Edge-Modified Phosphorene Antidot Nanoflakes and Their van der

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C: Energy Conversion and Storage; Energy and Charge Transport

Edge Modified Phosphorene Antidot Nanoflakes and Their van der Waals Heterojunctions for Solar Cell Applications Moumita Kar, Ritabrata Sarkar, Sougata Pal, and Pranab Sarkar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05307 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 8, 2019

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The Journal of Physical Chemistry

Edge Modified Phosphorene Antidot Nanoflakes and their van der Waals Heterojunctions for Solar Cell Applications Moumita Kar,† Ritabrata Sarkar,‡ Sougata Pal,‡ and Pranab Sarkar∗,† Department of Chemistry, Visva-Bharati University, Santiniketan- 731235, India, and Department of Chemistry, University of Gour Banga, Malda- 732103, India E-mail: [email protected]

Abstract Using the density functional method, we demonstrate that edge modified phosphorene antidot nanoflakes (PANFs) and their van der Waals heterojunctions are the new entity for novel light-electricity conversion. We have herein studied H, OH and CN edge passivated PANFs (PANF− H, PANF− OH and PANF− CN) of different lengths and pore sizes. Our study reveals that irrespective of the length and pore size, PANF− OH and PANF− CN show localized HOMO and LUMO charge densities in nanoscale. This localization causes spatial charge separation, i.e., well-defined electron-hole puddle and thus has promise to be used in solar cell. The edge decoration effect of PANFs offers a versatile route to design PANF− H/PANF− CN and PANF− H/PANF− OH type-II staggered heterojunctions where hole and electron charge carriers are localized on distinct donor and acceptor PANFs, respectively and thus shows spatial charge separation which reduce the electron-hole recombination and in turn, prolongs carrier lifetime. Our results predict that the edge modified PANFs based heterojunctions show ∗ To

whom correspondence should be addressed University ‡ Gour Banga University † Visva-Bharati

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photoconversion efficiency of upto 4.6%, making it competitive with other two dimensional heterojunction based solar cells.

1. INTRODUCTION Concerning about the global warming and high price of fossil fuels, the developement of environmentally clean alternative energy resources have to be explored to meet our urgent demand. One of the promising approaches as an alternative resource is solar energy conversion, i.e., conversion of sunlight to electricity. To be an efficient solar cells, the system must possess desirable band gap for absorbing sunlight and high carrier mobility for enhancing the charge carrier conduction. Recently, two dimensional (2D) materials hold great promise due to their semiconducting band gap, high on/off current ratio, high carrier mobility and tunable electronic properties as well as noticeable thermal and mechanical stabilities. 1–9 But some intrinsic defect limits its application toward electronic and optoelectronic devices. As for example graphene, a layer of carbon atoms in a 2D hexagonal lattice, has high carrier mobility upto 106 cm2 /Vs but its lack of band gap restrict its application. 10,11 On the other hand, inorganic analogous of graphene, the transition metal dichalcogenides (TMDCs) such as MoS2 12–15 has a direct band gap of ∼2 eV, high on/off current ratio upto 108 but its carrier mobility of 200 cm2 /Vs is very lower than what we actually desired. Recently, phosphorus analogue of graphene (phosphorene), in which each phosphorus (P) atom is coordinated with three neighbouring P atoms in a hexagonal puckered honeycomb network, is experimentally isolated from the bulk phosphorus. 16 Phosphorene has received great attention due to the balance between semiconducting band gap and high on/off current ratio of TMDCs and high carrier mobility of graphene. For instance, phosphorene is a direct band gap semiconductor of 1.5 eV with carrier mobility of 1000 cm2 /Vs. 16–20 The band gap value of phosphorene is highly desirable for sunlight absorption. When phosphorene is deposited in nanoelectronics, it shows drain current modulation upto 105 . 17,21 Phosphorene 22 also shows extraordinary optical properties which distinguishes it from other 2D materials such as graphene 23 and MoS2 . 24 So, phosphorene

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exhibits remarkable electronic and optical properties which are much more superior as compared to graphene and MoS2 . Therefore, phosphorene is promising 2D material for excitonic solar cell applications. Nanostructures arisen from reducing the dimensionality from phosphorene to nanoribbons and quantum dots exhibits tunable electronic properties due to the quantum confinement effect. 25 Again one may achieve tunability by varying the size, shape and edge passivating atoms or groups of the nanostructures. 26–28 They have shown noticeable thermal, 29 mechanical 30 stabilities and intriguing electronic, optical properties 31–34 as well as usages in various fields such as solar energy conversion, 35–38 photothermal cancer therapy, 39 thermoelectric devices, 40 gas sensing, 41–44 Li-ion battery, 45,46 spintronics, 47 electrocatalytic 48 and photocatalytic 49 applications. Recently, phosphorene based heterojunctions such as MoS2 /phosphorene, 50,51 phosphorene/TiO2 52 and phosphorene/WS2 53 are well studied from both experimental and theoretical point of view for solar cell applications. Heterojunctions consisting of two different types of 2D materials as donor and acceptor are found to be highly efficient as solar cells. 54–59 Two different types of material considering as donor and acceptor in heterojunctions form type-II staggered band alignment where valence band and conduction band are localized on two independent materials and thus shows spatial separation of electron and hole charge carriers which is required for efficient light-electricity interconversion. Recently, Hu et al. 60 have shown that the phosphorene nanoflakes based heterojunctions show very high power conversion efficiency, making it special among other 2D heterojunctions. Same group further predict that the phosphorene nanoribbons based heterojunctions are also extremely efficient for photovoltaic and photocatalytic water splitting. 49 Very recently, in 2017, Cupo et al. 61 have synthesized a new nanomaterial derived from phosphorene, phosphorene antidot lattice which is the periodic array of the pores or holes etched in the phosphorene sheet. In this paper, we have studied the dimensionality reduced form of phosphorene anitidot lattice, i.e., zero dimensional phosphorene antidot nanoflakes (PANFs). Recently, Sun et al. 39 have synthesized phosphorene nanoflakes using sonification method by breaking the phosphorene nanosheets into smaller nanoparticles. Therefore, one may expect that PANFs can

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be easily fabricated from phosphorene antidot lattices. In this article, by using density functional theory, we have studied the detailed electronic structure of edge passivating PANFs by varying the size of the pore for a constant length of PANF and by varying the length of the PANF for a constant pore size and also by varying the edge passivating atoms or groups to explore their novel applications in photovoltaics. Next, we propose a new type of heterojunction using different edge modified PANFs. As the heterojunctions are made from the PANFs with different type of edge passivation, the heterojunctions can be easily synthesized without no lattice mismatch. By studying detailed electronic structure of heterojunctions as a function of nanoflake length, pore size as well as edge passivating atoms or groups of constituent nanoflakes, we search for a better heterojunction systems which may open up new possibilities for designing efficient solar energy harvesting devices based on PANFs and their heterojunctions.

2. COMPUTATIONAL DETAILS In this study, we have employed self-consistent-charge density-functional tight-binding (SCCDFTB) method for the electronic structure calculations of edge modified PANFs and their heterojunctions. The SCC-DFTB method is one of the suitable method for studying the systems having large number of atoms and is discussed in detail elsewhere. 62–65 Since the synthesized phosphorene antidot lattice is rectangular with circular pore, we herein model rectangular nanoflakes with circular pore. The PANF is defined by the pore size (R) and length (L), as [L, R] (Figure 1). The L and R carry the unit of Å. We have herein chosen PANFs of different lengths and different pore sizes such as [46.9, 8.8], [46.9, 13.8], [59.9, 8.8] and [59.9, 13.8] which contain 542, 458, 962 and 876 phosphorus atoms, respectively. When phosphorene antidot lattice is cut into nanoflakes, edge passivation is required to stabilize the nanoflakes. Experimentally and theoretically, the hydrogen (H), 32,66 hydroxyl (OH), 28 amino (NH2 ), 67,68 halogen atoms (F, Cl), 28,60 pseudohalogen groups (CN, OCN) 49 are extensively used for edge passivation. Herein we passivate the PANFs with H, electron donating group such as OH and electron

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withdrawing group such as CN. The PANFs passivated by H, OH, CN are denoted by PANF− H, PANF− OH, PANF− CN, respectively. The Slater-type orbitals (STOs) as basis sets and Perdew-Burke-Ernzerhof (PBE) 69 exchange correlation energy functional are employed in our study. The geometry optimizations are performed with the conjugate gradient algorithm, until the forces became smaller than 0.0005 eV/Å. As covalent interaction gives lateral stability of the 2D materials, van der Waals interaction is sufficient to keep the stacking together. The van der Waals interaction is included through LennardJones potential 70 between each pair of atoms and the requisite parameters can be taken from the Universal Force Field (UFF). 71 All the calculations have been performed with the DFTB+ program package. 72 We have calculated the optical properties using density functional theory (DFT) as implemented in the SIESTA 73 with the PBE exchange correlation functional and double-ζ plus polarization function (DZP) basis set and norm-conservative Troullier-Martins pseudo-potentials (PP). 74 A real mesh cutoff of 250 Ry is used for all the calculations. The conjugate gradient method is used to optimize the system untill the forces become less than 0.005 eV/Å. In this context, it is to be noted that we have calculated the optical properties of edge passivated [35, 8.8]PANF and their heterojunctions. We have analyzed the optical absorption of PANFs and their heterostructures by calculating the imaginary dielectric function which is directly related to the optical absorption at a definite frequency. 75,76 For the complex dielectric function ε(ω) = ε 1 (ω) + iε 2 (ω), the imaginary part ε 2 (ω) which is obtained from the Fermi’s golden rule as follows, 77 2 2 4π 2 e2 h¯ d~k|Wi j (~k)| δ [ω − ωi j (~k)] ε 2 (ω) = 2 2 ∑ 3 m ω i∈V B, j∈CB (2π)

Z

(1)

where the integral is over all states and the transition matrix element Wi j (~k) = < ϕ j (~k)|~e . ~p | ϕi (~k) >, where ~e and ~p denote the polarization vector and the momentum operator. Here, h¯ ωi j (~k) = E j - Ei and ω represents the frequency of the electromagnetic radiation.

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3. RESULTS AND DISCUSSION 3.1. Electronic Structures of PANFs The energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of edge passivated PANFs of different pore sizes and of different nanoflake lengths are shown in Figure 2. For a constant length of the PANF, the increase in the pore radius increases the band gap and also for a constant pore radius, the increase in the length of the nanoflake decreases the band gap, arisen from the well-known quantum confinement effect. Cupo et al. have reported similar observation for phosphorene antidot lattice. 61 The band gap of PANF− OH, PANF− CN are almost equal and less than PANF− H. Although the band gap of PANF− OH and PANF− CN are almost same, the absolute HOMO and LUMO energy levels are different. The shifting of HOMO and LUMO energy of PANF− OH are upward as compared to PANF− H, whereas the shifting of HOMO and LUMO energy of PANF− CN are downward as compared to PANF− H. So, the electron donating group, OH and electron withdrawing group, CN passivation show opposite shifting of HOMO and LUMO energy levels of PANFs. A similar type of phenomenon for edge modified phosphorene nanoflakes and nanoribbons has already been observed by Hu et al. 49,60 Figure 2 shows that OH passivation shifts the HOMO and LUMO energies up respectively by about 0.22-0.35 eV and 0.02-0.06 eV from those of H passivation, whereas CN passivation shifts the HOMO and LUMO energies down respectively by about 0.68-1.06 eV and 0.93-1.34 eV from those of H passivation. Therefore, the shifting of HOMO and LUMO energy levels of PANF− CN is greater than those of PANF− OH. This difference of energy shifts is because of the stronger dipole moment of P+ -(CN)− polar covalent bond in PANF− CN than P− -(OH)+ in PANF− OH. Now we are interested to see the spatial position of the frontier orbitals in different edge modified PANFs. The calculated HOMO and LUMO charge densities of one representative nanoflake ([46.9, 13.8]PANF) for each type of passivation is shown in Figure 3 and for other systems which we have herein studied are shown in Figures S1-S3 of Supporting Information. From the figures,

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it is anticipated that irrespective of the nanoflake length and pore size, PANF− OH and PANF− CN yield localized and spatially separated charge densities, i.e., electron-hole puddle within the layers. The charge localization is caused by the electric potential fluctuation which is induced by the polar covalent bond such as P− -(OH)+ and P+ -(CN)− in PANF− OH and PANF− CN, respectively. Due to the smaller dipole moment of P− -(H)+ polar bond in PANF− H, the electric potential within the layer is not so strong to localize and therefore separate the HOMO and LUMO charge densities. The charge localization and consequently spatial charge separation in PANF− OH and PANF− CN flakes reduces the electron-hole recombination rate and causes longer lifetime of charge carriers. The researchers have shown that the charge localization and separation play a significant role in carrier lifetimes from both experimental 78,79 and theoretical 80 point of view. The photovoltaic performance depends on light absorption properties as well as spatial charge separation. From the band gap values as shown in Figure 2 we see that the PANF− OH and PANF− CN nanoflakes with larger size of nanoflake length and smaller size of pore size are more favourable for absorption visible light of longer wavelength and thus they are more promising in solar cell. It is worth to mention here that the synthesis of larger PANFs is experimentally easy task than that of smaller ones. Therefore, solar-radiation absorption and charge separation make PANF− OH and PANF− CN as indispensable candidate in solar cells, making competitive with edge modified phosphorene nanoflakes. 60

3.2. Electronic Structures of Heterojunctions of PANFs Having studied the detailed electronic structures of the edge passivated PANFs, we explore the electronic structure of heterojunctions consisting of two different types of edge modified PANFs. The individually opposite shifting of HOMO/LUMO energy levels of PANF− CN and PANF− OH with respect to PANF− H as shown in Figure 2 allow to design PANF− H/PANF− CN and PANF− OH/PANF− H heterojunctions which may serve as a type-II donor-acceptor interface, as schematically illustrated in Figure 4. The type-II band alignment makes easy to collect electron and hole carrier at the interface and capable of generating spatial charge separation which is attributed to the longer lifetime 7



when it is used in solar cells. 58,81,82 The optimized structures for one representative PANF− H/PANF− CN and PANF− OH/PANF− H heterostructures are shown in Figure 5. In optimized structure, the top layer is shifted from the bottom layer by half of the unit cell of the phosphorene that means the edge of the puckered hexagon of top layer is on the center of the puckered hexagon of the bottom layer. The interlayer distance in our different optimized heterobilayers is about 4 Å (see Table 1). It is important to recognize here that, since both the donor and acceptor constituents of heterojunctions are derived from phosphorene antidot with different type of edge passivation, the edge passivated PANFs based heterojunctions can be easily fabricated without worry about the lattice-mismatch. The heterostructures show lower band gap values than those of the corresponding constituent PANFs through band alignment process of type-II system (Table S1 of Supporting Information). As for example, taking [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− CN and [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− OH heterobilayers, we see that they exhibit band gap values 2.22 and 2.64 eV, respectively. These values are smaller than corresponding constituent PANF monolayers (3.27, 2.97 and 2.99 eV for [46.9, 13.8]PANF− H, [46.9, 13.8]PANF− OH and [46.9, 13.8]PANF− CN, respectively). The reduced band gap in heterojunctions increase the range of absorption energies and makes it easier to absorb visible light of longer wavelength for achieving greater light harvesting efficiency. Furthermore, the interlayer interaction 83,84 and charge transfer 85 in heterojunctions may lead to new optical transition due to the overlap of electronic states of two layers and may enhance the optical absorption of heterojunctions. In order to examine this presumption about optical activity, the imaginary part of dielectric function of heterostructure and the each individual PANFs are shown in Figure 6. Figure shows that both PANF− H/PANF− OH and PANF− H/PANF− CN heterojunctions show wider absorption range and enhanced optical absorption as compared to each constituent PANFs. The type-II band alignment of PANF− OH/PANF− H and PANF− H/PANF− CN heterojunctions is clear from the HOMO and LUMO charge densities as shown in Figure 7 and Figures S4-S6 of Supporting Information. Figures show that the HOMO and LUMO of heterobilayers

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are localized on two different constituent monolayers. In particular, for PANF− OH/PANF− H heterojunctions, HOMO is localized on PANF− OH layer and LUMO is localized on PANF− H layer and on the other hand, for PANF− H/PANF− CN heterojunctions, HOMO is localized on PANF− H layer and LUMO is localized on PANF− CN layer. This type of localization is also reflected from projected density of states (PDOS) of heterobilayers, separating the contributions of two constituent PANF monolayers as shown in Figure 8 and Figure S7 of Supporting Information. Figures show that the HOMO and LUMO states of heterobilayers are contributed by two different monolayers. Another interesting result from the HOMO and LUMO charge densities plot of different heterobilayers as shown in Figure 7 and Figures S4-S6 of Supporting Information is that the localization of HOMO and LUMO on two different monolayers of heterobilayers are not just one above the other, rather they are spatially separated at large distance. So, the localization of HOMO and LUMO on distinct donor and acceptor PANF, respectively indicates spatial charge separation can be effectively achieved at the heterojunction interface. It causes drastic reduction of electronhole recombination and prolongs carrier lifetime which makes them potential candidate in solar cell applications. From a practical perspective, it is worth to mention here that, to achieve spatial charge separation at the interface, the photoexcited electron and hole charge carriers should must overcome the coulomb interaction which is defined by exciton binding energy. Choi et al. 86 have reported the exciton binding energy of phosphorene monolayer is about 0.5 eV. Although the exciton binding energy is quite large, it is much smaller than other popular 2D material such as graphene, silicon carbide, boron nitride and transition metal dichalcogenides. 86 However, phosphorene bilayer and phosphorene nanoribbons based heterobilayers show exciton binding energy of about 0.22 eV. 49 The interlayer interaction energy might play an important role to reduce the exciton binding energy. Furthermore, the additional substrate interfacing with phosphorene reduces the exciton binding energy significantly due to the surrounding screening effect. It shows 0.14 eV for phosphorene and 0.11 eV for phosphorene bilayer. 49 The additional substrate interfacing not only reduce the exciton binding energy but also protect phosphorene from aerial degradation. Moreover, the energy

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offsets between donor and acceptor LUMO and donor and acceptor HOMO facilitate to overcome the coulomb interaction between electron and hole pairs. 87 From the large energy offsets of our proposed PANF− H/PANF− CN and PANF− OH/PANF− H heterojunctions (Figure 2), we expect that the exciton will be dissociated into free electron and hole charge carriers at the heterojunction interface. The large density of hole and electron acceptor states in distinctive donor and acceptor PANFs (Figure 8 and Figure S7 of Supporting Information) facilitate the hole and electron transfer processes, respectively between donor and acceptor monolayers. In addition to electron transfer, hole transfer dynamics plays an important role in heterostructures solar cells. 88,89 The electron and hole transfer processes in opposite directions are likely to result in LUMO and HOMO states, respectively in heterobilayer. Therefore, we may expect that, after photoexcitation, dissociation and charge transfer processes, the electron and hole charge carriers are finally localized on LUMO and HOMO of heterojunctions, respectively. It is important to calculate the photoconversion efficiency (PCE) of the type-II heterostructures which are studied in this work. The maximum PCE is estimated using the following formula: 19,60,90 η=

βFF Voc Jsc = Psolar

0.65(Egd − 4Ec − 0.3) R∞ 0

R ∞ P(¯hω)d(¯hω) Egd

P(¯hω)d(¯hω)

h¯ ω

(2)

where Jsc , Voc , βFF , and Psolar are the short circuit current, the open-circuit voltage, the fill factor and the incident solar radiation, respectively. Here, 0.65 is the fill-factor, 60,90 Edg (in eV) is the band gap of donor, 4Ec (in eV) is energy difference of donor and acceptor LUMO and 0.3 eV is an empirical factor for energy conversion kinetics. P(¯hω) 91 (expressed in Wm−2 eV−1 ) is the solar energy flux at photon energy h¯ ω. The (Edg -4Ec - 0.3) term is the open circuit voltage and the integral in the numerator is calculated using a limit of external quantum efficiency (EQE) of 100%. The Psolar in the denominator is the integrated AM 1.5 solar energy flux, which amounts to 1000 Wm−2 . 60,90 Note that the PCE value depends on Edg and 4Ec . The donor band gap, conduction band offsets and the corresponding calculated PCE values for different heterojunctions are shown in Table 2. Our calculation shows that the edge modified phosphorene antidot nanoflakes based heterojunctions show maximum PCE of upto 4.6%. This maximum PCE value is comparable to 10


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the previously reported system such as MoS2 /Si (5.23%), 92 phosphorene/WSe2 (4%), 93 GaSe/InS (4.85%), 94 phosphorene/WS2 (6%) 93 heterojunction solar cells.

4. CONCLUSION In summary, using the self-consistent charge density functional tight binding method, we shed some light on electronic structure of H, OH and CN edge passivated phosphorene antidot nanoflakes (PANFs) and their van der Waals heterojunctions to explore their applicability in solar energy conversion devices. Our study reveals that irrespective of the nanoflake length and pore size, PANF− OH and PANF− CN exhibit localized HOMO and LUMO charge densities. The localized charge density causes charge separation, i.e., electron-hole puddle in the nanoflakes. It reduces the electron-hole recombination and thereby has promise to be used them in solar cell applications. On the other hand, the strong edge decoration effect of PANFs has promise to design type-II donoracceptor interface such as PANF− H/PANF− CN and PANF− OH/PANF− H heterojunctions where hole and electron charge carriers are localized on donor and acceptor region, respectively and thus shows spatial charge separation. This spatial charge separation reduce the electron-hole recombination effectively and prolongs the carrier lifetime, thereby leading their optimal performance in solar energy conversion process. Moreover, the heterostructures result wider absorption range and enhanced optical absorption as compared to constituent PANFs. Our proposed phosphorene antidot nanoflakes based heterojunction solar cells exhibit maximum photoconversion efficiency of upto 4.6% which makes it competitive with other previously reported heterojunction solar cells. Besides photovoltaic applications, edge modified PANFs and their heterojunctions envisage a new opportunity to manipulate optoelectronic devices such as light emitting diodes (LEDs) with tunable band gap simply by varying the size of the nanopore or varying the length of the nanoflakes resulting from quantum confinement effect and also by varying the edge passivating atoms or groups arising from edge dipole effect. We hope that our theoretical findings will motivate the experimentalists to fabricate fascinating edge modified PANFs and their heterostructures for wide-range applications

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including photovoltaics and optoelectronics.

Supporting Information Available HOMO and LUMO charge densities of PANF− H, PANF− CN, PANF− OH nanoflakes of different length and pore size; HOMO and LUMO charge densities and projected density of states (PDOS) of PANF− H/PANF− CN and PANF− H/PANF− OH heterojunctions depending on nanoflake length and pore size; The calculated band gap of different heterostructures. This material is available free of charge via the Internet at http://pubs.acs.org/.

ACKNOWLEDGMENTS The authors would like to thank the DST NanoMission, New Delhi, for financial support through research grant [Ref. No. SR/NM/NS-1005/2016]. Moumita Kar is grateful to CSIR, New Delhi for the award of Senior Research Fellowship (SRF) [CSIR Sanction No. 09/202(0056)/2016EMR-I]. Ritabrata Sarkar is thankful to CSIR for SRF.

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AM1.5G

spectrum

was

taken

from

the

NREL

website:

http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.html. (92) Tsai, M.-L.; Su, S.-H.; Chang, J.-K.; Tsai, D.-S.; Chen, C.-H.; Wu, C.-I.; Li, L.-J.; Chen, L.J.; He, J.-H. Monolayer MoS2 heterojunction solar cells. ACS Nano 2014, 8, 8317–8322. (93) Ganesan, V. D. S.; Linghu, J.; Zhang, C.; Feng, Y. P.; Shen, L. Heterostructures of phosphorene and transition metal dichalcogenides for excitonic solar cells: A first-principles study. App. Phys. Lett. 2016, 108, 122105. (94) Rawat, A.; Ahammed, R.; Sharma, D.; Jena, N.; Mohanta, M. K.; De Sarkar, A. Solar Energy Harvesting in Type II van der Waals Heterostructures of Semiconducting Group III Monochalcogenide Monolayers. J. Phys. Chem. C 2019, 123, 12666–12675.

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Table 1: Equilibrium interlayer distance (d0 in Å) between two PANFs in different heterostructures. Heterostructures of PANFs [46.9, 8.8]PANF− H/[46.9, 8.8]PANF− CN [46.9, 8.8]PANF− H/[46.9, 8.8]PANF− OH [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− CN [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− OH [59.9, 8.8]PANF− H/[59.9, 8.8]PANF− CN [59.9, 8.8]PANF− H/[59.9, 8.8]PANF− OH [56.9, 13.8]PANF− H/[59.9, 13.8]PANF− CN [56.9, 13.8]PANF− H/[59.9, 13.8]PANF− OH

d0 4.08 4.13 4.07 4.13 4.15 4.13 4.17 4.14

Table 2: The band gap of the donor (Edg in eV), conduction band offsets (4Ec in eV) and PCE of different heterostructures. Heterostructures of PANFs [46.9, 8.8]PANF− H/[46.9, 8.8]PANF− CN [46.9, 8.8]PANF− H/[46.9, 8.8]PANF− OH [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− CN [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− OH [59.9, 8.8]PANF− H/[59.9, 8.8]PANF− CN [59.9, 8.8]PANF− H/[59.9, 8.8]PANF− OH [56.9, 13.8]PANF− H/[59.9, 13.8]PANF− CN [56.9, 13.8]PANF− H/[59.9, 13.8]PANF− OH

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Edg 3.16 2.90 3.27 2.97 3.07 2.84 3.10 2.90

4Ec 1.09 0.06 1.34 0.05 0.93 0.06 1.01 0.02

PCE 1.4% 4.1% 1.0% 3.5% 1.9% 4.6% 1.6% 4.2%


Figure 1: Geometric structure of [46.9, 13.8]PANF. LX and LY are the lengths of PANF along armchair and zigzag directions, respectively. L is average of these two values and R is the radius of the nanopore. The orange balls denote phosphorus atoms.

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Figure 2: HOMO-LUMO energy levels and band gap of edge passivated (a) [46.9, 8.8]PANFs, (b) [46.9, 13.8]PANFs, (c) [59.9, 8.8]PANFs, (d) [59.9, 13.8]PANFs.

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Figure 3: HOMO and LUMO charge densities of (a) [46.9, 13.8]PANF− H, (b) [46.9, 13.8]PANF− OH, (c) [46.9, 13.8]PANF− CN. The red surface represent HOMO and blue surface represent LUMO. The iso-surface value is 0.005 e/Å3 . The orange, red, cyan, blue and white balls denote phosphorus, oxygen, carbon, nitrogen and hydrogen atoms, respectively.

Figure 4: Schematic illustration of type-II donor-acceptor band alignment in (a) PANF− H/PANF− CN and (b) PANF− H/PANF− OH heterojunctions. Photoexcitation (yellow arrow) and electron and hole transfer (blue arrow) processes are shown. 4Ec and 4Ev represent the conduction and valence band offsets, respectively.

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Figure 5: (a) Top and (b) side views of optimized [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− CN. (c) Top and (d) side views of [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− OH heterojunctions. The orange, red, cyan, blue and white balls denote phosphorus, oxygen, carbon, nitrogen and hydrogen atoms, respectively.

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Figure 6: Calculated imaginary part of dielectric function (a) [35, 8.8]PANF− H, [35, 8.8]PANF− CN, [35, 8.8]PANF− H/[35, 8.8]PANF− CN and (b) [35, 8.8]PANF− H, [35, 8.8]PANF− OH, [35, 8.8]PANF− H/[35, 8.8]PANF− OH systems.

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Figure 7: HOMO and LUMO charge densities of [46.9, 8.8]PANF− H/[46.9, 8.8]PANF− CN [(a) top and (b) side views] and [46.9, 8.8]PANF− H/[46.9, 8.8]PANF− OH [(c) top and (d) side views]. The red surface represent HOMO and blue surface represent LUMO. The iso-surface value is 0.005 e/Å3 . The orange, red, cyan, blue and white balls denote phosphorus, oxygen, carbon, nitrogen and hydrogen atoms, respectively.

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Figure 8: Projected density of states (PDOS) of (a) [46.9, 8.8]PANF− H/[46.9, 8.8]PANF− CN, (b) [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− CN, (c) [46.9, 8.8]PANF− H/[46.9, 8.8]PANF− OH, (d) [46.9, 13.8]PANF− H/[46.9, 13.8]PANF− OH heterojunctions. The Fermi energy is shifted to zero as indicated by dashed black line.

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Graphical TOC Entry

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Edge-Modified Phosphorene Antidot Nanoflakes and Their van der

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