High-Performance Ternary Polymer Solar Cells Enabled by a New Narrow Bandgap Nonfullerene Small Molecule Acceptor with a Higher LUMO Level.

Obtaining a large open-circuit voltage (VOC ) and high short-circuit current density (JSC ) simultaneously is important in improving power conversion efficiency (PCE) of organic photovoltaics. The ternary strategy with using a higher lowest unoccupied molecular orbital (LUMO) level nonfullerene acceptor (NFA) guest can achieve increased VOC , yet JSC is decreased or maintained, so it's still a challenge to offer increased VOC and JSC values concurrently via the newly presented VOC -increased ternary strategy. To overcome this issue, a new narrow bandgap NFA TT-S-4F is reported by introducing 3,6-dimethoxylthieno[3,2-b]thiophene (TT) as π-spacers to connect electron-rich core with terminal groups, so as to upshift the LUMO level and extend π-system. When adding 10% TT-S-4F into binary system based on PTB7-Th:IEICO-4F, the higher-LUMO-level of TT-S-4F, the increased charge mobilities, the reduced trap-assisted combination loss, and a finer nanofiber structure and increased phase separation size are obtained, which simultaneously promotes JSC , VOC , and fill factor (FF), thus obtaining an optimal PCE (12.5% vs 11.5%). This work illustrates that an extending conjugated backbone with large π-spacers and inclusion of alkylthiophenyl side-chains is a concept to synthesize NFA guests for use on the VOC -increased ternary strategy that enables to realize simultaneously increased JSC , VOC , and FF.

Quaternary Solar Cells with 12.5% Efficiency Enabled with Non-Fullerene and Fullerene Acceptor Guests to Improve Open Circuit Voltage and Film Morphology.

Designing the bulk-heterojunction structure (binary, ternary and quaternary) is of great fundamental interest for relaxing the trade-off between open circuit voltage (Voc ) and short circuit current density (   Jsc ). Herein, a new quaternary blended material system is reported with a nonfullerene and PC71 BM as the third and fourth component of PBDB-T:ITCT, leading to simultaneously increased Voc and Jsc and maintained fill factor (FF). The guest IT-T-IC has upshifted LUMO energy level helping to obtain a higher Voc . The further addition of 0.2 PC71 BM as the fourth component yields an even higher Voc because the LUMO of PC71 BM is higher than that of ITCT. Interestingly, the blend of PC71 BM leads to the formation of an unprecedented neuron-like morphology, which acts as new centers not only performing light absorption and charge separation but also charge transport through their surrounding donor and acceptor fibers. The increased hole and electron mobilities and the reduced bimolecular loss results in an even larger Jsc and FF. These results indicate that a combination using a structurally similar higher-LUMO-level non-fullerene acceptor and PCBM is a simple yet effective quaternary material approach to simultaneously increase Voc and Jsc while maintaining FF, improving final device performance.

A Cation-Exchange Approach for the Fabrication of Efficient Methylammonium Tin Iodide Perovskite Solar Cells.

Tin-based halide perovskite materials have been successfully employed in lead-free perovskite solar cells, but the overall power conversion efficiencies (PCEs) have been limited by the high carrier concentration from the facile oxidation of Sn2+ to Sn4+ . Now a chemical route is developed for fabrication of high-quality methylammonium tin iodide perovskite (MASnI3 ) films: hydrazinium tin iodide (HASnI3 ) perovskite film is first solution-deposited using presursors hydrazinium iodide (HAI) and tin iodide (SnI2 ), and then transformed into MASnI3 via a cation displacement approach. With the two-step process, a dense and uniform MASnI3 film is obtained with large grain sizes and high crystallization. Detrimental oxidation is suppressed by the hydrazine released from the film during the transformation. With the MASnI3 as light harvester, mesoporous perovskite solar cells were prepared, and a maximum power conversion efficiency (PCE) of 7.13 % is delivered with good reproducibility.

Molecular Quadripod as a Noncovalent Interfacial Coupling Reagent for Forming Immobilized Coordination Assemblies.

A pyrene-cored molecular quadripod 1,3,6,8-tetra(di( p-pyrid-4-ylphenyl)amino)pyrene (TAPyr) is presented as a noncovalent interfacial coupling reagent for the immobilization of coordination assemblies. This bench-stable molecule is readily available and has a quadripod shape with four pyridine legs and four pyridine handles on the top exterior. By a simple and short dipping procedure under ambient conditions, TAPyr is firmly immobilized on electrode surfaces in an upright fashion as probed by electrochemical, absorption spectral, atomic force microscopy, and scanning tunneling microscopy analysis. Using Pd(PhCN)2Cl2 as a metallolinker, 4-ferrocenylpyridine, a pyridine-terminated monoruthenium complex 1, and a diruthenium complex 2 with two pyridine ends have been grafted onto the ITO/TAPyr surface. The obtained thin films exhibit good electrochemical stability that is comparable or superior to those prepared by the state-of-the-art Si-O-Sn covalent functionalization. Appealing electrochromism is demonstrated with the thin films of ruthenium complexes on ITO.

Tunable Self-Assembly and Morphology-Dependent Photoconductivity of a Donor-Acceptor-Structured Diruthenium Complex.

A donor-acceptor-structured diruthenium complex, 1(PF6)4, that contains an electron-deficient bridging ligand and electron-rich distal diarylamines modified with long aliphatic chains has been synthesized. By varying the solvent environments and assembly conditions, we obtained three different self-assembled nanostructures of 1(PF6)4, including zero-dimensional nanospheres, one-dimensional nanofibers, and thin films with interconnected nanowire networks. These structures were investigated by scanning electron microscopy, transmission electron microscopy, dynamic light scattering, X-ray diffraction, and atomic force microscopy (AFM) analysis. Conductive AFM analysis shows that the nanowire networks exhibit a high conductivity of 0.023 S/cm and an enhanced photoconductivity of 0.59 S/cm under visible light irradiation.

Tuning morphology and photovoltaic properties of diketopyrrolopyrrole-based small-molecule solar cells by taloring end-capped aromatic groups.

In this article, we selected BDT­DPP­BDT (DPP = diketopyrrolopyrrole and BDT = 4,8-di-2-(2-ethylhexyl)-thienyl-benzo[1,2-b:4,5-b']dithiophene) as the model backbone and end-capped it with hydrogen, octyl 2-cyano-3-(thiophen-2-yl)acrylate (CNR), and 2-hexylbithiophene (HTT), respectively, forming three small molecule donors: BDB, CNRBDB and HTTBDB. Introduction of a polar and planar electron-withdrawing unit of CNR to both ends of the BDB backbone enhances the hole mobility from 4.14 × 10(−4) to 7.75 × 10(−3) cm(2) V(−1) s(−1) and raises the fill factor from 27 to 57% when blended with PC71BM. This is associated with the PC71BM phase size decreasing from 70 to 20 nm. When the electron-donating unit of HTT with poorer planarity is linked to both ends of the BDB backbone, both donor and acceptor phase sizes are decreased to 20 nm. The short-circuit current density is greatly improved from 4.22 to 9.66 mA cm(−2), and the fill factor is enhanced to 46%. Overall, this work demonstrates that the end-capped aromatic groups play an important role in tuning the phase size and photovoltaic properties of DPP-based small molecule solar cells.

Impact of molecular solvophobicity vs. solvophilicity on device performances of dimeric perylene diimide based solution-processed non-fullerene organic solar cells.

Because of their outstanding molecular optoelectronic properties, perylene diimides (PDIs) are promising alternatives to the commonly used PCBM. However, the overly strong aggregation ability, poor solution-processability and compatibility of PDIs severely limit their photovoltaic applications. We turned to borrowing the amphiphile concept to improve these supramolecular properties. Practically, we fine-tuned the molecular solvophobicity with respect to the molecular solvophilicity, e.g. F(solvophob/solvophil), by changing the number of the weakly solvophobic 2-methoxyethoxyl (EG) groups in the bay-region of the thienyl-bridged dimeric PDI backbone, forming three PDI dimers of Bis-PDI-T (0 EG), Bis-PDI-T-EG (2 EG) and Bis-PDI-T-di-EG (4 EG) (Scheme 1). The photovoltaic properties using these dimers as the solution-processed non-fullerene electron-acceptor and P3HT as the electron-donor were investigated via the device configuration of ITO/PEDOT:PSS/P3HT:PDI dimer/Ca/Al. Bis-PDI-T exhibited overly strong aggregation ability and very poor solution-processability, which severely limited compatibility, giving a very poor power conversion efficiency (PCE) of 0.007%. When two EG groups were attached at the 1,1'-positions, the resulted Bis-PDI-T-EG showed dramatically reduced aggregation ability, improved solution-processability, compatibility and proper phase separation. Small sized phases (∼20 nm) dominated in the active layer and the best PCE was increased to 0.39%. When four solvophobic EG functions were introduced, affording Bis-PDI-T-di-EG with excellent supramolecular properties, particularly, the improvement of the phase separation with an increased phase size of 24 nm and the enhanced electron and hole mobilities, by 2-4 times, with respect to that of Bis-PDI-T-EG. The best PCE was further enhanced to 0.88%. After using 1-chloronaphthalene as the co-solvent of 1,2-dichlorobenzene to further improve the compatibility, the PCE was improved further up to 0.41% for Bis-PDI-T, 0.76% for Bis-PDI-T-EG and 1.54% for Bis-PDI-T-di-EG.

Chromism based on supramolecular H-bonds.

Normal solvatochromic phenomena are induced by different polarities of the ground and excited states of a compound when it is dissolved in a solvent. A compound such as the perylene diimide (PDI) derivative, which has a small difference in the dipole moments of the excited and ground states, generally shows a weak color change. Herein, we found that a dilute dichloromethane (DCM) solution of the PDI derivative 1,6,7,12-tetra(4-tert-butylphenoxyl) PDI (1) with a typical concentration of 1 × 10(-5) M distinctly changed colour from red to dark blue with a distinct red-shift of both the absorption (Δλ(a)max = 32 nm) and the fluorescence (Δλ(f)max = 45 nm) when 50,000 equivalents of trifluoroacetic acid (TFA) were added. Such a new chromism originates from the stronger decrease of the energy level of the LUMO than that of the HOMO after the step-by-step H-bonding of TFA with the PDI chromophore: firstly, the imide C[double bond, length as m-dash]O functionality, then the bridged -O- and finally the TFA molecules undergo H-bonding, forming a highly polar TFA shell around the PDI molecule, as proved by the concentration variable UV-vis absorption, fluorescence, (1)H NMR, (13)C NMR, and NOE spectra, cyclic voltammetry, and quantum chemical calculations. The degree of the solution's color change (Δλ(a)max/Δλ(f)max) depends (1) on the number of the bay-substituted 4-n-butylphenoxyl groups: it amounts to 25/38 and 17/22 nm for 1,7-bis(4-tert-butylphenoxyl) PDI (2) and the bay-unsubstituted PDI 3, respectively, and (2) on the polarity of the -OH functionality: in HOOC-CX3, for example, the value of Δλ(a)max/Δλ(f)max of PDI 1 amounts to 9.5/17 nm for trichloroacetic acid (TClA, X = Cl) and 0/3.6 nm for acetic acid (AA, X = H). The protons are necessary for the chromism, and thus ethyl trifluoroacetic acid ester, EtOTFA, cannot produce any obvious red-shifting of the absorption and fluorescence for 1-3. However, 2,2,2-trifluoroethanol (TFEtOH) produces an obvious red-shift.

Fluorination Enhances NIR-II Emission and Photothermal Conversion Efficiency of Phototheranostic Agents for Imaging-Guided Cancer Therapy.

Phototheranostics with second near-infrared (NIR-II) imaging and photothermal effect have become a burgeoning biotechnology for tumor diagnosis and precise treatment. As important parameters of phototheranostic agents (PTAs), fluorescence quantum yield (QY) and photothermal conversion efficiency (PCE) are usually considered as a pair of contradictions that is difficult to be simultaneously enhanced. Herein, a fluorination strategy for designing A-D-A type PTAs with synchronously improved QY and PCE is proposed. Experimental results show that the molar extinction coefficient (ε), NIR-II QY, and PCE of all fluorinated PTAs nanoparticles (NPs) are definitely improved compared with the chlorinated counterparts. Theoretical calculation results demonstrate that fluorination can maximize the electrostatic potential difference by virtue of the high electronegativity of fluorine, which may increase intra/intermolecular D-A interactions, tighten molecule packing, and further promote the increase of ε, ultimately leading to simultaneously enhanced QY and PCE. In these PTA NPs, FY6-NPs display NIR-II emission extended to 1400 nm with the highest NIR-II QY (4.2%) and PCE (80%). These features make FY6-NPs perform well in high-resolution imaging of vasculature and NIR-II imaging-guided photothermal therapy (PTT) of tumors. This study develops a valuable guideline for constructing NIR-II organic PTAs with high performance.

A self-assembly phase diagram from amphiphilic perylene diimides.

Supramolecular forces govern self-assembly and further determine the final morphologies of self-assemblies. However, how they control the morphology remains hitherto largely unknown. In this paper, we have discovered that the self-assembled nanostructures of rigid organic semiconductor chromophores can be finely controlled by the secondary forces by fine-tuning the surrounding environments. In particular, we used water/methanol/hydrochloric acid to tune the environment and observed five different phases that resulted from versatile molecular self-assemblies. The representative self-assembled nanostructures were nanotapes, nanoparticles and their 1D assemblies, rigid microplates, soft nanoplates, and hollow nanospheres and their 1D assemblies, respectively. The specific nanostructure formation is governed by the water fraction, R(w), and the concentration of hydrochloric acid, [HCl]. For instance, nanotapes formed at low [HCl] and R(w) values, whereas hollow nanospheres formed when either the HCl concentration is high, or the water fraction is low, or both. The significance of this paper is that it provides a useful phase diagram by using R(w) and [HCl] as two variables. Such a self-assembly phase diagram maps out the fine control that the secondary forces have on the self-assembled morphology, and thus allows one to guide the formation toward a desired nanostructure self-assembled from rigid organic semiconductor chromophores by simply adjusting the two key parameters of R(w) and [HCl].

Intermolecular electron transfer promoted by directional donor-acceptor attractions in self-assembled diketopyrrolopyrrole-thiophene films.

The photophysics of a symmetric triad consisting of two bithiophene (BT) units covalently linked to a central diketopyrrolopyrrole unit (DPP) has been investigated both in dichloromethane and in the thin film. The DPP-BT film exhibits a red-shifted low-energy absorption band compared to its solution, which is indicative of efficient π-π interactions in the solid-state phase. The steady-state and time-resolved fluorescence results revealed that the photoluminescence was subjected to severe emission quenching when DPP-BT changes from its solution phase to its film form. Further femtosecond transient absorption studies clarified that rapid intermolecular electron transfer accounts for the considerable fluorescence quenching event. The structural characterization of DPP-BT nanobelts, based on GIXRD and SAED patterns, suggested that the composite may be self-assembled into a slipped face-to-face configuration in the film, providing compact interlayer D-A interactions. As a result, intermolecular electron transfer is promoted by the favorable donor-acceptor attractions between the adjacent molecules. Moreover, this packing configuration provides a moderate channel for charge transportation. The hole mobility, which was measured based on a single-belt field-effect transistor, was found to be around 0.07 cm(2) V(-1) s(-1). Our observation reveals the role of spatial orientation in photophysical processes and the consequential semiconductor performance, providing guidance for the development and self-assembly of new opto-electronic molecules.

Fine-tuning device performances of small molecule solar cells via the more polarized DPP-attached donor units.

Three solution-processable small molecules of DPPT, DPPSe and DPPTT were synthesized by Stille coupling through attaching donor units of thiophene (T), selenophene (Se) and thieno[3,2-b]thiophene (TT) to the diketopyrrolopyrrole (DPP) core, respectively. Replacement of the T donors with the more polarized Se units results in a balance between the a and b direction packing and an obvious increase of the power conversion efficiency (PCE) from 1.90% to 2.33% with the increase of the short-circuit current (I(sc)) from 5.59 to 5.81 mA cm(-2) and the open-circuit voltage (V(oc)) from 0.78 V to 0.86 under the small molecule/acceptor ratio of 3 : 1. However, introduction of the conjugation-enlarged TT groups (versus the T units) leads to a decrease of the PCE, down to 1.70%, with a significant decrease of the fill factor (FF) (38% versus 44%), due to its poor film-forming characteristics.

Photoswitchable nanoprobes offer unlimited brightness in frequency-domain imaging.

A single probe has limited brightness in time-domain imaging and such limitation frequently renders individual molecules undetectable in the presence of interference or complex cellular structures. However, a single photoswitchable probe produces a signal, which can be separated from interference or noise using photoswitching-enabled Fourier transformation (PFT). As a result, the light-modulated probes can be made super bright in the frequency domain simply by acquiring more cycles in the time domain.

Self-assembled hollow nanospheres strongly enhance photoluminescence.

We report that two molecular building blocks differ only by two protons, yet they form totally different nanostructures. The protonated one self-organized into hollow nanospheres (~200 nm), whereas the one without the protons self-assembled into rectangular plates. Consequently, the geometrically defined nanoassemblies exhibit radically different properties. As self-assembly directing units, protons impart ion-pairing and hydrogen-bonding probabilities. The plate-forming nanosystem fluoresces weakly, probably due to energy transfer among chromophores (Φ < 0.2), but the nanospheres emit strong yellow fluorescence (Φ ≈ 0.58-0.85).

Visible-light induced activation of persulfate by self-assembled EHPDI/TiO2 photocatalyst toward efficient degradation of carbamazepine.

Removal of pharmaceutical and personal care products from wastewater is very important in water treatment process. Combining photocatalysis with persulfate (PS) could be a good solvent for this problem. Novel perylene diimide derivative (EHPDI) was designed and synthesized. Furthermore, self-assembled EHPDI/TiO2 composite photocatalyst (EPT) was prepared and applied in activating persulfate (PS) under visible light to enhance the photodegradation of pollutants. The presence of the alkyl side chain 2-ethylhexyl optimizes the self-assembly process, enabling the composite material to achieve high performance under low EHPDI loading. Various methods were used to detect the physical and chemical characteristics of EPT. Carbamazepine (CBZ) was chosen to be the model pollutant to study the removal efficiency of EPT/PS system under visible light. Within 30 min, 5.0 mg/L CBZ could be almost completely degraded, and the removal ratio of TOC was 75.2% within 60 min. The SO4-, OH, O2-, 1O2, and h+ were proved to be involved in the removal of CBZ by EPR and quenching experiments. Then, other typical pollutants were degraded by this EPT/PS system, demonstrating this system is suitable for degrading different pollutants. Besides, the degradation paths of CBZ were proposed by HPLC/MS. Finally, the EPT showed excellent recyclability and stability.

Structural Cutting of Non-fullerene Acceptors by Chlorination: Effects of Substituent Number on Device Performance.

Effects of chlorination on photovoltaic performance of organic solar cells are yet largely unclear though it is emerging as a special yet effective strategy to design highly efficient non-fullerene acceptors (NFAs). Herein, a bi-chlorine-substituted NFA with regioregularity, namely, bichlorinated dithienothiophen[3.2-b]- pyrrolobenzothiadiazole (BTP-2Cl-δ), is synthesized and compared to the non-chlorinated BTP and tetra-chlorine-substituted BTP-4Cl to study the effects of Cl number on the photovoltaic performance. From BTP to BTP-2Cl-δ and BTP-4Cl, the three molecules show gradually red-shifted absorption peaks, narrowed band gaps, and lowered highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Polymer solar cells are fabricated using PM6 as the donor and the three small molecules as the acceptors. From BTP to BTP-2Cl-δ, efficiencies (8.8 vs 15.4%) are significantly enhanced due to the better film morphology and strong crystallization of the BTP-2Cl-δ-based device, giving rise to boosted fill factors (FFs) and short-circuit current densities (JSC's). From BTP-2Cl-δ to BTP-4Cl, although JSC's (24.3 vs 25.0 mA cm-2) are slightly elevated due to the higher crystallinity of BTP-4Cl, leading to improved exciton dissociation and collection efficiencies, FFs (71.1 vs 68.0%) are obviously decreased owing to the unfavorable film morphology, unbalanced hole-electron mobilities, and higher charge recombination in BTP-4Cl-based devices. As such, the efficiency of the BTP-2Cl-δ-based device (15.4%) is superior to that of the BTP-4Cl-based device (14.5%). This work elucidates a design strategy by cutting the numbers of substituent chlorine to obtain desired energy levels and crystallization with optimal performance.

Lanthanide MOFs for inducing molecular chirality of achiral stilbazolium with strong circularly polarized luminescence and efficient energy transfer for color tuning.

We present herein an innovative host-guest method to achieve induced molecular chirality from an achiral stilbazolium dye (DSM). The host-guest system is exquisitely designed by encapsulating the dye molecule in the molecule-sized chiral channel of homochiral lanthanide metal-organic frameworks (P-(+)/M-(-)-TbBTC), in which the P- or M-configuration of the dye is unidirectionally generated via a spatial confinement effect of the MOF and solidified by the dangling water molecules in the channel. Induced chirality of DSM is characterized by solid-state circularly polarized luminescence (CPL) and micro-area polarized emission of DSM@TbTBC, both excited with 514 nm light. A luminescence dissymmetry factor of 10-3 is obtained and the photoluminescence quantum yield (PLQY) of the encapsulated DSM in DSM@TbTBC is ∼10%, which is close to the PLQY value of DSM in dilute dichloromethane. Color-tuning from green to red is achieved, owing to efficient energy transfer (up to 56%) from Ln3+ to the dye. Therefore, this study for the first time exhibits an elegant host-guest system that shows induced strong CPL emission and enables efficient energy transfer from the host chiral Ln-MOF to the achiral guest DSM with the emission color tuned from green to red.

Improving ternary blend morphology by adding a conjugated molecule into non-fullerene polymer solar cells.

The use of ternary polymer solar cells (PSCs) is a promising strategy to enhance photovoltaic performance while improving the fill factor (FF) of a device, but is still a challenge due to the complicated morphology. Herein, ternary PSCs are fabricated via adding the conjugated small molecule p-DTS(FBTTh2)2 into a well-known blended film, PTB7-Th:IEICO-4F. The ternary blend morphology and device characterization reveal that the addition of p-DTS(FBTTh2)2 can improve crystallinity and optimize morphology, leading to the FF of the optimized device increasing to 73.69%. In combination with the advantages of an ultra-narrow bandgap material, IEICO-4F, with a broad optical absorption spectrum, the optimized ternary solar cell exhibits a high short-circuit current-density (J SC) of 25.22 mA cm-2. The best power conversion efficiency (PCE) is 12.84% for this optimized ternary device with 10 wt% p-DTS(FBTTh2)2 in the donors. This work indicates that incorporating a small molecule with high crystallinity into host binary non-fullerene PSCs would give an active layer with high crystallinity, thus greatly enhancing the FFs and PCEs of PSCs.

Effects of Halogenation in B ← N Embedded Polymer Acceptors on Performance of All-Polymer Solar Cells.

Halogenation, for example, fluorination and chlorination, is an effective strategy to regulate the performance of organic photovoltaic materials. Although fluorination has been widely applied to polymer acceptors, systematic studies on the comparison of nonhalogenated, fluorinated, and chlorinated polymer acceptors have been a blank to now. Herein, a B ← N embedded electron-deficient unit (A), namely, BNIDT was copolymerized with three electron-rich units (D), that is, benzodithiophene (BDT), fluorinated BDT, and chlorinated BDT to obtain three D-A polymers of BN-BDT, BN-BDT-F, and BN-BDT-Cl, respectively. The three polymers exhibit similar LUMOs of ca. -3.77 eV, whereas the HOMOs are remarkably decreased from BN-BDT (-5.46 eV) to BN-BDT-F (-5.71 eV) and further slightly lowered to BN-BDT-Cl (-5.74 eV). All-polymer solar cells (all-PSCs) were fabricated using PBDB-T as the donor and the three B ← N-based polymers as the acceptors. The efficiencies of all-PSCs were significantly promoted from nonhalogenated BN-BDT (1.60%) to fluorinated BN-BDT-F (3.71%) and further elevated to chlorinated BN-BDT-Cl (4.23%). Device characterizations revealed that halogenation on the polymer acceptors leads to enhanced hole-transfer driving forces and better donor/acceptor miscibility, for example, smaller domain sizes and root-mean-square roughness (rms) values, which further gives rise to higher and more balanced hole/electron mobilities and efficient physical processes, for example, efficient exciton dissociation and collection and weaker recombination losses in halogenated devices. This work demonstrates that the photovoltaic performance of nonhalogenated polymer acceptors can be remarkably boosted by fluorination and further enhanced by chlorination. This is the first systematic study on the halogenated polymer acceptors by comprehensively comparing nonhalogenated, fluorinated, and chlorinated ones.

Ion Exchange/Insertion Reactions for Fabrication of Efficient Methylammonium Tin Iodide Perovskite Solar Cells.

The low toxicity, narrow bandgaps, and high charge-carrier mobilities make tin perovskites the most promising light absorbers for low-cost perovskite solar cells (PSCs). However, the development of the Sn-based PSCs is seriously hampered by the critical issues of poor stability and low power conversion efficiency (PCE) due to the facile oxidation of Sn2+ to Sn4+ and poor film formability of the perovskite films. Herein, a synthetic strategy is developed for the fabrication of methylammonium tin iodide (MASnI3) film via ion exchange/insertion reactions between solid-state SnF2 and gaseous methylammonium iodide. In this way, the nucleation and crystallization of MASnI3 can be well controlled, and a highly uniform pinhole-free MASnI3 perovskite film is obtained. More importantly, the detrimental oxidation can be effectively suppressed in the resulting MASnI3 film due to the presence of a large amount of remaining SnF2. This high-quality perovskite film enables the realization of a PCE of 7.78%, which is among the highest values reported for the MASnI3-based solar cells. Moreover, the MASnI3 solar cells exhibit high reproducibility and good stability. This method provides new opportunities for the fabrication of low-cost and lead-free tin-based halide perovskite solar cells.