Interpreting interfacial semiconductor-liquid capacitive characteristics impacted by surface states: a theoretical and experimental study of CuGaS2.

Semiconductor-liquid interfaces are essential to the operation of many energy devices. Crucially, the operational characteristics of such devices are dependent upon both the flat band potential and doping concentration present in their solid-state semiconducting region. Traditionally, capacitive "linear" Mott-Schottky plots have often been utilized to extract these two parameters. However, significant concentrations of surface states within semiconductor-liquid junctions can give rise to strong non-linearities that prevent an effective linearity-based analysis. In this work, we detail a theoretical approach for estimating both the doping concentration and flat band potential from the capacitive characteristics of semiconductor-liquid junctions heavily impacted upon by surface states. Our theoretical approach is applied to CuGaS2 immersed in an aqueous electrolyte, for which excellent convergent values of the doping concentration and flat band potential are obtained across a wide range of impedance measurement frequencies. The results suggest a marked improvement over a linearity-based approach that could assist the analysis of many types of semiconductor-liquid junctions subject to high concentrations of surface states.

Triplet Energy Transfers in Well-Defined Host-Guest Porphyrin-Carboxylate/Cluster Assemblies.

The dyes (5-(4-carboxylphenyl)-10,15,20-tritolylporphyrinato)zinc(II) (MCP) and (5,15-bis(4-carboxylphenyl)-15,20-ditolylporphyrinato)zinc(II) (DCP), as their sodium salts, were used to form assemblies with the unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)], dppm = (Ph2P)2CH2) via ionic CO2(-)···Pd3(2+) interactions. The photophysical properties in their triplet states were studied. The position of the T1 state of [Pd3(2+)] (∼8190 cm(-1)) has been proposed using DFT computations and was corroborated by the presence of a Tn → S0 delayed emission at 680-700 nm arising from a T1-T1 annihilation process at 77 K. The static quenching of the near-IR phosphorescence of the dyes at 785 nm (T1 → S0) was observed. Thermodynamically poor reductive and oxidative driving forces render the photoinduced electron transfer quenching process either inoperative or very slow in the T1 states. Instead, slow to medium T1-T1 energy transfer ((3)dye*···[Pd3(2+)] → dye···(3)[Pd3(2+)]*) operates through a Förster mechanism exclusively with kET values of ∼1 × 10(5) s(-1) on the basis of transient absorption measurements at 298 K.

Electron-Transfer Kinetics within Supramolecular Assemblies of Donor Tetrapyrrolytic Dyes and an Acceptor Palladium Cluster.

9,18,27,36-Tetrakis[meso-(4-carboxyphenyl)]tetrabenzoporphyrinatozinc(II) (TCPBP, as a sodium salt) was prepared in order to compare its photoinduced electron-transfer behavior toward unsaturated cluster Pd3(dppm)3(CO)(2+) ([Pd3(2+)]; dppm = Ph2PCH2PPh2 as a PF6(-) salt) with that of 5,10,15,20-tetrakis[meso-(4-carboxyphenyl)]porphyrinatozinc(II) (TCPP) in nonluminescent assemblies of the type dye···[Pd3(2+)]x (x = 0-4; dye = TCPP and TCPBP) using femtosecond transient absorption spectroscopy. Binding constants extracted from UV-vis titration methods are the same as those extracted from fluorescence quenching measurements (static model), and both indicate that the TCPBP···[Pd3(2+)]x assemblies (K14 = 36000 M(-1)) are slightly more stable than those for TCPP···[Pd3(2+)]x (K14 = 27000 M(-1)). Density functional theory computations (B3LYP) corroborate this finding because the average ionic Pd···O distance is shorter in the TCPBP···[Pd3(2+)] assembly compared to that for TCPP···[Pd3(2+)]. Despite the difference in the binding constants and excited-state driving forces for the photoinduced electron transfer in dye*···[Pd3(2+)] → dye(•+)···[Pd3(•+)], the time scale for this process is ultrafast in both cases (<85 fs). The time scales for the back electron transfers (dye(•+)···[Pd3(•+)] → dye···[Pd3(2+)]) occurring in the various observed species (dye···[Pd3(2+)]x; x = 0-4) are the same for both series of assemblies. It is concluded that the structural modification on going from porphyrin to tetrabenzoporphyrin does not greatly affect the kinetic behavior in these processes.

Are the orientation and bond strength of the RCO2(-)···M link key factors for ultrafast electron transfers?

The photo-induced electron transfers in the "straight up" ionic assemblies [Pd3(2+)]···MCP and [Pd3(2+)]···DCP···[Pd3(2+)] ([Pd3(2+)]* → MCP or DCP) are ultrafast (<85 fs) indicating that it is not necessary to have a strong coordination bond or a bent geometry to obtain fast electron injection in porphyrin-containing DSSCs.

TiO2@C core-shell nanoparticles formed by polymeric nano-encapsulation.

TiO2 semiconducting nanoparticles are known to be photocatalysts of moderate activity due to their high band-gap and high rate of electron-hole recombination. The formation of a shell of carbon around the core of TiO2, i.e., the formation of TiO2@C nanoparticles, is believed to partly alleviate these problems. It is usually achieved by a hydrothermal treatment in a presence of a sugar derivative. We present here a novel method for the formation of highly uniform C shell around TiO2 nanoparticles. For this purpose, TiO2 nanoparticles were dispersed in water using an oligomeric dispersant prepared by Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization. Then the nanoparticles were engaged into an emulsion polymerization of acrylonitrile, resulting in the formation of a shell of polyacrylonitrile (PAN) around each TiO2 nanoparticles. Upon pyrolysis, the PAN was transformed into carbon, resulting in the formation of TiO2@C nanoparticles. The structure of the resulting particles was elucidated by X-Ray diffraction, FTIR, UV-VIS and Raman spectroscopy as well as TEM microscopy. Preliminary results about the use of the TiO2@C particles as photocatalysts for the splitting of water are presented. They indicate that the presence of the C shell is responsible for a significant enhancement of the photocurrent.

An organic redox electrolyte to rival triiodide/iodide in dye-sensitized solar cells.

Dye-sensitized solar cells (DSCs) have achieved impressive conversion efficiencies for solar energy of over 11% with an electrolyte that contains triiodide/iodide as a redox couple. Although triiodide/iodide redox couples work efficiently in DSCs, they suffer from two major disadvantages: electrolytes that contain triiodide/iodide corrode electrical contacts made of silver (which reduces the options for the scale up of DSCs to module size) and triiodide partially absorbs visible light. Here, we present a new disulfide/thiolate redox couple that has negligible absorption in the visible spectral range, a very attractive feature for flexible DSCs that use transparent conductors as current collectors. Using this novel, iodide-free redox electrolyte in conjunction with a sensitized heterojunction, we achieved an unprecedented efficiency of 6.4% under standard illumination test conditions. This novel redox couple offers a viable pathway to develop efficient DSCs with attractive properties for scale up and practical applications.

CoS supersedes Pt as efficient electrocatalyst for triiodide reduction in dye-sensitized solar cells.

We report an efficient nonplatinized flexible counter electrode for dye-sensitized solar cells. In combination with a solvent-free ionic liquid electrolyte, we have demonstrated a approximately 6.5% cell with an amphiphilic ruthenium polypyridyl photosensitizer showing excellent stability measured under prolonged light soaking at 60 degrees C. Compared to the Pt deposited PEN film, the CoS deposited PEN film shows higher electrocatalytic activity for the reduction of triiodide. This is expected to have an important practical consequence on the production of flexible low-cost and lightweight thin film DSC devices based on the plastic matrix.