Enhanced intraband absorption in two-step photon upconversion solar cells with a double-tunnel-junction structure

Two-step photon upconversion solar cells (TPU-SCs) belong to the class of solar cells that in principle can exceed the Shockley–Queisser limit for single-junction solar cells. A TPU-SC basically consists of a wide-gap and a narrow-gap semiconductor layer, where intraband absorption of infrared (IR) photons by electrons accumulated at the conduction-band-edge discontinuity leads to an increase in the output current and voltage. This IR-induced upconversion process enables a better utilization of the broad solar spectrum, and quantum dots (QDs) can be added to improve the intraband transition rates in actual devices. In this study, we added an n–p–n double-tunnel-junction structure to an $\mathrm{A}{\mathrm{l}}_{0.3}\mathrm{G}{\mathrm{a}}_{0.7}\mathrm{A}\mathrm{s}$/GaAs TPU-SC including a QD layer to further enhance the contribution of intraband absorption to the output current. The double-tunnel-junction structure should suppress carrier recombination at the heterointerface and improve the extraction efficiency of electrons after intraband absorption. We performed two-color excitation experiments using IR and 784-nm light, and we particularly found that 784-nm light causes a significant amount of intraband transitions in this device. By employing rate equations, we clarify that this result demonstrates a higher intraband absorption efficiency due to the double-tunnel-junction structure. This highlights the potential of double-tunnel-junction structures for the realization of high-efficiency TPU-SCs.