Braden Bills

Electrophoretic deposited TiO(2) pigment-based back reflectors for thin film solar cells.

Highly reflective coatings with strong light scattering effect have many applications in optical components and optoelectronic devices. This work reports titanium dioxide (TiO(2)) pigment-based reflectors that have 2.5 times higher broadband diffuse reflection than commercially produced aluminum or silver based reflectors and result in efficiency enhancements of a single-junction amorphous Si solar cell. Electrophoretic deposition is used to produce pigment-based back reflectors with high pigment density, controllable film thickness and site-specific deposition. Electrical conductivity of the pigment-based back reflectors is improved by creating electrical vias throughout the pigment-based back reflector by making holes using an electrical discharge / dielectric breakdown approach followed by a second electrophoretic deposition of conductive nanoparticles into the holes. While previous studies have demonstrated the use of pigment-based back reflectors, for example white paint, on glass superstrate configured thin film Si solar cells, this work presents a scheme for producing pigment-based reflectors on complex shape and flexible substrates. Mechanical durability and scalability are demonstrated on a continuous electrophoretic deposition roll-to-roll system which has flexible metal substrate capability of 4 inch wide and 300 feet long.

Substrate dependence of surface passivation using atomic layer deposited dielectrics

This paper presents low temperature surface passivation using atomic layer deposited aluminium oxide (Al<inf>2</inf>O<inf>3</inf>) and hafnium oxide (HfO<inf>2</inf>) thin films on different silicon substrates. Al<inf>2</inf>O<inf>3</inf> and HfO<inf>2</inf> thin films were deposited on p (111) and p (100) Si substrates with different ALD cycles, respectively. Capacitance-voltage (CV) measurement was executed on Al/oxide/Si structures. CV measurements were used to determine the thickness of dielectric layers and threshold voltage (V<inf>th</inf>) of Al/oxide/Si structures. The CV experiments suggest that the growth rate of ALD oxides is higher on (111) Si surface than that of the (100) substrate. Further, it was observed that the fixed charge density at the interface of Al<inf>2</inf>O<inf>3</inf>/Si and HfO<inf>2</inf>/Si is larger for (100) Si surface.

Effect of Tunnel Recombination Junction on Crossover Between the Dark and Illuminated Current–Voltage Curves of Tandem Solar Cells

The tunnel recombination junction (TRJ) quality of an amorphous silicon (a-Si)-based tandem junction solar cell can have a significant impact on its performance, where crossover between the dark and illuminated current–voltage curves of a tandem cell indicates a TRJ with poor quality. Therefore, it is critical to be able to understand the TRJ characteristics in order to optimize the tandem solar cell performance. A simulation approach requiring a few data points was used to quantify the TRJ quality. The uniform field collection model was modified to account for parasitic losses due to the TRJ, which are a voltage drop across the TRJ and a series resistance due to the TRJ. Simulation results using the TRJ model are in good agreement with experimental data. The TRJ model can be a useful tool to aid in the optimization of a-Si-based multijunction solar cells.

Overview of atomic layer deposited metal oxides for treating nanoporous TiO 2 photoelectrode for dye sensitized solar cells

Atomic layer deposited (ALD) ultrathin metal oxides, HfO2, Al2O3 and SiO2, were used to treat the surface of nanoporous TiO2 for dye sensitized solar cell (DSSC) application. X ray photoelectron spectroscopy was performed to probe the growth of metal oxides on TiO2 surface. ALD grown metal oxide layers on TiO2 surface did not block the porosity as observed in atomic force microscopic studies. Conformal surface coverage was achieved as shown by high resolution transmission electron microscopic analysis on ALD grown TiO2 and the thickness was about 2 nm. It was observed that performance of DSSC was significantly enhanced by ALD metal oxide surface treatment. The enhanced DSSC performance asserted that electron injection from the dye to TiO2 was not blocked while the electron trap from the conduction band of TiO2 to surface states was significantly suppressed and hence the improved performance. Further, it was noticed that the thickness of ALD metal oxide layers play a vital role in the photovoltaic performance of DSSCs.

Light scattering behavior of oxide nanoparticles

Titanium dioxide (TiO2) has high optical refractive index and is transparent to visible light. No strong reflection is expected from a single layer of continuous TiO2 film. In this work, we report the unusually high reflection from a thick layer of TiO2 nanoparticles. TiO2 nanoparticles of different sizes (~ 21 nm and ~ 400 nm) were deposited on indium tin oxide (ITO) coated glass substrate by electrophoretic process in atmosphere. It was observed that the film characteristics such as morphology and thickness were affected by the electrophoretic process conditions like voltage, deposition time, and electrolyte concentration. The electrolyte had the most significant effect on the deposition rate of the nanoparticles. The film thickness was proportional to the deposition time, which in turn determined the diffused reflectance of the TiO2 particle films. It was observed that the 400 nm TiO2 nanoparticle films lead to much stronger light scattering as compared to the 21 nm particles. The diffuse reflectance was compared to sputtering deposited Ag/ZnO back reflectors that are used in thin film solar cells. The TiO2 nanoparticle film showed higher reflectance, making it a potential candidate to replace the unstable Ag/ZnO back reflector.

Aqueous electrolyte system for porous silicon using electrochemical anodization

Electrochemical anodization on n-type silicon was performed with silicon as anode and platinum as cathode in a weak-acid aqueous electrolyte containing orthophosphoric acid and ammonium fluoride. Anodization was carried out in two different modes, i.e., potentiostatic and galvanostatic with the voltage and current density of 5-80 V and 10-50 mA/cm2 respectively. Anodization time was varied from 5 min to 10 hours. Morphology of the anodized samples was studied using scanning electron microscopy (SEM), revealing the formation of pores with uniform distribution throughout the silicon substrate. The pore size and density were controllable by varying the anodization voltage and current. Results showed that, in a broad spectrum range of 400-1100 nm, the total reflectance (sum of diffused and specular reflectance) of the porous silicon was about 10% compared to >30% of the as-received silicon wafer. The porous silicon is promising for solar cell applications due to the low reflection loss.

Electron transport in dye sensitized solar cells with TiO 2 /ZnO core-shell photoelectrode

This paper presents the electron transport in dye sensitized solar cell (DSSC) with TiO<inf>2</inf>/ZnO core-shell photoelectrode. Sol-gel processed ZnO was used to form the shell layer on TiO<inf>2</inf> nanoparticles. DSSC with TiO<inf>2</inf>/ZnO core-shell photoelectrode resulted in 32 % enhancement in the overall photoconversion efficiency compared to DSSC with conventional TiO<inf>2</inf> photoelectrode. Dark, illuminated current-voltage characteristics (I–V) and external quantum efficiency (EQE) of the DSSC with TiO<inf>2</inf>/ZnO core-shell photoelectrode suggest that formation of ZnO shell on TiO<inf>2</inf> nanoparticles can effectively prevent the electron back reaction from the conduction band of the mesoporous TiO<inf>2</inf> to the surface states and eventually suppressed the recombination of the photoexcited electrons with either a hole available in electrolyte or oxidized dye molecules. The enhancement in short circuit density (J<inf>SC</inf>) and open circuit voltage (V<inf>OC</inf>) confirmed the suppressed density and activity of surface states at TiO<inf>2</inf>/ZnO/dye/electrolyte interfaces compared to TiO<inf>2</inf>/dye/electrolyte interfaces.

Characterization of an Ultra Thin Dense Hafnium Oxide Compact Layer with Electrochemical Impedance Spectroscopy for Dye-Sensitized Solar Cell Application

The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the photoelectrode back into the electrolyte solution. An atomic layer deposited (ALD) hafnium oxide (HfO 2 ) ultra thin, a few nanometers, compact layer was grown on the surface of the transparent conducting oxide (TCO) and its effects on the performance of DSSCs were studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy (EIS) measurements. Further, the theory of electron recombination at the TCO/electrolyte interface was developed and used to explain the improved DSSC performance with an ALD HfO 2 compact layer.

Role of Mesoporous TiO 2 Surface States and Metal Oxide Treatment on Charge Transport of Dye Sensitized Solar Cells

Photovoltaic performance of dye sensitized solar cell (DSSC) was enhanced by 19 and 69 % compared to untreated DSSC by treating the nanoporous titanium dioxide (TiO 2 ) by ultra thin Aluminum oxide (Al 2 O 3 ) and Hafnium oxide (HfO 2 ) grown by atomic layer deposition method. Activation energy of dark current, obtained from the temperature dependent current-voltage (I-V-T), of the untreated DSSC was 1.03 eV on the other hand the DSSCs with Al 2 O 3 and HfO 2 surface treatment showed 1.27 and 1.31 eV respectively. A significant change in the activation energy of dark current, over 0.24 eV for Al 2 O 3 treatment and 0.28 eV in case of HfO 2 treatment, suggest that density and activity of surface states on nanoporous TiO 2 was suppressed by ALD grown metal oxides to result improved photovoltaic performance. Further the enhanced DSSC performance was confirmed by external quantum efficiency measurement in the wavelength range of 350-750 nm.

Analysis of carrier transport in dye sensitized solar cells using temperature dependent dark I–V measurements

This paper presents carrier transport in dye sensitized solar cell (DSSC) using dark current-voltage-temperature (I-V-T) measurements. Dark I–V measurements were performed in 30°C to 85°C range and the dark saturation current of the DSSC was extracted. The measurements revealed that the activation energy of the dark current lies in 1.18–1.27 eV range. This suggests that the charge injection from TiO2 into liquid electrolyte is dominated by electron capture process from TiO2 conduction band into the surface states with energy levels close to the electrolytes redox potential.