The replacement of common fullerene derivatives with neat-C70 could be an effective approach to restrain the costs of organic photovoltaics and increase their sustainability. In this study, bulk-heterojunction solar cells made of neat-C70 and low energy-gap conjugated polymers, PTB7 and PCDTBT, are thoroughly investigated and compared. Upon replacing PC70BM with C70, the mobility of positive carriers in the donor phase is roughly reduced by 1 order of magnitude, while that of electrons is only slightly modified. It is shown that the main loss mechanism of the investigated neat-C70 solar cells is a low mobility-lifetime product. Nevertheless, PCDTBT:C70 devices undergo a limited loss of 7.5%, compared to the reference PCDTBT:PC70BM cells, reaching a record efficiency (4.44%) for polymer solar cells with unfunctionalized fullerenes. The moderate efficiency loss of PCDTBT:C70 devices, due to an unexpected excellent miscibility of PCDTBT:C70 blends, demonstrates that efficient solar cells made of neat-fullerene are possible. The efficient dispersion of C70 in the PCDTBT matrix is attributed to an interaction between fullerene and the carbazole unit of the polymer.
Improving the dispersion properties of unfunctionalized fullerenes in photovoltaic blends is a key factor for replacing the commonly employed butyric acid methyl ester derivatives (PBCM and PC 71 BM), with economical and light harvesting advantages. We consider here the effects of a pyrene derivative (PyBB) as dispersant of neat fullerenes (C 60 or C 70 ) in photovoltaic blends prepared with a low energy gap conjugated polymer (PTB7) as electron donor. The morphological and spectroscopic properties in the presence and absence of PyBB were evaluated by AFM, emission fluorescence, and light induced EPR, and the deep trap density of states was calculated in the two cases. The electrical properties of the devices prepared with or without PyBB were investigated and compared. The intimate interaction of fullerene/PyBB aromatic species revealed by the spectroscopic analysis is in agreement with the enhanced dispersion of the three-component blends. At the same time the deep trap density of states is varied, and the mobility of negative charge carriers is reduced in films prepared with PyBB dispersant, frustrating the beneficial effect of PyBB on the blend morphology.
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