Expanding the Perspective of Polymeric Selective Contacts in Photovoltaic Devices Using Branched Polyethylenimine

Abstract

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.

Polymeric Interlayer in CdS-Free Electron-Selective Contact for Sb2Se3 Thin-Film Solar Cells

“…It is especially significant in the series resistance and the ideality factor, and it appears to be independent of the buffer layer. This is consistent with the reported decrease in the specific contact resistance and the increase in passivation [ 36 , 39 , 50 ]. The incorporation of b-PEI depletes the band of the interlayer.…”

“…The initially found work function of 4.11 eV for c-Si is reduced to 3.28 eV when the b-PEI film is incorporated and to 3.08 eV after the 10 nm thick semitransparent Al layer is deposited on top of the b-PEI. The obtained UPS spectra is consistent with the reported increase in passivation of Si/b-PEI after Al metallization [ 39 ]. This would suggest that aluminum presence promotes the orientation of the dipoles.…”

“…In polyelectrolytes such as polyaniline and polypyrrole, the C-O signal is sometimes associated with surface oxidation of some sort [ 37 , 38 ]. However, local composition analysis by means of high-resolution transmission electron microscopy (HRTEM) combined with electron energy loss spectroscopy (EELS) of the b-PEI films suggests that the nitrogen and oxygen specimens could be from a bilayer structure [ 39 ]. This picture is consistent with the interaction of the cationic polymer (PEI + ) via the sp 3 -hybridized nitrogen specimens and their respective counterions from the solvent (EtOx − ).…”

Poly(amidoamine) Dendrimer as an Interfacial Dipole Modification in Crystalline Silicon Solar Cells

“…It forms a dipole layer at the interface with protonated nitrogen as positive and ethanolate from the counterion condensation as the negative counterpart. This occurrence was proven in our previous work on polyethilimine …”

“…Organic semiconductors are currently being widely investigated as a result of their specific features, like low-production cost, limited processing time, lightweightness, and mechanical flexibility. They are utilized in visible lasers, light-emitting diodes, solar cells, and optical amplifiers. − Dendrimers are a new class of organic semiconductors that are well-defined multivalent non-dispersed macromolecules. Dendrimers show some advantages over other organic compounds as a result of better solubility determined by their functional groups, nanoscaled size, and low viscosity. − Dendrimers have been studied in the past decade as an emerging material in photovoltaics.…”