Optically Transparent FTO-Free Cathode for Dye-Sensitized Solar Cells

The woven fabric containing electrochemically platinized tungsten wire is an affordable flexible cathode for liquid-junction dye-sensitized solar cells with the I₃^–/I^– redox mediator and electrolyte solution consisting of ionic liquids and propionitrile. The fabric-based electrode outperforms the thermally platinized FTO in serial ohmic resistance and charge-transfer resistance for triiodide reduction, and it offers comparable or better optical transparency in the visible and particularly in the near-IR spectral region. The electrode exhibits good stability during electrochemical loading and storage at open circuit. The dye-sensitized solar cells with a C101-sensitized titania photoanode and either Pt–W/PEN or Pt–FTO cathodes show a comparable performance

Phase Segregation in Cs‑, Rb- and K‑Doped Mixed-Cation (MA)_<i>x</i>(FA)_1–<i>x</i>PbI₃ Hybrid Perovskites from Solid-State NMR

Hybrid (organic–inorganic) multication lead halide perovskites hold promise for a new generation of easily processable solar cells. Best performing compositions to date are multiple-cation solid alloys of formamidinium (FA), methylammonium (MA), cesium, and rubidium lead halides which provide power conversion efficiencies up to around 22%. Here, we elucidate the atomic-level nature of Cs and Rb incorporation into the perovskite lattice of FA-based materials. We use ¹³³Cs, ⁸⁷Rb, ³⁹K, ¹³C, and ¹⁴N solid-state MAS NMR to probe microscopic composition of Cs-, Rb-, K-, MA-, and FA-containing phases in double-, triple-, and quadruple-cation lead halides in bulk and in a thin film. Contrary to previous reports, we have found no proof of Rb or K incorporation into the 3D perovskite lattice in these systems. We also show that the structure of bulk mechanochemical perovskites bears close resemblance to that of thin films, making them a good benchmark for structural studies. These findings provide fundamental understanding of previously reported excellent photovoltaic parameters in these systems and their superior stability

Phase Segregation in Potassium-Doped Lead Halide Perovskites from ³⁹K Solid-State NMR at 21.1 T

Organic–inorganic lead halide perovskites are a promising family of light absorbers for a new generation of solar cells, with reported efficiencies currently exceeding 22%. A common problem of solar cells fabricated using these materials is that their efficiency depends on their cycling history, an effect known as current–voltage (<i>J</i>–<i>V</i>) hysteresis. Potassium doping has recently emerged as a universal way to overcome this adverse phenomenon. While the atomistic origins of <i>J</i>–<i>V</i> hysteresis are still not fully understood, it is essential to rationalize the atomic-level effect of protocols that lead to its suppression. Here, using ³⁹K MAS NMR at 21.1 T we provide for the first time atomic-level characterization of the potassium-containing phases that are formed upon KI doping of multication and multianion lead halide perovskites. We find no evidence of potassium incorporation into 3D perovskite lattices of the recently reported materials. Instead, we observe formation of a mixture of potassium-rich phases and unreacted KI. In the case of Br-containing lead halide perovskites doped with KI, a mixture of KI and KBr ensues, leading to a change in the Br/I ratio in the perovskite phase with respect to the undoped perovskite. Simultaneous Cs and K doping leads to the formation of nonperovskite Cs/K lead iodide phases

Phase Segregation in Potassium-Doped Lead Halide Perovskites from ³⁹K Solid-State NMR at 21.1 T

Organic–inorganic lead halide perovskites are a promising family of light absorbers for a new generation of solar cells, with reported efficiencies currently exceeding 22%. A common problem of solar cells fabricated using these materials is that their efficiency depends on their cycling history, an effect known as current–voltage (<i>J</i>–<i>V</i>) hysteresis. Potassium doping has recently emerged as a universal way to overcome this adverse phenomenon. While the atomistic origins of <i>J</i>–<i>V</i> hysteresis are still not fully understood, it is essential to rationalize the atomic-level effect of protocols that lead to its suppression. Here, using ³⁹K MAS NMR at 21.1 T we provide for the first time atomic-level characterization of the potassium-containing phases that are formed upon KI doping of multication and multianion lead halide perovskites. We find no evidence of potassium incorporation into 3D perovskite lattices of the recently reported materials. Instead, we observe formation of a mixture of potassium-rich phases and unreacted KI. In the case of Br-containing lead halide perovskites doped with KI, a mixture of KI and KBr ensues, leading to a change in the Br/I ratio in the perovskite phase with respect to the undoped perovskite. Simultaneous Cs and K doping leads to the formation of nonperovskite Cs/K lead iodide phases

Phase Segregation in Cs‑, Rb- and K‑Doped Mixed-Cation (MA)_<i>x</i>(FA)_1–<i>x</i>PbI₃ Hybrid Perovskites from Solid-State NMR

Hybrid (organic–inorganic) multication lead halide perovskites hold promise for a new generation of easily processable solar cells. Best performing compositions to date are multiple-cation solid alloys of formamidinium (FA), methylammonium (MA), cesium, and rubidium lead halides which provide power conversion efficiencies up to around 22%. Here, we elucidate the atomic-level nature of Cs and Rb incorporation into the perovskite lattice of FA-based materials. We use ¹³³Cs, ⁸⁷Rb, ³⁹K, ¹³C, and ¹⁴N solid-state MAS NMR to probe microscopic composition of Cs-, Rb-, K-, MA-, and FA-containing phases in double-, triple-, and quadruple-cation lead halides in bulk and in a thin film. Contrary to previous reports, we have found no proof of Rb or K incorporation into the 3D perovskite lattice in these systems. We also show that the structure of bulk mechanochemical perovskites bears close resemblance to that of thin films, making them a good benchmark for structural studies. These findings provide fundamental understanding of previously reported excellent photovoltaic parameters in these systems and their superior stability

Reduction in the Interfacial Trap Density of Mechanochemically Synthesized MAPbI₃

Organo-lead halide perovskites have emerged as promising light harvesting materials for solar cells. The ability to prepare high quality films with a low concentration of defects is essential for obtaining high device performance. Here, we advance the procedure for the fabrication of efficient perovskite solar cells (PSCs) based on mechanochemically synthesized MAPbI₃. The use of mechano-perovskite for the thin film formation provides a high degree of control of the stoichiometry and allows for the growth of relatively large crystalline grains. The best device achieved a maximum PCE of 17.5% from a current–voltage scan (<i>J–V</i>), which stabilized at 16.8% after 60 s of maximum power point tracking. Strikingly, PSCs based on MAPbI₃ mechanoperovskite exhibit lower “hysteretic” behavior in comparison to that comprising MAPbI₃ obtained from the conventional solvothermal reaction between PbI₂ and MAI. To gain a better understanding of the difference in <i>J–V</i> hysteresis, we analyze the charge/ion accumulation mechanism and identify the defect energy distribution in the resulting MAPbI₃ based devices. These results indicate that the use of mechanochemically synthesized perovskites provides a promising strategy for the formation of crystalline films demonstrating slow charge recombination and low trap density

Cation Dynamics in Mixed-Cation (MA)_<i>x</i>(FA)_1–<i>x</i>PbI₃ Hybrid Perovskites from Solid-State NMR

Mixed-cation organic lead halide perovskites attract unfaltering attention owing to their excellent photovoltaic properties. Currently, the best performing perovskite materials contain multiple cations and provide power conversion efficiencies up to around 22%. Here, we report the first quantitative, cation-specific data on cation reorientation dynamics in hybrid mixed-cation formamidinium (FA)/methylammonium (MA) lead halide perovskites. We use ¹⁴N, ²H, ¹³C, and ¹H solid-state MAS NMR to elucidate cation reorientation dynamics, microscopic phase composition, and the MA/FA ratio, in (MA)_<i>x</i>(FA)_1–<i>x</i>PbI₃ between 100 and 330 K. The reorientation rates correlate in a striking manner with the carrier lifetimes previously reported for these materials and provide evidence of the polaronic nature of charge carriers in PV perovskites

Cation Dynamics in Mixed-Cation (MA)_<i>x</i>(FA)_1–<i>x</i>PbI₃ Hybrid Perovskites from Solid-State NMR

Mixed-cation organic lead halide perovskites attract unfaltering attention owing to their excellent photovoltaic properties. Currently, the best performing perovskite materials contain multiple cations and provide power conversion efficiencies up to around 22%. Here, we report the first quantitative, cation-specific data on cation reorientation dynamics in hybrid mixed-cation formamidinium (FA)/methylammonium (MA) lead halide perovskites. We use ¹⁴N, ²H, ¹³C, and ¹H solid-state MAS NMR to elucidate cation reorientation dynamics, microscopic phase composition, and the MA/FA ratio, in (MA)_<i>x</i>(FA)_1–<i>x</i>PbI₃ between 100 and 330 K. The reorientation rates correlate in a striking manner with the carrier lifetimes previously reported for these materials and provide evidence of the polaronic nature of charge carriers in PV perovskites

Effect of Extended π‑Conjugation of the Donor Structure of Organic D–A−π–A Dyes on the Photovoltaic Performance of Dye-Sensitized Solar Cells

Two new D–A−π-spacer–A organic dyes, <b>KM-10</b> and <b>KM-11</b>, containing a benzothiadiazole unit in a π-spacer and a cyanoacrylic acid as an acceptor have been synthesized and tested as sensitizers in dye-sensitized solar cells. Structural variations of the donor moiety, i.e., π-extension of the diphenylamine electron-donating groups, gave rise to different photovoltaic efficiencies7.1% for KM-10 and 8% for KM-11despite having comparable absorption properties. A detailed investigation, including transient photocurrent and photovoltage decay measurement, transient absorption spectroscopy, and quantum chemical methods, provided important conclusions about the nature of the substitution on the photovoltaic properties of dyes

Effect of Cs-Incorporated NiO_<i>x</i> on the Performance of Perovskite Solar Cells

The effect of Cs-incorporated NiO_<i>x</i> on perovskite solar cells with an inverted structure was investigated, where NiO_<i>x</i> and PCBM were used as selective contacts for holes and electrons, respectively. It was found that the generation of an Ni phase in an NiO_<i>x</i> layer was significantly suppressed by employing cesium. Furthermore, Cs-incorporated NiO_<i>x</i> enabled holes to be efficiently separated at the interface, showing the improved photoluminescent quenching and thus generating higher short-circuit current. The effect of Cs incorporation was also prominent in the inhibition of recombination. The recombination resistance of Cs-incorporated NiO_<i>x</i> was noticeably increased by more than three-fold near the maximum power point, leading to a higher fill factor of 0.78 and consequently a higher power conversion efficiency of 17.2% for the device employing Cs-incorporated NiO_<i>x</i>