Organic-inorganic lead halide perovskite solar cells (PSCs) have generated considerable interest in recent years because of their high-power conversion efficiency. However, while converting solar light to electrical power has reported efficiencies exceeding 20 %, operational stability remains a concern, and therefore is preventing the widespread application of these devices. Several previously conducted studies have reported that perovskite solar cells such as those based on methyl ammonium lead iodide perovskite (CH3NH3PbI3 or MAPbI3) degrade rapidly when being operated under ambient operating conditions (e.g. in the presence of moisture and oxygen). The exact reason for the relatively poor stability of perovskite solar cells has yet to be fully elucidated. To better understand the reasons behind the poor stability and the rapid degradation of the PSCs in presence of moisture and oxygen, a new in-depth study was made and is the key focus of this thesis.
Oxygen and light induced degradation has recently been shown to be an important pathway for CH3NH3PbI3 perovskite films. This oxygen-light degradation process involves the generation of highly energetic electrons while the films are exposed to both visible light and oxygen. These charges can then react with oxygen in the atmosphere generating a reactive oxygen species (superoxide, O2-) which reacts with the methyl ammonium cation and causes CH3NH3+ to deprotonate. The degradation and O2- generation process is monitored from comparing Ultraviolet Visible Light Spectroscopy (UV-vis), Photoluminescence (PL) and Transient Absorption Spectroscopy (TAS) measurements with the exposure time. The results concluded from these three set of experiments indicate two main findings: 1.) oxygen induced degradation is an important loss pathway for both films and fully functioning devices and 2.) the drop rate in the yield of charge transfer at metal oxide / CH3NH3PbI3 / hole transporting material HTM heterojunctions was found to be less when the MAPbI3 was deposited on an electron accepting substrate; this was assigned to the presence of another kinetic pathway that can successfully compete with electron transfer to oxygen to form superoxide and therefore reduce the degradation rate.
After these two findings are established, the thesis aims to set a design criteria for electron accepting layer within heterojunction perovskite based solar cells to minimise light and oxygen induced degradation while the devices are functioned. The three main points addressed are:
1.) Improving air stability by maximising the electron acceptor-perovskite light absorber interface using mesoporous titiania (TiO2) films.
2.) Slowing down light and oxygen induced degradation by using tin oxide (SnO2) electron extraction layers; this being due to the low conduction band edge of SnO2 as compared to TiO2 and zinc oxide (ZnO).
3.) Applying (a) and (b) to a more oxygen-light stable lead trihalide perovskite photoabsorber, caesium formamidinium lead trihalide perovskite.
By incorporating the three above points to the lead trihalide perovskite solar cell, the optimised solar cell revealed itself to be over six times more stable than the controlled device while measured under identical conditions.
The studies also present an additional source of electron extraction process in order to improve light and oxygen induced degradation of perovskite films. This further stability enhancement of caesium formamidinium lead perovskite (CsFAPbI3) films and solar cells was made by doping the absorber solution with PC70BM. When an appropriate amount of PC70BM dopant (0.2 mg of PC70BM in 1 ml of DMSO) was blended with the absorber prior to the annealing process, the dopant acts as an intermediate source to assist the harvested charges to be extracted from the absorber layer successfully. In addition, the optimised PC70BM doped CsFAPbI3 device lasts over two times longer than the un-doped CsFAPbI3 while having similar initial light harvesting performances (71 hours compared with 38 hours, respectively).
This effect of the post annealing temperature (up to 100 oC) was incorporated during the existing oxygen and light degradation experiment. Hence, the longevity of each perovskite based film and device consisted of MAPbI3, CsFAPbI3, or caesium formamidinium lead perovskite CsFAPb(IBr)3 was compared. This study revealed that CsFAPb(IBr)3 films exhibited the highest degree of photo-stability and thermal-stability compared to CsFAPbI3 and MAPbI3 films. When CsFAPb(IBr)3 films were exposed to light and oxygen , CsFAPb(IBr)3 films showed almost no changes in their absorbance, superoxide formation rate and hole recombination lifetime in CsFAPb(IBr)3/spiro-OMeTAD over 120 hours. In addition, CsFAPb(IBr)3 based solar cell devices exhibited higher initial power conversion efficiencies and better both oxygen-light and thermal stability compared to CsFAPbI3 and MAPbI3 devices. The operational time of CsFAPb(IBr)3 is maximised with the appropriate amount of cation and anion substitutions.