V.A. Trukhanov

Effect of doping on performance of organic solar cells

Conventional models of planar and bulk heterojunction organic solar cells have been extended by introducing doping in the active layer. We have studied the performance of organic solar cells as a function of dopant concentration. For bulk heterojunction cells, the modeling shows that for the most studied material pair (poly-3-hexylthiophene, P3HT, and phenyl-C61-butyric acid methyl ester, PCBM) doping decreases the short-circuit current density (JSC), fill factor (FF) and efficiency. However, if bulk heterojunction cells are not optimized, namely, at low charge carrier mobilities, unbalanced mobilities or non-ohmic contacts, the efficiency can be increased by doping. For planar heterojunction cells, the modeling shows that if the acceptor layer is n doped, and the donor layer is p doped, the open-circuit voltage, JSC, FF and hence the efficiency can be increased by doping. Inversely, when the acceptor is p doped, and the donor is n doped; FF decreases rapidly with increasing dopant concentrations so that the current-voltage curve becomes S shaped. We also show that the detrimental effect of nonohmic contacts on the performance of the planar heterojunction cell can be strongly weakened by doping.

Fill factor in organic solar cells can exceed the Shockley-Queisser limit

The ultimate efficiency of organic solar cells (OSC) is under active debate. The solar cell efficiency is calculated from the current-voltage characteristic as a product of the open-circuit voltage (VOC), short-circuit current (JSC), and the fill factor (FF). While the factors limiting VOC and JSC for OSC were extensively studied, the ultimate FF for OSC is scarcely explored. Using numerical drift-diffusion modeling, we have found that the FF in OSC can exceed the Shockley-Queisser limit (SQL) established for inorganic p–n junction solar cells. Comparing charge generation and recombination in organic donor-acceptor bilayer heterojunction and inorganic p–n junction, we show that such distinctive properties of OSC as interface charge generation and heterojunction facilitate high FF, but the necessary condition for FF exceeding the SQL in OSC is field-dependence of charge recombination at the donor-acceptor interface. These findings can serve as a guideline for further improvement of OSC.

Novel indolin-2-one-substituted methanofullerenes bearing long n-alkyl chains: synthesis and application in bulk-heterojunction solar cells

Summary An easy, high-yield and atom-economic procedure of a C60 fullerene modification using a reaction of fullerene C60 with N-alkylisatins in the presence of tris(diethylamino)phosphine to form novel long-chain alkylindolinone-substituted methanofullerenes (AIMs) is described. Optical absorption, electrochemical properties and solubility of AIMs were studied. Poly(3-hexylthiophene-2,5-diyl) (P3HT)/AIMs solar cells were fabricated and the effect of the AIM alkyl chain length and the P3HT:AIM ratio on the solar cell performance was studied. The power conversion efficiencies of about 2% were measured in the P3HT/AIM devices with 1:0.4 P3HT:AIM weight ratio for the AIMs with hexadecyl and dodecyl substituents. From the optical and AFM data, we suggested that the AIMs, in contrast to [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), do not disturb the P3HT crystalline domains. Moreover, the more soluble AIMs do not show a better miscibility with the P3HT crystalline phase.

Non-fullerene acceptors for organic solar cells

Solar cells based on organic semiconductor molecules are a promising alternative to conventional silicon photocells owing to their low cost, simple production, and good mechanical properties. Effective organic photocells are based on a heterojunction using an active layer consisting of two different organic semiconductors, one of which is an electron donor, while the other is an acceptor. Progress in organic photovoltaics is related to the development of new donor materials, while fullerene derivatives are commonly used as acceptors. The advantages and disadvantages of fullerene compounds for organic solar cells are discussed in this review, the principles of their operation are briefly considered, and the most successful new non-fullerene acceptors are described. The application of latter acceptors has made it possible to fabricate organic solar cells with an efficiency of about 2–4%.

Long-Range Exciton Transport in Brightly Fluorescent Furan/Phenylene Co-oligomer Crystals

The design of light-emitting crystalline organic semiconductors for optoelectronic applications requires a thorough understanding of the singlet exciton transport process. In this study, we show that the singlet exciton diffusion length in a promising semiconductor crystal based on furan/phenylene co-oligomers is 24 nm. To achieve this, we employed the photoluminescence quenching technique using a specially synthesized quencher, which is a long furan/phenylene co-oligomer that was facilely implanted into the host crystal lattice. Extensive Monte-Carlo simulations, exciton–exciton annihilation experiments and numerical modelling fully supported our findings. We further demonstrated the high potential of the furan/phenylene co-oligomer crystals for light-emitting applications by fabricating solution-processed organic light emitting transistors.

Monolayer organic field effect phototransistors: photophysical characterization and modeling

Оrganic field-effect transistors (OFET) can combine photodetection and light amplification and, for example, work as phototransistors. Such organic phototransistors can be used in light-controlled switches and amplifiers, detection circuits, and sensors of ultrasensitive images. In this work, we present photophysical characterization of well-defined ultrathin organic field-effect devices with a semiconductive channel based on Langmuir-Blodgett monolayer film. We observe clear generation of photocurrent under illumination with a modulated laser at 405 nm. The increase of photocurrent with the optical modulation frequency indicates the presence of defect states serving as traps for photogenerated carriers and/or the saturation of charge concentration in the thin active layer. We also propose a simple one-dimensional numerical model of a photosensitive OFET. The model is based on the Poisson, current continuity and drift-diffusion equations allows future evaluation of the photocurrent generation mechanism in the studied systems.