Nature Commun. 4, 2334 (2013)

Solar cells convert solar energy into electrical signals. Alongside silicon-based photovoltaic devices, solar cells made from blends of conjugated polymer and fullerenes are being developed because of their inherently lower production costs. In these devices, the absorption of photons generates bound hole–electron pairs, which have to overcome Coulomb attraction to be separated into free electrons and holes. These charge carriers can subsequently be collected at electrodes to generate a photocurrent. The mechanism for charge separation is, however, not well understood, hindering optimization of material parameters. Vidmantas Gulbinas of Vilnius University and colleagues have now identified diffusion as the driving mechanism for charge separation.

The researchers examined charge-carrier dynamics in P3HT:PCBM solar cells on a timescale of pico- to nanoseconds. Using time-resolved electric field-induced second harmonic and Monte Carlo simulations they identified the respective contributions of drift and diffusion to carrier dynamics and charge separation. The displacement distances of the carriers was found to be very small on a subpicosecond timescale, ruling out the hypothesis of initial long-range carrier separation. Gulbinas and colleagues also find that charge separation is driven by fast diffusion rather than by drift in the applied electric field, up to distances at which the Coulomb attraction is overcome. The results highlight the importance of optimizing the carrier mobility, which is proportional to the diffusion coefficient, to fabricate efficient organic solar cells.