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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

Organic optoelectronics calls for materials combining bright luminescence and efficient charge transport. The former is readily achieved in isolated molecules, while the latter requires strong molecular aggregation, which usually quenches luminescence. This hurdle is generally resolved by doping the host material with highly luminescent molecules collecting the excitation energy from the host. Here, a novel concept of molecular self-doping is introduced in which a higher luminescent dopant emerges as a minute-amount byproduct during the host material synthesis. As a one-stage process, self-doping is more advantageous than widely used external doping. The concept is proved on thiophene-phenylene cooligomers (TPCO) consisting of four (host) and six (dopant) conjugated rings. It is shown that <1% self-doping doubles the photoluminescence in the TPCO single crystals, while not affecting much their charge transport properties. The Monte-Carlo modeling of photoluminescence dynamics reveals that host-dopant energy transfer is controlled by both excitonic transport in the host and host-dopant Förster resonant energy transfer. The self-doping concept is further broadened to a variety of conjugated oligomers synthesized via Suzuki, Kumada, and Stille crosscoupling reactions. It is concluded that self-doping combined with improved excitonic transport and host-dopant energy transfer is a promising route to highly luminescent semiconducting organic single crystals for optoelectronics.