Hybrid-PV Berlin, 05-2013 Inorganic, Organic and Hybrid Solar Cells: How different are they? with Pabitra K. Nayak Work done in part in collaboration with A. Kahn and J. Bisquert Cahen group WIS 2013 The Photovoltaic (PV) effect: Generalized picture • Metastable high and low energy states contact econtact one electron energy Absorber p+ space • Absorber transfers charges into high and low energy state • Driving force brings charges to contacts • Selective contacts ~~ photosynthesis (1) cf. e.g., Green, M.A., Photovoltaic principles. Physica E, 14 (2002) 11-17 Cahen group WIS 2013 Current Types of PV Cells Primarily based on solid-state electronic material systems • Elemental Semiconductors (non) concentrator; single-& multijunction – Single or multi-crystal – Polycrystalline films – Amorphous thin film • Inorganic Compound Semiconductors homo- & • hetero-junction; photoelectrochem; MIS-inversion Si ,Ge (Ga,In)(As,P) – Single crystal – Polycrystalline thin film Cu(In,Ga)Se2 CdTe Organic, Excitonic (molecules, polymer;hybrid) – Interpenetrating network P3HT, PCBM; – Polycrystalline thin film Porphyrin ++ – Nanocrystalline; dye+TiO2 dye- or SC-sensitized ……………………………………………………… (ZnO) Cahen group WIS 2013 How efficient can they be? Cahen group WIS 2013 1954 2013 Chapin Fuller Pearson Cahen group WIS 2013 “THE”(?) LIMIT: Shockley-Queisser* (SQ) detailed balance, photons-in = electrons-out + photons-out; on earth, @ RT, for single absorber / junction; cf. also Duysens (1958) “The path of light in photosynthesis” Brookhaven Symp. Biol. 11, 18-25 photosynthesis SQ Limit 30 GaAs 25 Prince, JAP 26 (1955) 534 Loferski, JAP 27 (1956) 777 Shockley & Queisser JAP (1961) c-Si Efficiency (%) * InP 20 CIGS CdTe 15 DSC a-Si 10 OPV 5 0.5 1.0 1.5 Band Gap (eV) Cahen group WIS 2013 2.0 2.5 Losses in PV cell Etendu; Photon entropy –TD 80 ~0.3eV @RT, lack of concentration Current (mA/cm2) 70 Carnot factor –TD 60 Eg Emission loss- (current) 50 40 30 < Eg not absorbed Electrical power out Current – Voltage Characteristics 20 >Eg thermalized 10 0 0 1 2 3 Energy (eV) After Hirst & Ekins-Daukes Prog.Photovolt:Res:Appl. (2010) Nayak, ……, Cahen., Energy Environ. Sci., 2012 ( Cahen group WIS 2013 4 3d generation PV: Present (?) Status ETA cells 12.3% 10.8% / WIS Cahen group WIS 2013 3d generation PV: Present (?) Status Cahen group WIS 2013 Maximal possible vs. experimental photocurrents 50 Max (Jsc 2 Current density(mA/cm ) DSSC JMP (a) = black dye, (b) = N719 Si 40 ) JSC & (c) = YD2-O-C8+Y123 OPV (a) = Mitsubishi, (b) = Konarka CIGS 30 InP GaAs CdTe 20 Natural PS <~10-2 mA/cm2 DSSC (a) DSSC OPV (a) DSSC (c) (b) a-Si OPV (b) 10 1.0 PKN, JB, DC, 2011, AM 1.5 Absorption Edge (eV) Cahen group WIS 2013 2.0 Thanx 2 Lee B 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. 70 2 Current density(mA/cm ) 60 50 40 30 \ 20 10 0 0.0 After Ron Milo, WIS PKN, JB, DC, 2011, AM 0.5 1.0 1.5 2.0 Absorption Edge (eV) Cahen group WIS 2013 2.5 3.0 qVMP / EG RT bandgap qVMP* Energy qVMP/EG abs. edge[eV] [eV] loss [%] Cell type (absorber) sc-Si 1.12 0.61 0.51 54 GaAs 1.42 1.00 0.42 70 InP 1.28 0.75 0.53 59 1.45 0.71 0.74 49 Cu(In~0.7Ga~0.3)Se ~1.15a 0.60 ~0.55 52 a-Si:H I SC IMP DSSC (black dye) ~1.75 0.70 ~1.05 ~40 ~0.75 ~1.3 0.53 ~0.9 PMAX ~1.6 0.69 ~1.0 ~1.75 0.73 ~1.65 a 0.67 ~0.98 ~1.65 a 0.68 b ~0.97 V ~1.8 1.05 VMP V(0.75) OC ~41 ~43 ~42 ~41 ~41 CdTe I (red N719) Ru-free porphyrin OPV Mitsubishi Konarka ( Belectric) Photosynthesis fuel a From EQE b Calculated * for best performing cell of given type Nayak et al. Adv. Mater., May 2011, updated Cahen group WIS 2013 (42) Theoretical (Shockley-Queisser) LOSS as function of minimal excitation energy So, what is “the” problem? [eV] qVoperational qVqVhνhv–- qV (= MP)[eV] operation(=MP) SQ- Limit Loss a-Si OPV Mitsubishi OPV Konarka 1.0 PS DSC-latest CuGaSe2 DSC-N719 0.8 CdTe DSC-Black (GaIn)P 0.6 CIGS C -Si InP ............... Includes basic add’l loss of disordered material GaAs 0.4 PS: natural photsynthesis 0.2 1.0 S-Q from R.Milo,WIS 1.2 1.4 1.6 Absorption Edge [eV] PKN, JB, DC, 2011, AM Cahen group WIS 2013 1.8 2.0 p/n vs. excitonic solar cells ORGANIC INORGANIC HIGH LOW Exciton binding energy ~ 10 meV • high dielectric constant • minority carrier device EF ~ 0.1-0.3 eV n • exciton splitting • includes jiggling & wiggling m*e4 EB (4 0 )2 2 2 2 dielectric constant from B. Kippelen, Georgia Tech • low dielectric constant Cahen group WIS 2013 Electron-hole pair: inorganic vs organic PV Organic Inorganic Exciton binding energy >> kT Exciton binding energy < kT → dissociation by E-field in the space charge region Binding energy ~ 10 meV Binding energy ~ 100 - 300 meV Energy (eV) CBM VBM P n Cahen group WIS 2013 → requires donor/ acceptor, (D/A) type structure Solar Cell (r)evolutions 1st generation Si 2nd generation CdTe, CIGS Single- crystalline poly-crystalline cm m 3d generation Organic (polymer/ small molecule) TiO2 nano crystalline ~ 20 nm grain size cheaper and simpler ? DISORDER HYBRID PV Cahen group WIS 2013 amorphous (a-Si:H;polymers) Why go HYBRID ? Best or worst of two worlds? Exchange between George Bernard Shaw or Anatole France (?) and dancer Isadora Duncan Cahen group WIS 2013 Estimated Energy Payback Times for Solar Cells Organic PV (OPV) Proc. IEEE PVSC 2010 A. Anctil et al., RIT, NREL courtesy David Ginley, NREL Cahen group WIS 2013 Bulk Organic Heterojunction PV Cells Metal contact Limited control over morphology Active Layer Hole-transport Transparent contact layer Transparent Substrate P3HT PCBM HYBRID Organic- Inorganic Heterojunction Metal oxides: • good electron acceptors cf. to polymers • good carrier mobility • solution-processable • chemically stable • mechanically robust • continuous networks can be deposited Cahen group WIS 2013 Dye-Sensitized Solar Cell © Frey lab, IIT-Technion 2013 OPV Materials of Interest 5.3 5.7 A. Kahn, Princeton U.; after Ratcliff et al. JPCL 2011 Cahen group WIS 2013 20 THX TO ARTEM BAKULIN Cahen group WIS 2013 Hybrid PV Cells; decreasing disorder Bulk heterojunction cell Dye-sensitized / ETA Cathode - D D A Anode Substrate Light OM Perovskite Cahen group WIS 2013 Si / Organic Hybrid Cells 5 0 J (mA/cm2) -5 -10 -15 FF Voc Jsc (100), with MeOH ML In-Ga back contact PEDOT by LOFO ! -20 -25 -30 -0.1 1015 cm-3, = 10.8% = 0.70 = 0.56 V = 27.5 mA/cm2 0.0 0.1 0.2 0.3 0.4 V (V) Cahen group WIS 2013 0.5 0.6 Ann Erickson, TBP Origin VOC … ordered The Importance ofofbeing Voc as J0 & J00 VOC kBT J SC n ln e J0 k :rate of charge transfer (s-1) B J 0 J 00 exp e k N DA e: electron charge (C) k BT NDA: surface density of DA complexes[cm-2] k Vif 2 2 ( G ) 0 2 exp 4 kBT 1 G0 Gibbs free energy reorganization energy, relaxation due to vibronic modes Vif electronic coupling after B. Kippelen, Georgia Tech Cahen group WIS 2013 Nayak et al., EES, 2012 Dynamic Disorder electron transfer & vibronic relaxation D+AEnergy (eV) D*A λ Energy (eV) ΔG* DA A- λ = λrel (1) + λrel (2) λrel (2) Vibronic Relaxation after electron transfer ΔG0 λrel (1) A Nuclear co-ordinate Loss = λrel (hole) + λrel (electron) Eg λrel (hole) ΔG0rec = ~150meV (UPS) λrel (electron) = ~150meV (DFT) ket = koexp -[(ΔG0+λ)2/4λkBT] Nuclear co-ordinate Cahen group WIS 2013 Nayak et al., EES, 2012 Nayak et al., EES, 2012 Static and Dynamic disorder After Kera, Yamane and Ueno Progress in Surface Science, 2009 Cahen group WIS 2013 Tail states Voc: crystalline vs. non-crystalline Amorphous CB Edge EFn- EFp VB Edge Energy (eV) Energy (eV) Crystalline Density of States CB Edge EFnEFp VB Edge Density of States Cahen group WIS 2013 a-Si tail state and Voc In case of a-Si:H,the carriers thermalize to the band tails after photogeneration. Energy (eV) An extra loss of ~300 mV occurs due to tail states. CB Edge 0.33eV EFn- EFp 0.44eV VB Edge Density of States Cahen group WIS 2013 T. Tiedje , APL 1982 Shockley-Queisser (SQ) Limit 35 S-Q Limit 25 SQ Limit 30 GaAs 25 c-Si 20 Efficiency (%) Calculated Efficiency (%) 30 15 InP 20 CIGS CdTe 15 10 a-Si DSC OPV 10 5 0.5 1.0 1.5 Band Gap (eV) 5 0 1.0 Nayak et al., EES, 2012 1.5 2.0 Band gap (eV) Cahen group WIS 2013 2.5 3.0 2.0 2.5 Extra Losses in Molecular Cells 35 + 80% EQE + Fill factor loss (n=2) + Tail state loss = 0.2eV + Vibronic loss = 0.25eV + Dielectric loss = 0.2eV + (Dielectric + vibronic) = 0.3eV S-Q Limit 25 SQ Limit 30 GaAs 25 c-Si 20 Efficiency (%) Calculated Efficiency (%) 30 15 InP 20 CIGS CdTe 15 10 a-Si DSC OPV 10 5 0.5 1.0 1.5 Band Gap (eV) 5 0 1.0 Nayak et al., EES, 2012 1.5 2.0 Band gap (eV) Cahen group WIS 2013 2.5 3.0 2.0 2.5 Summary • Oxides may not oxidize, but their surfaces can …. and • inorganic surfaces are nastier than organic ones …. and • interfaces is where it is at ….. but • most inorganic semiconductors are more ordered than organic ones • which leaves the question if “ordnung muss sein”? Cahen group WIS 2013