In laser manufacturing operations, accurate measurement of laser power is important for product quality, operational repeatability, and process validation. Accurate real-time measurement of high-power lasers, however, is difficult. Typical thermal power meters must absorb all the laser power in order to measure it. This constrains power meters to be large, slow and exclusive (that is, the laser cannot be used for its intended purpose during the measurement). To address these limitations, we have developed a different paradigm in laser power measurement where the power is not measured according to its thermal equivalent but rather by measuring the laser beam’s momentum (radiation pressure). Very simply, light reflecting from a mirror imparts a small force perpendicular to the mirror which is proportional to the optical power. By mounting a high-reflectivity mirror on a high-sensitivity force transducer (scale), we are able to measure laser power in the range of tens of watts up to ~ 100 kW. The critical parameters for such a device are mirror reflectivity, angle of incidence, and scale sensitivity and accuracy.
We will describe our experimental characterization of a radiation-pressure-based optical power meter. We have tested it for modulated and CW laser powers up to 92 kW in the laboratory and up to 20 kW in an experimental laser welding booth. We will describe present accuracy, temporal response, sources of measurement uncertainty, and hurdles which must be overcome to have an accurate power meter capable of routine operation as a turning mirror within a laser delivery head.
We have developed an all-laser processing technique by means of two industrially-relevant continuous-wave fiber lasers operating at 1070 nm. This approach is capable of both substrate heating with a large defocused beam and material processing with a second scanned beam, and is suitable for a variety of photovoltaic applications. We have demonstrated this technique for rapid crystallization of thin film (~10 μm) silicon on glass, which is a low cost alternative to wafer-based solar cells. We have also applied this technique to wafer silicon to control dopant diffusion at the surface region where the focused line beam rapidly melts the substrate that then regrows epitaxially. Finite element simulations have been used to model the melt depth as a function of preheat temperature and line beam power. This process is carried out in tens of seconds for an area approximately 10 cm2 using only about 1 kW of total optical power and is readily scalable. In this paper, we will discuss our results with both c-Si wafers and thin-film silicon.
We review prior and on-going works in using laser annealing (LA) techniques in the development of chalcogenide-based [CdTe and Cu(In,Ga)(S,Se)2] solar cells. LA can achieve unique processing regimes as the wavelength and pulse duration can be chosen to selectively heat particular layers of a thin film solar cell or even particular regions within a single layer. Pulsed LA, in particular, can achieve non-steady-state conditions that allow for stoichiometry control by preferential evaporation, which has been utilized in CdTe solar cells to create Ohmic back contacts. Pulsed lasers have also been used with Cu(In,Ga)(S,Se)2 to improve device performance by surface-defect annealing as well as bulk deep-defect annealing. Continuous-wave LA shows promise for use as a replacement for furnace annealing as it almost instantaneously supplies heat to the absorbing film without wasting time or energy to bring the much thicker substrate to temperature. Optimizing and utilizing such a technology would allow production lines to increase throughput and thus manufacturing capacity. Lasers have also been used to create potentially low-cost chalcogenide thin films from precursors, which is also reviewed.
We investigate how post-deposition laser annealing can be used to improve structural and electronic quality of room-temperature deposited CdTe. We use continuous-wave, 1064 nm laser light to anneal CdTe solar cell stacks prior to back contact deposition. Sub-bandgap optical absorption measurements by photothermal deflection spectroscopy show a reduction of sub-bandgap defects due to the annealing process. Since the 1064 nm light is only partially absorbed, in situ monitoring of the transmitted light during laser annealing gives real-time information about changes in the material. These results reveal an evolution of electronic defect annealing and surface roughness modification with laser exposure time. This hypothesis is supported by electron microscopy. Two distinct annealing regimes emerge: one at low laser power where electronic defect annealing saturates after about one minute exposure and another at high power where structural defects are annealed after several minutes exposure. Temperatures reached during laser annealing are estimated by finite element modeling of the thermal transport due to heat generation from optical absorption.
Presented here are the results of a three dimensional, finite element simulation that models pulsed, ultraviolet (UV) laser annealing of polycrystalline CdTe. The model considers heat generated by the absorption of a 25 ns, 248 nm laser pulse normally incident to a 5 μm thick CdTe thin film deposited on a polycrystalline alumina substrate. In particular, focus is on the spatial and temporal distribution of temperature from laser fluences that achieve a sub-melting condition. The model shows that there are very large temperature gradients both in depth and in-plane directions. These predictions, as well as the onset of melting, are confirmed with cross sectional scanning electron microscopy. Additionally, the model predicts that the heat generated dissipates rapidly after the pulse has ended. This has implications if pulse trains are to be used experimentally.
Silicon wire arrays have been synthesized through a two-step metal-assisted electrode-less etching from an n-type silicon
wafer with (100) orientation. Field Emission Scanning Electron microscope (FESEM), Ultra violet-Visible-Near infrared
(UV-VIS-NIR) spectrophotometer and Resonance-coupled photoconductivity decay (RCPCD) have been used to
characterize the morphological, optical, and electrical properties of Si wires at varying etching times. The reflectivity of
the wire arrays decreased with increasing etching time because of light scattering from the micro-roughness of the
Silicon wire surfaces. The effective carrier lifetime decreased with increasing wire length due to the increased surface
area. We also created smoother wire surfaces by thermal oxidation followed by HF dipping. From FESEM cross
sectional images and reflectivity results, this treatment removes the micro-roughness, but the effective lifetime is lower
than the as-grown wire arrays. A photoluminescence peak observed only in the smoother wires suggests that the lower
effective lifetime is due to the diffusion of residual Ag atoms from the wire surfaces into the bulk during the thermal
Nanomaterials have the potential to revolutionize photovoltaics with the promise of new physics, novel architectures
and low cost synthesis. Silicon quantum dots, relative to their II-VI counterparts, are understudied
due to the difficulty of solution synthesis and chemical passivation. However, silicon is still an attractive solar
cell material, providing an optimal band gap, low toxicity, and a very solid body of physical understanding of
bulk silicon to draw from. We have synthesized silicon quantum dots with plasma enhanced chemical vapor
deposition, and have developed a method for chemical passivation of these silicon quantum dots that can be
used on particles created in a variety of ways. This versatile method utilizes oxidation via wet chemical etch and
subsequent siloxane bond formation. The attachment of a silane to the SiOx shell leads to stability of the silicon
core for over a month in air, and individual particles can be seen with TEM; thus a stable, colloidal suspension
is formed. The future for this technique, including increasing quantum yield of the particles by changing the
nature of the oxide, will be discussed.
Amorphous silicon carbide alloys are being investigated as a possible top photovoltaic layer in photoelectochemical
(PEC) cells used for water splitting. In order to be used as such, it is important that the effect that varying carbon
concentration has on bonding, and thus the electronic and optical properties, is well understood. The samples being
studied are silicon rich films with between 6 and 11 atomic percent of carbon. Electron spin resonance (ESR)
experiments, including light-induced ESR (LESR), were performed to study defects from dangling bonds which occur
dominantly at the silicon atoms in these films. Spin densities resulting from silicon dangling bonds varied between 1016
and 1017 spins/cm3. Lastly, to test the validity of these materials being used for devices we prepared pin structured solar
cells with the films being studied used as the absorber layer.