Carrier Dynamics in AlxGa1-xAs/InAs-Based Photon Up-Conversion Solar Cells with a Doubled-Heterointerface

The concept of two-step photon up-conversion solar cells was proposed aiming to increase the conversion efficiency by the enhancement of the intraband excitation caused by below-bandgap photon absorption. The solar cells mainly consist of two different bandgap semiconductors, namely
$\text{Al}_{0.3}\text{Ga}_{0.7}\text{As}$
and GaAs, which create a heterointerface where InAs quantum dots exist. The experimental results indicated that the solar cells enhance photocurrent with additional below-bandgap photoirradiation without degradation of photovoltage. The solar cells, however, showed no photocurrent enhancement when the direct excitation (interband excitation) photon energy is higher than
$\text{Al}_{0.3}\text{Ga}_{0.7}$
As bandgap. In this work, we further developed the solar cells aiming to enhance the intraband transition by adding a larger bandgap semiconductor of
$\text{Al}_{0.7}\text{Ga}_{0.3}\text{As}$
forming two heterointerfaces, specifically the heterointerface at
$\text{Al}_{0.7}\text{Ga}_{0.3}\text{As}$
/
$\text{Al}_{0.3}\text{Ga}_{0.7}\text{As}$
and the heterointerface at
$\text{Al}_{0.3}\text{Ga}_{0.7}\text{As}$
/GaAs. The JV characteristics of our solar cells exhibit clear enhancement of short-circuit photocurrent with slight increases in photovoltage. The photocurrent enhancement detected under 484-nm interband excitation proves that the intraband transition occurs at the heterointerface of direct and indirect semiconductors because the 484-nm photons are almost absorbed by
$\text{Al}_{0.7}\text{Ga}_{0.3}$
As and
$\text{Al}_{0.3}\text{Ga}_{0.7}\text{As}$
. The enhancement detected under 784-nm interband excitation suggests that photogenerated electrons in GaAs absorb two infrared photons in order to overcome the heterointerfaces. This was proved by intraband excitation power-dependent gained photocurrent data. A notable superlinear feature was observed at a small 784-nm intraband excitation power density. The superlinear characteristics suggest that a photogenerated electron provided by GaAs absorbs more than one infrared photon. Therefore, we conclude that the three-step photoexcitation was achieved in our solar cells including one interband excitation and two intraband excitations. This might open the door to further development of photon up-conversion solar cells.