Florian Köhler

Role of the dopant aluminum for the growth of sputtered ZnO:Al investigated by means of a seed layer concept

This work elucidates the effect of the dopant aluminum on the growth of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films by means of a seed layer concept. Thin (<100 nm), highly doped seed layers and subsequently grown thick (∼800 nm), lowly doped bulk films were deposited using a ZnO:Al2O3 target with 2 wt. % and 1 wt. % Al2O3, respectively. We investigated the effect of bulk and seed layer deposition temperature as well as seed layer thickness on electrical, optical, and structural properties of ZnO:Al films. A reduction of deposition temperature by 100 °C was achieved without deteriorating conductivity, transparency, and etching morphology which renders these low-temperature films applicable as light-scattering front contact for thin-film silicon solar cells. Lowly doped bulk layers on highly doped seed layers showed smaller grains and lower surface roughness than their counterpart without seed layer. We attributed this observation to the beneficial role of the dopant aluminum that induces ...

Hot-wire chemical vapor deposition prepared aluminum doped p-type microcrystalline silicon carbide window layers for thin film silicon solar cells

Al-doped p-type microcrystalline silicon carbide (µc-SiC:H) thin films were deposited by hot-wire chemical vapor deposition at substrate temperatures below 400 °C. Monomethylsilane (MMS) highly diluted in hydrogen was used as the SiC source in favor of SiC deposition in a stoichiometric form. Aluminum (Al) introduced from trimethylaluminum (TMAl) was used as the p-type dopant. The material property of Al-doped p-type µc-SiC:H thin films deposited with different deposition pressure and filament temperature was investigated in this work. Such µc-SiC:H material is of mainly cubic (3C) SiC polytype. For certain conditions, like high deposition pressure and high filament temperature, additional hexagonal phase and/or stacking faults can be observed. P-type µc-SiC:H thin films with optical band gap E04 ranging from 2.0 to 2.8 eV and dark conductivity ranging from 10−5 to 0.1 S/cm can be prepared. Such transparent and conductive p-type µc-SiC:H thin films were applied in thin film silicon solar cells as the window layer, resulting in an improved quantum efficiency at wavelengths below 480 nm.

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...

Role of oxygen and nitrogen in n-type microcrystalline silicon carbide grown by hot wire chemical vapor deposition

N-type microcrystalline silicon carbide (μc-SiC:H(n)) deposited by hot wire chemical vapor deposition provides advantageous opto-electronic properties for window layer material in silicon-based thin-film solar cells and silicon heterojunction solar cells. So far, it is known that the dark conductivity (σd) increases with the increase in the crystallinity of μc-SiC:H(n)films. However, due to the fact that no active doping source is used, the mechanism of electrical transport in these films is still under debate. It is suggested that unintentional doping by atmospheric oxygen (O) or nitrogen (N) contamination plays an important role in the electrical transport. To investigate the impact of O and N, we incorporated O and N in μc-SiC:H(n) films and compared the influence on the microstructural, electronic, and optical properties. We discovered that, in addition to increasing the crystallinity, it is also possible to increase the σd by several orders of magnitude by increasing the O-concentration or the N-conc...

Highly transparent and conductive p-type microcrystalline silicon carbide window layers for thin film silicon solar cells

Transparent and conductive microcrystalline silicon carbide (μc-SiC:H) thin films are an excellent window layer for thin film solar cells. For amorphous silicon based solar cells, p-type conductive μc-SiC:H window layers were deposited by the hot-wire chemical vapor deposition (HWCVD) technique. Monomethylsilane (MMS) highly diluted in hydrogen was used as the SiC source in favor of SiC deposition in a stoichiometric form. Aluminum (Al) introduced from Trimethylaluminum (TMAl) was used as the p-type dopant. In this report, the optoelectronic properties of p-type μc-SiC:H thin films prepared with different deposition pressure and filament temperature were investigated. By managing the deposition parameters, materials with optical gap E04 ranging from 2.0 eV to 2.8 eV and dark conductivity ranging from 10-5 S/cm to 0.1 S/cm were prepared. Such p-type μc-SiC:H thin films were applied as the window layer in amorphous silicon thin film silicon solar cells. Taking advantage of the high transparency of μc-SiC:H window layer, improved quantum efficiency was obtained at the short wavelength below 500 nm.