Photovoltaic Materials

Transcription

1 18 th WIEN2k Workshop PennState University USA 2011 Photovoltaic Materials Xavier Rocquefelte Institut des Matériaux Jean-Rouxel(UMR 6502) Université de Nantes, FRANCE

2 JOINT EXP. & THEO. APPROACH From the experimental side: - Synthesis and XRD refinement: A. Lafond, P. Deniard, C. Guillot-Deudon, S. Jobic - NMR spectroscopy: M. Paris From the theoretical side: - Geometry optimization: VASP code - Analysis, simulation of spectroscopies: WIEN2k code THANKS TO 1- CIGSe study: MIOPS GROUP (head: S. Jobic) Marc Souilah PhD student 2- (In,Al) 2 S 3 study: Vincent Jouenne - 6 month project (master student)

3 Structure of electricityproduction Annual average growth rate (European Union) Solar 60 Geothermal 3.1% Wind 26.6% 50 Biomass.% 0 30 Wind Solar 55.% Hydraulic -0.5% Marine Energies -1.% Non-renewable waste 3.3% Nuclear 0.1% Fossil 1.7% 0-10

4 Structure of electricityproduction Average growth rate (European Union) Solar Geothermal -0.8% Wind.2% Biomass 5.% Solar 82.1% Hydraulic.7% Marine Energies -1.2% Wind Non-renewable waste.8% Nuclear 0.% Fossil -1.8% 0-10

5 Why solar energy is of interest? Amount of solar energy received on earth annually

6 Why solar energy is of interest? Amount of solar energy received on earth annually Annual consumption of energy on earth The amount of solar energy received on earth annually is 50,000 times greater than the total energy produced and consumed each year by the population

7 Why solar energy is of interest? Amount of solar energy received on earth annually Coil Stocks Annual consumption of energy on earth

8 Why solar energy is of interest? Amount of solar energy received on earth annually Gaz Oil Coil Stocks Annual consumption of energy on earth

9 Why solar energy is of interest? Amount of solar energy received on earth annually Gaz Oil Coil Uranium Stocks Annual consumption of energy on earth

10 18 th WIEN2k Workshop PennState University USA 2011 How a solar panel works? (solar to electrical power conversion)

11 Whichpart of the solarradiation spectrumisusedby PV? Solar radiation at the top of the atmosphere 2,5 2,0 Spectral Irradiance (W/m 2 /nm) 1,5 1,0 0, Wavelength (nm)

12 Whichpart of the solarradiation spectrumisusedby PV? Solar radiation at the top of the atmosphere Solar radiation at sea level 2,5 Spectral Irradiance (W/m 2 /nm) 2,0 1,5 1,0 0,5 0 O 3 O 2 H2 O H 2 O Absorption of solar radiations by atmosphere molecules (O 2, O 3 and H 2 O) H 2 O H2 O CO2 H 2 O Wavelength (nm)

13 Whichpart of the solarradiation spectrumisusedby PV? Solar radiation at the top of the atmosphere 2,5 Solar radiation at sea level Portion of the solar radiation used by a Si solar cell 2,0 Spectral Irradiance (W/m 2 /nm) 1,5 1,0 0,5 O 2 H2 O H 2 O 0 O 3 H 2 O H2 O CO2 Solar cell H 2 O Wavelength (nm)

14 Basic principles of solar to electric power conversion Solar Cell

15 Basic principles of solar to electric power conversion Solar Cell

16 Basic principles of solar to electric power conversion Solar Cell

17 Basic principles of solar to electric power conversion Solar Cell

18 Basic principles of solar to electric power conversion Solar Cell

19 Basic principles of solar to electric power conversion n-type Solar Cell p-type

20 Basic principles of solar to electric power conversion Solar Cell - + n-type p-type p-n junction

21 18 th WIEN2k Workshop PennState University USA 2011 Whichingredientsto have an efficient solarcell?

22 Which ingredients to have an efficient solar cell? A good absorber material (optimal band gap) NOT ONLY!

23 Which ingredients to have an efficient solar cell? A good absorber material (optimal band gap) NOT ONLY! parameters at least must be controled: 1. Light transmission up to the absorber layer (structure of the cell) 2. Light absorption of the absorber layer (optimal band gap) 3. Electron-hole separation (nature of the p-n junction). Charge carrier mobility and collection to the front and back contacts Illustration with a CIGS TF PV cell i.e. a Cu(In,Ga)Se 2 Thin Film Photovoltaic cell One of the more promising 2 nd generation PV cell for which NREL has reported a cell efficiency of 20% in 2008 NREL: National Renewable Energy Laboratory

24 Schematic representation of a CIGSe thin film PV cell On a soda lime glass substrate Glass substrate

25 Schematic representation of a CIGSe thin film PV cell On a soda lime glass substrate a molybdenum thin-film is sputter-deposited Mo (back contact) Glass substrate

26 Schematic representation of a CIGSe thin film PV cell Then the absorber material CIGSe (p-type) is co-evaporated CIGSe absorber (p-type) Mo (back contact) Glass substrate

27 Schematic representation of a CIGSe thin film PV cell And recovered by a chemical-bath-deposited CdS window recovered by a sputtered-deposited ZnO:Al window (n-type) ZnO:Al window CdS buffer layer CIGSe absorber Mo (back contact) Glass substrate N.B.: ZnO:Al is a Transparent Conducting Oxyde (TCO)

28 Schematic representation of a CIGSe thin film PV cell Finally, the device is terminated by e-beam-evaporated Ni/Al grids (front contact) and an MgF 2 antireflection coating Ni/Al grids (front contact) MgF 2 ZnO:Al window CdS buffer layer CIGSe absorber Mo (back contact) Glass substrate

29 Schematic representation of a CIGSe thin film PV cell On illumination 1. Light transmission up to the CIGSe layer Ni/Al grids (front contact) MgF 2 ZnO:Al window CdS buffer layer E g 10.8 ev 3. ev 2. ev Refractive index CIGSe absorber 1.2 ev 2.9 Mo (back contact) Glass substrate

30 Schematic representation of a CIGSe thin film PV cell On illumination 1. Light transmission 2. Light absorption by the CIGSe layer The light absorption in CIGSe lead to the photogeneration of electron-hole pairs E g value of the absorber material is essential to have an efficient solar to electrical power conversion Optimal band gap high efficiency

31 Atomic structure of the CIGSe compounds Structural parentage: From diamond to chalcopyrite structures Grimm-Sommerfeld rule: valence electrons/atom «diamond» structure III IV V VI Si IV I II B Al C Si N P O S III-V GaAs II-VI ZnSe = = Cu Ag Zn Cd Ga In Ge Sn As Sb Se Te II-IV-V 2 ZnGeAs 2 I-III-VI 2 CuInSe = 1++6 = 3

32 Si Si Si Si Si IV Si IV III V VI II I P As N Sb S Se O Te Al Ga B In Zn Cd Cu Ag Si Ge C Sn IV III V VI II I P As N Sb S Se O Te Al Ga B In Zn Cd Cu Ag Si Ge C Sn P As N Sb S Se O Te Al Ga B In Zn Cd Cu Ag Si Ge C Sn Atomic structure of the CIGSe compounds

33 Atomic structure of the CIGSe compounds III IV V VI B C N O I II Al Si P S Cu Zn Ga Ge As Se Ag Cd In Sn Sb Te Si IV Se 2- Zn 2+ II-VI ZnSe CdS

34 Atomic structure of the CIGSe compounds III IV V VI B C N O I II Al Si P S Cu Zn Ga Ge As Se Ag Cd In Sn Sb Te Si IV Cu + In 3+ II-VI ZnSe Se 2- I-III-VI 2 CuGaSe 2 CuInSe 2

35 Searchof the materialexhibitingan optimal band gap for PV 35 AM1.5 (Air Mass): indicate the thickness of atmosphere that solar radiation is crossing (sun at 8 ) 30 29% Efficiency (%) Black-body limit Optimal band gap for a single-junction PV cell: Eg 1. ev AM Band gap (ev)

36 Searchof the materialexhibitingan optimal band gap for PV Efficiency (%) Si GaAs CIGSe InP CISe CdTe CIGSSe CIS a-si:h Cu 2 S CGSe Black-body limit AM Band gap (ev)

37 Searchof the materialexhibitingan optimal band gap for PV 35 FIRST-GENERATION single-junction solar cells 30 Efficiency (%) Si InP GaAs Black-body limit AM Band gap (ev)

38 Searchof the materialexhibitingan optimal band gap for PV 35 SECOND-GENERATION single-junction solar cells 30 Efficiency (%) CISe Black-body limit AM CGSe Band gap (ev)

39 Searchof the materialexhibitingan optimal band gap for PV 35 SECOND-GENERATION single-junction solar cells 30 Efficiency (%) CISe 10 Possibility to adjust the band gap: E g =f(x) in CuIn 1-x Ga x Se 2 5 CIGSe? Black-body limit CGSe AM Band gap (ev)

40 Searchof the materialexhibitingan optimal band gap for PV 35 SECOND-GENERATION single-junction solar cells 30 Efficiency (%) CISe CIGSe Black-body limit AM However, the cell efficiency drops down if E g > 1.2 ev in CIGSe 5 CGSe Band gap (ev)

41 Search of the material exhibiting an optimal band gap for PV Why the efficiency drops down for Eg > 1.2 ev in CIGSe system? Efficiency (%) E g (ev) Appears to be related to the copper depletion at the surface of the CIGSe thin-film Is crystallography helpful to solve such problem (study of copper poor CIGSe phases)? Variation of the chemical composition: introduction of non-stoichiometry phase transistions?

42 18 th WIEN2k Workshop PennState University USA 2011 Cristallographicstudyof CIGSe PhDThesisof Marc Souilah(IMN)

43 Ternary diagram: from CuInSe 2 to CuGaSe 2 The x-direction controls the proportion of In and Ga in the trivalent crystallographic site Ga 2 Se 3 CuGaSe 2 x Chemical Formula: CuIn 1-x Ga x Se 2 CuGa 0.5 In 0.5 Se 2 Cu + M 3+ (In 3+ or Ga 3+ ) Se 2- Cu 2 Se CuInSe 2 In 2 Se 3

44 Ternary diagram: towards copper-poor compounds The z - direction controls the proportion of Cu, V and 3+ Cu M Cu Ga 2 Se 3 CuGa 0.5 In 0.5 Se 2 Ga 1 In 1 Se Cu 3 (M 3+ ) 3 Se 6 (M 3+ ) Se 6 Charge balance along z: x Ga 1 In 1 Se 3 3 Cu + 1 M V Cu Chemical Formula: Cu 1-z 2z/3 (In 1-x Ga x ) 1+z/3 Se 2 CuGa 0.5 In 0.5 Se 2 z If z > 0, then the compound is copper poor and non-stoichiometric Cu 2 Se In 2 Se 3

45 Ternary diagram: towards copper-poor compounds The z - direction controls the proportion of Cu, V and 3+ Cu M Cu Ga 2 Se 3 Chemical Formula: Cu 1-z 2z/3 (In 1-x Ga x ) 1+z/3 Se 2 z = 0.2 x Ga 1 In 1 Se 3 Chemical Formula: Cu (In 1-x Ga x ) 1.08 Se 2 x = 0.5 Cu Ga 0.5 In 0.5 Se 2 CuGa 0.5 In 0.5 Se 2 z Cu 2 Se z = 0.2 In 2 Se 3

46 Comparison of the powder X-Ray diagrams of 2 compounds 16 Stoichiometric Copper poor 1 Intensity (a.u) 10 8 x = 0.5, z=0.2 Cu Ga 0.5 In 0.5 Se 2 6 x = 0.5, z= θ ( ) Cu 1 Ga 0.5 In 0.5 Se 2 Forbidden I-2d reflections appears in the non-stoichiometric compound (z = 0.2) Single-crystal refinement + TEM Stannite-type structure (I-2m) M. Souilah, A. Lafond, N. Barreau, C. Guillot-Deudon, and J. Kessler Applied physics letters 92, (2008)

47 Conclusions of the structural investigation Ga 2 Se 3 Chalcopyrite structure type (I-2d) Stannite structure type (I-2m) Multi-phases compound Chalcopyrite structure Stannite structure Cu 2 Se In 2 Se 3 z = 0 z > 0

48 Conclusions of the structural investigation Chalcopyrite structure Ga 2 Se 3 Stannite structure Chalcopyrite structure type (I-2d) Stannite structure type (I-2m) Multi-phases compound Stability of the copper poor compounds? Impact of the Cu vacancies on the atomic structure type? Cu 2 Se In 2 Se 3 z = 0 z > 0

49 18 th WIEN2k Workshop PennState University USA 2011 DFT geometry optimization of stoichiometric and non-stoichiometric CIGSe phases

50 DFT geometry optimization From X-ray diffraction and TEM we know that: - Stoichiometric (S) phases (z = 0) have a chalcopyrite structure-type (I-2d) - Non-stoichiometric (NS) phases (z 0) which are significantly copper poor have a stannite structure-type (I-2m) Our calculations: - Geometry optimization using the VASP code Pseudopotential: PAW Basis set: Plane Wave Cutoff Energy of 500 ev Functional: Generalized Gradient Approximation (PBE) Optimization: Atomic positions using the experimental cell parameters Supercell treatment: a 3b c cells (about 190 atoms) to have a random distribution of In Cu, V and Ga Cu and M in "S" models (S1and S2) 3+ Cu in "NS" models (NS1and NS2)

51 DFT geometry optimization Supercell treatment: a 3b c cells (about 190 atoms) Cu + M 3+ Se 2- Crystallographic unit cell a 3b c supercell For x = 0.5 Ga/In = 0.5 For z = 0 Stoichiometric For z 0 Non-stoichiometric Chemical Formula: Cu 8 (In 2 Ga 2 )Se 96 How non-stoichiometry is treated in our calculation?

52 DFT geometry optimization How non-stoichiometry is treated in our calculation? Starting from the stoichiometric composition: Cu 8 (In 2 Ga 2 )Se 96 We should remove Cu + ions respecting the following charge balance: Charge balance along z: 3 Cu + 1 M V Cu Chemical Formula: Cu 1-z 2z/3 (In 1-x Ga x ) 1+z/3 Se 2 3 Cu + 1 M V Cu 6 Cu + 2 M 3+ + V Cu 9 Cu + 3 M V Cu Cu + M V Cu z = 3/8 = z = 6/8 = 0.5 z = 9/8 = z = /8 = 0.25

53 DFT geometry optimization How non-stoichiometry is treated in our calculation? Starting from the stoichiometric composition: Cu 8 (In 2 Ga 2 )Se 96 We should remove Cu + ions respecting the following charge balance: Charge balance along z: 3 Cu + 1 M V Cu Chemical Formula: Cu 1-z 2z/3 (In 1-x Ga x ) 1+z/3 Se 2 9 Cu + 3 M V Cu z = 9/8 = For z = 0 Stoichiometric Chemical Formula: Cu 8 (In 2 Ga 2 )Se 96 For x = 0.5 Ga/In = 0.5 For z = Non-Stoichiometric Chemical Formula: Cu 39 6 (In 2 Ga 2+3 )Se 96

54 DFT geometry optimization Crystallographic cell a b c a c 16 atoms / cell Cu + M 3+ 8 Se 2-

55 DFT geometry optimization Crystallographic cell a b c a Supercell a 3b c c 16 atoms / cell 16 = 192 atoms / cell Cu + 8 Cu + Site a (I-2d) M 3+ 8 M 3+ Site b (I-2d) 8 Se 2-96 Se 2- (not shown)

56 DFT geometry optimization b a Crystallographic cell a b c a Supercell a 3b c c 16 atoms / cell 16 = 192 atoms / cell Cu + 8 Cu + Site a (I-2d) M 3+ 8 M 3+ Site b (I-2d) 8 Se 2-96 Se 2- (not shown)

57 DFT geometry optimization Crystallographic cell a b c a Supercell a 3b c b a GRC c GRC: Global Row Charge 16 atoms / cell 16 = 192 atoms / cell Rows containing Cu + : GRC = 1 = Rows containing M 3+ : GRC = 3 = Cu + 8 Cu + Site a (I-2d) M 3+ 8 M 3+ Site b (I-2d) 8 Se 2-96 Se 2- (not shown)

58 DFT geometry optimization Crystallographic cell a b c a c 16 atoms / cell Supercell a 3b c Cu + 8 Cu + Site a (I-2d) M 3+ 8 M 3+ Site b (I-2d) 8 Se 2-96 Se 2- (not shown) b 16 = 192 atoms / cell a GRC

59 Structural filiations from I-2d and I-2m I-2d Site a (I-2d) Site b (I-2d) Positions = 8 = 8 S1 and S2 models 8 Cu+ 2 Ga 3+ 2 In 3+

60 Structural filiations from I-2d and I-2m I-2d Site a (I-2d) Site b (I-2d) Positions = 8 = 8 S1 and S2 models 8 Cu+ 2 Ga 3+ 2 In 3+ I-2m 1/2 1/2 Positions NS1 model Site 2a (I-2m) 2 = 2 2 Cu + 0 V Cu Site 2b (I-2m) 2 = 2 2 In 3+

61 Structural filiations from I-2d and I-2m I-2d Site a (I-2d) Site b (I-2d) Positions = 8 = 8 S1 and S2 models 8 Cu+ 2 Ga 3+ 2 In 3+ I-2m 1/2 1/2 1/2 1/2 Site 2a (I-2m) Site d (I-2m) Site 2b (I-2m) Positions 2 = 2 = 8 2 = 2 NS1 model 2 Cu + 0 V Cu 15 Cu + 2 Ga3+ 6 V Cu 2 In 3+ 3 Ga Cu 9 Cu + 3 M V Cu z = 9/8 =

62 Structural filiations from I-2d and I-2m I-2d Site a (I-2d) Site b (I-2d) Positions = 8 = 8 S1 and S2 models 8 Cu+ 2 Ga 3+ 2 In 3+ I-2m 1/2 1/2 1/2 1/2 Site 2a (I-2m) Site d (I-2m) Site 2b (I-2m) Positions 2 = 2 = 8 2 = 2 NS2 model 21 Cu + 3 V Cu 18 Cu + 2 Ga3+ 3 V Cu 2 In 3+ 3 Ga Cu 9 Cu + 3 M V Cu z = 9/8 =

63 Stoichiometric models Chalcopyrite structure-type (Space group I-2d) S1 Model GRC S2 Model GRC Cu In Ga V Cu Ga Cu

64 Stoichiometric models Chalcopyrite structure-type (Space group I-2d) Stoichiometric Models Stannite structure-type (Space group I-2m) S1 Model S2 Model NS1 Model NS2 Model GRC GRC GRC GRC Cu In Ga V Cu Ga Cu

65 Stoichiometric models Chalcopyrite structure-type (Space group I-2d) Stoichiometric Models Stannite structure-type (Space group I-2m) S1 Model S2 Model NS1 Model NS2 Model GRC GRC GRC GRC E(S1-S2)=0meV 3 3 E(NS1-NS2)=5.9eV Cu In Ga V Cu Ga Cu

66 Analysis and Optical simulations Our calculations - WIEN2k code - Necessity to have a correct description of the d-states of Cu, In and Ga atoms LDA+U Illustration of our approach: 1) Choice of the k-mesh for the optical simulations 2) Choice of the U eff values comparison with XPS experiments 3) DFT optical simulations comparison with ellipsometry data

67 Optical properties: Diffusion and Absorption Diffusion Absorption n Complex index of refraction: 1.5 N = n+ i k k Energy (ev) Energy (ev) Kramers-Kronig ε 1 ε 2 Complex dielectric function: 1.5 ε= ε 1 + i ε ε 1 = n 2 k Energy (ev) ε 2 = 2nk Energy (ev)

68 Comparison between DOS and XPS (choice of U eff ) 10 5 Cu(3d) XPS DOS DOS lissée Binding Energy (ev) In(d) Binding Energy (ev) Binding Energy (ev) Binding Energy (ev) LDA LDA+U: Cu(d) LDA+U: In(d) LDA+U: Cu(d) & In(d) Comparison theory vs experiment * for CuInSe 2 Cu: U eff = ev In: U eff = 10.9 ev XPS DOS DOS+broad. * J. C. Rife, R. N. Dexter, P. M. Bridenbaugh and B. W. Veal, Physical Review B, 16(10) (1977) 91. U eff = U-J

69 Comparison between DOS and XPS (choice of U eff ) 10 5 XPS DOS DOS lissée 10 5 GGA+U calc. calc. XPS Cu: Cu: U eff ev DOS eff = ev Ga: Ga: U eff ev DOS lissé eff = 7 ev In: In: U eff 10.9 ev eff = 10.9 ev Binding Energy (ev) Binding Energy (ev) XPS DOS DOS+broad CuInSe 2 LDA+U: Cu(d) & In(d) CuGaSe 2 LDA+U: Cu(d) & Ga(d)

70 Optical properties simulation: Number of k points 1.8 CuInSe 2 LDA calculations (WIEN2k) 1.6 Extinction coefficient (k) Exp. data Energie (ev)

71 Optical properties simulation: Number of k points Extinction coefficient (k) CuInSe 2 LDA calculations (WIEN2k) #kpts in Full Brillouin zone 500k Exp. data Energie (ev)

72 Optical properties simulation: Number of k points Extinction coefficient (k) CuInSe 2 LDA calculations (WIEN2k) #kpts in Full Brillouin zone 500k 1500k Exp. data Energie (ev)

73 Optical properties simulation: Number of k points Extinction coefficient (k) CuInSe 2 LDA calculations (WIEN2k) #kpts in Full Brillouin zone 500k 1500k 3000k Exp. data Energie (ev)

74 Optical properties simulation: Number of k points Extinction coefficient (k) CuInSe 2 LDA calculations (WIEN2k) #kpts in Full Brillouin zone 500k 1500k 3000k 5000k Exp. data Energie (ev)

75 Optical properties simulation: Number of k points Extinction coefficient (k) Exp. LDA (SO = 1eV) SO CuInSe Energy (ev) SO = scissors operator

76 Optical properties simulation: GGA+U correction CuInSe 2 Exp. LDA+U : Cu(d) & In(d) (SO = 0.75 ev) Extinction coefficient (k) SO Energy (ev) SO = scissors operator

77 Relation between non-stoichiometry and band gap Effect of the non-stoichiometry on the evolution of the band gap M 3+ s.p/se p S1 model (I-2d) NS2 model (I-2m) 600 Total DOS Total DOS Energy (ev ) Se p/cu d E 1 2 F Energy (ev) DOS calculations and optical simulations using the WIEN2k code M. Souilah, X. Rocquefelte, A. Lafond, C. Guillot-Deudon, J.-P. Morniroli, J. Kessler Thin Solid Films 517 (2009)

78 Relation between non-stoichiometry and band gap 1.5 Partial densities of states for Ga Ga and Se Se Partial DOS (a.u.) Ga Ga Se Energy (ev) Observed both in S1 (I-2d) and NS2 (I-2m) models

79 Relation between non-stoichiometry and band gap 1.5 Partial densities of states for Ga Ga and Se Se Ga Ga 1.0 Se Energy (ev) Observed only in NS2 (I-2m) models

80 Relation between non-stoichiometry and band gap 1.5 Both in S1 model (I-2d) and NS2 model (I-2m) 1.5 In NS2 model (I-2m) only Partial DOS (a.u) Ga Ga Se Ga Cu Se E 2 Energy (ev) F Se coord. 2 M 3+ 2 Cu E 2 Energy (ev) F Se coord. 3 M 3+ 1 Cu + Ga Ga Ga Cu

81 CIGSe conclusion To meet the 20% EU goal of renewable energies in 2020: Crucial to improve the efficiencies of the 2 nd generation solar cells (thin-film technology) One of the more promising absorber materials: Cu(In,Ga)Se 2 Its structure has been reinvestigated showing for the very first time a phase transition from chalcopyrite to stannitestructure-types. Preliminary DFT results The geometry optimizations evidenced the role played by the pairdefects (2V Cu + Ga Cu ) in the stabilisationof the stannitestructure-type The related densities of states show that the effect of the non-stoichiometry is: -to reduce the band gap - to change significantly the band character near the Fermi level

82 18 th WIEN2k Workshop PennState University USA 2011 XRD-NMR-DFT combinedstudyof the buffer material(in,al) 2 S 3

83 Schematic representation of a CIGSe thin film PV cell On illumination 1. Light transmission / 2. Light absorption 3. e-h separation (role of the p-n junction) Ni/Al grids (front contact) MgF 2 ZnO:Al (n-type) CdS buffer layer CIGSe absorber (p-type) Mo (back contact) Glass substrate

84 How to separate the photogeneratedelectron-hole pair? Use of a single p-n homojunction Si p-doped SCR Si n-doped e Energy (ev) hν h + E CB E F E VB Position ( µm) E VB : Energy of the top of the valence band SCR: Space Charge Region E CB : Energy of the bottom of the conduction band

85 How to separate the photogeneratedelectron-hole pair? Use of a single p-n heterojunction CIGSe p-doped SCR e ZnO:Al n-doped Energy (ev) hν E VB E CB h +recombination E F E CB E VB Position ( µm) E VB : Energy of the top of the valence band SCR: Space Charge Region E CB : Energy of the bottom of the conduction band

86 How to separate the photogeneratedelectron-hole pair? Use of a single p-n heterojunction + a buffer layer CIGSe p-doped e CdS ZnO:Al n-doped E CB Energy (ev) hν E VB h + E CB E F E CB E VB E VB Position ( µm) E VB : Energy of the top of the valence band SCR: Space Charge Region E CB : Energy of the bottom of the conduction band

87 Thin-film CIGSe solar cell and its related energy diagram Glass substrate Molybdenum hν CIGSe p-doped e CdS ZnO:Al n-doped E CB Energy (ev) hν E VB h + E CB E F E CB E VB E VB Position ( µm)

88 Evolution of the band gap as a function of Al content in thin-films compounds 2.25 Eg (ev) Expected behaviour Aim for the alternative layer: Eg 2. ev (CdS) Saturation of the band gap (Eg 2.15 ev) Is it due to a structural change (phase transition)? x in (In 1-x Al x ) 2 S 3

89 Structural evolutions in (In 1-X Al x )S 3 α+β β α α + γ x (Al) Ordered defect spinel structure In vacancies are randomly distributed on the Td sites Tetragonal I 1 /amd Cubic Fd-3m Structural evolution?

90 Atomic structure of β-in 2 S 3 Tetragonal phase (β) I 1 /amd a = Å c = Å 16 f.u. / unit cell In 3+ Oh site (8c) In 3+ Oh site (16h) In 3+ Td site (8e) S 2- site (16h) c a b

91 Atomic structure of β-in 2 S 3 Tetragonal phase (β) I 1 /amd a = Å c = Å 16 f.u. / unit cell In 3+ Oh site (8c) In 3+ Oh site (16h) In 3+ Td site (8e) S 2- site (16h) c a b

92 Atomic structure of β-in 2 S 3 Tetragonal phase (β) I 1 /amd a = Å c = Å 16 f.u. / unit cell In 3+ Oh site (8c) In 3+ Oh site (16h) In 3+ Td site (8e) S 2- site (16h) c a b

93 Atomic structure of β-in 2 S 3 Tetragonal phase (β) I 1 /amd a = Å c = Å 16 f.u. / unit cell In 3+ Oh site (8c) In 3+ Oh site (16h) In 3+ Td site (8e) S 2- site (16h) c a b

94 Atomic structure of β-in 2 S 3 Tetragonal phase (β) I 1 /amd a = Å c = Å 16 f.u. / unit cell In 3+ Oh site (8c) In 3+ Oh site (16h) In 3+ Td site (8e) S 2- site (16h) c a b

95 Atomic structure of β-in 2 S 3 Tetragonal phase (β) I 1 /amd a = Å c = Å 16 f.u. / unit cell In 3+ Oh site (8c) In 3+ Oh site (16h) In 3+ Td site (8e) S 2- site (16h) c a b

96 Atomic structure of β-in 2 S 3 Tetragonal phase (β) I 1 /amd a = Å c = Å 16 f.u. / unit cell In 3+ Oh site (8c) In 3+ Oh site (16h) In 3+ Td site (8e) S 2- site (16h) Ordered defect spinel structure a c b c a

97 Atomic structure of β-in 2 S 3 Tetragonal phase (β) I 1 /amd a = Å c = Å 16 f.u. / unit cell In 3+ Oh site (8c) occ.: 1 In 3+ Oh site (16h) occ.: 1 In 3+ Td site (8e) occ.: 1 S 2- site (16h) occ.: 1 V In site a occ.: 1 Ordered defect spinel structure a c b c a

98 Structural evolutions in (In 1-X Al x ) 2 S 3 α+β β α α + γ x (Al) Ordered defect spinel structure In vacancies are randomly distributed on the Td sites Tetragonal I 1 /amd Cubic Fd-3m Structural evolution?

99 Structural evolutions in (In 1-X Al x ) 2 S 3 α phase: Ordered In a β phase: Disordered In a c c

100 Structural evolutions in (In 1-X Al x ) 2 S 3 Cubic phase (α) Fd-3m a = Å 8 f.u. / unit cell b c a In 3+ Oh site (16d) occ.: 1 In 3+ Td site (8a) occ.: 2/3 S 2- site (32e) occ.: 1 1/3 V In are randomly distributed on this site

101 Origin of the band gap saturation in (In 1-x Al x ) 2 S 3 Joint experimental & theoretical study Powder and single crystal X-ray diffraction DFT geometry optimization Relative Energies Atomic structure (extended probe) - Site preference of Al NMR spectroscopy EFG calculations NMR signal attribution Local structure around Al 3+ ions Diffuse Reflectance DOS from DFT Understanding of the band gap variation upon Al substitution

102 27 Al NMR data of (In 1-x Al x ) 2 S 3 powder samples 27 Al MAS spectrum of (In 1-x Al x ) 2 S 3 for 20% of Al 3+ 2D 27 Al 3QMAS spectrum of (In 1-x Al x ) 2 S 3 for 20% of Al 3+ Intensity (a.u.) Tetrahedral environments Octahedral environments x=0.20 Isotropic dimension (ppm) (ppm) (ppm) (ppm) MAS dimension (ppm) Al nuclear spin I = 5/2 quadrupolar nuclei Sensitive to the Electric Field Gradient (EFG): V zz V yy V xx Quadrupolar coupling constant: C Q = e.q.v zz h η Q = (V xx V yy ) with Q the Al quadropolar moment Assymetry parameter: V zz

103 27 Al NMR data of (In 1-x Al x ) 2 S 3 powder samples 27 Al MAS spectrum of (In 1-x Al x ) 2 S 3 for 20% of Al 3+ 2D 27 Al 3QMAS spectrum of (In 1-x Al x ) 2 S 3 for 20% of Al 3+ Intensity (a.u.) Tetrahedral environments Octahedral environments x=0.20 Isotropic dimension (ppm) (ppm) (ppm) (ppm) MAS dimension (ppm) Al nuclear spin I = 5/2 quadrupolar nuclei Sensitive to the Electric Field Gradient (EFG): V zz V yy V xx Quadrupolar coupling constant: C Q = e.q.v zz h η Q = (V xx V yy ) with Q the Al quadropolar moment Assymetry parameter: V zz

104 27 Al NMR data of (In 1-x Al x ) 2 S 3 powder samples Intensity (a.u.) 27 Al MAS spectrum of (In 1-x Al x ) 2 S 3 for 20% of Al 3+ Tetrahedral environments Octahedral environments x= D 27 Al 3QMAS spectrum of (In 1-x Al x ) 2 S 3 for 20% of Al (ppm) MAS dim Al 3+ observed in 3 different Td sites - 1 site with C Q.8 MHz and η Q sites with C Q 0-2 MHz

105 27 Al NMR data of (In 1-x Al x ) 2 S 3 powder samples 27 Al MAS spectrum of (In 1-x Al x ) 2 S 3 for 20% of Al 3+ 2D 27 Al 3QMAS spectrum of (In 1-x Al x ) 2 S 3 for 20% of Al 3+ Intensity (a.u.) Tetrahedral environments Octahedral environments x=0.20 Isotropic dimension (ppm) (ppm) (ppm) (ppm) MAS dimension (ppm) Al 3+ observed in 2 different Oh sites, at least - 1 site with C Q 5 MHz and η Q site with C Q MHz and η Q 0

106 27 Al NMR data of (In 1-x Al x ) 2 S 3 powder samples 27 Al MAS spectrum of (In 1-x Al x ) 2 S 3 for % and 20% of Al 3+ Tetrahedral environments Octahedral environments C Q (MHz) η Q x = 0.0 x = 0.20 Intensity (a.u.) x=0.0 x=0.20 Td(1) Td(2) Td(3) % 58% (ppm) 0-50 Oh(1) Oh(2) % 2% At x = 0.0, no Td(1) line is observed Al 3+ first occupy the Oh sites and then fill the Td sites Could we attribute these NMR signals to specific atomic arrangements?

107 Towards a full description of the local structure of Al 3+ ions in (In 1-x Al x ) 2 S 3 DFT geometry optimization of a model compound (diluted limit): In 63 Al 1 S 96 x = 1/6 = 0.016

108 Towards a full description of the local structure of Al 3+ ions in (In 1-x Al x ) 2 S 3 DFT geometry optimization of a model compound (diluted limit): In 63 Al 1 S 96 x = 1/6 = Unit cell of the periodic array

109 Towards a full description of the local structure of Al 3+ ions in (In 1-x Al x ) 2 S 3 DFT geometry optimization of a model compound (diluted limit): In 63 Al 1 S 96 x = 1/6 = Defects periodicity is lost

110 Towards a full description of the local structure of Al 3+ ions in (In 1-x Al x ) 2 S 3 DFT geometry optimization of a model compound (diluted limit): In 63 Al 1 S 96 x = 1/6 = NMR (EFG) Local probe

111 Towards a full description of the local structure of Al 3+ ions in (In 1-x Al x ) 2 S 3 DFT geometry optimization of a model compound (diluted limit): In 63 Al 1 S 96 x = 1/6 = NMR (EFG) Local probe

112 Towards a full description of the local structure of Al 3+ ions in (In 1-x Al x ) 2 S 3 DFT geometry optimization of a model compound (diluted limit): In 63 Al 1 S 96 x = 1/6 = SITES 5 STRUCTURAL MODELS 5 signatures 1- Atomic relaxation around the Al 3+ defect 2- Proximity with the In vacancies 3- Relative energies of the Al 3+ defects (after relaxation) - DFT calculation of 23 Al NMR parameters (C Q and η Q ) 5- Analysis of the densities of states and discussion of the band gap evolution upon Al substitution DFT calculations details: supercell of 162 atoms to avoid any artificial interactions between the defects.

113 Atomic structure of β-in 2 S 3 Oh(8c) Oh(16h) Tetragonal phase (β) I 1 /amd a = Å c = Å 16 f.u. / unit cell Td(8e) Td(a) In 3+ Oh site (8c) In 3+ Oh site (16h) In 3+ Td site (8e) S 2- site (16h) Ordered defect spinel structure a c b c a

114 Oh(16h) Oh(8c) Td(8e) Td(a) Td(a)' d (Å) V In Al a = 1. V In Al a =.7

115 Oh(16h) Oh(8c) Td(8e) Td(a) Td(a)' d (Å) V In Al a = 1. V In Al a =.7 Relax. Relax. Relax. Relax. Relax. d (Å)

116 Oh(16h) Oh(8c) Td(8e) Td(a) Td(a)' E (mev) V In Al a = 1. Å V In Al a =.7 Å d (Å)

117 Oh(16h) Oh(8c) Td(8e) Td(a) Td(a)' C Q (MHz) η Q E (mev) V In Al a = 1. Å V In Al a =.7 Å d (Å)

118 Oh(16h) Oh(8c) Td(8e) Td(a) Td(a)' Oh(2) Oh(1) Td(2/3) Td(1) C Q (MHz) -.1 () +.3 (5) -0.2 (0-1) (.8) η Q 0.0 (0) 0.6 (0.6) (0.3) E (mev) V In Al a = 1. Å V In Al a =.7 Å d (Å)

119 27 Al NMR data attribution for (In 1-x Al x ) 2 S 3 Oh(8c) Oh(16h) Tetrahedral environments Octahedral environments Td(8e) Intensity (a.u.) Td(8e) Oh(8c) x=0.0 Td(a) Oh(16h) (ppm) 0-50 c a b

120 27 Al NMR data attribution for (In 1-x Al x ) 2 S 3 Oh(8c) Oh(16h) Tetrahedral environments Octahedral environments Td(8e) Td(a) Intensity (a.u.) x=0.0 x= (ppm) 0-50 c a b

121 27 Al NMR data attribution for (In 1-x Al x ) 2 S 3 Oh(8c) 20% of Al 3+ Oh(16h) Tetrahedral environments Octahedral environments Td(8e) Td(a) Intensity (a.u.) x=0.0 x= (ppm) 0-50 a c b ppm

122 27 Al NMR data attribution for (In 1-x Al x ) 2 S 3 Oh(8c) 20% of Al 3+ Oh(16h) Tetrahedral environments Octahedral environments Td(8e) Td(a) Intensity (a.u.) x=0.0 x= (ppm) 0-50 a c b ppm

123 27 Al NMR data attribution for (In 1-x Al x ) 2 S 3 Oh(8c) 20% of Al 3+ Oh(16h) Tetrahedral environments Octahedral environments Td(8e) Intensity (a.u.) x=0.0 x=0.20 Td(a) Td(a) (ppm) 0-50 a c b ppm

124 Discussion of the band gap evolution in (In 1-x Al x ) 2 S 3 Oh(8c) Oh(16h) Td(8e) Td(a) c For x = 0.0: 3 sites are occupied by Al 3+ ions Densities of States (States/eV/f.u) ENERGY (ev) ENERGY (ev) Oh(16h) Oh(8c) Td(8e) E = 0meV E = +7meV E = +170meV a b ENERGY (ev)

125 Discussion of the band gap evolution in (In 1-x Al x ) 2 S 3 Oh(8c) Oh(16h) Td(8e) Td(a) c For x = 0.20: The Td site (a) which is empty in the tetragonal phase is occupied by Al 3+ ions Densities of States (States/eV/f.u) Td(a)' ENERGY (ev) Td(a) ENERGY (ev) E = +2797meV V In Al a = 1. Å E = +65meV V In Al a =.7 Å a b

126 Conclusions (In,Al) 2 S 3 / conclusion Discussions - An approach combining NMR to XRD and DFT allows to identify the atomic arrangement of a defect when it differs from the host lattice (different element). Al 3+ ions first occupy the Oh sites and the Td(8e) site and only for higher concentration the Td(a) site The occupation of all the sites by Al 3+ ions do not leads to a significant decrease of the band gap, except for the Td(a) site in specific situations (distance with In vacancies) Next steps for this study: - growing of (In,Al) 2 S 3 thin-films with additional elements (Cu +, Na + ) to fill the Td(a) sites and avoid the occupation by Al 3+ ions. - more calculations taking into account other defects (effect of the vacancies) and concentrations of defects, chemical shifts (WIEN2k)

127 General conclusion The accurate knowledge of the atomic structure of a compound is crucial to properly: - understand the emergence of a property - simulate a property - tune a property The present period allows us to go towards the accurate characterization of the chemical nature of defects embedded in complex materials (ternary, quaternary compounds) Need the combination of various techniques from both experimental and theoretical sides

128 Thank you for your attention Nantes in few pictures