Photovoltaic Heterojunctions of Fullerenes with MoS2 and WS2

Letter pubs.acs.org/JPCL

Photovoltaic Heterojunctions of Fullerenes with MoS2 and WS2 Monolayers Li-Yong Gan, Qingyun Zhang, Yingchun Cheng,* and Udo Schwingenschlögl* PSE Division, KAUST, Thuwal 23955-6900, Kingdom of Saudi Arabia ABSTRACT: First-principles calculations are performed to explore the geometry, bonding, and electronic structures of six ultrathin photovoltaic heterostructures consisting of pristine and B- or N-doped fullerenes and MoS2 or WS2 monolayers. The fullerenes prefer to be attached with a hexagon parallel to the monolayer, where B and N favor proximity to the monolayer. The main electronic properties of the subsystems stay intact, suggesting weak interfacial interaction. Both the C60/MoS2 and C60/WS2 systems show type-II band alignments. However, the built-in potential in the former case is too small to effectively drive electron−hole separation across the interface, whereas the latter system is predicted to show good photovoltaic performance. Unfortunately, B and N doping destroys the type-II band alignment on MoS2 and preserves it only in one spin channel on WS2, which is unsuitable for excitonic solar cells. Our results suggest that the C60/WS2 system is highly promising for excitonic solar cells. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

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features similar to those of MoS2, it is feasible to fabricate both monolayer C60/MoS2 and C60/WS2 photovoltaic heterojunctions. For exploring the potential of these hybrid systems, it is vital to understand the interfacial band alignment as it determines the photovoltaic efficiency. In the present work, we use density functional theory to simulate the electronic structures and energy band alignments at the C60/MoS2 and C60/WS2 interfaces. Boron (B)- and nitrogen (N)-doped fullerenes (C59B and C59N) are also considered because the doping remarkably alters the energy levels near the Fermi energy, in particular, the band gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). We demonstrate that C60/MoS2 forms a type-II30 photovoltaic heterojunction with a small builtin potential, whereas C59N/MoS2 and C59B/MoS2 show type-I band alignments, leading to narrow fundamental band gaps and thus extended optical absorption ranges. However, the three composites are not beneficial for exciton separation across the interface.5 On the other hand, C60/WS2 forms a type-II heterojunction with a large built-in potential that promises an excellent photovoltaic performance. First-principles calculations are performed using the Vienna ab initio simulation package with the spin-polarized Perdew, Burke, and Ernzerhof generalized gradient approximation. For pristine MoS2 and WS2 monolayers, we obtain the same optimized lattice parameter of 3.18 Å. Our hybrid models consist of a fullerene molecule on one side of a monolayer in a 5 × 5 supercell, comprising 60 + 3 × 5 × 5 = 135 atoms in total. Because the configurations are asymmetric, dipole corrections are required.31

olar cell technology, being considered as one of the best options for clean energy production, has developed into two main branches.1 The first uses conventional bulk inorganic semiconductors such as Si, GaAs, and abundant chalcogenides,2 creating free charge carriers without intermediate steps upon light absorption. The latter is based on a hybrid donor−acceptor composite forming an excitonic solar cell,3 in which electron− hole pairs are generated and simultaneously separated owing to discontinuities of the electron affinity and ionization potentials across the donor−acceptor interface. Considerable efforts have been dedicated to the development of highly efficient excitonic solar cells.1,4 In addition to a high optical absorption in the visible range, efficient separation and inhibited recombination of the excitons at the donor−acceptor interface are important to improve the photovoltaic performance.5,6 The two-dimensional monolayers of semiconducting transition-metal dichalcogenides exhibit intriguing electronic,7,8 optical,9−12 and mechanical13−15 properties. Particularly, MoS2 and WS2 have a direct band gap of ∼2.0 eV16,17 and possess a sufficiently high in-plane carrier mobility18,19 and thus are suitable for direct application in photovoltaics20−22 as well as indirect photocatalyzed hydrogen evolution reactions.23,24 Moreover, the band gap, electron affinity, and ionization potential can be well engineered by applying mechanical strain17,20,25 or forming Mo1−xWxS2 alloys.26,27 Therefore, ultrathin excitonic solar cells with tunable optical band gaps and band offsets at the donor−acceptor interface have great potential. Recently, hybrid C60/MoS2 crystals have been synthesized and characterized experimentally.28,29 Due to the strong electron affinity of the C60 molecule, it has been suggested that the system can provide in solar cells high quantum yields. It has been demonstrated that the van der Waals interaction assembles the component materials.28 Because WS2 possesses semiconducting © 2014 American Chemical Society

Received: February 17, 2014 Accepted: March 31, 2014 Published: March 31, 2014 1445

dx.doi.org/10.1021/jz500344s | J. Phys. Chem. Lett. 2014, 5, 1445−1449

The Journal of Physical Chemistry Letters

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Figure 1. (a) Energy level alignment with respect to the vacuum level. The HOMOs (VBMs) and LUMOs (CBMs) are shown by solid and dashed lines, respectively, where red and blue colors stand for the spin majority and minority levels. Spin-polarized density of states of (b) MoS2, (c) WS2, (d) C60, (e) C59B, and (f) C59N. The vacuum level is set to zero, and the Fermi energy is indicated by vertical dashed lines. In (e) and (f), the B and N contributions are shown by dotted lines (amplified by a factor of 5). The corresponding spin density is plotted as the inset.

Figure 2. Interface structures, (a) top and (b) side view, where C60 is represented by hexagons.

Table 1. Binding Energy (EB), Rotation Energy Barrier (ΔEB), and Interface Separation (D) of C60 As Well As C59B and C59N on MoS2 and WS2 Monolayers MoS2 S-atop M-atop bridge hollow C59B C60N

WS2

EB (eV)

ΔEB (eV)

D (Å)

EB (eV)

ΔEB (eV)

D (Å)

−0.84 −0.67 −0.69 −0.72 −0.84 −0.83

0.03 0.06 0.01 0.06

2.91 3.08 3.04 3.17 2.85 2.94

−0.97 −0.80 −0.81 −0.84 −1.05 −0.98

0.04 0.07 0.02 0.06

2.89 3.07 3.04 3.17 2.85 2.93

In each configuration, the c axis is 40 Å long, resulting in a vacuum thickness of at least 17 Å. A cutoff energy of 500 eV and a 4 × 4 × 1 k-mesh are found to be sufficient for accurate results. The geometry is optimized until all residual forces remain below 0.01 eV/Å. Because of the absence of strong chemical bonding, a damped van der Waals correction is employed.32,33

Figure 3. Relative binding energy of C59B and C59N on MoS2 and WS2 when the doping site approaches the monolayer. 1446



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Figure 4. Charge density differences for the (a) C60/MoS2, (b) C59B/MoS2, (c) C59N/MoS2, (d) C60/WS2, (e) C59B/WS2, and (f) C59B/WS2 systems. The yellow and cyan regions represent charge accumulation and depletion, respectively.

Figure 5. Spin-polarized density of states of the (a) C60/MoS2, (b) C59B/MoS2, (c) C59N/MoS2, (d) C60/WS2, (e) C59B/WS2, and (f) C59N/WS2 systems. Black and red colors refer to the monolayer and fullerene subsystems. The vacuum level is set to zero, and the Fermi energy is depicted by vertical dashed lines.

visible light can excite electrons from the valence band maximum (VBM) to the conduction band minimum (CBM) of MoS2 and WS2 as well as from the HOMO to the LUMO of C60. Substitution of B or N for a single C atom in C60 breaks the degeneracy of the C60 HOMO and LUMO, respectively, as shown in Figure 1e and f. Both C59B and C59N show spin

As a starting point, the electronic properties of MoS2, WS2, C60, C59B, and C59N are studied. We align the band edges with respect to the vacuum level; see Figure 1a. According to Figure 1b−d, the energy band gaps of MoS2, WS2, and C60 are 1.73, 1.91, and 1.64 eV, respectively, in good agreement with the experimental values of 1.80, 1.99, and 1.90 eV.9,17,34 Obviously, 1447



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charge transfer off of the fullerene. Still, both B and N doping enhance the charge redistribution (see Figure 4b, c, e, and f) and thus the interfacial interaction (compare Table 1). However, we find different behaviors; N doping results in similar features as those discussed previously for the undoped case (charge depletion mainly from the fullerene), whereas for B doping, we observe a more complex redistribution pattern. Projected densities of states of the hybrid systems are shown in Figure 5. Due to the weak interfacial interaction, the electronic properties of the two components are largely intact; see the results in Figure 1. The densities of states demonstrate that the undoped systems form type-II photovoltaic heterojunctions (Figure 5a and d). The situation is more complicated under doping due to the lifted spin degeneracy (Figure 5b, c, e, and f). Different band alignments are found for the two spin channels. The VBM/HOMO and CBM/LUMO band offsets and fundamental band gaps are listed in Table 2, where the gap is defined as the energy difference between the VBM of the donor and the LUMO of the acceptor (type-II heterojunction). Both the C60/MoS2 and C60/WS2 systems show semiconducting features with band gaps of 1.62 and 1.48 eV. Generally, the ability to efficiently drive electron−hole separation at the interface and inhibit charge recombination is governed by the built-in potential, which is defined as the CBM/ LUMO offset. According to our results, electron−hole pairs can be separated at the C60/MoS2 interface, but the built-in potential is only 0.11 eV, which is too small to compete with the exciton binding energy.5 In addition, the VBM/HOMO offset is even smaller, only 0.01 eV, resulting in inefficient charge separation. B and N doping narrows the fundamental band gap and extends the optical absorption ranges. However, the type-II band alignment is lost, and a type-I alignment is formed instead; see the opposite signs in Table 2. Thus, doping does not improve the photovoltaic performance. In contrast, a large built-in potential of 0.43 eV is obtained for the C60/WS2 system, resulting in a remarkable driving force for the electron−hole separation and thus making the system promising for excitonic solar cells. B and N doping breaks the degeneracy of the HOMO and LUMO, respectively, giving rise to a type-I band alignment in one spin channel while preserving the type-II alignment in the other; see Figure 5e and f and Table 2. As a result, the photovoltaic efficiency is reduced. In conclusion, we have performed first-principles calculations to investigate the electronic states and energy level alignments of six ultrathin photovoltaic heterojunctions, C60/MoS2, C60/WS2, C59B/MoS2, C59B/WS2, C59N/MoS2, and C59N/WS2. It is found that C60 prefers to sit at the S-atop site with a hexagon parallel to the monolayer. The total energy is minimized when the dopant in the fullerene is close to the monolayer. All of the interfacial interactions are found to be weak such that the main electronic features of the components are preserved. Both the C60/MoS2 and C60/WS2 systems give rise to type-II heterojunctions. However, the built-in potential in the former case is too small to achieve an efficient electron−hole separation. In contrast, the latter system has a large built-in potential and thus is expected to yield excellent photovoltaic performance. B and N doping of the fullerene results in a type-I band alignment for MoS2, while for WS2, a type-I alignment is obtained in one spin channel and a type-II alignment in the other, which is unsuitable for excitonic solar cells. We note that the same methodology that recently has been established for fabricating C60/MoS2 crystals is also promising to achieve C60/WS2 crystals.28

Table 2. Band Offset of the VBM/HOMO (ΔEV) and CBM/ LUMO (ΔEC) As Well As the Fundamental Band Gap (Eg) spin majority

C60/MoS2 C59B/MoS2 C59N/MoS2 C60/WS2 C59B/WS2 C59N/WS2 a

spin minority

ΔEV (eV)

ΔEC (eV)

Eg (eV)

ΔEV (eV)

ΔEC (eV)

Eg (eV)

0.01 −0.39 −1.35 0.17 −0.22 −1.19

0.11 0.03 0.06 0.43 0.39 0.39

1.62a 1.31 0.32 1.48a 1.31 0.33

0.009 −0.08 −0.13 0.17 0.11 0.04

0.114 1.05 0.19 0.43 1.40 0.54

1.62a 0.60 1.41 1.48a 0.51a 1.37a

Type-II band alignment.

polarization with a magnetic moment of 1.00 μB.35 The lifting of the degeneracy results in narrowing of the fundamental band gaps in the two spin channels (slightly and clearly visible, respectively), potentially generating new optical transitions.36 It is found that C60 favors to have a C hexagon aligned parallel to the monolayer rather than a pentagon for all sites by about 80 meV. Therefore, only such geometries are studied, namely, Satop, Mo-atop (or W-atop), bridge, and hollow, as depicted in Figure 2a. The binding energy EB is calculated by subtracting the total energies of the fullerene and the monolayer from that of the hybrid system. The obtained binding energies and interlayer distances (see Figure 2b) are listed in Table 1. For C60 interacting with both MoS2 and WS2, we find the order (from most to least favorable) S-atop, hollow, bridge, Mo-atop (or W-atop), where the binding is stronger by 0.13 eV for WS2 in all four cases. The preference of the S-atop site agrees with the experimental C60/ MoS2 structure.28 In addition to the distance between the bottom of the C60 molecule and the monolayer, the rotation with respect to an axis perpendicular to the monolayer also alters the binding strength. The potential energy surface as a function of this angle shows that the rotation barriers are less than 0.1 eV. In particular, tiny rotation barriers for the energetically favorable S-atop site suggest that the C60 molecules can rotate freely at room temperature for both monolayers, similar to the situation on metal surfaces.37 For the energetically favorable configurations, we shift the doping site closer and closer to the monolayer to obtain Figure 3 for the relative binding energies. In each case, it is favorable to have the dopant close to the monolayer. The potential energy surfaces for N doping are quite flat, suggesting low site sensitivity, whereas the sensitivity is remarkably enhanced for B doping, with a significant energy jump between the closest and second closest cases. We next visualize the charge redistribution at the interfaces by subtracting the charge of the hybrid system from that of the isolated components. Figure 4a and d demonstrates that charge depletes mostly from the bottom of the C60 molecule and accumulates at the interface. Perturbation is also visible within the monolayer, while almost no redistribution affects the C60 molecule farther away from the interface. The induced charge transfer, which is estimated by integrating planar-averaged charge density differences along the z axis, amounts to 0.055 and 0.044 | e| on MoS2 and WS2, respectively. Therefore, the weak interaction does not preclude charge transfer, similar to hybrid systems formed by physisorption on MoS2.38 The charge transfer is stronger on MoS2 than that on WS2 because the band offset to C60 is smaller according to the energy level diagram in Figure 1a. Substitution of B and N, respectively, for C in C60 results in charge deficiency and excess and correspondingly less and more 1448



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.C.). *E-mail: [email protected] (U.S.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a KAUST CRG grant, and computational resources were provided by KAUST HPC. REFERENCES

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Photovoltaic Heterojunctions of Fullerenes with MoS2 and WS2

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