Epitaxie und Charakterisierung von neuartigen Halbleiter-Heterostrukturen auf Basis von unterschiedlichen Strukturtypen des Siliziums

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Fundamental consideration of junction formation strategies for phosphorus-doped emitters with J0e < 10 fA/cm2

This work shows the potential of further optimization of phosphorus-doped emitters in p-type silicon solar cells. We investigate the impact of different combinations of phosphorus doping profiles and surface passivation qualities on the saturation current density J0e by considering boundary conditions based on published experimental data. Our simulation study shows that there are two possible ways to achieve J0e values below 10 fA/cm2. One is the reduction of the electrically active phosphorus concentration nsurf at the surface beneath 2×1019 cm-3 and simultaneously reducing the surface recombination velocity Sp to below 103 cm/s. The other contrarily increases nsurf to values of up to 1×1021 cm-3 while ensuring full activation of all phosphorus dopants. In the latter case, J0e values below 10 fA/cm2 seem possible, even for Sp = 107 cm/s which is equal to the thermal velocity

High time resolution measurement of solar irradiance onto driving car body for vehicle integrated photovoltaics

Vehicle integrated photovoltaic (VIPV) systems have much different requirements on maximum power tracking compared to stationary setups. The occurrence of fast changes between full irradiance and shading are demanding. To evaluate the specific impact of these conditions on the specifications of VIPV systems, we conduct high resolution measurements of the incident irradiance onto a car body while driving. We investigate the influence of environmental conditions like weather, season and building density in an urban environment on measured irradiance on the roof and the sides of a vehicle. We find that weather conditions have the highest impact on the measured irradiance on the roof, while the relative irradiance on the side depends more heavily on the season. We also find that changes in irradiance occur predominantly at frequencies below 1 Hz, but changes with 100 Hz or more can occur in certain situations, with a tendency toward higher frequencies for sunny weather. This must be considered in maximum power point tracker design

A Combination of Ion Implantation and High-Temperature Annealing: The Origin of the 265 nm Absorption in AlN

The commonly observed absorption around 265 nm in AlN is hampering the outcoupling efficiency of light-emitting diodes (LEDs) emitting in the UV-C regime. Carbon impurities in the nitrogen sublattice (CN) of AlN are believed to be the origin of this absorption. A specially tailored experiment using a combination of ion implantation of boron, carbon, and neon with subsequent high-temperature annealing allows to separate the influence of intrinsic point defects and carbon impurities regarding this absorption. Herein, the presented results reveal the relevance of the intrinsic nitrogen-vacancy defect VN. This is in contradiction to the established explanation based on CN defects as the defect causing the 265 nm absorption and will be crucial for further UV-LED improvement. Finally, in this article, a new interpretation of the 265 nm absorption is introduced, which is corroborated by density functional theory (DFT) results from the past decade, which are reviewed and discussed based on the new findings

Rear side dielectrics on interdigitating p+-(i)-n+ back-contact solar cells − hydrogenation vs. charge effects

Polysilicon-on-oxide (POLO) passivating contacts and interdigitated back-contact (IBC) cell technologies have recently attracted a lot of interest as candidates for the implementation in the next generation of solar cells. An IBC cell with POLO junctions for both polarities-a POLO2-IBC cell-has to electrically isolate the highly defective p+ and n+ poly-Si regions on the rear side of the cell to avoid parasitic recombination. Inserting an initially undoped, intrinsic (i) region between the p+ and n+ poly-Si regions was demonstrated to successfully prevent the parasitic recombination in the transition region of ISFH's 26.1%-efficient POLO2-IBC cell. In order to further improve the conversion efficiency towards 27%, we apply hydrogen-donating dielectric layer stacks to the p+-(i)-n+ POLO interdigitating rear side to enhance the passivation quality of the POLO junctions. We indeed show a significant improvement of POLO junctions on symmetrical full-Area homogenously doped reference samples, but when we apply a hydrogen-donating layer stack on the p+-(i)-n+ POLO interdigitating rear side, we observe a strong degradation in the performance of the POLO2-IBC cell. We attribute this to the formation of a conductive channel between the p+ and n+ poly-Si regions due to the strong negative charge density of the hydrogen-donating layer stack

Recombination Behavior of Photolithography-free Back Junction Back Contact Solar Cells with Carrier-selective Polysilicon on Oxide Junctions for Both Polarities

We report on ion-implanted, inkjet patterned back junction back contact silicon solar cells with POLysilicon on Oxide (POLO) junctions for both polarities – n+ doped BSF and p+ doped emitter. The recombination behavior is investigated at two different processing stages: before and after trench separation of p+ and n+ regions within polysilicon (poly-Si). Before trench separation, we find a systematic dependence of the recombination behavior on the BSF index, i.e. the p+n+-junction meander length in the poly-Si. Obviously, recombination at the p+n+-junction in the poly-Si limits the implied open circuit voltage Voc,impl. at one sun illumination and the implied pseudo fill factor pFFimpl. to 695 mV and 80%, respectively. After trench isolation, however, Voc,impl (pFFimpl.) values increase up to 730 mV (85.5%), corresponding to a pseudo-efficiency of 26.2% for an assumed short circuit current density Jsc of 42 mA/cm2. We demonstrate a photolithography-free back junction back contacted solar cell with p-type and n-type POLO junctions with an in-house measured champion efficiency of 23.9% on a designated area of 3.97 cm2. This efficiency is mainly limited by the imperfect passivation in the undoped trench regions and at the undoped front side.EU/FP7/60849

Introducing pinhole magnification by selective etching: Application to poly-Si on ultra-thin silicon oxide films

Carrier selective junctions formed by polycrystalline silicon (poly-Si) on ultra-thin silicon oxide films are currently in the spotlight of silicon photovoltaics. We develop a simple method using selective etching and conventional optical microscopy to determine the pinhole density in interfacial oxide films of poly-Si on oxide (POLO)-junctions with excellent electrical properties. We characterize the selective etching of poly-Si versus ultra-thin silicon oxide. We use test structures with deliberately patterned openings and 3 nm thin oxide films to check the feasibility of magnification by undercutting the interfacial oxide. With the successful proof of our concept we introduce a new method to access the density of nanometer-size pinholes in POLO-junctions with excellent passivation properties

Firing stability of tube furnace-annealed n-type poly-Si on oxide junctions

Stability of the passivation quality of poly-Si on oxide junctions against the conventional mainstream high-temperature screen-print firing processes is highly desirable and also expected since the poly-Si on oxide preparation occurs at higher temperatures and for longer durations than firing. We measure recombination current densities (J0) and interface state densities (Dit) of symmetrical samples with n-type poly-Si contacts before and after firing. Samples without a capping dielectric layer show a significant deterioration of the passivation quality during firing. The Dit values are (3 ± 0.2) x 1011 and (8 ± 2) x 1011 eV/cm2 when fired at 620°C and 900°C, respectively. The activation energy in an Arrhenius fit of Dit versus the firing temperature is 0.30 ± 0.03 eV. This indicates that thermally induced desorption of hydrogen from Si-H bonds at the poly-Si/SiOx interface is not the root cause of depassivation. Postfiring annealing at 425°C can improve the passivation again. Samples with SiNx capping layers show an increase in J0 up to about 100 fA/cm2 by firing, which can be attributed to blistering and is not reversed by annealing at 425°C. On the other hand, blistering does not occur in poly-Si samples capped with AlOx layers or AlOx/SiNy stacks, and J0 values of 2–5 fA/cm2 can be achieved after firing. Those findings suggest that a combination of two effects might be the root cause of the increase in J0 and Dit: thermal stress at the SiOz interface during firing and blistering. Blistering is presumed to occur when the hydrogen concentration in the capping layers exceeds a certain level

Parasitic Absorption in Polycrystalline Si-layers for Carrier-selective Front Junctions

We investigate the optical properties of n- and p-type polycrystalline silicon (poly-Si) layers. We determine the optical constants n and k of the complex refractive index of polycrystalline silicon by using variable-angle spectroscopic ellipsometry. Moreover, we investigate the effect of different doping levels in the poly-Si on free carrier absorption (FCA). Thereby, we demonstrate that the FCA in poly-Si can be described by a model developed for crystalline silicon (c-Si) at a first approximation. The optical properties of hydrogenated amorphous silicon layers (a-Si:H) are also investigated as a reference. With ray tracing simulations the absorption losses of poly-Si and of the a-Si:H layers are quantified with respect to the film thickness. Based on this approach we find that the short-circuit current density losses due to parasitic absorption of poly-Si layers are significantly lower when compared to a-Si:H layers of the same thickness. For example the short-circuit current density loss due to a 20 nm thick p-type poly-Si layer is around 1.1 mA/cm2, whereas a 20 nm thick p-type a-Si:H layer leads to a loss of around 3.5 mA/cm2.BMWi/032570

Transient electrical characteristics of silicon heterojunction solar cells under fast transient illumination

For vehicle integrated photovoltaics VIPV there are possibly specific requirements on the speed of maximum power point tracking. However, a certain delay in the response of solar cells to fast irradiance changes might impact those requirements on the MPPT. The subject of this work is therefore the investigation of the transient electrical behaviour of state of the art silicon heterojunction solar cells. We use a fast switchable LED array and an oscilloscope to investigate the cell voltage of silicon heterojunction solar cells under transient illumination. We find that these cells have a switch off delay that is smaller than one millisecond when operated at or close to the maximum power point. This can be assumed to be faster than the time scale on that irradiances change typically occur on a car body while driving. We also find that, after switching off the light source, the transient cell voltage does not show a simple capacitive behaviour, but a more complex characteristic. A theoretical analysis shows that this behaviour can be explained by the nonlinear dependency of the diffusion and depletion capacitance and the minority carrier lifetime on the cell voltag