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Numerical simulations and advanced characterization of organic solar cells

: Spies, Annika
: Weber, Eicke R.; Reiter, Günter


Freiburg/Brsg., 2018, 214 pp.
Freiburg/Brsg., Univ., Diss., 2017
URN: urn:nbn:de:bsz:25-freidok-157800
Fraunhofer ISE ()

Organic solar cells (OSC) have reached power conversion efficiencies above 10% by continuously improving material properties and device architectures. However, the efficiency remains still significantly lower than the thermodynamic limit of 33% for an ideal solar cell. This thesis deals with the identification of the most important efficiency limitations of OSC and its implications for future design guidelines. These limitations are investigated on the basis of numerical drift-diffusion simulations as well as accompanying experimental verification thus bridging the gap between theoretical modeling and experimental results. The benefit of electrical simulations is the possibility to obtain information about the charge carrier motion and distribution within the photoactive layer normal to the electrode surface which is experimentally not accessible. Hence, it offers the opportunity to investigate characteristic microscopic effects and to evaluate their impact on a macroscopic level. Real material systems will be investigated and compared to the theoretically calculated drift-diffusion results. One intrinsic limitation is the low charge carrier mobility for many common organic semiconductors that are applied in OSC as photoactive materials which are several orders of magnitude lower compared to inorganic semiconductors. From literature it is known that this leads to a non-ohmic but charge carrier density dependent transport resistance. An analytical model is developed that describes this transport resistance and gives the opportunity to make reliable efficiency predictions as a function of mobility. Further, it is shown that the original Shockley equation, which could not be applied to low mobility materials, can be corrected for the contribution of the transport resistance enabling its applicability on JV-curves of OSC. Additionally, in a sensitivity analysis the limitations of this analytical model with respect to different parameters, e.g. recombination coefficient, effective density of states or imbalanced mobilities are investigated and evaluated. The selectivity of the charge collecting electrodes is another important aspect in the development of new materials and device architectures. The impact of contact selectivity is investigated by varying the electrodes workfunction for three different photoactive blend systems. The influence of charge carrier mobility in general but also with respect to asymmetry between the mobility of electrons and holes is discussed. Therefore, characterization techniques, i.e. photo- and electroluminesence, JV-curves and charge extraction are applied and compared to simulation results. It is shown that for imperfect selectivity the VOC is not a good figure-of-merit to obtain any information about the bulk recombination due to the low charge carrier mobilities. It is emphasized that this is a result of the large gradients in the electrochemical potential of majority charge carriers in close proximity to the non-selective contact. The experimental results can be reliably reproduced quantitatively by drift-diffusion simulations by solely changing the workfunction and thus the selectivity of the contact. By analyzing the contact selectivity a reverse open-circuit voltage effect was observed in the temperature and illumination dependence of the VOC which also revealed in transient characterization experiments. This effect is investigated in detail in a comprehensive study by comparing different photoactive blend systems, layer stacks, electrode materials and interfacial materials to electrical simulations. Two different explanatory approaches are proposed, namely a p-doping of the photoactive layer or a vertical phase separation that reproduce the experimental results and rely on the same physical origin, which is a distinct change in the ratio of the conductivity of majority and minority charge carriers. Additionally, several common blend systems are characterized in detail by determining the loss factors quantitatively. A limitation in all figures-of-merit, i.e. JSC, VOC and FF are observed and design guidelines are proposed and discussed to overcome these limitations. For the in-depth characterization of the JSC losses, EQE measurements as well as reflection and transmission measurements are carried out and the optical constants are determined for optical simulations using a RCWA. The VOC losses are further investigated by applying a theoretical model known from literature that takes the absorption and emission via the CT-state into account. This offers the possibility to explore the VOC reduction related to the donor-acceptor structure of the photoactive layer. The reduction in FF is investigated by applying a modified Suns-VOC method that enables to determine the maximum possible FF without the influence of charge carrier transport.