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Seed-induced multistep and one-step hydrothermal methods have been successfully used by Lee et al. [ 31 ] and Wu et al. [ 15 ] to fabricate hyperbranched TiO 2 nanostructures with improved light harvesting in DSSCs owing to their larger surface area. However, these synthesis methods have resulted in a low density of nanofi bers, [ 15,31 ] and have suffered from slow carrier transport due to the presence of structural defects, including at grain boundaries between the nanofi ber trunks and nanorod branches of the hyperbranched ETM. Electrospinning of metal oxide nanofi bers has recently emerged as a potentially inexpensive, rapid, facile, and versatile route to growing 1D TiO 2 nanomaterials on a variety of substrates, [ 40,41 ] and has been investigated as a 1D material in the context of DSSCs. [42][43][44] However, the combination of electrospun TiO 2 fi bers with hydrothermally grown TiO 2 branches has not been investigated in the contexts of DSSCs or PSCs. [ 45 ] In this communication, we introduce a unique and scalable multistage electrospinning and hydrothermal route for the development of 3D hyperbranched anatase TiO 2 nanorodnanofi ber arrays as electron transporting materials. The hyperbranched ETM with optimal electron transport and carrier lifetime leads to highly effi cient mesostructured perovskite (CH 3 NH 3 PbI 3 ) solar cells with an average power conversion effi ciency (PCE avg ) of 15.03% and a maximum power conversion effi ciency (PCE max ) of 15.50%. Increasing the thickness of the hyperbranched ETM from 0.6 µm to ≈29 µm led to highly effi cient DSSCs as well, with PCE max = 11.22%. These remarkable performances were possible thanks to the development of 3D hyperbranched nanofi ber-nanorod arrays made of high quality anatase TiO 2 with few defects and capable of transporting electrons rapidly and over long distances, minimizing recombination losses. Light harvesting was also found to be signifi cantly enhanced due to light scattering effects of the hyperbranched architecture, leading to signifi cant performance boosts in DSSCs. This work demonstrates remarkable advantages in using hyperbranched ETMs for highly effi cient, largearea, and low-cost hybrid photovoltaics.In Figure 1 a, we show scanning electron micrographs (SEM) of nanofi bers obtained by electrospinning of a fi rst layer without subsequent hydrothermal synthesis of nanorod branches (E 1 H 0 ). We have carefully selected the electrospinning conditions, such as the applied voltage, the solution viscosity, and fl ow rate, in order to obtain nanofi bers assembled from small TiO 2 nanoparticles (40-50 nm diameter) and exhibiting significant internal porosity within the nanofi bers (see the Supporting Information). The formation of mesoporous nanofi bers is also an important requirement to obtaining hierarchically mesostructured nanofi ber-nanorod arrays during the subsequent Solution-processed hybrid thin fi lm photovoltaic technologies, such as perovskite solar cells (PSCs), dye sensitized solar cells (DSSCs), and colloidal quantum dot...

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The realization of stable p-type nitrogen-doped ZnO thin films with durable and controlled growth is important for the fabrication of nanoscale electronic and optoelectronic devices. ZnO thin films codoped with tantalum and nitrogen (Ta, N-ZnO) were fabricated by using the electrospraying method at an atmospheric pressure. X-ray diffraction (XRD) studies demonstrated that all the prepared films were polycrystalline in nature with hexagonal wurtzite structure. In addition, a shift in the XRD patterns was observed, and the crystal orientation was changed at a certain amount of nitrogen (>6 at.%) in the starting solution. Analysis of X-ray diffraction patterns and X-ray photoelectron spectra revealed that nitrogen which was combined with the zinc atom (N-Zn) was successfully doped into the ZnO crystal lattice. It was also observed that 2 at.% tantalum and 6 at.% nitrogen (2 at.% Ta and 6 at.% N) were the optimal dopant amounts to achieve the minimum resistivity of about 9.70 × 10(-5) Ω cm and the maximum transmittance of 98% in the visible region. Consequently, the field-emission characteristics of such a Ta, N-ZnO emitter can exhibit the higher current density of 1.33 mA cm(-2), larger field-enhancement factor (β) of 4706, lower turn-on field of 2.6 V μm(-1), and lower threshold field of 3.5 V μm(-1) attributed to the enhanced conductivity and better crystallinity of films. Moreover, the obtained values of resistivity were closest to the lowest resistivity values among the doped ZnO films as well as to the indium tin oxide (ITO) resistivity values that were previously studied. We confirmed that the tantalum and nitrogen atoms substitution in the ZnO lattice induced positive effects in terms of enhancing the free carrier concentration which will further improve the electrical, optical, and field-emission properties. The proposed electrospraying method was well suitable for the fabrication of Ta, N-ZnO thin films at optimum conditions with superior electrical, optical, and field-emission characteristics, implying the potential applications as both a transparent electrode and field-emission (FE) devices.

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