Morphology and Thermal Stability of the Active Layer in Poly(p-phenylenevinylene)/Methanofullerene Plastic Photovoltaic Devices

Abstract

The morphology of composite thin films consisting of a conjugated polymer (poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene], MDMO-PPV) and methanofullerene ([6,6]-phenyl C61 butyric acid methyl ester, PCBM), which are used as the active layer in polymer photovoltaic devices, has been extensively studied using transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). Composite MDMO-PPV:PCBM films have been prepared with PCBM concentrations varying from 20 to 90 wt %. PCBM-rich clusters are clearly observed in TEM bright-field mode when the PCBM concentration is increased to ca. 75 wt % in the composite film. The SAED analysis shows that these clusters consist of many PCBM nanocrystals with random crystallographic orientations. Furthermore, we show that these nanocrystals are also present in the homogeneous matrix at PCBM concentrations below 75 wt %. Annealing of the blend films has been performed at temperatures between 60 and 130 °C for different times. In all cases, but especially when the annealing temperature is above the glass transition temperature of MDMO-PPV (∼80 °C), PCBM molecules show high diffusion mobility, resulting in accelerated phase segregation and in the formation of large PCBM single crystals in the film. The observed phase segregation, even at temperatures as low as 60 °C, indicates that the thermal stability of MDMO-PPV:PCBM films will likely limit the long-term performance of solar cells based on these materials.

On the morphology of polymer‐based photovoltaics

“…Devices with different compositions were fabricated and a maximum in efficiency was seen when the PCBM concentration was $80 wt %, mainly due to the significantly increased J sc and fill factor, FF, [see that thermal annealing of the MDMO-PPV:PCBM induced crystallization of PCBM, which will affect the efficiency and stability of the devices. 27,28 These observations directly link the device efficiency to the morphology, clearly demonstrating the necessity of research on the morphology of BHJ OPVs. These initial findings, though, have bearing on the findings that are emerging on the more efficient low bandgap polymers of current interest.…”

Methanofullerene elongated nanostructure formation for enhanced organic solar cells

“…However, Z-contrast images, HRTEM and EELS indicated that small "crystalline" pathways are well dispersed throughout the polymer matrix. Yang et al [10] suggested this in a different system that the reported in this work; they observed the formation of PCBM single crystals by using SAED patterns. We have corroborated the existence of these PCBM nanocrystals using HRTEM.…”

Hydrogen bonds in PC61BM solids

“…Therefore tremendous efforts have been paid to optimizing the morphologies and crystal structures of polymer/PC 61 BM blends in the past years. The efforts include optimizing spin-coating parameters, selecting suitable solvent, [5][6][7] use of solvent additives [8][9][10][11] and thermal annealing, [12][13][14] which greatly helped to increase the power conversion efficiency reaching 11%. 15 Considering all the efforts being empiric, we believe that fundamental understanding of the inter-molecular interaction of PC 61 BM pure material is desired for more effective design of OSC.…”