Organic Charge Transport Research Articles

Crystalline and Amorphous Interface Simulations of Donor-Acceptor Blends.

In an organic solar cell, exciton dissociation and charge transport to generate current depend on interface and bulk morphology, respectively, and their rates dictate device performance. Blend miscibility and processing determine the final morphology. We investigate the blend miscibility of P3HT and O-IDTBR, employing our recently developed "push-pull" computational technique, and explore its effect on nanoscale morphology. The resulting Flory-Huggins χ parameter is primarily enthalpic, and the blend exhibits UCST behavior. To directly simulate an equilibrated morphology in such a blend requires large (30 nm) systems and long (10 μs) simulations, which we attain with our virtual site coarse graining technique. In the resulting amorphous phase-separated state, O-IDTBR swells P3HT by about 15%, but does not percolate within the P3HT-rich phase. The interfacial profile depends on crystallinity: the amorphous interface is several nanometers wide, whereas an acceptor crystal induces order in the adjacent donor polymer, forming a sharp interface quite similar to a crystal-crystal interface. The wide amorphous interface promotes more donor-acceptor contacts with a wide range of relative orientations; however, the acceptors do not form a percolating network, which will likely lead to charge recombination. A crystal interface exhibits fewer donor-acceptor contacts with primarily face-on orientation that are more likely to charge-separate because of instant access to the preferred domain.

Carrier transport layer engineering of Cs2TiI2Br4 halide double perovskite solar cell via SCAPS 1D: Approaching the Shockley-Queisser limit

All-inorganic Cs2TiIxBr(6-x)-based perovskite solar cells (PSCs) are recently attracting a lot of attention for their tunable bandgaps, earth abundance, non-toxicity, and ultra-stability. Among the Cs2TiIxBr(6-x) family of materials, Cs2TiI2Br4 with a bandgap of ∼1.38 eV has the potential to be an excellent single junction solar cell material with a theoretically higher Shockley-Queisser limit of power conversion efficiency (PCE). Its excellent optoelectronic properties make it a potential candidate for being the highest-performing PSC from the Cs2TiIxBr(6-x) family. In our study, a total of eight hole transport materials (P3HT, PTAA, Spiro-OMeTAD, PEDOT:Pss, CuSCN, CuI, NiO, and MoO3) and six electron transport materials (PCBM, TiO2, CdS, SnO2, ZnO, and IGZO) were investigated to select suitable charge transport materials. The defect densities of interface and absorber, different absorber layer thicknesses, several metal work functions, series-shunt resistance, and temperature were investigated to derive the conditions for optimum performance. After thorough investigation, we derived four novel devices with the combination of all the organic and inorganic charge transport materials to provide optimum performance. Among them, the combination of inorganic SnO2 and CuSCN as electron and hole transport layer respectively achieved the highest PCE of 23.41 %.

Multi-layer transparent electrodes for high performance bifacial perovskite solar cells

Bifacial perovskite solar cells have gained significant interest due to their high light-harvesting capabilities. In this study, a multilayer transparent conducting film (MLTCF) composed of indium tin oxide (ITO)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Ag/ITO is developed as a top electrode for bifacial perovskite solar cells (PSCs). By applying BCP at the bottom of the ITO layer as an adhesion promoter, the Volmer–Weber mechanism of island growth pattern for Ag under thickness threshold conditions is successfully suppressed. This results in ultra-thin (8 nm) and ultra-smooth Ag films with a relatively low sheet resistance of 5.5 Ω/sq and high transmittance of 85.8 % at a typical wavelength of 550 nm. Furthermore, a molybdenum oxide (MoO3) overlayer is evaporated onto the soft organic charge transport layer as a protective buffer to prevent sputtering damage during the deposition of ITO and Ag films. Consequently, bifacial PSCs exhibit an optimized power conversion efficiency (PCE) of 20.3% for single-side illumination from the glass side, and equivalent PCEs of 23.6 %, 25.2 %, 26.8 % with albedo of 0.2, 0.3 and 0.4, respectively. This work paves the way for the development of highly efficient bifacial thin-film solar cells.

Hole and electron transport materials: A review on recent progress in organic charge transport materials for efficient, stable, and scalable perovskite solar cells

Charge transporting materials are essential for fabricating stable and efficient perovskite solar cells. The high-temperature processing, surface defects, and low mobility are common issues in inorganic charge transport materials which can hinder the fabrication of low-cost, efficient, and stable perovskite solar cells. Dopants increase the processing cost of the hole transport materials. Surface defects are observed in the high-temperature processed inorganic electron transport materials. There is a need to develop organic compounds for charge transportation in perovskite solar cells. Herein, the advancements in the organic materials for charge transport in the perovskite solar cells are reviewed. The low-cost processing, better solubility, efficient charge mobility, tunable molecular orbitals, and better stability of organic compounds are some properties for their utilization as charge transport materials in perovskite solar cells. Organic small molecules, polymers, and phthalocyanine compounds can be utilized as dopant-free hole transport materials. Fullerene and non-fullerene derivatives like C60, C70, azaacenes, indacenodithiophene, polymers, and rylene diimides are promising electron transport materials. The functionality, engineering, charge transport properties, and device characteristics of different organic materials are discussed. The review will offer a forecast for improving device stability and efficiency in terms of architecture, engineering, and materials to realize the commercialization of perovskite solar cells soon.

Interfacial Engineering of Quantum Dots–Metal–Organic Framework Composite Toward Efficient Charge Transport for a Short‐Wave Infrared Photodetector

AbstractShort‐wave infrared (SWIR) quantum dot (QD)‐converted photodetectors are one of the promising devices used in artificial intelligence and automatic navigation systems. However, compared to semi‐conductor‐type photodetectors, they suffer from poor chemical stability and weak electronic conductivity. In this study, a PbS QD and conductive metal–organic framework (MOF) hybrid composite is designed with SWIR detection capability. The Fourier‐transform infrared spectroscopy confirms the surface modification of PbS QDs with MOF. X‐ray absorption near‐edge structures spectra reveal the chemical bonding between MOF and PbS QDs. Furthermore, 2D grazing‐incidence small‐ and wide‐angle X‐ray scattering is utilized to unveil the particle stacking and the preferred orientation of the PbS@MOF thin film. By integrating the PbS@MOF thin film directly on top of the graphene field effect transistor (FET) device, the chemical stability and photoelectric properties of graphene FET are enhanced, and these are investigated using a focus ion beam with a high‐resolution transmission electron microscope. This study offers a strategy to simultaneously improve the chemical stability and the photoelectronic properties of the nanomaterials as well as contribute to the advancement of a QD‐converted SWIR photodetector.

Unleaded all-inorganic KSnI3 perovskite solar cell: A computational study

The implementation and commercialization of perovskite solar cells (PSCs) is hindered due to lead, which has a toxic nature. Although the PSCs based on the organic charge transport layers (CTLs) have achieved tremendous improvement in efficiency, they are not stable due to the structure of organic materials. In the present work, a theoretical study of the effect of inorganic CTLs on a KSnI3-based PSC is carried out using SCAPS software. The defect concentration of the layers and the interfaces, doping density and thickness of the layers, shunt, and series resistance of the device is optimized. Carbon is suggested as an affordable alternative to the state of art back contact material (BCM), gold. The stability of device performance with temperature is also established. The optimized solar cell (SC) showed a power conversion efficiency (PCE) of 9.28% and an excellent quantum efficiency(QE) in the visible region(99.59% at 400 nm to 78.58% at 650 nm). This study demonstrates the potential of KSnI3 as a perovskite material to achieve toxicity-free renewable energy using inorganic charge transport layers.

Low Damage Scalable Pulsed Laser Deposition of SnO2 for p–i–n Perovskite Solar Cells

Pulsed laser deposition (PLD) has already been adopted as a low damage deposition technique of transparent conducting oxides on top of sensitive organic charge transport layers in optoelectronic devices. Herein, SnO2 deposition is demonstrated as buffer layer in p–i–n perovskite solar cells (PSCs) via wafer‐scale (4 inch) PLD at room temperature. The PLD SnO2 properties, its interface with perovskite/C60, and device performance are compared to atomic layer deposited (ALD) SnO2, i.e., state‐of‐the‐art buffer layer in perovskite‐based single junction and tandem photovoltaics. The PLD SnO2‐based solar cells exhibit on par efficiencies (17.8%) with that of SnO2 fabricated using ALD. The solvent‐free room temperature processing and wafer‐scale approach of PLD open up possibilities for buffer layer formation with increased deposition rates while mitigating potential thermal or physical damage to the top organic layers. This is a promising outlook for fully physical vapor‐processed halide PSCs and optoelectronic devices requiring low thermal budget.

The impact of Ag/Co nanocomposite on organic charge transport medium for improved photocurrent in polymer solar cell

AbstractTuning charge transport properties using silver‐cobalt bimetallic nanoparticles is investigated to enhance the photo‐current collection in polymer solar cells. The silver‐cobalt nanoparticles were not only augmenting the charge transport processes but also expected to trap light by way of reflection. The polymers blend consisting of poly‐(3‐hexylthiophene) (P3HT):[6,6]‐phenyl C61 butyric acid methyl ester (PC61BM) is used in the fabrication of polymer solar cells. The experimental results indicated that the solar cells' performance depended on the concentration of Ag/Co in the hole transport layer (HTL). Nevertheless, the high‐power conversion efficiency value of up to 4.21% was observed, at the concentration of 0.1% by weight, which is much higher than pristine solar cells due to improved measured photocurrents from the devices. This is a significant improvement in PCE by 66.4% compared to the undoped transport medium. Ag/Co nanocomposite is found to be more stable under ambient laboratory conditions.

Effect of organic charge transport layers on unleaded KSnI3 based perovskite solar cell

The implementation and commercialization of perovskite solar cells (PSCs) is hindered due to lead, which has a toxic nature. The inorganic charge transport layers (CTLs) require complex deposition techniques, which are not very cost friendly and the reported efficiencies of PSC with organic CTLs are higher than those with inorganic CTLs. In the present work, a theoretical study of the effect of organic CTLs on a KSnI3 based PSC is carried out using SCAPS software. The defect concentration of the layers and the interfaces, doping density and thickness of the layers, and the shunt and series resistance of the device is optimized. Carbon is suggested as an affordable alternative to state-of-the-art back contact material (BCM), gold. The stability of this device with temperature is also established. The optimized solar cell showed an excellent fill factor (FF) of 80.80% with a power conversion efficiency (PCE) of 10.83% and a very good quantum efficiency (QE) in the visible region.

An investigation of defects, band-offset, and Schottky barrier height for boosting the performance of formamidinium mixed cation mixed halide based perovskite solar cell: Theoretical approach

The phase stability of FAPbI3 based perovskite solar cells (PSCs) has been a serious concern. The black stable α-phase of FAPbI3 transits into a hexagonal non-perovskite yellow phase because of a large tolerance factor (>1). Many studies reported that the incorporation of Cs+ and Br− ions into the FAPbI3 could tune the tolerance factor resulting in improved thermal and phase stability of the device. In this work, we have optimized FA0.85Cs0.15Pb(I0.85Br0.15)3(FAPbIBr) based PSCs using various organic and inorganic charge transport layers (CTLs). This work aims to explain the collection and transportation of charge carriers at perovskite/CTL and hole transport layer (HTL)/back contact interfaces by optimizing energy band levels. Additionally, the impact of Schottky barrier height at the HTL/back contact interface has been investigated. Simulation results reveal that ZnO (as ETL) and SrCu2O2(as HTL) demonstrate superior efficiency of 19.02% among all the investigated CTLs.

Organic electronic applications and charge transport mechanism in novel discotic liquid crystals

ABSTRACTTriphenylene-based pentaalkynylbenzene dyads and naphthophenanthridine derivativediscotic mesogens have been proven a backbone-like utility for modern optoelectronic applications. Herein, we present the charge transport and organic electronic applications of naphthophenanthridine derivatives and a dimer composed of pentaalkynylbenzene (PA) and triphenylene (TP) discotic liquid crystals (DLCs). Naphthophenanthridine derivatives exhibit a hexagonal columnar (Colh) phase; whereas, triphenylene-pentaalkynylbenzene dyads show a columnar centred rectangular (Colr) phase and transform into Colh phase on doping the pure compounds with an electron-acceptor 2,4,7-trinitrofluorenone (TNF) in a 2:1 TNF/compound ratio. The ambipolar charge transport behaviour of the compounds mentioned above has been investigated by the time-of-flight technique. Naphthophenanthridine derivative renders an ambipolar charge transport, showing temperature-independent electron and hole mobility of the order of 3 × 10−4 cm2/Vs; whereas, TP-PA dimer yields an ambipolar charge carrier mobility of order 10−3 cm2/Vs. The phenazine-fused triphenylene DLC shows unipolar hole mobility of the order of 10−4 cm2/Vs; whereas, hydrogen-bonded Hpz-C9-TP DLCs exhibit ambipolar charge mobility of the order of 10−2 cm2/Vs. The high ambipolar charge carrier mobility in the investigated DLC compounds makes them suitable for fabricating organic semiconducting electronic devices.

Void of lead and non-carcinogenic germanium based RbGeI3 PSC using organic charge transport layers: towards a clean and green future

Void of lead and non-carcinogenic germanium based RbGeI3 PSC using organic charge transport layers: towards a clean and green future

Vapor Phase Infiltration Improves Thermal Stability of Organic Layers in Perovskite Solar Cells

Despite the rapid increase in power conversion efficiency (PCE) of perovskite solar cells (PSCs) over the past decade, stability remains a major roadblock to commercialization. This work shows vapor phase infiltration (VPI) as a tool to create hybrid organic–inorganic layers that improve the stability of organic charge transport layers, such as hole-selective spiro-OMeTAD in PSCs and in other organic electronic devices. Using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and grazing incident wide-angle X-ray scattering (GIWAXS), we identify that infiltration of TiOx via VPI hinders the crystallization of the spiro-OMeTAD layer by likely preventing the π–π stacking of the molecules. Infiltrated PSCs retained more than 80% of their original efficiency after an operando stability test of 200 h at 75 °C, double the efficiency retained by devices without infiltration, in which the efficiency rapidly decreases in the first 50 h. This work provides a blueprint for using VPI to stabilize organic charge transport layers via prevention of π–π stacking leading to deleterious crystallization that shortens device lifetimes.

Efficient and Thermally Stable All‐Perovskite Tandem Solar Cells Using All‐FA Narrow‐Bandgap Perovskite and Metal‐oxide‐based Tunnel Junction

AbstractCommercialization of all‐perovskite tandem solar cells requires thermally stable narrow‐bandgap (NBG) perovskites and tunnel junction. However, the high content of methylammonium (MA) and organic hole transport layer used in NBG perovskite subcell undermine the thermal stability of all‐perovskite tandems. Here, thermally stable mixed lead‐tin NBG perovskite solar cells (PSCs) are developed by using only formamidinium (FA) for the A‐site cation. Solution‐processed indium tin oxide nanocrystals (ITO NCs) are deployed further to replace the conventional organic charge transport layer. Meanwhile, the ITO NCs layer simultaneously functions as a recombination layer in the tunnel junction, which simplifies the architecture of all‐perovskite tandem devices. The thermally stable all‐FA Pb‐Sn PSCs achieve a high power conversion efficiency (PCE) of 21.0%. With the thermally stable all‐FA NBG perovskite and optimized tunnel junction, a stabilized PCE of 26.3% is further obtained in all‐perovskite tandems. The unencapsulated tandem devices maintain >90% of their initial efficiencies after 212 h aging at 85 °C in the N2 atmosphere. The strategies herein offer a crucial step toward efficient and thermally stable all‐perovskite tandem solar cells.

Theoretical analysis of all-inorganic solar cells based on numerical simulation of CsGeI3/CsPbI3 with p-p+ built-in electric field

With the development of perovskite solar cells (PSCs) technology, inorganic perovskite led by CsPbI3 gradually shows the photovoltaic performance of organic perovskite cells. However, this is still based on the organic charge transport layer. CsPbI3 with all-inorganic structure still need to be further studied. Therefore, in this study, an all inorganic CsGeI3/CsPbI3 laminated structure with CsGeI3 instead of organic HTL is introduced, and numerical simulation is carried out through SCAPS-1D (Solar Cells Capability Simulator). It is found that the two perovskite layers, which are both p-type materials, have shown excellent efficiency in the p-p+ built-in electric field. In this paper, PSCs are optimized with absorbing layer thickness, doping concentration, defect density, metal electrode work function and so on. The best thickness is determined according to the different light absorption properties of Pb2+ and Ge2+, and the toxicity of the original single-layer lead based perovskite is reduced. Through the comprehensive analysis of band bending, built-in electric field and recombination rate, we explained the reason why the parameters increase with the growth of doping concentration (NA). In terms of NA regulation, it is found through simulation that when NA of the two absorption layers is at 1015 to 1018 cm−3, the photogenerated carriers are driven by the strong p-p+ built-in electric field to migrate well in the flat energy band, which significantly improves the photovoltaic performance of PSCs. For the defect density the charge transport layer, the PSC absorption characteristics, and the metal electrodes, we made a choice based on the photovoltaic performance and process level. Finally, the power conversion efficiency of the optimized PSCs reached 24.06 % and the fill factor is 86.19 %. This study proved that p-p+ structure constructed by lead germanium mixed metal has great potential for inorganic perovskite solar cells with Cs as a cation in the future.

Ultrahigh-resolution full-color perovskite nanocrystal patterning for ultrathin skin-attachable displays.

High-definition red/green/blue (RGB) pixels and deformable form factors are essential for the next-generation advanced displays. Here, we present ultrahigh-resolution full-color perovskite nanocrystal (PeNC) patterning for ultrathin wearable displays. Double-layer transfer printing of the PeNC and organic charge transport layers is developed, which prevents internal cracking of the PeNC film during the transfer printing process. This results in RGB pixelated PeNC patterns of 2550 pixels per inch (PPI) and monochromic patterns of 33,000 line pairs per inch with 100% transfer yield. The perovskite light-emitting diodes (PeLEDs) with transfer-printed active layers exhibit outstanding electroluminescence characteristics with remarkable external quantum efficiencies (15.3, 14.8, and 2.5% for red, green, and blue, respectively), which are high compared to the printed PeLEDs reported to date. Furthermore, double-layer transfer printing enables the fabrication of ultrathin multicolor PeLEDs that can operate on curvilinear surfaces, including human skin, under various mechanical deformations. These results highlight that PeLEDs are promising for high-definition full-color wearable displays.

High-Efficiency Tandem White Perovskite Light-Emitting Diodes by Using an Organic/Inorganic Intermediate Connector

The performance of monochromatic perovskite light-emitting diodes (PeLEDs) has undergone rapid development in recent years, while white PeLEDs are still in their infancy. The tandem devices prepared by connecting light-emitting units of different colors with intermediate connectors comprise one of the promising approaches for realizing white light emission. The intermediate connector plays a crucial role in determining the effectiveness of the light-emitting units. In this work, all-perovskite-based white LEDs are assembled by employing sky-blue (490 nm) and red (690 nm) perovskite emitters as active layers. With mixed p-type and n-type organic charge transport materials as the intermediate connector, the highest external quantum efficiency (EQE) of the device was 1.4%, the electroluminescence (EL) peak position of the blue light part was shifted to 512 nm, and the red light part was shifted to 673 nm. In contrast, with organic and inorganic charge transport materials as the intermediate connector, the devices exhibited better charge generation/injection capabilities and a record EQE of 8.5%. The EL peak position of the device prepared from the stored film was not shifted, implying that the organic/inorganic layer effectively suppresses halide migration. Our work paves the way for highly efficient tandem white PeLEDs.

Manipulate organic crystal morphology and charge transport

Organic electronics has become one of the fastest developing fields with exciting advances in discovering novel organic semiconductors, enhancing charge carrier mobility, and exploring high-performance electronic device applications. Nevertheless, the solution-induced growth of small molecular organic semiconductors provokes predominant crystal misorientation, thermal cracks and grain boundary, which poses a tough challenge to maneuver the crystal alignment towards device fabrication. In this article, we conduct an in-depth review of external force-based, additive-based, and binary solvent-based techniques to effectively control the organic semiconductor crystallization, thin-film morphology, and crystal orientations. We first introduce the recent progress and the various challenges of flexible electronics, and discuss how these issues are correlated to charge carrier mobility variations. Next, we discuss the miscellaneous external force alignment methods based on the categories of slot-die coating, blade coating, substrate patterning, and air flow. Furthermore, we investigate the additive-based methods for controlling crystal growth and orientations, including polymeric additives, small molecular additives, and nanostructured additives. Finally, we discuss the binary solvent-based methods to control the crystal morphology and dimensionality. By correlating the charge transport characteristics with the intrinsic merits of those unique methods, this article shares a useful insight into maximizing the performance of organic semiconductors from a new perspective.

Hysteresis in hybrid perovskite indoor photovoltaics.

Halide perovskite indoor photovoltaics (PV) are a viable solution to autonomously power the billions of sensors in the huge technology field of the Internet of Things. However, there exists a knowledge gap in the hysteresis behaviour of these photovoltaic devices under indoor lighting conditions. The present work is the first experimental study dedicated to exploring the degree of hysteresis in halide perovskite indoor photovoltaic devices by carrying out both transient J–V scan and steady state maximum power point tracking (MPPT) measurements. Dependence of hysteresis on device architecture, selection of electron transporting layers and the composition of the perovskite photoactive layers were investigated. Under indoor illumination, the p-i-n MAPbI3-based devices show consistently high power conversion efficiency (PCE) (stabilized PCE) of greater than 30% and negligible hysteresis behaviour, whereas the n-i-p MAPbI3 devices show poor performance (stabilized PCE ∼ 15%) with pronounced hysteresis effect. Our study also reveals that the n-i-p triple cation perovskite devices are more promising (stabilized PCE ∼ 25%) for indoor PV compared to n-i-p MAPbI3 due to their suppressed ion migration effects. It was observed that the divergence of the PCE values estimated from the J–V scan measurements, and the maximum power point tracking method is higher under indoor illumination compared to 1 Sun, and hence for halide perovskite-based indoor PV, the PCE from the MPPT measurements should be prioritized over the J–V scan measurements. The results from our study suggest the following approaches for maximizing the steady state PCE from halide perovskite indoor PV: (i) select perovskite active layer composition with suppressed ion migration effects (such as Cs-containing triple cation perovskites) and (ii) for the perovskite composition such as MAPbI3, where the ion migration is very active, p-i-n architecture with organic charge transport layers is beneficial over the n-i-p architecture with conventional metal oxides (such as TiO2, SnO2) as charge transport layers.This article is part of the theme issue ‘Developing resilient energy systems’.

Illumination‐Driven Energy Level Realignment at Buried Interfaces between Organic Charge Transport Layers and a Lead Halide Perovskite

Tremendous progress in employing metal halide perovskites (MHPs) in a variety of applications, especially in photovoltaics, has been made in the past decade. To unlock the full potential of MHP materials in optoelectronic devices, an improved understanding of the electronic energy level alignment at perovskite‐based interfaces is required. This particularly pertains to such interfaces under device operation conditions, e.g. under illumination with visible light such as in a solar cell. Herein, it is revealed that the energy level alignment at the buried interface between a double cation lead halide perovskite film and charge‐selective organic transport layers changes upon white light illumination. This is found from photoemission experiments performed with the samples in dark and under illumination, and the interfacial energy level shift is reversible. The underlying mechanism is attributed to the accumulation of one charge carrier type within the perovskite film at the interface under illumination, as a result of the charge‐selective nature of the organic layer. The fact that the interfacial energy level alignment at MHP‐based junctions under illumination can differ from that in dark is to be taken into account to fully rationalize device characteristics.