HIGH-EFFICIENT SOLID-STATE PEROVSKITE SOLAR CELL WITHOUT LITHIUM SALT IN THE HOLE TRANSPORT MATERIAL

Nano, Volume 09, Issue 05, July 2014.

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. Up to now, it is essential to add lithium bis(Trifluoromethanesulfonyl)Imide (LiTFSI) to the organic hole transport materials (HTM) to get an efficient perovskite solar cells. However, in this paper, an efficiency of 10.9% (AM 1.5 illumination of 100 mW/cm2) was achieved by applying with a higher mobility hole conductor PTAA in the absence of LiTFSI. Initial device stability test showed that the solar cell can have a relatively higher stability without LiTFSI.

DONGQIN BI et al

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ORGANOMETAL HALIDE PEROVSKITE PHOTOVOLTAICS: A DIAMOND IN THE ROUGH

Nano, Volume 09, Issue 05, July 2014.

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 solar cells based on organolead halide perovskite light absorbers have been considered as an emerging photovoltaic technology because of superb photovoltaic performance and very low cost. Since the first trial of perovskite as a sensitizer in dye-sensitized solar cell structure in 2009, efficiency was improved from 3.8% to 6.% by optimizing perovskite coating condition in 2011. However, perovskite has drawback in the liquid based sensitized solar cell because it tends to dissolve in liquid electrolyte. In 2012, all-solid-state perovskite solar cell with long-term durability and higher efficiency of 9.7% was reported. This finding sparks the beginning of perovskite solar cell era. As of the end of 2013, the certified efficiency of 16.2% was achieved from perovskite solar cell. Based on learning curve, higher efficiency approaching 20% is expected.

MICHAEL GRÄTZEL et al

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MODULATING CH3NH3PbI3 PEROVSKITE CRYSTALLIZATION BEHAVIOR THROUGH PRECURSOR CONCENTRATION

Nano, Volume 09, Issue 05, July 2014.

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. Despite rapid increase of efficiency in solid-state perovskite solar cells, there has been limited exploration into the morphological and crystallographic characteristics of this kind of device. Here we show how the concentration of perovskite precursor plays a crucial role in CH3NH3PbI3-based devices. The optical analysis reveals an absorbance increase and a shift of photoluminescence responses towards lower energies with increasing concentratio. Additionally, the crystallographic study shows a gradual decrease of the perovskite lattice parameters. These differences determine the photovoltaic performance of the solar cells.

KUNWU FU et al

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EFFECT OF THE CHROMOPHORES STRUCTURES ON THE PERFORMANCE OF SOLID-STATE DYE SENSITIZED SOLAR CELLS

Nano, Volume 09, Issue 05, July 2014.

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. Sensitizers based on triphenylamine with thiophene linkers and different alkyl chain in the donor units have been used on Solid State Dye Solar Cells, to check the main factors that control the recombination, the capacitance and the efficiency of the cells. Identifying that the design of dyes with a bulky moieties in the donor part of the structure are the key factor to reduce the recombination and improve the efficiency of the device.

HAINING TIAN et al

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EFFECT OF SUBSTITUENTS IN THE IMIDAZOLIUM RING ON THE PERFORMANCE OF SOLID-STATE DYE-SENSITIZED SOLAR CELLS

Nano, Volume 09, Issue 05, July 2014.

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 performance of solid-state dye-sensitized solar cells can be tuned by varying the substituents in the imidazolium ring of the solid ionic conductor. The power conversion efficiency has been improved gradually by replacing the alkyl group with the ester group and further with the hydroxyethyl group due to the significant increase in short-circuit photocurrent.

JUAN LI et al

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TiO2 SUB-MICROSPHERES AS A BI-FUNCTIONAL SCATTERING LAYER FOR HIGH-PERFORMANCE DYE-SENSITIZED SOLAR CELLS

Nano, Volume 09, Issue 05, July 2014.

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 sub-microspheres play multiple roles in enhancing dye adsorption and light-scattering to enhance the performance of dye-sensitized solar cells. In this work, the well-defined TiO2 sub-microspheres with anatase granular-like nanocrystals are facilely and reproducibly prepared in high yield by combining hydrolytic process with solvothermal treatment. The obtained sub-microspheres with appropriate pore diameter and relatively high specific surface area result in a larger amount of dye adsorption.

YONG DING et al

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INVESTIGATION OF DYE-REGENERATION KINETICS AT DYE-SENSITIZED p-TYPE CuCrO2 FILM/ELECTROLYTES INTERFACE WITH SCANNING ELECTROCHEMICAL MICROSCOPY

Nano, Volume 09, Issue 05, July 2014.

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. Dye-regeneration kinetics on CuCrO2 mesoporous film can be investigated by scanning electrochemical microscopy with feedback mode. Various organic sensitizers in combination with iodine-based and thiolate-based electrolytes are used to understand the effect of sensitizers and redox shuttles on regeneration process.

GETACHEW ALEMU et al

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EFFECT OF THIOPHENE IN BITHIAZOLE-BRIDGED SENSITIZERS ON THE PERFORMANCE OF DYE-SENSITIZED SOLAR CELLS

Nano, Volume 09, Issue 05, July 2014.

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 Jsc. Since the Jsc dropped sharply from 15.26 mAcm-2 to 4.52 mAcm-2, the photovoltaic conversion efficiency decreased dramatically from 7.86% to 1.93%. In this paper, we investigated the effect of thiophene unit between donor and bithiazole bridge on the performance of DSSCs. The results showed that thiophene unit enhanced the photovoltaic conversion efficiency from 7.12% of BT-T2 to 7.51% of TBT-T2 for triphenylamine-based sensitizers, but decreased dramatically from 7.86% of BT-T3 to 1.93% of TBT-T3 for indoline-based sensitizers. The reason may be that the introduction of thiophene unit suppressed the intramolecular charge transfer in the conjugation system of indoline-based sensitizer, which led to the low Jsc.

XIAOYU ZHANG et al

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EFFECTS OF TiO2 PARTICLE SIZE ON THE PERFORMANCE OF DYE-SENSITIZED SOLAR CELLS USING IONIC LIQUID ELECTROLYTES

Nano, Volume 09, Issue 05, July 2014.

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. Different-sized nanocrystalline–TiO2 particles have been used for the optimization of photovoltaic effects of dye-sensitized solar cells (DSC) using an ionic–liquid (IL) electrolyte. Ru dye (Z907) was used for the IL-DSC optimization. The particle size of the nanocrystalline–TiO2 film greatly affected the photovoltaic characteristics, particularly for the ionic–liquid electrolyte due to limitation of the photocurrent by I-/I-3-diffusion. The optimized electrode for IL-DSC had a 15 µm thickness using a 27 nm diameter of nanocrystalline–TiO2 particles.

SEIGO ITO et al

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ENHANCED INTERCONNECTION OF TiO2 NANOPARTICLES USING ATOMIC LAYER DEPOSITION FOR FLEXIBLE DYE-SENSITIZED SOLAR CELLS WITH PLASTIC SUBSTRATES

Nano, Volume 09, Issue 05, July 2014.

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 develop low temperature sintering method using atomic layer deposition (ALD) to induce tight interconnection of TiO2 nanoparticles in plastic dye-sensitized solar cells. The ALD-induced TiO2 thin layers on porous films cause better interconnection between TiO2 particles, which can account for the enhanced photocurrent. Consequently, the ALD-treated solar cells show enhanced power conversion efficiency by 33%, compared to that without ALD-treatment.

BOEUN KIM et al

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