Dajiang Zheng

High efficiency perovskite solar cells: from complex nanostructure to planar heterojunction

Perovskite solar cells have garnered great attention in recent years as promising high performance next-generation solar cells with long-term stability at low cost. Since the seminal work of Miyasaka and others in 2009, the power conversion efficiency (PCE) of perovskite-based dye-sensitized solar cells (DSSCs) has rapidly increased from 3.8% to 15% over the past four years, exceeding the highest efficiency of conventional organic dye-sensitized DSSCs. Recently, the perovskite has been demonstrated to act successfully as an active layer in simple planar-heterojunction solar cells with no need of complex nanostructured DSSC architectures, leading to an attractively high PCE of 15.4% at a competitive low manufacturing cost. In this Feature Article, we aim to review the recent impressive development in perovskite solar cells, and discuss the prognosis for future progress in exploiting perovskite materials for high efficiency solar cells.

Hierarchically structured nanotubes for highly efficient dye-sensitized solar cells.

Hierarchical TiO2 nanotube arrays grown on Ti foil are yielded by subjecting electrochemically anodized, vertically oriented TiO2 nanotube arrays to hydrothermal processing. The resulting DSSCs exhibit a significantly enhanced power conversion efficiency of 7.24%, which is a direct consequence of the synergy of higher dye loading, superior light-scattering ability, and fast electron transport.

Optimized porous rutile TiO2 nanorod arrays for enhancing the efficiency of dye-sensitized solar cells

Highly ordered rutile TiO2 nanorod arrays (NRAs) are promising architectures in dye-sensitized solar cells (DSCs). However, the efficiency of DSCs based on such photoanodes is still relatively low, largely due to the limited internal surface area. Herein, we report that highly oriented rutile TiO2 NRAs with film thickness up to ∼30 μm was developed by a facile hydrothermal method. More importantly, an optimized porous rutile TiO2 NRAs with a large internal surface area was fabricated on the FTO (fluorine-doped tin oxide) substrate via a secondary hydrothermal treatment and when applied as the photoanodes in DSCs, a record efficiency of 7.91% was achieved.

One‐Dimensional Densely Aligned Perovskite‐Decorated Semiconductor Heterojunctions with Enhanced Photocatalytic Activity

By using one-dimensional rutile TiO(2) nanorod arrays as the structure-directing scaffold as well as the TiO(2) source to two consecutive hydrothermal reactions, densely aligned SrTiO(3) -modified rutile TiO(2) heterojunction photocatalysts are crafted for the first time. The first hydrothermal processing yielded nanostructured rutile TiO(2) with the hollow openings on the top of nanorods (i.e., partially etched rutile TiO(2) nanorod arrays; denoted PE-TNRAs). The subsequent second hydrothermal treatment in the presence of Sr(2+) transforms the surface of partially etched rutile TiO(2) nanorods into SrTiO(3) nanoparticles via the concurrent dissolution of TiO(2) and precipitation of SrTiO(3) while retaining the cylindrical shape (i.e., forming SrTiO(3) -decorated rutile TiO(2) composite nanorods; denoted STO-TNRAs). The structural and composition characterizations substantiate the success in achieving STO-TNRA nanostructures. In comparison to PE-TNRAs, STO-TNRA photocatalysts exhibit higher photocurrents and larger photocatalytic degradation rates of methylene blue (3.21 times over PE-TNRAs) under UV light illumination as a direct consequence of improved charge carrier mobility and enhanced electron/hole separation. Such 1D perovskite-decorated semiconductor nanoarrays are very attractive for optoelectronic applications in photovoltaics, photocatalytic hydrogen production, among other areas.

Hierarchically Structured Microspheres for High-Efficiency Rutile TiO2-Based Dye-Sensitized Solar Cells

Peachlike rutile TiO2 microsphere films were successfully produced on transparent conducting fluorine-doped tin oxide substrate via a facile, one-pot chemical bath route at low temperature (T = 80-85 °C) by introducing polyethylene glycol (PEG) as steric dispersant. The formation of TiO2 microspheres composed of nanoneedles was attributed to the acidic medium for the growth of 1D needle-shaped building blocks where the steric interaction of PEG reduced the aggregation of TiO2 nanoneedles and the Ostwald ripening process. Dye-sensitized solar cells (DSSCs) assembled by employing these complex rutile TiO2 microspheres as photoanodes exhibited a light-to-electricity conversion efficiency of 2.55%. It was further improved to a considerably high efficiency of 5.25% upon a series of post-treatments (i.e., calcination, TiCl4 treatment, and O2 plasma exposure) as a direct consequence of the well-crystallized TiO2 for fast electron transport, the enhanced capacity of dye loading, the effective light scattering, and trapping from microstructures.

Garden-like perovskite superstructures with enhanced photocatalytic activity

By subjecting amorphous flower-like TiO2 to a facile hydrothermal synthesis in the presence of Sr(2+), garden-like perovskite SrTiO3 superstructures were achieved. The amorphous TiO2 was preformed using ZnO flowers as templates. Different three-dimensional SrTiO3 architectures were coexisted in the garden, including SrTiO3 flowers composed of several hollow sword-shaped petals, many sheet-shaped petals or numerous flake-shaped petals, and SrTiO3 grass consisting of a number of long blades. These SrTiO3 superstructures were simultaneously grown on fluorine-doped tin oxide (FTO) substrates. On the basis of a comprehensive study on the effects of growth time, temperature, initial concentrations of precursor, and pH, the formation of these various hierarchical architectures was attributed primarily to the dissolution of amorphous TiO2 and precipitation of perovskite crystals, followed by the Ostwald ripening process of perovskite nanocrystals and self-organization of perovskite building blocks. Interestingly, this approach can be readily extended to create other perovskite structures, including dendritic BaTiO3 and nest-like CaTiO3, as well as PbTiO3 transformed from plate-like pyrochlore Pb2Ti2O6 after post-thermal treatment. Garden-like SrTiO3 superstructures showed a superior photocatalytic performance when compared to other as-prepared semiconductors and perovskite materials (i.e., ZnO, TiO2, BaTiO3, CaTiO3 and PbTiO3), probably due to their intrinsic photocatalytic activity and special garden-like features with a coexistence of various structures that significantly facilitated the adsorption and diffusion of methyl blue (MB) molecules and oxygen species in the photochemical reaction of MB degradation.

Nonepitaxial growth of uniform and precisely size-tunable core/shell nanoparticles and their enhanced plasmon-driven photocatalysis

The ability to synthetically tune the size, shape, composition and architecture of inorganic nanostructures offers enormous opportunities to explore the fundamental structure–property relationships that occur uniquely at the nanoscale, and engineer greater functionality and design complexity into new material systems. Core/shell nanoparticles represent an important class of nanostructured materials that have garnered considerable interest. The success in producing core/shell nanoparticles with strictly controlled core diameter and shell thickness and tailoring their material properties relies crucially on the epitaxial growth of the shell material over the highly curved surface of the spherical core. However, effective methods to yield such high-quality core/shell nanoparticles are comparatively few and limited in scope. Here, we develop a robust nonepitaxial growth strategy to create uniform plasmonic/semiconducting core/shell nanoparticles with precisely controlled dimensions by capitalizing on amphiphilic star-like triblock copolymers as nanoreactors. The diameter of the plasmonic core and the thickness of the semiconductor shell can be independently and precisely regulated by tailoring the molecular weights (i.e., the lengths) of the inner and intermediate blocks of star-like triblock copolymers, respectively. The successful crafting of plasmonic/semiconducting core/shell nanoparticles was corroborated by the composition and structural characterizations. These functional nanoparticles exhibited largely improved photocatalytic activities, which can be attributed to the localized surface plasmon-mediated light harvesting enhancement of the plasmonic core and the built-in internal electric field. This nonepitaxial growth strategy offers new levels of tailorability in the dimensions, compositions and architectures of nanomaterials with engineered functionalities for applications in catalytic, electronic, optic, optoelectronic and sensory materials and devices.

Dual-functional semiconductor-decorated upconversion hollow spheres for high efficiency dye-sensitized solar cells

Upconversion/semiconductor submicron hollow spheres composed of inner NaxGdFyOz:Yb/Er shell and outer TiO2 shell (denoted NaxGdFyOz:Yb/Er@TiO2) were, for the first time, crafted by exploiting colloidal carbon spheres as the scaffold. The hollow spheres were then incorporated into the TiO2 nanoparticle film photoanode to yield dye-sensitized solar cells (DSSCs) with improved performance. The implementation of NaxGdFyOz:Yb/Er@TiO2 hollow spheres in DSSCs imparted the light trapping due to the light scattering from submicron hollow spheres, and the harvesting of near infrared solar photons by the upconversion material (i.e., dual functionalities), thereby resulting in an increased short-circuit current density Jsc, and thus an improved power conversion efficiency PCE. The electrochemical impedance spectroscopy measurements were performed to scrutinize the interfacial charge transfer characteristics of DSSCs. The measurements revealed that when NaxGdFyOz:Yb/Er hollow spheres without the deposition of TiO2 shell were integrated in the photoanode, a high charge transfer resistance was found. In stark contrast, the judicious decoration of NaxGdFyOz:Yb/Er hollow spheres with a thin layer of TiO2 shell markedly improved the contact between the resulting NaxGdFyOz:Yb/Er@TiO2 shell/shell hollow spheres and the TiO2 nanoparticle film photoanode, leading to a much decreased charge transfer resistance. Taken together, compared to the PCE of 6.81% for the pristine device, the DSSC assembled with the introduction of 8 wt% NaxGdFyOz:Yb/Er@TiO2 hollow spheres in the photoanode exhibited an optimal PCE of 7.58% and a maximum short-circuit current density Jsc of 18.72 mA cm−2 under AM 1.5G one sun illumination, corresponding to 11.31% performance enhancement. As such, the implementation of upconversion submicron hollow materials in photoanode may stand out as an intriguing strategy to improve the device performance of DSSCs.

Rational design of hybrid dye-sensitized solar cells composed of double-layered photoanodes with enhanced power conversion efficiency

A uniquely structured dye-sensitized solar cell was fabricated by assembling two photoanodes and one counter electrode in a single compartment. The two photoanodes have complementary roles in absorbing solar light at different wavelengths. The power conversion efficiency of the hybrid cell can reach 6.6%, which is significantly higher than that of the single cell. The rational design of the hybrid cell does not need an interconnecting layer as that is used in conventional tandem solar cells, leading to higher power conversion efficiency.

Branched TiO2 Nanorods Covered with TiO2 Nanosheets for Harvesting Solar Energies in Dye-sensitized Solar Cells

Conference Name:Symposium on Photovoltaics for the 21st Century 7 held during the 220th Meeting of the Electrochemical-Society (ECS). Conference Address: Boston, MA. Time:OCT 09-14, 2011.