Ralf Schmitz

Microcrystalline silicon for large area thin film solar cells

Abstract We present a comprehensive study of microcrystalline silicon (μc-Si:H) solar cells prepared by plasma-enhanced chemical vapour deposition (PECVD) at 13.56 MHz excitation frequency. In the first step the cell development was performed in a small area PECVD reactor showing the relationship between the deposition process parameters and the resulting solar cell performance. Focus was on the influence of deposition pressure, electrode distance and the application of a pulsed plasma on high rate deposition of solar cells. Subsequent up-scaling to a substrate area of 30×30 cm 2 confirmed the suitability of the process for large area reactors. The influence of i-layer deposition parameters on solar cell performance was studied directly in p–i–n cells prepared on textured ZnO. Solar cell efficiencies up to 9% were achieved at deposition rates of 5–6 A/s for the i-layer using high plasma powers. Applied as bottom cell in a-Si:H/μc-Si:H tandem cells a stable cell efficiency of 11.2% could be obtained. The excellent homogeneity was proven by the realization of first modules with an aperture area of 689 cm 2 and an active area initial efficiency of 10.3% (stable: 8.9%) using an established base technology for laser patterning and back contact sputtering at RWE Solar GmbH.

The high-intensity reflectometer of the Jülich Centre for Neutron Science: MARIA

MARIA is a world class vertical sample reflectometer dedicated to the investigation of thin films in the fields of magnetism, soft matter and biology. With the elliptical vertically focusing guide and a wavelength resolution of Δλ/λ = 10%, the non-polarized flux at the sample position amounts to 1.2 × 108u2005nu2005(su2005cm2)−1. Besides the polarized and non-polarized reflectivity mode for specular and off-specular reflectivity measurements, MARIA can also be used to carry out grazing-incidence small-angle neutron scattering investigations.

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.

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.