Thermal and Evolved Gas Analysis as a Tool for Investigating Processing Conditions

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Oral, Alice Williams, presentation 196
Publication date: 5th February 2015

Hybrid organic / inorganic perovskites, such as alkylammonium lead trihalides (MAPbX₃), have become materials of substantial interest as the light-harvester in photovoltaic devices.  For solution deposition of MAPbI_3-xCl_x, which has produced PV devices boasting efficiencies exceeding 19 %,¹there is currently debate about the structure and composition of the resulting material, with suggestion that there is no chlorine present; it has also been proposedthat processing conditions and environment might affect the final material.^2,3 Thermal methods, such as thermogravimmetric analysis (TGA), differential scanning calorimetry (DSC) and simultaneous thermal analysis (STA), facilitate measurement of subtle thermal changes of a material as a function of temperature, and can be used to demonstrate the effect of sample preparation conditions on the resulting material.  Concurrent, real-time measurement of evolved gases via  FTIR⁴gives further information about what is lost from the sample.  Samples of MAPbI_3-xCl_x(40 wt. % in DMF) were annealed under N₂ in situ (i.e. within an STA) then subjected to a temperature ramp.  Evolved gases were monitored throughout using FTIR, which was equipped with a high-temperature gas cell and coupled to the STA via a heated transfer line. During annealing, only DMF was observed by FTIR.  The subsequent temperature ramp revealed a significant amount of residual solvent, with only DMF being seen until ca. 230 °C, when HCl and CH₃NH₂ appeared.⁴  These results were reproducible for samples that had been prepared in a number of ways, including pre-drying at 30 °C for 15 hours and then annealing at 100 °C for 90 minutes.⁴  This illustrates that Cl is not completely absent from the annealed material. DSC can also be used to demonstrate the effect of changing processing atmosphere.  Samples of MAPb_3-xCl_x annealed under dry N₂ and ambient air (RH ca. 55 %) at 100 °C were scanned over the range 20 °C – 120 °C (Fig. 1).  The sample annealed in air (Fig. 1, black) shows a small endotherm at ca. 78 °C, whereas the sample annealed in N₂ (red) shows no such feature.  The feature could be a result of trapped water, or even a structurally different material resulting from the presence of moisture during annealing.  Further study by STA-FTIR in combination with structural determination and device analysis will be used to probe this further.
Figure 1: DSC of MAPbI3-xClx annealed in ambient air (black) and dry N2 (red)
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