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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.