Gunnar Schöpe

Texture etched ZnO:Al coated glass substrates for silicon based thin film solar cells

ZnO:Al films were r.f.- and d.c.-magnetron sputtered on glass substrates from ceramic (ZnO:Al2O3) and metallic (Zn:Al) targets, respectively. The initially smooth films exhibit high transparencies (T≥83% for visible light including all reflection losses) and excellent electrical properties (ρ=2.7–6×10−4 Ω cm). Depending on their structural properties these films develop different surface textures upon post deposition etching in diluted HCl. The light scattering properties of suitable films can be controlled over a wide range simply by varying the etching time. Moreover, the electrical properties are not affected by the etching process. Thus, within certain limits a separate optimization of the electro-optical and light scattering properties is possible. Amorphous silicon (a-Si:H) based solar cells prepared on these new texture etched ZnO-substrates show high quantum efficiencies in the long wavelength range demonstrating an effective light trapping. First a-Si/a-Si stacked solar cells were realized with initial efficiencies exceeding 10%.

The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells

This study addresses the material properties of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films and their application as front contacts in silicon thin-film solar cells. Optimized films exhibit high conductivity and transparency, as well as a surface topography with adapted light-scattering properties to induce efficient light trapping in silicon thin-film solar cells. We investigated the influence on the ZnO:Al properties of the amount of alumina in the target as well as the substrate temperature during sputter deposition. The alumina content in the target influences the carrier concentration leading to different conductivity and free carrier absorption in the near infrared. Additionally, a distinct influence on the film growth of the ZnO:Al layer was found. The latter affects the surface topography which develops during wet-chemical etching in diluted hydrochloric acid. Depending on alumina content in the target and heater temperature, three different regimes of etching behavior have been i...

Modified Thornton model for magnetron sputtered zinc oxide: film structure and etching behaviour

ZnO:Al films were prepared on glass substrates with different sputter techniques from ceramic ZnO:Al2O3 target as well as metallic Zn:Al targets using a wide range of deposition parameters. Independent of the sputter technique, sputter pressure and substrate temperature were found to have a major influence on the electrical and structural properties of the ZnO:Al films. With an increasing deposition pressure, we observed a strong decrease in the carrier mobility and also an increase of the etching rate. The surface morphology obtained after etching of RF sputtered ZnO:Al systematically changes from crater-like to hill-like surface appearance with increasing pressure. The correlation of sputter parameters, film growth and structural properties is discussed in terms of a modified Thornton model.

Optimization of the electrical properties of magnetron sputtered aluminum-doped zinc oxide films for opto-electronic applications

Abstract Magnetron sputtered ZnO:Al films are promising candidates as front electrode in a variety of opto-electronic devices. Here we report on efforts to obtain highly conductive and transparent ZnO:Al films using different deposition conditions for RF, DC and MF (mid frequency) sputtering. Investigations were made to see the effect of target doping concentration (TDC), film thickness, sputter pressure and deposition temperature. RF sputtering from ceramic targets yields low resistivities between 3 and 5×10−4 Ω cm for target doping concentrations between 4 and 0.5%. With decreasing TDC to 0.5% carrier mobilities up to 44 cm2/Vs were obtained, accompanied by the extension of the region of high transmission to the near infrared, due to a reduction in free carrier absorption and corresponding shift in plasma wavelength. DC and MF sputtering from metallic targets yielded similar low resistivities at deposition rates up to 200 nm/min. An analysis of mobility (μ) data of all films as function of the corresponding carrier densities (N) showed that the μ–N values obtained in this study are in the vicinity to limits suggested in the literature.

Development of highly efficient thin film silicon solar cells on texture-etched zinc oxide-coated glass substrates

Abstract ZnO films prepared by magnetron sputtering on glass substrates and textured by post-deposition chemical etching are applied as substrates for p–i–n solar cells. Using both rf and dc sputtering, similar surface textures can be achieved upon etching. Excellent light trapping is demonstrated by high quantum efficiencies at long wavelengths for microcrystalline silicon solar cells. Applying an optimized microcrystalline/amorphous p-layer design, stacked solar cells with amorphous silicon top cells yield similarly high stabilized efficiencies on ZnO as on state-of-the-art SnO 2 (9.2% for a-Si/a-Si). The efficiencies are significantly higher than on SnO 2 -coated float glass as used for module production.

State-of-the-art mid-frequency sputtered ZnO films for thin film silicon solar cells and modules

This article reports on the use of ZnO films in silicon thin film p-i-n solar cells and modules. It summarizes the status in the final phase of a joint research project aiming at the development of high-quality ZnO/glass substrates feasible for an industrial solar module production. The samples were prepared by reactive mid-frequency (mf) sputtering on large area (60×100 cm 2 ) glass sheets using low-cost metallic Zn:Al targets. These ZnO films exhibit resistivities down to 2.6×10 -4 Ω cm and high optical transmittance. Variation of the sputter pressure leads to films with significantly differing etching behavior in diluted acids. ZnO layers prepared in the high pressure regime develop strongly textured light scattering surfaces after etching, which is necessary to obtain highly efficient solar cells. Initial efficiencies of small area amorphous silicon (a-Si:H) cells on texture-etched ZnO-films prepared by mf-sputtering on 60×100 cm 2 soda-lime glass (3 mm thick) range from 8 to 9% (highest efficiency 9.2%, i-layer thickness 350 nm). First 0.6 m 2 modules on ZnO prove the principal applicability of the films for an industrial manufacturing process.

Upscaling of texture-etched zinc oxide substrates for silicon thin film solar cells

Abstract Large area (320×400 mm2) glass/ZnO-films were prepared by high-rate d.c. magnetron sputtering from ceramic targets and compared to lab-type r.f.- and m.f.-sputtered ZnO. The very uniform and initially smooth films exhibit excellent electrical and optical properties (resistivity ≤5×10−4 Ωcm, transmission >80% for visible light and 1500-nm thick films). Upon etching in diluted hydrochloric acid they develop a surface texture. Independent of sputter technique (d.c. or r.f.) and substrate size, higher substrate temperatures and lower sputter gas pressures have a similar influence on the film structure and lead to more robust and etch-resistant films. Showing excellent light scattering properties, amorphous silicon pin solar cells prepared on these large area glass/ZnO samples exhibit initial efficiencies up to 9.2%, proving the viability of sputtered and texture-etched ZnO as TCO-substrate for industrial solar module production.

Influence of the laser parameters on the patterning quality of thin-film silicon modules

An analysis of the monolithical series connection of silicon thin-film modules with metal back contact fabricated by high speed laser ablation will be presented. Optically pumped solid state lasers with wavelengths of 1064 nm and 532 nm were used for the patterning steps. The near infrared laser is applied to pattern the TCO (P1) while the green laser is used for the ablation of the silicon layer stack (P2) and the back contact layer stack (P3). The influence of various laser parameters on the performance of amorphous and microcrystalline silicon modules was studied. In particular the back contact patterning and the Si removal can significantly affect the module efficiency. Non-optimized patterning conditions for P2 can lead to a high contact resistance, while the ablation of the ZnO/Ag back contact system can introduce shunts at the laser scribed line. Therefore, a criterion for flakeless patterning will be briefly introduced and the influence of flakeless back contact patterning on the electrical behavior of silicon single junction cells will be discussed.