#1Terry Lou Solar cell - aka photovoltaic (PV) cell Dye-Sensitized Solar Cells Convert light energy into electricity (vs solar collector - thermal) "Solar PV development will continue to break records, with annual additions reaching 162 GW by 2022 – almost 50% higher than the pre-pandemic level of 2019" (IEA, 2020) Primary energy supply rate = 18.55 TW (2017); - Electric power generation = 2.92 TW (2017); - Solar PV power = 0.4 TW (2017) c.f. Solar power reaches the Earth surface = 89,000 TW (1.36 kW/m²) Exponential Growth of Solar PV (in GW) 1,000 gigawatts 100 2017: 404 GW 10 1 2018: 508 GW (tent.) Traditional PV cell Working principle: Baran Group Meeting Jun 5, 2021 Absorption of light generates e-/h+ pair at p-n junction Separation of charge carriers (e- towards n-type; h* towards p-type) Injection of carriers into external circuit 1st generation crystalline silicon (c-Si) Monocrystalline (mono-Si) highest single-cell efficiency n = 26.7% Multicrystalline (multi-Si): n = 23.3% 2nd generation thin-film solar cells electrode(minus) Light surface reflective coating electrode(plus)/ n-type semiconductor p-type semiconductor underside reflective coating Amorphous Si (a-Si), Cadmium telluride (CdTe), Copper indium gallium selenide (CIGS) Flexible and low cost; lower efficiency in general (n = 15-20%) Semiconductor MIZIRANAW 0 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 3rd generation - tandem cells - p-type (“positive”) doped with trivalent atoms (B, Ga, In) - n-type ("negative") doped with pentavalent atoms (P, As) - Fermi level (EF): energy required to add one electron to the body → hypothetical energy level of e- Conduction band - LUMO - - Band gap HOMO-LUMO gap Valence band - HOMO EF- B D D D D DDD Metal Semimetal p-type intrin. n-type Semiconductor Insulator Multi-junction solar cells; best n = 47.1% (4-junction) Dye-sensitized (DSSC), organic (OSC), and perovskite solar cells (PSC) Overcome theoretical efficiency limit of 33.7% Monocrystalline Polycrystalline Global Market Share by PV Technology from 1990 to 2013 100% T 100% Amorphous Thin-film ribbon-Si 75% - 50% 25%- mono-Si 75% multi-Si 50% 0% 1990 2000 2010 25% 0% 1#2Terry Lou Assessment of a solar cell Power conversion efficiency (PCE; n) = Jsc VocFF SC Pin FF = Pmax(seVoc) SC Jsc short-circuit photocurrent density (V=0) Voc open-circuit photovoltage (J=0) SC = FF = "fill factor" P = intensity of incident light Current (mA) 4.0 3.5 55 Dye-Sensitized Solar Cells 3.0 Voc = 0.7210 V 2.5 Imax 3.55 mA Jsc 20.53 mAcm-2 2.0 Fill Factor 70.41% 1.5 Vmax = 0.546 V 10 1.0 Isc = 3.82 mA 0.5 Pmax 1.94 mW Efficiency 10.4 % 0.0 -0.5 -0.2 0.0 0.2 0.4 0.6 0.8 Voltage (V) Dye-sensitized solar cell (aka Grätzel cell) Invented by Michael Grätzel in 1985 State-of-the-art device reported in 1991 achieved n = 7.1% at 1 sun; n = 12% in diffuse daylight; IPCE 97% (520 nm) Baran Group Meeting Separate light absorption from charge carrier transport Composition (n-type) Working electrode (-ve; anode) Transparent conducting oxide glass (TCO) Semiconductor Photosensitizer (dye) Jun 5, 2021 == 122°C Michael Grätzel (1944-) Professor at EPFL (Lausanne, CH) Glass TCO TiO2 with dye in P = maximum power point max Incident photon to converted electron (IPCE) represents quantum efficiency at a particular wavelength Standard Conditions: AM 1.5G (1 sun) (42° angle of elevation of the sun; Irradiance Shockley-Queisser limit Maximum n = 33.7% at band gap of 1.34 eV at AM 1.5G for a single-junction solar cell - - Spectrum losses Blackbody radiation Recombination of e-/h+ = 1000 W m-2) Max Efficiency (%) 30 20 20 10 - Impedance (resistance; fill factor) For infinite stack of p-n junctions, theoretical limit for n = 68.7% (1 sun) JACS 1985, 107, 2988-2990; Nature 1991, 353, 737-740. 1 2 Bandgap (eV) Electrolyte Counter electrode (+ve; cathode) (Catalyst) TCO glass UNI-T CE DT830C 3 33 Electrolyte (I/13) TCO Glass 2 +#3Terry Lou Working principle of DSSC (1) Photo-excitation of dye (2) Injection of e- to semiconductor conduction band (CB) (3) e-transport through CB, TCO and external circuit (4) Electrolyte reduced at cathode (5) Dye regenerated by electrolyte (6) Cell voltage corresponds to AV between CB edge energy (Ec) and redox potential of electrolyte Unproductive pathway (→ lower n) - E vs NHE (V -0.5- Conducting glass TiO2 Injection Ec Dye-Sensitized Solar Cells Dye Electrolyte Cathode 2 S* 1 6 Maximum voltage hv Red Ox Mediator 5 Interception Diffusion 1.0 S%/S+ 3 0.5- Relaxation of dye molecule at excited state Recombination of e- in CB or TCO with electrolyte or oxidized dye - Trapping of e in semiconductor 10-3 s 10-13 s 10-12 s 10-11 10-12 s 10-8 s 10-2 s hv 104 s 12/I- 10-6 s 13/I- 10-5 s FTO TiO2 Sensitizer Redox mediator Strategies in optimizing DSSC - C Increase light absorbance Increase AV (by raising Ę or redox potential of electrolyte) Reduce recombination Reduce internal resistance (TCO, porosity of semiconductor and electrolyte) Achieve high stability and turnover of dye and electrolyte Materials development Transparent Conducting Oxide (TCO) Baran Group Meeting Jun 5, 2021 Indium tin oxide (ITO) – highly conductive, less chemically stable Fluorine-doped tin oxide (FTO) - less conductive, highly chemically stable Coated on glass or plastic substrates Semiconductor Nanostructured metal oxide Film thickness ~10 μm; nanoparticle diameter 10-30 nm; porosity 50-60% High porosity facilitates dye adsorption and regeneration, but reduces particle coordination and increases dead ends for e- transport (<1% dead ends in a 50% porous film; 31% in a 75% porous film) TiO2 - anatase > rutile Preparation of TiO 2 nanoparticles: . • • • Hydrolysis of Ti(OR) 4, then hydrothermal growth and crystallization Acid-catalyzed: better dye adsorption Base-catalyzed: slower recombination AcOH commonly used Deposition onto conducting glass: • Nanoparticle formulated in paste with polymer additives • Doctor blading or screen printing, then sintered at >450 °C • Porosity controlled by amount of polymer Coating of ultrapure TiO 2 shell (~1 nm) by treating with TiCl4 ZnO higher e mobility but lower chemical stability Other metal oxides (SnO 2, Zn2SnO 4, SrTiO 3, Nb2O5, etc.) are less efficient 3#4Terry Lou Materials development (Cont'd) Photosensitizer (Dye) - Strong absorption at visible region and near-IR - Dye-Sensitized Solar Cells Anchoring group to strongly bind onto semiconductor surface (-COOH, -H2PO 3, -SO 3 H, etc.) Excited state energy E(S*) > Ec TBAOOC COOH .COOH HOOC COOTBA 8=0= COOH -N=G=S Baran Group Meeting Jun 5, 2021 HOOC 8=0=4= COOH N719 n = 10.2%; enhanced Voc; reference dye for studies COOH N749 (Black dye) n = 10.4%; IPCE spectrum extended to 920 nm N886 n = 5.9%; IPCE spectrum up to 900 nm, max = 40% Redox potential of oxidized dye more +ve than electrolyte Avoid aggregation (unable to inject e- to semiconductor) by structure optimization or addition of coadsorbers (e.g. 4-tert-butylpyridine (TBP), deoxycholic acids, acetic acid) High turnover number (photostable, electrochemically and thermally stable). HOOC COOH HOOC соон COONa H13 R R R R R 0 0. M M M M unidentate bidendate chelating bidentate bridging H-bonded H-bonded Ru complexes HOOC HOOC COOH 72+ HOOC HOOC COOH (Desilvestro, 1985) First reported DSSC IPCE 44% (460 nm) COOH 2Cr HOOC HOOC HOOC (O'Regan, 1991) n = 7.1%; IPCE 97% (520 nm) HOOC COOH Z907 S=C=N* -Z== CHie n = 7%; improved stability .COOH HOOC 8=0=N- N3 (Grätzel, 1993) OOH HOOC n = 10%; IPCE spectrum up to 800 nm COOH COOH (Bessho, 2009) n = 10.1%; replaced -NCS Z910 -2=0=0 n = 10.2%; excellent thermal stability Summary: HOOC C101 Cg H₁3 n = 11%; retain >90% performance after 1,000 h (1 sun, 60 °C) - MLCT (d→л*) upon photo-excitation Dicarboxylate-bpy provides good anchoring electronic coupling of the dye* with TiO2 CB Second bpy can be modified with EDW to broaden the absorption spectra and to increase the molar extinction coefficient -NCS as EDG to raise the HOMO energy of the complex, leading to a red-shifted absorption, but is the weakest part of Ru complex 4#5- - Terry Lou Materials development (Cont'd) Photosensitizer (Dye) Dye-Sensitized Solar Cells Other metal complexes were studied, e.g. Os, Re, Fe, Pt and Cu Poor absorption, slow e- transfer with electrolyte and fast recombination are common challenges Porphyrins COOH Organic dyes Baran Group Meeting Jun 5, 2021 Structurally diverse molecules can be designed and synthesized. Lower cost and environmental hazards than metal complexes Higher extinction coefficient than Ru complexes Most adopted D-π-A design Coumarins HOOC -NH HN COOH CN CO₂H CN COOH COOH H2TCPP (Wamser, 2000) n = 3%; IPCE max = 55% low efficiency due to aggregation (Officer, 2007) -COOH HOOC n = 7.1%; 3.6% in solid state DSSC; Optimize energy level at ẞ-position C343 n = 0.9%; narrow absorption range; fast injection (200 fs) Indolines NKX-2311 n = 6%; absorption red-shifted vs C343 NKX-2677 n = 8.1%; thiophene extend π- conjugation; avoid isomerization, improve stability CN .COOH Donor π-bridge Acceptor CN .COOH (Tan, 2009) n = 4.3%; D-π-A design for intramolecular charge separation (Tan, 2009) n = 5.1%; n-Hex suppress charge recombination and dye aggregation D149 n=9%; rhodanine red-shift absorption COOH D205 n = 9.5% (N719, 11.2%) COOH 5#6Terry Lou Organic dyes (Cont’d) Triarylamines NC -COOH Dyenamo Yellow (L0) n = 3.3% (N719, 7.7%) Amax = 386 nm (€ = 33,800 М-1 cm-1) For N749 (black dye): Amax = 605 nm (€ = 7,500 M-1 cm-1) NC -СООН Dye-Sensitized Solar Cells CN .COOH дающи D5 Baran Group Meeting Jun 5, 2021 COOH n-C6H130 CN n = 7.2%; 3% in solid state DSSC; Amax = 458 nm (€ = 38,000 M-1 cm-1) NC здовая C217 n = 9.8%; 8.1% in ionic liquid; retain >96% efficiency after 1,000 h (1 sun, 60 °C) COOH n = 5.1% (N719, 6%); IPCE = 85% (400 nm) max = 441 nm ( = 33,000 М-1 cm-1) D11 CN COOH С6Н130 -ОС 6 Н 13 C6H13 C6H13 CN COOH COOH С6Н130 С6Н130 JK-2 n = 8%; dimethylfluorene improved photo- and thermal stability; red shift by extended thiophene bridge JK-46 n = 8.6%; 7% in ionic liquid; excellent stability; alkyl chain enhanced tolerance towards water C205 n = 8.3%; 3,4-ethylenedioxythiophene (EDOT) further red shift vs thiophene Y123 n = 10.3%; ехсeptional stability; retain >90% efficiency after 3,000 h (1 sun, 85°C) CO 6#7Terry Lou Other dyes DOCK✗CDON (Zhai, 2005) 5 n = 3.4%; dye aggregation due to л-л interaction Pekot (Ohshita, 2008) n = 0.12%; formation of Ti-O-Si bonds Dye-Sensitized Solar Cells Transparent photovoltaic (TPV) HOOC N I C16H33 Co-sensitizing (Cocktail dyes) C4H9Q C4H9O C4H9O XY1b C6H13Q C4H9O XY1b+Y123 C6H130 C6H13Q Y123 C6H130 COOH CN CN COOH COOH Absorbance / Emission (a.u.) 1.0 0.8- 0.6. 0.4 Baran Group Meeting Jun 5, 2021 NC TiO2 (Ooyama, 2007) n = 1%; -COOH only serve for anchoring (carbazole dye) CN S NC Donor Acceptor C16H33 VG20-C16 (Sauvage, 2021) n = 3.1%; 76% average visible transmittance (AVT) Electrolyte Redox couple + solvent + additive 1/13 most commonly used / electrolyte 0.2 0.0 400 500 600 700 800 900 Wavelength (nm) Co(bpy), electrolyte Cation affects V. oc, e.g. Li*<<Na*<K+ (Li* binds to TiO 2) Other redox couple: Co 3+/2+, Cu2+/+, SCN-/(SCN)2, Br/Br2 Common challenge: slow mass transport, chemical instability, corrosion Mediators can be added to speed up dye regeneration, e.g. 4-tert-butylpyridine (TBP), triphenylamine (TPA), N-methyl- benzimidazole (NMBI), guanidinium thiocyanate (GuSCN) Common solvent: water, EtOH, ACN, valeronitrile, propylene carbonate... Major issues with liquid electrolyte: HOOC -N Anchor -S NC • (Sun, 2008) Volatility of solvent, leakage, toxicity CN • Sealant reduce active area of PV module (Grätzel, 2018) n = 13.1% (1 sun); 32% (1,000 lux) (1 sun 110,000 lux) Highest efficiency for DSSC reported; Best ambient light efficiency of all PVs Joule 2018, 2, 1108-1117; JACS Au 2021, 1, 409–426. n = 0.05%; p-type dye lonic liquids, gel, and polymer electrolyte, albeit less efficient 7#8Terry Lou Solid-state (ss) DSSC - Incomplete penetration into nanoporous TiO2 Dye-Sensitized Solar Cells - Poor electronic contact with dye - Thin layer of TiO 2 is needed, hence reduce light absorption - Fast charge recombination between e in TiO 2 and h* in HTM Low conductivity of HTMs Hole transporting material (HTM) - - Conducting polymer, e.g. PEDOT Hole-conducting molecules, e.g. spiro-MeOTAD Quasi-solid-state electrolyte • • Polymer swollen with liquid electrolyte Overcome volatility & leakage problem p-Type semiconductor, e.g. CsSnl 2.95 F0.05 Dried out electrolyte ("Zombie cells") DSSCs normally died upon evaporation of redox electrolyte MeO MeO Retention of performance of dried-out cells was observed using [Cu(dmp)2]2+/+ (Gerrit, 2015) OMe OMe PEDOT OMe OMe Spiro-MeOTAD ⚫ Dried out [Cu(tmby)2]2+/+ gave the highest ssDSSC efficiency of 11.7% (Zhang, 2018) "electrolyte solvent was evaporated in a dry box by keeping the holes on the counter electrode unsealed for a period of more than one week" -OMe -OMe Counter electrode Baran Group Meeting Jun 5, 2021 TCO-coated glass (or conducting polymer) as substrate Commonly spray-coated with Pt (as catalyst and high conductivity) Carbon nanotube, graphene, C 60, PEDOT were also used p-Type DCCS Best n < 1% Rather unexplored NiO commonly used as electrode (lower VB is preferred) counter electrode electrolyte dye D*/D* (a) (b) hv NiO film CB -1 Tandem pn-DSSC Theoretical limit of n = 43% p-DSSC as photocathode Photocurrent from two electrodes have to be matched e cb e LUMO e hv 0 EI/13 VB 1 I/I3* D/D+ 2 E load V vs NHE TiO2 HOMO N719 1/13 e LUMO hv HOMO Dye 3 vb NiO (Nattestad, 2010) n = 2.4%; 8#9Terry Lou Module development - - - Sandwich or monolithic design Dye-Sensitized Solar Cells Interconnection, cell sealing and encapsulation take up space, hence lowering module area efficiency Module area efficiency = active area (efficiency × area) ÷ module area G24 Innovation Ltd., UK. Flexible Z-interconnected sandwich modules with module area efficiency 2.2% P-design Z-design W-design M-design + O + 0 + Legend Glass substrate TCO ΤΙΟΣ Electrolyte Current collectors Insulation/cell-to-cell seal Porous separator Porous counter electrode Hermetic backsheet Discussion Baran Group Meeting Jun 5, 2021 30 years of development, sluggish improvement in efficiency n = 10% in 1993, and n = 13.1% in 2018 Excellent indoor efficiency n = 32% (Grätzel, 2018) Development of near-IR dye for transparent photovoltaic (TPV) Low cost and easy manufacturing (e.g. by roll-to-roll printing) Est. material cost $0.78/Wp in 2009, similar to thin-film PVs Development of tandem pn-DSSC remained challenging Tandem cells with n-DSSC and other PV technology was reported Emerging new PV technology based on similar concept: • Organic solar cell (OSC) . Perovskite solar cell (PSC) • Quantum dot solar cell (QDSC) From a solar cell to a PV System MAY Fraunhofer ISE, Germany. 90 cm² Z-type sandwich modules with module area efficiency 3% The Institute of Plasma Physics, China. 3,600 cm² panel consisting 12 DSSC modules, with module efficiency of 5.9 % Helpful resources Nature 2001, 414, 338–344. Chem Rev 2010, 110, 6595-6663. Nanoscale Res Lett 2018, 13, 381. Front Chem 2019, 7, 77. Electricity Meter AC Isolator Fusebox Inverter Battery PV-System Charge Controller Generation Meter DC Isolator Cabling Mounting Tracking System J Inorg Organomet Polym 2021, 31, 1894-1901. Solar Module Solar Cell Solar Panel Solar Array 6#10Varian (216x) Varian (205x) Stanford (140x) Varian 20 24 20 (T.J. Watson A- Research Center) IBM UNSW Stanford UNSW ARCO Terry Lou 52 48 44 40 90 36 88 Best Research-Cell Efficiencies I I Cell Efficiency (%) T 32 I Multijunction Cells (2-terminal, monolithic) LM lattice matched MM metamorphic IMM inverted, metamorphic Three-junction (concentrator) Three-junction (non-concentrator) A Two-junction (concentrator) ▲ Two-junction (non-concentrator) Four-junction or more (concentrator) Four-junction or more (non-concentrator) Single-Junction GaAs ▲ Single crystal A Concentrator Thin-film crystal Crystalline Si Cells Single crystal (concentrator) ■Single crystal (non-concentrator) Multicrystalline Silicon heterostructures (HIT) Thin-film crystal Thin-Film Technologies O CIGS (concentrator) CIGS ○ CdTe O Amorphous Si:H (stabilized) Emerging PV O Dye-sensitized cells O Perovskite cells. Perovskite/Si tandem (monolithic) Organic cells Organic tandem cells Inorganic cells (CZTSSe) Quantum dot cells (various types) Perovskite/CIGS tandem (monolithic) Dye-Sensitized Solar Cells Baran Group Meeting Jun 5, 2021 ONREL Transforming ENERGY Boeing- Spectrolab (LM, 364x) Sharp (IMM, 302x) Solar Junc (LM, 942x) Soitec (4-J, 297x) NREL (6-J,143x) FhG-ISE/ Soitec 回 47.1% 화 Spectrolab (MM, 299x) (MM, 454x) FhG-ISE SpireSemicon (MM, 406x) Boeing-Spectrolab Boeing-Spectrolab (MM,179x) (MM, 240x) NREL (IMM) NREL NREL Solar Junc (IMM, 325.7x) (LM, 418x) Soitec (4-J, 319x) NREL NREL (4-J, 327x) Boeing- Spectrolab (5-J) 44.4% V NREL (6-J) Boeing- Spectrolab Spectrolab NREL/ Boeing- Spectrolab Boeing- Spectrolab NREL (IMM) Sharp (IMM) 39.2% 37.9% Sharp (IMM) NREL (38.1x) Sharp (IMM) 35.5% A FhG-ISE Spectrolab Spectrolab NREL Japan Energy Spectrolab NREL Radboud Univ. Varian AA IES-UPM (1026x) FhG-ISE (117x) Alta Alta SunPower (large-area) FhG-ISE (232x) NREL (467x) NREL NREL (MM) LG NREL Alta NREL (258x) LG Alta Devices 32.9% 30.5% A 29.5% Panasonic NREL SunPower (96x) Amonix (92x) -A Panasonic Kopin UNSW Radboud U. UNSW UNSW UNSW FhG-ISE Alta Panasonic LG Kaneka Solexel ZSW Spire UNSW Sanyo Sanyo NREL (14.7x) FhG-ISE UNSW Sanyo Sanyo UNSW UNSW/ Georgia NREL Sanyo Eurosolare (14x) Georgia Georgia Tech Westing- Spire Tech Tech Varian UNSW house ロ NREL NREL NREL NREL NREL NREL NREL NREL 8 KRICT U. Stuttgart Sandia 16 RCA U. So. No. Carolina Florida Matsushita NREL Mobil Solar State U. Solarex Solarex NREL U. Stuttgart Boeing Euro-CIS NREL UniSolar UniSolar UniSolar (aSi/ncSi/ncSi). Mitsubishi 12 Boeing Kodak Kodak ARCO ARCO IBM HKUST Kodak Boeing Sharp Photon Energy EPFLO Matsushita Kodak AMETEK Boeing Boeing EPFL IBM IBM Konarka Solarex ARCO 8 Monosolar UniSolar U.of Maine 8 Solarmer Heliatek NREL (15.47SW NREL ZSW EMPA (Flex poly). ISFH FhG-ISE First Solar Solexel GE First Solar LG EPFLO NIMS Sharp First Solar Solibro HZB Oxford PV 29.1% Oxford PV FhG-ISE KRICT/MIT KRICT/MIT& Korea U (tie) UNIST Stanford/ASU SolarFrontier ISCAS UCLA KRICT/UNIST FhG-ISE First Solar -EPFL GE SolarFron Univ.of Queensland KRICT AIST IBM UCLA-Sumitomo NREL HZB Jinko Solar Trina Solar Canadian Solar UNIST SJTU/BUAA City U HK/UW SJTU-UMass SCUT-CSU ICCAS O EPFL EPFL -SCUT/eFlexPV UCLA -ICCAS UCLA 25.5% O 24.2% 23.4% 23.3% O 23.3% Alta ALG 27.8% A Oxford PV 27.6% Kaneka 26.7% ISFH EPFL 26.1% 22.1% 21.2% ▼ 18.2% 18.1% ◇ Trina AIST Raynergy Tek of Taiwan 14.2% 14.0% 13.0% 12.6% Phillips 66 U.Toronto MIT U. Toronto Boeing RCA EPFL U.of Maine EPFL Groningen NREL/Konarka Konarka U. Linz UCLA Sumi- tomo U. Toronto (PbS-QD) DSSC 0 4 1975 RCA Plextronics Heliatek RCA RCA RCA RCA RCA 1980 Siemens U. Linz U. Dresden U. Linz NREL (ZnO/PbS-QD) (Rev. 01-04-2021) 1985 1990 1995 2000 2005 2010 2015 2020 10