Impurities are of large concern in inorganic semiconductor technology as they strongly influence the electrical and optical characteristis of electronic devices. However, purity is often not discussed in the organic semiconductor community, although almost every group has eperiences with strong batch-to-batch variations on the performance on organic solar cells and some factors, e.g. the residual content of metal catalysts used during synthesis of the materials are know to dentrimental influences the electrical characteristics. Additional problems might arise due to contaminations of the processing solvents or the in-situ-formation of degradation products due to photoelectrical or photooxidative stress. Despite the intrinsic importance to know these impurities, the qualitiative and even more the quantitative analysis is extremely difficult due to 1) the similar nature of the impurities and the semiconductor, i.e. comprising the same elements (C, H, N, S, O, Cl, F...) and 2) due to their very low concentration in the active layer.

This work focuses on a systematic investigation of the impurities of bulk heterojunction organic solar cells by means of pyrolysis (Py) gas chromatography (GC) coupled to a high-resolution time-of-flight (HRTOF) mass spectrometer (MS). We thereby have chosen one of the currently standard materials combination PM6/Y6 in the device architecture ITO/PEDOT:PSS/PM6:Y6/PNDIT-F3N-Br/Ag. 1-chloronaphthalene was used as additve. We thereby analysed the single materials (PEDOT:PSS, PM6, Y6, PNDIT-F3N) as reference, but the main focus was set on the characterisation of the complete device, as additional impurities might be introduced during additves and solvents during device fabrications.

Thermogravimetric analysis (TGA) revealed that PM6 – which has the highest thermal stability - limits the thermal range for Py-GC/MS to temperatures above appr. 450 °C. Consequently, pre-investigations were performed between 500 and 750 °C. A pyrolysis temperature of 550 °C was found to the best compromise with respect to intensity and diversity of characteristic pyrolysis products. The pyrolsis of all materials of a solar cells at once leads to a complex mixture of volatile products and thermal fragementation species, which were separated to a large extent by GC and then analysed by MS. However, the main challenge was to identify impurities in the ppm range, which were hidden under the many fragmentation products of the semiconductors. It was possible to identify even the additive 1-chloronaphthalene in the ppm range and it was found, that its concentration is strongly dependent on the processing conditions. Additionally even impurities such as dichloronaphthalene or remains of the metal catalysts, organic phosphin oxides, could be detected.

A further interesting aspect is, that already in the early stages some degradation products related to defluorination of the organic semiconductor had been identified. An unexpected observation was the fact, that the change in aggregation due to an thermal annealing step leads to a significant change in the degradation pattern of the Y6 acceptor whciht might be related to differences in the thermal stability.