Xiaobao Xu

Hole selective NiO contact for efficient perovskite solar cells with carbon electrode.

In this study, we communicate an investigation on efficient CH3NH3PbI3-based solar cells with carbon electrode using mesoporous TiO2 and NiO layers as electron and hole selective contacts. The device possesses an appreciated power conversion efficiency of 14.9% under AM 1.5G illumination. The detailed information can be disclosed with impedance spectroscopy via tuning the interfaces between CH3NH3PbI3 and different charge selective contacts. The results clearly show charge accumulation at the interface of CH3NH3PbI3. The NiO is believed to efficiently accelerate charge extraction to the external circuit. The extracted charge could improve photovoltaic performance by shifting hole Fermi level down, achieving a high device photovoltage. A fast interfacial recombination at the interface of CH3NH3PbI3/electron selective contact layer (mesoporous TiO2), occurring in millisecond domains, is the critical issue for charge carrier recombination loss.

The optoelectronic role of chlorine in CH3NH3PbI3(Cl)-based perovskite solar cells.

Perovskite photovoltaics offer a compelling combination of extremely low-cost, ease of processing and high device performance. The optoelectronic properties of the prototypical CH3NH3PbI3 can be further adjusted by introducing other extrinsic ions. Specifically, chlorine incorporation has been shown to affect the morphological development of perovksite films, which results in improved optoelectronic characteristics for high efficiency. However, it requires a deep understanding to the role of extrinsic halide, especially in the absence of unpredictable morphological influence during film growth. Here we report an effective strategy to investigate the role of the extrinsic ion in the context of optoelectronic properties, in which the morphological factors that closely correlate to device performance are mostly decoupled. The chlorine incorporation is found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals. Further optimization according this protocol leads to solar cells achieving power conversion efficiency of 17.91%.

Electrochemically Reduced Graphene Oxide Multilayer Films as Efficient Counter Electrode for Dye-Sensitized Solar Cells

We report on a new counter electrode for dye-sensitized solar cells (DSCs), which is prepared using layer-by-layer assembly of negatively charged graphene oxide and positively charged poly (diallyldimethylammonium chloride) followed by an electrochemical reduction procedure. The DSC devises using the heteroleptic Ru complex C106TBA as sensitizer and this new counter electrode reach power conversion efficiencies of 9.5% and 7.6% in conjunction with low volatility and solvent free ionic liquid electrolytes, respectively. The new counter electrode exhibits good durability (60°C for 1000 h in a solar simulator, 100 mW cm−2) during the accelerated tests when used in combination with an ionic liquid electrolyte. This work identifies a new class of electro-catalysts with potential for low cost photovoltaic devices.

Hydrothermal synthesis of ultrasmall CuCrO2 nanocrystal alternatives to NiO nanoparticles in efficient p-type dye-sensitized solar cells

In this study, we present a facile hydrothermal synthesis of ultrasmall delafossite CuCrO2 nanocrystals, with a typical size of 15 nm × 5 nm, and a high surface area of 87.86 m2 g−1, thermally stable up to 400 °C. The screen-printed CuCrO2 films sintered at different temperatures and under different atmospheres present different optical transmittances, tunable BET surface areas, dye adsorbing amounts, and tunable hole transport rates and hole recombination kinetics, which allows the optimization of the performance of p-type dye sensitized solar cells (DSSCs) based on CuCrO2 photocathodes. At optimized conditions, an open circuit voltage of 102 mV, a short circuit density of 0.491 mA cm−2, a fill factor of 0.398 and an overall photoconversion efficiency of 0.0194% were finally achieved for a coumarin 343 dye sensitized CuCrO2 solar cell. This record is acceptably high in comparison to a DSSC based on an NiO photocathode prepared and tested in similar experimental conditions. The light harvesting properties, charge collection capability, and flat-band potential of CuCrO2 photocathodes and the NiO photocathode have been critically compared.

Zinc Porphyrins with a Pyridine-Ring-Anchoring Group for Dye-Sensitized Solar Cells

Anchoring groups are extremely important in controlling the performance of dye-sensitized solar cells (DSCs). The design and characterization of sensitizers with new anchoring groups, in particular non-carboxylic acid groups, has become a recent focus of DSC research. Herein, new donor-π-acceptor zinc-porphyrin dyes with a pyridine ring as an anchoring group have been designed and synthesized for applications in DSCs. Photophysical and electrochemical investigations demonstrated that the pyridine ring worked effectively as an anchoring group for the porphyrin sensitizers. DSCs that were based on these new porphyrins showed an overall power-conversion efficiency of about 4.0 % under full sunlight (AM 1.5G, 100 mW cm(-2)).

A Power Pack Based on Organometallic Perovskite Solar Cell and Supercapacitor

We present an investigation on a power pack combining a CH3NH3PbI3-based solar cell with a polypyrrole-based supercapacitor and evaluate its performance as an energy pack. The package achieved an energy storage efficiency of 10%, which is much higher than that of other systems combining a PV cell with a supercapacitor. We find a high output voltage of 1.45 V for the device under AM 1.5G illumination when the CH3NH3PbI3-based solar cell is connected in series with a polypyrrole-based supercapacitor. This system affords continuous output of electric power by using CH3NH3PbI3-based solar cell as an energy source mitigating transients caused by light intensity fluctuations or the diurnal cycle.

Remarkable photocurrent of p-type dye-sensitized solar cell achieved by size controlled CuGaO2 nanoplates

In this paper, we report the successful hydrothermal synthesis of CuGaO2 nanoplates with a critically small size and deposition methods to fabricate effective p-type semiconductive photocathodes in dye-sensitized solar cells (DSCs). Based on an efficient P1 dye and an iodide electrolyte, the optimal CuGaO2 photocathode has achieved remarkably high photocurrent density, up to 2.05 mA cm−2. To the best of our knowledge, this is the highest achieved by nanocrystalline p-type semiconductors besides NiO. The light harvesting, charge collection in CuGaO2 and NiO based DSCs have been compared. Owing to size control of the CuGaO2 nanoplates and subsequent mechanical pressing for photocathode film deposition, the light harvesting efficiency of the CuGaO2 photocathode is greatly increased to a comparable level to that of the NiO reference. Another noteworthy feature is the remarkably high charge collection efficiency of the CuGaO2 photocathode, which should benefit from the nature of delafossite oxides with a high conductivity leading to a much higher hole diffusion coefficient in the DSC system. The open-circuit voltage is 199.3 mV, about twice as high as that of the NiO reference, benefiting from the valence band position shifting from −5.15 eV for NiO to −5.29 eV for CuGaO2versus the vacuum level.

Recent progress in efficient hybrid lead halide perovskite solar cells

Abstract The efficiency of perovskite solar cells (PSCs) has been improved from 9.7 to 19.3%, with the highest value of 20.1% achieved in 2014. Such a high photovoltaic performance can be attributed to optically high absorption characteristics and balanced charge transport properties with long diffusion lengths of the hybrid lead halide perovskite materials. In this review, some fundamental details of hybrid lead iodide perovskite materials, various fabrication techniques and device structures are described, aiming for a better understanding of these materials and thus highly efficient PSC devices. In addition, some advantages and open issues are discussed here to outline the prospects and challenges of using perovskites in commercial photovoltaic devices.

Low-Temperature TiOx Compact Layer for Planar Heterojunction Perovskite Solar Cells

Here, we demonstrate an effective low-temperature approach to fabricate a uniform and pinhole-free compact TiO2 layer for enhancing photovoltaic performance of perovskite solar cells. TiCl4 was used to modify TiO2 for efficient charge generation and significantly reduced recombination loss. We found that a TiO2 layer with an appropriate TiCl4 treatment possesses a smooth surface with full coverage of the conductive electrode. Further studies on charge carrier dynamics confirmed that the TiCl4 treatment improves the contact of the TiO2/perovskite interface, facilitating charge extraction and suppressing charge recombination. On the basis of the treatment, we improved the open circuit voltage from 1.01 V of the reference cell to 1.08 V, and achieved a power conversion efficiency of 16.4%.

Working Mechanism for Flexible Perovskite Solar Cells with Simplified Architecture

In this communication, we report an efficient and flexible perovskite solar cell based on formamidinium lead trihalide (FAPbI3) with simplified configuration. The device achieved a champion efficiency of 12.70%, utilizing direct contact between metallic indium tin oxide (ITO) electrode and perovskite absorber. The underlying working mechanism is proposed subsequently, via a systematic investigation focusing on the heterojunction within this device. A significant charge storage has been observed in the perovskite, which is believed to generate photovoltage and serves as the driving force for charge transferring from the absorber to ITO electrode as well. More importantly, this simplified device structure on flexible substrates suggests its compatibility for scale-up fabrication, which paves the way for commercialization of perovskite photovoltaic technology.