Open Access

Theresa Linderl

• Organic photovoltaic cells (OPVCs) are an upcoming technology in the field of renewable energy sources. The possibility to produce lightweight, flexible and semitransparent modules of different color and an extremely short energy payback time fulfills all requirements for a solar cell technology that can be used complementary to the already established silicon based modules. Within the first years of research on OPVCs the increase in efficiency could mainly be achieved by the choice of material combinations that cover the solar spectrum to a larger degree and thus increase the generated photocurrent. However, within the last years, the focus of research on OPVCs has more and more changed towards understanding the comparatively low open-circuit voltage VOC at a given energy gap Eopt of the light absorbing material. Some reasons for this large bandgap-voltage offset can be found in the working principle of OPVCs. Especially the required heterojunction between the electron donating andOrganic photovoltaic cells (OPVCs) are an upcoming technology in the field of renewable energy sources. The possibility to produce lightweight, flexible and semitransparent modules of different color and an extremely short energy payback time fulfills all requirements for a solar cell technology that can be used complementary to the already established silicon based modules. Within the first years of research on OPVCs the increase in efficiency could mainly be achieved by the choice of material combinations that cover the solar spectrum to a larger degree and thus increase the generated photocurrent. However, within the last years, the focus of research on OPVCs has more and more changed towards understanding the comparatively low open-circuit voltage VOC at a given energy gap Eopt of the light absorbing material. Some reasons for this large bandgap-voltage offset can be found in the working principle of OPVCs. Especially the required heterojunction between the electron donating and the electron accepting material is responsible for a large part of these energy losses. At this donor/acceptor interface so called charge transfer (CT) states are formed, where the electron is situated on the acceptor material and the hole on the donor side. Both charge carriers are still Coulombically bound in the CT exciton. These CT states were found to play a crucial role in the working principle of OPVCs only a few years ago. On the one hand these CT states play an important role concerning the separation of excitons into free charge carriers and thus in the generation of the photocurrent. On the other hand, for OPVCs the energy of these CT states that is smaller than the optical gap Eopt of the absorbing materials, is considered to be the upper limit for VOC. However, so far only little is known about how the energy and form of the distribution of CT states is affected by parameters like the morphology of the donor and acceptor layers and the energetic properties directly at the interface. This work contributes to this discussion with a special focus on CT states and associated energy losses. To create a comprehensive picture about the correlations between local morphology at the donor/acceptor interface, the electronic properties within the solar cell stack and the distribution of CT states, several donor/acceptor material combinations are investigated by means of different measurement techniques. Especially the comparison of the crystalline growing diindenoperylene (DIP) and the amorphous growing tetraphenyldibenzoperiflanthene as donor materials in combination with the fullerene C60 as acceptor material are used to investigate the influence of the donor morphology on the distribution of the CT states. A combination of sensitive energy level measurements, temperature dependent emission and absorption spectra and time-resolved photoluminescence reveals a higher amount of exponentially distributed gap-states for solar cells based on the crystalline growing DIP. Thus, a clear connection between the distribution of occupied states of the donor layer and the distribution of CT states is established. Furthermore, the comparison of solar cells with different hole injection layers reveals a strong influence on VOC that is attributed to different band conditions of the energy levels within the solar cell due to a strongly different workfunction of the respective hole injection layers. Additionally non-fullerene acceptor materials are investigated that show a thermalization within the distribution of CT states towards states with the lowest energy, which leads to large energy losses for these material combinations. Moreover, the influence of molecular orientation and electronic coupling at the donor/acceptor interface is investigated by -sexithiophene /DIP solar cells prepared on substrates at different temperatures. Simulations of temperature dependent VOC measurements with a modification of the Shockley-Queisser theory suggest that a temperature dependent competition between recombination via optical and CT gap leads to the observed differences in the temperature dependent VOC measurements. A transition temperature Ttr is introduced that separates the temperature regimes, where recombination across the CT gap (T< Ttr) and the optical gap (T> Ttr) dominates. These simulations are verified by temperature dependent electroluminescence spectra. Finally energy losses are reviewed and several strategies to reduce energy losses are suggested. On exemplary solar cells that are discussed within this work, the influence of these strategies on the different loss channels and the total energy loss of the respective solar cells is discussed.…