Toward mechanically robust and intrinsically stretchable organic solar cells: Evolution of photovoltaic properties with tensile strain

This paper describes the effect of tensile strains of up to 20% on the photovoltaic properties of organic solar cells based on two different conjugated polymers and their blends with (6,6)-phenyl C-61 butyric acid methyl ester (PCBM). The two conjugated polymers are poly(3-hexylthiophene) (P3HT) and a donor-acceptor polymer whose repeat unit comprises a diketo pyrrolo-pyrrole moiety, thiophene, thienothiophene, and thiophene (DPPT-TT). The stretchable substrate, transparent electrode, and top electrode are, respectively, poly(dimethylsiloxane) (PDMS), poly(3,4-ethylenedioxythiophene):poly(s-tyrenesulfonate) (PEDOT:PSS), and eutectic gallium-indium (EGaln). Analysis of the results of mechanical testing using a buckling based metrology, atomic force and optical microscopy, and linear and cyclic applications of strain yielded six major conclusions. (i) The identity of the conjugated polymer significantly affects the response of the solar cells to strain. Blending of PCBM in bulk heterojunction films stiffens P3HT more than it stiffens DPPT-TT, as determined by buckling-based measurements of elastic moduli. (ii) The greater elastic modulus of P3HT:PCBM than that of DPPT-TT:PCBM is correlated with a greater tendency of P3HT:PCBM to fracture under strain. (iii) PEDOT:PSS has the effect of an adhesion layer of the bulk heterojunction films to PDMS; its presence increases the crack-onset strain from <2% to 8% for P3HT:PCBM and from 6% to 12% for DPPT-TT:PCBM. (iv) Cracks dominate the characteristics of the devices under strain. The differences in the response to strain between devices comprising P3HT:PCBM and DPFT-TT:PCBM are correlated to the unequal brittleness of the polymer:fullerene films. (v) Devices based on both P3HT and DPPT-TT exhibit increased open-circuit voltage with strain, which is greatest for devices containing P3HT. These effects are attributed to disruption of the gallium oxide "skin" that covers the top electrode, EGaln. (vi) Wave-like buckles appear in the films upon stretching <= 10% for the first time. These buckles permit reversible stretchability. (C) 2012 Elsevier B.V. All rights reserved.