Generalized picture 1

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1 one electron energy contact contact The Photovoltaic (PV) effect: Generalized picture 1 Absorber e - High energy state Metastable high and low energy states Absorber transfers charges into high and low energy state Low energy state p + Driving force brings charges to contacts Selective contacts space ~~ biological photosynthesis (1) Inspired by Ross, JAP-1967; scheme after Green, Physica E, (2002)

2 * self-repairing (TD!) **possible selfrepair (non) concentrator; single-& multijunction Current Types of PV Cells Primarily based on solid-state electronic material systems homo- junction hetero-junction Elemental Semiconductors Inorganic Compound Semiconductors Organic, Excitonic (molecules,polymer;hybrid) Interpenetrating network Mesoporous framework Si,Ge (Ga,In)(As,P) Cu(In,Ga)Se 2 * CdTe P3HT-PCBM Porphyrin dye+tio 2

3 Single p-n junction solar cell e - Energy e - h n h n p - type n - type useable photo - voltage ( qv ) h + O. Niitsoo space

4 Current (ma/cm 2 ) Losses in PV cell : 1 photon 1 (e - - h + ) pair E g Etendu; Photon entropy TD lack of concentration Carnot factor TD Emission loss- (current) < E g not absorbed Electrical power out >E g thermalized Current Voltage Characteristics Energy (ev) After Hirst & Ekins-Daukes Prog.Photovolt:Res:Appl. (2010)

5 Shockley-Queisser* (SQ) Limit * detailed balance, photons-in = electrons-out + photons-out; on RT, for single absorber / junction; Efficiency (%) Radiative Limit c-si CIGS GaAs CdTe InP Perovskite GaInP Prince, JAP 26 (1955) 534 Loferski, JAP 27 (1956) 777 Shockley & Queisser JAP (1961) 15 DSC 10 CZTSS PbS OPV CZTS a-si Sb 2 S Band Gap (ev) * cf. also Duysens (1958) The path of light in photosynthesis Brookhaven Symp. Biol. 11, 18-25

6 Current density(ma/cm 2 ) Maximum possible vs. experimental photocurrents 50 (J sc Max ) 40 Si J SC J MP CZTSS CIGS 30 InP GaAs CdTe Perovskite 20 CZTS OPV DSSC GaInP 10 a-si Absorption Edge (ev) Nayak et al. Adv. Mater. (2011) and (2014) updated

7 Current density(ma/cm 2 ) Geometrical illustration of solar spectrum loss due to over-potential Consider ~ 1 ev or 2 ev absorption edge Assume 1 ev overpotential shifts reference energy from 0 to 1 ev small red rectangle gives new optimal energy value \ 10 After Ron Milo, WIS PKN, JB, DC, 2011, AM Absorption Edge (ev)

8 qv hν qv operation(=mp) [ev] Operational Loss (ev) Shockley-Queisser ( ) and experimental ( ) LOSS as function of minimal excitation energy CZTSS CIGS c-si SQ- Limit Loss InP CZTS CdTe GaAs OPV DSSC a-si Perovskite PS CdZnTe GaInP Includes basic add l loss of disorder... CH 3 NH 3 Pb(Br 0.4 I 0.6 )? 0.2 PS: natural photsynthesis S-Q from R.Milo,WIS Absorption Edge (ev) Nayak et al. Adv. Mater., ,3-2014; updated

9 1 st generation Si Solar Cell (r)evolutions 2 nd generation CdTe, CIGS 3 d generation Organic (polymer/ small molecule) TiO 2 Single- crystalline cm poly-crystalline m nano crystalline ~ 20 nm grain size cheaper and simpler? DISORDER amorphous (a-si:h;polymers) HYBRID PV?

10 Hybrid PV Cells; decreasing disorder Bulk heterojunction OPV cell Cathode - D A Dye-sensitized / ETA cell Anode Substrate Light OM Perovskite cell?

11 ybrid Organic-Inorganic / Metal Halide / Pb Halide CH 3 NH 3 PEROVSKITE Photovoltaic Cells PbI 6 octahedron with Gary Hodes Funding by Helmsley Trust, Israel Ministry of Science, Israel Nat l. Nano-Initiative

12 Evolution of ( cm 2 ) hybrid I-O Perovskite solar cell performance from Nayak & Cahen Adv. Mater also G. Hodes, Science , updated

13 Hybrid Organic-Inorganic Metal Halide PV started as unstable Dye Cells mp-tio 2 ; CH 3 NH 3 PbI 3; Liquid Junction cells Kojima et. al. JACS ma/cm V FF=0.57 CE=3.81%

14 CH 3 NH 3 PbI(Cl) is the poster child halide perovskite Nat. Comm 2014

15 20 and seems remarkably versatile! Current density/ ma.cm -2 0 Voltage / V 1.0 mp-tio 2 ; CH 3 NH 3 PbI 3; >15 %; Planar; thermally evaporated CH 3 NH 3 PbI 3-x Cl x with HTM Nature 2013 Current density/ ma.cm mp-tio 2 ; 2 step (solution) deposition of CH 3 NH 3 PbI 3 with organic HTM; Nature % Voltage / V 1.0 ~17 % PCE for 2 step MAPbI 3 deposition on thin mp-tio 2 (and 19.7% by UCLA) and with solvent annealing++ even higher Materials are made differently; cells have different designs! 14 Voltage / V 0.8 No HTM - JACS, Energy Environ. Sci ~ 14% 0

16 What makes a perovskite-type structure.. CN=12,6/(6 1 =4+2) A is a 12 coordinate large size cation (Cs, RNH 3 ; R: CH 3 ) B is a 6 coordinated small size cation (Pb, Sn, Ge) is a 6 coordinated anion (halide, Cl, Br, I, SCN) MONOVALENT 16 I. Lubomirsky

17 What is so special about these new PV materials? X = Cl -, Br -, I - B = Pb, Sn, Ge... A = ORGANIC (CH 3 NH 3+ ) ABX 3 Solution processable (< 140 C). Chemically tunable. (Very) strong optical absorption } Sharp optical absorption onset }~ GaAs High crystalline( electronic) quality} Auto-surface passivation? Auto-repair?

18 MAPbI 3 : an ideal absorber? Band gap ~ ev Very strong absorption Dopable.. Good transport (μτ) properties Low [interface states] in gap Small transport barrier across grain boundaries Composed of earth-abundant and available elements Low-cost processing BUT.. stability, scalability, toxicity? V. Stevanovic

19 Some key points about the 3D Sn/Pb perovskites... Direct band gap and p - p transition (I p to Sn/Pb p) Ultrahigh absorption Enables ultrathin absorbers. Photogenerated carriers do not need to travel far. s-p coupling enhances the dispersion of the upper VB Both electrons and holes have small effective masses and high mobilities Contributes to long L d W.-J. Yin et. al., Adv. Mater. 2014, 26, avid Mitzi (IBM Duke)

20 What are the most important properties of the hybrid lead halide perovskites? Low recombination rate simple low-t preps

21 How organic-inorganic films are deposited: Self assembly greatly facilitates the formation of the organic-inorganic perovskites They will even form in a mortar and pestle upon grinding at room temperature. ( CuInSe 2 ) 1. Spin coating, slit casting, printing or spray coating of solutions 2. Melt processing 3. Vacuum evaporation 4. Two-step coating Solution or vapor phase avid Mitzi (IBM Duke)

22 Device structure: Evolution of device structure Devices now look a lot more like traditional TF solar cells N.-G. Park, Mater. Today 8, 506 (2014). avid Mitzi (IBM Duke)

23 preparation of these HOI-Perovskites Let s take the most common one - CH 3 NH 3 PbI 3 Basic idea: PbI 2 + CH 3 NH 3 I CH 3 NH 3 PbI 3 CH 3 NH 2 + HI CH 3 NH 3 + PbI 6 octahedron PbI 2 CH 3 NH 3 PbI 3

24 Perovskite preparation CH 3 NH 3 I + PbI 2 CH 3 NH 3 PbI 3 (dissolved in organic solvents) OR 2-step process PbI 2 spin coat treat with CH 3 NH 3 I spin coat anneal (ca. 100 C) OR

25 Flat junction device with high efficiency: Spiro-OMeTAD = 2,2,7,7 -tetrakis-(n,n-di-pmethoxyphenylamine)9,9 -spirobifluorene M. Liu et. al., Nature 501, 395 (2013). avid Mitzi (IBM Duke)

26 So, fine, but Why / How can the perovskites be such good OE (PV) materials? Large Large crystals crystals (formed (formed at at low low temperature temperature from from solution) solution) Strong, sharp absorption with little tailing OK mobilities Long lifetimes Long diffusion/drift lengths Intrinsic defects are in the bands or close to band edges Low trap density

27 Large crystals (formed at low temperature from solution) CH 3 NH 3 PbBr 3 CH 3 NH 3 PbI 3 1 mm 1 mm Y. Tidhar et al. J. Amer. Chem. Soc., 136, , (2014).

28 Why are the perovskites so good? Large Large crystals crystals (formed (formed at at low low temperature temperature from from solution) solution) Strong, sharp absorption with little tailing OK mobilities Long lifetimes Long diffusion/drift lengths Intrinsic defects are in the bands or close to band edges Low trap density

29 RT Optical Absorption of Solar Cell Materials MAPbI 3 has strong, sharp absorption with little tailing Very sharp absorption onset Wolf et al. J. Phys. Chem. Lett. 5, 1035, (2014) Cells give high qv OC / E g CH 3 NH 3 PbI 3 ~1.15 V; Eg = 1.6 ev; CH 3 NH 3 PbBr V; Eg = 2.3e V

30 Connection between absorption and band structure Electronic structure of MAPbI 3 Pb(p) I(p) Pb I Pb(s) V. Stevanovic

31 Connection between absorption and band structure Pb(p) E C Cd(s) E G E G Pb(s)+I(p) E V S(p) MAPbI 3 CdS p-p transitions involve a higher JDOS than p-s transitions; therefore stronger absorption

32 Maybe good for spectral splitting / tandem cells Bandgaps CH 3 NH 3 PbI ev (~ 800 nm) CH 3 NH 3 PbBr ev (~ 540 nm) HC(NH 2 ) 2 PbI ev (~840 nm) CH 3 NH 3 SnI 3 ~1.2 ev (~ 1030 nm) (can combine these) Tandem cells GaInP 2 E g = ev up to 1.45 V V OC

33 SHORT NATURE BREAK 5 min please

34 Why are the perovskites so good? Large Large crystals crystals (formed (formed at at low low temperature temperature from from solution) solution) Strong, sharp absorption with little tailing OK mobilities Long lifetimes Long diffusion/drift lengths Intrinsic defects are in the bands or close to band edges Low trap density

35 Long diffusion/drift lengths and lifetimes Diffusion/drift lengths depend on: 1. Diffusion coefficient/mobility 2. Lifetime of charges Diffusion length average distance a charge moves Drift length average distance a charge moves in an electric field (per V) Mobilities Mobilities are good but for CH 3 NH 3 PbI 3 not exceptional

36 Why are the perovskites so good? Large Large crystals crystals (formed (formed at at low low temperature temperature from from solution) solution) Strong, sharp absorption with little tailing OK mobilities Long lifetimes Long diffusion/drift lengths Intrinsic defects are in the bands or close to band edges Low trap density

37 Long lifetimes particularly important GaAs: 10 ns typical good value of minority carrier lifetime; for close to intrinsic GaAs, typically ~1 µs and up to > 10 µs PbS: ca. 1 µs radiative lifetime CsPbBr 3: e- lifetime (minority carrier) = 2.5 µs (Stoumpos) MeAPbI 3: 100 ns (PL) [Stranks] 10s of µs (THz/µwave) [Ponseca] 5 µs (from diff. lengths) [Savenije]

38 Why are the lifetimes so long? Absence of charge traps in the bandgap It has been suggested that the presence of ferroelectric domains may assist charge separation and reduce recombination. (Ferroelectric domain boundaries (J.M. Frost et al., NanoLett., 14, 2584 (2014); S. Liu et al., J. Phys. Chem. Lett., 6, 693 (2015)) but.. CsPbBr 3 is cubic (not ferroelectric), yet has long lifetime Long lifetimes maybe due to absence of charge traps in the bandgap

39 long diffusion lengths Charges can move 1 µm or more Using EBIC (electron beam-induced current) on cell cross-section we measured diffusion/drift lengths > 1 µm in CH 3 NH 3 PbI 3 and showed mechanism of iodide cell action was p-i-n

40 Scheme of EBIC Measurement HTM Hole transport material ETM Electron transport material

41 n-type n-type p-type From EBIC Profile to Working Mechanism Mono-exponential decay p-type y y 0 Ae x ( x ) 0 L n intrinsic c

42 FTO/TiO 2 HTL HTL EBIC of p-i-n MAPbI 3 (Cl) SE image MAPbI 3 (Cl) EBIC image Drift/diffusion length: electrons L n = 1.9 ± 0.1 μm hole L p = 1.5 ± 0.2 μm FTO/ TiO 2

43 EBIC of p-n MAPbBr 3 (Cl) Diffusion length for electrons : L n = nm

44 Solar Cell: p-n p-i-n junction p-type n-type Intrinsic - I Low charge carrier density region sandwiched between two high charge carrier density regions High electronic quality The p and the n contacts induce a field in the material, extending throughout the whole layer. p - i i - n

45 Important differences between the p-n and p-i-n cells load E C V E F E V p-n junction cell p-i-n cell 1 vs. 2 junctions Intrinsic semiconductor in p-i-n should have higher mobility and, particularly, longer lifetime than higher doped p, n semiconductors.

46 long diffusion lengths Measured L D from mobility and lifetime: MeAPbBr µm MeAPbI µm (N = 2x10 10 /cm 3 ) Low trap-state density and long carrier diffusion in lead trihalide perovskite single crystals D. Shi et al., Science (2015).

47 Very long diffusion lengths - how do we define them? Electron-hole diffusion lengths >175 μm in solution-grown CH 3 NH 3 PbI 3 single crystals Dong et al., Sciencexpress, /science.aaa5760 (2015) We report that the diffusion lengths in CH 3 NH 3 PbI 3 single crystals grown by a solutiongrowth method can exceed 175 μm under 1 sun illumination and exceed 3 mm under weak light for both electrons and holes. Diffusion length: the average length a carrier moves between generation and recombination. The electrons generated in the very thin perovskite layer near the Au anode must traverse the whole crystal to be collected by the Ga cathode, indicating that the electron diffusion length in MSCs is greater than the crystal thickness (~3 mm). L D n n n

48 Other examples of long diffusion lengths DSSC Single crystal PEC counter electrode S x 2- CdSe GaIn

49 long diffusion lengths? hν semiconductor h + e - barrier contact Ohmic contact

50 Why are the perovskites so good? Large Large crystals crystals (formed (formed at at low low temperature temperature from from solution) solution) Strong, sharp absorption with little tailing OK mobilities Long lifetimes Long diffusion/drift lengths Intrinsic defects are in bands or close to band edges Low trap density

51 contact potential difference [mv] HEIGHT [nm] Films are polycystalline. How bad is that? S. Mukhopadhyay AFM SKPFM Edri et al,, Nat. Comm 2014 Shown by us for I and I(Cl) Confirmed by several other groups, since 0 X [μm] (almost) no potential difference across the grain boundaries X [μm]

52 {E G /q V OC } vs. Urbach energy for PV absorber RT CH 3 NH 3 PbBr 3 (2.3eV) De Wolf et al., J. Phys. Chem. Lett. 2014, 5, ; (also Sadhanala et al., ibidem pp )

53 One important consequence of low trap densities e - Explains (very) high V OC / Eg : E C CH 3 NH 3 PbI V; Eg = 1.6 V CH 3 NH 3 PbBr V; Eg = 2.3 V E V Intrinsic defects are in the bands or close to the band edges Yin et al. APL (2014); Kim et al. JPCL (2014)) Low trap density /cm 3 (D. Shi et al., Science, 347, 6221, (2015)) 4x10 10 /cm 3 (Q. Dong et al., Science, 347, 6225, (2015))

54 How important is the ORGANIC part for the Hybrid Organic-Inorganic Metal Halide PEROVSKITE Photovoltaic Cells? CH 3 NH 3 PbI 6 octahedron

55 CsPbBr 3 Mobility 1000 cm 2 /V-s Charge lifetime 2.5 µs mp-tio 2 /CsPbBr 3 /PTAA/Au

56 CsPbBr 3

57 How important is the HTM for the Metal Halide PEROVSKITE Photovoltaic Cells? Hole Transport Material

58 CsPbBr 3 HTM-free CsPbBr 3 cell mp-tio 2 /CsPbBr 3 /Au Mp-Al 2 O 3 /CsPbBr 3 /Au So, the organic cation is not an essential part of these perovskite cells

59 ETL/perovskite jctn. dominates L. Barnea-Nehoshtan et al. J. Phys. Chem. Lett. 5, (2014)

60 BUT. Don t ignore environmental issues How readily can Pb reach the surroundings in event of breakage/fire? Two questions: Quite readily as PbI 2 is (sparingly) soluble in water. What is the effect of this Pb on the environment? But. not as bad as it may first seem.

61 Rain on Pb-Perovskite based solar panels Pb compounds?

62 1 m 2 panel contains 1 g Pb Consider a large solar field 1 cm 70 ppm Pb Natural Pb levels in soil: <10-30 ppm for rural soil; ppm for urban areas

63 Medium sized coal-burning power station, generating 1 GW, emits over 20 years (20 GW-yr energy supplied) kg of Pb to the air. Perovskite cell equivalent to this continuous environmental contamination;, assume 1 gm Pb/m 2 of 15% efficient (solar to AC electrical power) in PV modules, required to provide 20 GW/yr equivalent continuous power Assume ~ 20% equivalent operation time, to convert peak to continuous power is a field containing ~30 tons of lead. Assume lower 100 kg value of Pb emission for coal-fired power station: If over 20 years less than one module out of 300 releases all its Pb to environment, there will be a reduction in Pb emission compared with best presently-available coal generation station.

64 Guessing the future of (HO)IP-based PV

65 Advantages and disadvantages of ybrid organic-inorganic metal halide perovskites for PV cells Advantages No rare/expensive elements - CH 3 NH 3 (Pb,Sn)(I,Br,Cl) 3 Cells can be efficient, cheap (money and energy-wise) High efficiencies for very small area cells after short time High V OC ev semiconductor 1.1 V V OC Disadvantages Environmental problems with Pb (esp. outside USA) Larger -area cells still large drops in efficiency Stability not yet clear (? Hysteresis; ΔG form ) Ion migration (CuInSe 2 )