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Bulk heterojunction organic solar cells continue to show steady photoconversion efficiency improvements. However, single component organic solar cells are a particularly attractive alternative due to the relative simplicity of device manufacture. It has been proposed that organic semiconductors with a high dielectric constant (≈10) could give rise to spontaneous free charge carrier generation upon photoexcitation. In this manuscript, factors are explored that affect the dielectric constant of organic semiconductors, particularly the optical-frequency dielectric constant. The properties of monomers, dimers and trimers of two isoelectronic families of materials that have acceptor units composed of one or two dicyanovinylbenzothiadiazole moieties and one to three donor units are compared. The donor components are composed of either fluorenyl or cyclopentadithiophene moieties with the same glycol-based solubilizing groups. It is found that chromophore planarity and orientation with respect to the substrate, and film density affect the optical and electronic properties of the materials, especially the high-frequency dielectric constant. The results also indicate that delocalization of the highest occupied and lowest unoccupied molecular orbitals is a critical factor. The dimer with two dicyanovinylbenzothiadiazole moieties and two dithienocyclopentadiene units is found to have the highest optical frequency dielectric constant and overall performance.

The microstructure of solution-processed organometallic lead halide perovskite thin films prepared by the “gas-assisted” method is investigated with synchrotron-based techniques. Using a combination of GIWAXS and NEXAFS spectroscopy the orientational alignment of CH3NH3PbI3 crystallites and CH3NH3+ cations are separately probed. The GIWAXS results reveal a lack of preferential orientation of CH3NH3PbI3 crystallites in 200–250 nm thick films prepared on both planar TiO2 and mesoporous TiO2. Relatively high efficiencies are observed for device based on such films, with 14.3% achieved for planar devices and 12% for mesoporous devices suggesting that highly oriented crystallites are not crucial for good cell performance. Oriented crystallites however are observed in thinner films (≈60 nm) deposited on planar TiO2 (but not on mesoporous TiO2) indicating that the formation of oriented crystallites is sensitive to the kinetics of solvent evaporation and the underlying TiO2 morphology. NEXAFS measurements on all samples found that CH3NH3+ cations exhibit a random molecular orientation with respect to the substrate. The lack of any NEXAFS dichroism for the thin CH3NH3PbI3 layer deposited on planar TiO2 in particular indicates the absence of any preferential orientation of CH3NH3+ cations within the CH3NH3PbI3 unit cell for as-prepared layers, that is, without any electrical poling.

The influence of the interface of the dielectric SiO2 on the performance of bottom-contact, bottom-gate poly(3-alkylthiophene) (P3AT) field-effect transistors (FETs) is investigated. In particular, the operation of transistors where the active polythiophene layer is directly spin-coated from chlorobenzene (CB) onto the bare SiO2 dielectric is compared to those where the active layer is first spin-coated then laminated via a wet transfer process such that the film/air interface of this film contacts the SiO2 surface. While an apparent alkyl side-chain length dependent mobility is observed for films directly spin-coated onto the SiO2 dielectric (with mobilities of ≈10−3 cm2 V−1 s−1 or less) for laminated films mobilities of 0.14 ± 0.03 cm2 V−1 s−1 independent of alkyl chain length are recorded. Surface-sensitive near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements indicate a strong out-of-plane orientation of the polymer backbone at the original air/film interface while much lower average tilt angles of the polymer backbone are observed at the SiO2/film interface. A comparison with NEXAFS on crystalline P3AT nanofibers, as well as molecular mechanics and electronic structure calculations on ideal P3AT crystals suggest a close to crystalline polymer organization at the P3AT/air interface of films from CB. These results emphasize the negative influence of wrongly oriented polymer on charge carrier mobility and highlight the potential of the polymer/air interface in achieving excellent “out-of-plane” orientation and high FET mobilities.

We utilize transient techniques to directly compare the operation of polymer/fullerene, polymer/nanocrystal, and polymer/polymer bulk heterojunction solar cells. For all devices, poly(3-hexylthiophene) (P3HT) is used as the electron donating polymer, in combination with either the fullerene derivative phenyl-C(61)-butyric acid methyl ester (PCBM) in polymer/fullerene cells, CdSe nanoparticles in polymer/nanocrystal cells, or the polyfluorene copolymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3- benzothiadiazole]-2,2-diyl) (F8TBT) in polymer/polymer cells. Transient photocurrent and photovoltage measurements are used to probe the dynamics of charge-separated carriers, with vastly different dynamic behavior observed for polymer/fullerene, polymer/polymer, and polymer/nanocrystal devices on the microsecond to millisecond timescale. Furthermore, by employing transient photocurrent analysis with different applied voltages we are also able to probe the dynamics behavior of these cells from short circuit to open circuit. P3HT/F8TBT and P3HT/CdSe devices are characterized by poor charge extraction of the long-lived carriers attributed to charge trapping. P3HT/PCBM devices, in contrast, show relatively trap-free operation with the variation in the photocurrent decay kinetics with applied bias at low intensity, consistent with the drift of free charges under a uniform electric field. Under solar conditions at the maximum power point, we see direct evidence of bimolecular recombination in the P3HT/PCBM device competing with charge extraction. Transient photovoltage measurements reveal that, at open circuit, photogenerated charges have similar lifetimes in all device types, and hence, the extraction of these long-lived charges is a limiting process in polymer/nanocrystal and polymer/polymer devices.

We utilize scanning transmission X-ray microscopy (STXM) to study the domain structure of polycrystalline films of the semiconducting polymer poly(9,9’-dioctylfluorene-co-benzothiadiazole) (F8BT). By taking several images at different orientations of the film with respect to the polarization of the X-ray beam, we are able to compute quantitative maps of molecular alignment/order and molecular orientation, including both the backbone direction and phenyl ring plane orientation, as well as the in-plane and out-of-plane components. We show that polycrystalline F8BT films consist of well-ordered micron-sized domains with the transition from one domain orientation to another characterized either by a smooth transition of orientation or by ∼ 200 nm wide disordered domain boundaries. The morphology of the disordered domain boundaries resemble the electroluminescence patterns observed previously in F8BT light-emitting field-effect transistors suggesting that charge trapping at these disordered domain boundaries facilitates charge recombination in ambipolar operation. A relatively narrow distribution of local average tilt angles is observed that correlates with film structure, with the ordered domains in general showing a higher tilt angle than the disordered domain boundaries. We also use secondary electron detection to image the surface domain structure of polycrystalline F8BT films and demonstrate that the polycrystalline structure extends to the film/air interface. Finally, we calculate ideal NEXAFS spectra corresponding to a perfect F8BT crystal oriented with the 1s – π* transition dipole moment parallel and perpendicular to the electric field vector of a perfectly linearly polarized X-ray beam.

The operation of resistive switches based on phase-separated blends of organic ferroelectrics and semiconductors depends significantly on the microstructure of such systems. A wide range of analysis techniques are used to characterize spin-coated films of the ferroelectric random copolymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], and the semiconducting polymer, regio-irregular poly(3-hexylthiophene) (rir-P3HT). The blend separates into amorphous rir-P3HT domains embedded in a crystalline P(VDF-TrFE) matrix. The rir-P3HT domains are continuous throughout the film, from the substrate/blend interface to the blend/air interface. We also investigate the rir-P3HT domain size and number as a function of composition and find - unexpectedly - a rather mono-disperse domain size distribution for a given rir-P3HT:P(VDF-TrFE) ratio. The domain size increases with rir-P3HT content, indicating that the solidification is not dominated by nucleation processes. Spinodal decomposition is therefore more likely to be responsible for the microstructure induced in the rir-P3HT:P(VDF-TrFE) blends. Since spinodal decomposition occurs spontaneously without the presence of a nucleation step, this can facilitate processing considerably, since the intricate control of nucleation processes (homogenous or heterogenous) is rendered unnecessary. Measurement of the lateral conductivity of the blends demonstrates that the rir-P3HT domains are electrically not connected, supporting the microstructural evidence. A perpendicular current through the film is measured using both Au and Ag electrodes as a function of blend composition. A model was used to interpret the electrical transport. The injection for Ag diodes poled into the ON-state preferentially occurs at the circumference of the rir-P3HT domains. An accumulation width over which the injection occurs is estimated to be of the order of a few hundred nm. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Through controlled annealing of planar heterojunction (bilayer) devices based on the polyfluorene copolymers poly(9,9-dioctylfluorene-co-bis(N,N′-(4,butylphenyl))bis(N,N′-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) we study the influence of interface roughness on the generation and separation of electron–hole pairs at the donor/acceptor interface. Interface structure is independently characterized by resonant soft X-ray reflectivity with the interfacial width of the PFB/F8BT heterojunction observed to systematically increase with annealing temperature from 1.6 nm for unannealed films to 16 nm with annealing at 200 °C for ten minutes. Photoluminescence quenching measurements confirm the increase in interface area by the three-fold increase in the number of excitons dissociated. Under short-circuit conditions, however, unannealed devices with the sharpest interface are found to give the best device performance, despite the increase in interfacial area (and hence the number of excitons dissociated) in annealed devices. The decrease in device efficiency with annealing is attributed to decreased interfacial charge separation efficiency, partly due to a decrease in the bulk mobility of the constituent materials upon annealing but also (and significantly) due to the increased interface roughness. We present results of Monte Carlo simulations that demonstrate that increased interface roughness leads to lower charge separation efficiency, and are able to reproduce the experimental current-voltage curves taking both increased interfacial roughness and decreased carrier mobility into account. Our results show that organic photovoltaic performance can be sensitive to interfacial order, and heterojunction sharpness should be considered a requirement for high performance devices.

Molecular weight is an important factor determining the morphology and performance of all-polymer solar cells. Through the application of direct arylation polycondention, a series of batches of a fluorinated naphthalene diimide-based acceptor polymer are prepared with molecular weight varying from M-n = 20 to 167 kDa. Used in conjunction with a common low bandgap donor polymer, the effect of acceptor molecular weight on solar cell performance, morphology, charge generation, and transport is explored. Increasing the molecular weight of the acceptor from M-n = 20 to 87 kDa is found to increase cell efficiency from 2.3% to 5.4% due to improved charge separation and transport. Further increasing the molecular weight to M-n = 167 kDa however is found to produce a drop in performance to 3% due to liquid-liquid phase separation which produces coarse domains, poor charge generation, and collection. In addition to device studies, a systematic investigation of the microstructure and photophysics of this system is presented using a combination of transmission electron microscopy, grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, photoluminescence quenching, and transient absorption spectroscopy to provide a comprehensive understanding of the interplay between morphology, photophysics, and photovoltaic performance.

Here the influence of annealing on the operational efficiency of all-polymer solar cells based on blends of the polymers poly(3-hexylthiophene) (P3HT) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) is investigated. Annealing of completed devices is found to result in an increase in power conversion efficiency from 0.14 to 1.20%, while annealing of films prior to top electrode deposition increases device efficiency to only 0.19% due to a lowering of the open-circuit voltage and short-circuit current. By studying the dependence of photocurrent on intensity and effective applied bias, annealing is found to increase charge generation efficiency through an increase in the efficiency of the separation of bound electron-hole pairs following charge transfer. However, unlike many other all-polymer blends, this increase in charge separation efficiency is not only due to an increase in the degree of phase separation that assists in the spatial separation of electron-hole pairs, but also due to an order of magnitude increase in the hole mobility of the P3HT phase. The increase in hole mobility with annealing is attributed to the ordering of P3HT chains evidenced by the red-shifting of P3HT optical absorption in the blend. We also use X-ray photoelectron spectroscopy (XPS) to study the influence of annealing protocol on film interface composition. Surprisingly both top and bottom electrode/blend interfaces are enriched with P3HT, with the blend/top electrode interface consisting of more than 95% P3HT for as-spun films and films annealed without a top electrode. Films annealed following top electrode deposition, however, show an increase in F8TBT composition to ∼15%. The implications of interfacial composition and the origin of open-circuit voltage in these devices are also discussed.

The charge transport and microstructural properties of five different molecular weight (MW) batches of the naphthalenediimide-thiophene copolymer P(NDI2OD-T2) are investigated. In particular, the field-effect transistor (FET) performance and thin-film microstructure of samples with MW varying from Mn = 10 to 41 kDa are studied. Unlike conventional semiconducting polymers such as poly(3-hexylthiophene) where FET mobility dramatically drops with decreasing molecular weight, the FET mobility of P(NDI2OD-T2)-based transistors processed from 1,2-dichlorobenzene is found to increase with decreasing MW. Using a combination of grazing-incidence wide-angle X-ray scattering, near-edge X-ray absorption fine-structure spectroscopy, atomic force microscopy, and resonant soft X-ray scattering, the increase in FET mobility with decreasing MW is attributed to the pronounced increase in the orientational correlation length (OCL) with decreasing MW. In particular, the OCL is observed to systematically increase from

In situ grazing incidence wide-angle X-ray scattering (GIWAXS) is used to study the in situ thermal behavior of solution-processed, high mobility core-expanded naphthalene diimide thin films. A series of three different molecules is studied where the side-chain branching position is systematically varied through the use of 2-, 3- and 4-branched N-alkyl chains. For all molecules, a number of different phases and their associated phase transitions are observed with heating up to 200 °C. In situ GIWAXS measurements allow following significant variations of packing in each phase including crystalline coherence length, orientation, d-spacing, and paracrystallinity, as well as, for the first time, thin film thermal expansion coefficients in both the in-plane and out-of-plane direction. Relating these parameters with device measurements of quenched films, a striking correlation is found between high field-effect mobilities and low in-plane thermal expansion coefficients. This relationship indicates that high in-plane thermal expansion coefficients are detrimental to in-plane charge transport due to the formation of nanoscale defects in the critical first few monolayers upon quenching.