Evolution of defect densities with height in a HPMC-SI ingot

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Hydrogen-related defects in hydrothermally grown ZnO studied by Fourier transform infrared spectroscopy

Hydrogen is an important impurity in ZnO, and it is believed to act as a shallow donor and to passivate acceptors in the material. H is readily associated with other defects in ZnO, forming complexes with characteristic localized vibrational modes (LVMs). The H-related peaks observed in the IR absorption spectra of ZnO is thus highly dependent on the concentration of other impurities and native defects. In this work, H-related defects in hydrothermally (HT) grown ZnO single crystals have been investigated by Fourier Transform Infrared Spectroscopy (FTIR), Secondary Ion Mass Spectrometry (SIMS) and four point resistivity measurements. Due to a high concentration of Li acceptors, the LVM of a OH-Li complex dominates the IR spectra of the as-grown samples. Several other H-related peaks are however also observed. The results presented in this work indicate that the 3577 cm-1 peak associated to the OH-Li defect exhibits a complex annealing dependency, which may be explained by a process involving diffusion and recapture of H. Also, dissociation of the defect occurs at substantially lower temperatures than the previously reported thermal stability of 1200 °C. The absorption cross section of the OH-Li signal has been estimated to be 1.27 x 10^(-17) cm. A group of IR absorption peaks at 4216, 4240 and 4246 cm-1 have also beenobserved in the IR spectra of as-grown samples. By comparison with SIMS measurements, the previous identification of these peaks as internal electronic transitions of substitutional Ni_(Zn) impurities has been verified. The absorption cross section of the peaks has been found to be 2.91 x 10^(-17) cm. Several other H-related peaks appeared in the IR spectra recorded after annealing of the samples in H2 and/or D2 atmospheres, caused by diffusion of H/D into the crystals. Two IR absorption peaks at 3303 and 3321 cm-1 were assigned to the LVMs of a defect complex labeled H-X, consisting of two O-H bonds associated to an unknown impurity atom. The 3321 cm-1 mode is oriented along the c-axis of ZnO, while the 3303 cm-1 mode is oriented at an angle with the c-axis. The H-X complex is thermally stable up to ~ 600 °C and the activation energy fordissociation was estimated to be 2.8 eV. An IR absorption peak at 2783 cm-1 was also observed after hydrogenation. This peak was assigned to the LVM of a defect complex labeled H-Y, involving a single O-H bond oriented at an angle with the c-axis. Also, a pair of peaks at 3347 and 3374 cm-1 were observed in both as-grown and hydrogenated samples after annealing at ~ 500 °C. These peaks were assigned to the LVMs of two O-H bonds associated to the same defect, labeled H-Z. The H-X defect is to our knowledge not reported in the literature. The H-Y and H-Z defects have previously been identified as OH-Ni_(Zn) and (OH)2-Cu_(Zn) complexes, respectively. However, the SIMS and IR absorption data presented in this work indicate that both these assignments should be revisited. The H2 and/or D2 gas anneals were also followed by a substantial drop in resistivity, which was found to be stable after annealing at 200 °C. The resistivity however increased markedly after subsequent annealing at higher temperatures (>500 °C). The increased carrier density after hydrogenation is presumably caused by a combination of thermally stable H donors like HO and H passivation of acceptors present in the as grown samples, forming neutral complexes like OH-LiZn

New methods for investigation of surface passivation layers for crystalline silicon solar cells

One of the main challenges of the c-Si PV industry is the implementation of high quality surface passivation layers. Effective surface passivation is needed in order to avoid large efficiency losses when moving towards thinner silicon wafers and to utilize the full potential of high quality Si material. To obtain a fundamental understanding of the surface recombination mechanisms and their impact on device performance, precise characterization methods and suitable physical models are of high importance. The main result of this work is the development of a new method for analyzing surface recombination for passivated Si substrates under varying surface band bending conditions: A photoluminescence imaging setup is used to measure the effective minority carrier lifetime of passivated Si wafers while applying an external bias to a metal electrode deposited onto the rear side passivation layer. The experimental measurements have been analyzed using an extended Shockley Read-Hall model with added recombination in the space charge region. This approach is shown to give valuable information about the passivation mechanisms. The measured data can be used to independently determine the fixed charge density under illumination (field-effect passivation) and the surface recombination velocity parameters for electrons and holes (chemical passivation) for a wide range of dielectric passivation layers. The results are in good agreement with both capacitance-voltage measurements and lifetime measurements after deposition of corona charges on the sample surface. A second major contribution from this work is the development of a modified version of the widely used simulation tool PC1D which runs from a command line, thus allowing for scripted simulations. This modified version of the program has also been used as a basis for a new user interface, which allows for improved visualization, multivariable analysis, optimization, fitting to experimental data and implementation of additional models. Through this work, several passivation processes have been developed and optimized, and excellent passivation quality has been demonstrated for a stack of a-Si:H and a-SiNx:H deposited by plasma enhanced chemical vapor deposition. Selected passivation layers have also been demonstrated as rear side passivation in a co-planar solar cell test structure with local rear contacts

Endogenous soiling rate determination and detection of cleaning events in utility scale PV plants

As the deployment rate of PV power plants continuesto soar, the need for robust, scalable methods for performanceanalytics increases. In this paper, we demonstrate the usefulness ofone approach for quantifying soiling rates in utility-scale PV powerplants endogenously, i.e., directly from the production data. Thetemperature corrected performance ratio, normalized to a cleanstate, is used to derive the soiling ratio (SR). Cleaning events, causedby either rain or manual cleaning, are automatically detected bypositive shifts in the running median of the SR time series. Soilingrates are then estimated by the rate of change of the SR betweenthe cleaning events, which is determined by linear regression. Themethod is validated on data from three utility-scale PV powerplants in the Middle East, yielding soiling rates that are in therange 0%–0.18%/day at least 50% of the time, with a median of 0.1%/day.publishedVersio

Modulating the field-effect passivation at the SiO2/c-Si interface: Analysis and verification of the photoluminescence imaging under applied bias method

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Photoluminescence imaging under applied bias for characterization of Si surface passivation layers

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Lifetime spectroscopy with high spatial resolution based on temperature- and injection dependent photoluminescence imaging

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Temperature dependent photoluminescence imaging calibrated by photoconductance measurements

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Identifying recombination parameters by injection-dependent lifetime spectroscopy on mc-silicon based on photoluminescence imaging

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