Interdigitated back-contacted crystalline silicon solar cells fully manufactured with atomic layer deposited selective contacts

The interdigitated back-contacted (IBC) solar cell concept has been extensively studied for single-junction cells and more recently as a good choice for three-terminal tandem devices. In this work, carrier-selective contacts based on transition metal oxides deposited by atomic layer deposition (ALD) technique are applied to IBC c-Si(n) devices. In the first part of the study, we develop a hole-selective contact based on thin ALD vanadium oxide (V2O5) layers without using an amorphous silicon interlayer. The ALD process has been optimised, i.e. number of ALD cycles and deposition temperature, as a trade-off between surface passivation and contact resistivity. Noticeable surface passivation with recombination current densities around 100 fA/cm2, as well as reasonable contact resistivity values below 250 mOcm2 are reached using 200 ALD V2O5 cycles deposited at a deposition temperature of 125 °C (~10 nm layer thickness). The optimised ALD V2O5-based contact is combined with both an ALD TiO2-based electron-selective contact and an excellent surface passivation in non-contacted regions provided by ALD Al2O3 films, to form a fully ALD IBC c-Si(n) solar cell scheme. Fabricated devices yield photovoltaic efficiencies and pseudo efficiencies, i.e. calculated without series resistance losses, of 18.6% and 21.1% respectively (3 cm × 3 cm device area). These results reveal the potential of the ALD technique to deposit transition metal oxide (TMO) films as selective contacts on high efficiency devices, paving the way of using low thermal-budget, low cost and highly scalable processes for a highly demanding IBC solar cell architecture in the photovoltaic industry.Peer ReviewedPostprint (updated version

Atomic layer deposition of vanadium oxide films for crystalline silicon solar cells

Transition metal oxides (TMOs) are promising materials to develop selective contacts on high-efficiency crystalline silicon solar cells. Nevertheless, the standard deposition technique used for TMOs is thermal evaporation, which could add potential scalability problems to industrial photovoltaic fabrication processes. As an alternative, atomic layer deposition (ALD) is a thin film deposition technique already used for dielectric deposition in the semiconductor device industry that has a straightforward up scalable design. This work reports the results of vanadium oxide (V2O5) films deposited by ALD acting as a hole-selective contact for n-type crystalline silicon (c-Si) solar cell frontal transparent contact without the additional PECVD passivating layer. A reasonable specific contact resistance of 100 mO cm2 was measured by the transfer length method. In addition, measurements suggest the presence of an inversion layer at the c-Si/V2O5 interface with a sheet resistance of 15 kO sq-1. The strong band bending induced at the c-Si surface was confirmed through capacitance–voltage measurements with a built-in voltage value of 683 mV. Besides low contact resistance, vanadium oxide films provide excellent surface passivation with effective lifetime values of up to 800 µs. Finally, proof-of-concept both-side contacted solar cells exhibit efficiencies beyond 18%, shedding light on the possibilities of TMOs deposited by the atomic layer deposition technique.Peer ReviewedObjectius de Desenvolupament Sostenible::7 - Energia Assequible i No ContaminantPostprint (updated version

Expanding the perspective of polymeric selective contacts in photovoltaic devices using branched polyethylenimine

This work studies the use of polymeric layers of polyethylenimine (PEI) as an interface modification of electron-selective contacts. A clearly enhanced electrical transport with lower contact resistance and significant surface passivation (about 3 ms) can be achieved with PEI modification. As for other conjugated polyelectrolytes, protonated groups of the polymer with their respective counter anions from the solvent create an intense dipole. In this work, part of the amine groups in PEI are protonated by ethanol that behaves as a weak Brønsted acid during the process. A comprehensive characterization including high-resolution compositional analysis confirms the formation of a dipolar interlayer. The PEI modification is able to eliminate completely Fermi-level pinning at metal/semiconductor junctions and shifts the work function of the metallic electrode by more than 1 eV. Induced charge transport between the metal and the semiconductor allows the formation of an electron accumulation region. Consequently, electron-selective contacts are clearly improved with a significant reduction of the specific contact resistance (less than 100 mO·cm2). Proof-of-concept dopant-free solar cells on silicon were fabricated to demonstrate the beneficial effect of PEI dipolar interlayers. Full dopant-free solar cells with conversion efficiencies of about 14% could be fabricated on flat wafers. The PEI modification also improved the performance of classical high-efficiency heterojunction solar cells.This research has been supported by the Spanish government through Grants PID2019-109215RB-C41, PID2019109215RB-C43, PID2020-115719RB-C21, and PID2020116719RB-C41 and funded by MCIN/AEI/10.13039/ 501100011033. Besides this the authors would like to thank Prof. Jordi LLorca for his expertise and helpful discussions of XPS results, as well as Dr. Rodrigo Fernández-Pacheco of the Laboratorio de Microscopias Avanzadas (LMA-INA) of Zaragoza for the HRTEM images and EDS and EELS analysis, and Guillaume Sauthier from ICN2 for his contribution through UPS measurements and discussions.Peer ReviewedPostprint (published version

Effect of permanent dipole layers in the electrical characteristics of electronic devices and solar cells

Postprint (author's final draft

Hole Transport Layer based on atomic layer deposited V2Ox films: paving the road to semi-transparent CZTSe solar cells

© 2021 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/This work explores the use of very thin transparent vanadium oxide films deposited by atomic layer deposition (ALD) technique as hole transport layer for CZTSe solar cells as alternative of opaque molybdenum based contacts. Contact resistivity between the CZTSe absorber and the ALD V2Ox contact was measured. In order to improve contact quality, a cleaning bath using hydrofluoric acid (HF) dip, was also analyzed and its influence on kesterite surface was studied. Elementary material characterization and composition analysis of the V2Ox layers was performed. Contact quality was assessed yielding contact resistivity values below 9 and 30 mOcm2 for ALD and thermal evaporated V2Ox films respectively. The proposed ALD V2Ox based hole transport layer was deposited onto a glass covered with a transparent conductive oxide forming part of the rear contact scheme of a vertical CZTSe solar cell with a conventional ITO/CdS stack as electron transport layer. The impact of subsequent thermal post-annealing treatments in the cell performance was also analyzed yielding efficiency up to 3.9% on a semi-transparent CZTSe solar cell without any additional optimization process. In this way, a CZTSe solar cell with both transparent electrodes has been demonstrated paving the way to obtain in the future high efficiency bifacial and/or semitransparent Building –integrated photovoltaic devices.Authors acknowledge the use of instrumentation as well as the technical advice provided by the National Facility ELECMI ICTS, node "Laboratorio de Microscopias Avanzadas" at Universidad de Zaragoza; moreover, this work has been supported by the Ecuadorian government grants SENESCYT (BECAS DE POSGRADOS INTERNACIONALES 2018), Spanish Government through projects IGNITE (ENE2017-87671-C3-2-R) - SCALED (PID2019-109215RB-C41) and the European Research Council Grant Horizon 2020 CUSTOM-ART project (Grant agreement ID: 952982) and Ministerio de Ciencia e Innovación, grant number PID2020-116719RB-C41Peer ReviewedPostprint (author's final draft

Polyethienimine interface dipole tuning for electron selective contacts

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This work studies the use of thin layers of polyethylenimine (PEI) as an interface film to produce electron selective contacts for photovoltaic applications in crystalline silicon. Generally, in conjugated polyelectrolytes such as PEI with a high Lewis basicity, charge is accumulated along the chain of the polymer and counter anions from the solvent create an intense dipole array. In this work, part of the amine groups in PEI are protonated by the solvent that behaves as a weak Bronsted acid during the process. The PEI band modification is able to eliminate Fermi level pinning at metal/semiconductor junctions as it shifts the work function of the metallic electrode by more than 1 eV. As a consequence, induced charge transport between the metal and the semiconductor forms an electron accumulation region and promotes enhanced selectivity.This research has been supported by Spanish government through Grants PID2019-109215RB-C41 (SCALED), PID2019-109215RB-C43, PID2020-116719RB-C41 (MATER ONE) and PID2020-115719RB-C21 (GETPV) and funded by MCIN/AEI/ 10.13039/501100011033. Besides this the work is also supported by the international Grants SENESCYT-2018 funded by Ecuadorian government.Peer ReviewedPostprint (author's final draft