Phenanthroline-Based Low-Cost and Efficient Small-Molecule Cathode Interfacial Layer Enables High-Performance Inverted Perovskite Solar Cells via Doctor-Blade Coating.

Perovskite solar cells (PSCs) have recently emerged as highly efficient and cutting-edge photovoltaic technology. In inverted PSCs, challenges are focused on the insufficient interface contact and energy level misalignment between the electron transport layer (ETL) and the metal electrode. Hence, the cathode interfacial layer (CIL) plays a crucial role in regulating energy levels and enabling charge extraction in PSCs. In this study, a low-cost phenanthroline derivative, 4,7-dimethoxy-1,10-phenanthroline (Phen-OMe), is developed as an efficient CIL between the PCBM and Ag electrodes. The incorporation of Phen-OMe not only improves the interfacial contact but also effectively reduces the work function (WF) of the Ag electrode, thus promoting charge dissociation and transport at the interface. Through utilizing a wide-band-gap perovskite with the band gap of 1.77 eV as the active layer by a simple, high-throughput, and low-cost doctor-blade coating process, the power conversion efficiency (PCE) is enhanced significantly from 16.11% of the control device to 18.61% of the device with Phen-OMe as the CIL. Interestingly, Phen-OMe shows a broad application as the CIL in PSCs and tandem solar cells (TSCs), resulting in a boosted efficiency of 22.29% in intermediate-band-gap PSCs and a PCE of 22.05% with a high open-circuit voltage (VOC) of 2.12 V in the perovskite/organic TSC. This achievement shows that Phen-OMe would be a potential candidate as low-cost and efficient CILs for PSCs.

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.

Dual-Strategy Tailoring Molecular Structures of Dopant-Free Hole Transport Materials for Efficient and Stable Perovskite Solar Cells.

Dopant-free hole transport materials (HTMs) are ideal materials for highly efficient and stable n-i-p perovskite solar cells (PSCs), but most current design strategies for tailoring the molecular structures of HTMs are limited to single strategy. Herein, four HTMs based on dithienothiophenepyrrole (DTTP) core are devised through dual-strategy methods combining conjugate engineering and side chain engineering. DTTP-ThSO with ester alkyl chain that can form six-membered ring by the S⋅⋅⋅O noncovalent conformation lock with thiophene in the backbone shows good planarity, high-quality film, matching energy level and high hole mobility, as well as strong defect passivation ability. Consequently, a remarkable power conversion efficiency (PCE) of 23.3 % with a nice long-term stability is achieved by dopant-free DTTP-ThSO-based PSCs, representing one of the highest values for un-doped organic HTMs based PSCs. Especially, the fill factor (FF) of 82.3 % is the highest value for dopant-free small molecular HTMs-based n-i-p PSCs to date. Moreover, DTTP-ThSO-based devices have achieved an excellent PCE of 20.9 % in large-area (1.01 cm2) devices. This work clearly elucidates the structure-performance relationships of HTMs and offers a practical dual-strategy approach to designing dopant-free HTMs for high-performance PSCs.

Mechanical milling processed highly luminescent Cs-Pb-Br perovskite emitters.

We report well-dispersed highly emitting perovskite emitters synthesized via the surfactant-assisted ball-milling method. Both the emitting peaks and the colour purity of the synthesized perovskite emitters can be effectively tuned through additive functionalization and precursor engineering.

Two-Second-Annealed 2D/3D Perovskite Films with Graded Energy Funnels and Toughened Heterointerfaces for Efficient and Durable Solar Cells.

The 2D/3D perovskite heterostructures have been widely investigated to enhance the efficiency and stability of perovskite solar cells (PSCs). However, rational manipulation of phase distribution and energy level alignment in such 2D/3D perovskite hybrids are still of great challenge. Herein, we successfully achieved spontaneous phase alignment of 2D/3D perovskite heterostructures by concurrently introducing both 2D perovskite component and organic halide additive. The graded phase distribution of 2D perovskites with different n values and 3D perovskites induced favorable energy band alignment across the perovskite film and boosted the charge transfer at the relevant heterointerfaces. Moreover, the 2D perovskite component also acted as a "band-aid" to simultaneously passivate the defects and release the residual tensile stress of perovskite films. Encouragingly, the blade-coated PSCs based on only ≈2 s in-situ fast annealed 2D/3D perovskite films with favorable energy funnels and toughened heterointerfaces achieved promising efficiencies of 22.5 %, accompanied by extended lifespan. To our knowledge, this is the highest reported efficiency for the PSCs fabricated with energy-saved thermal treatment just within a few seconds, which also outperformed those state-of-the-art annealing-free analogues. Such a two-second-in-situ-annealing technique could save the energy cost by up to 99.6 % during device fabrication, which will grant its low-coast implementation.

Additive-Stabilized Emission Centers for Blue Perovskite Light-Emitting Diodes.

The performance of the blue perovskite light-emitting diodes (PeLEDs) is limited by the low photoluminescence quantum yields (PLQYs) and the unstable emission centers. In this work, we incorporate sodium bromide and acesulfame potassium into a quasi-2D perovskite to control the dimension distribution and promote the PLQYs. Benefiting from the efficient energy cascade channel and passivation, the sky-blue PeLED has an external quantum efficiency of 9.7% and no shift of the electroluminescence center under operation voltages from 4 to 8 V. Moreover, the half lifetime of the devices reaches 325 s, 3.3 times that of control devices without additives. This work provides new insights into enhancing the performance of blue PeLEDs.

Phosphotungstate-Based Anode Interfacial Material for Constructing High-Performance Polymer Solar Cells with a Fill Factor over 80.

In the field of organic solar cells (OSCs), the interfacial layer plays the role of enhancing carrier extraction/transportation, inhibiting their recombination, etc. In contrast to the wide variety of cathode interfacial materials with good modification ability, much less effort has been reported for anode interfacial materials. In this study, we report a polyoxometalate-based inorganic molecular cluster, zinc phosphotungstate (Zn3P2W24O80, denoted ZnPW), as an anode interfacial layer. Based on the PM6/EH-HD-4F/L8-BO-F ternary system, the device with ZnPW modification achieved a high power conversion efficiency (PCE) and a fill factor of up to 18.67 and 80.29%, respectively, which are higher than the counterpart device (PCE of 18.01%) with PEDOT/PSS as the anode interfacial layer. Detailed studies revealed that under the modification of ZnPW, the devices obtained promoted light absorption and suitable energy level matching between the active layer and the electrode, reduced contact resistance, and suppressed charge recombination. In addition, the ZnPW-modified devices had improved photostability and storage stability compared to PEDOT/PSS-modified devices. Our work shows that the polyoxometalate-based inorganic nanocluster ZnPW has great advantages in enhancing the device performance and stability of OSCs.

Secondary Anti-Solvent Treatment for Efficient 2D Dion-Jacobson Perovskite Solar Cells.

Surface defects-mediated nonradiative recombination plays a critical role in the performance and stability of perovskite solar cells (PSCs) and surface post-treatment is widely used for efficient PSCs. However, the commonly used surface passivation strategies are one-off and the passivation defect ability is limited, which can only solve part of the defects in the topmost surface area. Here, a secondary anti-solvent strategy is proposed to further reduce surface defects based on conventional surface passivation for the first time. Based on this, the crystallization quality of 2D Dion-Jacobson perovskite is enhanced and the surface defects density is further reduced by nearly two orders. In addition, a gradient structure of perovskite with n = 2 phases located at the top of the film and 3D-like phases located at the bottom of the film can also be obtained. The modulated perovskite film boosts the efficiency of 2D perovskites (n = 5) up to 19.55%. This strategy is also very useful in other anti-solvent processed perovskite dipping systems, which paves a promising avenue for minimizing surface defects toward highly efficient perovskite devices.

Dopant-Free Bithiophene-Imide-Based Polymeric Hole-Transporting Materials for Efficient and Stable Perovskite Solar Cells.

The development of hole-transport materials (HTMs) with high mobility, long-term stability, and comprehensive passivation is significant for simultaneously improving the efficiency and stability of perovskite solar cells (PVSCs). Herein, two donor-acceptor (D-A) conjugated polymers PBTI and PFBTI with alternating benzodithiophene (BDT) and bithiophene imide (BTI) units are successfully developed with desirable hole mobilities due to the good planarity and extended conjugation of molecular backbone. Both copolymers can be employed as HTMs with suitable energy levels and efficient defect passivation. Shortening the alkyl chain of the BTI unit and introducing fluorine atoms on the BDT moiety effectively enhances hole mobility and hydrophobicity of the HTMs, leading to improved efficiency and stability of PVSCs. As a result, the organic-inorganic hybrid PVSCs with PFBTI as the HTM deliver a power conversion efficiency (PCE) of 23.1% with enhanced long-term operational and ambient stability, which is one of the best efficiencies reported for PVSCs with dopant-free polymeric HTMs to date. Moreover, PFBTI can be applied in inorganic PVSCs and perovskite/organic tandem solar cells, achieving a high PCE of 17.4% and 22.2%, respectively. These results illustrate the great potential of PFBTI as an efficient and widely applicable HTM for cost-effective and stable PVSCs.

High-Performance Deep Red Colloidal Quantum Well Light-Emitting Diodes Enabled by the Understanding of Charge Dynamics.

Colloidal quantum wells (CQWs) have emerged as a promising family of two-dimensional (2D) optoelectronic materials with outstanding properties, including ultranarrow luminescence emission, nearly unity quantum yield, and large extinction coefficient. However, the performance of CQWs-based light-emitting diodes (CQW-LEDs) is far from satisfactory, particularly for deep red emissions (≥660 nm). Herein, high efficiency, ultra-low-efficiency roll-off, high luminance, and extremely saturated deep red CQW-LEDs are reported. A key feature for the high performance is the understanding of charge dynamics achieved by introducing an efficient electron transport layer, ZnMgO, which enables balanced charge injection, reduced nonradiative channels, and smooth films. The CQW-LEDs based on (CdSe/CdS)@(CdS/CdZnS) ((core/crown)@(colloidal atomic layer deposition shell/hot injection shell)) show an external quantum efficiency of 9.89%, which is a record value for 2D nanocrystal LEDs with deep red emissions. The device also exhibits an ultra-low-efficiency roll-off and a high luminance of 3853 cd m-2. Additionally, an exceptional color purity with the CIE coordinates of (0.719, 0.278) is obtained, indicating that the color gamut covers 102% of the International Telecommunication Union Recommendation BT 2020 (Rec. 2020) standard in the CIE 1931 color space, which is the best for CQW-LEDs. Furthermore, an active-matrix CQW-LED pixel circuit is demonstrated. The findings imply that the understanding of charge dynamics not only enables high-performance CQW-LEDs and can be further applied to other kinds of nanocrystal LEDs but also is beneficial to the development of CQW-LEDs-based display technology and related integrated optoelectronics.

Doping Compensation Enables High-Detectivity Infrared Organic Photodiodes for Image Sensing.

Infrared organic photodiodes have gained increasing attention due to their great application potentials in night vision, optical communication, and all-weather imaging. However, the commonly occurring high dark current and low detectivity impede infrared photodetectors from portable applications at room temperature. Herein, an efficient and generic doping compensation strategy is developed to improve the detectivity of infrared organic photodiodes. A series of n-type organic semiconductors is investigated, and it is found that doping compensation strategy not only reduces the trap density of states and dark currents, but also restrains the nonradiative recombination with improved charge transport and collection. As a result, an ultralow noise spectral density of 8 × 10-15 A Hz-1/2 as well as a high specific detectivity over 1013 Jones in 780-1070 nm is achieved at room temperature. More importantly, the high-performance infrared organic photodiodes can be successfully applied in high-pixel-density image arrays without patterning sensing layers. These findings provide important compensation design insights that will be crucial to further improve the performance of infrared organic photodiodes in the future.

Molecularly Engineered Interfaces in Metal Halide Perovskite Solar Cells.

Perovskite solar cells (PSCs) have emerged as a promising candidate for next-generation thin-film photovoltaic technology owing to their excellent optoelectronic properties and cost-effectiveness. To gain the full potential of device performance, an in-depth understanding of the surface/interface science is an urgent need. Here, we present a review of molecularly engineered studies on interface modifications of PSCs. We elaborate a systematic classification of the existing optimization techniques employed in molecularly engineered perovskite and interface materials and analyze the insights underlying the reliability issues and functional behaviors. The achievements allow us to highlight the crucial strengths of molecular design for further tailoring of the interfacial properties, mitigating the nonradiative losses, optimizing the device performance, and retarding the degradation process of PSCs. Finally, the remaining challenges and potential development directions of molecularly engineered interfaces for high-performance and stable PSCs are also proposed.

Architecturing 1D-2D-3D Multidimensional Coupled CsPbI2 Br Perovskites toward Highly Effective and Stable Solar Cells.

Despite the rapid development of CsPbIx Br3- x (0 ≤ x ≤ 3) inorganic perovskite solar cells, associated with their superior thermal stability, their low moisture stability limits their commercial deployment. In this study, 1D-2D-3D multidimensional coupled perovskites are prepared by means of an in situ self-integration approach. This pioneering method allows incorporating thus far unreported 1D-Tpy2 Pb3 I6 and 2D-TpyPb3 I6 (Tpy; terpyridine) perovskites. Heterojunction perovskites demonstrate superior stability against water in comparison with control 3D CsPbI2 Br, which is related to the hydrophobicity of low-dimension (LD) perovskites. Remarkably, the spontaneous involvement of LD perovskites can adjust/reconstruct the interfacial structure. This modification allows releasing the residual strain, establishing effective charge transfer channels that increase the carrier transport ability. Accordingly, 1D-2D-3D hybrid CsPbI2 Br perovskite solar cells demonstrate a stabilized power conversion efficiency as high as 16.1%, which represents a very significant improvement, by a factor of 43%, with respect to control 3D CsPbI2 Br perovskite solar cell. Equally importantly, the multidimensional coupled perovskite solar cells exhibit extraordinary stability, well above 1000 h in ambient atmosphere.

Graded 2D/3D Perovskite Heterostructure for Efficient and Operationally Stable MA-Free Perovskite Solar Cells.

Almost all highly efficient perovskite solar cells (PVSCs) with power conversion efficiencies (PCEs) of greater than 22% currently contain the thermally unstable methylammonium (MA) molecule. MA-free perovskites are an intrinsically more stable optoelectronic material for use in solar cells but compromise the performance of PVSCs with relatively large energy loss. Here, the open-circuit voltage (Voc ) deficit is circumvented by the incorporation of ß-guanidinopropionic acid (ß-GUA) molecules into an MA-free bulk perovskite, which facilitates the formation of quasi-2D structure with face-on orientation. The 2D/3D hybrid perovskites embed at the grain boundaries of the 3D bulk perovskites and are distributed through half the thickness of the film, which effectively passivates defects and minimizes energy loss of the PVSCs through reduced charge recombination rates and enhanced charge extraction efficiencies. A PCE of 22.2% (certified efficiency of 21.5%) is achieved and the operational stability of the MA-free PVSCs is improved.

Dopant-Free Organic Hole-Transporting Material for Efficient and Stable Inverted All-Inorganic and Hybrid Perovskite Solar Cells.

Designing new hole-transporting materials (HTMs) with desired chemical, electrical, and electronic properties is critical to realize efficient and stable inverted perovskite solar cells (PVSCs) with a p-i-n structure. Herein, the synthesis of a novel 3D small molecule named TPE-S and its application as an HTM in PVSCs are shown. The all-inorganic inverted PVSCs made using TPE-S, processed without any dopant or post-treatment, are highly efficient and stable. Compared to control devices based on the commonly used HTM, PEDOT:PSS, devices based on TPE-S exhibit improved optoelectronic properties, more favorable interfacial energetics, and reduced recombination due to an improved trap passivation effect. As a result, the all-inorganic CsPbI2 Br PVSCs based on TPE-S demonstrate a remarkable efficiency of 15.4% along with excellent stability, which is the one of the highest reported values for inverted all-inorganic PVSCs. Meanwhile, the TPE-S layer can also be generally used to improve the performance of organic/inorganic hybrid inverted PVSCs, which show an outstanding power conversation efficiency of 21.0%, approaching the highest reported efficiency for inverted PVSCs. This work highlights the great potential of TPE-S as a simple and general dopant-free HTM for different types of high-performance PVSCs.

Highly efficient all-inorganic perovskite solar cells with suppressed non-radiative recombination by a Lewis base.

All-inorganic perovskite solar cells (PVSCs) have drawn increasing attention because of their outstanding thermal stability. However, their performance is still inferior than the typical organic-inorganic counterparts, especially for the devices with p-i-n configuration. Herein, we successfully employ a Lewis base small molecule to passivate the inorganic perovskite film, and its derived PVSCs achieved a champion efficiency of 16.1% and a certificated efficiency of 15.6% with improved photostability, representing the most efficient inverted all-inorganic PVSCs to date. Our studies reveal that the nitrile (C-N) groups on the small molecule effectively reduce the trap density of the perovskite film and thus significantly suppresses the non-radiative recombination in the derived PVSC by passivating the Pb-exposed surface, resulting in an improved open-circuit voltage from 1.10 V to 1.16 V after passivation. This work provides an insight in the design of functional interlayers for improving efficiencies and stability of all-inorganic PVSCs.

Dopant-Free Squaraine-Based Polymeric Hole-Transporting Materials with Comprehensive Passivation Effects for Efficient All-Inorganic Perovskite Solar Cells.

Development of high-performance dopant-free hole-transporting materials (HTMs) with comprehensive passivation effects is highly desirable for all-inorganic perovskite solar cells (PVSCs). Squaraines (SQs) could be a candidate for dopant-free HTMs as they are natural passivators for perovskites. One major limitation of SQs is their relatively low hole mobility. Herein we demonstrate that polymerizing SQs into pseudo two dimensional (2D) p-π conjugated polymers could overcome this problem. By rationally using N,N-diarylanilinosquaraines as the comonomers, the resulting polysquaraine HTMs not only exhibit suitable energy levels and efficient passivation effects, but also achieve very high hole mobility close to 0.01 cm-2 V-1 s-1 . Thus as dopant-free HTMs for α-CsPbI2 Br-based all-inorganic PVSCs, the best PCE reached is 15.5 %, outperforming those of the doped-Spiro-OMeTAD (14.4 %) based control devices and among the best for all-inorganic PVSCs.

Dual Interfacial Design for Efficient CsPbI2 Br Perovskite Solar Cells with Improved Photostability.

A synergic interface design is demonstrated for photostable inorganic mixed-halide perovskite solar cells (PVSCs) by applying an amino-functionalized polymer (PN4N) as cathode interlayer and a dopant-free hole-transporting polymer poly[5,5'-bis(2-butyloctyl)-(2,2'-bithiophene)-4,4'-dicarboxylate-alt-5,5'-2,2'-bithiophene] (PDCBT) as anode interlayer. First, the interfacial dipole formed at the cathode interface reduces the workfunction of SnO2 , while PDCBT with deeper-lying highest occupied molecular orbital (HOMO) level provides a better energy-level matching at the anode, leading to a significant enhancement in open-circuit voltage (Voc ) of the PVSCs. Second, the PN4N layer can also tune the surface wetting property to promote the growth of high-quality all-inorganic perovskite films with larger grain size and higher crystallinity. Most importantly, both theoretical and experimental results reveal that PN4N and PDCBT can interact strongly with the perovskite crystal, which effectively passivates the electronic surface trap states and suppresses the photoinduced halide segregation of CsPbI2 Br films. Therefore, the optimized CsPbI2 Br PVSCs exhibit reduced interfacial recombination with efficiency over 16%, which is one of the highest efficiencies reported for all-inorganic PVSCs. A high photostability with a less than 10% efficiency drop is demonstrated for the CsPbI2 Br PVSCs with dual interfacial modifications under continuous 1 sun equivalent illumination for 400 h.