Session D39: Focus Session: Materials and Applications for Solar Energy II

Intermediate- band solar cells: future prospects and challenges

The intermediate band solar cell is a novel type of solar cell with the potential of exceeding the limiting efficiency of single gap solar cells. Its principle of operation relies on the existence of a material characterized by an electronic band (intermediate band) located within the semiconductor bandgap. This intermediate band allows the absorption of two below-band gap energy photons to produce one electron-hole pair and is engineered to prevent introducing non-radiative recombination mechanisms in the cell. This basic principle of operation has been recently experimentally demonstrated in devices employing InAs/GaAs quantum dots. In this system, the intermediate band arises from the energy states associated to the confinement of the electrons in the dots. The challenges for the future are: a) to produce devices in which the intermediate band effect is enhanced, b) to identify and synthesize other intermediate band material systems, maybe different from the ones based on quantum dots and c) to produce high efficiency devices that allow the production of photovoltaic electricity at competitive costs.   

Materials characterization and optimization in silicon heterojunction solar cells using spectroscopic ellipsometry

Silicon heterojunction solar cells (SHJ) utilize very thin (3-5 nm) layers of amorphous silicon deposited on the surface of crystal silicon to produce very high efficiency solar cells with low temperature processing. Our research team has used hot wire chemical vapor deposition (HWCVD) to fabricate SHJ solar cells on p-type FZ silicon with efficiencies as high as 18.2{\%}. The best cells are deposited on textured (100) silicon substrates where the texturing process creates pyramidal facets with (111) crystal faces. One of the key factors in maximizing the efficiency of our SHJ devices is the process of optimizing the material properties of the amorphous silicon (a-Si) layers used to create the junction and back contact in these cells. Such optimization is technically challenging because of the difficulty in measuring the properties of extremely thin layers. This difficulty is compounded by the fact that the properties of such amorphous layers are substrate- and thickness-dependent. We report in this study how the substrate temperature and substrate orientation affect the structural, optical, and electronic properties of the a-Si layers used in our SHJ devices, and how these properties affect the final device performance.   

Characterization of Grain Boundaries in Polycrystalline Photovoltaic Devices using Near-Field Scanning Optical Microscopy.

Polycrystalline thin film PV devices have the potential to reduce the cost per watt for commercial photovoltaic, but, their lower efficiency compared to their counter parts and lack of stability have prevented their widespread adoption. There is a need for a more fundamental understanding of these PV devices. A near-field scanning optical microscope (NSOM) has been built to optically and electrically characterize polycrystalline thin film PV devices. The NSOM is presently being used in air and at room temperature to perform spatially resolved photocurrent measurements using a broad range of visible excitation wavelengths on planar PV devices. Results from the front side illuminated planar CdTe devices show between a 5 and 10{\%} increase in the generated photocurrent between the grains supporting the idea of charge separation at the grain boundary. Unlike previous studies, these photocurrent measurements have been decoupled from the topographical cross talk typically common to NSOM measurements. The authors gratefully acknowledge support from the National Science Foundation under Grant No. DMR-0103945 and samples provided by the University of Toledo.   

Theory of photo-conversion in polycrystalline silicon

We developed a three--dimensional analytical formalism of photo-conversion in polycrystalline silicon based solar cells. Polycrystalline Si was modeled by representing the grains as parallelepipeds or cylinders, considering spatial dependence of generation and recombination of electron-hole pairs both in the bulk and at the grain boundaries. We calculated spectral dependence of the short circuit current and open circuit voltage averaged over the grain. The recombination of the photo carriers at the grain boundary was described by introducing the effective diffusion length, responsible for the attenuation of excess electron--hole pairs. The recombination dependence on the bulk diffusion length, grain size and effective recombination velocity at the boundaries were derived and discussed. The short circuit current, open circuit voltage and photo-conversion efficiency in polycrystalline Si are in good agreement with the experimental data available. The research was supported by the Centre for Materials and Manufacturing/Ontario Centres of Excellence (OCE/CMM) ``Sonus/PV Photovoltaic Highway Traffic Noise Barrier'' project.   

Photovoltaic applications of hydrogenated amorphous silicon thin films grown by the Saddle Field Glow Discharge Method

Thin film hydrogenated amorphous silicon (a-Si:H) is widely used for photovoltaic solar cells. We present a combined theoretical and experimental study of the thin a-Si:H films for efficient and inexpensive solar cells, grown by the Saddle Field Glow Discharge Method. The type of solar cell studied is glass/SnO$_{2}$/p-i-n Si:H/Al. We investigated the mechanism of hydrogen diffusion inside the film, its relation to the bonding within the amorphous silicon network. Hydrogen diffusion in a-Si:H was modeled using first-principles finite temperature molecular dynamics. Optimization of the solar cells was performed based on the experimental diffusion coefficients, carrier mobilities, parameters of the p-i-n structures, and electron band structure (defect distribution inside the gap). An analytical model to optimize photo-conversion efficiency of a-Si:H based solar cells with contact grid has been developed. The research was supported by the Centre for Materials and Manufacturing/Ontario Centres of Excellence (OCE/CMM) ``Sonus/PV Photovoltaic Highway Traffic Noise Barrier'' project.   

Simple method to examine the work function of transparent conducting oxide for traditional and organic based photovoltaics

Transparent conducting oxides (TCOs) are key components in both traditional and organic based optoelectronic devices. In photovoltaic applications in which TCOs are employed as transparent electrical contacts, the matching of the TCO work function to that of the active material is critical to device performance. We report the adaptation of a commercial electrostatic voltmeter to measure the work function of In-Zn-O and other TCO materials relevant to photovoltaics. The applicability of this technique to high-throughput combinatorial studies of compositionally graded TCO libraries will be presented. We will also examine correlations between the observed work function and other material properties in these TCO libraries. The relationship between device performance and the measured work function will also be assessed.   

Combinatorial Development of Amorphous Mixed Metal Oxide Transparent Conductors

We are using combinatorial approaches to optimize both amorphous In-Zn-O (a-IZO) and amorphous Zn-Sn-O (a-ZTO) transparent conductors for photovoltaic applications. Compositionally-graded combinatorial samples (``libraries'') are deposited by co-sputtering onto 2"x2" glass substrates at temperatures ranging from room-temperature to 500 \r{ }C. Three to five libraries are generally required to cover the full composition range for a binary tie-line, such as from In2O3 to ZnO. For IZO, we have found that IZO films deposited in Ar at 100 \r{ }C are amorphous for films with 65 to 85 cation{\%} In, with a maximum conductivity of 3000 S/cm at 80 cation{\%} In and an RMS roughness of 0.4 nm. Subsequent sequential annealing experiments in both Ar and air show that a-IZO films are structurally, electrically and optically quite robust for anneals up to 500 or 600 \r{ }C. For a-ZTO, the best conductivity obtained to date for an amorphous ZTO film is 200 S/cm for films grown at 400 \r{ }C with 35 cation{\%} Zn.   

Optimization of amorphous In-Zn-O (IZO) transparent conductor sputtered at ambient temperature

Amorphous indium zinc oxide (IZO) is an n-type transparent conducting oxide (TCO) that offers high electrical conductivity, visible-spectrum transparency, smoothness and ease of deposition, all properties of interest for photovoltaic and optoelectronic applications. Previous work has shown that magnetron-sputtered IZO is amorphous over the metals-only composition range $\sim $55 to 85 atomic {\%} indium. In this work, five different single-composition targets spanning the amorphous range were used to sputter thin films at ambient temperature with varying oxygen content in the sputter gas. In addition, highly resistive films were deposited to make field-effect thin-film transistors. The resistivity, carrier concentration, and hall mobility, as well as the optical transmission and reflection for $\lambda $=300-900 nm light were measured for each film. The conductivity was tunable from $\sim $2.5 x 10$^{3}$ S/cm to $\sim $10$^{-3}$ S/cm depending on the amount of oxygen present. Generally, increasing the oxygen or lowering the indium content lowers the carrier concentration, while increasing the indium content increases the electron mobility. For thin-film transistors, a low carrier concentration but high mobility is desired.   

Transparent Conducting ZnO Thin Films Doped with Al and Mo

Transparent conducting oxide (TCO) thin films are a vital part of photovoltaic cells, flat-panel displays, and electrochromic windows. ZnO-based TCOs, due to the relative abundance of Zn, may reduce production costs compared to those of the prevalent TCO In$_{2}$O$_{3}$:Sn (ITO). Undoped ZnO, ZnO:Al (0.5, 1, and 2 wt.{\%} Al$_{2}$O$_{3})$, and ZnO:Mo (2 wt.{\%}) films were deposited by RF magnetron sputtering. Optimal deposition temperature was found to be 200$^{o}$C. Controlled incorporation of H$_{2}$ in the Ar sputtering ambient increased mobility of undoped ZnO significantly to 48 cm$^{2}$V$^{-1}$s$^{-1}$. H$_{2}$ also appears to catalyze ionization of dopants. This enabled lightly doped ZnO:Al to provide comparable conductivity to the standard 2 wt.{\%}-doped ZnO:Al while demonstrating reduced infrared absorption. Mo was found to be an n-type dopant of ZnO, though material properties did not match those of ZnO:Al. Scattering mechanisms were investigated using temperature-dependent Hall measurements and the method of four coefficients. This abstract is subject to government rights.   

A theoretical study on native point defects and dopants in cuprous oxide

We have performed a first-principle study on the electronic structures, atomic configurations, and formation energies of native point defects in cuprous oxide, i.e. vacancies$(V_{Cu} ,V_O )$, interstitials $(Cu_i ,O_i )$ and antisite defects $(Cu_O ,O_{Cu} )$ by using Density Function Theory based VASP package with PAW potentials. We have carefully studied the formation of native point defects under different chemical environments and Fermi level positions. We have also calculated the electronic structures of dopants such as F, Cl, N, Ca and Mg in the cuprous oxide crystal. Their formation at different chemical environments and Fermi level positions will be presented as well.   

Thin film preparation of the p-type transparent semiconductor Cu$_{3}$TaS$_{4}$

Thin films of a new wide band gap p-type semiconductor Cu$_{3}$TaS$_{4}$ (CTS) are prepared by PLD deposition of Cu and Ta metal multilayers and subsequent ex-situ rapid thermal processing in a sulfur environment. X-ray diffraction confirmed the presence of single phase CTS. 275 nm thick CTS films on fused SiO$_{2}$ substrates show reflection-corrected transmission $>$70{\%} over the range 400-700 nm, with an optical band gap near 2.8 eV. The electrical resistivity of undoped CTS thin films is $\sim $ 5 Ohm cm. These properties indicate that CTS thin films may find application in transparent electronics.   

Properties of a potential transparent p-type semiconductor Cu$_{3}$TaQ$_{4}$ (Q = S or Se)

Physical, optical and electrical properties of powder samples of the Cu$_{3}$TaQ$_{4}$ (Q = S or Se) series are investigated to determine the potential as a transparent semiconductor. The series crystallizes in a P-43m sulvanite structure. The sulfide has a lattice parameter of a = 5.5036(4) {\AA} and the selenide has a lattice parameter of a = 5.6535(7) {\AA}. The optical band gaps are 2.75eV for Cu$_{3}$TaS$_{4}$ and 2.36eV for Cu$_{3}$TaSe$_{4}$. Seebeck coefficients of +27$\mu $V/K for Cu$_{3}$TaS$_{4}$ and +24$\mu $V/K for Cu$_{3}$TaSe$_{4}$ confirm that both materials are p-type. FLAPW band structure calculations indicate that the band gap is indirect.   

Transparent conductive BaCuTeF thin films by pulsed laser deposition

Transparent p-type carrier conductive BaCuTeF thin films are reported. Undoped BaCuTeF films obtained \textit{in-situ} by pulsed laser deposition in UHV exhibit maximum conductivities of 50-55 S/cm on fused silica substrates. The polycrystalline films deposited at various temperatures up to 600$^{\circ}$C are single phase with optical band gap of about 3 eV and 70{\%} average transparency in the visible and near-IR optical ranges. BaCuTeF films deposited on single crystal MgO substrates are highly oriented.