Investigation of the processing parameters for efficient organic solar cells

Abstract

The field of organic photovoltaics (OPV) has been attracting great scientific attention over the last few years. New fields of applications are feasible as OPVs are lightweight, flexible and have the potential for truly low fabrication cost. At present, so-called bulk heterojunction structures based on blends of a conjugated polymer as electron donor and a soluble fullerene derivative as acceptor represent the OPV material system with the highest power conversion efficiency reported until now. This thesis addresses several critical parameters in the processing issues of polymer:fullerene based organic solar cells. Primarily, a sedulous investigation concerning deposition parameters of an electron blocking-hole transporting interfacial layer (Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) [PEDOT:PSS]) was performed. The conductivity and thickness effect of the layer on overall device performance was analyzed. Then the optically active layer consisting of the reference poly-3-hexylthiophene and phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend material system was investigated. Parameters such as composition ratio, layer thickness and annealing conditions were examined in order to achieve the reference organic solar cells for the newly founded Molecular Electronics and Photonics research unit with optimized and highly reproducible power conversion efficiency (PCE). The processing steps described in this thesis yielded P3HT:PCBM devices with repeatable and consistent PCE performance higher than 3 % and now the process is used as a standard method to provide the reference solar cells in the Molecular Electronics and Photonics laboratory. During the processing steps of P3HT:PCBM, a 21 kink in the shape of the current density/voltage (J/V) characteristics was observed. Devices of different ratios of P3HT to PCBM were fabricated and sent for further studies at the Jaume University in Spain. The s-shaped characteristics were explained by capacitance measurements related to the presence of defect bands exhibiting Gaussian shape located at E approximate to 0.38 eV above the highest occupied molecular orbital level of the P3HT. After establishing the reference process for P3HT:PCBM based solar cells new synthesized conjugated polymers donors were investigated in terms of their potential for organic solar cell applications. A perfluoro poly[(9,9-dioctylfluorenyl-2,7-diyl)- alt-5,5- (4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (APFO-3) derivative and different fluorescent boron-dipyrromethene conjugated polymers (BODIPY) were characterized. Absorption and photo luminescent spectra as well as current density characteristics of the fabricated photovoltaic devices were compared to commercially available materials. Despite the exhibited low mobility and PCE values (in the range of 0.2-1.2 %), some of the newly synthesized polymers, like PBT and PBTT, are promising electron donors for further synthetic trials due to their high absorption coefficient and high photovoltage (over 1 V). Furthermore a systematic study of inkjet printing processing parameters for organic solar cells applications was followed. Optimum parameters for inkjet printing active layer were firstly identified, proving that the viscosity of the inkjet formulation, substrate temperature, drop spacing and the height of the droplet in relation to the surface are critical factors to achieving high quality inkjet-printed polymer-fullerene 22 based active layers. The last part of the chapter focuses on replacing the expensive and brittle indium tin oxide (ITO) with an inkjet printed silver grid architecture. Investigation of an inkjet-printed silver nanoparticle grid combined with different conductivity PEDOT:PSS was performed. An ultimate control of the design requirements of current collecting grid based on the proposed inkjet-printed process to accurately control the uniformity and dimensions of the silver nanoparticle based grid was achieved. The performed measurements revealed higher transparency of the printed Ag grid when compared to different thicknesses of ITO. As a result a record power conversion efficiency of 1.96 % is achieved for ITO-free P3HT:PCBM based organic solar cells using the combination of PEDOT:PSS/inkjet printed nanoparticles based current collecting grids.