• Energy Research

Lead in perovskite solar cells is a potential environmental and health hazard if it is released from accidentally damaged panels. Now, the encapsulation of perovskite solar cells with self-healing polymers is shown to significantly reduce the risk of lead leakage from hail impact under a variety of weather conditions.

We demonstrate the interconnection of silicon solar cells with evaporated aluminum back contacts using an aluminum foil which is attached to a silicone encapsulant. The aluminum-based mechanical and electrical laser interconnection (AMELI) process forms laser weld spots using single laser pulses. These laser welds resist high mechanical stresses and have a low electrical contact resistivity. No solder, conductive adhesives, or Ag-pastes are required for interconnection. We find the electrical contact resistivity to be below ρc = 0.01 mΩ·cm2. The contact resistance is constant under accelerated aging of 300 humidity-freeze cycles. With a tensile testing machine, we measure tear-off stresses in the perpendicular direction of up to 380 kPa for our laser weld spots. We present a proof-of-concept module which consists of five n-type back-junction back-contact solar cells with a conversion efficiency of 20.4%. The unchanged fill factor FF and open circuit voltage Voc verify a damage- and loss-free interconnection which is supported by electroluminescence measurements.

Solar cell cracks in wafer based silicon solar modules are a well-known problem. In order to identify the origin of cracks and thus lay the foundation for the inhibition of crack formation, we provide for the first time a statistic crack distribution in photovoltaic (PV) modules. We evaluate electroluminescence images of PV modules tested at the ISFH with respect to cracks. The results of the static load test and the “as delivered” PV modules are compared. Additionally we perform a simulation of the strain distribution on a glass plate subjected to a uniform mechanical load and supported at the edges, which is used as a measure for the relative mechanical load on the individual cells. The measured crack distribution correlates well with the stress distribution calculated by the simulation. “As delivered” PV modules show an average of 6% of broken cells per PV module. The analysis of the spatial distribution and orientation of micro cracks in PV modules offers valuable insight into the causes of micro cracks if the PV module is subject to a uniform mechanical load. It lays the foundation for PV module developments that reduce the risk of cracks, as well as for statistical power loss assessment. 26th European Photovoltaic Solar Energy Conference and Exhibition; 3290-3294

We show the degradation of the front surface passivation by rear-side laser processing of thin silicon solar cells when using a laser with a pulse length of 8 ps. 45-μm-thick back-contact back-junction monocrystalline silicon solar cells show an energy conversion efficiency of 18.8% without rear-side laser processing, whereas they show only 7.5% with an additional rear-side laser process step for contact separation. This low efficiency is due to the degradation of the front surface passivation, which is confirmed by quantum efficiency measurements. The internal quantum efficiency at short wavelength is 0.88 without laser processing, whereas it is only 0.33 with the rear-side laser process step.

The solar industry is introducing p‐type monofacial passivated emitter and rear cells (PERC) into mass production. However, the efficiency of p‐type PERC cells is subject to light‐induced degradation (LID). In this paper, we introduce a novel solar cell design which we name BiCoRE as abbreviation of “bifacial co‐diffused rear emitter.” The BiCoRE cell process is very similar to the high volume proven PERC process sequence, but uses LID stable n‐type wafers. A boron silicate glass (BSG) silicon nitride (SiNz) stack at the rear side of the BiCoRE cells acts as protection layer against texturing and POCl3 diffusion, as boron dopant source during the POCl3 co‐diffusion as well as passivation layer. The rear contacts are formed by laser contact opening (LCO) and screen printing of an Al finger grid similar to the recently introduced PERC+ solar cells. The Al finger grid enables bifaciality and results in up to 8.5 μm deep aluminum back surface fields (Al‐BSFs) and up to 21.1% conversion efficiency obtained with n‐type reference solar cells. The multifunctional BSG/SiNz stack demonstrates up to 20.6% conversion efficiency with BiCoRE solar cells. When illuminated from the rear side, the BiCoRE cells exhibit conversion efficiencies up to 16.1% which corresponds to a bifaciality of 78%.

Abstract The porous silicon (PSI) process is a wafering method to fabricate high quality kerfless crystalline Si wafers by epitaxial wafer growth on porous Si and subsequent detachment from a reusable substrate wafer. The process yield is a key parameter for the economic viability of the PSI process. We experimentally demonstrate the detachment of 59 out of 62 PSI wafers with a size of 10 × 10 cm2, and separation layer etch current densities of 105–120 mA/cm2 for electrochemically etching the porous Si, and for substrate wafers with a resistivity of 15.7–16.9 mΩcm. We discuss the statistics of how to deduce a detachment probability from this. From our experiments, we determine a detachment yield of at least 88% with an error probability of 5%. The demonstration of a 99% detachment yield with an error probability of 5% would require at least 300 successfully detached wafers with no failed detachment. Samples have a minority carrier density ranging from 1 to 1.7 ms before any external gettering, which demonstrates the high electric quality of the PSI wafers.

The fabrication of thin solar cells by kerfless wafering techniques offers a high potential for the reduction of photovoltaic costs. We present an experimental setup for the exfoliation of thin crystalline silicon foils from a silicon substrate induced by the difference in thermal expansion coefficient of the silicon and an aluminum stressor layer at moderate temperatures. A moving temperature gradient across the substrate controls the crack propagation parallel to the silicon surface. We measure and simulate the spatial temperature distribution during thermal treatment and find that the direction of crack propagation is controlled by the temperature distribution. We detach foils with an area of 19.6 cm 2 with thickness values ranging from 50 to 80 μm within one layer. The foils have a smooth surface with some irregularities near the edge.

Kerfless silicon wafers epitaxially grown on porous silicon (PSI) and subsequently detached from the growth substrate are a promising candidate for reducing the cost of the silicon wafer, which is particularly important for silicon photovoltaics. However, the carrier lifetime of these epitaxial wafers has to be at least as high as that of today's standard Czochralski (Cz)-grown wafers in order to become competitive. Here, we compare the measured lifetimes of n -type epitaxial silicon wafers that grow on PSI and epitaxial silicon wafers that grow on nonporous surfaces of epi-ready wafers. The latter are subsequently ground to have free-standing epitaxial wafers. Gettering improves the carrier lifetime of the ground wafers up to 4.2 ms. In contrast, PSI wafers show regions with effective lifetimes of 4.5 ms, even without gettering. This lifetime value is a factor of four larger than lifetimes of Cz wafers which are typically employed in today's PERC solar cells. We model the lifetime measurements with three Shockley–Read–Hall (SRH) defects: two defects that exist in the PSI and in the epi-ready wafer and a third defect that is only present in the epi-ready wafer.

Laser welding of thin Al layers offers a silver-free and highly flexible option for the interconnection of Al-metallized solar cells. Welding requires the melting of the Al layers in order to form a reliable electrical and mechanical contact. Here, we investigate the process driving the melt front of the aluminum, which is attached to a transparent substrate, toward the interface between the two Al layers. In experiments, we observe two different mechanisms depending on the thickness of the irradiated layer. In the case of Al layers thinner than 5 μm, a melt-through of the Al-layer is observed, whereas for thicker layers, thermal expansion causes a breakage of the surface and ejection of molten Al, which enables the contact formation. Using simulations that are based on the finite-element method, we instigate both mechanisms. The simulation results match the experimental observations within the measurement uncertainty. In case of thin layers, the simulation shows that the process is limited by thermal diffusion. For thicker Al layers, the onset of melting on the irradiated side initiates the breakage of the surface and the ejection of the aluminum.

We use the fluorescence effect of the lamination material of photovoltaic (PV) modules to detect cracks in wafer-based solar cells in a power plant. For this purpose, the PV modules are irradiated by ultraviolet (UV) light, and the fluorescence light is measured by a camera. The measurement is realized in the dark. This new application of the fluorescence method allows new insight into cracks of a huge amount of PV modules during service life without remounting or touching the PV modules. We found that the frequency distribution of so-called cross cracks is almost homogenous in the PV modules. These cracks are frequently induced by crumbs or needle-shaped production equipment and not introduced after production. We show that the measured distribution of “cross cracks” in the PV modules fits to the binominal frequency distribution, as expected for production-induced cell failures. The measured crack frequency distribution for other crack types is compared with a finite-element simulation of a simplified PV module. We find that the lateral crack distribution correlates with the simulated strain distribution induced by module vibrations. In total, we found that 4.1% of the solar cells in the PV modules show at least one crack.