Photogenerated current by two-step photon excitation in ZnTeO Intermediate Band Solar Cells

Tooru Tanaka1,2, Masaki Miyabara1, Yasuhiro Nagao1, Katsuhiko Saito1, Qixin Guo1, Mitsuhiro Nishio1, Kin M Yu3, Wladek Walukiewicz3 1Department of Electrical and Electronic Engineering, Saga University, Saga, Japan /2PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Japan /3Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, United States

Highly mismatched ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC) because a narrow, O-derived intermediate band IB (E-) is formed well below the conduction band CB (E+) edge of the ZnTe. Recently, we demonstrated the growth of ZnTeO layers by molecular beam epitaxy (MBE) and showed both E- and E+ transitions with different O content in the layers are in excellent agreement with the band anti-crossing model. For an operational IBSC, it is essential to show the evidence of photocurrent induced by two-step photon excitation (TPE) via the IB. Here, we present results of systematic experimental studies providing an evidence for the production of photocurrent by TPE process via IB in ZnTeO-based IBSC structure. Two types of the devices, with and without a blocking layer for the IB, were prepared. The device with a blocked IB exhibited small external quantum efficiency (EQE) in photon energy range in which electron transitions from valence band VB to IB take place, implying the electron accumulation in IB. The presence of photogenerated current by the TPE process was confirmed by measuring the EQE curves with a simultaneous irradiation of infrared light. The enhancement of EQE was observed as a result of electron transition from VB to conduction band via IB. The ratio of the enhancement of EQE to the EQE without IR illumination at 2 eV, which implies the ratios of optical transition rate from IB to CB with respect to the direct escape rate, were as high as ~1. The high optical transition rate from IB to CB clearly indicates the high potential of this material for the application of IBSC.