H. Keppner

Complete microcrystalline p‐i‐n solar cell—Crystalline or amorphous cell behavior?

Complete μc‐Si:H p‐i‐n solar cells have been prepared by the very high frequency glow discharge method. Up to now, intrinsic μc‐Si:H has never attracted much attention as a photovoltaic active material. However, an efficiency of 4.6% and remarkably high short circuit current densities of up to 21.9 mA/cm2 due to an enhanced absorption in the near‐infrared could be obtained. First light‐soaking experiments indicate no degradation for the entirely μc‐Si:H cells. Voltage‐dependent spectral response measurements suggest that the carrier transport in complete μc‐Si:H p‐i‐n cells may possibly be cosupported by diffusion (in addition to drift).

On the Way Towards High Efficiency Thin Film Silicon Solar Cells by the “Micromorph” Concept

Note: IMT-NE Number: 222 Reference PV-LAB-CONF-1996-008 Record created on 2009-02-10, modified on 2017-05-10

Device grade microcrystalline silicon owing to reduced oxygen contamination

As‐deposited undoped microcrystalline silicon (μc‐Si:H) has in general a pronounced n‐type behavior. Such a material is therefore often not appropriate for use in devices, such as p‐i‐n diodes, as an active, absorbing i layer or as channel material for thin‐film transistors. In recent work, on p‐i‐n solar cells, this disturbing n‐type character had been successfully compensated by the ‘‘microdoping’’ technique. In the present letter, it is shown that this n‐type behavior is mainly linked to oxygen impurities; therefore, one can replace the technologically delicate microdoping technique by a purification method, that is much easier to handle. This results in a reduction of oxygen impurities by two orders of magnitude; it has, furthermore a pronounced impact on the electrical properties of μc‐Si:H films and on device performance, as well. Additionally, these results prove that the unwanted donor‐like states within μc‐Si:H are mainly due to extrinsic impurities and not to structural native defects.

Towards high-efficiency thin-film silicon solar cells with the “micromorph” concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as “micromorph” tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 × 1016/cm3 could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on μc-Si : H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 A/s for microcrystalline silicon could be achieved.

Thin Film Silicon Solar Cells: A Review and Selected Trends

A case is developed for considering silicon as the prime medium-term candidate for semiconductor photovoltaic cells; the reason is based on other materials not being abundantly available, highly toxic, or very expensive. Crystalline silicon solar cells, however, have excellent efficiencies according to the data presented. Thus, thin-film silicon solar cells are considered as the main point for large-scale energy applications in the future. Further, novel approaches to further improve the stable efficiencies of the cells are also listed.

Preparation of pyrite films by plasma‐assisted sulfurization of thin iron films

Pyrite films were prepared using the pure elements as source materials: thin iron films were evaporated on quartz substrates and exposed to a sulfur plasma. The process was controlled by a transmission measurement. X‐ray spectroscopy was used to characterize the films and preliminary optical and electrical measurements were carried out.

Potential of VHF-Plasmas for Low-Cost Production of a-Si:H Solar Cells

Abstract Compared to the use of the standard glow discharge technique the production of amorphous silicon solar cells by the very high frequency glow discharge (VHF-GD) bears yet additional cost reduction potentials: Using VHF-GD at excitation frequencies higher than 13.56 MHz, a more efficient dissociation of silane gas is obtained; thus, higher deposition rates are achieved; this reduces considerably the deposition time for intrinsic amorphous and microcrystalline silicon layers. Furthermore, by itself and even more so, in combination with argon dilution, VHF-GD technique improves silane utilisation and leads, thus, to further cost reduction. Finally, by combining the VHF-GD technique and the “micromorph” concept “real” tandem cells (i.e. a superposition of two cells with distinctly different band gaps) can be deposited at low temperatures without the use of expensive germane gas.

Decorating Parylene-Coated Glass with ZnO Nanoparticles for Antibacterial Applications: A Comparative Study of Sonochemical, Microwave, and Microwave-Plasma Coating Routes

A glass substrate, coated with a Parylene film, was coated with ZnO by three different methods: ultrasound, microwave, and microwave-plasma irradiation. These coating modes are simple, efficient, and environmentally friendly one-step processes. The structure of the coated products was characterized and compared using methods such as XRD, HR-SEM, EDS, RBS, and optical spectroscopy. Coating by ZnO nanoparticles was achieved for all three approaches. The products were found to differ in their particle sizes, coating thickness, and depth of penetration. All of the ZnO-Parylene-glass composites demonstrated a significant antibacterial activity against Escherichia coli (Gram negative) and Staphylococcus aureus (Gram positive) strains.

Origins of atmospheric contamination in amorphous silicon prepared by very high frequency (70 MHz) glow discharge

The authors have studied the effect of plasma power, reactor outgassing rates, and of silane purity on the oxygen, carbon, and nitrogen contents of amorphous silicon material prepared by the very high frequency (70 MHz) glow discharge technique. The silane purity could be optionally enhanced by the application of a getter‐based silane gas purifier. It was found that oxygen incorporation was enhanced at lower deposition rates, whereas the nitrogen and carbon film contamination were unaffected. The deposition rate dependence of the incorporation is in excellent agreement with a proposed model. Apart from the effects of plasma power on the incorporation probability, the reactor outgassing rate and the purity of the silane gas itself were identified as the main contamination sources for the atmospheric contaminants in the deposited films. At the low outgassing rate, at least around one‐half of the oxygen detected in the a‐Si:H material originates from the silane gas. Due to the reduced outgassing rate and an ...

Structural properties and electronic transport in intrinsic microcrystalline silicon deposited by the VHF-GD technique

A series of microcrystalline samples was deposited by the very high frequency glow discharge (VHF-GD) technique, with various input powers while keeping all the other parameters of deposition constant. The goal was to correlate transport and structural properties and avoid as much as possible the problem of a variation of the Fermi level between the samples. The observed decrease of the photoconductivity and of the product mobility-lifetime of hole (as measured by time of flight, TOF) with the increase of the power was surprisingly not connected to the structural properties, which remain approximately unchanged, but with a surface contribution to the transport properties.