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The bulk-heterojunction concept was originally developed to allow for a large active interfacial area between the donor and acceptor materials in organic solar cells. [ 1 ] The energy offsets between the lowest unoccupied molecular orbital (LUMO) levels of the donor and acceptor are tuned to effi ciently dissociate photogenerated excitons, i.e. an offset on the order of > 0.5 eV is effi ciently promoting electron transfer to the acceptor and prohibiting back-transfer to the donor. However, after dissociation the charge carriers are still Coulombically bound at zero external fi eld within the Coulomb radius r c = e 2 /4 π ε ε 0 kT , where e is the electron charge, ε ( ε 0 ) is the relative (absolute) dielectric permittivity, k the Boltzmann constant and T is the temperature. The formation of a ground-state charge transfer (CT) state opens up (non-)radiative recombination channels at the interface, where the Coulombically bound carriers can eventually recombine both geminately and non-geminately, causing recombination losses in solar cells.Recent reports on charge transfer state formation in the ground state of the polymer/fullerene mixtures suggests that the CT state arises from a wave function overlap of the polymer and fullerene molecules, [2][3][4][5][6][7] whereby a new inter-bandgap charge transfer complex state is formed at the donor-acceptor interface. Experimental evidence show that this CT state has an energy lower than the bandgap of both the donor and the acceptor materials. [ 8 , 2 , 3 ] Two routes for populating the CT states have been reported. The fi rst is by relaxation from singlet states formed via either above-bandgap excitation or injection from contacts. [ 9 , 10 ] Here the charge pair migrates to a donor-acceptor interface and minimizes its energy by populating the CT state at the interface. The second route for populating a CT complex is by direct optical excitation using subbandgap light. [ 11 , 12 ] Besides providing a recombination channel for Coulombically bound electron-hole pairs at donor-acceptor interfaces, there is also evidence of charge generation via the CT state. In regioregular-(rr) poly (3-hexylthiophene):[6,6]-phenyl-C 61 -butyric acid methyl ester (P3HT:PCBM) a substantial number of sub-bandgap generated charges escape from the interface and can contribute to the solar cell photocurrent. [ 11 , 13 ] The CT state is also shown to be closely linked to the open circuit voltage of bulk heterojunction solar cells. [ 13 ] The main obstacle for materials with low dielectric constants (and consequently a large r c ) to be operational in effi cient solar cells is avoiding geminate recombination of the Coulombically bound charge pairs. The carrier generation in homogeneous polymeric semiconductors is of Onsager-type, which means that the generation is governed by the Brownian motion of the geminate pair within their mutual Coulomb potential. The criterion for this process is that the hopping distance is much shorter than the Coulomb radius. If a geminate pair can separate this di...

Solid contact potassium-selective electrodes with the internal ion-to-electron transduction layer composed of plasticized poly(vinyl chloride) (PVC) and 2-20% (m/m) of polyaniline (PANI) nanoparticles, with the mean particle size of 8 nm, have been studied in this paper. UV-vis measurements in pH buffer solutions between pH 0 and 12 show that the electrically conducting emeraldine salt (ES) form of PANI has exceptionally good pH stability. Membranes of PANI nanoparticles were mainly in the ES form even at pH 12, in contrast to electrochemically prepared PANI(Cl) films, which are converted completely to the nonconducting form already at pH 6. Long-term UV-vis measurements with the PANI membranes in contact with aqueous buffer solution at pH 7.5 showed no degradation of the ES form. The PANI nanoparticles are homogenously mixed in the PVC-based solid contact (SC) layer. Only the uppermost part of the SC layer is to a minor extent dissolved in the outer potassium-selective PVC membrane. This enabled the preparation of geometrically well-defined inner SC layers, thus improving the reproducibility of the solid contact electrodes and resulting in good mechanical strength between the inner and outer membranes.

Magneto-electrical measurements were performed on diodes and bulk heterojunction solar cells (BHSCs) to clarify the role of formation of coulombically bound electron-hole (e-h) pairs on the magnetoresistance (MR) response in organic thin film devices. BHSCs are suitable model systems because they effectively quench excitons but the probability of forming e-h pairs in them can be tuned over orders of magnitude by the choice of material and solvent in the blend. We have systematically varied the e-h recombination coefficients, which are directly proportional to the probability for the charge carriers to meet in space, and found that a reduced probability of electrons and holes meeting in space lead to disappearance of the MR. Our results clearly show that MR is a direct consequence of e-h pair formation. We also found that the MR line shape follows a power law-dependence of B 0.5 at higher fields.

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The fi eld-effect transistor possesses an important function as a current and voltage switcher in various electronic applications. Ever since the fi rst reports on organic fi eld-effect transistors (OFETs) were published, [ 1 ] extensive research has been carried out in order to improve the device performance and, hence, lower the power consumption. The OFET is a three terminal device consisting of the drain, source and gate electrodes. The electronic current path between the drain and the source (i.e., the active channel) is situated at the semiconductor/insulator interface and the current is modulated by applying a gate voltage. The insulator is typically an inorganic or an organic dielectric. A high capacitance over the dielectric is desirable for low-voltage operation. [ 2 ] There are two standard methods to increase the capacitance: 1) by using an insulator with a high dielectric constant, which is, for example, accomplished with inorganic oxides, [ 3 ] and 2) by reducing the thickness of the dielectric. This has been achieved by utilizing ultrathin cross-linked polymeric insulators. [ 4 ] Indeed, low-voltage and high-performing OFETs have been demonstrated with these dielectrics. The downside is that they are often also thin and brittle to use on, e.g., bendable surfaces. The electrolyte gated OFET constitutes a specifi c class of OFETs. [ 5 ] The device structure is identical to the OFET. However, the dielectric is replaced with an electrolyte as an ion conducting insulator. In such a structure, cations and anions assemble at opposite interfaces when applying the gate voltage. The resulting capacitance over the electrolyte can be several orders of magnitude higher than in ordinary dielectrics. [6][7][8] Thus, a high charge density is generated at the semiconductor/electrolyte-interface already at low voltages, and the electrolyte gated OFET can, therefore, be operated within 3 V, occasionally also with high fi eld-effect mobilities. [ 7 ] Recently we demonstrated the possibility of using ion conducting membranes in order to manufacture electrolyte gated OFETs (MemFETs) operating at 1 V. [ 9 ] The major advantages with membranes are their thickness, robustness and chemical resistance. The thickness of used membranes varied between 50-150 μ m and especially fully and partially fl uorinated membranes resist common organic solvents, such as chloroform. Hence, the membrane can easily be sandwiched within a complex geometry, such as the transistor. The concept of using ion conducting membranes (50-150 μ m thick) for gating low-voltage (1 V) organic fi eld-effect transistors (OFETs) is attractive due to its low-cost and large-area manufacturing capabilities. Furthermore, the membranes can be tailor-made to be ion conducting in any desired way or pattern. For the electrolyte gated OFETs in general, the key to low-voltage operation is the electrolyte "insulator" (the membrane) that provides a high effective capacitance due to ionic polarization within the insulator. Hydrous ion conducting membranes are easy to pro...

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