Methyl ammonium lead iodide perovskite (MAPbI3) has recently emerged as one of the most promising next generation solar cell materials. Laboratory scale solar cells have recently reached 20% efficiency and further improvements are expected. The high extinction coefficient, favorable bandgap, and large carrier diffusion length allow this material to efficiently collect solar radiation and transport the charges for collection in an external circuit. It can be deposited by simple solution based methods, which will allow low production costs and simple scale up.
These solar cells are typically fabricated in a device structure where the MAPbI3 layer is sandwiched between the electron and hole transporting layers. For high solar cell performance, the energy levels of the MAPbI3 valence band maximum (VBM) and conduction band minimum (CBM) need to be favorably aligned with the energy levels of charge transporting layers. To this end, ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy and X-ray photoemission spectroscopy have previously been employed to characterize the MAPbI3 VBM and CBM energy levels. However, MAPbI3 has been shown to exhibit significant temperature dependent changes in its structure and certain optoelectronic properties, and yet all previous studies thus far have measured the energy levels of the VBM and CBM at room temperature only. Solar cells under typical terrestrial operating conditions have been shown to reach high temperatures (above 60 oC) due to thermalization loss induced heating. Therefore, characterizing the temperature dependence of the VBM and CBM levels of MAPbI3is important to predict and optimize the solar cell performance under various operating temperatures as well as to obtain deeper insights on the temperature dependent structure-property relationships of this material.
Our combined UPS, absorbance, and photoluminescence data show that the VBM and CBM shift down in energy by 110 meV and 77 meV, respectively, as temperature increases from 28 oC to 85 oC. Density functional theory calculations using slab structures show that the decreased orbital splitting due to thermal expansion is a major contribution to the experimentally observed shift in energy levels. These results enable rational design to optimize solar cell performance under operating conditions with continued sunlight exposure and increased temperature, as well as providing a deeper understanding of the relationship between atomic structure and electronic properties in lead halide perovskites.