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Unravelling the mechanism of photoinduced charge transfer process in bilayer heterojunction Hao Jin, Jianwei Li, Yadong Wei, Ying Dai, and Hong Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07138 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018
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ACS Applied Materials & Interfaces
Unravelling the mechanism of photoinduced charge transfer process in bilayer heterojunction Hao Jin,† Jianwei Li,† Yadong Wei,∗,† Ying Dai,‡ and Hong Guo†,¶ †Shenzhen Key Laboratory of Advanced Thin Films and Applications, College of Physics and Energy, Shenzhen University, Shenzhen 518060, People’s Republic of China ‡School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, People’s Republic of China ´ H3A ¶Centre for the Physics of Materials and Department of Physics, McGill University, Montreal 2T8, Canada E-mail:
[email protected] Abstract
way for designing of next-generation devices for light detecting and harvesting.
Charge transfer is a fundamental process that determines the performance of solar cell devices. Although great efforts have been made, the detailed mechanism of charge transfer process across the two-dimensional van der Waals (vdW) heterostructure remains elusive. Here, based on the ab initio nonadiabatic molecular dynamics (NAMD) simulation, we model the photoinduced charge transfer dynamics at the InSe/InTe vdW heterostructures. Our results show that carriers can follow either R-scheme or Z-scheme transfer path, depending on the coupling between the interlayer states at the band edge positions. In addition, the charge transfer dynamics can be effectively controlled by the external parameters, such as strains and interlayer stacking configurations. The predicated electron-hole recombination lifetime in R-scheme is up to 1.4 ns, while it is shortened to 1.2 ps in Z-scheme transfer path. The proposed R-scheme and Z-scheme are further verified by the quantum transport simulations based on the DFT method combined with the nonequilibrium Green’s functions (NEGF-DFT). The analysis reveals that the system dominated by the Z-scheme shows better performance, which can be attributed to the built-in electric field that facilitates the charge transfer. Our work may pave the
Keywords 2D heterostructure, solar cell, charge transfer dynamics, built-in electric field, type-II band alignment, Z-scheme
1 Introduction So far, solar energy is the most abundant and cleanest renewable energy source, which attracts tremendous interests. To harness solar energy, the most common strategy is through the formation of heterostructure between two dissimilar semiconductors at the interface, which is able to facilitate fast charge separation. 1,2 Compared with conventional heterostructures, particular attention has been devoted to ultrathin layered heterostructures. 3–6 These heterostructures are composed of two-dimensional (2D) semiconductors held together by van der Waals (vdW) interactions, and hold potential applications in fields such as photovoltaic cells, 3,7 photodetectors, 8,9 and light emitters. 10 As shown in Figure 1(a), many of them form type II heterostructures, where the staggered
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Z-scheme systems have been demonstrated, 17,18 there is still a lack of solid evidence that verifies the proposed scheme. Moreover, the key parameters which determine the charge transfer process remain elusive. Therefore, it is of significance to investigate the charge transfer dynamics via fundamental studies. In this work, based on the nonadiabatic molecular dynamics (NAMD) simulation, the charge transfer process at the InSe/InTe vdW heterostructures is studied in details. Our results demonstrate that either R-scheme or Z-scheme can dominate the charge transfer process at the interface. The predicated electron-hole recombination lifetime varies from 1.2 ps to 1.4 ns depending on their intrinsic electronic structures. Further analysis reveals that the charge transfer dynamics is governed by the coupling between the interlayer states at the band edge positions, which can be tuned by the strains or interlayer stacking arrangements. To better understand the detailed mechanism of photoinduced charge transfer process, we calculate the photoinduced current by DFT method combined with the nonequilibrium Green’s functions (NEGF-DFT). We find that the photocurrents under different schemes move in opposite directions, in agreement with our previous predictions. The performance of the device is then evaluated using a two-probe model. The results indicate that Zscheme system shows promising photovoltaic properties with the energy conversion efficiency η up to 2.08%, which is about one order of magnitude larger as compared with that of regular Type-II heterostructures. Based on these findings, our work provides a fundamental understanding of the charge transfer process in 2D vdW heterostructures, which paves the way for designing of novel nano-devices for optoelectronic and photovoltaic applications.
band alignment occurs at the interface. Upon illumination, the photoinduced current is generated and carriers migrate through the heterojunction region. R-Scheme
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Z-Scheme e− -
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hν h+ +
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Figure 1: Schematic diagram of the energy levels involved in the charge transfer process at the 2D InSe/InTe vdW interface. (a) R-scheme, and (b) Z-scheme charge transfer paths. The green (red) dashed line indicates interlayer coupling states of the InSe (InTe) side. It is known that charge transfer is a fundamental process which controls electron-hole recombination and hence plays a significant role in determining the performance of photovoltaic devices. 11,12 In the past several decades, great efforts have been made to understand charge transfer dynamics in 2D vdW heterostructures. 13–15 Figure 1(a) illustrates the regular scheme (R-scheme) reported in previous studies. 1,4,14 When the junction is formed, electrons flow from the conduction band with higher energy level (namely InTe) to the conduction band with lower energy level (i.e. InSe), while holes transfer from the valence band with lower energy level to the valence band with higher energy level. It is believed that the electron-hole recombination rate is low in this case. Apart from the R-scheme system, the concept of Z-scheme is proposed in recent experiments, which is originated from the natural photosynthetic process. 1,16 As shown in Figure 1(b), coupling states are present within the vdW interlayer, which can serve as the mediated states that enable carriers to migrate directly between the two semiconductors. Moreover, the photoinduced current in Z-scheme transfers in the opposite way as compared with that in R-scheme system. We have to point out that though some examples of
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Results
Indium monochalcogenides are a kind of layered semiconducting materials, which consist of four covalently bonded X-In-In-X (X=Se or Te) sublayers. 19,20 Compared to transition metal dichalcogenides (TMDs) and black phosphorus (BP), indium monochalcogenides show higher carrier mobilities and better environmental stability, 20–22
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ACS Applied Materials & Interfaces
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Figure 2: Band structures of InSe/InTe vdW heterostructures (a) with ε=-3% compressive strain, (b) without strain, and (c) with ε=3% tensile strain. The Fermi level is set to zero. which can be applied in electronic and optoelectronic applications. 19,22,23 Unlike TMDs, monolayer InX shows an indirect bandgap (see Figure S2 in Supporting Information). Nevertheless, an indirect-direct bandgap transition can be realized if the compressive strain is applied for InSe monolayer. 20 The studied model system in this work is InSe/InTe vdW heterostructure with the most stable AB (3R) stacking configuration (see Figure S1 in Supporting Information). The calculated interlayer distance between InSe and InTe monolayers is in the range of 2.93∼3.13Å depending on the applied strains, which is in consistence with previous reported vdW systems. 24 Based on the orbital characteristics, the band structures are plotted using different colors, which are defined as the ratio (W) of the states contributed by the InSe side to the total states of the heterostructure for each energy band. If W=1 (W=0), the energy band is plotted using red (blue) color, which means such band is primarily contributed by the InSe (InTe) layer. When the energy band is contributed by both sides, i.e. 0
