Novel Excitonic Solar Cells in Phosphorene–TiO2 Heterostructures

Letter pubs.acs.org/JPCL

Novel Excitonic Solar Cells in Phosphorene−TiO2 Heterostructures with Extraordinary Charge Separation Efficiency Liujiang Zhou,*,†,¶ Jin Zhang,‡,¶ Zhiwen Zhuo,§ Liangzhi Kou,∥ Wei Ma,‡ Bin Shao,† Aijun Du,∥ Sheng Meng,*,‡,⊥ and Thomas Frauenheim† †

Bremen Center for Computational Materials Science, University of Bremen, Am Falturm 1, 28359 Bremen, Germany Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China § Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China ∥ School of Chemistry, Physics and Mechanical Engineering Faculty, Queensland University of Technology, Garden Point Campus, Brisbane, Queensland 4001, Australia ⊥ Collaborative Innovation Center of Quantum Matter, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: Constructing van der Waals heterostructures is an efficient approach to modulate the electronic structure, to advance the charge separation efficiency, and thus to optimize the optoelectronic property. Here, we theoretically investigated the phosphorene interfaced with TiO2(110) surface (1L-BP/TiO2) with a type-II band alignment, showing enhanced photoactivity. The 1L-BP/TiO2 excitonic solar cell (XSC) based on the 1L-BP/TiO2 exhibits large built-in potential and high power conversion efficiency (PCE), dozens of times higher than conventional solar cells, comparable to MoS2/WS2 XSC. The nonadiabatic molecular dynamics simulation shows the ultrafast electron transfer time of 6.1 fs, and slow electron−hole recombination of 0.58 ps, yielding >98% internal quantum efficiency for charge separation, further guaranteeing the practical PCE. Moreover, doping in phosphorene has a tunability on built-in potential, charge transfer, light absorbance, as well as electron dynamics, which greatly helps to optimize the optoelectronic efficiency of a XSC. mobility (>1000 cm2/V·s), a direct band gap of ∼2.0 eV,11,12 that significantly remedy graphene’s shortcoming in optoelectronic devices. More importantly, the electronic and optoelectronic properties can be modulated through a variety of ways, such as doping,13−15 point and line defects,16 layer-by-layer17 or heterostructures mapping,18,19 chemical adsorption,20 strain,21 and external electric field,22 further broadening its widespread applications in optoelectronic devices, such as light-emitting diodes, field effect transistors, and solar cells.12,23,24 Using a van der Waals (vdW) heterostructure, formed by stacking 2D atomic monolayers, is one of the best approaches

G

raphene, a two-dimensional, one-atom-thick honeycomblike layered material, possesses intriguing electronic and mechanical properties and is highly desired in the field of next generation of faster and smaller electronic devices.1,2 However, the feature of zero band gap makes it unsuitable for the controlled and reliable transistor operation and consequently limits its widespread applications in optoelectronic devices.3 Therefore, it is highly desired to open an energy gap in graphene. Subsequently, a whole new class of 2D materials has been studied and prepared in experiment, such as the transition metal dichalcogenides, 4 − 7 transition-metal carbides (MXenes)8,9 or halides,10 monolayer black phosphorus (1LBP) (termed phosphorene),11 and so forth. Among them, phosphorene has demonstrated extraordinary properties, including its highly anisotropic effective masses, high electron © XXXX American Chemical Society

Received: February 29, 2016 Accepted: May 4, 2016

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DOI: 10.1021/acs.jpclett.6b00475 J. Phys. Chem. Lett. 2016, 7, 1880−1887

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The Journal of Physical Chemistry Letters

Figure 1. (a) Schematic drawing of the interfaces between phosphorene and TiO2(110) surface. The optimized interface between 1L-BP and TiO2(110) surface (denoted as 1L-BP/TiO2): (b) top view; (c) side view. Gray, P; red, O; skyblue, Ti. The distance between 1L-BP and TiO2(110) in (b) is inserted. (d) Charge density difference for a 1L-BP/TiO2(110) based on PBE level. The yellow region represents charge accumulation, and the cyan region indicates charge depletion; the isosurface value is 0.0004 e/Å3. (e) Density of states (DOS) for 1L-BP/TiO2 interface. The Fermi level is set to zero.

especially interfaced with TiO2 crystals, on photovoltaic application are still unexplored. Because of the low level of valence band maximum (VBM) of TiO2, phosphorene interfaced with TiO2 (1L-BP@TiO2) may form an efficient XSC where phosphorene is used as electron donor and TiO2 as electron acceptor. By utilizing the large-scale density functional theory and GW + Bethe−Salpeter equation (BSE) calculations, we first systemically investigate the heterostructure consisting of phosphorene monolayer interfaced with TiO2(110) surface (1L-BP/TiO2) (Figure 1a). In our models, we fix the lattice constants of TiO2 substrate and change the lattice constant of phosphorene (a = 3.321 Å, b = 4.583 Å) to adjust to those of TiO2. The lattice mismatch is only about 2.5%, producing a very slight modulation of light absorption, which suggests the negligible impact on the main conclusions. 1L-BP/TiO2 shows a type-II band alignment, and large built-in potential for carrier separation. The constructed XSC based on 1L-BP/TiO2 possesses a PCE of about 1.5% and ultrahigh power density of ∼16.7 MW/L at the atomical level, dozens of times higher than conventional GaAs (290 kW/L) and Si (5.9 kW/L) solar cells, and comparable with MoS2/WS2 XSC. Nonadiabatic molecular dynamics simulation provides important insights into the charge-separation and electron−hole recombination processes, showing an ultrafast electron transfer time, and slow electron−hole recombination time, further guaranteeing the practical PCE. Moreover, doping in phosphorene layer has a tunability on built-in potential, charge transfer, light absorbance, as well as electron dynamics. Figure 1b and c show the optimized 1L-BP/TiO 2 heterostructure at the atomical level. The interaction between phosphorene and the TiO2 surface determines the lightabsorption, charger transfer, band alignment and electron transfer, as well as electron−hole recombination process. Phosphorene layer almost maintains its quadrangular pyramid as isolated one, indicating that the electron system keeps intact. The equilibrium distance between the phosphorene and the top of the TiO2(110) surface is calculated to be 2.75 Å, slightly

to protect the active layers against environmental contamination without affecting their electrical performance or to modulate the band offsets at the interfaces so as to provide a highly effective approach for the manipulation of charge carriers.18,25−27 Such vdW heterostructures can be widely used to enhance the electron−holes separation when used as an excitonic solar cells (XSC). In the XSC, power conversion efficiency (PCE) depends critically on the interface band alignment between donor and acceptor materials. The type-II band alignment between interfaces is a prerequisite to achieve the efficient electron−hole pairs (excitons) separation, which has been achieved in various 2D layered transition metal dichalcogenides-,28 carbon-, or silicon-based heterostructures3 and so forth. Although such phosphorene-based heterostructures are widely reported in experiment and theory, type-II band alignment heterostructures are extremely scarce, only being BP/MoS2,19,29,30 BP/GaAs31 as well as black−red phosphorus heterostructures.32 Therefore, desirable type-II phosphorene-based heterostructures preferably with a large built-in potential for driving electrons and holes are still lacking and deserve to be explored in experiment and theory. TiO2 normally possesses a wide band gap and excellent electrical conductance and has been widely used as a promising photocatalysis and photovoltaic materials33−35 due to the fact that the generated photoexcited electrons can either be readily channeled to create electricity directly in solar cells or be used to drive water splitting for hydrogen production. However, the large energy gap of pristine TiO2 (3.0 eV for rutile and 3.2 eV for anatase) limits its actual efficiency to generate photoexcited electrons and the subsequential dissociation of excitons. With these above factors in mind, interfacing wide-band gap TiO2 with phosphorene may offer a straightforward approach to overcome these shortcomings in light absorption, so as to increase the light absorption efficiency via harvesting solar spectrum in a wider frequency range and also to enhance the exciton separation efficiency. Layered BP interfaced with TiO2 nanoparticles with a type-II band alignment shows novel photocatalytic performances over traditional graphene−TiO2 hybrid.36,37 However, studies of phosphorene−TiO2 hybrids, 1881

DOI: 10.1021/acs.jpclett.6b00475 J. Phys. Chem. Lett. 2016, 7, 1880−1887

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Figure 2. (a) Schematic type-II Band alignment at the 1L-BP/TiO2(110) interface. (ΔEV) and ΔEC are referred to Valence and conduction band offsets. (b) Absorbance of three heterostructures based on PBE level, 1L-BP/TiO2, Al- and Cl@1L-BP/TiO2, as well as TiO2(110) surface, overlapped to the incident AM1.5G solar flux. (c) Variation of VBM and CBM with respect to the vacuum level for phosphorene layers from G0W0 (red) and G0W0 plus BSE (GW+BSE) (“o”) calculations, as well as for the TiO2 structures from HSE06. Gray lines: experimental VBM and CBM energies. (d) VB offsets (ΔEV) and CB offsets (ΔEC) based on GW+BSE and DFT calculations.

remaining the same in that donor−acceptor coupling is not strong enough in the absence of covalent bonding. Large phosphorene states are localized within the TiO2(110) gap, indicating the dominant electron transition is from O−2p states at the valence band (VB) to Ti-3d states at the CB under ultraviolet (UV) irradiation. More importantly, the electron can be excited from the VB of phosphorene to its CB under visible light irradiation, and then, this photogenerated electron is injected into the CB of TiO2. The 1L-BP/TiO2 interface has an effective band gap of 0.26 eV and shows the type-II band alignment with a built-in potential (CBM offsets, denoted as ΔEC) of about 0.25 eV (Figure 2a). The large CBM (0.25 eV) and VBM (1.8 eV) offsets (ΔEV) can drive electrons to migration from phosphorene CBM to TiO2 CBM, and hole transfer between VBMs, indicating the possible highly efficient charge separation. These features, including type-II band alignment, large built-in potentials, enable phosphorene/TiO2 to hold the potential capability of forming highly effective XSC. The charge-transfer hybrid is also expected to mediate photocatalytic activities under visible light. To confirm this effect, the independent particle absorbance spectra of phosphorene/TiO2 and TiO2(110) substrate are calculated with the random phase approximation (DFT-RPA) due to the large supercell, as shown in Figure 2b. For pure TiO2, the optical absorption due to intrinsic transition from the O−2p to Ti−3d states mainly occurs in the visible light and UV region with an energy larger than 2.5 eV. Upon formation of an interface, the optical absorption edges extend to the low-energy range, displaying the obvious enhanced photoactivity under the visible and near-infrared light irradiation. In high-energy region (>2.5 eV), 1L-BP/TiO2 interface also displays better light absorption performance than that of TiO2 substrate. Thus, the enhanced light absorption in the whole region ( 1L-BP/TiO2 > Cl@1L-BP/TiO2. When considering the excitonic effect, as shown in Figure 3c−f, the first prominent peaks are located at 1.62 and 1.62 eV for Al@ 1L-BP and Cl@1L-BP, corresponding to two bright excitions with Eb of 0.82 and 0.85 eV, respectively, slightly larger than that in pure phosphorene (0.75 eV). What’s more, it is noteworthy that the Eb in pure and doped phosphorene layers show a linear scaling as a function of the quasiparticle (QP) gap (equivalent to G0W0 gap) (Figure 3g), in accordance with previous findings in pure or chemical fictionalized or strained 2D materials,46 further extending the applicability of the scaling relationship into doped 2D materials. The Jabs of Al@1L-BP along two directions are larger than that in pure phosphorene, whereas the Jabs of Cl@1L-BP has the oppositive effect, showing hole doping can enlarge the Jabs, which helps to optimize the light absorption efficiency of phosphorene via doping. The photoinduced electron injection, relaxation, and electron−hole recombination of the hybrid system affect the exciton lifetime, and in turn, solar cell current and performance. Previous studies mainly focus on dye sensitized solar cell (DSSC), in which photoexcited electron transfer to TiO2 plays a crucial role in the performance of the DSSC.52−55 The trajectory surface hopping method,56−58 one of the most popular approaches capable of capturing the essential physics and simultaneously remaining computationally less expensive, includes electron−nuclear correlations and successfully depicts the rates of quantum transitions upward and downward in energy,54 which has been widely applied to the simulation of the light-induced nonequilibrium process in various photovoltaic systems, including graphene−TiO2,59 MoS2/MoSe2,60 organic−inorganic hybrid photovoltaic heterojunctions.61 Using trajectory surface hopping methods implemented within the time-dependent Kohn−Sham theory (see the details in Supporting Information),54,62 the simulation of photoinduced electron transfer (ET) dynamics within interfaces was calculated and presented in Figure 4a. Within the 1L-BP/ TiO2 interface, about 52% of the photoexcited states are localized on TiO2 after photoexcitation. The ET process is dominated by the adiabatic process with a small portion of nonadiabatic process. By the exponential fitting, the photoinduced ET from phosphorene into the TiO2 surface occurs on a 6.1 fs time scale, which is on the same order of magnitude as conventional DSSC,52−55 and much shorter than the electron injection time (τinj) (about 160 fs) in vdW’s graphene/TiO2 interface.33,59 Such ultrafast ET efficiency would be significantly beneficial for the practical application in XSC. The electron−hole recombination at the 1L-BP/TiO2 interface occurs by a nonradiative transition of the photoexcited electron from the CB of TiO2 to VBM of phosphorene layers. The electron−hole recombination dynamics of injected electrons are shown in Figure 4b. In the beginning, almost all electrons are localized on TiO2 substrate and then transfer back to the VBM of phosphorene. The electron−hole recombination can take place in a recombination time (τrec) of 0.58 ps, which is much slower than that of τinj. The large τrec can reduce the energy losses, which is very beneficial for maintaining the photocurrent and finally achieving the high PCE in phosphorene/TiO2 XSC. On the basis of above results, the internal quantum efficiency (IQE, namely the fraction of absorbed photons extracted as carriers within interface) was 1884

DOI: 10.1021/acs.jpclett.6b00475 J. Phys. Chem. Lett. 2016, 7, 1880−1887

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manner65,66 could be carried out to maximize the interface area, which may improve the PCE by a factor of dozens of times compared to that in GaAs (Table 2). It is noteworthy that after doping in phosphorene, although the fluctuations of the efficiencies of electron injection and electron−hole recombination are present (Figures S4 and 4b), the IQEs also still maintain the ultrahigh value of 98.9 and 99.3% for Al@ and Cl@1L-BP/TiO2, respectively, almost the same to intact 1LBP/TiO2 interface. The evaluated PCEs are 1.71 and 1.22% and the PD are 17.1 and 12.2 MW/L for Al@ and Cl@1L-BP/ TiO2, respectively. The tunability of PCE and charge separation efficiency via doping offers a promising route to engineer the phosphorene-based XSC. In summary, we have systemically investigated the electronic and optical properties of heterostructure consisting of phosphorene interfaced with TiO2(110) surface. Such a interface shows a type-II band alignment, enhanced photoactivity and large built-in potential. The excitonic solar cell (XSC) based on phosphorene−TiO2 heterostructure is constructed, where the phosphorene is served as the donor and the TiO2(110) as the acceptor. Such a XSC shows a high power conversion efficiency (PCE) of 1.67% and ultrahigh power density, dozens of times higher than conventional GaAs solar cells, comparable to that of MoS2/WS2 XSC. The nonadiabatic molecular dynamics within the time-dependent density functional theory framework shows ultrafast electron transfer of 6.1 fs and slow electron−hole recombination of 0.58 ps, further ensuring the practical PCE. Moreover, doping in phosphorene has a well tunability on the optical band gap, excitonic binding energy, light absorbance, and exciton dynamics as well as the final PCE in a XSC. These features, including the tunable optical band gap, type-II interface band alignment, high optical absorbance, and large PCE, enable phosphorene-based heterostructures to be promising for nextgeneration flexible optoelectronic devices. The results presented here may stimulate further efforts on the rational design of future solar cell devices based on the combinations of 2D materials and 3D wide band gap semiconductors.

Figure 4. (a) Electron transfer dynamics of photoexcited states phosphorene. The solid black, red and blue lines represent the total adiabatic and nonadaiabatic electron transfer. The total, adiabatic (AD), and nonadiabatic (NA) ET are indicated by the black solid, red solid, and blue solid lines, respectively. The dashed lines show the exponential fits of each lines. (b) Electron−hole recombination dynamics from phosphorene CBs to TiO2 VBM. Intact, Al-doped, and Cl-doped interfaces are indicated in solid black, blue, red lines, respectively. The dashed lines are fitted by linear functions.

estimated by using τrec as the upper limit of carrier lifetimes based on the ratio of τinj and τrec, IQE = 1 − τinj/τrec. The calculated IQE of 98.9% indicates a ultrahigh charge separation efficiency for 1L-BP/TiO2 XSC. Although the thermodynamic efficiency limit for thermal carriers in the absence of nonradiative recombination is set through the Schockley−Quisser limit,63 the practical PCE is more useful than ultimate thermodynamic limits when they come to practical implementation in XSC. We compute the PCE under AM1.5G illumination by dividing the product JSC × VOC × FF through the incident power of 100 mW/cm2.64 Using an mediate open circuit voltage VOC = 0.6 V, FF values of 0.65, the PCE values of 1.67% is achieved in 1L-BP/TiO2 XSC. Combined with the obtained PCE value as derived above, we estimate the 1L-BP/TiO2 XSC with a thickness of 1.0 nm would achieve a power density of 16.7 MW/L, higher by approximately 1−3 orders of magnitude compared to existing solar cells, such as 1 μm thick GaAs with a power density of 290 kW/L, and 35 μm thick Si of 5.9 kW/L.28 Such a PCE is comparable to MoS2/graphene and MoS2/WS2 bilayer ultrathin photovoltaic (PV) with the PCE of ∼0.1−1.5%.28 Although the PCE at atomistic level is much lower than conventional GaAs or Si PV devices with PCEs more than 20%, thicker multilayer stacking (50−100 nm thick) in a bulk heterojunction

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00475. Methods, simulation details, PDOS, and electron dynamics of Al@1L-BP/TiO2 and Cl@1L-BP/TiO2 heterostructures. (PDF)

Table 2. Optical Band Gaps of Donor Material, and Absorbed Photon Fluxa Jaabs, Flux Jzabsb, Total Jabs under AM1.5G Solar Illumination, PCE, and Power Density PD for the 1L-BP/TiO2, Al@1L-BP/TiO2, and Cl@1L-BP/TiO2c material

Eog (eV)

Jaabs

Jzabs

Jabs (mA/cm2)

phosphorene Al-doped Cl-doped Si GaAs

1.28 1.64 1.62 1.11 1.42

4.30 4.41 3.15

2.02 2.53 1.01

3.16 3.47 2.08 0.1 0.3

thickness 1 1 1 35 1

nm nm nm μm μm

PCE

PD (kW/L)

1.67% 1.71% 1.22% ∼29% 20.6%

16 700 17 100 12 200 5.9 290

a

Incident light polarized along the armchair direction. bAlong zigzag direction. cComputed using eq 1 with the absorbance values in Figure 3a−f. Jabs quantifies the flux of absorbed photons, converted to units of equivalent electrical current. The same quantities are also shown for 1 nm thick representative bulk materials in ultrathin PV, taken from the literature.28 1885

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AUTHOR INFORMATION

Corresponding Authors

*(L.Z.) E-mail: [email protected]. Tel.: +49 421 21862354. *(S.M.) E-mail: [email protected]. Tel.: +86 10 82649396. Author Contributions ¶

L.Z. and J.Z. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.Z. and B.S. acknowledge financial support from BremenTRAC−COFUND fellowships, cofinanced by the Marie Curie Program of the European Union. A.D, L.Z., and T.F. acknowledge financial support from Australian Technology Network (ATN) and Deutscher Akademischer Austausch Dienst (DAAD) German Academic Exchange Service. This work is partially financially supported by MOST (grant 2012CB921403) and NSFC (grant 11222431). We thank Gang Lu at California State University, Northridge for help with electronic dynamics calculations. The support of the Supercomputer Center of Northern Germany (HLRN Grant No. hbp00027) is also acknowledged.

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DOI: 10.1021/acs.jpclett.6b00475 J. Phys. Chem. Lett. 2016, 7, 1880−1887

Letter

The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.6b00475 J. Phys. Chem. Lett. 2016, 7, 1880−1887