Both top and bottom substrates are typically made of glass in order for the solar energy to be able to strike the photosensitizer molecules. However, transparency is only mandatory in one of them. Nevertheless, when both substrates are transparent, it offers added value to numerous applications (facades, windows, roof panels, etc.). For high performance cells, along with transparency (transmittance higher than 80 %) the supporting substrate must also have very low sheet resistance (< 15 Ω/sq). Traditionally a transparent conductive oxide layer (TCO) is deposited on the top of the glass surface in order to extract electrons and transport them onto a outer circuit. One of the most used TCO in electronic industry is indium-tin oxide (ITO). However, ITO’s resistance increases dramatically above 450 ºC in air, the temperature required for the preparation of the photoelectrode . As an alternative, fluorine-doped tin oxide, SnO 2 :F (FTO) is used. Efforts have been made to find
The morphology of the films was assessed by field-emission scanning electron microscopy (JEOL JSM 6700F). Film thicknesses and surface profiles were measured with a Taylor Hobson Frontoy Lorsurf 50 profilometer. For the electrical characterization of the DSCs, characteristic parameters were obtained by illuminating the samples with an AM 1.5 solar simulator (SolarLight XPS400) at 100 mW cm 2 . The current– voltage (j–V) data were acquired with an Autolab PGSTAT-30 potentiostat–galvanostat. Electrochemical impedance spectro- scopy (EIS) was carried out by using a Solartron impedance meter (model SI 1287). The impedance spectra from the sealed DSCs were measured in a two-electrode configuration in the dark using a bias voltage of 0.8 V with a 20 mV AC perturbation over a frequency range of 10 mHz to 1 MHz.
In this work, three dyesolarcells were assembled using ZnO electrodeposited at three different potentials to observe the effect these changes had on the conversion efficiency of the cells, besides the effects on the morphology, optical and structural ZnO film characteristics. The electrodepositions were carried out at a low temperature, below the usually tested temperatures, without any catalyst or additive, a simple aqueous zinc nitrate solution, on fluoride doped tin oxide layer synthesized in laboratory and used as the working electrode in the ECD process.
Platinum counter electrodes were obtained by DC magnetron sputtering of a pure platinum target varying only the deposition time. In order to select best sputtering parameters to obtain opti- mized counter electrodes for DSSC application, the ﬁlms were characterized by proﬁlometry and elipsometry to measure the thickness of the ﬁlms; four point electrical resistance to evaluate the sheet resistance; spectrophotometry to evaluate the trans- parency; and electrochemical impedance spectroscopy to evaluate the electrode response to iodine redox couple. The ﬁlms responses were compared to commercial platinum paint commonly used as counter electrode for dyesolarcells. For samples obtained by sput- tering for less than 30 s (Sample 4), the sheet resistance presented an impeditive value (from approximately 15–50 /䊐) as well as better optical transmittance. In samples prepared for 30, 150, and 300 s (thickness equal to 24.8, 122.0, and 319.0 nm, respectively) the values of sheet resistance were below substrate resistance and revealed good charge transfer efﬁciency.
The investigated nanostructures of TiO 2 , NaTiNT and Nanoribbons deposited successfully on FTO, through the electrophoresis technique. The shift on the absorption spectrums of the films to the regions near the visible, after the thermic treatment, became noticeable for the NaTiNT and Nanoribbons nanostructures, probably due to the structural changes, which happened after the collapse of these nanostructures. The results of the cells J-V curves demonstrated that the cell with the NaTiNT film, after thermal treatment, presented
In this work we report DSSCs based on gallium-modiﬁed zinc oxide electrodes (ZnO:Ga) with improved efﬁciencies in comparison to pure ZnO based solarcells. In order to have more clear evidence of the gallium modiﬁcation effect on the photovoltaic properties and on the ﬁlm energetics, transient absorption spectroscopy (TAS) was used as a tool to probe the charge recombination dynamics in these ﬁlms.
Organic photovoltaics are made of electron donor and electron acceptor materials rather than semiconductor p-n junctions. The molecules forming the electron donor region of organic p-v cells, where exciton electron-hole pairs are generated, are generally conjugated polymers processing delocalized π electrons that result from carbon p orbital hybridization. These π electrons can be excited by light in or near the visible part of the spectrum from molecules highest occupied molecular orbital(HOMO) to the lowest unoccupied molecular orbital(LUMO), denoted by π- π* transition. The energy band gap between these orbitals determines which wavelength of light can be absorbed. Unlike in an inorganic crystalline PV cell material, with its band structure and delocalized electrons, excitons in organic photovoltaics are strongly bound with an energy between 0.1 and 1.4ev .This strong binding occurs because electronic wave functions in organic molecules are more localized, and electrostatic attraction can thus keep an electron and a hole together as an exiction. The electron and hole can be separated by providing an interface across which the chemical potential of electron decreases. The material that absorbs the photon is the donor, and the material which acquires the electron is the acceptor. In Fig.3, the polymer chain is the donor and the fullerene is the acceptor. After dissociation, the electron and hole may be still joined as a “geminate pair”, and an electric field is then required to separate them.
It is possible to list a number of desirable properties for a photosensitizer: strong light absorption in visible and near-IR region (for efficient light harvesting); good solubility in organic solvents (for easy deposition from stock solutions in few hours or less); presence of suitable peripheral anchoring ligands such as –COOH (to promote the effective interaction of the dye with the oxide surface and thus the coupling of donor and acceptor levels); suitable disposition of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the dye molecule (to permit quantitative injection of charges from the electronically excited state); good thermal stability and good chemical stability (to retain the chemical identity over repetitive oxido-reduction cycles). 57
Electrochemical impedance spectroscopy (EIS) was employed to analyze charge transport kinetics in the cell. Usually in EIS study, the solar cell is perturbed by a small AC voltage signal. In this work, the DSSCs were perturbed by with a signal of amplitude 10 mV with var- ying frequency (10 mHz-10 MHz). According to the DSSC response, Nyquist and Bode curves were obtained for the DSSCs sensitized by safflower, Medicago sativa, and Ros marinus extracts. Nyquist and Bode curves are illustrated in Figs. 4 and 5, respectively. The impedance parameters were calculated from Figs. 4 and 5 and listed in Table 2 in which R S is the charge recombination re-
pared at different sputtering pressures are shown in Figure 2. Figure 2A shows the surface SEM images and Figure 2B shows the cross-sectional SEM images. From the Figure 2A, it can be seen that the rough surface has been obtained for all the films. Indeed, a rough surface is desirable for solar energy application because it allows the light that enters into the solarcells through the TCO layer to scatter efficiently so as to enhance absorption. It can also be seen that the structure is totally different for the films prepared at different sputtering pressures. For the film prepared at pressure of 0.7 Pa, only a close-packed compact structure can be observed. The surface view shows the various sizes of angular grains and no voids between the grains can be observed. For the films prepared at pressure higher than 1 Pa, the structure becomes different. Ball of yarn shaped grains and the voids between these grains are observed. The cross-sectional views show that these films have the nanorod structures. The TEM images shown that the surface of these nanorods is very rough ( Meng et al., 2011a ), which is favorable for the dye adsorption. From the surface view, it also can be seen that the voids between the grains become big
first approaches to convert low-energy starting mate- rials into high-energy products, such as H 2 O into H 2 and O 2 , in homogeneous cells. Although ingenious, the drawback was the low efficiencies (light-to- elec- trical conversion efficiency < 0.5%) due to the fast recombination of the photoproducts in the solution (Connolly 1981). The answer seemed to be the use of photoelectrochemical cells, in which a semi- conductor electrode was the light absorber with the band bending separating the reduction and oxidation sites. However, these cells displayed low efficiency conversion of visible light into redox energy, and were limited to the band-gap of the semiconductors employed (Connolly 1981, Wrighton 1983). The use of photostable wide band-gap semiconductors would require high-energy light to create electron- hole pairs and dye sensitization was of limited utility because of sub-monolayer coverage and low absorp- tivities, although it presented advantages over direct band to band excitation as in conventional solarcells due to the reduction of electron-hole recombination. The development of mesoporous membrane type film with large surface areas prepared from nanosized colloidal semiconductor dispersion caused a remarkable growth in the field (Brown Jr. et al. 1999). Dye sensitization of nanostructured wide band-gap semiconductors has led to an extension of their photoresponse into the visible region and to efficient conversion of solar energy into electric- ity in photoelectrochemical devices (O’Reagan & Grätzel 1991). Similarly to chlorophyll molecules, adsorbed dyes act as light absorbing antenna to mimic the photosynthetic process by promot- ing photoinduced charge separation in an organized molecular structure on the nanometric scale.
In this paper, nitrogen-doped TiO 2 thin ilms deposited at diferent doping levels by DC reactive sputtering were used for fabrication of dye-sensitized solarcells. he efect of nitrogen incorporation on the general properties of the ilms, as well as, its efect on the working principle of the solarcells was investigated. he results indicate that the substitutional nitrogen is incorporated into TiO 2 structure from reaction with suboxides created by oxidation of the nitride layers so that the control of the reactive gas mixture, during ilm deposition, plays a fundamental role to achieve high doping levels. he incorporation of nitrogen particles in the ilm lattice shits eiciently the absorption coeicient of the ilms toward the visible and near-infrared regions. However, excessive nitrogen incorporation becomes the ilm amorphous and leads to the appearance of excessive electron acceptor states like oxygen vacancies, interstitial nitrogen, and hydroxyl radicals. Characterization of the cells shows that nitrogen incorporated in the ilm structure increases the short-circuit current density due to the photoexcitation of nitrogen states in the visible part of solar spectrum. On other hand, it decreases the open-circuit voltage and increases the dark current due to the combination of several parallel efects. Among them the main ones are the increase of the electron acceptor states and the surface area of the doped ilms. his latter efect increases the contact area in the solid- liquid interface and, as consequence, the back reaction in the regions in which the dye molecules were not chemisorbed.
fabrication, and long electron lifetimes. However, it also shows low electron mobility, high density of electronic trap states below the conduction band, and it requires high-temperature processing, which presents a serious problem for flexible solarcells production.  For that, a zinc oxide (ZnO) layer was introduced as an alternative ETL, which presented better electron mobility than titanium oxide and low temperature preparation. However, its chemical and thermal instability made this material a poor solution. A tin dioxide (SnO 2 ) layer has also
In the early days of photovoltaics, Becquerel’s research was in fact motivated by photography. Actually, there is an interesting convergence of photography and photoelectrochemistry since both phenomena are based on a photoinduced charge separation in a liquid‐solid interface. However, before being aware of such similarities, and following the experiments developed by Becquerel, Vogel discovered in 1873 that silver halide emulsions sensitized by a dye result in an extended photosensitivity to longer wavelengths. 12 Four years later, J. Moser was the first to report the dye‐sensitized photovoltaic effect. 13 Nevertheless, the modern photoelectrochemistry was only envisaged as an interesting topic for the research community after the works developed by Brattain and Garret 14 and mainly with the first detailed electrochemical and photoelectrochemical studies on the semiconductor‐electrolyte interface undertook by Gerischer. 15 Since then several attempts were made to use dye‐sensitized photoelectrochemical cells to convert sunlight into electricity. However, the efficiency of those devices was very low, well below 1 %, mainly due to the poor light harvesting and instability of the dyes employed. In 1991, Brian O’Regan and Michael Grätzel described for the first time a three dimensional (bulk) heterojunction applied to the fabrication of DSCs. This new device was based on the use of semiconductor films consisting of nanometre‐sized TiO 2 particles, together with newly developed charge‐transfer dyes. These authors
2.6. Theoretical Calculations. To obtain further insight into the distinct di ﬀerences in the structures, electronic properties, and photovoltaic performance of dyes 3a −b, a combined DFT and time-dependent DFT (TD-DFT) computational study was performed. Dyes 3a and 3b can have several di ﬀerent isomers, depending on the relative arrangement of some of their units. These can have di ﬀerent degrees of conjugation and possibly very di ﬀerent electronic structures. A total of twelve isomers of dye 3a and four isomers of dye 3b were studied, resulting from the consideration of both E and Z con ﬁgurations and the diﬀerent conformational arrangements between the carboxylic acid moiety, the cyano group, and the adjacent thienothiophene spacer in dye 3a and between the rhodanine ring and the neighboring thienothio- phene moiety in dye 3b. The structures and energies were determined for these isomers at the DFT/Becke three parameter Lee −Yang−Parr (B3LYP) level of theory, taking into account the bulk solvent e ﬀects of chloroform; the
Organic thick film solarcells were considered to be impossible due to low mobility of charge carriers and small exciton diffusion lengths (typically between 20 nm to 60 nm) in most of the organic materials including small organic molecules and conjugated conducting polymers as compared with their inorganic counterparts. Inorganic thick film solarcells are existing. There are several advantages of thick film based solarcells over thin film ones such as easy fabrication and lower cost and hence are expected to be more suitable for industry. In fact, this is one of the biggest advantage that Gratzel solarcells (Popularly known as dye sensitized solarcells) have (in which the typical porous titania thick films are made using Dr. Blade technique or screen printing technique). In contrast, the organic solarcells demonstrated until now are all thin film devices having typical active layer thickness less than 300 nm. One of the common strategies used to increase the efficiency of such cells is to reduce the active layer thickness (i.e. to make it more thin). Very often, it again gives rise to problems such as pin-holes in the films and shorts. One way to avoid such pin-holes in the thin films and shorts in the devices is to fabricate the devices in clean room or in vacuum conductions or using special environments / conditions, which is again contrary to the main objectives of organic solarcells research. Due to these reasons, organic thick film solarcells which could be fabricated by as simple method as that in Gratzel cells are highly desirable. In the present work undertaken, our aim was to demonstrate an organic thick film solar cell and not towards high efficiency. We believe
This electron donor group delocalizes the pyrylium charge to the styryl moiety allowing adsorption of GK4 through the oxygen in position 7. Alternatively, adsorption can take place through the amino group. The absorption maximum of this photoanode occurs at 574 nm (Figure 10) and is blue-shifted relative to the dye in solution (636 nm), suggesting involvement of the nitrogen lone pair in Ti(IV) binding. For styrylflavylium GK5 there are also two possibilities: The first, in analogy to what seems to be happening in compound GK4, adsorption occurs via the nitrogen atom; the second involves Ti(IV) complexation through the catechol moiety. Catechol groups have proved to be particularly efficient in Ti(IV) binding in natural [9,25] as well as in synthetic flavylium dyes [10–13]. In support of this reasoning, absorption spectra of the two GK5 photoanodes, either soaked in water or in acidified ethanol, are not blue-shifted in comparison with the dye in solution, suggesting that the nitrogen is poorly involved in the adsorption process that would be guaranteed by the catechol. Nevertheless, contribution of the amino group in the adsorption to TiO 2 cannot be excluded if we consider that the