Electrohydrodynamically Assisted Deposition of Efficient Perovskite Photovoltaics

DOI: 10.1002/admi.201500762 temperature, and solution processability makes it possible for the realization of a high throughput, roll-to-roll production process of high effi ciency photovoltaics, thus ending the gap between academic prototypes and industrial standards. While the prospect of harnessing these advantageous properties makes perovskites ideal candidates for next generation solar cells, there are roadblocks hindering widespread deployment. Two of the major challenges are: (a) the lack of a proper processing route that is compatible with in-line production while generating thin fi lms with both the material quality and morphology required to effectively harness solar irradiation and extraction of charge carriers at interfaces and (b) effi cient management of the environmentally toxic element, e.g., lead (Pb). Thus far, spin-coating represents one of the most widely employed production routes in solution processed perovskite photovoltaics. Convective fl ow during spinning and the subsequent evaporation process induce rapid and strong ionic interactions between metal cations and organic anions, leading to the formation of well-crystallized structure. However, the overall morphology is neither homogenous nor continuous over the entire functional area unless lengthy thermal annealing, [ 11 ] sequential deposition, [ 12 ] multistep solvent infi ltration at interfaces, [ 13 ] post solvent engineering, [ 14,15 ] or hot casting [ 16 ] are implemented. The need for additional processing steps adversely interrupts the fabrication process and creates extra manufacturing complication and cost in scaling up. Further, excessive amounts of perovskite precursor solutions are wasted as a result of centrifugal forces, thus generating unwanted Pb wastes. Built upon the successful demonstration of printable polymer photovoltaics, [ 17 ] spray coating techniques have been revisited to circumvent these formidable challenges as they potentially enable a glass-in-module-out processing technique for cost-effective and scalable production of large-area photovoltaics (the amount of solute directly scales with the volume of the precursor solutions deposited on the substrates). In essence, fi ne droplets containing perovskite precursors with a polydispersed distribution of diameters are iteratively deposited onto the substrates until a desired thickness is achieved. [ 18,19 ] When incoming droplets meet the surface at a non-zero contact angle, the contact line is pinned to its initial Organic–inorganic perovskites that combine the strength of both chemical worlds have emerged as tantalizing candidates for next generation photovoltaics. Here, the electrohydrodynamically assisted continuous liquid interface propagation as a general, and potentially scalable nanomanufacturing route toward synthesizing high quality perovskite thin fi lms in a rapid and high throughput fashion is reported. This strategy conceptually mimics the advantageous self-organizing features of emulsion droplets where the use of a binary solvent system, concurrently and continuously, initiates a three-stage process of coalescence, spreading, and merging, thus optimizing thin fi lm morphology upon deposition without the needs for additional engineering steps. The resulting perovskite thin fi lm not only exhibits a smooth topology with the root mean square roughness of only a few nm but also reveals hybrid morphology where micrometer-sized grains intersperse between interconnected and continuous crystalline networks. This gives rise to the highest power conversion effi ciency of 16.50% and average 14.68%; representing a nearly twofold increase compared to that of conventional spray-pyrolysis approach. As a fi nal critical aspect, the proposed strategy contributes new insights to effi ciently managing the environmentally hazardous lead during processing, signifi cantly reducing the amount by two orders of magnitude compared to that of spin-coating to achieve the same thin fi lm thickness.