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A power conversion efficiency of 3.4% with an open-circuit voltage of 1 V was recently demonstrated in a thin film solar cell utilizing fullerene C 60 as acceptor and a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor ͓K. Schulze et al., Adv. Mater. ͑Weinheim, Ger.͒ 18, 2872 ͑2006͔͒. This prompted us to systematically study the energy-and electron transfer processes at the oligothiophene:fullerene heterojunction for a homologous series of these oligothiophenes. Cyclic voltammetry and ultraviolet photoelectron spectroscopy data show that the heterojunction is modified due to tuning of the highest occupied molecular orbital energy for different oligothiophene chain lengths, while the lowest unoccupied molecular orbital energy remains essentially fixed due to the presence of electron-withdrawing end groups ͑dicyanovinyl͒ attached to the oligothiophene. Use of photoinduced absorption ͑PA͒ allows the study of the electron transfer process at the heterojunction to C 60. Quantum-chemical calculations performed at the density functional theory and/or time-dependent density functional theory level and cation absorption spectra of diluted DCVnT provide an unambiguous identification of the transitions observed in the PA spectra. Upon increasing the effective energy gap of the donor-acceptor pair by increasing the ionization energy of the donor, photoinduced electron transfer is eventually replaced with energy transfer, which alters the photovoltaic operation conditions. The optimum open-circuit voltage of a solar cell is thus a trade-off between efficient charge separation at the interface and maximized effective gap. It appears that the open-circuit voltages of 1.0-1.1 V in our solar cell devices have reached an optimum since higher voltages result in a loss in charge separation efficiency.

The aim of this article is to investigate the origin of the open circuit voltage ͑V oc ͒ in organic heterojunction solar cells. The studied devices consist of buckminsterfullerene C 60 as acceptor material and an oligophenyl-derivative 4 , 4Ј-bis-͑N , N-diphenylamino͒quaterphenyl ͑4P-TPD͒ as donor material. These photoactive materials are sandwiched between indium tin oxide and p-doped hole transport layers. Using two different p-doped hole transport layers, the built-in voltage of the solar cells is independently changed from the metal contacts. The influence of the built-in voltage on the V oc is investigated in bulk and planar heterojunctions. In bulk heterojunctions, in which doped transport layers border directly on the photoactive blend layer, V oc cannot exceed the built-in voltage significantly. Though, in planar heterojunctions, V oc is identical with the splitting of quasi-Fermi levels at the donor-acceptor interface and is thus primarily determined by the difference of the lowest unoccupied molecular orbital of C 60 and the highest occupied molecular orbital of 4P-TPD. In planar heterojunctions, the open circuit voltage can exceed the built-in voltage. Furthermore, the investigations show that the efficiency of organic solar cells can be improved by using p-doped charge transport layers with optimized energy level alignment to the active materials. The optimized planar heterojunction shows a fill factor of up to 65.5% and a V oc of 0.95 V. For solar cells with insufficient energy level alignment between the photoactive layer system and the hole transport layer, a reduced V oc in bulk heterojunction cells and a characteristic S shape of the I-V characteristics in planar heterojunction cells are observed.