Transient Absorption for Characterization of Quantum Dot Intermediate Band Solar Cells
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Transient Absorption for Characterization of Quantum Dot Intermediate Band Solar Cells Praveen Kolla1, Andrew Norman2 and Steve Smith1,* 1
South Dakota School of Mines and Technology, Rapid City, South Dakota. 2 *
NREL, Golden, Colorado.
Corresponding Author: [email protected]
ABSTRACT We use transient absorption methods to characterize the sequential two-photon absorption in a quantum-dot super-lattice 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 photo-current 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. INTRODUCTION The intermediate band solar cell (IBSC) allows sequential absorption of low-energy photons which would otherwise not be absorbed by conventional solar cells, thereby significantly increasing the achievable efficiency over the theoretical efficiency limit for a single junction solar cell, commonly known as the Shockley–Queisser limit [1]. Based on thermodynamic arguments similar to the those used in deriving the Shockley–Queisser result, the IBSC has a theoretical efficiency as high as 63%, well above current state of the art tandem cells [2]. However, the details of realizing a device which satisfies the contingencies upon which such efficiency is theoretically possible have not been achieved. The Quantum Dot IBSC (QD-IBSC), consisting of a superlattice of self-assembled quantum dots embedded in a higher band-gap absorber, has been proposed as a means of achieving a working IBSC, and has been studied both experimentally and theoretically [3-5].While evidence of the sequential absorption process has been confirmed [4], significant improvements in efficiency have not been realized. Further, such measurements generally do not reveal the mechanisms which limit the device, and many details of the electronic structure of the QD-IBSC remain unknown. In this work, we report our adaptation of transient absorption methods to investigate the dynamics of the carrier populations associated with the below-gap QD states in a QD-IBSC, with the inten
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