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Blends of semiconducting (SC) and ferroelectric (FE) polymers have been proposed for applications in resistive memories and organic photovoltaics (OPV). For OPV, the rationale is that the local electric field associated with the dipoles in a blend could aid exciton dissociation, thus improving power conversion efficiency. However, FE polymers either require solvents or processing steps that are incompatible with those required for SC polymers. To overcome this limitation, SC (poly(3-hexylthiophene)) and FE (poly(vinylidene fluoride-trifluoroethylene)) components are incorporated into a block copolymer and thus a path to a facile fabrication of smooth thin films from suitably chosen solvents is achieved. In this work, the photophysical properties and device performance of organic solar cells containing the aforementioned block copolymer consisting of poly(vinylidene fluoride-trifluoroethylene): P(VDF-TrFE), poly(3-hexylthiophene): P3HT and the electron acceptor phenyl-C 61 -butyric acid methyl ester: [60]PCBM are explored. A decrease in photovoltaic performance is observed in blends of the copolymer with P3HT:[60]PCBM, which is attributed to a less favorable nanomorphology upon addition of the copolymer. The role of lithium fluoride (the cathode modification layer) is also clarified in devices containing the copolymer, and it is demonstrated that ferroelectric compensation prevents the ferroelectricity of the copolymer from improving photovoltaic performance in SC-FE blends., (© 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The morphology of the active layer in polymer:fullerene solar cells is a key parameter for the performance. We compare bilayer poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butyric acid methyl ester (P3HT/PCBM) solar cell devices produced from orthogonal solvents before and after thermal annealing with P3HT:PCBM bulk heterojunction solar cells produced from a single solvent. By comparing the spectral shape and magnitude of the experimental and theoretically modeled EQEs we show that P3HT/PCBM bilayers made via orthogonal solution processing do not lead to bilayers with a sharp interface but that partial intermixing has occurred. Thermal annealing of these diffusive P3HT/PCBM bilayers leads to increased mixing but does not result in the same mixed bulk heterojunction morphology that is obtained when P3HT and PCBM are cast simultaneously from single solution. For thicker layers, the annealed bilayers significantly outperform the bulk heterojunction devices with the same nominal composition and same total thickness.

The power conversion efficiency of solar cells based on a conjugated polymer (donor) and a fullerene derivative (acceptor) is very sensitive to the morphology of the active layer. One detrimental feature, which is often encountered in non-optimal morphologies, is the occurrence of fullerene blobs in a finely mixed matrix containing both donor and acceptor material. Here, the effects of such fullerene blobs are studied in detail with a three-dimensional drift-diffusion model. It includes the effects of exciton diffusion and quenching; space-charge; recombination, generation, drift and diffusion of charge carriers; and the injection/extraction of carriers at the contacts. The influence of blob size and shape, and matrix composition are quantified. The latter has the strongest effect on the overall efficiency, as most of the current is transported through the mixed phase. The total current flowing out of the solar cell can be split up in a part which comes from the interfacial region between the acceptor phase and the mixed phase, and a part that stems from the mixed phase itself. Depending on the bias voltage and the morphology, one or the other contribution is dominant. Finally, it is shown how both contributions can be computed with a simple one-dimensional drift-diffusion simulator. [ABSTRACT FROM AUTHOR]

Three different theoretical approaches are presented to identify pathways to organic solar cells with power conversion efficiencies in excess of 20%. A radiation limit for organic solar cells is introduced that elucidates the role of charge-transfer (CT) state absorption. Provided this CT action is sufficiently weak, organic solar cells can be as efficient as their inorganic counterparts. Next, a model based on Marcus theory of electronic transfer that also considers exciton generation in both the electron donor and electron acceptor is used to show how reduction of the reorganization energies can lead to substantial efficiency gains. Finally, the dielectric constant is introduced as a central parameter for efficient solar cells. By using a drift-diffusion model, it is found that efficiencies of more than 20% are within reach. [ABSTRACT FROM AUTHOR]

The 'phonon-glass electron-crystal' concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly 'phonon glasses'. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being 'electron crystals'. Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic 'PGEC' thermoelectric material. This thermoelectric material exhibits an excellent electrical conductivity of >10 S cm−1 and an ultralow thermal conductivity of <0.1 Wm−1K−1, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use 'arm-shaped' double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of organic semiconductors as thermoelectric materials. Achieved high thermoelectric figure of merit (ZT) in organic thermoelectric materials remains a challenge due to their low packing order and poor host/dopant miscibility. Here, the authors report side chain-engineered n-doped fullerene derivatives with record ZT >0.3 for organic thermoelectrics. [ABSTRACT FROM AUTHOR]

The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell. Identifying materials that are able to transport excitons over longer distances can help advancing our understanding and lead to solar cells with higher efficiency. Here, we measure the exciton diffusion length in a wide range of nonfullerene acceptor molecules using two different experimental techniques based on photocurrent and ultrafast spectroscopy measurements. The acceptors exhibit balanced ambipolar charge transport and surprisingly long exciton diffusion lengths in the range of 20 to 47 nm. With the aid of quantum-chemical calculations, we are able to rationalize the exciton dynamics and draw basic chemical design rules, particularly on the importance of the end-group substituent on the crystal packing of nonfullerene acceptors. The short-range diffusion length of organic semiconductors severely limits exciton harvesting and charge generation in organic bulk heterojunction solar cells. Here, the authors report exciton diffusion length in the range of 20 to 47 nm for a wide range of non-fullerene acceptors molecules. [ABSTRACT FROM AUTHOR]

We study the thickness dependence of the performance of bulk heterojunction solar cells based on poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] as electron donor and [6,6]-phenyl C61 butyric acid methyl ester as electron acceptor. Typically, these devices have an active layer thickness of 100 nm at which only 60% of the incoming light is absorbed. Increasing device thickness results in a lower overall power conversion efficiency, mainly due to a lowering of the fill factor. We demonstrate that the decrease in fill factor and hence device efficiency is due to a combination of charge recombination and space-charge effects. [ABSTRACT FROM AUTHOR]

Bimolecular recombination in organic semiconductors is known to follow the Langevin expression, i.e., the rate of recombination depends on the sum of the mobilities of both carriers. We show that this does not hold for polymer/fullerene bulk heterojunction solar cells. The voltage dependence of the photocurrent reveals that the recombination rate in these blends is determined by the slowest charge carrier only, as a consequence of the confinement of both types of carriers to two different phases. [ABSTRACT FROM AUTHOR]

A typical feature of polymer/fullerene based solar cells is that the current density under short-circuit conditions (Jsc) does not scale exactly linearly with light intensity (I). Instead, a power law relationship is found given by Jsc∝Iα, where α ranges from 0.85 to 1. In a number of reports this deviation from unity is speculated to arise from the occurrence of bimolecular recombination. We demonstrate that the dependence of the photocurrent in bulk heterojunction solar cells is governed by the build-up of space-charge in the device as a consequence of a difference in electron- and hole mobility. We have verified this for an experimental model system in which the mobility difference can be tuned from one to three orders of magnitude by changing the annealing treatment. [ABSTRACT FROM AUTHOR]

The performance of organic bulk heterojunction solar cells is strongly dependent on the donor/acceptor morphology. Morphological parameters, such as the extent and the composition of donor- and acceptor-rich domains, influence both the charge generation and the charge transport throughout the active layer. This work focuses on a polymer:fullerene system based on a small bandgap diketopyrrolopyrrole–quinquethiophene alternating copolymer (PDPP5T) mixed with [6,6]-phenyl-C 71 -butyric acid methyl ester ([70]PCBM) that is capable of efficiencies higher than 6%. By changing the processing conditions, the morphology can be varied from a coarse separated morphology, with fullerene domains (blobs) embedded in a polymer-rich matrix, to a completely mixed layer. The charge carrier transport and the strength of the bimolecular recombination in PDPP5T:[70]PCBM blends with different morphologies and fullerene concentrations are experimentally characterized. The large difference in electron and hole mobility and the electric field dependency of the electron mobility are identified as the causes that limit the performance of devices with low [70]PCBM content. These effects are not present if the concentration of [70]PCBM is increased while keeping a fine phase separation by the addition of ortho -dichlorobenzene as a cosolvent. For the phase-separated blends, a model is proposed, based on a drift–diffusion approach that combines electrical and morphological parameters; with this model the contribution of each phase to the total current is quantified. Under operating conditions, most of the current comes from the interfacial region between the phases, with holes traveling through the matrix and the electrons through the blobs. This device model consistently connects morphological features to overall device performance. [ABSTRACT FROM AUTHOR]