H. Kampwerth

Advanced luminescence based effective series resistance imaging of silicon solar cells

A technique for fast and spatially resolved measurement of the effective series resistance of silicon solar cells from luminescence images is introduced. Without compromising the speed of existing luminescence based series resistance imaging methods, this method offers significant advantages in that it is more robust against variations in local diode characteristics. Lateral variations in the series resistance of an industrial screen printed multicrystalline silicon solar cell obtained from this method show excellent correlation with a Corescan measurement and are also shown to be unaffected by lateral variations in the diode properties.

Spatially resolved silicon solar cell characterization using infrared imaging methods

We present a comprehensive overview over infrared imaging techniques for (electrical) silicon solar cell characterization. Recent method development in local series resistance imaging is reviewed in more detail and new results in local breakdown investigations on multicrystalline (mc) silicon solar cells are reported. We observe local junction breakdown sites on industrial mc-cells at reverse voltages as low as −7V and breakdown in great areas of the cell at voltages around −14V. As these breakdown sites (as well as local shunts) can cause hot spots which can damage the cell and the module, we also present an ultra-fast, simple and quantitative method for hot-spot detection. Typical measurement times in the order of 10 milliseconds are achieved.

Spatially resolved photoluminescence imaging of essential silicon solar cell parameters

We present a method to image the following spatially resolved parameters of a solar cell: voltage V xy , current density J xy , power density P xy , efficiency η xy , series resistance R s,xy , fill factor FF xy and dark saturation current densities of a two diode model J 01,xy and J 02,xy . The algorithm determines this set of self- consistent parameters by using a minimum of five electrical biased photoluminescence images.

Lifetime analysis of laser crystallized silicon films on glass

Only recently, the quality of liquid phase crystallized silicon directly on glass substrates made a huge leap towards the quality of multi-crystalline wafers with open circuit voltages well above 600 mV. In this paper, we investigate the material quality in order to identify the factors limiting further performance improvements. We employ photoluminescence imaging on a state of the art test structure with lifetime calibration by transient photoluminescence. The resulting lifetime map is converted into an effective diffusion length map and the origin of regions with short lifetimes is investigated with electron backscattering and transmission electron microscopy. High local dislocation densities in areas with dissociated coincidence site lattice boundaries were found to be responsible for the localised quenching of the photoluminescence signal.

Transient photoluminescence from silicon wafers: Finite element analysis

This paper presents an accurate and practical mathematical model of time-resolved photoluminescence (PL) response from silicon wafers generated by fast repetitive excitation pulses. The model is valid under low level injection condition and takes into account the depth dependence of carrier generation, diffusion, and surface recombination. Finite element analysis is employed for the carrier density and PL computations. By comparing computational results with results obtained from PC1D (a computer program solving fully coupled nonlinear equations for quasi-one-dimensional carrier transportation in crystalline semiconductor devices, especially focusing on photovoltaic devices), the validity of this method is confirmed. Early stage application and the limitations of this method have been studied, and future work has been proposed.

Separation algorithm for bulk lifetime and surface recombination velocity of thick silicon wafers and bricks via time-resolved photoluminescence decay

We present a method to simultaneously determine bulk and surface recombination properties using time-resolved photoluminescence (PL) decay. The lifetime separation algorithm makes use of the analytical expression of the asymptotic separation of two time-resolved PL decays corresponding to different excitation wavelengths as well as that of the ratio of two steady-state PL intensities excited by the two different wavelengths. Detailed experimental methods of measuring these two terms are presented and the effect of signal-to-noise ratio is discussed to determine the applicability of this algorithm.

The effect of rear surface passivation layer thickness on high efficiency solar cells with planar and scattering metal reflectors

Rear surface reflector of solar cell is designed to improve light collection capacity by allowing the low energy photons to go through multiple bounces inside the solar device before escaping. In this paper, we investigate the thickness effect of rear SiO2 surface passivation layer on both optical and electrical properties of front-planar high efficiency PERT (Passivated Emitter and Rear Totally-Diffused) solar cells. Two kinds of metal reflectors are fabricated: the conventional planar reflectors by evaporated Al and the novel scattering reflectors by self assembled Ag nanoparticles. We find that the thickness dependence of rear SiO2 layer (from 8 nm - 134 nm) on photocurrent shows an asymmetry for planar and scattering reflectors, moreover, the scattering reflectors perform much better than the planar reflectors under all tested SiO2 thicknesses. A maximum current enhancement (calculated from wavelength 900 nm to 1200 nm) of 12.1% is presented for planar reflector with 134 nm SiO2 film, and 18.4% for scattering reflector with the optimized 19 nm rear SiO2 film. Additionally, by adding a detached metal mirror, the maximum current enhancement from scattering reflector jumps to 27.0%. Effective optical path length Z is calculated to study the light trapping (optical properties) under various SiO2 thicknesses for both reflectors. Diffusion length L is calculated to track the electrical performance. It is shown that thicker SiO2 is of benefit for both optical and electrical properties when planar Al reflector is used. However, for scattering reflectors, thinner SiO2 is preferable for optical enhancement, but thicker SiO2 is desirable for electrical gain. 19 nm SiO2 is found to be the best choice for cells with scattering reflectors, considering both effects.

Measurement of internal optical reflection characteristics of solar cell back reflectors

The optimization work of high-efficient solar cells includes light-trapping schemes for the weakly absorbed near-infrared (NIR) photons. The light scattering properties of the back-reflector are here of particular interest. Novel back reflector designs that employ plasmonic and multi-stack interference effects require especially an experimental verification of the numerical simulations. Unfortunately, a good measurement of these scatter properties is nontrivial due to the refractive index of silicon and its resulting angle of total reflection. On planar wafers, light can therefore only be coupled in and out of silicon in a quite narrow angle, impossible to measure wider angles. Therefore we present an experimental setup to measure the angular resolved scatter properties of back-reflectors for IR photons.