Blade-Coating (100)-Oriented α-FAPbI3 Perovskite Films via Crystal Surface Energy Regulation for Efficient and Stable Inverted Perovskite Photovoltaics.

Photoactive formamidinium lead triiodide (α-FAPbI3) perovskite has dominated the prevailing high-performance perovskite solar cells (PSCs), normally for those spin-coated, conventional n-i-p structured devices. Unfortunately, α-FAPbI3 has not been made full use of its advantages in inverted p-i-n structured PSCs fabricated via blade-coating techniques owing to uncontrollable crystallization kinetics and complicated phase evolution of FAPbI3 perovskites. Herein, a customized crystal surface energy regulation strategy has been innovatively developed by incorporating 0.5 mol% of N-aminoethylpiperazine hydroiodide (NAPI) additive into α-FAPbI3 crystal-derived perovskite ink, which enabled the formation of phase-pure, highly-oriented α-FAPbI3 films. We deciphered the phase transformation mechanisms and crystallization kinetics of blade-coated α-FAPbI3 perovskite films via combining a series of in-situ characterizations. Interestingly, the strong chemical interactions between the NAPI and inorganic Pb-I framework help to reduce the surface energy of (100) crystal plane by 42%, retard the crystallization rate and lower the formation energy of α-FAPbI3. The resultant blade-coated inverted PSCs based on (100)-oriented α-FAPbI3 perovskite films realized promising efficiencies up to 24.16% (~26.5% higher than that of the randomly-oriented counterparts), accompanied by improved operational stability. This result represented one of the best performances reported to date for FAPbI3-based inverted PSCs fabricated via scalable deposition methods.

Synergic Electron and Defect Compensation Minimizes Voltage Loss in Lead-Free Perovskite Solar Cells.

Sn perovskite solar cells have been regarded as one of the most promising alternatives to the Pb-based counterparts due to their low toxicity and excellent optoelectronic properties. However, the Sn perovskites are notorious to feature heavy p-doping characteristics and possess abundant vacancy defects, which result in under-optimized interfacial energy level alignment and severe nonradiative recombination. Here, we reported a synergic "electron and defect compensation" strategy to simultaneously modulate the electronic structures and defect profiles of Sn perovskites via incorporating a traced amount (0.1 mol %) of heterovalent metal halide salts. Consequently, the doping level of modified Sn perovskites was altered from heavy p-type to weak p-type (i.e. up-shifting the Fermi level by ∼0.12 eV) that determinately reducing the barrier of interfacial charge extraction and effectively suppressing the charge recombination loss throughout the bulk perovskite film and at relevant interfaces. Pioneeringly, the resultant device modified with electron and defect compensation realized a champion efficiency of 14.02 %, which is ∼46 % higher than that of control device (9.56 %). Notably, a record-high photovoltage of 1.013 V was attained, corresponding to the lowest voltage deficit of 0.38 eV reported to date, and narrowing the gap with Pb-based analogues (∼0.30 V).

Near-Stoichiometric and Homogenized Perovskite Films for Solar Cells with Minimized Performance Variation.

Mixed-cation, small band-gap perovskites via rationally alloying formamidinium (FA) and methylammonium (MA) together have been widely employed for blade-coated perovskite solar cells with satisfied efficiencies. One of the stringent challenges lies in difficult control of the nucleation and crystallization kinetics of the perovskites with mixed ingredients. Herein, a pre-seeding strategy by mixing FAPbI3 solution with pre-synthesized MAPbI3 microcrystals has been developed to smartly decouple the nucleation and crystallization process. As a result, the time window of initialized crystallization has been greatly extended by 3 folds (i.e. from 5 s to 20 s), which enables the formation of uniform and homogeneous alloyed-FAMA perovskite films with designated stoichiometric ratios. The resultant blade-coated solar cells achieved a champion efficiency of 24.31 % accompanied by outstanding reproducibility with more than 87 % of the devices showing efficiencies higher than 23 %.

Multidentate Chelation Heals Structural Imperfections for Minimized Recombination Loss in Lead-Free Perovskite Solar Cells.

Tin-based perovskite solar cells (Sn-PSCs) have emerged as promising environmentally viable photovoltaic technologies, but still suffer from severe non-radiative recombination loss due to the presence of abundant deep-level defects in the perovskite film and under-optimized carrier dynamics throughout the device. Herein, we healed the structural imperfections of Sn perovskites in an "inside-out" manner by incorporating a new class of biocompatible chelating agent with multidentate claws, namely, 2-Guanidinoacetic acid (GAA), which passivated a variety of deep-level Sn-related and I-related defects, cooperatively reinforced the passivation efficacy, released the lattice strain, improved the structural toughness, and promoted the carrier transport of Sn perovskites. Encouragingly, an efficiency of 13.7 % with a small voltage deficit of ≈0.47 V has been achieved for the GAA-modified Sn-PSCs. GAA modification also extended the lifespan of Sn-PSCs over 1200 hours.

Interfacial Linkage and Carbon Encapsulation Enable Full Solution-Printed Perovskite Photovoltaics with Prolonged Lifespan.

Simplified perovskite solar cells (PSCs) were fabricated with the perovskite layer sandwiched and encapsulated between carbon-based electron transport layer (ETL) and counter electrode (CE) by a fully blade-coated process. A self-assembled monolayer of amphiphilic silane (AS) molecules on transparent conducting oxide (TCO) substrate appeals to the fullerene ETL deposition and preserves its integrity against the solvent damage. The AS serves as a "molecular glue" to strengthen the adhesion toughness at the TCO/ETL interface via robust chemical interaction and bonding, facilitating the interfacial charge extraction, increasing PCEs by 77 % and reducing hysteresis. A PCE of 18.64 % was achieved for the fully printed devices, one of the highest reported for carbon-based PSCs. AS-assisted interfacial linkage and carbon-material-assisted self-encapsulation enhance the stability of the PSCs, which did not experience performance degradation when stored at ambient conditions for over 3000 h.

Correlating alkyl chain length with defect passivation efficacy in perovskite solar cells.

We, for the first time, correlated the alkyl chain length of amine molecules with the defect passivation efficacy, either on the surfaces or at grain boundaries of perovskite films. Blade-coated perovskite solar cells with long-chain amine passivation achieved an efficiency of 21.5%, accompanied by a small voltage loss of 0.35 V.

Bicomponent Random Approach for the Synthesis of Donor Polymers for Efficient All-Polymer Solar Cells Processed from A Green Solvent.

All-polymer solar cells (all-PSCs) can offer unique merits of high morphological stability to thermal and mechanical stress. To realize their full potential as flexible or wearable devices, it is highly desirable that the all-PSCs can be fabricated from a green solvent with simple post-treatment to avoid thermal annealing on the flexible substrate. This posed a severe challenge on material design to tune their properties with suitable solubility, aggregation, and morphology. To address this challenge, here, a simple bicomponent random approach on a D-A-type polymer donor was developed by just varying the D-A molar ratio. Under this approach, a series of new random polymers PBDTa-TPDb with different molar ratios of the D component of 2D-benzo[1,2-b:4,5-b']dithiophene (BDT) and A component of thieno[3,4-c]pyrrole-4,6-dione (TPD) were designed and synthesized. The energy levels, light absorption, solubility, and packing structure of random donors PBDTa-TPDb were found to vary substantially with the various D-A molar ratios. The devices based on PBDTa-TPDb/P(NDI2HD-T) were fabricated to explore the synergistic effects of the processing solvent and composition of D-A-type random polymers. The results show that nanoscale morphology, balanced miscibility/crystallinity of blend, and photovoltaic properties could be rationally optimized by tuning the composition of random donors. As a result, as-cast all-PSC-based optimal donor PBDT5-TPD4 achieves the best power conversion efficiency (PCE) of 8.20% processed from a green solvent, which performs better than that the reference polymer (PCE: 6.41%). This efficiency is the highest value for all-PSCs from BDT-TPD-based donors. Moreover, the optimized devices were relatively insensitive to the thickness of the active layer and exhibited good stability.

Synthesis of Bithiophene-Based D-A1-D-A2 Terpolymers with Different A2 Moieties for Polymer Solar Cells via Direct Arylation.

A series of bithiophene (2T)-based D-A1-D-A2 terpolymers with different A2 moieties were prepared via direct arylation reaction. In these terpolymers, pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) was selected as the first electron-accepting (A1) moiety, 2,1,3-benzothiadiazole (BT) or fluorinated benzothiadiazole (FBT) or octyl-thieno[3,4-c]pyrrole-4,6-dione (TPD) or 2,1,3-benzoselendiazole (SeT) was selected as the second electron-accepting (A2) moiety, while bithiophene with hexyl side chain was used as the electron-donating moiety. The UV-vis absorption, electrochemical properties, blend film morphology, and photovoltaic properties were studied to explore the effects of the A2 moiety. It is shown that these terpolymer films exhibit broad absorption (350⁻1000 nm), full width at half-maximum of more than 265 nm and ordered molecular packing. Varying the A2 moiety could affect the energy levels and blend film morphology leading to different polymer solar cell (PSC) performances of these (2T)-based D-A1-D-A2 terpolymers. As a result, the highest Jsc of 10.70 mA/cm² is achieved for Polymer 1 (P1) with BT as A2 moiety, while the higher highest occupied molecular orbital (HOMO) level limits the open circuit voltage (Voc) and leads to a power conversion efficiency (PCE) of 3.46%.