Date of Award
Doctor of Philosophy (PhD)
Nano-structured columnar films synthesized by the aerosol chemical vapor deposition (ACVD) system are unique, and have proved to be useful for fabricating solar cells and in other applications. The film formation by ACVD process involves three main phenomenon - aerosol formation and growth, deposition, and restructuring. In this work, thin film formation by the ACVD process is simulated by combining three models - (A) particle formation in gas phase by atrimodal model, (B) particle deposition onto heated substrate by a Brownian dynamics model and (C) sintering on the heated substrate by a multi-particle geometric sintering model (MPGSM). Modelling and simulation done in this work gives insights into working of the ACVD. Analysis suggests that a balance between arrival rate and sintering rate must be maintained to obtain the desired morphology by the ACVD process.
Since the titanium dioxide films synthesized by ACVD cannot absorb light in visible regions, various biological and biomimetic sensitizers have been explored. Natural sensitizers and reaction centers, which have high absorption coefficient and remarkable quantum efficiency, have been characterized in this work by electrospray-scanning mobility particle sizer (ES-SMPS) and deposited. The stability and retention of the photoactivity for the same has been demonstrated in this work. PSI which is a light harvesting complex with reaction center has been deposited by electrospray onto nanostructured TiO2 columns, synthesized by ACVD. The deposited PSI adheres due to dissolution of surfactant in the electrolyte and the performance is characterized using a photoelectrochemical cell. The orientation of PSI, due to linker free deposition and adhesion, has resulted in the highest photo-current observed for PSI based photoelectrochemical cells.
In order to improve the biological sensitizer chlorosome, mimics of chlorosomes are synthesized by self-assembly of synthetic and natural dyes in aerosolized droplet. We have developed a single-step method for the self-assembly of synthetic chlorin molecules (analogs of native bacteriochlorophyll c) in aerosolized droplets, containing a single solvent and two solvents, to synthesize bio-mimetic light harvesting structures. In the single-solvent approach, assembly is promoted by a concentration driven process due to evaporation of the solvent. Although assembly is thermodynamically favorable, the kinetics of self-assembly play an important role and this was demonstrated by varying the initial concentration of the pigment monomer. The kinetic limitations can be overcome by the use of a two solvent technique, which can also alter the size of self-assembled structures. The self-assembly of dye in aerosolized droplets has been extended to the naturally occurring bacteriochlorophyll c. The absorption and fluorescence of the assembled BChl c were demonstrated to be comparable to the naturally occurring chlorosomes. Finally the films were characterized by GISAXS and they show a scattering pattern characteristic of lamellar structures, which are present in chlorosomes. This technique of aerosol based assembly will enable the use of these materials as biomimetic sensitizers for solar cells.
Dewey Holten, Palghat Ramachandran, Venkat Subramanian