Charge separation in molecular donor acceptor heterojunctions

Institute of Physics 13 July 2009 Charge separation in molecular donor acceptor heterojunctions Jenny Nelson, James Kirkpatrick, Jarvist Frost, Panagiotis Keivanidis, Clare Dyer-Smith, Jessica Benson-Smith Department of Physics, Imperial College London James Durrant, Tracey Clarke Department of Chemistry, Imperial College London 1

Molecular photovoltaic materials Easily processable e.g. from solution Abundant, non-toxic materials Tune properties via chemical design dye 1 nm conjugated molecule Excited states are localised: limited charge and exciton mobility 1 nm 2 Konarka Technologies conjugated polymer

Device structures Inorganic semiconductor + E B ~ 0.01 ev Spontaneous charge pair generation - n p active region Molecular semiconductor +- E B ~ 0. 1-0.5 ev Charges hard to dissociate dead region Cannot copy inorganic PV device structures! 3

Donor-acceptor solar energy converters C 60 Conjugated polymer Electron acceptor Electron donor 4

Device structures Donor-acceptor bulk heterojunction devices Domain size ca. 10 nm. ~ exciton diffusion length Al cathode Donor-Acceptor blend e - h + Active layer can be 100s of nm - limited by charge diffusion length ITO anode Glass substrate Both components deposited from same solution 5

Device structures Donor-acceptor bulk heterojunction devices Al cathode Donor-Acceptor blend e - h + ITO anode Glass substrate Both components deposited from same solution 6

Key steps in photocurrent generation 1. Photon absorption 2 + 3 + 4 5 2. Exciton diffusion 1 5 Exciton decay 3. Exciton dissociation geminate charge pair Other excited states e.g. triplets 4. Geminate charge pair separation Geminate charge pair recombination 5. Charge transport to contacts 7 Non geminate charge pair recombination Current generation

Key challenges LUMO ΔE e LUMO E g ΔECS ΔΕ F = ev oc 0 e E g b HOMO donor HOMO acceptor inc ( E) Abs( E) de J sc Current density / ma cm -2-10 -20-30 -40-50 P3HT:PCBM solar cell η = 5 % High efficiency silicon cell η = 24 % E g E ΔΕ F -60 0.0 0.2 0.4 0.6 0.8 Voltage / V 8

Key challenges LUMO ΔE e LUMO E g ΔECS ΔΕ F = ev oc e E g b HOMO donor HOMO acceptor inc ( E) Abs( E) de J sc LUMO Level Donor [ ev ] -4.0-3.8-3.6-3.4-3.2 2.00 Lower donor HOMO level 4.00 3.00 5.00 8.00 6.00 T 10.00 7.00 9.00 1.00 Reduce optical gap E -3.0 2.00 2.8 2.4 2.0 1.6 1.2 Band Gap [ ev ] E g ΔΕ F 9

Reducing the donor optical gap: an example P3HS R R Acceptor (PCBM) LUMO Level Donor [ ev ] -4.0-3.8-3.6-3.4-3.2-3.0 P3HT 2.00 4.00 3.00 P3HS (Merck) 5.00 8.00 6.00 T 2.00 10.00 S 7.00 9.00 1.00 2.8 2.4 2.0 1.6 1.2 Band Gap [ ev ] Se R n S n PCE = 3.0% EQE / % 60 50 40 30 20 10 0 Se P3HS device P3HT device 300 500 700 Wavelength / nm Disappointing performance of low gap selenophene polymer related to low efficiency of charge pair generation: n RMS:20.62nm PCE = 2.7% A. Ballantyne et al, Adv. Mater. 19, 4544 (2007) 10

Outline Molecular photovoltaic devices Process of charge separation Factors that influence charge separation competing excited states driving force chemical structure microstructure Modelling the process excited states and energies how charge separation can occur escape from bimolecular recombination dynamics of charge separation 11

Outline Molecular photovoltaic devices Process of charge separation Factors that influence charge separation competing excited states driving force chemical structure microstructure Modelling the process excited states and energies how charge separation can occur escape from bimolecular recombination dynamics of charge separation 12

Process of charge pair generation Energy A 3 A D* 3 D (D + A ) * D +, A ΔE 3 (D + A ) Γ = J h 2 π λkt exp 2 [ ( ΔE + λ) / 4λkT ] +- + - 13

Measurement of charge generation Transient optical spectroscopy 1.0 Relative ΔOD 0.8 0.6 0.4 0.2 pump pulse I(λ, t) Detector 0.0 800 1000 1200 Wavelength (nm) Nature of photoexcited state Probe light t Sample Δ Absorbance (*10-3 ) 0.5 0.4 0.3 0.2 0.1 ΔO.D (λ, t) lifetime 0.0 10-5 10-4 10-3 10-2 10-1 10 0 Time / s yield 14

Time scales of events 10 2 10 0 seconds 10-2 10-4 10-6 10-8 10-10 10-12 10-14 Phosphorescence lifetime Non geminate (bimolecular) charge recombination Charge carrier extraction from an organic PV device Molecular (segmental) motion in a polymer matrix Fluorescence lifetime Geminate (monomolecular) charge recombination Energy migration in a polymeric matrix, eg in poly(fluorene) vibrational relaxation of an electronically excited state photo-induced electron transfer reaction ultrafast fluorescence ultrafast TAS ns ms TAS 15 10-16 10-18 revolution period of the electron in Bohr s model not accessible

Outline Molecular photovoltaic devices Process of charge separation Factors that influence charge separation competing excited states driving force chemical structure microstructure LUMO HOMO E g ΔE e ΔE CS LUMO Modelling the process energies of excited states charge transfer excited states means of charge separation dynamics of charge separation donor HOMO acceptor (PCBM) 16

Influence of ΔE CS on charge separation TFMO PFO tris- N bis- PCBM H 17 C 8 C 8 H 17 TFB H 17 C 8 C 8 H 17 n F8BT PCBM PCBM OMe n N S N H 17 C 8 C 8 H 17 n Energy / ev 2.2 5.2 2.3 ΔE CS 5.3 ΔE CS ΔE CS 3.5 5.8 5.9 ΔE CS 3.7 6.1 3.6 6.0 3.5 5.9 ΔE CS 1.5 ev 1.6 ev 2.1 ev 2.2 ev 17

Influence of ΔE CS on charge separation % External Quantum Efficiency (EQE) 20 18 16 14 12 10 8 6 4 2 80% PCBM:Polyfluorene Devices 80% PCBM/F8BT 80% PCBM/PFO 80% PCBM/TFMO 0 300 350 400 450 500 550 600 wavelength (nm) Small ΔE CS leads to photocurrent generation Delta OD (Normalized) 1.0 0.8 0.6 0.4 0.2 0.0 600 700 800 900 F8BT:PCBM 5% PFO:PCBM 5% TFMO:PCBM 80% wavelength (nm) Large ΔE CS leads to appearance of transient PCBM triplet spectrum, small ΔE CS leads to polarons. Jessica Benson-Smith, Dalton Trans. (submitted) 18

Influence of ΔE CS on charge separation ΔE CS J sc =0.006mA/cm 2, PCBM triplet 0.00020 2.0x10-4 0.00015 1.5x10-4 ΔOD 0.00010 1.0x10-4 0.00005 5.0x10-5 0.00000 600 800 1000 1200 1400 Wavelength (nm) ΔE CS J sc =0.7mA/cm 2, no triplet Clare Dyer-Smith, 2009 19

Influence of ΔE CS on charge separation LUMO ΔE e LUMO Lowest neutral excited state limits charge pair generation LUMO ΔE e LUMO E g ΔE CS E g ΔE CS Energy (ev) (a) 3 2 1 S 1 T 1 HOMO low IP donor S 1 T1 ET HOMO acceptor (PCBM) CS ΔE CS Polaron Pair CTC RD LUMO Level Donor [ ev ] -4.0-3.8-3.6-3.4-3.2-3.0 (at low PCBM concentrations) 5.00 4.00 2.00 3.00 8.00 10.00 9.00 7.00 6.00 T 1.00 2.00 2.8 2.4 2.0 1.6 1.2 Band Gap [ ev ] Energy (ev) (b) 3 2 1 S 1 HOMO high IP donor S 1 T1 T 1 ET RD ISC HOMO acceptor (PCBM) Polaron Pair CTC ΔE CS Polymer PCBM Polymer:PCBM Blend Polymer PCBM Polymer:PCBM Blend Small ΔE CS : HOMO D -LUMO A < 3 PCBM Charge pair formation is most favourable Large ΔE CS : HOMO D -LUMO A > 3 PCBM PCBM triplet formation is most favourable 20

Influence of ΔE CS on charge separation: Transitional case Red F : PCBM H 17 C 8 C 8 H 17 50% n -co- S S N N N N * N n * * n * * S S n * 40% 5% 5% Me Normalized ΔAbsorbance 1.2 1.0 0.8 0.6 0.4 0.2 50 wt% PCBM: RedF 5 wt% PCBM: RedF PCBM triplet Normalized ΔAbsorbance 1 10 5 wt% PCBM/RedF 50 wt% PCBM/RedF 0.0 600 700 800 900 1000 Wavelength (nm) 10-7 10-6 10-5 10-4 10-3 polymer PCBM (4) (2) 10 0 10-1 10-2 10-3 Time (s) polymer PCBM - + + - - Lowest neutral excited state limits charge pair generation, but influenced by microstructure - + + - - 21

Emissive charge transfer (CT) states Polyfluorenes blended with F and non F silole acceptors Slow red shifted emission assigned to CT state Fluorinated silole acceptors show: less CT state emission Polymer ΔE cs Silole F F N N N N S Relative CT emission 4 3 2 1 0 Silole blends F-silole blends 2 2.1 2.2 2.3 2.4 2.5 ΔE CS 22

Emissive charge transfer (CT) states Polyfluorenes blended with F and non F silole acceptors Slow red shifted emission assigned to CT state Fluorinated silole acceptors show: Polymer Silole less CT state emission ΔE cs higher polaron yield higher photocurrent F N N N N F S Emissive CT states are a loss pathway 8.00E-009 Silole F-silole 0.05 0.04 F silole TFB:Silole TFMO:Silole PFB:Silole TFB:F-silole TFMO:F-silole PFB:F-silole Corrected polaron yield 7.00E-009 6.00E-009 5.00E-009 EQE 0.03 0.02 0.01 Silole 4.00E-009 0.00 23 2.10 2.15 2.20 2.25 2.30 2.35 2.40 Energy gap 300 350 400 450 Wavelength

Outline Molecular photovoltaic devices Process of charge separation Factors that influence charge separation competing excited states driving force chemical structure microstructure LUMO HOMO E g ΔE e ΔE CS LUMO Modelling the process energies of excited states charge transfer excited states means of charge separation dynamics of charge separation donor HOMO acceptor (PCBM) 24

Influence of ΔE e on charge separation LUMO E g HOMO electron donor ΔE e ΔE CS LUMO HOMO electron acceptor ΔE e needs to exceed Coulombic binding energy of geminate charge pair Correlation suggests need large ΔE e (> 0.6 ev) for charge genaration ΔOD (1 μs) 1x10-4 1x10-5 10-6 P(Se 10 Se 0 Se 10 ) P(Se 10 T 0 Se 10 ) P(T 8 T 8 T 0 ) P(T 12 SeT 12 ) P(T 12 T 12 TT 0 ) P3HS P(T 0 T 0 TT 16 ) P(T 12 NpT 12 ) 0.5 0.6 0.7 0.8 0.9 1.0 -ΔG CS rel / ev ΔE e P(T 0 TT 16 ) P3HT P(T 10 PhT 10 ) H. Ohkita et al., J. Am. Chem. Soc. 130, 3030 (2008) 25

Influence of ΔE e on charge separation LUMO E g HOMO electron donor ΔE e ΔE CS LUMO HOMO electron acceptor ΔE e needs to exceed Coulombic binding energy of geminate charge pair Correlation suggests need large ΔE e (> 0.6 ev) for charge genaration LUMO Level Donor [ ev ] -4.0-3.8-3.6-3.4-3.2-3.0 2.00 4.00 3.00 5.00 8.00? 6.00 T 2.00 10.00 7.00 9.00 1.00 2.8 2.4 2.0 1.6 1.2 Band Gap [ ev ] H. Ohkita et al., J. Am. Chem. Soc. 130, 3030 (2008) What about other donor : acceptor combinations? 26

Influence of ΔE e on charge separation Alternative acceptors cause charge generation at lower ΔE e than P3HT:PCBM 10-4 ΔOD (1 μs) 10-5 10-6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -ΔG CS rel / ev P. Keivanidis et al, 2009 27

Influence of ΔE e on charge separation Alternative acceptors cause charge generation at lower ΔE e than P3HT:PCBM Alternative donors with D-A character capable of efficient charge generation.. 28 HOMO LUMO ΔOD (1 μs) 10-4 10-5 10-6 Charge separation efficiency depends on specific chemical structure at interface 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -ΔG CS rel / ev S C 6 H 13 n T. M.Clarke et al.,chem. Comm. in press (2008)

Outline Molecular photovoltaic devices Process of charge separation Factors that influence charge separation competing excited states driving force chemical structure microstructure Modelling the process energies of excited states charge transfer excited states means of charge separation dynamics of charge separation 29

Effect of microstructure on current generation S n P3HT 5 0.30 Absorption (a.u.) 2.0 1.5 1.0 0.5 30 wt% PCBM J SC (ma cm -2 ) 4 3 2 1 absorbance 0.25 0.20 0.15 0.10 0.05 0.00 300 400 500 600 700 800 900 Wavelength 50 wt% 300 400 500 600 700 800 wavelength (nm) 0 0 20 40 60 80 100 PC 61 BM P3HT wt% P3HT Photocurrent generation in P3HT:PCBM blends is sensitive to composition and annealing Optical absorbance increases on annealing but not by enough to explain J sc 30

Influence of microstructure on charge separation PCBM P3HT -+ - + - Well dispersed PCBM: excitons dissociate but high probability of prompt charge recombination. Normalised ΔOD 900nm at 30 μs (a.u) 80 70 60 50 40 30 20 10 0 As-spun Annelaed 20 30 40 50 60 70 80 90 100 + + - PCBM P3HT P3HT wt% - Larger PCBM domains, ordered packing: most excitons dissociate, geminate charges able to escape each other P3HT PCBM Exciton dissociation inefficient. P. E. Keivanidis at al., 2009 Photocurrent generation correlates to charge separation yield Local microstructure influences efficiency of pair separation. 31

Effect of microstructure on charge separation T. M. Clarke et al., Adv. Funct. Mater.18, 4029 (2008) Charge generation yield (1 us) increases on annealing phase segregation reduces (bimolecular) recombination coefficient molecular ordering increases efficiency of geminate pair separation ΔOD/(1-T) 510nm 10-4 10-5 annealed at 140 C as cast 10-6 10-5 10-4 10-3 10-2 time (sec) Fraction of holes remaining 10 0 10-1 10-2 + 2d w Planar barrier Interdigitated + 10-8 10-7 10-6 10-5 10-4 Time [s] Tom Hammant Need to study excited state dynamics at shorter times 32

Questions What are the stages and intermediates in charge separation? What controls the energies of different excited states? How does chemical structure affect the process? How do local molecular ordering and phase separation affect the process? How do extinction coefficients of excited states evolve? How do charges separate anyway? 34

Outline Molecular photovoltaic devices Process of charge separation Factors that influence charge separation competing excited states driving force chemical structure microstructure Modelling the process energies of excited states charge transfer excited states means of charge separation dynamics of charge separation 35

Calculations of excited states and energies Excited states calculated using DFT at B3lyp 6-31g* level plus TDDFT singlet at same level. Good agreement with calculation of anion using SCF. Find excited state energies and oscillator strengths Identify contributions of MOs to excited states Simulate absorption spectrum by broadening each excited state energy Example: PCBM. Five quasi degenerate HOMOs, three LUMOs oscillator strength James Kirkpatrick and Jarvist Frost wavelength / nm 36

Calculations of excited states and energies Validation of excited state calculations against absorption spectra or electrochemical reduction spectra Jarvist Frost 37

Calculations of excited states and energies Use of excited state calculations in molecular design Jarvist Frost 38

Calculations of excited states and energies Study of charge transfer excited states in complex of oligo- CPDTBT + PCBM CT character appears at very short separations Influenced by specific position of acceptor James Kirkpatrick HOMO 39 LUMO

How can charge separation occur anyway? 1 D* Energy Energy of P + + PCBM - 0.3 1 ev Nearest neighbour separation (1-2 nm) Spatial separation of P + PCBM - 40

How can charge separation occur? Coulomb binding energy for point charges at ~ 1 nm separation in medium with ε = 3 is >> kt What effects could reduce the effective binding energy? Effect of polarisable medium Spatial distribution of charge Entropic contribution All these effects help reduce effective binding energy, but none large enough.. Model system: cubic lattice of isotropically polarizable dipoles. Charge pair placed on this lattice at some separation. Dipoles moved self consistently. Effect of solvation is to reduce binding at close separations. 41

Effect of vibrationally hot excited states D* (D + A ) * D +, A ΔE 3 D Energy 3 (D + A ) james.kirkpatrick99@imperial.ac.uk 42

Effect of vibrationally hot excited states 43 james.kirkpatrick99@imperial.ac.uk

Effect of vibrationally hot excited states Faster charge separation achieved as a result of slow cooling of the excited pair state james.kirkpatrick99@imperial.ac.uk 44

Conclusions Photocurrent generation in polymer:fullerene solar cells relies on charge separation at a donor-acceptor interface The efficiency of charge separation depends on driving force chemical structure microstructure (domain size and order) competing excited states (triplets and CT states) No simple guidelines exist to describe the process Need new methods to address: nature and energy of excited states stages involved in charge separation dynamics of charge separation dynamics of excited states during charge separation Thank you for your attention! 45