Energy Band Alignment by Solution-Processed Aluminum Doping Strategy toward Record Efficiency in Pulsed Laser-Deposited Kesterite Thin-Film Solar Cell.

Kesterite-based Cu2ZnSnS4 (CZTS) thin-film photovoltaics involve a serious interfacial dilemma, leading to severe recombination of carriers and insufficient band alignment at the CZTS/CdS heterojunction. Herein, an interface modification scheme by aluminum doping is introduced for CZTS/CdS via a spin coating method combined with heat treatment. The thermal annealing of the kesterite/CdS junction drives the migration of doped Al from CdS to the absorber, achieving an effective ion substitution and interface passivation. This condition greatly reduces interface recombination and improves device fill factor and current density. The JSC and FF of the champion device increased from 18.01 to 22.33 mA cm-2 and 60.24 to 64.06%, respectively, owing to the optimized band alignment and remarkably enhanced charge carrier generation, separation, and transport. Consequently, a photoelectric conversion efficiency (PCE) of 8.65% was achieved, representing the highest efficiency in CZTS thin-film solar cells fabricated by pulsed laser deposition (PLD) to date. This work proposed a facile strategy for interfacial engineering treatment, opening a valuable avenue to overcome the efficiency bottleneck of CZTS thin-film solar cells.

Structure, Morphology, and Photoelectric Performances of Te-Sb2Se3 Thin Film Prepared via Magnetron Sputtering.

Antimony selenide (Sb2Se3) has been widely investigated as a promising absorber material for photovoltaic devices. However, low open-circuit voltage (Voc) limits the power conversion efficiency (PCE) of Sb2Se3-based cells, largely due to the low-charge carrier density. Herein, high-quality n-type (Tellurium) Te-doped Sb2Se3 thin films were successfully prepared using a homemade target via magnetron sputtering. The Te atoms were expected to be inserted in the spacing of (Sb4Se6)n ribbons based on increased lattice parameters in this study. Moreover, the thin film was found to possess a narrow and direct band gap of approximately 1.27 eV, appropriate for harvesting the solar energy. It was found that the photoelectric performance is related to not only the quality of films but also the preferred growth orientation. The Te-Sb2Se3 film annealed at 325 °C showed a maximum photocurrent density of 1.91 mA/cm2 with a light intensity of 10.5 mW/cm2 at a bias of 1.4 V. The fast response and recovery speed confirms the great potential of these films as excellent photodetectors.

Fundamental Physical Characterization of Sb2Se3-Based Quasi-Homojunction Thin Film Solar Cells.

A new type of solar cell based on Cu-doped (p-type) and I-doped (n-type) Sb2Se3 has been designed and fabricated using magnetron sputtering with two different thicknesses of absorber. The overall objective is for better understanding the charge recombination mechanism, especially at the interface region. The investigation has been specifically performed using IMPS (intensity modulated photocurrent spectroscopy), IMVS (intensity modulated photovoltage spectroscopy), and diode characterizations. It has been found that an increase of the absorber thickness leads to a shorter carrier lifetime, but longer diffusion length and lower trap density, resulting in significantly better performance. Furthermore, it is demonstrated that trap-assisted recombination does not affect the short-circuit current density (Jsc), but significantly decreases the open-circuit voltage (Voc). As a result, an encouraging power conversion efficiency (PCE) of 2.41%, fill factor (FF) of 41%, Jsc of 20 mA/cm2, and Voc of 294 mV are obtained. Most importantly, key parameters for further increasing the PCE have been identified.

Global sensitivity analysis of bandpass and antireflection coating manufacturing by numerical space filling designs.

We present the effectiveness of global sensitivity analyses of optical coatings manufacturing to assess the robustness of filters by computer experiments. The most critical interactions of layers are determined for a 29 quarter-wave layer bandpass filter and for an antireflection coating with eight non-quarter-wave layers. Two monitoring techniques with the associated production performances are considered, and their influence on the interactions classification is discussed. Global sensitivity analyses by numerical space filling designs give clues to improve filter manufacturing against error effects and to assess the potential robustness of the coatings.

Kerr spatial solitons in chalcogenide waveguides.

Kerr spatial solitons are observed in slab chalcogenide waveguides at near-IR wavelengths. Waveguides are realized either by electron-beam evaporation or rf sputtering of a Ge-Sb-S compound deposited on oxidized silicon wafer. The Kerr coefficient of the thin film is evaluated to be 5 x 10(-18) m(2)/W from the experimentally required soliton power at 1.5 mum. Limitations due to material photosensitivity are revealed.

Manufacturing of an absorbing filter controlled with a broadband optical monitoring.

This paper deals with a broadband optical monitoring set up useful for the manufacturing of absorbing coatings. The monitoring strategy consists in simultaneous measurements of transmittance and reflectance over a large spectral range. The resulting analysis allows then to determine the real time deposited thickness. A stage of design correction is possible after the deposition and analysis of each layer. This method has potential for thin metallic layers coatings. We then describe layer after layer the strategy for the control and manufacturing of a filter with given colorimetric properties.

Laser damage resistance of hafnia thin films deposited by electron beam deposition, reactive low voltage ion plating, and dual ion beam sputtering.

A comparative study is made of the laser damage resistance of hafnia coatings deposited on fused silica substrates with different technologies: electron beam deposition (from Hf or HfO(2) starting material), reactive low voltage ion plating, and dual ion beam sputtering. The laser damage thresholds of these coatings are determined at 1064 and 355 nm using a nanosecond pulsed YAG laser and a one-on-one test procedure. The results are associated with a complete characterization of the samples: refractive index n measured by spectrophotometry, extinction coefficient k measured by photothermal deflection, and roughness measured by atomic force microscopy.

Chalcogenide coatings of Ge15Sb20S65 and Te20As30Se50.

Chalcogenide coatings are investigated to obtain either optical components for spectral applications or optochemical sensors in the mid-infrared. The deposition of Ge(15)Sb(20)S(65) and Te(20)As(30)Se(50) chalcogenide glasses is performed by two physical techniques: electron-beam and pulsed-laser deposition. The quality of the film is analyzed by scanning electron microscopy, atomic force microscopy, and energy dispersive spectroscopy to characterize the morphology, topography, and chemical composition. The optical properties and optical constants are also determined. A CF(4) dry etching is performed on these films to obtain a channeled optical waveguide. For a passband filter made by electron-beam deposition, cryolite as a low-refractive-index material and chalcogenide glasses as high-refractive-index materials are used to favor a large refractive-index contrast. A shift of a centered wavelength of a photosensitive passband filter is controlled by illumination time.

Optical modeling of organic solar cells based on CuPc and C60.

We have investigated the influence of the poly(3,4-ethylenedioxythiophene)-blend-poly(styrene-sulfonate) (PEDOT:PSS) layer on the short-circuit current density (J(sc)) of single planar heterojunction organic solar cells based on a copper phthalocyanine (CuPc)-buckminsterfullerene (C(60)) active layer. Complete optical and electrical modeling of the cell has been performed taking into account optical interferences and exciton diffusion. Comparison of experimental and simulated external quantum efficiency has allowed us to estimate the exciton diffusion length to be 37 nm for the CuPc and 19 nm for the C(60). The dependence of short-circuit current densities versus the thickness of the PEDOT:PSS layer is analyzed and compared with experimental data. It is found that the variation in short-circuit current densities could be explained by optical interferences.

Interest of broadband optical monitoring for thin-film filter manufacturing.

Broadband optical monitoring for thin-film filter manufacturing is more and more developed thanks to better performances of spectrometers with array detectors. We compare this optical monitoring with turning point monitoring and quartz monitoring of different designs. The sensitivity to thickness errors and to refractive index errors is evaluated. We show that real time determination of deposited thickness is a valuable criterion. We also present our experimental setup of transmittance and reflectance broadband optical monitoring. The use of a 400-1000 nm range combined with a signal-to-noise ratio of ~2500 in transmittance and 1000 in reflectance permits us to expect the manufacturing of high-performance non-quarter-wave designs. A first manufacturing of an 18-layer non-quarter-wave high-pass filter is provided.

Optical characterization of an unknown single layer: Institut Fresnel contribution to the Optical Interference Coatings 2004 Topical Meeting Measurement Problem.

We present the characterizations performed at the Institut Fresnel for the Measurement Problem of the Optical Interference Coatings 2004 Topical Meeting. A single layer coated on a fused-silica substrate of unknown composition and parameters is analyzed in terms of optogeometrical parameters, uniformity, and scattering. We determine the refractive index and the average thickness of the coating, then provide the localized determination of the thickness with a 2 mm spatial resolution. Topography measurements include atomic force microscopy and angle-resolved scattering measurements. These results are completed thanks to a Taylor Hobson noncontact 3D surface profiler.

Ultranarrow bandpass hybrid filter with wide rejection band.

The theoretical study and the experimental realization of an ultranarrow bandpass filter, joining a fiber Bragg grating and a dielectric mirror directly deposited at the extremity of the fiber tip, is presented. The features of such a filter are in very good accordance with the results of theoretical simulations.

Piezoelectric tantalum pentoxide studied for optical tunable applications.

Piezoelectric transparent thin films are of great interest for use in tunable filters. We present experimental results on Ta2O5 single layers coated on fused-silica substrates with an electron-beam deposition process. Above 450 degrees C, coatings change from an amorphous to a polycrystallized structure. When this structure shows a preferred orientation matching the piezoelectric tensor of the Ta2O5 crystal and the external electric field, variation in the piezoelectric layer thickness is expected. We detail experimental results in terms of optical (spectrophotometric and scattering measurements) and nonoptical characterizations (x-ray diffraction and scanning electron microscopy). Then the resultant thickness variation under oscillating applied voltage is measured with an extrinsic Fabry-Perot interferometer setup.

Index determination of opaque and semitransparent metallic films: application to light absorbers.

Index-determination methods based on reflectance and transmittance measurements are developed for both opaque and semitransparent metallic films. Results are given concerning chromium and nickel layers manufactured by electron-beam deposition. To take account of the evolution of the optical constants versus layer thickness, an inhomogeneous layer model is used, which permits us to obtain a good agreement between measurements and calculations. Results are applied to the design and manufacture of light absorbers for which accurate index knowledge is required. Measured absorption is higher than 0.999 on both broadband and monochromatic components.