J.-M. Wagner

Understanding junction breakdown in multicrystalline solar cells

Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 Ω cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between −4 and −9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between −9 and −13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond −13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade materia...

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.

Efficiency limits of Si/SiO2 quantum well solar cells from first-principles calculations

In order to investigate the applicability of new photovoltaic absorber materials, we show how to use first-principles calculations combined with device simulations to determine the efficiency limits of solar cells made from SiO2/Si superlattices and from coaxial ZnO/ZnS nanowires. Efficiency limits are calculated for ideal systems according to the Shockley–Queisser theory but also for more realistic devices with finite mobilities, nonradiative lifetimes, and absorption coefficients. Thereby, we identify the critical values for mobility and lifetime that are required for efficient single junction as well as tandem solar cells.

Incorporation of Si in δ‐doped GaAs studied by local vibrational mode spectroscopy

Raman scattering by local vibrational modes has been used to study the incorporation of Si in single δ‐doped GaAs layers. Placing the doping spike at different depths underneath the surface, a depth profile of the dopant concentration incorporated on lattice sites has been obtained. For samples grown by molecular beam epitaxy under conditions which are known to lead to a significant broadening of the doping spike, the applied Raman technique reveals the incorporation of Si on Ga sites with a broadening of the Si distribution in excess of 20 nm.

Physical mechanisms of breakdown in multicrystalline silicon solar cells

We have identified at least five different local breakdown mechanisms according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under reverse bias. These are (1) early pre-breakdown (strongly negative TC, low slope), (2) edge breakdown (positive TC, low slope, no EL), (3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550 nm defect luminescence), (4) strong defect-induced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and (5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at high reverse bias. In addition to the local breakdown sites there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Since defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication, we propose defect level-induced avalanche instead of trap-assisted tunneling to be responsible for this breakdown mechanism.

Free‐exciton luminescence in GaSb quantum wells confined by short‐period AlSb‐GaSb superlattices

Photoluminescence and photoluminescence excitation measurements at temperatures ranging from 4 to 300 K were performed on 9.2‐nm GaSb quantum wells confined by AlSb‐GaSb short‐period superlattices. In the temperature range 4–200 K the photoluminescence from the high‐quality molecular beam epitaxially grown samples is dominated by free‐exciton emission. This assignment is confirmed by photoluminescence excitation spectroscopy which reveals an energy separation of 11.5 meV between n=1 heavy‐hole and n=1 light‐hole free excitons for the 9.2‐nm wells. At low temperatures the photoluminescence line is shifted by only 7.5 meV to low energy from the heavy‐hole excitonic peak observed in the excitation spectrum. This small value of the Stokes shift indicates the absence of impurity‐related trapping of excitons.

Effect of spatial localization of dopant atoms on the spacing of electron subbands in δ‐doped GaAs:Si

Using Raman spectroscopy we have investigated the spacing of the electron subbands in nominally δ‐doped GaAs structures which show a considerable spread of the silicon dopant atoms along the growth direction. For optical excitation in resonance with the E0+Δ0band gap, spin‐density intersubband excitations are observed. For excitation in resonance with the E1 band gap we find a strong enhancement of scattering by collective intersubband plasmon‐phonon modes. The measured energy spacings between the electron subbands deviate significantly from what is expected for ideal δ doping. Self‐consistent electronic subband calculations taking into account the spread of the dopant atoms along the growth direction, in contrast, yield a good quantitative agreement between calculated and measured subband spacings. This demonstrates the potential of intersubband Raman spectroscopy for the analysis of the spatial localization of dopant atoms in δ‐doped structures.

Photoluminescence from rapid thermal annealed and pulsed-laser-annealed, ion-implanted Si

Low‐temperature photoluminescence studies of ion‐implanted and rapid thermal annealed or pulsed‐laser‐annealed Si are reported. The samples were implanted with As, P, Sb, or B. The luminescence spectra of the pulsed‐laser‐annealed samples show strong sharp luminescence lines from radiation induced defects, whereas in samples implanted with As, P, or B and rapidly annealed with an arc lamp a very clean spectrum without any defect luminescence is observed. This indicates a very low defect concentration in the lamp annealed material. In Sb‐implanted lamp‐annealed samples, however, a broad defect luminescence band appears as the temperature is raised and which varies in shape as a function of the annealing temperature. This band is probably due to Sb agglomerates.

Absolute efficiency and dispersion of Raman scattering by phonons in silicon

Abstract We have measured the absolute Raman efficiency for first order scattering of the optical zone center phonon with two different incident photon energies (1.16 and 0.94 eV) by comparison with the Raman scattering from diamond. A value significantly lower than the experimental data published previously in this region has been found. For the first order scattering no dispersion of the scattering efficiency has been observed at the indirect gap, in contrast to the second order scattering which shows a pronounced resonance behaviour in this energy range.

Defect-Induced Breakdown in Multicrystalline Silicon Solar Cells

We have identified at least five different kinds of local breakdown according to the temperature coefficient (TC) and slope of their characteristics and electroluminescence (EL) under a reverse bias. These are 1) early prebreakdown (negative TC, low slope), 2) edge breakdown (positive TC, low slope, no EL), 3) weak defect-induced breakdown (zero or weakly negative TC, moderate slope, 1550-nm defect luminescence), 4) strong defect-induced breakdown (zero or weakly negative TC, moderate slope, no or weak defect luminescence), and 5) avalanche breakdown at dislocation-induced etch pits (negative TC, high slope). The latter mechanism usually dominates at a high reverse bias. The defects leading to the etch pits are investigated in detail. In addition to the local breakdown sites, there is evidence of an areal reverse current between the dominant breakdown sites showing a positive TC. Defect-induced breakdown shows a zero or weakly negative TC and also leads to weak avalanche multiplication. It has been found recently that it is caused by metal-containing precipitates lying in grain boundaries.