Yannick Riesen

Light-induced Voc increase and decrease in high-efficiency amorphous silicon solar cells

High-efficiency amorphous silicon (a-Si:H) solar cells were deposited with different thicknesses of the p-type amorphous silicon carbide layer on substrates of varying roughness. We observed a light-induced open-circuit voltage (Voc) increase upon light soaking for thin p-layers, but a decrease for thick p-layers. Further, the Voc increase is enhanced with increasing substrate roughness. After correction of the p-layer thickness for the increased surface area of rough substrates, we can exclude varying the effective p-layer thickness as the cause of the substrate roughness dependence. Instead, we explain the observations by an increase of the dangling-bond density in both the p-layer—causing a Voc increase—and in the intrinsic absorber layer, causing a Voc decrease. We present a mechanism for the light-induced increase and decrease, justified by the investigation of light-induced changes of the p-layer and supported by Advanced Semiconductor Analysis simulation. We conclude that a shift of the electron quasi-Fermi level towards the conduction band is the reason for the observed Voc enhancements, and poor amorphous silicon quality on rough substrates enhances this effect.

Nanocrystalline Silicon Carrier Collectors for Silicon Heterojunction Solar Cells and Impact on Low-Temperature Device Characteristics

Silicon heterojunction solar cells typically use stacks of hydrogenated intrinsic/doped amorphous silicon layers as carrier selective contacts. However, the use of these layers may cause parasitic optical absorption losses and moderate fill factor (FF) values due to a high contact resistivity. In this study, we show that the replacement of doped amorphous silicon with nanocrystalline silicon is beneficial for device performance. Optically, we observe an improved short-circuit current density when these layers are applied to the front side of the device. Electrically, we observe a lower contact resistivity, as well as higher FF. Importantly, our cell parameter analysis, performed in a temperature range from -100 to +80 °C, reveals that the use of hole-collecting p-type nanocrystalline layer suppresses the carrier transport barrier, maintaining FF s in the range of 70% at -100 °C, whereas it drops to 40% for standard amorphous doped layers. The same analysis also reveals a saturation onset of the open-circuit voltage at -100 °C using doped nanocrystalline layers, compared with saturation onset at -60 °C for doped amorphous layers. These findings hint at a reduced importance of the parasitic Schottky barrier at the interface between the transparent electrodes and the selective contact in the case of nanocrystalline layer implementation.

Temperature dependence of hydrogenated amorphous silicon solar cell performances

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.

Class AAA LED-Based Solar Simulator for Steady-State Measurements and Light Soaking

Recent improvements in light-emitting diode (LED) technology has allowed for the use of LEDs for solar simulators with excellent characteristics. In this paper, we present a solar simulator prototype fully based on LEDs. Our prototype has been designed specifically for light soaking and current-voltage (I(V)) measurements of amorphous silicon solar cells. With 11 different LED types, the spectrum from 400 to 750 nm can be adapted to any reference spectrum-such as AM1.5g-with a spectral match corresponding to class A+ or better. The densely packed LEDs provide power densities equivalent to 4 suns for AM1.5g or 5 suns with all LEDs at full power with no concentrator optics. The concept of modular LED blocks and electronics guarantees good uniformity and easy up-scalability. Instead of cost-intensive LED drivers, low-cost power supplies were used with current control, including a feedback loop on in-house developed electronics. This prototype satisfies the highest classifications (better than AAA from 400 to 750 nm) with an illuminated area of 18 cm × 18 cm. For a broader spectrum, the spectral range could be extended by using other types of LEDs or by adding halogen lamps. The space required for this can be saved by using LEDs with higher power or by reducing the maximum light intensity.

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.

High Spatial Resolution of Thin-Film-on-ASIC Particle Detectors

Thin-film-on-ASIC (TFA) detectors are monolithic pixel devices that consist of amorphous silicon detecting diodes directly deposited on readout electronics. This paper presents a characterization of the TFA spatial resolution using the electron-beam-induced current (EBIC) technique, in which pixel pads patterned in microstrips were swept by the beam. We measured the spatial resolution for different configurations and thicknesses of the TFA active layer with different beam energies, currents and sweep speeds. We observed that the generated electron-hole pairs are collected most efficiently when the beam is over the microstrips. This better collection efficiency gives a larger signal than off the strips, and thereby enabled us to distinguish strips as small as 0.6 μm wide which are spaced by 1.4 μm gaps. This high spatial resolution was obtained even though microvoids in the amorphous silicon layer-induced by an ASIC morphology as rough as 2 μm-were observed in the detector cross section, thus demonstrating the potential of the TFA architecture even with non-planar readout electronics.

Demand Side Management for Enhanced Integration of Photovoltaics into Grid

Demand side management could be a very effective way to shift electric consumption of households to the maximum production hours of photovoltaic (PV) systems. A field study on more than 100 households were performed to evaluate the share of electric consumption that could be easily shifted. Assuming a fixed time window for the PV production (namely 11:00 to 15:00) the share of consumption already in this window was found to be around 20 % while the flexible consumption outside of that window was ca. 5% (not including hot water heating). Information to households was found insufficient to push households to change behaviour and shift consumption demonstrates that information is not sufficient. However, financial incentive households were able to shift ca. 3% of their consumption, Hot water heating at mid-day (rather than during the night) is shown to be a very effective demand side management measure.