We report on a series of organic solar cells based on heterojunctions of oligothiophene derivatives with varying
chain length and C60 fullerenes. Devices are based on either p-i-n or p-i-i structure. In the first the intrinsic
photovoltaic active layer is sandwiched between a p-type and n-type doped organic wide-gap layer for hole and
electron transport respectively. In the latter the electron transport layer is replaced by a thin layer of wide-gap
material as exciton blocker. Through optimization of transport and absorber layers we are able to reach in
devices with single heterojunctions an open circuit voltage <i>V</i><sub>oc</sub> of about 1V, a short circuit current density <i>J</i><sub>sc</sub>
of about 5.6mA/cm<sup>2</sup> and a fill factor <i>FF</i> above 50% under an AM1.5 illumination with 1000W/m<sup>2</sup>. However,
still only a small part of the available solar spectrum is used.
Thus, based on these materials stacked solar cells have been made to further improve the light absorption.
The thickness of each layer is optimized using optical simulations to match the currents delivered by each of the
solar cells in the stack. Through the incorporation of a very efficient recombination zone between the stacked
solar cells the resulting <i>V</i><sub>oc</sub> nearly reaches the sum of the <i>V</i><sub>oc</sub> of the two serially connected solar cells.
Recently, we have demonstrated an open circuit voltage of 1.0V and a power conversion efficiency of 3.4% in thin
film solar cells, utilizing a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor and
C<sub>60</sub> as acceptor. Stimulated by this result, we systematically study the energy and electron transfer processes
taking place at the oligothiophene:fullerene heterojunction along a homologous series of these oligothiophenes.
The heterojunction is modified by tuning the HOMO level using different oligothiophene chain lengths, while
the LUMO level is essentially fixed by the choice of the acceptor-type end-groups (dicyanovinyl) attached to
the oligothiophene. We study electron transfer at the heterojunction to C<sub>60</sub> using photoinduced absorption.
The observed transitions are unambiguously identified by TD-DFT calculations. With increasing the effective
energy gap of the donor-acceptor pair, charge carrier dissociation following the photoinduced electron transfer is
eventually replaced by recombination into the triplet state, which alters the photovoltaic operation conditions.
Therefore, the optimum open-circuit voltage of a solar cell is a trade-off between an efficient charge separation at
the interface and a maximized effective gap. We conclude that values between 1.0 and 1.1 V for the open-circuit
voltage in our solar cell devices present an optimum, as higher voltages were only achieved with concomitant
losses in charge separation efficiency.
In this work, we report on efficient heterojunction organic solar cells containing a new oligothiophene derivative α,α'-bis-(2,2-dicyanovinyl)-quinquethiophene (DCV5T) as donor (D) and fullerene C<sub>60</sub> as acceptor (A). The oligothiophene carries electron withdrawing substituents which increase the ionization energy and even more strongly the electron affinity. In thin films, the absorption is significantly broadened compared to solution and the optical gap is reduced to 1.77 eV. Nevertheless, the material shows strong fluorescence with low Stokes shift (peak at 1.71 eV), i.e. low energy loss upon reorganisation in the excited state.
At the heterointerface between the low band-gap oligothiophene and fullerene C<sub>60</sub>, photogenerated excitons from both materials are efficiently separated into electrons on the LUMO of C<sub>60</sub> and holes on the low-lying HOMO of the oligothiophene. This step involves only low energetic losses since both the HOMO and the LUMO offset of the two materials are below 0.6 eV, close to the expected exciton binding energy. We can thus reach high open circuit voltages of up to 1.0 V. The most efficient solar cells with power efficiencies around 4 % are obtained when the photoactive heterojunction is embedded between a p-doped hole transport layer on the anode side and a combination of a thin exciton blocking layer and aluminium on the cathode side. However, due to the high ionization energy of the oligothiophene (approx. (5.6 ± 0.1) eV), hole injection from any anode or hole transport layer is difficult and the IV curves thus show a characteristic S-shape which reduces the fill factor FF. It is found that the actual FF sensitively depends on the work function of the p-doped hole transport layer, that can be influenced by doping.