Efficient and Stable Mesoscopic Perovskite Solar Cells Using PDTITT as a New Hole Transporting Layer

Stability is the main challenge in the field of organic–inorganic perovskite solar cells (PSCs). Finding low‐cost and stable hole transporting layer (HTL) is an effective strategy to address this issue. Here, a new donor polymer, poly(5,5‐didecyl‐5H‐1,8‐dithia‐as‐indacenone‐alt‐thieno[3,2‐b]thiophene) (PDTITT), is synthesized and employed as an HTL in PSCs, which has a suitable band alignment with respect to the double‐A cation perovskite film. Using PDTITT, the hole extraction in PSCs is greatly improved as compared to commonly used HTLs such as 2,2′,7,7′‐tetrakis[N,N‐di(4‐methoxyphenyl)amino]‐9,9′‐spirobifluorene (spiro‐OMeTAD), addressing the hysteresis issue. After careful optimization, an efficient PSC is achieved based on mesoscopic TiO₂ electron transporting layer with a maximum power conversion efficiency (PCE) of 18.42% based on PDTITT HTL, which is comparable with spiro‐OMeTAD‐based PSC (19.21%). Since spiro‐based PSCs suffer from stability issue, the operational stability in the PSC with PDTITT HTL is studied. It is found that the device with PDTITT retains 88% of its initial PCE value after 200 h under illumination, which is better than the spiro‐based PSC (54%).     

Multifunctional Phosphorus‐Containing Lewis Acid and Base Passivation Enabling Efficient and Moisture‐Stable Perovskite Solar Cells

Multiple‐cation lead mixed‐halide perovskites (MLMPs) have been recognized as ideal candidates in perovskite solar cells in terms of high efficiency and stability due to decreased open‐circuit voltage loss and suppressed yellow phase formation. However, they still suffer from an unsatisfactory long‐term moisture stability. In this study, phosphorus‐containing Lewis acid and base molecules are employed to improve device efficiency and stability based on their multifunction including recombination reduction, phase segregation suppression, and moisture resistance. The strong fluorine‐containing Lewis acid treatment can achieve a champion PCE of 22.02%. Unencapsulated and encapsulated devices retain 63% and 80% of the initial efficiency after 14 days of aging under 75% and 85% relative humidity, respectively. The better passivation of Lewis acid implies more halide defects than Pb defects at the MLMP surface. This unbalanced defect type results from phase segregation that is the synergistic effect of Cs and halide ion migrations. Identifying defect type based on different passivation effects is beneficial to not only choose suitable passivators to boost the efficiency and slow down the moisture degradation of MLMP solar cells, but also to understand the mechanism of defect‐assisted moisture degradation.     

In Situ Growth of 2D Perovskite Capping Layer for Stable and Efficient Perovskite Solar Cells

2D halide perovskites have recently been recognized as a promising avenue in perovskite solar cells (PSCs) in terms of encouraging stability and defect passivation effect. However, the efficiency (less than 15%) of ultrastable 2D Ruddlesden–Popper PSCs still lag far behind their traditional 3D perovskite counterparts. Here, a rationally designed 2D‐3D perovskite stacking‐layered architecture by in situ growing 2D PEA₂PbI₄ capping layers on top of 3D perovskite film, which drastically improves the stability of PSCs without compromising their high performance, is reported. Such a 2D perovskite capping layer induces larger Fermi‐level splitting in the 2D‐3D perovskite film under light illumination, resulting in an enhanced open‐circuit voltage (V_oc) and thus a higher efficiency of 18.51% in the 2D‐3D PSCs. Time‐resolved photoluminescence decay measurements indicate the facilitated hole extraction in the 2D‐3D stacking‐layered perovskite films, which is ascribed to the optimized energy band alignment and reduced nonradiative recombination at the subgap states. Benefiting from the high moisture resistivity as well as suppressed ion migration of the 2D perovskite, the 2D‐3D PSCs show significantly improved long‐term stability, retaining nearly 90% of the initial power conversion efficiency after 1000 h exposure in the ambient conditions with a high relative humidity level of 60 ± 10%.     

Highly Stable and Efficient Perovskite Solar Cells with 22.0% Efficiency Based on Inorganic–Organic Dopant‐Free Double Hole Transporting Layers

Most of the high performance in perovskite solar cells (PSCs) have only been achieved with two organic hole transporting materials: 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (Spiro‐OMeTAD) and poly(triarylamine) (PTAA), but their high cost and low stability caused by the hygroscopic dopant greatly hinder the commercialization of PSCs. One effective alternative to address this problem is to utilize inexpensive inorganic hole transporting layer (i‐HTL), but obtaining high efficiency via i‐HTLs has remained a challenge. Herein, a well‐designed inorganic–organic double HTL is constructed by introducing an ultrathin polymer layer dithiophene‐benzene (DTB) between CuSCN and Au contact. This strategy not only enhances the hole extraction efficiency through the formation of cascaded energy levels, but also prevents the degradation of CuSCN caused by the reaction between CuSCN and Au electrode. Furthermore, the CuSCN layer also promotes the formation of a pinhole‐free and compact DTB over layer in the CuSCN/DTB structure. Consequently, the PSCs fabricated with this CuSCN/DTB layer achieves the power conversion efficiency of 22.0% (certified: 21.7%), which is among the top efficiencies for PSCs based on dopant‐free HTLs. Moreover, the fabricated PSCs exhibit high light stability under more than 1000 h of light illumination and excellent environmental stability at high temperature (85 °C) or high relative humidity (>60% RH).     

Multi-Site Intermolecular Interaction for In Situ Formation of Vertically Orientated 2D Passivation Layer in Highly Efficient Perovskite Solar Cells

Surface passivation via 2D perovskite is critical for perovskite solar cells (PSCs) to achieve remarkable performances, in which the applied spacer cations play an important role on structural templating. However, the random orientation of 2D perovskite always hinder the carrier transport. Herein, multiple nitrogen sites containing organic spacer molecule (1H-Pyrazole-1-carboxamidine hydrochloride, PAH) is introduced to form 2D passivation layer on the surface of formamidinium based (FAPbI₃) perovskite. Deriving from the interactions between PAH and PbI₂, the defects of FAPbI₃ perovskite are effectively passivated. Interestingly, due to the multiple-site interactions, the 2D nanosheets are found to grow perpendicularly to the substrate for promotion of charge transfer. Therefore, an impressive power conversion efficiency of 24.6% and outstanding long-term stability are achieved for the 2D/3D perovskite devices. The findings further provide a perspective in structure design of novel organic halide salts for the fabrication of efficient and stable PSCs.     

Combustion Synthesized Zinc Oxide Electron‐Transport Layers for Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) have advanced rapidly with power conversion efficiencies (PCEs) now exceeding 22%. Due to the long diffusion lengths of charge carriers in the photoactive layer, a PSC device architecture comprising an electron‐ transporting layer (ETL) is essential to optimize charge flow and collection for maximum performance. Here, a novel approach is reported to low temperature, solution‐processed ZnO ETLs for PSCs using combustion synthesis. Due to the intrinsic passivation effects, high crystallinity, matched energy levels, ideal surface topography, and good chemical compatibility with the perovskite layer, this combustion‐derived ZnO enables PCEs approaching 17–20% for three types of perovskite materials systems with no need for ETL doping or surface functionalization.     

Multifunctional Enhancement for Highly Stable and Efficient Perovskite Solar Cells

With a certified efficiency as high as 25.2%, perovskite has taken the crown as the highest efficiency thin film solar cell material. Unfortunately, serious instability issues must be resolved before perovskite solar cells (PSCs) are commercialized. Aided by theoretical calculation, an appropriate multifunctional molecule, 2,2-difluoropropanediamide (DFPDA), is selected to ameliorate all the instability issues. Specifically, the carbonyl groups in DFPDA form chemical bonds with Pb²⁺ and passivate under-coordinated Pb²⁺ defects. Consequently, the perovskite crystallization rate is reduced and high-quality films are produced with fewer defects. The amino groups not only bind with iodide to suppress ion migration but also increase the electron density on the carbonyl groups to further enhance their passivation effect. Furthermore, the fluorine groups in DFPDA form both an effective barrier on the perovskite to improve its moisture stability and a bridge between the perovskite and HTL for effective charge transport. In addition, they show an effective doping effect in the HTL to improve its carrier mobility. With the help of the combined effects of these groups in DFPDA, the PSCs with DFPDA additive achieve a champion efficiency of 22.21% and a substantially improved stability against moisture, heat, and light.     

Multifunctional Cross-Linked Polyurethane Polymer as Interface Layer for Efficient and Stable Perovskite Solar Cells

In the past decade, perovskite solar cells (PSCs) have made remarkable progress in improving power conversion efficiency (PCE). In order to further improve the photovoltaic performance and long-term stability of PSCs, the interface layer is essential. A multifunctional cross-linked polyurethane (CLPU) is designed and synthesized via the spontaneous quaternization of polyurethane and 1, 6-diiodohexane on the surface of the perovskite layer. CLPU layer cannot only effectively induce secondary crystallization and passivate the surface defects of perovskite, reduce the non-radiative recombination, but also effectively block the moisture invasion. By this strategy, Cs_0.05FA_0.95PbI₃ PSCs with excellent reproducibility, is realized, achieving a PCE of 23.14% with an open-circuit voltage of 1.11 V, a short-circuit current density of 25.69 mA cm⁻², and a fill factor of 0.81. In addition, the unencapsulated devices show enhanced stability in 35 ± 5% relative humidity (RH) near 3000 h and in 65 ± 5% RH over 700 h. This study provides valuable insights into the role of CLPU interface layer in PSCs, which are essential for the design of high-performance devices.     

Self‐Adhesive Macroporous Carbon Electrodes for Efficient and Stable Perovskite Solar Cells

Carbon electrode are a low‐cost and great potential strategy for stable perovskite solar cells (PSCs). However, the efficiency of carbon‐based PSCs lags far behind compared with that of state‐of‐the‐art PSCs. The poor interface contact between the carbon electrode and the underlying layer dominates the performance loss of the reported carbon‐based PSCs. In this respect, a sort of self‐adhesive macroporous carbon film is developed as counter electrode by a room‐temperature solvent‐exchange method. Via a simple press transfer technique, the carbon film can form excellent interface contact with the underlying hole transporting layer, remarkably beneficial to interface charge transfer. A power conversion efficiency of up to 19.2% is obtained for mesoporous‐structure PSCs, which is the best achieved for carbon‐based PSCs. Moreover, the device exhibits greatly improved long‐term stability. It retains over 95% of the initial efficiency after 1000 h storage under ambient atmosphere. Furthermore, after aging for 80 h under illumination and maximum power point in nitrogen atmosphere, the carbon‐based PSC retains over 94% of its initial performance.     

Hole‐Transporting Materials Incorporating Carbazole into Spiro‐Core for Highly Efficient Perovskite Solar Cells

Hole‐transporting materials (HTMs) play a significant role in hole transport and extraction for perovskite solar cells (PeSCs). As an important type of HTMs, the spiro‐architecture‐based material is widely used as small organic HTM in PeSCs with good photovoltaic performances. The skeletal modification of spiro‐based HTMs is a critical way of modifying energy level and hole mobility. Thus, many spiro alternatives are developed to optimize the spiro‐type HTMs. Herein, a novel carbazole‐based single‐spiro‐HTM named SCZF‐5 is designed and prepared for efficient PeSCs. In addition, another single‐spiro HTM SAF‐5 with reported 10‐phenyl‐10H‐spiro[acridine‐9,9′‐fluorene] (SAF) core is also synthesized for comparison. Through varying from SAF core to SCZF core as well as comparing with the classic 9,9′‐spiro‐bifluorene, it is found that the new HTM SCZF‐5 exhibits more impressive power conversion efficiency (PCE) of 20.10% than SAF‐5 (13.93%) and the commercial HTM spiro‐OMeTAD (19.11%). On the other hand, the SCZF‐5‐based device also has better durability in lifetime testing, indicating the newly designed SCZF by integrating carbazole into the spiro concept has good potential for developing effective HTMs.     

2D Perovskite Seeding Layer for Efficient Air‐Processable and Stable Planar Perovskite Solar Cells

Despite the record power conversion efficiencies, inverted perovskite solar cells (PSCs) are still looking to overcome the challenge of moisture instability. This is mitigated by introducing 2D perovskite at the base of a 3D perovskite via coating of ethylenediamine bications on top of the hole transport layer of p–i–n planar configured devices. The cations induce thin 2D perovskite growth beneath the 3D perovskite to create a 2D/3D hybrid active layer. This 2D layer in turn acts as a template for the growth of relatively large grains compared to that of pure 3D perovskite films. This stems from the merging of grain boundaries. The hydrophobicity of the 2D/3D perovskite film consequently improves, as evidenced by a large contact angle of 93.1°, compared to 68.9° for the 3D perovskite film. Because there are fewer defects sourced from grain boundaries, the air‐processed 2D/3D perovskite devices yield a high power conversion efficiency of 15.02%, compared to 13.10% from 3D perovskite devices. When stored in moderately humid environment of 55% relative humidity, the 2D/3D devices exhibit longer stabilities, with 75% of their power conversion efficiencies maintained after 150 h, compared to a total loss in efficiency for 3D device in the same time frame.     

Bonding Strength Regulates Anchoring-Based Self-Assembly Monolayers for Efficient and Stable Perovskite Solar Cells

Anchoring-based self-assembly (ASA) has emerged as a material-saving and highly scalable strategy to fabricate charge-transporting monolayers for perovskite solar cells (PSCs). However, the interfacial hole-extraction and electron-blocking performances are highly dependent on the compactness of the ASA monolayers, which has been largely ignored though it is very crucial to the efficiency and stability of PSCs. Here, strategically designed hole-transporting molecules with different anchoring groups are incorporated to investigate the effect of bonding strength on monolayer quality and correlate these with the performance of p-i-n structured PSCs. It is unraveled that the anchoring groups with a stronger bonding strength are advantageous for improving the assembly rate, density, and compactness of ASA monolayer, thus enhancing charge collection and suppressing interfacial recombination. The prototypical PSCs based on optimal ASA monolayer achieve a high power conversion efficiency (PCE) of 21.43% (0.09 cm²). More encouragingly, when enlarging the device area by tenfold, a comparable PCE of 20.09% (1.0 cm²) can be obtained, suggesting that the ASA strategy is practically useful for scaling-up. The robust anchoring of the ASA monolayer also enhances devices stability, retaining 90% of initial PCE after three months. This study provides important insights into the ASA charge-transporting monolayers for efficient and stable PSCs.     

Organic Ionic Plastic Crystals as Hole Transporting Layer for Stable and Efficient Perovskite Solar Cells

Organic ionic plastic crystals (OIPCs) are synthesized through a simple metal‐free, cost‐effective approach. The strategized synchronization of electron‐rich phenoxazine with benzimidazolium iodide (OIPC‐I) and bromide (OIPC‐Br) salts lead to enhanced hole mobility and conductivity of OIPCs which is suitable for an efficient alternative to conventional organic hole transporting materials (HTMs) for stable perovskite solar cells (PSCs). The fabricated PSCs with OIPC‐I as hole transporting layer yielded a power conversion efficiency of 15.0% and 18.1% without and with additive (Li salt) respectively, which are comparable with spiro‐OMeTAD based devices prepared under similar conditions. Furthermore, the PSCs with OIPCs show good stability compared to the spiro‐OMeTAD with or without additives. Here, first time benzimidazolium‐based OIPCs have been used as an alternative organic HTM for perovskite solar cells, which opens a window for the design of effective OIPCs for highly efficient PSCs with long‐term stability.     

Polyfluorene Derivatives are High‐Performance Organic Hole‐Transporting Materials for Inorganic−Organic Hybrid Perovskite Solar Cells

Photovoltaics based on organic?inorganic perovskites offer new promise to address the contemporary energy and environmental issues. These solar cells have so far largely relied on small‐molecule hole transport materials such as spiro‐OMeTAD, which commonly suffer from high cost and low mobility. In principle, polyfluorene copolymers can be an ideal alternative to spiro‐OMeTAD, given their low price, high hole mobility and good processability, but this potential has not been explored. Herein, polyfluorene derived polymers‐TFB and PFB, which contain fluorine and arylamine groups, are demonstrated and can indeed rival or even outperform spiro‐OMeTAD as efficient hole‐conducting materials for perovskite solar cells. In particular, under the one‐step perovskite deposition condition, TFB achieves a 10.92% power conversion efficiency that is considerably higher than that with spiro‐OMeTAD (9.78%), while using the two‐step perovskite deposition method, about 13% efficient solar cells with TFB (12.80%) and spiro‐OMeTAD (13.58%) are delivered. Photo­luminescence reveals the efficient hole extraction and diffusion at the interface between CH₃NH₃PbI₃ and the hole conducting polymer. Impedance spectroscopy uncovers the higher electrical conductivity and lower series resistance than spiro‐OMeTAD, accounting for the significantly higher fill factor, photocurrent and open‐circuit voltage of the TFB‐derived cells than with spiro‐MeOTAD.     

Multifunctional Synthesis Approach of In:CuCrO2 Nanoparticles for Hole Transport Layer in High‐Performance Perovskite Solar Cells

While there are very limited studies of doped ternary metal oxide based hole transport materials, a multifunctional synthesis approach of In doped CuCrO₂ nanoparticles (NPs) as efficient hole transport layers (HTLs) including simplifying the synthesis requirements is proposed, enabling doping and achievement of treatment‐free HTLs. Remarkably, compared with conventional methods for synthesizing CuCrO₂ NPs, the newly proposed azeotropic promoted approach dramatically reduces the reaction time by 90% and the calcination temperature by one‐third, which not only promotes high throughput production but also reduces power consumption and cost in synthesis. Equally important, indium is successfully doped into CuCrO₂, which is fundamentally difficult in low temperature processes. The In doping offers less d–d transition of Cr³⁺ and p‐type doping characteristics for improving HTL transmittance and conductivity, respectively. Interestingly, In doped CuCrO₂ HTL with these improvements can be achieved by a simple ambient‐condition process and exhibits thermal stability up to 200 °C, which allows perovskite solar cells (PSCs) to achieve a power conversion efficiency of 20.54%. Meanwhile, the devices show good repeatability and photostability. Consequently, the work contributes to establishing a simple approach to realize pristine and doped multinary oxides based HTL for the development of practical and high performing PSCs.     

Molecular Configuration Engineering in Hole-Transporting Materials toward Efficient and Stable Perovskite Solar Cells

The development of hole-transporting materials (HTMs) that can passivate defects in perovskite is of great significance in improving the efficiency and long-term stability of perovskite solar cells. To date, the investigation on HTMs mainly focus on exploring new structures, while molecular configuration is seldomly concerned. In this work, two small molecules are developed as HTMs with benzil and phenanthrene quinone as the core structure, respectively. With similar structure and the same defect passivation groups, whereas, the two molecules exhibit different configurations, thus distinct properties. Compared to 3,6-bis(3,6-bis(bis(4-methoxyphenyl)amino)-9H-carbazol-9-yl)phenanthrene-9,10-dione (PQ) with a rigid core structure, the benzil group in 1,2-bis(4-(3,6-bis(bis(4-methoxyphenyl)amino)-9H-carbazol-9-yl)phenyl)ethane-1,2-dione (DB) is flexible and can adjust molecular configuration to efficiently interact with the underlying perovskite material, which is confirmed from both experimental results and theoretical simulations. The DB-based device exhibits a high power conversion efficiency of 22.21% with excellent long-term stability, superior to the PQ-based device (20.22%). This study demonstrates that molecular configuration engineering will directly affect the properties of hole transport materials, as well as their interactions with perovskite, which should also be taken into consideration when devising HTMs.     

One-Source Strategy Boosting Dopant-Free Hole Transporting Layers for Highly Efficient and Stable CsPbI2Br Perovskite Solar Cells

All-inorganic perovskites have emerged as promising photovoltaic materials due to their superior thermal stability compared to their organic–inorganic hybrid counterparts. However, the inferior film quality and doped hole transport layer (HTL) have a strong tendency to degrade the perovskite under high temperatures or harsh operating conditions. To solve these problems, a one-source strategy using the same polymer donor material (PDM) to simultaneously dope CsPbI₂Br perovskite films via antisolvent engineering and fabricating the HTL is proposed. The doping assists perovskite film growth and forms a top–down gradient distribution, generating CsPbI₂Br with enlarged grain size and reduced defect density. The PDM as the HTL suppresses the energy barrier and forms favorable electrical contacts for hole extraction, and assemble into a fingerprint-like morphology that improves the conductivity, facilitating the creation of a dopant-free HTL. Based on this one-source strategy using PBDB-T as PDM, the CsPbI₂Br perovskite solar cell with a dopant-free HTL achieves a power conversion efficiency (PCE) of 16.40%, which is one of the highest PCEs reported among all-inorganic CsPbI₂Br pero-SCs with a dopant-free HTL. Importantly, the devices exhibit the highest thermal stability at 85 °C and operational stability under continuous illumination even with Ag as the top electrode and present good universality.     

Multifunctional Small Molecule as Buried Interface Passivator for Efficient Planar Perovskite Solar Cells

The improvement of power conversion efficiency (PCE) and stability of the perovskite solar cell (PSC) is hindered by carrier recombination originating from the defects at the buried interface of the PSC. It is crucial to suppress the nonradiative recombination and facilitate carrier transfer in PSC via interface engineering. Herein, P-biguanylbenzoic acid hydrochloride (PBGH) is developed to modify the tin oxide (SnO₂)/perovskite interface. The effects of PBGH on carrier transportation, perovskite growth, defect passivation, and PSC performance are systematically investigated. On the one hand, the PBGH can effectively passivate the trap states of Sn dangling bonds and O vacancies on the SnO₂ surface via Lewis acid/base coordination, which is conducive to improving the conductivity of SnO₂ film and accelerating the electron extraction. On the other hand, PBGH modification assists the formation of high-quality perovskite film with low defect density due to its strong interaction with PbI₂. Consequently, the PBGH-modified PSC exhibits a champion power conversion efficiency (PCE) of 24.79%, which is one of the highest PCEs among all the FACsPbI₃-based PSCs reported to date. In addition, the stabilities of perovskite films and devices under high temperature/humidity and light illumination conditions are also systematically studied.     

Modulation on Electrostatic Potential of Passivator for Highly Efficient and Stable Perovskite Solar Cells

The perovskite layer contains a large number of charged defects that seriously impair the efficiency and stability of perovskite solar cells (PSCs), thus it is essential to develop an effective passivation strategy to heal them. Based on theoretical calculations, it is found that enhancing the electrostatic potential of passivators can improve passivation effect and adsorption energy between charged defects and passivators. Herein, an electrostatic potential modulation (EPM) strategy is developed to design passivators for highly efficient and stable PSCs. With the EPM strategy, 1-phenylethylbiguanide (PEBG) and 1-phenylbiguanide (PBG) are designed. It is found that the charge distribution and electrostatic potential of phenyl- and phenylethyl- substituent on the biguanide are significantly enhanced. The N atom directly bonding to the phenyl group shows larger positive charge than that bonding to the phenylethyl group. The modulated electrostatic potential makes PBG bind stronger with the defects on perovskite surface. Based on the effective passivation of EPM, a champion efficiency of 24.67% is realized and the device retain 91.5% of its initial PCE after ≈1300 h. The promising EPM strategy, which provides a principle of passivator design and allows passivation to be controllable, may advance further optimization and application of perovskite solar cells toward commercialization.