Sang Bok Kim

Enhancing the efficiency of SnS solar cells via band-offset engineering with a zinc oxysulfide buffer layer

SnS is a promising earth-abundant material for photovoltaic applications. Heterojuction solar cells were made by vapor deposition of p-type tin(II) sulfide, SnS, and n-type zinc oxysulfide, Zn(O,S), using a device structure of soda-lime glass/Mo/SnS/Zn(O,S)/ZnO/ITO. A record efficiency was achieved for SnS-based thin-film solar cells by varying the oxygen-to-sulfur ratio in Zn(O,S). Increasing the sulfur content in Zn(O,S) raises the conduction band offset between Zn(O,S) and SnS to an optimum slightly positive value. A record SnS/Zn(O,S) solar cell with a S/Zn ratio of 0.37 exhibits short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) of 19.4 mA/cm2, 0.244 V, and 42.97%, respectively, as well as an NREL-certified total-area power-conversion efficiency of 2.04% and an uncertified active-area efficiency of 2.46%.

Atomic layer deposited zinc tin oxide channel for amorphous oxide thin film transistors

Bottom-gate thin film transistors with amorphous zinc tin oxide channels were grown by atomic layer deposition. The films maintained their amorphous character up to temperatures over 500 °C. The highest field effect mobility was ∼13 cm2/V·s with on-to-off ratios of drain current ∼109–1010. The lowest subthreshold swing of 0.27 V/decade was observed with thermal oxide as a gate insulator. The channel layers grown at 170 °C showed better transistor properties than those grown at 120 °C. Channels with higher zinc to tin ratio (∼3–4) also performed better than ones with lower ratios (∼1–3).

Band alignment of SnS/Zn(O,S) heterojunctions in SnS thin film solar cells

Band alignment is critical to the performance of heterojunction thin film solar cells. In this letter, we report band alignment studies of SnS/Zn(O,S) heterojunctions with various compositions of Zn(O,S). Valence band offsets (VBOs) are measured by femtosecond laser pump/probe ultraviolet photoelectron spectroscopy (fs-UPS) from which conduction band offsets (CBOs) are calculated by combining with band gaps obtained by optical transmission/reflection measurements. The SnS/Zn(O,S) heterojunctions with S/Zn ratios of 0.37 and 0.50 have desirable small positive CBOs, while a ratio of 0.64 produces an undesirable large positive CBO. The results are consistent with the device performance of SnS/Zn(O,S) solar cells.

Nitrogen-doped cuprous oxide as a p-type hole-transporting layer in thin-film solar cells

We demonstrate the potential of a nitrogen-doped cuprous oxide (Cu2O:N) film as a p-type hole-transporting layer for photovoltaic devices. To reduce back-contact resistance and create an electron-reflecting back surface field, high carrier density and appropriate work function are desired for the layer. Its electrical and optical properties can be appropriately tuned via nitrogen-doping to create a semi-transparent tunnel junction to a back-contact. We fabricate Cu2O-based heterojunction thin-film solar cells and insert a 20 nm-thick Cu2O:N hole-transporting layer between a silver back-contact and a Cu2O light-absorbing layer. The insertion of a 20 nm-thick Cu2O:N layer results in sizeable enhancements of fill-factor and power conversion efficiency of the solar cells. Cu2O:N thin-films may also be useful in other photovoltaic material systems, improving their back-contact properties as well as widening the range of possible back-contact materials.

Incorporation of Fe3O4 Nanoparticles into Organometallic Coordination Polymers by Nanoparticle Surface Modification

Surface-modified Fe(3)O(4) nanoparticles (NPs) can be obtained by substituting [(eta(5)-semiquinone)Mn(CO)(3)] for oleylamine surface protecting groups. The resulting NP can function as a nucleus or template to generate crystalline coordination polymers that contain superparamagnetic Fe(3)O(4) NPs. Hybridized magnetic properties can be obtained by introducing paramagnetic metal nodes, such as Mn(2+), into the polymers (see picture).

π-Bonded quinonoid transition-metal complexes

Coordination of the carbocyclic ring of hydroquinones to electrophilic transition-metal fragments such as Mn(CO)3+ and Rh(COD)+ produces stable π-bonded η6-complexes that are activated to facile reversible deprotonation of the –OH groups. The deprotonations are accompanied by electron transfer to the transition metal, which acts as an internal oxidizing agent or electron sink. With manganese as the metal, the resulting η5-semiquinone and η4-quinone complexes have been used to synthesize one- two- and three-dimensional polymeric metal–organometallic coordination networks. With rhodium as the metal, the π-quinonoid complexes have been demonstrated to play a unique role in multifunctional C–C coupling catalysis and in the synthesis of new organolithium reagents. Both classes of π-quinonoid complexes appear to have significant applications in nanochemistry by providing an excellent vehicle for templating the directed self-assembly of nanoparticles into functional materials.

Atomic layer deposition of tin oxide with nitric oxide as an oxidant gas

Atomic layer deposition (ALD) of tin oxide (SnO2) thin films was achieved using a cyclic amide of Sn(II) (1,3-bis(1,1-dimethylethyl)-4,5-dimethyl-(4R,5R)-1,3,2-diazastannolidin-2-ylidene) as a tin precursor and nitric oxide (NO) as an oxidant gas. Film properties as a function of growth temperature from 130–250 °C were studied. Highly conducting SnO2 films were obtained at 200–250 °C with the growth per cycle of ∼1.4 A/cycle, while insulating films were grown at temperatures lower than 200 °C. Conformal growth of SnO2 in holes of aspect-ratios up to ∼50:1 was successfully demonstrated.

Patterned Monolayers of Neutral and Charged Functionalized Manganese Arene Complexes on a Highly Ordered Pyrolytic Graphite Surface

Complex patterns: The arene manganese tricarbonyl complexes [Mn(eta(5)-2,5-didodecoxy-1,4-semiquinone)(CO)(3)] and [Mn(eta(6)-1,4-dioctyloxybenzene)(CO)(3)] BF(4) form patterned monolayers on the surface of highly ordered pyrolytic graphite (HOPG), as a result of hydrogen-bonding, hydrophobic, and electrostatic interactions, leading to an ordered 2D array of manganese atoms or ions.

Synthesis of Calcium(II) Amidinate Precursors for Atomic Layer Deposition through a Redox Reaction between Calcium and Amidines

Abstract We have prepared two new CaII amidinates, which comprise a new class of ALD precursors. The syntheses proceed by a direct reaction between Ca metal and the amidine ligands in the presence of ammonia. Bis(N,N′‐diisopropylformamidinato)calcium(II) (1) and bis(N,N′‐diisopropylacetamidinato)calcium(II) (2) adopt dimeric structures in solution and in the solid state. X‐ray crystallography revealed asymmetry in one of the bridging ligands to afford the structure [(η2‐L)Ca(μ‐η2:η2‐L)(μ‐η2:η1‐L)Ca(η2‐L)]. These amidinate complexes showed unprecedentedly high volatility as compared to the widely employed and commercially available CaII precursor, [Ca3(tmhd)6]. In CaS ALD with 1 and H2S, the ALD window was approximately two times wider and lower in temperature by about 150 °C than previously reported with [Ca3(tmhd)6] and H2S. Complexes 1 and 2, with their excellent volatility and thermal stability (up to at least 350 °C), are the first homoleptic CaII amidinates suitable for use as ALD precursors.

DC and RF characterizations of AlGaN/GaN MOSHEMTs with deep sub-micron T-gates and atomic layer epitaxy MgCaO as gate dielectric

High performance deep sub-micron T-gate AlGaN/GaN MOSHEMTs are demonstrated using lattice matched ALE MgCaO as gate dielectric. The 120 nm-Lg MOSHEMT has an I_DMAX of 1.2 A/mm, R_on of 1.5 Ω·mm, a f_t/f_max of 101/150 GHz, with negligible hysteresis and I_G, showing the promise as a GaN MOS technology. The work at Purdue University is supported by AFOSR and the work at Harvard University is supported by ONR.