染料敏化太阳能电池以其低成本高效率引起了人们广泛的关注,其中的光诱导电荷转移过程对太阳能电池的光电转化效率起着重要作用.本工作中我们以两种香豆素类染料分子7-羟基香豆素-4-乙酸(HCA)和7-N,N-二甲胺基香豆素-4-乙酸(DMACA)为例,从理论上研究了它们的几何结构和电子吸收光谱;并将其吸附在TiO2表面上,计算了它们与TiO2表面之间电荷转移的重组能、耦合强度和驱动力,进而计算了电荷转移速率.结果表明,DMACA分子中二甲胺基在第一激发态的旋转能垒约为0.08 eV,因此DMACA分子在第一激发态时很容易发生扭转.通过对HCA-TiO2/DMACA-TiO2体系中电子转移过程的研究,发现尽管两者重组能相似,但前者耦合强度和驱动力比后者小,使前者的电子转移速率略小于后者.当DMACA-TiO2体系中二甲胺基在激发态发生扭转后,耦合强度略微减小,但由于驱动力减小,重组能增大,电子注入速率明显降低.因此,本工作不仅合理地解释了实验现象,而且也提供了一种理论预测染料分子-半导体界面上电子转移的可行性方法.%Dye-sensitized solar cells (DSSCs) have attracted extensive attention due to low cost and relative high efficiency.The photo induced charge transfer process is very important to the efficiency of DSSCs.In this work, we take two coumarins, i.e., 7-hydroxy coumarin 4-acetic acid (HCA) and 7-N,N-dimethyl amino coumarin 4-acetic acid (DMACA) as examples, and investigate their geometries and electronic absorption spectra theoretically.Furthermore, the three parameters controlling electron transfer rate, i.e., the reorganization energy, electronic coupling strength, and driving force, between the two coumarins and TiO2, are calculated to obtain electron transfer rate.It shows that rotation barrier of-N(CH3)2 group in DMACA in the first excited state is predicted to be about 0.08 eV.So this group is very easy to twist.For HCA-TiO2 and DMACA-TiO2 systems, the reorganization energy is similar, but the electronic coupling strength and driving force in the former are smaller than those in the latter.As a result, the electron transfer rate in the former is a little smaller than that in the latter.When-N(CH3)2 group in DMACA-TiO2 system rotates, the electronic coupling strength decreases slightly, and the driving force decreases, and the reorganization energy increases, leading to a drastically decrease in the electron injection rate.Therefore, this work not only explains the experimental phenome-na, but also provides one practicable approach to predict the electron transfer process between dye molecules and semiconductors.
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