Henning Schulte-Huxel

High-Efficiency Cells From Layer Transfer: A First Step Toward Thin-Film/Wafer Hybrid Silicon Technologies

Future low-cost Si photovoltaics shall combine the high-efficiency potential of ultrathin monocrystalline Si films with the low cost per area of the Si-thin-film photovoltaics. The literature describes various techniques for fabricating ultrathin monocrystalline Si films with no need for sawing wafers. Layer transfer using epitaxy on porous Si and subsequent layer separation is one option. We demonstrate an independently confirmed aperture efficiency of 19.1% for a 4-cm2-sized layer transfer cell with a thickness of 43 μm. This cell has a passivated emitter and rear contact structure with an Al2O3-surface passivation by atomic layer deposition and lasered contact openings. Highly efficient thin crystalline solar cells have to be integrated into modules. We also report on laser bonding of Si cells to a metalized carrier for module integration.

Aluminum-Based Mechanical and Electrical Laser Interconnection Process for Module Integration of Silicon Solar Cells

In this paper, an interconnection method for the module integration of silicon solar cells by laser microwelding of the Al-metalized rear side of the solar cell to a metalized substrate is introduced. This laser microwelding process forms a direct mechanical and electrical connection between two Al-layers without the need for any soldering, conductive adhesives, or Ag-pastes. With a tensile tester, we measure tear-off stresses of up to 303 kPa for our laser weld spots. Furthermore, carrier lifetime measurements show that no defects are induced into the Si-crystal by the laser process over a wide range of laser pulse energies and number of laser pulses. In order to demonstrate the applicability of this laser-based interconnection method, we present a proof-of-concept module consisting of five n-type back-junction back-contact solar cells with a conversion efficiency of 20.0%.

Optimized Interconnection of Passivated Emitter and Rear Cells by Experimentally Verified Modeling

Recent reports about new cell efficiency records are highlighting the continuing development of passivated emitter and rear cells (PERC). Additionally, volume production has started, forming the basis for cutting edge solar modules. However, transferring the high efficiency of the cells into a module requires an adaptation of the conventional front metallization and of the cell interconnection design. This paper studies and compares the module output of various cell interconnection technologies, including conventional cell interconnection ribbons and wires. We fabricate solar cells and characterize their electrical and optical properties. From the cells, we build experimental modules with various cell interconnection technologies. We determine the optical and electrical characteristics of the experimental modules. Based on our experimental results, we develop an analytical model that reproduces the power output of the experimental modules within the measurement uncertainty. The analytical model is then applied to simulate various cell interconnection technologies employing halved cells, optical enhanced cell interconnectors, and multiwires. We also consider the effect of enhancing the cell-to-cell spacing. Based on the experimentally verified simulations, we propose an optimized cell interconnection for a 60-PERC module that achieves a power output of 323 W. Our simulations reveal that wires combined with halved cells show the best module performance. However, applying light-harvesting structures to the cell interconnection ribbons is an attractive alternative for upgrading existing production lines.

High-Efficiency Modules With Passivated Emitter and Rear Solar Cells—An Analysis of Electrical and Optical Losses

We process a photovoltaic (PV) module with 120 half passivated emitter and rear cells that exhibits an independently confirmed power of 303.2 W and a module efficiency of 20.2% (aperture area). The cells are optimized for operation within the module. We enhance light harvesting from the inactive spacing between the cells and the cell interconnect ribbons. Additionally, we reduce the inactive area to below 3% of the aperture module area. The impact of these measures is analyzed by ray-tracing simulations of the module. Using a numerical model, we analyze and predict the module performance based on the individual cell measurements and the optical simulations. We determine the power loss due to series interconnection of the solar cells to be 1.5%. This is compensated by a gain in current of 1.8% caused by the change of the optical environment of the cells in the module. We achieve a good agreement between simulations and experiments, both showing no cell-to-module power loss.

Reduced Module Operating Temperature and Increased Yield of Modules With PERC Instead of Al-BSF Solar Cells

We demonstrate a reduced operating temperature of modules made from passivated emitter rear cells (PERCs) compared with modules made from cells featuring an unpassivated fullarea screen-printed aluminum rear side metallization aluminum back surface field (Al-BSF). Measurements on specific test modules fabricated from p-type silicon PERC and Al-BSF solar cells reveal a 4 °C lower operating temperature for the PERC module under 1400 W/m2 halogen illumination, if no temperature control is applied. For detailed analysis of the temperature effect, we perform a 3-D ray tracing analysis in the spectral range from 300 to 2500 nm to determine the radiative heat sources in a photovoltaic (PV) module. We combine these heat sources with a 1-D finite element method model solving the coupled system of semiconductor, thermal conduction, convection, and radiation equations for module temperature and power output. The simulations reveal that the origin of the reduced temperature of the PERC modules is a higher efficiency, as well as a higher reflectivity, of the cells rear side mirror. This reduces the parasitic absorptions in the rear metallization and increases the reflection for wavelengths above 1000 nm. This operating temperature difference is simulated to be linear in intensity. The slope depends on the spectral distribution of the incoming light. Under 1000 W/m2 in AM1.5G, our simulations reveal that the operating temperature difference is about 1.7 °C. The operating temperature can be lowered another 3.2 °C, if all parasitic absorption for wavelengths longer than 1200 nm can be prevented. Standard testing conditions applying a temperature control to the module do not show this effect of enhanced performance of the PERC modules. Yield calculations for systems in the field will thus systematically underestimate their electrical power output unless the inherently lower operating temperature of PERC modules is taken into account.

Module interconnection of both sides-contacted silicon solar cells by screen-printing

We demonstrate the module interconnection by means of screen printing. The metallization paste is printed over the edge of five heterojunction solar cells. This connects the frontside of a cell to an underlying Al-patterned rear-side contact of the neighboring cell. All cell interconnections are realized in a single printing step. Encapsulated modules reach open circuit voltages up to 683 mV per cell and designated area conversion efficiencies up to 17.0 %. The designated module area is 82 cm2. We eliminate shunts that originate from laser scribing to reach this efficiency. Quantitative dark lock in thermography images and I-V curve modeling of the shunting losses lead the way.

Analysis of Thermal Processes Driving Laser Welding of Aluminum Deposited on Glass Substrates for Module Interconnection of Silicon Solar Cells

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.

Effect of UV illumination on the passivation quality of AlO x /c-Si interfaces

We report on the stability of the c-Si surface passivation quality by aluminum oxide (AIO<inf>x</inf>), silicon nitride (SiN<inf>y</inf>), and AlO<inf>x</inf>/SiN<inf>y</inf>, stacks under UV illumination. Low-temperature annealed AlO<inf>x</inf> shows a weak degradation during UV illumination, with surface recombination velocities (SRVs) of 25 cm/s after a UV dose of 275 kWh/m². This degradation is less pronounced compared to that of fired SiN<inf>y</inf> layers with an SRV of 117 cm/s. After a firing step, the AlO<inf>x</inf> layer show even an improvement during UV illumination, resulting in stabilized SRVs of down to 1 cm/s. The improvement is mainly due to an increase of the negative fixed charge density in the AlO<inf>x</inf> layer up to a large value of −1.2×10¹³ cm⁻².

Laser microwelding of thin Al layers for interconnection of crystalline Si solar cells: analysis of process limits for ns and μs lasers

Leibniz University of Hanover, Institute for Solid-State Physics, Appelstrasse 2,Hannover 30167, GermanyAbstract. We investigated a laser welding process for contacting aluminum-metallized crystal-line silicon solar cells to a 10-μm-thick aluminum layer on a glass substrate. We analyzed thethreshold for laser-induced damage in dependence on the solar cell metallization thickness byapplying the process on SiNx passivated lifetime samples. In addition, we measured themechanical failure stress of the laser welds by perpendicular tear-off. We applied two typesof laser processes; one used single or multiple 20-ns-laser pulses at 355 nm with fluencesbetween 12 and 40 J∕cm

Yield analysis and comparison of GaInP/Si and GaInP/GaAs multi-terminal tandem solar cells

We present a yield analysis of tandem devices consisting of GaInP top cells on Si or GaAs bottom cells with different terminal configurations. Inputs are the I-V and external quantum efficiency of the individual subcells and the irradiance-dependent module temperature of the bottom cell. Our model calculates the temperature of the tandem module by taking into account the performance, spectral working range and luminescent coupling of the different tandem devices, enabling an irradiance- and weather-dependent yield analysis for these modules. We apply the model to compare two types of two junction devices, a GaInP/GaAs monolithically grown tandem device, and a GaInP top cell stacked on a Si bottom cell, the present two best dual junction devices. When the subcells are series connected both technologies perform equally well. The performance of the GaInP/Si can be significantly improved relatively by 5.8% using 3-terminal (3T) devices with a back-contacted bottom cell instead of a 2T configuration, showing a...