Resumen de Tracing degradation effects in organic solar cells
Remarkable progress has been made in the field of organic solar cells (OSCs) over the last decade. Polymer based OSCs have achieved power conversion efficiencies of more than 10% already and 15% is within reach. Polymer chemistry has played a significant part in this achievement and will be the key to the further developments. On the other hand, the underlying mechanisms in the photovoltaic conversion process are still under debate in polymer solar cells. A critical issue to be improved in organic solar cells is their operational lifetime. Current-voltage characterization is the most direct method to monitor degradation induced performance loss in organic solar cells. But an I-V curve gives only a rough estimation to the degradation mechanism. The external quantum efficiency (EQE) of an OSC is dependent on efficiency of absorption (¿abs), efficiency of exciton dissociation (¿diss), efficiency of polaron pair separation (¿sep) and efficiency of charge extraction (¿extr). These processes are happening in an OSC in a wide range of time scales varying from few femtoseconds to milliseconds.
The effects of chemical degradation and thermal stress on the active layer of OSCs are still poorly understood. Therefore this thesis attempts to shed more light on the effects of light induced photooxidation and thermal annealing on a model polymer system, P3HT and a series of new low optical band gap materials containing fullerene. Transient opto-electrical measurements are excellent tools for identifying the degradation routes. Transient absorption spectroscopy (TAS) studies give access to the time-resolved population of excitons and charged species, and can be performed in films of the active layer as well as in complete devices. It is the only technique which is capable of addressing all relevant time scales in which loss processes can occur, from femtoseconds to milliseconds. When combined with matrix analysis tools like global and target analysis, it is able to provide quantitative information on the dynamics of all relevant photo-excited states, such as singlet excitons, triplets, charged states, charge transfer states etc. This allows to identify the reasons for performance loss of organic solar cells. In addition to TAS, complementary transient electrical techniques like transient photo-voltage (TPV), transient photo-current (TPC), charge extraction by linearly increasing voltage (CELIV) were also used to monitor degradation induced performance losses in organic solar cell.
The main goal of this thesis has been to study the photophysics of pristine and degraded organic solar cells by means of time-resolved techniques. Chapter 1 gives an introduction to the world of organic solar cells and the fundamental photophysics of OSCs upon photon absorption followed by detailing the major degradation triggers affecting the performance of OSCs. Chapter 2 deals with the materials studied in this thesis and experimental techniques used to probe the photophysics of organic solar cells.
In Chapter 3, the photophysics of pristine P3HT:PCBM solar cells and thin films are studied. Different time-resolved techniques, namely transient absorption spectroscopy (TAS) and transient photovoltage (TPV), have been used to unravel excited state dynamics of P3HT:PCBM blends. Microsecond TAS results suggested an intensity dependent bimolecular recombination dynamics of charge carriers. This chapter also investigates the effect of the highly conductive PEDOT:PSS interlayer on the charge carrier transport. It is found that the presence of highly conductive PEDOT:PSS facilitates the lateral movement of charge carriers, in addition to vertical diffusion through the bulk.
In Chapter 4, the effects of photooxidation on P3HT and Si-PCPDTBT solar cells were investigated by means of TAS and charge extraction by linearly increasing voltage (CELIV). A drastic reduction of short circuit current (Jsc) was observed already at very low levels of photooxidation. Before any appreciable changes in absorption occurs, photo-oxidation leads to a strong reduction in the recombination and extraction of charge carriers due to a decrease of charge carrier mobility and an increase in the background carrier concentration, whereas the generation of charge carriers is not affected. Extraction is shown to be retarded even more strongly than recombination, possibly by a reduction of the extraction field by the background carriers.
In Chapter 5, effects of oxygen-induced degradation in KP115:PCBM in the presence of light is explained by means of time-resolved transient techniques. Studies unveiled that the huge loss in Jsc is due to combined effects of reduction in the effective mobility and formation of isolated donor-acceptor islands. Interestingly, the photooxidation of these materials does not alter the recombination order and creates no background carriers. It is also observed that even in the pristine case the charge extraction is not complete, indicating the morphology is not optimum.
Chapter 6 deals with the thermal degradation effects, which are as critical for the operational lifetime of the devices as the effects of photooxidation but in contrast to the latter cannot be controlled by encapsulation but rather depend on the morphology evolution of the active layer. Therefore, we studied a series of BDTDPP polymer solar cells having different sidechains, which provides the opportunity to investigate different morphologies but very similar electronic structure. Immediate triplet formation even in the pristine case seems a major problem in these solar cells and thermal annealing increase the triplet yield, reducing the performance of the devices. Thermal degradation is found to strongly reduce the ultrafast charge generation, while the diffusion-mediated delayed pathway also shows reduced charge generation efficiency. This could be due to the reduction of the ultrafast pathway with the growth of donor-rich domains. The reduced charge carrier yield of the delayed pathway is attributed to exciton quenching processes like quenching at molecularly dissolved acceptors in the donor-rich phase. Microsecond transient measurements show that additionally, part of the observed electrical performance loss is due to an increased charge carrier extraction and recombination time, indicating their reduced mobility. These processes are less pronounced in solar cells employing polymers with alkyl side chains which demonstrate an improved stability of the blend under thermal stress.
Finally, conclusive remarks on the studied solar cells and outlook are presented in Chapters 7 and 8.
