Design of conduction band structure of TiO2 electrode using Nb doping for highly efficient dye‐sensitized solar cells

A barrier layer of undoped TiO₂ was deposited on the Nb‐doped TiO₂ electrode to suppress the recombination at the Nb‐doped TiO₂/dye–electrolyte interface for highly efficient dye‐sensitized solar cells (DSCs). The Nb content in TiO₂ was varied in a range of 0.7–3.5 mol% to modify the TiO₂ energy‐band structure. Nb‐doped TiO₂/dye interfaces were characterized by a combination of ultraviolet photoemission spectroscopy and optical absorption spectroscopy measurements, allowing the determination of the conduction band minimum (CBM) of the TiO₂ electrode and the lowest unoccupied molecular orbital of the N719 dye. The lowering of TiO₂ CBM by Nb doping induced the increase in short‐circuit current of DSCs. However, open‐circuit voltage and fill factor are decreased, and this result was ascribed to the enhanced recombination at the Nb‐doped TiO₂/dye–electrolyte interface. The effect of doping on charge transport in DSCs was analyzed using electrochemical impedance spectroscopy. We have shown that by introducing of TiO₂ barrier layer, the Nb doping content, which results in DSC highest efficiency, can be increased because of the suppression of the dopant‐induced recombination. The energy conversion efficiency of the solar cells increased from 7.8% to 9.0% when undoped TiO₂ electrode is replaced with electrode doped with 2.7 mol% of Nb because of the improvement of the electron injection and collection efficiencies. The correlation between the electronic structure of the TiO₂ electrode, charge transfer characteristics, and photovoltaic parameters of DSCs is discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Fabrication of screen‐printing pastes from TiO2 powders for dye‐sensitised solar cells

A preparation technique of TiO₂ screen‐printing pastes from commercially‐available powders has been disclosed in order to fabricate the nanocrystalline layers without cracking and peeling‐off over 17 µm thickness for the photoactive electrodes of the dye‐sensitised solar cells. A conversion efficiency of 8·7% was obtained by using a single‐layer of a semi‐transparent‐TiO₂ film. A conversion efficiency of 9·2% was obtained by using double‐layers composed of transparent and light‐scattering TiO₂ films for a photon‐trapping system. Copyright © 2007 John Wiley & Sons, Ltd.     

Manipulation of light harvesting for efficient dye‐sensitized solar cell by doping an ultraviolet light‐capturing fluorophore

An organic fluorophore is doped into a mesoporous TiO₂ photoelectrode to absorb ultraviolet light and convert it to green light for more efficient light harvesting of N719 dye. This fluorescence conversion enables the absorption of additional green light by dye molecules by means of Förster resonance energy transfer between fluorescent compound donor and N719 dye acceptor. Owing to close fit between the emission peak of fluorophore and the absorption peak of N719 dye, the Förster resonance energy transfer effect enhances the incident photon to current conversion efficiency of the dye‐sensitized solar cells based on fluorophore‐doped TiO₂ photoelectrodes. Improved power conversion efficiency (8.03–8.13%) is also achieved for the fluorophore‐doped (10⁻⁴ M) dye‐sensitized solar cells compared with a cell without the doping of fluorophore (7.63%). Copyright © 2013 John Wiley & Sons, Ltd.     

Sensitization of zinc oxide photoanode with an indoline dye

A dye‐sensitized solar cell (DSC) made of nanoporous ZnO film on aluminum‐doped zinc oxide (ZnO/AZO) transparent substrate has higher solar‐to‐electric energy conversion efficiency than a DSC consisting of nanoporous ZnO film deposited on conventional fluorine‐doped tin oxide (ZnO/FTO) transparent substrate. The ZnO/AZO DSC gave an overall conversion efficiency of 7.2% whereas the ZnO/FTO yielded a conversion efficiency of 4.5%. The film‐substrate orientation and higher light harvesting of the nanoporous ZnO film on the AZO after heating in air are mainly attributed to the higher energy conversion efficiency of the ZnO/AZO DSC. Copyright © 2012 John Wiley & Sons, Ltd.     

Zr‐Doped Indium Oxide (IZRO) Transparent Electrodes for Perovskite‐Based Tandem Solar Cells

Parasitic absorption in transparent electrodes is one of the main roadblocks to enabling power conversion efficiencies (PCEs) for perovskite‐based tandem solar cells beyond 30%. To reduce such losses and maximize light coupling, the broadband transparency of such electrodes should be improved, especially at the front of the device. Here, the excellent properties of Zr‐doped indium oxide (IZRO) transparent electrodes for such applications, with improved near‐infrared (NIR) response, compared to conventional tin‐doped indium oxide (ITO) electrodes, are shown. Optimized IZRO films feature a very high electron mobility (up to ≈77 cm² V^?1 s^?1), enabling highly infrared transparent films with a very low sheet resistance (≈18 Ω □^?1 for annealed 100 nm films). For devices, this translates in a parasitic absorption of only ≈5% for IZRO within the solar spectrum (250–2500 nm range), to be compared with ≈10% for commercial ITO. Fundamentally, it is found that the high conductivity of annealed IZRO films is directly linked to promoted crystallinity of the indium oxide (In₂O₃) films due to Zr‐doping. Overall, on a four‐terminal perovskite/silicon tandem device level, an absolute 3.5 mA cm^?2 short‐circuit current improvement in silicon bottom cells is obtained by replacing commercial ITO electrodes with IZRO, resulting in improving the PCE from 23.3% to 26.2%.     

Metal-free indoline-dye-sensitized TiO2 nanotube solar cells

Titanium dioxide nanotubes were directly fabricated from commercial P25 TiO₂ via alkali hydrothermal transformation. The prepared titanate nanotubes were successfully used as an electrode material for dye-sensitized solar cells (DSCs). A metal-free organic dye (indoline dye D102) was used as a sensitizer. The used indoline dye D102 is of high purity (?98%) and high absorption coefficient (67,500 L mol⁻¹ cm⁻¹ at 501 nm). The TiO₂ pastes were prepared with PEG (Mw 20,000) and as-made TiO₂ nanotubes or P25 powders. Titania thin films were grown by screen printing method. High conversion efficiencies of light to electricity of around 9.8% and 7.6% under illumination of simulated AM1.5 sunlight (100 mW/cm²) were achieved with P25 and TiO₂ nanotube cells, respectively. The fill factor of DSCs based on TiO₂ nanotubes increased in comparison with that of DSCs based on TiO₂ nanoparticles. The electron transport and dye adsorption properties in both titanate nanotube and P25 electrodes were evaluated in terms of photovoltaic characteristics of the fabricated cells. The related mechanisms were discussed. The study provides a promising method for the development of high-efficiency and low-cost DSCs.     

Enhanced Performance of I2‐Free Solid‐State Dye‐Sensitized Solar Cells with Conductive Polymer up to 6.8%

An iodine‐free solid‐state dye‐sensitized solar cell (ssDSSC) is reported here, with 6.8% energy conversion efficiency—one of the highest yet reported for N719 dye—as a result of enhanced light harvesting from the increased transmittance of an organized mesoporous TiO₂ interfacial layer and the good hole conductivity of the solid‐state‐polymerized material. The organized mesoporous TiO₂ (OM‐TiO₂) interfacial layer is prepared on large‐area substrates by a sol‐gel process, and is confirmed by scanning electron microscopy (SEM) and grazing incidence small‐angle X‐ray scattering (GISAXS). A 550‐nm‐thick OM‐TiO₂ film coated on fluorine‐doped tin oxide (FTO) glass is highly transparent, resulting in transmittance increases of 8 and 4% compared to those of the bare FTO and conventional compact TiO₂ film on FTO, respectively. The high cell performance is achieved through careful control of the electrode/hole transport material (HTM) and nanocrystalline TiO₂/conductive glass interfaces, which affect the interfacial resistance of the cell. Furthermore, the transparent OM‐TiO₂ film, with its high porosity and good connectivity, exhibits improved cell performance due to increased transmittance in the visible light region, decreased interfacial resistance ( Ω ), and enhanced electron lifetime ( τ ). The cell performance also depends on the conductivity of HTMs, which indicates that both highly conductive HTM and the transparent OM‐TiO₂ film interface are crucial for obtaining high‐energy conversion efficiencies in I₂‐free ssDSSCs.     

Processing and characterization of a TiO2 paste based on small particle size powders for dye‐sensitized solar cell semi‐transparent photo‐electrodes

The formulation and the preparation of a TiO₂ paste for dye‐sensitized solar cell technology are proposed. The TiO₂ paste is characterized in terms of rheology, morphology, cross section, specific surface area, and average pore size. A conversion efficiency of 7.42% without any chemical treatment or scattering layer was obtained using the optimal thickness of 11 µm as carried out in this work. The results over different batches of the paste confirm the stability and the reproducibility. The characterization of the TiO₂ semi‐transparent film is completed by investigating the dye adsorption saturation time after cyclic dye dipping steps through UV–Vis spectra measurements. Copyright © 2011 John Wiley & Sons, Ltd.     

CdTe solar cell in a novel configuration

Polycrystalline thin‐film CdTe/CdS solar cells have been developed in a configuration in which a transparent conducting layer of indium tin oxide (ITO) has been used for the first time as a back electrical contact on p‐CdTe. Solar cells of 7·9% efficiency were developed on SnO_x:F‐coated glass substrates with a low‐temperature (<450°C) high‐vacuum evaporation method. After the CdCl₂ annealing treatment of the CdTe/CdS stack, a bromine methanol solution was used for etching the CdTe surface prior to the ITO deposition. The unique features of this solar cell with both front and back contacts being transparent and conducting are that the cell can be illuminated from either or both sides simultaneously like a ‘bi‐facial’ cell, and it can be used in tandem solar cells. The solar cells with transparent conducting oxide back contact show long‐term stable performance under accelerated test conditions. Copyright © 2004 John Wiley & Sons, Ltd.     

The use of Ti meshes with self‐organized TiO2 nanotubes as photoanodes of all‐Ti dye‐sensitized solar cells

This paper reports a simple and facile method for directly growing self‐organized TiO₂ nanotubular arrays around the whole Ti mesh by electrochemical anodization in organic electrolytes and their application in all‐Ti dye‐sensitized solar cells (DSSCs). Compared with the traditional fluorine‐doped tin oxide (FTO)‐based DSSC and the backside illuminated DSSC, this type of DSSC showed advantages such as low resistance, cheap fabrication cost and enhanced sunlight utilization. Different thicknesses of nanotubular array layers were investigated to find their influence on the photovoltaic parameters of the cell. We also considered three types of meshes as the substrates of anodes and found that the cell with 6 openings/mm² exhibited the highest conversion efficiency of 5.3%. The area of the cell had only a little impact on the photovoltaic performances. Copyright © 2010 John Wiley & Sons, Ltd.     

Enhanced short circuit current density of dye-sensitized solar cells aided by Sr,V co-doped TiO2 particles

This study comes up with a straightforward method for preparing uniform electrodes containing Sr,V co-doped TiO₂ particles for dye-sensitized solar cells (DSCs) applications. The spherical particles with the average diameter around 2.5 µm are assembled from small nanoparticles with the average grain size of 60 nm. X-ray diffraction (XRD) reveals that the introduction of dopants not only inhibits the growth of rutile phase, but also results in smaller primary crystallites, improving the surface area and dye adsorption ability of the electrodes. X-ray photoelectron spectroscopy (XPS) showed that Sr²⁺ and V⁵⁺ ions are well incorporated into the titania crystal lattice without forming specific Strontium and Vanadium compositions. UV–visible spectra show that the co-doped TiO₂ films have lower band gap energy than that of undoped-TiO₂, extending the absorption of TiO₂ into visible region. Isolated energy levels in band structure of TiO₂ as well as local lattice distortions due to dopants introduction are the parameters enhanced the short circuit current density of the cells. The TiO₂ DSC co-doped with 0.075 at% Sr and 1.5 at% V (i.e., S7V15 cell) had the highest circuit current density and power conversion efficiency of 18.57 and 7.76%, respectively, as a result of less recombination, which is demonstrated by electrochemical impedance spectroscopy (EIS).     

Cold isostatic pressing technique for producing highly efficient flexible dye‐sensitised solar cells on plastic substrates

One of the biggest challenges for making dye‐sensitised solar cells (DSCs) on plastic substrates is the difficulty in making good quality nanoporous TiO₂ films with both good mechanical stability and high electrical conductivity. Cold isostatic pressing (CIP) is a powder compaction technique that applies an isostatic pressure to a powder sample in all directions. It is particularly suitable for making thin films on plastic substrates, including non‐flat surfaces. Cold isostatically pressed nanocrystalline TiO₂ electrodes with excellent mechanical robustness were prepared on indium tin oxide (ITO)‐coated polyethylene naphthalate (PEN) substrates in the absence of organic binders and without heat treatment. The morphology and the physical properties of the TiO₂ films prepared by the CIP method were found to be very compatible with requirements for flexible DSCs on plastics. This room‐temperature processing technique has led to an important technical breakthrough in producing high efficiency flexible DSCs. Devices fabricated on ITO/PEN films by this method using standard P‐25 TiO₂ films with a Ru‐complex sensitiser yielded a maximum incident photon‐to‐current conversion efficiency of 72% at the wavelength of 530 nm and showed high conversion efficiencies of 6.3% and 7.4% for incident light intensities of 100 and 15 mW cm⁻², respectively, which are the highest power conversion efficiencies achieved so far for any DSC on a polymer substrate using the widely used, commercially available P‐25 TiO₂ powder. Copyright © 2011 John Wiley & Sons, Ltd.     

Solution‐Processed Metallic Nanowire Electrodes as Indium Tin Oxide Replacement for Thin‐Film Solar Cells

Solution processed silver nanowire (Ag NW) films are introduced as transparent electrodes for thin‐film solar cells. Ag NW electrodes were processed by doctor blade‐coating on glass substrates at moderate temperatures (less than 100 °C). The morphological, optical, and electrical characteristics of these electrodes were investigated as a function of processing parameters. For solar‐cell application, Ag NW electrodes with an average transparency of 90% between 450 and 800 nm and a sheet resistivity of ≈10 Ω per square were chosen. The performance of poly(3‐hexylthiophen‐2,5‐diyl):[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT:PCBM) solar cells on Ag NW electrodes was found to match the performance of otherwise identical cells on indium tin oxide. Overall, P3HT:PCBM solar cells with an efficiency of 2.5% on transparent Ag NW electrodes have been realized.     

All‐Solution‐Processed Indium‐Free Transparent Composite Electrodes based on Ag Nanowire and Metal Oxide for Thin‐Film Solar Cells

Fully solution‐processed Al‐doped ZnO/silver nanowire (AgNW)/Al‐doped ZnO/ZnO multi‐stacked composite electrodes are introduced as a transparent, conductive window layer for thin‐film solar cells. Unlike conventional sol–gel synthetic pathways, a newly developed combustion reaction‐based sol–gel chemical approach allows dense and uniform composite electrodes at temperatures as low as 200 °C. The resulting composite layer exhibits high transmittance (93.4% at 550 nm) and low sheet resistance (11.3 Ω sq^‐1), which are far superior to those of other solution‐processed transparent electrodes and are comparable to their sputtered counterparts. Conductive atomic force microscopy reveals that the multi‐stacked metal‐oxide layers embedded with the AgNWs enhance the photocarrier collection efficiency by broadening the lateral conduction range. This as‐developed composite electrode is successfully applied in Cu(In_1‐x,Ga_x)S₂ (CIGS) thin‐film solar cells and exhibits a power conversion efficiency of 11.03%. The fully solution‐processed indium‐free composite films demonstrate not only good performance as transparent electrodes but also the potential for applications in various optoelectronic and photovoltaic devices as a cost‐effective and sustainable alternative electrode.     

Performance analysis of dye solar cell with additional TiO2 layer under different light Intensities

Deployment of dye solar cells (DSCs) for building integration application would require a highly efficient solar cell that work well in diffused light. In order to improve the efficiency of dye solar cell, an additional layer of ultrathin anatase titanium dioxide (TiO₂) has been deposited for strengthening the adhesion of the porous TiO₂-based photo electrode to the conductive transparent substrate, which can lead to an enhancement in electron transportation. Fabricated cells of 1 cm² area were tested under different light intensities (100, 33 and 10 mW cm⁻²) and characterized by scanning electron microscopy (SEM), Raman spectroscopy and electrochemical impedance spectroscopy (EIS). Analysis showed an increment in overall quantum conversion efficiency (η), as high as 35% compared to the standard cell without the additional layer of TiO₂. EIS analysis has proven that the additional ultrathin anatase layer has improved the collection efficiency (Φ_COLL) as the result of the enhancement in both electron transport and lifetime within the porous TiO₂ film which translated into better conversion efficiency of DSCs.     

A new type of counter electrode for dye sensitized solar cells based on solution processed SnO2 and activated carbon

A new type of nano-composite material is investigated for use as the counter electrode of a dye sensitized solar cell. The proposed material consists of SnO₂ and activated carbon, which can be deposited using a cost-effective sol–gel technique. The effects of the activated carbon concentration on the structural and optoelectronic properties were investigated using scanning electron microscopy, X-ray diffraction, UV–vis spectroscopy and four probe measurements. It was found that the addition of activated carbon reduces resistivity and optical transmittance, and suppresses fluorine doped tin oxide (FTO) growth. The nanocomposite material was integrated into a dye sensitized solar cell and evaluated using cyclic voltammetry and J–V characteristics under solar light illumination, and demonstrated power conversion efficiency ~2.35%.     

Material development for dye solar modules: results from an integrated approach

In this paper, we report on the outcome of a German network project conducted with 12 partners from universities and research institutes on the material development of dye solar cells (DSC). We give an overview in the field and evaluate the concept of monolithic DSC further with respect to upscaling and producibility on glass substrates. We have developed a manufacturing process for monolithic DSC modules which is entirely based on screen printing. Similar to our previous experience gained in the sealing of standard DSC, the encapsulation of the modules is achieved in a fusing step by soldering of glass frit layers. For use in monolithic DSC, a platinum free, conductive counter electrode layer, showing a charge transfer resistance of R_CT < 1·5 Ω cm², has been realized by firing a graphite/carbon black composite under an inert atmosphere. Glass frit sealed monolithic test cells have been prepared using this platinum‐free material. A solar efficiency of 6% on a 2·0 cm² active cell area has been achieved in this case. Various types of non‐volatile imidazolium‐based binary ionic liquid electrolytes have been synthesized and optimized with respect to diffusion‐limited currents and charge transfer resistances in DSC. In addition, quasi‐solid‐state electrolytes have been successfully tested by applying inorganic (SiO₂) physical gelators. For the use in semi‐transparent DSC modules, a polyol process has been developed which resulted in the preparation of screen printed, transparent catalytic platinum layers showing an extremely low charge transfer resistance (0·25 Ω cm²). Copyright © 2008 John Wiley & Sons, Ltd.     

27%‐Efficiency Four‐Terminal Perovskite/Silicon Tandem Solar Cells by Sandwiched Gold Nanomesh

Multijunction/tandem solar cells have naturally attracted great attention because they are not subject to the Shockley–Queisser limit. Perovskite solar cells are ideal candidates for the top cell in multijunction/tandem devices due to the high power conversion efficiency (PCE) and relatively low voltage loss. Herein, sandwiched gold nanomesh between MoO₃ layers is designed as a transparent electrode. The large surface tension of MoO₃ effectively improves wettability for gold, resulting in Frank–van der Merwe growth to produce an ultrathin gold nanomesh layer, which guarantees not only excellent conductivity but also great optical transparency, which is particularly important for a multijunction/tandem solar cell. The top MoO₃ layer reduces the reflection at the gold layer to further increase light transmission. As a result, the semitransparent perovskite cell shows an 18.3% efficiency, the highest reported for this type of device. When the semitransparent perovskite device is mechanically stacked with a heterojunction silicon solar cell of 23.3% PCE, it yields a combined efficiency of 27.0%, higher than those of both the sub‐cells. This breakthrough in elevating the efficiency of semitransparent and multijunction/tandem devices can help to break the Shockley–Queisser limit.     

A stacked chalcopyrite thin‐film tandem solar cell with 1.2 V open‐circuit voltage

CuGaSe₂ (CGS) thin films were prepared on tin‐doped indium oxide (ITO) coated soda‐lime glass substrates by thermal co‐evaporation to fabricate transparent solar cells. The films consisted of columnar grains with a diameter of approximately 1 μm. Some deterioration of the transparency of the ITO was observed after deposition of the CGS film. The CGS solar cells were electrically connected in series with Cu(In,Ga)Se₂ (CIGS) solar cells and mechanically stacked on the CIGS cells to construct tandem cells. The tandem solar cell with the CGS cell as the top cell showed an efficiency of 7.4% and an open‐circuit voltage of 1.18 V (AM 1.5, total area). Copyright © 2003 John Wiley & Sons, Ltd.     

Large area dye‐sensitized solar cells: material aspects of fabrication

Dye‐sensitized photoelectrochemical solar cells of large area are fabricated using highly conducting and optically transparent glass consisting of an inner layer of indium‐tin oxide and an outer layer of fluorine doped tin oxide. A method is described for the deposition of nanocrystalline films of TiO₂ consisting of large and small median size particles (30 and 5 nm, respectively) which promote porosity and light scattering. Incorporation of trace quantities of magnesium oxide into TiO₂ increased the efficiency of the cells. The energy conversion efficiency of a cell (AM 1·5, 1000 W m⁻² simulated sunlight) of area 21 cm² was found to be 7·2% compared to 5·6% in the absence of magnesium oxide. The mechanisms by which the magnesium oxide improves the cell performance are discussed. Copyright © 2006 John Wiley & Sons, Ltd.