For nanomaterials, surface chemistry can dictate fundamental material properties, including charge-carrier lifetimes, doping levels, and electrical mobilities. In devices, surface defects are usually the key limiting factor for performance, particularly in solar-energy applications. Here, we develop a strategy to uniformly and selectively passivate defect sites in semiconductor nanomaterials using a vapor-phase process termed targeted atomic deposition (TAD). Because defects often consist of atomic vacancies and dangling bonds with heightened reactivity, we observe-for the widely used p-type cathode nickel oxide-that a volatile precursor such as trimethylaluminum can undergo a kinetically limited selective reaction with these sites. The TAD process eliminates all measurable defects in NiO, leading to a nearly 3-fold improvement in the performance of dye-sensitized solar cells. Our results suggest that TAD could be implemented with a range of vapor-phase precursors and be developed into a general strategy to passivate defects in zero-, one-, and two-dimensional nanomaterials.
The synthesis, X-ray structures and photophysics of ten complexes of CuX (X = I or Br) with bridging N-substituted and N,N'-disubstituted piperazines (Pip) are presented. Depending on the steric demand of the Pip substituents, the complexes fall into four categories: (CuX)(4)(Pip)(2), which are networks of linked Cu(4)X(4) cubane units, (CuX)(2)(Pip), which are chains of linked Cu(2)X(2) rhombs, and (CuX)(2)(Pip)(2) or (CuX)(4)(Pip)(4), which are simple rhomboid dimers and cubane tetramers. A combination of spectroscopic studies and DFT calculations was used to investigate the luminescence of the products. The results suggest that the relatively high energy emission seen in dimers is due to cluster-centred (XMLT/metal-centred) excitations for the aliphatic amines and MLCT (d →π*) for aromatic amines, and low energy emission seen in the tetramers is the result of cluster-centred transitions. The (CuI)(2)(Pip) complexes act as sensor materials, undergoing irreversible reaction with aliphatic and aromatic amines (Nu) in the vapour state, irreversibly producing cubanes (CuI)(4)Nu(4), with corresponding production of long wavelength emission.
A p-type metal oxide with high surface area and good charge carrier mobility is of paramount importance for development of tandem solar fuel and dye-sensitized solar cell (DSSC) devices. Here, we report the synthesis, hierarchical morphology, electrical properties, and DSSC performance of mesoscale p-type NiO platelets. This material, which exhibits lateral dimensions of 100 nm but thicknesses less than 10 nm, can be controllably functionalized with a high-density array of vertical pores 4−6, 5−9, or 7−23 nm in diameter depending on exact synthetic conditions. Thin films of this porous but still quasi-two-dimensional material retain a high surface area and exhibit electrical mobilities more than 10-fold higher than comparable films of spherical particles with similar doping levels. These advantages lead to a modest, 20−30% improvement in the performance of DSSC devices under simulated 1-sun illumination. The capability to rationally control morphology provides a route for continued development of NiO as a high-efficiency material for tandem solar energy devices.
A series of metal-organic networks of CuSCN were prepared by direct reactions with substituted pyridine and aliphatic amine ligands, L. Thiocyanate bridging is seen in all but 1 of 11 new X-ray structures. Structures are reported for (CuSCN)L sheets (L = 3-chloro- and 3-bromopyridine, N-methylmorpholine), ladders (L = 2-ethylpyridine, N-methylpiperidine), and chains (L = 2,4,6-collidine). X-ray structures of (CuSCN)L(2) are chains (L = 4-ethyl- and 4-t-butylpyridine, piperidine, and morpholine). A unique N-thiocyanato monomer structure, (CuSCN)(3-ethylpyridine)(3), is also reported. In most cases, amine ligands are thermally released at temperatures <100 °C. Strong yellow-to-green luminescence at ambient temperature is observed for the substituted pyridine complexes. High solid state quantum efficiencies are seen for many of the CuSCN-L complexes. Microsecond phosphorescence lifetimes seen for CuSCN-L are in direct contrast to the nanosecond-lifetime emission of CuSCN. MLCT associated with pyridine π* orbitals is proposed as the excitation mechanism.
Localized
trap states, which are deleterious to the performance
of many solar-energy materials, often originate from the under-coordinated
bonding associated with defects. Recently, the concept of targeted
atomic deposition (TAD) was introduced as a process that permits the
passivation of trap states using a vapor-phase precursor that selectively
reacts with only the surface defect sites. Here, we demonstrate the
passivation of nickel oxide (NiO) with the TAD process using diborane
gas for selective, low-temperature deposition of boron (B) under continuous
flow in a chemical vapor deposition (CVD) system. NiO is a ubiquitous
cathode material used in dye-sensitized solar cells (DSSCs), organic
photovoltaic devices, and organo-lead halide perovskite solar cells.
The deposition of B at 100 °C is shown to follow first-order
kinetics, exhibiting saturation at a B to Ni atomic ratio of ∼10%.
Electrochemical measurements, combined with first-principles calculations,
indicate that B passivates Ni vacancy defects by partially saturating
the bonding of the oxygen atoms adjacent to the vacancy. p-Type DSSCs
were fabricated using TAD-treated NiO and show a modest improvement
in photovoltaic performance metrics. The results highlight the potential
ubiquity of TAD passivation with a range of atomic precursors and
vapor-phase processes.
The
performance of dye-sensitized solar cells (DSSCs) depends on
the properties and interactions of three fundamental components: the
semiconductor, chromophore, and electrolyte. For the electrolyte,
the dependence of DSSC performance on the identity and valence state
of the spectator cation has not been well studied in p-type semiconductor
systems, although the effects of these species in n-type TiO2 devices are significant, producing large shifts in semiconductor
flat-band potential, charge-transfer kinetics, photocurrent, and open-circuit
voltage (V
OC). Here, we vary the spectator
cation in p-type NiO DSSCs and demonstrate an increase in V
OC by over 50% with two common redox couples.
Using optimal cations, we achieved high V
OC values without a significant reduction in photocurrent. Mott–Schottky
analysis and electrochemical impedance spectroscopy reveal that the
cation can shift the flat-band potential of NiO by nearly 1 V and
substantially alter the lifetime of charge carriers and charge-transfer
resistance at the semiconductor–electrolyte interface. Differences
between the anionic and cationic redox couples employed suggest favorable
and unfavorable interactions, respectively, with divalent cations
at the NiO surface, causing increases and decreases in charge carrier
recombination rate constants. Our results highlight the complex interaction
between the semiconductor and electrolyte solution and indicate that
varying the cation should yield immediate improvements in device metrics
for most p-type DSSC systems.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.