices that incorporate planar-heterojunctions (PHJ) and bulk heterojunctions (BHJ) between a donor (p-type semiconductor) and an acceptor (n-type semiconductor) material. The article pays particular attention to the design and development of molecular materials and their performance in corresponding devices. In recent years, a substantial amount of both, academic and industrial research, has been directed towards organic solar cells, in an effort to develop new materials and to improve their tunability, processability, power conversion efficiency, and stability. On the eve of commercialization of organic solar cells, this review provides an overview over efficiencies attained with small molecules/oligomers in OSCs and reflects materials and device concepts developed over the last decade. Approaches to enhancing the efficiency of organic solar cells are analyzed.

Small molecule organic semiconductors on the move: promises for future solar energy technology.

Our objective in this Account is 3-fold. First, we provide an overview of the optical and electronic processes that take place in a solid-state organic solar cell, which we define as a cell in which the semiconducting materials between the electrodes are organic, be them polymers, oligomers, or small molecules; this discussion is also meant to set the conceptual framework in which many of the contributions to this Special Issue on Photovoltaics can be viewed. We successively turn our attention to (i) optical absorption and exciton formation, (ii) exciton migration to the donor−acceptor interface, (iii) exciton dissociation into charge carriers, resulting in the appearance of holes in the donor and electrons in the acceptor, (iv) charge-carrier mobility, and (v) charge collection at the electrodes. For each of these processes, we also describe the theoretical challenges that need to be overcome to gain a comprehensive understanding at the molecular level. Finally, we highlight recent theoretical advances, ...

Molecular Understanding of Organic Solar Cells: The Challenges

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.

Organic Photodiodes: The Future of Full Color Detection and Image Sensing

Light and current generation functions from a rubrene/fullerene heterostructure are integrated into an efficient organic dual device (see figure). Electroluminescence, with the characteristic rubrene color, has a turn-on voltage less than 1 V. 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. The half-bandgap electroluminescence turn-on voltage is interpreted as an Auger fountain effect.

Rubrene/Fullerene Heterostructures with a Half‐Gap Electroluminescence Threshold and Large Photovoltage

A new, strongly absorbing narrow optical gap small molecule acceptor YF25 is reported. When YF25 is blended with the archetypal donor P3HT, significant photocurrent red of the polymer optical gap is produced in addition to the normal response generated by the donor polymer. Using EQE and TRMC measurements we show that the additional photocurrent red of the donor edge is derived from excitation of the electron acceptor and subsequent hole-transfer to P3HT.

A narrow optical gap small molecule acceptor for organic 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.

/pdf/efficient-flexible-and-thermally-stable-pentacene-c60-small-3pojixju75.pdf

Efficient flexible and thermally stable pentacene/C60 small molecule based organic solar cells

We report fabrication of a TiO2 interconnected network structure for photovoltaic applications, which was obtained using polystyrene-block-polyethylene oxide diblock copolymer as the templating agent. The synthetic method is simple and highly reproducible. The pore size of the structure is controlled by the amount of Ti precursor provided. The heterojunction solar cells consisting of a TiO2 porous network structure and poly (2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) showed improved performance with a short circuit current of 3.25mA∕cm2 under AM 1.5 solar illumination. The achieved maximum external quantum efficiency for optimum MEH-PPV thickness was 34%.

/pdf/titania-bicontinuous-network-structures-for-solar-cell-3fokqehqnv.pdf

Ajay K. Pandey

Papers

Organic Photodiodes: The Future of Full Color Detection and Image Sensing

Rubrene/Fullerene Heterostructures with a Half‐Gap Electroluminescence Threshold and Large Photovoltage

A narrow optical gap small molecule acceptor for organic solar cells

Efficient flexible and thermally stable pentacene/C60 small molecule based organic solar cells

Titania bicontinuous network structures for solar cell applications