Trap-Limited Recombination and Energy Offset at the Carbon Nanotube – C60 Interface

We study photophysics and free carrier recombination in (7,5) semiconducting single walled carbon nanotube (s-SWCNT)/C60 fullerene heterojunction diodes over the temperature range 4.5-300 K. We characterize the temperature dependence of the magnitude and spectral peak position of the external quantum efficiency. Appling the Marcus theory description of energy transfer at the interface, we estimate that the reorganization energy and change in Gibbs free energy are relatively equal in magnitude, and predict the high exciton dissociation rate at the s-SWCNT/C60 heterojunction to be relatively resistant to changes in temperature. We show that the current-voltage characteristics in forward bias at all temperatures are best fit to a two-diode model in which traps dictate recombination at the heterointerface. From the diode ideality factor, we calculate the trap depth for holes in (7,5) s-SWCNT films to be 150 meV. We measure the temperature dependence of the open-circuit voltage, short-circuit current density, and fill factor of the devices. Open-circuit voltage strongly depends on temperature, rising to a maximum of 0.82 V at 20 K. Meanwhile, fill factor decreases precipitously below 150 K to a fluence-independent value of 28%. We are able to verify our two-diode model by successfully recreating the temperature and fluence dependence of the open-circuit voltage, and suggest that decreasing fill factor is consistent with a temperature-dependent polaron pair recombination rate at the heterointerface. Finally, we calculate the interfacial energy gap of the s-SWCNT/C60 heterojunction using two methods and find agreement, estimating 0.90 eV by examining the current-voltage characteristics in the dark and 0.86 eV by extrapolating the open-circuit voltage to its 0 K value. These insights provide a greater understanding of the processes that take place at the heterointerface of s-SWCNT/C60 solar cells, guiding further research to improve the efficiency of these devices.