Evidence for near-Surface NiOOH Species in Solution-Processed NiO_x Selective Interlayer Materials: Impact on Energetics and the Performance of Polymer Bulk Heterojunction Photovoltaics

Abstract

The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiO x ) holeselective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O 2 -plasma treatment of the asdeposited NiO x films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni 2 O 3 , resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O 2 -plasma treated NiO x films are compared using both electron-only and BHJ devices. O 2 -plasma-treated NiO x interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiO x interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiO x films and the resultant chemical reactions at the oxide/ organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime.

Water Interaction with Sputter-Deposited Nickel Oxide on n-Si Photoanode: Cryo Photoelectron Spectroscopy on Adsorbed Water in the Frozen Electrolyte Approach

“…The main feature at 529.4 eV is assigned to the O 2− lattice oxygen. 23,28 For the pristine layer the broader structure at higher binding energies is assigned to Ni 2 O 3 , but upon water exposure also hydroxylated nickel oxide species appear in this binding energy region. 23,28 Initially, a strongly attenuated silicon oxide peak and minor carbon oxide contaminations are present around 533.0 eV.…”

“…Additionally, the broader feature at 855.7 eV is known to include contributions from hydroxylated species, namely Ni(OH) 2 (Ni 2+ ) 24,28,29,37 and NiOOH (Ni 3+ ) 15,24,40 that are all summarized under the term h-NiO x . Unfortunately, further stoichiometric phases cannot be deconvoluted here due to their complex signature.…”

Electrodeposited NiO anode interlayers: Enhancement of the charge carrier selectivity in organic solar cells

“…Also deposition methods such as pulsed laser deposition, sol-gel route, or sputtering play a determining influence [9,18]. We consider here that UVO treatment followed in our experiments produces similar effects as those occurring after O 2 -plasma treatment [18].…”

Interfacial Layer Engineering for Performance Enhancement in Polymer Solar Cells

“…The performance enhancement of NiO-based cells is attributed to the favorable energy band alignment, interface passivation, crystallinity, smooth surface, and optical transparency of NiO IFL [117]. The results indicate that NiO is an excellent p-type semiconductor as anode IFL for high-performance PSCs [118].…”