Paper is ubiquitous in everyday life and a truly low-cost substrate. The use of paper substrates could be extended even further, if electronic applications would be applied next to or below the printed graphics. However, applying electronics on paper is challenging. The paper surface is not only very rough compared to plastics, but is also porous. While this is detrimental for most electronic devices manufactured directly onto paper substrates, there are also approaches that are compatible with the rough and absorptive paper surface. In this review, recent advances and possibilities of these approaches are evaluated and the limitations of paper electronics are discussed.

Several spectroscopic methods were applied to study the characteristic properties of the electronic excitations in thin films of regioregular and regiorandom polythiophene polymers. In the regioregular polymers, which form two-dimensional lamellar structures, increased interchain coupling strongly influences the traditional one-dimensional electronic properties of the polymer chains. The photogenerated charge excitations (polarons) show two-dimensional delocalization that results in a relatively small polaronic energy, multiple absorption bands in the gap where the lowest energy band becomes dominant, and associated infrared active vibrations with reverse absorption bands caused by electron-vibration interferences. The relatively weak absorption bands of the delocalized polaron in the visible and near-infrared spectral ranges may help to achieve laser action in nanocrystalline polymer devices using current injection.

INTRODUCTION TO CHARGE CARRIER TRANSPORT IN ORGANIC SOLAR CELLSIncreasing energy consumption and rising energy prices in the world forces to look for energy alternatives, one of the most promising being the photovoltaic solar energy conversion. Various concepts and device architectures of organic solar cells have been actively studied for more than 30 years. [1][2][3][4][5][6][7][8] Efficiencies, routinely exceeding 4-5% have been reached in thin-film organic solar cells today. From purely academic point of view, the research of organic solar cells is interesting due to novel photophysical phenomena, whereas technologically low fabrication costs due to roll-toroll printing possibilities drive the economic point of view. There are four main important processes which might limit the power conversion efficiency of photovoltaic devices: 9 1. Light absorption in the film. 2. Free charge carrier generation. 3. Charge transport to the opposite electrodes and extraction by the electrodes. 4. Carrier recombination.When the photoexcitation (an exciton) is created after the photon energy is absorbed in the material, mobile charge carriers must be created by splitting the exciton into a free electron and hole. Therefore, donor and acceptor blends are used in the organic photovoltaics to facilitate photoinduced charge transfer. If the exciton reaches the donor-acceptor interface, the electron can then be transferred to the material with lower lying Lowest Unoccupied Molecular Orbit (LUMO) if I D * − E A − Coulomb < 0, where I D * is the ionization potential of the excited donor, E A the electron affinity of the acceptor, and Coulomb summarizes all the electrostatic interactions including the exciton binding energy and all polarizations. The important parameters here are the exciton diffusion length and the distance between the donor and acceptor phases.Furthermore, both these charge carriers must be transported to the opposite electrodes and reach them prior to recombination. If after photoinduced charge transfer the electron and hole are still bound by the Coulomb potential, then typically for low mobility materials, they cannot escape from each others attraction and will finally recombine. However, the excitons can be split into free electrons and holes when the carrier dissipation distance is larger than the Coulomb radius. To fulfill this condition the Coulomb field must be screened or charge carrier hopping distance must be larger than the Coulomb radius (for low mobility materials it is unrealistic). 10 In this case mobile charge carriers can be transported to the contacts either by carrier diffusion or electric field induced drift. In order to have unity quantum efficiency for charge extraction, one needs to fulfill the condition that the charge carrier transit time t tr is much smaller than the carrier lifetime τ (t tr τ). The carrier transit time t tr = d/µE is determined by the charge carrier mobility µ, sample thickness d, and the electric field E inside the film. If the photocurrent is governed by the carrier dri...

Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly‐3‐alkyl thiophene with regioregular order, which forms self‐assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain‐like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long‐lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long‐lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano‐type interference mechanism, using the amplitude mode model.

Bimolecular charge carrier recombination has been clarified in bulk-heterojunction solar cells based on a blend of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene using the time-of-flight method. We show how bimolecular recombination influences the charge carrier transport, how it limits the efficiency of low-mobility solar cells, and how to estimate the bimolecular recombination coefficient. We found that bimolecular recombination in these efficient photovoltaic materials is orders of magnitude slower as compared to Langevin recombination expected for low-mobility materials. This effect is inherent to the nanomorphology of the bicontinuous interpenetrating network creating separate pathways for electrons and holes, and paves the way for the fabrication of bulk-heterojunction solar cells where bimolecular recombination is not the limiting factor.