Opto-Electronic Review Advances 2020, Vol. 3, No. 6 DOI: 10.29026/oea.2020.190038 Printing photovoltaics by electrospray Xinyan Zhao1,2 and Weiwei Deng2* Solution processible photovoltaics (PV) are poised to play an important role in scalable manufacturing of low-cost solar cells. Electrospray is uniquely suited for fabricating PVs due to its several desirable characteristics of an ideal manufac- turing process such as compatibility with roll-to-roll production processes, tunability and uniformity of droplet size, capa- bility of operating at atmospheric pressure, and negligible material waste and nano structures. This review begins with an introduction of the fundamentals and unique properties of electrospray. We put emphasis on the evaporation time and residence time that jointly affect the deposition outcome. Then we review the efforts of electrospray printing polymer solar cells, perovskite solar cells, and dye sensitized solar cells. Collectively, these results demonstrate the advantages of electrospray for solution processed PV. Electrospray has also exhibited the capability of producing uniform films as well as nanostructured and even multiscale films. So far, the electrospray has been found to improve active layer morphology, and create devices with efficiencies comparable with that of spin-coating. Finally, we discuss challenges and research opportunities that enable electrospray to become a mainstream technique for industrial scale production. Keywords: photovoltaics; electrospray; solution process; polymer solar cells; perovskite solar cells; dye sensitized solar cells Zhao X Y, Deng W W. Printing photovoltaics by electrospray: a review. Opto‐Electron Adv 3, 190038 (2020). mobilities, efficient high light absorptions, and long Introduction charge-carrier diffusion lengths1,2. Moreover, lead halide Photovoltaics (PV) are devices that directly convert the perovskites are synthesized from inexpensive and clean, renewable and abundant solar energy into electric- earth-abundant materials by solution process at relatively ity. In the past decade, solution processible photovoltaics low temperatures (<150 °C). Within ten years, the PCEs (SPPV) has emerged as a strong contestant for low cost of perovskite solar cells have surged from 3.8% to a certi- renewable energy technology as compared with copper fied 23.7%3–5. Meanwhile, polymer solar cells have also indium gallium diselenide (CIGS) and GaAs thin film experienced a breakthrough after the PCEs hovering near solar cells or polycrystalline silicon solar cells, which suf- 10% from 20002010. The surge of new donor materials fer from high cost due to the expensive and demanding and non-fullerene acceptor (NFA) has pushed the PCEs clean room or vacuum environments. Three main types beyond 16%6–8. In comparison, the interest in DSSC has of solution processible solar cells are: polymer solar cells, faded considerably because the film is much thicker (10 perovskite solar cells, as well as dye sensitized solar cells micro meters vs ~100 nm for polymer or ~300 nm for (DSSC). Truly remarkable progresses have been achieved perovskite) and other challenge such as the sealing of in each of the three types of cells, all of which have liquid phase of dye9. But it is the push to search for solid achieved power conversion efficiencies (PCEs) beyond state dye that helped the discovery of perovskite solar 14%. For example, lead halide perovskites have attracted cells. For the sake of completeness, we still include DSSCs immense attention as PV materials owing to their many in the scope of this review. ideal optoelectronic properties such as high ambipolar Compared to silicon based solar cells, the manufactur- 1Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology (SUSTech), Shenzhen 518055, China. 2Department of Mechanics and Aerospace Engineering, Southern University of Science and Technology (SUSTech), Shenzhen 518055, China. *Correspondence: W W Deng, E-mail: [email protected] Received: 4 November 2019; Accepted: 8 January 2020; Published: 22 June 2020 190038‐1 © 2020 Institute of Optics and Electronics, Chinese Academy of Sciences. All rights reserved. Opto-Electronic Advances https://doi.org/10.29026/oea.2020.190038 ing process for solution-based photovoltaics is greatly Finally, we present summary and outlook of electrospray simplified, since devices can be fabricated at low temper- printing of photovoltaics. ature and atmospheric pressure from minute amounts of low-cost, abundant raw materials. Moreover, solution Fundamentals of electrospray as a material processible solar cells are naturally capable of imple- processing technique menting roll-to-roll processes, using flexible substrates, and being more environmentally friendly. Therefore, so- Overview of the cone-jet mode electrospray lution processible solar cells promise vast reduction in Electrospray is a fluid dynamic phenomenon in which the manufacturing cost and are attractive from an industrial electrohydrodynamic stress deforms the liquid meniscus manufacturing process perspective. into a conical shape with a fine jet emanating from the To make SPPVs commercially viable, one of the re- apex of the cone. This elegant process can generate maining challenges is to find a low cost and scalable monodispersed droplets of a few nm to 100 μm. The su- manufacturing process to replace spin coating, the labor- perior capability of generating charged droplets in the atory scale standard technique. Two categories of scalable nanometer range helped Prof. John B. Fenn to pioneer the 27 techniques for making thin films from liquid phase pre- electrospray ionization mass spectroscopy (ESI-MS) that cursor have been investigated (i) continuous phase based eventually brought him the Nobel Prize in Chemistry in methods including doctor blade or shearing plate coat- 2002. A typical electrospray can be established by supply- ing10–13 and slot-die coating14–16; and (ii) dispersed phase ing a liquid with certain electric conductivity through a or droplet based method, such as inkjet printing17–19 and capillary charged to a voltage of a few kVs (Figs. 1 (a) and spray coating20–23, which are able to deposit films 1(b)). The liquid at the capillary end takes a conical shape, 28 conformally on non-flat substrate and are much more termed Taylor-cone , which is due to the balance be- tolerant to the surface curvature, roughness and defects. tween surface tension and electric stress norm at the liq- Most spray techniques are pneumatically based and need uid-gas interface (Fig. 1(a)). The electric shear stress ac- intense gas flow to atomize, disperse, or/and blow-dry the celerates the liquid near the free surface, accelerating the liquid solution20,24. The rapid gas flow in the pneumatic bulk liquid from nearly zero velocity at the cone base spray may blow away droplets containing active layer to >10 m/s at the cone apex. This operating mode is gen- 29 precursors and cause significant waste of expensive mate- erally known as the cone-jet mode . For a comprehensive rials. Electrospray, on the other hand, relies on electric review on the cone-jet electrospray, readers are referred to 30,31 fields solely to create quasi-monodisperse charged drop- Ref. lets25. The electrostatic attraction force between the sub- We shall clarify the terms of printing and deposition. strate and droplet effectively minimize material waste26. Printing suggests the process can pattern the materials on In addition, electrospray also has the ability of working in the substrate without the need of a mask, while deposi- non-vacuum environments, natural compatibility to tion usually does not involve active patterning. Although roll-to-roll process, and added benefits from the electrical this review is focused on printing photovoltaic devices, field including less material loss and nano structures. The the principles and ideas are also applicable to printing of electrospray is also found to improve the alignment and many types of thin film optoelectrical and semiconduct- orientation of the polymer molecules, and overall device ing devices. Electrospray can provide both printing and performance in certain cases. Therefore, electrospray has deposition by positioning the substrate at different dis- become a promising tool in SPPV fabrication (Table 1). tance from the Taylor cone. When such distance is on the This review begins with a concise introduction of the order of millimeter or is comparable with the cone diam- electrospray fundamentals to enable the readers to un- eter, it is termed as near-field and the jet has not broken derstand the unique properties and advantages offered by up or the droplets have not diverged too much from the the electrospray. This section also serves as a brief tutorial center line. Near-field is suitable for printing. When the of electrospray in the context of material processing. substrate is centimeters away from the emitter, it is con- Then the effort and progress of electrospray fabrication of sidered as far-field and the droplets have sufficient time polymer solar cells, perovskite solar cells and DSSCs are to repel each other and from a plume to cover a circular reviewed. These results illustrate the simplicity of fabri- area of a few millimeter to centimeters in diameter. cating those mainstream types of solution processible PVs. Far-field is suitable for material deposition (Fig. 1(a)). 190038‐2 © 2020 Institute of Optics and Electronics, Chinese Academy of Sciences. All rights reserved. Opto-Electronic