Charge Transport In Materials Research Articles

Strategies for high‐performance perovskite solar cells from materials, film engineering to carrier dynamics and photon management

AbstractIn recent years, halide perovskite solar cells (HPSCs) have attracted a great attention due to their superior photoelectric performance and the low‐cost of processing their quality films. In order to commercialize HPSCs, the researchers are focusing on developing high‐performance HPSCs. Many strategies have been reported to increase the power conversion efficiency and the long‐term stability of HPSCs over the past decade. Herein, we review the latest efforts and the chemical‐physical principles for preparing high‐efficiency and long‐term stability HPSCs in particular, concentrating on the perovskite materials, technologies for perovskite films, charge transport materials and ferroelectric effect to reduce the carrier loss, and photon management via plasmonic and upconversion effects. Finally, the key issues for future researches of HPSCs are also discussed with regard to the requirements in practical application.image

Enhanced performance of GeSe thin-film solar cells via bifacial charge transport materials design

Owing to non-toxic, earth abundant and stable properties, GeSe has become a promising photovoltaic absorber material in thin-film solar cells (TFSCs). Currently GeSe film is mainly prepared by rapid thermal sublimation (also called one-step method), where the evaporation source and substrate are heated together, thus it is difficult to regulate their temperatures independently, leading to the re-sublimation of GeSe film and diffusion of Cd element. In this work, a two-step method is introduced to prepare the GeSe film. Moreover, a synergic interface design is applied to enhance the performance of GeSe TFSCs. On one hand, the conductivity of CdS buffer layer is increased by post-annealing to reduce the series resistance and promote the electron transfer from GeSe to CdS, validated via linear sweep voltammetry curves. On the other hand, Cu2O is inserted as a hole transport layer for the first time. The Cu2O layer has sufficiently improved the hole extraction efficiency and suppressed the charge recombination, demonstrated by transient photovoltage spectroscopy and electrochemical impedance spectroscopy. By incorporating these bifacial modifications, the power-conversion efficiency (PCE) of the device has been improved by about four times (0.16%–0.63%), comparable to the highest PCE among all the reported GeSe TFSCs fabricated via a two-step method.

Efficient NIR Perovskite Light-Emitting Diodes Enabled by Incorporating an Anthracene Derivative as a Bifunctional Electron Transport Layer

Exploring multifunctional charge transport materials with well-matched energy-levels alignment and compatible interfaces is deemed a potential approach toward high-performance perovskite light-emitting diodes (PeLEDs). Herein, an anthracene derivative, 1-[4-(10-[1,1′-biphenyl]-4-yl-9-anthracenyl)phenyl]-2-ethyl-1H-benzimidazole (BAEBi), is demonstrated as an efficient electron tranport layer (ETL) as well as an effective surface passivation agent for near-infrared (NIR) quasi two-dimensional (Q-2D) PeLEDs. Owing to the promising electron injection and transport capability of BAEBi, an improved charge-carrier balance is attained in rather hole-dominant PeLEDs. Meanwhile, BAEBi assisted to significantly mitigate the perovskite/ETL interfacial defects, rendering a pinhole-free smooth film with quite low roughness and ameliorated exciton lifetime. Consequently, BAEBi-containing NIR Q-2D PeLEDs manifested a low turn-on voltage of 2.5 V with a decent external quantum efficiency (EQE) of 9.2% and a peak radiance of 127 W sr–1 m–2; these values are, respectively, ∼1.5- and 15-fold higher compared to that of a commonly used ETL, 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi). Importantly, the BAEBi-incorporating champion PeLED exhibited significantly curtailed efficiency roll-off and prolonged operational stability. These findings reveal an ingenious solution to address the charge imbalance issue and suppress the interfacial defects in PeLEDs that would spur further development of this emerging technology toward efficient solid-state lighting and high-resolution displays.

Structural, Electronic, and Charge Transport Properties of New Materials based on 2-(5-Mercapto-1,3,4-Oxadiazol-2-yl) Phenol for Organic Solar Cells and Light Emitting Diodes by DFT and TD-DFT

This work reports on the density functional theory (DFT) and its time-dependent extension (TD-DFT) study of the structural, electronic, and charge transport properties of 2-(5-mercapto-1,3,4-oxadiazol-2-yl) phenol (MODP) and some of its transition M2+ complexes (M = Fe, Co, Cu, Ni, Zn, Pd, Pt). Reorganization energy, integral charge transfer, mobility, open circuit voltage, and electronic properties of these compounds have been calculated by employing the global hybrid functional PBE0 in conjunction with the Karlsruhe basis set def2-TZVP. Results show that MODP and its transition metal complexes are good electron donors for organic solar cells (OSC) owing to their relatively higher HOMO and LUMO energies compared to the prototypical (6, 6)-phenyl-C61-butyric acid methyl ester (PCBM). Energy gaps ranging between 2.502 and 4.455 eV, energy driving forces (∆EL-L) ranging between 2.08 and 2.44 eV, and large open circuit voltages ( V OC ) ranging from 1.12 to 2.05 eV were obtained. The results also revealed that MODP and its Pd(II) and Pt(II) complexes could serve as ambipolar charge transport materials owing to their very small reorganization energies, integral charge transfers, high rate charge transfers, and mobilities. All studied molecules showed OSC donor and hole/electron transport characteristics required by organic light-emitting diodes (OLEDs). Based on these results, new ways for designing charge transport materials for OLEDs as well as donor materials in OSCs are proposed.

Stability-limiting heterointerfaces of perovskite photovoltaics.

Optoelectronic devices consist of heterointerfaces formed between dissimilar semiconducting materials. The relative energy-level alignment between contacting semiconductors determinately affects the heterointerface charge injection and extraction dynamics. For perovskite solar cells (PSCs), the heterointerface between the top perovskite surface and a charge-transporting material is often treated for defect passivation1-4 to improve the PSC stability and performance. However, such surface treatments can also affect the heterointerface energetics1. Here we show that surface treatments may induce a negative work function shift (that is, more n-type), which activates halide migration to aggravate PSC instability. Therefore, despite the beneficial effects of surface passivation, this detrimental side effect limits the maximum stability improvement attainable for PSCs treated in this way. This trade-off between the beneficial and detrimental effects should guide further work on improving PSC stability via surface treatments.

Coordination polymers for emerging molecular devices

Conductive coordination polymers are hybrid materials with the potential to be implemented in the next generation of electronic devices, owing to several desirable properties. A decade ago, only a few scattered examples exhibiting conductivity existed within this class of materials, yet today groups of coordination polymers possess electrical conductivities and mobilities that rival those of inorganic semiconductors. Many currently emerging energy harvesting and storage technologies are limited by the use of inefficient, unstable, and unsustainable charge transport materials with little tunability. Coordination polymers, on the other hand, offer great electrical properties and fine-tunability through their assembly from molecular building blocks. Herein, the structure–function relationship of these building blocks and how to characterize the resulting materials are examined. Solution processability allows devices to step away drastically from conventional fabrication methods and enables cheap production from earth abundant materials. The ability to tune the electrical and structural properties through modifications at the molecular level during the material synthesis stages allows for a large design space, opening the door to a wide spectrum of applications in environmentally friendly technologies, such as molecular wires, photovoltaics, batteries, and sensors. Sustainable, high-performing charge transport materials are crucial for the continued advance of emerging molecular technologies. This review aims to provide examples of how the promising properties of coordination polymers have been exploited to accelerate the development of molecular devices.

Centimetre-scale perovskite solar cells with fill factors of more than 86 per cent.

Owing to rapid development in their efficiency1 and stability2, perovskite solar cells are at the forefront of emerging photovoltaic technologies. State-of-the-art cells exhibit voltage losses3-8 approaching the theoretical minimum and near-unity internal quantum efficiency9-13, but conversion efficiencies are limited by the fill factor (<83%, below the Shockley-Queisser limit of approximately90%). This limitation results from non-ideal charge transport between the perovskite absorber and the cell's electrodes5,8,13-16. Reducing the electrical series resistance of charge transport layers is therefore crucial for improving efficiency. Here we introduce a reverse-doping process to fabricate nitrogen-doped titanium oxide electron transport layers with outstanding charge transport performance. By incorporating this charge transport material into perovskite solar cells, we demonstrate 1-cm2 cells with fill factors of >86%, and an average fill factor of 85.3%. We also report a certified steady-state efficiency of 22.6% for a 1-cm2 cell (23.33% ± 0.58% from a reverse current-voltage scan).

Framework structure materials in photovoltaics based on perovskites 3D

The inorganic and hybrid organic / inorganic perovskites based on lead halide and derived materials (alloys) having the formula ABX3 are emerging as a new and innovative family of absorber materials for the conversion of solar energy into electricity in photovoltaic systems. Thanks to their composition, perovskites and derivatives have exceptional optical, electrical and structural properties with high absorption of light in the visible solar spectrum and good mobility of charge carriers generated by photons facilitating the extraction of electric current. Due to these important properties, perovskite solar cells combine a high efficiency of conversion of light into electricity with great ease of preparation and synthesis at very low costs via simple deposition techniques. In this article we review the structural and optoelectronic properties of perovskites 3D exhibiting photovoltaic properties, we are also interested in the operating principle of perovskite-based solar cells, charge transport materials and associated architectures. Simple fabrication techniques and issues with stability and hysteresis are also discussed.

Toward More Efficient Organic Semiconductors: The Relationship between Morphology, Charge Transport, and Photophysical Properties

In recent years, materials science has been facing an electronics revolution due to the fast progress in the development of organic semiconductors. In this context, the current study attempts to tune the photophysical properties of polycyclic aromatic hydrocarbons (PAHs) by engineering their morphology and size. Four families of PAHs─oligoacenes, circulenes, triangulenes, and sunflowers─are considered to investigate the evolution of their optoelectronic properties with size and morphology. Based on the calculated properties, several quantitative structure–property relationships have been established for the prediction of reorganization energy. It is found that morphologies like circulene and triangulene that are less aromatic and have smaller energy gaps achieve smaller reorganization energy and thus are better candidates for charge transport materials. For all considered PAHs, the excitation energies of the strongest low-lying electronic transition (β peak) span in the visible region and demonstrate the size tunability. The β peak of some oligoacene and triangulene families exhibits plasmonic behavior that is important for the fabrication of lighter optical devices compared with metallic systems. We have also explored the effect of sulfur heteroatoms on the structural, electronic, and optical properties of circulene and sunflower morphologies.

High-performance Ruddlesden-Popper two-dimensional perovskite solar cells via solution processed inorganic charge transport layers.

Two-dimensional (2D) layered halide perovskites have been shown to enable improved long-term stability in comparison to the well-known three-dimensional hybrid organic-inorganic halide perovskites. The optoelectronic properties of the 2D perovskites are strongly influenced by the chemical nature of the charge transport layer. In this work, we fabricated Ruddlesden-Popper 2D perovskite solar cells (PSCs) using solution processed inorganic NiOx and a C60 : C70 (1 : 1) mixture as the hole and electron transport layers, which significantly improved the performance of the 2D PSCs. Time resolved photoluminescence measurements indicate the shortened lifetime of excitons, which demonstrates the excellent charge extraction properties. The PSCs based on these inorganic charge transport materials (CTMs) exhibit an average power conversion efficiency (PCE) of 14.1%, which is higher than that (12.3%) of PSCs using organic CTMs of poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS) and phenyl-C61-butyric acid methyl ester (PCBM). Compared with PEDOT:PSS and PCBM based cases, the PSCs using inorganic CTMs also show improved long-term stability, with the PCE degradation significantly suppressed from 20% to 12% after a measurement of 15 days. The best PSCs using NiOx and C60 : C70 show a high PCE of 14.4%, with a stable power output and negligible hysteresis.

Numerical study of inorganic Cs2TiBr6 solar cells with a double hole transport layers

The proposed Cesium Titanium Bromide (Cs2TiBr6) has piqued curiosity as an absorber layer for the next-generation renewable energy. This is especially due to their important properties such as high stability and nontoxicity. Nevertheless, the performance of solar cells based on this double-perovskite is still lower than methylammonium lead halide perovskite solar cells, which limited their commercialization in the real world. Up till now, the highest efficiency of the experimentally constructed PSC has been 3.28%, where Cs2TiBr6 was utilized as an absorber layer and both inorganic (TiO2) and organic (P3HT, C60) as charge transport materials. . In this work, Numerical modeling tools were used to optimize the active layer with the novel architecture Au /P3HT/CuSbS2/ Cs2TiBr6/TiO2/FTO using solar cell capacitance simulator (SCAPS-1D). The results obtained using double hole transport layers provide a higher efficiency for Cs2TiBr6 perovskite solar cell compared with the conventional structure.

Numerical development of lead-free Cs2TiI6-based perovskite solar cell via SCAPS-1D

Because of the toxicity and stability concerns, commercialization of lead-based perovskite solar cells (PSCs) is limited. Solar cells made entirely of Ti-based all-inorganic perovskite could be a viable answer to these issues. This paper is a theoretical paper on a perovskite solar cell (PSC) based on Cs2TiI6 using all-inorganic charge transport materials. We proposed a high performance perovskite solar cell (PSC) according to variables such as charge transport materials and its optimal thicknesses, absorber thickness, absorber defect density and interface defect density and working temperature. The optimal absorber thickness, Hole transport layer (HTL) thickness, and Electron transport layer (ETL) thickness are 500 nm, 50 nm, and 10 nm, respectively. After analyzing the other factors, we ended up with a high-performance PSC with a power conversion efficiency of 22.5% at room temperature and 22.84% at 270 K. These results are useful for the conception and manufacture of PSCs.

Exploration of charge transport materials to improve the radiation tolerance of lead halide perovskite solar cells

Towards the application of perovskite solar cells (PSCs) in space, we extensively investigated the effects of electron beam irradiation on binary-mixed PSCs with various hole- and electron-transport materials.

Copper coordination polymers with selective hole conductivity

Low-dimensional copper coordination polymers represent an alternative, unprecedented class of charge transport materials, comprised of molecular building blocks.

Optimization based on computational analysis of lead-free inverted planar photo-voltaic device structure using halide based double perovskite material

Owing to the appealing optoelectrical, mechanical, and physical properties, configurable bandgaps, good performance on opaque devices, and low manufacturing cost, Perovskites solar cells are found to be the best alternatives for semi-transparent photo-voltaic (PV) applications. The high absorption coefficient, high carrier mobility, and long carrier diffusion length are inherent features of many organic and inorganic halide perovskite solar cells. Most of the promising perovskite materials contain Lead (Pb) and hence they are toxic. Besides toxicity issues, the stability of these perovskite structures is another key challenge in the process of its commercial production. The performance of various lead-free active PV materials was studied and identified that Cesium–Titanium Halide materials can be a suitable alternate for lead-based perovskite materials. These promising Cs2TiX6 materials have a tunable bandgap between 1.4 eV and 2.3 eV and have balanced electron and hole diffusion lengths. Most of the conventional PSC structure consists of regular planar N-I-P structure and the commercial applications of such devices are bounded due to very high J-V hysteresis and the need for high temperature during fabrication. The primary objective of this work is to optimize and develop an eco-friendly, highly efficient, Cesium-Titanium based novel inverted PV cell structure with all in-organic charge transport materials. The solar cell simulation program, SCAPS – 1D simulator, was used for conducting the numerical study and tested various device structures for obtaining maximum efficiency. The variations in the performance of PV cell structure were analyzed while using assorted absorber materials and inorganic charge transport materials. After the SCAPS-1D based simulation study, a high-performance PSC structure, FTO / MoO3 / Cs2TiBr6 / SnO2 / Au, was obtained. The proposed inverted PV cell structure can produce an excellent power conversion efficiency of 15.68 %, short-circuit current density of 16.59 mA/cm2, an open circuit voltage of 1.1 V, and a fill-factor of 85.76 %.

Linear and second‐order nonlinear optical properties of non‐fullerene acceptor derivatives with A–D–A structure

AbstractIn this paper, in order to study the relationship between structure and performance, four new non‐fullerene acceptor (NFA) derivatives were designed based on the two reported NFA molecules BO‐4Cl and BTP‐S2 by replacing the benzothiadiazole unit with a less‐electron deficient benzotriazole unit and inserting another ethylene double bond between the central core and the terminal groups. Density functional theory (DFT) and Time‐dependent DFT were applied to investigate linear and nonlinear optical (NLO) properties, such as electronic structure, electronic absorption, reorganization energy, and the second‐order NLO properties. The investigation demonstrates that they are all narrow bandgap derivatives, the absorption spectrum extends to the near‐infrared region and using two ethylene double bond is the most effective way to reduce the energy gap, redshift the maximum absorption peak and the middle absorption band, and enhance second‐order NLO response. Considering the smaller electron and hole reorganization energy and the larger static first hyperpolarizability value, the studied NFA derivatives have great potential to become ambipolar charge transport materials and large second‐order NLO materials.

Numerical simulation of novel lead-free Cs3Sb2Br9 absorber-based highly efficient perovskite solar cell

In this paper, a novel Cs 3 Sb 2 Br 9 perovskite is presented for the absorber layer of perovskite solar cells (PSC). A planar structure as ITO/TiO 2 /Absorber/Spiro-OMeTAD/Au with Cs 3 Sb 2 Br 9 as a light harvesting material is numerically simulated using SCAPS-1D simulation software. Initially, the optimization is performed on the various material parameters of absorber layer such as thickness, bulk defect density, relative permittivity and bandgap. The obtained optimal values for these parameters are 1000 nm, 1 × 10 12 cm −3 , 10, and 2.0 eV, respectively. Finally, the proposed PSC structure with optimized absorber layer is achieved the maximum power conversion efficiency (PCE) of 15.69% whereas V OC as 1.31 V, J SC as 13.67 mA/cm 2 , FF as 87.61%, and QE of ∼100% at 390 nm wavelength near the visible range of solar spectrum. The obtained results of numerical simulations with Cs 3 Sb 2 Br 9 as absorber layer endorse the novel proposed PSC material as the main body of PSC and opens a better route towards the development of a cost effective, highly efficient (Pb) lead-free and environment friendly PSC. • A novel Cs 3 Sb 2 Br 9 absorber layer material is proposed for design of high efficiency lead-free perovskite solar cell. • Numerical analysis and optimization of novel absorber in proposed PSC structure is performed using SCAPS-1D to investigate the effect of bulk defect density, absorber thickness, energy bandgap and permittivity variations on PSC parameters. • A planar structure consisting of charge transport materials TiO 2 and Spiro-OMeTAD used as ETL and HTL, is simulated for the analysis. • The proposed PSC is promising and may offer the PCE of 15.69% with V OC of 1.31 V , J SC of 13.67 mA/cm 2 and FF of 86.78%.

Inkjet printed organic light-emitting diodes employing organometal-halide perovskite as hole transport layer

Due to their narrow spectrum and high photoluminescence quantum yield, organic-inorganic hybrid perovskite materials have become an important emitter for light-emitting diodes (LED). In addition to the perovskite emitters, other perovskite materials such as methyl lead ammonium chloride (MAPbCl3) with high charge mobility can potentially be used as excellent charge transport materials. In this work, phosphorescence LED devices in which MAPbCl3 was employed as hole transport layer (HTL) was designed and fabricated by inkjet printing (IJP) process. Ethanolamine was added to the poly(3,4-ethylenedioxythiophene):poly-(styrenesulfonate) hole injection layer (HIL) to control the crystallization process and to suppress the surface defects of MAPbCl3. In addition, polyethylene oxide was doped into MAPbCl3 to improve the printability and the quality of film formation. The ‘blurred interface’ concept was successively applied to enable for the first time the IJP of three layers (HIL, HTL and emitter layers) in the LEDs. The fabricated multilayer LEDs achieved the maximum external quantum efficiency of 8.9%, maximum current efficiency of 30.8 cd A−1, and maximum power efficiency of 10.7 lm W−1. A 40 × 40 mm2 OLED light emitting device was successfully fabricated by IJP technology.

Structural Templating of an Organic Solar Cell Absorber by Ellagic Acid To Tune Its Aggregation, Molecular Orientation, and Optical Properties

Structural templating with homogeneous template layers is one of the strategies for controlling the orientation of small molecular absorbers in the photoactive layer of an organic solar cell to increase its power conversion efficiency. A main challenge thereby is the energetic alignment of the template molecules to the photoactive and charge-transporting materials. In the present study, the effects of a cluster-like template layer of ellagic acid (EA) on the morphology and optical properties of side-chain-substituted dicyanovinyl quaterthiophene (DCV4T-Et2) thin films are discussed. In the monolayer regime, J-aggregation of DCV4T-Et2 is confirmed. Insertion of the EA template layer leads to an improved aggregation behavior and formation of J-aggregates in DCV4T-Et2 films near the EA interface. The orientation of DCV4T-Et2 molecules in 30 nm thick films changes from “edge-on” to “face-on” due to a π–π interaction between the flat-lying EA molecules and the DCV4T-Et2 molecules. The face-on orientation by templating is preserved in blend layers with C60, and a considerable increase in the crystallinity of the DCV4T-Et2 phase in the blend is induced. Organic solar cells based on templated DCV4T-Et2:C60 active layers exhibit more than a 50% increase in the efficiency compared to nontemplated active layers. The short-circuit current density and the fill factor are significantly improved. Although the energetic alignment of EA is not ideal, no additional open-circuit voltage losses were observed with templating, due to the cluster-like morphology of the EA layer. Our results demonstrate a cluster-like templating approach with the novel template molecule EA to tailor the molecular orientation, crystallinity, and consequently optical properties of organic semiconducting molecules without significant energetic losses favorable for use in organic electronics.

Chemical bath deposition of AgBiS2 films for visible and X-ray detection

AgBiS2 possesses high absorption coefficient from ultra-violet to near-infrared, decent charge transport and superior materials stability, which has emerged as a promising candidate in the field of optoelectronics. However, efficient AgBiS2-based devices are mostly based on AgBiS2 quantum dots and precursor methods, which require either long preparation time and complex synthetic steps or inert atmosphere protection. To address these issues, we developed a novel and facile chemical bath deposition approach to deposit highly crystallized, uniform AgBiS2 thin films with decent optoelectronic properties. We also fabricated AgBiS2 heterojunction photodetectors based on this method, and achieved high responsivity covering the whole range of visible and near-infrared, fast response speed of ∼960 ns, and high specific detectivity of 8 × 1011 Jones and outstanding device stability. More importantly, we first demonstrated the device performance under X-ray illumination and observed decent sensitivity, implying great potential for low-cost, solution-processable X-ray detectors.