The demand for low-cost, nontoxic photovoltaics (PV) has brought to the development of new materials and processing techniques. In particular, organic solar cells (OSCs) composed by a blend of fullerenes and organic polymers are among the most promising technologies for next generation PV. The main drawbacks of these devices are:
(i) thermal instability
(ii) low power conversion efficiency (PCE) (~10% vs >20% for Si-based and perovskite solar cells)
OSCs are unstable at high temperatures since phase separation between donor and acceptor may occur, with consequent losses in photocurrent: in fact, heat can promote PC61BM diffusion throughout the polymer phase resulting in aggregation and formation of large fullerene crystals. Here we show that blends formed by the low band gap polymer PTB7 and the larger PC71BM fullerene can exhibit excellent thermal stability at specific concentrations, due to the formation of a highly stable blend. The enthalpy of mixing ΔH/Vm of the blend [Caddeo2013] is studiedby molecular dynamics simulations (MD) as a function of the fullerene mass ratio, and we find deep local minima in ΔH/Vm corresponding to experimentally observed highly stable blends [Dkhil2016].
Among the factors which limit the PCE in OSCs, recombination plays a key role. Usually defect-assisted, it can be reduced by improving the quality of the active layer. The presence of PC71BM in the donor-rich domain gives rise to defect states facilitating recombination. We propose a ternary blend which includes the acceptor PC71BM and two donor polymers of the PTB7 family, to realize ultrastable blends with reduced recombination. By MD simulations, we calculate the energy of formation of polymer/PC71BM systems [Bellani2015] and we provide physical insight into the mechanisms at the origin of the improved performances [Caddeo2017].
[Dkhil2016] S. B. Dkhil et al., Adv. En. Mater. 2016, 1601486
[Caddeo2013] C. Caddeo, A. Mattoni, Macromolecules 2013, 46 (19),8003-8008
[Bellani2015] S. Bellani et al.,J. Mater. Chem. B, 2015, 3, 6429
[Caddeo2017] C. Caddeo, J. Ackermann et al,in preparation