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Organic photovoltaics (OPV) have attracted great interest as a solar cell technology with appealing mechanical, aesthetical, and economies-of-scale features. To drive OPV toward economic viability, low-cost, large-scale module production has to be realized in combination with increased top-quality material availability and minimal batch-to-batch variation. To this extent, continuous flow chemistry can serve as a powerful tool. In this contribution, a flow protocol is optimized for the high performance benzodithiophene-thienopyrroledione copolymer PBDTTPD and the material quality is probed through systematic solar-cell evaluation. A stepwise approach is adopted to turn the batch process into a reproducible and scalable continuous flow procedure. Solar cell devices fabricated using the obtained polymer batches deliver an average power conversion efficiency of 7.2 %. Upon incorporation of an ionic polythiophene-based cathodic interlayer, the photovoltaic performance could be enhanced to a maximum efficiency of 9.1 %., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We examine the significance of hot exciton dissociation in two archetypical polymer-fullerene blend solar cells. Rather than evolving through a bound charge transfer state, hot processes are proposed to convert excitons directly into free charges. But we find that the internal quantum yields of carrier photogeneration are similar for both excitons and direct excitation of charge transfer states. The internal quantum yield, together with the temperature dependence of the current-voltage characteristics, is consistent with negligible impact from hot exciton dissociation.

The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.

Triplet exciton (TE) formation pathways are systematically investigated in prototype bulk heterojunction (BHJ) 'super yellow' poly(p-phenylene vinylene) (SY-PPV) solar cell devices with varying fullerene compositions using complementary optoelectrical and electrically detected magnetic resonance (EDMR) spectroscopies. It is shown that EDMR spectroscopy allows the unambiguous demonstration of fullerene triplet production in BHJ polymer:fullerene solar cells. EDMR triplet detection under selective photoexcitation of each blend component and of the interfacial charge transfer (CT) state reveals that low lying fullerene TEs are produced by direct intersystem crossing from singlet excitons (SEs). The direct CT-TE recombination pathway, although energetically feasible, is kinetically suppressed in these devices. However, high energy CT states in the CT manifold can contribute to the population of the fullerene triplet state via a direct CT-SE conversion. This undesirable energetic alignment could be one of the causes for the severe reduction in photocurrent observed when the open-circuit voltage of polymer:fullerene solar cells is pushed to 1.0 V or beyond. [ABSTRACT FROM AUTHOR]

The influence of the interface of the dielectric SiO2 on the performance of bottom-contact, bottom-gate poly(3-alkylthiophene) (P3 AT) 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 P3 AT nanofibers, as well as molecular mechanics and electronic structure calculations on ideal P3 AT crystals suggest a close to crystalline polymer organization at the P3 AT/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. [ABSTRACT FROM AUTHOR]

ABSTRACT For an increased lifetime of polymer:fullerene bulk heterojunction (BHJ) solar cells, an understanding of the chemical and morphological degradation phenomena taking place under operational conditions is crucial. Phase separation between polymer and fullerene induced by thermal stress has been pointed out as a major issue to overcome. While often the effect of thermal stress on the morphology of polymer:fullerene BHJ is investigated in the darkness, here we observe that light exposure slows down fullerene crystallization and phase separation induced at elevated temperatures. The observed photo-stabilizing effect on active layer morphology is quite independent on the polymer and is attributed to light-induced dimerization of the fullerene. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1209-1214 [ABSTRACT FROM AUTHOR]

Defining appropriate annealing temperatures and times is vitally important for increasing the efficiency of bulk heterojunction solar cells by favoring the crystallinity of the polymer-fullerene blend components. In order to better understand the annealing process, the isothermal crystallization of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C-butyric acid methyl ester (PCBM) blend investigated by means of rapid heating cooling calorimetry (RHC). Isothermal crystallization experiments at temperatures in between the glass transition and melting, within the temperature range of 70-150 °C, can successfully be performed since RHC permits cooling at a sufficiently high rate in order to prevent crystallization during cooling. Crystallization isotherms were determined from the subsequent melting behavior of the blend. They were measured for a wide set of annealing temperatures and times, and the evolution of the crystallization rate with temperature is compared for annealing from the glassy state and from the melt state. [ABSTRACT FROM AUTHOR]

The power conversion efficiency of polymer:fullerene bulk heterojunction solar cells depends on the generated photocurrent and photovoltage. Here we show, using the thermodynamic theory of detailed balance, that the photovoltage in particular is limited by the presence of polymer:fullerene material interaction, resulting in the formation of a weak donor–acceptor charge transfer complex (CTC). Excited CTCs, or charge transfer (CT) states, are visible in highly sensitive measurements of the absorption and photovoltaic action spectrum, or in photoluminescence and electroluminescence measurements. It is shown that photovoltaic and electroluminescent actions of the polymer:fullerene CTC are related by a reciprocity relation. This relation reproduces the measured open-circuit voltage (Voc) of the photovoltaic device under solar conditions. Also, the temperature and illumination intensity dependence of Voc is reproduced by the theory. Assuming perfect conditions for charge generation and recombination, a maximum obtainable Voc value in function of polymer:fullerene CTC properties is derived. [ABSTRACT FROM PUBLISHER]

The effect of poly(3-alkylthiophene) (P3AT) crystallinity in (nanofiber P3AT):PCBM photovoltaic devices on the energy of the charge-transfer state (ECT) and on the open-circuit voltage (Voc) is investigated for poly(3-butythiophene), poly(3-pentylthiophene) and poly(3-hexylhiophene). P3AT crystallinity, expressed as the crystalline nanofiber mass fraction f to the total P3AT mass in the spin-coating dispersion, is varied between ∼0.1 and ∼0.9 by temperature control. ECT, as obtained by Fourier-transform photocurrent spectroscopy decreased with f as ECT=ECT0-0.2f eV. Alkyl side-chain length only influences ECT0. Voc relates to ECT as Voc=ECT/q-0.6 V. [ABSTRACT FROM AUTHOR]

Two push-pull type conjugated polymers - PIDTT−BTz and PIDTT−DTBTz , based on the ladder-type donor unit indacenodithieno[3,2- b ]thiophene (IDTT) and bithiazole (BTz) as acceptor component - are designed and synthesized for photovoltaic applications. The polymers exhibit relatively high optical gaps of ~2.0 eV with strong absorption in the range of 400–600 nm, rendering them of particular interest for the harvesting of indoor light and/or multijunction devices. Electrochemical investigations indicate a lower highest occupied molecular orbital energy level (−5.44 eV) for PIDTT−BTz as compared to PIDTT−DTBTz (−5.36 eV), enabling to achieve a higher open-circuit voltage. Under solar illumination, the best power conversion efficiency (5.1%) is achieved for the combination PIDTT−DTBTz :PC 71 BM (compared to 4.6% for PIDTT−BTz :PC 71 BM). Image 1 • High gap copolymers based on indacenodithieno[3,2- b ]thiophene and bithiazole are synthesized. • The polymers are applied in bulk heterojunction polymer:fullerene solar cells. • Power conversion efficiencies up to 5.1% are achieved under solar illumination. • The binary blends are attractive for multijunction and ternary blend devices. • The presented polymer solar cells are of particular interest for indoor photovoltaics. [ABSTRACT FROM AUTHOR]

A significant number of organic electronic devices rely on blends of electron-donating and electron-accepting molecules. In bulk heterojunction organic photovoltaics, the nanoscopic phase behavior of the two individual components within the photoactive layer has a major impact on the charge separation and charge transport properties. For polymer:fullerene solar cells, it has been hypothesized that an increased accessibility of the electron-deficient monomer unit in push-pull type low bandgap polymers allows for fullerene 'docking'. The close proximity of electron donor and acceptor molecules enables more efficient charge transfer, which is beneficial for the device efficiency. With this in mind, we synthesized a series of PBDTTPD [poly(benzodithiophene-thienopyrroledione)] low bandgap copolymers with varying side chains. Solar cells were fabricated for all polymers and the device characteristics were compared. The combination of proton wideline solid-state NMR (ssNMR) relaxometry and sensitive external quantum efficiency (sEQE) measurements was shown to provide essential information on donor-acceptor interactions and phase separation in bulk heterojunction organic photovoltaics. The reduced charge transfer state absorption and the observed phase separation of crystalline PC 71 BM domains for the polymers containing the most accessible methyl-TPD unit indicate a diminished contact between donor and acceptor, leading to a loss in performance. Image 1 • PBDTTPD low bandgap copolymers are synthesized. • The polymers are applied in bulk heterojunction organic solar cells. • Steric control of the fullerene approach is realized through side chain variation. • Efficiencies go down when introducing short methyl (TPD) side chains. • ssNMR relaxometry and sEQE give detailed insights on the nanoscale organization. [ABSTRACT FROM AUTHOR]

Abstract Fullerene-free organic photovoltaics have recently reached impressive power conversion efficiencies above 14% for single junctions, increasing their competitiveness with respect to alternative thin-film technologies. In most record devices, electron-donating conjugated polymers are combined with novel generation small molecule acceptors. All-polymer organic solar cells, on the other hand, still lag behind in efficiency, although they have specific advantages in terms of ink formulation and long-term operational stability. Another point of attention is the synthetic complexity of the active layer materials, notably on the side of the new acceptor molecules. Therefore, the present study focuses on the implementation of the stable and cost-effective perylene diimide structure as the key component of high-performance electron-accepting polymers. The synthesis, structural and optoelectronic characterization of four push-pull type copolymers containing the electron-deficient bis(perylene diimide) (bis-PDI) unit is reported, as well as the photovoltaic analysis of these acceptor materials in combination with a well-known donor polymer (PTB7-Th). The acceptor polymers differ in the electron-rich part of the alternating push-pull structure and their solar cell power conversion efficiencies range from 3.2 to 4.7%. The maximum efficiency - the best performance achieved with a bis-PDI polymer so far - is obtained for the structurally most simple polymer, containing merely thiophene as the electron-rich building block. Controlled degradation under blue light in air is monitored by the bleaching of the relevant UV–Vis absorption bands, demonstrating high stability for the bis-PDI-thiophene containing polymers as compared to some prototype small molecule acceptors (FBR and ITIC). Graphical abstract Image 1 Highlights • A series of electron acceptor polymers based on bis(perylene diimide) is prepared. • All-polymer solar cells are fabricated in combination with PTB7-Th. • The best efficiency (4.7%) is obtained for the structurally most simple polymer. • The acceptor polymers have a low synthetic complexity and high photostability. [ABSTRACT FROM AUTHOR]

Although research in the field of organic photovoltaics (OPV) still merely focuses on efficiency, efforts to increase the sustainability of the production process and the materials encompassing the device stack are of equally crucial importance to fulfil the promises of a truly renewable source of energy. In this study, a number of steps in this direction are taken. The photoactive polymers all contain an electron-deficient building block inspired on the natural indigo dye, bay-annulated indigo, combined with electron-rich thiophene and 4 H -dithieno[3,2- b :2′,3′- d ]pyrrole units. The synthetic protocol (starting from indigo) is optimized and the final materials are thoroughly analyzed. MALDI-TOF mass spectrometry provides detailed information on the structural composition of the polymers. Best solar cell efficiencies are obtained for polymer:fullerene blends spin-coated from a pristine non-halogenated solvent ( o -xylene), which is highly recommended to reduce the ecological footprint of OPV and is imperative for large scale production and commercialization. [ABSTRACT FROM AUTHOR]

Abstract: Hybrid solar cells based on metal oxides in combination with semiconducting polymers are offering potential as future low-cost and upscalable photovoltaics. However, the use of toxic solvents such as chlorobenzene, chloroform or toluene for the polymer processing, remains a drawback. In an effort to attain the summit of environmentally friendly device fabrication, in this work, fully water-based hybrid solar cells are fabricated. Metal oxide nanorod arrays, prepared via an aqueous synthesis, are used as electron acceptors, while the need for toxic solvents in polymer processing is circumvented by replacing the widely used poly(3-hexylthiophene-2,5-diyl) (P3HT) with its water-soluble counterpart poly(3-(potassium-4-hexanoate)thiophene-2,5-diyl) (P3PHT). Comparing two deposition techniques and two metal oxides leads to a promising efficiency of 0.2% for drop casted P3PHT in combination with rutile TiO2 nanorod arrays, and demonstrates the possibility to develop fully ‘green’ nanorod based hybrid solar cells. [Copyright &y& Elsevier]

Abstract: A new approach is presented in order to improve the thermal stability of polymer: [6-6]-phenyl C61 butyric acid methyl ester (PCBM) bulk heterojunction solar cells. The central idea in this approach is the use of a polymer with high glass transition temperature (T g), well above the normal operating temperatures of the devices. In this paper, a PPV-derivative with a T g of 150°C was used as an electron donor and the thermal stability of the obtained solar cells was compared with solar cells based on the reference material poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with a T g of 45°C. The use of the material with higher glass transition temperature resulted in a significant improvement of the thermal stability of the photovoltaic parameters. Furthermore, a systematic transmission electron microscope (TEM) study demonstrates that the better thermal stability of performance coincides with a more stable active layer morphology. Both improvements are attributed to the reduced free movement of the electron donor material (PCBM) within the active layer of the solar cell. [Copyright &y& Elsevier]

An analytical model has been developed to estimate the short-circuit current density of conjugated polymer/fullerene bulk heterojunction solar cells. The model takes into account the solvent-dependent molecular morphology of the donor/acceptor blend, which was revealed by transmission electron microscopy. Field-effect transistors based on single and composite organic layers were fabricated to determine values for the charge carrier mobilities of such films. These values served as input parameters of the model. It is shown that the difference in short-circuit current density that was measured between toluene-cast and chlorobenzene-cast conjugated polymer/fullerene photovoltaic cells (Appl. Phys. Lett. 78 (2001) 841) could be very well simulated with the model. Moreover, the calculations illustrate how increasing the hole and electron mobilities in the photoactive blend can improve the overall short-circuit current density of the solar cell. [Copyright &y& Elsevier]

We examine the significance of hot exciton dissociation in two archetypical polymer-fullerene blend solar cells. Rather than evolving through a bound charge transfer state, hot processes are proposed to convert excitons directly into free charges. But we find that the internal quantum yields of carrier photogeneration are similar for both excitons and direct excitation of charge transfer states. The internal quantum yield, together with the temperature dependence of the current-voltage characteristics, is consistent with negligible impact from hot exciton dissociation. [ABSTRACT FROM AUTHOR]