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-C61-butyric acid methyl ester, PCBM) doping decreases the short-circuit current density (JSC), fill factor (FF), and efficiency. However, if bulk heterojunction cells are not optimized, namely, at low charge carrier mobilities, unbalanced mobilities, or nonohmic 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, JSC, 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.

Intermolecular donor-acceptor charge transfer complex (CTC) formed in the electronic groundstate 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 ∼1580cm−1 reaching 5cm−1 with partial electron transfer from MEH-PPV to TNF amounting ∼0.2e−. We suggest that polymer conjugated segments can form the CTC of variable composition MEH-PPV:TNF=1:X, where X⩽0.5 is per MEH-PPV monomer unit. Our Raman data indicate that MEH-PPV conjugated segments involved in the CTC become more planar; however, their conjugation length seemingly does not change.