Alkaline-earth metal embedded expanded phthalocyanine nanosheets with direct band gaps and high power conversion efficiency

Abstract

In this study, the geometrical structures and electronic and optical properties of a class of novel 2D semiconducting M₂Pc (M = Be, Mg, Ca, Sr and Ba) sheets have been systematically investigated using the density functional theory approach. The M₂Pc monolayers are direct-gap semiconductors with band gaps ranging from 1.2 to 1.4 eV at the HSE06 level. By applying biaxial strains within the range of −5% to 10%, the band gaps of M₂Pc monolayers can be effectively tuned from 1.09 to 1.83 eV. The feature of a direct gap can be maintained in the broad strain range. Interestingly, M₂Pc monolayers exhibit remarkable absorbance coefficients as large as 10⁵ cm⁻¹ with prominent peaks encompassing the infrared to ultraviolet domains. Unexpectedly, a series of heterostructures (M₂Pc/TMD and M₂Pc/GaN) with type-II band alignment were constructed for solar cells with power conversion efficiency (PCE) as high as ∼23.61%. The moderate direct band gaps, high absorption coefficients and PCE pave the way towards potential diverse applications of M₂Pc monolayers in nanoelectronics, optics, optoelectronics, etc.