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Abstract: Pad printing as a technique for preparing the active layer in polymer solar cells is presented. The technique employs a silicone rubber stamp to pick up the motif from a gravure plate and transfer it to the substrate. The strengths and limitations of pad printing are discussed and polymer solar cells prepared by pad printing are presented. Devices were prepared on indium tin oxide substrates but in principle the entire photovoltaic device comprising front and back electrodes, barrier layers and active layer could be printed with no need for vacuum steps. The device geometry comprises a spin coated transparent zinc oxide front electrode, a pad printed active layer based on a bulk heterojunction of the thermocleavable polymer poly(3-(2-methylhexyloxycarbonyl)thiophene-co-thiopene) (P3MHOCT) and zinc oxide nanoparticles, spin coated PEDOT:PSS and finally a manually cast thermally cured silver paste back electrode. The P3MHOCT was converted to poly(3-carboxy-dithiophene) (P3CT) in situ by heating the film to for a brief period. The entire printing and device preparation was carried out in the ambient atmosphere and the devices obtained had a good stability in air during storage and operation. [Copyright &y& Elsevier]

Abstract: A complete polymer solar cell module prepared in the ambient atmosphere using all-solution processing with no vacuum steps and full roll-to-roll (R2R) processing is presented. The modules comprise five layers that were prepared on a 175-μm flexible polyethyleneterephthalate (PET) substrate with an 80-nm layer of transparent conducting indium–tin oxide (ITO). The ITO layer was first patterned by screen printing an etch resist followed by etching. The second layer was applied by either knife-over-edge (KOE) coating or slot-die coating a solution of zinc oxide nanoparticles (ZnO-nps) followed by curing. The second layer comprised a mixture of the thermocleavable poly-(3-(2-methylhexan-2-yl)-oxy-carbonyldithiophene) (P3MHOCT) and ZnO-nps and was applied by a modified slot-die coating procedure, enabling slow coating speeds with low viscosity and low surface tension ink solutions. The third layer was patterned into stripes and juxtaposed with the ITO layer. The fourth layer comprised screen-printed or slot-die-coated PEDOT:PSS and the fifth and the final layer comprised a screen-printed or slot-die-coated silver electrode. The final module dimensions were 28cm×32cm and presented four individual solar cell modules: a single-stripe cell, a two-stripe serially connected module, a three-stripe serially connected module and finally an eight-stripe serially connected module. The length of the individual stripes was 25cm and the width was 0.9cm. With overlaps of the individual layers this gave a width of the active layer of 0.6cm and an active area for each stripe of 15cm2. The performance was increased ten fold compared to mass-produced modules employing screen printing for all five layers of the device. The processing speeds employed for the R2R processed layers were in the range of 40–50mh−1. Finally a comparison is made with the state of the art represented by P3HT–PCBM as the active layer and full R2R solution processing using slot-die coating. [Copyright &y& Elsevier]

Abstract: A polymer solar cell that can be stored under ambient conditions (25°C and 35±5% relative humidity) in the dark for 6 months without noticeable degradation in performance is presented. The active layer is based on low-cost materials and is free from fullerenes. No vacuum steps are required for processing the device that employs an inverted device geometry, where the active layers in the device comprise a transparent cathode based on solution processed zinc oxide, an active layer based on a bulk heterojunction of zinc oxide nanoparticles and poly-(3-carboxydithiophene) (P3CT), a PEDOT:PSS layer and finally a printed silver based anode. No encapsulation was employed and the devices were robust and not sensitive to mechanical handling of the active layer and back electrode. The accelerated lifetime in air defined as 80% of the initial performance at continuous illumination (1000Wm−2, AM1.5G, 72±2°C, ambient atmosphere, 35±5% humidity) was typically 100h and the devices were tested for 150h. When keeping the same conditions and lowering the temperature, stable operation for hundreds of hours was possible. In terms of long-term stability, this performance is inferior to inorganic photovoltaics but the technology compares well and competes with small batteries in terms of capacity. The device efficiency more than doubled upon decreasing the incident light intensity from 1000 to 100Wm−2. [Copyright &y& Elsevier]

Abstract: The synthesis and characterization of a number of so-called thermo-cleavable solvents are described with their application in all-air, all-solution and all-screen-printed polymer solar cells. These solvents were developed to meet some requirements for printing techniques such as long “open time” combined with fast drying on heating that cannot be attained with the usual solvents used for conjugated polymers. The new solvents have low volatility at ambient conditions, but decompose thermally at 130–180°C to low-boiling and highly volatile products. Characterization by thermogravimetric analysis (TGA) and high-temperature NMR established the onset temperature of decomposition, the rate of the reaction and the nature of the products. Printing experiments with inks based on these solvents together with conjugated polymers are exemplified for polymer solar cell devices to show how they enable large-scale production of polymer solar cells using screen printing. Screen-printed solar cells are still very inferior to state of the art P3HT/PCBM technology, but it is our view that it is necessary to explore these printing technologies if polymer solar cells are to ever become commercial products. [Copyright &y& Elsevier]

Abstract: Multilayer tandem polymer solar cells were prepared by solution processing using thermocleavable polymer materials that allow for conversion to an insoluble state through a short thermal treatment. The problems associated with solubility during application of subsequent layers in the stack were efficiently solved. Devices comprised a transparent front cathode based on solution processed zinc oxide nanoparticles, a large band gap active layer based on a bulk heterojunction between zinc oxide and poly(3-carboxydithiophene) (P3CT) followed by a layer of PEDOT:PSS processed from water. The second cell in the stack employed a zinc oxide front cathode processed on top of the PEDOT:PSS layer from an organic solvent, a low band gap active layer based on a bulk heterojunction between zinc oxide and the novel poly(carboxyterthiophene-co-diphenylthienopyrazine) (P3CTTP) followed by a layer of PEDOT:PSS again processed from water and finally a printed silver electrode. The devices were prepared without the use of fullerenes and vacuum steps and employ only thermal treatments and orthogonal solvents. The devices exhibited operational stability in air without any form of encapsulation. [Copyright &y& Elsevier]

Abstract: Continuous operation of a polymer photovoltaic device under accelerated conditions for more than 1 year has been demonstrated (8760h at 72°C, 1000Wm−2, AM1.5, under vacuum). Formation of hydrogen-bonded networks is proposed to be responsible for the long lifetime and high stability observed in photovoltaic devices employing polythiophene substituted with carboxylic-acid moieties under oxygen free conditions. 1H and 13C solid-state NMR, IR, and ESR spectroscopy of unmodified and isotopically labeled polythiophenes were studied. Distances between the isotopically labeled carboxylic acid carbon atoms were measured by 13C solid-state magic-angle-spinning (MAS) NMR using symmetry-based double-quantum (2Q) dipolar recoupling. This revealed the presence of 13C–13C distances of 3.85Å, which correspond to the C–C distance in hydrogen-bonded carboxylic acid dimers. In spite of the presence of carboxylic groups in the polymer as demonstrated by 13C CP/MAS NMR and IR spectroscopy, the absence of carboxylic protons in solid state 1H NMR spectra indicate that they are mobile. We link the extraordinary stability of this system to the rigid nature, cross-linking through a hydrogen-bonded network and a partially oxidized state. [Copyright &y& Elsevier]