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

Two-level Metallization and Module Integration of Point-contacted Solar Cells

AbstractWe present a module integration process for back junction back contact (BJBC) solar cells featuring point contacts to the back surface field (BSF). We apply two metallization layers. A first metal layer of aluminum is deposited onto the rear side of the cell and carries the current extracted from the polarity with the larger surface area fraction, e.g. from the emitter. The second metallization layer is an Al layer on a transparent substrate that we laser-weld to the small and point-shaped regions of the other polarity, e.g. the BSF region. We use a polymer for insulation between the two metal layers. The Al layer on the substrate also serves for cell interconnection, i.e., it enables module integration. Such an interconnection structure halves the fill factor losses due to the metallization. First proof-of-principle modules show a shunt free interconnection, no laser-induced damage, and an energy conversion efficiency of up to 20.7%

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

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

Rear side dielectrics on interdigitating

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