Morphological, thermal, and electrical characterization of organic photovoltaic blends

Abstract

The major objective of this thesis was to correlate morphology to the electrical properties of bulk heterojunction solar cells. Since at the beginning of this work P3HT:PCBM solar cells gave the best efficiencies, the focus was on this type of blend. First, an extensive morphology study was needed since it is generally known that morphology influences the properties of bulk heterojunction solar cells. Secondly, the relationship between morphology and electrical properties was still inefficiently known and therefore a few options were explored how to correlate them.
First, the P3HT:PCBM blend was studied morphologically with a wide variety of techniques, i.e. transmission electron microscopy (TEM), X-Ray diffraction (XRD), atomic force microscopy (AFM), confocal fluorescence microscopy and optical microscopy. Different blend ratios and various annealing conditions were studied.
In order to get a clear idea of how and why this morphology of the P3HT:PCBM blend is formed a preliminary state diagram was composed. Differential scanning calorimetry (DSC) was used for studying melting and crystallization behaviour while modulated temperature differential scanning calorimetry (MTDSC) was used for glass transition determination. First the pure P3HT and PCBM was investigated followed by different blend compositions. The Tg of pure P3HT was determined at 18.7°C and a melting and crystallization temperature of 209°C and 179°C respectively, values which are in good correspondence with literature reports. For the first time, a Tg for PCBM, i.e. 130°C, could be found. A double melting peak, a small one at 266°C and a larger one at 287°C, was encountered for PCBM which can originate from two crystal forms, i.e. triclinic and monoclinic, present in pristine PCBM. With increased PCBM contents the Tg shifts from the Tg of P3HT till the Tg of PCBM, meaning that all blend compositions are in a homogeneous phase, down to a scale of 10 nm. Only phase separation induced by the crystallization of either P3HT or PCBM could be found, no intrinsic liquid-liquid phase separation is observed.
An elaborate morphological study was now performed. In order to link this morphology to local electrical properties different SPM techniques were applied. In this work, the opportunity to use EFM and C-AFM has been provided. Interestingly, the non-contact scanning procedure inherent to EFM provides a non-destructive scanning guarantee on such soft materials as organic semiconductors. In C-AFM, this major issue was handled using very soft probes. C-AFM was specifically attractive for its ability to map sample conductivity and local I-V profiling. The traditional EFM set-up was adapted successfully in order to probe local lateral conductivity differences, while the C-AFM is used to probe transversal conductivity differences and to analyse charge transport mechanisms.It is demonstrated that the combination of AFM/EFM/CAFM can be a powerful approach to obtain complementary information on local morphological and electrical properties of blended organic material systems.

Date of Award	21 Dec 2007
Original language	English
Supervisor	Guy Van Assche (Jury), Jean Manca (Promotor), J. D'haen (Jury), Dirk Vanderzande (Jury), C. Van Haesendonck (Jury), O. Douhéret (Jury), A. Gadisa Dinku (Jury) & Marc D'olieslaeger (Co-promotor)