4 Chapter Exciton dynamics in rubrene thin films
4.7 TrPL of rubrene thin film with different thickness and different excitation
Fig. 4-9 shows the TrPL intensity at 2 ns window with different rubrene thickness (20, 100, and 200 nm) under room temperature by different excitation pulse energy (0.00329, 0.049, 136.8, and 1256.6 µJ / cm2). τ1values are shown in Table 4.5.
One can see that it decreases with increasing pulse energy due to SSA, as shown in Sec. 4.5. And it shows similar characteristics with different rubrene thickness, which means SSA is independent to rubrene film thickness.
85
(a) (b)
(c) (d)
Fig. 4-9 Normalized TrPL fluorescence decay response for 20, 100, 200 nm rubrene under (a) 0.00329, (b) 0.049 (c) 136.8 (d) 1256.6 µJ / cm2 excitation density in 2.0 ns window.
Table. 4-5 τ1 for different thickness under different excitation density.
Excitation density (µJ / cm2)
0.00329 0.049 136.8 1256.6
20 nm 0.819 ns 0.757 ns 0.312 ns 0.314 ns
100 nm 0.830 ns 0.730 ns 0.310 ns 0.251 ns
200 nm 0.809 ns 0.723 ns 0.299 ns 0.266 ns
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τ2 values were characterized by delayed fluorescence in the long time scale, which can be derived from Eq. 2, as shown in Eq. 4-7:
T
kfus N 1
2≈ ×
τ (4-7)
Considering the rubrene thickness at 100 nm (as we discussed the short time scale in Sec. 4-5), we can obtain τ2 value from the long time scale, as shown in Fig. 4-10.
We can see that τ2 value decreased and saturated as the excitation pulse energy increased. The first decrease came from the increase in triplet exciton density NT. When the power density was larger than 1256.6 µJ / cm2 , τ2 saturated due to strong SSA which limited singlet exciton density and hence triplet exciton density.
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Fig. 4-10 Normalized TrPL fluorescence decay response for 20, 100, 200 nm rubrene under (a) 0.00329, (b) 0.049 (c) 136.8 (d) 1256.6 µJ / cm2 excitation density in 2.0 µs window.
Table. 4-6 τ2 for different thickness under different excitation density in long time scale.
Excitation density (µJ / cm2)
0.00329 0.049 136.8 1256.6
20 nm 1.785 s 1.109 µs 0.906 µs 0.982 µs
100 nm 2.061 µs 1.792 µs 1.026 µs 1.016 µs
200 nm 2.183 µs 2.237 µs 1.084 µs 1.150 µs
Fig. 4-11 (a) τ1 and (b) τ2 values of 20, 100, and 200 nm rubrene thin films.
Comparing the τ2 at the same pulse energy with different rubrene thickness (20, 100, and 200 nm), we can see that this value decreased with thinner rubrene film. In this case, non-germinate recombination rate increased due to exciton spatial confinement. To further understand such mechanism, a series rubrene thin film with the thickness of 5 to 300 nm was fabricated with the excitation energy density of 43 μJ/cm2. As the thickness of rubrene increased, τ1 almost kept the same while τ2
88
increased, as shown in Fig. 4-12 and Table 4-5. It can be attributed to caging effect, which triplet exciton were caged by thin rubrene film. The schema of this idea is shown in Fig. 4-12 (d). The decrement of τ2 in thinner film is cause by the increment of kfus term in equation 4-7.
Fig. 4-12 Normalized TrPL fluorescence decay response for different thickness of rubrene for (a) 2 ns window, (b) 2 μs window, (c) τ2 versus thickness and (d) schema of caging effect.
Table. 4-7 τ2 for different thickness of rubrene under 43 μJ/cm2 Thickness
(nm)
5 7 10 15 20 100 200 300
τ2(μs) 0.811 1.008 1.254 1.444 1.322 1.674 1.836 1.582
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5 Chapter 5 Conclusion
5.1 Summary
In chapter 3 of this dissertation, we demonstrated OSC device performances based on the four electron donor materials, DTCPB, DTCTB, DTCPBO, and DTCTBO with D-A-A configurations and C60 and C70 as electron acceptor materials.
Although we have tried different device configurations, such as planar heterojunction and donor buffer layer, it is interesting that the OSCs with simple bulk heterojunction structure exhibited highest PCE with our electron donor materials. The highest PCE (6.55%) was achieved by using DTCPB and C70 as electron donor and acceptor materials, with VOC, JSC and FF of 0.89 V, 11.12 mA/cm2, and 65.63%, respectively.
In chapter 4, we obtained the krad of rubrene dissolved in toluene solution as 0.088 ns-1 and ksf under different temperature without the influence of SSA. By fitting the ksf under different temperature, we obtained the activation energy for amorphous rubrene is 24 meV. And thinner rubrene (5 nm) exhibited higher fusion rate, which resulted from higher probability for non-germinate recombination. Such phenomenon was also observed when increasing the input power to the thicker film (100 nm rubrene at 43.4μJ/cm2).
5.2 Future work
The performance of bulk OSC could be possibly improved by the following concepts by: (1) layer structure engineering by adding more functional layers (such as buffer layer(s) to reduce the recombination or enhance the cavity effect), and (2)
90
engineering the mixing ratio at different position to adjust the exciton and carrier dynamics.
Although we can qualitatively explain our experimental results of fusion process, a complete and quantitative model is needed and more simulation works need to be done for clarifying germinate and non-germinate recombination process, as well as exciton diffusion dynamics in thin and thick rubrene films.
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Appendix
Appendix. I Morphology for four single cyano groups electron donor materials
In this section, we measure the roughness of these four compound. D/A interface is important for device and material engineering because a suitable phase separations would be pursued especially for thermal deposition OSCs. Here, four compounds were deposited on quartz with 50 nm, 1 anstrong per second to measured under AFM which shown in Fig. A-1 .
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Fig. A-1 AFM image for roughness and phase for (a) DTCTB (b) DTCPB (c) DTCBPO (d) DTCTBO
Table. A-1 AFM roughness and phase for DTCTB, DTCPB, DTCBPO and DTCTBO.
DTCTB DTCPB DTCPBO DTCTBO
Roughness
(nm) 0.800 0.776 0.816 0.949
Phase (%) 0.67 0.71 2.97 0.6
The results for these four compounds were shown in Table. A-1. DTCPB show the lowest value and DTCTBO gives the harshest value among these four. Anyway, these four materials didn't show any aggregation and island in AFM image and the roughness were always lower than 1 nm, which implied a very amorphous situation can be formed. This is good to hear because a smoother roughness would be better when mix with acceptor, the coverage for electron donor to acceptor could be better and led to an increment for D/A interface, so the exciton can be well dissociated. For the comparison between BO and BT block, BO will slightly higher than BT. This might be result in a serious recombination for BO block. And the topography of thiophene groups also exhibit a little bit increase compare to phenylene goups.
However, these small variations between their morphologies are hard to identify with final PCE accurately. In our future work, it will be clear to mix with acceptor with different mixing ratio and to observe the topography..
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Appendix. II Anisotropic characteristics of four single cyano groups electron donor materials
Molecules who equipped the anisotropic characteristic show a different refractive index or absorbance physical properties when the light is incident along to a different direction or polarization of its optical axis, as oppose to isotropic one. These kinds of materials always show the rod like shapes because light will be encountered at a different speed when it illuminate along at short (or long) axis and also called birefringence property. As shown in equation A, the slacken of light speed is due to the different refractive index, root of εr*μr, which εr and μr are the relative permittivity and permeability of the material. That means the optical constants can be separated and identified into the ordinary (in-plane) polarization, no + iko and the extraordinary (out-of-plane) polarization, ne + ike. When the difference between ordinary and extraordinary becomes larger, the chromophore is more approach like rod shape. By the effort of Hao-Wu Lin’s group, D-A-A system with dicyanoethylene based materials do shows an anisotropic property which the value no is larger than ne. This implied that the molecular is laid on the substrate during self-packing which will help to harvest photon with the horizontal polarization more effectively.
v = c
n and
x2+ y2 n0 +z2
ne = 1 … … … . (A) For the single cyan group substituent, all the neat film samples were deposited onto the silicon substrates with 1 angstrom/sec and measured by an angle-variation reflective ellipsometry SOPRA GESP5 as presented in the Fig. B-1.
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Fig. B-1 Anisotropic characteristics of (a) DTCPB, (b) DTCTB in thin film and (c) packing imaginary representation.
From Fig. B-1 (a) and (b), DTCPB and DTCTB also show an anisotropic characteristic. The difference between no and ne (ko and ke) exhibited a different value that give the evidence for rod-like shape of the materials during the packing. The worth thing to mentioned is the value of ne (ke) is larger than the no (ko), dissimilar from the dicyanoethylene based molecules reported before. Geographically, the chromophore is standing straight on the substrate during the packing. This might be resulted from the changes of molecular length by replacing into single cyno one. A simple imaginary representation was shown in Fig. B-1 (c)
95
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