Surface Regulation via Carboxylate Polymer for Efficient and Stable CsPbI2Br Perovskite Solar Cells.

CsPbI2Br perovskite solar cell (PSC) is a promising candidate for high-efficiency single-junction and tandem solar cells. However, due to the numerous surface defects of the CsPbI2Br film and the mismatch of energy levels at the CsPbI2Br/charge transport layer interface, the power conversion efficiency (PCE) of CsPbI2Br PSC is still significantly lower than the theoretical limits. To alleviate those issues, in this work, a carboxylate-based p-type polymer, TTC-Cl, is employed to modify the surface of CsPbI2Br layer. TTC-Cl can interact with uncoordinated Pb2+, thereby mitigating surficial defects of CsPbI2Br film and reducing non-radiative recombination losses. Furthermore, TTC-Cl also improves the band properties of the CsPbI2Br thin film surface, rendering it more p-type, which facilitates hole transport. Consequently, the CsPbI2Br PSCs with TTC-Cl modification achieve a remarkable PCE of 17.81%, which is notably higher than that of counterpart without TTC-Cl (15.87%). Moreover, CsPbI2Br PSCs with TTC-Cl modification also exhibit better stability. This work highlights the importance of surface regulation via carboxylate polymer for further enhancing the performance of CsPbI2Br PSCs.

Recent Progress on Cross-Linkable Fullerene-based Electron Transport Materials for Perovskite Solar Cells.

Fullerene-based derivatives are frequently used as electron transport materials (ETMs) and interface buffers for perovskite solar cells (PSCs) due to their excellent properties, including high electron affinity and mobility, low recombination energy, tunable energy levels, and solution processability. However, significant challenges arise because fullerene derivatives tend to aggregate and dimerize, which reduces exciton dissociation and charge transport capacity. Additionally, their chemical compatibility with perovskite absorbers facilitates halide diffusion and degradation of PSCs. This overlap causes delamination and dissolution during device fabrication, hindering the performance enhancement of fullerene-based PSCs. To address these issues, researchers have developed cross-linkable fullerene materials. These materials have been shown to not only significantly improve the power conversion efficiency (PCE) of PSCs but also effectively enhance the device stability. In this review, we summarized recent research progress on cross-linkable fullerene derivatives as ETMs for PSCs. We systematically analyze the impact of these cross-linked ETMs on device performance and long-term stability, focusing on their molecular structures and working mechanisms. Finally, we discuss the future challenges that need to be overcome to advance the application of cross-linkable fullerene materials in PSCs.

Robust Imidazole-Linked Covalent Organic Framework Enabling Crystallization Regulation and Bulk Defect Passivation for Highly Efficient and Stable Perovskite Solar Cells.

The low crystallinity of the perovskite layers and many defects at grain boundaries within the bulk phase and at interfaces are considered huge barriers to the attainment of high performance and stability in perovskite solar cells (PSCs). Herein, a robust photoelectric imidazole-linked porphyrin-based covalent organic framework (PyPor-COF) is introduced to precisely control the perovskite crystallization process and effectively passivate defects at grain boundaries through a sequential deposition method. The 1D porous channels, abundant active sites, and high crystallization orientation of PyPor-COF offer advantages for regulating the crystallization of PbI2 and eliminating defects. Moreover, the intrinsic electronic characteristics of PyPor-COF endow a more closely matched energy level arrangement within the perovskite layer, which promotes charge transport and thereby suppresses the recombination of photogenerated carriers. The champion PSCs containing PyPor-COF achieved power conversion efficiencies of 24.10% (0.09 cm2) and 20.81% (1.0 cm2), respectively. The unpackaged optimized device is able to maintain its initial efficiency of 80.39% even after being exposed to air for 2000 h. The device also exhibits excellent heating stability and light stability. This work gives a new impetus to the development of highly efficient and stable PSCs via employing COFs.

Dual-Site Molecular Dipole Enables Tunable Interfacial Field Toward Efficient and Stable Perovskite Solar Cells.

The interfacial management in perovskite solar cells (PSCs), including mitigating the carrier transport barrier and suppressing non-radiative recombination, still remains a significant challenge for efficiency and stability enhancement. Herein, by screening a family of fluorine (F) terminated dual-site organic dipole molecules, the study aims to gain insight into the molecular dipole array toward tunable interfacial field. Both experimental and theoretical results reveal that these functional interfacial dipole molecules can effectively anchor on perovskite surface through Lewis acid-base interaction. In addition, the tailored side-chain with terminated F atoms allows for altering and constructing a well matched perovskite/Spiro-OMeTAD interfacial contact. As a result, the inserting dual-site organic dipole array effectively modulates the interface to deliver a gradient energy level alignment, facilitating carrier extraction and transport. The optimal dual-site dipole trifluoro-methanesulfonamide mediated N-i-P PSCs achieve the highest efficiency of 25.47%, together with enhanced operational stability under 1000 h of the simulated 1-sun illumination exposure. These findings are believed to provide insight into the design of dual-site molecular dipole with sufficient interfacial tunability for perovskite-based optoelectronic devices.

Towards High-Performance Inverted Mesoporous Perovskite Solar Cell by Using Bathocuproine (BCP).

Perovskite solar cells (PSCs) are considered the most promising photovoltaic devices to replace silicon-based solar cells because of their low preparation cost and high photoelectric conversion efficiency (PCE). Reducing defects in perovskite films is an effective means to improve the efficiency of PSCs. In this paper, a lead chelator was selected and mixed into hole transport layers (HTLs) to design and prepare mesoporous PSCs with the structure of ITO/PTAA(BCP)/Al2O3/PVK/PCBM/BCP/Ag, and its modification effect on the buried interface at the bottom of the perovskite layer in the mesoporous structure was explored. The experimental results show that in the presence of mesoporous alumina, the lead chelator can still play a role in modifying the bottom of the perovskite film. The use of lead chelator as passivation material added to the HTL can effectively reduce the residue of dimethyl sulfoxide (DMSO) and decrease the defects at the bottom of the perovskite film, which dramatically improves the device performance. The PCE of the device is increased from 18.03% to 20.78%, which is an increase of 15%. The work in this paper provides an effective method to enhance the performance of PSCs.

Computational screening two-dimensional conjugated microporous polymer applied in perovskite solar cells.

A two-dimensional (2D) conjugated microporous polymer with a structure of 2D nanosheets has been synthesized. Theoretical calculations and experimental results reveal that the Fermi level of this 2D polymer aligns well with perovskite absorber, and its conduction band is high enough to block electron transport to the anode. This 2D polymer is used to modify the hole transport layer, significantly improving its photoelectric properties, including enhanced hole mobility, matched energy level, and reduced recombination. Furthermore, the 2D polymer exhibits a mesoporous structure, allowing perovskite to fill into its loose framework, increasing the hole export area and providing a large hole transport flux. As a result, the efficiency of inverted perovskite solar cells enhances to 24.64% from 21.17% of control device without 2D conjugated microporous polymer. Given that this material can be synthesized on a large scale, this work has significant implications for the future development of 2D polymers in perovskite solar cells, potentially accelerating industrialization.

Over 21% Efficient Cesium Lead Triiodide Solar Cell Enabled by Molten Salt Accelerating the Crystallization Processes.

High-quality CsPbI3 with low defect density is indispensable for acquiring excellent photoelectric performance. Meticulous regulation of the CsPbI3 crystal growth processes is both feasible and efficacious in enhancing the quality of perovskite films. In this study, the cesium formate (CsFo) is introduced. On one hand, its low melting point can induce the crystallization processes at a low level of energy consumption. On the other hand, the pseudo-halide anion can participate in the passivation of iodide vacancies, as the formate anion exhibits a relatively higher affinity with iodide vacancies compared to other halides. Consequently, the introduction of CsFo enhances the quality of CsPbI3 thin films by altering the crystallization process and curbing defect formation. As a result, a steady-state output efficiency of 21.23% and an open-circuit voltage (Voc) as high as 1.25 V are achieved, with both parameters ranking among the highest for this type of solar cell.

Effects of Antisolvent Treatment on Copper(I) Thiocyanate Hole Transport Layer in n-i-p Perovskite Solar Cells.

Copper(I) thiocyanate (CuSCN) is considered an efficient HTL of low cost and with high stability in perovskite solar cells (PSCs). However, the diethyl sulfide solvent used for CuSCN preparation is known to cause damage to the underlying perovskite layer in n-i-p PSCs. Antisolvent treatment of CuSCN during spin-coating can effectively minimize interfacial interactions. However, the effects of antisolvent treatment are not sufficiently understood. In this study, the effects of five different antisolvents were investigated. Scanning electron microscopy and X-ray diffraction analyses showed that the antisolvent treatment improved the crystallinity of the CuSCN layer on the perovskite layer and reduced damage to the perovskite layer. However, X-ray and ultraviolet photoelectron spectroscopy analyses showed that antisolvent treatment did not affect the chemical bonds or electronic structures of CuSCN. As a result, the power conversion efficiency of the PSCs was increased from 14.72% for untreated CuSCN to 15.86% for ethyl-acetate-treated CuSCN.

Energy Level Tuning in CsPbBr3 Perovskite Solar Cells through In Situ-Polymerized PEDOT Hole Transport Layer.

The all-inorganic CsPbBr3 perovskite solar cells exhibit excellent stability against humidity and thermal conditions as well as relatively low production cost, rendering them a gradually emerging research hot spot in the field of photovoltaics. However, the absence of a hole transport layer (HTL) in its common structure and the substantial energy level difference of up to 0.6 eV between the highest occupied molecular orbital (HOMO) level of CsPbBr3 and the work function of the carbon electrode have emerged as the primary factor limiting the improvement of its power conversion efficiency (PCE). In this work, the monomer 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) is spin-coated onto the surface of the CsPbBr3 film directly and then subjected to annealing; DBEDOT undergoes in situ polymerization to form poly(3,4-ethylenedioxythiophene) (PEDOT), which aims to ameliorate the issue of excessive energy level difference between CsPbBr3 and the carbon electrode, and to facilitate the extraction and transport efficiency of holes between the CsPbBr3 perovskite and the carbon electrode. Compared to the pristine device, the PCE of the device based on in situ polymerization is enhanced and achieves a maximum efficiency of 9.81%. Furthermore, the unencapsulated devices based on in situ polymerization maintain 95.9% of their original efficiency after 40 days of stability testing.

Efficient Perovskite Solar Cells by Employing Triphenylamine-Functionalized Azadipyrromethene Dyes as Dopant-Free Hole-Transporting Materials and Bidentate Surface Passivating Agents.

In this study, a series of dopant-free, low-cost hole-transporting materials (HTMs) based on triphenylamine-functionalized azadipyrromethene dyes 1-3 (TPA-ADPs 1-3) were designed and synthesized. The properties of these new HTMs were investigated by optical spectroscopy, cyclic voltammetry, thermogravimetric analysis, differential scanning calorimetric, atomic force microscopy, and X-ray diffraction, as well as theoretical calculations. The results indicated that the TPA-ADPs 1-3 presented well-matched energy levels with perovskite, higher hole mobility, as well as more effective defect passivation at the perovskite/HTM interface by the coordination interaction between the ADP moiety and the undercoordinated Pb2+. The n-i-p perovskite solar cells (PSCs) employing HTMs 1-3 as well as doped Spiro-OMeTAD were fabricated and characterized. The TPA-ADP 1-based PSCs exhibited the best performance with a champion power conversion efficiency (PCE) of 22.13% and an fill factor of 0.81, which was superior to that of the devices based on the doped Spiro-OMeTAD. Long-term device performance studies indicated that the TPA-ADP 1-based PSCs maintained 80% of the initial PCE after 1800 h of storage in the ambient condition of 40-60% RH, which was also higher than the stability of doped Spiro-OMeTAD-based devices under the same conditions.

Enhanced Interfacial Modification by Ordered Discotic Liquid Crystals for Thermotolerance Perovskite Solar Cells.

Traditionally used phenylethylamine iodide (PEAI) and its derivatives, such as ortho-fluorine o-F-PEAI, in interfacial modification, are beneficial for perovskite solar cell (PSC) efficiency but vulnerable to heat stability above 85 °C due to ion migration. To address this issue, we propose a composite interface modification layer incorporating the discotic liquid crystal 2,3,6,7,10,11-hexa(pentoxy)triphenylene (HAT5) into o-F-PEAI. The triphenyl core in HAT5 promotes π-π stacking self-assembly and enhances its interaction with o-F-PEAI, forming an oriented columnar phase that improves hole extraction along the one-dimensional direction. HAT5 repairs structural defects in the interfacial layer and retains the layered structure to inhibit ion migration after annealing. Ultimately, our approach increases the efficiency of solar cells from 23.36% to 25.02%. The thermal stability of the devices retains 80.1% of their initial efficiency after aging at 85 °C for 1008 hours without encapsulation. Moreover, the optimized PSCs maintained their initial efficiency of 82.4% after aging under one sunlight exposure for 1008 hours. This study provides a novel strategy using composite materials for interface modification to enhance the thermal and light stability of semiconductor devices.

Highly flexible and transparent colorless polyimide substrate sandwiched between plasma polymerized fluorocarbon and InGaTiO for high performance flexible perovskite solar cells.

We integrated transparent antireflective coatings and transparent electrodes onto flexible colorless polyimide (CPI) substrates to fabricate high-performance flexible perovskite solar cells. Multifunctional PPFC/CPI/IGTO substrates were fabricated by sputtering the optimal plasma-polymerized fluorocarbon (PPFC) antireflective coating and InGaTiO (IGTO) electrode films on both sides of the CPI substrate. By applying PPFC with a low refractive index (1.38) as an antireflective coating, the transparency of the PPFC/CPI/IGTO substrate increased by an additional 1.2%. In addition, owing to the amorphous characteristics of the PPFC and IGTO layers, the PPFC/CPI/IGTO substrate showed constant sheet resistance and transmittance change even after 10,000 cycles during the bending tests. The flexible perovskite solar cells, fabricated on the PPFC/CPI/IGTO substrate, exhibited an increase in current density of 1.48 mA/cm2 after the deposition of the PPFC antireflective coating. These results confirmed that the PPFC/CPI/IGTO substrate was durable against high-temperature treatment, flexible, and exhibited excellent electrical characteristics. This enhanced the efficiency and durability of the flexible perovskite solar cells. Moreover, the hydrophobic PPFC layer allowed the self-cleaning of inflexible perovskite solar cells. Given these attributes, the PPFC/CPI/IGTO structure has been recognized as a good choice for multifunctional substrates of flexible perovskite solar cells, presenting the potential for enhancing performance.

We have confirmed the durability of PPFC/CPI/IGTO substrates against high-temperature treatment, their flexibility, transparency, and their exceptional electrical properties, suggesting them as a prime selection for FPSCs.

High Open-Circuit Voltage Wide-Bandgap Perovskite Solar Cell with Interface Dipole Layer.

Wide-bandgap perovskite solar cells (PSCs) with high open-circuit voltage (Voc) represent a compelling and emerging technological advancement in high-performing perovskite-based tandem solar cells. Interfacial engineering is an effective strategy to enhance Voc in PSCs by tailoring the energy level alignments between the constituent layers. Herein, n-type quinoxaline-phosphine oxide-based small molecules with strong dipole moments is designed and introduce them as effective cathode interfacial layers. Their strong dipole effect leads to appropriate energy level alignment by tuning the work function of the Ag electrode to form an ohmic contact and enhance the built-in potential within the device, thereby improving charge-carrier transport and mitigating charge recombination. The organic interfacial layer-modified wide-bandgap PSCs exhibit a high Voc of 1.31 V (deficit of <0.44 V) and a power conversion efficiency (PCE) of 20.3%, significantly improved from the device without an interface dipole layer (Voc of 1.26 V and PCE of 16.7%). Furthermore, the hydrophobic characteristics of the small molecules contribute to improved device stability, retaining 95% of the initial PCE after 500 h in ambient air.

Halogen-Functionalized Hole Transport Materials with Strong Passivation Effects for Stable and Highly Efficient Quasi-2D Perovskite Solar Cells.

The performance of quasi-two-dimensional (Q-2D) perovskite solar cells (PSCs) strongly depends on the interface characteristics between the hole transport material (HTM) and the perovskite layer. In this work, we designed and synthesized a series of HTMs with triphenylamine-carbazole as the core structure and modified end groups with chlorine and bromine atoms. These HTMs show deeper highest occupied molecular orbital energy levels than commercial HTMs. This reduced energy band mismatch between the HTM and perovskite layer facilitates efficient charge extraction at the interface. Moreover, these HTMs containing halogen atoms on the end groups could form halogen bonding with the Pb2+ ions at the buried interface of the perovskite layer, effectively passivating defects to suppress nonradiative recombination. Additionally, halogen bonding also contributes to the formation of vertically oriented perovskite crystals with a high quality. By incorporation of chlorohexane-substituted HTMs, the resultant Q-2D PSCs exhibited the highest power conversion efficiency of 21.07%. Furthermore, the devices show improved stability, retaining 97.2% of their initial efficiency after 1100 h of continuous illumination.

Enhancing the Humidity Stability of Perovskite Films through Interfacial Modification with Differentiated Hydrophilic Organics.

Preparing high-quality perovskite films is a decisive step toward realizing highly efficient and stable perovskite solar cells (Pero-SCs). Water is a key factor affecting the stability of the Pero-SCs. Here, the widely used water adsorbents chitosan, sorbitol, and sodium hyaluronate (NaHA) were used as hydrophilic layers on the upper interface of the perovskite to form a barrier against water. The water adsorbents also passivated defects on the surface of the perovskite active layer due to their -OH and -COOH functional groups. The NaHA-modified devices showed the best power conversion efficiency (PCE) (PCE = 21.74%). Although the NaHA-modified Pero-SCs showed optimal photovoltaic performance, the stability of the modified devices decreased due to the strong water adsorption ability of NaHA, while with moderate water adsorption ability sorbitol-modified devices exhibited good stability and PCE. The devices were tested in the dark and room temperature at different humidity levels for 800 h. At low humidity (25% ± 5% RH), the PCEs of the sorbitol- and NaHA-modified devices were maintained at 80% and 71% of the initial values, respectively. At high humidity (75% ± 5% RH), the PCE was maintained at 64% and 23% of the initial values, respectively. This work provides an avenue to select adsorbents with suitable water absorption ability as the interface modification layer, thus reducing the water erosion of perovskite films and obtaining highly stable inverted Pero-SCs.

Development of Lightweight and Flexible Perovskite Solar Cells on Ultrathin Colorless Polyimide Foils.

The development of Perovskite Solar Cells (PSCs) on flexible substrates marks a significant advancement in thin-film photovoltaic technology. However, current state-of-the-art research predominantly utilizes Poly(ethylene terephthalate) (PET) substrate, which limits the deployment to less challenging environments. To address this limitation, we explore the fabrication of inverted PSCs on colorless polyimide (CPI) substrates that can withstand harsh environmental conditions. We employed a sequential sputtering technique to deposit indium tin oxide (bottom electrode) and nickel oxide (hole transport layer) as a base stack for the perovskite. This base layer was further enhanced by incorporating MeO-2PACz into the hole transport bilayer, significantly improving the NiOx interface, and thereby enhancing the efficiency of the devices. The PSCs fabricated on CPI demonstrated a power conversion efficiency (PCE) of 15.52% and a remarkable power-to-weight ratio (PWR) of 4.39 W/g, which is five times higher than that of devices on PET (0.87 W/g). Moreover, the active stack developed in this study can be used on any transparent substrate, showing its broader application potential.

Tetrapodal Hole-Collecting Monolayer Materials Based on Saddle-Like Cyclooctatetraene Core for Inverted Perovskite Solar Cells.

Hole-collecting monolayers have greatly advanced the development of positive-intrinsic-negative perovskite solar cells (p-i-n PSCs). To date, however, most of the anchoring groups in the reported monolayer materials are designed to bind to the transparent conductive oxide (TCO) surface, resulting in less availability for other functions such as tuning the wettability of the monolayer surface. In this work, we developed two anchorable molecules, 4PATTI-C3 and 4PATTI-C4, by employing a saddle-like indole-fused cyclooctatetraene as a π-core with four phosphonic acid anchoring groups linked through propyl or butyl chains. Both molecules form monolayers on TCO substrates. Thanks to the saddle shape of a cyclooctatetraene skeleton, two of the four phosphonic acid anchoring groups were found to point upward, resulting in hydrophilic surfaces. Compared to the devices using 4PATTI-C4 as the hole-collecting monolayer, 4PATTI-C3-based devices exhibit a faster hole-collection process, leading to higher power conversion efficiencies of up to 21.7 % and 21.4 % for a mini-cell (0.1 cm2) and a mini-module (1.62 cm2), respectively, together with good operational stability. This work represents how structural modification of multipodal molecules could substantially modulate the functions of the hole-collecting monolayers after being adsorbed onto TCO substrates.

Bottom Contact Engineering for Ambient Fabrication of >25% Durable Perovskite Solar Cells.

The bottom contact in perovskite solar cells (PSCs) is easy to cause deep trap states and severe instability issues, especially under maximum power point tracking (MPPT). In this study, sodium gluconate (SG) is employed to disperse tin oxide (SnO2) nanoparticles (NPs) and regulate the interface contact at the buried interface. The SG-SnO2 electron transfer layer (ETL) enabled the deposition of pinhole-free perovskite films in ambient air and improved interface contact by bridging effect. SG-SnO2 PSCs achieved an impressive power conversion efficiency (PCE) of 25.34% (certified as 25.17%) with a high open-circuit voltage (VOC) exceeding 1.19 V. The VOC loss is less than 0.34 V relative to the 1.53 eV bandgap, and the fill factor (FF) loss is only 2.02% due to the improved contact. The SG-SnO2 PSCs retained around 90% of their initial PCEs after 1000 h operation (T90 = 1000 h), higher than T80 = 1000 h for the control SnO2 PSC. Microstructure analysis revealed that light-induced degradation primarily occurred at the buried holes and grain boundaries and highlighted the importance of bottom-contact engineering.

Inorganic-Derived 0D Perovskite Induced Surface Lattice Arrangement for Efficient and Stable All-Inorganic Perovskite Solar Cells.

The inverted inorganic CsPbI3 perovskite solar cells (PSCs) are prospective candidates for next-generation photovoltaics owing to inherent robust thermal/photo-stability and compatibility for tandems. However, the performance and stability of the inverted CsPbI3 PSCs fall behind the n-i-p counterparts due to poor energetic alignment and abundant interfacial defect states. Here, an inorganic 0D Cs4PbBr6 with a good lattice strain arrangement is implemented as the surface anchoring capping layer on CsPbI3. The Cs4PbBr6 perovskite induces enhanced electron-selective junction and thus facilitates efficient charge extraction and effectively inhibits non-radiative recombination. Consequently, the CsPbI3 PSCs with Cs4PbBr6 demonstrate the highest power conversion efficiency (PCE) of CsPbI3-based inverted PSCs, reaching 21.03% PCE from a unit cell and 17.39% PCE from a module with a 64 cm2 aperture area. Furthermore, the resulting devices retain 92.48% after 1000 h under simultaneous 1-sun and damp heat (85 °C / 85% relative humidity) environment.

Crown Ethers with Different Cavity Diameters Inhibit Ion Migration and Passivate Defects toward Efficient and Stable Lead Halide Perovskite Solar Cells.

Organic-inorganic hybrid metal halide perovskite solar cells have been considered as one of the most promising next-generation photovoltaic technologies. Nevertheless, perovskite defects and Li+ ionic migration will seriously affect the power conversion efficiency and stability of the formal device. Herein, we designed two crown ether derivatives (PC12 and PC15) with different cavity diameters, which selectively bind to different metal cations. It is found that PC15 in perovskite precursor solution can actively regulate the nucleation and crystallization processes and passivate the uncoordinated Pb2+ ions, while PC12 at the interface between the perovskite layer and hole-transporting layer can effectively inhibit the migration of Li+ ions and reduce nonradiative recombination losses. Therefore, PC12 and PC15 can act as "lubricant" and defect passivators, as well as inhibitors of ion migration, when they are synergistically applied at the surface and bulk of perovskite layer. Consequently, the optimized device achieved a champion efficiency of 24.8% with significantly improved humidity, thermal, and light stability.