Evaporation of CH₃NH₃PbI₃ layers for photovoltaic applications has the advantage of forming fully-covered, pin-hole free films. However, control of the evaporation of the organic precursor is still challenging. Here, a hybrid process using evaporation of inorganic precursor and subsequent ultrasonic spray-coating of the organic precursor offers a solution. The main merits of spray-coating are controllable deposition, film uniformity for large area substrates, and its compatibility with non-planar surfaces.

We managed to simultaneously control the evaporation of methylammonium iodide (CH₃NH₃I) and lead iodide (PbI₂) in a vacuum chamber, resulting in flat and homogeneous CH₃NH₃PbI₃ perovskite layers. Devices composed of a double layer of TiO₂/PCBM as ETL and co-evaporated CH₃NH₃PbI₃ achieved power conversion efficiencies (PCEs) of 17.1% (reverse scan) and 13.4% (forward scan) with a stabilized PCE of over 16%. [1] Moreover, conformal perovskite layers were successfully deposited on top of textured silicon with full surface coverage. Additionally, our process is low-temperature, so that it can be combined with heterojunction silicon bottom solar cells allowing for high voltages tandem devices.

Apart from the traditional co-evaporation method, the hybrid method (evaporation of PbI₂/spray of CH₃NH₃I) demonstrates a fast conversion of PbI₂ into CH₃NH₃PbI₃ in comparison to the co-evaporation. This method can be applied to realize more complex mixed perovskite composition which showed higher stability, reproducibility that CH₃NH₃PbI₃. Also, no toxic solvents are used. With this method, we could obtain high quality perovskite absorbers and a maximum PCE of 14.3% (reverse scan). This combination of evaporation and spray coating is also well-suited for an up-scaling towards industrial mass production.

The next focus of CH₃NH₃PbI₃ is the replacement of Pb due to environmental concerns. Toxic lead can be replaced by Bismuth (Bi). [2] Current Bi based solar cells suffer from poor reproducibility in device fabrication because of the non-uniformity of the film prepared by the solution process. Bi-based films produced by spin-coating are less uniform and form pin-holes which cause leakage currents that strongly influence the PCEs of the devices. Enhancing the photocurrent and the open circuit voltage could be achieved by the evaporation method. In addition to the improved uniformity of the films, the evaporation method allows to use solvent free materials resulting in less impurities (than the solution method). By using our evaporation system, we could first of all prove our ability to evaporate the new absorber (Bi-based) and, secondly, deposit highly crystalline and homogeneous films on the entire area of FTO substrates.