Heterojunction Films Research Articles

Single-layer organic photovoltaics fabricated via solution-based electrical doping of ternary bulk heterojunction films

The commercial viability of organic photovoltaics (OPVs) can be improved by simplifying their device geometry and easing fabrication complexity. Here, we demonstrate that solution-based p-type electrical doping of ternary bulk heterojunction (BHJ) films, which comprise 2 donor polymers and 1 fullerene acceptor (2D:1A), enables the realization of efficient single-layer OPVs. Systematic and detailed investigations of the optoelectronic characteristics of films with varying donor ratios, and their photovoltaic performance, demonstrate p-type electrical doping via post-process immersion into a 12-molybdophosphoric acid hydrate (PMA) solution, resulting in a reduced trap density and charge recombination without significantly changing the BHJ morphology. Furthermore, PMA doping of films comprising optimized ternary blend compositions and polyethylenimine enables the demonstration of single-layer OPVs with economic top electrode metals and a high level of performance under outdoor and indoor illumination conditions. These PMA-doped 2D:1A BHJ films are an attractive platform to reduce the efficiency-cost gap and accelerate the commercialization of OPVs for emerging applications.

Efficient inverted PM6:Y6 solar cells with tuned lateral phase separation and vertical composition distribution via bis(5-chlorothiophen-2-yl)alkane additives

Finely controlling the morphology of bulk heterojunction (BHJ) films by additives provides a critical and versatile tool to boost performance of solution processed polymer solar cells (PSCs). In great need are efficient additives that can optimize the arrangement of energetically paired donor and acceptor materials to propel individual photoelectric conversion steps close to unity either in conventional or inverted PSCs. Alternative to the paradigm additives such as 1-chloronaphthlene (CN) etc. for highly efficient conventional PSCs with Y6-type acceptors, herein, bis(5-chlorothiophen-2-yl)alkanes (BCTA-m) with 3 ∼ 6 methylene units (m = 3, 4, 5 and 6) were incorporated as solvent additives to fabricate inverted PM6:Y6 PSCs. The lateral phase separation and vertical composition distribution between PM6 and Y6 were discovered tunable by varying the length (m) of alkanes. The BCTA-4 device offers the maximum power conversion efficiency (PCE) of 15.59%, which is superior to the ones processed from the other BCTA-m (m = 3, 5 and 6) additives and comparable to that of the CN reference (15.62%). The outperformed exciton generation and dissociation, charge transport and suppressed charge recombination are evidently observed within the BCTA-4 device. Furthermore, the lateral phase-separated domain sizes between the pure Y6 (2Rg-Y6) and the PM6:Y6 mixture (ξ) get an optimum ratio. The overwhelmingly vertical distribution of PM6 near the anode is also confirmed in the BCTA-4 device over the ones processed from the other three analogues. This work demonstrates the potential of BCTA-4 additive to realize high-performance inverted PSCs based on Y6-type acceptors.

Detrimental Effects of “Universal” Singlet Photocrosslinkers in Organic Photovoltaics

The use of singlet photocrosslinkers based on diazirine or azide has been investigated for organic photovoltaics (OPVs) to stabilize the morphology of mixed bulk heterojunction (BHJ) films. The photocrosslinking reactions in BHJs have major detrimental effects on the performance of OPVs. This is in striking contrast to previous reports on organic field-effect transistors, where these singlet crosslinkers can solidify the films without affecting the charge transport properties. Based on the OPV results with the layer-by-layer-deposited films and the transferred films, we found that the crosslinking reactions had greater effects on the non-fullerene acceptor than on the donor polymer. The photoluminescence measurements suggested that the crosslinkers could react with the π-conjugated backbone of the non-fullerene acceptor to form trace amounts of byproducts that cause severe exciton quenching. These results indicate that the use of the “universal” singlet photocrosslinkers in OPVs requires careful selection of the organic semiconducting materials and crosslinkers.

In-situ electrochemical-ion-exchange synthesis of S-scheme 1D/2D BiPO4/BiOBr heterojunction film from Bi plate with highly efficient photocatalytic CO2 reduction activity

In-situ electrochemical-ion-exchange synthesis of S-scheme 1D/2D BiPO4/BiOBr heterojunction film from Bi plate with highly efficient photocatalytic CO2 reduction activity

Exceptional visible-light photoelectrocatalytic activity of dual Z-scheme Bi@BiOI-Bi2O3/C3N4 heterojunction for simultaneous remediation of Cr(VI) and phenol

Developing highly efficient and stable photocatalysts remains a major challenge for the remediation of environmental pollutants. In this work, the Bi0 decorated BiOI-Bi2O3/C3N4 heterojunction (Bi@BiOI-Bi2O3/C3N4) film was fabricated through ultrasonic stripping, I− etching and in situ UV-reduction processes and then characterized thoroughly by various analytical techniques. The characteristics of simultaneous mitigation of phenol and Cr(VI) were evaluated over Bi@BiOI-Bi2O3/C3N4 photoanode under visible light. The results exhibited that both phenol and Cr(VI) were removed completely by the photoanode at 2.5 V within 1.5 h, superior to our previous report. The synergy of the surface plasmon resonance (SPR) effect of Bi0 and ternary heterojunction accelerated the separation and transfer of photo-induced charge carrier and thus heavily promoted the removal efficiency. Moreover, the excellent stability of this photoanode was hold with no considerably activity attenuation after 4 cycles. Finally, a dual Z-scheme charge transfer process was presented. This work offers an attractive pathway to construct highly active photoelectrode with promising application for simultaneous remediation of organics and heavy metals in wastewater.

Efficient and stable organic solar cells enabled by multicomponent photoactive layer based on one-pot polymerization

Degradation of the kinetically trapped bulk heterojunction film morphology in organic solar cells (OSCs) remains a grand challenge for their practical application. Herein, we demonstrate highly thermally stable OSCs using multicomponent photoactive layer synthesized via a facile one-pot polymerization, which show the advantages of low synthetic cost and simplified device fabrication. The OSCs based on multicomponent photoactive layer deliver a high power conversion efficiency of 11.8% and exhibit excellent device stability for over 1000 h (>80% of their initial efficiency retention), realizing a balance between device efficiency and operational lifetime for OSCs. In-depth opto-electrical and morphological properties characterizations revealed that the dominant PM6-b-L15 block polymers with backbone entanglement and the small fraction of PM6 and L15 polymers synergistically contribute to the frozen fine-tuned film morphology and maintain well-balanced charge transport under long-time operation. These findings pave the way towards the development of low-cost and long-term stable OSCs.

Smooth and Compact All‐Inorganic Perovskite Based Heterojunction Films Enabled by Nanocomposite‐Buffered Thermal Imprint for Highly Sensitive X‐Ray Detection

AbstractChemical vapor deposition (CVD) is a promising way to fabricate perovskite polycrystalline films for optoelectronic device applications. However, it remains challenging to prepare thick yet smooth and compact all‐inorganic perovskite films by this method, which is of paramount importance for efficient X‐ray detection. In this work, a nanocomposite‐buffered thermal imprint strategy is proposed to improve the surface roughness and eliminate pinholes in CVD‐processed cesium‐lead‐bromide perovskite films with thickness above 40 µm, which is realized by spray‐coating a soft layer composed of organic semiconductors and perovskite quantum dots on top of the perovskite layer and then perform the thermal imprint to prepare the heterojunction films. It is demonstrated that the thermal imprint process can reduce the dark current and noise, enhance the carrier transport properties, as well as improve the spatial uniformity of the devices. Consequently, the optimal detectors show a large sensitivity up to 2 × 104 µC Gyair−1 cm−2 and a very low detection limit of 15 nGyair s−1 that represents one of the most sensitive polycrystalline perovskite X‐ray detectors to date, which highlights the great potential of thermal‐imprinted heterojunction films for realizing high performance X‐ray detection and imaging.

Bias dependent NDR in TiO2/NiO heterojunction diodes

TiO2/NiO thin film heterojunction diodes are fabricated using electron beam evaporation and DC sputtering techniques. Three different thicknesses of NiO (58, 82, and 160 nm) are combined with a TiO2 film of about 80 nm thick. The common device configuration is FTO/TiO2/NiO/Au with a 4 mm sq. device area. The thickness dependent diode I-V characteristics were analyzed; with the increase in thickness, the diode knee voltage, series resistance, and ideality factor have reduced. All the devices exhibit a high diode ideality factor, which means the diode currents are not limited by drift, diffusion, or recombination at the space charge region. The high ideality factor points to the presence of interface states, spatial inhomogeneities, or tunneling in the devices. Further, the bias dependent negative differential resistance (NDR) in I-V characteristics and the presence of an anomalous peak in C-V characteristics were analyzed, which claimed the presence of interfacial trap states, defects, or tunneling in the diodes. The Fowler-Nordheim tunnel model was applied to the devices and the field enhancement factors were calculated as −14.3, −9.3, and −9.2 for the diodes in the increasing order of thickness. The tunneling phenomenon was confirmed as trap-assisted indirect tunneling. Also, the temperature dependence of NDR was studied.

High-efficiency photocathodic protection performance of novel MnIn2S4/TiO2 n-n heterojunction films for Q235 carbon steel in chloride-containing simulated concrete pore solution

In this work, the n-type TiO2 nanotube film was modified successfully by n-type MnIn2S4 nanoparticles with one-step hydrothermal method, and a novel MnIn2S4/TiO2 heterojunction material with excellent photoelectrochemical properties was constructed firstly. Owing to the modification, the light response range of MnIn2S4/TiO2 nanocomposites was extended to the visible light region. Benefiting from the presence of type II n-n heterojunction with built-in electric field effects in the nanocomposites, the photogenerated carriers recombination rate decreased significantly. In addition, the photocathodic protection effects of MnIn2S4/TiO2 (MIS/T) photoanodes on Q235 carbon steel (Q235 CS) immersed in simulated concrete pore solution containing chloride were investigated. Under visible light irradiation, the cathodic protection current density of MIS/T-2 photoanode was found to be more than 30 μA/cm2 and over 10 times that of T photoanode. The MIS/T-2 photoanode has good long-term photochemical stability and maintains excellent photocathodic protection effect on Q235 CS (OCP value are about - 870 mV (vs. SCE)) under visible light irradiation. The amount of MnIn2S4 nanoparticles deposited on the TiO2 film could affect the photoelectrochemical properties of MnIn2S4/TiO2 n-n heterojunctions and an appropriate number of the nanoparticles, corresponding to the best cathodic protection performance was revealed.

Unraveling Its Intrinsic Role of CH3NH3Cl Doping for Efficient Enhancement of Perovskite Solar Cells from Fine Insight by Ultrafast Charge‐Transfer Dynamics

The addition of CH3NH3Cl (MACl) in perovskite precursor has become one of the most effective strategies for enhancing the photovoltaic performance of perovskite solar cells (PSCs). To further determine its relevant intrinsic modification mechanism, a series of PSCs with variant MACl contents are prepared. Apart from the analysis of crystal morphology and defect states, molecular‐level photophysical processes related closely to photovoltaic performance are systematically investigated by transient absorption (TA) and time‐resolved photoluminescence spectroscopy. Promisingly, by a diffusion‐coupled charge‐transport model via global fitting of TA spectra, the kinetic of perovskite/SnO2 heterojunction films is resolved into four distinct photophysical processes. Among the processes, as the MACl concentrations increase, the charge carriers’ bulk diffusion in perovskite and interfacial transfer in perovskite/SnO2 heterojunction accelerate simultaneously, while the back charge recombination from SnO2 to perovskite decelerates, which correlates closely with larger grains featuring fewer grain boundaries and defect sites of perovskite induced by MACl doping. The aforementioned modified charge dynamics constitute the origin of the excellent optoelectronic properties in the resultant device, which exhibits an optimal conversion efficiency of 23.6%.

Fabrication of Porous Hydrophilic CN/PANI Heterojunction Film for High-Efficiency Photocatalytic H2 Evolution

The modulation of surface wettability and morphology are essential to optimize the photocatalytic H2 evolution activity of graphitic carbon nitride (CN)-based photocatalysts. In this work, the porous hydrophilic CN/PANI heterojunction film was prepared via interfacial polymerization and loaded on a porous PCL substrate. The construction of the type-II CN/PANI heterojunction enabled an overall spectrum response and the efficient separation and transportation of photoexcited charge carriers. The fabricated CN/PANI solid-state film in comparison with its powder counterpart elevated the utilization efficiency and maintained the long-term stability of photocatalyst. The porous morphology and hydrophilic surface increased the surface area and enhanced the surface wettability, favoring water-molecule adsorption and activation. The as-prepared CN/PANI heterojunction film exhibited photocatalytic H2 production activity up to 3164.3 μmol·h−1·g−1, which was nearly 16-fold higher than that of pristine CN (569.1 μmol·h−1·g−1).

Exploring the Effects of Crystal Facet Orientation at the Heterojunction Interface on Charge Separation for Photoanodes

As one of the most effective strategies to promote the spatial separation of charges, constructing heterojunction has received extensive attention in recent years. However, it remains unclear whether the crystal facet orientation (CFO) at the heterojunction interface is contributory to charge separation. Herein, three types of TiO2/CdS heterojunction films with different CFOs at the heterojunction interface were produced by adjusting the CdS CFO through in situ conversion. Among them, the TiO2/CdS film with a mixed CdS CFO showed the maximum photocurrent density and charge separation efficiency. In contrast, the TiO2/CdS film with a uniform CdS (100) (CdS-100) performed worst. According to the results of experimentation and DFT calculation, these three types of TiO2/CdS films varied significantly in electron transport time. This is attributable to the different Fermi levels of CdS CFO and the formation of different built-in electric fields upon coupling with TiO2. The rise in the Fermi level of CdS can increase the driving force required for charge migration at the heterojunction interface. Additionally, a stronger built-in electric field is conducive to charge separation. To sum up, these results highlight the significant impact of CFO at the heterojunction interface on charge separation.

Extracting energetic disorder in organic solar cells using percolation models

The energetic disorder σ describes the energy state distribution in organic semiconducting materials. In organic solar cells (OSCs), energetic disorder is an important parameter for evaluating the charge transport behavior, and it is strongly correlated with the device performance. Thus far, a widely used approach for extracting energetic disorder values in OSCs is the Gaussian disorder model (GDM), in which the disorder values can be extracted by fitting the slope of ln μ ∼ 1 T 2 , where μ is the charge mobility and T is the temperature. Herein, we demonstrate the potential of the percolation approach to evaluate the energetic disorder values in OSCs and compare them with the data obtained using the GDM approach. Two typical non-fullerene acceptor (NFA)-based bulk heterojunction (BHJ) films, with PTB7-Th:ITIC and PM6:Y6, were selected as the model systems. When the percolation models were adopted in the two BHJ films, the energetic disorder values extracted from the Grünewald/Thomas and Nenashev percolation models gave similar results for electron transport in the PTB7-Th:ITIC and PM6:Y6 BHJ films. This work successfully demonstrates the feasibility of microresistance analysis in BHJ systems and the application potential of the percolation model for extracting energetic disorders in OSCs.

Construction of chiral-2D/3D perovskite heterojunction films for efficient circularly polarized light detection

A chiral-2D/3D perovskite heterojunction is prepared via intermittent spin-coating of the chiral-2D ligands on top of a 3D perovskite film, which shows enhanced circularly polarized light detection performance as compared to the chiral-2D based ones.

The impact of film deposition and annealing on the nanostructure and dielectric constant of organic semiconductor thin films.

The strategy of using a bulk-heterojunction light-absorbing layer has led to the most efficient organic solar cells. However, optimising the blend morphology to maximise light absorption, charge generation and extraction can be challenging. Homojunction devices containing a single component have the potential to overcome the challenges associated with bulk heterojunction films. A strategy towards this goal is to increase the dielectric constant of the organic semiconductor to ≈10, which in principle would lead to free charge carrier generation upon photoexcitation. However, the factors that affect the thin film dielectric constants are still not well understood. In this work we report an organic semiconductor material that can be solution processed or vacuum evaporated to form good quality thin films to explore the effect of chromophore structure and film morphology on the dielectric constant and other optoelectronic properties. 2,2'-[(4,4,4',4'-Tetrakis{2-[2-methoxyethoxy]ethyl}-4H,4'H-{2,2'-bi[cyclo-penta[2,1-b:3,4-b']dithiophene]}-6,6'-diyl)bis(methaneylylidene)]dimalononitrile [D(CPDT-DCV)] was designed to have high electron-affinity end groups and low ionisation-potential central moieties. It can be processed from solution or be thermally evaporated, with the film morphology changing from face-on to a herringbone arrangement upon solvent or thermal annealing. The glycol solubilising groups led to the static dielectric constant (taken from capacitance measurements) of the films to be between 6 and 7 (independent of processing conditions), while the optical frequency dielectric constant depended on the processing conditions. The less ordered solution processed film was found to have the lowest optical frequency dielectric constant of 3.6 at 2.0 × 1014 Hz, which did not change upon annealing. In contrast, the more ordered evaporated film had an optical frequency dielectric constant 20% higher at 4.2 and thermal annealing further increased it to 4.5, which is amongst the highest reported for an organic semiconductor at that frequency. Finally, the more ordered evaporated films had more balanced charge transport, which did not change upon annealing.

A paradigm study of polymer donor diluted bulk heterojunction films for application in semitransparent organic photovoltaics

More J-type polymer aggregations and the prioritized polymer reduction from its pure domain help to achieve high LUE in ST-OSCs during the polymer donor diluting process.

Regulation of solar cell performance by cadmium sulfide/copper-based thin film heterojunction annealing under different atmospheres

Efficient copper based thin film solar cells usually use inorganic n-type semiconductor material CdS as the buffer layer. Therefore, the interface quality and energy band matching between the buffer layer and the absorption layer are crucial to the collection and utilization of carriers. Heat treatment can promote the mutual diffusion of interface elements, the migration of ions in the material, and the change of defect state, and the appropriate temperature will change the Cu-Zn ordering degree in the absorption layer, so as to improve the efficiency of the solar cells. Based on the optimization of CdS basic process, the strategy of annealing CdS/copper-based thin film heterojunction in sulfur atmosphere further improves the quality of CdS thin film, and is applied to copper-based solar cells to regulate the p-n heterojunction energy band gap matching of copper-based thin film cells. The results show that the annealing of CdS thin film in sulfur-containing inert atmosphere can effectively improve the crystal quality of CdS thin film and inhibit the non-radiative recombination loss caused by defect trapping at the interface of CZTS/CdS heterojunction, and the open-circuit voltage of the device can significantly increase to 718 mV. In addition, annealing CZTS/CdS heterojunction in S/Ar atmosphere can effectively improve the p-n heterojunction energy band gap matching, which not only improves the electron transmission, but also reduces the carrier recombination, thus improving the <i>V</i><sub>oc</sub> and FF of the device. Besides, the oxygen element in CdS film can be replaced by sulfur element in sulfur atmosphere to improve the quality of CdS film, and thus enhancing the short-wave absorption of solar cell device. Therefore, in terms of device efficiency, the efficiency of CZTS solar cell based on sputtering method increases from 3.47% to 5.68%, which is about twice that of non-annealing treatment. Its device structure is Glass/Mo/CZTS/CdS/i-ZnO/Al:ZnO/Ni/Al, providing a reliable process window for copper based thin film solar cell devices to achieve high open-circuit voltage. Meanwhile, this study strongly demonstrates the importance of annealing atmosphere selection for CdS quality and energy band matching of CZTS/CdS heterojunction. In addition to interface interdiffusion, the composition and crystallinity of thin film material are controlled.

Fluorination of Terminal Groups Promoting Electron Transfer in Small Molecular Acceptors of Bulk Heterojunction Films.

The fluorination strategy is one of the most efficient and popular molecular modification methods to develop new materials for organic photovoltaic (OPV) cells. For OPV materials, it is a broad agreement that fluorination can reduce the energy level and change the morphology of active layers. To explore the effect of fluorination on small molecule acceptors, we selected two non-fullerene acceptors (NFA) based bulk heterojunction (BHJ) films, involving PM6:Y6 and PM6:Y5 as model systems. The electron mobilities of the PM6:Y5 and PM6:Y6 BHJ films are 5.76 × 10-7 cm2V-1s-1 and 5.02 × 10-5 cm2V-1s-1 from the space-charge-limited current (SCLC) measurements. Through molecular dynamics (MD) simulation, it is observed that halogen bonds can be formed between Y6 dimers, which can provide external channels for electron carrier transfer. Meanwhile, the "A-to-A" type J-aggregates are more likely to be generated between Y6 molecules, and the π-π stacking can be also enhanced, thus increasing the charge transfer rate and electron mobility between Y6 molecules.

Intrinsic polarization-sensitive organic photodetector with self-assembled all-polymer heterojunction

Intrinsic polarization-sensitive photodetectors (IPPDs) have attracted considerable attention in recent years due to their simplicity in configuration, making them ideal candidates for compact and integrated polarization-sensitive sensing and imaging systems. Photoactive films with intrinsic optical anisotropy are necessary for IPPDs. This study reports an achievement of photoactive films based on all-polymer heterojunction films with in-plane optical anisotropy using a simple bottom-up self-assembly method. Both the donor (TQ1) and acceptor (N2200) polymers have the same spatial orientation with distinct anisotropy, approaching a dichroic ratio (DR) of 8. Polarization-sensitive light absorption is due to the uniaxially oriented polymer chains, which are dominated by lamellar packing with edge-on orientation. For IPPDs based on this anisotropic all-polymer heterojunction film, a photocurrent anisotropy was found with a polarized photocurrent ratio of 2.6. The detectivity of these IPPDs was found to be 1.9 × 1011 Jones (@ ∼600 nm, 0 V bias). Our work shows that oriented polymer donor–acceptor films fabricated using bottom-up self-assembly have great potential in applications, such as polarization detection.

Band offset measurement at the MAPbBr3/Al2O3 heterointerface by X-ray photoelectron spectroscopy

Recently, Al2O3 has been demonstrated to be an effective material type for perovskite photoelectric device performance improvement. It is therefore essential to investigate the interfacial electronic characteristics and the energy-level diagrams of perovskite/Al2O3 heterojunctions in depth. In this research, we fabricated an MAPbBr3 microcrystal/Al2O3 film heterojunction and determined the band offset by performing X-ray photoelectron spectroscopy measurements. The resulting valence band offset of ΔEv = 0.87 ± 0.01 eV and the conduction band offset of ΔEc = 3.03 ± 0.01 eV indicated occurrence of a type-I band alignment at the MAPbBr3/Al2O3 heterointerface. The carrier confinement effect was reflected in the photoluminescence spectra obtained for the heterojunction sample. This work will be valuable for optimization of the energy level alignment of perovskite/metallic oxide heterojunctions during the design of photoelectric devices.