Development towards fully solution processed OPV devices

evaporation. When looking to move towards large volume production, the top interfacial layer and electrode ideally should also be solution processed as well.

As well as being used as a hole transport layer in regular architecture devices, PEDOT:PSS has also been used as a interfacial layer in inverted structures. However, additives have to be added to the PEDOT:PSS solution in order to improve the wettability of this layer on the polymer:fullerene blend.186-188 Improvements in the material properties during the manufacturing process of PEDOT:PSS (Clevios HTL Solar, PEDOT:PSSSolar) has seen it emerge as an

effective interfacial layer between the photoactive layer and Al top electrode without the need for further treatment (Figure 4.15). P3HT:PCBMDCB was used as the

156 photoactive layer which was spin coated onto ITO/ZnO electrodes. The devices had the following structure: ITO/ZnO/P3HT:PCBMDCB/PEDOT:PSSSolar/Al.

Figure 4.15 – J-V curves under 1 sun illumination (solid lines) and in the dark (dashed lines) of inverted P3HT:PCBMDCB devices on ITO/ZnO electrodes. Either MoOx or PEDOT:PSSSolar as an interfacial layer.

The key devices parameters are shown in Table 4.10. Devices utilising a MoOx

interfacial layer produced a Jsc of 11.46 mA cm-2, a Voc of 0.52 V and a FF of 0.55

resulting in a PCE of 3.06 %. Those devices using a PEDOT:PSSSolar hole

transporting layer had a lower PCE of 2.45 % with a Jsc of 9.03 mA cm-2, Voc of 0.52

V and a FF of 0.56. The difference in device performance is due to the lower Jsc

obtained by devices using PEDOT:PSSSolar, most likely due to the thicker and less

transparent PEDOT:PSSSolar layer. With further processing optimisation of the

PEDOT:PSSSolar layer the Jsc could be further improved. The Voc obtained for both

sets of devices are comparable to what was achieved in Section 4.2.2 and the FF for both anode interfacial materials is also similar.

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Table 4.10 – OPV device parameters for inverted P3HT:PCBM devices comparing different anode

interfacial layers.

Interlayer Jsc (mA cm-2) Voc (V) FF PCE (%)

MoOx 11.46 0.52 0.55 3.06

PEDOT:PSSSolar 9.03 0.52 0.56 2.45

4.4.3 Summary

This section has highlighted the feasibility of casting the photoactive P3HT:PCBM layer out of non-halogenated solvents. Inverted P3HT:PCBM devices using different non-halogenated solvents were seen to produce PCEs comparable to those using DCB as the solvent. P3HT:PCBMTol devices were even seen to have a higher

PCE than P3HT:PCBMDCB devices in the inverted architecture unlike, the poor

performance seen for regular architecture devices using P3HT:PCBMTol in Section

4.3.1.

In a step to move towards a fully solution processed device, the MoOx hole

extraction layer was replaced by PEDOT:PSSSolar in inverted structures. These

P3HT:PCBMDCB devices showed promising device performance and with further

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4.4 Conclusions

With OPV devices attracting significant attention as a promising cheap and alternative renewable energy source; attention is needed as to how this low-cost potential will be reached. In order to meet the rapid production speeds required a predominantly ambient atmosphere is favourable with the majority of the individual layers being solution processed from environmentally friendly solvents via a roll-to- roll process. The beginning of this chapter addressed the possibility of fabricating a P3HT:PCBMDCB device entirely under ambient conditions. In air a huge decrease in

the device performance was seen, largely due to reductions in Jsc and FF. Careful

consideration of the post-spin coating conditions enabled the device performance of the air deposited devices to reach that of those spin coated under N2. However, it

was discovered that annealing had to be carried out under N2.

A comparison of PEDOT:PSSHTL and MoOx interfacial layers as the hole extracting

layer in regular devices highlighted the importance of selecting the appropriate material for the processing environment. A reduction in device performance was seen for regular devices using a MoOx hole extracting layer. This was attributed to

poor charge extraction due to exposing the layer to air. Inverted devices using an ambient processed ZnO electron extracting layer and MoOx hole transporting layer

(not exposed to air) displayed good OPV performance.

Replacing the DCB casting solvent with toluene resulted in decreased device performance in regular architecture devices. The decrease was attributed to the low absorption and poorer fullerene solubility in toluene. This led to severe phase segregation and large aggregate formation which has been reported to hinder charge

159 generation and increase recombination. To improve the fullerene solubility 1- methylnaphthalene was used as an additive in the toluene parent solvent. The device performance decreased with this addition and the formation of a kink suggested there was a problem with charge extraction.

Inverted OPV devices have shown promise to lending themselves to possible large scale fabrication as they can be processed in air from non-halogenated solvents and show favourable device performance compared to DCB based devices. Finally, the use of PEDOT:PSSSolar as a hole transporting layer for inverted devices was

compared to those using thermally evaporated MoOx. The main difference in device

performance was due to a lower Jsc seen for those using the thicker PEDOT:PSSSolar

layer, future work should allow this layer to be further optimised and so improve device performance. The next step would be to investigate the use of Ag inks (either as a full layer or grid) or even PEDOT:PSSADD as a top electrode to produce a fully

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