Interfacial Energy Level Alignment Research Articles

Fabrication of Efficient and Simple-Structured Perovskite Solar Cells Using a Multifunctional Biocolina Surface Treatment.

The electron transport layer (ETL)-free perovskite solar cells (PSCs) have gained significant interest by simplifying the manufacture process and reducing the time/energy required for the fabrication of ETLs. Unfortunately, the performance of these ETL-free PSCs still lags behind those of the conventional counterparts due to the slow electron extraction and undesired interfacial charge recombination loss at the buried interface. In this work, a facile and multifunctional biocolina thin layer is incorporated on the bottom electrodes to regulate the interface energy level alignment by forming an interface dipole layer, resulting in a suppressed nonradiative recombination and an improved charge extraction. Furthermore, the biocolina thin layer possess the capability to passivate the surface defects within the perovskite films while simultaneously facilitate the formation of perovskite crystals. Consequently, a remarkable enhancement in photovoltaic performance is observed in the biocolina-based ETL-free PSCs with an increase from 15.96% to an outstanding 20.01%. Additionally, the biocolina extends the stability and relieves the hysteresis effect through the interface defect passivation and inhibition of interface charge accumulation. This research contributes to the development of cost-effective, simplified designs for highly efficient ETL-free PSCs by modifying the bottom electrodes.

Evaporated Nickel Oxide Films with Slow Annealing and Interface Modification for Perovskite Solar Cells.

Electron-beam-evaporated nickel oxide (NiOx) films are known for their high quality, precise control, and suitability for complex structures in perovskite (PVK) solar cells (PSCs). However, untreated NiOx films have inherent challenges, such as surface defects, relatively low intrinsic conductivity, and shallow valence band maximum, which seriously restrict the efficiency and stability of the devices. To address these challenges, we employ a dual coordination optimization strategy. The strategy includes low heating rate annealing of NiOx films and using an aminoguanidine nitrate spin coating process on the surfaces of NiOx films to strategically modify NiOx films itself and the interface of NiOx/PVK. Under the synergistic effect of this dual optimization method, the quality of the films is significantly improved and its p-type characteristics are enhanced. At the same time, the interface defects and energy level alignment of the films are effectively improved, and the charge extraction ability at the interface is improved. The combined treatment significantly improved the efficiency of inverted PSCs, from 17.85% to 20.31%, and enhanced device stability under various conditions. This innovative dual-coordinated optimization strategy provides a clear and effective framework for improving the performance of NiOx films and inverted PSCs.

Minimizing Voltage Losses via Synergistically Reducing Hetero-Interface Energy Offset for High Efficiency Perovskite Solar Cells.

The open circuit voltage (VOC) losses at multiple interfaces within perovskite solar cells (PSCs) limit the improvements in power conversion efficiency (PCE). Herein, a tailored strategy is proposed to reduce the energy offset at both hetero-interfaces within PSCs to decrease the VOC losses. For the interface of perovskite and electron transport layer where exists a mass of defects, it uses the pyromellitic acid to serve as a molecular bridge, which reduces non-radiative recombination and energy level offset. For the interface of perovskite and hole transport layer, which includes a passivator of PEAI, the detrimental effect (negative shift of work function) of PEAI passivation and optimizing the interface energy level alignment are neutralized by incorporating (2-(4-(bis(4-methoxyphenyl)amino)phenyl)-1-cyanovinyl)phosphonic acid. Owing to synergistically reduced hetero-interface energy offset, the PSCs achieve a PCE of 25.13%, and the VOC is increased from 1.134 to 1.174V. In addition, the resulting PSCs possess enhanced stability, the unencapsulated PSCs can maintain ≈96% and ≈97% of their initial PCE after 2000h of aging under ambient conditions and 210h under operation conditions.

Carrier Transport in Low‐Dimensional Tin Perovskite Solar Cells

Recently, the emerging of low‐dimensional tin perovskite solar cells (TPSCs) by introducing bulky organic spacers has attracted extensive attention. It can not only inhibit the oxidation of Sn2+, but also reduce the ion migration and self‐doping effect. Thanks to these advantages, the TPSCs have achieved an impressive power conversion efficiency (PCE) of over 15% recently. However, the introduction of organic spacers impedes the carrier transport and thus limits the attainable PCE. Therefore, it is important to understand the carrier‐transport mechanism in low‐dimensional perovskite to develop more efficient and stable TPSCs. In this review, the latest progress of carrier transport in low‐dimensional TPSCs is summarized in detail. First, the characteristics of carrier transport in low‐dimensional tin perovskites are discussed. Then, the strategies to improve carrier transport are discussed, mainly from the aspects of crystal orientation, crystallization kinetics, defects, interface energy level alignment, quantum well effect, and exciton binding energy. Finally, the future challenges and prospects are expounded to prepare high‐performance and stable low‐dimensional TPSCs.

Flexible Indoor Perovskite Solar Cells by In Situ Bottom-Up Crystallization Modulation and Interfacial Passivation.

A robust perovskite-buried interface is pivotal for achieving high-performance flexible indoor photovoltaics as it significantly influences charge transport and extraction efficiency. Herein, a molecular bridge strategy is introduced utilizing sodium 2-cyanoacetate (SZC) additive at the perovskite-buried interface to simultaneously achieve in situ passivation of interfacial defects and bottom-up crystallization modulation, resulting in high-performance flexible indoor photovoltaic applications. Supported by both theoretical calculations and experimental evidences, it illustrates how SZCs serve as molecular bridges, establishing robust bonds between SnO2 transport layer and perovskite, mitigating oxygen vacancy defects and under-coordinated Pb defects at interface during flexible fabrication. This, in turn, enhances interfacial energy level alignment and facilitates efficient carrier transport. Moreover, this in situ investigation of perovskite crystallization dynamics reveals bottom-up crystallization modulation, extending perovskite growth at the buried interface and influencing subsequent surface recrystallization. This results in larger crystalline grains and improved lattice strain of the perovskite during flexible fabrication. Finally, the optimized flexible solar cells achieve an impressive efficiency exceeding 41% at 1000 lux, with a fill factor as high as 84.32%. The concept of the molecular bridge represents a significant advancement in enhancing the performance of perovskite-based flexible indoor photovoltaics for the upcoming era of Internet of Things (IoT).

Aqueous solution processed transition-metal-doped ZnO as electron injection layers for efficient inverted organic light emitting diodes

Zinc oxide (ZnO) has attracted lots of attention to be used as interfacial materials in organic optoelectronic devices owing to its attractive properties. In this work, a series of transition-metal-doped ZnO thin films were prepared by using a facile aqueous solution process, thereafter these films were employed as electron injection layers (EILs) combined with the tris-(8-hydroxyquinoline) aluminum (Alq3) emitting layer to construct the inverted organic light-emitting diodes (IOLEDs). The transition-metal-doped ZnO EIL-based IOLEDs demonstrate markedly improved device performance compared to the reference neat ZnO EIL-based IOLED. However, different transition-metal ions have much different doping effect on the device performance. CoCl2 doped ZnO EIL based IOLED device possesss an optimal current efficiency of 5.52 cd A−1, which is increased about 40% compared to the device with the neat ZnO as EIL. The enhancement might be attributed to the increased electron mobility and favorable interfacial energy level alignment of the transition-metal doped ZnO EIL for more efficient electron injection. Our findings may provide a promising strategy to improve the electron injection properties of ZnO film in the OLED devices.

Interfacial defects passivation and energy level alignment with small molecule pyridine material for efficient and stable inverted perovskite solar cells

Unavoidable charge defects at the interface are major bottlenecks in improving the performance of inverted perovskite solar cells (PSCs). Interfacial engineering for interfacial defect passivation is a promising approach for minimizing interfacial defects. In this study, the small molecule 2,6-lutidine (2,6-Lu), incorporating [6,6]-phenyl C61-butyric acid methyl ester (PCBM), was established as an electron transport layer (PCBM: 2,6-Lu ETL) to minimize charge defects at the upper interface of the perovskite. After a series of experimental tests, we confirmed that the –N– groups in 2,6-Lu could immobilize uncoordinated Pb2+ to passivate charge defects. Furthermore, the designed ETL induced a favorable energy level between the perovskite and ETL layers, boosting charge transfer and reducing charge recombination. Therefore, the inverted PSCs with the designed ETL exhibited a remarkable power conversion efficiency of 19.93 % with an outstanding long-term ambient stability of 30 d, which was ascribed to interfacial defect passivation and suitable energy level alignment.

Photoelectron Spectroscopic Study of the Interfacial Electronic Structures of Metal‐Ion Containing Polyelectrolytes on ITO Substrates

AbstractResearch in the field of organic electronics has witnessed dramatic improvements in device performance over the past several decades through an ever‐improving understanding of electron and hole movement and the development of new interfacial materials. In this study, a type of interfacial material that relies on ionic charges comprising metal:poly(styrenesulfonate) (PSS) polyelectrolytes are synthesized and investigated as structural analogs of the ubiquitously used poly(3,4‐ethylenedioxythiophene:polystyrenesulfonate) (PEDOT:PSS) hole transport layer, in order to investigate correlations between metal cation ions and the cationic PEDOT component. The metal ions selected for this study include Li, Mg, V, Mn, Co, Ni, Cu, Zn, Pd, Ag, Cs, and Pb ions. To analyze the interfacial energy level alignment, electronic band structure, and band bending at the Indium tin oxide (ITO)/metal:PSS interface, X‐ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) are employed. Alkali (earth) and post‐transition metals show deep highest occupied molecular orbital (HOMO) levels and low work function (WF) due to Fermi level balance, implying poor hole transport. Remarkably, Cu:PSS displays a unique electronic structure, suggesting potential as a hole transport layer with increased WF and low hole injection barrier. Period 5 transition metals mirror PEDOT:PSS trends, and Ag:PSS holds the potential to form effective ohmic contacts.

Ionic liquid tailoring defect/interface-induced recombination loss toward efficient Dion-Jacobson quasi-2D perovskite solar cells

Dion-Jacobson (DJ) quasi-two-dimensional (quasi-2D) perovskites have emerged as a potential photovoltaic material due to their versatile n-layer variation and superior stable structure. However, compared to their three-dimensional counterparts, the power conversion efficiency (PCE) of quasi-2D perovskite solar cells (PSCs) reported to date still lags behind the detailed balance (DB) limit for single-junction PSCs, which derives from the presence of non-radiative recombination loss. Herein, we demonstrated that the defect/interface-induced recombination loss in DJ quasi-2D PSCs could be significantly suppressed by introducing a green ionic liquid methylammonium acetate (MAAc) into (BDA)(MA)4Pb5I16 (n = 5) (BDA, 1,4-butanediamine) perovskite precursor. Both the film quality of 2D perovskites and the interfacial property of perovskite/charge transporting layer (CTL) were improved after introducing MAAc in precursor solution. Correspondingly, a champion PCE of 18.76 % with a remarkable open-circuit voltage (Voc) of 1.24 V was achieved. Through the quantitative loss analysis, which involved the calculation of photovoltaic parameters relative to DB limit and quasi-Fermi level splitting values, the improvement of PCE and Voc by introducing MAAc was mainly attributed to the reduction of defects in perovskite films and at perovskite/CTL interfaces, and the optimization of interfacial energy-level alignment. The present work provides a green solvent strategy for improving the performance of quasi-2D PSCs, and offers an insight into the fundamental physics of non-radiative recombination loss tailored by ionic liquid.

Boosting Cu─In─Zn─S-based Quantum-Dot Light-Emitting Diodes Enabled by Engineering Cu─NiOx/PEDOT:PSS Bilayered Hole-Injection Layer.

The imbalance of charge injection is considered to be a major factor that limits the device performance of cadmium-free quantum-dot light-emitting diodes (QLEDs). In this work, high-performance cadmium-free Cu─In─Zn─S(CIZS)-based QLEDs are designed and fabricated through tailoring interfacial energy level alignment and improving the balance of charge injection. This is achieved by introducing a bilayered hole-injection layer (HIL) of Cu-doped NiOx (Cu─NiOx)/Poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT:PSS). High-quality Cu─NiOx film is prepared through a novel and straightforward sol-gel procedure. Multiple experimental characterizations and theoretical calculations show that the incorporation of Cu2+ ions can regulate the energy level structure of NiOx and enhance the hole mobility. The state-of-art CIZS-based QLEDs with Cu─NiOx/PEDOT:PSS bilayered HIL exhibit the maximum external quantum efficiency of 6.04% and half-life time of 48min, which is 1.3 times and four times of the device with only PEDOT:PSS HIL. The work provides a new pathway for developing high-performance cadmium-free QLEDs.

Side chain modulated carbazole-based bifunctional hole-shuttle interlayer simultaneously improves interfacial energy level alignment and defect passivation in high-efficiency perovskite solar cells

Perovskite solar cells have reached a power conversion efficiency over 26.1 %, and the interface engineering between perovskite and hole transport layer (HTL) is crucial for achieving high performance. There exists a noticeable research gap when it comes to designing an interlayer layer that can both hold defect passivation and hole transportability. To bridge this gap, we have designed and synthesized two functional molecules with carbazole cores and side chain modifications, namely CVE-Br and CVE-DPA. Beyond the suitable energy levels of the carbazole group, CVE-Br can intercalate into perovskite lattice, create a low-dimensional perovskite, and further minimize defects. Instead of forming low dimensional structure, CVE-DPA molecule wraps around the perovskite. That makes it passivate perovskite defects and facilitate hole transport without the limitation of carrier transportation. As a result, the power conversion efficiency of perovskite solar cells with CVE-DPA can achieve 22.05 %. The hydrophobicity of CVE-DPA confers the corresponding devices to retain 87 % of the initial efficiency after 1000 h.

Interfacial Energy Level Alignment and Defect Passivation by Using a Multifunctional Molecular for Efficient and Stable Perovskite Solar Cells

AbstractTin oxide (SnO2) is currently the dominating electron transport material (ETL) used in state‐of‐the‐art perovskite solar cells (PSCs). However, there are amounts of defects distributed at the interface between ETL and perovskite to deteriorate PSC performance. Herein, a molecule bridging layer is built by incorporating 2,5‐dichloroterephthalic acid (DCTPA) into the interface between the SnO2 and perovskites to achieve better energy level alignment and superior interfacial contact. The multifunctional molecular bridging layer not only can passivate the trap states of Sn dangling bonds and oxygen vacancies resulting in improved conductivity and the electron extraction of SnO2 but also can regulate the perovskite crystal growth and reduce defect‐assisted nonradiative recombination due to its strong interaction with undercoordinated lead ions. As a result, the DCTPA‐modified PSCs achieve champion power conversion efficiencies (PCEs) of 23.25% and 20.23% for an active area of 0.15 cm2 device and 17.52 cm2 mini‐module, respectively. Moreover, the perovskite films and PSCs based on DCTPA modification show excellent long‐term stability. The unencapsulated target device can maintain over 90% of the initial PCE after 1000 h under ambient air. This strategy guides design methods of molecule bridging layer at the interface between SnO2 and perovskite to improve the performance of PSCs .

Interfacial electronic properties of hybrid interfaces

A detailed understanding of interfacial electronic properties of organic–inorganic hybrid junctions is crucial for their technological applications. In case of organic–inorganic hybrid junctions information about interfacial charge transfer, band bending and work function is crucial for efficient charge carrier movement across interfaces. In this context, investigation of filled and empty starts near Fermi level is relevant in molecular electronics to know electron affinity, and charge injection barriers. A multi-technique approach using ultraviolet photoemission and Inverse photoemission spectroscopy provides quantitative information about frontier occupied and unoccupied states near Fermi level, thereby providing crucial information about energy level alignment. Inverse photoemission spectroscopy technique is particularly helpful for the study of the electronic properties at metal–molecule interfaces and thus provides information about transport gap for conjugated organic molecules. Organic molecular beam epitaxy has been extensively utilized to deposit conjugated organic molecular thin films on various oxide, transition metal dichalcogenides and metallic substrates. Novel approaches have been investigated explored in order to control the interfacial energy level alignment by introducing intermediate electron donor and electron acceptor organic molecular layers across interfaces to tune the work function of electrodes. This review summarizes recent advances in exploiting combined approach using photoemission and inverse photoemission spectroscopy for the investigation of electronic properties of conjugated organic molecular interfaces.

Regulating Orientational Crystallization and Buried Interface for Efficient Perovskite Solar Cells Enabled by a Multi‐Fluorine‐Containing Higher Fullerene Derivative

AbstractPerovskite films prepared by the solution process usually result in irregular grain orientation and rich buried interface defects, hindering the further improvement of device performance. Herein, multi‐fluorine‐containing C60‐ and C70 (higher fullerene)‐porphyrin derivatives, F60PD and F70PD, are synthesized and pre‐buried to modify the SnO2/perovskite heterointerface. The F70PD modification layer provides a better perovskite quality and more effective electron transporting capability compared to the corresponding F60PD, with the F70PD being more effective in regulating the perovskite growth, passivating the buried interface defects, and optimizing the interface energy level alignment. Consequently, the F70PD‐based device delivers superior efficiency and stability than the control and F60PD‐based devices. The F70PD‐based device yields a champion efficiency of 24.09% with negligible hysteresis. Meanwhile, due to the increased activation energy of ion migration, the F70PD‐based device maintains 80% of its initial efficiency after operating at the maximum power point for 1620 h. This study highlights the potential of designing higher fullerene materials for buried interface to further improve the perovskite solar cells’ performance.

Applications of Metal Oxide Charge Transport Layers in Perovskite Solar Cells

Metal oxide (MO) charge transport layers (CTLs) are widely used for fabricating highly efficient and stable perovskite solar cells (PSCs) due to their superior stability, material and preparation cost, light transmission, and charge selection. However, the complex surface states, unbalanced carrier mobility, and variable energy band structure determined by MOs can lead to additional interfacial charge recombination and transport losses within the device, which limit further improvements in device performance. Extensive research has been conducted to address these challenges. In this review, an overview of current popular MO‐CTLs and their preparation methods for PSCs are provided. Interface regulation strategies, such as passivating interface defects, modulating interface energy level alignment, and improving interface contact are also discussed, which can enhance the performance of PSCs. Meanwhile, the commonly used dopants and doping strategies for optimizing the charge transport properties of CTLs are also discussed.

Synergic Electron and Defect Compensation Minimizes Voltage Loss in Lead‐Free Perovskite Solar Cells

AbstractSn perovskite solar cells have been regarded as one of the most promising alternatives to the Pb‐based counterparts due to their low toxicity and excellent optoelectronic properties. However, the Sn perovskites are notorious to feature heavy p‐doping characteristics and possess abundant vacancy defects, which result in under‐optimized interfacial energy level alignment and severe nonradiative recombination. Here, we reported a synergic “electron and defect compensation” strategy to simultaneously modulate the electronic structures and defect profiles of Sn perovskites via incorporating a traced amount (0.1 mol %) of heterovalent metal halide salts. Consequently, the doping level of modified Sn perovskites was altered from heavy p‐type to weak p‐type (i.e. up‐shifting the Fermi level by ~0.12 eV) that determinately reducing the barrier of interfacial charge extraction and effectively suppressing the charge recombination loss throughout the bulk perovskite film and at relevant interfaces. Pioneeringly, the resultant device modified with electron and defect compensation realized a champion efficiency of 14.02 %, which is ~46 % higher than that of control device (9.56 %). Notably, a record‐high photovoltage of 1.013 V was attained, corresponding to the lowest voltage deficit of 0.38 eV reported to date, and narrowing the gap with Pb‐based analogues (~0.30 V).

Synergic Electron and Defect Compensation Minimizes Voltage Loss in Lead-Free Perovskite Solar Cells.

Sn perovskite solar cells have been regarded as one of the most promising alternatives to the Pb-based counterparts due to their low toxicity and excellent optoelectronic properties. However, the Sn perovskites are notorious to feature heavy p-doping characteristics and possess abundant vacancy defects, which resulted in under-optimized interfacial energy level alignment and severe nonradiative recombination. Here, we reported a synergic "electron and defect compensation" strategy to simultaneously modulate the electronic structures and defect profiles of Sn perovskites via incorporating a traced amount (0.1 mol%) of heterovalent metal halide salts. Consequently, the doping level of modified Sn perovskites was altered from heavy p-type to weak p-type (i.e. up-shifting the Fermi Level by ~ 0.12 eV) that determinately reducing the barrier of interfacial charge extraction and effectively suppressing the charge recombination loss throughout the bulk perovskite film and at relevant interfaces. Pioneeringly, the resultant device modified with electron and defect compensation realized a champion efficiency of 14.02%, which is ~46% higher than that of control device (9.56%). Notably, a record-high photovoltage of 1.013 V was attained, corresponding to the lowest voltage deficit of 0.38 eV reported to date, and narrowing the gap with Pb-based analogues (~ 0.30 V).

Ionic Accumulation as a Diagnostic Tool in Perovskite Solar Cells: Characterizing Band Alignment with Rapid Voltage Pulses.

Despite record-breaking devices, interfaces in perovskite solar cells are still poorly understood, inhibiting further progress. Their mixed ionic-electronic nature results in compositional variations at the interfaces, depending on the history of externally applied biases. This makes it difficult to measure the band energy alignment of charge extraction layers accurately. As a result, the field often resorts to a trial-and-error process to optimize these interfaces. Current approaches are typically carried out in a vacuum and on incomplete cells, hence values may not reflect those found in working devices. To address this, a pulsed measurement technique characterizing the electrostatic potential energy drop across the perovskite layer in a functioning device is developed. This method reconstructs the current-voltage (JV) curve for a range of stabilization biases, holding the ion distribution "static" during subsequent rapid voltage pulses. Two different regimes are observed: at low biases, the reconstructed JV curve is "s-shaped", whereas, at high biases, typical diode-shaped curves are returned. Using drift-diffusion simulations, it is demonstrated that the intersection of the two regimes reflectsthe band offsets at the interfaces. This approach effectively allows measurements of interfacial energy level alignment in a complete device under illumination and without the need for expensive vacuum equipment.

Constructing Interfacial Gradient Heterostructure Enables Efficient CsPbI3 Perovskite Solar Cells and Printed Minimodules.

Severe nonradiative recombination originating from interfacial defects together with the pervasive energy level mismatch at the interface remarkably limits the performance of CsPbI3 perovskite solar cells (PSCs). These issues need to be addressed urgently for high-performance cells and their applications. Herein, we demonstrate an interfacial gradient heterostructure based on low-temperature post-treatment of quaternary bromide salts for efficient CsPbI3 PSCs with an impressive efficiency of 21.31% and an extraordinary fill factor of 0.854. Further investigation reveals that Br- ions diffuse into the perovskite films to heal undercoordinated Pb2+ and inhibit Pb clusters formation, thus suppressing nonradiative recombination in CsPbI3 . Meanwhile, a more compatible interfacial energy level alignment resulting from Br- gradient distribution and organic cations surface termination has been also achieved, hence promoting charge separation and collection. Consequently, we also demonstrate the printed small-size cell with an efficiency of 20.28% and 12-cm2 printed CsPbI3 minimodules with a record efficiency of 16.60%. Moreover, the unencapsulated CsPbI3 films and devices exhibit superior stability. This article is protected by copyright. All rights reserved.

Multifunctional Aminoglycoside Antibiotics Modified SnO2 Enabling High Efficiency and Mechanical Stability Perovskite Solar Cells

AbstractSnO2 as an electron transport layer (ETL) has been widely used in regular planar perovskite solar cells (PSCs) owing to its high optical transmittance, less photocatalytic activity, and low‐temperature processing. However, SnO2‐based PSCs still face many challenges which greatly impair their efficiency and stability of PSCs. Herein, a novel and effective multifunctional modification strategy is proposed by incorporating streptomycin sulfate (STRS) molecules with multiple functional groups into SnO2 ETL. STRS can significantly suppress SnO2 nanoparticle agglomeration, improve the electronic property of SnO2, as well as reduce nonradiative recombination. At the same time, interfacial residual tensile stress is released and the interfacial energy level alignment becomes more matched. As a result, the STRS‐modified PSCs achieve a higher efficiency of 22.89% compared to 20.61% of the control device and exhibit a hysteresis‐free feature. The humidity and thermal stability of PSCs based on STRS‐SnO2 are significantly improved. Furthermore, the efficiency of flexible devices increased from 19.74% to 20.79%, and the devices still maintain >80% of initial PCE after 4500 bending cycles with a bend radius of 5 mm. This study provides a low‐cost, facile, and efficient strategy for achieving high efficiency and stability in PSCs.