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. 2023 Jun 14;15(23):28705-28715.
doi: 10.1021/acsami.3c01930. Epub 2023 Jun 3.

Elimination of Interface Energy Barriers Using Dendrimer Polyelectrolytes with Fractal Geometry

E Ros  1 , T Tom  2   3 , P Ortega  1 , I Martin  1 , E Maggi  1 , J M Asensi  2   3 , J López-Vidrier  2   3 , E Saucedo  1 , J Bertomeu  2   3 , J Puigdollers  1 , C Voz  1
Affiliations

Elimination of Interface Energy Barriers Using Dendrimer Polyelectrolytes with Fractal Geometry

E Ros et al. ACS Appl Mater Interfaces. .

Abstract

In this work we study conjugated polyelectrolyte (CPE) films based on polyamidoamine (PAMAM) dendrimers of generations G1 and G3. These fractal macromolecules are compared to branched polyethylenimine (b-PEI) polymer using methanol as the solvent. All of these materials present a high density of amino groups, which protonated by methoxide counter-anions create strong dipolar interfaces. The vacuum level shift associated to these films on n-type silicon was 0.93 eV for b-PEI, 0.72 eV for PAMAM G1 and 1.07 eV for PAMAM G3. These surface potentials were enough to overcome Fermi level pinning, which is a typical limitation of aluminium contacts on n-type silicon. A specific contact resistance as low as 20 mΩ·cm2 was achieved with PAMAM G3, in agreement with the higher surface potential of this material. Good electron transport properties were also obtained for the other materials. Proof-of-concept silicon solar cells combining vanadium oxide as a hole-selective contact with these new electron transport layers have been fabricated and compared. The solar cell with PAMAM G3 surpassed 15% conversion efficiency with an overall increase of all the photovoltaic parameters. The performance of these devices correlates with compositional and nanostructural studies of the different CPE films. Particularly, a figure-of-merit (Vσ) for CPE films that considers the number of protonated amino groups per macromolecule has been introduced. The fractal geometry of dendrimers leads to a geometric increase in the number of amino groups per generation. Thus, investigation of dendrimer macromolecules seems a very good strategy to design CPE films with enhanced charge-carrier selectivity.

Keywords: Fermi-level pinning; conjugated polyelectrolytes; dendrimer; dipole; dipole film; electronic transport; solar cells.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Schematic representation of PAMAM dendrimers for consecutive generations (top), Schematic view of the different geometry from linear polymers to branched, dendrons and finally dendrimers (bottom).
Figure 2
Figure 2
Atomic Force Microscopy topographic images of CPE films based on PAMAM G1 and G3 spin-coated on silicon substrates.
Figure 3
Figure 3
Tauc plot to calculate the bandgap of CPE films based on b-PEI, PAMAM G1 and G3. These curves are calculated from transmittance measurements of the films deposited on sapphire substrates.
Figure 4
Figure 4
UPS spectra of n-type silicon substrates coated by CPE films of b-PEI, PAMAM G1 and G3 (up). Work function values of each sample calculated according to WF = hν – (Ecutoff – Eonset) – qVbias (eq 1).
Figure 5
Figure 5
Specific contact resistance as a function of the film thickness for conjugated polyelectrolytes based on b-PEI, PAMAM G1 and G3. In all cases a minimum thickness is needed to surpass Fermi level pinning and reduce the contact resistance. The contact resistance increases for thicker layers, suggesting that the main transport mechanism is direct electron tunneling.
Figure 6
Figure 6
Deconvolution of nitrogen signal in XPS spectra measured on CPE films based on b-PEI, PAMAM G1 and G3. This analysis evaluates the fraction of amino groups protonated in these films.
Figure 7
Figure 7
Schematic to explain the figure-of-merit Vσ and its relation to surface potential introduced by conjugated polyelectrolyte films.
Figure 8
Figure 8
Thermal dependence of the contact resistance measured on the thinner CPE films of b-PEI, PAMAM G1 and G3 (top). Energy barrier of each same deduced from its thermal dependence (eq 4) plotted versus reciprocal Vσ (bottom).
Figure 9
Figure 9
HRTEM cross-section images comparing CPE films of PAMAM G1 and G3 dendrimers. On the right side, EELS compositional analysis by EELS to detect nitrogen and oxygen content in the films.
Figure 10
Figure 10
Electrical characteristics (JV curves) measured under AM1.5 illumination for the solar cells implementing electron-selective contacts based on CPE films of the materials studied in this work.
Figure 11
Figure 11
Photovoltaic parameters versus the figure-of-merit Vσ for the proof-of-concept solar cells fabricated in this work. These results confirm that Vσ is representative for the quality of the different CPE films to be used as electron-selective contacts.
See this image and copyright information in PMC

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