Multifunctional Role of Nanostructured CdS Interfacial Layers in Hybrid Solar Cells.

We demonstrate here a multifunctional application of CdS layers with nanotextured and nanowire morphology in four types of hybrid solar cells, i.e., (i) nanocrystal-polymer cell, (ii) nanocrystal-organic donor-acceptor bulk heterojunction (BHJ) inverted cell, (iii) nanocrystal-dye sensitized solid state cell and (iv) nanocrystal-dye sensitized electrochemical cell. The role of CdS layer in each type of the above cells has been elucidated and the photovoltaic (PV) performance of the PV cells has been compared. It is shown that CdS acts as acceptor in the cells of types (i) and (iii), while it plays the role of an electron-selective (hole-blocking) layer to direct electrons from the organic counterpart to anode in the cases (ii) and (iv). Morphology of the CdS layer makes a noticeable effect on the PV performance. In particular, the nanowire array demonstrated an improved efficiency of collection of charge carriers as compared with the continuous textured surface due to the increased organic-CdS interface area in PV cells of practically all types. It is demonstrated that the same nanocrystal-dye structure can operate either as PV cell of type (iii) or PV cell of type (iv).

Nanorods and nanotubes for solar cells.

Nanorods and nanotubes as photoactive materials as well as electrodes in photovoltaic cells have been launched a few years ago, and the literature in this field started to appear only recently. The first steps have shown both advantages and disadvantages of their application, and the main expectation associated with their effective charge transport has not been realized completely. This article aims to review both the first and the recent tendencies in the development and application of nanorod and nanotube materials in photovoltaic cells. Two basic techniques of synthesis of crystalline nanorod structures are described, the top-down and bottom-up approaches, respectively. Design and photovoltaic performance of solar cells based on various semiconductor nanorod materials, such as TiO2, ZnO, CdS, CdSe, CdTe, CuO, Si are presented and compared with respective solar cells based on semiconductor nanoparticles. Specific of synthesis and application of carbon nanotubes in photovoltaic devices is also reviewed.

Processes of molecular association and excimeric emission in protonated N,N-dimethylformamide.

Formation of associates of N,N-dimethylformamide (DMF) molecules was studied to clarify their role in photoluminescent activity of protonated DMF solutions. The association of DMF molecules was observed in dilute aqueous solutions at concentrations of DMF above approximately 4x10(-2) M. The association is enhanced when the CO bond of the DMF molecule is activated by protonation with hydrochloric acid, which leads to appearance of an excimeric emission at approximately 530 nm. The excitation spectrum of the excimeric emission showed the excitation maximum in the region of the absorption of DMF associates, which is a first evidence of a more complex mechanism of excimer formation originating from excitation of associated rather than monomeric molecules in the ground state. A simple approach was provided to evaluate a number of molecules in the excimer structure. An original theory has been developed, and it was calculated that the DMF excimer has a dimeric nature. A model of the excimer formation was proposed, which suggests that a hydrogen-bonded associate is an intermediate form leading to the excimeric structure upon excitation. It was observed that DMF possesses also a monomeric emission with the emission maximum at approximately 385 nm, which was attributed to the intramolecular charge-transfer process. It has been found that the change in structure of the DMF associates via the liquid-solid phase transition affects both excitation and emission bands of excimers, so that the excimeric emission shifts to the blue region and intermixes with the emission of DMF monomers.

Anomalous green emission and energy transfer in the n,n-dimethylformamide/hydrochloric acid/europium chloride system.

The unusual green photoluminescence (PL) of N,N-dimethylformamide (DMF)/hydrochloric acid (HCl)/europium chloride (EuCl3) solutions discovered earlier was investigated in more detail to clarify the emission mechanism. It was revealed that the DMF/HCl pair alone can yield a green PL band under UV excitation, and the emission has features of that of excimers. The addition of EuCl3 salt to the solution further stimulates the green emission. The quantum yield of the line emission of Eu3+ ions at 592 and 612 nm is also affected by the presence of HCl in the solution. Both the green emission band and Eu3+ emission lines possess a common channel of excitation at approximately 280 nm. This channel is the only source for the green emission band and an additional source for the Eu3+ emission lines, which can also be stimulated through a conventional Eu3+ excitation channel at 394 nm. The common excitation channel was found to be time-dependent, and its excitation maximum gradually shifts to longer wavelengths. Changes in the PL profiles of europium ions were also observed depending on the presence of HCl and the solution aging.

Solvent-induced organization of squaraine dyes in solution capillary layers and adsorbed films.

Unusual behavior of indolenine and hydroxyphenyl squaraines has been observed in solution capillary layers and adsorbed films. The confined solutions showed anomalous aggregation of squaraine molecules in contrast to their monomer behavior in the bulk solutions of the same concentration, along with formation of a macroscopic cell-like structure in the confined solution layer, with the diameter of cells being 3-5 microm. The aggregate structure, as observed through electronic absorption spectra, was strongly dependent on the chemical structure of squaraine used and solvent used, and it also was different from squaraine aggregates observed in aqueous solutions and films prepared by vacuum evaporation. It has been found that indolenine squaraine is capable of forming H-aggregates in confined dimethylformamide solutions and hydroxyphenyl squaraine is capable of forming J-aggregates in confined dimethylformamide solutions and adsorbed films. The results were compared with pseudoisocyanine, which forms J-aggregates in aqueous bulk solutions readily; however, no J-aggregates have been found in their capillary layers. The interplay of dye-dye, dye-surface, and dye-solvent interactions resulting in the above effects is discussed.