In many photovoltaic materials the impact of disorder and band tails is discussed controversially and it is unclear whether extended band states or localized tail states dominate charge carrier dynamics.
Here, we integrate the effect of band tails on charge carrier distribution, transport and recombination into one narrative and deduce the consequences for solar cells.
To this end, we probe photoexcited charge carriers in a kesterite-semiconductor as a model system over their full life span from femtoseconds to microseconds by time-resolved terahertz spectroscopy and photoluminescence spectroscopy.
We find that occupation of the band tails is strongly temperature-dependent. Above a characteristic temperature charge carrier are activated out of Urbach band tails. Then charge carriers occupy dominantly shallow Gaussian band tails in the probed kesterite thin film. At room temperature we directly observe the thermalisation of charge carriers into these tail states within the first 2 ps. The occupation of these tail states reduces the quasi-Fermi level splitting and the open circuit voltage of a potential solar cell by ca. 80 meV.
The mobility of the charge carriers decreases to 144 cm2/Vs while they thermalize into the tail states. This mobility value is at least one order of magnitude lower than in a theoretical tail-free kesterite-semiconductor. Further, the mobility at terahertz frequencies indicates that charge carriers localize in tail states with a typical spatial extend of 11 nm.
The recombination kinetics of photoexcited charge carriers can be described within the Shockley-Read-Hall model of defect assisted recombination. Unambiguous signs of an influence of band tails are not measured at room temperature, but secondary effects from the affected carrier distribution and transport are discussed. The non-radiative recombination is found to be the major bottleneck for a potential solar cell and reduces the open circuit voltage of a potential solar cell by 270 meV.
However, below the characteristic temperature of 200 K charge carrier relax by multiple-trapping into the deep Urbach tail and lead to a strongly injection-dependent charge carrier distribution, transport and recombination as previously observed for amorphous semiconductors.