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Vacancy doping and charge transport in Bi2S3 nanoparticle films for photovoltaic applications
(2019)
Native point defect doping via thermal treatment is an easy and promising method to tune the electrical transport properties of semiconductors made for renewable-energy conversion. In this study, we investigate the vacancy doping of the lowly toxic semiconductor Bi2S3
using electrical conductivity as well as thermoelectric power measurements. We enhance the electrical conductivity of bismuth sulfide nanoparticle layers by more than four orders of magnitude by a stepwise thermal treatment in a moderate temperature range (300–480 K). Via thermoelectric power measurements we attribute this enhancement to an increase in charge-carrier mobility by two orders of magnitude and to an increase in charge-carrier density by more than two orders of magnitude. We find that the energetic position of the electron-doping sulfur vacancies of bismuth sulfide nanoparticles is significantly shallower than previously reported for bulk material. Subsequently, we implement Bi2S3
nanoparticles doped with sulfur vacancies by thermal annealing in photovoltaic devices using P3HT as an electron donor molecule. We find that annealing up to 383 K yields the best compromise between improving charge-carrier transport and increasing defect densities.
The thermal stability of thick (≈4 μm) plasma-grown hydrogenated amorphous silicon (a-Si:H) layers on glass upon application of a rather rapid annealing step is investigated. Such films are of interest as precursor layers for laser liquid-phase crystallized silicon solar cells. However, at least half-day annealing at T ≈550 °C is considered to be necessary so far to reduce the hydrogen (H) content and thus avoid blistering and peeling during the crystallization process due to H. By varying the deposition conditions of a-Si:H, layers of rather different thermal stability are fabricated. Changes in the surface morphology of these a-Si:H layers are investigated using scanning electron microscopy and profilometry measurements. Hydrogen effusion, secondary-ion mass spectrometry (SIMS) depth profiling, and Raman spectroscopy measurements are also carried out. In summary, amorphous silicon precursor layers are fabricated that can be heated within 30 min to a temperature of 550 °C without peeling and major surface morphological changes. Successful laser liquid-phase crystallization of such material is demonstrated. The physical nature of a-Si:H material stability/instability upon application of rapid heating is studied.
Double layers of deuterated and hydrogenated amorphous silicon (a-Si:H) on glass are heated in the ambient by scanning with a green (532 nm) continuous wave laser. The hydrogen diffusion length in the laser spot is obtained from the deuterium (D)–hydrogen (H) interdiffusion measured by secondary ion mass spectrometry (SIMS), the temperature in the laser spot is obtained by calculation. Under certain conditions, detachment of the deuterated layer from the hydrogenated layer is observed in the SIMS depth profiles, visible by rising oxygen and carbon signals at the D/H interface attributed to in-diffusion of atmospheric gas species like water vapor, oxygen, and carbon oxide. Stacks involving both undoped and boron-doped a-Si:H films show disintegration. The results suggest that the parameters leading to the disintegration effects are the presence of a plane of reduced material cohesion at the D/H interface, a sizeable H diffusion length and a rather high heating rate. Herein, it is likely considered that the observed layer disintegration process is involved in the peeling of a-Si:H films upon fast heating. Furthermore, the results show that rapid laser heating can be used to detect planes of reduced material cohesion which may compromise the electronic properties of a-Si:H-based stacks.
The application of thin underdense hydrogenated amorphous silicon (a-Si:H) films for passivation of crystalline Si (c-Si) by avoiding epitaxy in silicon heterojunction (SHJ) solar cell technology has recently been proposed and successfully applied. Herein, the microstructure of such underdense a-Si:H films, as used in our silicon heterojunction solar cell baseline, is investigated mainly by Raman spectroscopy, effusion, and secondary ion mass spectrometry. In H effusion experiments, a low-temperature (near 400 °C) effusion peak which has been attributed to the diffusion of molecular H2 through a void network is seen. The dependence of the H effusion peaks on film thickness is similar as observed previously for void rich, low substrate-temperature a-Si:H material. Solar cells using underdense a-Si:H as i1-layer with a maximum efficiency of 24.1% are produced. The passivation quality of the solar cells saturates with increasing i1-layer thickness. The fact that with such underdense material combined with a following high-quality i2-layer, instead of only high-quality a-Si:H with a low defect density direct on the c-Si substrate, good passivation of c-Si solar cells is achieved, which demonstrates that in the passivation process, molecular hydrogen plays an important role.
In silicon heterojunction solar cell technology, thin layers of hydrogenated amorphous silicon (a-Si:H) are applied as passivating contacts to the crystalline silicon (c-Si) wafer. Thus, the properties of the a-Si:H is crucial for the performance of the solar cells. One important property of a-Si:H is its microstructure which can be characterized by the microstructure parameter R based on Si─H bond stretching vibrations. A common method to determine R is Fourier transform infrared (FTIR) absorption measurement which, however, is difficult to perform on solar cells for various reasons like the use of textured Si wafers and the presence of conducting oxide contact layers. Here, it is demonstrated that Raman spectroscopy is suitable to determine the microstructure of bulk a-Si:H layers of 10 nm or less on textured c-Si underneath indium tin oxide as conducting oxide. A detailed comparison of FTIR and Raman spectra is performed and significant differences in the microstructure parameter are obtained by both methods with decreasing a-Si:H film thickness.
For application as precursor layers for silicon solar cells fabricated by laser liquid phase crystallization, thick amorphous silicon films on glass are of interest. However, for hydrogenated amorphous silicon (a-Si:H) precursor layers containing about 10 at.% hydrogen, hydrogen needs to be removed prior to liquid phase crystallization to avoid bubble formation and peeling. For this purpose, an at least 12 hours annealing procedure up to 550°C is considered necessary thus involving long process time and high costs. In this article, we investigate the use of thick hot wire grown a-Si:H films which turn out to need considerably less time for dehydrogenation than dense plasma-grown a-Si:H. The dehydrogenation process is studied by depth profiles of hydrogen concentration and medium range order (MRO) using Raman spectroscopy analysis at etch pits. The results show already at an annealing temperature of 450°C the disappearance of all detectable H in the substrate-near part and the complete removal of H at 550°C after about 4 hours annealing. We attribute this rather fast hydrogen removal to the formation of interconnected voids primarily in the substrate-near range. In the same range of the film, we find a correlation between hydrogen concentration and medium range order suggesting that a silicon network reconstruction due to hydrogen out-diffusion causes an observed decrease of reciprocal MRO. The results stress the importance of void-related microstructure in the a-Si:H for hydrogen removal at a rather low annealing temperature and short annealing time. Our results suggest that hot wire a-Si:H films which can be grown with a high deposition rate and a rather pronounced void-related microstructure may be well suited as economic precursor layers.
Liquid silanes can be used for low-cost, fast deposition of hydrogenated amorphous silicon (a-Si:H) as an alternative to state-of-the-art deposition processes such as plasma enhanced chemical vapor deposition or electron beam evaporation. However, liquid silane deposition techniques are still in their infancy. In this paper, we present a new version of the atmospheric pressure chemical vapor deposition technique designed to improve the reproducibility of a-Si:H deposition. With this new tool, we explore ways to improve the quality of the material. The films can be prepared using pure trisilane as a precursor; frequently, however, trisilane is diluted with cyclooctane for better handling and process control. Currently, the influence of this dilution on the film quality is not well understood. In our work, we investigate and compare both precursor strategies. This paper presents a comprehensive analysis of the effects of cyclooctane dilution, deposition temperature, process duration, and precursor amount on the structure stoichiometry and electronic properties of the resulting films. The analysis was performed using a range of techniques, including Fourier transform infrared spectroscopy, electronic spin resonance spectroscopy, Raman spectroscopy, ellipsometry, secondary ion mass spectrometry, and conductivity measurements. For films deposited with pure silane, we found a low oxygen (O) and carbon (C) impurity incorporation and an adjustable H content up to 10%, resulting in a photosensitivity of up to 104. Dependent on the dilution and deposition temperature, the films deposited with cyclooctane dilution showed various amounts of C incorporation, culminating in an a-Si:H/a-SiC:H structure for high temperatures and dilutions. High purity a-Si:H films as a-Si:C:H films are promising for application in solar cells and transistors either as an amorphous functional layer or as a precursor for recrystallization processes, e.g., in TOPCon solar cell technology.
Synthesis and Characterization of Nitrogen-Doped Crystallized SiC Films from Liquid Precursors
(2025)
Silicon carbide (SiC) is an established material for photovoltaics and other semiconductor devices due to its wide band gap and high thermal stability. Traditional deposition systems for thin, doped SiC layers are often costly and complex. This study investigates the use of 1,4-disilabutane as a low-cost liquid precursor with a rather low decomposition temperature for the deposition of hydrogenated amorphous silicon carbide (a-SiC:H) films at atmospheric pressure. Nitrogen doping was achieved using 1,1,3,3-tetramethyldisilazane. The films were characterized by Fourier-transform infrared spectroscopy, Raman spectroscopy, secondary ion mass spectrometry, and conductivity measurements. Optimizing the deposition temperature maximized the Si–C bond density. Crystallization was induced by annealing at temperatures between 800 and 1100 °C, resulting in a three-order-of-magnitude increase in conductivity. The highest conductivity achieved was 0.03 S cm–1 for crystalline, N-doped SiC films. This cost-effective method for producing highly conductive, crystalline SiC films offers significant potential for industrial applications.
