Thermal Post-deposition Treatment Research Articles

Gradient doping in Cu2ZnSnSe4 by temperature and potential induced defect steering

Kesterite materials are among the most promising emerging photovoltaic absorbers, despite the number of challenging issues this technology presents. The use of soft thermal post-deposition treatments is key to improving the CdS/kesterite interface quality. Thermal treatments can result in a low-temperature phase transition which affects the optoelectronic properties. In this work, the effects of applied voltage during a full device thermal post-deposition treatments above the critical temperature of the phase transition are explored. The applied voltage modifies the formation energy and drives in-depth migration of ionized defects, which can generate a shallow doping density gradient. Supporting the experimental findings, the effects of a shallow doping density gradient on the current–voltage curves and the external quantum efficiency are modelled using drift–diffusion calculations. The presence of bulk recombination centers in the modelling is a key aspect to precisely reproduce the experimental results. The shallow doping density gradient in opposite directions precisely matches the experimental results for opposite voltage polarizations. The effects on the band structure of the device are presented proving this as a promising strategy for improving charge carrier selectivity.This is the first time that a defect engineering approach has been proposed and successfully proven to control shallow doping density profiling and improve the charge carrier selectivity of kesterite devices. In this sense, the results and their thorough physical interpretation presented in this work will potentially open new perspectives in other photovoltaic technologies as well as in the field of materials engineering.

Reactive Deposition Versus Strong Electrostatic Adsorption (SEA): A Key to Highly Active Single Atom Co-Catalysts in Photocatalytic H2 Generation.

In recent years, the use of single atoms (SAs) has become of a rapidly increasing significance in photocatalytic H2 generation; here SAnoble metals (mainly Pt SAs) can act as highly effective co-catalysts. The classic strategy to decorate oxide semiconductor surfaces with maximally dispersed SAs relies on "strong electrostatic adsorption" (SEA) of suitable noble metal complexes. In the case of TiO2 - the classic benchmark photocatalyst - SEA calls for adsorption of cationic Pt complexes such as [(NH3 )4 Pt]2+ which then are thermally reacted to surface-bound SAs. While SEA is widely used in literature, in the present work it is shown by a direct comparison that reactive attachment based on the reductive anchoring of SAs, e.g., from hexachloroplatinic(IV) acid (H2 PtCl6 )leads directly to SAs in a configuration with a significantly higher specific activity than SAs deposited with SEA - and this at a significantly lower Pt loading and without any thermal post-deposition treatments. Overall, the work demonstrates that the reactive deposition strategy is superior to the classic SEA concept as it provides a direct electronically well-connected SA-anchoring and thus leads to highly active single-atom sites in photocatalysis.

Metal-insulator multilayer precursors for enhanced Sm2+ absorption and Tm2+ luminescence in SiAlO thin films prepared by magnetron sputtering

Silicon-aluminum-oxigen (SiAlO) coatings doped with Sm2+ and prepared by reactive magnetron co-sputtering of Si, Al, and Sm targets, are attractive for luminescence solar concentrator applications but suffer from the low absorption between 300 and 600 nm. This article reports that the main cause of low absorption is a high concentration of undesired Sm3+. This finding is supported by optical transmittance, photoluminescence emission and excitation characterization, and X-ray photoelectron spectroscopy data of the Sm's 3d5/2 edge. We present an alternative deposition process for obtaining Sm doped SiAlO layers with enhanced Sm2+ absorption by incorporating Sm through the use of multilayer thin-film precursors composed of metallic Sm and SiAlO layers. After thermal post-deposition treatments, diffusion and reaction of the metallic Sm layers with the SiAlO host results in coatings showing the characteristic 5d → 4f transitions of Sm2+ in the region between 250 and 600 nm which were not detectable in Sm-doped single layers. This same deposition strategy produces Tm doped SiAlO coatings with Tm2+‘s characteristic luminescence at 1132 nm when the SiAlO host is in the mullite composition region. The photoluminescence excitation spectrum of Tm2+ is compared to phosphor with similar composition and covers the range between 300 and 700 nm.

In- and through-plane conductivity of 8YSZ films produced at room temperature by aerosol deposition

Aerosol deposition is a unique spray coating process to deposit dense ceramic films directly at room temperature just by spraying a dry powder, without any binder or additional sintering step needed. Therefore, it is a very promising coating method for functional materials that may be used to produce a variety of electrochemical devices, solid oxide fuel cells or gas sensors. However, functional properties like the electrical conductivity significantly depend on the microstructure of the produced film. Consequently, anisotropic film properties may occur even for isotropic functional materials. In this work, films of oxide ion-conducting yttria-stabilized zirconia (YSZ) were produced by aerosol deposition. The orientation-dependent electrical conductivity was measured using two different electrode configurations: one for the in-plane conductivity and another one for through-plane measurements. A slight anisotropy was observed, attributed to the present film morphology. In particular, the grain boundaries influenced the total conductivity. Furthermore, a thermal post-deposition treatment significantly affected the total conductivity of aerosol-deposited YSZ films.

Charge Carrier Generation and Extraction in Hybrid Polymer/Quantum Dot Solar Cells

Here we investigate charge carrier generation and extraction processes in hybrid polymer/nanocrystal solar cells by means of time-resolved optical and photoelectrical techniques. We addressed the role of both poly(3-hexylthiophene) and colloidal arenethiolate-capped PbS quantum dots, which constitute the hybrid composite nanomaterial, in the photoinduced processes most relevant to device operation by changing the compositional ratio and applying chemical and thermal postdeposition treatments. The carrier generation processes were found to be wavelength-dependent: excitons generated in the polymer domains led to long-lived weakly bound charge pairs upon electron transfer to PbS nanocrystals; whereas charge carrier generation in the nanocrystal domains is highly efficient, although effective separation required the application of external electric field. Overall, charge carrier generation was found efficient and almost independent of the strength of applied electric field; therefore, competition between sep...

Thermal post-deposition treatment effects on nanocrystalline hydrogenated silicon prepared by PECVD under different hydrogen flow rates

In this paper, hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited on mono-crystalline silicon substrate by plasma enhanced chemical vapor deposition (PECVD) under different hydrogen flow rates followed by a thermal treatment in an infrared furnace at different temperature ranging from 300 to 900°C. The investigated structural, morphological and optoelectronic properties of samples were found to be strongly dependent on the annealing temperature. Raman spectroscopy revealed that nc-Si:H films contain crystalline, amorphous and mixed structures as well. We find that post-deposition thermal treatment may lead to a tendency for structural improvement and a decrease of the disorder in the film network at moderate temperature under 500°C. As for annealing at higher temperature up to 900°C induces the recrystallization of the film which is correlated with the grain size and volume fraction in the layer. We demonstrate that high annealing temperature can lead to a decrease of silicon⿿hydrogen bonds corresponding to a reduction of the amorphous matrix in the layer promoting the formation of covalent Si⿿Si bonds. The effusion of the hydrogen from the grown film leads to increase its density and therefore induces a decrease in the thickness of the layer. For post-deposition thermal treatment in temperature range under 700°C, the post-deposition anneal seems to be crucial for obtaining good passivation quality as expressed by a minority carrier lifetime of 17μs, as it allows a significant reduction in defect states at the layer/substrate interface. While for a temperature higher than 900°C, the lifetime reduction is obtained because of hydrogen effusion phenomenon, thus a tendency for crystallization in the grown film.

Classification of semiconducting polymeric mesophases to optimize device postprocessing

ABSTRACTSemiconducting polymers form a variety of phases and mesophases that respond differently to postdeposition solvent or thermal treatments. Here it is shown that classification of these materials into their appropriate mesophases can be a useful tool to optimize their thermal postdeposition treatments. Calorimetry is used to quantify differences between materials having similar molecular structures, using a well‐established framework based on the kinetics and thermodynamics of phase changes. By way of example, this classification scheme is used to identify differences in three polymers, poly(3‐hexylthiophene‐2,5‐diyl) and two isomeric bithiophene–thienothiophene copolymers. It is demonstrated that poly(3‐hexylthiophene) is a “normal” polymer crystal and that the two bithiophene–thienothiophene copolymers have liquid crystalline phases. The different phase structure is notable in light of the molecular similarity of the three polymers and has an impact on the thermal postprocessing conditions that maximize field effect charge carrier mobility in thin film transistor devices. Strong superheating effects are demonstrated for the two bithiophene–thienothiophene copolymers and the impact on annealing is demonstrated using grazing incidence X‐ray diffraction. Some suggestions are also put forth for what post‐processing should be employed for each class of polymer. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1641–1653

Properties of the c-Si/Al2O3 interface of ultrathin atomic layer deposited Al2O3 layers capped by SiNx for c-Si surface passivation

This work presents a detailed study of c-Si/Al2O3 interfaces of ultrathin Al2O3 layers deposited with atomic layer deposition (ALD), and capped with SiNx layers deposited with plasma-enhanced chemical vapor deposition. A special focus was the characterization of the fixed charge density of these dielectric stacks and the interface defect density as a function of the Al2O3 layer thickness for different ALD Al2O3 deposition processes (plasma-assisted ALD and thermal ALD) and different thermal post-deposition treatments. Based on theoretical calculations with the extended Shockley–Read–Hall model for surface recombination, these interface properties were found to explain well the experimentally determined surface recombination. Thus, these interface properties provide fundamental insights into to the passivation mechanisms of these Al2O3/SiNx stacks, a stack system highly relevant, particularly for high efficiency silicon solar cells. Based on these findings, it was also possible to improve the surface passivation quality of stacks with thermal ALD Al2O3 by oxidizing the c-Si surface prior to the Al2O3 deposition.

Reaction kinetics during the thermal activation of the silicon surface passivation with atomic layer deposited Al2O3

The excellent surface passivation of crystalline silicon provided by Al2O3 requires always an activation by a thermal post-deposition treatment. In this work, we present an indirect study of the reaction kinetics during such thermal activation treatments for Al2O3 synthesized by atomic layer deposition. The study was performed for Al2O3 deposited at varying temperatures, which results in different micro-structures of the films and, in particular, different hydrogen concentrations. The effective carrier lifetime was measured sequentially as a function of the annealing time and temperature. From these data, the reaction rate Ract and the activation energy EA were extracted. The results revealed a rather constant EA in the range of 1.4 to 1.5 eV, independent of the deposition temperature. The reaction rate, however, was found to increase with decreasing deposition temperature, which correlates with an increasing amount of hydrogen being incorporated in the Al2O3 films. This is a strong indication for an interface hydrogenation that takes place during the thermal activation, which is limited by the amount of hydrogen provided by the Al2O3 layer.

Al2O3/TiO2 stack layers for effective surface passivation of crystalline silicon

For silicon surface passivation, we investigate stack layers consisting of a thin Al2O3 layer and a TiO2 capping layer deposited by means of thermal atomic layer deposition (ALD). In this work, we studied the influence of different thermal post-deposition treatments and film thickness for the activation of passivating ALD Al2O3 single layers and Al2O3/TiO2 stack layers. Our experiments show a substantial improvement of the passivation for the Al2O3/TiO2 stack layers compared to a thin single Al2O3 layer. For the stacks, especially with less than 10 nm Al2O3, a TiO2 capping layer results in a remarkably lower surface recombination. Effective fixed charge density of Al2O3/TiO2 stack layers increases after TiO2 deposition and O2 annealing. It is also demonstrated that the enhanced surface passivation can be mainly related to a remarkably low interface defect density of 1.1 × 1010 eV−1 cm−2, whereas post-TiO2 heat treatment in O2 ambience is not beneficial for the passivation of silicon, which is attributed to increasing interface defect density of stack layers.

Boron Emitter Passivation With Al$_{\bf 2}$O $_{\bf 3}$ and Al$_{\bf 2}$O$_{\bf 3}$/SiN$_{\bm x}$ Stacks Using ALD Al$_{\bf 2}$O $_{\bf 3}$

Thin layers of Al2O3 are known to feature excellent passivation properties on highly boron-doped silicon surfaces. In this paper, we present a detailed study of the passivation quality of Al2O3 single layers and stacks of Al2O3 and antireflection SiNx on boron-doped emitters, where the Al2O3 was deposited by plasma-assisted atomic layer deposition and the SiNx by plasma-enhanced chemical vapor deposition. The passivation quality was studied for different atomic layer deposition temperatures, as a function of the Al2O3 layer thickness, as well as on samples with planar and random pyramids textured surfaces. These investigations were performed for different boron emitter diffusions, such as shallow, industrial emitters with high surface concentrations, as well as driven-in emitters with low surface concentrations. For all these variations, we compared systematically different thermal post-deposition treatments to activate the Al2O3 passivation, i.e., annealing processes at moderate temperatures and short high-temperature processes, as required for firing printed metal contacts. Therefore, symmetrically processed p+np + samples were fabricated, which were characterized with the photoconductance decay technique to determine emitter saturation current densities. Finally, the longtime stability of the Al2O3/SiNx stacks with planar and textured surfaces was investigated with an accelerated ultraviolet (UV) exposure experiment, miming about 34 month of outdoor performance.

Ag Doped Zinc Tin Oxide as Cathode for Organic Photovoltaic Cells

A thin Ag layer embedded between layers of zinc tin oxide (ZTO) are compared to cells using an indium tin oxide electrode was investigated for inverted organic bulk heterojunction solar cells employing a multilayer electrode. ZTO/Ag/ ZTO (ZAZ) electrode is the preparation at room temperature, a high transparency in the visible part of the spectrum, and a very low sheet resistance comparable to treated ITO without the need for any thermal post deposition treatment as it is necessary for ITO. The In-free ZAZ electrodes exhibit a favorable work function of 4.3 eV and are shown to allow for excellent electron extraction even without a further interlayer. This renders ZAZ a perfectly suited bottom electrode for inverted organic solar cells with simplified cell architecture.

Low-Temperature, Solution-Processed MoOx for Efficient and Stable Organic Solar Cells

Sol-gel processed MoO(x) (sMoO(x)) hole-extraction layers for organic solar cells are reported. A Bis(2,4-pentanedionato)molybdenum(VI)dioxide/isopropanol solution is used and only a moderate thermal post deposition treatment at 150 °C in N(2) ambient is required to achieve sMoO(x) layers with a high work-function of 5.3 eV. We demonstrate that in P3HT:PC(60)BM organic solar cells (OSCs) our sMoO(x) layers lead to a high filling factor of about 65% and an efficiency of 3.3% comparable to that of reference devices with thermally evaporated MoO(3) layers (eMoO(3)). At the same time, a substantially improved stability of the OSCs compared to devices using a PEDOT:PSS hole extraction layer is evidenced.

Improving the electrical and optical properties of DC-sputtered ZnO:Al by thermal post deposition treatments

Thermal post deposition treatments are applied to DC-sputtered aluminum-doped zinc oxide (ZnO:Al) films and lead to a significant improvement of the electrical properties. Protective layers of amorphous silicon are used to protect the films from degradation during the high temperature treatment. Annealing for 6 hours at 500°C leads to a carrier mobility of 48cm2/Vs at a carrier concentration of 5.5·1020cm−3. Furthermore, improvements in the optical as well as in the electrical properties are possible at the same time compared to the as-deposited film. This is achieved by carrying out two thermal treatments to the ZnO:Al film, one prior to the capping with the protective layer and one afterwards. A series of samples with different carrier concentrations allows us to draw conclusions on the specific electrical transport properties.