Organic-based light harvesting electronic devices

Transcription

1 Organic-based light harvesting electronic devices Maddalena Binda Organic Electronics: principles, devices and applications Milano, November 26-29th, 2013

2 Organic-based light harvesting devices From signal detection to power generation... OPTICAL COMMUNICATIONS Light source Optical Fiber Photodetector SENSORISTICS IMAGING REMOTE CONTROL

3 Organic-based light harvesting devices From signal detection to power generation... Top Electrode Active material Bottom transparent electrode Glass High speed Photogeneration efficiency: EQE CW operation Spectral selectivity Bias voltage allowed High signal-to-noise ratio Broad responsivity Bias voltage not allowed Hardiness in critical conditions

4 Light sensors: Organic PhotoDetectors (OPD)

5 Light sensors: sensitivity We want to go from this......to this: signal noise High signal-to-noise ratio (SNR) Increase signal Increase EQE Reduce parasitic effects like e.g. leakage current Al Vbias Active material ITO+PEDOT:PSS Glass M. Binda et al. Appl. Phys. Lett. 98 (7), , ITO -4.0eV -4.7eV eV -5eV Al

6 Light sensors: sensitivity We want to go from this......to this: signal noise High signal-to-noise ratio (SNR) Increase signal Increase EQE Reduce parasitic effects like e.g. leakage current Al Vbias Active material ITO -4.7eV - MEH-PPV eV Al MEH-PPV ITO Glass M. Binda et al. Appl. Phys. Lett. 98 (7), , Cross-linked -5eV

7 I dark [A/cm 2 ] EQE [%] Light sensors: sensitivity M. Binda et al., Appl. Phys. Lett. 98 (7), , n without PPV with PPV without PPV 10 10n with PPV 1n 1 100p 0,1 1 V [V] Wavelength [nm] SEE ALSO: P. E. Keivanidis et al., Appl. Phys. Lett. 94, , X. Gong et al., Science 325, 1665, Reduced leakage current while EQE is conserved SNR

8 [cm 2 /Vs] Normalized photocurrent Light sensors: response speed E.g. APPLICATION IN OPTICAL DATA TRANSMISSION Play with: device geometry > Reduced interelectrode spacing active material composition 1E-3 1E-5 1E-7 1E-9 holes electrons AlkSQ:PCBM 1:1 GlySQ:PCBM 1:1 GlySQ:PCBM 1:3 M. Binda et al., Organic Electronics 10 (2009) ,0 0,8 0,6 0,4 0,2 0,0 Fast reponse is required! AlkSQ:PCBM 1:1 GlySQ:PCBM 1:1 GlySQ/PCBM 1: Time [ s] BW~10kHz Adv. Mater. 2013, 25,

9 Organic (and hybrid) Solar Cells

10 Power Conversion Efficiency of a solar cell LOAD FF V V mpp oc I I mpp sc Power Conversion Efficiency (PCE)

11 Series and Shunt resistance of a solar cell Rs Photocurrent flow Rs Rsh We want: Rs low Rsh high Leakage paths: metal infiltration, pinholes,... poorly conductive paths in the semiconductor, contact resistance, resistivity of the electrode,...

12 Power Conversion Efficiency of a solar cell FF V V mpp oc I I mpp sc Power Conversion Efficiency (PCE)

13 Power Conversion Efficiency of a solar cell Important notes on cell characterization: 1. The solar emission must be reproduced 2. A standard path must be adopted air-mass: optical path through the Earth athmosphere scattering absorption Standard AM1.5-1 sun AM1.5 z=48.2º (middle latitudes) L0 z L AM X X L L 0 1 cos L 0 =zenit path length at see level L=effective path length z ~1000W/m 2 Es. z=0 AM1

14 Power Conversion Efficiency of a solar cell Different parameters must be simoultaneously optimized: FF PCE V P OC IN But they are indeed strongly correlated!!! I SC

15 Open Circuit Voltage (V OC ) Debated... See: Adv. Funct. Mater. 11, 5, Adv. Funct. Mater. 2011, 21, Adv. Mater. 2010, 22, Nat. Materials, 8, , 2009 LUMO, A empirical V oc =(1/q)(HOMO D -LUMO A 0.3) HOMO, D D A at solar intensity Adv. Mater. 18, 789 (2006)

16 Open Circuit Voltage (V OC ): recombination empirical V oc =(1/q)(HOMO D -LUMO A 0.3) Jnet(x)=0 EFn *Boltzmann approx., no disorder EFh? Voc,max=HOMO D -LUMO A qvoc=efh-efe G=R(x) G=Rgeminate G=Rlangevin G=RSRH... PHYSICAL REVIEW B 84, (2011) Es. R=Rlangevin=γnh(x)ne(x) * qvoc E g k B T ln G N N h Recombination Logaritmic dependence on G, often reported in the literature e

17 Open Circuit Voltage (V OC ): energetic disorder not the complete story...egergetic disorder plays a role!!! Recombination via localized states Carriers relaxed into the tail of the DOS reduce the quasi-fermi level splitting LUMO EFe EFh qvoc const. mk B T ln G HOMO HOMO D -LUMO A -ΔE m>1 Example. PHYSICAL REVIEW B 84, (2011) Hp. Langevin modified for disorder, exponential tail distribution

18 Origin of the V OC : role of the electrodes MIM: Metal Intrinsic Metal model Short-circuit condition Open-circuit condition or flat band condition Built-in field due to metals workfunction mismatch No net current flow. Voc given by the difference between the metal workfunctions J PHOTO : photogenerated carriers need a driving force Internal Electric Field Drift current Charge concentration gradient Diffusion current ideal BHJ

19 Origin of the V OC : role of the electrodes MIM: Metal Intrinsic Metal model Short-circuit condition Open-circuit condition or flat band condition Built-in field due to metals workfunction mismatch built-in voltage V oc,max =(1/q)(ΦM1-ΦM2) Vs V oc,max =(1/q)(HOMO D -LUMO A ΔE) J PHOTO : photogenerated carriers needs a driving force No net current flow. Voc given by the difference between the metal workfunctions Internal Electric Field Drift current Charge concentration gradient Diffusion current ideal BHJ

20 Origin of the V OC : role of the electrodes Contacts have little effect if I choose them right! Fermi-level pinning PHYSICAL REVIEW B 84, (2011) J. Appl. Phys., Vol. 94, No. 10, 15 November eV 1.4eV Ag Au «effective» due to interf. dipole Au nominal

21 Origin of the V OC : role of the electrodes V oc can exceed the built-in of the contacts if the bulk-heterojunction is not so ideal Waldauf et al., J. Appl. Phys. 99, (2006) Compex, more realistic device architectures provide asymmetry to the system (i.e. blocking layers) Current flow can be driven by diffusion! D. Cheyns, Phys. Rev. B 77, (2008).

22 Improving the V OC : active material 1. D/A energy level alignment 2. Reducing recombinations J photo Voc DONOR ACCEPTOR?? material level Increase phase separation: slow down the rate at which charges meet each other... But TRADE OFF with charge generation (reduced D/A interface)!!! TRADE OFF with charge dissociation!!!

23 Improving the V OC : device BLOCKING LAYERS FOR SELECTIVE COLLECTION AT THE ELECTRODES Dr. Carvelli

24 Improving the V OC : tandem cells Voc,6 Voc,5 Voc,4 Voc,3 Voc,2 Voc,1 Renee Janssen-University of Eindhoven, Plastic Electronics 2011.

25 Tandem solar cells for increased V oc S. Sista et al., Adv. Mater. 22, 380, Equivalent Voc= sum of single cells Voc Equivalent Jsc= Jsc of the worst cell in the stack REVIEW ART.: Energy Environ. Sci., 2013,6,

26 Short-circuit current (J sc ) EQE Light absorption Charge photogeneration efficiency Charge transport and collection EQE= Number of collected charges/s Number of incoming photons/s n c /s = = abs ed cc n v /s Active material Device (architecture) level

27 Short-circuit current (J sc ): light absorption active material Extending the responsivity to low energy photons As much as half of the energy of the solar spectrum is carried by long wavelengths photons! ΔLUMO Voc DONOR ACCEPTOR Progress in Polymer Science 38 (2013) optimum bandgap

28 Red-light responsivity EXAMPLES 1: POLYMERS J. Peet et al., Nature Materials 6, (2007) PCE>5% PCPDTBT-PCBM PDDTT-PCBM PDDTT X. Gong et al., Science, 325, 25, Other works: Muhlbacher et al., 2006; Kooistra et al., 2006; Yao et al., 2006; Soci et al., 2007; Wang et al., 2008; Hou et al., 2008)

29 Red-light responsivity EXAMPLES 2: SMALL MOLECULES SQUARAINES G. Wei et al., ACSNano 4, 4, REVIEW: L. Beverina et al., Eur. J. Org. Chem. 2010,

30 Short-circuit current (J sc ): light absorption - device Maximum of the stationary optical field inside the active region Requirements: Transparent Electron transporter Conduction band lower than LUMO of acceptor and higher than cathode Fermi level η=6% (certified by NREL) Park et al., Nat. Phot. 3 (2009) 297

31 Short-circuit current (J sc ): photogeneration and charge transport BULK-HETEROJUNCTION OPTIMAL FOR EXCITON DISSOCIATION......BUT REQUIRES CAREFUL CONTROL OF MORPHOLOGY! Charges percolation must be allowed! How? 1. Additives (solvents mixtures): selective solubility One phase still in solution while the other is already in solid state Higher degree of phase separation, higher cristallinity A. de Sio et al., 95, , 2011.

32 Short-circuit current (J sc ): photogeneration and charge transport BULK-HETEROJUNCTION OPTIMAL FOR EXCITON DISSOCIATION......BUT REQUIRES CAREFUL CONTROL OF MORPHOLOGY! Charges percolation must be allowed! How? 2. Post deposition treatments: thermal annealing X. Fan et al., Appl. Phys. A, XRD, increased P3HT crystallinity

33 Fill Factor (FF) Going towards the ideal comb-like structure No continuous paths between the electrodes for leakage carriers... Higher Shunt resistance...but continuous and directional paths for photogenerated carriers: Higher mobility Lower recombination Low series resistance

34 Hybrid devices: organic/inorganic Inorganic nanostructured semiconductors exploited in combination with organics for their Optical properties CdSe, CdS, PbSe, PbS, nanocrystals Morphological and electrical properties 1 2 Metal oxides: TiO2, ZnO Strong and tunable absorption Tunable shape A.C. Arango et al., Nano Letters 9, (2009) B. Sun et al., J. Appl. Phys., 2005, 97, D.M.N.M. Dissanayake, Nanotechnology 20 (2009) Morphologically stable Higher mobilities Transparent High dielectric constant K. Coakley, Adv. Funct. Mater., 2003, 13, 301. S.S. Li, J. Am. Chem. Soc. 2011, 133, S.D. Oosterhout, Nat. Mater. 2009, 8, Recent review paper: Energy Environ. Sci., 2013,6,

35 Hybrid devices: organic/inorganic Inorganic nanostructured semiconductors exploited in combination with organics for their Optical properties CdSe, CdS, PbSe, PbS, nanocrystals Tunable absorption High dielectric constant Tunable shape A.C. Arango et al., Nano Letters 9, (2009) B. Sun et al., J. Appl. Phys., 2005, 97, D.M.N.M. Dissanayake, Nanotechnology 20 (2009) Morphological and electrical properties 1 2 solution processed! Metal oxides: TiO2, ZnO Colloidal NCs Solubility! Morphologically stable Higher mobilities Transparent High dielectric constant K. Coakley, Adv. Funct. Mater., 2003, 13, 301. S.S. Li, J. Am. Chem. Soc. 2011, 133, S.D. Oosterhout, Nat. Mater. 2009, 8, Recent review paper: Energy Environ. Sci., 2013,6,

36 Hybrid devices: heterojunction structure BHJ Mimicking comb-like Planar HJ enabled by high morphological stability after thermal post-processing: organic ligands removal, nanoparticles sintering ZnO, TiO2, Al2O3

37 Nanocrystals based hybrid solar cells Optical properties Quantum confinement effect: Extreme spectral tunability by tuning the dots dimension MEG? Multiple Exciton Generation Debated... EQE>100% reported! Science, Vol. 334 no pp , 2011.

38 Nanocrystals based hybrid solar cells Charge transport interparticle hops Charge trapping at surface states Distance between nanocrystals (hopping sites) direct transport intra-crystal careful tuning of capping ligands! (shorter, conductive,...) Nano Lett., 2011, 11,

39 Nanocrystals based hybrid solar cells blended with polymer for extended absorption

40 Nanocrystals based hybrid solar cells blended with polymer for extended absorption Effectiveness of D/A interface? Energy transfer between polymer and NCs? Poor charge transport?

41 Metal oxides based hybrid solar cells Polymer-MetalOxide Solution processed! Dye-sensitized TiO 2 TiO 2 Dye-doped with perovskites Mesoporous TiO2 TiO2/Al203

42 Metal oxides based hybrid solar cells Polymer-MetalOxide Solution processed! Dye-sensitized polymer TiO 2 Dye monolayer HTM TiO 2 Dye-doped with perovskites Mesoporous TiO2 Perovksite layer HTM TiO2/Al203

43 Metal oxides based hybrid solar cells polymer - metal oxide polymer Poor performance: PCE ~ 0.5% < 1 μm TiO A Possible explanation: 2 - Polymer infiltration inside the mesoporous scaffold - Poor efficiency of charge generation at D/A interface D Improvement with interlayers chemisorbed onto TiO2 surface to: - Induce order in the polymer phase at the interface (Energy Environ. Sci., 2012, 5, ) - Mediate charge transfer process (monolayer of a standard organic acceptor molecule) (Adv. Funct. Mater. 2012, 22, ) PCE ~ 1%

44 Metal oxides based hybrid solar cells Dye-sensitized (solid state) PCE ~ 7% HTM - - Dye monolayer TiO 2 μm + + ETM Dye HTM Electrode 1 Electrode 2 HTM: spiroometad molecule Light absorption and charge transport are separated - No exciton diffusion losses Extended interface for charge transfer, readily available - Dye s charge transport properties are not involved in the process - High surface mesoporous TiO2 ensures good light absorption from the dye monolayer New dye with higher absorption Room for improvement: Charge transport in TiO2 and HTM REVIEW ARTICLE: H.J. Snaith, Adv. Mater. 2007, 19,

45 Metal oxides based hybrid solar cells Dye-doped based on perovskites Perovksite layer HTM TiO2/Al203 PCE ~ 15 % Record efficiency (not certified) Science, 338, , 2012 Nature, 501 (7467), pp Organometal halide perovskite A=CH 3 NH 3 B=Pb X=Cl, I Solution processed from precursor Polycrystalline morphology

46 Metal oxides based hybrid solar cells Dye-doped based on perovskites Very new... many things to be understood: - charge transport: features and mobility values? - primary photoexcitation: excitonic or not? - role of morphology - stability......but strong panchromatic absorption + HTM TiO2/Al203 long range crystallinity (>100 nm) Efficient migration of the species (excitons, charges)