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Charge Transport Phenomena in OPVs

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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)

- 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

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

- 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%

- 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

- 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)

- 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

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

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

PRL (2005)

- 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

- 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 ILP1Vsat ILP0

2) Jph ILP3/4Vsat ILP1/2

εr=2.6; μ=1.2x10-7 cm2/V s

=0.5ms; G=1.56x1027 m-3s-1

- 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

Critical Evidence of SCL

- ½ Power regime: JILP1/4
- Sat. Regime: JILP
- No discussion of why

- VsatILP1/2

- Used lowest temp. due to largest ½ power regime
- Fit highest ILP for G & modeled the rest
- Vsat from intersection of linear fits

- 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?

- 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

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).

- 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

- 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

- 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)

- 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