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.
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