Abstract

The Bulk photovoltaic effect (BPVE) has long been believed to hold enormous potential for the designing of the next-generation solar cell components of ultra-high light energy conversion efficiency. Different from the conventional photovoltaic effect, BPVE does not depend on the internally built electric field created by the PN-junction of two semiconductors. With its nature being non-linear response to light excitation, theoretical study of BPVE falls into a special case of second-harmonic generation (SHG) where the alternating external field outputs a DC current response. The lack of centrosymmetry lies at the center of the onset mechanism of BPVE . Here I will focus on one type of BPVE current called shift current. Two types of approaches of studying shift current were applied to moiré twisted multilayer graphene layers and Janus transition-metal dichalcogenides, respectively. The moiré systems are shown to have a special moiré amplification effect where the amplitude of shift current response is expected to be inversely proportional to the square of the twist angle. The response to light is limited to infrared regime, with giant and various responses to far infrared light and stable large response to mid-infrared excitations. In the Janus TMD system, steady shift current response to vis-UV light was confirmed via TDDFT /MPBT calculations. Through comparative studying by different treatment of interactions in the system, it is shown that interaction and correlation have substantial influences on the shift current response, where we remark particularly the importance of excitonic states.