Browsing by Author "Tsai, Cheng-Min"

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Ag Doping of Organometal Lead Halide Perovskites: Morphology Modification and p-Type Character
(2017-02-23)
Shahbazi, Saeed
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Tsai, Cheng-Min
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Narra, Sudhakar
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Wang, Chi-Yung
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Shiu, Hau-Shiang
We report a simple synthetic approach to grow uniform CH3NH3PbI3 perovskite (PSK) layers free of pinholes via ciE varied portions of silver iodide (AgI) added to the precursor solution. XRD/EDS elemental mapping experiments demonstrated nearly uniform Ag distribution inside the perovskite film. When the 1% AgI- assisted perovskite films were fabricated into a p-i-n planar device, the a) photovoltaic performance was enhanced by similar to 30% (PCE increased from 9.5% to 12.0%) relative to the standard cell without added AgI. . Measurement of electronic properties using a hall setup indicated that perovskite films show p-type character after Ag doping, whereas the film is n-type without Ag. Transients of photoluminescence of perovskite films with and without AgI additive deposited on glass, p-type (PEDOT:PSS), and n-type (TiO2) contact layers were recorded with a time-correlated single-photon counting (TCSPC) technique. The TCSPC results indicate that addition of AgI inside perovskite in contact with PEDOT:PSS accelerated the hole-extraction motion whereas that in contact with TiO2 led to a decelerated electron extraction, in agreement with the trend observed from the photovoltaic results. The silver cationic dopant inside the perovskite films had hence an effect of controlling the morphology to improve photovoltaic performance for devices with p-i-n configuration.
Control of Crystal Structures and Optical Properties with Hybrid Formamidinium and 2-Hydroxyethylammonium Cations for Mesoscopic Carbon-Electrode Tin-Based Perovskite Solar Cells
(2018-09-01)
Tsai, Cheng-Min
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Lin, Yu-Pei
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Pola, Murali Krishna
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Narra, Sudhakar
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Jokar, Efat
Alcohol-based bifunctional ammonium cations, 2-hydroxyethylammonium (HEA(+)), HO(CH2)(2)NH3+, were introduced into formamidinium (FA(+)) tin-based perovskites (HEA(x)FA(1-x)Snl(3); x = 0-1) to absorb light in carbon-based mesoscopic solar cells. We found that HEAP cations play a key role to control the crystal structures, the lattice structures altered from orthorhombic (x = 0) to rhombohedral (x = 0.2-0.4) with greater symmetry. When x was increased to 0.6-1.0, tin and iodide vacancies were formed to generate 3D-vacant perovskites (HEA(x)FA(1-x)Sn(0.6)I(2.33), x >= 0.6) with a tetragonal structure. Tin-based perovskites in this series were fabricated into mesoporous solar cells using one-step drop-cast (DC), two-step solvent-extraction (SE), and SE + 3% ethyl-enediammonium diiodide (EDAI(2)) as an additive. After optimization of device performance with the SE + 3% EDAI2 approach, the HEA(0.4)FA(0.6)SnI(3) (HEAT = 40%) device gave the best photovoltaic performance with J(sc) = 18.52 mA cm(-2)) V-OC 371 mV, FF = 0.562, and overall efficiency 17 = 3.9% after the device was stored for a period of 340 h.
Control of preferred orientation with slow crystallization for carbon-based mesoscopic perovskite solar cells attaining efficiency 15%
(2017-01-14)
Tsai, Cheng-Min
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Wu, Guan-Wei
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Narra, Sudhakar
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Chang, Hung-Ming
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Mohanta, Nayantara
To produce mesoscopic carbon-electrode-based solar cells free of a hole-conduction layer, we developed a simple one-step drop-cast method (temperature 20 degrees C, relative humidity 50%) to grow uniform and compact perovskite nanocrystals via slow crystallization (SC). X-ray diffraction (XRD) results indicated that perovskite nanocrystals produced inside the mesoporous TiO2/Al2O3/C layers according to our SC approach with N-methyl-2-pyrrolidone (NMP) as a precursor solvent showed a preferred orientation at facet (004) with large crystalline grains, whereas those produced with other solvents or via thermal annealing (TA) methods showed greater intensity at facet (220) with small crystals. After the SC (NMP) procedure was completed, additional TA treatment would change the preferred orientation from (004) back to facet (220), which would break the connectivity and lose the uniformity of the crystals, causing the grains to become significantly smaller. Transient photoluminescence (PL) decay profiles of the SC devices made with four precursor solvents, NMP, g-butyrolactone (GBL), dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), were recorded to show the trend of charge separation kinetics of the perovskite crystals inside the mesoporous films, which is consistent with the trend of the corresponding device performance showing the same order. The SC (NMP) device attained the best efficiency of power conversion (PCE), 15.0%, with average value (13.9 +/- 0.5%), which is much superior to those devices from either the traditional one-step TA (DMF) method (5.2 +/- 1.0%) or the traditional sequential TA method (10.1 +/- 0.7%). This work emphasizes the significance of the control of preferred orientation of perovskite nanocrystals using the SC approach, to obtain high-performance carbon-based mesoscopic solar cells with excellent reproducibility and stability.
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
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Li, Wei
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Li, Feng
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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.
Formation of Stable Tin Perovskites Co-crystallized with Three Halides for Carbon-Based Mesoscopic Lead-Free Perovskite Solar Cells
(2017-10-23)
Tsai, Cheng-Min
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Mohanta, Nayantara
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Wang, Chi-Yung
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Lin, Yu-Pei
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Yang, Yaw-Wen
We synthesized and characterized methylammonium (MA) mixed tri-halide tin perovskites (MASnIBr(2-x)Cl(x)) for carbon-based mesoscopic solar cells free of lead and holetransporting layers. Varied SnCl2/SnBr2 ratios yielded tin perovskites with three halides (I, Br, and Cl) co-crystallized inside the tin-perovskite. When the SnCl2 proportion was >= 50% (x >= 1), phase separation occurred to give MAS-nI(3-y)Br(y) and MASnCl(3-z)Br(z) in the stoichiometric proportions of their precursors, confirmed by XRD. A device with MASnIBr(1.8)Cl(0.2) (SnCl2= 10%) showed the best photovoltaic performance: J(SC)= 14.0 mAcm(-2), V-OC= 380 mV, FF= 0.573, and PCE= 3.1%, and long-term stability. Electrochemical impedance spectra (EIS) show superior charge recombination and dielectric relaxation properties for the MASnIBr(1.8)Cl(0.2) cell. Transient PL decays showed the intrinsic problem of tinbased perovskites with average lifetimes less than 100 ps.
Functionalization of Graphene Oxide Films with Au and MoOx Nanoparticles as Efficient p-Contact Electrodes for Inverted Planar Perovskite Solar Cells
(2018-09-12)
Bhosale, Sumit S.
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Jokar, Efat
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Fathi, Amir
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Tsai, Cheng-Min
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Wang, Chi-Yung
A graphene oxide (GO) film is functionalized with metal (Au) and metal-oxide (MoOx) nanoparticles (NPs) as a hole-extraction layer for high-performance inverted planar-heterojunction perovskite solar cells (PSCs). These NPs can increase the work function of GO, which is confirmed with X-ray photoelectron spectra, Kelvin probe force microscopy, and ultraviolet photoelectron spectra measurements. The down-shifts of work functions lead to a decreased level of potential energy and hence increased V-oc of the PSC devices. Although the GO-AuNP film shows rapid hole extraction and increased V-oc, a J(sc) improvement is not observed because of localization of the extracted holes inside the AuNP that leads to rapid charge recombination, which is confirmed with transient photoelectric measurements. The power conversion efficiency (PCE) of the GO-AuNP device attains 14.6%, which is comparable with that of the GO-based device (14.4%). In contrast, the rapid hole extraction from perovskite to the GO-MoOx layer does not cause trapping of holes and delocalization of holes in the GO film accelerates rapid charge transfer to the indium tin oxide substrate; charge recombination in the perovskite/GO-MoOx interface is hence significantly retarded. The GO-MoOx device consequently shows significantly enhanced V-oc and J(sc), for which its device performance attains PCE of 16.7% with great reproducibility and enduring stability.
Interfacial Engineering with Cross-Linkable Fullerene Derivatives for High-Performance Perovskite Solar Cells
(2017-11-08)
Kang, Tin
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Tsai, Cheng-Min
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Jiang, Yu-He
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Gollavelli, Ganesh
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Mohanta, Nayantara
Two fullerene derivatives with styryl and oxetane cross-linking groups served as interfacial materials to modify an electron-transporting layer (ETL) of TiO2, doped with Au nanoparticles, processed under low-temperature conditions to improve the performance of perovskite solar cells (PSC). The cross-linkable [6,6]-phenyl-C-61-butyric styryl dendron ester was produced via thermal treatment at 160 degrees C for 20 min, whereas the cross-linkable [6,6]-phenyl-C-61-butyric oxetane dendron ester (C-PCBOD) was obtained via UV-curing treatment for 45 s. Both cross-linked fullerenes can passivate surface-trap states of TiO2 and have also excellent surface coverage on the TiO2 layer shown in the corresponding atomic force microscopy images. To improve the crystallinity of perovskite, we propose a simple co-solvent method involving mixing dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) in a specific ratio (DMF/DMSO = 90/10). The fullerene derivative layer between the ETL and perovskite layers significantly improved electron extraction and suppressed charge recombination by decreasing the density of traps at the ETL surface. A planar PSC device was fabricated with the configuration indium tin oxide/TiO2 (Au)/C-PCBOD/perovskite/spiro-OMeTAD/Au to attain a power conversion efficiency (PCE) of 15.9%. The device performance was optimized with C-PCBOD as an interfacial mediate to modify the surface of the mesoporous TiO2 ETL; the C-PCBOD-treated device attained a significantly enhanced performance, PCE 18.3%. Electrochemical impedance spectral and photoluminescence decay measurements were carried out to understand the characteristics of electron transfer and charge recombination of the perovskite/TiO2 samples with and without a fullerene interfacial layer.
Interfacial Investigation on Printable Carbon-Based Mesoscopic Perovskite Solar Cells with NiOx/C Back Electrode
(2017-08-02)
Behrouznejad, Fatemeh
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Tsai, Cheng-Min
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Narra, Sudhakar
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Diau, Eric W. -G.
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Taghavinia, Nima
Solar cells with high efficiency, low cost, and high stability are the target for the new generation of solar cells. A fully printable perovskite (CH3NH3PbI3) solar cell (PSC) with device architecture FTO/TiO2/Al2O3/NiOx/C is fabricated in the current research as a low-cost and relatively stable structure and is investigated to determine how different fabrication factors such as the thickness of the insulating spacer layer (Al2O3) or treatments such as heat and UV-O-3 treatments can affect the interfacial properties of this multilayer mesoporous structure. X-ray photoelectron spectra (XPS) show that UV-O-3 treatment increases the Ni3+ (Ni2O3) phase on the surface of the black nickel oxide layer leading to better charge extraction and increasing open-circuit voltage (V-OC) up to 0.945 V. We observe improved CH3NH3PbI3 formation inside the mesoporous layers by the PbI2 penetration at a higher temperature. Impedance spectral together with current-voltage measurements show the effect of thickness for the insulator layer in the internal and interfacial resistances and photovoltaic characteristics of the cell. The best performance of the carbon-based PSC attains power conversion efficiency of 12.1% with the thickness of the Al2O3 layer at 450 nm.
Low-temperature, simple and efficient preparation of perovskite solar cells using Lewis bases urea and thiourea as additives: stimulating large grain growth and providing a PCE up to 18.8%
(2018-01-01)
Hsieh, Cheng-Ming
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Liao, Yung-Sheng
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Lin, Yan-Ru
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Chen, Chih-Ping
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Tsai, Cheng-Min
We demonstrated that two Lewis bases - urea and thiourea - acted as efficient additives for CH3NH3(MA)PbI3-xClx and MAPbI(3) perovskite solar cells (PSCs) and observed a significant increase in PCE for the MAPbI(3) devices in the presence of 1% urea with a remarkable PCE of 18.8% using an extremely low annealing temperature (85 degrees C).
Robust Tin-Based Perovskite Solar Cells with Hybrid Organic Cations to Attain Efficiency Approaching 10%
(2019-01-01)
Jokar, Efat
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Chien, Cheng-Hsun
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Tsai, Cheng-Min
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Fathi, Amir
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Diau, Eric Wei-Guang
The stability of a tin-based perovskite solar cell is a major challenge. Here, hybrid tin-based perovskite solar cells in a new series that incorporate a nonpolar organic cation, guanidinium (GA(+)), in varied proportions into the formamidinium (FA(+)) tin triiodide perovskite (FASnI(3)) crystal structure in the presence of 1% ethylenediammonium diiodide (EDAI(2)) as an additive, are reported. The device performance is optimized at a precursor ratio (GAI:FAI) of 20:80 to attain a power conversion efficiency (PCE) of 8.5% when prepared freshly; the efficiencies continuously increase to attain a record PCE of 9.6% after storage in a glove-box environment for 2000 h. The hybrid perovskite works stably under continuous 1 sun illumination for 1 h and storage in air for 6 days without encapsulation. Such a tin-based perovskite passes all harsh standard tests, and the efficiency of a fresh device, 8.3%, is certified. The great performance and stability of the device reported herein attains a new milestone for lead-free perovskite solar cells on a path toward commercial development.
Role of Tin Chloride in Tin-Rich Mixed-Halide Perovskites Applied as Mesoscopic Solar Cells with a Carbon Counter Electrode
(2016-12)
Tsai, Cheng-Min
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Wu, Hui-Ping
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Chang, Sun-Tang
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Huang, Chi-Feng
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Wang, Chia-Hsin
We report the synthesis and characterization of alloyed Sn Pb methylammonium mixed-halide perovskites (CH3NH3SnyPb1-yI3-xClx) to extend light harvesting toward the near infrared region for carbon-based mesoscopic solar cells free of organic hole-transport layers. The proportions of Sn in perovskites are well controlled by mixing tin chloride (SnCl2) and lead iodide (Pbl(2)) in varied stoichiometric ratios (y = 0-1). SnCl2 plays a key role in modifying the lattice structure of the perovskite, showing anomalous optical and optoelectronic properties; upon increasing the concentration of SnC2, the variation of the band gap and band energy differed from those of the Snl(2) precursor. The CH3NH3SnyPb1-yI3-xClx devices showed enhanced photovoltaic performance upon increasing the proportion of SnCl2 until y = 0.75, consistent with the corresponding potential energy levels. The photovoltaic performance was further improved upon adding 30 mol % tin fluoride (SnF2) with device configuration FTO/TiO2/Al2O3/NiO/C, producing the best power conversion efficiency, 5.13%, with great reproducibility and intrinsic stability.