Stefan Muthmann

Liquid hydridosilane precursor prepared from cyclopentasilane via sonication at low temperatures without the action of light.

We report on a liquid hydridosilane precursor ink prepared via the ultrasonically induced ring-opening polymerisation of cyclopentasilane (Si5H10) without irradiation by ultraviolet light. The sonication is carried out in N2 atmosphere at temperatures between 20 and 75°C. We use size exclusion chromatography (SEC) to show polymer growth and estimate molecular mass with increasing sonication time. In combination with UV-vis transmission measurements, further SEC analysis is used to compare solutions subjected to either purely thermal or ultrasonic treatment at the same process temperature and for the same duration. Our findings provide strong evidence showing that the initiation of the polymerisation is sonocatalytic in nature and not thermic due to the macroscopic temperature of the solution. The liquid precursor is used to produce homogeneous hydrogenated amorphous silicon (a-Si:H) thin films via spin coating and pyrolytic conversion. The optoelectronic properties of the films are subsequently improved by hydrogen radical treatment. Fourier transform infrared spectroscopy (FTIR) is used to determine a compact film morphology and electrical conductivity measurements show that the layers attain a light-to-dark photosensitivity ratio of 2×103 making them suitable for application in optoelectronic devices.

Structural Order and Staebler–Wronski Effect in Hydrogenated Amorphous Silicon Films and Solar Cells

The structure of hydrogenated amorphous silicon films is investigated by Raman spectroscopy and X-ray diffraction. Raman spectroscopy probes the phonon density of states, whereas X-ray diffraction measures the distribution of the electron density. Yet, both methods can yield information on the microstructure of the material represented by certain parameters like, e.g., the position or the width of the transverse optical phonon or the width of the first scattering peak. Interdependences between these parameters are investigated and evaluated. A correlation was found between the structural disorder and the relative efficiency loss caused by the Staebler-Wronski effect for intrinsic films applied as absorbing layers in solar cells. This correlation could be used to estimate the solar cell degradation without time-consuming light-soaking experiments.

New insight into the microstructure and doping of unintentionally n-type microcrystalline silicon carbide

Microcrystalline silicon carbide (μc-SiC:H) deposited by hot wire chemical vapor deposition (HWCVD) and plasma-enhanced chemical vapor deposition (PECVD) provide advantageous opto-electronic properties, making it attractive as a window layer material in silicon thin-film and silicon heterojunction solar cells. However, it is still not clear which electrical transport mechanisms yield dark conductivities up to 10−3 S/cm without the active use of any doping gas and how the transport mechanisms are related to the morphology of μc-SiC:H. To investigate these open questions systematically, we investigated HWCVD and PECVD grown layers that provide a very extensive range of dark conductivity values from 10−12 S/cm to 10−3 S/cm. We found out by secondary ion mass spectrometry measurements that no direct correlation exists between oxygen or nitrogen concentrations and high dark conductivity σd, high charge carrier density n, and low activation energy Ea. Higher σd seems to rise from lower hydrogen concentrations o...

Interplay between crystallinity profiles and the performance of microcrystalline thin-film silicon solar cells studied by in-situ Raman spectroscopy

The intrinsic microcrystalline absorber layer growth in thin-film silicon solar-cells is investigated by in-situ Raman spectroscopy during plasma enhanced chemical vapor deposition. In-situ Raman spectroscopy enables a detailed study of the correlation between the process settings, the evolution of the Raman crystallinity in growth direction, and the photovoltaic parameters η (solar cell conversion efficiency), JSC (short circuit current density), FF (fill factor), and VOC (open circuit voltage). Raman spectra were taken every 7 nm of the absorber layer growth depending on the process settings. The Raman crystallinity of growing microcrystalline silicon was determined with an absolute error of approximately ±5% for total absorber layer thicknesses >50 nm. Due to this high accuracy, inherent drifts of the Raman crystallinity profiles are resolvable for almost the entire absorber layer deposition. For constant process settings and optimized solar cell device efficiency Raman crystallinity increases during t...

In Situ Current Determination of a-Si/μc-Si Tandem Solar Cells via Transmission Measurements During Silicon PECVD

In situ optical transmission measurements performed during thin-film silicon plasma-enhanced chemical vapor deposition (PECVD) are presented. Hereto, the plasma emission was used as light source. With this setup information about thickness, crystallinity and absorption characteristic of the growing intrinsic silicon thin film can be obtained. By integrating the intrinsic layers in solar cells with p-i-n configuration, the layer information gained in situ during the PECVD process can be directly correlated to the generated short-circuit current of the solar cell. The intention of this paper is to show that, by using these transmission measurements for the estimation of solar cell currents, an in situ current matching of stacked a-Si/μc-Si tandem devices is possible, which is a useful extension of the process control techniques.

Highly controlled microcrystalline silicon growth using in-situ Raman spectroscopy

The growth of PV grade microcrystalline silicon using plasma enhanced chemical vapor deposition is realized in a narrow process window. Thus, the stability and the controllability of deposition processes is challenging. Process drifts occur between two different deposition runs or within a single deposition run, which can lead to microcrystalline silicon material that is not optimal for thin-film silicon device applications. In the present work, we use in-situ Raman spectroscopy during silicon deposition to study the growth with high temporal and depth resolution in growth direction. It is shown that with a homogeneous crystallinity profile in growth direction the solar device conversion efficiency was increased from 7.29% to 7.67%. Hence, in this paper it is demonstrated that using in-situ Raman spectroscopy is suitable for highly controlled microcrystalline silicon processing.

In-situ absorption measurements for solar cell current determination during thin-film silicon PECVD

Process control is very important in the fabrication of high quality thin-film silicon solar cells. Solar cell parameters like film thickness, crystalline volume fraction or conductivity are usually measured in the back end of an industrial production line using ex-situ techniques. At the back end of solar module production the most of the money has been spent already and detrimental effects on the system performance like process drifts during the fabrication have not been detected online. Measuring in-situ, during the deposition of the thin silicon layers, utilizes the advantage that the investments in the front end of the process are still at low level. Additionally, the possibility of an active process control and hence the optimization of the solar cell is given, by monitoring process parameters in real time. In recent studies we performed transmission measurements during silicon deposition, in which the plasma emission was used as light source. It was shown, that deposition rate and the crystalline volume fraction of microcrystalline silicon layers and the roughness of ZnO:Al substrates can be detected with high accuracy using only a single optical setup. Additionally, the setup convinces with its simplicity for the use as process control which makes it interesting for the industrial mass production. In this paper we show that using the transmission measurements the absorption characteristic of the growing silicon thin film can be estimated. It can be seen that a direct correlation between the measured absorption of intrinsic absorber layers and the resulting solar cell current is possible.