Interfacial Charge Recombination Research Articles

Fabrication of Efficient and Simple-Structured Perovskite Solar Cells Using a Multifunctional Biocolina Surface Treatment.

The electron transport layer (ETL)-free perovskite solar cells (PSCs) have gained significant interest by simplifying the manufacture process and reducing the time/energy required for the fabrication of ETLs. Unfortunately, the performance of these ETL-free PSCs still lags behind those of the conventional counterparts due to the slow electron extraction and undesired interfacial charge recombination loss at the buried interface. In this work, a facile and multifunctional biocolina thin layer is incorporated on the bottom electrodes to regulate the interface energy level alignment by forming an interface dipole layer, resulting in a suppressed nonradiative recombination and an improved charge extraction. Furthermore, the biocolina thin layer possess the capability to passivate the surface defects within the perovskite films while simultaneously facilitate the formation of perovskite crystals. Consequently, a remarkable enhancement in photovoltaic performance is observed in the biocolina-based ETL-free PSCs with an increase from 15.96% to an outstanding 20.01%. Additionally, the biocolina extends the stability and relieves the hysteresis effect through the interface defect passivation and inhibition of interface charge accumulation. This research contributes to the development of cost-effective, simplified designs for highly efficient ETL-free PSCs by modifying the bottom electrodes.

Phosphate-based ionic liquids for advanced interface modification in perovskite photovoltaics: Novel insights for ionic liquid molecule design

Interface modification stands out as a crucial method to enhance the performance of perovskite solar cells. This study explores the utilization of the ionic liquid 1-Methyl-3-propylimidazolium phosphate (MPIMPH), containing phosphate (PO43−) groups, as a modifier for SnO2/perovskite interfaces. Detailed investigations delve into the specific impact of IL MPIMPH on defects in tin dioxide, perovskite defects, perovskite crystallization, morphology, and overall device performance. Initially, Density Functional Theory (DFT) is employed to evaluate the defect passivation potential of different anions on tin vacancies within the SnO2 matrix, revealing the exceptional defect passivation ability of the PO43− group. Chemical interactions between ILs MPIMPH and SnO2, as well as perovskite, are confirmed, establishing a fundamental basis for interface defect passivation. ILs-based interface modification enhances interface contact, influencing the lead iodide conversion into perovskite and perovskite crystallization, ultimately leading to the formation of high-quality perovskite thin films. ILs also effectively suppress interfacial charge recombination and optimize the energy alignment of the SnO2/perovskite interface to enhance carrier extraction. Benefiting from the interface modification with ILs, the optimized device achieved an impressive device efficiency of 24.24 % with remarkable durability. This work provides an novel aspect for designing ionic liquids to enhance interface modification, contributing to upscaling and application of perovskite photovoltaics.

Improving perovskite solar cell efficiency and prolonging durability via interface modification with guanidinium iodide by post treatment method

The performance of perovskite solar cells (PSCs) can be significantly compromised by the presence of halide vacancies on the perovskite surface, leading to defects and a decline in stability and efficiency. Effectively addressing this challenge, the application of Guanidinium iodide (GAI) as a post dopant treatment (POST) has demonstrated success. GAI (POST) not only reduces surface defects but also minimizes halide deficiencies and mitigates losses due to interfacial charge recombination. When incorporating a GAI additive at a solution concentration of 1.5 mg/ml in the modification layer with the perovskite composition Cs0.1FA0.8MA0.1PbI3, the PSC showed a notable power conversion efficiency (PCE) of 20.7%, surpassing the 16.3% efficiency of the control device. Moreover, the adapted device demonstrated an impressive fill factor (FF) of 80.1%. Notably, the enhanced film of the modified device maintained over 80% of its initial efficiency for more than 312 h, highlighting the prospect of creating affordable, effective, and durable PSCs.

Elucidating Interfacial Hole Extraction and Recombination Kinetics in Perovskite Thin Films

Hybrid organic–inorganic perovskite solar cells (PSCs) are receiving huge attention owing to their marvelous advantages, such as low cost, high efficiency, and superior optoelectronics characteristics. Despite their promising potential, charge-carrier dynamics at the interfaces are still ambiguous, causing carrier recombination and hindering carrier transport, thus lowering the open-circuit voltages (Voc) of PSCs. To unveil this ambiguous phenomenon, we intensively performed various optoelectronic measurements to investigate the impact of interfacial charge-carrier dynamics of PSCs under various light intensities. This is because the charge density can exhibit different mobility and charge transport properties depending on the characteristics of the charge transport layers. We explored the influence of the hole transport layer (HTL) by investigating charge transport properties using photoluminescence (PL) and time-resolved (TRPL) to unveil interfacial recombination phenomena and optoelectronic characteristics. We specifically investigated the impact of various thicknesses of HTLs, such as 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD), and poly(triaryl)amine (PTAA), on FA0.83MA0.17Pb(Br0.05I0.95)3 perovskite films. The HTLs are coated on perovskite film by altering the HTL’s concentration and using F4-TCNQ and 4-tert-butylpyridine (tBP) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSi) as dopants both for spiro-OMeTAD and PTAA. These HTLs diversified the charge concentration gradients in the absorption layer, thus leading to different recombination rates based on the employed laser intensities. At the same time, the generated charge carriers are rapidly transferred to the interface of the HTL/absorption layer and accumulate holes at the interface because of inefficient capacitance and mobility differences caused by differently doped HTL thicknesses. Notably, the charge concentration gradient is low at lower light intensities and did not accumulate holes at the HTL/absorption layer interface, even though they have high charge mobility. Therefore, this study highlights the importance of interfacial charge recombination and charge transport phenomena to achieve highly efficient and stable PSCs.

Multi‐site Passivation of ZnO Metal Oxides via Biomolecules for Efficient and Highly Stable Organic Solar Cells

Comprehensive SummaryZnO nanoparticles (nps) among metal oxide (MOs) are proven to be essential electron transporting layers (ETLs) applied in organic solar cells (OSCs). However, intrinsic defects, interfacial charge recombination, and catalytic behavior towards the active layer restrict the applications of ZnO nps for efficient and long‐term stable OSCs. The commonly available biomolecule cytidine 5'‐monophosphate (CMP‐OH) with phosphonic acid, its salt cytidine 5'‐monophosphate disodium salt (CMP‐ONa) with a phosphate group as an anchoring group and conjugated terminal functional in both analogous molecules provide carrier transfer bridge at bottom interface of the active layer. Systematized theoretical investigations and characterizations have discovered the multi‐site coordination of CMP‐OH towards acceptor molecules and ZnO nps. The dual‐side alignment of CMP analogous molecules hinders interfacial charge recombination and enhances charge transfer potential at once. Inevitably, PM6:L8‐BO‐based OSCs with modified ETL obtain 18.13% efficiency, 12% higher than that of unmodified nps. Besides higher efficiency, CMP‐OH‐based OSC devices illustrate remarkably improved thermal stability for 500 h at 85 °C with 72% of initial PCE and operation stability for 2000 h with 90.1% of initial PCE. This work reveals the passivation mechanism of multi‐anchoring groups towards MOs and single‐functional groups towards the active layer to optimize the interface for efficient and highly stable OSCs.

Modulating Interfacial Charge Transfer Behavior through the Construction of a Hetero-Interface for Efficient Photoelectrochemical Water Splitting

Surface-coupled transition metal oxyhydroxide (TMOOH) on semiconductor (SC)-based photoanodes are effective strategies for improving photoelectrochemical (PEC) performance. However, there is a substantial difference between the current density and theoretical value due to the inevitable interfacial charge recombination of SC/TMOOH. Here, we employ BiVO4/FeNiOOH as a model, constructing the BiVO4/MnOx/CoOx/FeNiOOH integrated system by introducing a novel hetero-interface regulation unit, i.e., MnOx/CoOx. As expected, the optimized integrated system demonstrates a photocurrent density as high as 5.0 mA/cm2 at 1.23 V versus the reversible hydrogen electrode (RHE) under 1 sun AM 1.5G illumination, accompanied by 12-h stability. The detailed electrochemical analysis and intensity modulated photocurrent spectroscopy (IMPS) have confirmed that the high PEC performance mainly originates from the hetero-interface structure, which not only suppresses the interfacial charge recombination by accelerating the photogenerated hole transfer kinetics from BiVO4 to FeNiOOH but promotes the kinetics of surface oxygen evolution reaction (OER). Notably, these findings can also be extended to other structures (CeOx/CoOx), reflecting its universality. This finding has provided a new insight into the highly efficient solar energy conversion in the SC/TMOOH system.

Zn-doped CdSSe composite quantum dots synthesized by a two-step hydrothermal ion exchange in sensitized solar cells

Metal ion doping is an effective strategy for enhancing the photoelectric properties of quantum dots (QDs). In this study, Zn-doped CdSSe composite QDs (ZnCdSSe) were synthesized using a two-step hydrothermal ion exchange method. The synthesis process involved the exchange reaction of Cd2+ with Zn2+, followed by the exchange reaction of Se2− with S2−, based on ZnS templates. The morphology, composition and crystalline phase of synthesized QDs were characterized using X-ray and electron-based techniques such as XRD, SEM, TEM, and XPS. The results indicate ZnCdSSe QDs were synthesized while preserving a similar morphology to that of ZnCdS. Performance tests demonstrated that quantum dot sensitized solar cells (QDSSCs) equipped with ZnCdSSe/TiO2 photoanodes achieved a power conversion efficiency (PCE) of 3.61% by optimizing the hydrothermal reaction temperature and time. This efficiency was 33.2% higher than that of CdSSe (2.17%) QDSSCs synthesized via a one-step hydrothermal exchange reaction using CdS templates. The significant enhancement in PCE can be attributed to improvements in light absorption and charge carrier transport. Notably, Zn doping reduced interface charge recombination and prolong electronic lifetime (τn), facilitating more effective charge collection in QDSSCs.

ZnIn2S4 Heterojunctions Constructed with In-MOF Precursor for Photocatalytic Hydrogen Evolution without Cocatalysts.

Znln2S4 has great prospects for photocatalytic water splitting to hydrogen by visible light. Herein, a novel Znln2S4-In-MOF (ZnInMS4) photocatalyst is elaborately synthesized by in situ method with In-MOF as the template and In3+ as the source. ZnInMS4 overcomes the fast interface charge recombination and a sluggish charge lifetime via the formed heterojunctions. Photoelectrochemical measurements reveal that the charge-transfer kinetics is enhanced since In-MOF is introduced to act as a reliable charge-transport channel. ZnInMS4 exhibits outstanding cocatalyst-free H2 evolution rate of 70 μmol h-1 under irradiation (λ > 420 nm), which is 3.2-fold higher than that of Znln2S4. In addition, the ZnInMS4 photocatalyst shows good stability in the 16 h continuous reaction. This work illustrates the feasibility of the MOF precursor instead of inorganic salts to directly synthesize photocatalysts with high performance.

Triisocyanate Derived Interlayer and High-Melting-Point Doping Promoter Boost Operational Stability of Perovskite Solar Cells.

Formamidinium lead triiodide serves as the optimal light-absorbing layer in single-junction perovskite solar cells. However, achieving operational stability of high-efficiency n-i-p type devices at elevated temperatures remains challenging. In this work, we implemented effective surface modifications on microcrystalline perovskite films. This involved the nucleophilic addition of formamidinium cations and coordination of residual PbI2 with triphenylmethane triisocyanate as well as subsequent polymerization. The in situ growth of a cross-linking network chemically anchored on the perovskite film in this approach effectively reduced trap densities, favorably altered surface work function, suppressing interface charge recombination and thus enhancing cell efficiency. Coupled with a high-melting-point air-doping promoter, we fabricated n-i-p type perovskite solar cells surpassing 25 % efficiency, demonstrating excellent operational stability at 65 °C.

Triisocyanate Derived Interlayer and High‐Melting‐Point Doping Promoter Boost Operational Stability of Perovskite Solar Cells

AbstractFormamidinium lead triiodide serves as the optimal light‐absorbing layer in single‐junction perovskite solar cells. However, achieving operational stability of high‐efficiency n‐i‐p type devices at elevated temperatures remains challenging. In this work, we implemented effective surface modifications on microcrystalline perovskite films. This involved the nucleophilic addition of formamidinium cations and coordination of residual PbI2 with triphenylmethane triisocyanate as well as subsequent polymerization. The in situ growth of a cross‐linking network chemically anchored on the perovskite film in this approach effectively reduced trap densities, favorably altered surface work function, suppressing interface charge recombination and thus enhancing cell efficiency. Coupled with a high‐melting‐point air‐doping promoter, we fabricated n‐i‐p type perovskite solar cells surpassing 25 % efficiency, demonstrating excellent operational stability at 65 °C.

Enhancing Stability and Efficiency of Inverted Inorganic Perovskite Solar Cells with In-Situ Interfacial Cross-Linked Modifier.

Inverted inorganic perovskite solar cells (PSCs) is potential as the top cells in tandem configurations, owing to the ideal bandgap, good thermal and light stability of inorganic perovskites. However, challenges such as mismatch of energy levels between charge transport layer and perovskite, significant non-radiative recombination caused by surface defects, and poor water stability have led to the urgent need for further improvement in the performance of inverted inorganic PSCs. Herein, the fabrication of efficient and stable CsPbI3-x Brx PSCs through surface treatment of (3-mercaptopropyl) trimethoxysilane (MPTS), is reported. The silane groups in MPTS can in situ crosslink in the presence of moisture to build a 3-dimensional (3D) network by Si-O-Si bonds, which forms a hydrophobic layer on perovskite surface to inhibit water invasion. Additionally, -SH can strongly interact with the undercoordinated Pb2+ at the perovskite surface, effectively minimizing interfacial charge recombination. Consequently, the efficiency of the inverted inorganic PSCs improves dramatically from 19.0% to 21.0% under 100mW cm-2 illumination with MPTS treatment. Remarkably, perovskite films with crosslinked MPTS exhibit superior stability when soaking in water. The optimized PSC maintains 91% of its initial efficiency after aging 1000 h in ambient atmosphere, and 86% in 800 h of operational stability testing.

Modulating charge oriented accumulation via interfacial chemical-bond on In2O3/Bi2MoO6 heterostructures for photocatalytic nitrogen fixation

Photocatalytic nitrogen fixation presents an eco-friendly approach to converting atmospheric nitrogen into ammonia (NH3), but the process faces challenges due to rapid interface charge recombination. Here, we report an innovative charge transfer and oriented accumulation strategy using an In-O-Mo bond-modulated S-scheme heterostructure composed of In2O3/Bi2MoO6 (In/BMO) synthesized using a simple electrostatic assembly. The unique interfacial arrangement with optimal photocatalyst configuration (3 % In/BMO) enabled enhanced photogenerated electron separation and transfer, leading to a remarkable nitrogen fixation rate of approximately 150.9 μmol·gcat−1·h−1 under visible light irradiation. The performance of the photocatalyst was 9-fold and 27-fold higher than that of its pristine components, Bi2MoO6 and In2O3, respectively. The experimental and theoretical evaluation deemed interfacial In-O-Mo bonds crucial for rapid transfer and charge-oriented accumulation. Whereas the generated internal electric field drove the spatial separation and transfer of photo-generated electrons and holes, significantly enhancing the photocatalytic N2-to-NH3 conversion efficiency. The proposed work lays the foundation for designing S-scheme heterostructures with highly efficient interfacial bonds, offering a promising avenue for substantial improvements in photocatalytic nitrogen fixation.

Efficient Electron Transfer across Strontium Titanate Decorated Graphene Oxide for High‐Performance Dye‐Sensitized Solar Cells and Photocatalytic Degradation of Dye and Antibiotic

AbstractUnderstanding synergistic interface features is a crucial step towards adjustable and controllable interfaces for photocatalytic and photovoltaic devices. In this work, a novel layer‐structured rGO hybrid with SrTiO3 (STO/rGO) based binary composite has developed via hydrothermal method. The highly stable STO/rGO is used to construct a dual‐functional platform for dye, antibiotic degradation and DSSC. The photocatalytic activities of STO/rGO composites were examined using Methylene Blue (MB) and Tetracycline (TC) degradation in an aqueous solution under white light irradiation. The catalyst containing STG‐3 exhibited outstanding photocatalytic degradation efficiency of 93.06 % and 95.23 % toward MB and TC, respectively. In addition, the hybrid composites‐based photoanode had the most outstanding efficiency of 3.74 % with an increase in JSC. The performance is far better than pristine STO. The improved efficiency is ascribed to the strong interconnectivity, excellent crystallinity, higher surface area and high porosity, which results in reduced interfacial charge recombination loss, outstanding dye adsorption and better electron transport. Furthermore, a predicted mechanism is provided to explain the higher performance of STO/rGO hybrid composites.

Interfacial modification using the cross-linkable tannic acid for highly-efficient perovskite solar cells with excellent stability

Although the performance of perovskite solar cells (PSCs) has been dramatically increased in recent years, stability is still the main obstacle preventing the PSCs from being commercial. PSC device instability can be caused by a variety of reasons, including ions diffusion, surface and grain boundary defects, etc. In this work, the cross-linkable tannic acid (TA) is introduced to modify perovskite film through post-treatment method. The numerous organic functional groups (–OH and CO) in TA can interact with the uncoordinated Pb2+ and I− ions in perovskite, thus passivating defects and inhibiting ions diffusion. In addition, the formed TA network can absorb a small amount of the residual moisture inside the device to protect the perovskite layer. Furthermore, TA modification regulates the energy level of perovskite, and reduces interfacial charge recombination. Ultimately, following TA treatment, the device efficiency is increased significantly from 21.31% to 23.11%, with a decreased hysteresis effect. Notably, the treated device shows excellent air, thermal, and operational stability. In light of this, the readily available, inexpensive TA has the potential to operate as a multipurpose interfacial modifier to increase device efficiency while also enhancing device stability.

Regulation on electron density distribution of organic molecule passivator enables efficient and stable perovskite solar cells

Interfacial trap-mediated nonradiative charge recombination is a dominated limit to improving the efficiency and stability of perovskite solar cells (PSCs). The ionic nature of perovskite crystal enables molecular passivation methods through interaction between functional groups and unsaturated sites. However, a lack of in-depth understanding of the passivation mechanism and function of every functional group is a great challenge for the development of versatile passivators and further improvement of the PSCs performance. Herein, three 2-mercaptobenzothiazole (MBT) ligands featuring different functional groups which directly linking to their conjugated structure are employed to passivate interfacial defects and regulate crystallization. It is found that the increased electron density on conjugated ring structure and coordination sites gives a positive effect on the defect passivation and crystal growth. The MBT organic molecule with electron-donating group OCH3 gives the best passivation effects, while the molecule with electron-withdrawing group NO2 presents negative impacts. Furthermore, it is also demonstrated that the MBT based ligands containing multi-coordination sites (N, S and O atoms) present a strong coordination capacity with uncoordinated Pb2+ ion at the surface or grain boundaries of perovskite films. As a result, the MBT-OCH3 device shows a strikingly improved efficiency of 21.04 % along with excellent long-term stability. Therefore, this work opens a new line through the strategies to improve photovoltaic performance by modulating the electronic configuration of the passivation molecules.

(Invited) Two-Dimensional Low-Toxicity Absorbers for Efficient and Air-Stable Indoor Photovoltaics

The toxicity and bioavailability of lead (Pb) in halide perovskites motivates the search of Pb-free perovskite-inspired materials (PIMs), which would replicate the great optoelectronic properties of the traditional Pb-based counterparts. Among them, pnictohalides employing antimony or bismuth are especially attractive due to their low toxicity and inherent air stability. Their wide bandgaps (⁓2 eV) can broaden the application scenario beyond single-junction solar cells, being specifically suitable for e.g., silicon-perovskite tandems, indoor photovoltaics (IPVs), and photocatalysis.Despite outstanding indoor power conversion efficiencies up to 60% have been theoretically predicted for PIM-based IPVs, this research is still in a very early stage. In this talk, I will highlight some of our key findings on two-dimensional Sb- and/or Bi- based two-dimensional pnictohalides for sustainable indoor light harvesting. The considerable presence of both intrinsic and surface defects in wide-bandgap PIMs is mostly responsible for the modest indoor power conversion efficiency (PCE(i)) reported so far up to ⁓5% at 1000 lux white LED illumination. I will discuss the effective defect mitigation strategies relying on compositional engineering that we proposed to significantly improve the PCE(i) up to nearly 10% (9.53%). Our extensive characterizations at both material and device level evidence the successful suppression of trap-assisted and interfacial charge recombination as the result of a reduced defect density within the PIM domains and/or at their surface.The importance of our findings for the energy research community (within and beyond halide perovskite) lies in promoting future compositional design effort to reduce the defects in the next-generation wide-bandgap absorbers for eco-friendly energy generation and consumption.

(Invited) Correlation of Exciton and Charge Carrier Dynamics with the Performance of Metal Halide Perovskite Solar Cells

Perovskite solar cells have been recognized as a newly emerging solar cell with the potential of achieving high efficiency with a low cost fabrication process. In particular, facile solution processed cell fabrication facilitated rapid development of optimum cell structure and composition. Over the last few years, the cell efficiency has exceeded 25%.Highly efficient charge transfer reactions in addition to high charge separation efficiency and swift charge transport with minimum charge recombination are required to improve solar cell performance. Intensive studies have been focused on monitoring photo-induced exciton formation and charge dissociation, charge transport and interfacial charge transfer dynamics including interfacial charge recombination in perovskite solar cells.[1-5] Several parameters have been identified to influence these charge carrier dynamics, and therefore the solar cell performance.[1-5] Clarifying these parameters is extremely important to understand the charge transfer mechanisms to further improve solar cell performance.In this presentation, we will present parameters controlling charge dissociation in metal halide perovskites and their charge separation and recombination dynamics at the perovskite interfaces employing a series of transient absorption and emission spectroscopies. Correlation of the key parameters controlling the exciton and charge carrier dynamics with the solar cell performance will be discussed [1-5].This work was partly supported by JSPS KAKENHI Grant (19H02813) and (22H02182), and the Collaborative Research Program of Institute for Chemical Research, Kyoto University (grant number 2023-45 and 2022-99), Japan. We would like to acknowledge supports from the Australia-Japan Foundation for the international collaborative project. We also acknowledge supports from ARC LIEF fund (LE200100051 and LE170100235), Australia and School of Engineering in RMIT University, and Forefront Research Center, Faculty of Science at Osaka University.

Wavelength‐Recognizable SbSI:Sb2S3 Photovoltaic Devices: Elucidation of the Mechanism and Modulation of their Characteristics

AbstractThe output of photovoltaic (PV) devices is mostly independent of the wavelength of the incident light, whereas an anomalous wavelength‐dependent photovoltaic effect (WDPE) has recently been observed in antimony chalcohalide‐chalcogenide (SbSI:Sb2S3) PVs. Remarkably, the open‐circuit voltage (VOC) exhibits a reversible change between low VOC for short wavelengths and high VOC for long wavelengths. Herein, this work presents i) insights into the underlying mechanisms of this phenomenon by electron spin resonance (ESR) measurements and ii) the switchable character of WDPE depending on the hole transport material (HTM). Operando ESR measurements with light irradiation revealed that the hole density in the HTM is significantly suppressed when the ultraviolet component is included in the irradiation light. This indicated interfacial charge recombination rather than hole transfer to the HTM under short‐wavelength light irradiation, providing a basis for understanding the mechanism of WDPE. Furthermore, the use of poly(triarylamine) as the HTM unexpectedly exhibit the opposite wavelength‐VOC dependence, where a low VOC is observed with long‐wavelength light. In addition, the introduction of a polar gas accelerated the response speed of these effects. These findings shed light on the expansion of unique wavelength‐responsive single junction devices.

C-Rich Carbon Nitride Conjugated Polymer Enabling Ion-Migration-Induced Precise Electrochromic Display.

The development of electrochromic (EC) displays has been in the challenge of displaying precise patterns, such as characters or high-resolution images of small size. High-performance EC materials as well as efficient, precise-display strategies are still urgent. To enable a microfactor-guided strategy for highly precise display, I3-/I- ion-migration-induced localized electrochromism is developed in an EC device based on the C-rich polymeric carbon nitride (CPCN). The CPCN material with an extended conjugated backbone of individual aromatic nuclei and heptazine rings has been reported possessing remarkable photorechargeable performance. Owing to the self-charging behavior, the CPCN exhibits color switching by the interfacial charge recombination with I3- ions in electrolyte and serves as the EC material with a coloration efficiency of 210.2 cm2 C-1 and an optical contrast of 48.6%. Material synthesis, electrode preparation, device design and fabrication, mechanism analysis, and performance evaluation of the CPCN-based EC display device are described.

Reducing charge-recombination losses in photovoltaic cells by spontaneous reconstruction of n/p homojunction in a monolithic perovskite film using black phosphorus nanosheets

Trap-assisted charge recombination and interfacial charge recombination have limited further improvements in the efficiency and stability of perovskite solar cells. We construct a novel two-dimensional (2D) black phosphorus nanosheet (BP NS)-modified perovskite model that utilizes an upgraded additive post-processing technique to introduce 2D BP NSs, comprising multiple functional groups and carrying a high charge mobility and high work function, into the absorber during Ostwald ripening. 2D BP NSs, while passivating grain boundaries and surface defects, successfully induced the in-situ formation of an intriguing n/p homojunction structure between the underlying bulk and the top in a monolithic perovskite film, i.e., a graded n/p-PVK-BP absorber. This resulted in the synergistic advantages of expanding the built-in electric field to facilitate the oriented transfer of photogenerated carriers, optimizing the energy level alignment, and minimizing the hole injection barrier at the perovskite/hole transporting layer interface, thereby accelerating the selective extraction/collection of interfacial charges and synchronously reducing nonradiative and interfacial charge recombination losses. The 2D BP NSs anchored at the grain boundaries and surface of p-type perovskite form a “fast path” for hole migration, achieving carrier transport balance at the top and bottom interfaces. The optimized n/p-PVK-BP device achieved an efficiency of 23.22 %, with negligible hysteresis. Owing to the synergistic effects of the various advantages of the graded n/p-PVK-BP homojunction absorber, the unencapsulated device exhibited good long-term storage and operational stability, providing potential for commercial conversion of perovskite-based optoelectronics. This work provides a simple homojunction structure design and performance optimization strategy for the simultaneous reduction of trap-assisted charge recombination and interfacial charge recombination losses and provides new insights for the development of more efficient and stable perovskite photovoltaic applications.