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Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching
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Image of FIG. 1.
FIG. 1.

Exciton density profile normalized to the incident flux , lifetime , and diffusion length as a function of position from the film surface calculated by solving Eq. (1). In the calculation, , (black lines), and (gray lines) for samples with (solid lines) and without (dotted lines) an exciton quenching layer. Inset: schematic diagram of the sample structure and coordinate system used in the diffusion analysis.

Image of FIG. 2.
FIG. 2.

Normalized PL spectra for CBP (filled circle), NPD (triangle), SubPc (filled square), DIP (x), PTCDA (hollow circle), and PtOEP (hollow square). CBP, NPD, and SubPc were excited at a wavelength of and DIP, PTCDA, and PtOEP were excited at 400 nm. The peak emission wavelengths are , 440 nm, 630 nm, 625 nm, and 700 nm for CBP, NPD, SubPc, DIP, and PTCDA, respectively. The PtOEP monomer and dimer emission peaks are located at and 790 nm, respectively. The normalized absorption spectrum (dotted black line) is also included to highlight the overlap with CBP and NPD emission.

Image of FIG. 3.
FIG. 3.

Measured (normal incidence) absorption coefficients as functions of wavelength for CBP (filled circle), NPD (hollow triangle), SubPc (filled square), PTCDA (hollow circle), PtOEP (hollow square), DIP standing up (light X), and DIP lying flat (black X). The absorption coefficient at for samples with DIP lying flat on the substrate is roughly three times larger than that for molecules oriented upright.

Image of FIG. 4.
FIG. 4.

PL excitation spectra for (a) SubPc at an emission wavelength of and (b) PtOEP at with (black line) and without (gray line) a quenching layer . For PtOEP, BCP is used as an exciton blocking layer. Excitation spectra were corrected for transmission losses and reflectance variations in the quenching and blocking layers. Absorption coefficients, adjusted for 45° incidence, are also included as dotted black lines for both materials.

Image of FIG. 5.
FIG. 5.

Quenching ratio versus absorption coefficient for (a) 600 nm DIP, (b) 450 nm PtOEP, (c) 400 nm PTCDA, (d) 300 nm SubPc, (e) 400 nm NPD, and (f) 400 nm CBP. Transmission losses and reflectance variations in the quenching and blocking layers were considered in determining from the excitation scans. The quenching data were fit using Eq. (3) except for NPD and CBP where Förster energy transfer was considered. Black and gray circles in (b) represent PtOEP at emission wavelengths of and 790 nm corresponding to monomer and dimer excitons, respectively. The DIP films grown on quartz (gray circles) and (0.5 nm) PTCDA/quartz (black circles) show x-ray diffraction peaks corresponding to standing and flat lying molecular orientations, respectively.

Image of FIG. 6.
FIG. 6.

X-ray scans for (a) 450 nm thick PtOEP grown on quartz, (b) 600 nm thick DIP grown on quartz, (c) 600 nm thick DIP grown on (0.5 nm) PTCDA/quartz, (d) 400 nm thick PTCDA grown on quartz, and (e) 300 nm thick SubPc grown on quartz. The x-ray data for NPD and CBP are identical to that of amorphous SubPc on quartz, showing no observable diffraction peaks.


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Table I.

Calculated quenching layer Förster radii and diffusion lengths for singlet (S) and triplet (T) excitons of crystalline (C.) and amorphous (Amorph.) films.

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Table II.

Average hopping distance , index of refraction , dipole orientation factors , Förster self radii , and calculated ( calc.) and measured diffusion lengths ( meas.).

Generic image for table
Table III.

Measured natural lifetime and calculated diffusivities from experimental diffusion lengths in Table I.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching