H. Straube

Quantitative evaluation of loss mechanisms in thin film solar cells using lock-in thermography

We describe the measurement and modeling of lock-in thermograms for three differently processed crystalline silicon on glass thin film silicon solar modules. For the purpose of defect impact evaluation, a bias series of lock-in thermograms for a single cell in each module is measured. The resulting images around maximum power point bias show pronounced Peltier heat redistribution inside the cell, which needs to be taken into account for quantitative evaluation of the thermography results. This is done using a finite differences electronics simulation of the current flow inside the module and convolution of the heat distribution patterns with the thermal blurring. The procedure makes it possible to extract relevant cell performance parameters like the area diode dark saturation current and nonlinear edge shunting current densities as well as to evaluate the relative impact of these on the efficiency under simulated illumination.

Estimation of heat loss in thermal wave experiments

The theoretical description of thermal wave experiments assumes harmonic heat introduction (sinusoidally modulated heating and cooling) and an adiabatic (thermally insulated from the environment) sample. In most experimental setups, however, only pulsed heat is introduced, and the mean sample temperature stabilizes at a value above ambient determined by the strength of the heat loss to the surroundings. The question arises as to what conditions have to be satisfied such that the assumption of an adiabatic sample is applicable to the experimental situation. In this contribution we present a treatment of heat-loss mechanisms (radiation, conduction/convection to air, and conduction to the sample holder) together with their implications for signal shape and temperature control of the sample. The treatment leads to a general mathematical criterion to decide whether heat conduction inside the sample can be considered to be adiabatic or not, depending on excitation frequency and spatial frequency content of the ...

Growth of Silicon Carbide Filaments in Multicrystalline Silicon for Solar Cells

This work introduces two different approaches to explain the growth of silicon carbide (SiC) filaments, found in the bulk material and in grain boundaries of solar cells made from multicrystalline (mc) silicon. These filaments are responsible for ohmic shunts. The first model proposes that the SiC filaments grow at the solid-liquid interface of the mc-Si ingot, whereas the second model proposes a growth due to solid state diffusion of carbon atoms in the solid fraction of the ingot during the block-casting process. The melt interface model can explain quantitatively the observed morphologies, diameters and mean distances of SiC filaments. The modeling of the temperature- and time-dependent carbon diffusion to a grain boundary in the cooling ingot shows that solid state diffusion based on literature data is not sufficient to transport the required amount of approximately 3.4  1017 carbon atoms per cm2 to form typical SiC filaments found in grain boundaries of mc-Si for solar cells. However, possible mechanisms are discussed to explain an enhanced diffusion of carbon to the grain boundaries.