© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com COMMUNICATION Highly Efficient Hybrid Photovoltaics Based on Hyperbranched Three-Dimensional TiO 2 Electron Transporting Materials Khalid Mahmood, Bhabani Sankar Swain, and Aram Amassian* Dr. K. Mahmood, Prof. A. Amassian Physical Sciences and Engineering Division, and Solar and Photovoltaic Engineering Research Center King Abdullah University of Science and Technology (KAUST) Thuwal 23955–6900, Saudi Arabia E-mail: aram.amassian@kaust.edu.sa Dr. B. S. Swain School of Advanced Materials Engineering Kookmin University Seoul 136–702, South Korea DOI: 10.1002/adma.201500336 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 den- sity of nanofibers, [15,31] and have suffered from slow carrier transport due to the presence of structural defects, including at grain boundaries between the nanofiber trunks and nanorod branches of the hyperbranched ETM. Electrospinning of metal oxide nanofibers has recently emerged as a potentially inexpen- sive, rapid, facile, and versatile route to growing 1D TiO 2 nano- materials on a variety of substrates, [40,41] and has been investi- gated as a 1D material in the context of DSSCs. [42–44] However, the combination of electrospun TiO 2 fibers 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 scal- able multistage electrospinning and hydrothermal route for the development of 3D hyperbranched anatase TiO 2 nanorod– nanofiber arrays as electron transporting materials. The hyper- branched ETM with optimal electron transport and carrier lifetime leads to highly efficient mesostructured perovskite (CH 3 NH 3 PbI 3 ) solar cells with an average power conversion efficiency (PCE avg ) of 15.03% and a maximum power conver- sion efficiency (PCE max ) of 15.50%. Increasing the thickness of the hyperbranched ETM from 0.6 μm to ≈29 μm led to highly efficient DSSCs as well, with PCE max = 11.22%. These remarkable performances were possible thanks to the develop- ment of 3D hyperbranched nanofiber–nanorod arrays made of high quality anatase TiO 2 with few defects and capable of transporting electrons rapidly and over long distances, mini- mizing recombination losses. Light harvesting was also found to be significantly enhanced due to light scattering effects of the hyperbranched architecture, leading to significant performance boosts in DSSCs. This work demonstrates remarkable advan- tages in using hyperbranched ETMs for highly efficient, large- area, and low-cost hybrid photovoltaics. In Figure 1a, we show scanning electron micrographs (SEM) of nanofibers obtained by electrospinning of a first layer without subsequent hydrothermal synthesis of nanorod branches (E 1 H 0 ). We have carefully selected the electrospinning condi- tions, such as the applied voltage, the solution viscosity, and flow rate, in order to obtain nanofibers assembled from small TiO 2 nanoparticles (40–50 nm diameter) and exhibiting signifi- cant internal porosity within the nanofibers (see the Supporting Information). The formation of mesoporous nanofibers is also an important requirement to obtaining hierarchically meso- structured nanofiber–nanorod arrays during the subsequent Solution-processed hybrid thin film photovoltaic technologies, such as perovskite solar cells (PSCs), dye sensitized solar cells (DSSCs), and colloidal quantum dot solar cells rely heavily on TiO 2 as the electron transporting material (ETM), with organic solar cells now also making use of the material as electron transporting layer (ETL). [1–9] In the particular cases of DSSCs and mesostructured PSCs, the ETL is typically engineered to be mesoporous in order to promote a large surface area, good loading of the absorber and to provide a continuous pathway for electron transport and extraction with minimal recombi- nation. [10–13] One-dimensional (1D) TiO 2 nanostructured ETLs (nanotubes, nanorods, and nanowires) were recently shown to provide larger internal surface area than the nanoparticulate structure, promoting higher loading of absorber and distinct light-scattering effects. [14–21] 1D nanostructured ETLs have also been found to offer efficient charge separation and electron transport, [10,22] making them promising candidates for efficient mesoporous PSCs and DSSCs. However, practical efforts to fab- ricate PSCs and DSSCs based on 1D TiO 2 (anatase) nanowire or nanotube arrays have not yielded performance improve- ments to date. [23–27] Substantial further developments are needed in the archi- tecture of mesostructured ETMs in order to achieve important photovoltaic performance improvements in conditions compat- ible with scalable manufacturing. We take the view that three- dimensional (3D) hyperbranched nanowire architectures with high surface area, low defect density, and a 3D interconnec- tion network for electron extraction can be the basis of effec- tive ETMs for mesostructured photovoltaics. Efforts have been deployed to develop 3D hierarchical branched nanowire archi- tectures, [28–35] as these are of great interest to a wide range of applications including photocatalysis, energy storage, and pho- tovoltaics. [36–38] Different methods of synthesizing 3D hyper- branched TiO 2 nanowires have included vapor deposition, [29] pulsed-laser deposition, [39] and hydrothermal methods. [24] Adv. Mater. 2015, DOI: 10.1002/adma.201500336 www.advmat.de www.MaterialsViews.com