To improve productivity in solar cell production, different approaches are developed. One includes the development of self-regulating manufacturing devices. As an aim the cell output is increased and the maintenance time is reduced. Our research focuses on an automated laser power regulation of a laser tool used for different process steps in fabricating interdigitated back contact solar cells. The development of a digital model of the laser tool using Automation Markup Language (AML) and converting it into an Open Platform Communications Unified Architecture (OPC UA) information model allows a self-regulation of the laser tool regarding the laser power. Integration of the model into an OPC UA Server and the combination with an OPC UA client even allows a continuous adjustment of process recipes.

bottom solar cells featuring an interdigitated back contact (IBC) based on laser processes can be integrated in highly efficient 3-Terminal perovskite/silicon tandem solar cells. While surface texturing of the Si bottom cell on the front side is essential for light trapping and thus enhanced absorption of long-wavelength light, it can hamper the conformal wet-chemical deposition of the perovskite top cell. Modification of our texture-etch allows a reduction of the pyramids size without affecting the light absorption in the Si cell. Our laser-processed double-side textured Si IBC cells reach efficiencies up to 22.6% under 1-sun illumination when using a non-optimized laser doping process. Perovskite layers deposited on these modified bottom cells cover their pyramids, which is necessary for shunt-free tandem devices. Since tandem operation requires a low-resistive electrical connection of the subcells, passivation properties of wet-processed SnO2 and TiO2 layers on Si are evaluated. To achieve higher tandem cell efficiencies, IBC solar cells with suitable passivating front contact layers need to be developed.

We investigate the conditions for the formation of crystal defects leading to residual stress after spot laser melting of monocrystalline silicon with microsecond laser pulses. With the help of micro-Raman spectroscopy and Secco defect etching, we find a sharp transition from dislocation free to dislocation rich recrystallization corresponding to a threshold laser pulse energy Ep, for a given laser pulse length τp and focus diameter df. Besides the dependence of the threshold Ep on τp and df, our experiments show a strong dependence on the crystal orientation. The f100g-oriented substrates resist laser pulses with a two times higher laser pulse energy than the {111}- or {110}-oriented substrates. Using electron backscatter diffraction (EBSD), we find evidence for the formation of grain boundaries parallel to the appearance of pores within the melt pool. The pores most likely form when the oxygen solubility in the melt and the resolidified material decreases during cool down and the excess oxygen leaves, forming vapor pores. Avoiding oxygen uptake from the environment by processing under vacuum conditions at p = 1 mbar ambient pressure, prevents both, pores and grain boundaries.

Laser processing, increasingly used for solar cell production, induces defects when choosing inappropriate process parameters. Besides the shape and the pulse energy of the laser, also the surface orientation of silicon substrate has a great influence on the defect formation. By applying a laser beam with a line focus exceeding the critical values such as line width and laser pulse energy, the development of dislocations is observed on (111)oriented wafers by transmission electron microscopy (TEM). As a result, the formed dislocations are arranged practically parallel to each other in planes parallel to the (111) surface. Their Burgers vectors lie within this plane too. Thus classical concepts of epitaxy hardly explain the dislocation formation. Alternative explanations have to take into account the excessively high temperature close to the melting point, the short time frames given by the laser pulse duration (100 ns) and the not yet analyzed inhomogeneous stress distribution. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Laser doping in industrial crystalline solar cells creates a selective emitter and thereby enhances the efficiency. Nevertheless, if not done carefully, the irradiation of the semiconductor introduces defects. We have suppressed defect formation by using a laser beam that is focused to a line with a width of only several micrometers. The maximal line width for a defect free recrystallization, however, depends on the surface orientation of the silicon. Using a transmission electron microscope, we find a dislocation-free recrystallized layer on (100)-oriented silicon wafers that are irradiated with a line focus smaller than or equal to 15 μm. For (111)-oriented surfaces, this holds for the use of a 5.2-μm-wide line focus. Wafers that are irradiated with a circular laser focus of diameter D = 36 μm show the formation of microcracks, but no hints of dislocations are found using transmission electron microscopy (TEM).