Stefan Haas

Electrical detection of electron spin resonance in microcrystalline silicon pin solar cells

Pulsed electrically detected magnetic resonance (pEDMR) was employed to study spin-dependent processes that influence charge transport in microcrystalline silicon (µc-Si:H) pin solar cells. Special emphasis was put on the identification of the signals with respect to the individual layers of the cell structure. To do this, we systematically modulated the morphology of the highly doped n- and p-layers from amorphous to microcrystalline. By combining the information obtained from low-temperature (T = 10 K) pEDMR spectra and from the deconvoluted time evolution of spectrally overlapping resonances, we found signals from conduction band tail states as well as phosphorus donor states in samples containing an amorphous n-type layer and a resonance associated with valence band tail states in samples with an amorphous p-layer. Moreover, several signals from the intrinsic microcrystalline absorber layers could be identified. An additional resonance at g = 1.9675(5), which has not been observed in EDMR before, was found. We assign this signal to shallow donors in the Al-doped ZnO layer, which is commonly used as transparent conducting oxide in thin-film solar cells. The experimental findings are discussed in the light of various spin-dependent transport mechanisms known to occur in the respective layers of the pin structure.

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.

Temperature and hydrogen diffusion length in hydrogenated amorphous silicon films on glass while scanning with a continuous wave laser at 532 nm wavelength

Rapid thermal annealing by, e.g., laser scanning of hydrogenated amorphous silicon (a-Si:H) films is of interest for device improvement and for development of new device structures for solar cell and large area display application. For well controlled annealing of such multilayers, precise knowledge of temperature and/or hydrogen diffusion length in the heated material is required but unavailable so far. In this study, we explore the use of deuterium (D) and hydrogen (H) interdiffusion during laser scanning (employing a continuous wave laser at 532 nm wavelength) to characterize both quantities. The evaluation of temperature from hydrogen diffusion data requires knowledge of the high temperature (T > 500 °C) deuterium-hydrogen (D-H) interdiffusion Arrhenius parameters for which, however, no experimental data exist. Using data based on recent model considerations, we find for laser scanning of single films on glass substrates a broad scale agreement with experimental temperature data obtained by measuring the silicon melting point and with calculated data using a physical model as well as published work. Since D-H interdiffusion measures hydrogen diffusion length and temperature within the silicon films by a memory effect, the method is capable of determining both quantities precisely also in multilayer structures, as is demonstrated for films underneath metal contacts. Several applications are discussed. Employing literature data of laser-induced temperature rise, laser scanning is used to measure the H diffusion coefficient at T > 500 °C in a-Si:H. The model-based high temperature hydrogen diffusion parameters are confirmed with important implications for the understanding of hydrogen diffusion in the amorphous silicon material.Rapid thermal annealing by, e.g., laser scanning of hydrogenated amorphous silicon (a-Si:H) films is of interest for device improvement and for development of new device structures for solar cell and large area display application. For well controlled annealing of such multilayers, precise knowledge of temperature and/or hydrogen diffusion length in the heated material is required but unavailable so far. In this study, we explore the use of deuterium (D) and hydrogen (H) interdiffusion during laser scanning (employing a continuous wave laser at 532 nm wavelength) to characterize both quantities. The evaluation of temperature from hydrogen diffusion data requires knowledge of the high temperature (T > 500 °C) deuterium-hydrogen (D-H) interdiffusion Arrhenius parameters for which, however, no experimental data exist. Using data based on recent model considerations, we find for laser scanning of single films on glass substrates a broad scale agreement with experimental temperature data obtained by measuring ...