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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


  • 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

  • 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


    • 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”




    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


    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 Solar Cells

    • 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 Solar Cells

    • 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 Solar Cells

    • 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
    Solar CellsField 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