Ultrafast photocurrent detection contradicts optical detection conclusions: Exciton diffusion contributes little to carbon nanotube device efficiency

Many conclusions about energy conversion in next-generation photovoltaic devices are gleaned indirectly from optical measurements of exciton dynamics, not directly from photocurrent itself. This method is problematic because optical measurements report on all excitons, not just productive ones. Using a new ultrafast photocurrent spectrometer, we compare exciton dynamics of semiconducting carbon nanotubes measured in films to those measured in devices using photoabsorption- and photocurrent-detected transient and two-dimensional spectroscopies. We find that photoabsorption detection greatly overestimates the importance of long-lived excitons for photovoltaic device performance. Excitons diffuse across nanotubes for picoseconds, but we find that the photocurrent is mostly created by excitons that diffuse little before dissociating at the electron transfer interface within 30 femtoseconds of being created. Thus, scientific conclusions reached from optical-only studies bear little importance to the performance of these devices, calling into question the processes thought critical for efficient photoconversion. This study points to the necessity for directly measuring photocurrent-generating exciton dynamics, not surmising them from optical spectroscopy alone.