Design, Synthesis And Optimization Of Light-harvesting Materials For Mesoscopic Solar Cells

Based on the difference of ligh-harvesting materials, mesoscopic solar cell (MSC) is classified as dye-sensitized solar cell (DSSC) and mesoscopic perovskite solar cell (MPSC). MSC attracts extensive attention on account of their advantages of low cost, ease of fabrication and high power conversion efficiency (PCE). However, conventional MSC usually employ noble metal (such as Pt, Ag and Au) as counter electrodes (CE), which limits their large-scale commercial applications. Consequently, the fully printable MSC based on substitution of noble metal electrode with low-cost carbon electrode possesses vast prospects in practical application.In this thesis, efforts were mainly devoted to investigate the light-harvesting materials based on fully printable MSC. The work mainly includes the following two aspects:(1) Three series of porphyrin sensitizers were designed and synthesized for DSSC; (2) The photovoltaic performance of hole-conductor-free fully printable MPSC was optimized via in situ Lewis acid and base passivation, solvent engineering and mixed-anion strategies. The main contents of this paper include:Two novel D-?-A porphyrin sensitizers (denoted as WH-C1 and WH-C2) through substituting the methoxy group on the donor of YD20 with a methyl and a hexyloxy group, respectively were designed and synthesized for dye-sensitized solar cells based on Pt CE. For comparison, YD20 was synthesized as a reference dye. The effects of different alkyl chains (methyl, methoxyl and hexyloxy groups) on the photophysical, electrochemical and photovoltaic performance were investigated systematically. The results indicate that the molar extinction coefficients of three dyes normally increase as the alkyl chain length increases. Furthermore, it is found that the incident photo-to-current conversion efficiency (IPCE), short circuit current (Jsc) and open circuit voltage (Voc) of DSSC based on WH-C1, YD20 and WH-C2 increase with the elongation of alkyl chains in the order of WH-C1? YD20<WH-C2. Accordingly, under standard global air mass 1.5 solar conditions, the optimized WH-C2-sensitized cell could produce a high PCE of 7.77%, with a Jsc of 13.10 mA cm-2, a Voc of 831.10 mV, and a fill factor (FF) of 0.70.Three novel porphyrin sensitizers with different electronegative moieties (coded as WH-C3 with 2,4,6-triphenyl-1,3,5-triazine, WH-C4 with carbazole and WH-C5 with triphenylamine) attached at the meso-position were designed and synthesized for monolithic DSSC based on mesoscopic carbon CE. The effects of these different electronegative moieties on the photophysical, electrochemical and accordingly photovoltaic performance of the corresponding devices were investigated systematically. Electrochemical measurements indicate that the HOMO and LUMO energy levels could be tuned through the introduction of different electronegative groups onto the backbone of D-?-A porphyrin molecules. Current-voltage characteristics indicate that the Jsc and Voc of the DSSC based on WH-C3, WH-C4 and WH-C5 increase as the electron-donating ability of their donors enhance in the order of WH-C3 o WH-C4 o WH-C5 and WH-C5-sensitized cells showed the best photovoltaic performance:a Jsc of 11.43 mA cm-2, a Voc of 633.84 mV, and a FF of 0.69, corresponding to an overall PCE of 5.00%.Four D-?-A porphyrin dye molecules with carboxylic acid at the para-position or meta-position of the benzene ring (coded as WH-C4, WH-Cl, WH-C6 and WH-C7, respectively) were designed and synthesized for monolithic DSSC with mesoscopic carbon CE. The effects of different anchoring group orientations on the photophysical, electrochemical and accordingly photovoltaic performance of the corresponding devices were investigated systematically. Significant optical and electrochemical differences were found for WH-C6 and WH-C7 with carboxylic acid at the meta-position of the benzene ring in comparison with WH-C4 and WH-C1 with carboxylic acid at the para-position of the benzene ring. The devices sensitized by WH-C6 and WH-C7 show inferior Voc and Jsc compared with those sensitized by WH-C4 and WH-C1. The above performance differences were explained by electrochemical impedance spectroscopy (EIS), UV-visible absorption spectrum and dye loading measurement.Efficient printable mesoscopic perovskite solar cells based on carbon counter electrodes were fabricated using a sequential deposition method. A nontoxic and low-cost additive of dicyandiamide (DCDA) with simultaneous Lewis acid and base properties was employed to in situ passivate the surface electron and hole trapping states of MAPbI3 perovskite crystals. It was found that the Voc and FF of the devices treated with DCDA were significantly enhanced. EIS and intensity modulated photovoltage spectroscopy (IMVS) analysis indicated DCDA treatment resulted in much longer electron lifetime and suppressed charge recombination for the devices. Under standard air mass 1.5 solar conditions, a PCE of 12.50% was achieved with the printable mesoscopic perovskite solar cells, which offered a wide prospect for low-cost photovoltaics.Although high PCE has already been achieved by solvent engineering strategy in traditional cell structures, systematic study of solvent effects on PSC has not been reported so far. Herein, solvent engineering strategy was designed rationally and was utilized to optimize the hole-conductor-free fully printable PSC based on pristine perovskite MAPbI3 via one-step deposition. The contact angle measurement shows that the polarity and viscosity of solvents possess a significant effect on the wettability of perovskite precursor solution for device films. XRD and FTIR results confirm the existence of different intermediate phases and reveal that the stability of intermediate phases from different solvents is affected by their coordination ability with PbI2, where coordination ability is determined by both electron density of S=O and C=O in solvents and steric hindrance of S=O and C=O. SEM indicates that the above wettability and the stability of intermediate phases influence the crysitallization and infiltration of perovskite in mesoporous films, and then affect the optical properties as confirmed by UV-visible absorption and steady-state PL results. Due to the suitable interplay and compromise of polarity, viscosity, wettability and coordination ability, the optimized DMF/DMSO (0.93:0.07)-based device achieved a promising PCE of 13.89% under 100 mW·cm-2 simulated sunlight illumination.A series of mixed-anion I- and BF4- perovskites MAPbI(3-x)(BF4)x were developed for hole-conductor-free fully printable MPSC. The absorption ability, filling, electrical properties and crystal form were characterized by UV, SEM, Hall effect measurement and XRD methods, respectively. The employment of BF4- in conventional pure iodide perovskite MAPbI3 significantly improved its optical and electrical properties, such as light harvesting ability, carrier concentration and conductivity, thus leading to an enhanced power conversion efficiency from 10.54% to 13.24%. Compared with MAPbI3 based device, the device fabricated with optimized mixed halide perovskite MAPbI2.95(BF4)0.05 showed increased interfacial and bulk recombination resistances, thus achieved remarkably improved Jsc (18.15 mA cm-2), Voc (957 mV) and FF (0.76).