Intermolecular donor-acceptor charge transfer complex (CTC) formed in the electronic ground state between poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and 2,4,7-trinitrofluorenone (TNF) has been investigated by Raman and optical absorption spectroscopies. Blending of MEH-PPV and TNF results in appearance of the CTC absorption band in the optical gap of the both components and in changes in the characteristic MEH-PPV Raman bands including shifts, change in bandwidth, and intensity. The experimental data are similar in films and solutions indicating the CTC formation in both. We associate the low-frequency shift of the strongest MEH-PPV Raman band at approximately 1580 cm(-1) reaching 5 cm(-1) with partial electron transfer from MEH-PPV to TNF amounting approximately 0.2e(-). We suggest that polymer conjugated segments can form the CTC of variable composition MEH-PPV:TNF=1:X, where X

Conventional models of planar and bulk heterojunction organic solar cells have been extended by introducing doping in the active layer. We have studied the performance of organic solar cells as a function of dopant concentration. For bulk heterojunction cells, the modeling shows that for the most studied material pair (poly-3-hexylthiophene, P3HT, and phenyl-C 61 -butyric acid methyl ester, PCBM) doping decreases the short-circuit current density (J SC ), fill factor (FF) and efficiency. However, if bulk heterojunction cells are not optimized, namely, at low charge carrier mobilities, unbalanced mobilities or non-ohmic contacts, the efficiency can be increased by doping. For planar heterojunction cells, the modeling shows that if the acceptor layer is n doped, and the donor layer is p doped, the open-circuit voltage, J SC , FF and hence the efficiency can be increased by doping. Inversely, when the acceptor is p doped, and the donor is n doped; FF decreases rapidly with increasing dopant concentrations so that the current-voltage curve becomes S shaped. We also show that the detrimental effect of nonohmic contacts on the performance of the planar heterojunction cell can be strongly weakened by doping. Phys. Rev. B. 84, 205318 (2011)PACS number(s): 72.20.Jv, 72.40.+w, 72.80.Le, 73.50.Pz

The donor-acceptor ground-state charge-transfer complex (CTC) formed in solution between a conjugated polymer, poly[methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV), and a low-molecular-weight organic acceptor, 2,4,7-trinitrofluorenone (TNF), is studied by optical absorption and Raman spectroscopy. The CTC absorption as a function of TNF content shows a threshold increase that is in conflict with the model commonly used for optical characterization of low-molecular-weight CTCs. The shift of MEH-PPV characteristic Raman band at 1585 cm(-1) also exhibits a threshold dependence upon TNF addition. We assign the threshold in both the absorption and Raman data to the CTC concentration. To describe the threshold in the terms of the common model, we extend it by introducing an association function instead of a constant. The association function of acceptor concentration has been calculated to be K(a) approximately 1.5-3 M(-1) below the threshold, to increase steeply up to K(a) approximately 6-7.5 M(-1) just after the threshold, and then to grow gradually up to K(a) approximately 40 M(-1). The CTC molar absorption coefficient has been found to be epsilon(CTC) = (12.7 +/- 0.6) x 10(3) M(-1) cm(-1) at 635 nm. We explain the threshold as a result of the positive feedback: the CTC formation induces planarizaton of conjugated polymer segments that in turn facilitates further CTC formation.

Single crystals of thiophene−phenelyne co-oligomers (TPCOs) have previously shown their potential for organic optoelectronics. Here we report on solution growth of large-area thin single-crystalline films of TPCOs at the gas−liquid interface by using solvent−antisolvent crystallization, isothermal slow solvent evaporation, and isochoric cooling. The studied co-oligomers contain identical conjugated core (5,5′diphyenyl-2,2′-bithiophene) and different terminal substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated films are molecularly smooth over areas larger than 10 × 10 μm 2 , which is of high importance for organic field-effect devices. The low-defect structure of the TPCO crystals is suggested from the monoexponential kinetics of the PL decay measured in a wide dynamic range (up to four decades) and from low crystal mosaicity assessed by microfocus X-ray diffraction. The TPCO crystal structure is solved using a combination of X-ray and electron diffraction. The terminal substituents affect the crystal structure of TPCOs, bringing about the formation of a noncentrosymmetric crystal lattice with a crystal symmetry Cc for the bulkiest trimethylsilyl terminal groups, which is unusual for linear conjugated oligomers. Comparing the different crystal growth techniques, it is concluded that the solvent−antisolvent crystallization is the most robust for fabrication of single-crystalline TPCOs films. The possible nucleation and crystallization mechanisms operating at the gas−solution interface are discussed.

Thiophene-phenylene co-oligomers (TPCOs) are among the most promising materials for organic light emitting devices. Here we report on record high among TPCO single crystals photoluminescence quantum yield reaching 60%. The solution-grown crystals are stronger luminescent than the vapor-grown ones, in contrast to a common believe that the vapor-processed organic electronic materials show the highest performance. We also demonstrate that the solution-grown TPCO single crystals perform in organic field effect transistors as good as the vapor-grown ones. Altogether, the solution-grown TPCO crystals are demonstrated to hold great potential for organic electronics.