Hole Transport Layer Research Articles

Controlling Polarization‐Induced Polaron Accumulation in OLEDs via Device Design

AbstractSpontaneous alignment of molecular electric dipole moments, known as spontaneous orientation polarization (SOP), is present in many organic electronic materials. The SOP‐induced field changes the internal voltage distribution within the device, altering polaron injection and accumulation. The important role of SOP in organic light‐emitting devices (OLEDs) has recently been emphasized with clear links between dipole alignment and device efficiency and operational lifetime. Prior work has thus sought to engineer SOP via changes in thin film composition or processing conditions. These efforts have generally been directed at the OLED electron transport layer, which can show strong SOP. This work takes a different approach, focusing instead on how changes in device architecture can be applied to tune the magnitude and dynamics of polaron accumulation in response to SOP. In this work, it is demonstrated that the introduction of SOP into the device emissive layer offers control over the spatial location of accumulation. In parallel, insertion of a spacer layer between the hole transport and emissive layers can be used to tune the polaron accumulation rate and density during device operation. Both of these architectural changes permit an increase in OLED efficiency and a pathway around the deleterious impact of SOP‐induced polaron accumulation.

Conformal SnO2 and NiOx charge transport layers for pragmatic air-processed perovskite solar cells

The industrialization of perovskite (PVK) solar cells (PSCs) necessitates simple manufacturing, cheapness, nontoxicity, high efficiency, and stability. We report an air-processed lead halide PSCs with cheap, nontoxic, stable NiOx and SnO2 as the hole and electron transport layers separately, to meet all the above commercial requests. By preparing the NiOx (water-dispersible) and SnO2 (oil-dispersible) quantum dots (QDs) with good conductivity and energy band matched well with that of PVK, we fabricate inverted PSCs (ITO/NiOx/PVK/SnO2/Ag) with efficiency up to 20.1/14.6% for active areas of 0.0707/1 cm2, under 1 sun simulated illumination with enhanced UV light ratio. The UV-light resistant PSCs reported here will impel their application in the high altitude areas with strong UV light. Importantly, the monodisperse size and dominant (110) plane of SnO2 QDs achieved by controlling the solvothermal parameters, realize the conformal deposition and defect prevention of SnO2, respectively, thus improving the PSC efficiency and stability.

High-performance wide-bandgap perovskite solar cells using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) as the electron-transport layer

We used work-function-tuned poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the electron-transport layer for wide-bandgap (1.74 eV) perovskite solar cells (PSCs). With the coating of polyethylenimine, the work function of PEDOT:PSS was modified from −5.03 to −4.05 eV, which enabled efficient electron transportation from perovskite absorber to cathode. With this technique, the standard-structured n-i-p PSCs showed an average power-conversion-efficiency (PCE) of 18.6 %, which is higher than that (17.5 %) of inverted PSCs with PEDOT:PSS as the hole-transport layer. Moreover, the long-term stability of the PSCs was enhanced with the PCE degradation significantly suppressed from 29.7 % to 15.6 % after a 10 days’ measurement in ambient.

Reaction Dynamics between Formamidinium Lead Iodide and Copper Oxide.

Copper oxide (CuOx) has been announced as a very promising hole-transporting layer for perovskite solar cells. However, in our previous work, we have shown that once a formamidinium lead triiodide (FAPI) perovskite is spin-coated on a spray-coated cuprous oxide (Cu2O) substrate, the Cu2O diffuses into and reacts with the FAPI film. In order to verify if the degradation products are related to the oxidation state of CuOx and/or its preparation method, in this work, we first prepared CuOx films by thermal oxidation at temperatures ranging from 120 to 300 °C. While increasing the process temperature, a transformation from copper I (Cu2O) to copper II (CuO) oxidation states was observed. For both oxidation states of copper, FAPI perovskite degradation was found; however, some alterations in the reaction products were noticed. In contrast to our expectations, the introduction of an ultrathin plasma-enhanced atomic layer deposited Al2O3 layer in between both films only partially blocked the CuOx migration into the FAPI film. It can be concluded that regardless of the chemical composition and/or preparation method of CuOx, the overlayered FAPI film gets decomposed. In order to use CuOx as a hole-transporting layer in solar cells, new strategies must be developed to limit these unwanted chemical reactions.

Regulation of the Buried Interface to Achieve Efficient HTL-Free All-Inorganic CsPbI2Br-Based Perovskite Solar Cells.

The large voltage loss (Vloss) mainly stems from the mismatch between the perovskite film and electron transport layer in CsPbI2Br-based all-inorganic perovskite solar cells (I-PSCs), which restricts the power conversion efficiency (PCE) of devices. To address this issue, potassium benzoate (BAP) is first introduced as a bifunctional passivation material to regulate the TiO2/CsPbI2Br interface, reduce the Vloss, and improve the photovoltaic performance of CsPbI2Br-based I-PSCs. Eventually, the champion PCE of CsPbI2Br-based I-PSCs without a hole transport layer modified by BAP (Target-PSCs) improves to 14.90% from the 12.14% of reference PSCs. The open-circuit voltage (Voc) increases to 1.27 V from the initial 1.14 V after BAP modification. A series of characterizations show that BAP modification can not only optimize the energy level alignment of I-PSCs but also passivize the surface defects caused by uncoordinated Cs+/Pb2+. Moreover, the Target-PSCs without encapsulation demonstrate better thermal stability, which can maintain 107.6% of the original PCE after annealing at 160 °C for 140 min in humid air. While the reference PSCs only maintain 76.5% of their initial PCE after annealing at the same process. This work provides a simple strategy to modify the buried interface and improve the performance of CsPbI2Br-based I-PSCs.

Enhancing SrZrS3 perovskite solar cells: A comprehensive SCAPS-1D analysis of inorganic transport layers

In the quest for sustainable energy, perovskite solar cells have emerged as promising candidates due to their high power conversion efficiencies and excellent optoelectronic properties. This study focuses on SrZrS3, a lead-free chalcogenide perovskite, and its integration with various inorganic transport layers for enhanced photovoltaic performance. Using SCAPS-1D simulation software, the effects of different electron transport layers (ETLs) and hole transport layers (HTLs) on device efficiency were systematically explored. The device with a-Si:H as the HTL and ZnS as the ETL (aSi-3) shows the highest efficiency of 20.01 % resulting from better energy band alignment and reduced recombination losses. This study highlights the importance of optimizing transport layers for enhancing SrZrS3-based solar cells, offering insights for developing high-performance, lead-free perovskite solar cells.

Numerical investigation of graphene derivatives as HTL in PBDB-T:NCBDT bulk heterojunction organic solar cell

The damaging effects of the most widely used PEDOT:PSS have led researchers to search for an alternative hole transport layer (HTL) in non-fullerene acceptor (NFA) based bulk heterojunction organic solar cells (BHJOSC’s). This work highlights the numerical simulation study of possible new approach of utilizing graphene, graphene oxide (GO), reduced graphene oxide (rGO), and p type doped graphene (p-graphene), as an alternative to PEDOT:PSS in non-fullerene acceptor based bulk heterojunction organic solar cell (NFABHJOSC) with PBDB-T as donor and NCBDT as acceptor using SCAPS1D. NCBDT is an emerging small-molecule non-fullerene acceptor (SM-NFA) whose bandgap can be tuned and reduced, making it a better option as an acceptor. Validation of the software is done by matching simulation results with experimental results. Diverse electron transport layers (ETL’s), including PDINO, ZnO, IGZO, TiO2, TiO2:gr (20%) composite, and TiO2:gr (10%) composite, are introduced, and device performance of various configurations is analysed. Optimization of the best device configuration ITO (4.8 eV)/rGO/PBDB-T:NCBDT/PDINO/Al gave an improved efficiency of 17.47%, fill factor (FF) of 77.45%, short circuit current density (Jsc) of 25.407 mA/cm2, and an open circuit voltage (Voc) of 0.8878 V.

Functionalized porphyrin as a carrier bridge and a passivator for perovskite solar cells

Interfacial modification becomes one of the most emerging strategies in the state-of-the-art perovskite solar cells (PSCs). Here, two porphyrin derivatives (p-MeOTPP, m-DMeOTPP) with different methoxy groups are employed as the modifiers between perovskite and the hole transporting layer (HTL). The interaction at both of perovskite/modifier and modifier/HTL interfaces, and the hole-transfer ability of the modifier molecule are comprehensively studied, which are jointly responsible for the significant improvement in the optoelectronic properties of the device. More importantly, discussion on a molecular level reveals the distinct roles of modifiers with highly similar structures. A highest power conversion efficiency (PCE) of 24.56% is achieved for m-DMeOTPP modified PSCs, which is attributed to a synergy of efficient passivation effect at perovskite/modifier interface, sufficient built-in potential at modifier/HTL interface and minimal reorganization energy of hole hopping process. p-MeOTPP has a stronger passivation ability, but causes unfavorable interfacial dipole at modifier/HTL interface and a large energy barrier when hole’s hopping, only resulting in a smaller enhancement. Additionally, both modifers improve the device stability by strongly suppress the immigration of iodine species and moisture penetration. This work presents a synergy mechanism for interfacial engineering to pursue PSCs with high efficiency and stability, and provides practical guidelines at a molecular level to design a modifier not only as an efficient passivator but also as a high-speed carrier bridge.

Enhanced Carrier Selectivity and Superior Passivation in Cost‐Effective Heterogeneous Hybrid Transport Layers for Next Generation Silicon Solar Cells

AbstractHybrid hole transport layers (HHTLs) consisting of‐a low‐cost, low‐temperature, dopant‐free, organic, hole‐selective poly(3,4‐ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) and an inorganic homogenous tunnel oxides (HoTOs), such as SiOx, TiOx, and AlOx have allowed significant improvements in carrier selectivity as well as in power and cost efficiency. These key parameters can be further improved by introducing heterogeneous tunnel oxides (HeTOs), formed of two different inorganic oxides. This work examines the potential of heterogeneous HHTLs and their impact on passivation as well as on the interfacial properties in silicon solar cells. HeTOs of SiOx/TiOx and SiOx/AlOx with negative fixed charge are studied and optimized. A heterogeneous HHTL with AlOx (>1.5 nm) resulted in a high lifetime (1.2 ms), low surface recombination velocity, and improved surface band bending at the interface due to the presence of higher negative fixed charge (1012 cm−2) within these layers. The passivation properties further improved on light soaking and annealing due to oxidation of PSS and AlOx/SiOx. Overall, these HHTLs demonstrated a hole selectivity (S10) of 12.5 and very high efficiencies up to of 26.1%, surpassing homogenous‐SiOx/PEDOT:PSS by 1.4%.

Inverted All-Inorganic Nanorod-Based Light-Emitting Diodes via Electrophoretic Deposition.

High performance and high stability in all-inorganic solution processed nanocrystal-based light-emitting diodes (LEDs) are highly attractive for large area devices compared to organic material-based LEDs. In this work, an inverted all-inorganic LED structure is designed to have an easy integration with thin-film transistors. Adopting robust inorganic materials such as Ni1-x O nanoparticle films as a hole transport layer (HTL) is beneficial for the performance of LED. Herein, we have optimized the HTL by introducing Mg into Ni1-x O to bridge the difference in energy offset between the nanorod emissive layer and the HTL, in addition to the advantages of low temperature solubility of Ni1-x O:Mg nanoparticles. Furthermore, CdSe/CdS-based nanorods via electrophoretic deposition (EPD) are amassed in a vertically aligned (VA-NR) fashion as an emissive layer to facilitate the carrier transportation. Fostering these approaches enabled an EQE of 1.2% of the fabricated device, establishing the viability for further development of efficient and highly stable nanocrystal-based LEDs.

Transient Leakage Electroluminescence of Quantum-Dot Light-Emitting Diodes.

Parasitic emission or leakage emission caused by electron leakage to a hole transport layer in quantum-dot light-emitting diodes (QLEDs) critically impacts device efficiency and operational stability. The buildup dynamics of such emission channels, however, was insufficiently researched. Herein, we investigate transient electroluminescence dynamics of leakage emission in red/green/blue (R/G/B) QLEDs and reveal notable contrast for R and G. In RQLEDs, leakage emission exhibits delayed turning-on than primary emission, which is attributed to much slower filling up from lower-energy electron states and the initial quenching by nonradiative recombination with trapped holes. For GQLEDs, leakage emission turns on much more concurrently, and emission preferably starts from higher energy states. For R/G QLEDs, under varied offset voltage modulation, the current efficiency of primary emission is invert-correlated to leakage emission, reconfirming leakage emission as a loss indicator.

Organic Salt Buffer Layer Enables High‐Performance NiOx‐Based Inverted Perovskite Solar Cells

The merits of a low‐cost fabrication process, suitable band structure, excellent wettability to perovskite precursor, and outstanding stability ensure NiOx as a hole transport material with beneficial characteristics to construct high‐performance perovskite solar cells (PSCs). However, direct contact between perovskite and NiOx causes delamination and chemical instability and thus results in poor carrier transport and short device lifespan. Here, we propose a solution for this issue by introducing an organic salt additive 4‐(trifluoromethyl) benzylammonium formate (TFMBAFa) in the perovskite precursor to passivate perovskite film and NiOx@(2‐(3,6‐dimethyl‐9H‐carbazol‐9‐yl) ethyl) phosphonic acid (Me‐2PACz) composited hole transport layer (HTL), and thus construct a buffer layer between perovskite‐HTL interface. The effective diminishing of NiOx/perovskite interfacial reactions and defects results in enhanced carrier transport. Consequently, the target device achieves simultaneous improvements in power conversion efficiency (24.2%), storage stability (T100 > 1400 h), thermal stability (T80 > 1000 h), and operational stability (T70 > 850 h), where T100, T80, and T70 refer to the retention of 100%, 80%, and 70% of initial PCE, respectively. This work provides an effective strategy to advance the performance of NiOx‐based inverted PSCs.

Performance optimization of FASnI3 based perovskite solar cell through SCAPS-1D simulation

The article provides a detailed look at the fabrication of a high-performance structure for FASnI3-based perovskite solar cells (PSCs). The FTO/CeO2/FASnI3/CuI/Au structure is designed using the Solar Cell Capacitance Simulator in One Dimension (SCAPS-1D) to investigate the fabricated PSC's performance. This investigation's main objective is to improve PSCs performance by using non-traditional Hole transport layer (HTL) and Electron transport layer (ETL) materials, such as CeO2 and CuI, which have both been the subject of limited research. Moreover, the investigation seeks to determine the impact of several perovskite layer characteristics, including bandgap (Eg), electron affinity (χ), acceptor density (NA), thickness (t), and defect density (Nt). Additionally, this study also investigates the effect of various back contact work functions. Significant improvements in solar cell parameters, such as power conversion efficiency (PCE) from 22.06% to 24.87% and current density (Jsc) from 26.0274 to 30.675 mA/cm2, were observed by optimizing the device's parameters. In contrast, the fill factor (FF) and open circuit voltage (Voc) decreased their values from 86.13% to 87.10% and 0.9843 to 0.9308 V, respectively. These findings show that our designed solar cell structure performs better than those with conventionally used HTLs and ETLs. Consequently, this study highlights the potential benefits of lead-free PSCs and presents fresh opportunities for their development and use in various solar applications.

Hierarchical Nanostructures CuBi2O4 Integrated with Polyaniline Nanofibrous for Boosting Hole Extraction in Carbon-Based Perovskite Solar Cells.

Toward commercialization of carbon-based perovskite solar cells (C-PSCs), it is crucial to innovatively design inorganic hole transport layer materials that excel in extracting and transporting charge carriers to promote their photoelectric conversion efficiency (PCE). In this work, a novel and high-connectivity CuBi2O4-polyaniline nanofibrous (CuBi2O4-PN) reticular structure is created by integrating CuBi2O4 hierarchical microspheres (CuBi2O4 MS) with polyaniline nanofibrous. The introduction of CuBi2O4-PN as a hole transport layer (HTL) notably enhances the contact quality of the devices and substantially reduces the surface defects of C-PSCs. In a comparative analysis under identical experimental conditions, MAPbI3 devices incorporating CuBi2O4-PN HTL demonstrated a PCE of 14.79%, achieving a 44.3% increase over the reference device (10.25%). CuBi2O4-treated C-PSCs retained 89.9% of their original PCE after 45 days in storage, and they demonstrated improved stability over a longer time frame. This remarkable improvement in device performance can be attributed to the effective suppression of nonradiative recombination and the enhancement of the carrier transfer process in the device. Additionally, the unique interconnected reticular structure of CuBi2O4-PN provides efficient pathways for hole transfer, significantly contributing to the enhanced efficiency of the device.

Efficient and stable perovskite solar cells based on multi-active sites 5-amino-1,3,4-thiadiazole-2-thiol modified interface

The highest certification efficiency of perovskite solar cells (PSCs) has reached 26.7 %. However, the high defect density on the surface of perovskite films prepared by low temperature solution method and the energy mismatch between the carrier transport layers and perovskite layer (PVK) greatly limit the performance improvement of PSCs. The introduction of passivating agent to modify the perovskite interface and grain boundary can reduce the defect density, coordinate the energy level effectively, and improve the efficiency and stability of devices. A Lewis base molecule 5-amino-1,3,4-thiadiazole-2-thiol (AMTD) with multiple active sites is introduced at the interface between PVK and hole transport layer (HTL). The electron-rich groups, such as = S, –S–, –NH2, –N on AMTD, passivate the positive electrical defects on the interface and grain boundary, and increase carrier transport efficiency. The interfacial energy level array is optimized to achieve more efficient charge transportation. In addition, the modified of AMTD has a significant protective effect on the perovskite, which inhibit the moisture erosion of in environment. Consequently, the AMTD-optimized device achieves a power conversion efficiency (PCE) of 24.13 %, compared to the efficiency of 21.62 % for pristine device. The stability of the devices is improved greatly.

Study on the enhancement of device performance by the action of carbonohydrazide at the buried bottom interface of inverted mesoporous perovskite solar cells

The interface of perovskite solar cells (PSCs) significantly influences their efficiency and stability. Researchers tend to pay more attention to the upper surface of the perovskite absorber layer and conduct relatively less research on the bottom buried interface, mainly because the bottom of the perovskite film is challenging to peel off, which increases the difficulty of the characterization of the observation and analysis. Defects at the bottom interface make it more challenging to optimize the treatment than the upper surface. For inverted PSCs, the interface located in direct contact between the light absorption layer of perovskite and the hole transport layer is the buried interface. The buried interface is the direct interface for transporting charge carriers of PSCs and is also the center of non-radiative compound enrichment. The defect density is higher than the defect density of perovskite film crystal. Due to the DMSO initially left in the commonly used perovskite precursor solution during the film formation process, evaporation during the annealing and crystallization process creates vacancy holes at the bottom of the perovskite layer film. These holes and crystal boundaries tend to produce many non-radiative recombinations. These holes are also the sites of photodecomposition, leading to reduced efficiency and stability in PSCs, ultimately impacting the overall performance of PSCs. The work adds an Al2O3 mesoporous layer on top of the hole transport layer (HTL), which reduces the direct contact area between PTAA and perovskite film. In the perovskite precursor solution, we incorporate a solid additive called Carbonohydrazide (CBH), this substance replaces some of the DMSO and fills in the voids at the bottom of the perovskite film that is left when the DMSO evaporates. This process helps to reduce non-radiative recombination and photodegradation caused by the voids at the bottom of the perovskite, leading to an improvement in the efficiency and stability of the cell. The optimization of the buried bottom interface has improved the cell’s average efficiency from 17.6 % to 19.7 %, an 11.9 % increase. The aperture area of the devices in this work is 0.048 cm2, and the photoelectric conversion efficiency of our device still reaches 80.64 % of the initial efficiency after 600 h of continuous heating in a glove box with a nitrogen atmosphere, maintaining a test temperature of 60 °C.

Loading Precursors into Self‐Assembling Contacts for Improved Performance and Process Control in Evaporated Perovskite Solar Cells

Organo‐lead‐halide perovskites are promising materials for solar cell applications with efficiencies now exceeding 26% for single junction, and over 33% for silicon tandem devices. Evaporation has proven viable for industrial scale‐up but presents challenges for perovskite materials. Perovskite precursor is introduced into self‐assembling MeO‐2PACz hole transport layers for application to 4 source perovskite coevaporation. This allows precursors that can be difficult to add via evaporation, like methylammonium chloride. These precursor molecules influence growth during evaporation, film behavior during annealing as measured by photoluminescence, and aid the conversion to perovskite as shown by X‐Ray diffraction. Devices have improved power conversion efficiency and stability compared to a control sample within the same evaporation. The best cells reach ≈21% efficiency and comparable performing ≈20% cells maintain their original efficiency after 1000 h of maximum power tracking at 25 °C. This process provides significant process flexibility for perovskite evaporation and requires no additional steps.

Enhanced performance of perovskite solar cell by optimization of thin film control parameters using Taguchi method

The objective of the study is to examine the influence of various thin film control parameters on the synthesis of carbon-based passivated perovskite solar cells. Passivation additives, namely MOF UiO-66-NH2 (20mg/ml) and inorganic additive KPF6 (1mg/ml), were applied to treat the perovskite solar cells simultaneously. A PSC device of a standard configuration FTO/TiO2/Perovskite layer/Additives/HTL/Carbon electrode is utilized for experimentation. The performance evaluation of the system was carried out using Taguchi method. The critical control parameters considered in the study are: type of perovskite; annealing temperature of perovskite layer; the number of perovskite coating layers; and the type of hole transport layer (HTL). Each parameter is allocated three levels. An orthogonal array was designed using the Taguchi technique to optimize the quality control parameters. The signal-to-noise (S/N) ratio and analysis of variance (ANOVA) were then utilized to determine the ideal combination of parameters. The analysis identified the significant contribution of each parameter to the efficiency of perovskite cells. Among these parameters, the type of perovskite (used in the synthesis process) was found to impact the fabrication and performance of the cells significantly. Furthermore, it was observed that the type of hole transport layer (HTL) employed in the device significantly influences its performance. The best combination for our experiment is found to have FA0.55MA0.25Cs0.20PbI3 as the perovskite-type, an annealing temperature of 120˚C, a double layer of perovskite coating and Spiro-OMeTAD as the HTL. The solar cell is developed for each experiment as well as the validation test.

Current density–voltage characteristics of exciplex-type organic light-emitting diodes expressed by a simple analytic equation

Exciplex-type bilayer organic light-emitting diodes (OLEDs) with ohmic contacts exhibited current density–voltage (J–V) characteristics that closely matched a simplified analytical model proposed by Nikitenko and Bässler. The analytical model is based on the following key assumptions: (i) complete hole–electron recombination at the interface between a hole transport layer (HTL) and an electron transport layer (ETL), (ii) ohmic contacts at the interfaces between metal electrodes and carrier transport layers, and (iii) electric-field-independent carrier mobilities in both HTL and ETL. The excellent matching shows that the simplified analytical model is sufficient to describe the J–V characteristics of the OLEDs. We also demonstrated that if the carrier mobility of one carrier transport layer is known, that of the other transport layer can be estimated using the equation derived by the simplified analytical model. The simplified analytical model provides a useful method to estimate carrier mobilities within carrier transport layers themselves in OLEDs.

Regulation of nucleation and crystallization for blade-coating large-area CsPbBr3 perovskite light-emitting diodes

Metal halide perovskite light-emitting diodes (PeLEDs) and large-area perovskite color conversion layers for liquid crystal display exhibit great potential in the field of illumination and display. Blade-coating method stands out as a highly suitable technique for fabricating large-scale films, albeit with challenges such as uneven nucleation coverage and non-uniformity crystallization process. In this work, we developed an in-situ characterization measurement system to monitor the perovskite nucleation, and crystallization process. By incorporating formamidine acetate (FAAc) into perovskite precursor solutions, the nucleation rate and nuclei density of perovskite were increased, leading to more uniform nucleation. In addition, we inserted a layer of [2-(9H-carbazol-9-yl)ethyl] phosphonic acid above the poly(9-vinylcarbazole) hole transport layer. This layer acts as an anchor for the perovskite nano-crystal nuclei formed in the precursor, enhancing the steric hindrance of the solute and subsequently slowing down the crystal growth rate, thereby improving crystal quality. Based on these improvements, large-area perovskite nano-polycrystalline films with significantly improved uniformity and enhanced photoluminescence quantum yield were obtained. A small-area PeLED (2 mm × 2 mm) with a maximum external quantum efficiency of 25.91% was realized, marking the highest record of PeLED prepared by blade-coating method to date. An ultra-large-area PeLED (5 cm × 7 cm) was also prepared, which is the largest PeLED prepared by the solution method reported so far.