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: 24.35). 09/2009; 42(11):1799-808. DOI: 10.1021/ar900145z
Source: PubMed

ABSTRACT 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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    • "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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    • "Importantly, although the bandgap energy of TiO 2 is rather large (for anatase 3.2 eV; ∼ 390 nm) and direct bandto-band excitation can therefore only be achieved by highenergy UV light, the utilization of TiO 2 is not confined to UV-light-driven applications. Drawing on the fundamental studies on dye sensitization of other wide bandgap semiconductors (ZnO, SnO 2 ) in the 1960s [55] [56] [57] [58] [59], attaching visiblelight-absorbing organic dyes to the surface of TiO 2 [60] [61] has led to fabrication of regenerative dye-sensitized solar cells with the overall solar conversion efficiencies exceeding 10% [62] [63] [64]. Other sensitization approaches utilize chromophores like semiconductor quantum dots [65] [66] [67] [68] [69] [70], plasmonic metal nanocrystals [71] [72] [73] [74] [75], simple coordination compounds like chloroplatinate (IV) complexes [29] [32] [76] or ferrocyanide ions [77] [78] [79], stable polymeric compounds [38, 39, 80–83], or metal ions (Cu 2+ , Fe 3+ ) grafted onto the TiO 2 surface [84] [85]. "
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    Advances in Physical Chemistry 01/2011; DOI:10.1155/2011/786759(1687-7985). DOI:10.1155/2011/786759
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    ABSTRACT: Dye-sensitized mesoscopic solar cell (DSC) has been intensively investigated as a promising photovoltaic cell. Redox electrolyte is important to determine the photovoltaic (PV) performance of the DSC devices, which has become the focus of this topic. In this contribution, recent advances in understanding and controlling of various redox couples are reviewed. Specially, we extend our discussion on the trends that enable iodide-free redox couples to be controllable and feasible for applications in the DSC with promising features.
    Chinese Science Bulletin 11/2012; 57(32). DOI:10.1007/s11434-012-5409-3 · 1.37 Impact Factor
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