Charge Carrier Extraction Research Articles

Surface dipole assisted charge carrier extraction in inverted architecture perovskite solar cells

Engineering the energetics of perovskite solar cells through the introduction of surface dipoles that assist with charge carrier extraction is a promising route to enhance the device performance without altering other device layers or fabrication parameters. In this work, we introduce four different derivatives of dicationic phosphonium-bridged ladder stilbenes (PYMC) in inverted perovskite solar cells with the device structure of ITO/Meo-2pacz/perovskite/PYMC/phenyl-C61-butyric acid methyl ester (PCBM)/bathocuproine/Ag. We show that the derivatives introduce a dipole at the perovskite/PCBM interface, which for derivatives with suitable energy levels can enhance the charge carrier extraction, leading to a quenched photoluminescence of perovskite thin films and an improved photovoltaic performance. As a result, both a higher average and maximum power conversion efficiency could be achieved and an overall better device reproducibility. This work highlights the significant potential of energetics engineering between perovskites and transport layers in perovskite solar cells for highly efficient photovoltaic devices.

SnO2 Passivation and Enhanced Perovskite Charge Extraction with a Benzylamine Hydrochloric Interlayer

Perovskite solar cells (PSCs) have gained much attention because of their expressive power conversion efficiency (PCE) of up to 25.5%. A good contact and a well-aligned energy level at the buried interfaces between electron transport layers (ETLs) and perovskite films play an essential role in promoting charge-carrier collection and suppressing nonradiative recombination. Currently, low-temperature-processed SnO2 thin films are widely used as the ETLs to achieve efficient and stable planar PSCs. However, fabricating proper SnO2/perovskite interfaces with a good contact and a well-aligned energy level is necessary but implies a great challenge. Herein, we modify the SnO2 ETL using benzylamine hydrochloride (BH), which is expected to facilitate the energy level alignment and to enhance perovskite crystallization. Moreover, the BH interlayer is found to effectively reduce the trap-state density and thereby improve the charge-carrier extraction between the ETL and the perovskite layer. Consequently, the PSC with BH modification yields a higher PCE, a lower hysteresis, and better stability than the device without a BH interlayer. This study highlights the key role of molecule modification of ETLs in designing efficient and stable PSCs.

Efficiency‐Enhanced Scalable Organic Photovoltaics Using Roll‐to‐Roll Nanoimprint Lithography

Light-trapping nanostructures have for decades been researched as a route to enhance the performance of organic solar cells (OSCs). Whereas the power conversion efficiencies (PCEs) of OSCs have reached above 18 %, industrially compatible devices made by scalable processing in air, using only nontoxic solvents and materials, have shown significantly lower performance values. Although light-trapping nanostructures may improve this, the methods for integrating the nanostructures are typically not compatible with industrial scale up. In this work, scalable, industrially compatible, nonfullerene-based OSCs are developed with integrated light-trapping nanostructures at the back electrodes in the devices. The OSCs are made by using scalable roll-to-roll (R2R) and sheet-to-sheet (S2S) processes and the nanostructures are made by using roll-to-plate (R2P) nanoimprint lithography. A fully scalable solution is thereby developed for industrially compatible nanostructured OSCs. The nanostructured devices show enhancements in PCE up to 25 % compared to reference cells, owing to an enhancement in the short-circuit current density (15 %) by enhanced absorption, and improved charge carrier extraction leading to an enhancement in the fill factor (7 %). Optical modeling is utilized to verify the optical effect of the nanostructures. The best devices attain a PCE of 6.5 %, which is the highest reported efficiency for air-processed slot-die coated ITO-free flexible PBDB-T : ITIC devices, here using nontoxic solvents.

Impact of the reverse bias voltage on the lifetime of polymer solar cells

We report an investigation into the impact of applying a large reverse bias voltage during current–voltage (J-V) sweeps on the degradation rate of solar cells based on poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6-6]-phenyl C71-butyric acid methyl ester (PC70BM), under air mass 1.5 simulated solar irradiation. The degradation rate was found to be significantly slower when devices were characterized using frequent J-V sweeps with a large reverse bias component (-7 V to 1 V), with a decrease in power conversion efficiency of 24% after ∼ 100 hrs of light exposure. In contrast, devices scanned from −1 V to 1 V degrade by 72% in the same time period. The decay in the photovoltaic performance was found to be related to an increase in the series resistance and a decrease in the shunt resistance of the devices over time. Further characterization of the bulk heterojunction layer under irradiation conditions indicates that this behavior might be caused by the decreasing mobility of the charge carriers due to the formation of the defects or traps in the layer, induced by the photo-oxidation process. The frequent application of a large reverse bias sweep most likely helps de-trap the charge carriers and slows down the device degradation. Impedance analysis indicated that during the decay of PCDTBT:PC70BM devices the bulk layer became more resistive with time, and applying large and frequent reverse bias sweeps during testing could significantly slow down this process. The charge carrier extraction time was found to increase from 7 μs to more than 400 μs after photo-degradation. These findings demonstrate that repeated application of a reverse bias voltage can affect device degradation, and therefore that the specific details of J-V characterization should be reported alongside device lifetime claims.

Utilizing the unique charge extraction properties of antimony tin oxide nanoparticles for efficient and stable organic photovoltaics

Simultaneously enhancing device performance and longevity, as well as balancing the requirements on cost, scalability, and simplification of processing, is the goal of interface engineering of organic solar cells (OSCs). In our work, we strategically introduce antimony (Sb 3+ ) cations into an efficient and generic n-type SnO 2 nanoparticles (NPs) host during the scalable flame spray pyrolysis synthesis. Accordingly, a significant switch of conduction property from an n-type character to a p-type character is observed, with a corresponding shift in the work function (WF) from 4.01 ± 0.02 eV for pristine SnO 2 NPs to 5.28 ± 0.02 eV for SnO 2 NPs with 20 mol. % Sb content (ATO). Both pristine SnO 2 and ATO NPs with fine-tuned optoelectronic properties exhibit remarkable charge carrier extraction properties, excellent UV resistance and photo-stability being compatible with various state-of-the-art OSCs systems. The reliable and scalable pristine SnO 2 and ATO NPs processed by doctor-blading in air demand no complex post-treatment. Our work offers a simple but unique approach to accelerate the development of advanced interfacial materials, which could circumvent the major existing interfacial problems in solution-processed OSCs. • Introducing antimony cations to effectively dope the n-type SnO 2 nanoparticles. • In-depth characterization of the cation doping effect and mechanisms. • Validation of the generic applicability of the p-type doped nanoparticles for organic photovoltaic applications. • Demonstrating efficient and stable organic solar cells based on state-of-the-art organic photovoltaic materials.

ITO/SnO2 Interface Defect Passivation via Atomic Layer Deposited Al2O3 for High‐Efficiency Perovskite Solar Cells

Indium tin oxide (ITO) substrate is widely used as a transparent electrode in perovskite solar cells (PSCs). However, the intrinsic defects, especially on the ITO surface, are one of the most key factors to restrict the power conversion efficiency (PCE) of PSCs. Herein, a facile method to passivate the defects of the ITO/SnO2 interface using an ultrathin aluminum oxide (Al2O3) layer through atomic layer deposition, of which the film thickness is exactly regulated, is first demonstrated. With the optimized film thickness, carrier recombination at the ITO/SnO2 interface is effectively suppressed within the three‐cycle Al2O3 layer, which results in an improved charge carrier collection efficiency from the SnO2 electron transporting layer to the ITO electrode. Furthermore, a thinner Al2O3 film (one cycle) does not passivate the interface defect effectively, and a thicker Al2O3 film (five cycles) retards the charge carrier extraction at the ITO/SnO2 interface. The average PCE of the three‐cycle Al2O3‐based PSC is 19.43%, among which the champion PCE is 20.24%, showing a significant improvement compared with the counterpart with an average PCE of 18.50%.

Two-/Three-Dimensional Perovskite Bilayer Thin Films Post-Treated with Solvent Vapor for High-Performance Perovskite Photovoltaics.

Perovskite photovoltaics (PPVs) using three-dimensional (3D) perovskites incorporated with two-dimensional (2D) perovskites have drawn great concentration in both academic and industrial sectors. Here, we report high performance of PPVs based on the 2D/3D perovskite bilayer thin film post-annealed with solvent vapor. The 2D/3D perovskite bilayer thin film post-annealed with solvent vapor possesses enlarged crystal size and crystallinity and blue-shifted photoluminescence compared to a 3D MAPbI3 thin film. Moreover, compared to the PPVs based on a 3D perovskite thin film, enlarged built-in potential, suppressed charge carrier recombination, boosted charge transport, and reduced charge carrier extraction time are observed from the PPVs based on the 2D/3D perovskite bilayer thin film post-annealed with solvent vapor. As a result, perovskite solar cells exhibit a power conversion efficiency of 22.13% and dramatically enhanced stability, and perovskite photodetectors show a photoresponsivity of 1.38 AW-1, detectivity of 6.52 × 1014 cm Hz1/2 W-1, and linear dynamic range of over 167 dB at room temperature. These results demonstrate that we develop a simple method to approach high-performance PPVs by the 2D/3D perovskite bilayer thin film.

Molecular dopants: Tools to control the electronic structure of metal halide perovskite interfaces

In the standard configurations of metal halide perovskite solar cell, the active layer, or absorber, follows a p-i-n or n-i-p electronic structure that is designed to enhance the separation and extraction of photo-induced charge carriers. The control of the Fermi level position across the film, between electron and hole transport layers, is therefore of paramount importance. Direct localized doping in metal halide perovskites being still elusive, the design of n-i-p and p-i-n structures has so far relied predominantly on surface and interface doping of the perovskite as well as on the control of the work function of the substrate and transport layers on which, or between which, the absorber is being placed. We provide here a short review of that work, emphasizing the fundamental studies of electronic structure performed on systems modified with organic molecular dopants. The review starts with a justification for the effectiveness of interface doping, based on the ability to move the Fermi level across the gap of the perovskite. We then review work done on the deposition of molecular oxidants and reductants on perovskite surfaces, including the mitigation of the surface states, and the impact of these dopants on energy level alignment with substrate and charge transport layers. The second part of the review focuses on the use of molecular dopants to either modify the work function of electron or hole transport layers to establish the boundary conditions for a p-i-n or n-i-p structure, or to enhance the conductivity of these layers in order to facilitate charge carrier extraction. Final considerations are also given on recent work on bulk doping of the perovskite layer with molecular dopants.

Elastic Lattice and Excess Charge Carrier Manipulation in 1D-3D Perovskite Solar Cells for Exceptionally Long-Term Operational Stability.

3D organic-inorganic hybrid halide perovskite solar cells (pero-SCs) inherently face severe instability issue due to ion migration under operational conditions. This ion migration inevitably results from the decomposition of ionic bonds under lattice strain and is accelerated by the existence of excess charge carriers. In this study, a 1D-3D mixed-dimensional perovskite material is explored by adding an organic salt with a bulk benzimidazole cation (Bn+ ). The Bn+ can induce 3D perovskite crystalline growth with the preferred orientation and form a 1D BnPbI3 perovskite spatially distributed in the 3D perovskite film. For the first time, the electro-strictive response, which has a significant influence on the lattice strain under an electric field, is observed in polycrystalline perovskite. The 1D-3D perovskite can effectively suppress electro-strictive responses and unbalanced charge carrier extraction, providing an intrinsically stable lattice with enhanced ionic bonds and fewer excess charge carriers. As a result, the ion migration behavior of the p-i-n 1D-3D based pero-SC is dramatically suppressed under operational conditions, showing ultra-long-term stability that retains 95.3% of its initial power conversion efficiency (PCE) under operation for 3072 h, and simultaneously achieving an excellent PCE with a hysteresis-free photovoltaic behavior.

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Durable Metal Heteroanionic Photocatalysts.

Heterogeneous photocatalysis provides a promising strategy to generate renewable fuels by harnessing solar energy. Metal heteroanionic photocatalysts have gained attention for their visible-light absorption; however, they are also plagued by photocorrosion, which limits their long-term use. Such photocorrosion occurs from photooxidation of the less electronegative nonoxide ions, leading to decomposition of the material's lattice. In this Perspective, we highlight emerging strategies to develop durable metal heteroanionic photocatalysts. We devote attention to the approaches taken for model metal oxynitrides, oxysulfides, and oxyhalide photocatalysts to provide a holistic framework. This analysis emphasizes the vital roles that interface engineering, charge carrier extraction, and crystal and electronic structure play in providing photodurability. We believe that through these approaches, durable and visible-light-absorbing artificial photosynthetic systems can be developed for a sustainable future.

Evaluation of hole mobility in non-peripherally alkyl-substituted tetrabenzotriazaporphyrin thin films by MIS-CELIV method

Charge carrier mobility measurement of donor materials for organic thin-film solar cells, 1,4,8,11,15,18,22,25-octaalkyltetrabenzotriazaporphyrin (CnTBTAPH2), was carried out by using metal-insulator-semiconductor charge carrier extraction by linearly increasing voltage (MIS-CELIV) method. The estimated carrier mobility, which is based on the hopping conduction process between the molecular columns composed with CnTBTAPH2, sequentially decreased depending on the alkyl-chain length. The alkyl-chain length dependence of the mobility was discussed by taking the evaluated activation energy and the inter-column spacing into consideration.

Single-Molecule Colocalization of Redox Reactions on Semiconductor Photocatalysts Connects Surface Heterogeneity and Charge-Carrier Separation in Bismuth Oxybromide.

The surface structure of semiconductor photocatalysts controls the efficiency of charge-carrier extraction during photocatalytic reactions. However, understanding the connection between surface heterogeneity and the locations where photogenerated charge carriers are preferentially extracted is challenging. Herein we use single-molecule fluorescence imaging to map the spatial distribution of active regions and quantify the activity for both photocatalytic oxidation and reduction reactions on individual bismuth oxybromide (BiOBr) nanoplates. Through a coordinate-based colocalization analysis, we quantify the spatial correlation between the locations where fluorogenic probe molecules are oxidized and reduced on the surface of individual nanoplates. Surprisingly, we observed two distinct photochemical behaviors for BiOBr particles prepared within the same batch, which exhibit either predominantly uncorrelated activity where electrons and holes are extracted from different sites or colocalized activity in which oxidation and reduction take place within the same nanoscale regions. By analyzing the emissive properties of the fluorogenic probes, we propose that electrons and holes colocalize at defect-deficient regions, while defects promote the selective extraction of one carrier type by trapping either electrons or holes. Although previous work has used defect engineering to enhance the activity of bismuth oxyhalides and other semiconductor photocatalysts for useful reductive half-reactions (e.g., CO2 or N2 reduction), our results show that defect-free regions are needed to promote both oxidation and reduction in fuel-generating photocatalysts that do not rely on sacrificial reagents.

Incorporation of Two-Dimensional WSe2 into MAPbI3 Perovskite for Efficient and Stable Photovoltaics.

Achieving reduced defect density and efficient charge carrier extraction plays a vital role for efficient and stable perovskite solar cells (PSCs). Over the course of technical development, it is desired to use one single material or approach to synergistically passivate defects and enhance the charge extraction. In this work, we developed an effective strategy for obtaining efficient and stable PSCs via incorporating quasi-monolayer two-dimensional WSe2 into the MAPbI3 perovskite layer. The addition of WSe2 helps with the formation of perovskite film with higher quality and also passivates the Pb-related defects through Pb-Se coordination bonding. MAPbI3/WSe2 shows a more matched energy-level alignment between the perovskite layer and hole transport layer for accelerated hole extraction. Consequently, the performances of PSCs significantly improved with power conversion efficiency increase from 19.2% to 21.2% after the incorporation of WSe2. Accordingly, the MAPbI3/WSe2-based PSCs exhibit well-improved photostability with suppression of Pb0 defect formation.

Nitrogen doped carbon quantum dots as Co-active materials for highly efficient dye sensitized solar cells

High-quality nitrogen-doped carbon quantum dots (NCQDs) are synthesized using domestic microwave-assisted pyrolysis method. These show excellent physiochemical and optical properties such as wide spectral adsorption, high charge carrier extraction, fast charge carrier transportation, and tuneable emission. These NCQDs are introduced in the dye sensitized solar cell (DSSC)/quantum dot sensitized solar cell (QDSSC) structure to improve the performance. The effect of synthesized NCQD as sensitizer, co-sensitizer, and co-photoactive layers is investigated for the DSSC/QDSSC structure. High photoconversion efficiency of 8.75% and photocurrent density of 18.13 mA/cm2 is achieved under one sun irradiation when NCQDs are used as co-photoactive layer. The obtained power conversion efficiency is approximately 55% and 99% better than NCQDs as co-sensitizer and sensitizer, respectively. The incorporation of the NCQDs in the photoactive layer synergically enhanced photo absorbance and reduced recombination between photoanode and electrolyte. A large number of anchoring sites for dye, highly conducting photoanode, fast charge carrier transportation, and inherent light-emitting photo-fluorescent property of NCQDs in mesoporous titania are understood to be responsible for this enhancement. The optimized weight ratio of citric acid and urea in the synthesis of NCQDs has provided the widened light response, low recombination rate, and high charge transport in the DSSC structure.

ELECTRONIC AND OPTICAL MODIFICATION OF ORGANIC-HYBRID PEROVSKITES

Renewed interest has brought significant attention to tune coherently the electronic and optical properties of hybrid organic–inorganic perovskites (HOIPs) in recent years. Tailoring the intimate structure–property relationship is a primary target toward the advancement of light-harvesting technologies. These constructive progresses are expected to promote staggering endeavors within the solar cells community that needs to be revisited. Several considerations and strategies are introduced mainly to illustrate the importance of structural stability, interfacial alignment, and photo-generated carriers extraction across the perovskite heterostructures. Here, we review recent strides of such vast compelling diversity in order to shed some light on the interplay of the interfacial chemistry, photophysics, and light-emitting properties of HOIPs via molecular engineering or doping approach. In addition, we outline several fundamental knowledge processes across the role of charge transfer, charge carrier extraction, passivation agent, bandgap, and emission tunability at two-dimensional (2D) level of HOIPs/molecule heterointerfaces. An extensive range of the relevant work is illustrated to embrace new research directions for employing organic molecules as targeted active layer in perovskite-based devices. Ultimately, we address important insights related to the physical phenomena at the active molecules/perovskites interfaces that deserve careful considerations. This review specifically outlines a comprehensive overview of surface-based interactions that fundamentally challenges the delicate balance between organic materials and perovskites, which promotes bright future of desired practical applications.

Incorporation of Zr-doped TiO2 nanoparticles in electron transport layer for efficient planar perovskite solar cells

Organic–inorganic perovskite solar cells (PSC) are gaining widespread recognition due to their higher performances and low-cost relative to other photovoltaic technologies. However, the efficient charge collection and rapid electron transportation are the key parameters desirable for the development of optimized electron transport layer (ETL) in PSC. In this research work, hydrothermally synthesized zirconium-doped titania nanoparticles (Zr-TiO2) were introduced as an ETL in a n–i–p planar configuration. The Zr-doped TiO2 nanoparticles were characterized for various morphological, structural and optical studies by using SEM, EDX, XRD, Raman, PL and UV-Vis spectroscopy techniques. The photocatalytic activity of newly synthesized Zr-TiO2 nanoparticles was investigated by CV, EIS and tafel measurements. The current–voltage characteristics (J-V) of the newly fabricated PSC devices were investigated under 1 sun condition. The superior power conversion efficiency (PCE) of 12.35 % was observed with 0.2 M Zr-doped TiO2 based ETL in PSC. The remarkable PCE in 0.2 M Zr-doped TiO2 nanoparticles-based PSC is attributed to significant enhancement in charge carrier extraction and due to less recombination reactions at interfaces.

Insights into Bulk‐Heterojunction Organic Solar Cells Processed from Green Solvent

The environmental impact of solution processed organic solar cells (OSCs) can be mitigated by introducing so‐called green solvents during the fabrication processes. However, the effects of such green solvents on the molecular‐level structures and optoelectronic properties lack in‐depth characterization. Here, insights into the structure–processing–property correlation of a PPDT2FBT:PC61BM bulk‐heterojunction (BHJ) system processed from a green solvent, ortho‐xylene (o‐XY), is investigated in comparison with the same blend processed from a traditional halogenated solvent, chlorobenzene (CB). The BHJ blends are characterized with various techniques probing at difference length scales, and an increased donor:acceptor (D:A) interfacial area as well as well‐mixed features in the bulk morphologies of the active layer are observed for the o‐XY processed BHJ blend. Furthermore, molecular‐level differences in the D–A intermolecular interactions at the BHJ interfaces in o‐XY and CB cast films are elucidated by 2‐dimensional solid‐state nuclear magnetic resonance (ssNMR) measurements and analysis. These results are consistent with the device properties, suggesting that the green‐solvent‐processed devices have longer charge carrier lifetimes and faster charge carrier extraction. The optimized PPDT2FBT:PC61BM devices processed from o‐XY can achieve a noteworthy higher power conversion efficiency (PCE) owing to a higher short‐circuit current density and fill factor.

Molecular tailor-making of zinc phthalocyanines as dopant-free hole-transporting materials for efficient and stable perovskite solar cells

Four Zinc (II) phthalocyanines (ZnPcs) derivatives, ZnPH13, ZnPH14, ZnPH15 and ZnPH22, decorated with elaborate molecular tailor-making on the phenyl-based peripheral substituents of Pc scaffolds are synthesized and acted as dopant-free hole-transporting materials (HTMs) in perovskite solar cells (PSCs). The detailed electrochemical, photophysical characteristics, theoretical geometric/electronic features, hole mobility, charge carrier extraction, collection and transporting dynamic processes of these HTMs are cautiously unveiled. The heuristic probe into the substantial relations and dependences between the Pc organic scaffold structure and properties at molecule level are investigated. The mixed-ion PSC utilizing the ZnPH22 containing the benzo[d] [1,3]dioxole peripheral substituent with cyclic/planar conformation presents a most competitive power conversion efficiency (PCE) of 18.3% associating with excellent reproducibility and superior long-period stability. These results reveal that the eventual solar cell performance can be decipherable through the intrinsic electronic and structural effects originating from the peripheral substituents modifications. This work thus exploits the new insights into the advanced molecular building of efficient phthalocyanines HTMs for high performance and stable PSCs.

(Invited) Ultrafast Pump-Probe and Single-Particle Photoluminescence Studies of Charge Carrier Recombination and Extraction Dynamics of the Caesium Lead Halide Perovskite Nanocrystals

The lead halide perovskites have emerged in recent years as the most promising materials in photovoltaic and photonic applications.1,2 As utilization of the full potential of these materials in these applications requires a thorough understanding of the pathways through which the photo-generated charge carriers relax and the rate of their relaxation and capture by other substances prior to recombination, we have been investigating the dynamics of charge carrier recombination and extraction in a variety of caesium lead halide perovskite (CsPbX3) nanocrystals employing femtosecond pump-probe, fluorescence upconversion and single-particle fluorescence techniques. In this talk, some of our recent studies, which throw important insight on these topics, will be presented.3-8