Efficient Charge Photogeneration by the Dissociation of PC₇₀BM Excitons in Polymer/Fullerene Solar Cells

The role of PC₇₀BM excitons in driving charge photogeneration in low bandgap polymer/fullerene bulk heterojunction solar cells has been studied. Both transient absorption spectroscopy of charge generation yields in blend films as a function of excitation energies and photocurrent quantum efficiency spectra of the corresponding devices indicate that charge generation in this system results primarily from direct optical excitation of PC₇₀BM. Blend composition studies of photocurrent density and photoluminescence quenching indicate that the efficiency of photocurrent generation is primarily determined by the limited efficiency of PC₇₀BM exciton diffusion to the polymer due to the formation of PC₇₀BM domains (≥5 nm). This limitation becomes more severe as the PC₇₀BM content is increased above 50%. Despite this limitation and despite the poor charge photogeneration from polymer excitons, organic solar cells fabricated using this photoactive blend layer yielded device photocurrents of 7.1 mA/cm², maximal EQEs of 41%, and a device efficiency of 3.1%