Nano

CH₃NH₃PbX(X = Br, I, Cl) perovskites have recently been used as light absorbers in hybrid organic–inorganic solid-state solar cells, with efficiencies above 15%. To date, it is essential to add Lithium bis(Trifluoromethanesulfonyl)Imide (LiTFSI) to the hole transport materials (HTM) to get a higher conductivity. However, the detrimental effect of high LiTFSI concentration on the charge transport, DOS in the conduction band of the TiO₂ substrate and device stability results in an overall compromise for a satisfactory device. Using a higher mobility hole conductor to avoid lithium salt is an interesting alternative. Herein, we successfully made an efficient perovskite solar cell by applying a hole conductor PTAA (Poly[bis(4-phenyl) (2,4,6-trimethylphenyl)-amine]) in the absence of LiTFSI. Under AM 1.5 illumination of 100 mW/cm², an efficiency of 10.9% was achieved, which is comparable to the efficiency of 12.3% with the addition of 1.3 mM LiTFSI. An unsealed device without Li⁺ shows interestingly a promising stability.

The sun finds a diamond in the rough, which is the organo-metal halide perovskite. Thanks to exceptional optoelectronic characteristics, solar cells employing perovskite demonstrated first a power conversion efficiency (PCE) of 9.7% in the middle of 2012, which rose steeply to an amazing 16% at the end of 2013. Perovskite-based photovoltaics have several advantages over conventional semiconductor p-n junction devices because high efficiency can be achieved from sub-micrometer-thick very cheap perovskite layers that can be formed by solution processing at temperatures below 150°C, rendering the perovskite solar cell versatile in its application. If photo- and thermal stability as well as tolerance to humidity can be achieved, commercial application on the large scale appear to be feasible.

Perovskite-based photovoltaic devices have recently achieved impressively high efficiencies beyond 15% and gained great interest. We show here the formation of perovskite cluster overlayer structures which consist of individual perovskite grains on top of mesoporous TiO₂ films, coexisting with the randomly distributed nanocrystals within the films. Perovskite solution concentration was found to play an important role in modulating the perovskite crystallization and cluster overlayer formation process. Absorbance increase in visible wavelength range and shift of photoluminescence (PL) responses of perovskite films due to the effect of precursor concentration change were observed and investigated in detail. The crystallographic analysis of the CH₃NH₃PbI₃ films shows a gradual decrease of the perovskite lattice parameters and shrinkage of unit volume as precursor solution concentration increases, which is correlated to the changes of optical properties. Finally, perovskite-based solar cell device performance was enhanced at higher precursor concentration.

The effect of metal-free chromophores on dye-sensitized solar cell performance is investigated. Solid state dye-sensitized solar cells (ssDSCs) using different molecular sensitizers based on triphenylamine (TPA) with thiophene linkers and different alkyl chain in the donor unit have been characterized using impedance spectroscopy (IS). We show that different molecular structures play a fundamental role on solar cell performance, by the effect produced on TiO₂ conduction band position and in the recombination rate. Dye structure and its electronic properties are the main factors that control the recombination, the capacitance and the efficiency of the cells. A clear trend between the performance of the cell and the optimization level of the blocking effect of the dye structure has been identified in the solid state solar cells with Spiro-OMeTAD hole conductor.

Three solid-state imidazolium iodides have been designed and synthesized for use as all-solid-state electrolytes in solid-state dye-sensitized solar cells (ssDSSCs). The effect of substituents in the imidazolium ring on the ionic conductivity and solar cell performance of ssDSSCs has been investigated. As compared to the methyl-ethyl-substituted imidazolium iodide, replacement of one alkyl group (the methyl group) with an ester group increases the ionic conductivity and solar cell performance significantly, and further replacement of the other alkyl group (the ethyl group) with a hydroxyethyl group further increases the ionic conductivity and solar cell performance significantly. A power conversion efficiency of 7.45% has been achieved under the irradiation of simulated AM1.5G solar light (100 mW cm^-2) with the ssDSSC using the hydroxyethyl and ester co-functionalized imidazolium iodide based solid-state electrolyte and a metal-free organic dye sensitizer.

The sub-microspheres play multiple roles in enhancing dye adsorption and light-scattering to improve the performance of dye-sensitized solar cells (DSSCs). In this work, the well-defined TiO₂ sub-microspheres with anatase granular-like nanocrystals are prepared in high yield by combining hydrolytic process with solvothermal treatment. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results indicated that plenty of rhombic nanoparticles with ~ 18 nm diameter having mutual contacts to neighboring nanoparticles were densely self-assembled into sub-microspheres, and abundant mesopores existed in the whole sub-microspheres with superior light scattering ability. The appropriate pore diameter and relatively high specific surface area of the as-obtained sub-microsphere result in a higher dye adsorption. As expected, by using the sub-microspheres as a scattering layer, a higher photovoltaic conversion efficiency of 10.15% is obtained for DSSCs.

The power conversion efficiency of p-type dye-sensitized solar cells (DSSC) is determined by the kinetics of hole injection and dye-regeneration reaction at the dye/electrolyte interface. In this work, the photochemical regeneration kinetics of dye adsorbed on CuCrO₂ mesoporous film was investigated by using scanning electrochemical microscopy with feedback mode. Organic P1 and C343 sensitizers in combination with iodide-based and thiolate-based electrolytes were selected to understand the effect of sensitizers and redox shuttles on dye-regeneration process. A fast regeneration kinetic rate constant was confirmed in thiolate-based sample compared with iodide-based electrolyte, indicating that the organic redox shuttle was an efficient mediator to optimize the performance of p-type DSSC.

In this paper, we have designed and synthesized four bithiazole-bridged sensitizers (BT-T2, TBT-T2, BT-T3 and TBT-T3) with triphenylamine and indoline as the donor segment and applied them to dye-sensitized solar cells (DSSCs). For triphenylamine-based sensitizers as BT-T2 and TBT-T2, adding one thiophene unit between triphenylamine donor and bithiazole moiety not only led to bathochromic shift of the maximum absorption and increase of molar extinction coefficient, but also enhanced the photovoltaic conversion efficiency from 7.12% of BT-T2 to 7.51% of TBT-T2. But for indoline-based sensitizers as BT-T3 and TBT-T3, adding one thiophene unit between indoline donor and bithiazole moiety resulted in hypochromatic shift instead of bathochromic shift. We employed the density functional theory (DFT) calculations to further investigate the influence of the thiophene unit on their optical and electronic properties and photovoltaic performance of corresponding DSSC devices. Given the results, a reasonable explanation is the introduction of thiophene unit suppressed the intramolecular charge transfer and charge separation in the conjugation system of indoline-based sensitizer, which led to the hypochromatic shift of the maximum absorption wavelength and finally the low J_sc. Since the J_sc dropped sharply from 15.26 mAcm^-2 to 4.52 mAcm^-2, the photovoltaic conversion efficiency decreased dramatically from 7.86% to 1.93%.

Different-sized nanocrystalline-TiO₂ particles have been used for the optimization of photovoltaic effects of dye-sensitized solar cells (DSCs) using an ionic-liquid (IL) electrolyte. Ru dye (Z907) was used for the IL-DSC optimization. The TiO₂ nanoparticle sizes and the thickness of nanocrystalline-TiO₂ electrodes ranged from 13 nm to 81 nm and 2 μm to 23 μm, respectively. The particle size of the nanocrystalline TiO₂ film greatly affected the photovoltaic characteristics, particularly for the IL electrolyte due to limitation of the photocurrent by -diffusion. The optimized electrode for IL-DSC had a 15 μm thickness using a 27 nm diameter of nanocrystalline-TiO₂ particles. In order to characterize the effect of the TiO₂ particle size on the photovoltaic effects of IL-DSCs, a scanning electron micrograph (surface and cross section of nanoparticles), BET specific surface area analysis, pore-size distribution analysis, photocurrent transient measurements, haze spectroscopy, photovoltaic measurements, incident-photon-to-current conversion efficiency spectroscopy and impedance measurement have been used.

In order to improve the interconnection of TiO₂ photoelectrode for the flexible dye-sensitized solar cells (DSSCs) with plastic substrates, thin TiO₂ layers are additionally introduced to the surface of main TiO₂ nanoparticles by atomic layer deposition (ALD) at low temperature. The ALD-induced TiO₂ thin layers on porous films effectively reduced the internal electrical resistance, leading to the facilitated electron transport in DSSCs. As a result, DSSCs with ALD-induced TiO₂ thin layers (thickness of ~ 1.5 nm) showed better power conversion efficiency of 3.09%, which is 33% enhancement compared with that without ALD-induced TiO₂ thin layers (2.32%).

We demonstrate a strategy for incorporating plasmon resonant metallic nanoparticles in the construction of the three-dimensional (3D) interwoven structured TiO₂ photoanodes. The 3D interwoven structure contained continuous TiO₂ skeleton and numerous interconnected macro/mesopores, which supplied effective straight path for electron transfer and high specific surface area for dye load. Localized surface plasmon resonance (SPR) was produced by the addition of gold nanoparticles (AuNPs), which increased the light absorption of the photoanodes more effectively. The synergistic effect of SPR with constructed TiO₂ nanostructures has been investigated, and was confirmed by field-emission scanning electron microscope (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-Vis absorption spectra, J–V characteristics, and electrochemical impedance spectroscopy (EIS) analysis. It was found that the range and strength of light absorption of TiO₂ photoanodes, the photon capture ability of the dye molecules, the photoelectric conversion efficiency were significantly increased while the electron transfer resistance decreased due to the incorporation of AuNPs compared to the P25 and Au-free photoanode. The related photoelectric performance enhancement mechanisms, and surface-plasmon resonances in dye-sensitized solar cells (DSSCs) with Au nanostructures were analyzed and discussed.