Otwin Breitenstein

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...

Microscopic lock-in thermography investigation of leakage sites in integrated circuits

The detection limit of infrared thermographic investigations can be improved down to 10 μK by using a highly sensitive high-speed infrared camera in an on-line averaging lock-in thermography system. Together with a microscope objective, this allows lock-in thermography to be used as a simple and sensitive technique to localize the sites of leakage currents and other heat sources in electronic components. The practical realization of a novel lock-in thermography system is described and both test measurements and practical applications are introduced. The detection limit for surface-near local heat sources in silicon is a few microwatts with a spatial resolution down to 5 μm. Leakage sites in several microelectronic structures are imaged and assigned to the layout of the integrated circuit by comparing direct images with lock-in ones. The direct comparison of an averaged and background-subtracted stationary thermogram with a lock-in one, both measured under similar conditions at the same sample, clearly dem...

Explanation of commonly observed shunt currents in c-Si solar cells by means of recombination statistics beyond the Shockley-Read-Hall approximation

The current-voltage (I–V) characteristics of industrially fabricated, crystalline silicon solar cells are often influenced by non-linear shunts that originate from localized, highly disturbed regions and cause ideality factors n > 2. We show that recombination within such locations needs model descriptions that go beyond the Shockley-Read-Hall (SRH) approximation, because the density of defects is so high that recombination does not occur via isolated, but coupled defect states. We use a variant of coupled defect level (CDL) recombination, the donor-acceptor-pair (DAP) recombination, but via deep levels (as opposed to shallow levels). With this model, we quantitatively reproduce the I–V curves of solar cells that we subjected to various degrees of cleaving, laser scribing or diamond scratching to form shunt locations in a controlled manner. The suggested model explains the transition from ideality factors n   2 when going from low to high defect densities. We also explain the non-saturating rever...

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.

Electrical properties of nominally undoped silicon nanowires grown by molecular-beam epitaxy

Single undoped Si nanowires were electrically characterized. The nanowires were grown by molecular-beam epitaxy on n+ silicon substrates and were contacted by platinum/iridium tips. I-V curves were measured and electron beam induced current investigations were performed on single nanowires. It was found that the nanowires have an apparent resistivity of 0.85Ωcm, which is much smaller than expected for undoped Si nanowires. The conductance is explained by hopping conductivity at the Si–SiO2 interface of the nanowire surface.

Material-induced shunts in multicrystalline silicon solar cells

By applying lock-in thermography imaging, light-beam-induced current imaging, electron-beam-induced current imaging at different stages of sample preparation, and infrared light microscopy in transmission mode, the physical nature of the dominant material-induced shunts in multicrystalline solar cells made from p-type silicon material has been investigated. It turns out that these shunts are due to silicon carbide (SiC) filaments, which grow preferentially in grain boundaries and cross the whole cell. These filaments are highly n-type doped, like the emitter layer on the surface of the cells. They are electrically connected both with the emitter and with the back contact, thereby producing internal shunts in the solar cell.

Can Luminescence Imaging Replace Lock-in Thermography on Solar Cells?

The purpose of this paper is a detailed comparison of selected luminescence and lock-in thermography (LIT) results on one exemplary sample and the drawing of corresponding conclusions. Our focus is on solar cells, but some investigations on wafers will be discussed as well. The comparison will help to decide which characterization tools are needed to solve technological problems. It will be demonstrated that luminescence imaging may widely replace LIT with respect to the analysis of recombination-active bulk defects, cracks, series resistance, and junction breakdown sites. However, some important investigations can be done only by LIT. LIT allows for a quantitative analysis of different kinds of leakage currents both under forward and under reverse bias, enabling a reliable analysis of local I-V characteristics. It is shown that LIT and luminescence imaging are complementary to each other and should be used in combination.

Potential-Induced Degradation (PID): Introduction of a Novel Test Approach and Explanation of Increased Depletion Region Recombination

In recent years, a detrimental degradation mechanism of solar cells in large photovoltaic fields called potential-induced degradation (PID) has been intensively investigated and discussed. Here, the module efficiency is decreasing down to a fractional part of their original efficiency. In this study, we introduce a PID test at a solar-cell level and for individual module components applicable as a tool for process control in industries and root cause analyses in science departments. Using the proposed method, one example analysis of a solar cell that is degraded by the PID tester is presented. It is shown that PID of the shunting type influences both the parallel resistance (Rp) and the depletion region recombination behavior (J02) of the solar cell. Increased recombination in the depletion region is caused by Na decorated stacking faults crossing the depletion region. This strongly influences recombination behavior in the depletion region, leading to an increased J02 and an ideality factor n2 > 2. However, the defects leave the base of the solar cell primarily unaffected, and hence, J01 recombination remains rather low. Based on these findings, a model for the shunting and the increased depletion region recombination behavior is discussed.

Observation of transition metals at shunt locations in multicrystalline silicon solar cells

By employing a combination of analytical tools including lock-in thermography and synchrotron-based x-ray fluorescencemicroscopy,transition metals have been identified at shunting locations in two types of low-cost multicrystalline silicon (mc-Si) solar cellmaterials: cast multicrystalline and ribbon growth on substrate (RGS). At a shunting location in the cast mc-Si cell, silver and titanium, both contact strip materials, have been identified at the shunting location, suggesting a process-induced error related to contact metallization. At a shunting location in the RGS cell, a material-specific shunting mechanism is described, involving channels of inverse conductivity type, where copper and iron are found. The possible roles of these metals in this shunting mechanism are discussed. These results illustrate the wide range of physical mechanisms involved with shunting in solar cells.

Electrothermal simulation of a defect in a solar cell

A local electrothermal simulation of a model solar cell is presented. A rigorous discussion of the heat dissipation mechanisms in a solar cell is performed, showing that the total dissipated heat splits into heating terms (thermalization, recombination, and Joule heat) and different Peltier cooling terms. Such simulations are important for interpreting lock-in thermography images of real solar cells. The simulated model cell consists of a circular noncontacted region surrounded by a grid line and a nonlinear edge shunt. Based on this simulation, a special lock-in thermography operation mode is proposed, which enables noncontacted regions in real solar cells to be imaged. Experimental results confirm the theoretical predictions.