High-Performance Flexible Perovskite Solar Cells by Using a Combination of Ultrasonic Spray-Coating and Low Thermal Budget Photonic Curing

Realizing the commercialization of high-performance and robust perovskite solar cells urgently requires the development of economically scalable processing techniques. Here we report a high-throughput ultrasonic spray-coating (USC) process capable of fabricating perovskite film-based solar cells on glass substrates with a power conversion efficiency (PCE) as high as 13%. Perovskite films with high uniformity, crystallinity, and surface coverage are obtained in a single step. Moreover, we report USC processing on TiO₂/ITO-coated polyethylene terephthalate (PET) substrates to realize flexible perovskite solar cells with a PCE as high as 8.1% that are robust under mechanical stress. In this case, a photonic curing technique was used to achieve a highly conductive TiO₂ layer on flexible PET substrates for the first time. The high device performance and reliability obtained by this combination of USC processing with optical curing appear very promising for roll-to-roll manufacturing of high-efficiency, flexible perovskite solar cells

Synthesis of Millimeter-Size Hexagon-Shaped Graphene Single Crystals on Resolidified Copper

We present a facile method to grow millimeter-size, hexagon-shaped, monolayer, single-crystal graphene domains on commercial metal foils. After a brief <i>in situ</i> treatment, namely, melting and subsequent resolidification of copper at atmospheric pressure, a smooth surface is obtained, resulting in the low nucleation density necessary for the growth of large-size single-crystal graphene domains. Comparison with other pretreatment methods reveals the importance of copper surface morphology and the critical role of the melting–resolidification pretreatment. The effect of important growth process parameters is also studied to determine their roles in achieving low nucleation density. Insight into the growth mechanism has thus been gained. Raman spectroscopy and selected area electron diffraction confirm that the synthesized millimeter-size graphene domains are high-quality monolayer single crystals with zigzag edge terminations

Unraveling the Fundamental Mechanisms of Solvent-Additive-Induced Optimization of Power Conversion Efficiencies in Organic Photovoltaic Devices

The realization of controllable morphologies of bulk heterojunctions (BHJ) in organic photovoltaics (OPVs) is one of the key factors enabling high-efficiency devices. We provide new insights into the fundamental mechanisms essential for the optimization of power conversion efficiencies (PCEs) with additive processing to PBDTTT-CF:PC₇₁BM system. We have studied the underlying mechanisms by monitoring the 3D nanostructural modifications in BHJs and correlated the modifications with the optical analysis and theoretical modeling of charge transport. Our results demonstrate profound effects of diiodooctane (DIO) on morphology and charge transport in the active layers. For small amounts of DIO (<3 vol %), DIO promotes the formation of a well-mixed donor–acceptor compact film and augments charge transfer and PCE. In contrast, for large amounts of DIO (>3 vol %), DIO facilitates a loosely packed mixed morphology with large clusters of PC₇₁BM, leading to deterioration in PCE. Theoretical modeling of charge transport reveals that DIO increases the mobility of electrons and holes (the charge carriers) by affecting the energetic disorder and electric field dependence of the mobility. Our findings show the implications of phase separation and carrier transport pathways to achieve optimal device performances

Surface-Induced Orientation Control of CuPc Molecules for the Epitaxial Growth of Highly Ordered Organic Crystals on Graphene

The epitaxial growth and preferred molecular orientation of copper phthalocyanine (CuPc) molecules on graphene has been systematically investigated and compared with growth on Si substrates, demonstrating the role of surface-mediated interactions in determining molecular orientation. X-ray scattering and diffraction, scanning tunneling microscopy, scanning electron microscopy, and first-principles theoretical calculations were used to show that the nucleation, orientation, and packing of CuPc molecules on films of graphene are fundamentally different compared to those grown on Si substrates. Interfacial dipole interactions induced by charge transfer between CuPc molecules and graphene are shown to epitaxially align the CuPc molecules in a face-on orientation in a series of ordered superstructures. At high temperatures, CuPc molecules lie flat with respect to the graphene substrate to form strip-like CuPc crystals with micrometer sizes containing monocrystalline grains. Such large epitaxial crystals may potentially enable improvement in the device performance of organic thin films, wherein charge transport, exciton diffusion, and dissociation are currently limited by grain size effects and molecular orientation