The impact of silicon solar cell architecture and cell interconnection on energy yield in hot & sunny climates

Extensive knowledge of the dependence of solar cell and module performance on temperature and irradiance is essential for their optimal application in the field. Here we study such dependencies in the most common high-efficiency silicon solar cell architectures, including so-called Aluminum back-surface-field (BSF), passivated emitter and rear cell (PERC), passivated emitter rear totally diffused (PERT), and silicon heterojunction (SHJ) solar cells. We compare measured temperature coefficients (TC) of the different electrical parameters with values collected from commercial module data sheets. While similar TC values of the open-circuit voltage and the short circuit current density are obtained for cells and modules of a given technology, we systematically find that the TC under maximum power-point (MPP) conditions is lower in the modules. We attribute this discrepancy to additional series resistance in the modules from solar cell interconnections. This detrimental effect can be reduced by using a cell design that exhibits a high characteristic load resistance (defined by its voltage-over-current ratio at MPP), such as the SHJ architecture. We calculate the energy yield for moderate and hot climate conditions for each cell architecture, taking into account ohmic cell-to-module losses caused by cell interconnections. Our calculations allow us to conclude that maximizing energy production in hot and sunny environments requires not only a high open-circuit voltage, but also a minimal series-to-load-resistance ratio.

Amorphous silicon oxide window layers for high-efficiency silicon heterojunction solar cells

Christophe Ballif, Stefaan De Wolf, Antoine Descoeudres, Johannes Peter Seif

In amorphous/crystalline silicon heterojunction solar cells, optical losses can be mitigated by replacing the amorphous silicon films by wider bandgap amorphous silicon oxide layers. In this article, we use stacks of intrinsic amorphous silicon and amorphous silicon oxide as front intrinsic buffer layers and show that this increases the short-circuit current density by up to 0.43 mA/cm2 due to less reflection and a higher transparency at short wavelengths. Additionally, high open-circuit voltages can be maintained, thanks to good interface passivation. However, we find that the gain in current is more than offset by losses in fill factor. Aided by device simulations, we link these losses to impeded carrier collection fundamentally caused by the increased valence band offset at the amorphous/crystalline interface. Despite this, carrier extraction can be improved by raising the temperature; we find that cells with amorphous silicon oxide window layers show an even lower temperature coefficient than reference heterojunction solar cells (-0.1%/K relative drop in efficiency, compared to -0.3%/K). Hence, even though cells with oxide layers do not outperform cells with the standard design at room temperature, at higher temperatures—which are closer to the real working conditions encountered in the field—they show superior performance in both experiment and simulation.

American Institute of Physics2014

Window Layers for Silicon Heterojunction Solar Cells

Johannes Peter Seif

Currently, crystalline silicon (c-Si) wafer-based solar cells dominate the photovoltaic market (80-90%). In this thesis we concentrate on silicon heterojunction (SHJ) solar cells that--in contrast to diffused homojunction cells--rely on the application of amorphous silicon (a-Si:H) thin films. Unlike standard homojunction devices, which are typically limited by their highly recombination-active semiconductor-to-metal contacts, SHJ devices exhibit excellent surface passivation enabled by intrinsic and doped a-Si:H films. These a-Si:H layers, however, entail drawbacks for optical performance and carrier transport, two topics that will be addressed in this work. To this end we investigate non-traditional materials for SHJ devices, with the goal of replacing the a-Si:H or the transparent electrodes. These materials include microcrystalline silicon (uc-Si:H) and organic semiconductors for contact formation; amorphous silicon suboxides (a-SiOx:H) for surface passivation; and transparent electrodes applied by atomic layer deposition (ALD) as protective layers against subsequent processing steps. Along with the optical and electrical properties of these materials, we study the impact on device performance associated with their deposition. For this we test the devices under standard testing conditions (25 °C) and at elevated temperatures closer to those encountered in the field. For the investigations on uc-Si:H layers, we vary process parameters (including temperature, pressure, power, excitation frequency and hydrogen dilution) as well as pre-treatments, gas variations and nucleation layers. We assess the suitability of these approaches for SHJ solar cells and apply selected measures in devices. Thereby we demonstrate a gain in short-circuit current density in the range of 0.5-1 mA.cm-2 and good fill factor values of up to 79.2% using either n- and p-type uc-Si:H layers. Furthermore, with the goal of reducing optical losses, we test wide-bandgap a-SiOx:H layers for passivation. In terms of current gain without negative side effects on transport, we argue that these layers are best applied to the electron-collecting contact--put at the front of the device--as their application to the hole-collecting contact introduces a transport barrier for holes. This barrier deteriorates the device performance at 25 °C, but shows a beneficial effect on the temperature coefficient of the device, yielding coefficients as low as -0.1%/°C. In some cases--compared to standard devices--devices with a-SiOx:H layers exhibit superior performance at elevated temperatures, which can be of interest in warmer climates. In parallel to these main topic we also study aluminium-doped zinc oxide (ZnO:Al) layers deposited by ALD as a protective layer against sputter-induced damage and organic semiconductors as transparent electrodes for the hole-collecting contact. In both cases we observe a gain in terms of surface passivation, which indicates that these materials may be beneficial for the contact formation in future device structures. In addition to these material-related investigations, we unravel the temperature-dependence of each individual cell parameter and present a brief comparison of state-of-the-art technologies and their respective temperature dependence.

EPFL2015

Temperature dependence of hydrogenated amorphous silicon solar cell performances

Christophe Ballif, Franz-Josef Haug, Yannick Samuel Riesen, Michael Elias Stückelberger

Thin-film hydrogenated amorphous silicon solar (a-Si:H) cells are known to have better temperature coefficients than crystalline silicon cells. To investigate whether a-Si:H cells that are optimized for standard conditions (STC) also have the highest energy yield, we measured the temperature and irradiance dependence of the maximum power output (Pmpp), the fill factor (FF), the short-circuit current density (Jsc), and the open-circuit voltage (Voc) for four series of cells fabricated with different deposition conditions. The parameters varied during plasma-enhanced chemical vapor deposition (PE-CVD) were the power and frequency of the PE-CVD generator, the hydrogen-to-silane dilution during deposition of the intrinsic absorber layer (i-layer), and the thicknesses of the a-Si:H i-layer and p-type hydrogenated amorphous silicon carbide layer. The results show that the temperature coefficient of the Voc generally varies linearly with the Voc value. The Jsc increases linearly with temperature mainly due to temperature-inducedbandgap reduction and reduced recombination. The FFtemperature dependence is not linear and reaches a maximum at temperatures between 15 °C and 80 °C. Numerical simulations show that this behavior is due to a more positive space-charge induced by the photogenerated holes in the p-layer and to a recombination decrease with temperature. Due to the FF(T) behavior, the Pmpp (T) curves also have a maximum, but at a lower temperature. Moreover, for most series, the cells with the highest power output at STC also have the best energy yield. However, the Pmpp (T) curves of two cells with different i-layer thicknesses cross each other in the operating cell temperature range, indicating that the cell with the highest power output could, for instance, have a lower energy yield than the other cell. A simple energy-yield simulation for the light-soaked and annealed states shows that for Neuchâtel (Switzerland) the best cell at STC also has the best energy yield. However, for a different climate or cell configuration, this may not be true.