Browsing by Author "Lu, Jianfeng"

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Diammonium and Monoammonium Mixed-Organic-Cation Perovskites for High Performance Solar Cells with Improved Stability
(2017-09-20)
Lu, Jianfeng
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Jiang, Liangcong
;
Li, Wei
;
Li, Feng
;
Pai, Narendra K.
Remarkable power conversion efficiencies (PCE) of metal-halide perovskite solar cells (PSCs) are overshadowed by concerns about their ultimate stability, which is arguably the prime obstacle to commercialization of this promising technology. Herein, the problem is addressed by introducing ethane-1,2-diammonium (+NH3(CH2)(2)NH3+, EDA(2+)) cations into the methyl ammonium (CH3NH3+, MA(+)) lead iodide perovskite, which enables, inter alia, systematic tuning of the morphology, electronic structure, light absorption, and photoluminescence properties of the perovskite films. Incorporation of < 5 mol% EDA(2+) induces strain in the perovskite crystal structure with no new phase formed. With 0.8 mol% EDA(2+), PCE of the MAPbI(3)-based PSCs (aperture of 0.16 cm(2)) improves from 16.7% +/- 0.6% to 17.9% +/- 0.4% under 1 sun irradiation, and fabrication of larger area devices (aperture 1.04 cm(2)) with a certified PCE of 15.2% +/- 0.5% is demonstrated. Most importantly, EDA(2+)/MA(+)-based solar cells retain 75% of the initial performance after 72 h of continuous operation at 50% relative humidity and 50 degrees C under 1 sun illumination, whereas the MAPbI(3) devices degrade by approximately 90% within only 15 h. This substantial improvement in stability is attributed to the steric and coulombic interactions of embedded EDA(2+) in the perovskite structure.
Metal-free organic dyes for TiO2 and ZnO dye-sensitized solar cells
(2016-01-07)
Selopal, Gurpreet Singh
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Wu, Hui-Ping
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Lu, Jianfeng
;
Chang, Yu-Cheng
;
Wang, Mingkui
We report the synthesis and characterization of new metal-free organic dyes (namely B18, BTD-R, and CPTD-R) which designed with D-p-A concept to extending the light absorption region by strong conjugation group of p-linker part and applied as light harvester in dye sensitized solar cells (DSSCs). We compared the photovoltaic performance of these dyes in two different photoanodes: a standard TiO2 mesoporous photoanode and a ZnO photoanode composed of hierarchically assembled nanostructures. The results demonstrated that B18 dye has better photovoltaic properties compared to other two dyes (BTD-R and CPTD-R) and each dye has higher current density (J(sc)) when applied to hierarchical ZnO nanocrystallites than the standard TiO2 mesoporous film. Transient photocurrent and photovoltage decay measurements (TCD/TVD) were applied to systematically study the charge transport and recombination kinetics in these devices, showing the electron life time (T-R) of B18 dye in ZnO and TiO2 based DSSCs is higher than CPTD-R and BTD-R based DSSCs, which is consistent with the photovoltaic performances. The conversion efficiency in ZnO based DSSCs can be further boosted by 35%, when a compact ZnO blocking layer (BL) is applied to inhibit electron back reaction.
Molecular Engineering of Organic Dyes with a Hole-Extending Donor Tail for Efficient All-Solid-State Dye-Sensitized Solar Cells
(2015-01-01)
Lu, Jianfeng
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Chang, Yu-Cheng
;
Cheng, Hsu-Yang
;
Wu, Hui-Ping
;
Cheng, Yibing
We report a new concept for the design of metal-free organic dyes (OD5-OD9) with an extended donor-pi-acceptor (D-pi-A) molecular framework, in which the donor terminal unit is attached by a hole-extending side chain to retard back electron transfer and charge recombination; the pi-bridge component contains varied thiophene-based substituents to enhance the light-harvesting ability of the device. The best dye (OD9) has a D-A-pi-A configuration with the hexyloxyphenylthiophene (HPT) side chain as a hole-extension component and a benzothiadiazole (BTD) internal acceptor as a pi-extension component. The co-sensitization of OD9 with the new porphyrin dye LW24 enhanced the light-harvesting ability to 800 nm; thus, a power conversion efficiency 5.5% was achieved. Photoinduced absorption (PIA) and transient absorption spectral (TAS) techniques were applied to account for the observed trend of the open-circuit voltage (V-OC) of the devices. This work provides insights into the molecular design, photovoltaic performance, and kinetics of charge recombination.
Novel porphyrin-preparation, characterization, and applications in solar energy conversion
(2016-03-07)
Lu, Jianfeng
;
Li, Hao
;
Liu, Shuangshuang
;
Chang, Yu-Cheng
;
Wu, Hui-Ping
Porphyrins have been demonstrated as one of the most efficient sensitizers in dye-sensitized solar cells (DSSC). Herein, we investigated a series of porphyrin sensitizers functionalized with various pi-spacers, such as phenyl for LD14, thiophene for LW4, thiophene-phenyl for LW5, and 2,1,3-benzothiadiazole (BTD)-phenyl for LW24. Photo-physical investigation by means of time-resolved fluorescence and nanosecond transient absorption spectroscopy revealed an accelerated inner charge transfer in porphyrins containing the BTD-phenyl pi-spacer. Implementation of an auxiliary electron-deficient BTD unit to the porphyrin spacer also results in a broad light-harvesting ability extending up to 840 nm, contributing to an enhanced charge transfer character from the porphyrin ring to the anchoring group. When utilized as a sensitizer in DSSCs, the LW24 device achieved a power conversion efficiency of 9.2%, higher than those based on LD14 or LW5 porphyrins (PCE 9.0% or 8.2%, respectively) but lower than that of the LW4 device (PCE 9.5%). Measurements of transient photovoltage decays demonstrate that the LW24 device features the up-shifted potential band edge of the conduction band of TiO2, but involves serious charge recombination in the dye/TiO2 interface. The findings provide insights into the molecular structure and the charge-transfer characteristics for designing efficient porphyrin sensitizers for DSSC applications.