Electrical and Electronic Engineering - Research Publications
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ItemAn ITO-Free Kesterite Solar Cell(WILEY-V C H VERLAG GMBH, 2024-02)Photovoltaic thin film solar cells based on kesterite Cu2 ZnSn(S, Se)4 (CZTSSe) have reached 13.8% sunlight-to-electricity conversion efficiency. However, this efficiency is still far from the Shockley-Queisser radiative limit and is hindered by the significant deficit in open circuit voltage (VOC ). The presence of high-density interface states between the absorber layer and buffer or window layer leads to the recombination of photogenerated carriers, thereby reducing effective carrier collection. To tackle this issue, a new window structure ZnO/AgNW/ZnO/AgNW (ZAZA) comprising layers of ZnO and silver nanowires (AgNWs) is proposed. This structure offers a simple and low-damage processing method, resulting in improved optoelectronic properties and junction quality. The ZAZA-based devices exhibit enhanced VOC due to the higher built-in voltage (Vbi ) and reduced interface recombination compared to the usual indium tin oxide (ITO) based structures. Additionally, improved carrier collection is demonstrated as a result of the shortened collection paths and the more uniform carrier lifetime distribution. These advances enable the fabrication of the first ITO-free CZTSSe solar cells with over 10% efficiency without an anti-reflective coating.
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ItemSilicon solar cells with passivating contacts: Classification and performance(WILEY, 2023-04-01)Abstract The year 2014 marks the point when silicon solar cells surpassed the 25% efficiency mark. Since then, all devices exceeding this mark, both small and large area, with contacts on both sides of the silicon wafer or just at the back, have utilized at least one passivating contact. Here, a passivating contact is defined as a group of layers that simultaneously provide selective conduction of charge carriers and effective passivation of the silicon surface. The widespread success of passivating contacts has prompted increased research into ways in which carrier‐selective junctions can be formed, yielding a diverse range of approaches. This paper seeks to classify passivating contact solar cells into three families, according to the material used for charge‐carrier selection: doped amorphous silicon, doped polycrystalline silicon, and metal compounds/organic materials. The paper tabulates their current efficiency values, discusses distinctive features, advantages, and limitations, and highlights promising opportunities going forth towards even higher conversion efficiencies.
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ItemVisible to Short-Wave Infrared Photodetectors Based on ZrGeTe4 van der Waals Materials(AMER CHEMICAL SOC, 2021-09-29)The self-terminated, layered structure of van der Waals materials introduces fundamental advantages for infrared (IR) optoelectronic devices. These are mainly associated with the potential for low noise while maintaining high internal quantum efficiency when reducing IR absorber thicknesses. In this study, we introduce a new van der Waals material candidate, zirconium germanium telluride (ZrGeTe4), to a growing family of promising IR van der Waals materials. We find the bulk form ZrGeTe4 has an indirect band edge around ∼0.5 eV, in close agreement with previous theoretical predictions. This material is found to be stable up to 140 °C and shows minimal compositional variation even after >30 days storage in humid air. We demonstrate simple proof-of-concept broad spectrum photodetectors with responsivities above 0.1 AW-1 across both the visible and short-wave infrared wavelengths. This corresponds to a specific detectivity of ∼109 cm Hz1/2 W-1 at λ = 1.4 μm at room temperature. These devices show a linear photoresponse vs illumination intensity relationship over ∼4 orders of magnitude, and fast rise/fall times of ∼50 ns, also verified by a 3 dB roll-off frequency of 5.9 MHz. As the first demonstration of photodetection using ZrGeTe4, these characteristics measured on a simple proof-of-concept device show the exciting potential of the ZrGeTe4 for room temperature IR optoelectronic applications.
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ItemConductive and Stable Magnesium Oxide Electron-Selective Contacts for Efficient Silicon Solar Cells(WILEY-V C H VERLAG GMBH, 2017-03-08)A high Schottky barrier (>0.65 eV) for electrons is typically found on lightly doped n‐type crystalline (c‐Si) wafers for a variety of contact metals. This behavior is commonly attributed to the Fermi‐level pinning effect and has hindered the development of n‐type c‐Si solar cells, while its p‐type counterparts have been commercialized for several decades, typically utilizing aluminium alloys in full‐area, and more recently, partial‐area rear contact configurations. Here the authors demonstrate a highly conductive and thermally stable electrode composed of a magnesium oxide/aluminium (MgOx/Al) contact, achieving moderately low resistivity Ohmic contacts on lightly doped n‐type c‐Si. The electrode, functionalized with nanoscale MgOx films, significantly enhances the performance of n‐type c‐Si solar cells to a power conversion efficiency of 20%, advancing n‐type c‐Si solar cells with full‐area dopant‐free rear contacts to a point of competitiveness with the standard p‐type architecture. The low thermal budget of the cathode formation, its dopant‐free nature, and the simplicity of the device structure enabled by the MgOx/Al contact open up new possibilities in designing and fabricating low‐cost optoelectronic devices, including solar cells, thin film transistors, or light emitting diodes.
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ItemLithium Fluoride Based Electron Contacts for High Efficiency n-Type Crystalline Silicon Solar Cells(WILEY-V C H VERLAG GMBH, 2016-07-20)Low‐resistance contact to lightly doped n‐type crystalline silicon (c‐Si) has long been recognized as technologically challenging due to the pervasive Fermi‐level pinning effect. This has hindered the development of certain devices such as n‐type c‐Si solar cells made with partial rear contacts (PRC) directly to the lowly doped c‐Si wafer. Here, a simple and robust process is demonstrated for achieving mΩ cm2 scale contact resistivities on lightly doped n‐type c‐Si via a lithium fluoride/aluminum contact. The realization of this low‐resistance contact enables the fabrication of a first‐of‐its‐kind high‐efficiency n‐type PRC solar cell. The electron contact of this cell is made to less than 1% of the rear surface area, reducing the impact of contact recombination and optical losses, permitting a power conversion efficiency of greater than 20% in the initial proof‐of‐concept stage. The implementation of the LiFx/Al contact mitigates the need for the costly high‐temperature phosphorus diffusion, typically implemented in such a cell design to nullify the issue of Fermi level pinning at the electron contact. The timing of this demonstration is significant, given the ongoing transition from p‐type to n‐type c‐Si solar cell architectures, together with the increased adoption of advanced PRC device structures within the c‐Si photovoltaic industry.
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ItemDopant-Free Partial Rear Contacts Enabling 23% Silicon Solar Cells(WILEY-V C H VERLAG GMBH, 2019-03-06)Abstract Over the past five years, there has been a significant increase in both the intensity of research and the performance of crystalline silicon devices which utilize metal compounds to form carrier‐selective heterocontacts. Such heterocontacts are less fundamentally limited and have the potential for lower costs compared to the current industry dominating heavily doped, directly metalized contacts. A low temperature (≤230 °C), TiOx/LiFx/Al electron heterocontact is presented here, which achieves mΩcm2 scale contact resistivities ρc on lowly doped n‐type substrates. As an extreme demonstration of the potential of this heterocontact, it is trialed in a newly developed, high efficiency n‐type solar cell architecture as a partial rear contact (PRC). Despite only contacting ≈1% of the rear surface area, an efficiency of greater than 23% is achieved, setting a new benchmark for n‐type solar cells featuring undoped PRCs and confirming the unusually low ρc of the TiOx/LiFx/Al contact. Finally, in contrast to previous versions of the n‐type undoped PRC cell, the performance of this cell is maintained after annealing at 350–400 °C, suggesting its compatibility with conventional surface passivation activation and sintering steps.
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ItemTemperature and Humidity Stable Alkali/Alkaline-Earth Metal Carbonates as Electron Heterocontacts for Silicon Photovoltaics(WILEY-V C H VERLAG GMBH, 2018-08-06)Abstract Nanometer scale interfacial layers between the metal cathode and the n‐type semiconductor play a critical role in enhancing the transport of charge carriers in and out of optoelectronic devices. Here, a range of nanoscale alkali and alkaline earth metal carbonates (i.e., potassium, rubidium, caesium, calcium, strontium, and barium) are shown to function effectively as electron heterocontacts to lightly doped n‐type crystalline silicon (c‐Si), which is particularly challenging to contact with common metals. These carbonate interlayers are shown to enhance the performance of n‐type c‐Si proof‐of‐concept solar cells up to a power conversion efficiency of ≈19%. Furthermore, these devices are thermally stable up to 350 °C and both the caesium and barium carbonates pass a standard 1000 h damp heat test, with >95% of their initial performance maintained. The temperature and humidity stable electron heterocontacts based on alkali and alkaline earth metal carbonates show a high potential for industrial feasibility and longevity for deployment in the field.
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ItemMicrochannel contacting of crystalline silicon solar cells(NATURE PORTFOLIO, 2017-08-22)There is tremendous interest in reducing losses caused by the metal contacts in silicon photovoltaics, particularly the optical and resistive losses of the front metal grid. One commonly sought-after goal is the creation of high aspect-ratio metal fingers which provide an optically narrow and low resistance pathway to the external circuit. Currently, the most widely used metal contact deposition techniques are limited to widths and aspect-ratios of ~40 μm and ~0.5, respectively. In this study, we introduce the use of a micropatterned polydimethylsiloxane encapsulation layer to form narrow (~20 μm) microchannels, with aspect-ratios up to 8, on the surface of solar cells. We demonstrate that low temperature metal pastes, electroless plating and atomic layer deposition can all be used within the microchannels. Further, we fabricate proof-of-concept structures including simple planar silicon heterojunction and homojunction solar cells. While preliminary in both design and efficiency, these results demonstrate the potential of this approach and its compatibility with current solar cell architectures.