Charge Recombination In Dye-sensitized Solar Cells Research Articles

Synthesis of TiO<sub>2</sub>-rGO Nanocomposite and its Application as Photoanode of Dye-Sensitized Solar Cell (DSSC)

Dye-sensitized solar cells (DSSC) are solar cells that has a great potential to be applied as renewable energy conversion. The major advantages of DSSC are the ease of fabrication process and low cost of production. Despite of these advantages, the efficiency of DSSC for converting light into electricity is still low. It is due to charge recombination in DSSC which limits the photoanode performance. Numerous efforts has been carried out to increase the efficiency of DSSC, one of which is by adding reduced graphene oxide (rGO) to titanium oxide (TiO2) to obtain TiO2-rGO nanocomposite. In this study, the synthesis of TiO2-rGO nanocomposites was carried out with concentration of rGO are 0.6, 0.8, and 1.0 wt% to amount of TiO2. We have done some characterizations to confirm the result of synthesized TiO2-rGO. UV-Vis measurement shows the addition of rGO has widened the absorption up to 400 nm. The FT-IR spectrum confirms that the rGO peaks appears at wavelength of 1400, 1600, dan 1700 cm-1 which exhibited the vibration C-O, C=C, and C=O stretching from COOH groups, respectively. The highest efficiency of DSSC with photoanode TiO2-rGO nanocomposite is 0.09% which was obtained from 0.8 wt% of rGO.

Bilayer films using broadband nanoparticles and mesoporous TiO2 for high efficient dye sensitized solar cells

The improvement of the power-conversion efficiency of dye-sensitized solar cells (DSCs) has slowed down in recent years due to limited light absorption of the dyes in the near-infrared region. In this study, we present a novel strategy that uses ultra-broadband core–shell Ag@TiO2 nanoparticles and mesoporous TiO2 particles to improve light absorption and reduce charge recombination in DSCs. The effect of the concentration of nanoparticles on both photovoltaic performance and charge recombination is studied. The results show that Ag@TiO2 nanoparticles can significantly improve the short-circuit current of DSCs and reduce the charge-transfer resistance at the TiO2-dye/electrolyte interface. These are key factors to achieve high power-conversion efficiency. A power-conversion efficiency of 8.34% is achieved by integrating core–shell NPs (0.6 wt%) with pure P25 TiO2 particles. This efficiency can be improved further (to 10.02%) by depositing a layer of mesoporous TiO2 particles, which increases the propagation distance of incoming light. The good interconnection between the TiO2 crystals of mesoporous particles and the faceted nanocrystal surface enhance electron transport. This work shows a new route to speed-up the transfer of photo-generated carriers, and to split electron-hole pairs more efficiently.

An Investigation of Surface States Energy Distribution and Band Edge Shifts in Solar Cells Based on TiO2 Submicrospheres and Nanoparticles

TiO2 submicrospheres were often used as photoanodes in dye-sensitized solar cells (DSSCs) due to the high internal surface area and great scattering properties. However, surface states in TiO2 submicrospheres limit the electron transport process. In this paper, an investigation of surface states in TiO2 submicrospheres and traditional TiO2 nanoparticles was conducted by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The chemical capacitance, electron transport resistance and recombination resistance were interpreted in a consistent framework. The electron transport and recombination behavior were also studied by open-circuit voltage decay measurements (OCVD) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). The results show that the TiO2 submicrospheres based films possess a larger number of surface states and a shallow energy distribution than TiO2 nanoparticles under the same film thickness. Furthermore, the fitted data indicate that the electron transport in TiO2 submicrospheres film was faster than in traditional nanoparticles film owing to their high electron concentration and shallow surface states energy distribution. The discussion highlights the location of surface states in the band gap region of TiO2 film, which plays an important role in electron transport and charge recombination in DSSCs.

Preparation of TiO2 nanoparticles by sparking technique for enhancing photovoltaic performance of dye-sensitized solar cells

The double-layer photoanodes fabricated from TiO2 nanoparticles (np-TiO2) and TiO2 powder (P25) for dye-sensitized solar cells (DSSCs) are reported. The np-TiO2 was deposited on FTO substrates by a sparking technique. The PT1 and PT2 DSSCs were composed of FTO/P25/np-TiO2/N719/electrolyte/Pt and FTO/np-TiO2/P25/N719/electrolyte/Pt, respectively. The Nyquist plot and equivalent circuit of impedance of the DSSCs are also explained and discussed. In this research, the PT1 DSSC with a 1 h sparking period has the highest efficiency of 3.62, 50.21% higher than that of the reference. The enhancement is explained by the increase of adsorption of dye molecules that lead to a remarkable improvement in short-circuit photocurrent (J sc). The pore size distribution with increasing the film thickness played a role in the penetration of the electrolyte, dye molecules and effective surface area. Moreover, a decrease in the interfacial resistance was detected in the P25/np-TiO2 double-layered photoanode, leading to fast charge transport and decreased charge recombination in DSSCs.

The Effect of Pyridyl Nitrogen Atom Position in Pyrido[3,4‐b]pyrazines in Donor‐Acceptor‐π‐Acceptor Dyes on Absorption, Energy Levels, and Photovoltaic Performances of Dye‐Sensitized Solar Cells

AbstractA donor‐acceptor‐π‐acceptor (D‐A‐π‐A)‐type organic dye (DTN‐1) incorporating a pyrido[3,4‐b]pyrazine (PP) unit with the pyridyl N atom adjacent to the anchoring group has been synthesized for use in dye‐sensitized solar cells (DSSCs). The maximum absorption wavelength of DTN‐1 was clearly red‐shifted compared with dye DT‐1, based on PP unit with the pyridyl N atom adjacent to the donor group. However, this change of structure has a negative effect on photovoltaic performances, and devices made with DTN‐1 only reached a power conversion efficiency of 6.10 % under AM1.5G irradiation compared with 8.57 % achieved by DT‐1. Density functional theory calculations suggest that DTN‐1 has a smaller oscillator strength, which is connected to its relatively low light‐harvesting efficiency. In addition, the results of electrochemical impedance spectroscopy (EIS) reveal that charge recombination in DSSCs based on DTN‐1 is more than that in the counterpart DT‐1, thus leading to a lower open‐circuit voltage (Voc).

Comparative Study on Pyrido[3,4-b]pyrazine-Based Sensitizers by Tuning Bulky Donors for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) with cobalt electrolytes have gained increasing attention. In this Research Article, two new pyrido[3,4-b]pyrazine-based sensitizers with different cores of bulky donors (indoline for DT-1 and triphenylamine for DT-2) were designed and synthesized for a comparative study of their photophysical and electrochemical properties and device performance and were also analyzed through density functional theory calculations. The results of density function theory calculations reveal the limited electronic communication between the biphenyl branch at the cis-position of N-phenylindoline and the indoline core, which could act as an insulating blocking group and inhibit the dye aggregation and charge recombination at the interface of TiO2/dye/electrolyte. As expected, DSSCs based on DT-1 with cobalt redox electrolyte gained a higher photoelectric conversion efficiency of 8.57% under standard AM 1.5 G simulated sunlight, with Jsc = 16.08 mA cm(-2), Voc = 802 mV, and FF = 0.66. Both electrochemical impedance spectroscopy (EIS) and intensity-modulated photovoltage spectroscopy (IMVS) suggest that charge recombination in DSSCs based on DT-1 is much less than that in their counterparts of DT-2, owing to the bigger donor size and the insulating blocking branch in the donor of DT-1.

ChemInform Abstract: Molecular Design Principle of All‐Organic Dyes for Dye‐Sensitized Solar Cells

AbstractReview: 70 refs.

Significant Enhancement of Open-Circuit Voltage in Indoline-Based Dye-Sensitized Solar Cells via Retarding Charge Recombination

Indoline dyes exhibit impressive short-circuit photocurrent (JSC) but show generally low open-circuit voltage (VOC) in dye-sensitized solar cells (DSCs). To retard charge recombination in DSCs, four indoline dyes (XS41, XS42, XS43, and XS44) featuring, respectively, dipropylfluorene, hexyloxybenzene, tert-butylbenzene, and hexapropyltruxene electron donors, have been engineered. The incorporation of bulky rigid groups (i.e., dipropylfluorene and hexapropyltruxene unit) can notably retard the charge recombination at the titania/electrolyte interface. Moreover, we have developed two organic dyes (TC1 and TC2) as alternative coadsorbents to chenodeoxycholic acid (CDCA). Interestingly, it is found that regardless of the dye selection coadsorption with TC2 shows an improved VOC as well as JSC in comparison with its TC1 analogues. Dependence of photovoltage on the structure of TC1/TC2 was also investigated. The results suggest that the change in VOC is likely correlated with the molecular matching between the d...

Molecular Design Principle of All‐organic Dyes for Dye‐Sensitized Solar Cells

All-organic dyes have shown promising potential as an effective sensitizer in dye-sensitized solar cells (DSSCs). The design concept of all-organic dyes to improve light-to-electric-energy conversion is discussed based on the absorption, electron injection, dye regeneration, and recombination. How the electron-donor-acceptor-type framework can provide better light harvesting through bandgap-tuning and why proper arrangement of acceptor/anchoring groups within a conjugated dye frame is important in suppressing improper charge recombination in DSSCs are discussed. Separating the electron acceptor from the anchoring unit in the donor-acceptor-type organic dye would be a promising strategy to reduce recombination and improve photocurrent generation.

Effect of the adsorbed concentration of dye on charge recombination in dye-sensitized solar cells

Charge recombination is the most important factor limiting the power conversion efficiency of dye-sensitized solar cells (DSCs). Lots of factors affecting the charge recombination in DSCs have been investigated except the adsorbed concentration of dye on the surface of TiO2 film. By electrochemical impedance spectroscopy (EIS) analysis, it is found that with the adsorbed concentration of dye decreasing, both charge transport resistance at the TiO2/electrolyte interface and electron life time within the TiO2 photoanode increase, which indicates that the charge recombination in DSC decreases. Owing to this effect, the DSC keeps the fill factor as high as 0.75–0.78 even though the TiO2 film thickness exceeds 20μm.

A ZnO nanorod layer with a superior light-scattering effect for dye-sensitized solar cells

ZnO nanorod (NR) structures have been fabricated using a precipitation method and applied as an effective scattering layer on the top of a ZnO nanoparticle (NP) electrode in a dye sensitized solar cell (DSC). This can effectively improve the light harvesting and enhance the photocurrent. Furthermore, the scattering layer decreases the charge recombination in DSCs, which is demonstrated by the results of the decreasing dark current and increasing recombination resistance. This leads to a remarkable 35% improvement in the conversion efficiency of the DSC using the ZnO nanorod scattering layer.

Charge Recombination in Dye-sensitized Solar Cells with Low Adsorbed Concentration of Dye

Photovoltaic conversion performances of dye-sensitized solar cells (DSCs) are significantly influenced by the interface charge recombination in DSCs. Lots of factors affecting the charge recombination, such as surface states of TiO2 and components of electrolytes, have been studied and dyes have been always ignored for the charge recombination in DSCs. Although the charge recombination occurring between the injection electrons and triiodide in electrolyte is calculated to take priority kinetically to the one between the injection electrons and oxidized dye molecules, dyes are not independent from the electrolyte related electron recombination. Instead by dye molecules themselves, the chance of injection electrons recaptured by triiodide in electrolyte could rise due to the increase of the adsorbed concentration of dye, which leads to the local concen- tration of triiodide increasing. In this paper, an effect of low charge recombination in DSCs with low adsorbed concentration of dye is observed. The adsorbed concentration of dye is defined as the adsorbed amount of dye in unit specific surface area of TiO2 films and adjusted by adsorbing similar amount of dye on the surface of TiO2 films with different film thickness. The influence of the adsorbed concentration of dye on the charge recombination in DSC is investigated by the electrochemical impedance spectroscopy (EIS) technology. It turns out that with the adsorbed concentration of dye decreasing, the electron lift time within TiO2 film and the interface resistance of TiO2/electrolyte increase significantly, which indicates the charge recombination in DSC decrease. Owing to this effect, with the TiO2 film thickness increasing from about 2 μm to 18 μm, the cells keep the fill factor (ff) as high as 0.72~0.80. And the energy conversion efficiency loss, which resulted from the in- crease of the active area of TiO2 photoanode from 0.25 cm 2 to 1 cm 2 , decreased from 34.7% to 19.6%.

Interface modification of 8-hydroxyquinoline aluminium with combined effects in quasi-solid dye-sensitized solar cells

In this paper, 8-hydroxyquinoline aluminium (Alq(3)) was used in interface modification of dye-sensitized solar cells (DSCs). Alq(3) was the first discovered interface modification material with combined effects of retarding charge recombination and Förster resonant energy transfer (FRET). Results of dark current curve and AC impedance showed that Alq(3) could retard charge recombination in DSCs. I-V curves showed that conversion efficiency increased with Alq(3) modification. Besides the interface modification effect, it was discovered that Alq(3) also acted as energy relay dye with the FRET effect between itself and N3, which increased photoresponse and electron injection. The application of Alq(3) with combined effects opened a new door to explore more novel multi-functional interface modification materials to improve the performance of DSCs.

Mg(OOCCH3)2 Interface Modification after Sensitization to Improve Performance in Quasi-solid Dye-Sensitized Solar Cells

In this paper, a simple yet efficient method is proposed to improve the performance of dye-sensitized solar cells (DSCs) by modification after sensitization using Mg(OOCCH(3))(2). With modification of Mg(OOCCH(3))(2), a blue shift of the absorption peak and optical band gap were observed in the UV-vis spectrum. As shown in the Fourier transform infrared spectrum, the intermolecular hydrogen bonding of N3 dye, which caused the aggregation of dye molecules, was weakened. As shown in the I-V characteristic, the conversion efficiency of the DSCs was improved by the treatment of Mg(OOCCH(3))(2). Furthermore, the charge recombination was retarded as evidenced by the decreased dark current and the slowed decay rate of the dye excited state, which were characterized by the I-V curve in dark and transient photovoltage spectra. The mechanism of this modification process was also proposed further. Modification with Mg(OOCCH(3))(2) facilitated the electron injection from the dye molecule to the conductive band of TiO(2) by raising the excited state energy level of the dye molecule. This energy level rising was evidenced by the results of the cyclic voltammetry test and the blue shift of the optical band gap. Furthermore, Mg(OOCCH(3))(2) worked as an insulating barrier layer at the sensitized TiO(2)/electrolyte interface, thereby retarding the charge recombination in DSCs.

Novel Carboxylated Oligothiophenes as Sensitizers in Photoelectric Conversion Systems

Novel carboxylated oligothiophenes with different thiophene units were designed and synthesized as photosensitizers in dye-sensitized solar cells (DSSCs) for efficient opto-electric materials. The introduction of -COOH into thiophene molecules can lead to a red shift of UV-visible absorption, increase light-harvesting efficiency, and enhance photoinduced charge transport by forming efficient covalent bonds to the substrate surface. A red shift of the absorption spectrum of oligothiophene is also achieved by the increase in the number of thiophene units. The DSSCs based on the oligomers have excellent photovoltaic performances. Under 100 mW cm(-2) irradiation a short-circuit current of 10.57 mA cm(-2) and an overall energy conversion efficiency of 3.36 % is achieved when pentathiophene dicarboxylated acid was used as a sensitizer. The incident photo-to-current conversion efficiency (IPCE) has a maximum as high as 80 %. In addition, photovoltage and photocurrent transients show that slow charge recombination in DSSCs is important for efficient charge separation and excellent photoelectric conversion properties of the oligomers. These initial and promising results suggest that carboxylated oligothiophenes are efficient photosensitizers.