Flexible amorphous and microcrystalline silicon tandem solar modules in the temporary superstrate concept

Encapsulated and series-connected amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) based thin film silicon solar modules were developed in the superstrate configuration using an aluminum foil as temporary substrate during processing and a commodity polymer as permanent substrate in the finished module. For the development of μc-Si:H single junction modules, aspects regarding TCO conductivity, TCO reduction, deposition uniformity, substrate temperature stability and surface morphology were addressed. It was established that on sharp TCO morphologies where single junction μc-Si:H solar cells fail, tandem structures consisting of an a-Si:H top cell and a μc-Si:H bottom cell can still show a good performance. Initial aperture area efficiencies of 8.2%, 3.9% and 9.4% were obtained for fully encapsulated amorphous silicon (a-Si:H) single junction, microcrystalline silicon (μc-Si:H) single junction and a-Si:H/μc-Si:H tandem junction modules, respectively.     

Vertically integrated amorphous silicon color sensor arrays

Large-area color sensor arrays based on vertically integrated thin-film sensors were realized. The complete color information of each color pixel is detected at the same position of the sensor array without using optical filters. Sensor arrays consist of amorphous silicon thin-film color sensors integrated on top of amorphous silicon readout transistors. The spectral sensitivity of the sensors is controlled by the applied bias voltage. The operating principle of the color sensor arrays and the influence of device design on spectral sensitivity are described. Furthermore, the image quality of the sensor arrays is analyzed by measurements of the line spread function and the modulation transfer function.     

Ambipolar microcrystalline silicon thin-film transistors

Hydrogenated microcrystalline silicon (µc-Si:H) has recently received significant attention as a promising material for thin-film transistors (TFTs) in large area electronics due to its high electron and hole charge carrier mobilities. We report on ambipolar TFTs based on microcrystalline silicon prepared by plasma-enhanced chemical vapor deposition at temperature of 160 °C with high electron and hole charge carrier mobilities of 40 cm²/Vs and 10 cm²/Vs, respectively. The ambipolar microcrystalline silicon TFTs provide a simple route in realizing large area integrated circuits at low cost. The electrical characteristics of the ambipolar microcrystalline silicon TFTs will be described and the first results on ambipolar inverters will be presented. The influence of the ambipolar TFT characteristics on the performance of the inverter will be also discussed.     

Silicon-based micro-Fourier spectrometer

A novel Fourier spectrometer based on a partly transparent thin-film detector in combination with a tunable silicon micromachined mirror was developed. The operation principle based on the detection of an intensity profile of a standing-wave by introducing a partly transparent detector in the standing-wave. Varying the position of the mirror results in a phase shift of the standing-wave and thus in a change of the optical intensity profile within the detector. The photoelectric active region of the sensor is thinner than the wavelength of the incoming light, so that the modulation of the intensity leads to the modulation of the photocurrent. The spectral information of the incoming light can be determined by the Fourier transform of the sensor signal. Based on the linear arrangement of the sensor and the mirror, the spectrometer facilitates the realization of one- and two-dimensional arrays of spectrometers combining spectral and spatial resolution. The operation principle of the spectrometer will be described and the influence of the detector design on the spectrometer performance will be discussed. A spectral resolution of down to 6 nm was achieved under real-time imaging conditions.     

基于高迁移率微晶硅的薄膜晶体管

近年来,微晶硅(μc-Si:H)被认为是一种制作 TFT 的有前景的材料.采用PECVD法,在低于200℃时制作了微晶硅TFTs,其制作条件类似于非晶态 TFTs.微晶硅 TFTs 器件的迁移率超过了 30 cm2/Vs,而阈值电压是 2.5 V.在长沟道器件(50～200 μm)中观测到了这种高迁移率.但对于短沟道器件(2 μm),迁移率就降低到了7 cm2/Vs.此外,该 TFTs 的阈值电压随着沟道长度的减少而增大.文章采用了一种简单模型解释了迁移率、阈值电压随着沟道长度的缩短而分别减少、增加的原因在于源漏接触电阻的影响.     

Optical properties of silicon-based thin-film solar cells in substrate and superstrate configuration

The optical properties of microcrystalline silicon substrate-type solar cells with a front ZnO thickness of 500–1000 nm as required for monolithic series connection in PV-modules are investigated. The surface texture of the front ZnO was varied to study the possibility to reduce reflection losses and to improve light trapping. Before encapsulation, certain textures of the front ZnO exhibit improved light coupling for substrate solar cells. For substrate solar cells encapsulated with EVA and a glass cover, however, additional texture of the front ZnO has hardly any effect. Encapsulation also removes the antireflection conditions in case that the front ZnO was originally designed as ARC. So far, the quantum efficiency of encapsulated substrate solar cells does not show the high level as observed in superstrate solar cells possibly due to parasitic absorption in the front and back contact and the not optimized texture of the reflector (substrate).     

Applying analytical and numerical methods for the analysis of microcrystalline silicon solar cells

The results of numerical device simulations for p–i–n diodes and the closed-form expression of the current–voltage characteristics developed for p–n diodes are compared. It is shown that the closed-form expression correctly predicts the functional relationship between material parameters and device performance of p–i–n diodes. The ideality factor between 1 and 2 is analyzed in detail. The effect of the defect density, the intrinsic carrier concentration, the mobility and the built-in potential on device performance are demonstrated. These insights are applied to analyze microcrystalline silicon thin-film solar cells deposited by chemical vapor deposition at temperatures below 250 °C.     

More insights from CPM and PDS: Charged and neutral defects in a-Si:H

CPM and PDS spectra of annealed and degraded a-Si:H are analyzed. Numerical simulations of CPM and PDS data using occupation statistics yield information on the energy distribution and the charge state of the defects. The simulations reveal the coexistence of charged and neutral defects resembling the predictions of the defect-pool model. Charged states dominate the defect densities of annealed and degraded a-Si:H. In the case of spatial homogeneous defect densities, different sensitivities of CPM and PDS on charged and neutral defects cause different defect absorptions detected by both methods. Spatially inhomogeneous defect densities caused, e.g. by voids or columnar growth are detected by combining CPM and PDS since PDS detects the total defect density while CPM favors regions with low defect densities.     

Enhanced infrared response of ultra thin amorphous silicon photosensitive devices with Ag nanoparticles

Small silver (Ag) nanoparticles (with diameter smaller than 50 nm) can lead to a resonant light absorption accompanied by an enhanced electromagnetic field in their vicinity when they are irradiated by light due to so-called localized surface plasmon. In order to study the influence of metal nanoparticles in thin-film silicon solar cells in more detail, a photosensitive test structure based on a-Si:H and consisting of a TCO/(nanoparticles)/i/n/TCO layer stack was realized. A higher quantum efficiency of the test structure is achieved for wavelengths longer than 800 nm compared to the structure without nanoparticles. As a-Si:H only efficiently absorbs light for wavelenghts of up to 800 nm, the enhanced photocurrent cannot be explained by improved light absorption in a-Si:H.