Optical spacing effect in single junction devices

Figure 4.14: Schematic of a ClAlPc:C60 BHJ single junction OPV device with an α-NPD:MoOx optical spacer layer.

To provide an experimental measurement of the optical spacing effect in OPV devices using α-NPD:MoOx, a simple BHJ single junction device was investigated. This allowed for the optical effects of the active layers to be probed without dealing with the hurdles of current matching in tandem OPV devices. Figure 4.14 shows the device structure used for this investigation; ITO / MoOx (5 nm) / ClAlPc (10 nm) /

117 ClAlPc:C60 (1:1) (20 nm) / C60 (10 nm) / BCP (8 nm) / Ag (0.3 nm) / α-NPD:MoOx (0-190 nm) / Al. The thin Ag layer acts as a recombination site for electrons in the BCP and holes in the α-NPD:MoOx (Section 3.2). This simulates the HTL effect of α-NPD:MoOx in a tandem OPV device. As the device is in operation, the photo- generated electrons that are extracted by the BCP layer are recombined at the Ag nanocluster sites by holes that are injected from the external circuit and travel through the α-NPD:MoOx layer.

Before fabricating the device a transfer matrix model (Section 2.2.3.3) was used to investigate the variation in absorptance within the device layers with spacer thickness. The absorptance of the photoactive layers of ClAlPc / ClAlPc:C60 (1:1) / C60 has been integrated over the AM1.5 solar spectrum (Figure 2.11b) and a plot of unity photocurrent (the JSC assumed with 100% IQE) versus spacer thickness is

displayed in Figure 4.15. The absorptance in the device will be dominated by the interference pattern created by the reflecting Al cathode which acts as a node for a Figure 4.15: Unity photocurrent as a function of spacer thickness for the device shown in Figure 4.14 calculated from a transfer matrix simulation.

14 12 10 8 6 4 U n it y Ph o to c u rre n t (mA cm -2 ) 200 150 100 50 0 Spacer Thickness (nm) Unity Photocurrent

Chapter 4 α-NPD as an Optical Spacer Layer

118 standing wave interference pattern. Figure 4.15 shows an increase in photocurrent of 30 % to a maximum at 30 nm spacer thickness as the photoactive layer is placed in an optical interference maximum. A further increase in spacer thickness reduces the photocurrent and a minimum is reached at 140 nm with a 65 % reduction in the original photocurrent. At thicknesses beyond 150 nm, the photocurrent starts to recover as the photoactive layers move towards the second interference maximum.

Figure 4.16: Performancecharacteristics of the ClAlPc / C60 BHJ device versus spacer thickess a)JSC

and PCEb)VOC and FF

The device performance characteristics for the fabricated devices are shown in Figure 4.16. The JSC varies substantially with spacer thickness because of the

119 interference pattern of the optical field within the active layers (Section 1.5). The device without an optical spacer layer has a JSC of 4.31 mA cm-2. On addition of a 20

nm α-NPD:MoOx spacer layer this increases to 5.25 mA cm-2

. This is the result of moving the active layers into the first optical interference maximum. Increasing the spacer thickness to 100 nm shows a rapid decrease in JSC reaching a minimum of

1.87 mA cm-2 when the active layers are placed in an interference minimum. The JSC

of the device recovers again with further spacer thickness reaching its original value of 4.37 mA cm-2 at 190 nm when the active layers are positioned close to the second maximum. The JSC of the experimental devices follows the sine wave interference

pattern expected from the optical model (Figure 4.15) although the peak positions vary slightly due to the inaccuracy of the model data and the simplification of the layers (Section 2.2.3.3). The VOC and FF are also displayed in Figure 4.16b showing

thickness independence on resistance losses in these devices (section 4.3.2) with the

VOC remaining around 0.8 V and the FF at ~ 0.4. The PCE of the devices therefore

varies with the JSC and an increase from 1.4 % to 1.6 % can be achieved by using the

optical spacer layer. Depending upon the active layer position in the optical field a large variation can be seen from 0.5 % to 1.6 %.

Chapter 4 α-NPD as an Optical Spacer Layer

120 Figure 4.17: EQE spectra for the ClAlPc / C60 device as a function of α-NPD:MoOx optical spacer layer thickness.

To investigate the origins of the variation in JSC, EQE measurements were carried

out at a range of spacer layer thicknesses, and are shown in Figure 4.17. Both the model (Figure 4.15) and the experimental JSC (Figure 4.16) suggest a wave like

change in JSC with spacer thickness and this is reflected in the EQE. The general

shape of the EQE follows the absorption spectra for the photoactive layers (Figure 3.1) with bands at 400-500 nm from the C60 and at 650-850 nm from the ClAlPc. There is a rise in the ClAlPc peak at 730 nm from 29 % to 38 % with small spacer thicknesses up to 40 nm which suggests this to be optimum position for ClAlPc absorptance. This does not however result in the overall highest JSC as the

contribution from C60 at 430 nm is reduced in the 40 nm spacer compared to the 20 nm spacer device to 6.9 % compared to 11.9 %. This suggests that the model of the device photocurrent could be improved by including the relative IQEs to calculate the photocurrent contributions of the two components. The peak at 730 nm decreases to a minimum of 16 % in the 100 nm spacer device when the photoactive layer is

121 placed in an optical interference minimum and recovers again back up to 31.1 % with a spacer thickness of 190 nm, higher than the reference device without an optical spacer layer.

In Section 1.5, wavelength-dependent optical interference was discussed as an important factor for optimising device absorptance. Figure 4.18 displays the integrated EQE photocurrent and the EQE at 430 nm and 730 nm (the peaks of C60 and ClAlPc) for the varying spacer thicknesses. It is clear from this plot that individual wavelength patterns do not follow the overall trend in JSC. The 730 nm

EQE peaks at 40 nm spacer thickness and reaches a minimum at 100 nm. However, the 430 nm peak shows the opposite trend, dipping at 40 nm and peaking at 100 nm. The overall JSC then becomes a combination of all of the different wavelengths

which each have maxima at different spacer thicknesses. Optimising device performance using an optical spacer therefore has to account for the whole spectral absorption range and not just the peak wavelength of the photoactive layer.

Figure 4.18: Integrated EQE photocurrent and peak wavelength EQE at 430 and 730 nm versus spacer thickness of ClAlPc:C60 BHJ device.

Chapter 4 α-NPD as an Optical Spacer Layer

122