Kinetic and Energetic Paradigms for Dye-Sensitized Solar Cells: Moving from the Ideal to the Real

Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom.
Accounts of Chemical Research (Impact Factor: 22.32). 09/2009; 42(11):1799-808. DOI: 10.1021/ar900145z
Source: PubMed


Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar cells. Their function is based on photoinduced charge separation at a dye-sensitized interface between a nanocrystalline, mesoporous metal oxide electrode and a redox electrolyte. They have been the subject of substantial academic and commercial research over the last 20 years, motivated by their potential as a low-cost solar energy conversion technology. Substantial progress has been made in enhancing the efficiency, stability, and processability of this technology and, in particular, the interplay between these technology drivers. However, despite intense research efforts, our ability to identify predictive materials and structure/device function relationships and, thus, achieve the rational optimization of materials and device design, remains relatively limited. A key challenge in developing such predictive design tools is the chemical complexity of the device. DSSCs comprise distinct materials components, including metal oxide nanoparticles, a molecular sensitizer dye, and a redox electrolyte, all of which exhibit complex interactions with each other. In particular, the electrolyte alone is chemically complex, including not only a redox couple (almost always iodide/iodine) but also a range of additional additives found empirically to enhance device performance. These molecular solutes make up typically 20% of the electrolyte by volume. As with most molecular systems, they exhibit complex interactions with both themselves and the other device components (e.g., the sensitizer dye and the metal oxide). Moreover, these interactions can be modulated by solar irradiation and device operation. As such, understanding the function of these photoelectrochemical solar cells requires careful consideration of the chemical complexity and its impact upon device operation. In this Account, we focus on the process by which electrons injected into the nanocrystalline electrode are collected by the external electrical circuit in real devices under operating conditions. We first of all summarize device function, including the energetics and kinetics of the key processes, using an "idealized" description, which does not fully account for much of the chemical complexity of the system. We then go on to consider recent advances in our understanding of the impact of these complexities upon the efficiency of electron collection. These include "catalysis" of interfacial recombination losses by surface adsorption processes and the influence of device operating conditions upon the recombination rate constant and conduction band energy, both attributed to changes in the chemical composition of the interface. We go on to discuss appropriate methodologies for quantifying the efficiency of electron collection in devices under operation. Finally, we show that, by taking into account these advances in our understanding of the DSSC function, we are able to recreate the current/voltage curves of both efficient and degraded devices without any fitting parameters and, thus, gain significant insight into the determinants of DSSC performance.

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    • "We found that for both solar cells, R rec decreases with increasing applied voltage, indicating an increased rate of recombination due to an upward shift of the quasi-Fermi level in TiO 2 . The trend is in good agreement with the data for a conventional solid-state DSC [12]. Fig. 7a shows that the plasma treatment causes an increase in Table 1 PV parameters of PSCs shown in Figs. "
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    ABSTRACT: This study reports for increasing the efficiency of perovskite solar cells (PSCs) by modifying the surface of a fluorine-doped indium tin oxide (FTO) substrate using an atmospheric pressure plasma treatment. Surface modification of the FTO film involved several challenges, such as control of the blocking layer uniformity, removal of pinholes, and deposition of a dense layer. This strategy allows the suppression of charge recombination at the interface between the FTO substrate and hole conductor. Electrochemical impedance spectroscopy analysis showed that the plasma treatment increased the charge transfer resistance between the FTO and hole conductor from 95.1 to 351.1 Ω, indicating enhanced resistance to the electron back reaction. Analyses of the open-circuit photovoltage decay revealed that modification of the surface of the FTO substrate by plasma treatment increased time constant from 6.44 ms to 13.5 ms. The effect is ascribed to suppression of the electron recombination rate. PSCs based on the newly developed electrode had 39% higher efficiency than reference devices. The obtained results provide direct evidence in favor of the developed strategy.
    Thin Solid Films 09/2015; 593. DOI:10.1016/j.tsf.2015.09.035 · 1.76 Impact Factor
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    • "The components of a DSSC have more or less been standardized and they are: a TiO 2 nanocrystalline film deposited on a SnO 2 :F transparent conductive electrode (negative electrode), a ruthenium bipyridyl derivative adsorbed and chemically anchored on TiO 2 nanocrystallites, an electrolyte bearing the I À /I 3 À redox couple and a platinized SnO 2 :F electrode (positive electrode). A large volume of the recent works on DSSC's is devoted to the study of the physicochemical state of the electrolyte [7] [8] [9]. This is dictated by some concern that has been expressed as to the long term photochemical stability of the devices due to leakage of the electrolyte caused sealing problems as well as stability and durability of liquid electrolytes. "
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    ABSTRACT: Six ruthenium(II) complexes as charge-transfer sensitizers for dye sensitized solar cells (DSSCs) are synthesized. The absorption and electrochemical properties of newly synthesized ruthenium-dye molecules contained one bipyridine (bpy) ligand with two carboxylic groups have been investigated. Among them, four ruthenium(II) complexes contain a second bpy ligand with branching and non-branching side groups containing C and H only and the remaining two ruthenium(II) complexes instead of a second bipyridine (bpy) ligand, they consisted of a pyridine (py) ligand with side groups containing –C–O–C–molecular group. Dye sensitized solar cells employing quasi-solid state electrolyte and the six ruthenium complexes are constructed and electrically characterized under standard conditions of light irradiance (1000 W/m2, AM 1.5). Their behavior is compared with that of commercially available ruthenium complex D907 in terms of current-voltage characteristic curves under simulated light and dark while electrochemical impedance spectroscopy showed comparable results for local resistance to charge transfer across the TiO2-electrolyte interface and free electron lifetimes for two bipyridine and commercial D907 complexes. The influence of molecular side groups into ruthenium-dye molecules is discussed in terms of the cells’ efficiency.
    Electrochimica Acta 02/2015; 160. DOI:10.1016/j.electacta.2015.01.195 · 4.50 Impact Factor
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    • "Also, the change of Voc may result from the difference of reverse saturation current J0. We have synthesized nanoporous ZnO spheres by hydrothermal method [16], and the nanostructural quality of porous ZnO spheres may vary from batch to batch, thus resulting in the difference of band offset, charge transfer mobilities, porosities, etc. [32,33]. "
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    ABSTRACT: A scattering layer is utilized by mixing nanoporous spheres and nanoparticles in ZnO-based dye-sensitized solar cells. Hundred-nanometer-sized ZnO spheres consisting of approximately 35-nm-sized nanoparticles provide not only effective light scattering but also a large surface area. Furthermore, ZnO nanoparticles are added to the scattering layer to facilitate charge transport and increase the surface area as filling up large voids. The mixed scattering layer of nanoparticles and nanoporous spheres on top of the nanoparticle-based electrode (bilayer geometry) improves solar cell efficiency by enhancing both the short-circuit current (J sc) and fill factor (FF), compared to the layer consisting of only nanoparticles or nanoporous spheres.
    Nanoscale Research Letters 06/2014; 9(1):295. DOI:10.1186/1556-276X-9-295 · 2.78 Impact Factor
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