The majority of PVs (~85%) in the market are based on crystalline silicon [1]. While silicon cells have the highest power conversion efficiencies (PCEs), with multi-junction versions reaching 46%, thin-film PVs had advantages by being easier to manufacture and compatible with a large range of substrates, but many require rare-earth elements [2]. There has been rapid progress in the development of perovskite-based thin film solar cells with cells with their power-conversion efficiencies (PCE) surpassing organic and dye-sensitized cells within a few years of development. The highest officially confirmed power-conversion efficiencies are at 22.1% [https://www.nrel.gov/pv/assets/images/efficiency-chart.png].
The most commonly used perovskites for this purpose are methylammonim lead triiodide (CH3NH3PbI3) and the mixed-halides CH3NH3PbI3-xAx with A being Cl or Br. These architectures have the advantage of production by low-temperature solution processing which makes them low-cost and allows for use of flexible substrates3. At the same time, they do not require rare materials. The high efficiencies are partially attributed to the fact that unlike organic materials, perovskites have high carrier mobilities, lifetimes and therefore diffusion lengths. This ensures that more of the generated free carriers reach the electrodes before recombining.
A planar heterojunction perovskite solar cell is composed of several layers: the active layer, electron transport layer, hole transport layer, and the electrodes. The free carriers are generated due to the photoelectric effect in the active layer (this is made of the perovskite material). The electron and hole transport layers act as selective transport layers that only allow one carrier type through. This results in carriers being seperated to opposite sides of the device. Each of these layers must be of good quality with minimum defects and maximum coverage of the previous layer in order to achieve maximal efficiency. Typically these layers are made from precursor solutions by spin-coating and then annealing.
I investigated the dependence of the precursor solutions, spin-coating and annealing techniques on the morphology of the TiO2 transport layer and perovskite active layer.