Interfacial Charge Carrier Recombination Research Articles

Collaborative role of co-doping and surface modifying for adjusting surface state of hematite to release the photoelectrochemical activity

Inhibition of bulk and interfacial charge carrier recombination is an urgent solution to release furthest the photoelectrochemical potential abilities of hematite photoelectrode. Herein, the Zr-Y dual metal co-doping engineering and Co/Fe dual metal-organic frameworks cocatalyst layer were employed for modifying hematite photoanode. The novel CoFe-MOF/Zr-Y:Fe2O3 nanoarray composite photoanode exhibits higher PEC activity, the photocurrent density at 1.23 V vs.RHE is increased by 4.7 times than that of the pristine α-Fe2O3. The gain in bulk charge separation efficiency was mainly attributed to the synergistic effect between the two doping ions to improve the electrical conductivity. Wherein, Zr4+ doping endows more free electrons in the bulk α-Fe2O3 and Y3+ introduction causes polaron mobility enhancement. For another, co-doping could reduce the recombination surface state, but form the intermediate surface state to promote hole transfer, thus effectively inhibiting the severe recombination of photogenerated electrons and holes. Furthermore, the CoFe-MOF cocatalyst coating with abundant active sites not only expedites the oxygen evolution reaction kinetics process, but also passivates the surface hole trapping state more thoroughly, resulting in efficient interfacial hole transfer and utilization. This work affords a new collaborative role of co-doping and surface modifying effective avenue for adjusting surface state of hematite to release the photoelectrochemical activity.

Polymer‐Wrapped Perovskite Single Crystal for Surface Passivation and Robust Stability

AbstractMetal halide perovskites (MHPs) emerging as a promising semiconducting material have shown superiority in optoelectronic applications. Currently, polycrystalline perovskite thin films are dominantly employed; while owing to the existence of grain boundary, the polycrystalline films typically have charge transport issues. To overcome these limitations, a perovskite single crystal (SC) is synthesized and applied owing to the more superior bulk charge transport properties compared with its polycrystalline counterpart. Nevertheless, due to the defective nature of the crystal surface, the surfaces of single crystals remain the bottleneck that not only induces stability issues but also causes interfacial charge carrier recombination. In this work, triple cation perovskite single crystals, FA0.8MA0.15Cs0.05PbI3 (denoted as FAMACs) are synthesized, and a facile surface modification strategy is reported for single crystals by polymer monolayer coupling, which shows high effectiveness for improving stability and reducing surface trap state density for single crystals. The polymer‐wrapped single crystals exhibit robust stability and a well‐passivated surface, which shows an order of magnitude lower trap density compared with those of unmodified single crystals and achieves a high responsivity of 5.069 A W−1 when used in photodetector applications.

Ternary Z-Scheme Ag-Embedded TiO2–Ag2S Nanojunction as a Novel Photoelectrochemical Converter for CD44 Detection

Nanoarrays (NAs) with stable signal output have become the most promising photoelectrochemical (PEC) biosensing substrate. However, a general issue is that interfacial charge-carrier recombination in a single-component semiconductor cannot be easily prevented, resulting in a low photocurrent density. Herein, a biosensor utilizing a Ag-embedded TiO2–Ag2S nanojunction (TiO2–Ag–Ag2S) as a signal converter was developed for the detection of CD44 protein─a transmembrane glycoprotein highly expressed in breast cancer cells. The ternary Z-scheme heterojunction was prepared by a distinctive scheme in which the Ag layer is introduced onto the surface of rutile TiO2 NAs by magnetron sputtering, whereas the Ag2S is rooted in the local sulfuration of Ag. With a sufficient density of oriented nanorods, TiO2–Ag–Ag2S exhibits a smooth photocurrent output and minimal variation among different batches; it is undoubtedly a satisfactory PEC sensing carrier, which enables highly specific identification of target CD44 on the surface of MDA-MB-231 cells due to DNA strand displacement reactions (SDRs) and host–guest recognition between hyaluronic acid (HA) and CD44. The biosensor shows a sensitive PEC response to CD44 over a wide range of 37 to 5.0 × 105 cells/mL. We can conclude that this approach will provide an alternative solution to breast cancer diagnosis.

Dual bulk and interface engineering with ionic liquid for enhanced performance of ambient-processed inverted CsPbI3 perovskite solar cells

All-inorganic cesium lead iodide (CsPbI3) perovskites with superior thermal stability are attractive candidates for perovskite solar cells (PSCs). Fabricating such inorganic PSCs in the ambient atmosphere is desirable for practical production, however, the challenge remains in inhibiting the phase transition of CsPbI3 in ambient air. Herein, we demonstrate a dual bulk and interface engineering using ionic liquid to stabilize CsPbI3 perovskite structure, thus enhancing the performance of ambient-processed inverted CsPbI3 PSCs. Such dual bulk and interface engineering is found effective not only in suppressing the bulk and interfacial charge carrier recombination and enhancing charge carrier transport and extraction, but also in protecting CsPbI3 crystal structure by leaving hydrophobic alkyl chains coverage at the boundary and surface to prevent phase transition caused by moisture from ambient air. The optimized device fully processed in the open air with relative humidity up to 55% exhibits remarkably enhanced efficiency and stability over the control device, with the efficiency increasing from 8.6% to 13.21%, and 92% efficiency maintaining after storage for 1680 h, which outperforms the control device with only 82% retaining after 648 h storage. We thus believe this work can provide an efficient alternative for the low-cost fabrication of ambient-processible PSCs.

Stable and efficient perovskite solar cells by discrete two-dimensional perovskites capped on the three-dimensional perovskites bilayer thin film

The combination of the three-dimensional (3D) perovskites and the two-dimensional (2D) perovskites to create the 2D/3D perovskites bilayer structure was demonstrated to be a facile way to realize stable and efficient perovskite solar cells (PSCs). In this study, we report stable and efficient PSCs based on the 2D/3D PEA2PbBr0.3I3.7/MAPbBr0.3I2.7 thin film, where PEA is phenethylammonium and MA is methylammonium (MA). Systematically studies demonstrate that the 2D PEA2PbBr0.3I3.7 crystals are discretely capped on the top of the 3D MAPbBr0.3I2.7 thin film. These discrete 2D PEA2PbBr0.3I3.7 crystals could improve water resistance due to the hydrophobic 2D PEA2PbBr0.3I3.7, but without sacrificing the optoelectronic properties of the 3D MAPbBr0.3I2.7. Moreover, the 2D MAPbBr0.3I3.7 crystals could elevate the build-in potential, suppress the interfacial charge carrier recombination, and reduce the dark current density of the PSCs based on the 2D/3D PEA2PbBr0.3I3.7/MAPbBr0.3I2.7 thin film. As a result, the PSCs based on the 2D/3D PEA2PbBr0.3I3.7/MAPbBr0.3I2.7 thin film exhibit over 23% power conversion efficiency and boosted stability. These results demonstrate that we have provided a facile way to engineer the 2D PEA2PbBr0.3I3.7 crystals on the surface of the 3D MAPbBr0.3I2.7 for approaching stable and efficient PSCs.

Suppressing PEDOT:PSS Doping-Induced Interfacial Recombination Loss in Perovskite Solar Cells.

PEDOT:PSS is widely used as a hole transport layer (HTL) in perovskite solar cells (PSCs) due to its facile processability, industrial scalability, and commercialization potential. However, PSCs utilizing PEDOT:PSS suffer from strong recombination losses compared to other organic HTLs. This results in lower open-circuit voltage (VOC) and power conversion efficiency (PCE). Most studies focus on doping PEDOT:PSS to improve charge extraction, but it has been suggested that a high doping level can cause strong recombination losses. Herein, we systematically dedope PEDOT:PSS with aqueous NaOH, raising its Fermi level by up to 500 meV, and optimize its layer thickness in p-i-n devices. A significant reduction of recombination losses at the dedoped PEDOT:PSS/perovskite interface is evidenced by a longer photoluminescence lifetime and higher magnitude of surface photovoltage, leading to an increased device VOC, fill factor, and PCE. These results provide insights into the relationship between doping level of HTLs and interfacial charge carrier recombination losses.

Metal-organic framework-tailored perovskite solar cells

Metal-organic frameworks (MOFs) with tailorable structures and building blocks have demonstrated their advantages in improving the long-term stability of perovskite solar cells (PSCs). However, the inferior conductivity of MOFs and their lack of strong chemical interaction with perovskites cause undesirable interfacial charge carrier recombination and then deteriorate the photovoltaic (PV) performance of PSCs. This perspective offers an insightful overview of the versatile functionalities and key merits of MOFs for stabilizing PSCs under various external stimuli in terms of MOF interlayers and MOF-perovskite heterostructures. To tackle the charge transport problem of MOFs, promising strategies are outlined to improve the intrinsic conductivity and chemical coordination of MOFs, with the aim of achieving long-term stable PSCs without compromising their PV performance. The current challenging issues and potential solutions are also discussed to provide a roadmap for MOF-tailored PSCs towards practical applications.

Layer-by-Layer Titanium (IV) Chloride Treatment of TiO2 Films to Improve Solar Energy Harvesting in Dye-Sensitized Solar Cells

A layer-by-layer titanium (IV) chloride treatment was applied on different layers of TiO2 in dye-sensitized solar cells (DSSCs). The effects were analysed and compared with standard untreated devices. A significant increase in short-circuit current density (JSC) was observed by employing layer-by-layer TiCl4 treatment of TiO2 in DSSCs. This increase of JSC is attributed to the increased inter-particle connectivity and increase in TiO2 nanoparticle size, resulting in better electron transfer and a lower charge carrier recombination rate. The DSSC fabricated with layer-by-layer-treated TiO2 achieved power conversion efficiency of 8.3%, which is significantly higher than the 6.7% achieved for the DSSC fabricated without TiCl4 treatment. Electrochemical impedance spectroscopy (EIS) was performed to assess the better performance of the device fabricated with TiCl4 treatment. Atomic force microscopy and surface roughness were studied to visualize and statistically determine the function of TiCl4 treatment on different layers of TiO2. Transient photocurrent and transient photovoltage measurements were also performed to gain insight into interfacial charge carrier recombination.

Carbon-Based All-Inorganic CsPbI2Br Perovskite Solar Cells Using TiO2 Nanorod Arrays: Interface Modification and the Enhanced Photovoltaic Performance

Recently, carbon-based all-inorganic perovskite solar cells (PSCs) caught the attention of photovoltaic researchers due to their remarkable stability. Herein, we report a carbon-based all-inorganic CsPbI2Br PSC using TiO2 nanorod arrays (TiO2 NRAs) as an electron transporting layer. The method of using TiO2 NRAs as an electron pathway has the advantage of increasing the electron–hole separation effectively and enables a thicker active perovskite layer to obtain a stronger absorbance. In addition, a water–alcohol soluble CsCl was used to modify the interface between TiO2 NRAs and the CsPbI2Br active layer to reduce interfacial recombination of charge carriers in the cells, and the CsCl interface-modified device showed a 10.6% increase in the open-circuit voltage. An optimized power conversion efficiency of 11.45% has been obtained, which is one of the superior performances reported for the carbon-based pure CsPbI2Br PSCs to date.

In-situ engineered MXene-TiO2/ BiVO4 hybrid as an efficient photoelectrochemical platform for sensitive detection of soluble CD44 proteins

Interfacial charge-carrier recombination is a bottle-neck issue restricting photoelectrochemical biosensors advancement in the wearable clinical electronics. In this study, we propose a simple approach to construct a highly efficient photoactive heterojunction capable of functioning as an active substrate in PEC biosensing of CD44 proteins. Taking the advantage of high photocatalytic activity of BiVO4, and biocompatible yet conductive 2D-Ti3C2Tx nanosheets, a workable heterojunction was constructed between in-situ formed TiO2 from the partially oxidized Ti3C2Tx and lysine functionalized BiVO4 (TiO2/MX-BiVO4). The interfacial arrangement was ideal for promoting fast charge transfer from photo-excited BiVO4 and TiO2 to Ti3C2Tx, constructing an energy level-cascade that permits minimal charge-carrier recombination besides robust photocatalytic redox activity. The PEC biosensor relies on the ligand-protein interaction, where hyaluronic acid was directly immobilized over TiO2/MX-BiVO4 based on the interactions between carboxyl of lysine and amino moieties of hyaluronic acid. The PEC biosensor response depends on the inhibition in the measured photo-oxidation current of mediator species, i.e., ascorbic acid after the addition of CD44 proteins. The superior photo-activity, and robust heterojunction arrangement, produced a sensitive signal capable of recognizing CD44 in the wide concentration window of 2.2 × 10−4 ng mL−1 to 3.2 ng mL−1 with a low-detection limit of 1.4 × 10−2 pg mL−1. The strong interaction between lysine functionalized BiVO4 and hyaluronic acid enabled biosensor to exhibit robust antifouling characteristics towards similar proteins such as PSA and NSE. The quantification of CD44 protein from real-blood serum samples further confirmed the biosensor's reliability for clinical application.

Highly conductive double perovskite oxides A2LuTaO6 (A = Ba, Sr, Ca) as promising photoanode material for dye sensitized solar cells

Poor electrical conductivity, interfacial charge carrier recombination, electrode degradation, etc., restrict practical implementation of the TiO2 based dye sensitised solar cells (DSSCs). In this respect, double perovskite oxides (DPOs) A2LuTaO6 (A = Ba, Sr, Ca) have been introduced as the promising DSSC photoanode. These solid-state synthesised DPOs exhibit a suitable wide band gap in the range of 2.94 to 4.07 eV, and 104 times higher order of electrical conductivity than TiO2. This study reveals that the crystal structure and hence the physical properties of these DPOs can also be easily modulated as required by varying its composition.

Energy level engineering of charge selective contact and halide perovskite by modulating band offset: Mechanistic insights

Mixed cation and anion based perovskites solar cells exhibited enhanced stability under outdoor conditions, however, it yielded limited power conversion efficiency when TiO2 and Spiro-OMeTAD were employed as electron and hole transport layer (ETL/HTL) respectively. The inevitable interfacial recombination of charge carriers at ETL/perovskite and perovskite/HTL interface diminished the efficiency in planar (n-i-p) perovskite solar cells. By employing computational approach for uni-dimensional device simulator, the effect of band offset on charge recombination at both interfaces was investigated. We noted that it acquired cliff structure when the conduction band minimum of the ETL was lower than that of the perovskite, and thus maximized interfacial recombination. However, if the conduction band minimum of ETL is higher than perovskite, a spike structure is formed, which improve the performance of solar cell. An optimum value of conduction band offset allows to reach performance of 25.21%, with an open circuit voltage (Voc) of 1231 mV, a current density Jsc of 24.57 mA/cm2 and a fill factor of 83.28%. Additionally, we found that beyond the optimum offset value, large spike structure could decrease the performance. With an optimized energy level of Spiro-OMeTAD and the thickness of mixed-perovskite layer performance of 26.56% can be attained. Our results demonstrate a detailed understanding about the energy level tuning between the charge selective layers and perovskite and how the improvement in PV performance can be achieved by adjusting the energy level offset.

Temperature dependent device characteristics of graphene/h-BN/Si heterojunction

Graphene-on-semiconductor heterojunction solar cell is an emerging class of photovoltaics (PV) with potential for efficient and reliable energy conversion systems. The interface between graphene and a lightly doped semiconductor plays a key role in charge-carrier separation and recombination dynamics. Owing to the low Schottky barrier height (ϕSBH)-induced interfacial charge carrier recombination, graphene-on-silicon (Si) heterojunction solar cells suffer from instability in power conversion efficiency over time. Therefore, it is critical to engineer the interface to enhance the barrier height by interfacing a chemically stable, insulating, and atomically thin layer. Further, the temperature dependent photovoltaic characteristics of such stacked architectures are unknown, and temperature dependent behavior is critical to understand the metal-insulator-semiconductor (MIS) junction behavior and photovoltaic phenomenon. Here, we have introduced hexagonal boron nitride (h-BN) as a tunneling interlayer in graphene-on-Si heterojunction solar cells, which enables the passivation of the chemical dangling bonds on the Si surface. The effect of temperature on the performance of a graphene/h-BN/Si PV cell is examined. Thin films of h-BN are directly synthesized on a lightly doped Si surface via a bottom-up chemical-surface-adsorption strategy followed by the transfer of a graphene monolayer. The 2D layer-on-2D layer-on-3D bulk semiconductor nanoarchitecture of graphene/h-BN/Si forms a MIS-type junction, where the h-BN acts as an electron-blocking layer to avoid interfacial charge carrier recombination. A four-fold increase in open-circuit voltage (Voc) is found for the graphene/h-BN/Si heterojunction cell (0.52 V) in contrast to the graphene/Si cell (0.13 V), which is due to the increase in the ϕSBH and hence built-in electric potential. Interestingly, the Voc linearly decreases by only ~4% with every 10 K increase in temperature. This work will lead to the evolution of new 2D/2D/3D nanoarchitectures for mechanically robust, high performance, and durable optoelectronic functionalities.

Solution-Processed Polymeric Thin Film as the Transparent Electrode for Flexible Perovskite Solar Cells.

In the past decade, perovskite solar cells (PSCs) were arising as a new generation of low-cost solar technology for renewable energy generation. More than 25% of power conversion efficiency (PCE) was reported from PSCs on the rigid indium tin oxide (ITO)/glass electrode. However, PSCs fabricated on flexible solution-processed transparent electrodes have still been a challenge to date. In this work, we report a solution-processed transparent polymeric thin film as the electrode for flexible solution-processed PSCs. The solution-processed polymeric thin film exhibits superior optical transparency and decent electrical conductivity. As compared with a PCE of 16.60% from PSCs on the ITO/glass substrate, PSCs on the solution-processed transparent polymeric electrode/glass substrate exhibit a PCE of 13.36% and PSCs on the solution-processed transparent polymeric thin-film/polyethylene terephthalate flexible substrate possess a PCE of 10.16%. Systematic studies demonstrate that poor electrical conductivity of the solution-processed transparent polymeric electrode and serious interfacial charge carrier recombination are responsible for low PCEs. Nevertheless, our results demonstrate that we provide a facile route to develop flexible PSCs by utilization of solution-processed polymeric thin films as the transparent electrodes.

Band Alignment Engineering Towards High Efficiency Carbon-Based Inorganic Planar CsPbIBr2 Perovskite Solar Cells.

Perovskite CsPbIBr2 is attracting ever-increasing attention for carbon-based, all-inorganic solar cells, owing to its well-balanced band gap and stability features. However, significant interfacial recombination of charge carriers in solar cells fabricated with this active layer, which is intrinsically associated with the unwanted conduction band misalignment between CsPbIBr2 and the commonly used TiO2 electron transport layer, has limited power conversion efficiency (PCE) values. Herein, we demonstrate successful conduction band alignment engineering at the TiO2 /CsPbIBr2 heterojunction by modifying TiO2 with CsBr clusters. Such modification triggers a beneficial increase in the conduction band minimum (CBM) of TiO2 from -4.00 to -3.81 eV and decreases the work function from 4.11 to 3.86 eV, thus promoting favorable band alignment at the heterojunction, suppressing recombination, and improving extraction and transport of charge carriers. As a result, the carbon-based, all-inorganic CsPbIBr2 solar cells exhibit over 20 % enhancement in average PCE. The champion device achieves a PCE of 10.71 %, a record among pure CsPbIBr2 -based cells, open-circuit voltage of 1.261 V, and excellent stability.

Surface modification of ZnO nanorods with CdS quantum dots for application in inverted organic solar cells: effect of deposition duration

Incorporating cadmium sulfide quantum dots (CdS QDs) onto ZnO nanorod (ZNRs) has been investigated to be an efficient approach to enhance the photovoltaic performance of the inverted organic solar cell (IOSC) devices based on ZNRs/poly (3-hexylthiophene) (P3HT). To synthesize CdS/ZNRs, different durations of deposition per cycle from 1 to 9 min were used to deposit CdS via SILAR technique onto ZNRs surface grown via hydrothermal method at low temperature on FTO substrate. In typical procedures, P3HT as donor polymer were spun-coating onto CdS/ZNRs to fabricate IOSC devices, followed by Ag deposition as anode by magnetron sputtering technique. Incorporation of CdS QDs has modified the morphological, structural, and optical properties of ZNRs. Incorporation of CdS QDs onto ZNRs also led to higher open circuit voltage (Voc) and short circuit current density (Jsc) of optimum ZNRs/CdS QDs devices due to the increased interfacial area between ZNRs and P3HT for more efficient exciton dissociation, reduced interfacial charge carrier recombination as a result of lower number of oxygen defects which act as electron traps in ZnO and prolonged carrier recombination lifetime. Therefore, the ZNRs/CdS QDs/P3HT device exhibited threefold higher PCE (0.55%) at 5 min in comparison to pristine ZNR constructed device (0.16%). Overall, our study highlights the potential of ZNRs/CdS QDs to be excellent electron acceptors for high efficiency hybrid optoelectronic devices.

Strategic PbS quantum dot-based multilayered photoanodes for high efficiency quantum dot-sensitized solar cells

In2S3 is employed as a new passivation layer in PbS quantum dot-based multilayered photoanodes to coalesce the improvement in photocurrent density (JSC) and open-circuit voltage. Suppression of detrimental interfacial charge carrier recombination in In2S3/PbS/TiO2 and In2S3/PbS/CdS/TiO2 multilayered photoanodes, attributed to the In2S3 passivation layer, leads to increased VOC in both photoanodes. In particular, In2S3/PbS/CdS/TiO2 multilayered photoanode exhibited a substantial improvement of ∼36% (from 3.2 to 4.3%) in power conversion efficiency, which is attributed to significant increase in JSC value of ∼26mA/cm2 by enhanced PbS loading and co-sensitizing effect of CdS along with substantially suppressed interfacial recombination. In-depth electrochemical impedance spectroscopic analysis revealed that the resistance against back-transfer of electrons to electrolyte can be subtly modulated by incorporating In2S3 deposition over PbS. Further, this multiple passivation layer turned out to be beneficial for improving photocorrosion of PbS in a polysulfide electrolyte.

High Performance Perovskite Hybrid Solar Cells with E-beam-Processed TiOx Electron Extraction Layer.

Perovskite hybrid solar cells (pero-HSCs) have drawn great attention in the last 5 years. The efficiencies of pero-HSCs have been boosted from 3.8% to over 20%. However, one of the bottlenecks for commercialization of pero-HSCs is to make a high electrical conductive TiOx electron extraction layer (EEL). In this study, we report high performance pero-HSCs with TiOx EEL, where the TiOx EEL is fabricated by electron beam (e-beam) evaporation, which has been proved to be a well-developed manufacturing process. The resistance of the e-beam evaporated TiOx EEL is smaller than that of sol-gel processed TiOx EEL. Moreover, the dark current densities and interfacial charge carrier recombination of pero-HSCs incorporated with e-beam processed TiOx EEL is also smaller than that of pero-HSCs incorporated with sol-gel processed TiOx EEL. All these result in efficient pero-HSCs with high reproducibility. These results demonstrate that our method provides a simple and facile way to approach high performance pero-HSCs.

Si/PEDOT:PSS core/shell nanowire arrays for efficient hybrid solar cells

A solution filling and drying method has been demonstrated to fabricate Si/PEDOT:PSS core/shell nanowire arrays for hybrid solar cells. The hybrid core/shell nanowire arrays show excellent broadband anti-reflection, and resulting hybrid solar cells absorb about 88% of AM 1.5G photons in the 300-1100 nm range. The power conversion efficiency (PCE) of the hybrid solar cell reaches 6.35%, and is primarily limited by direct and indirect interfacial recombination of charge carriers.

Effect of heat and light on the performance of dye-sensitized solar cells based on organic sensitizers and nanostructured TiO 2

The dye-sensitized solar cells based on the functionalized organic sensitizer, 3-(5-(5-(4-(bis(4-(hexyloxy)-phenyl)amino)phenyl)thiophen-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (D21L6) exhibit comparable stability to the ruthenium sensitizers under 60 °C heat stress and AM 1.5 light illumination where the overall efficiency remained at above 90% of the initial value after 1000 h. However, at 80 °C heat stress without light soaking the overall efficiency of dye-sensitized solar cell remained at 80% of the initial value after 1000 h. The main reason for loss of performance was a decrease in open circuit photovoltage (Voc) under light soaking conditions and in photocurrent (Jsc) under heat stress, respectively. In this regard, interfacial charge carrier recombination and change of capacitance due to aging under light soaking or heat stress were studied.