University of Chinese Academy of Sciences, Beijing , People s Republic of China,

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1 SiC 2 Siligraphene and Nanotubes: Novel Donor Materials in Excitonic Solar Cell Liu-Jiang Zhou,, Yong-Fan Zhang, Li-Ming Wu *, State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian , People s Republic of China, University of Chinese Academy of Sciences, Beijing , People s Republic of China, Department of Chemistry, Fuzhou University, Fujian , People s Republic of China. *Corresponding Author liming_wu@fjirsm.ac.cn (L.M.W.).

2 CACULATION METHODS Particle Swarm Optimization Algorithm for 2D SiC 2 Sheets System. The structure predictions were carried out with the aid of the particle swarm optimization (PSO) 1 methodology as implemented in CALYPSO (the Crystal structure AnaLysis by Particle Swarm Optimization) code. 2 This method has been widely used to search stable 2D nanostructures, such as B x C y compounds, 3 nitrogen-graphene alloys, 4 boron monolayers. 5, 6 The number of formula units per simulation cell (z) was set as 1 to 3. The population size and the number of generations were set to be 30. Two stable phases, g-sic 2 siligraphene and pt-sic 2 silagraphene, 7 were obtained at the 4 th generation with z = 2 and the 13 th generation with z = 1, respectively. The new g-sic 2 siligraphene is 0.41 ev/atom lower in energy than the pt-sic 2 silagraphene. 7 And their binding energies are 6.46 and 6.05 ev/atom, respectively. Density Functional Theory Calculations. The local structural relaxations and electronic band structure calculations were performed in the framework of the density functional theory within the generalized gradient approximation (GGA) 8 parametrized by Perdew, Burke, and Ernzerhof (PBE) 9 as the exchange-correlation function as implemented in the VASP code. 10 The electronic configurations for C and Si were 2s 2 2p 2 and 3s 2 3p 2, respectively. Since DFT methods often underestimate the band gap, the screened exchange hybrid density functional by Heyd, Scuseria, and Ernzerhof (HSE06) 11, 12 was adopted to correct the PBE band gaps. The plane-wave cutoff energy of 500 ev and the appropriate Monkhorst-Pack k-meshes were used to ensure the convergence of all the enthalpy calculations to 10-5 ev. A Gaussian smearing with a width of σ = 0.1 ev was used for the occupation of the electronic levels. The k-point

3 grids of and were used for the sheet and nanotubes, respectively. In the geometry optimization, the atoms are allowed to relax until forces on atoms are < 0.02 ev/å. A supercell length of 15 Å along the z direction was taken to eliminate the interaction between the g-sic 2 derivatives and their periodic replicas. The phonon calculations were carried out by using the density functional perturbation theory (DFPT) 13 as implemented in the PHONOPY code. 14 To evaluate the stability of the g-sic 2 siligraphene and derivative nanotubes, the binding energy, E b was calculated as followed: E b = (xe Si + ye C E SixCy ) / (x+y) where E Si, E C, and E SixCy were the energies of Si, C, and Si x C y nanomaterials. According to this definition, those with larger binding energies were energetically more favorable. Molecular dynamics (MD) simulations. To assess the thermal stability of g-sic 2 siligraphene, the melting point was estimated by using the ab initio molecular dynamic (MD) simulations with the consideration of the vibration effects of the nucleus. The temperature was controlled by using the Nosé algorithm 15 in the NVT ensemble. Simulations lasted for 5 ps with a time step of 1 fs at the temperature of 2000, 3000, and 3500 K, were carried out. Calculations of Optical Properties. The optical absorption spectra were calculated by using the GW approximation plus the random phase approximation (RPA) 16 17, 18 or Bethe-Salpeter equation (BSE) of the two-particle Green s function with/without considering the electron-hole effects, as implemented in BerkelyGW package. 17, 19 First, the electronic ground state with DFT in the LDA 20 was calculated.

4 Second, the quasiparticle energy within the GW approximation for the electron self-energy was obtained. 42, 45 Finally, the coupled electron-hole excitation energies and optical spectra were evaluated by solving the Bethe-Salpeter equation (BSE) of the two-particle Green s function. 17, 18 To obtain the converged optical spectra, in this work, the kernel K eh of g-sic 2 siligraphene was computed on a sparse k-point with 0.02 Å -1 spacing and then interpolated 18 onto a denser k-point grid with 0.01 Å -1 spacing. Three valence bands and three conduction bands were included to calculate the optical absorption spectrum. More calculation details could refer to reference. 21 Calculation of power conversion efficiency. The practical upper limit of the power conversion efficiency (η) for such a XSC system can be estimated according to 22, 23 the type-ii alignment. d 0.65 ( Eg- Ec-0.3) d P( hϖ ) d( hϖ ) J scjocβ FF E g η= = Psolar P( hϖ ) d( hϖ ) 0 (1) Where 0.65 is the fill factor (β FF ), P( hϖ ) is the AM1.5 solar energy flux (expressed in W.m -2.eV -1 ) 24 at the photo energy ( h ϖ ), E c is the conduction band offset between the donor and acceptor, and J oc is the maximum open circuit voltage calculated as the effective interface gap ( E d g E ), c 22, 25 which accounts for the energy conversion kinetics. β FF is the fill factor, J sc in the numerator is an integration in the limit external quantum efficiency of 100 %, 26 and the integration in the denominator is the incident solar radiation. The standard AM1.5G solar spectrum P solar = 1000 W/m 2 is applied. 23

5 Figure S1. The phonon dispersion of g-sic 2 siligraphene. Γ, K and M correspond to the (0, 0, 0), (0.333, 0.666, 0) and (0, 0.5, 0) k-points in the first Brillouin zone, respectively.

6 Figure S2 Top and side view of a snapshot of the g-sic 2 siligraphene at 5 ps of the ab initio molecular dynamics simulation in the NVT ensemble. The optimized g-sic 2 siligraphene was used as the initial structure. The temperature of the system was controlled at (a) 2000, (b) 3000 K and (c) 3500 K. The estimated melting temperature is between 3000 and 3500 K.

7 Figure S3. (a): Side and top views of the optimized structures of (4, 4) and (8, 0) g-sic 2 nanotubes. (b): The PBE band structures of (4, 4) and (8, 0) g-sic 2 nanotubes. The black dashed line denotes the Fermi energy. Figure S4. The Forcite analysis of bond distribution for (a): 2D g-sic 2 siligraphene, (b): (3, 0), (8, 0) g-sic 2 nanotubes and (c): (2, 2), (4, 4) g-sic 2 nanotubes. Different C C and Si C bonds locate at different positions. Bin size is References (1) Wang, Y.; Lv, J.; Zhu, L.; Ma, Y. Phys Rev B 2010, 82,

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