Uncertainty of the coefficient of band-to-band absorption of crystalline silicon at near-infrared wavelengths

We present data of the coefficient of band-to-band absorption of crystalline silicon at 295 K in the wavelength range from 950 to 1350 nm and analyze its uncertainty. The data is obtained from measurements of reflectance and transmittance as well as spectrally resolved photoluminescence measurements and spectral response measurements. A rigorous measurement uncertainty analysis based on an extensive characterization of our setups is carried out. We determine relative uncertainties of 4% at 1000 nm, increasing to 22% at 1200 nm and 160% at 1300 nm, and show that all methods yield comparable results. © 2014 AIP Publishing LLC

Impact of contacting geometries on measured fill factors

The fill factor determined from a measured current-voltage characteristic of a bare solar cell depends on the number and positions of the electrical contacting probes. Nine different geometries for contacting the front side busbars are used to measure the current-voltage (I-V) characteristics of a 5 busbar industrial-type passivated emitter and rear totally diffused (PERT) solar cell under standard testing conditions. The fill factors of the measured I-V characteristics vary from 78.5 %abs to 80.6 %abs. We further measure the contacting resistance of 3 different contacting probes to estimate the sensitivity of measurements with different contacting geometries on random resistance variations. The contacting resistance is 60 mΩ for nine-point probes and 80 mΩ for four- and single-point probes. We determine the magnitude of contacting resistance variations from measurements at different probe positions to be ±30 mΩ. Using this variation, we perform numerical simulations and find a larger sensitivity on random resistance variations for tandem- (pairs of current- and sense probes) compared to triplet (one sense- between two current probes) configurations. The corresponding fill factor deviation is approximately 0.1%abs for tandem configurations when the contacting resistances of up to two current probes are altered. The sensitivity for triplet configurations is negligible

Determination of the collection diffusion length by electroluminescence imaging

The electroluminescence emission of crystalline silicon solar cells at near-bandgap wavelengths is investigated. We show that the intensity of the emitted luminescence at near-bandgap wavelengths is directly proportional to the collection diffusion length Lc which is a measure of bulk and rear surface recombination properties and determines the short circuit current of a solar cell illuminated with light of near-bandgap wavelengths. We provide experimental evidence for the determination of Lc by carrying out electroluminescence measurements on a set of 15 specially prepared monocrystalline silicon solar cells with different thicknesses. Moreover, we demonstrate and discuss the applicability of the proposed method to obtain images of the collection diffusion length Lc of multicrystalline silicon solar cells. The values determined by electroluminescence imaging coincide with values obtained from spectrally resolved quantum efficiency measurements with a relative accuracy of 13 %.German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt

Identifying the location of recombination from voltage-dependent quantum efficiency measurements

This paper investigates process-induced variations of the open-circuit voltage (Voc) using voltage-dependent quantum efficiency measurements. By means of device modelling we show that this method is able to explain the Voc difference of two solar cells, even if they show identical electrical behaviour under short-circuit condition. This paper furthermore explains how the origin of Voc variations can be classified into emitter, base and rear of the solar cell. The simulation results have been experimentally verified with industrial-type passivated emitter and rear cells (PERC) cells made from p-type Czochralski wafers. The proposed analysis method is an attractive way for monitoring Voc variations of solar cells in industrial mass production since there is no need for specially prepared test structures. © 2017 The Authors. Published by Elsevier Ltd

Hong-Ou-Mandel dip using photon pairs from a PPLN waveguide

We experimentally observed a Hong-Ou-Mandle dip with photon pairs generated in a periodically poled reverse-proton-exchange lithium niobate waveguide with an integrated mode demultiplexer at a wavelength of 1.5 um. The visibility of the dip in the experiment was 80% without subtraction of any noise terms at a peak pump power of 4.4 mW. The new technology developed in the experiment can find various applications in the research field of linear optics quantum computation in fiber or quantum optical coherence tomography with near infrared photon pairs.Comment: 5 Pages, 2 figure

Experimental setup for camera-based measurements of electrically and optically stimulated luminescence of silicon solar cells and wafers

We report in detail on the luminescence imaging setup developed within the last years in our laboratory. In this setup, the luminescence emission of silicon solar cells or silicon wafers is analyzed quantitatively. Charge carriers are excited electrically (electroluminescence) using a power supply for carrier injection or optically (photoluminescence) using a laser as illumination source. The luminescence emission arising from the radiative recombination of the stimulated charge carriers is measured spatially resolved using a camera. We give details of the various components including cameras, optical filters for electro- and photo-luminescence, the semiconductor laser and the four-quadrant power supply. We compare a silicon charged-coupled device (CCD) camera with a back-illuminated silicon CCD camera comprising an electron multiplier gain and a complementary metal oxide semiconductor indium gallium arsenide camera. For the detection of the luminescence emission of silicon we analyze the dominant noise sources along with the signal-to-noise ratio of all three cameras at different operation conditions. © 2011 American Institute of Physics

Luminescence emission from forward- and reverse-biased multicrystalline silicon solar cells

We study the emission of light from industrial multicrystalline silicon solar cells under forward and reverse biases. Camera-based luminescence imaging techniques and dark lock-in thermography are used to gain information about the spatial distribution and the energy dissipation at pre-breakdown sites frequently found in multicrystalline silicon solar cells. The pre-breakdown occurs at specific sites and is associated with an increase in temperature and the emission of visible light under reverse bias. Moreover, additional light emission is found in some regions in the subband-gap range between 1400 and 1700 nm under forward bias. Investigations of multicrystalline silicon solar cells with different interstitial oxygen concentrations and with an electron microscopic analysis suggest that the local light emission in these areas is directly related to clusters of oxygen. © 2009 American Institute of Physics

PV module current gains due to structured backsheets

We evaluate the optical performance of PV modules with respect to an increase in short circuit current density. Our evaluation is based on the combination of ray tracing simulations and measurements on test modules with four types of backsheets: Two of them are structured, the third is white and diffusively reflecting and the fourth reflects no light. Under normal incidence, structured backsheets reflect incoming light at an angle that causes total internal reflection at the glass/air interface, which guides the light to the solar cell surface. Three different irradiance conditions are studied: a) standard testing conditions (STC) with light incident perpendicular to the module surface, b) variation in the angle of incidence and c) light source with mean annual distribution of angles of incidence. Using the measured refractive index data in ray tracing simulations we find a short circuit current density (Jsc) gain of up to 0.9 mA/cm2 (2.3%) for monofacial cells and a structured backsheet, when compared to a white backsheet with diffuse reflection. For bifacial cells we calculate an even larger Jsc increase of 1.4 mA/cm2 (3.6%). The Jsc increase is larger for bifacial cells, since some light is transmitted through the cells and thus more light interacts with the backsheet. Our optical loss analysis reveals the best performance in STC for edge-aligned Ag grooves. This structure reduces absorption losses from 1.8 mA/cm2 to 0.3 mA/cm and reflection losses from 0.7 mA/cm to 0 mA/cm. This trend also holds under various angles of incidence as confirmed consistently by Jsc measurements and ray racing simulations. Simulations using an annual light source emitting a mean annual distribution of angles of incidence reveal grooves in both orientations edge alignment and east-west alignment achieve similar current gains of up to 1.5% for mono- and of 2.5% for bifacial cells compared to modules with white back sheets. This indicates that for modules with light guiding structures such as these backsheets optimization for STC differs from optimization for annul yield