Green Alex N M, Palomares Emilio, Haque Saif A, Kroon Jan M, Durrant James R
Center for Electronic Materials and Devices, Department of Chemistry, Imperial College of Science, Technology, and Medicine, Exhibition Road, London SW7 2AZ, United Kingdom.
J Phys Chem B. 2005 Jun 30;109(25):12525-33. doi: 10.1021/jp050145y.
We report a comparison of charge transport and recombination dynamics in dye-sensitized solar cells (DSSCs) employing nanocrystalline TiO(2) and SnO(2) films and address the impact of these dynamics upon photovoltaic device efficiency. Transient photovoltage studies of electron transport in the metal oxide film are correlated with transient absorption studies of electron recombination with both oxidized sensitizer dyes and the redox couple. For all three processes, the dynamics are observed to be 2-3 orders of magnitude faster for the SnO(2) electrode. The origins of these faster dynamics are addressed by studies correlating the electron recombination dynamics to dye cations with chronoamperometric studies of film electron density. These studies indicate that the faster recombination dynamics for the SnO(2) electrodes result both from a 100-fold higher electron diffusion constant at matched electron densities, consistent with a lower trap density for this metal oxide relative to TiO(2), and from a 300 mV positive shift of the SnO(2) conduction band/trap states density of states relative to TiO(2). The faster recombination to the redox couple results in an increased dark current for DSSCs employing SnO(2) films, limiting the device open-circuit voltage. The faster recombination dynamics to the dye cation result in a significant reduction in the efficiency of regeneration of the dye ground state by the redox couple, as confirmed by transient absorption studies of this reaction, and in a loss of device short-circuit current and fill factor. The importance of this loss pathway was confirmed by nonideal diode equation analyses of device current-voltage data. The addition of MgO blocking layers is shown to be effective at reducing recombination losses to the redox electrolyte but is found to be unable to retard recombination dynamics to the dye cation sufficiently to allow efficient dye regeneration without resulting in concomitant losses of electron injection efficiency. We conclude that such a large acceleration of electron dynamics within the metal oxide films of DSSCs may in general be detrimental to device efficiency due to the limited rate of dye regeneration by the redox couple and discuss the implications of this conclusion for strategies to optimize device performance.
我们报告了采用纳米晶TiO₂和SnO₂薄膜的染料敏化太阳能电池（DSSC）中电荷传输和复合动力学的比较，并探讨了这些动力学对光伏器件效率的影响。金属氧化物薄膜中电子传输的瞬态光电压研究与电子与氧化敏化剂染料和氧化还原对复合的瞬态吸收研究相关。对于所有这三个过程，观察到SnO₂电极的动力学快2-3个数量级。通过将电子复合动力学与染料阳离子相关的研究以及薄膜电子密度的计时电流法研究，探讨了这些更快动力学的起源。这些研究表明，SnO₂电极更快的复合动力学既源于在匹配电子密度下高100倍的电子扩散常数，这与该金属氧化物相对于TiO₂较低的陷阱密度一致，也源于SnO₂导带/陷阱态密度相对于TiO₂的300 mV正移。与氧化还原对更快的复合导致采用SnO₂薄膜的DSSC暗电流增加，限制了器件的开路电压。与染料阳离子更快的复合动力学导致氧化还原对使染料基态再生的效率显著降低，这通过该反应的瞬态吸收研究得到证实，并且导致器件短路电流和填充因子损失。通过对器件电流 - 电压数据的非理想二极管方程分析证实了这种损失途径的重要性。添加MgO阻挡层被证明可有效减少与氧化还原电解质的复合损失，但发现无法充分延缓与染料阳离子的复合动力学，以实现有效的染料再生而不伴随电子注入效率的损失。我们得出结论，由于氧化还原对染料再生速率有限，DSSC金属氧化物薄膜中电子动力学如此大幅加速通常可能对器件效率不利，并讨论了这一结论对优化器件性能策略的影响。
