A II -B IV -C V 2 TERNARY SEMICONDUCTORS FOR PHOTOVOLTAICS

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1 A II -B IV -C V ERARY SEMICODUCORS FOR PHOOVOLAICS A. V. Krivosheeva, V. L. Shaposhnikov, V. E. Borisenko Belarusian State University of Informatics and Radioelectronics (Minsk, Belarus) F. Arnaud D Avitaya, J.-L. Lazzari Centre Interdisciplinaire de anoscience de Marseille (Marseille, France)

2 MOIVAIO Search for alternative and renewable sources of energy requires materials of low cost with new electronic, transport and optical properties; Stable II-IV-V compounds weakly lattice mismatched with silicon, germanium or gallium arsenide can be potential candidates for photovoltaic devices; Some of II-IV-V compounds may replace more expensive CuInSe in photovoltaic applications.

3 APPROACH - o find materials with a lattice constant, closest to the silicon, gallium arsenide or germanium substrate - o determine the optical properties (dielectric constants and absorption coefficients) of II-IV-V semiconductors PROBLEMS Materials for solar cells should possess high absorption coefficient and have large gap to absorb mass of the visible light spectrum; Be- and most of Mg-containing compounds were not experimentally synthesized. heir electronic and optical properties are still unknown.

4 MEHOD GGA-PAW approximation (VASP code) structural optimization and calculation of lattice parameters FLAPW method (WIEK code) simulation of band structures and optical properties Chalcopyrite type tetragonal crystal structure with 6 atoms was used for simulation

5 SRUCURE A II B IV A II B IV C V

6 SRUCURE heoretically computed lattice constants of some chalcopyrites with variation of group IV element 6. BeXP 6. BeXAs a, Ang.8.6 GaAs MgXP MgXAs ZnXP. Si ZnXAs CdXP. CdXAs. Si Ge Sn X

7 Be-compounds

8 ELECROIC PROPERIES - BeSiAs =.7 ev - BeGeAs =.6 ev DOS, states/ev BeSiAs otal DOS BeGeAs Г BeSiP =. ev Г BeGeP =.8 ev PDOS, states/ev BeSiAs Projected DOS Si-s As-p BeGeAs Ge-s As-p Г - Г Valence band near Fermi level is formed primary by p states of As. Conduction band near the gap is formed primary by s electrons of Si or Ge Band structures and DOS demonstrate semiconducting nature of Be-compounds. o experimental data is available.

9 OPICAL PROPERIES ε ε 6 8 BeSiAs BeSiP 6 8 BeGeAs 6 8 BeGeP 6 8 8π e Dielectric function ( ) ( ) i ( ) ε ω = ε ω + ε ω ε m ω n n n Ω k k ( ω) = ( ) δ( ω) Pn n k En En h ( ) ε ( ω) = + P π ( ω ) ω Equal starts and similar shape of ε curves for both directions of light polarization anisotropy of ε is practically absent. Slightly different location of the main peaks for E a and E c at higher photon energies (about ev) Direct gap and rapid increase of ε curves at about. ev in BeSiAs and BeSiP All Be-containing compounds are characterized by low value of dipole matrix element of first direct transition (at the Gamma point), thus they have low oscillator strength and thereby they are not suitable for light emitting devices. ε ω ω dω d k π

10 OPICAL PROPERIES BeSiAs BeGeAs Absorption coefficient α, cm BeSiP BeGeP. Markvart, Luis Castañer, Practical handbook of photovoltaics: fundamentals and applications.- Elsevier (), p α, cm α, cm - BeSiAs BeGeAs BeSiP BeGeP 6 8 Absorption coefficients of BeGeAs demonstrate similar order of magnitude with CuInSe

11 Mg-compounds

12 ELECROIC PROPERIES MgSiP MgSiAs =.7 ev à MgGeP =.6 ev à MgGeAs Magnesium-containing compounds are found to be direct-gap semiconductors, except MgGeP which is quasi-direct gap semiconductor. Here the VBM is located at the Γ-point whereas the CBM at the Γ-point and Γ- direction have practically equal values. - =. ev - =.7 ev à - Ã

13 OPICAL PROPERIES ε MgSiP 6 8 MgGeP MgSiAs 6 8 MgGeAs Dielectric function he dielectric functions of Mg-compounds show more anisotropic behavior especially for the photon energies higher than ev. Both Sicontaining direct-gap compounds are characterized by low value of the dipole matrix element of the first direct transition as well as MgGeP. MgGeAs is found to have a large value of dipole matrix element (.9 for E c) and its ε spectra is characterized by abrupt threshold and strong increase at about.6 ev that corresponds to the value of estimated band gap. ε

14 OPICAL PROPERIES MgSiP MgSiAs Absorption coefficient α, cm MgSiP has the lowest value of the absorption coefficient while other Mg IV V compounds possess the values comparable with GaAs and GaP. Moreover, the values of α are higher than those for CuInSe especially at photon energies higher than ev. MgGeP MgGeAs α, cm

15 Zn-compounds

16 ELECROIC PROPERIES ZnSiAs =.89 ev Г ZnSiP ZnGeAs ~. ev Г ZnGeP Band gap behavior of ZnGe{P-As-Sb} compounds Eg, ev ZnGe{P-As-Sb} calc. ref. P As Sb atomic number ZnSiP : Eg=.8 (d) - - =.8 ev - - =.6 ev ZnSiAs : Eg=.89 (d) ZnGeP : Eg=.6 (qd) ZnGeAs : Eg=. (i) - Г - Г

17 OPICAL PROPERIES ZnSiAs ZnGeAs ε 6 8 ZnSiP 6 8 ZnGeP ZnSiAs compound is characterized by a large value of dipole matrix element (.77 for E c) and its ε spectrum is described by strong increase at about ev Similar behaviour can be observed for ZnGeP ( Pzz =.) with ε curve starting at. ev in spite of the fact that this compound was found to have a quasi-direct gap ε

18 OPICAL PROPERIES ZnSiAs ZnGeAs Absorption coefficient α, cm ZnSiP ZnGeP. Markvart, Luis Castañer, Practical handbook of photovoltaics: fundamentals and applications.- Elsevier (), p α, cm α, cm - ZnSiAs ZnGeAs ZnSiP ZnGeP 6 8 Absorption coefficients of ZnSiAs and ZnGeP demonstrate similar order of magnitude with CuInSe

19 Cd-compounds

20 ELECROIC PROPERIES CdSiAs CdGeAs Band gap behaviour of CdGe{P-As-Sb} compounds =.8 ev Г CdSiP ~. ev Г CdGeP Eg, ev.6 CdGe{P-As-Sb}. calc..8 ref... P As Sb atomic number - - =. ev - - =.9 ev DOS, states/ev 8 6 CdGeAs - Г - Г - -

21 OPICAL PROPERIES CdSiAs CdGeAs ε Cd-based materials provide similar optical properties as compared with Zn-based ones. ε 6 8 CdSiP 6 8 CdGeP CdSiAs is characterized by a large value of dipole matrix element (. for E c) and its ε spectrum is described by strong increase at about. ev. CdGeP has a Pzz value of., its ε curve starts at about.6 ev

22 OPICAL PROPERIES CdSiAs CdGeAs Absorption coefficient α, cm CdSiP CdGeP. Markvart, Luis Castañer, Practical handbook of photovoltaics: fundamentals and applications.- Elsevier (), p α, cm α, cm - CdSiAs CdGeAs CdSiP CdGeP 6 8 Absorption coefficients of CdSiAs and CdGeP demonstrate similar order of magnitude with CuInSe

23 GEERAL REMARKS One of the possible ways to increase the quantum efficiency of solar cell is multilayered heterostructures consisting of semiconducting materials with different band gaps. For photovoltaic applications we are interested in stable compounds having a direct gap (absorption coefficients and effective masses are higher for direct gaps as compared to indirect ones). If we want eliminate expensive, rare, dangerous or too toxic elements it is better do not consider Cd-based and P containing compounds, as phosphorous is highly volatile, red-phosphorous burns, and phosphine gas is highly toxic. Among materials of II-IV-V class arsenic compounds are easier to be obtained experimentally in comparison with nitrogen and phosphorous based ternaries. Zn(Si, Sn)As family should possess lattice constants close to GaAs one and have band gap suitable for photovoltaics.

24 GEERAL REMARKS heoretical and experimental gap values Compound Band gap, ev Calc. Exp. ransition Compound Band gap, ev Calc. Exp. ransition BeSiP. d MgSiP.7. d BeSiAs.7 d MgSiAs.6 d BeGeP.8 i MgGeP. qd BeGeAs.6 i MgGeAs.7 d ZnSiP.8. d CdSiP.. d ZnSiAs.89.7 d CdSiAs.8.6 d ZnGeP.6. qd CdGeP.9.8 d ZnGeAs..8 d CdGeAs.. i Blue fields good lattice matching with Si, yellow fields with Ge and GaAs MgSiP, ZnSiP, ZnSiAs, CdSiP, CdSiAs, CdGeP, having direct gaps with the values more than ev and possessing good matching with Si, Ge and GaAs substrates, can find application in photovoltaics and may compete with the more expensive CuInSe

25 GEERAL REMARKS Band-gap values in dependence of variation of group IV element. MgXP ; ZnXP ; CdXP, ev... MgXAs ; ZnXAs ; CdXAs Si Ge Sn X

26 GEERAL REMARKS Averaged reflectance spectra of II IV V ternaries in comparison with GaAs, GaP and CuInSe λ, nm 8 6. MgSnAs ZnSnAs R Photon energy, ev ZnSiAs CdGeAs CdSiAs CuInSe GaAs GaP Averaged absorption coefficients of II IV V ternaries in comparison with GaAs, GaP and CuInSe 8 λ, nm 8 6 α, cm Photon energy, ev

27 COCLUSIOS All investigated A II -B IV -C V materials are found to be semiconductors; Most of materials (except Be-containing compounds) have the direct-band gap located at the Γ-point with the gaps ranging from about. to. ev. he band gaps in P-containing compounds are higher than those in As-containing ones; An increase of the atomic number of the group IV elements leads to the decrease of the energy of the start point of ε curve for all chalcopyrites studied; he static dielectric function (ε ()) remains practically the same for both light polarizations having the same order of magnitude in these materials; MgGeAs, MgSnP, MgSnAs, ZnSiAs, ZnGeP, ZnSnP, CdSiAs and CdGeP were found to have valuable dipole matrix elements of the first direct transitions that make them promising for light-emission applications; ZnSiAs and ZnSnAs seem to be the most suitable for photovoltaic applications as they have large absorption coefficients, optimal band gaps and lattice constants matched to GaAs substrate.

28 hank you for your attention!