This item is non-discoverable
A tale of materials to devices: hydrogenation of interfaces for high open-circuit voltage silicon heterojunction solar cells
Date
2022
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
With solar energy expecting to reach Terawatt scale, it has a major role in decarbonizing the energy sector and phasing out of conventional fuels. The price/kilowatt for solar energy is already lower than coal, with solar plus batteries now being cost-competitive with natural gas for dispatchable peaking applications. Currently solar cells made from silicon wafers dominate with more than 95% of the market share. However, despite their maturity they still have potential for further improvements which are directly addressed in our current research. The existing silicon solar cells use high temperature processing steps (800 to 1000°C) to form the junctions and contacts. These steps lead to regions of higher recombination losses which reduce the open circuit voltage (Voc). One approach to eliminate these recombination losses is Silicon Heterojunction (SHJ) solar cell in which the junction and contact regions are deposited on the silicon wafer surface using a process involving plasma deposition at much lower temperature of <300 °C thereby reducing thermal budget. Also, the hetero-interface between crystalline (c-Si) and amorphous (a-Si:H) silicon results in solar cells with higher Voc. A major factor which limits the efficiency and Voc of the SHJ cells is the annihilation (recombination) of electronic charges at the interfaces when they interact with the defects formed at the surface of the silicon wafer due to unterminated Si bonds. This can be overcome by neutralizing (passivating) those defect states which act as recombination centers for the charge carriers. ☐ A major portion of my doctoral research seeks to reduce this electrical power loss by using hydrogen atoms to passivate the defects from unterminated Si bonds at the surface using hydrogen plasma treatment (HPT). We are developing the complete cell process using direct current (d.c.) plasma enhanced chemical vapor deposition (PECVD) which can be easily integrated to production line and has simpler equipment design. In this work, we have developed two HPT methods namely Post and Intermediate HPT that can help achieve excellent a-Si:H/c-Si interface passivation. These methods done using d.c. PECVD has relatively easier process control and a short processing time of ~30 seconds. To optimize the parameters of HPT we have used Optical Emission Spectroscopy (OES) as an in-situ diagnostic tool to co-relate the excited hydrogen species in the plasma to the minority carrier lifetime of the test structures. Our state-of-the-art test structures having ultra-thin (~10 nm) a-Si:H films treated with Post HPT passivating both sides of Si wafer has implied Voc of 755 mV which is close to the theoretical maximum of ~760 mV for 150 um Si wafer. The proof of concept has been demonstrated in Front junction SHJ solar cells with 18.7% efficiency and Voc of 729 mV; with our champion cell having 20.1% efficiency. The Intermediate HPT method also has comparable iVoc of 746 mV in test structures and 706 mV in Front SHJ devices with 17.7% efficiency. The bonding configuration of silicon-hydride in the films with different HPT methods are studied using Fourier Transform Infrared Spectroscopy (FTIR) and Raman spectroscopy is used to check any hydrogen induced crystallization in the films. We have also investigated the mechanism of HPT and annealing using advanced characterization. The stability of SHJ with HPT a-Si:H films and their potential contribution to Light and elevated temperature-induced degradation (LeTID) are also studied by performing accelerated degradation testing at elevated temperature and one-sun illumination conditions. The results are one of the first experimental demonstration of effect of hydrogen in long-term degradation for SHJ solar cells having extrinsically hydrogenated a-Si:H layers. ☐ After proving the proof of concept in Front junction solar cells we then applied the HPT technique to a-Si:H passivation in Interdigitated Back Contact (IBC) solar cell structures which have the highest efficiency of any single junction Si solar cell. Along with reducing the interface defect states, we also focused on developing a non-lithography IBC architecture which can be industrially viable. We have tried achieving this by patterning the doped region using masked deposition during PECVD and forming contacts by laser ablation. We thus seek to increase the Voc and efficiency of these promising solar cells while keeping them manufacturable by applying both scientific and engineering approaches to minimize the impact of surface defects. Our initial trials resulted in devices with Voc <300 mV and efficiency ~4%. A detailed study has been done in this work to investigate the cause of this poor Voc in spite of the same devices having implied Voc >700 mV before metallization. We optimized this architecture using multiple versions of this structure with different sequence of deposition, etching methods and patterning approaches. We found that one of the major deterrents in achieving high Voc is the plasma leakage under the mask during PECVD deposition. Our efforts lead to devices having Voc >650 mV and efficiency of 15.4%. The challenges to a low-cost scalable architecture and our methodology to overcome barriers at each stage have been elucidated. Additionally, we have explored the application of the hydrogen plasma in the broader field of photonics by using it as a proton exposure source for 2D materials. We investigated the plasma damage caused to semiconducting 2D material like molybdenum disulfide (MoS2) and how a graphene layer can be used as an encapsulant layer to prevent its degradation.
Description
Keywords
2D materials, Heterojunction, Hydrogen plasma treatment, Interdigitated back contact, Silicon solar cell