Control of morphology is a key issue in order to improve the performance of organic bulk heterojunction solar
cells. Solar cells consisting of a blend of regioregular P3HT (poly(3-hexylthiophene)) and PCBM ([6-6]-phenyl
C<sub>61</sub> butyric acid methyl ester) have demonstrated the highest efficiencies until now (up to 5 %). This
performance was achieved by applying a post-production annealing, which is considered to induce a dual
crystallization behavior. In order to control and tune the morphology, the phase behavior needs to be
described in terms of the underlying fundamental thermodynamics. Hence, it is essential to obtain a phase
diagram of the blend. In this study, the state diagram of P3HT:PCBM blends is measured by means of
standard and modulated temperature differential scanning calorimetry (DSC). For the first time, the glass
transition (T<sub>g</sub>) of PCBM could be determined. All blends evidenced a single T<sub>g</sub>, indicating an homogeneous
blend is formed. Phase separation is thus only induced from crystallization and no "intrinsic" phase separation
is occurring in the blend.
This paper investigates the thermal stability of organic bulk heterojunction solar cells, with a special focus on the thermal ageing of both photovoltaic parameters and morphology of the active layer. The photovoltaic parameters of a set of bulk heterojunction solar cells were determined by IV-characterization and their bulk morphology was investigated with transmission electron microscopy (TEM). A link could be made between the degradation of the short circuit current under a thermal treatment and the corresponding change in bulk morphology. A possible improvement of the thermal stability of bulk heterojunction solar cells is presented through the use of a polymer with higher glass transition temperature.