Improvement in Solid-State Dye Sensitized Solar Cells by <i>p</i>‑Type Doping with Lewis Acid SnCl₄

The Lewis acid SnCl₄ is employed as a <i>p</i>-type dopant for 2,2′,7,7′-tetrakis­(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) for the solution process in solid-state dye sensitized solar cell. The UV–vis absorption spectra and time-resolved photoluminescence (PL) spectra are used to investigate the doping level of spiro-OMeTAD with a <i>p</i>-type dopant, indicating the strong molecular acceptor of SnCl₄. X-ray photoelectron spectra (XPS) exhibiting close energy shifts of the Fermi level toward HOMO are observed when adding Li salt or SnCl₄. A significant enhancement in fill factor of the photovoltaic devices, corresponding to the power conversion efficiency, is observed when doping with SnCl₄. This is attributed to the low charge transport resistance of the hole transport film and high hole injection efficiency from the hole transport material to the counter electrode

Heavily n‑Dopable π‑Conjugated Redox Polymers with Ultrafast Energy Storage Capability

We report here the first successful demonstration of a “π-conjugated redox polymer” simultaneously featuring a π-conjugated backbone and integrated redox sites, which can be stably and reversibly n-doped to a high doping level of 2.0 with significantly enhanced electronic conductivity. The properties of such a heavily n-dopable polymer, poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} (P­(NDI2OD-T2)), were compared <i>vis-à-vis</i> to those of the corresponding backbone-insulated poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-<i>alt</i>-5,5′-[2,2′-(1,2-ethanediyl)­bithiophene]} (P­(NDI2OD-TET)). When evaluated as a charge storage material for rechargeable Li batteries, P­(NDI2OD-T2) delivers 95% of its theoretical capacity at a high rate of 100C (72 s per charge–discharge cycle) under practical measurement conditions as well as 96% capacity retention after 3000 cycles of deep discharge–charge. Electrochemical, impedance, and charge-transport measurements unambiguously demonstrate that the ultrafast electrode kinetics of P­(NDI2OD-T2) are attributed to the high electronic conductivity of the polymer in the heavily n-doped state

Hole-Conductor-Free Mesoscopic TiO₂/CH₃NH₃PbI₃ Heterojunction Solar Cells Based on Anatase Nanosheets and Carbon Counter Electrodes

A hole-conductor-free fully printable mesoscopic TiO₂/CH₃NH₃PbI₃ heterojunction solar cell was developed with TiO₂ nanosheets containing high levels of exposed (001) facets. The solar cell embodiment employed a double layer of mesoporous TiO₂ and ZrO₂ as a scaffold infiltrated by perovskite as a light harvester. No hole conductor or Au reflector was employed. Instead, the back contact was simply a printable carbon layer. The perovskite was infiltrated from solution through the porous carbon layer. The high reactivity of (001) facets in TiO₂ nanosheets improved the interfacial properties between the perovskite and the electron collector. As a result, photoelectric conversion efficiency of up to 10.64% was obtained with the hole-conductor-free fully printable mesoscopic TiO₂/CH₃NH₃PbI₃ heterojunction solar cell. The advantages of fully printable technology and the use of low-cost carbon-materials-based counter electrode and hole-conductor-free structure provide this design a promising prospect to approach low-cost photovoltaic devices

Efficient Compact-Layer-Free, Hole-Conductor-Free, Fully Printable Mesoscopic Perovskite Solar Cell

A compact-layer-free, hole-conductor-free, fully printable mesoscopic perovskite solar cell presents a power conversion efficiency of over 13%, which is comparable to that of the device with a TiO₂ compact layer. The different wettability of the perovskite precursor solution on the surface of FTO and TiO₂ possesses a significant effect on realizing efficient mesoscopic perovskite solar cell. This result shows a promising future in printable solar cells by further simplifying the fabrication process and lowering the preparation costs

A Multifunctional Bis-Adduct Fullerene for Efficient Printable Mesoscopic Perovskite Solar Cells

Printable mesoscopic perovskite solar cells (PMPSCs) have exhibited great attractive prospects in the energy conversion field due to their high stability and potential scalability. However, the thick perovskite film in the mesoporous layers challenges the charge transportation and increase grain boundary defects, limiting the performance of the PMPSCs. It is critical not only to improve the electric property of the perovskite film but also to passivate the charge traps to improve the device performance. Herein we synthesized a bis-adduct 2,5-(dimethyl ester) C₆₀ fulleropyrrolidine (bis-DMEC₆₀) via a rational molecular design and incorporated it into the PMPSCs. The enhanced chemical interactions between perovskite and bis-DMEC₆₀ improve the conductivity of the perovskite film as well as elevate the passivation effect of bis-DMEC₆₀ at the grain boundaries. As a result, the fill factor (FF) and power conversion efficiency (PCE) of the PMPSCs containing bis-DMEC₆₀ reached 0.71 and 15.21%, respectively, significantly superior to the analogous monoadduct derivative (DMEC₆₀)-containing and control devices. This work suggests that fullerene derivatives with multifunctional groups are promising for achieving high-performance PMPSCs

Boron-Doped Graphite for High Work Function Carbon Electrode in Printable Hole-Conductor-Free Mesoscopic Perovskite Solar Cells

Work function of carbon electrodes is critical in obtaining high open-circuit voltage as well as high device performance for carbon-based perovskite solar cells. Herein, we propose a novel strategy to upshift work function of carbon electrode by incorporating boron atom into graphite lattice and employ it in printable hole-conductor-free mesoscopic perovskite solar cells. The high-work-function boron-doped carbon electrode facilitates hole extraction from perovskite as verified by photoluminescence. Meanwhile, the carbon electrode is endowed with an improved conductivity because of a higher graphitization carbon of boron-doped graphite. These advantages of the boron-doped carbon electrode result in a low charge transfer resistance at carbon/perovskite interface and an extended carrier recombination lifetime. Together with the merit of both high work function and conductivity, the power conversion efficiency of hole-conductor-free mesoscopic perovskite solar cells is increased from 12.4% for the pristine graphite electrode-based cells to 13.6% for the boron-doped graphite electrode-based cells with an enhanced open-circuit voltage and fill factor

The Influence of the Work Function of Hybrid Carbon Electrodes on Printable Mesoscopic Perovskite Solar Cells

In printable mesoscopic perovskite solar cells (PSCs), carbon electrodes play a significant role in charge extraction and transport, influencing the overall device performance. The work function and electrical conductivity of the carbon electrodes mainly affect the open-circuit voltage (<i>V</i>_OC) and series resistance (<i>R</i>_s) of the device. In this paper, we propose a hybrid carbon electrode based on a high-temperature mesoporous carbon (m-C) layer and a low-temperature highly conductive carbon (c-C) layer. The m-C layer has a high work function and large surface area and is mainly responsible for charge extraction. The c-C layer has a high conductivity and is responsible for charge transport. The work function of the m-C layer was tuned by adding different amounts of NiO, and at the same time, the conductivities of the hybrid carbon electrodes were maintained by the c-C layer. It was supposed that the increase of the work function of the carbon electrode can enhance the <i>V</i>_OC of printable mesoscopic PSCs. Here, we found the <i>V</i>_OC of the device based on hybrid carbon electrodes can be enhanced remarkably when the insulating layer has a relatively small thickness (500–1000 nm). An optimal improvement in <i>V</i>_OC of up to 90 mV could be achieved when the work function of the m-C was increased from 4.94 to 5.04 eV. When the thickness of the insulating layer was increased to ∼3000 nm, the variation of <i>V</i>_OC as the work function of m-C increased became less distinct

Interaction of Organic Cation with Water Molecule in Perovskite MAPbI₃: From Dynamic Orientational Disorder to Hydrogen Bonding

Microscopic understanding of interaction between H₂O and MAPbI₃ (CH₃NH₃PbI₃) is essential to further improve efficiency and stability of perovskite solar cells. A complete picture of perovskite from initial physical uptake of water molecules to final chemical transition to its monohydrate MAPbI₃·H₂O is obtained with in situ infrared spectroscopy, mass monitoring, and X-ray diffraction. Despite strong affinity of MA to water, MAPbI₃ absorbs almost no water from ambient air. Water molecules penetrate the perovskite lattice and share the space with MA up to one H₂O per MA at high-humidity levels. However, the interaction between MA and H₂O through hydrogen bonding is not established until the phase transition to monohydrate where H₂O and MA are locked to each other. This lack of interaction in water-infiltrated perovskite is a result of dynamic orientational disorder imposed by tetragonal lattice symmetry. The apparent inertness of H₂O along with high stability of perovskite in an ambient environment provides a solid foundation for its long-term application in solar cells and optoelectronic devices