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, n_o + ik_o 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 n_o is larger than n_e. 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

n^e = 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 n_o and n_e (k_o and k_e) 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 n_e (k_e) is larger than the n_o (k_o), 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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