and renewable or earth abundant materials. [ 1,2 ] For photovoltaic panels, the energy recovery time is the length of operation of a panel required to generate the energy used to manufacture the panel. In thin fi lm inorganic panels, this time has dropped to about one year. [ 3 ] One of the revolutionary appeals of roll-to-roll manufacturing of organic photovoltaics (OPV) is the potential to achieve energy recovery times as low as 10 d. [ 4 ] Tremendous advances have recently been made in OPV effi ciencies, [ 5 ] all based on optimization of the bulk-heterojunction (BHJ) motif. [ 6,7 ] The performance of a BHJ requires the optimization of both its molecular-scale order and its nanometer-scale domain structure. [ 8 ] In general, the optimized structure is not at equilibrium. Many techniques have been employed for BHJ fi lm optimization, [ 9 ] including solvent choice and post deposition thermal [ 10 ] or vapor annealing. [ 11 ] Recently, formulations using low volatility liquid additives to achieve higher effi ciencies without thermal treatment have become popular.  Although additives such as 1,8-octanedithiol (ODT) and 1-chloronaphthalene (CN) have almost ubiquitous benefi cial effects, there is no consensus on either the origin or mechanism of their effi cacy. In some systems, additives cause smaller domains, [ 15,16 ] while in others they cause larger domains. [ 12,17 ] The effi cacy of ODT and diiodooctane has been attributed to selective solvation of the fullerene, enabling improved polymer order. CN, however, is typically a good solvent for both polymer and fullerene and thus must act via a different mechanism.We recently employed real-time optical techniques to study the mechanism by which the additives ODT and CN infl uence the solidifi cation of the poly(3-hexylthiophene):phenyl-C61butyric-acid-methyl-ester (P3HT:PCBM) BHJ. [ 18 ] Both additives promoted a greater than 5× improvement in device effi ciency over the additive-free fi lm in the absence of thermal annealing. Post-deposition characterization with ultraviolet-visible absorption spectroscopy (UV-Vis) and grazing incidence X-ray diffraction (GIXD) established that the additives increased the polymer's local order and crystallinity. Additionally, energy-fi ltered transmission electron microscopy (EF-TEM) revealed that the The most successful active fi lm morphology in organic photovoltaics is the bulk heterojunction (BHJ). The performance of a BHJ arises from a complex interplay of the spatial organization of the segregated donor and acceptor phases and the local order/quality of the respective phases. These critical morphological features develop dynamically during fi lm formation, and it has become common practice to control them by the introduction of processing additives. Here, in situ grazing incidence X-ray diffraction (GIXD) and grazing incidence small angle X-ray scattering (GISAXS) studies of the development of order in BHJ fi lms formed from the donor polymer poly(3-hexylthiophene) and acceptor phenyl-C61-butyric acid methyl e...
A consensus is emerging that mixed phases are present in bulk heterojunction organic photovoltaic (OPV) devices. Significant insights into the mixed phases have come from bilayer stability measurements, in which an initial sample consisting of material pure layers of donor and acceptor is thermally treated, resulting in swelling of one layer by the other. We present a comparative study of the stability of polymer/fullerene bilayers using two common OPV polymer donors poly(3-hexylthiophene), P3HT, and poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, and four fullerene acceptors phenyl-C61-butyric acid methyl ester, phenyl-C71-butyric acid methyl ester, PCBM bis-adduct, and indene C60 bis-adduct. Using in situ spectroscopic ellipsometry to characterize the quasi-steady state behavior of the films, we find that the polymer glass transition temperature (T g) is critical to the bilayer stability, with no significant changes occurring below T g of the high T g PCDTBT. Above the polymer T g, we find the behavior is irreversible and most consistent with swelling of the polymer by the fullerene, constrained by tie chains in the polymer network and influenced by the rubbery dynamics of the mixed region. The swelling varies significantly with the nature of the fullerene and the polymer. Across the eight systems studied, there is no clear relationship between swelling and OPV device performance. The relationship between the observed swelling and the underlying fullerene–polymer miscibility is explored via Flory–Rehner theory.
A series of well-defined poly(3-hexylthiophene)s (P3HT) of different molecular weight (MW) and high regioregularity was investigated for charge transport properties in ascast and melt-crystallized films. The semicrystalline structure of the P3HT was characterized by X-ray scattering and Atomic force microscopy. Crystallization by cooling from the melt led to a substantial increase in crystallinity and a stronger alignment of the crystals in comparison to as-cast films. The increase in crystallinity went along with an increase in hole mobility of up to an order of magnitude as measured by the space charge limited current method. Additionally, the hole mobility depended on the long period of P3HT lamellae and consequently on the MW. In compliance with the long period, the charge carrier mobility first increased with the MW before decreasing again at the onset of chain folding. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 943-951
While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 10 16 cm 23 , resulting in space-charge regions ,100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent.T he record power conversion efficiency (PCE) achieved by polymer:fullerene solar cells has increased considerably in the past 4 years to a record published value of 9.2% 1 for a single bulk heterojunction and efficiencies of 10.6% for tandem solar cells 2 . This is despite the fact that organic semiconductors are known to be both structurally and electronically disordered, have lower dielectric constants inhibiting separation of the photogenerated excitonic species and have charge carrier mobilities orders of magnitude lower than inorganic semiconductors.Whilst charge mobilities are low in organic semiconductors and collection losses have been shown to limit the fill factor (FF) 3-5 and short circuit current density (J SC ) 6-10 of certain devices, low mobilities do not necessarily prevent devices from performing efficiently. However the lower charge mobilities and diffusion coefficients in organic semiconductors do mean that diffusion alone is insufficient for charge carrier collection and drift must account for a large proportion of the generated photocurrent. Additionally, polymer:fullerene solar cells are not intentionally doped like their inorganic counterparts or like many small molecule solar cells 11 and therefore rely on selective contacts and the difference in work function between electrodes for efficient charge collection. However, several studies have found evidence for unintentional doping  and discussed the consequences for device behaviour 6, . Whilst the origin of this doping is unclear 15 , its effects on photovoltaic performance can be substantial; however many recent analyses of device performance neglect doping 8, despite the fact that the influence of doping and the electric field on charge carrier collection is well known for a long time 34 and wellstudied for instance in the field of quantum dot photovoltaics 35,36 .In this paper, we address the...
Recent demonstration of mobilities in excess of 10 cm2 V–1 s–1 have energized research in solution deposition of polymers for thin film transistor applications. Due to the lamella motif of most soluble, semiconducting polymers, the local mobility is intrinsically anisotropic. Therefore, fabrication of aligned films is of interest for optimization of device performance. Many techniques have been developed to control film alignment, including solution deposition via directed flows and deposition on topologically structured substrates. We report device and detailed structural analysis (ultraviolet–visible absorption, IR absorption, near-edge X-ray absorption (NEXAFS), grazing incidence X-ray diffraction, and atomic force microscopy) results from blade coating two high performing semiconducting polymers on unpatterned and nanostructured substrates. Blade coating exhibits two distinct operational regimes: the Landau–Levich or horizontal dip coating regime and the evaporative regime. We find that in the evaporative deposition regime, aligned films are produced on unpatterned substrates with the polymer chain director perpendicular to the coating direction. Both NEXAFS and device measurements indicate the coating induced orientation is nucleated at the air interface. Nanostructured substrates produce anisotropic bottom contact devices with the polymer chain at the buried interface oriented along the direction of the substrate grooves, independent of coating regime and coating direction. Real time studies of film drying establish that alignment occurs at extremely high polymer volume-fraction conditions, suggesting mediation via a lyotropic phase. In all cases the final films appear to exhibit high degrees of crystalline order. The independent control of alignment at the air and substrate interfaces via coating conditions and substrate treatment, respectively, enable detailed assessment of structure–function relationships that suggest the improved performance of the nanostructure aligned films arise from alignment of the less ordered material in the crystallite interphase regions.
Comprehensive studies of the influence of the formulation additive 1,8-diiodooctane on the structural evolution of small molecule BHJ films with GIWAXS and GISAXS are presented.
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