Lecturer, Electrical Engineering
H.S.Philip Wong, Postdoctoral Faculty Sponsor
Limitation of Optical Enhancement in Ultra-thin Solar Cells Imposed by Contact Selectivity
2018; 8: 8863
Ultra-thin crystalline silicon (c-Si) solar cell suffers both from poor light absorption and minority carrier recombination at the contacts resulting in low contact selectivity. Yet most of the research focuses on improving the light absorption by introducing novel light trapping technique. Our work shows that for ultra-thin absorber, the benefit of optical enhancement is limited by low contact selectivity. Using simulation we observe that performance enhancement from light trapping starts to saturate as the absorber scales down because of the increase in probability of the photo-generated carriers to recombine at the metal contact. Therefore, improving the carrier selectivity of the contacts, which reduces the recombination at contacts, is important to improve the performance of the solar cell beyond what is possible by enhancing light absorption only. The impact of improving contact selectivity increases as the absorber thickness scales below 20 micrometer (μm). Light trapping provides better light management and improving contact selectivity provides better photo-generated carrier management. When better light management increases the number of photo-generated carriers, better carrier management is a useful optimization knob to achieve the efficiency close to the thermodynamic limit. Our work explores a design trade-off in detail which is often overlooked by the research community.
View details for PubMedID 29891992
- Carrier-selective interlayer materials for silicon solar cell contacts JOURNAL OF APPLIED PHYSICS 2018; 123 (14)
Contact Selectivity Engineering in a 2 mum Thick Ultrathin c-Si Solar Cell Using Transition-Metal Oxides Achieving an Efficiency of 10.8.
ACS applied materials & interfaces
In this paper, the integration of metal oxides as carrier-selective contacts for ultrathin crystalline silicon (c-Si) solar cells is demonstrated which results in an 13% relative improvement in efficiency. The improvement in efficiency originates from the suppression of the contact recombination current due to the band offset asymmetry of these oxides with Si. First, an ultrathin c-Si solar cell having a total thickness of 2 mum is shown to have >10% efficiency without any light-trapping scheme. This is achieved by the integration of nickel oxide (NiOx) as a hole-selective contact interlayer material, which has a low valence band offset and high conduction band offset with Si. Second, we show a champion cell efficiency of 10.8% with the additional integration of titanium oxide (TiOx), a well-known material for an electron-selective contact interlayer. Key parameters including Voc and Jsc also show different degrees of enhancement if single (NiOx only) or double (both NiOx and TiOx) carrier-selective contacts are integrated. The fabrication process for TiOx and NiOx layer integration is scalable and shows good compatibility with the device.
View details for PubMedID 29124928
) Due to UV/Ozone Treatment.
ACS applied materials & interfaces
2017; 9 (20): 17201-17207
Drastic reduction in nickel oxide (NiOx) film resistivity and ionization potential is observed when subjected to ultraviolet (UV)/ozone (O3) treatment. X-ray photoemission spectroscopy suggests that UV/O3 treatment changes the film stoichiometry by introducing Ni vacancy defects. Oxygen-rich NiOx having Ni vacancy defects behaves as a p-type semiconductor. Therefore, in this work, a simple and effective technique to introduce doping in NiOx is shown. Angle-resolved XPS reveals that the effect of UV/O3 treatment does not only alter the film surface property but also introduces oxygen-rich stoichiometry throughout the depth of the film. Finally, simple metal/interlayer/semiconductor (MIS) contacts are fabricated on p-type Si using NiOx as the interlayer and different metals. Significant barrier height reduction is observed with respect to the control sample following UV/O3 treatment, which is in agreement with the observed reduction in film resistivity. From an energy band diagram point of view, the introduction of the UV/O3 treatment changes the defect state distribution, resulting in a change in the pinning of the Fermi level. Therefore, this work also shows that the Fermi level pinning property of NiOx can be controlled using UV/O3 treatment.
View details for DOI 10.1021/acsami.7b01629
View details for PubMedID 28447776
- Si Heterojunction Solar Cells: A Simulation Study of the Design Issues IEEE TRANSACTIONS ON ELECTRON DEVICES 2016; 63 (12): 4788-4795
Si Heterojunction Solar Cells: A Simulation Study of the Design Issues
IEEE Transactions on Electron Devices
2016; 63 (12): 4788 - 4795
View details for DOI 10.1109/TED.2016.2613057
- Schottky barrier height reduction for holes by Fermi level depinning using metal/nickel oxide/silicon contacts APPLIED PHYSICS LETTERS 2014; 105 (18)