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Optical optimization of polyfluorene-fullerene blend photodiodes
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Image of FIG. 1.
FIG. 1.

Chemical formulas for the constituents in the blend. (1) is, DiO-PFDTBT, the alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(-di-2-thienyl--benzothiadiazole)]. (2) is the fullerene derivative [6,6]-phenyl--butyric acid methyl ester (PCBM).

Image of FIG. 2.
FIG. 2.

The structure assumed for the simulation. Interfaces are assumed to be sharp and layers to be homogeneous. Note that is calculated from the ITO (indium tin oxide)-PedotPSS [poly(3,4-ethylenedioxythiophene), polystyrene sulfonic acid] interface and have the unit in nanometer. The glass substrate is many times thicker than any other layer in the stack.

Image of FIG. 3.
FIG. 3.

Optics in the structure. is reflectance and is transmittance. Star (*)-marked quantities refer to the glass substrate unmarked to the stack. The derivation was made general keeping a finite although it is later assumed that is zero due to thick enough aluminum. The angle of incidence is . Normal incidence is used throughout the study.

Image of FIG. 4.
FIG. 4.

and for the blend of 4:1 weight ratio PCBM and the copolymer DiO-PFDTBT from the ellipsometric investigation in the wavelength interval . is parallel to the surface plane (ordinary direction) and is normal to the sample plane (extraordinary).

Image of FIG. 5.
FIG. 5.

Comparison of anisotropy between a pure copolymer (lbpford,lbpfeord) and the blend (blendordi,blendextra). The quantity given is .

Image of FIG. 6.
FIG. 6.

as a function of wavelength. For is for the blend layer lower than PCBM as well as PFDTBT.

Image of FIG. 7.
FIG. 7.

diagram for a number of wavelengths. The figures are formed by the Wiener limits. The star indicates the blend. The extended arcs pass through origo. In all cases the Wiener limits are violated.

Image of FIG. 8.
FIG. 8.

Absorbed power per unit area, i.e., as a function of and for a zone comprising all of the blend layers and extending into the layer. and are fixed to 70 and , respectively.

Image of FIG. 9.
FIG. 9.

Impact of variation in and illustrated as a difference in . The most varying surface corresponds to PedotPSS. It is the difference in calculated for and . The less varying surface corresponds to ITO for and . ITO thickness variation is and that in PedotPSS , still the figure gives that the PedotPSS difference is almost 30 for some geometries and only 12 for ITO.

Image of FIG. 10.
FIG. 10.

for the geometry , , , and . Irradiated wavelengths are 300 (---), 550 (—), and (……). As follows from the optimization part this set corresponds to the optimal geometry.

Image of FIG. 11.
FIG. 11.

Sum of for . The structure is .

Image of FIG. 12.
FIG. 12.

Redistribution of the incoming irradiation on reflection and layerwise absorption. The structure is . The is so thin it is not seen in the diagram. The curve is free from differences due to different number of irradiated photons.

Image of FIG. 13.
FIG. 13.

Simulated (dashed) and experimental (line) measured reflectance for a solar cell. The structures for the simulated curves are , (100, 90, 180, 1), and .

Image of FIG. 14.
FIG. 14.

(unbroken line), (dashed line), and (dotted) as functions of wavelength. is an upper limit for is experimentally determined. It is not relevant below . The structure is for the simulation corresponding to an optimal blend thickness. It is assumed that the experiment corresponds to (100, 90, 160, 1).


Generic image for table
Table I.

Some extremal values in and for the blending: PCBM PFDTBT 4:1 weight parts.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Optical optimization of polyfluorene-fullerene blend photodiodes