Neutral-Color Semitransparent Organic Solar Cells with All-Graphene Electrodes

Graphene has been considered as a promising material for transparent electrodes due to its advantages including ultrahigh carrier mobilities, high optical transmittance, excellent mechanical flexibility, and good stability. Solar cells with all-graphene electrodes are potentially low-cost, high-performance, and environmental friendly, which however have not been realized until now. Here, we report the fabrication of semitransparent organic photovoltaics (OPVs) with graphene transparent electrodes as both cathode and anode, which can absorb light from both sides with the power conversion efficiency up to 3.4%. Meanwhile, the OPVs have a neutral color and show the transmittance of ∼40% in the visible region, making them suitable for some special applications, such as power-generating windows and building integrated photovoltaics. This work demonstrates the great potential of graphene for the applications in carbon-based optoelectronic devices

Detection of Bisphenol A Using DNA-Functionalized Graphene Field Effect Transistors Integrated in Microfluidic Systems

Bisphenol A (BPA) detection has attracted much attention recently for its importance to food safety and environment. The DNA-functionalized solution-gated graphene transistors are integrated in microfluidic systems and used for recycling detections of BPA for the first time. In the presence of BPA, both single- and double-stranded DNA molecules are detached and released from the graphene surface in aqueous solutions, leading to the change of device electrical performance. The channel currents of the devices change monotonically with the concentration of BPA. Moreover, the devices modified with double-stranded DNA are more sensitive to BPA and show the detection limit down to 10 ng/mL. The highly sensitive label-free BPA sensors are expected to be used for convenient BPA detections in many applications

The Application of Highly Doped Single-Layer Graphene as the Top Electrodes of Semitransparent Organic Solar Cells

A single-layer graphene film with high conductance and transparency was realized by effective chemical doping. The conductance of single-layer graphene was increased for more than 400% when it was doped with Au nanoparticles and poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid). Then semitransparent organic solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) were fabricated with single-layer graphene and indium tin oxide (ITO) as the top and bottom electrodes, respectively. The performance of the devices was optimized by tuning the active layer thickness and doping the single-layer graphene electrodes. The maximum efficiency of 2.7% was observed in the devices with the area of 20 mm² illuminated from graphene electrode under the AM1.5 solar simulator. It is notable that all of the devices showed higher efficiency from the graphene than ITO side, which was attributed to the better transmittance of the graphene electrodes. In addition, the influence of the active area of the organic solar cell on its photovoltaic performance was studied. We found that, when the active areas increased from 6 to 50 mm², the power conversion efficiencies decreased from 3% to 2.3% because of the increased series resistances and the decreased edge effect of the devices

Enhancing Efficiency and Stability of Perovskite Solar Cells through Nb-Doping of TiO₂ at Low Temperature

The conduction band energy, conductivity, mobility, and electronic trap states of electron transport layer (ETL) are very important to the efficiency and stability of a planar perovskite solar cell (PSC). However, as the most widely used ETL, TiO₂ often needs to be prepared under high temperature and has unfavorable electrical properties such as low conductivity and high electronic trap states. Modifications such as elemental doping are effective methods for improving the electrical properties of TiO₂ and the performance of PSCs. In this study, Nb-doped TiO₂ films are prepared by a facile one-port chemical bath process at low temperature (70 °C) and applied as a high quality ETL for planar PSCs. Compared with pure TiO₂, the Nb-doped TiO₂ is more efficient for photogenerated electron injection and extraction, showing higher conductivity, higher mobility, and lower trap-state density. A PSC with 1% Nb-doped TiO₂ yielded a power conversion efficiency of more than 19%, with about 90% of its initial efficiency remaining after storing for 1200 h in air or annealing at 80 °C for 20 h in a glovebox

Dithiafulvenyl Unit as a New Donor for High-Efficiency Dye-Sensitized Solar Cells: Synthesis and Demonstration of a Family of Metal-Free Organic Sensitizers

This work identifies the dithiafulvenyl unit as an excellent electron donor for constructing D−π–A-type metal-free organic sensitizers of dye-sensitized solar cells (DSCs). Synthesized and tested are three sensitizers all with this donor and a cyanoacrylic acid acceptor but differing in the phenyl (<b>DTF-C1</b>), biphenyl (<b>DTF-C2</b>), and phenyl–thiopheneyl–phenyl π-bridges (<b>DTF-C3</b>). Devices based on these dyes exhibit a dramatically improved performance with the increasing π-bridge length, culminating with DTF-C3 in η = 8.29% under standard global AM 1.5 illumination

Synthesis and Properties of Dithiafulvenyl Functionalized Spiro[fluorene-9,9′-xanthene] Molecules

Two spiroannulated molecular structures with dithiafulvenyl units functionalized at the 2,2′,7,7′- (<b>SFX-DTF1</b>) and 2,3′,6,′7- (<b>SFX-DTF2</b>) positions of a spiro­[fluorene-9,9′-xanthene] core were synthesized. Studies revealed the hole mobility was significantly influenced by the dithiafulvenyl functionalized positions in the molecular structure. To explore their primary applications as hole-transporting materials in perovskite solar cells, <b>SFX-DTF1</b>-based devices exhibited a power conversion efficiency of 10.67% without the use of p-type dopants, yielding good air stability

CO₂ Plasma-Treated TiO₂ Film as an Effective Electron Transport Layer for High-Performance Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have received great attention because of their excellent photovoltaic properties especially for the comparable efficiency to silicon solar cells. The electron transport layer (ETL) is regarded as a crucial medium in transporting electrons and blocking holes for PSCs. In this study, CO₂ plasma generated by plasma-enhanced chemical vapor deposition (PECVD) was introduced to modify the TiO₂ ETL. The results indicated that the CO₂ plasma-treated compact TiO₂ layer exhibited better surface hydrophilicity, higher conductivity, and lower bulk defect state density in comparison with the pristine TiO₂ film. The quality of the stoichiometric TiO₂ structure was improved, and the concentration of oxygen-deficiency-induced defect sites was reduced significantly after CO₂ plasma treatment for 90 s. The PSCs with the TiO₂ film treated by CO₂ plasma for 90 s exhibited simultaneously improved short-circuit current (<i>J</i>_SC) and fill factor. As a result, the PSC-based TiO₂ ETL with CO₂ plasma treatment affords a power conversion efficiency of 15.39%, outperforming that based on pristine TiO₂ (13.54%). These results indicate that the plasma treatment by the PECVD method is an effective approach to modify the ETL for high-performance planar PSCs

Magnetic Field-Assisted Perovskite Film Preparation for Enhanced Performance of Solar Cells

Perovskite solar cells (PSCs) are promising low-cost photovoltaic technologies with high power conversion efficiency (PCE). The crystalline quality of perovskite materials is crucial to the photovoltaic performance of the PSCs. Herein, a simple approach is introduced to prepare high-quality CH₃NH₃PbI₃ perovskite films with larger crystalline grains and longer carriers lifetime by using magnetic field to control the nucleation and crystal growth. The fabricated planar CH₃NH₃PbI₃ solar cells have an average PCE of 17.84% and the highest PCE of 18.56% using an optimized magnetic field at 80 mT. In contrast, the PSCs fabricated without the magnetic field give an average PCE of 15.52% and the highest PCE of 16.72%. The magnetic field action produces an ordered arrangement of the perovskite ions, improving the crystallinity of the perovskite films and resulting in a higher PCE

High-Performance, Self-Powered Photodetectors Based on Perovskite and Graphene

An ideal photodetector must exhibit a fast and wide tunable spectral response, be highly responsive, have low power consumption, and have a facile fabrication process. In this work, a self-powered photodetector with a graphene electrode and a perovskite photoactive layer is assembled for the first time. The graphene electrode is prepared using a solution transfer process, and the perovskite layer is prepared using a solution coating process, which makes the device low cost. Graphene can form a Schottky junction with TiO₂ to efficiently separate/transport photogenerated excitons at the graphene/perovskite interface. Unlike the conventional photovoltaic structure, in this photodetector, both photogenerated electrons and holes are transported along the same direction to graphene, and electrons tunneled into TiO₂ are collected by the cathode and holes transported by graphene are collected by the anode; therefore, the photodetector is self-powered. The photodetector has a broad range of detection, from 260 to 900 nm, an ultrahigh on–off ratio of 4 × 10⁶, rapid response to light on–off (<5 ms), and a high level of detection of ∼10¹¹ Jones. The high performance is primarily due to the unique charge-transport property of graphene and strong light absorption properties of perovskite. This work suggests a new method for the production of self-powered photodetectors with high performance and low power consumption on a large scale

Bifunctional Hydroxylamine Hydrochloride Incorporated Perovskite Films for Efficient and Stable Planar Perovskite Solar Cells

Research on the addition of suitable materials into perovskite film for improved quality is important to fabricate efficient and stable perovskite solar cells. An attempt to enhance the quality of perovskite is performed by incorporation of a bifunctional hydroxylamine hydrochloride (HaHc) into pristine perovskite solution. On the one hand, the chloride ion in HaHc changes the crystallization kinetic and defect state of the perovskite film and a high-quality perovskite film with larger grain size and lower defect density is obtained. Perovskite solar cell (PSC) with HaHc additive exhibit a power conversion efficiency (PCE) of 18.69% with less hysteresis, which is obviously higher than that of pristine cells (16.85%). On the other hand, the hydroxyl group in HaHc can form a strong hydrogen bond with iodide ion in perovskite film to impede the decomposition of the film when under thermal annealing or storing in air. As a result, the PSCs with HaHc additive show superior thermal and air stability to the pristine devices. These results indicate that the addition of HaHc in perovskite film can greatly improve the performance of PSCs as well as their thermal and air stability