André Taylor

The team reporting on this research was led by André D. Taylor, a professor of chemical and biomolecular engineering at NYU Tandon, and post-doctoral associate Jaemin Kong.

Perovskite solar cells have progressed in recent years with rapid increases in power conversion efficiency (from 3% in 2006 to 25.5% today), making them more competitive with silicon-based photovoltaic cells. However, a number of challenges remain before they can become a competitive commercial technology.
One of these challenges involves inherent limitations in the process of p-type doping of organic hole-transporting materials within the photovoltaic cells.

This process, wherein doping is achieved by the ingress and diffusion of oxygen into hole transport layers, is time intensive (several hours to a day), making commercial mass production of perovskite solar cells impractical. The Tandon team, however, discovered a method of vastly increasing the speed of this process through the use of carbon dioxide instead of oxygen.

In perovskite solar cells, doped organic semiconductors are normally required as charge-extraction interlayers situated between the photoactive perovskite layer and the electrodes. The conventional means of doping these interlayers involves the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI), a lithium salt, to spiro-OMeTAD, a π-conjugated organic semiconductor widely used for a hole-transporting material in perovskite solar cells, and the doping process is then initiated by exposing spiro-OMeTAD:LiTFSI blend films to air and light. Besides being time consuming, this method largely depends on ambient conditions. By contrast, Taylor and his team reported a fast and reproducible doping method that involves bubbling a spiro-OMeTAD:LiTFSI solution with carbon dioxide (CO2) under ultraviolet light.

They found that the CO2 bubbling process rapidly enhanced electrical conductivity of the interlayer by 100 times compared to that of a pristine blend film, which is also approximately 10 times higher than that obtained from an oxygen bubbling process. The CO2 treated film also resulted in stable, high-efficiency perovskite solar cells without any post-treatments.

The lead author Jaemin Kong explained that “Employing the pre-doped spiro-OMeTAD to perovskite solar cells shortens the device fabrication and processing time. Further, it makes cells much more stable as most detrimental lithium ions in spiro-OMeTAD:LiTFSI solution were stabilized to lithium carbonate, created while the doping of spiro-OMeTAD happened during CO2 bubbling process. The lithium carbonates end up being filtered out when we spincast the pre-doped solution onto the perovskite layer. Thus, we could obtain fairly pure doped organic materials for efficient hole transporting layers.”

Moreover, the team found that the CO2 doping method can be used for p-type doping of other π-conjugated polymers, such as PTAA, MEH-PPV, P3HT, and PBDB-T. Taylor said the team is looking to push the boundary beyond typical organic semiconductors used for solar cells.

"We believe that wide applicability of CO2 doping to various π-conjugated organic molecules stimulates research ranging from organic solar cells to OLEDs and OFETs even to thermoelectric devices that all require controlled doping of organic semiconductors,” and added that “Since this process consumes a quite large amount of CO2 gas during the process, it can be also considered for CO2 capture and sequestration study in the future. We are hoping that the CO2 doping technique could be a stepping stone for overcoming existing challenges in organic electronics and beyond.”
 

Jason Lipton, a Ph.D. candidate under the guidance of Taylor, was lead author. Elisa Riedo (chemical and biomolecular engineering)  and researchers from Drexel University and the Brookhaven National Laboratory also participated.

The proliferation and miniaturization of electronics in devices, wearables medical implants and other applications has made technologies for blocking electromagnetic interference (EMI) especially important, while making their implementation more challenging. While EMI can cause disruptions in communication in critical applications, resulting in potentially disastrous consequences, traditional EMI shields require large thicknesses to be effective, hampering design flexibility. 

One solution resides in MXenes, a family of 2D transition metal carbides, nitrides, and carbonitrides with potential for blocking EMI demonstrate high conductivity and excellent EMI shielding properties. The key to the commercialization of these materials is industry-scale manufacturing.

A multi-institution research team led by Andre ́ D. Taylor, professor of chemical and biomolecular engineering at the NYU Tandon School of Engineering demonstrated a novel approach to MXene fabrication that could lead to methods for at-scale production of MXene freestanding films: drop-casting onto pre-patterned hydrophobic substrates. Their method led to a 38% enhancement of EMI shielding efficiency over conventional methods. The work suggests that micropatterned MXene films, prepared using a method that is scalable and allows for high throughput, can be readily used in EMI shielding, energy storage, and optoelectronics applications.

The team cast aqueous dispersions of MXene nanosheets (with the formula Ti3C2Tx) on hydrophobic polystyrene substrates and dried them. After drying, the resulting free-standing films could be easily peeled off, a method demonstrating a variety of advantages over the conventional vacuum-assisted filtration method with regards to time efficiency, operation simplicity, and surface smoothness. 

The drop-casting method allows for modulation of micrometer-scale 3D patterns on the film surface by utilizing pre-patterned substrates (such as a vinyl record, retroreflective packaging, and retroreflective tape). 

The research, “Scalable, Highly Conductive, and Micropatternable MXene Films for Enhanced Electromagnetic Interference Shielding,” is published in the first-anniversary issue of the Cell Press publication Matter. 

Perovskite solar cells have seen massive improvements over the last few years. But despite big increases in power conversion efficiency, fill factors – one of the important characteristics in need of optimization – have still hovered around 80 percent, limiting the capacity for solar energy.

Thanks to a team led by Associate Professor André D. Taylor, that fill factor has been pushed up to 85 percent. Using a polymer-capped solvent-annealing process, they enhanced open-circuit voltage without sacrificing short-circuit current, creating better perovskite cells with improved output and a longer lifespan than current models.

The research team included NYU Tandon Postdoctoral Research Associates Jaemin Kong and Jason A. Röhr, along with colleagues from Yale University, Brown University, Brookhaven National Laboratory and the Korea Research Institute of Chemical Technology, and received funding from several groups including the National Science Foundation and the Office for Naval Research. 

They found that during the solvent-annealing, the perovskite surface flattens and the perovskite grains agglomerate into micrometer-sized clusters having enlarged α-phase crystallites, while the δ-phase simultaneously disappears. The optimized structure enhances efficiency from 18.2 percent to 19.8 percent reliably, creating more stable and better solar cells.