We report on the latest progress in the field of organic p-i-n tandem solar cells. The results of tandem solar cells with an efficiency of 9.8% are shown (certified by Fraunhofer ISE) with an active area of about 1.1 cm<sup>2</sup>. These solar cells show a promising intrinsic stability: a relative reduction of 5.7% of its initial power conversion efficiency was measured when stored at 85°C for 2400 hours. Additionally, we present a small OPV module with an active area of 122cm<sup>2</sup> showing an efficiency of with 9%, and an excellent low light behavior. Furthermore, we present the latest results on optimized tandem solar cells showing a power conversion efficiency of 10.7 % (measured by SGS, accredited and independent testing facility, active area of 1.1cm<sup>2</sup>).
We report on the latest progress in the field of organic p-i-n tandem solar cells. The external quantum efficiencies of a tandem solar cell with two complementary absorbing bulk heterojunctions with an efficiency of 6.07% (certified by Fraunhofer ISE) with an active area of about 2cm<sup>2</sup> is analyzed. These solar cells are extremely stable: a reduction of only 3% of its initial power conversion efficiency was measured when stored at 85°C for 5000 hours. The solar cell does not show any reduction in efficiency when stored under continuous illumination of a tungsten lamp corresponding to 1.5 suns for 5000 hours. Furthermore, we present the latest results on optimized tandem solar cells showing a power
conversion efficiency of 7.66 % (certified by Fraunhofer ISE, active area of 1.1cm<sup>2</sup>).
We report on latest progress in the field of p-i-n type tandem solar cells. An optimized tandem cell architecture with two
complementary absorbing bulk heterojunctions leads to a certified power conversion efficiency of 5.9% on 2 cm<sup>2</sup> active
area. Moreover, we show that p-i-n type tandem solar cells can be extremely stable: Extrapolated lifetimes corresponding
to more than 30 years of sun illumination have been achieved. Furthermore, we show that efficiency and stability only
slightly decrease when transferring the cell architecture to large serially interconnected modules of more than 100 cm²
The transparent electron transport material NTCDA (1,4,5,8-naphthalenetetracarboxylic dianhydride) was examined
in order to find a suitable substitute for C60 which is today often used in small molecular organic solar
cells as transport layer. Due to its wide band gap, NTCDA does not absorb in the visible range and is furthermore
exciton blocking. By doping with AOB (acridine orange base), its conductivity was raised to about
1 • 10<sup>-4</sup>S/cm. It can therefore simultaneously be used as electron transport material and optical spacer in p-i-n
type solar cells, leading to power conversion efficiencies of up to 2.83%. Additionally, an investigation of the
surface morphology using AFM was performed.
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.
To realize organic solar cells with high performance, we developed a novel way of stable n-doping using cationic dyes in electron transport materials. In our approach, the volatile donors are created in-situ from stable precursor compounds. Using the cationic dye pyronin B (PyB) as a model precursor, we carried out conductivity and field effect measurements to characterize the properties of doped naphtalene tetracarboxylic dianhydride (NTCDA) thin film. The results show a strong increase in n-type conductivity. Combined FTIR, UV/VIS/NIR and mass spectroscopic measurements suggest the formation of leuco pyronin B during sublimation of pyronin B chloride, and a subsequent charge transfer between dopant and matrix providing free electrons, which increase the n-type conductivity.