The role of defects in organic semiconductors for solar cells

The aim of this dissertation was to find out the impact of various degradation agents on defect state generation and degradation in OSCs. We studied the role of defect states on OSC samples exposed to: oxygen, humidity, solar radiation as well as the role of the intrinsic degradation. We obtained the reproducible PCE of 3 % of referential OSC based on the P3HT:PCBM active layer by optimization of OSC fabrication processes. This value was achieved for the P3HT:PCBM weight ratio of 1.5:1 and the active layer thickness of 120 nm. We investigated the presence of defect states in OSCs with the isothermal Q-DLTS method. All the external degradation factors led to the increase of defect state concentration with the emission rate of about 100 ms^-1. Defect states were found even in nondegraded OSCs. The concentration of defect states was correlated with the basic OSC parameters. The obtained J_SC correlation implies the 3 types of dominant degradation processes: 1. The generation of defect states in the active layer, 2. the degradation of cathode-organic interface, 3. the segregation of P3HT and PCBM phases. The degradation of nonencapsulated samples took place in two stages in ambient and humid atmosphere. The first stage was determined by the defect state generation due to oxygen and water diffusion into the active layer. The second stage is mostly related to the cathode-organic interface, which is more pronounced at the degradation in humid air. These states are mostly located at cathode-organic interface in nonencapsulated OSCs, where the charge is captured by states located above HOMO of P3HT. A similar correlation was obtained for the first degradation stage of OSCs with the active layer degraded before the cathode evaporation, albeit the degradation processes were much slower. These results suggest a significant impact of the cathode-organic interface on the OSC degradation. The photodegradation of encapsulated OSCs was accompanied by the sudden PCE decrease and the increase of defect states. This can be ascribed to the photoinduced chemical reaction of impurities (e.g. oxygen molecules) with organic semiconductor. The PCE degradation proceeded much slower in the next degradation stage. The observed drop of defect states is probably caused by temperature-induced phase segregation. We found two types of defect states induced by photo-oxidation of the active layer. The deeper states with the activation energy of about 200 meV dominated during the first 10 minutes of the photo-oxidation, but these states diminished in the following degradation stage. The shallower states with the activation energy of 60 meV increased monotonously during the whole observed degradation period. The correlations of J_SC with both types of states imply that the J_SC degradation is related to the shallower states. Absorbance spectra analysis confirmed that the J_SC decrease is related to the exciton recombination, whilst the impact of the exciton generation decrease is much less significant. The intrinsic degradation was accompanied by simultaneous decrease of both the J_SC and the defect state concentration. This indicates the generation of morphological changes of heterojunction or the presence of defect states with emission rate beyond the spectrometer range. The obtained results offer valuable knowledge about the impact of various degradation agents on the generation of defect states and about the influence of these states on OSC electrical parameters. This dissertation represents a contribution to the understanding of degradation mechanisms as well as to the stability improvement of OSC, which is the main problem of large-scale OSC production. (Author)