ZnO ETL Research Articles

Enhanced Performance of Inverted Perovskite Quantum Dot Light-Emitting Diode Using Electron Suppression Layer and Surface Morphology Control.

The energy level offset at inorganic layer-organic layer interfaces and the mismatch of hole/electron mobilities of the individual layers greatly limit the establishment of balanced charge carrier injection inside the emissive layer of halide perovskite light-emitting diodes (PeQLEDs). In contrast with other types of light-emitting devices, namely OLEDs and QLEDs, various techniques such as inserting an electron suppression layer between the emissive and electron transport layer have been employed as a means of establishing charge carrier injection into their respective emissive layers. Hence, in this study, we report the use of a thin layer of Poly(4-vinylpyridine) (PVPy) (an electron suppression material) placed between the emissive and electron transport layer of a halide PeQLEDs fabricated with an inverted configuration. With ZnO as the electron transport material, devices fabricated with a thin PVPy interlayer between the ZnO ETL and CsPbBr3 -based green QDs emissive layer yielded a 4.5-fold increase in the maximum observed luminance and about a 10-fold increase in external quantum efficiency (EQE) when compared to ones fabricated without PVPy. Furthermore, the concentration and coating process conditions of CsPbBr3 QDs were altered to produce various thicknesses and film properties which resulted in improved EQE values for devices fabricated with QDs thin films of lower surface root-mean-square (RMS) values. These results show that inhibiting the excessive injection of electrons and adjusting QDs layer thickness in perovskite-inverted QLEDs is an effective way to improve device luminescence and efficiency, thereby improving the carrier injection balance.

Enhancing the Efficiency of Inverted Organic Solar Cells with Treatment Techniques: Numerical and Experimental Study

The combination of UV-ozone and Al-doping enhancements in the ZnO ETL layer was studied for fullerene-based (P3HT:PCBM) and nonfullerene-based (J71:m-ITIC) solar cells. The study focused on morphological and interfacial defects and their relation to the electrical and optical properties of different Al-doped ZnO layers. The study also investigated the surface recombination velocity at the interface of the ZnO/active layer, as well as the carrier lifetime and diffusion length of the active layer theoretically. The most effective Al-doping dosage was 3% for P3HT:PCBM and 2% for J71:m-ITIC, attributed to the varying sensitivity of the active layer materials to morphological enhancement.

Impact of Al on ZnO Electron Transport Layer in Perovskite Solar Cells

Perovskite solar cells have shown remarkable performance and improvements in terms of solar cell efficiency. The ETL material is one of the important components in perovskite solar cells in conducting electrons to produce current. Here, ZnO was used as ETL material in a perovskite solar cell using the SCAPS 1D simulation software. The ZnO ETL showed poor cell efficiency due to its reaction with the perovskite material. A small amount of Al doped into ZnO was introduced to enhance the physiochemical properties of the ZnO against perovskite materials. Al concentrations were varied between 1 and 4 mol% to observe the effect on cell efficiency. Compared with a conventional ZnO ETL solar cell with 0 mol% Al perovskite, the Al-doped based solar cell showed better performance. Meanwhile, perovskite solar cells with 1 mol% Al-doping and appropriate layer thickness showed the best cell performance in improving the charge transport mechanism, resulting in increased cell efficiency. Thus, the parameters studied can be a guide in the fabrication process.

Performance analysis of WSe2-based bifacial solar cells with different electron transport and hole transport materials by SCAPS-1D

In recent years, solar cells made of tungsten diselenide (WSe2) have received comprehensive consideration because of their good photoelectric properties. The planar WSe2-based heterojunction solar cell with a preliminary device structure of Au/WSe2/electron transport layer (ETL)/FTO/Al was designed and investigated numerically by SCAPS-1D. CdS ETL is widely used in thin film solar cells (TFSCs). Due to environmental issues and the low band gap (2.42 eV) of CdS ETL, an alternative to CdS ETL was being explored for WSe2 solar cells. In this work, the photovoltaic (PV) performance of the WSe2-based TFSCs with different ETLs were simulated, analyzed and compared, in an attempt to track down a suitable substitute for the CdS ETL. In addition to CdS ETL, ZnO, TiO2 and SnO2 ETLs were independently used to simulate the PV performance of WSe2-based TCSCs. In the wake of analyzing the J-V curves of different cell configurations, SnO2 ETL yielded the best results with PCE of 27.14 % for the single-junction WSe2/SnO2 TFSC. Then, our simulation predicted that the PV performance of the WSe2 device can be improved significantly by using N doped Cu2O as a hole transport layer (HTL). The optimized WSe2 device with SnO2 ETL and Cu2O:N HTL showed an improved PCE of 33.84 % with very good performance stability at higher temperature. Furthermore, this article proposes to use the Au/Cu2O:N/WSe2/SnO2/FTO/Al heterojunction solar cell in bifacial mode and PV performance of the proposed bifacial device have been also studied using SCAPS-1D. Bifacial WSe2 device leads to enhanced PV performance with bifaciality factor for PCE is 83.64 %. Bifacial gain of the proposed device under simultaneous irradiation of 1 sun from the front and 20 % of 1 sun from back side is found to be 13.95 %. Our simulation predicts that the proposed WSe2 bifacial solar cell is capable of converting solar energy into electricity with an efficiency of about 38.38 %.

Effects of vertically aligned ZnO nanorods surface morphology on the ambient-atmosphere fabricated organic solar cells

The present study evaluates the effects of ZnO nanorods electron transport layer morphology synthesized using the low temperature chemical bath method on the performance of organic solar cells. The photoactive layers consisted of P3HT as electron donor and PCBM as electron acceptor. Two ZnO nanorods electron transport layers were studied, one with a smooth surface and a decorated surface. Well-defined ZnO nanorods with perfect hexagonal shapes could be synthesized by chemical bath deposition. Structural characterization confirmed that the vertically aligned ZnO nanorods were successfully obtained in pure form. Performance analysis revealed that the use of decorated ZnO nanorods increased the overall performance relative to the uncoated ZnO nanorods, attributed to the possible increase of light paths on the ZnO nanorods. The best device was obtained from annealed decorated ZnO nanorods ETL having PCE of 7.38%, Voc of 0.63 V, Jsc 18.44 mAcm−2 and FF of 0.60. Solar cells made from decorated ZnO nanorods ETL presented high Rsh of 1311.67 Ω.cm2 and 1497.23 Ωcm2 for as-synthesized and annealed ZnO ETL, respectively. In general, the strategy of decorating ZnO nanorods ETL is an important strategy to fabricate highly efficient organic solar cells and thus may have good potential for many other organic solar cells comprising other polymers apart from P3HT. Even though the efficiency obtained is low compared to the PCE achieved so far, this study gives the observation that may be possibly useful in the further improving PCE of already achieved high efficiencies.

An efficient organic and inorganic hybrid interlayer for high performance inverted red cadmium-free quantum dot light-emitting diodes.

The efficiency and device lifetime of quantum dot light-emitting diode (QLED) devices suffer from the charge imbalance issue resulting from excess electron injection from the ZnO electron transport layer (ETL) to the quantum dot (QD) emissive layer (EML). Herein we report 1,3-bis(9-phenyl-1,10-phenanthrolin-2-yl)benzene (BPPB) small molecule ETL blended ZnO as an interlayer in inverted red indium phosphide (InP) based QLEDs to slow down the ZnO ETL mobility. The device with ZnO : BPPB (15 wt%) shows 16.7% external quantum efficiency and 595 h lifetime (T50) at 1000 cd m−2. Insertion of a thin hybrid interlayer reduces the electron injection to match the charge balance in the QD layer and also suppresses the interfacial exciton quenching between the ZnO ETL and QD EML.

Inorganic Lead-Free B-γ-CsSnI3 Perovskite Solar Cells Using Diverse Electron-Transporting Materials: A Simulation Study.

B-γ-CsSnI3 perovskite solar cells (PSCs) are simulated employing diverse electron-transporting layers (ETLs, including TiO2, ZnO, SnO2, GaN, C60, and PCBM), and a comparative study has been made. Both regular and inverted planar structures are simulated. Effects of the thickness of absorbers and ETLs, doping of ETLs, and interface trap states on the photovoltaic performance are studied to optimize the device structures. The regular structures have larger short-circuit current density (Jsc) than the inverted structures, but the inverted structures have larger fill factor (FF). All of the simulated optimal PSCs have similar open-circuit voltages (Voc) of ∼0.96 V. The PSCs with TiO2 ETLs have the best photovoltaic performance, and the optimum structure exhibits the highest efficiency of 20.2% with a Voc of 0.97 V, Jsc of 29.67 mA/cm2, and FF of 0.70. The optimal PSCs with ZnO, GaN, C60, and PCBM ETLs exhibit efficiencies of 17.88, 18.09, 16.71, and 16.59%, respectively. The optimal PSC with SnO2 ETL exhibits the lowest efficiency of 15.5% in all of the simulated PSCs due to its cliff-like band offset at the SnO2/CsSnI3 interface. Furthermore, the increase of interface trap density and capture cross section is found to reduce the photovoltaic performance of PSCs. This work contributes to designing and fabricating CsSnI3 PSCs.

Facile preparation of a polymer-ZnO composite colloid as an electron transport layer and its effects on inverted polymer solar cells

A polymer-ZnO colloidal solution was prepared as the organic-inorganic composite electron transport layer of inverted polymer solar cells (i-PSCs) by successive modification of ZnO particles with organic dispersant and polyethyleneimine (PEI), which offers an alternative to previously used complicated processes such as high temperature annealing or multi-layer coatings. The modification of ZnO surfaces with 2-(2-methoxyethoxy) acetic acid (MEA) provides colloidal stability and prevents aggregation of ZnO particles after adding PEI into the ZnO colloidal solution. The amount of incorporated PEI was increased to ~10 wt%, and the cell performance parameters of Jsc and fill factor (FF) were enhanced proportional to PEI content because of its contribution in decreasing the work function of ZnO ETL. By fabrication of i-PSCs with the modified ZnO ETL, the power conversion efficiency (PCE) was increased by ~40% from 5.10% to 7.51% in the ITO/ETL/PV-D4610:PC60BM/MoO3/Ag structure. The multi-functionalized ZnO ETL provides a more facile process for preparation of an organic-inorganic composite ETL to improve the performance of i-PSCs.

Improvement of the interfacial contact between zinc oxide and a mixed cation perovskite using carbon nanotubes for ambient-air-processed perovskite solar cells

(a) The sandwich structure of the planar device based on the ZnO ETL and fully-processed in ambient air. (b) Significant improvement in the current density of the PSCs after using 1D carbon nanotubes in the ZnO ETLs.

Improve the oxide/perovskite heterojunction contact for low temperature high efficiency and stable all-inorganic CsPbI2Br perovskite solar cells

Oxide/perovskite heterojunction contact is important for realizing high performance and stable perovskite solar cells (PSCs). However, the development of ZnO/perovskite heterojunction contact is plagued by the poor chemical compatibility at ZnO/organic-inorganic perovskite interface. Nevertheless, the ZnO/inorganic perovskite heterojunction contact exhibits potential applications in all-inorganic PSCs. In this study, all-inorganic CsPbI2Br PSCs based on ZnO/perovskite heterojunction contact have been successfully realized at low temperature process below 150 °C and delivered an excellent power efficiency conversion of 14.78% with a high open-circuit voltage (Voc) of 1.21 V and a truly extraordinary fill factor (FF) of 81.42%. Compared to SnO2-based PSCs, the better band alignment between ZnO ETL and perovskite contributed larger built-in potential, resulting in superior electron extraction capability and effective interfacial recombination suppression. The better perovskite film quality and improved interface contact of ETL/perovskite contributed to better air and thermal stability of ZnO-based perovskite devices. These researches have proved the unlimited possibilities for ZnO as ETL in all-inorganic perovskite solar cells.

Functionalization of fullerene by polyethylene glycol toward promoted electron transport in inverted polymer solar cells

Abstract A novel polyethylene glycol-functionalized fullerene derivative (C60-PEGA) was synthesized by a facile one-step nucleophilic addition reaction. C60-PEGA possessed good solubility in methanol and was estimated as C60-(C8H18N)13H13O with average PEG moiety of 13 by 1H NMR, FT-IR and X-ray photoelectron spectroscopy (XPS). C60-PEGA was applied as an ETL to construct inverted bulk heterojunction polymer solar cells (inverted BHJ-PSCs) based on photoactive layers of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3, 4-b]thiophene-2-carboxylate]: [6,6]-phenyl-C71-butyric acid methyl ester (PTB7-Th: PC71BM), which achieve the best PCE of 9.25%, surpassing that of reference device based on the ZnO ETL (8.61%). The higher ETL performance of C60-PEGA ETL in BHJ-iPSC device relative to that of the ZnO ETL was attributed to the increase of electron mobility and effective electron transport from the active layer to the ITO cathode because that the reduced work function (WF) of ITO via the modification of C60-PEGA leads to the increase of short-circuit current density (Jsc) and consequent PCE.

Interlayer Modification Using Eco-friendly Glucose-Based Natural Polymers in Polymer Solar Cells

Applying biomaterials in optoelectronic devices has recently received great research interests since they not only possess economic benefits but also can facilitate the sustainable development of technology. We herein investigated the effectiveness of glucose-based biopolymers as zinc oxide surface modifiers in inverted OPVs by rationally studying chitosan, methyl-cellulose, and dextrin. Owing to the proper side-group and configurational modification, these three biopolymers possess better solution processability and film-formation capability than the pristine cellulose. Besides, their abundant availability in the environment renders them to be easily accessible and more economical as compared to other commonly used polymeric interlayers. Our results reveal the critical structure–performance relationship of these glucose-based biopolymers and their derived OPVs. In particular, the “β-type” glucose-based polymer, methyl-cellulose, was demonstrated as the most efficient modifying interlayer for ZnO ETL, whi...

Electrospun 3D composite nano-flowers for high performance triple-cation perovskite solar cells

Three dimensional (3-D) flower-shaped SnO2-TiO2 nano-structure has been synthesized by electro-spinning and incorporated on top of sol-gel ZnO ETL to fabricate highly efficient (highest efficiency: 17.25%) triple-cation (methyl ammonium, formamidinium and rubidium cations) based perovskite solar cell (PSC). The flower-based PSCs demonstrate superior photovoltaic performance compared to control ZnO or one-dimensional (1-D) fiber-shaped nano-structure ETL based devices. Nano-structured ETLs passivate the interstitial trap sites in pristine ZnO by intercalation of metal atoms in host ZnO lattice matrix and increase the n-type conductivity of the (nano-structured) ETL films by reducing the functional groups on ZnO surface. The accumulated ions at the perovskite/ETL interface are also well-distributed and hence the accumulation capacitance is significantly reduced in nano-structured ETL based PSCs, due to the branch-structured ETL network. Moreover, the nano-flower based PSC demonstrates superior charge transfer property, compared to nano-fiber based PSC owing to enhanced material crystallinity and higher effective surface area of 3-D nano-flower network, with respect to 1-D nano-fiber structure. The photo-current hysteretic phenomena are also most suppressed in nano-flower based PSC, due to mitigated electrode polarization mechanism in it. Adding to the merits, PSCs incorporating nano-flower ETL demonstrate enhanced device stability compared to the control devices, retaining about 92% of its initial efficiency even after a month. The enhanced device stability with nano-flower based PSC is contributed by the lower hydrophilicity, lower extent of functional surface hydroxyl group and lower content of vacant interstitial trap sites of the respective ETL film.

High Efficiency Low-Temperature Processed Perovskite Solar Cells Integrated with Alkali Metal Doped ZnO Electron Transport Layers

Herein, we achieved, air-stable low-temperature processed PSC (L-PSC) using alkali-metal modified ZnO ETLs. Using a simple chemical alkali-metal modification method, the surface defects of the ZnO were effectively passivated. As a result, the interfacial decomposition reactions were suppressed, while raising the Fermi energy level and enhancing electron mobility. The improved interfacial charge transfer and internal electric field in the developed L-PSC using K modified ZnO (ZnO-K) exhibited an improved power conversion efficiency (PCE) of 19.90% with negligible hysteresis, while a pristine ZnO based L-PSC exhibited a PCE of 16.10% with significant hysteresis. The ZnO-K based L-PSC also exhibited remarkably higher long-term air-storage stability (91% retention after 800 h) than pristine ZnO based L-PSCs (36% retention after 800 h) due to the suppressed decomposition reactions. The PCE and air stability of our L-PSC with the modified ZnO are among the highest reported for PSCs processed at ≤150 °C.

Interface dipole for remarkable efficiency enhancement in all-solution-processable transparent inverted quantum dot light-emitting diodes

We report our efforts to develop high performing all-solution-processable transparent inverted QD-LEDs by interposing an interface dipole between the ZnO ETL and the quantum dot light-emitting layer.

High crystallization of a multiple cation perovskite absorber for low-temperature stable ZnO solar cells with high-efficiency of over 20.

ZnO as a promising electron transport layer (ETL) to TiO2 for perovskite solar cells (PSCs) has achieved a power conversion efficiency (PCE) of 18.9%; however, this is still lower than that obtained for TiO2-based PSCs (higher than 20%). Herein, we report the fabrication of high-efficiency methylammonium (MA) and Cs co-alloyed formamidinium (FA) triple cation perovskite based ZnO PSCs via delicate control of the cation compositions and annealing temperatures. By virtue of structural, morphological, spectral and electrochemical characterizations and analysis, we found that the incorporation of MA and Cs into FA perovskite enables the formation of a highly crystalline black phase perovskite with reduced surface roughness, which inhibits charge recombination and promotes electron transfer at the ZnO/perovskite/spiro-OMeTAD interfaces and hence improves Jsc and FF values of the cell. As a result, the ZnO PSC based on MA0.1FA0.75Cs0.15PbI3 annealed at 110 °C achieved a PCE as high as 20.09%, exceeding the previous highest efficiency recorded for ZnO ETL based PSCs. The optimized MA0.1FA0.75Cs0.15PbI3 material demonstrated excellent reproducibility and long-term cell durability under ambient conditions within 1000 h. Particularly, the incorporation of a small amount of Br into the triple cation perovskite, i.e., MA0.1FA0.75Cs0.15PbI2.9Br0.1 led to a further enhancement in PCE of up to 20.44%, which is comparable with the best-performing MA and Cs-containing FA-based lead halide TiO2 PSCs.

Efficient organic solar cells employing ytterbium ion-doped zinc oxide as cathode transporting layer

Abstract Rare earth ions possess highly conductive, magnetic, electrochemical and luminescent properties, multiple valence electrons and long lifetime of excited state, thus could be alternatives as dopant to tune and promote the cathode interlayer property in solar cells. In this work, we study the modification of cathode interlayer by rare earth ion doping, based on the model of Yb doped ZnO by sol-gel method. A relative low doping concentration (below 1%) can improve the electron transport of ZnO and the solar cell performance. This is also confirmed by the obtained photocurrent density, excitons generation rate and electron mobility of device. The best PCE 11.04% (with VOC 0.940 V, JSC 16.58 mW cm−2, FF 70.7%) is achieved with 0.5% Yb doping, based on PBDB-T:IT-M active layer, compared to the reference PCE of 10.19% with pure ZnO ETL. While with concentration over 1%, the Yb doping decreases the device performance.

Role of organic interfacial modifiers in inverted polymers solar cells: An in-depth analysis of perylene vs fullerene organic modifiers

Interfacial issues can significantly restrict the performance of photovoltaic devices by exacerbating the charge recombination channels, macroscopic phase separation, and providing a non-ideal contact for selective extraction of charges particularly in photovoltaic devices using organic and inorganic materials together. Organic interfacial modifiers (IMs) are often used to mitigate these issues by modifying the organic-inorganic interface. In order to extricate the role of these IMs on the photovoltaic performance we have made a comprehensive study on the application of perylene-based and fullerene small molecules having different molecular origin as organic IMs on ZnO electron extracting layers in inverted BHJs photovoltaic devices. We report an elaborate study on the electronic and surface altering properties of these IMs and correlated their effect on the different PV performance parameters of the inverted BHJ solar cells employing P3HT: PCBM photoactive layer. Our investigations demonstrate the role of these organic IMs in reducing the ZnO cathode work function and increasing its electron transportation property along with the passivation of superficial traps states present on ZnO which helps in selective extraction of charge carriers from the devices and minimize the recombination losses. These different aspects of IMs compete and their balanced effect decides the final outcome. As a result, we obtain a substantial improvement in the device performance with power conversion efficiency (PCE) of 3.0% for the C70/ZnO cathode device which shows over 60% improvement in contrast to the devices without any ZnO surface modification. The present investigation intents to exhibit the feasibility of vacuum sublimated organic small molecules in performance improvement in BHJ solar cells utilizing the ZnO ETLs and contrast their efficacy for the purpose rather than setting any benchmark device performance although the efficiencies obtained are typical for the active layer used in the study.

High‐Efficiency Organic Tandem Solar Cells With Effective Transition Metal Chelates Interconnecting Layer

Organic tandem solar cells have two sub‐cells vertically connected via a tunnel junction (or interconnecting layer, ICL), which the ICL plays a critical role of determining the performance of the tandem cell. In this manuscript, we report a new design of the ICL consisting of zirconium (IV) acetylacetonate (Zr‐Acac) as the electron transport layer. The Zr‐Acac has a suitable work function to pin the Fermi‐level of the ICL and hence, can effectively behave as the charge recombination function for the tandem device. Comparing to the reference device consisting of ZnO ETL layer, the Zr‐Acac ICL can effectively enhance the efficiency by 20%, from 8.4 to 10.1% power conversion efficiencies. More importantly, the Zr‐Acac ETL layer functions effectively without thermal annealing, which not only simplifies the tandem solar cell processing, but also widens the front sub‐cell material selection.

Controlled Ostwald ripening mediated grain growth for smooth perovskite morphology and enhanced device performance

Here we report, a novel two-step dipping technique via post-immersion polar solvent engineering for controlled secondary grain growth (Ostwald Ripening) to fabricate efficient mixed organic cation based MA0.6FA0.4PbI3 perovskite solar cell (PSC) in conjunction with low temperature (140°C) processed sol-gel ZnO ETL for full process compatibility with flexible substrates. The reported MTD-SE method (stands for Modified Two Step Dipping - Solvent Engineering) limits the grain coarsening effect during post-immersion stage of two-step dipping method and provides substantially smooth perovskite surface morphology for enhanced charge transport properties compared to conventional two-step techniques by means of controlled Ostwald Ripening process. The grain coarsening process and concomitant irregular grain size distribution are judiciously controlled by increasing the chemical potential or free energy change (ΔG) of the system at the post-immersion. The photovoltaic performance and photo-current hysteresis phenomena of the reported MTD-SE PSC have been compared with PSCs fabricated with conventional two-step techniques, incorporating 2-Propanol or ethyl alcohol as dipping solvents. The enhanced device performance of MTD-SE PSCs is correlated with the conducive role of the evenly distributed grain boundaries in them, which act as carrier dissociation interfaces and carrier transport pathways to charge selective contacts for superior charge separation and extraction properties. Adding to the merits, MTD-SE PSCs also demonstrate significantly suppressed photo-current hysteretic behaviour which has been elucidated in the context of faster ion migration kinetics with the increased grain boundaries, which exhibit higher ionic diffusivity. The favourable ion migration kinetics with MTD-SE PSC have also been comprehensively analysed from the frequency-dependent capacitive spectra.