Transient absorption for characterization of intermediate band solar cells

We use transient absorption methods to characterize the sequential two-photon absorption in a quantum-dot superlattice based intermediate band solar cell (QD-IBSC). Using collinear, orthogonally polarized beams generated from an Optical Parametric Oscillator (OPO) at varying time delay, tuned stepwise from 1050nm to 1250nm, we use the solar cell photocurrent as a direct measure of the transient absorption by measuring the differential photocurrent as a function of time delay between two energetically degenerate, ~ 100fs pulses. For comparison, we measure the pulse autocorrelation in the same geometry using a GaAsP photodiode, where all observed photocurrent is derived from instantaneous two-photon absorption. Our measurements show that at high intensity, the measurement is dominated by instantaneous two photon absorption, with a simultaneous sequential two-photon photocurrent which persists beyond the pulse overlap. Our measurements demonstrate the method can reveal carrier dynamics in a working QD-IBSC, and their dependence on energy. The method could potentially give details of the band structure formed in the QD-IBSC. Such knowledge may benefit device development and future designs of IBSCs based on QD superlattices or alternative intermediate band materials or device structures.