Multi-scale Characterization of laser-induced defects in the production of heterojunction photovoltaic cells

Background incl. aims Silicon-based HeteroJuncTion solar cells (HJT or SHJ) are a family of photovoltaic cells based on the heterojunction formed between two materials with different bandgaps. They are hybrid devices combining the technology of classical crystalline silicon-based cells and thin-film cells. HJT are now a well-established reality as they guarantee high efficiency and mass production [1], however they suffer of slight losses of the cell integrity when they are cut and assembled with a shingled structure. In this study, a mass-produced HJT cell surface was scribed for half of its thickness employing an ns-IR laser. Subsequently, the cell is separated in sub-cells by mechanically cleaving. This process is both stressful for the cell structure due to the high level of thermalization and is also locally removing the passivation layer of the newly cut edges. These mechanisms induce a drop in performance at the edges of the cell, hindering the advantages of shingled technology [2]. In this study, defects induced by the cut were characterized using different multi-scale techniques to identify the technological solution that would allow the cell structure to be protected as much as possible. Methods The sample object of the study is a HJT cell with a bulk structure based on n-type silicon and a multi-layer passivation surface composed of 3 thin layers of intrinsic amorphous silicon doped with hydrogen (15-20 nm), n-doped amorphous silicon doped with hydrogen (15-20 nm) and indium thin oxide (ITO) (70-80 nm) provided by Applied Materials. The cut was performed with an ns-IR laser (Rofin Powerline F50) on an entire wafer (156,75 mm side) on several lines, while a second pristine wafer was used as a comparison. The techniques used were X-Ray Diffraction (Bruker D8 ADVANCE), Raman and Photo-Luminescence spectroscopy (Renishaw InViaTM), Scanning Electron Microscopy (Zeiss Auriga) equipped with EdX Spectroscope (Bruker Quantax) and with Focused Ion Beam (Physics d’Orsay Cobra) and Atomic Force Microscopy (Oxford Instruments Cypher VRS). Results The analyses, focused on the detection and identification of defects in the pristine and post-cut wafer, were initially concentrated on the physical-chemical characterization of the basic structure. The XRD analyses show that after laser cut, the ITO main peak disappears or consistently decreases its intensity, thus indicating a loss of crystallinity or even a detachment from the surface. Moreover, the silicon beneath ITO layers seems affected by the high energy laser treatment and in particular the strongest peak of Si (400) shows an increase in the strained component. In fact the pristine HJT sample shows, by means of XRD, a double peak of Si (400) with two relative maximum values at 30.3° and 30.4° (with a Mo tube). The relative amount of strained silicon (i.e. the intensity of the peak at lower angle and therefore higher interplanar distance) appears increased, such behaviour might indicate an annealing of the sample due to the high energy treatment. Through Raman and PhotoLuminescence (PL) analyses, it was then possible to assess the presence of crystalline defects within the silicon of both the pristine and post-cut wafers, with a clear splitting of the silicon peak in the case of Raman spectroscopy and a decrease in the Band To Band transition of the silicon in the case of PhotoLuminescence spectroscopy. The morphology and chemical composition of these defects were then studied using scanning electron microscopy and atomic force microscopy. The surface structure is composed of square-based pyramids necessary to maximize the active surface of the cell. The pristine wafer presents mechanical defects derived from the production phase which cause the removal of the surface layer of ITO and thus an initial random decrease in the cell's performance. Once the laser cut has been performed, however, a complete destruction of the structure can be seen in the proximity of the cut. Moving away from the cut up to about 1 mm, a gradual improvement of the integrity of the structure can be seen until its complete recovery. Afterwards, in addition to the vast damage to the structures around the cut area due to the very high temperatures, the presence of silicon-based particles of various sizes scattered throughout the sample was observed causing a masking effect which decreases the cell’s efficiency. Finally, viscoelastic and EDX maps were made on both the intact and cut zones unveiling that the ITO is not completely ablated; on the contrary, it tends to follow the reorganization of the silicon-based substrate, as also demonstrated through FIB-SEM. The viscoelastic maps revealed that the hardness of the sample strongly depends on the size of the ITO layer and, in the areas away from the cut, there is also a clear directional trend in hardness and thus in ITO thickness. This variation in thickness, although not critical, causes a heterogeneity that makes it easier to ablate the ITO in areas close to the cut. Conclusion A complete characterization and identification of defects within HJT cells (both pristine and post-cut) was conducted. Several defectivities, of structural, mechanical and thermal nature were identified and characterized. The defects on the pristine wafer show that there is still room to optimize the synthesis of these materials in order to improve their final performances. Finally, through the study of the cut-induced defects, it was possible to demonstrate the causes of the decrease in the performances of this type of cell after the cutting and shingling process, something which has been already widely reported in literature.