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 o...