Oleksandr Astakhov

Performance of p- and n-side illuminated microcrystalline silicon solar cells following 2 MeV electron bombardment

The impact of defects on the performance of p- and n-side illuminated microcrystalline silicon solar cells is investigated. The absorber layer spin density NS is controlled over some two orders of magnitude by electron bombardment and subsequent annealing steps. At increased NS (between 3 × 1016 and 1018 cm−3), performance of n-side illuminated cells is much more strongly reduced relative to p-side illuminated cells, particularly with regard to short circuit current density. Quantum efficiency measurements indicate a corresponding strong asymmetry in wavelength-dependence, which has been successfully reproduced by numerical device simulations.

Silicon Thin Film Powder Samples for Electron Spin Resonance Investigation: Role of Substrate and Preparation Procedure

Different methods to prepare powder samples of amorphous and microcrystalline thin film silicon material deposited on glass and metal substrates for application in electron spin resonance (ESR) experiments are compared. The powder is needed to provide sufficient sample mass for ESR experiments which otherwise suffer from low signal-to-noise ratio for thin film silicon material deposited on glass with thickness of typically 1 µm and spin densities of less than 1016 cm-3, all in view of the geometrical constraints of ESR cavities. As temporary substrates, Al and Mo foils and glass covered with ZnO are used and we compare the influences of the different substrates, the removal procedure and the exposure time to the etchant or ambient on the ESR properties of the silicon films. We describe reliable sample preparation and annealing procedures under which all three temporary substrates yield very similar ESR properties for the different silicon materials.

Variation in Absorber Layer Defect Density in Amorphous and Microcrystalline Silicon Thin Film Solar Cells with 2 MeV Electron Bombardment

The effect of the defect density in hydrogenated amorphous and microcrystalline silicon (a-Si:H and µc-Si:H) absorber layers on the performance of thin film solar cells was investigated. The defect density was varied reproducibly over more than two orders of magnitude by 2 MeV electron bombardment and subsequent thermal annealing. Considerable quantitative and qualitative differences were observed for the dependences of the cells parameters on the defect densities of a-Si:H and µc-Si:H. The experimental data suggest further possible improvement of µc-Si:H based solar cells with further reduced defect densities, while for a-Si:H based solar cells, a saturation of performance is observed below a defect density of about 1016 cm-3. Moreover, the experimental data provide an excellent database for numerical simulation over a range unavailable so far particularly in µc-Si:H based solar cells.

Multi-frequency EDMR applied to microcrystalline thin-film silicon solar cells

Pulsed multi-frequency electrically detected magnetic resonance (EDMR) at X-, Q- and W-Band (9.7, 34, and 94GHz) was applied to investigate paramagnetic centers in microcrystalline silicon thin-film solar cells under illumination. The EDMR spectra are decomposed into resonances of conduction band tail states (e states) and phosphorus donor states (P states) from the amorphous layer and localized states near the conduction band (CE states) in the microcrystalline layer. The e resonance has a symmetric profile at all three frequencies, whereas the CE resonance reveals an asymmetry especially at W-band. This is suggested to be due to a size distribution of Si crystallites in the microcrystalline material. A gain in spectral resolution for the e and CE resonances at high fields and frequencies demonstrates the advantages of high-field EDMR for investigating devices of disordered Si. The microwave frequency independence of the EDMR spectra indicates that a spin-dependent process independent of thermal spin-polarization is responsible for the EDMR signals observed at X-, Q- and W-band.

Thin-film silicon solar cells fabricated at low temperature: A versatile technology for application on transparent flexible plastic substrates and in integrated photoelectrochemical water splitting modules

Amorphous silicon (a-Si:H) solar cells in p-i-n configuration were developed at a low deposition temperature of 140 °C, suitable for application on transparent flexible plastic substrates. Deteriorated electronic properties of the p-layer with decreasing temperature were identified as the main reason for reduced solar cell performance. Optimization of the p-layer properties resulted in an efficiency of 8.2 % for a solar cell fabricated entirely at 140 °C. As a parallel application scenario, a-Si:H/a-Si:H tandem solar cells are designed for application in integrated photoelectrochemical water splitting modules. Here we benefit from the increased open circuit voltages with values around 1.9 V which provides ample margin for possible overpotential losses in water splitting modules.

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...

The electrically detected magnetic resonance microscope: Combining conductive atomic force microscopy with electrically detected magnetic resonance

We present the design and implementation of a scanning probe microscope, which combines electrically detected magnetic resonance (EDMR) and (photo-)conductive atomic force microscopy ((p)cAFM). The integration of a 3-loop 2-gap X-band microwave resonator into an AFM allows the use of conductive AFM tips as a movable contact for EDMR experiments. The optical readout of the AFM cantilever is based on an infrared laser to avoid disturbances of current measurements by absorption of straylight of the detection laser. Using amorphous silicon thin film samples with varying defect densities, the capability to detect a spatial EDMR contrast is demonstrated. Resonant current changes as low as 20 fA can be detected, allowing the method to realize a spin sensitivity of 8×10(6)spins/√Hz at room temperature.

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...

The electrically detected magnetic resonance microscope

We present the design and implementation of a scanning probe microscope, which combines electrically detected magnetic resonance (EDMR) and (photo-)conductive atomic force microscopy ((p)cAFM). The integration of a 3-loop 2-gap X-band microwave resonator into an AFM allows the use of conductive AFM tips as a movable contact for EDMR experiments. The optical readout of the AFM cantilever is based on an infrared laser to avoid disturbances of current measurements by absorption of straylight of the detection laser. Using amorphous silicon thin film samples with varying defect densities, the capability to detect a spatial EDMR contrast is demonstrated. Resonant current changes as low as 20 fA can be detected, allowing the method to realize a spin sensitivity of 8×10(6)spins/√Hz at room temperature.

The relationship between hydrogen and paramagnetic defects in thin film silicon irradiated with 2 MeV electrons

After irradiation of hydrogenated amorphous and microcrystalline silicon (a-Si:H and μc-Si:H) with 2 MeV electrons at 100 K, we observe satellite-like components close to the dominating electron spin resonance (ESR) signal of these materials. The satellites overlap with the commonly observed dangling bond resonance and are proposed to originate from a hyperfine interaction with the nuclear magnetic moment of hydrogen atoms in a-Si:H and μc-Si:H. Our present study is focused on the verification of this hypothesis. Equivalent hydrogenated and deuterated a-/μc-Si:H/D materials have been investigated with ESR before and after 2 MeV electron bombardment. From the difference between ESR spectra of hydrogenated and deuterated samples we identify the doublet structure in the ESR spectra as a hyperfine pattern of hydrogen-related paramagnetic centers. The observations of H-related paramagnetic centers in a-/μc-Si:H are of particular interest in view of metastability models of a-Si:H, which include H-related complexes as precursors for the stabilization of the metastable Si dangling bonds. The nature of the observed center is discussed in the light of known H-related complexes in crystalline Si and suggested H-related dangling bonds in a-Si:H.