Bobby To

Optimization of CBD CdS process in high-efficiency Cu(In,Ga)Se2-based solar cells

Abstract We present an optimization of the CdS chemical bath deposition process as applied to high-efficiency Cu(In,Ga)Se2 photovoltaic thin-film absorber materials. Specifically, we investigated deposition time (thickness), bath temperature (65, 80 and 90°C) and a Cd2+ partial-electrolyte treatment of the chalcopyrite absorber prior to CdS deposition. We found that thinner CdS layers (grown on as-deposited absorbers) allowing more light to reach the junction are not necessarily conducive to higher short-circuit current density. Device performance was found to be dependent on the CdS layer thickness, but rather independent of the growth temperature. On the other hand, devices prepared from absorbers subjected to a Cd2+ partial electrolyte treatment show that the device performance dependence on CdS thickness is somewhat alleviated, and devices incorporating thinner CdS layers are possible without loss of parameters, such as open-circuit voltage and fill factor.

Transparent Conductive Single-Walled Carbon Nanotube Networks with Precisely Tunable Ratios of Semiconducting and Metallic Nanotubes

We present a comprehensive study of the optical and electrical properties of transparent conductive films made from precisely tuned ratios of metallic and semiconducting single-wall carbon nanotubes. The conductivity and transparency of the SWNT films are controlled by an interplay between localized and delocalized carriers, as determined by the SWNT electronic structure, tube-tube junctions, and intentional and unintentional redox dopants. The results suggest that the main resistance in the SWNT thin films is the resistance associated with tube-tube junctions. Redox dopants are found to increase the delocalized carrier density and transmission probability through intertube junctions more effectively for semiconductor-enriched films than for metal-enriched films. As a result, redox-doped semiconductor-enriched films are more conductive than either intrinsic or redox-doped metal-enriched films.

Nanostructured black silicon and the optical reflectance of graded-density surfaces

We fabricate and measure graded-index “black silicon” surfaces and find the underlying scaling law governing reflectance. Wet etching (100) silicon in HAuCl4, HF, and H2O2 produces Au nanoparticles that catalyze formation of a network of [100]-oriented nanopores. This network grades the near-surface optical constants and reduces reflectance to below 2% at wavelengths from 300 to 1000 nm. As the density-grade depth increases, reflectance decreases exponentially with a characteristic grade depth of about 1/8 the vacuum wavelength or half the wavelength in Si. Observation of Au nanoparticles at the ends of cylindrical nanopores confirms local catalytic action of moving Au nanoparticles.

Combinatorial studies of Zn-Al-O and Zn-Sn-O transparent conducting oxide thin films

Abstract In this work, we discuss the development of combinatorial deposition and analysis tools for the investigation of and the optimization of transparent conducting oxides. Library deposition by co-sputtering followed by optical analysis is shown to be a facile way to achieve these goals. Initial work focused on Zn-Al-O libraries with low Al contents as a test case. Subsequent work has focused on the ZnO-SnO2 tie line. Local maxima in the composition dependence of the conductivity were found for Zn/Sn ≈2:1 (Zn2SnO4) and Zn/Sn ≈1:1 (ZnSnO3). For these two representative stoichiometries, constant composition films have also been grown by pulsed laser deposition.

Carrier separation and transport in perovskite solar cells studied by nanometre-scale profiling of electrical potential

Organometal–halide perovskite solar cells have greatly improved in just a few years to a power conversion efficiency exceeding 20%. This technology shows unprecedented promise for terawatt-scale deployment of solar energy because of its low-cost, solution-based processing and earth-abundant materials. We have studied charge separation and transport in perovskite solar cells—which are the fundamental mechanisms of device operation and critical factors for power output—by determining the junction structure across the device using the nanoelectrical characterization technique of Kelvin probe force microscopy. The distribution of electrical potential across both planar and porous devices demonstrates p–n junction structure at the TiO2/perovskite interfaces and minority-carrier diffusion/drift operation of the devices, rather than the operation mechanism of either an excitonic cell or a p-i-n structure. Combining the potential profiling results with solar cell performance parameters measured on optimized and thickened devices, we find that carrier mobility is a main factor that needs to be improved for further gains in efficiency of the perovskite solar cells.

Efficient heterojunction solar cells on p-type crystal silicon wafers

Efficient crystalline silicon heterojunction solar cells are fabricated on p-type wafers using amorphous silicon emitter and back contact layers. The independently confirmed AM1.5 conversion efficiencies are 19.3% on a float-zone wafer and 18.8% on a Czochralski wafer; conversion efficiencies show no significant light-induced degradation. The best open-circuit voltage is above 700 mV. Surface cleaning and passivation play important roles in heterojunction solar cell performance.

Understanding the Formation and Temperature Dependence of Thick-Film Ag Contacts on High-Sheet-Resistance Si Emitters for Solar Cells

Physical and electrical properties of screen-printed Ag thick-film contacts were studied and correlated to understand and achieve good-quality ohmic contacts to high-sheet-resistance emitters for solar cells. Analytical microscopy and surface analysis techniques were used to study the Ag-Si contact interface of three different screen-printed Ag pastes (A, B, and PV168) subjected to high (∼835°C) and conventional (740-750°C) temperature firing conditions. At ∼750°C firing, all three pastes failed on a 100 Ω/□ emitter because of incomplete etching of the silicon nitride film (PV168), an irregular small distribution of regrown Ag crystallites (paste A), or an excessive diffusion of Ag into the p-n junction (paste B). At a firing temperature of ∼835°C, paste A gave a lower open-circuit voltage because of the diffusion of Al from the glass frit into the emitter region. Paste B failed because of the formation of very large (0.3-1 μm) Ag crystallites that shunted the p-n junction. Of the three pastes, the PV168 paste from DuPont gave the best contact quality on a 100 Ω/□ emitter with a solar cell fill factor of 0.782 only after annealing in a hydrogen atmosphere.

Phase Identification and Control of Thin Films Deposited by Co-Evaporation of Elemental Cu, Zn, Sn, and Se

Kesterite thin films [(i.e., Cu2ZnSn(S,Se)4 and related alloys] have been the subject of recent interest for use as an absorber layer for thin film photovoltaics due to their high absorption coefficient (>104 cm−1), their similarity to successful chalcopyrites (like CuInxGa1−xSe2) in structure, and their earth-abundance. The process window for growing a single-phase kesterite film is narrow. In this work, we have documented, for our 9.15%-efficient kesterite co-evaporation process, (1) how appearance of certain undesirable phases are controlled via choice of processing conditions, (2) several techniques for identification of phases in these films with resolution adequate to discern changes that are important to device performance, and (3) reference measurements for those performing such phase identification. Data from x-ray diffraction, x-ray fluorescence, Raman scattering, scanning electron microscopy, energy dispersive spectroscopy, and current-voltage characterization are presented.

Deposition and properties of CBD and CSS CdS thin films for solar cell application

Abstract We deposited cadmium sulfide (CdS) thin films using the chemical-bath deposition (CBD) and close-spaced sublimation (CSS) techniques. The films were then treated in CdCl 2 vapor at 400 °C for 5 min. The CSS CdS films had hexagonal structure, and good crystallinity. The CdCl 2 treatment did not produce major changes, but there was a decrease in the density of planar defects. The untreated CBD CdS films had cubic structure and poorer crystallinity than the CSS films. After the CdCl 2 treatment, these films recrystallized to the hexagonal phase, resulting in better crystallinity and a lower density of planar defects. The conformal coverage and the presence of bulk oxygen are the key issues in making the CBD films more suitable for photovoltaic applications.

The electrical, optical and structural properties of InxZn1−xOy (0 ⩽ x ⩽ 1) thin films by combinatorial techniques

Indium–zinc-oxide (IZO) compositional libraries were deposited with dc magnetron sputtering onto glass substrates at 100 °C and analysed with high throughput, combinatorial techniques. The composition range from 4 to 95 at% In for Zn was explored. A peak in conductivity with σ > 3000 (Ω cm)−1 was observed at an indium content of ~70%. The mobility exceeded 30 cm2 (V s)−1 and the carrier concentrations were greater than 8 × 1020 cm−3. Crystalline phases were observed for In concentrations less than 45% and greater than 80% with an intermediate amorphous region. The low indium content films have a zinc oxide type structure with a ZnO (002) spacing ranging from ~2.61 to 2.85 A for 4% In and 45% In, respectively. For indium contents between 82% and 95%, the In2O3 (222) spacing varied from 2.98 to 2.99 A. Regardless of the composition or the degree of crystallinity, all films showed high optical transparency with the transmission >80% across the visible spectrum.