B. Rech

Absorption loss at nanorough silver back reflector of thin-film silicon solar cells

Absorption losses at a nanorough silver back reflector of a solar cell were measured with high accuracy by photothermal deflection spectroscopy. Roughness was characterized by atomic force microscopy. The observed increase of absorption, compared to the smooth silver, was explained by the surface plasmon absorption. Two series of silver back reflectors (one covered with thin ZnO layer) were investigated and their absorption related to surface morphology.

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.

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.

Highly efficient microcrystalline silicon solar cells deposited from a pure SiH4 flow

A time-resolved optical emission spectroscopic study identified transient behavior of the excited SiH emission in a parallel plate SiH4∕H2 plasma. The transient behavior could be prevented by filling the background gas with H2 prior to plasma ignition. Applying this condition, state-of-the-art microcrystalline silicon (μc-Si:H) could be deposited irrespective of the applied H2 flow, ultimately demonstrated by a 9.5% efficient solar cell deposited from pure SiH4. The results are discussed in terms of SiH4 back diffusion: an initial diffusion flux of SiH4 from the reactor’s dead volume back into the plasma.

Recent developments of silicon thin film solar cells on glass substrates

Abstract Among various semiconductor technologies applied in photovoltaics, thin-film technologies offer several attractive features, both technically and economically. Recent developments of silicon thin-film devices predominantly based on amorphous silicon (a-Si), are described. Particular emphasis is placed on the importance of the substrates that consist of glass coated with a transparent conductive oxide (TCO) to serve as front contact layer of the photovoltaic film combination. The electrical conductivity and optical absorption of the TCO substantially affect the energy conversion efficiency of solar modules. An essential enhancement of the light absorption within the photoactive a-Si layer is derived from a surface texture of the TCO, that provides index-grading for lower front reflection, and light scattering to result in light trapping due to total internal reflections within the a-Si. The requirements for optimal TCO properties are reviewed in the context of their availability. Additionally, novel cell structures involving microcrystalline silicon, are expected to lead to higher efficiencies. Thin-film technologies, led by those based on a-Si, will take on an increased role in module production worldwide, which is consistent with favorable manufacturing cost expectations.

Solution of the ZnO/p contact problem in a-Si:H solar cells

Abstract This paper addresses the problem of preparing a good p-layer contact with zinc oxide as TCO. Our approach was to deposit pin cells with different p-layer recipes on ZnO coated SnO2:F and on uncoated SnOi2:F in one run, in order to obtain a direct comparison of the interface properties of the two TCO materials under the condition of as equal as possible surface morphology. The pin cells prepared on the ZnO surface exhibit a lower fill factor (FF). Our experiments demonstrate that the hydrogen interaction with the ZnO surface plays the most decisive role for the ZnO/p contact. We explain the observed effects using a band diagram of the ZnO/p interface and show that the accumulation layer at the ZnO surface — caused by atomic hydrogen in the plasma — is responsible for the low FF in pin cells. Based on this model the contact problem is solved by introducing a μc-n-Si intralayer between ZnO and p-layer resulting in an identical high FF on both ZnO and SnO2 substrates.

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.

Oxygen and nitrogen impurities in microcrystalline silicon deposited under optimized conditions: Influence on material properties and solar cell performance

The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (μc-Si:H) has been systematically investigated. Single μc-Si:H layers and complete μc-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of ∼1.2 μm thick μc-Si:H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2×1019 to 2×1019 cm−3; in the case of nitrogen contamination the critical impurity level is lower ([N]critical=6×1018–8×1018 cm−3). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of μc-Si:H films. It was observed that the both contaminant types act as donors. Efficiency losses due to oxygen or nitrogen impurities are attributed to fill factor decreases and to a reduced external quantum efficiency at wavelengths of >500 nm. In the case of an air leak during the μc...

Temperature stability of ZnO:Al film properties for poly-Si thin-film devices

The crystallization of thin silicon films at temperatures between 425 and 600°C was investigated on glass substrates coated with Al-doped zinc oxide (ZnO:Al). Bare ZnO:Al layers degrade at the crystallization temperatures used. A silicon layer on top, however, efficiently prevents deterioration. The resistivity was even found to drop from 4.3×10−4Ωcm for the as deposited ZnO:Al to 2.2×10−4Ωcm in the case of aluminium induced crystallization and to 3.4×10−4Ωcm for solid phase crystallization. The temperature-stable conductivity of ZnO:Al films coated with Si opens up appealing options for the production of polycrystalline silicon thin-film solar cells with transparent front contacts.

Transient depletion of source gases during materials processing: a case study on the plasma deposition of microcrystalline silicon

Transient depletion of source gases can play an important role in materials processing, particularly during the initial phase of thin film synthesis in which nucleation takes place and the interface is formed. In this paper, we present a zero-order analytical model that allows an estimation of the magnitude and timescale of transient depletion. The model is based on a lumped particle balance for a processing region and reactor volume that are coupled via a directive feed gas flow and diffusive transport. To illustrate the model, an experimental case study is presented on transient depletion during the parallel plate radio-frequency SiH4+H2 plasma deposition of microcrystalline silicon for solar cells. The SiH4 steady-state depletion was experimentally determined by mass spectrometry, deposition rate and optical emission spectroscopy measurements. The transient depletion of the SiH4 was monitored by time-resolved optical emission spectroscopy measurements. Model and experiment are in good agreement. The implications for materials processing and thin film synthesis, as well as methods to control transient depletion, are discussed.