Theoretical Studies On Triphenylamine-based Organic D-π-A Sensitizers For Dye-Sensitized Solar Cell

Dyes-sensitized solar cell (DSSC), as a promising renewable energy conversion device for solving "Terawatt Challenge", is receiving an ever-increasing amount of attention due to their flexibility, easy preparation, environmental-friendly, low cost and high efficiency. Currently, the power conversion efficiency (PCE) of DSSC still lags behind that of the traditional silicon-based solar cells, which will retard their large-scale commercial application. As a key component of DSSC, sensitizers play an important role in affecting the lighting harvesting efficiency, the interfacial charge transfer process and ultimately the overall PCE. However, the experimental design and optimization of sensitizers for the improvement of PCE is still a grand challenge because of the complicated and intricate interfacial charge transfer process. Importantly, quantum chemical calculations could provide insight into the influences of DSSC components on the PCE at a molecular level, which is crucial for us to better understanding the operational mechanism of DSSC. Thus, quantum chemical calculations manifest themselves as a reliable and suitable avenue to boost the performance of DSSC.In comparison with ruthenium polypyridyl dyes, metal-free organic dyes with Donor-π-linker-Acceptor (D-π-A) structural motif have the desirable advantages such as strong molar absorptivity, low cost, synthetic flexibility in molecular design. Herein, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations on triphenylamine (TPA)-based sensitizers were carried out to demonstrate the effects of sensitizers on photocurrent density (Jsc) and open-circuit photovoltage (Voc) in an attempt to shed light on the structure-property relationships and thus to provide theoretical guidelines for the further optimization and design of organic sensitizers. The details are described in the following:1. Considering the low PCE of p-type DSSC, the important influences of π-linkers of sensitizers on the performance of DSSC were firstly explored to improve its performance. The difference in performance for DSSCs based on experimentally reported TPA-based organic p-type sensitizers with different π-linkers were first theoretically rationalized. Then, we also designed and evaluated a series of p-type sensitizers with different thiophene derivatives as π-linkers. In particular, the charge transfer index (DCT) upon photoexcitation was calculated to qualitatively evaluate the charge recombination between the photoinjected hole and the reduced sensitizers. Our calculated results indicated that the trend of DCT is in good line with the experimental value of the electron lifetime, an important parameter pertaining to the open-circuit photovoltage. And, the theoretically designed dye4with dithienothiophene may be a promising candidate for the high-performance p-type sensitizer based on the large DCT, the red-shifted absorption and the improved light harvesting efficiency.2. The effects of different dithiophene rigidifying modes in TPA-based organic sensitizers on the performance of DSSC were investigated. The absorption of sensitizers, the density of state (DOS) and partial DOS of (TiO2)38before and after sensitizer adsorption and the intermolecular interaction of electron acceptor I2in the electrolyte with sensitizer were computed by DFT and TDDFT calculations to extract and evaluate the light harvesting, conduction band energy shift and charge recombination process. It is found that the rigidification via phosphine oxide and methylene group are promising rigidifying modes, which is conducive to improve the light harvesting ability and to slow down the charge recombination, respectively.3. DFT and TDDFT calculations were systematically carried out to shed light on the important role played by planarization of TPA donor and the extended conjugation of π-linker in improving the performance of TPA-based organic sensitizers. The calculated results demonstrated that the remarkable increase in incident photon-to-current conversion efficiency (IPCE) could be mainly ascribed to the decreased exciton binding energy due to the planarization of TPA donor and the extended conjugation of π-linker. And, the red-shifted absorption due to the planarization of TPA donor and the extended conjugation of π-linker is conducive to enhance the spectrum matching with solar photon-flux spectrum. Thus, the improved IPCE and the superior spectrum matching both lead to significant increase in Jsc. In addition, the electrostatic potential (ESP) of these sensitizers mapped onto0.001a.u. electron density isosurface was also calculated to determine the potential binding sites for electron acceptor I2. Then, the effects of the planarization of TPA donor and the extended conjugation of π-linker on open-circuit photovoltage were rationalized based on the analysis of the interaction of dye-I2adducts, which could affect the local concentration of I2in proximity of semiconductor surface. In particular, the calculated nonlinear optical (NLO) properties are found to be correlated well with the photocurrent response of sensitizers.