Charge Carrier Extraction Research Articles

Synergistic Impact of Passivation and Efficient Hole Extraction on Phase Segregation in Mixed Halide Perovskites

AbstractThe interface between the hole transport layer (HTL) and perovskite in p‐i‐n perovskite solar cells (PSCs) plays a vital role in the device performance and stability. However, the impact of this interface on the vertical phase segregation of mixed halide perovskite remains insufficiently understood. This work systematically investigates the impact of chemical and electronic properties of HTL on vertical halide segregation of mixed‐halide perovskites. This work shows that incorporating a poly[bis(4‐phenyl) (2,4,6‐trimethylphenyl) amine] (PTAA)/CuIxBr1‐x bilayer as the HTL significantly suppresses light‐induced vertical phase segregation in MAPb(I0.7Br0.3)3. This work uses grazing‐incidence X‐ray diffraction (GIXRD) to capture the depth‐resolved composition change of MAPb(I0.7Br0.3)3 at the interface and within the bulk under illumination. By changing the illumination direction and the electronic properties of HTL, this work elucidates the roles of charge carrier extraction and interfacial defects on vertical phase segregation. The PTAA/CuIxBr1‐x bilayer, with its synergistic passivation and efficient hole extraction ability, stabilizes the interface and bulk of the mixed halide perovskite layer and prevents phase segregation. This work underscores that synergetic passivation and efficient hole extraction pack a more powerful punch for arresting the vertical phase segregation in mixed‐halide perovskite.

Fabrication of High-Efficiency Perovskite Solar Cells using Benzodithiophene-Based Random Copolymeric Hole Transport Material

The design of donor-acceptor based random copolymers-type hole transporting materials (HTMs) are important for achieving superior performance of perovskite solar cells (PSCs) with high durability. In this work, a 2-alkylthienyl-substituted benzodithiophene (BDTT)-based random copolymer (denoted as RCP-BDTTPD), containing 2-ethylhexylthiophene-substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT as an electron-donor; M1) and two different side-chain functionalized thieno[3,4-c]-pyrrole-4,6-dione as the electron-acceptors (M2 and M3), is prepared and applied as an efficient interfacial HTM for PSCs. The optical, electrochemical, and electronic properties of RCP-BDTTPD is shown to be structurally and energetically viable to serve as HTM for PSCs. The RCP-BDTTPD has deeper highest occupied molecular orbitals (HOMO; -5.53 eV) and lowest unoccupied molecular orbitals (LUMO; -3.57 eV) energy levels. This is shown to be energetically suitable for realizing better compatibility with Cs-containing formamidinium/methylammonium (FAMA) mixed-cation perovskite as light absorber having HOMO energy level (-5.85 eV). The RCP-BDTTPD possessing gradient band alignment with perovskite, which is shown to be highly significant for the extraction of charge carriers, resulting in higher hole mobility of PSCs. RCP-BDTTPD delivered a reasonably good Voc of 1.10 V and higher Jsc of 19.01 mAcm−2 and, champion power conversion efficiency (PCE) up to 15.30% with hole mobility (1.34×10−3 cm2V−1s−1) and high durability (Encapsulated cell retention of its PCE about 98% over 16 hours under harsh environment: Temp. ∼85 °C, RH∼85%). This work demonstrating a potential application of RCP-BDTTPD based HTMs for the fabrication of high-performance PSCs with high durability as well as low cost.

Pyro-phototronic Effect Enhanced Self-Powered Photoresponse in Lead-Free Hybrid Perovskite.

Pyro-phototronic effect (PPE) can significantly boost the performance of hybrid perovskite (HPs) photodetectors due to the effective modulation of photogenerated charge carrier separations, transportation, and extraction. However, there are few reports on the application of PPE in lead-free HPs. Herein, a polar lead-free HP (1,3-BMACH)BiBr5 [1,3-BMACH = 1,3-bis(aminomethyl)cyclohexane] is synthesized and realized broadband self-powered photoresponse from X-rays to near-infrared (NIR) through the PPE. Particularly, this light-induced PPE in lead-free HPs breaks the limitation of the optical band gap, making them suitable for broadband self-powered photodetection. Under 405 nm illumination, compared with a purely photovoltaic system, Iphoto+pyro is boosted by 3100%, and R and D* are both enhanced by 260% after coupled PPE. It is particularly interesting that an obvious X-ray-induced PPE phenomenon is also observed, which endows (1,3-BMACH)BiBr5 with a high sensitivity of 154 μC Gy-1 cm-2 and a low detection limit of 307 nGy s-1 under the self-powered mode. The implementation of light-induced PPE from X-rays to NIR in lead-free HPs provides a new approach for constructing environmentally friendly, broadband, and self-powered optoelectronic devices in the future.

ZnO@NiO core-shell heterojunction photoanode modified with NiOOH co-catalyst for high-performance self-powered photoelectrochemical-type UV photodetector

ZnO, a promising photoanode material, faces significant challenges in achieving high-efficient photoelectrochemical (PEC) type UV photodetectors due to inadequate charge separation and sluggish surface reaction kinetics. Constructing heterojunction nanostructures by coupling of p-NiO to n-ZnO nanowire arrays with well-aligned energy band positions presents a feasible strategy for enhancing PEC-type photodetector performance. Herein, we demonstrate that the designed ZnO@NiO core–shell nanowire arrays form a spatial charge transfer channel, facilitating efficient generation and extraction of charge carriers. The exceptional catalytic properties of NiOOH, derived from the transformation of NiO in an aqueous alkaline environment, serve as a potent co-catalyst, effectively accelerating the kinetics of the surface oxygen evolution reaction. Benefiting from the synergistic effect of the heterojunction and cocatalyst, the optimized photoanode achieves a high responsivity of 438.36 mA/W, a large detectivity of 4.85 × 1012 Jones, and rapid response time of 150.05/150.26 ms, while simultaneously exhibiting long-time PEC stability. This work highlights the critical role of nanostructure design in developing high-performance self-powered UV photodetectors.

Research on the Fabrication and X‐Ray Detection Performance of MAPbBr3‐Based 3D/2D Heterojunctions

Perovskite materials, renowned for their high mean atomic number, charge carrier mobility–lifetime product, and X‐ray absorption coefficient, have emerged as exceptional candidates for the fabrication of high‐sensitivity X‐ray detectors. Herein, a BA2PbBr4/MAPbBr3 heterojunction is epitaxially grown, with ion migration effectively suppressed through the passivation of surface defects of the 3D heterojunction. Additionally, the presence of an internal electric field within the heterojunction facilitated the extraction and accumulation of photoinduced charge carriers, further enhancing device sensitivity (5964.05 μC Gy−1 cm−2). The fabricated 2D/3D perovskite heterojunction devices show outstanding self‐driven X‐ray detection performance at 0 V mm−1 bias (1195.5 μC Gy−1 cm−2), surpassing that of α‐Se detectors (20 μC Gy−1 cm−2). The proposed method for the fabrication of self‐driven detectors is relatively simple and is therefore expected to contribute to the commercialization of perovskite‐based X‐ray detectors.

Surface Lattice Engineering Enables Efficient Inverted Perovskite Solar Cells

AbstractState‐of‐the‐art inverted perovskite solar cells (PSCs) have exhibited considerable promise for commercialization due to their prospective stability. However, the intricate crystallization of halide perovskite, especially for multi‐component perovskites, not only distorts the surface lattice from its ideal form but also introduces numerous unsaturated dangling bonds to form surface defects, which can easily lead to reduced stability and poor performance. Herein, a surface lattice engineering is developed by coupling surface unsaturated ions and regulating ion bonding lengths/angles to achieve efficient and stable inverted PSCs. The renovated surface lattice not only eliminates shallow/deep level defects on the surface of perovskite, but also enhances photo/thermal stability of the materials. Moreover, the surface lattice engineering contributes to uniform potential surface, and improves energy‐level alignment at the interfaces of the perovskite and C60 carrier transport layer, enhancing charge carrier extraction and transportation. Finally, the champion PSC delivers an impressive efficiency of 25.82% (certified 25.5%). Moreover, these PSCs exhibit excellent operational stability, retaining 94% initial efficiency after more than ≈1 000h maximum power point test.

Lead-Free Cs2AgBiBr6/TiO2 S-Scheme Heterojunction for Efficient Photocatalytic Antibiotic Rifampicin Degradation.

Exploring efficient and stable halide perovskite-based photocatalysts is a great challenge due to the balance between the photocatalytic performance, toxicity, and intrinsic chemical instability of the materials. Here, the environmentally friendly lead-free perovskite Cs2AgBiBr6 confined in the mesoporous TiO2 crystal matrix has been designed to enhance the charge carrier extraction and utilization for efficient photocatalytic rifampicin degradation. The as-prepared Cs2AgBiBr6/TiO2 catalyst was stable in air for over 500 days. An S-scheme heterojunction was formed between the (004) plane of Cs2AgBiBr6 and the (101) plane of TiO2 through the Bi-O-Br bonds. The built-in electric field at the interface efficiently promoted the photoinduced charge separation and carrier extraction. The Cs2AgBiBr6/TiO2-200 showed a 92.83% degradation efficiency of rifampicin within 80 min under simulated sunlight illumination (AM 1.5G 100 mW cm-2). This work offers an effective way for the construction of halide perovskite-based photocatalysts with high photocatalytic performance, good stability, and low toxicity simultaneously.

Enhanced Performance and Stability of Perovskite Solar Cells Through Surface Modification with Benzocaine Hydrochloride.

Current development of inverted p-i-n perovskite solar cells (PSCs), with nickel oxide as the hole transport layer, is progressing toward lower net costs, higher efficiencies, and superior stabilities. Unfortunately, the high density of defect-based traps on the surface of perovskite films significantly limits the photoelectric conversion efficiency and operational stability of perovskite solar cells. Finding cost-effective interface modifiers is crucial for the further commercial development of p-i-n PSCs. In the present work, we report a passivation strategy using a multifunctional molecule, benzocaine hydrochloride (BHC), which is shown to reduce defect density and enhance the photovoltaic performance and stability of the resultant p-i-n PSCs. It has been revealed that BHC strongly interacts with perovskite precursor components and triggers the evolution of the perovskite absorber film morphology and enables improved surface energy level alignment, thus promoting charge carrier transport and extraction. These properties are beneficial for improving open-circuit voltage (VOC) and fill factor (FF). Our results show that the photoelectric conversion efficiency (PCE) of p-i-n PSCs with nickel oxide as the hole transport layer increased from an initial 20.0% to 22.1% after being passivated with BHC, and these passivated devices also exhibited improved stability. DFT calculations reveal the unusual ability of the BHC passivant to improve band alignment while also preventing the accumulation of holes at the interface. In this work, the advantages of BHC passivation are demonstrated by linking theoretical calculations with optical and electrical characterizations.

Over 19 % efficienct ternary organic solar cells enabled by tuning charge behaviors through quinoxalineimide-based Y-type acceptors

Attaining a balanced charge carrier generation is a significant challenge in organic photovoltaics (OPVs). To enhance device performance, the ternary strategy has emerged as an effective approach. In this study, two Y6 derivatives, QIP-4F and QIP-4Cl, containing quinoxalineimide (QI) building blocks fused with thienylthiophene as the central unit were incorporated into the PM6:L8-BO system. The quinoxalineimide units reduce the energy losses in charge carrier extraction and non-radiative charge recombination. Furthermore, the increasing steric hindrance in the central block impacts phase separation and ordered intermolecular packing, which enhances the charge transport and minimizes the charge recombination. Ultimately, the PM6:L8-BO:QIP-4Cl device achieved a champion efficiency of 19.05%, offering a promising direction for enhancing the photovoltaic performance of OPVs.

Air-stable and UV-NIR broadband photodetectors utilizing graphene and core/shell quantum dots hybrid heterostructure

Photodetectors based on PbSe quantum dots (QDs) are commonly used for light detection in the near-infrared (NIR) region. Nevertheless, the performance of photodetectors based on PbSe QDs is constrained by ineffective carrier transfer and poor photo and thermal stabilities. Herein, a promising strategy that harnesses PbSe/PbS core/shell QDs structures is developed and demonstrated to enhance the long-term stability of photodetectors, further combining with graphene to form hybrid heterojunctions that effectively promote carrier transfer. As a result, the devices exhibit a broad photoresponse to the incident light at range of 365–1250 nm, and also possess a high photosensitivity of 1.5 × 104 A/W and a high detectivity of 4.0 × 1013 Jones. Moreover, the lifetime of graphene-PbSe/PbS core/shell QDs hybrid photodetector was 9 times greater than that of uncoated QDs devices. The enormous boost might be attributed to the passivation and protection provided by the core–shell structure, and graphene’s efficient extraction of charge carriers.

Passivating perovskite surface defects via bidentate chelation for high-performing solar cells

Surface defects seriously damage carrier transport by forming non-radiative recombination centers on the perovskite film, which negatively affect the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Here we present a strong chelate coordination bond to anchor the surface of the perovskite film. Employing Bathophenanthroline (BPhen) as a π-conjugated bidentate chelator, the uncoordinated Pb2+ can be firmly anchored accompanying with the reduced surface recombination sites, giving rise to suppressed non-radiative recombination and enhanced charge carrier extraction. With the BPhen, the perovskite film shows an approximately threefold longer carrier lifetime than the control device, leading to a champion PCE of 24.09 % and a high stable performance with retained over 80 % of its initial efficiency after 1000 h exposed to relative humidity (RH) of 65 ± 5 %.

Synthesis of VS4 and VO on carbon nanofibers for back interface modification in HTL free CsPbBr3-based perovskite solar cells

Carbon-based hole transport layer (HTL)-free all-inorganic perovskite solar cells (C-PSCs) exhibit advantages of simplified preparation procedure, high stability, and low cost, showcasing significant application potential. Nonetheless, the development of C-PSCs requires overcoming challenges such as the misalignment of energy levels between carbon counter electrode and the CsPbBr₃ perovskite, abundant defects at the perovskite/carbon back interface, and the carbon counter electrode's low conductivity. Herein, VS₄ and VO-deposited carbon nanofibers (CNF), termed VS₄@CNF and VO@CNF respectively, have been synthesized to adjust the work function and improve the conductivity of the carbon counter electrode. CsPbBr₃-based C-PSCs employing VS₄@CNF and VO@CNF as counter electrodes yield power conversion efficiencies of 7.88 % and 8.80 %, indicating great PCE enhancements of 15.4 % and 28.8 % as compared with the device with unmodified CNF counter electrodes. This improvement is attributed to the optimized conductivity and adjusted work function of the counter electrodes, which can optimize the energy level alignment, thereby enhancing charge carrier extraction and transport while effectively inhibiting recombination. Furthermore, VS₄@CNF and VO@CNF demonstrate a notable passivation effect on defects at the back interface. This study offers a valuable path for the design of novel carbon counter electrodes and the modification of the back interface in carbon-based all-inorganic C-PSCs.

Enhancing the Efficiency and Stability of Perovskite Solar Cells Using Chemical Bath Deposition of SnO2 Electron Transport Layers and 3D/2D Heterojunctions.

Chemical bath deposition (CBD) is an effective technique used to produce high-quality SnO2 electron transport layers (ETLs) employed in perovskite solar cells (PSCs). By optimizing the CBD process, high-quality SnO2 films are obtained with minimal oxygen vacancies and close energy level alignment with the perovskite layer. In addition, the 3D perovskite layers are passivated with n-butylammonium iodide (BAI), iso-pentylammonium iodide (PNAI), or 2-methoxyethylammonium iodide (MOAI) to form 3D/2D heterojunctions, resulting in defect passivation, suppressing ion migration and improving charge carrier extraction. As a result of these heterojunctions, the power conversion efficiency (PCE) of the PSCs increased from 21.39% for the reference device to 23.70% for the device containing the MOAI-passivated film. The 2D perovskite layer also provides a hydrophobic barrier, thus enhancing stability to humidity. Notably, the PNAI-based device exhibited remarkable stability, retaining approximately 95% of its initial efficiency after undergoing 1000-h testing in an N2 environment at room temperature.

Integral morphology and structure design of poly (heptazine imide) for efficient utilization of visible light generated charge carriers in proton reduction reactions

AbstractSufficient utilization of visible‐light generated charge carriers in proton reduction reactions is of great significance for the development of effective solar‐fuel technologies. Achieving simultaneous bulk rapid transfer and surface efficient extraction of charge carriers is still very challenging. Herein, it is found for the first time ammonium persulfate (APS) can significantly influence polymerization processes of C3N4 (CN) from melamine to poly (heptazine imide) (PHI) under the simultaneous oxygen doping and etching effect of SO42−. PHI with high crystallinity, porous structure, and in‐situ oxygen doping was therefore obtained through one‐step APS‐assisted salt strategy. Benefiting from sufficient visible‐light absorption and upshifted conduction band originating from regulated electronic structure and optimized morphology through APS modification, the as‐prepared PHI achieved a H2 evolution activity of 3274.23 μmol h−1 g−1 (λ > 420 nm), which is appropriately 148 and 19 times that of conventional and crystalline CN. This work opens up new opportunities for efficient photocatalysis.

Bi-Directional Modification to Quench Detrimental Redox Reactions and Minimize Interfacial Energy Offset for NiOX/Perovskite-Based Solar Cells.

The quality of the buried heterojunction of nickel oxide (NiOX)/perovskite is crucial for efficient charge carrier extraction and minimizing interfacial non-radiative recombination in inverted perovskite solar cells (PSCs). However, NiOX has limitations as a hole transport layer (HTL) due to energy level mismatch, low conduction, and undesirable redox reactions with the perovskite layer, which impede power conversion efficiency (PCE) and long-term stability. In this study, para-amino 2,3,5,6-tetrafluorobenzoic acid (PATFBA) is proposed as a bifacial defect passivator to tailor the NiOX/perovskite interface. The acid group and adjacent fluorine atoms of PATFBA effectively passivate NiOX surface defects, thereby improving its Ni3+/Ni2+ ratio, hole extraction capability, and energy band alignment with perovskite, while also providing active sites for homogenous nucleation. Meanwhile, the amine and adjacent fluorine atomsstabilize the buried perovskite interface by passivating interfacial defects, resulting in higher crystalline perovskite films with supressed non-radaitive recombination. Furthermore, the PATFBA buffer layer prevents redox reactions between Ni3+ and perovskite.These synergistic bi-directional interactions lead to optimized inverted PSCs with a PCE of 20.51% compared to 16.89% for pristine devices and the unencapsulated PATFBA-modified devices exhibit outstanding thermal and long-term stability. This work provides a new engineering approach to buried interfaces through the synergy of functional groups.

Engineering the active layer of lead-free perovskite (CH3NH3SnI3) solar cells using numerical simulation

We conduct a thorough numerical simulation to examine the impact of the thickness, defect density, and doping density of the active layer on the photovoltaic performance of the lead-free CH3NH3SnI3 perovskite solar cell (PSC). We observe that increasing the thickness of the active layer initially from 100 nm to 400 nm improved the power conversion efficiency (PCE) from 10.4% to 11.6%. However, further increasing the thickness to 800 nm resulted in a slight decline in PCE to 11.1%. This unexpected trend can be attributed to the high carrier mobility of charges in the CH3NH3SnI3 perovskite, which enables fast extraction of charge carriers, offsetting losses due to charge recombination. Increasing active layer trap density substantially declines the PCE from 11.5% at 1014 cm−3 to 7.5% at 1018 cm−3, as a result of the noticeable drop in open-circuit voltage (VOC) and fill factor (FF) with a growing defect density due to the enhancement in trap-assisted recombination. This is backed by a striking reduction in the shunt resistance upon increasing the defect density. Raising the active layer doping firstly enhances the PCE, reaching a peak value of 12.5% at the active layer doping density of 1017 cm−3, after which the PCE decreases as the doping density continues to increase. We explain these observations by energy level diagrams deduced at various doping levels.

Nanoscale phase management of the 2D/3D heterostructure toward efficient perovskite solar cells

The stabilization of the formamidinium lead iodide (FAPbI3) structure is pivotal for the development of efficient photovoltaic devices. Employing two-dimensional (2D) layers to passivate the three-dimensional (3D) perovskite is essential for maintaining the α-phase of FAPbI3 and enhancing the power conversion efficiency (PCE) of perovskite solar cells (PSCs). However, the role of bulky ligands in the phase management of 2D perovskites, crucial for the stabilization of FAPbI3, has not yet been elucidated. In this study, we synthesized nanoscale 2D perovskite capping crusts with <n> = 1 and 2 Ruddlesden-Popper (RP) perovskite layers, respectively, which form a type-II 2D/3D heterostructure. This heterostructure stabilizes the α-phase of FAPbI3, and facilitates ultrafast carrier extraction from the 3D perovskite network to transport contact layer. We introduced tri-fluorinated ligands to mitigate defects caused by the halide vacancies and uncoordinated Pb2+ ions, thereby reducing nonradiative carrier recombination and extending carrier lifetime. The films produced were incorporated into PSCs that not only achieved a PCE of 25.39% but also maintained 95% of their initial efficiency after 2000 h of continuous light exposure without encapsulation. These findings underscore the effectiveness of a phase-pure 2D/3D heterostructure-terminated film in inhibiting phase transitions passivating the iodide anion vacancy defects, facilitating the charge carrier extraction, and boosting the performance of optoelectronic devices.

Nanometer Control of Ruddlesden-Popper Interlayers by Thermal Evaporation for Efficient Perovskite Photovoltaics.

Solution-processed Ruddlesden-Popper (RP) interlayers in lead halide perovskite solar cells (PSCs) present processing challenges due to fast film formation and uncontrolled growth of phases and layer thickness at interfaces. In this work, an alternative, solvent-free, thermal co-evaporation process is developed to deposit RP interlayers. The method provides precise control on interlayer thickness and enables understanding its role on charge-carrier extraction. Studying RP film growth reveals the development of heterointerfaces when deposited on three-dimensional (3D) perovskite layers. This allows a large thickness window with an optimum between 20 nm and 40 nm to improve the optoelectronic properties of the underlying 3D perovskite. Solar cells using evaporated interlayers achieve power conversion efficiency of 21.6%, compared to 19.6% for untreated devices, driven by improvements in the open-circuit voltage and fill factor. This work sheds light on the importance of phase and thickness control of passivation layers, which ultimately determine the solar cell performance in state-of-the-art PSCs.

Investigation of the effect of oxygen flow modulation on ITO film properties to improve the performance of SHJ solar cells

Photovoltaics offer a sustainable solution for the growing energy needs of the world, while reducing environmental impact. Transparent Conductive Oxides (TCOs), which are essential in photovoltaic technology, offer high light transmittance and effective charge carrier extraction. This research focuses on the impact of oxygen (O2) flow rates in the deposition of indium tin oxide (ITO) films on n-type crystalline silicon (c-Si) substrates by DC magnetron sputtering. The structural, optical, and electrical properties were analyzed. Based on the results, the optimal O2 ratio was determined and used in the fabrication of SHJ solar cells, achieving an efficiency of 18.6%.

Construction Au/FAPbI3 Schottky Heterojunction towards a High-Speed Electron Transfer Channel for High-Performance Perovskite Quantum Dot Solar Cells.

Formamidinium lead triiodide quantum dot (FAPbI3 QD) exhibits substantial potential in solar cells due to its suitable band gap, extended carrier lifetime, and superior phase stability. However, despite great attempts toward reconfiguring the surface chemical environment of FAPbI3 QDs, achieving the optimal efficiency of charge carrier extraction and transfer in cells remains a challenge. To circumvent this problem, we selectively introduced Au/FAPbI3 Schottky heterojunctions by reducing Au+ to Au0 and subsequently anchoring them on the surface of FAPbI3 QDs, which acts as a light-harvesting layer and establishes high-speed electron transfer channels (Au dot ↔ Au dot). As a result, the champion photoelectric conversion efficiency of solar cells reached 13.68%, a significant improvement over 11.19% of that of FAPbI3-based solar cells. The enhancement is attributed to efficient and directed electron transfer as well as a more aligned energy level arrangement. This work constructed Au/FAPbI3 QD Schottky heterojunctions, providing a viable strategy to enhance QD electron coupling for high-performance optoelectronic applications.