(meh-ppv) - astronomyboehmelab/icans 23_sang-yun.lee...(meh-ppv) (c) observing spin-rabi nutation...

Introduction

Recombination in organic semiconductors

Photoluminescence after resonant excitation

Recombination and dissociation processes occur via polaronpairs (excitonic precursor pairs). Weakly spin coupled singlet and triplet polaron pairs form due to Coulomb interaction. These pairs are two spin-½ systems which can dissociate spin-dependently (ds, dt) into free charge carriers, and recombine (ks,kt) into excitons. Spin-lattice relaxation (kSL) between singlet and triplet pairs can therefore have an influence on conductivity and light emission. Magnetic resonance [1] allows us to change these processes experimentally and thus, to investigate their nature.

Organic semiconductors are used for various optoelectronic devices including organic light emitting diodes, flat panel displays, solar cells and magneticfield sensors. Spin-dependent electronic transitions (especially recombination) are an important factor for the efficiency of these devices. In order toenhance our understanding of these processes we conducted pulsed Optically Detected Magnetic Resonance (pODMR) measurements on MEH-PPV.

S.-Y. Lee, S.-Y. Paik, D. R. McCamey and C. BoehmeDepartment of Physics and Astronomy, University of Utah, Salt Lake City, UT, 84112, USA

Change in Photoluminescence after a 128 ms pulse with B10.55 mT

(a) Magnetic field (B0) dependence of photoluminescence (PL) changes following abrief coherent microwave pulse – one can clearly observe a response at magnetic fieldsaround B0 345.5mT.(b) The B0-dependence can be fit well with a double exponential function that is basedon the pair model.(c) The broad and narrow Gaussian peaks can be assigned to positive and negativepolarons, respectively.

0 200 400 600 800 1000

336

338

340

342

344

346

348

350

352

354

B0 (

mT

)

t (ms)

-440PL/PL

0 (10

-5)

(a) (b)

Spin-dependent transitions of exciton precursor pairs in poly[2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylene vinylene]

(MEH-PPV)

(c)

Observing spin-Rabi nutation using a spin pump/ PL probe detection scheme

(a) PL amplitude singlet to tripletchange of the spin ensemble.

By integrating the transient for a fixedtime following a microwave pulse oflength t, the Rabi nutation can beobserved.

(b) Integrated PL changes as a function ofB0 and the excitation pulse length t.

(a) (b)

G1=62.0(6) ms G2=830(40) ms

0 200 400 600 800 1000

0

2

4

P

L/P

L0(1

0-4

)

t (ms)

PL/PL0=I

1e

-G1t+I

2e

-G2t

at 345.5 mT

1.98 2.00 2.02

0

1

2

3

P

L/P

L0 (

10

-4)

g-value

At 40 msG = 1.67(6) mTg = 2.0021(2)

T=5K

Plot of the B0 field dependence of Fouriertransformed spin Rabi nutation transients,FFT[N(t,B0)](a) at B1=0.55 mT, and(b) at B1=1.54 mT.

Solid and dashed curves are Rabi frequencies andSpin pair partners rotating together, causingbeating, due to the strong B1 field.

Strong agreement with the theoretical predictionfor a weakly coupled [2] and disordered [3] spinensemble is seen.

(a)

(b)

(c)

B1=0.55 mT

Conclusion

References

2

0

2

1 )2()( mwiii fBB jibeat

V. Rajevac et al., Phys. Rev. B 74, 245206 (2006)

[2]

[3] C. Michel et al., Phys. Rev. B 79, 052201 (2009)

B1=1.54 mT

(c) B1 dependence of Rabi frequencies Red line: = B1 (weakly coupled pairs). Blue line: =2B1 (strongly coupled pairs). The beating component (=2B1) is highly visible when B1 > Bhyper 1.32 mT (the difference in the hyperfine fields seen by two pair partners).

• The recombination model explainspODMR of MEH-PPV. Experiments indicatethe system is comprised of weakly coupled,disordered pairs.• Exciton precursor (polaron) pairs in MEH-PPV are weakly coupled via hyperfinecoupling to surrounding hydrogen nuclei.• Spin beating in MEH-PPV observed.• The exciton precursor pairs can only beweakly exchange coupled since the beatfrequency vanishes at low B1 fields.

B0

B1

Apparatus for pODMR

[1] D. R. McCamey et al., Nature Mater. 7,723 (2008)

G = 3.5(2) mTg = 2.0019(6)

t = 40 ms

Optically detected Rabi nutations

Observation of spin-beating of weakly coupled pairs

tteIeIPLPL 21

210/GG

342 345 348

0

50

100

150

200

250

B0 (mT)

Pu

lse

le

ng

th (

ns

)

-0.1

1

[N(t

)](B

0)

(Arb

. U

nit

s)