Enabling Continuous Processing of Perovskite Solar Cells
Authors: Druffel Thad 
Published: February 2020

The perovskite solar cell since its introduction in 2010 has received a great deal of attention with efficiencies exceeding 20%, the highest for a solution deposited device. This is significant since typical manufacturing of solar cells requires high temperatures and expensive vacuum processing greatly increasing costs. The perovskite avoids these expensive processes, although current state of the art techniques will be difficult to scale, especially into a continuous roll-to-roll manufacturing platform. However, the depsoition and heat treatment of the perovskite solar cell as currently described is very difficult to scale. A new technique, intense pulsed light (IPL) is very scalable and has been shown to improve the crystallinity of semiconductors.

In the past decade, the IPL technique has gained a large amount of interest within the printed electronics industry. The IPL technique delivers high-energy light in a very short duration over a large processing area, heating thin films containing photosensitive materials. The method is analogous to additive manufacturing techniques that utilize infrared lasers to locally sinter powders; however, IPL uses a broader spectrum of light and the processing area is far larger improving the feasibility for manufacturing economically. A module of series connected perovskite solar cells could include multiple rectangular cells in series similar to a dye-sensitized solar cell shown in the Figure. The IPL processing area is 20 mm x 300 mm, and anneals the perovskite absorber in approximately 1 millisecond; therefore is ideally suited for implementation into a continuous roll-to-roll platform of series connected cells.

Creating larger perovskite crystals and improving the surface coverage of perovskite films have become great topics of discussion in perovskite research. The deposition of the perovskite layer is typically accomplished using slution phase processing. The deposited perovskite films must undergo an annealing process but are limited to temperatures below 150 oC due to sublimation of the methylammonium iodide cation. At the lower temperatures, it is very difficult to achieve the film uniformity required for an acceptable device even at very long processing times. The IPL technique has been used to not just aneeal the perovskite thin films, but actually sinter the deposited crystals. The IPL process subjects the films to temperatures far exceeding the 150 oC maximum, but for very short durations (less than 1 ms). Thus the high vapor pressure cation does not sublimate. The process also does not induce high temperatures onto the substrate, making it applicable to flexible devices.

In this work, CH3NH3PbI3 perovskite films were successfully sintered using IPL. Perovskite films were subjected to varying energy intensities, and their resulting surface coverage, crystal size, and performances were recorded. X-ray diffraction (XRD) studies show the chemical composition is unchanged and the perovskite structure is not damaged during the process. This work can be used to envision a promising scalable method to mass-produce perovskite solar cells as shown in the Figure. Individual cells measuring 60 cm2 can be sequentially deposited using traditional printing techniques and connected in series using a conductive trace. The perovskite layer would be subjected to a very fast pulse of light to sinter the material, resulting in a smooth large grain film.

The thin films are analyzed using scanning electron microscopy, photoluminescense spectroscopy, ultraviolet-visible spectroscopy and fabricated devices are built and the performance metrics are reported. The paper will also describe how the work can be incorporated into roll-to-roll applications and the remaining steps to get the technology to a scalable platform.

Figure 1