Investigations on Structural, Morphological and Optical Characteristics of Metal-Oxides Based Charge Extraction Layers Fabricated Via Low-Temperature Colloidal Dispersion Route for Inverted Hybrid Solar Cells /

Abstract

Hybrid Solar Cells (HSCs) are a type of advanced 3G solar PV technology which combine the attractive features of both organic solar cells such as economic production, flexibility and light weight with high power conversion efficiency as well as tremendous stability of inorganic semiconductors. Unlike conventional device structure, an inverted configuration utilizes a back metal electrode with high work function which can resist oxidation, thus enabling highly stable devices. Recently, much attention is being paid to fabrication of hybrid solar cells via low temperature, solution-phase processing techniques. In this work, attempts have been made to fabricate charge extraction layers, using colloidal dispersions of inorganic metal-oxides, for an inverted configuration HSC based on a P3HT:ZnO bulk heterojunction (BHJ) nanocomposite photoactive layer. Colloidal synthetic methods are promising for large batch production and commercial applications. Colloidal suspensions of zinc oxide and molybdenum trioxide are prepared in organic solvents via wet chemistry precipitation route at temperatures below 100°C utilizing inexpensive and non-toxic precursors. Subsequently, the dispersions are spincoated to deposit respectively the electron and hole extraction layer. Next, film quality in terms of various physical parameters such as coverage, adhesion, uniformity and porosity, is investigated as well as optimized via microscopy based characterization tests. XRD analysis confirms pure and crystalline nature of pristine zinc oxide and molybdenum trioxide nanoparticles with a particle size of 17nm and 33nm respectively. TG-DTA reveals excellent thermal stability of the as-synthesized nanoparticles, but a three times higher ratio of surface adsorbed moieties on molybdenum trioxide molecules. Unlike zinc oxide, for the spincoated molybdenum trioxide nanoparticle film, initial observations via optical microscopy demonstrate poor coverage of the coating on the substrate. Particle size analysis reveals that particle agglomeration in the molybdenum trioxide colloidal dispersion due to surface charge density effects as revealed by TG-DTA, lead to poor film coverage and adhesion. Also, particle size analysis shows that exposure to air accelerates aggregation of zinc oxide nanoparticles. Therefore, storage of zinc oxide dispersion under air-tight conditions, results in stable dispersion even without use of stabilizing ligands. iv Moreover, a detailed study on structural, morphological and optical characteristics of zinc oxide nanoparticle film, spincoated from its colloidal dispersion, is carried out in collaboration with Arizona State Univeristy (ASU), USA. Additionally, comparisons of surface morphology of zinc oxide nanoparticle films deposited via two different techniques i.e. spincoating as well as spray coating of colloidal dispersion, is carried out. TEM images display uniform hexagonal shaped zinc oxide nanoparticles with a diameter of 15nm. SEM as well as AFM results indicate that zinc oxide colloidal dispersion with an optimal concentration of about 50mg/ml results in a better quality spincoated nanoparticle film exhibiting minimal particle aggregation, fewer pin holes and greater uniformity across surface, capable of preventing short-circuits. The zinc oxide nanoparticle film deposited via in-house manufactured spray coater, exhibits extremely high surface roughness and micrometer sized pits which deems it unsuitable for use as an electron extraction layer. UV-Vis spectrophotometry shows that zinc oxide nanoparticle film is transparent in the visible region of spectrum in which the silicon wafer absorbs as well as it can function as an anti-reflection coating to allow improved absorption of the solar spectrum by silicon. Therefore, it is deemed suitable for use as an electron extraction layer when spincoated via the colloidal dispersion route. Eventually, an inverted configuration HSC is fabricated. UV-Vis absorption spectroscopy of the photoactive component materials blend indicates a bandgap of 2.2eV for P3HT. The wavelengths of photons of solar spectrum which are absorbed range from 300nm to 550nm. Finally, the electrical performance of inverted HSC, utilizing zinc oxide nanoparticle film and a P3HT:ZnO BHJ nanocomposite photoactive layer, is analyzed by obtaining a current-voltage characteristic curve under AM1.5G illumination.