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Charge Transport Phenomena in OPVs. Papers by the Paul W. M. Blom Group University of Groningen, The Netherlands. Outline. Background Review PRL 2004: Photocurrent Generation in BHJ PRL 2005: Space-Charge Limited Photocurrent Fundamental Question: Internal E-Field?

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charge transport phenomena in opvs

Charge Transport Phenomena in OPVs

Papers by the Paul W. M. Blom Group

University of Groningen, The Netherlands

outline
Outline
  • Background Review
  • PRL 2004: Photocurrent Generation in BHJ
  • PRL 2005: Space-Charge Limited Photocurrent
  • Fundamental Question: Internal E-Field?
  • Adv. Func. Mater. Charge Transport in P3HT:PCBM BHJ(Jaewook’s paper – Using the model)
concept review of opv ops
Concept Review of OPV Ops
  • Photons absorbed by polymer (primarily), creating excitons
  • Excitons to D/A interface within 10-20nm = μexciton * lifetimeexciton
  • Ultrafast (45fs) charge transfer occurs → “bound polaron pair” or “charge transfer state”
    • Metastable state lasting micro to milliseconds
  • Something causes this to either separate into e + h or (gemenately) recombine producing photoluminescence.
    • Thermalization or Brownian motion\E-field thought to be the cause of sep.
  • Charges travel via speeds determined by μe,h [m2/Vs]
    • Caused by internal E-Field (drift), population imbalance (diffusion)
  • Charges are either collected at electrodes or get stuck at another interface (go back to #4, but now called non-gemenate).

Barker, Ramsdale, & Greenham, PRB (2003).

photocurrent generation in polymer fullerene bulk heterojunctions

Photocurrent Generation in Polymer-Fullerene Bulk Heterojunctions

V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, and P.W.M. Blom, PRL (2004)

conditions definitions
Conditions/Definitions
  • Materials: PPV polymer & PCBM BHJ (1:4) wt%
  • Light used: “white halogen lamp” 800W/m2 in N2 atmosphere
  • PhotocurrentJph=JL - JDCurrent generated by the light
  • Compensation voltage V0Voltage where Jph=0
  • Internal Field: E=(V0-V)/L
    • V = applied voltage
    • L = thickness of the active layer
  • Most graphs x-axes are V0-V: Start at V0 and go NEGATIVE.
  • Generation Rate: G(T,E) Rate at which charge carriers are created (disassociated & make it to the electrodes)
  • Result for this system: G(V=0) only 60%
analytical theory
Analytical Theory
  • Bracketed term adds effects of carrier diffusion
    • Fixes low-field behavior
    • Calculated by Sokel & Hughes, JAP (1982)
    • μ-independent (assumes no recombination)
  • G is a function of Temp & Field
    • Gmaxmeasured via large reverse bias
    • P is prob. of charge sep. at interface

Jph=eGL →

Using Const. G = Bad

p t e calc from onsager braun theory
P(T,E) calc. from Onsager-Braun theory
  • Onsager: oppositely charged ions undergoing Brownian motion in electrolyte w/Coulomb & E-field don’t recombination
  • Braun: finite lifetime of charge transfer state
    • kD rate of disassociation, kF rate of recombination, kRrate of recapture
    • a initial sep. of bound pair, b=e3E/8πεk2T2, EB e-h binding energy
    • kR=e‹μ›/‹ε›(‹› = spatially averaged mobility and dielectric constant)
  • Goliber & Perlstein, J. Chem Phys (1984)
    • Adds distribution function for a
    • F(x) distribution of e-h distances
    • NF normalization

Jph=eGL →

Using Const. G = Bad

Jph=eG(T,E)·L only

results using full model
Results using full model

Activation Energies

Dotted line is result at room temp

Assumes “blocking contacts”

no band bending at interface

No space-charge effects

No recombination?!?

Solid lines are numerical model

Steady-state charge distribution for Ohmic contacts using Poisson’s & continuity equation, including diffusion and recombination at interface

Jph(E)

  • Arrhenius plot: J(1/T)
  • Claim activation energies contain “combined effects of the distribution of binding energies, the temperature dependence of both charge carrier mobility and decay rate kF-1 as well as the effect of applied field”

0.01V

~0.9V

P(V=0)~60%

space charge limited photocurrent

Space-Charge Limited Photocurrent

V. D. Mihailetchi, J. Wildeman, and P.W. M. Blom

PRL (2005)

confirms scl theory in an opv
Confirms SCL Theory in an OPV
  • Drift lengths:
    • If one is less than thickness L, recomb. occurs
    • & if one is << than other, SPACE CHARGE!!
  • Space Charge Region:
    • L1 = wh
    • All Jphfrom this region
    • V1 ≈ V if imbalance large
  • Space Charge LimitMott-Gurney (circ. 1940)
    • J V2/L3
    • if imbalance too large (>> 100x)

Concepts: After Separation

3 limits of charge transport
3 Limits of Charge Transport
  • Drift length imbalance
  • Space charge
  • Total thickness L1 → L
    • Saturation at high V
    • Solve L-eqs. for V(G): Diff G-dep.

Major Assumption:

G  ILP (Incident Light Power)

Refs show true for “non-SCL” devices

1) Jph ILP1 Vsat ILP0

2) Jph ILP3/4 Vsat  ILP1/2

εr=2.6; μ=1.2x10-7 cm2/V s

=0.5ms; G=1.56x1027 m-3s-1

preliminary results
Preliminary Results
  • BEH1MBM3-PPV:PCBM (1:4)
  • Use Reg. Rand. To reduce mobility (increase difference)
  • Diff. increases at lower temp.
  • Get ½ power dep. on V in mid-region
  • Low-V is diffusion limited
  • High-V is saturation

RT

important results
Important Results

Critical Evidence of SCL

  • ½ Power regime: JILP1/4
  • Sat. Regime: JILP
    • No discussion of why
  • VsatILP1/2
  • Used lowest temp. due to largest ½ power regime
  • Fit highest ILP for G & modeled the rest
  • Vsat from intersection of linear fits
questions to answer
Questions to answer
  • What do activation energies mean in organics?
    • My experience is in inorganics w/doping: Ea gives energy in the band gap of the conductive state
  • What’s the J-dep. in the saturation regime from?
  • Is G always proportional to ILP?
the internal e field
The Internal E-Field
  • Blom and Greenham have different definitions?!
  • What is the definition?
    • No drift current?
    • No photo current?
    • No current at all?
  • Why do we only show effectsof external E-fields in diagram?

Diffusion from dissociated excitons

Barker, Ramsdale, & Greenham, PRB (2003).

charge transport photocurrent generation in p3ht pcbm bhj solar cells

Charge Transport & Photocurrent Generation in P3HT:PCBM BHJ Solar Cells

V. D. Mihailetchi, HangxingXie, Bert de Boer, L. Jan Anton Koster, and Paul W. M. Blom

Adv. Func. Mater. (2006)

Uses full model for P3HT:PCBM

(Numerical, but has all components discussed above – just uses Poisson’s equation to achieve steady state).

parameters of experiment
Parameters of Experiment
  • P3HT:PCBM BHJ (1:1) Annealing study to understand mechanism for 10-fold improvement in PCE
    • All annealing for 4min
    • Max PCE=3.5% annealing >110C
  • Measures μe,h(Tannealing) – discussed by Jaewook last time
    • μh improves 3 orders of mag., μe improves by 1
  • Absorption of 1.5AM spectrum increases 60%
  • DSC shows feature at ~125°C & claims this is Tg
    • Don’t believe it!
  • Uses above theory to show that improvement mostly caused by increase in μh, eliminating space charge
    • Probably due to higher degree of P3HT crystallinity when annealed above 110C
preliminary data
Preliminary Data
  • BHJ initially lower mobility than pure P3HT films
  • High MW films crystallize w/out annealing, so annealing does nothing to mobility
  • Annealing improves both carriers
    • Holes back up to pure P3HT values
    • Electrons improved even further, but gap btwn the two is closed.
  • As cast & poorly annealed shows J(V1/2) (space charge)
  • Annealed at 120C shows no space charge
    • low-V J(V) diffusion-limited
    • high-V saturation regime
evidence of scl
Evidence of SCL
  • High illumination expands SCL region to the short circuit point, reducing FF & PCE
  • Lower illumination reduces space charge buildup
  • Further evidence for SCL at 0.1V via J(ILP3/4)
  • At 3.0V (saturation regime) back to simple μ-limited current where J(ILP1)
  • Devices annealed >110C not μ-limited nor SCL: J(V1,ILP1)
field and temp fitting numerical simulations
←Field and Temp. FittingNumerical Simulations↓
  • Uses eqn. on slide 6 w/G(E,T)to fit Charge Transfer separation and lifetime
    • a=1.8nm and kF-1=50 μs, both larger than PPV
  • Uses Poisson eqn. to numerically calc JL-V w/just μp and Gmaxmeasured values for each annealing temp.
  • Disassociation prob.=90% above 110C
    • I think: P=90% for lower tempsif anneal longer
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