Monolithic Series Connection Of Thin-film Solar Modules Using Direct Patterning With A Nanosecond UV Laser

Thin-film silicon solar modules are usually monolithically series connected.To achieve monolithic interconnection the deposition processes are alternatedwith patterning processes. For the patterning of thin-film modules laser scribingis often used. In the commonly used laser scribing methods for thin-film siliconsolar modules the laser beam is incident through the (transparent) substrate toremove the films. However, for modules with opaque substrates the thin-filmlayers cannot be removed with a laser beam incident through the substrate. Alsofor some transparent plastic substrates a substrate incidence is undesirable. Inthese cases a laser beam incident directly from the iflm side is required. The laserscribing of thin-film solar modules from the iflm side faces several challenges likediiffculties of producing abrupt edges of scribing grooves, and plume shielding.The monolithic series connection is commonly achieved with three laser pattern?ing processes, PI, P2, and P3. In this work we will investigate the patterning ofeach of these three patterning processes from the iflm side with a nano-secondpulsed ultraviolet (UV) laser with a wavelength of355nm. This laser was chosenbecause light of this wavelength is strongly absorbed in all layers that need to beablated for the series connection.Most work on film-side scribing of thin-film solar cells focused on the struc?tural properties of the lines. However, in this work we also investigated therelevant electrical properties required for the series connection, and put these results in relation to the structural investigations.We investigated P1laser lines for various transparent conductive oxides (TCOs) on glass. The standard process for the PI was with an infrared (IR) laser. In this work we demonstrated that the ablation threshold for the UV laser is much lower. Furthermore, we show that the difference in ablation threshold originates in the higher absorption coefficient for the UV laser, which results in a cleaner ablation with less sign of molten and re-solidified material on the sur-face of the lines. In addition to these structural investigations we examined the electrical properties of the PI line which are relevant to the monolithic series con-nection. The PI line is an isolation line, meaning that the line must isolate two adjacent parts of the TCO electrode. In order to investigate the leakage currents induced by the P1line we deposited highly conductive μc-Si:H p-layers as they are used in state of the art thin-film silicon solar cells. From this we demonstrate that with the film-side processes we were able to achieve equally good P1lines as with conventional glass side processes. We demonstrated the high quality film-side P1lines by comparing mini-modules prepared with conventional and film-side P1lines.The function of the P2lines is to achieve a low-ohmic contact between the fron and back electrodes. We investigated the contact resistance of P2lines in a-Si:H layers from the film side with a ns pulsed UV laser for thin-film solar modules. We compared the contact resistance for several scribing methods and find that a low contact resistance is only achieved for double scribing methods (i.e. scribing the same line twice). Furthermore, we find that for such double scribing methods the alignment between the laser spots of the first and second sub-scribes is critical for good-quality contacts. In order to analyze these results in more detail, we examined the morphology and chemical composition at the surface of the laser lines using scanning electron microscopy, X-ray photoelec-tron spectroscopy and secondary ion mass spectrometry. From this analysis, we conclude that a good alignment between the first and second scribes results in less re-deposition of silicon in the form of SiO2on the surface, which explains differences in contact resistance found for the various scribing methods. As a good alignment between the two sub-scribes is difficult to obtain, these double scribing methods are not attractive for industrial application. We developed a new scribing method for which alignment between the two scribes is not critical, and demonstrate that we can obtain high-quality contacts with this method.We investigated laser scribing of P3isolation lines for a-Si:Ⅱ/μc-Si:H solar modules from the film side. In order to study the P3line electrically we developed a method characterize the relevant electrical properties. With our new method we can distinguish between leakage currents directly across the P3line, leakage currents along the laser line side walls and monitor changes in conductivity of the TCO underneath the line. We systematically varied scribing parameters. The properties of the P3lines where then electrically analyzed with the devel-oped method. Furthermore, the P3lines were analyzed using Scanning Electron Microscopy images. As with our characterization method we can distinguish be-tween various paths for the leakage current we could observe that a high leakage current directly across the P3lines is present when the back contact is not fully removed. The best P3lines have the back contact fully removed but an incom-plete removal of the silicon in the line. When the silicon layers are fully removed we observe an increase in the leakage current which we could ascribe to currents flowing along the sidewalls of the laser lines. The best P3laser line we obtained exhibits a very low leakage current density of only1.5μAcm-1at1V (current per unit laser line length). Finally, we demonstrate all three patterning steps from the film-side in a thin-film mini-module. The film side mini-module has an equal performance compared to a mini-module processed with well developed substrate side patterning processes.