Enhanced performance in inverted polymer solar cells employing microwave-annealed sol-gel ZnO as electron transport layers

In this work, we propose a facile microwave-assisted approach for annealing sol-gel derived ZnO films to serve as electron transport layers (ETLs) for inverted bulk heterojunction polymer solar cells. We have demonstrated an impressive enhancement in performance for devices based on a poly (3-hexylthiophene) (P3HT): (6,6)-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) system employing the microwave-annealed ZnO (ZnO (MW)) ETLs in comparison to the cases using the conventional hotplate-annealed ZnO (ZnO (HP)) ones. The better electron transport in the device with the ZnO (MW) ETL is mainly ascribed to the preferable interfacial contact as evidenced by the morphology characteristics. Furthermore, the comprehensive analyses conducted from the light intensity dependent photocurrent and photovoltage measurements, the capacitance-voltage characteristics, and the alternating current impedance spectra suggest that the utilization of the ZnO (MW) ETLs can effectively suppress trap-assisted recombination as well as charge accumulation at the interface between P3HT: PC₆₁BM layers and ZnO layers, which is responsible for the enhanced device performance.     

Solution‐Processable Hole‐Generation Layer and Electron‐Transporting Layer: Towards High‐Performance,Alternating‐Current‐Driven,Field‐Induced Polymer Electroluminescent Devices

The effect of solution‐processed p‐type doping of hole‐generation layers (HGLs) and electron‐transporting layer (ETLs) are systematically investigated on the performance of solution‐processable alternating current (AC) field‐induced polymer EL (FIPEL) devices in terms of hole‐generation capability of HGLs and electron‐transporting characteristics of ETLs. A variety of p‐type doping conjugated polymers and a series of solution‐processed electron‐transporting small molecules are employed. It is found that the free hole density in p‐type doping HGLs and electron mobility of solution‐processed ETLs are directly related to the device performance, and that the hole‐transporting characteristics of ETLs also play an important role since holes need to be injected from electrode through ETLs to refill the depleted HGLs in the positive half of the AC cycle. As a result, the best FIPEL device exhibits exceptional performance: a low turn‐on voltage of 12 V, a maximum luminance of 20 500 cd m^?2, a maximum current and power efficiency of 110.7 cd A^?1 and 29.3 lm W^?1. To the best of the authors' knowledge, this is the highest report to date among FIPEL devices driven by AC voltage.     

A Regularity-Based Fullerene Interfacial Layer for Efficient and Stable Perovskite Solar Cells via Blade-Coating

The electron transport layer (ETL) plays a crucial part in extracting electron carriers while optimizing the interfacial contact of perovskite/electrode in planar heterojunction perovskite solar cells (PVSCs). Despite various ETLs being designed for efficient PVSCs, there exists hardly any research on the effect of molecular stacking order on device performance. Herein, poly(ethylene-co-vinyl acetate) (EVA) is employed as the [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) solution additive. The strong binding energy between EVA with PC₆₁BM promotes the molecular stacking order of ETLs, which alleviates the morphology inhomogeneity, possesses a matched energy level, blocks ion migration, and improves the water–oxygen barrier of perovskite devices. The blade-coated MAPbI₃-based PVSCs achieve a power conversion efficiency (PCE) of 19.32% with positive reproducibility and negligible hysteresis, as well as maintain 90% and 80% of the initial PCE after storage under inert and ambient conditions (52% humidity) for 1500 h without encapsulation. This strategy also improves the champion PCE of CsFAMA-based PVSCs to 20.33%. These findings demonstrate that the regulation of molecular stacking order is a valid approach to optimize interfacial charge-carrier recombination in PVSCs, which meet the demand for high-performance ETL in large-area PVSCs and improve the upscaling of the fabrication technology toward practical applications.     

Prolonged lifetime of polymer solar cells with amphiphilic monolayers modified cathodes

The short lifetime and low stability of polymer solar cells (PSCs) devices limit their feasibility for commercial use. Modification of the interfacial electron-transport layers (ETL) has been demonstrated as an effective way to enhance power conversion efficiency (PCE) and device stability. In this work, two types of monolayers consisting of amphiphilic molecules (sodium stearate or sodium oleate - a major constituent of “soap”) are introduced as novel ETLs in polymer: PCBM based PSCs. Significant improvement of PCE was demonstrated and an extended operational lifetime by 5–25 times was achieved. We attributed the improved performance to the interface modification by the amphiphilic molecular layers. The amphiphilic interfacial layers established a better contact between the active layer and the cathode by reducing the roughness and forming a compact dipole at the interface, which facilitates charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency and stability. This versatile interface modification approach has shown to be an immediate and promising means to improve the performance of PSCs.     

Atomic layer deposition of ZnO electron transporting layers directly onto the active layer of organic solar cells

Interlayers in organic solar cells (OSCs) are used to reduce energy barriers for charge injection/extraction, act as optical spacers, introduce carrier selectivity and increase organic/contact compatibility. To date, the most widely used inorganic interlayers are metal oxides such as TiO₂ and ZnO. However, these materials require harsh deposition conditions that could damage the organic active layers, and hence are generally used in inverted devices. Here we show, for the first time, that judicious selection of materials and processing conditions allow the use of an atomic layer deposition (ALD) system to deposit thin conformal ZnO interlayers on bulk heterojunctions (BHJs). ALD-ZnO interlayers were utilized as electron transporting layers (ETLs) in OSCs and compared to similar devices with solution deposited ZnO nanoparticle (np) ETLs. OSCs with ALD-ZnO ETLs exhibited higher photocurrent densities, J_sc, but lower open circuit voltages, V_oc. The low V_oc is associated with the presence of pinholes and an offset between the ALD-ZnO and PC₇₀BM electron conducting states. This offset results from traps and acceptor sites generated during the low temperature ALD process. To recover the V_oc we introduced a fluorinated phosphonic acid (PA) additive to the blend. We suggest that the additive migrates to the film surface, interacts with the ZnO to produce a denser layer and to passivate traps, effectively improving the device shunt resistance and energy level alignment and increasing V_oc. Overall, the devices with PA and ALD-ZnO ETLs possess significantly higher power conversion efficiencies (PCEs) than those with np-ZnO ETLs. For example, the champion ALD-ZnO device PCE is 3.5%, while that with np-ZnO is 2.75%.     

Efficient and hysteresis-less pseudo-planar heterojunction perovskite solar cells fabricated by a facile and solution-saving one-step dip-coating method

Rough dense sol-gel-derived titanium dioxide (TiO₂) electron-transport layers (ETLs) and smooth organolead halide perovskite (PVK) films for pseudo-planar heterojunction perovskite solar cells (P-PH PVKSCs) were fabricated by a facile one-step dip-coating method. The highly compact TiO₂ ETLs and uniform PVK films endow the device a high power conversion efficiency (PCE) of over 11%, which was nearly identical to that of a reference device (12%) fabricated by conventional spin-coating. Furthermore, the device showed no pronounced hysteresis when tested by scanning the voltage in a forward and backward direction, showing the potential of facile and waste-free dip-coating in replacing of spin-coating for large area perovskite solar cells preparation. Lastly, the hysteresis was compared and discussed and models regarding the abnormal hysteresis, roll-over and current peak phenomena were proposed as well.     

New Solution‐Processable Electron Transport Materials for Highly Efficient Blue Phosphorescent OLEDs

Several new solution‐processable organic semiconductors based on dendritic oligoquinolines were synthesized and were used as electron‐transport and hole‐blocking materials to realize highly efficient blue phosphorescent organic light‐emitting diodes (PhOLEDs). Various substitutions on the quinoline rings while keeping the central meta‐linked tris(quinolin‐2‐yl)benzene gave electron transport materials that combined wide energy gap (>3.3 eV), moderate electron affinity (2.55‐2.8 eV), and deep HOMO energy level (<‐6.08 eV) with electron mobility as high as 3.3 × 10^?3 cm² V^?1 s^?1. Polymer‐based PhOLEDs with iridium (III) bis(4,6‐(di‐fluorophenyl)pyridinato‐N,C^2′)picolinate (FIrpic) blue triplet emitter and solution‐processed oligoquinolines as the electron‐transport layers (ETLs) gave luminous efficiency of 30.5 cd A^?1 at a brightness of 4130 cd m^?2 with an external quantum efficiency (EQE) of 16.0%. Blue PhOLEDs incorporating solution‐deposited ETLs were over two‐fold more efficient than those containing vacuum‐deposited ETLs. Atomic force microscopy imaging shows that the solution‐deposited oligoquinoline ETLs formed vertically oriented nanopillars and rough surfaces that enable good ETL/cathode contacts, eliminating the need for cathode interfacial materials (LiF, CsF). These solution‐processed blue PhOLEDs have the highest performance observed to date in polymer‐based blue PhOLEDs.     

Influences of charge of conjugated polymer electrolytes cathode interlayer for bulk-heterojunction polymer solar cells

Two fluorene-based conjugated polymer electrolyte (CPE) poly[(9,9-bis(6′-(N,N,N-trimethylammonium)hexyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFNBr) and poly[9,9-bis(4′-sulfonatobutyl)fluorene-alt-2,7-(9,9-dioctylfluorene)] sodium salt (PFSO₃Na), bearing amine groups and anionic sulfonate groups on side chains respectively, are synthesized and applied as cathode interlayer in polymer solar cells. Both of the hydrophilic CPEs can well modify the interfacial properties and allow ohomic contact between the activelayer and cathode. The opposite charges exert great influence on the effective work function of cathode and interfacial interaction through the orientation of the interfacial dipole at the active layer/metal electrode interface, subsequently influence the resulting device performance. Compared with the cationic PFNBr, PFSO₃Na with anionic sulfonate groups can dramatically reduce the work function of Al by accumulation of the polar groups at the PFSO₃Na/Al interface to induce more favorable the interfacial dipole. The better energy alignment for electron extraction and transportation at active layer/Al interface is confirmed by a significant enhancement of V_OC. The better wettability and morphology of PFSO₃Na on the active layer and the more effective motion of sodium counterion further modify the barrier to facilitate electron extraction and transportation. Moreover, 14% and 22% performance enhancement can also be achieved respectively, when PFNBr and PFSO₃Na are used as interlayers for low bandgap poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT)-based solar cells.     

退火处理对锑化铟MIS器件C-V特性的影响

采用原子层沉积技术制备Al2O3薄膜作为InSb材料介电层,制备了MIS器件,研究了金属化后不同退火温度对界面特性的影响.利用C-V测试表征了MIS(metal-insulator-semiconductor)器件的界面特性,结果表明Al2O3介电层引入了表面固定正电荷,200℃和300℃退火处理可有效减小慢界面态密度...     

Review on the Application of SnO2 in Perovskite Solar Cells

SnO₂ has been well investigated in many successful state‐of‐the‐art perovskite solar cells (PSCs) due to its favorable attributes such as high mobility, wide bandgap, and deep conduction band and valence band. Several independent studies show the performances of PSCs with SnO₂ are higher than that with TiO₂, especially in device stability. In 2015, the first planar PSCs were reported with a power conversion efficiency over 17% using a low temperature sol‐derived SnO₂ nanocrystal electron transport layer (ETL). Since then, many other groups have also reported high performance PSCs based on SnO₂ ETLs. SnO₂ planar PSCs show currently the highest performance in planar configuration devices (21.6%) and are close to the record holder of TiO₂ mesoporous PSCs, suggesting their high potential as ETLs in PSCs. The main concerns with the application of SnO₂ as ETL are that it suffers from degradation in high temperature processes and that its much lower conduction band compared to perovskite may result in a voltage loss of PSCs. Here, notable achievements to date are outlined, the unique attributes of SnO₂ as ETLs in PSCs are described, and the challenges facing the successful development of PSCs and approaches to the problems are discussed.     

Inverted-structure polymer solar cells fabricated by sequential spraying of electron-transport and photoactive layers

Inverted-structure polymer solar cells (I-PSCs) containing sequentially sprayed electron-transporting layers (ETLs) and photoactive layers were fabricated. Low-temperature sol–gel-derived ZnO thin films were used as the ETLs and films of a poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend were used as the photoactive layers. Nanoripples-containing ZnO ETLs could be successfully fabricated by controlling the spraying rate of the ZnO precursor solution and the subsequent annealing conditions. The P3HT/PCBM active layers sprayed on the ZnO ETLs were optimized using a unique solvent-assisted post-deposition treatment, namely, the sprayed solvent overlayer (SSO) treatment. The power conversion efficiency (PCE) of the I-PSCs based on the optimized ETLs and active layers was as high as 3.55%, which is comparable to that reported for I-PSCs fabricated using the conventional spin-coating method. The sprayed I-PSCs also exhibited high environmental stability, maintaining ∼80% of their PCE even after 40 days of aging in air under ambient conditions without encapsulation. The I-PSCs based on the P3HT/PCBM photoactive layers optimized using the SSO treatment displayed much higher stability than those based on photoactive layers optimized using a conventional thermal annealing treatment. This result indicated that the SSO treatment is a suitable post-deposition treatment method for improving the morphological stability of P3HT/PCBM active layers. Further, the fabrication technique investigated in this study is a high-throughput low-temperature one and is suitable for fabricating high-stability PSCs.     

Interface Dipole Induced Field-Effect Passivation for Achieving 21.7% Efficiency and Stable Perovskite Solar Cells

Organolead halide hybrid perovskite solar cells (PSCs) have become a shining star in the renewable devices field due to the sharp growth of power conversion efficiency; however, interfacial recombination and carrier-extraction losses at heterointerfaces between the perovskite active layer and the carrier transport layers remain the two main obstacles to further improve the power conversion efficiency. Here, novel field-effect passivation has been successfully induced to effectively suppress the interfacial recombination and improve interfacial charge transfer by incorporating interfacial polarization via inserting a high work function interlayer between perovskite and holes transport layer. The charge dynamics within the device and the mechanism of the field-effect passivation are elucidated in detail. The unique interfacial dipoles reinforce the built-in field and prevent the photogenerated charges from recombining, resulting in power conversion efficiency up to 21.7% with negligible hysteresis. Furthermore, the hydrophobic interlayer also suppresses the perovskite decomposition by preventing the moisture penetration, thereby improving the humidity stability of the PSCs (>91% of the initial power conversion efficiency (PCE) after 30 d in 65 ± 5% humidity). Finally, several promising research perspectives based on field-effect passivation are also suggested for further conversion efficiency improvements and photovoltaic applications.     

Effects of ultraviolet–ozone treatment on organic-stabilized ZnO nanoparticle-based electron transporting layers in inverted polymer solar cells

Electron transporting layers (ETLs) in inverted polymer solar cells (I-PSCs) were fabricated by spin coating a colloidal dispersion of ZnO nanoparticles (NPs), and the effects of ultraviolet–ozone (UVO) treatment on the ZnO NP ETLs were investigated. The brief UVO treatment (<5 min) could considerably improve the performance of the resulting I-PSCs (∼30% increase in power conversion efficiency); whereas, excessive UVO treatment (>10 min) caused significant degradation. The characterization of the ZnO ETLs as a function of the UVO treatment duration revealed that brief treatment can remove the residual organic stabilizer molecules on the surface of the ZnO films by UV induced decomposition mechanism. However, excessive treatment can generate additional defects on/within the ZnO films, which can induce charge recombination. This effect was further confirmed by the thermal treatment of the ZnO ETLs at a high temperature (280 °C) at which the organic surfactants could be removed. Flexible I-PSCs were also fabricated using indium doped tin oxide coated plastic substrates and the usefulness of the room temperature UVO treatment was further confirmed in view of its potential applicability in flexible devices.     

In‐Situ Switching from Barrier‐Limited to Ohmic Anodes for Efficient Organic Optoelectronics

Injection and extraction of charges through ohmic contacts are required for efficient operation of semiconductor devices. Treatment using polar non‐solvents switches polar anode surfaces, including PEDOT:PSS and ITO, from barrier‐limited hole injection and extraction to ohmic behaviour. This is caused by an in‐situ modification of the anode surface that is buried under a layer of organic semiconductor. The exposure to methanol removes polar hydroxyl groups from the buried anode interface, and permanently increases the work function by 0.2–0.3 eV. In the case of ITO/PEDOT:PSS/PBDTTT‐CT:PC71BM/Al photovoltaic devices, the higher work function promotes charge transfer, leading to p‐doping of the organic semiconductor at the interface. This results in a two‐fold increase in hole extraction rates which raises both the fill factor and the open‐circuit voltage, leading to high power conversion efficiency of 7.4%. In ITO/PEDOT:PSS/F8BT/Al polymer light‐emitting diodes, where the organic semiconductor's HOMO level lies deeper than the anode Fermi level, the increased work function enhances hole injection efficiency and luminance intensity by 3 orders of magnitude. In particular, hole injection rates from PEDOT:PSS anodes are equivalent to those achievable using MoO₃. These findings exemplify the importance of work function control as a tool for improved electrode design, and open new routes to device interfacial optimization using facile solvent processing techniques. Such simple, persistent, treatments pave the way towards low cost manufacturing of efficient organic optoelectronic devices.     

Correlations of impedance-voltage characteristics and carrier mobility in organic light emitting diodes

The correlation of accumulation charges at the interfaces of organic layers and carrier mobility in organic light emitting devices (OLEDs) were investigated through the impedance versus voltage (Z-V) characteristics of devices. The properties of devices with various combinations of cathode structures, HTLs and ETLs were investigated to understand the impedance transition in Z-V characteristics of OLEDs. It was observed that there is an extra impedance transition before devices turn on when the hole mobility in the HTL is much greater than the electron mobility in the ETL in the devices, which makes the Z-V characteristics a potential tool to compare the electron mobility in ETL and hole mobility in HTL.     

Doped/Undoped A1-A2 Typed Copolymers as ETLs for Highly Efficient Organic Solar Cells

The electron transport layer (ETL) is a critical component in achieving high device performance and stability in organic solar cells. Conjugated polyelectrolytes (CPEs) have become an attractive alternative due to film-forming properties and ease of preparation. However, p-type CPEs generally exhibit poor charge mobility and conductivity, incorporation of electron-withdrawing units forming alternated D-A conjugated backbone can make up for these deficiencies. Herein, the ratio of electron withdrawing moieties are further increased and two poly(A1-alt-A2) typed PIIDNDI-Br and PDPPNDI-Br based on the combination of naphthalene diimide (NDI) with isoindigo (IID) or diketopyrrolopyrrole (DPP) via direct arylation polycondensation are synthesized. These CPEs possess excellent alcohol solubility, a suitable lowest unocuppied molecular orbital energy level, and work function tunability. Surprisingly, the incorporation of IID and DPP units generate distinct self-doping behaviors, which are confirmed by UV–vis absorption and ESR spectra. However, no matter doped or undoped, both CPEs present better charge-transporting properties and conductivity when utilized as ETLs. The PIIDNDI-Br and PDPPNDI-Br display good universal compatibility with the blend of PM6:Y6 and PM6:L8-BO, and PCEs of 18.32% and 18.36% are obtained, respectively, which also present excellent storage stability. In short, the combination of two different acceptors demonstrates an efficient strategy to design highly efficient ETLs for high performance photovoltaic devices.     

Investigation of turn-on voltage shift in organic ferroelectric transistor with high polarity gate dielectric

Large positive shifts of turn-on voltage V_to were observed in ferroelectric organic thin film transistor using P(VDF-TrFE) copolymer (57–43 mol%) as gate insulator during OFF to ON state sweeping. The shift of the transfer characteristic up to +25 V is attributed to the accumulation of mobile charge carriers (holes) in pentacene layer even during the device OFF state. The observed phenomena were first discussed on the basis of a negative surface potential created by the dipole field of a polar dielectric and trap states in an organic semiconductor layer. It was however found that these were unable to fully address the observed strong V_to shift due to the presence of large polarization in the P(VDF-TrFE) layer. A mechanism of negative polarization-compensating charges which are injected to the insulator region next to the semiconductor layer was proposed and examined to understand the phenomenon. The turn-on voltage is found to change with different magnitude of positive voltage pulses, and corresponds to different amount of charges injected for compensation. Time measurement of drain current shows a transient decaying behavior when gate bias is switched from positive to negative polarity which confirms the trapping of negative charges in the insulator.     

PEO作阴极修饰层提高有机光伏电池的稳定性

采用聚氧化乙烯(PEO)作为聚合物太阳能电池的阴极修饰层,以P3HT:PCBM为活性层制备了聚合物本体异质结太阳能电池。考察了PEO的厚度对器件光伏性能及稳定性的影响。比较了加入PEO修饰层前后器件的稳定性,研究了采用PEO修饰层前后器件电阻的差异。结果表明:加入PEO作为阴极修饰层后器件的光电性能(JSC,VOC,FF,PCE)均有明显提高,而器件的串联电阻Rs则有了明显降低。没有阴极修饰层的器件的初始光电转换效率为1.92%,90 h后衰减为初始值的5%;而加入PEO修饰层后初始光电转换效率为3.36%,90 h后仅衰减为初始值的20%,光电转换效率提高了75%,稳定性提高了3倍。     

Temperature dependent subthreshold model of long channel GAA MOSFET including localized charges to study variations in its temperature sensitivity

In this paper, degrading effects of hot carrier damage/radiation damage/process damage induced interface localized charges on the temperature sensitivity of the Cylindrical Gate All Around (GAA) MOSFET are investigated. A temperature dependent numerical model is developed for GAA MOSFET including interface localized charges and the results so obtained are validated with the simulation results of ATLAS 3D device simulator. Results show that subthreshold region is the most affected region in both the cases i.e. (1) in presence of localized charges and (2) under temperature variation. Also degrading effects of localized charges are found to be more pronounced at low temperatures. Apart from electrical performance degradation, localized charges change the temperature sensitivity of the device i.e. change in temperature coefficient of the drain current and zero crossover point (gate bias corresponding to zero temperature coefficient).     

A mobility model for submicrometer MOSFET simulations includinghot-carrier-induced device degradation

The model presented includes the quantum effects of electrons in the inversion layer proposed by S.A. Schwarz and S.E. Russek (1983) and the surface scattering effects due to the interfacial charges. By comparison with experimental data from scaled MOSFETs, the limitation of K. Yamaguchi's (1983) mobility model in submicrometer device simulations is implied, while the quantum channel broadening effects have been proven significant in turn. In addition, it is shown that the modeling of the screening effect of Coulomb scattering plays an important role in simulating the hot-carrier-induced MOSFET degradation. The model can predict the current-voltage characteristics within 5% accuracy for scaled MOSFETs down to 0.5-μm, as well as the degradation of electrical characteristics due to hot-carrier effects for submicrometer MOSFETs