Self-Functionalization Behind a Solution-Processed NiO_x Film Used As Hole Transporting Layer for Efficient Perovskite Solar Cells

Abstract

Fabrication of solution-processed perovskite solar cells (PSCs) requires the deposition of high quality films from precursor inks. Frequently, buffer layers of PSCs are formed from dispersions of metal oxide nanoparticles (NPs). Therefore, the development of trustable methods for the preparation of stable colloidal NPs dispersions is crucial. In this work, a novel approach to form very compact semiconducting buffer layers with suitable optoelectronic properties is presented through a self-functionalization process of the nanocrystalline particles by their own amorphous phase and without adding any other inorganic or organic functionalization component or surfactant. Such interconnecting amorphous phase composed by residual nitrate, hydroxide, and sodium ions, proved to be fundamental to reach stable colloidal dispersions and contribute to assemble the separate crystalline nickel oxide NPs in the final film, resulting in a very homogeneous and compact layer. A proposed mechanism behind the great stabilization of the nanoparticles is exposed. At the end, the self-functionalized nickel oxide layer exhibited high optoelectronic properties enabling perovskite p-i-n solar cells as efficient as 16.6% demonstrating the pertinence of the presented strategy to obtain high quality buffer layers processed in solution at room temperature.

Ligand‐Free, Highly Dispersed NiOx Nanocrystal for Efficient, Stable, Low‐Temperature Processable Perovskite Solar Cells

“…Because the chemical composition of the surface of the NiO x film strongly influences the interfacial energy level alignment with active layers as well as the photovoltaic performance of the final device, X‐ray photoelectron spectroscopy was used to examine the surface composition of the E‐NiO x nanocrystal (Figure c). The X‐ray photoelectron spectrum for E‐NiO x in the binding energy range of the Ni 2p 3/2 signal could be separated into three distinct peaks: one at 853.8 eV attributed to Ni 2+ , which is associated with the NiO octahedral bonding of cubic rock salt NiO; one at 855.7 eV attributed to vacancy‐induced Ni 3+ ion or Ni 2 O 3 ; and a broad peak at 861.1 eV attributed to a shake‐up process . The corresponding O 1s spectrum contained two major peaks at 529.3 and 531.0 eV, which were assigned to O bonded within a regular oxide crystal (O 2− ) and oxygen atoms in positions adjacent to Ni vacancies (O (def)) within the oxide structure, respectively (Figure d) .…”

“…In contrast, signals at 3448 cm −1 (‐OH), 1630 cm −1 (H 2 O), and 1381 cm −1 (NO 3 − ) were detected in the spectrum for W‐NiO x , an indication that there were water molecules, hydroxyl groups, and other functional ligands on the surface of W‐NiO x nanoparticle. We found that these functional ligands allowed the W‐NiO x powder to produce a stable dispersion in deionized water without the addition of a surfactant or dispersant (Figure S1, Supporting Information) . When the annealing temperature was increased to 400 °C to remove surface ligands, the ability of W‐NiO x powder to disperse in water was markedly reduced.…”

“…Choy et al have fabricated a W‐NiO x film by using the same method to produce a high‐performance, flexible PSC with a PCE of 14.5% . Jaramillo et al have elucidated the mechanism underlying the stabilization of solution‐processed W‐NiO x nanoparticles and have used that knowledge to produce a rigid PSC with a PCE of 16.6% and null hysteresis . Recently, Jen et al developed a benzoic acid‐derivative self‐assembled monolayer interfacial modification method that was used to produce rigid and flexible, W‐NiO x ‐based, inverted PSCs with a PCE of 18.4 and 16.2%, respectively .…”

“…[30] Jaramillo et al have elucidated the mechanism underlying the stabilization of solution-processed W-NiO x nanoparticles and have used that knowledge to produce a rigid PSC with a PCE of 16.6% and null hysteresis. [31] Recently, Jen et al developed a benzoic acid-derivative self-assembled monolayer interfacial modification method that was used to produce rigid and flexible, W-NiO x -based, inverted PSCs with a PCE of 18.4 and 16.2%, respectively. [32] Unfortunately, the stability of PSCs made by using low-temperature W-NiO x remains inferior to that of devices fabricated with high-temperature-processed chargetransporting layers.…”

“…The X-ray photoelectron spectrum for E-NiO x in the binding energy range of the Ni 2p 3/2 signal could be separated into three distinct peaks: one at 853.8 eV attributed to Ni 2þ , which is associated with the Ni─O octahedral bonding of cubic rock salt NiO [30] ; one at 855.7 eV attributed to vacancy-induced Ni 3þ ion or Ni 2 O 3 ; and a broad peak at 861.1 eV attributed to a shake-up process. [31] The corresponding O 1s spectrum contained two major peaks at 529.3 and 531.0 eV, which were assigned to O bonded within a regular oxide crystal (O 2À ) and oxygen atoms in positions adjacent to Ni vacancies (O (def)) within the oxide structure, respectively ( Figure 2d). [35] A Fourier transform-IR analysis was also performed to examine the chemical groups at the surface of the nanocrystal (Figure 2e).…”