Hybrid Perovskite Solar Cells

1 Hybrid Perovskite Solar Cells Annamaria Petrozza "ORGANIC ELECTRONICS : principles, devices and applications" Milan, Novembre 27 th, 2015

Perovskite Crystal with ABX 3 stoichiometry A X B I-V-O3, II-IV-O3 and III-III-O3 e.g KTaO3, SrTiO3 and GdFeO3 I-II-X3, e.g. CsSnI3, CH3NH3PbI3, 2

It is not only a matter of stoichiometry. A X B The Goldschmidt tolerance factor: R A RX t 2( R B RX ) t< 0.7 the perovskite falls apart t >1 towards 2D structures 3

Playing with Dimensionality CH 3 NH 3 PbI 3 (CH 3 NH 3 ) 2 PbI 4 (C 10 H 21 NH 3 ) 2 PbI 4 (CH 3 NH 3 ) 4 PbI 6 *H 2 O 4

Dielectric Confinement CH 3 NH 3 PbI 3 (CH 3 NH 3 ) 2 PbI 4 (C 10 H 21 NH 3 ) 2 PbI 4 (CH 3 NH 3 ) 4 PbI 6 *H 2 O Ishihara, J. Lum, 1994. 5 5

Organo-Metal Halide Crystalline Perovskite ABX 3 B = (CH 3 NH 3+ ) x X= I -, Cl -, Br - A= Pb 2+ 6

Organo-Metal Halide Crystalline Perovskite Filip et al, Nat Comm, 2014 Eric Hoke et al, Chem Sci, 2015 7

Organo-Metal Halide Crystalline Perovskite 8

CH3NH3 + :Orientational disorder and Polarizability Hydrogen Bonds Dipole nearly free to move 9

Modular Structure 10

Prone to a variety of processing methods 11

Excitonic Solar Cells Type II Hetero-Junction

Voc (V) Intrinsic Loss in Excitonic Solar Cells 1.2 Voc = BG -0.2eV -0.4eV -0.6eV 1.0 0.8 Si CIGS GaAs CdTe DSC MSSC OPV asi 0.6 ncsi CZTSS 0.4 1.0 1.2 1.4 1.6 1.8 Bandgap (ev) 13

Hybrid Crystals in DSSC Devices CH 3 NH 3 PbI 3 perovskite as light antenna in the DSSC concept e- injection h u Hole transfer H. S. Kim et al. Sci Rep. 2: 591 (2012)

Hybrid Crystals in Hybrid Solar Cells Silver HTL Perovskite role: Absorber Electron-transporter FTO TiO2 Al2O3 Glass M.Lee et al. Science 338, 643 (2012)

Which is their strength? 16

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Optoelectronic Devices Zhu et al, Nat Mat, 2015 Tan et al, Nat Nanotech, 2014 18

Designing the Device Architecture HTM TiO2/Al2O3 scaffold+ Perovskite HTM Perovkite capping layer HTM Perovkite capping layer J. Ball, EES, 2013 Nano-structured vs Thin Film 19

Designing the Device Architecture Light Absorption Charge Generation Photo-carriers Transport

Designing the Device Architecture Light Absorption Charge Generation Photo-carriers Transport

Silicon Light Absorption GaAs Dyes/conjugated polymers In direct band-gap Phonon assisted um range sun light penetration depth thick solar cells no light emission direct band-gap excitonic effects at absorption edge good light penetration depth localized states excitonic effects large absorption cross-section/ efficient carrier recombination

Silicon Light Absorption GaAs Dyes/conjugated polymers In direct bandgapphonon assisted um range visible light penetration dept thick solar cells no light emission direct band-gap excitonic effects at absorption edge good light penetration depth localized states excitonic effects large absorption cross-section/ efficient carrier recombination

Optical Density (a.u.) Silicon Light Absorption Hybrid Perovskites Dyes/conjugated polymers 1.0 4.2K 290K In direct bandgapphonon assisted um range visible light penetration dept thick solar cells no light emission 0.5 1.50 1.53 1.56 1.59 1.62 1.65 1.68 1.71 1.74 1.77 1.80 Energy (ev) direct band-gap excitonic effects at absorption edge good light penetration depth localized states excitonic effects large absorption cross-section/ efficient carrier recombination

Exciton..is a quasi-particle that represents a collective excited state of an ensable of atoms or molecules. It is represented by a wavepacket for which we can define mass and speed which transports Energy 25

How many kind of excitons?? Wannier-Mott Frenkel Binding energy Delocalization Oscillator strength Triplet Singlet Lifetime 26

Frenkel Exciton Solids made by weakly interacting units (e.g organic crystals, inter-molecular coupling weaker than the intra-molecular ones). General case of a Molecular Excitation (DSSC) The wavefunction squared amplitude is the probability of finding the excited state in the lattice. G. Lanzani, The Photophysics behind Photovoltaics and Photonics Wiley-VCH E u g De e 27

Wannier-Mott Solids with tightly bounded atoms Low Screening Coulomb attraction does not allow the generation of FREE e-h pairs Hydrogenoid system. Center of Mass/Exciton radius (LARGER than E the lattice constant) k e k h R X r h r e O 2 2 Eb K X EK X, n Eg 2 n m m e G. Lanzani, The Photophysics behind Photovoltaics and Photonics Wiley-VCH h E 1 K E g

Why do we need to know the details? Energy spent in the exciton dissociation

Why do we need to know the details? Energy spent in the exciton dissociation Absoprtion cross section enhancement X 1 ( a B 3 )

Why do we need to know the details? Energy spent in the exciton dissociation Absoprtion cross section enhancement Elliott s Theory (1963)

Exciton Vs Free Charges at the Thermodynamic Equilibrium Total excitation density Excitons n n FC n exc PV regime @ RT n n FC exc Free charges 2 k ( h Law of mass action T B ) 3/ 2 2 e Eb k T Saha-Langmiur equation B D Innocenzo et al, Nat Comm. 5, 3586, 2014 x n FC n 32

Exciton Binding Energy Exciton Reduced mass E B Dielectric constant 4 2 e 2 2 2 e 2 a 2 B 2 e a B 2 e memh m m h Bohr Radius e The golden rule: the Bohr orbital frequency (E b /h) must be compared with the optical phonon frequency

Dielectric constant (ε): A measure of a substance s ability to insulate charges from each other.

Screening in solids: basic concept E 0 METAL DIELECTRIC E=0 E d E d = E 0 ε

Screening mechanisms Electronic Polarizability (Electron Cloud Distortion) 10 15 s -1 Dipole Re Orientation (Langevin Mechanism) 10 8-10 10 s -1 Ions Displacement 10 10-10 0 s -1 (Optical Phonons)

Screening mechanisms

Dielectric constant of GaAs. Why it is so easy e 0 2 e e 0 (T) = 12.4 + 0.00012*T LO = 36meV TO = 38meV

Dielectric constants of CH3NH3PbI3, real and imaginary parts

Dielectric constants of CH3NH3PbI3, real and imaginary parts 1KHz

(I) Direct Measurement of Exciton Binding Energy in CH 3 NH 3 PbI 3 Exciton Binding Energy hd D u k 1 u exp( k E B b T ) E b (upper limit)= 50meV Limitations: It is assumed that exciton-phonon interaction induces only exciton dissociation though in a limited range it assumes the exciton binding energy constant in T D Innocenzo et al, Nat Comm. 5, 3586, 2014 41

(I) Direct Measurement of Exciton Binding Energy in CH 3 NH 3 PbI 3 D 1 T 2 1 T 2 2 1 T 1 1 k k T 0 T 1 k T e T E B k B T 42

(II) Direct Measurement of Exciton Binding Energy in CH 3 NH 3 PbI 3 Numerical modelling of band-edge absorption by using Elliot s theory of Wannier excitons E b = 25meV Note: This simple formalism does not consider the frequency dependance of the exciton-phonon interaction Saba et al, Nat Comm. 5, 5049, 2014 43

(III) Direct Measurement of Exciton Binding Energy in CH 3 NH 3 PbI 3 E( B) Eg ( N 1/ 2) ( eb ) m* E n E g R n At 4K m*= 0.1m ; E b = 16meV Miyata et al, Nature Physics 11, 582 587 (2015) e 9 44

(III) Direct Measurement of Exciton Binding Energy in CH 3 NH 3 PbI 3 E( B) Eg ( N 1/ 2) ( eb ) m* E n E g R n At 161K E b 10 mev Miyata et al, Nature Physics 11, 582 587 (2015) e > 9 45

Raman I (norm.units) Organic-Inorganic Interplay: Raman Spectroscopy as a probe 1.0 Meso Flat 0.8 0.6 0.4 0.2 0.0 60 80 100 120 140 160 180 200 220 240 260 280 300 Raman shift (cm-1) Grancini G et al, JPC Letters, 5, 3836, 2014

Light Absorption Charge Generation Photo-carriers Transport

Exciton Vs Free Charges Total excitation density at the Thermodynamic Equilibrium n n FC n Free charges exc Excitons n n FC exc 2 k ( h 2 B T ) 3/ 2 e Eb k T B x n FC n Saha-Langmuir equation 48

Are there excitons around? MAPbI 3 Room Temperature, in the typical PV regime ph <10 16 cm -3 : free carriers only THz conductivity spectra are Drude-like in accordance with the presence of free charge carriers. Wehrefenning et al, Adv Mater, 26, 1584, 2014, Milot et al, Adv Funct Mater, 2015 DOI: 10.1002/adfm.201502340, PL dynamics are dictated by bimolecular recombination processes. Yamada, Y J. Am. Chem. Soc. 2014, 136, 11610. Stranks, Phys. Rev. Appl. 2014, 2 034007. fs-ta spectra show band filling effect and Varnshi shift. Kamat et al, Nature Photonics, 2014; Grancini&Kandada et al, Nature Photonics 2015 The deal for PV is: we exploit the presence of the exciton transition for light harvesting without paying in charge dissociation

x (n FC /n TOT ) Are there excitons around? MAPbI 3 Low Temperature, Exciton population is detectable. 1.0 0.8 0.6 290K 170K 70K 0.4 5 mev 0.2 0.0 10 14 10 15 10 16 10 17 10 18 10 19 10 20 10 21 10 22 Excitation Density (cm -3 ) THz spectra probe localization effects as a consequence of exciton formation below 80K. Milot et al, Adv Funct Mater, 2015 DOI: 10.1002/adfm.201502340, fs-ta spectra show a modulation of the photo-bleach as a result of exciton-exciton interaction. Grancini&Kandada et al, Nature Photonics 2015 WARNING: The Exciton binding energy increases (the dielectric constant decreases) when cooling down! Miyata et al, Nature Physics 11, 582 587 (2015)

Local order/ microstructure MAPbI 3 D Innocenzo et al, Nat Comm. 5, 3586, 2014 Grancini&Kandada et al, Nature Photonics 2015

Designing the Device Architecture Light Absorption Charge Generation Photo-carriers Transport

O.D. PL (norm.) Photo-carriers Diffusion Length 0.8 0.6 CH 3 NH 3 PbI 3-x Cl x /PMMA 1.0 0.4 0.2 0.5 0.0 500 600 700 800 Wavelength (nm) 0.0 53

Electron-Hole Diffusion Lengths By Photoluminescence Quenching + - + - Glass Silica Selective Quencher Diffusion constant Natural decay rate (no quencher) 54

Electron-Hole Diffusion Lengths By Photoluminescence Quenching DT (n e +n h ) CB VB Science, 342,341, 2013 55

PL (norm.) PL (norm.) Electron-Hole Diffusion Lengths By Photoluminescence Quenching 10 0 10-1 CH 3 NH 3 PbI 3 PMMA Spiro-OMeTAD PCBM 10 0 10-1 CH 3 NH 3 PbI 3-x Cl x PMMA Spiro-OMeTAD PCBM 10-2 0 10 20 30 40 50 Time after excitation (ns) 10-2 0 50 100 150 200 Time after excitation (ns) Perovskite Species D (cm 2 s -1 ) L D (nm) CH 3 NH 3 PbI 3-x Cl x Electrons 0.042 ± 0.016 1094 ± 210 Holes 0.054 ± 0.022 1242 ± 250 CH 3 NH 3 PbI 3 Electrons 0.017 ± 0.009 117 ± 29 Science, 342,341, 2013 Holes 0.012 ± 0.005 96 ± 22 56

Time-resolved Photoluminescence Stranks et al Yamada, Y J. Am. Chem. Soc. 2014, 136, 11610. Stranks, Phys. Rev. Appl. 2014, 2 034007. Saba, M.; Nat. Commun. 2014, 5 No. 5049. D Innocenzo, J. Am. Chem. Soc. 2014, 136 (51), pp 17730 1773 Deschler, J. Phys. Chem. Lett. 2014, 5, 1421. 57 Deschler, et al

Time-resolved Photoluminescence D Innocenzo et al. JACS, 2014, 136 (51), pp 17730 1773 58

Spectral position (nm) Time-resolved Photoluminescence 782 780 778 776 774 772 770 768 766 764 762 760 Band Edge Position cwpl Peak Position 100 1000 10000 Avarage crystallite size (nm) D Innocenzo et al. JACS, 2014, 136 (51), pp 17730 1773 R i rad n ( E ( E G G ) ) E CBB E G ph ( e ) ( e ) v ( e ) 1 e 1 ph (e ) k B ( e ) de T de Filippetti, et al. J.Phys.Chem, 2014, 59 doi:10.1021/jp507430x

Measuring Charge-Transport in Perovskites Terahertz/Microwave Spectroscopy Simple sample preparation Only informative on short length-scale μ ~ 10 cm 2 /Vs Time-of-flight Relevant over longer length-scale Time-scales difficult to measure in thin-film μ ~ 25 cm 2 /Vs C. Wehrenfennig et al., Adv. Mat. (2013) Space-charge limited current Relevant to optoelectronic devices Needs highly-selective non-limiting contacts μ ~ 25-165 cm 2 /Vs Q. Dong et al., Science (2015) Hall-effect Simultaneously get free-carrier density Like THz, only band mobility obtained μ ~ 66 cm 2 /Vs Q. Dong et al., Science (2015) C. Stoumpos et al., Inorg. Chem. (2013)

Take-home Message The room temperature structure of MAPbX3 is a fluctuating structure where titling and distortion of the octahedral networks and rotations and polarizability of the molecular dipole can strongly affect the optoelectronic properties of the semiconductor.

Open Questions Which is the secret of the low recombination rate Elucidation of the photo-carriers cooling Role of phonons Nature of carriers, localization vs delocalization Carriers transport mechanism Vs Structural Properties

Technology

Technology

Technology Zhang et al, Mater. Horiz., 2015, 2, 315 322

Perovskite PV at IIT p I n Spiro MeOTAD MAPbI 3 TiO x /PCBM

Perovskite PV at IIT Technology Development Fotovoltaico leggero e flessibile, a basso costo di produzione e energetico 17.6 % p I n Aumento efficienza fotovoltaico tradizionale Processi a bassa temperatura < 120 C Celle «TANDEM» 67

Further Developments Interface Engineering Stability Toxicity..