Interfacial Engineering for High-Efficiency Nanorod Array-Structured Perovskite Solar Cells

Selective Control of Novel TiO2 Nanorods: Excellent Building Blocks for the Electron Transport Layer of Mesoscopic Perovskite Solar Cells

Novel TiO2 nanorods (NRs) with various lengths of 70-200 nm and uniform widths of 46-48 nm are selectively synthesized by a solvothermal reaction under a basic environment. The length of TiO2 NRs is reproducibly tuned by varying the concentration of tetramethylammonium hydroxide (TMAH), while the NRs in the pure anatase phase are grown in the [001] direction, caused by the preferential binding affinity of TMAH to the TiO2 (101) facet. TiO2 NRs of various lengths are then applied to form the electron transporting layer (ETL) of mesoscopic perovskite solar cells (PSCs). We found that PSC devices with NRs exhibit superior photovoltaic (PV) performance to those with conventional 46 nm-sized TiO2 nanoparticles (NP46). Particularly, the PSC with TiO2 NRs of 110 nm length (NR110) exhibits the optimum PV conversion efficiency (PCE): the average PCE is 22.64% with a VOC of 1.137 V, a JSC of 24.60 mA·cm-2, and a FF of 80.96%, while the champion PCE is 23.18%. In addition, the PSC with NR110 (PSC-NR110) reveals significantly improved long-term stability in air with a relative humidity of 40-50%. In 1000 h, its PCE is reduced by only 9% whereas that of PSC with NP46 decreases by 25%. The PSC properties analyzed by impedance spectroscopy and J-V curve measurements under dark conditions and at various light intensities provide evidence that PSC-NR110 has fewer defects and shows significantly reduced charge recombination. We discuss the advantages of NR structures in preparing the ETL of PSC devices and also explain why the charge recombination is suppressed.

A Morphological Study of Solvothermally Grown SnO2 Nanostructures for Application in Perovskite Solar Cells

Tin(IV) oxide (SnO₂) nanostructures, which possess larger surface areas for transporting electron carriers, have been used as an electron transport layer (ETL) in perovskite solar cells (PSCs). However, the reported power conversion efficiencies (PCEs) of this type of PSCs show a large variation. One of the possible reasons for this phenomenon is the low reproducibility of SnO₂ nanostructures if they are prepared by different research groups using various growth methods. This work focuses on the morphological study of SnO₂ nanostructures grown by a solvothermal method. The growth parameters including growth pressure, substrate orientation, DI water-to-ethanol ratios, types of seed layer, amount of acetic acid, and growth time have been systematically varied. The SnO₂ nanomorphology exhibits a different degree of sensitivity and trends towards each growth factor. A surface treatment is also required for solvothermally grown SnO₂ nanomaterials for improving photovoltaic performance of PSCs. The obtained results in this work provide the research community with an insight into the general trend of morphological changes in SnO₂ nanostructures influenced by different solvothermal growth parameters. This information can guide the researchers to prepare more reproducible solvothermally grown SnO₂ nanomaterials for future application in devices.

Introducing Postmetalation Metal-Organic Framework to Control Perovskite Crystal Growth for Efficient Perovskite Solar Cells

A novel lead-containing metal-organic framework (Pb-MOF) is synthesized through postmetalation of MOF-525. Postmetalation renders lead ions bound with the organic linker of MOF-525, which can serve as nucleation points to promote perovskite crystallization. The introduction of lead postmetalated MOF-525 (Pb-MOF) as a scaffold layer between compact TiO₂ (c-TiO₂) layer and perovskite layer promotes perovskite crystal growth in enlarging crystal grain size with better crystallinity, hence decreasing defect sites in the perovskite layer. Postmetalation of MOF-525 with lead ions allows MAPbI₃ to form a solid crystal structure to facilitate the charge separation between electron transport layer (ETL) and light-harvesting layer so as to resolve the issue of possible vacancies present in MOFs. As a result, the champion perovskite solar cell (PSC) with the introduction of Pb-MOF exhibits a power conversion efficiency (PCE) of 20.87% and better stability (86% PCE retention after 40 days), outperforming the pristine PSC (16.85% PCE, with 52% retention after 40 days) and MOF-525-introduced PSC (18.61% PCE, with 76% retention after 40 days).

Effect of ligand groups on photoexcited charge carrier dynamics at the perovskite/TiO2 interface

The work proposed here aims to describe the dynamics of photoexcited charge carriers at the interface between the perovskite and electron transport layer (ETL) in perovskite solar cells (PSCs) and the effect that the interface morphology has on these dynamics. This is done in an effort to further develop the understanding of these materials so that their chemical composition and morphology may be better utilized to improve PSCs by means of increasing the power conversion efficiency (PCE), maximizing the chemical stability of PSCs to lengthen their lifespan, finding the cheapest and easiest materials to synthesize which have beneficial properties in photovoltaics, etc. This is done by using density functional theory to model the interface and open system Redfield theory to describe the charge carrier dynamics. We find that the charge transfer characteristics at the perovskite/ETL interface depend greatly on the choice of ligands adsorbed on the ETL that act as a bridge between the perovskite and ETL. The two ligand choices discussed here go so far as to determine whether the system will undergo a Förster energy transfer or a Dexter energy transfer upon photoexcitation.

First principles modeling of excited state dynamics of charge carriers at the interface between the perovskite and electron transport layer in perovskite solar cells identifies an effect of the interface morphology onto efficiency of charge transfer.

Enhanced Photovoltaic Performance of BiSCl Solar Cells Through Nanorod Array

BiSCl single-crystalline nanofibers were synthesized by a facile one-pot solvothermal approach for the first time. BiSCl possesses a double chain type structure and grows readily along the c-axis, resulting the fibrous morphology. UV/Vis absorption spectroscopy revealed that BiSCl nanofibers exhibit a strong light absorption in a wavelength range from UV to visible light, corresponding to a bandgap of 1.96 eV. Ultraviolet photoelectron spectroscopy and density functional theory calculations revealed that BiSCl is a direct n-type semiconductor with valence band maximum and conduction band minimum located at 6.04 and 4.08 eV below the vacuum level, respectively. To investigate the photovoltaic performance, the homogeneous thin film of BiSCl-nanorod array was fabricated on a TiO₂ porous film by a modified solvothermal process, where the nanorod array is oriented vertically to the surface of the TiO₂ porous film. A proper band alignment of BiSCl-based solar cells with an architecture of fluorine-doped tin oxide (FTO)/TiO₂ /BiSCl/(I₃ ^- /I^- )/Pt gave a PCE of 1.36 % and a relatively large short-circuit photocurrent density of 9.87 mA cm^-2 for the first time. The preliminary photovoltaic study result revealed a potential possibility of BiSCl-nanorod array as a light absorber for solar cells that can be fabricated by the low-cost solution process.

Nonradiative Relaxation Dynamics of a Cesium Lead Halide Perovskite Photovoltaic Architecture: Effect of External Electric Fields

Lead halide perovskites have attracted much attention as an active material in solar cells. In this first-principles study, we consider a cesium lead halide perovskite slab interfacing with electron transport and hole transport layers, relevant to the practical photovoltaic architecture. We apply external electric fields normal to the surface of the perovskite slab and explore the induced changes onto optoelectronic properties. It is found that the bandgap increases linearly and the conductivity diminishes exponentially with decreasing electric field strengths. Furthermore, we study the influence of electric fields onto nonradiative relaxation of photoexcited electrons and holes using the reduced density matrix in the formalism of Redfield theory. Our calculations provide relaxation rates and relaxation pathways, illustrating the mechanisms of modulations of electric field strengths onto charge carrier dynamics. Our results show that holes have longer lifetimes than electrons at various external electric fields. It is also found that the patterns of charge carrier dynamics depend on the direction of external electric fields. Specifically, in comparison with the system under zero field, our findings show that (i) the positive electric field facilitates the relaxation of electrons and holes and (ii) the negative electric field facilitates the relaxation of electrons but inhibits the relaxation of holes.

Stoichiometry-Controlled MoxW1-xTe2 Nanowhiskers: A Novel Electrocatalyst for Pt-Free Dye-Sensitized Solar Cells

Novel polymorphic Mo_xW_1-xTe₂-based counter electrodes possess high carrier mobility, phase-dependent lattice distortion, and surface charge density wave to boost the charge-transfer kinetics and electrocatalytic activity in dye-sensitized solar cells (DSSCs). Here, we report the syntheses of stoichiometry-controlled binary and ternary Mo_xW_1-xTe₂ nanowhiskers directly on carbon cloth (CC), denoted by Mo_xW_1-xTe₂/CC, with an atmospheric chemical vapor deposition technique. The synthesized Mo_xW_1-xTe₂/CC samples, including 1T'-MoTe₂/CC, T_d-WTe₂/CC, T_d-Mo_0.26W_0.73Te_2.01/CC, and 1T'- & T_d-Mo_0.66W_0.32Te_2.02/CC, were then employed as different counter electrodes to study their electrochemical activities and efficiencies in DSSCs. The photovoltaic parameter analysis manifests that Mo_xW_1-xTe₂/CCs are more stable than a standard Pt/CC in the I^-/I₃^- electrolyte examined by cyclic voltammetry over 100 cycles. A 1T'- & T_d-Mo_0.66W_0.32Te_2.02/CC-based DSSC can achieve a photocurrent density of 16.29 mA cm^-2, a maximum incident photon-to-electron conversion efficiency of 90% at 550 nm excitation, and an efficiency of 9.40%, as compared with 8.93% of the Pt/CC counterpart. Moreover, the 1T'- & T_d-Mo_0.66W_0.32Te_2.02/CC shows lower charge-transfer resistance (0.62 Ω cm²) than a standard Pt/CC (1.19 Ω cm²) in electrocatalytic reactions. Notably, Mo_xW_1-xTe₂ nanowhiskers act as an electron expressway by shortening the path of carrier transportation in the axial direction from a counter electrode to electrolytic ions to enhance the reaction kinetics in DSSCs. This work demonstrates that the nanowhisker-structured 1T'- & T_d-Mo_0.66W_0.32Te_2.02/CC with high carrier mobility and robust surface states can serve as a highly efficient counter electrode in DSSCs to replace the conventional Pt counter electrode for electrocatalytic applications.

Exfoliated Fluorographene Quantum Dots as Outstanding Passivants for Improved Flexible Perovskite Solar Cells

Flexible perovskite solar cells (PSCs) are currently one of the most attractive flexible thin-film photovoltaic technologies. Despite achieving remarkable progress in power conversion efficiencies (PCEs), flexible PSCs have not yet kept pace with rigid PSCs. Defect passivation is of crucial importance to further enhance the PCEs of flexible PSCs. Here, highly dispersed fluorographene quantum dots (FGQDs) are exfoliated from graphite fluoride with the aid of stirring and sonication and used to passivate the grain boundaries and surface of the perovskite films for high-performance flexible PSCs. Photoluminescence spectroscopy (PL) and time-resolved PL decays indicate that the FGQDs are beneficial for suppressing carrier recombination. Space-charge-limited current measurements prove that the passivated perovskite film exhibits reduced trap densities. As a result, a best PCE of 20.40% is achieved from the flexible PSCs, owing to significantly reduced charge recombination. Moreover, the champion device delivers an outstanding steady-state PCE exceeding 20%. The flexible PSCs with the FGQDs also exhibit enhanced thermal stability and environmental stability. Our work not only highlights the importance of passivating the defects within the perovskite films for high-efficiency flexible PSCs but also offers a promising future for the commercialization of flexible PSCs.

Advances in Template Prepared Nano-Oxides and their Applications: Polluted Water Treatment, Energy, Sensing and Biomedical Drug Delivery

The nano-oxide materials with special structures prepared by template methods have a good dispersion, regular structures and high specific surface areas. Therefore, in some areas, improved properties are observed than conventional bulk oxide materials. For example, in the treatment of dye wastewater, the treatment efficiency of adsorbents and catalytic materials prepared by template method was about 30 % or even higher than that of conventional samples. This review mainly focuses on the progress of inorganic, organic and biological templates in the preparation of micro- and nano- oxide materials with special morphologies, and the roles of the prepared materials as adsorbents and photocatalysts in dye wastewater treatment. The characteristics and advantages of inorganic, organic and biological template are also summarized. In addition, the applications of template method prepared oxides in the field of sensors, drug carrier, energy materials and other fields are briefly discussed with detailed examples.

A Bi2Te3 Topological Insulator as a New and Outstanding Counter Electrode Material for High-Efficiency and Endurable Flexible Perovskite Solar Cells

Inverted flexible perovskite solar cells (PSCs) typically employ expensive metals as the counter electrodes, which are brittle and corrodible by perovskite, leading to a sharp performance drop under continuous bending, air exposure, thermal stress, or light illumination and eventually retard the commercialization. Herein, a low-cost Bi₂Te₃ counter electrode was employed by using a simple thermal evaporation process. The resultant device achieved an excellent power conversion efficiency of 18.16%, which was among the highest reported efficiencies, much higher than the reference Ag PSC (15.90%). The improvement should be attributed to the intrinsic suppressed electron backscattering in a Bi₂Te₃ topological insulator. Simultaneously, the Bi₂Te₃ device obtained a significantly improved mechanical flexibility and long-term operational stability. The present strategy will help to open up a new avenue for future commercialization of flexible photovoltaic applications.

Carbon nitride nanoplatelet photocatalysts heterostructured with B-doped carbon nanodots for enhanced photodegradation of organic pollutants

Decorating electron-accepting materials on carbon nitride (C₃N₄) is a promising strategy to construct heterostructure catalysts for improved photocatalytic abilities. In this study, B-doped carbon-dots (B-C-dots) decorated C₃N₄ (C₃N₄/B-C-dots) catalysts were fabricated through the surface deposition. The benefits from integration of B-C-dots and C₃N₄ are four folds: (i) increasing surface area; (ii) improving visible light absorption; (iii) promoting the transfer of photoinduced carriers; and (iv) reducing the recombination of photoinduced carriers. The optimum photocatalytic activity of B-C-dots/C₃N₄ for Rhodamine B (Rh B) (or tetracycline hydrochloride (TC)) degradation was about 7.21 (6.56) and 4.80 (4.35) times higher than that of C₃N₄ and C-dots/C₃N₄, respectively, exhibiting both remarkable stability and repeatability. Moreover, enhanced photocatalytic activity of C₃N₄/B-C-dots could also be attributed to the type-II heterojunction formed between C₃N₄ and B-C-dots caused by B doping.

Polymer spacer tunable Purcell-enhanced spontaneous emission in perovskite quantum dots coupled to plasmonic nanowire networks

Bright and fast emission of perovskite quantum dots has been demonstrated by using a polymer spacer to regulate the exciton–plasmon coupling.