Metal halide perovskite solar cells have drawn a great deal of attention in the photovoltaic research community due to their high efficiency and simple manufacturing process. However, these cells are facing issues towards commercialization, such as the need to achieve high stability and develop a manufacturing method to enable both the reproducible production of high-performance devices and their scaling-up. This is the quest in front of us when we started this work.
Similar to other kinds of thin film photovoltaics, a delicate control of film quality of perovskite layers is key to achieving both superior performance and high reproducibility. So far, high efficiency perovskite solar cells mainly adopt perovskite films with thickness ranging from 400 to 800 nm and most of these cells are small area devices. We thought that thicker films are easier to process and therefore offer better controllability in large area production of optoelectronics device when compared with thinner ones, because thick films tend to more tolerant for film thickness fluctuation and / or pinholes, which are almost inevitable when fabricating large scale solar cells in a realistic setting. This point should be also valid for perovskite solar cells, especially in terms of large area devices. In addition, thicker film can further enhance light absorption of the devices and potentially lead to higher efficiencies. However, thick perovskite films over 1 μm for perovskite solar cells have been found to be generally less efficient than the devices based on thinner films due to poor film quality. Thus, the key is to developing a method that is capable of depositing thick films but still keeping good film quality.
Our group – the Energy Materials and Surface Sciences Unit (EMSSU) at OIST - has been interested in development of a low cost and facile process to deposit high quality perovskite films for large area photovoltaic applications. To achieve this goal, we learned from previous reported perovskite formation methods and developed an improved version of the solid-gas perovskite formation method. First, we chose high-quality hydrogen lead triiodide crystals as the starting material to take full advantage of their fully coordinated lead-iodide structure to reduce iodide-vacancy and improve film quality. Second, we partially substituted iodide ions by chlorine ions by reacting hydrogen lead triiodide with methylammonium chloride, and a suitable substitution ratio can enhance charge carrier mobility and diffusion length in perovskite films. Third, we used methylamine gas based gas-solid perovskite formation to transform the chlorine incorporated hydrogen lead triiodide films into high quality perovskite films, and this gas-solid reaction led to film morphology improvement and defect healing. To check whether this method works, we implemented the resultant over 1 μm perovskite films prepared by our method into perovskite solar cells. It was exciting to see that the resultant devices not only achieved high efficiency, but also delivered excellent device performance reproducibility. The comprehensive characterization results presented in our work reveal that our method enables deposition of over 1 μm thick perovskite films with excellent film quality. One added benefit of our method is that since the film is thicker and with high quality, the outer-most layer naturally serves as protection for the inner film, which results in significantly improved decice stability. The stability test shows that the T80 lifetime under continuous operation conditions exceeds 1600 h, which almost doubles what is obtained for the solar cells fabricated using the convential method. Furthermore, we checked the compatibility of our method with larger area devices. The resultant solar modules also exhibited high efficiency and excellent reproducibility.
Written by Zonghao Liu and Yabing Qi
The paper in Nature Communications is here: go.nature.com/2OWtXPD