Performance improvement of organic solar cells incorporated with metallic nanoparticles

Abstract

Organic solar cells (OSCs) have shown great promise in becoming the next

generation of renewable energy due to its low cost, simple manufacturing process and

flexibility. A method of efficiency improvement in OSCs is by incorporating metallic

nanoparticles (NPs). While various reports have reported that incorporation of NPs

improve OSC efficiencies due to the Localized Surface Plasmon Resonance (LSPR)

effect, the investigations have lacked depth and a detailed investigation is necessary to

fully understand the device mechanisms of these OSCs.

In this thesis, we first investigate OSCs incorporating Au NPs into the hole

collection Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)

layer. Our theoretical and experimental results show that the very strong near field

around Au NPs due to LSPR mainly distributes laterally along the PEDOT:PSS rather

than vertically into the adjacent active layer, leading to minimal enhancement of light

absorption in the active layer. With optical effects proven to be minor contributors to

device performance improvements, we investigate the electrical properties of the

OSCs and obtain insights into the detailed device mechanisms. Improvements in

power conversion efficiency (PCE) of solar cells are found to originate from the

enlarged active layer/PEDOT:PSS interfacial area and improved PEDOT:PSS

conductivity. At high NP concentrations, reduced exciton quenching at donor/acceptor

junctions is found to cause PCE deterioration.

Next, the effects of Au NPs incorporated into the active layer of OSCs with a

newly synthesized donor polymer are investigated in detail. Our experimental and

theoretical results both show that LSPR introduced by the NPs can enhance the light

absorption in the active layer of OSCs because the strong LSPR near field mainly

distributes laterally along the active layer. Combined with our previous study, our

results strongly suggest that NPs have to be incorporated in the active layer in order to

harvest light by the LSPR effect. Meanwhile, our results show that the electrical

properties of NPs improve at low concentration of NPs. When NP concentration is

increased, the electrical properties deteriorates and counter-diminish the optical

enhancement from LSPR and reduces the overall performance improvement.

Finally, we demonstrate efficiency improvement in OSCs by ~22% through

incorporating Au NPs into all polymer layers. Au NPs are found to have distinct

mechanisms in improving device performance when incorporated in different polymer

layers. Our results indicate that the efficiency improvement is the accumulated effects

of incorporating NPs in the individual layers and that coupling is not observed in this

device configuration.

On the whole, our findings highlight the importance that both optical and

electrical properties need to be studied and optimized simultaneously for achieving

enhancement in PCE of OSCs. We have carried out a detailed study on incorporating

NP in various layers and our results are highly useful for the design of high efficiency

OSCs incorporating metallic NPs.