TRANSIENT ABSORPTION FOR CHARACTERIZATION OF INTERMEDIATE BAND SOLAR CELLS P. Kolla 1 , A. Norman 2 , and S. Smith 1 1 Nanoscience and Nanoengineering, South Dakota School of Mines and Technology Rapid City, SO 57701 2 National Renewable Energy Laboratory Golden, CO 80401 ABSTRACT We use transient absorption methods to charaterize the sequential two-photon absorption in a quantum-dot super latt . ice ba � ed intermediate band solar cell (QD-IBSC). USing collinear, orthogonally polarized beams generated f � om an Optical Parametric Oscillator (OPO) at varying time delay, tuned stepwise from 1050nm to 1250nm, we use the solar cell photocurrent as a diret measure of the transient absorption by measuring the diferential 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 obseved 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 dep � ndence 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 strutures. INTRODUCTION The intermediate band solar cell (IBSC) allows sequential absorption of low-energy photons which would otherwise not be absorbed by a single junction solar cell [1). Based on thermodynamic arguments, the IBSC has a theoretical eficiency as high as 63%, well above current state of the art �ande � cells . [2]. However, the details of realizing a device which satisfies the contingencies upon which such eficiency 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 eficiency 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 superlattice IBSC remain unknown. 978-1-4244-5892-9/10/$26.00 ©2010 IEEE 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 intention of revealing details of the intermediate band which are inaccessible to purely time-integrated methods. SAMPLES STUDIED As shown in figure 1, the QD-IBSCs used in our experiments consist of 50 periods of self-assembled InxGa 1 -xAs quantum dots grown by MOCVD under Stranski-Krastanov growth conditions, and embedded in a GaAs 1 -xPx p-i-n diode structure. These devices were developed at NREL, characterized by photoluminescence and I-V measurements, and were shown to be good working diodes with a room temperature QD luminescence peak energy around 1.04lm. This was confirmed by tuning our laser excitation near 1.05lm where strong absorption and corresponding photocurrent was obseved. Au grid bar 200 nm n+ GaAs n - 2 x 10'9 30 nm n GalnP, n - 2 X 10'9 100 nm p GaAs, P- 2 x 10 18 P+ GaAs (311)8 substrate P - 8 X 10'8 Au contact }SO x 10nm Gas•.•8SP•.• ,l 6.1 Mlln •.41Ga.... As QDs Figure 1 Representative QD-IBSC device structure used in our experiments. A 50 period quantum dot superlatice is embedded in a p-i-n diode. EXPERIMENTAL METHODS Under pulsed illumination, absorption and transmission of light is proportional to the occupation of carriers within the available density of states. Femtosecond lasers can produce intense pulses of light much shorter than the 001780