Studies on surface treatments of TiO2 electron transport layer for efficient CH3NH3PBL3 perovskite photovoltaic cells.

Although crystalline-Si based solar cells have been dominated the markets, the high manufacturing cost is addressed as an issue to be overcome for further spread of photovoltaic modules. Organometallic halide perovskite (OHP) solar cells are the other candidates because of their high-power conversion efficiencies (PCEs) and low manufacturing costs. In typical PSCs, an n-type bilayer composed of dense and porous titanium oxide (TiO2) films are usually used as a blocking layer to protect shunts between a transparent conductive oxide (TCO) and a hole transport material (HTM). The bilayer is also required to achieve effective electron transportation for the OHP material to the TCO layer.

The porous TiO2 (pTiO2) layer contains large amounts of hydroxyl groups and adsorbed water molecules on the surface. Since the organometallic halide perovskite materials used in solar cells are known to react with water, these surface hydroxyl groups and adsorbed water molecules should affect structural and electric properties of OHPs as well as the heterointerface between TiO2 and OHP layers. Thus, the main objective of this study is clarification of effects of these surface hydroxyl groups and adsorbed water molecules in the porous TiO2 (pTiO2) on the solar cell performance.

Firstly, dependences of photovoltaic properties of solar cells based on a OHP compound, CH3NH3PbI3, on structural and physicochemical properties of the pTiO2 layer were examined. This study was carried out by using pTiO2 obtained by annealing a spin-coated pTiO2 precursor film at different temperatures between 400 ºC and 700 ºC: varying the annealing temperature resulted in formation of pTiO2 films with different water contents and porosities. For the use of solar cell, the optimum annealing temperature was ound to be 550ºC: the highest PCE of 8.8 % was obtained in the series of this study. Structural analyses of used pTiO2 films suggested that the pTiO2 film obtained by the 550ºC-annealing had adequate porous nature and sufficient removal of surface hydroxyl groups, leading to efficient electron transport form the OHP (i.e., CH3NH3PbI3) layer to TCO through the TiO2 bilayer.

Fabrication of pTiO2 film by applying a low temperature annealing (below 400 ºC) is more favorable in view of the use of flexible/lightweight polymer substrate. However, these low-temperature-processed pTiO2 films were not adequate for the solar cell application due to the presence of large amounts of surface hydroxyl groups. Thus, reduction of surface hydroxyl groups of pTiO2 films obtained by annealing at relatively low temperatures (300 ºC and 400 ºC) were attempted by applying a TiCl4 post-treatment. As expected, the treatment led to the reduction of water content in these pTiO2 films: PCEs of solar cells showed the best conversion efficiencies of 4.5% (for the pTiO2 film obtained by the 300ºC-annealing) and 6.2% (for the pTiO2 film obtained by the 400ºC-annealing).

Finally, effects of the TiCl4 post-treatment on structural and photoelectrochemical properties of the pTiO2 film obtained by the 550ºC-annealing were investigated in order to improve solar cell performance of the above-discussed solar cell without the TiCl4 treatment. The effects of treatment in different concentration of TiCl4 (20, 50, 80 and 100 mM) on the amount of surface hydroxyl groups and electron transfer property were investigated. The enhancement of CH3NH3PbI3 solar cell performance was obtained from TiCl4 concentration (up to 50 mM), suggesting that the great importance of TiCl4 surface treatment in removing significant amounts of surface physisorbed water and sufficient formation of new TiO2 particles interconnection. With the application of these surface treatment, the PCE value improved up to 14.9 % at the optimum concentration of TiCl4 (50 mM).