We present intermediate-band solar cells manufactured using quantum dot technology that show for the first time the production of photocurrent when two sub-band-gap energy photons are absorbed simultaneously. One photon produces an optical transition from the intermediate-band to the conduction band while the second pumps an electron from the valence band to the intermediate-band. The detection of this two-photon absorption process is essential to verify the principles of operation of the intermediate-band solar cell. The phenomenon is the cornerstone physical principle that ultimately allows the production of photocurrent in a solar cell by below band gap photon absorption, without degradation of its output voltage.

Intermediate band solar cells (IBSCs) fabricated to date from In(Ga)As/GaAs quantum dot arrays (QD-IBSC) exhibit a quantum efficiency (QE) that extends to below bandgap energies. However, the production of sub-bandgap photocurrent relies often on the thermal and/or tunneling escape of carriers from the QDs, which is incompatible with preservation of the output voltage. In this work, we test the effectiveness of introducing a thick GaAs spacer in addition to an InAlGaAs strain relief layer (SRL) over the QDs to reduce carrier escape. From an analysis of the QE at different temperatures, it is concluded that escape via tunneling can be completely blocked under short-circuit conditions, and that carriers confined in QDs with an InAlGaAs SRL exhibit a thermal escape activation energy over 100 meV larger than in the case of InAs QDs capped only with GaAs.

The characteristics of intermediate band solar cells containing 10, 20, and 50 InAs quantum dot (QD) layers embedded in otherwise “standard” (Al,Ga)As solar cell structures have been compared. The short-circuit current densities of the cells decreased and the quantum efficiencies of the devices showed a concomitant reduction in the minority carrier lifetime in the p emitters with increasing number of QD layers. Dislocations threading up from the QDs toward the surface of the cells, and revealed by bright field scanning transmission electron microscopy, are the most likely cause of the deterioration in the electrical performance of the cells.

The intermediate-band solar cell (IBSC) has been proposed as a device whose conversion efficiency can exceed the 40.7% limiting value of single-gap cells. It utilizes the so-called intermediate-band material, characterized by the existence of a band that splits an otherwise conventional semiconductor bandgap into two sub-bandgaps. Two important criteria for its operation are that the carrier populations in the conduction, valence, and intermediate-bands are each described by their own quasi-Fermi levels, and that photocurrent is produced when the cell is illuminated with below-bandgap-energy photons. IBSC prototypes have been manufactured from InAs quantum dot structures and analyzed by electroluminescence and quantum efficiency measurements. We present evidence to show that the two main operating principles required of the IBSC are fulfilled.

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