Plasmon as a mechanism to improve performance of bulk-heterojunction organic solar cells.

Abstract

Narrow optical absorption band of fullerene based organic photovoltaics (OPVs) is one of the challenges to produce efficient polymer based solar panels that compete with inorganic counter part. There have been efforts to mitigate the challenges in the past but not enough to overcome all the problems. Plasmon light trapping using metal nano-particles (NPs) incorporated into the organic films is one of the mechanisms being investigated intensively in recent years. Excited plasmon meta nano-partices can dephase in number of ways that could assist in improving photon harvesting as well as charge transport processes in thin film organic solar cells without compromising the optimum thickness of the photoactive medium. The most investigated metal NPs are gold (Au) and silver (Ag) NPs due to their excellent plasmonic and light scattering properties. However, due to their scarcity and cost, several other metallic NPs have also been considered alternative options. Furthermore, mono metal NPs tend to possess high scattering coefficients but low absorption coefficients or vice versa. As a result, several nano-composites of NPs with differing scattering and absorption coefficients have been synthesized and studied. In this study, we have investigated the effect of inexpensive, solution processed and environmentally friendly metal NPs in organic solar cell based poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend solar absorber. Metal nanocomposites using metals such as zinc (Zn), copper (Cu), Ag, manganese (Mn) and sulphur (S) were synthesized via simple low temperature colloidal chemistry. These nano-composites were bimetallic (copper coated with silver, Cu@Ag), zinc sulphide (ZnS), zinc oxide (ZnO) coated with Ag (ZnO:Ag) and zinc sulphide doped with manganese (ZnS:Mn). The size, chemical composition and morphology of the synthesized NPs were studied using high resolution scanning electron microscopy (HRSEM), high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS/EDX). X-ray diffraction (XRD) was used to study their crystalline structure. The optical properties of the NPs in deionized water were studied using UV spectroscopy (UV-Vis). SEM analysis of the NPs showed various structures which mainly consisted of core shell type particles agglomerated together to form large clusters of nano-composites. EDX chemical composition analysis showed no significant impurities. This was consistent with the XRD which showed no diffraction peaks corresponding to impurity. HRTEM analysis confirmed the formation of a core-shell type structure for Cu@Ag with Cu core. Inter-planar (d-spacing) obtained via HRTEM compared very well with those obtained via XRD analysis. The NPs were incorporated either within the photoactive layer or the hole transport layer (HTL) of the solar structure. Significant enhancements on the optical and electrical properties of the OPV devices with NPs were observed when compared to pristine devices. Different NP concentrations were investigated ranging between (1 - 10 ) wt% relative to the absorber blend molecules. In some cases, the effect of solvent additives such as dimethyl sulfoxide (DMSO) at 5 wt% was used together with NPs in the HTL to boost the charge transport processes. The enhanced optical absorption, and electrical properties observed as increased current-densities (J), fill-factors and charge carrier mobilities resulted to improved power conversion efficiencies (PCEs) with increases exceeding 100 % when compared to pristine devices. The open-circuit voltages for all devices remained relatively unchanged. The enhancements in the optical and electrical properties which resulted to improved PCEs were attributed to strong light trapping through scattering by NPs and local surface plasmon resonance (LSPR) excitation on the metal-semiconductor interface. Light scattering at different angles into or within the photo-active layer increases its effective optical path length and hence more photons are absorbed. The thesis presents a series of experimental investigations on recently fabricated thin film organic solar cells with/without metal NPs at various concentration.