R. Brendel

Recombination current and series resistance imaging of solar cells by combined luminescence and lock-in thermography

We perform recombination current and series resistance imaging on large-area crystalline silicon solar cells using a combined analysis of camera-based dark lock-in thermography (DLIT) and electroluminescence (EL) imaging. The solar cells are imaged both by DLIT and EL under identical operating conditions. The quantitative analysis of the DLIT measurement produces an image of the local heating power and the EL picture results in an image of the local cell voltage. Combining the two images pixel by pixel allows us to calculate images of the local recombination current and the local series resistance of the solar cell.

Solar cell efficiency and carrier multiplication in Si1−xGex alloys

Crystalline Si1−xGex compounds offer the possibility for tuning the electronic energy band structure with the chemical composition of the alloy in order to adapt the material for devices utilizing the energy of solar photons at an optimum. We concentrate on the efficiency enhancement due to carrier multiplication by impact ionization. We calculate the internal quantum efficiency and the possible solar cell efficiency for this material system. The number of impact-generated charge carriers is obtained by a simulation of the competing carrier–carrier and carrier–photon scattering processes. These calculations show that the wave vector dependence of the scattering processes is unimportant for good agreement between theoretical and experimental quantum efficiencies in Si and Ge. Finally, we calculate solar cell efficiencies under the ideal assumption of unity collection efficiency and radiative recombination only. Impact ionization enhances the theoretical conversion efficiency by 0.5 percentage point; this i...

Quantum efficiency analysis of thin-layer silicon solar cells with back surface fields and optical confinement

Thin-layer silicon solar cells utilize surface textures to increase light absorption and back surface fields to prevent recombination at the silicon-substrate interface. We present an analytical model for the internal quantum efficiency that accounts for light trapping and also considers carrier generation and recombination in back surface fields or substrates. We introduce a graphical representation of experimental data, the so-called Parameter-Confidence-Plot, which allows one to draw maximum information on diffusion lengths and surface recombination velocities from quantum efficiency measurements. The analysis is exemplified for state of the art thin-layer silicon solar cells with and without back surface fields.

Sensitivity and transient response of microwave reflection measurements

Microwave reflection measurements are widely used for the characterization of minority‐carrier lifetimes in semiconductors. A theoretical description of the technique is presented. The approach is based on a dielectric multilayer model that accounts for experimental parameters such as microwave frequency, sample thickness and doping, and the distance to an optional reflector behind the sample. With a new definition of the sensitivity in transient microwave reflection measurements, the most sensitive configuration is investigated for a given semiconductor thickness and conductivity. Good agreement between the theoretical simulation and measurements is demonstrated. The model is also used for calculating microwave reflection transients from the excess carrier decay after pulsed laser excitation. It is found that the reflected microwave power mirrors the carrier decay if three criteria are fulfilled: The carrier generation must be homogeneous; low‐injection conditions are required; and the reflector must be ...

Dynamic carrier lifetime imaging of silicon wafers using an infrared-camera-based approach

We present a calibration-free dynamic infrared carrier lifetime mapping technique, yielding images of the carrier lifetime of multicrystalline silicon wafers within seconds. Images of the infrared emission of the sample under test are taken directly after switching on a monochromatic illumination source and after steady-state conditions have been established in the sample. Making use of the proportionality between the infrared emission and the free carrier density inside the sample, the carrier lifetime is calculated from the signal ratio of these two images by an analytical method. We achieve an excellent agreement when comparing our results with carrier lifetime mappings obtained by the microwave-detected photoconductance decay technique.

Thermodynamic efficiency limits for semiconductor solar cells with carrier multiplication

Abstract Our recent quantum efficiency measurements showed that more than one electron/hole pair per absorbed photon can be created in a solar cell. Thermodynamic consideration of carrier multiplication leads to new efficiency limits for photovoltaic energy conversion. An efficiency of 43% is theoretically possible for cells which are illuminated by the suns unconcentrated black body radiation. For sun light of full concentration, the new limit is 85%. These ideal values are thermodynamically possible with a single semiconductor which makes optimum use of carrier multiplication and shows radiative recombination only. The theoretical description of the thermodynamics of radiative recombination in a cell with carrier multiplication leads us also to a novel mathematical description of the saturation current density.

Sol—gel coatings for light trapping in crystalline thin film silicon solar cells

Abstract An increase of light absorption by light trapping is a key issue for the design of thin film solar cells from crystalline silicon. According to our numerical work, the deposition of crystalline silicon layers of thickness, W = 4 μ m, on textured glass substrates doubles the cell current for facet angles, α = 75°, and texture periods p μ m, without the need for anti reflection coatings. We demonstrate the fabrication of such micron-sized light traps by embossing of sol—gel glasses.

Ultrathin crystalline silicon solar cells on glass substrates

We fabricate thin crystalline silicon solar cells with a minority carrier diffusion length of 0.6±0.2 μm by direct high-temperature chemical vapor deposition on glass substrates. This small diffusion length does not allow high cell efficiencies with conventional cell designs. We propose a new cell design that utilizes submicron thin silicon layers to compensate for low minority carrier diffusion lengths. According to theoretical modeling, our design exhibits excellent light trapping properties and allows for 10% efficiency at an optimum cell thickness of 0.4 μm only. This submicron range of cell thicknesses was formerly thought to require direct band gap semiconductors.

On the data analysis of light‐biased photoconductance decay measurements

The use of bias light is common practice today in photoconductance decay (PCD) measurements to analyze semiconductor samples with injection‐level dependent recombination parameters (i.e., surface recombination velocity and/or bulk lifetime). Recently, it has been shown on theoretical grounds that the previously reported recombination parameters from light‐biased PCD experiments are not the actual properties of the investigated sample, but so‐called differential recombination parameters [R. Brendel, Appl. Phys. A 60, 523 (1995)]. In the present article the theory relevant to light‐biased PCD measurements is discussed in detail and subsequently applied to monocrystalline silicon wafers with nitride and oxide passivated surfaces in order to verify the deviations between the differential and actual surface recombination velocities. Special emphasis is paid to the experimental fact that the injection level cannot be reduced below a minimum value due to signal‐to‐noise problems.

On the thickness dependence of open circuit voltages of p-n junction solar cells

Abstract A reduction of the thickness of solar cells with low surface recombination is known to result in enhanced open circuit voltages, provided that the short circuit current can be maintained sufficiently high by light trapping schemes. Lower volume recombination is generally assumed to cause this effect. We offer another interpretation: The voltage increases because thinning the cell at constant short circuit current enhances the minority carrier generation rate per unit volume and hence the steady state carrier concentration. Thermodynamically, an increased carrier concentration is equivalent to a reduction of the entropy production per photon, thus leading to larger voltage.