Dye-sensitized Heterojunction Research Articles

Investigation of Dye Regeneration Kinetics in Sensitized Solar Cells by Scanning Electrochemical Microscopy

Sensitizers are responsible for the light harvesting and the charge injection in dye-sensitized solar cells (DSSCs). A fast dye-regeneration process is necessary to obtain highly efficient DSSC devices. Herein, dye-regeneration rates of two DSSC device types, that is, the reduction of immediately formed photo-oxidized sensitizers (ruthenium complex C106TBA and porphyrin LD14, k(ox)') by iodide ions (I(-)) and [Co(bpy)3](2+), and the oxidation of formed photo-reduced sensitizers (organic dye P1, k(re)') by triiodide ions (I3(-)) and the disulfide dimer (T2) are investigated by scanning electrochemical microscopy (SECM). We provide a thorough experimental verification of the feedback mode to compare the kinetics for dye-regeneration by using the above mentioned mediators. The charge recombination at the dye/semiconductor/electrolyte interface is further investigated by SECM. A theoretical model is applied to interpret the current response at the tip under short-circuit conditions, providing important information on factors that govern the dynamics of dye-regeneration onto the dye-sensitized heterojunction.

D–π–A structured porphyrins for efficient dye-sensitized solar cells

In this study, new push–pull alkoxy-wrapped zinc porphyrin dyes with intramolecular donor–π–acceptor structures have been designed and synthesized for dye-sensitized solar cells (DSCs). The linkers based on thiophene or 2,3-dihydrothieno[3,4-b][1,4] dioxine with cyanoacetic acid can broaden the spectral response of porphyrins into the near-IR region (∼850 nm), which is mainly attributed to the cyanoacetic acid group. However, porphyrins with cyanoacrylic acid as an anchoring group lead to a faster charge recombination rate at the dye-sensitized heterojunction interface, which lowers the device photovoltaic performance. By using porphyrins with a rigid π-linker feature structure that is 5-ethynylthiophene-2-carboxylic acid, highly efficient DSC devices with a power conversion efficiency of 9.5% can be obtained. Spectral, electrochemical, photovoltage transient decay and impedance measurements are performed to reveal the influence of π-conjugated linkers and anchoring groups upon the optoelectronic features of porphyrin dyes in DSCs.

Lithium salts as “redox active” p-type dopants for organic semiconductors and their impact in solid-state dye-sensitized solar cells

Lithium salts have been shown to dramatically increase the conductivity in a broad range of polymeric and small molecule organic semiconductors (OSs). Here we demonstrate and identify the mechanism by which Li(+) p-dopes OSs in the presence of oxygen. After we established the lithium doping mechanism, we re-evaluate the role of lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI) in 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9'-Spirobifluorene (Spiro-OMeTAD) based solid-state dye-sensitized solar cells (ss-DSSCs). The doping mechanism consumes Li(+) during the device operation, which poses a problem, since the lithium salt is required at the dye-sensitized heterojunction to enhance charge generation. This compromise highlights that new additives are required to maximize the performance and the long-term stability of ss-DSSCs.

The origin of an efficiency improving “light soaking” effect in SnO2 based solid-state dye-sensitized solar cells

We observe a strong “light-soaking” effect in SnO2 based solid-state dye-sensitized solar cells (SDSCs). Both with and without the presence of UV light, the device's short-circuit photocurrent and efficiency increase significantly over 20–30 minutes, until steady-state is achieved. We demonstrate that this is not due to improved charge collection and investigate the charge generation dynamics employing optical-pump terahertz-probe spectroscopy. We observe a monotonic speeding-up of the generation of free-electrons in the SnO2 conduction band as a function of the light-soaking time. This improved charge generation can be explained by a positive shift in the conduction band edge or, alternatively, an increase in the density of states (DoS) at the energy at which photoinduced electron transfer occurs. To verify this hypothesis, we perform capacitance and charge extraction measurements which indicate a shift in the surface potential of SnO2 of up to 70 mV with light soaking. The increased availability of states into which electrons can be transferred justifies the increase in both the charge injection rate and ensuing photocurrent. The cause for the shift in surface potential is not clear, but we postulate that it is due to the photoinduced charging of the SnO2 inducing a rearrangement of charged species or loss of surface oxygen at the dye-sensitized heterojunction. Understanding temporally evolving processes in DSCs is of critical importance for enabling this technology to operate optimally over a prolonged period of time. This work specifically highlights important changes that can occur at the dye-sensitized heterojunction, even without direct light absorption in the metal oxide.

Dye‐Sensitized Back‐Contact Solar Cells

sensitized solar cells (DSCs), [ 2 ] are based on low-cost materials and can be fabricated by means of simple printing techniques. Two transparent fl uorine-doped tin oxide (FTO) coated glass substrates are conventionally used as collecting electrodes to extract photogenerated charge carriers from the dye-sensitized junction. These substrates account for a major fraction of the fabrication costs while also compromising the DSC performance due to their limited transmission and conductivity. Here, we demonstrate that back-contact solar cell confi gurations, successfully developed to increase the performance of silicon solar cells, can be used to address these limitations. For the fi rst time, we report the successful fabrication of a monolithic back-contact dye-sensitized solar cell (BC-DSC) based on an array of two interdigitated fi nger electrodes located on one common substrate. In photovoltaic cells, light absorption results in the generation of charge carriers of opposite sign, which are subsequently collected by two charge selective contacts. For silicon solar cells, [ 1 ] as well as for dye-sensitized solar cells (DCSs) [ 2 ] those collecting electrodes are most commonly located on opposite sides of the junction in a co-planar arrangement, above and below the photoactive junction. Consequently at least one of the electrodes needs to combine transparent and conducting properties, compromising both, optical transmission as well as electrical conductivity. Placing both collecting electrodes onto one common face of the photovoltaic junction provides the opportunity to use opaque electrode materials and to illuminate the junction through the opposite, optically unobstructed face. Such back-contact concepts have successfully been applied to increase the performance of silicon solar cells. [ 3 ] Charge collection in these devices is facilitated by a set of two interdigitated fi nger electrodes, co-located on the backside of the silicon wafer. [ 3 ] In this work we use analogue concepts to construct dye-sensitized back-contact solar cells that use interdigitated electrodes to effi ciently extract photogenerated positive and negative charge carries from an overlying dye-sensitized heterojunction. The potential advantages of BC-DSCs over conventional DSCs are that (1) optical transmission losses can be avoided, (2) highly conductive electrode materials can be used for charge collection, (3) opaque substrate materials can be applied, and (4) auxiliary wiring and interconnections can be integrated onto one common substrate to facilitate the fabrication of interconnected solar cell modules and their integration into electronic circuits. The most common DSC device confi guration is schematically shown in Figure 1a . The DSC electrodes consist of an FTO glass plate coated with a dye-sensitized mesoporous TiO 2 layer and an additional platinized FTO substrate sandwiched together. The cell gap is fi with an electrolyte solution that contains a redox couple such as iodide/triodide. An appropriate spacer thickness ensures that direct contact of the mesoporous TiO 2 and the opposing platinized FTO glass (counter electrode) is avoided. Dye excitation by visible light is generally followed by ultrafast electron injection into the semiconductor electrode (TiO 2 ). These photogenerated electrons show free diffusion

Efficient and Stable Solid‐State Dye‐Sensitized Solar Cells Based on a High‐Molar‐Extinction‐Coefficient Sensitizer

The high-molar-extinction-coefficient heteroleptic ruthenium dye, cis-Ru (4,4'-bis(5-octylthieno[3,2-b] thiophen-2-yl)-2,2'-bipyridine) (4,4'-dicarboxyl-2,2'-bipyridine) (NCS)(2), exhibits an AM 1.5 solar (100 mW cm(-2))-to-electric power-conversion efficiency of 4.6% in a solid-state dye-sensitized solar cell (SSDSC) with 2,2', 7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9,9'-spirobifluorene (spiro-MeOTAD) as the organic hole-transporting material. These SSDSC devices exhibit good durability during accelerated tests under visible-light soaking for 1000 h at 60 degrees C. This demonstration elucidates a class of photovoltaic devices with potential for stable and low-cost power generation. The electron recombination dynamics and charge collection that take place at the dye-sensitized heterojunction are studied by means of impedance and transient photovoltage decay techniques.

1/f Noise in dye-sensitized solar cells and NIR photon detectors

All electronic devices are plagued with 1/f noise originating from many causes. The most important factors contributing to 1/f noise in a semiconductor is believed to be recombination of carriers and their trapping at defects and impurity sites. Adsorption of moisture and electron acceptor molecules enhances the intensity of 1/f noise. Amazingly, some molecular species that strongly chelate to the semiconductor surface, suppress 1/f noise owing to passivation of the recombination sites. Thus in addition to sensitization, the dye adsorbed on the nanocrystallites plays a key role in mitigation of recombinations. For this reason dye-sensitized heterojunctions could also find application as low noise NIR photon detectors. Experiments conducted with oxide semiconductors (TiO2, ZnO, SnO2) indicate that the mode of binding of dyes at specific sites determines the extent to which the recombination and 1/f noise are suppressed. The transport of electrons in a nanocrystalline matrix is diffusive with a diffusion coefficient D depending on the trapping and detrapping processes. Thus passivation of trapping sites by the adsorbed dye is expected to increase the response time which can be expressed as τ∼L2/D, where L=thickness of the nanocrystalline film. Measurement techniques and construction of a dye-sensitized NIR photon detector will be discussed.

Heteropolyacid-impregnated PVDF as a solid polymer electrolyte for dye-sensitized solar cells

Heteropolyacid (HPA)-impregnated polyvinylidene fluoride (PVDF) with iodine/iodide was prepared as a new polymer electrolyte for bio-mimicking natural photosynthesis. With this new polymer electrolyte, dye-sensitized solar cell was fabricated using N3 dye-adsorbed over TiO2 nanoparticles (photoanode) and conducting carbon cement coated on conducting glass (photocathode). The fabricated cell generates high open circuit voltage (VOC 426 mV) and short circuit current (ISC 3.90 mA) upon illumination with visible light. It is also demonstrated that the polymer electrolytes prevent the back-electron transfer reactions taking place in dye-sensitized hetero-junctions and are highly promising for solar energy conversion to electricity.

Room temperature growth of CuO nanorod arrays on copper and their application as a cathode in dye-sensitized solar cells

Vertically aligned CuO nanorod arrays on copper electrodes with diameters of ∼5 nm and lengths of hundreds of nanometers were synthesized successfully using a simple wet chemical method at room temperature. The composition, morphology, and growth conditions of the nanorods were characterized. We also show that the as-prepared CuO nanorod arrays are promising hole-transport media of dye-sensitized hetero-junctions for solar energy conversion.

Molecular Rectification: Application in Dye-Sensitized Solar Cells

A dye-sensitized heterojunction of configuration n-TiO2/PD-CuPC-MV/p-CuSCN (where PD = 3,4-pyridinedicarboxylic acid anchored to TiO2, CuPC = copper(II) phthallocyanine tetrasulfonic acid ionically linked to PD, and MV = Methyl Violet complexed to CuPC) is developed to demonstrate the applicability of molecular rectification to dye-sensitized solar cells as a strategy of suppressing recombination. Short-circuit photocurrent, open-circuit voltage, energy conversion efficiency, and incident photon to photocurrent conversion of this system are higher than that of the heterojunctions of configurations n-TiO2/PD-MV/p-CuSCN, n-TiO/CuPC-MV/p-CuSCN, and n-TiO2/MV/p-CuSCN. The impressively high rectification ratio and the mode of anchorage of CuPC toTiO2 are suggested as the cause of superior photovoltaic action of the cell TiO2/PD-CuPC-MV/p-CuSCN.

Charge Separation in Solid-State Dye-Sensitized Heterojunction Solar Cells

Dye-sensitized nanocrystalline solar cells are presently under intensive investigation, as they offer an attractive alternative to conventional p--n junction devices. Solid-state versions have been described where the electrolyte present in the pores of the malodorous oxide film is replaced by a large band gap p-type semiconductor. In this way, a solid-state heterojunction of very large contact area is formed. Light is absorbed by the dye that is located at the interface. Upon excitation, the dye injects electrons into the conduction band of the oxide and is regenerated by hole injection into the p-type conductor. High incident photon-to-electric current conversion efficiencies have been achieved recently with a cell consisting of a dye-derivatized mesoporous TiO{sub 2} film contacted by a new organic hole conductor. The great advantage of such systems with regard to conventional p--n junctions is that only majority carriers are involved in the photoelectric conversion process. Moreover, these are generated by the dye precisely at the site of the junction where the electric field is maximal, enhancing charge separation. Photoelectric conversion by conventional solar cells involves minority carriers whose lifetime is restricted due to recombination. As they are generated throughout the semiconductor and away from the junction, expensive high-purity materialsmore » are required in order to maintain the minority carrier diffusion length at a level where current losses are avoided. While the dynamics of photoinduced redo processes in photoelectrochemical systems have been studied in great detail, little is known about the electron-transfer dynamics in solid-state sensitized junctions. Here the authors report for the first time on the direct observation of photoinduced, interfacial charge separation across a dye-sensitized solid-state heterojunction by means of picosecond transient absorption laser spectroscopy.« less

Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies

Solar cells based on dye-sensitized mesoporous films of TiO2 arelow-cost alternatives to conventional solid-state devices1. Impressive solar-to-electrical energy conversion efficiencies have been achieved with such films when used in conjunction with liquid electrolytes2. Practical advantages may be gained by the replacement of the liquid electrolyte with a solid charge-transport material. Inorganic p-type semiconductors3,4 and organic materials5,6,7,8,9 have been tested in this regard, but in all cases the incident monochromatic photon-to-electron conversion efficiency remained low. Here we describe a dye-sensitized heterojunction of TiO2 with the amorphous organic hole-transport material 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene (OMeTAD; refs. 10 and 11). Photoinduced charge-carrier generation at the heterojunction is very efficient. A solar cell based on OMeTAD converts photons to electric current with a high yield of 33%.

Large Enhancement in Photocurrent Efficiency Caused by UV Illumination of the Dye-Sensitized Heterojunction TiO2/RuLL‘NCS/CuSCN: Initiation and Potential Mechanisms

Wide-band-gap, dye-sensitized, heterojunctions of the composition n-TiO2/Ru-dye/p-CuSCN undergo large increases in efficiency when subjected to low-power UV illumination (<40 mW/cm2) for periods between 10 and 30 min. Ru-dye refers to any of several ruthenium polypyridyl dyes. The UV illumination increases the incident-photon-to-current-efficiency for light absorbed by the dye by a factor of 5−10 and increases the open circuit voltage by 100−300 mV. This beneficial effect is stable for months after the UV illumination is terminated. Investigation of the effects of varying the UV wavelength and the applied bias show that the reaction(s) responsible for the increase in efficiency begin with the oxidation of some interface species by a photoexcited hole in the TiO2 valence band. Treatment of the TiO2/dye/CuSCN cells with oxidized thiocyanate in solution causes a similar increase in the efficiency of dye sensitization. Results from TiO2 and CuSCN photoelectrochemical cells are also presented. The combined res...