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 mΩ·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.
As optoelectronic devices continue to improve, control over film thickness has become crucial, especially in applications that require ultra-thin films. A variety of undesired effects may arise depending on the specific growth mechanism of each material, for instance a percolation threshold thickness is present in Volmer-Webber growth of materials such as silver. In this paper, we explore the introduction of aluminum in silver films as a mechanism to grow ultrathin metallic films of high transparency and low sheet resistance, suitable for many optoelectronic applications. Furthermore, we implemented such ultra-thin metallic films in Dielectric/Metal/Dielectric (DMD) structures based on Aluminum-doped Zinc Oxide (AZO) as the dielectric with an ultra-thin silver aluminum (Ag:Al) metallic interlayer. The multilayer structures were deposited by magnetron sputtering, which offers an industrial advantage and superior reliability over thermally evaporated DMDs. Finally, we tested the optimized DMD structures as a front contact for n-type silicon solar cells by introducing a hole-selective vanadium pentoxide (V2O5) dielectric layer.
Dielectric/metal/dielectric structures based on vanadium pentoxide with a thin silver interlayer have been optimized to replace traditional transparent electrodes. As would be expected, there is a trade-off in the metal thickness to achieve simultaneously high transparency and low sheet resistance. It has been demonstrated that an ultrathin gold seed prevents the tendency of silver to form clusters. This wetting effect reduces the metal thickness needed to form a continuous film, which leads to a higher averaged transmittance and very low sheet resistance. On the other hand, vanadium pentoxide on silicon forms a high quality hole-selective contact. Thus, these structures can be used as an all-in-one transparent electrode and selective contact for a new kind of heterojunction solar cells. This concept has been proved in a 13.3% efficient solar cell fabricated on n-type silicon wafers. Besides dopant-free, the complete fabrication route did not require any sputtered transparent electrode.
Development of carrier selective contacts for crystalline silicon solar cells has been recently of great interest towards the further expansion of silicon photovoltaics. The use of new electron and hole selective layers has opened an array of possibilities due to the low-cost processing and non-doping contacts. Here, a non-doped heterojunction silicon solar cell without the use of any intrinsic amorphous silicon is fabricated using Deoxyribonucleic acid (DNA) as the electron transport layer (ETL) and transition metal oxide V2O5 as the hole transport layer. The deposition and characterization of the DNA films on crystalline silicon have been studied, the films have shown a n-type behaviour with a work function of 3.42 eV and a contact resistance of 28 mΩ cm 2 . This non-doped architecture has demonstrated a power conversion efficiency of 15.6%, which supposes an increase of more than 9% with respect to the cell not containing the biomolecule, thus paving the way for a future role of nucleic acids as ETLs.
High open-circuit voltage in Sb2Se3 thin-film solar cells is a key challenge in the development of earth-abundant photovoltaic devices. CdS selective layers have been used as the standard electron contact in this technology. Long-term scalability issues due to cadmium toxicity and environmental impact are of great concern. In this study, we propose a ZnO-based buffer layer with a polymer-film-modified top interface to replace CdS in Sb2Se3 photovoltaic devices. The branched polyethylenimine layer at the ZnO and transparent electrode interface enhanced the performance of Sb2Se3 solar cells. An important increase in open-circuit voltage from 243 mV to 344 mV and a maximum efficiency of 2.4% was achieved. This study attempts to establish a relation between the use of conjugated polyelectrolyte thin films in chalcogenide photovoltaics and the resulting device improvements.
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