Vertically aligned 2D/3D Pb-Sn perovskites with enhanced charge extraction and suppressed phase segregation for efficient printable solar cells

摘要： The concept of mixed 2D/3D heterostructures has emerged as an effective method in improving the stability of lead halide perovskite solar cells (PSCs), which is, however, rarely reported in lead-tin (Pb-Sn) mixed perovskite devices. Here, we report a scalable process for depositing mixed 2D/3D Pb-Sn perovskite solar cells that deliver remarkably enhanced efficiency and stability compared to their 3D counterparts. The incorporation of a small amount (3.75%) of an organic cation 2-(4-fluorophenyl)ethyl ammonium iodide (FPEAI) induces the growth of highly orientated Pb-Sn perovskite crystals perpendicularly aligned to the substrate. Moreover, for the first time, phase segregation is observed in pristine 3D Pb-Sn perovskite, which is suppressed due to the presence of the 2D perovskites. Accordingly, a high current density of 28.42 mA cm-2 is obtained due to markedly enhanced spectral response and charge extraction. Eventually, mixed 2D/3D Pb-Sn perovskite devices with a bandgap of 1.33 eV yield efficiencies as high as 17.51% and in parallel exhibit good stability.

Potassium Induced Phase Stability Enables Stable and Efficient Widea??Bandgap Perovskite Solar Cells

摘要： The incorporation of potassium can remarkably stabilize wide-bandgap perovskites with a high Br content by the synergistic effect of the formation of 2D K2PbI4 at the grain boundaries and the interstitial occupancy in the perovskite lattices, which can effectively reduce the trap density and inhibit ion migration, thus suppressing the nonradiative recombination and photoinduced phase segregation.

Multifunctional Phosphorusa??Containing Lewis Acid and Base Passivation Enabling Efficient and Moisturea??Stable Perovskite Solar Cells

摘要： 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.

Impact of Polymer Backbone Fluorination on the Charge Generation/Recombination Patterns and Vertical Phase Segregation in Bulk Heterojunction Organic Solar Cells

摘要： Incorporating fluorine (–F) substituents along the main-chains of polymer donors and acceptors is an effective strategy toward efficient bulk-heterojunction (BHJ) solar cells. Specifically, F-substituted polymers often exhibit planar conformations, leading to favorable packing, and electronic coupling. However, the effects of fluorine substituents on the charge generation and recombination characteristics that determine the overall efficiency of BHJ active layers remain critically important issues to examine. In this report, two PBDT[2X]T polymer analogs –poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-thiophene] [PBDT[2H]T] and its F-substituted counterpart poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluoro-thiophene] [PBDT[2F]T]—are studied to systematically examine how –F substituents impact the blend morphology, charge generation, carrier recombination and extraction in BHJ solar cells. Considering the large efficiency differences between PBDT[2H]T- and PBDT[2F]T-based BHJ devices, significant emphasis is given to characterizing the out-of-plane morphology of the blend films as vertical phase-separation characteristics are known to have dramatic effects on charge transport and carrier extraction in polymer-fullerene BHJ solar cells. Herein, we use electron energy loss spectroscopy (EELS) in tandem with charge transport characterization to examine PBDT[2X]T-fullerene blend films. Our analyses show that PBDT[2H]T and PBDT[2F]T possess very different charge generation, recombination and extraction characteristics, resulting from distinct aggregation, and phase-distribution within the BHJ blend films.

Ba-induced phase segregation and band gap reduction in mixed-halide inorganic perovskite solar cells

摘要： All-inorganic metal halide perovskites are showing promising development towards efficient long-term stable materials and solar cells. Element doping, especially on the lead site, has been proved to be a useful strategy to obtain the desired film quality and material phase for high efficient and stable inorganic perovskite solar cells. Here we demonstrate a function by adding barium in CsPbI2Br. We find that barium is not incorporated into the perovskite lattice but induces phase segregation, resulting in a change in the iodide/bromide ratio compared with the precursor stoichiometry and consequently a reduction in the band gap energy of the perovskite phase. The device with 20 mol% barium shows a high power conversion efficiency of 14.0% and a great suppression of non-radiative recombination within the inorganic perovskite, yielding a high open-circuit voltage of 1.33 V and an external quantum efficiency of electroluminescence of 10?4.

Corrigendum: Impact of Polymer Backbone Fluorination on the Charge Generation/Recombination Patterns and Vertical Phase Segregation in Bulk Heterojunction Organic Solar Cells

Structural Origins of Light-Induced Phase Segregation in Organic-Inorganic Halide Perovskite Photovoltaic Materials

摘要： Light-induced phase segregation in mixed-halide perovskite photovoltaic materials results in the formation of low-band-gap regions that limit the voltage of devices. This work explores the dependence of this light instability on crystal structure and maps it across the cubic-tetragonal solvus in the (CsyFA1?y)Pb(BrxI1?x)3 phase diagram.

Wide‐Bandgap Perovskite/Gallium Arsenide Tandem Solar Cells

摘要： Gallium arsenide (GaAs) photovoltaic (PV) cells have been widely investigated due to their merits such as thin-film feasibility, flexibility, and high efficiency. To further increase their performance, a wider bandgap PV structure such as indium gallium phosphide (InGaP) has been integrated in two-terminal (2T) tandem configuration. However, it increases the overall fabrication cost, complicated tunnel-junction diode connecting subcells are inevitable, and materials are limited by lattice matching. Here, high-efficiency and stable wide-bandgap perovskite PVs having comparable bandgap to InGaP (1.8–1.9 eV) are developed, which can be stable low-cost add-on layers to further enhance the performance of GaAs PVs as tandem configurations by showing an efficiency improvement from 21.68% to 24.27% (2T configuration) and 25.19% (4T configuration). This approach is also feasible for thin-film GaAs PV, essential to reduce its fabrication cost for commercialization, with performance increasing from 21.85% to 24.32% and superior flexibility (1000 times bending) in a tandem configuration. Additionally, potential routes to over 30% stable perovskite/GaAs tandems, comparable to InGaP/GaAs with lower cost, are considered. This work can be an initial step to reach the objective of improving the usability of GaAs PV technology with enhanced performance for applications for which lightness and flexibility are crucial, without a significant additional cost increase.

Comparison of ZnO buffer layers prepared by spin coating or RF magnetron sputtering for application in inverted organic solar cells

摘要： We compared the electrical, optical, structural, and morphological properties of radio-frequency (RF) magnetron-sputtered ZnO and solution-processed ZnO nanoparticle (NP) buffer layers on ITO cathodes for use in inverted polymer solar cells (IPSCs). Continuous sputtering resulted in integration of the ZnO buffer layer in the ITO cathodes, which were then used as transparent cathodes for IPSCs. Although the electrical, optical, and morphological properties as well as work function of RF-sputtered ZnO film were similar to those of solution-processed ZnO NP film, the power conversion efficiency (PCE) of IPSCs with an RF-sputtered ZnO buffer layer was much lower than that of IPSCs with a solution-processed ZnO NP buffer layer due to vertical phase segregation of the organic active layer. However, intentional bias sweeping of IPSCs with an RF-sputtered ZnO buffer layer improved performance due to diffusion of PC70BM through the PV-D4610 donor layer and formation of a suitable heterojunction structure. Based on transmission electron microscope examination and dark current-voltage curves, we suggest a possible mechanism to explain the difference in behavior of RF-sputtered ZnO and solution-processed ZnO NP buffer layers in IPSCs.

Controlling the Phase Segregation in Mixed Halide Perovskite through Nanocrystal Size

摘要： Mixed halide perovskites are one of the promising candidates in developing solar cells and LEDs, among others applications due to their tunable optical properties. Nonetheless, photoinduced phase segregation, by forming segregated Br-rich and I-rich domains, limits the overall applicability. We tracked the phase segregation with increasing crystalline sizes of CsPbBr3-xIx and their photoluminescence under continuous-wave laser irradiation (405 nm, 10 mW cm-2), and observed the occurrence of the phase segregation from the threshold size of 46 ± 7 nm. This results have an outstanding agreement with the diffusion length (45.8 nm) calculated also experimentally from the emission lifetime, segregation rates. Furthermore, through Kelvin probe forced microscopy, we confirmed the correlation between the phase segregation and the reversible halide ion migration among grain center/boundaries. These results open a way to achieve segregation-free mixed halide perovskites and improve their performances in optoelectronic devices.