Production of Photocurrent due to Intermediate-to-Conduction-Band Transitions: A Demonstration of a Key Operating Principle of the Intermediate-Band Solar Cell

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.

Figure 1

Basic operation of an intermediate-band solar cell (a) structure of the intermediate-band material showing the absorption processes involved [1]; (b) intermediate-band material sandwiched between $p$ and $n$ type semiconductors showing the relation between voltage and quasi-Fermi level separation [2].

Figure 2

Structure of the quantum dot intermediate-band solar cells used in the experiments.

Figure 3

Photocurrent produced in the QD-IBSC samples as a function of the energy of the photons of the primary light source. Curve 1 (response to primary) represents the photocurrent produced when pumping with the chopped primary source only ($V=-1.5\mathrm{V}$, $T=4.2\mathrm{K}$). Curve 2 (noise background) is the photocurrent measured when the IR source is off while the chopper, located in front the of the IR source, is kept spinning ($V=0\mathrm{V}$, $T=36\mathrm{K}$). Curve 3 (IR on) is the photo-generated current when the IR source is turned on and chopped ($V=0\mathrm{V}$, $T=36\mathrm{K}$). Device junction area is 4 mm [2].

Figure 4

Description of the experimental setup.

Figure 5

Simplified band gap diagram of the QD-IBSC in equilibrium and under 0.6 V forward bias.