Showing papers by "Alex K.-Y. Jen published in 2016"

Pinhole-Free and Surface-Nanostructured NiOx Film by Room-Temperature Solution Process for High-Performance Flexible Perovskite Solar Cells with Good Stability and Reproducibility

Abstract: Recently, researchers have focused on the design of highly efficient flexible perovskite solar cells (PVSCs), which enables the implementation of portable and roll-to-roll fabrication in large scale. While NiOx is a promising material for hole transport layer (HTL) candidate for fabricating efficient PVSCs on a rigid substrate, the reported NiOx HTLs are formed using different multistep treatments (such as 300–500 °C annealing, O2-plasma, UVO, etc.), which hinders the development of flexible PVSCs based on NiOx. Meanwhile, the features of nanostructured morphology and flawless film quality are very important for the film to function as highly effective HTL of PVSCs. However, it is difficult to have the two features coexist natively, particularly in a solution process that flawless film will usually come with smooth morphology. Here, we demonstrate the flawless and surface-nanostructured NiOx film from a simple and controllable room-temperature solution process for achieving high performance flexible PVSCs...

Dopant-Free Hole-Transporting Material with a C3h Symmetrical Truxene Core for Highly Efficient Perovskite Solar Cells

Abstract: Herein we present a new structural design of hole-transporting material, Trux-OMeTAD, which consists of a C3h Truxene-core with arylamine terminals and hexyl side-chains. This planar, rigid, and fully conjugated molecule exhibits excellent hole mobility and desired surface energy to the perovskite uplayer. Perovskite solar cells fabricated using the p-i-n architecture with Trux-OMeTAD as the p-layer, show a high PCE of 18.6% with minimal hysteresis.

Enhanced Efficiency and Stability of Inverted Perovskite Solar Cells Using Highly Crystalline SnO2 Nanocrystals as the Robust Electron-Transporting Layer

Abstract: Highly crystalline SnO2 is demonstrated to serve as a stable and robust electron-transporting layer for high-performance perovskite solar cells. Benefiting from its high crystallinity, the relatively thick SnO2 electron-transporting layer (≈120 nm) provides a respectable electron-transporting property to yield a promising power conversion efficiency (PCE)(18.8%) Over 90% of the initial PCE can be retained after 30 d storage in ambient with ≈70% relative humidity.

Stable Low‐Bandgap Pb–Sn Binary Perovskites for Tandem Solar Cells

Abstract: A low-bandgap (1.33 eV) Sn-based MA0.5 FA0.5 Pb0.75 Sn0.25 I3 perovskite is developed via combined compositional, process, and interfacial engineering. It can deliver a high power conversion efficiency (PCE) of 14.19%. Finally, a four-terminal all-perovskite tandem solar cell is demonstrated by combining this low-bandgap cell with a semitransparent MAPbI3 cell to achieve a high efficiency of 19.08%.

Defect Passivation of Organic–Inorganic Hybrid Perovskites by Diammonium Iodide toward High-Performance Photovoltaic Devices

Abstract: The polycrystalline feature of solution-processed perovskite film and its ionic nature inevitably incur substantial crystallographic defects, especially at the film surface and the grain boundaries (GBs). Here, a simple defect passivation method was exploited by post-treating CH3NH3PbI3 (MAPbI3) film with a rationally selected diammonium iodide. The molecular structure of the used diammonium iodide was discovered to play a critical role in affecting the phase purity of treated MAPbI3. Both NH3I(CH2)4NH3I and NH3I(CH2)2O(CH2)2NH3I (EDBE) induce three-dimensional (3D) to two-dimensional (2D) perovskite phase transformation during the treatment while only NH3I(CH2)8NH3I (C8) successfully passivates perovskite surface and GBs without forming 2D perovskite because of the elevated activation energy arising from its unique anti–gauche isomerization. Defect passivation of MAPbI3 was clearly confirmed by scanning Kelvin probe microscopy (SKPM) and time-resolved photoluminescence (TRPL) studies, which results in th...

Highly Efficient Organic Solar Cells with Improved Vertical Donor–Acceptor Compositional Gradient Via an Inverted Off‐Center Spinning Method

Abstract: A novel, yet simple solution fabrication technique to address the trade-off between photocurrent and fill factor in thick bulk heterojunction organic solar cells is described. The inverted off-center spinning technique promotes a vertical gradient of the donor-acceptor phase-separated morphology, enabling devices with near 100% internal quantum efficiency and a high power conversion efficiency of 10.95%.

Rigidifying Nonplanar Perylene Diimides by Ring Fusion Toward Geometry-Tunable Acceptors for High-Performance Fullerene-Free Solar Cells.

Abstract: Rigid fused perylene diimide (PDI) dimers bridged with heterocycles exhibit superior photovoltaic performance compared to their unfused semiflexible analogues. Changing the chalcogen atoms in the aromatic bridges gradually increases the twist angles between the two PDI planes, leading to a varied morphology in which the one bridged by thiophene achieves a balance and shows the best efficiency of 6.72%.

Current Challenges and Prospective Research for Upscaling Hybrid Perovskite Photovoltaics.

Abstract: Organic–inorganic hybrid perovskite photovoltaics (PSCs) are poised to push toward technology translation, but significant challenges complicating commercialization remain. Though J–V hysteresis and ecotoxicity are uniquely imposing issues at scale, CH3NH3PbI3 degradation is by far the sharpest limitation to the technology’s potential market contribution. Herein, we offer a perspective on the practical market potential of PSCs, the nature of fundamental PSC challenges at scale, and an outline of prospective solutions for achieving module scale PSC production tailored to intrinsic advantages of CH3NH3PbI3. Although integrating PSCs into the energy grid is complicated by CH3NH3PbI3 degradation, the ability of PSCs to contribute to consumer electronics and other niche markets like those organic photovoltaics have sought footing in rests primarily upon the technology’s price point. Thus, slot die, roll-to-roll processing has the greatest potential to enable PSC scale-up, and herein, we present a perspective o...

Stabilized wide bandgap perovskite solar cells by tin substitution

Abstract: Wide bandgap MAPb(I1–yBry)3 perovskites show promising potential for application in tandem solar cells. However, unstable photovoltaic performance caused by phase segregation has been observed under illumination when y is above 0.2. Herein, we successfully demonstrate stabilization of the I/Br phase by partially replacing Pb2+ with Sn2+ and verify this stabilization with X-ray diffractometry and transient absorption spectroscopy. The resulting MAPb0.75Sn0.25(I1–yBry)3 perovskite solar cells show stable photovoltaic performance under continuous illumination. Among these cells, the one based on MAPb0.75Sn0.25(I0.4Br0.6)3 perovskite shows the highest efficiency of 12.59% with a bandgap of 1.73 eV, which make it a promising wide bandgap candidate for application in tandem solar cells. The engineering of internal bonding environment by partial Sn substitution is believed to be the main reason for making MAPb0.75Sn0.25(I1–yBry)3 perovskite less vulnerable to phase segregation during the photostriction under ill...

Rational Design of Dipolar Chromophore as an Efficient Dopant-Free Hole-Transporting Material for Perovskite Solar Cells

Abstract: In this paper, an electron donor–acceptor (D-A) substituted dipolar chromophore (BTPA-TCNE) is developed to serve as an efficient dopant-free hole-transporting material (HTM) for perovskite solar cells (PVSCs). BTPA-TCNE is synthesized via a simple reaction between a triphenylamine-based Michler’s base and tetracyanoethylene. This chromophore possesses a zwitterionic resonance structure in the ground state, as evidenced by X-ray crystallography and transient absorption spectroscopies. Moreover, BTPA-TCNE shows an antiparallel molecular packing (i.e., centrosymmetric dimers) in its crystalline state, which cancels out its overall molecular dipole moment to facilitate charge transport. As a result, BTPA-TCNE can be employed as an effective dopant-free HTM to realize an efficient (PCE ≈ 17.0%) PVSC in the conventional n-i-p configuration, outperforming the control device with doped spiro-OMeTAD HTM.

Modulation of PEDOT:PSS pH for Efficient Inverted Perovskite Solar Cells with Reduced Potential Loss and Enhanced Stability

Abstract: Inverted p-i-n perovskite solar cells (PVSCs) using PEDOT:PSS as the hole-transporting layer (HTL) is one of the most widely adopted device structures thus far due to its facile processability and good compatibility for high throughput manufacturing processes. However, most of the PEDOT:PSS-based CH3NH3PbI3 PVSCs reported to date suffered an inferior open-circuit voltage (VOC) (0.88–0.95 V) compared to that (1.05–1.12 V) obtained for common CH3NH3PbI3 PVSCs, revealing a severe potential loss issue. Herein, we describe a simple method to alleviate this problem by tuning the pH value of PEDOT:PSS with a mild base, imidazole. Accompanied by the pH modulation, the blended imidazole concurrently tailors the surface texture and electronic properties of PEDOT:PSS to promote the quality and crystallization of the perovskite film deposited on top of it and enable better energy-level alignment at this corresponding interface. Consequently, the PVSC using this modified PEDOT:PSS HTL yields an enhanced power conversi...

Improved efficiency and stability of Pb–Sn binary perovskite solar cells by Cs substitution

Abstract: Partially replacing Pb with Sn in organic–inorganic lead halide perovskites has been proven as a promising approach to reduce environmental toxicity and develop low bandgap (as low as 120 eV) perovskite solar cells (PVSCs) beneficial for constructing perovskite-based tandem solar cells In this work, we demonstrated that partially replacing MA+ or FA+ with Cs+ in a Pb–Sn binary perovskite system can effectively retard the associated crystallization rate to facilitate homogenous film formation, subsequently resulting in enhanced device performance and stability, especially for high Sn-containing compositions The representative MA09Cs01Pb05Sn05I3 PVSC with a low Eg of 128 eV not only achieves an improved efficiency up to 1007% but also possesses much improved thermal and ambient stability as compared to the pristine MAPb05Sn05I3 PVSC showing poorer efficiency (636%) and stability Similarly, when Cs was introduced into FAPb1−xSnxI3 perovskite, enhanced performance was observed, affirming its general applicability and beneficial role in mediating the crystal growth and film formation of Pb–Sn binary perovskites

Hexaazatrinaphthylene Derivatives: Efficient Electron-Transporting Materials with Tunable Energy Levels for Inverted Perovskite Solar Cells.

Abstract: Hexaazatrinaphthylene (HATNA) derivatives have been successfully shown to function as efficient electron-transporting materials (ETMs) for perovskite solar cells (PVSCs). The cells demonstrate a superior power conversion efficiency (PCE) of 17.6 % with negligible hysteresis. This study provides one of the first nonfullerene small-molecule-based ETMs for high-performance p-i-n PVSCs.

A Low‐Temperature, Solution Processable Tin Oxide Electron‐Transporting Layer Prepared by the Dual‐Fuel Combustion Method for Efficient Perovskite Solar Cells

Abstract: Although tin oxide (SnO2) has been employed recently as an efficient electron-transporter to realize highly efficient organometal halide perovskite solar cells (PVSCs), it is still quite challenging to apply it through facile solution-based synthesis at low enough temperature ( 15% with negligible hysteresis can be achieved in PVSC. This demonstrates the great potential of applying this dual-fuel combustion process to improve processability and charge-transporting properties of metal oxides for organic electronics applications.

A Low-Temperature, Solution-Processable Organic Electron-Transporting Layer Based on Planar Coronene for High-performance Conventional Perovskite Solar Cells

Abstract: A low-temperature, solution-processable organic electron-transporting material (ETM) is successfully developed for efficient conventional n-i-p perovskite solar cells (PVSCs). This ETM can show a high efficiency over 17% on rigid device and 14.2% on flexible PVSC. To the best of our knowledge, this efficiency is among the highest values reported for flexible n-i-p PVSCs with negligible hysteresis thus far.

High Performance Optical Modulator Based on Electro-Optic Polymer Filled Silicon Slot Photonic Crystal Waveguide

Abstract: Silicon-organic hybrid integrated devices have emerging applications ranging from high-speed optical interconnects to photonic electromagnetic-field sensors. Silicon slot photonic crystal waveguides (PCWs) filled with electro-optic (EO) polymers combine the slow-light effect in PCWs with the high polarizability of EO polymers, which promises the realization of high-performance optical modulators. In this paper, a high-speed, power-efficient, low-dispersion, and compact optical modulator based on an EO polymer filled silicon slot PCW is presented. Lattice-shifted PCWs are utilized to engineer the photonic band diagram and thus enable an 8 nm-wide low-dispersion spectrum range, which is over an order of magnitude wider than that in modulators based on non-band-engineered PCWs and ring-resonators. A small voltage-length product of Vπ × L = 0.282 V × mm measured at 100 KHz is achieved by slow-light enhancement, corresponding to an unprecedented record-high effective in-device EO coefficient (r33) of 1230 pm/V among silicon-organic hybrid modulators. Excluding the slow-light effect, the actual in-device r33 is estimated to be 98 pm/V. By engineering the RC time constant via silicon doping and also utilizing a backside gate technique, the 3-dB modulation bandwidth of the device is measured to be 15 GHz. In addition, the RF power consumption of the modulator is estimated to be 24 mW at 10 GHz, and the estimated energy consumption for potential digital modulations is approximately 94.4 fJ/bit at 10 Gb/s.

Enhanced Ambient Stability of Efficient Perovskite Solar Cells by Employing a Modified Fullerene Cathode Interlayer

Abstract: A novel fullerene cathode interlayer is employed to facilitate the fabrication of stable and efficient perovskite solar cells. This modified fullerene surfactant significantly increases air stability of the derived devices due to its hydrophobic characteristics to enable 80% of the initial PCE to be retained after being exposed in ambient condition with 20% relative humidity for 14 days.

Highly crystalline Zn2SnO4 nanoparticles as efficient electron-transporting layers toward stable inverted and flexible conventional perovskite solar cells

Abstract: In this work, we have successfully utilized a facile hydrothermal method to prepare crystalline Zn2SnO4 nanoparticles (ZSO NPs) and applied them as an efficient electron-transporting layer (ETL) via a simple room-temperature solution process for perovskite solar cells (PVSCs). The superior semiconducting properties of this ZSO-based ETL enable an efficient (PCE: 17.7%) inverted p–i–n PVSC to be fabricated with respectable ambient stability. It can retain over 90% of its original PCE after being stored under ambient conditions for 14 days under 30 ± 5% relative humidity. Moreover, it also facilitates the fabrication of efficient conventional n–i–p PVSCs with a PCE of ∼14.5% and ∼11.4% achieved on a glass/ITO rigid substrate and PEG/ITO flexible substrate, respectively.

Large Grained Perovskite Solar Cells Derived from Single-Crystal Perovskite Powders with Enhanced Ambient Stability.

Abstract: In this study, we demonstrate the large grained perovskite solar cells prepared from precursor solution comprising single-crystal perovskite powders for the first time. The resultant large grained perovskite thin film possesses a negligible physical (structural) gap between each large grain and is highly crystalline as evidenced by its fan-shaped birefringence observed under polarized light, which is very different from the thin film prepared from the typical precursor route (MAI + PbI2).

Highly Sensitive Built-In Strain Sensors for Polymer Composites: Fluorescence Turn-On Response through Mechanochemical Activation.

Abstract: A new class of rationally designed mechanophores is developed for highly sensitive built-in strain sensors in polymer composites. These mechanophores are designed to regenerate the π-conjugation pathway between the electron donor and electron acceptor by force-induced cleavage of the covalent bond to form a fluorescent dipolar dye.

Optical Enhancement via Electrode Designs for High‐Performance Polymer Solar Cells

Abstract: To capture the essence of the rapid progress in optical engineering exploited in high-performance polymer solar cells (PSCs), a comprehensive overview focusing on recent developments and achievements in PSC electrode engineering is provided in this review. To date, various kinds of electrode materials and geometries are exploited to enhance light-trapping in devices through distinct optical strategies. In addition to the widely used nanostructured electrodes that induce plasmonic-enhanced light absorption, planar ultra-thin metal films also have attracted significant attention due to their remarkably reflective transparent properties that beget efficient optical microcavities. These microcavities confine incident light with resonant frequencies between two reflective electrodes due to optically coherent interference, boosting the light absorption of thin-film PSCs while maintaining efficient charge dissociation and extraction. After reviewing the challenges in developing high-performance microcavity-enhanced PSCs (MCPSCs), we discuss strategies to improve MCPSC performance further to showcase the potential of harnessing microcavity resonance effects in thin-film PSCs.

Design rules for the broad application of fast (<1 s) methylamine vapor based, hybrid perovskite post deposition treatments

Abstract: While organo-metal halide perovskite photovoltaics have seen rapid development, growth of high quality material remains a challenge. Herein, we report a facile post deposition treatment utilizing coordination between methylamine (CH3NH2) vapor and CH3NH3PbI3 perovskite that rapidly improves film quality, enhancing power conversion efficiency (PCE) by ∼9%. We further comprehensively analyze the physical impact of this process with regard to the material's optoelectronic properties and its detailed microstructural changes. Connecting this with an analysis of the source of organo-metal halide perovskite reactivity toward the vapor as well as phase behavior as a function of CH3NH2 vapor pressure and time, we provide design rules for the broad, rational extension of this process to new systems and scales.

PCBM-doped electro-optic materials: investigation of dielectric, optical and electro-optic properties for highly efficient poling

Abstract: Fullerenes are ubiquitously popular in organic electronic materials and devices. The high electron affinity, electron mobility and percolated networks for electron transport of fullerene derivatives, such as PCBM, have established them as excellent electron acceptors and transport materials in organic solar cells and electronic devices. It is intriguing to utilize these electronic properties and molecular three-dimensional networks to explore their potential applications in new electronic or optical devices. In this work, PCBM was doped into organic electro-optic (EO) materials and their surface morphology, photophysical properties, dielectric properties as well as optical properties (refractive index) were systematically investigated. It was found that the dielectric constant and refractive index of the doped materials were significantly enhanced. Based on temperature-dependent dielectric constant measurements, the relation between relative microscopic dipole moment and dielectric properties was established. It revealed that, at the poling temperature, the dipole moment of chromophores in the PCBM-doped film P1/PCBM was higher than that of the conventional EO film P1. This enhanced microscopic property of chromophores in P1/PCBM well accounted for the improved poling results in electric field induced poling. A larger EO coefficient (197 pm V−1versus 133 pm V−1) and figure-of-merit n3r33 (1002.9 versus 632.2), as well as a higher order parameter (15.7% versus 10.6%) and birefringence were achieved for the PCBM-doped film P1/PCBM, demonstrating the significant potential of PCBM to be used in organic EO materials and devices.

Modulate Organic-Metal Oxide Heterojunction via [1,6] Azafulleroid for Highly Efficient Organic Solar Cells

Abstract: By creating an effective π-orbital hybridization between the fullerene cage and the aromatic anchor (addend), the azafulleroid interfacial modifiers exhibit enhanced electronic coupling to the underneath metal oxides. High power conversion efficiency of 10.3% can be achieved in organic solar cells using open-cage phenyl C61 butyric acid methyl ester (PCBM)-modified zinc oxide layer.

Development of Self‐Doped Conjugated Polyelectrolytes with Controlled Work Functions and Application to Hole Transport Layer Materials for High‐Performance Organic Solar Cells

Abstract: A series of anionic self-doped conjugated polyelectrolytes (CPEs) by copolymerization of a 1,4-bis(4-sulfonatobutoxy)benzene moiety with different counter monomers of thiophene, bithiophene, and terthiophene is reported. The CPEs show high conductivity of ≈10−4 S cm−1 due to being self-doped in a neutral state and exhibit excellent hole transporting property in the out-of-plane direction, compared with poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS). Moreover, the CPE incorporating a less electron-donating unit from terthiophene to thiophene exhibits a higher work function and therefore, PhNa-1T incorporating thiophene shows a relatively high work function of 5.21 eV than 4.97 eV of PEDOT:PSS. This can induce a higher internal field in the solar cell device, facilitating efficient charge collection to the electrode. As a result, polymer solar cell devices incorporating the CPEs as a hole transporting layer achieve enhanced photovoltaic performances from those of the conventional PEDOT:PSS-based devices. The solar cell efficiency reaches up to 9.89%, which is among the highest values demonstrated by PCE-10-based normal-type organic solar cells.

Abnormal Current–Voltage Hysteresis Induced by Reverse Bias in Organic–Inorganic Hybrid Perovskite Photovoltaics

Abstract: In this study, reverse bias (RB)-induced abnormal hysteresis is investigated in perovskite solar cells (PVSCs) with nickel oxide (NiOx)/methylammonium lead iodide (CH3NH3PbI3) interfaces. Through comprehensive current–voltage (I–V) characterization and bias-dependent external quantum efficiency (EQE) measurements, we demonstrate that this phenomenon is caused by the interfacial ion accumulation intrinsic to CH3NH3PbI3. Subsequently, via systematic analysis we discover that the abnormal I–V behavior is remarkably similar to tunnel diode I–V characteristics and is due to the formation of a transient tunnel junction at NiOx/CH3NH3PbI3 interfaces under RB. The detailed analysis navigating the complexities of I–V behavior in CH3NH3PbI3-based solar cells provided here ultimately illuminates possibilities in modulating ion motion and hysteresis via interfacial engineering in PVSCs. Furthermore, this work shows that RB can alter how CH3NH3PbI3 contributes to the functional nature of devices and provides the first...

Effects of Counterions with Multiple Charges on the Linear and Nonlinear Optical Properties of Polymethine Salts

Abstract: Counterions with multiple charges have been used to organize polymethines into multichromophore salt complexes. The intramolecular Coulombic interactions between multiply charged counterions and polymethines can play a significant role in modifying intermolecular interactions (i.e., aggregation). Here, we report a detailed study of the linear and nonlinear optical properties of such complexes with dications and hexacations over a large range of molecular concentrations. Our results have demonstrated that, despite strong intramolecular interactions, the preorganization of chromophores into the multichromophore salt constructs with multiple charges can provide a steric repulsive effect that can mitigate intermolecular interactions in the solid state. This results in a more efficient translation between microscopic and macroscopic optical properties for highly polarizable polymethines, which is essential for all optical signal switching.