Surface May Not Form an Efficient Photovoltaic Heterojunction

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Letter 2

Fullerene Interfaced with TiO (110) Surface May not Form Efficient Photovoltaic Heterojunctions: FirstPrinciples Investigation of Electronic Structures Run Long, Ying Dai, and Baibiao Huang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz401124w • Publication Date (Web): 24 Jun 2013 Downloaded from http://pubs.acs.org on June 24, 2013

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The Journal of Physical Chemistry Letters

Fullerene Interfaced with TiO2 (110) Surface May not Form Efficient Photovoltaic Heterojunctions: First-Principles Investigation of Electronic Structures Run Long 1, a), Ying Dai2, Baibiao Huang2 1

School of Physics, Complex & Adaptive Systems Laboratory, University College Dublin, Belfield, Dublin 4, Ireland

2

School of Physics, State Key Lab of Crystal Materials, Shandong University, Jinan 250100, P.R. China

Abstract: Electronic structures of C60 fullerene interfaced with TiO2 (110) surface, on several types of hybrid organic/inorganic composites, have been investigated using density functional theory. For the C60/TiO2 surface, a type-II heterojunction can form while could be inefficient since small driving force cannot lead to efficient charge separation due to weak donor-acceptor coupling. The charge transfer from C60 to TiO2 can be enhanced by doping, however, photoexcited electron-hole pair is expected to occur rapid recombination. Interestingly, charge transfer can take place for both directions across organic/inorganic interface with either high amount or medium value when C60 is covalent linking to TiO2 surface with and without hydrogen termination. It is because hydrogen atom alters the work function of the surface in the former case. In both cases, the strong hybridization between C2p and Ti3d leads to energy levels mismatch between organic and inorganic species. These observations indicate that, in a photovoltaic heterojunction based on fullerene, the interfaces are inefficient due to either small built-in potential or unfavorable energy band alignment.

Keywords: TiO2, Fullerene, Photovoltaic Heterojunction, Built-in Potential, Charge Separation

a)

Corresponding author’s email : [email protected]

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Nano-structured materials provide exciting possibilities for photovoltaic applications1-3 due to the tunable optical and electronic properties via changing size and shape. The most promising application is nano-structured materials interfaced with other metal oxide to form excitonic solar cell (XSC).2 Compared to conventional bulk semiconductor solar cells, XSC has a distinguishing character that charge carriers are generated and simultaneously separated across an interface. Therefore, efficient dissociation and inhibition recombination of exciton at the donor-acceptor interface are desirable to achieve high-performance photovoltaic devices. Here, electronic properties, especially energy band alignment between donor and acceptor species, of this interface determine the production of free charge carriers and inhibition of recombination rate. TiO2, as a wide band gap semiconductor, has been widely used as promising photocatalysis4-7 and photovoltaic8-10 materials for generation photo-excited electrons, which can either be channeled to create electricity directly in solar cells or else be used to drive water splitting for generation hydrogen.11 However, the large energy gap of pristine TiO2 limits its efficiency since light absorption and the consequent photo-excitation of electron-hole pairs take place when the incident photons matches or exceeds the band gap. Narrowing the TiO2 band gap offers a straightforward approach to increase its efficiency via harvesting longer solar spectrum. Although doping with impurity atoms is an effective way to reduce the band gap,12-19 dopantmediated recombination of photoexcitation electron-hole pair in the lattice restricts its practical applications. As an alternative way, composites TiO2 with other materials can overcome these shortcomings. Examples include molecular chromospheres,20,21nobel metal,22 semiconductor quantum dots,23-25 and nanoscale carbon materials.26-31 The above-mentioned materials often possess very different properties, for example, localized vs. delocalized electronic states, soft organic vs. hard inorganic materials, high- vs. low-frequency vibrational modes, good vs. poor conductors of electricity, and so on. Even with the same acceptor TiO2, the donor-acceptor coupling strength and energy levels alignment may change due to substantial differences in the geometrical and electronic properties of the acceptor species and then the donor-acceptor interfaces. Recently, a number of groups have synthesized and characterized experimentally TiO2/fullerene composites for photocatalysis32-34 and solar energy applications.35 A diversity of architectures is being investigated, including both mechanical mixtures32-34,36 of fullerene and


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TiO2 are covalently linked composites.35 By linking C60 to the TiO2 with a carboxylate group COOH, Jang and co-workers35 investigated the photovoltaic structure and reported to achieve high quantum efficiency. Functionalizing with porphyrin can further enhance the quantum yield; however, this situation is beyond the current work. The current letter reports density functional theory (DFT) simulations of the electronic structures in a series of fullerene/TiO2 composites, including C60 and B-doped C60 (B@C60) physisorption onto TiO2, and covalent linking C60 to TiO2. The calculated density of states (DOS) displays that the band gap of TiO2 reduces slightly while many C60 fullerene states localize within the energy gap. These states extend the optical absorption edge into visible-light region; potentially increase the photo-excited electron transfer flux. In particular, the transfer can be enhanced in the presence of B doping since B donates electrons to C60 due to small electronegative. Covalent link can further drive charge transfer due to the stronger donoracceptor coupling. However, the surface hydrogen alters the work function of inorganic species and induces inverse charge transfer from the TiO2 surface to C60. In the content of photovoltaic solar cells, the charge separation plays a key role in determining the solar-to-electricity efficiency. Although the C60/TiO2 forms a type-II heterojunction, the small built-in potential cannot efficiently drive the electron-hole pairs dissociation at the interface, and may depress the photovoltaic performance. Also, charge should take rapid recombination across B@C60/TiO2 interface since a metallic characteristic exists. For covalent linking C60 to TiO2 surface, a substantial hybridization between C2p and Ti3d states induce an unsatisfied energy level alignment, making the composites are not good photovoltaic heterojunction.

To construct the interface between TiO2 and fullerene, we chose a stoichiometric rutile TiO2 (110) slab because it is the most stable one among the rutile low index surfaces. The periodically repeated slab contained a 144-atom (6×2) surface composed of six atomic layers if TiO2 with three bottom layers fixed at bulk position. The vacuum depth is as large as 20 Å for both system C60/TiO2 and B@C60/TiO2 models while 25 Å for covalent link system that are sufficient to separate the interaction between periodic images. In order to test whether the selected geometries are energetically favorable, we carried out geometry optimization for a variety of configurations when C60 is interfaced with TiO2 surface with or without doping and covalent link. We chose the ground state geometries possessing the lowest total energy for each



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configuration to investigate the electronic properties. The density functional theory (DFT) calculations are carried out by using the Vienna ab initio Simulation Package (VASP) code,37,38 implementing the projector augmented wave (PAW) pseudopotentials.37 The Perdew-BurkeErnzerhof parameterization of the generalized gradient approximation39 was adopted for the exchange-correlation potential. The electron wave function was expanded in plane-wave cutoff energy of 400 eV. We carried out DFT calculations with a Monkhorst-Pack40 4 k-point in the Brillouin zone) for geometry optimization while much denser 34 k-point for electronic structure calculations with the DFT+U41 scheme due to the presence of 3d electrons of Ti ions. The geometry optimization stops until the residual forces were below 0.01 eV/Å. The onsite U = 6.0 eV and J = 0.5 eV parameters were applied to the Ti 3d electrons.42-44

Figure 1 depicts the optimized geometries of the simulated systems with van der Waals interaction. Both the first and second architecture involves the TiO2 (110) surface, weakly interacting with the pristine C60 and B@C60, Figure 1a-b. These systems provide the simplest way to fabricate such a kind of hetero-interface. The hybrid organic/inorganic photovoltaic material combing a B-doped C60 and a TiO2 surface is considered for the first time. The principle idea beyond the encapsulation of the boron atom is to increase the intermolecular interaction and therefore to enhance the electron transfer. C60 possesses a negative charge, which should induce stronger electrostatic interactions with the TiO2 surface. For both cases, the optimized geometries of C60 and TiO2 essentially the same, while the separation between them varies a little, Figure 1. In particular, the B atom moves closer to TiO2 surface in B@C60/TiO2 composites. This will give rises to a stronger donor-acceptor interaction and facilitate charge transfer across the interfaces. An analysis from Bader charge partition scheme indicates a strong 0.14 electron transfers from the B@C60 to TiO2 surface, whereas only 0.04 electron transfers from C60 to TiO2 is observed. To understand the origin of such a charge transfer at interface, work functions for individual TiO2(110) surface and C60 and B@C60 calculated by aligning the Fermi level to the vacuum level. The obtained values were 6.62, 5.64, and 4.64 eV, respectively, indicating that electron is inclined to escape the binding from B@C60 into TiO2 surface. In addition, it should note that dipole correction might artificially favor charge transfer. Therefore, repeating calculations of the Bader charge in the presence and absence of dipole correction illuminate that the interface charge transfer changes little.


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The calculated density of states (DOS) show that energy gaps for individual C60 and TiO2 are 1.64 and 2.27 eV. The former agrees well with experimental value of 1.9 eV,45 and the latter is substantially improved compared to pure DFT calculations46 despite it is lower than the experimental band gap 3.0 eV,47 Figures 2a and 2b. Obviously, the intrinsic photo-excitation of an electron from the O 2p states located on the TiO2 valence band maximum (VBM) to Ti 3d states resided on the TiO2 conduction band minimum (CBM) requires a photon energy to match or exceed TiO2 energy gap. However, excited an electron from the lowest unoccupied orbitals (LUMO) of fullerene to the highest occupied molecular orbital (HOMO) can be achieved under visible-light irradiation. Introduction B into C60 breaks the degeneracy of the C60 LUMO, Figure 2c. It also demonstrates that B states mix well with the LUMO of C60. The orbital hybrid is beneficial for B donating electron to C60 and enhances intermolecular electrostatic interactions compared to pristine C60. To explore charge redistribution across the organic/inorganic interface, we calculated the three-dimensional charge density difference by subtracting the electronic charge of a hybrid fullerene/TiO2 composites from the standalone fullerene and TiO2 (110) surface, Figure 3. Figure 3a displays that charge redistribution mostly takes place at the C60/TiO2 interface region and relax a bit into TiO2 bulk states, whereas almost no charge transfer on C60 farther away from the interface due to weak mechanical mixtures. In contrast, for B@C60/TiO2 interface, charge redistribution can be seen in the whole C60 species because B-doping enhances the intermolecular interaction since C60 can attract electron from boron atom due to larger electronegative of carbon atom than boron atom. Furthermore, charge relaxes significantly into the bulk state of TiO2 compared to C60/TiO2 composites, Figure 3b. Part c of Figure 3 plotted the planar averaged charge density difference along the direction perpendicular to the TiO2 (110) surface, which offers quantitative results of charge redistribution. The positive values represent electron accumulation and negative values indicate electron depletion. This plot further depicts that much more efficient charge transfer occurs in B@C60/TiO2 composites than in C60/TiO2 system. However, the performance of photovoltaic heterojunction is not only dependence of charge transfer, but also depends on charge separation at the interface, which is tightly related to the energy band alignment between donor and acceptor species. The calculated DOSs of the above two composites characterize the interface electronic properties and energy levels alignment in detail, Figure 4 and Table I. Part a and b of Figure 4



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shows that the each component of the combined DOS changes slightly compared to individual DOS, Figure 3, indicating the organic/inorganic interaction is indeed weak. Figure 4c shows the projected DOS (PDOS) of C2p in the case of C60/TiO2, well-separated C 2p states with the host Ti 3d states could not couple each other significantly, giving rise to smaller amount of charge transfer. Figure 4d depicts the orbital mixing in the case of B@C60/TiO2: B 2p states significantly hybridize with C 2p states Ti 3d states, lead to wave function extend closer to TiO2 surface and couple more to it, impel significant charge transfer. This analysis supports the results from Bader charge and charge density difference, Figure 3. According to optical selection rule, the following process prevails other possibilities under visible light irradiation since the host energy gap of TiO2 reduces tiny relative to pure TiO2, an absorbed photon promotes an electron form the organic ground state to its LUMO and then the excited electron is injected into the TiO2 CB. As shown by the state-of-the-art time-domain non-adiabatic molecular dynamics48-50 simulation for many systems,42,51-53 the motions of nuclei play a key role in electron transfer, which can proceed by both adiabatic and non-adiabatic mechanisms. Regardless of the mechanism, an efficient exciton dissociation at the interface is quiet important, which is determined significantly by the energy levels alignment of the organic and inorganic species. Figure 4 displays that C60/TiO2 and B@C60/TiO2 form type-II, type-III photovoltaic heterojunctions, respectively. The corresponding offset of CBM/LUMO, VBM/HOMO, and the band gap is listed in Table I. C60/TiO2 system exist a 1.34 eV band gap and show semiconducting characteristic and B@60/TiO2 possesses metallic behavior. In particular, the offset of CBM/LUMO, so-called built-in potential, is the key to drive charge separation. In principle, electron-hole pairs can successfully separate between C60 and TiO2 interface, making C60/TiO2 composites as promising photovoltaic materials. How efficient of the charge separation is tightly related to built-in potential, which plays a key role in driving the hole away from the interface and inhibiting charge recombination. However, our calculated built-in potential is only 0.09 eV, Table I and Figure 4a. This potential is too small to compete to the exciton binding energy; thereby making charge separation across the interface is inefficient. More seriously, injected electron should take place rapid recombination in a metallic B@C60/TiO2 heterojunction, Figure 4b, indicating B-doping cannot optimize the performance of the heterojunction while decrease its efficiency. The insets of Figures 4a and b show the orbital density of HOMO/VBM and LUMO/CBM for both C60 or B@C60 and TiO2, which emphasize the electron/hole donor and


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acceptor states. The orbital densities clearly show the components of key energy levels and coincide with the analysis of DOS. Both insets demonstrate O2p and Ti 3d orbitals construct the VBM and CBM of TiO2, respectively, which represent electron and hole acceptor states. Electron donor states delocalized from C60 and B@C60 into TiO2 slab in both cases, favoring electron injection. However, both the heterojunctions can provide excellent hole conductivity since the photoexcited electron injects into TiO2 CB, leaves a hole on fullerene ground states, and electron on TiO2 VBM is capable of traveling to C60. The electrostatic potential could offer additional information on how the presence of C60 and B@C60 would affect local charge distribution when interface is formed. Figure 5 shows the potential near the interface, subtracted by an individual TiO2 (110) and other three organic species. Contrary to the case of C60/TiO2 (cf. Figure 5a), where a shallow change of potential, while a strong potential drop was observed on the B@C60/TiO2. The presence of the B impurity causes this attractive region for negative charges on the TiO2, Figure 5b. Figure 5c further depicts the profile change due to charge transfer. It clearly demonstrates that potential increasing at the TiO2 part and decreasing at fullerene part becomes much more profound in the presence of B doping. Therefore, the interface attracts the electron rather the hole and it is not beneficial for electron-hole pair dissociation across the interface. The above discussions indicate that organic/inorganic composites cannot form efficient photovoltaic heterojunction because of weak electrostatic interactions. Typically, covalent connection can highly enhance the donor-acceptor coupling and optimize energy band alignment. Therefore, we used a COOH ligand covalently links pristine C60 and TiO2, as implemented in the recent experimental study.35 The connection is via two O-Ti bonds and the hydrogen atom either dissociates onto the bridging oxygen atom of TiO2 surface or removes from the supercell, as shown in Figures 6a and e of optimized geometries that two composites form stable interfaces. The calculated DOS demonstrates that the composites facilitate charge transfer because organic and inorganic states highly mix each other, indicating wave functions should couple significantly between C60 fullerene and TiO2 via covalent interactions. This strong interaction is reflected and distinguished by the PDOS in detail, as shown in the inset of DOS. With hydrogen in the supercell, that C2p and 3d states of two uncoordinated Ti atoms hybridized substantially at the TiO2 CB (cf. Figure 6b). Contrarily, C2p interact slightly with the two uncoordinated Ti 3d states without hydrogen in the simulation cell (cf. Figure 6f). The observation implies that wave



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function should overlap significantly in former case, which is beneficial for driving charge transfer. The Bader charge analysis reveals that 0.71 electron inversely transfers from TiO2 to C60 whereas 0.12 electron transfers from C60 to TiO2. Inverse electron transfer may originate from the different electronic properties at the interface since hydrogen is a strong electron donating element. It will alter the work function of the two components in the presence of hydrogen on the TiO2 surface. The calculated work function is 5.60 eV for organic species and 4.53 eV for inorganic species with H-terminated, indicating that electron is inclined to escape from TiO2 surface to C60. The phenomena were also observed experimentally between carbon nanotube and TiO2 surface.27,54 However, the strong interaction induces the energy levels mismatch between the organic and inorganic species destroy the type-II heterojunction existing in C60/TiO2 system, Figure 4a. In order to distinguish the charge transfer direction, we plotted the charge difference for the two types of interface in part c and g of Figure 6. Figure 6c shows that electrons indeed distribute largely onto C60 with H termination, inducing electrostatic potential inverse distribution, supported by electrostatic potential difference plot, Figure 6d. As expected, electrons accumulate heavily TiO2 surface without H termination, responsible for that the electrostatic potential has direction from TiO2 to C60 Figures 6 g and h. Therefore, the electron transfer in C60-TiO2 composites can proceed in both directions, depending on interfacial properties.

To conclude, we employ the density functional theory calculations to investigate the electronic properties and energy levels alignment between fullerene and TiO2 surface. Our theoretical results suggest that C60 interfaced TiO2 in both mechanical mixture and covalent linking cannot form efficient photovoltaic heterojunction. For C60/TiO2 system, the small built-in potential is unable to drive effective charge separation at the surface. And charge will take place rapid recombination due to the metallic characteristic in the presence of B doping. B-doping also results in charge transfer increasing since hybridization of B 2p states with C2p and Ti3d states leads to enhancement of intermolecular electrostatic interaction. Interestingly, charge transfer occurs inversely from TiO2 to C60 when the covalent link is connection to each other in the presence of hydrogen atom, and the charge transfer greatly enhances compared to covalent link in the absence of hydrogen atom and mechanical mixture cases due to the strong hybridization


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between C2p and Ti3d states. Unfortunately, the strong interaction induces the unfavorable energy levels alignment and cannot optimize the performance of photovoltaic heterojunction.

Acknowledgements RL thanks the Science Foundation Ireland (SFI) SIRG Program (Grant number 11/SIRG/E2172) and the Irish Center for High End Computing for the provision of computational resources. YD and BBH acknowledge the National Basic Research Program of China (973 program, No. 2013CB632401) and the National Natural Foundation of Shandong Province under Grant number ZR2011AM009.

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Table 1 Energies parameters (in eV) of the interfaced TiO2 (110) heterojunctions for the C60 and B@C60a. C60/TiO2 (110)

B@C60/TiO2(110)

CBM/LUMO offset

0.09

N/A

VBM/HOMO offset

0.93

2.21

KS gap

1.34

N/A

a

CBM/LUMO (VBM/HOMO) offsets are the energy differences between the effective TiO2

CBM (VBM) and C60 and PCBM LUMO (HOMO) for the semiconducting tube, while it refers to the effective TiO2 CBM (VBM) relative to the Fermi level for the B@C60.


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Figure Captions Figure 1 Optimized interface structures (a) C60/TiO2 (110) interface and (b) B@C60/TiO2 (110) interface. The green, gray, red and cyan balls represent B, C, O, and Ti atoms, respectively. Figure 2 Density of States of (DOS) for individual species (a) TiO2 (110) surface, (b) C60, and (c) B@C60. It shows that intrinsic photo-excitation of TiO2 occurring from O 2p to Ti 3d has to require a large photon energy. And the presence of B inside C60 breaks the degeneracy of the lowest unoccupied molecular orbital (LUMO). Figure 3 Charge density differences for (a) C60/TiO2 (110) interface and (b) B@C60/TiO2 (110) interface. The yellow region represents charge accumulation and the cyan region indicates charge depletion; isosurface value is 0.00011 e/ Å3. (c) Corresponding planar averaged charge density difference for above three cases is plotted in black and red blue lines, respectively, as a function of position in the z-direction. The plots show that B-doping greatly enhances charge transfer from organic to inorganic species. Figure 4 Density of States (DOS) (a) C60/TiO2 (110) interface and (b) B@C60/TiO2 (110) interface. The DOS of organic and inorganic species are plotted in red and black lines, respectively. The corresponding necessary projected density of states (PDOS) are plotted in (c) and (d). The Fermi level is set to zero. DOS shows formation of type-II heterojunction between C60 andTiO2 (110) interface, whereas the built-in potential is too small to drive efficient charge separation, Table I. B@C60/TiO2 composites form a type-III heterojunction, which indicates that charge occurs rapid recombination at the interface. The orbital densities of the key energy level are shown in the inset of (a) and (b). Absorption of photon onto C60 promotes an electron to its excited states which are resonance with TiO2 conduction band (CB) and then injects into TiO2 CB. This process extends the optical absorption edge of the organic/inorganic composites into visible-light region; potentially enhance electron transfer rate.

Figure 5 Induced change in the electrostatic potential by the interface formation, ∆V(r)=V(r)VX(r)-VTiO2(r), at the fixed ionic positions near the interface. Here X denotes C60 and B@C60,



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respectively, and thereby (a) and (b) represents C60/TiO2 (110) interface and B@C60/TiO2 (110) interface. The potential plots indicate B-doping induces significant change of electrostatic potential compared to pristine C60. Figure 6. Top panel with hydrogen termination from left to right frame: (a) the optimized structure of C60 covalently links to TiO2 in the presence of hydrogen atom; (b) the calculated DOS for TiO2 and C60 are plotted in black and red line, respectively; (c) the charge difference, and (d) electrostatic potential difference, respectively. The corresponding plots without hydrogen termination were shown in bottom panel (e)-(h).The charge difference shows that charge transfer occurs inversely from TiO2 surface to C60 and induces the electrostatic potential inverse at the interface. The inset of DOS shows the hybridization between C 2p and Ti 3d states facilitates charge transfer and induces energy band mismatch.


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 5



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Figure 6


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Surface May Not Form an Efficient Photovoltaic Heterojunction

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