Field-induced exciton dissociation in PTB7-based organic solar cells

Abstract

The physics of charge separation in organic semiconductors is a topic of ongoing research of relevance to material and device engineering. Herein, we present experimental observations of the field and temperature dependence of charge separation from singlet excitons in PTB7 and ${\mathrm{PC}}_{71}\mathrm{BM}$, and from charge-transfer states created across interfaces in $\mathrm{PTB}7/{\mathrm{PC}}_{71}\mathrm{BM}$ bulk heterojunction solar cells. We obtain this experimental data by time-resolving the near infrared emission of the states from 10 K to room temperature and electric fields from 0 to $2.5\phantom{\rule{4pt}{0ex}}{\mathrm{MV}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}}^{\ensuremath{-}1}$. Examining how the luminescence is quenched by field and temperature gives direct insight into the underlying physics. We observe that singlet excitons can be split by high fields, and that disorder broadens the high threshold fields needed to split the excitons. Charge-transfer (CT) states, on the other hand, can be separated by both field and temperature. Also, the data imply a strong reduction of the activation barrier for charge splitting from the CT state relative to the exciton state. The observations provided herein of the field-dependent separation of CT states as a function of temperature offer a rich data set against which theoretical models of charge separation can be rigorously tested; it should be useful for developing the more advanced theoretical models of charge separation.