Dibromobianthryl ordering and polymerization on Ag(100)

We study the interaction between dibromobianthryl (DBBA) and the Ag(100) surface using scanning tunneling microscopy and density functional theory. DBBA is prochiral on adsorption and forms racemic domains with molecular rows aligned with the substrate nearest-neighbor [011] and [0 ̄11] directions. Deposition at elevated temperature leads to the formation of disordered meandering graphene nanowires of constant width

(Invited) Band Line-up of High-k Oxides on GaN

We present comprehensive experimental work on TixAl1-xOy (with x = 9%, 16%, 25%, 36%, 100%) and GaxAl1-xOy (x = 5%, 20%, 80% and 95%) fabricated using atomic layer deposition with the aim of achieving favorable band alignment with GaN for device applications. The permittivity, k, has been found to be enhanced from ~10 for 9% Ti to 76 for TiO2, but brings unfavorable band line-up and a small conduction band offset (&lt; 0.1 eV) with GaN for all Ti% studied. On the other hand, GaxAl1-xOy (x = 5%, 20%) films show substantial increase of the band gap from 4.5 eV for Ga2O3 to 5.5 eV for x = 5% Ga and 6.0 eV for x = 20% Ga in mixed oxides and a strong suppression of leakage current in associated metal insulator semiconductor (MIS) capacitors.</jats:p

Density Functional Theory and Experimental Determination of Band Gaps and Lattice Parameters in Kesterite Cu2ZnSn(SxSe1-x)4

The structures and band gaps of copper-zinc-tin selenosulfides (CZTSSe) are investigated for a range of anion compositions through experimental analysis and complementary first-principles simulations. The band gap was found to be extremely sensitive to the Sn-anion bond length, with an almost linear correlation with the average Sn-anion bond length in the mixed anion phase Cu2ZnSn(S x Se1-x)4. Therefore, an accurate prediction of band gaps using first-principles methods requires the accurate reproduction of the experimental bond lengths. This is challenging for many widely used approaches that are suitable for large supercells. The HSE06 functional was found to predict the structure and band gap in good agreement with the experiment but is computationally expensive for large supercells. It was shown that a geometry optimization with the MS2 meta-GGA functional followed by a single point calculation of electronic properties using HSE06 is a reasonable compromise for modeling larger supercells that are often unavoidable in the study of point and extended defects

Colloidal dual-band gap cell for photocatalytic hydrogen generation

We report that the internal quantum efficiency for hydrogen generation in spherical, Pt-decorated CdS nanocrystals can be tuned by quantum confinement, resulting in higher efficiencies for smaller than for larger nanocrystals (17.3% for 2.8 nm and 11.4% for 4.6 nm diameter nanocrystals). We attribute this to a larger driving force for electron and hole transfer in the smaller nanocrystals. The larger internal quantum efficiency in smaller nanocrystals enables a novel colloidal dual-band gap cell utilising differently sized nanocrystals and showing larger external quantum efficiencies than cells with only one size of nanocrystals (9.4% for 2.8 nm particles only and 14.7% for 2.8 nm and 4.6 nm nanocrystals). This represents a proof-of-principle for future colloidal tandem cell

Direct Silicon Heterostructures With Methylammonium Lead Iodide Perovskite for Photovoltaic Applications

We investigated the formation of photovoltaic (PV) devices using direct n-Si/MAPI (methylammonium lead tri-iodide) two-sided heterojunctions for the first time (as a possible alternative to two-terminal tandem devices) in which charge might be generated and collected from both the Si and MAPI. Test structures were used to establish that the n-Si/MAPI junction was photoactive and that spiro-OMeTAD acted as a “pinhole blocking” layer in n-Si/MAPI devices. Two-terminal “substrate” geometry devices comprising Al/n-Si/MAPI/spiro-OMeTAD/Au were fabricated and the effects of changing the thickness of the semitransparent gold electrode and the silicon resistivity were investigated. External quantum efficiency and capacitance–voltage measurements determined that the junction was one-sided in the silicon—and that the majority of the photocurrent was generated in the silicon, with there being a sharp cutoff in photoresponse above the MAPI bandgap. Construction of band diagrams indicated the presence of an upward valence band spike of up to 0.5 eV at the n-Si/MAPI interface that could impede carrier flow. Evidence for hole accumulation at this feature was seen in both Kelvin-probe transients and from unusual features in both current–voltage and capacitance–voltage measurements. The devices achieved a hysteresis-free best power conversion efficiency of 2.08%, V OC 0.46 V, J SC 11.77 mA/cm2, and FF 38.4%, demonstrating for the first time that it is possible to create a heterojunction PV device directly between the MAPI and n-Si. Further prospects for two-sided n-Si/MAPI heterojunctions are also discussed

Natural Band Alignments and Band Offsets of Sb2Se3 Solar Cells

Sb2Se3 is a promising material for use in photovoltaics, but the optimum device structure has not yet been identified. This study provides band alignment measurements between Sb2Se3, identical to that used in high-efficiency photovoltaic devices, and its two most commonly used window layers, namely, CdS and TiO2. Band alignments are measured via two different approaches: Anderson’s rule was used to predict an interface band alignment from measured natural band alignments, and the Kraut method was used in conjunction with hard X-ray photoemission spectroscopy to directly measure the band offsets at the interface. This allows examination of the effect of interface formation on the band alignments. The conduction band minimum (CBM) of TiO2 is found by the Kraut method to lie 0.82 eV below that of Sb2Se3, whereas the CdS CBM is only 0.01 eV below that of Sb2Se3. Furthermore, a significant difference is observed between the natural alignment- and Kraut method-determined offsets for TiO2/Sb2Se3, whereas there is little difference for CdS/Sb2Se3. Finally, these results are related to device performance, taking into consideration how these results may guide the future development of Sb2Se3 solar cells and providing a methodology that can be used to assess band alignments in device-relevant systems