A. Morales-Vilches

Low Surface Recombination in Silicon-Heterojunction Solar Cells With Rear Laser-Fired Contacts From Aluminum Foils

In this study, an approach to create laser-fired contacts from aluminum foils is studied on p-type silicon-heterojunction solar cells. This alternative approach consists of the use of aluminum foils instead of evaporated layers as a metal source and rear electrode for the laser-firing process. A q-switched infrared laser (1064 nm) was employed to create the local point contacts. Quasi-steady-state photoconductance measurements evidenced a limited degradation in the surface passivation quality during the laser-firing process. Heterojunction solar cells fabricated with these rear contacts reached a best conversion efficiency of 18% with a remarkable open-circuit voltage of 690 mV. These values were very close to those of reference devices fabricated with evaporated aluminum layers. This result suggests a similar effect on the rear surface passivation by both contact strategies. However, external quantum efficiency curves revealed a better response from devices with a rear aluminum foil in the near infrared. Optical measurements indicate that this effect can be related to a higher internal reflection at the back surface. Consequently, laser-fired contacts from aluminum foils appear to be a fast and convenient solution for the rear contact of high-efficiency silicon solar cells.

Progress in silicon heterojunction solar cell fabrication with rear laser-fired contacts

Silicon Heterojunction (SHJ) solar cells are one of the most promising alternatives for high efficiency industrially feasible solar cells. The structure of these devices is based on hydrogenated amorphous silicon (a-Si:H) layers deposited at low temperature on crystalline silicon (c-Si) substrates. This fabrication process reduces the thermal stress on the substrate and is compatible with thinner wafers. In this work, we present our recent progress in the fabrication of SHJ solar cells on p-type c-Si wafers. The deposition conditions of hydrogenated amorphous silicon-carbon (a-SiCx:H) layers obtained by Plasma Enhanced Chemical Vapor Deposition (PECVD) are optimized. We have also applied a novel laser-firing process to contact the rear side of the fabricated devices. In this way, solar cells with point contacts through rear passivating layers can be fabricated without any photolithographic step. Recently, our group has obtained a remarkable conversion efficiency of 17.2 % on 1 cm2 SHJ solar cells fabricated in a fully low temperature process.

Study of the Surface Recombination Velocity for Ultraviolet and Visible Laser-Fired Contacts Applied to Silicon Heterojunction Solar Cells

In this study, we investigate the effect of the laser-firing process on the back surface passivation of p-type silicon heterojunction solar cells. For that purpose, two different nanosecond laser sources radiating at ultraviolet (UV) (355 nm) and visible (532 nm) wavelengths are employed. First, we optimize the laser-firing process in terms of the electrical resistance of locally diffused point contacts. Specific contact resistance values as low as 0.91 and 0.57 mΩ·cm2 are achieved for the visible and ultraviolet laser sources, respectively. In addition, the impact of the laser-firing process on the rear surface passivation is studied by analyzing the internal-quantum-efficiency curves of complete devices. Low surface recombination velocities in the range of 300 cm/s are obtained for the ultraviolet laser with a 1% fraction of contacted area. This value increases to about 700 cm/s for the visible laser, which indicates a significantly higher recombination at the contacted area. The best heterojunction solar cells with rear laser-fired contacts are obtained for the ultraviolet laser and reached a 17.5% conversion efficiency.

Interdigitated back contact silicon heterojunction solar cells: Towards an industrially applicable structuring method

We report on the investigation and comparison of two different processing approaches for interdigitated back contacted silicon heterojunction solar cells: our photolithography-based reference procedure and our newly developed shadow mask process. To this end, we analyse fill factor losses in different stages of the fabrication process. We find that although comparably high minority carrier lifetimes of about 4 ms can be observed for both concepts, the shadow masked solar cells suffer yet from poorly passivated emitter regions and significantly higher series resistance. Approaches for addressing the observed issues are outlined and first solar cell results with efficiencies of about 17 % and 23 % for shadow masked and photolithographically structured solar cells, respectively, are presented.

ZnO:Al/a-SiOx front contact for polycrystalline-silicon-on-oxide (POLO) solar cells

Polycrystalline-silicon-on-oxide (POLO) junctions and related contacting schemes have shown their capability to facilitate high efficiencies for solar cells with passivating selective contacts [1-3]. In this work the front contacting of two-side contacted POLO cells with sputtered aluminum-doped zinc oxide (ZnO:Al) has been investigated. Different approaches were followed to obtain good lifetimes in cell precursors and keep high Voc values in finished cells. Degradation in minority carrier lifetime and implied Voc (iVoc) was observed after the ZnO:Al sputtering deposition. In order to recover the passivation, various thermal treatments were applied. The necessity to implement a protecting layer to cap the ZnO:Al/poly-Si structures during the annealing treatment to prevent a fill factor degradation in finished cells was observed. Initially an intrinsic a-Si:H layer was used as a temporary protecting layer. However, during the decapping process, to remove the amorphous layer, lifetime and iVoc are significantly degraded. Therefore a permanent a-SiOx protecting layer was implemented for maintaining good passivation (Voc = 710 mV). This layer has the additional benefit of improving the optical (AR) behaviour on finished cells (increasing Jsc by 1.5%). The best cell reached a conversion efficiency of 21.7 %.Polycrystalline-silicon-on-oxide (POLO) junctions and related contacting schemes have shown their capability to facilitate high efficiencies for solar cells with passivating selective contacts [1-3]. In this work the front contacting of two-side contacted POLO cells with sputtered aluminum-doped zinc oxide (ZnO:Al) has been investigated. Different approaches were followed to obtain good lifetimes in cell precursors and keep high Voc values in finished cells. Degradation in minority carrier lifetime and implied Voc (iVoc) was observed after the ZnO:Al sputtering deposition. In order to recover the passivation, various thermal treatments were applied. The necessity to implement a protecting layer to cap the ZnO:Al/poly-Si structures during the annealing treatment to prevent a fill factor degradation in finished cells was observed. Initially an intrinsic a-Si:H layer was used as a temporary protecting layer. However, during the decapping process, to remove the amorphous layer, lifetime and iVoc are significa...

A critical analysis on the role of back surface passivation for a-Si/c-Si heterojunction solar cells

Back surface passivation is a well-known method to reduce carrier recombination and hence improves the efficiency of crystalline silicon solar cells. In this manuscript, we critically analyze the role of this process for a-Si/c-Si heterojunction solar cells through a combination of device fabrication, multiple characterization techniques, and modeling. Curiously, our experimental results indicate that dark current characteristics of these devices do not scale in accordance with the improvements in carrier lifetime achieved through back surface passivation. Our results indicate these puzzling experimental results could be due to the possibility that carrier injection from crystalline silicon base significantly contributes to the dark current of these devices. This result has obvious and significant implications towards understanding the device physics and efficiency optimization of a-Si/c-Si heterojunction devices.