Control of blend morphology at the microscopic scale is critical for optimizing the power conversion efficiency of plastic solar cells based on blends of conjugated polymer with fullerene ...derivatives. In the case of bulk heterojunctions of regioregular poly(3-hexylthiophene) (P3HT) and a soluble fullerene derivative (6,6-phenyl C61-butyric acid methyl ester, PCBM), both blend morphology and photovoltaic device performance are influenced by various treatments, including choice of solvent, rate of drying, thermal annealing and vapour annealing. Although the protocols differ significantly, the maximum power conversion efficiency values reported for the various techniques are comparable (4-5%). In this paper, we demonstrate that these techniques all lead to a common arrangement of the components, which consists of a vertically and laterally phase-separated blend of crystalline P3HT and PCBM. We propose a morphology evolution that consists of an initial crystallization of P3HT chains, followed by diffusion of PCBM molecules to nucleation sites, at which aggregates of PCBM then grow.
Free‐standing serum‐albumin mats can transport protons over millimetre length‐scales. The results of photoinduced proton transfer and voltage‐driven proton‐conductivity measurements, together with ...temperature‐dependent and isotope‐effect studies, suggest that oxo‐amino‐acids of the protein serum albumin play a major role in the translocation of protons via an “over‐the‐barrier” hopping mechanism. The use of proton‐conducting protein mats opens new possibilities for bioelectronic interfaces.
The use of copper thiocyanate (CuSCN) as a universal solution‐processable and highly transparent hole‐transporting layer in organic bulk‐heterojunction photovoltaic cells is demonstrated. When CuSCN ...is employed as a replacement for the commonly used poly(3,4‐ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), organic solar cells with maximum power conversion efficiency of 8%, are realized; this value is 1.27 times higher than that for optimized control cells based on PEDOT:PSS.
We study the appearance and energy of the charge transfer (CT) state using measurements of electroluminescence (EL) and photoluminescence (PL) in blend films of high-performance polymers with ...fullerene acceptors. EL spectroscopy provides a direct probe of the energy of the interfacial states without the need to rely on the LUMO and HOMO energies as estimated in pristine materials. For each polymer, we use different fullerenes with varying LUMO levels as electron acceptors, in order to vary the energy of the CT state relative to the blend with 6,6-phenyl C61-butyric acid methyl ester (PCBM). As the energy of the CT state emission approaches the absorption onset of the blend component with the smaller optical bandgap, E opt,min ≡ min{E opt,donor; E opt,acceptor}, we observe a transition in the EL spectrum from CT emission to singlet emission from the component with the smaller bandgap. The appearance of component singlet emission coincides with reduced photocurrent and fill factor. We conclude that the open circuit voltage V OC is limited by the smaller bandgap of the two blend components. From the losses of the studied materials, we derive an empirical limit for the open circuit voltage: V OC ≲ E opt,min/e – (0.66 ± 0.08)eV.
We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt % ...1-(3-methoxycarbonyl)propyl-1-phenyl-6,6C61 (PCBM). Three polymers are observed to give amorphous films, attributed to a nonplanar geometry of their backbone while the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (∼100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt % PCBM resulted in 70−90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer+ and PCBM- polarons, assayed by the appearance of a long-lived, power-law decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over 2 orders of magnitude depending upon the polymer employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned, for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find an excellent correlation between the free energy difference for charge separation (ΔG CS rel) and yield of the long-lived charge generation, with efficient charge generation requiring a much larger ΔG CS rel than that required to achieve efficient PL quenching. We suggest that this observation is consistent with a model where the excess thermal energy of the initially formed polaron pairs is necessary to overcome their Coulombic binding energy. This observation has important implications for synthetic strategies to optimize organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend films, a large ΔG CS rel (or LUMO level offset) is required to achieve efficient charge dissociation.
An effective method for reducing interelectrode shorting in silver nanowire (AgNW) based organic solar cells is reported. The method is applied to standard and inverted devices based on ...poly(3‐hexylthiophene) and 6,6‐phenyl‐C61‐butyric acid methyl ester. The best results are obtained using an inverted architecture with a 200 nm buffer layer of nanostructured titania (TiOx) on top of the AgNWs, yielding power conversion efficiencies of up to 3.5%.
Low-cost photovoltaic energy conversion using conjugated molecular materials has become increasingly feasible through the development of organic 'bulk heterojunction (BHJ)' structures, where ...efficient light-induced charge separation is enabled by a large-area donor-acceptor interface. The highest efficiencies have been achieved using blends of poly(3-hexylthiophene) (P3HT) and a fullerene derivative, but performance depends critically on the material properties and processing conditions. This variability is believed to be influenced by the self-organizing properties of P3HT, which means that both optical and electronic properties are sensitive to the molecular packing. However, the relationship between molecular nanostructure, optoelectronic properties of the blend material and device performance has not yet been demonstrated. Here we focus on the influence of polymer regioregularity (RR) on the molecular nanostructure, and hence on the resulting material properties and device performance. We find a strong influence of RR on solar-cell performance, which can be attributed to enhanced optical absorption and transport resulting from the organization of P3HT chains and domains. Further optimization of devices using the highest RR material resulted in a power conversion efficiency of 4.4%, even without optimization of electrodes.
Copper thiocyanate (CuSCN) is introduced as a hole‐injection/hole‐transport layer (HIL/HTL) for solution‐processed organic light‐emitting diodes (OLEDs). The OLED devices reported here with CuSCN as ...HIL/HTL perform significantly better than equivalent devices fabricated with a PEDOT:PSS HIL/HTL, and solution‐processed, phosphorescent, small‐molecule, green OLEDs with maximum luminance ≥10 000 cd m‐2, maximum luminous efficiency ≤50 cd A‐1, and maximum luminous power efficiency ≤55 lm W‐1 are demonstrated.
We present a study of dark air-exposure degradation of organic solar cells based on photoactive blends of the conjugated polymer, poly2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene ...(MDMO-PPV) with 6,6-phenyl C
61-butyric acid methyl ester (PCBM). Photovoltaic devices were fabricated on indium tin oxide (ITO) glass with or without a layer of poly (3,4-ethylenedioxythiophene):poly(4-styrene sulfonate) (PEDOT:PSS), and were studied without encapsulation. Photovoltaic performance characteristics were measured as a function of time for different ambient conditions (under white light irradiation and in the dark, and under air, dry oxygen and humid nitrogen atmospheres). It was found that a key cause of degradation under air exposure is light independent and results from water adsorption by the hygroscopic PEDOT:PSS layer. Measurements of the charge mobility and hole injection after air exposure showed that the degradation increases the resistance of the PEDOT:PSS/blend layer interface.
The use of fullerenes with two or more adducts as acceptors has been recently shown to enhance the performance of bulk‐heterojunction solar cells using poly(3‐hexylthiophene) (P3HT) as the donor. The ...enhancement is caused by a substantial increase in the open‐circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open‐circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multiadducts like bis‐PCBM (bis adduct of Phenyl‐C61‐butyric acid methyl ester) or the indene C60 bis‐adduct ICBA. Here we investigate the reasons for this decrease in photocurrent. We find that it can be attributed partly to a loss in charge generation efficiency that may be related to the LUMO‐LUMO and HOMO‐HOMO (highest occupied molecular orbital) offsets at the donor‐acceptor heterojunction, and partly to reduced charge carrier collection efficiencies. We show that the P3HT exhibits efficient collection due to high hole and electron mobilities with mono‐ and multiadduct fullerenes. In contrast the less crystalline polymer Poly9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl (PCDTBT) shows inefficient charge carrier collection, assigned to low hole mobility in the polymer and low electron mobility when blended with multiadduct fullerenes.
This article discusses the reasons for the decreased photovoltaic performance of absorber layers where polymers other than P3HT are blended with fullerene multiadducts like ICBA and bis‐PCBM. It is found that the reduced performance is due to a mixture of reduced charge generation due to the smaller band offsets and reduced electron mobilities.
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