Semiconductor performance of phthalocyaninato lead complex and its nonperipheral substituted derivatives for organic field effect transistors: Density functional theory calculations

Density functional theory (DFT) calculations were carried out to investigate the semiconductor performance for the organic field effect transistor (OFET) of PbPc, PbPc(α-OC₂H₅) ₄, and PbPc(α-OC₅H₁₁)₄ {Pc^2- = dianion of phthalocyanine; [Pc(α-OC₂H ₅)₄]^2- = dianion of 1, 8, 15, 22-tetraethoxyphthalocyanine; [Pc(α-OC₅H₁₁) ₄]^2- = dianion of 1, 8, 15, 22-tetrakis(3-pentyloxy) phthalocyanine} in terms of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy, ionization energy (IE), electron affinity (EA), and their reorganization energy (λ) during the charge-transport process. On the basis of Marcus electron transfer theory, transfer integral (t) and field effect transistor (FET) properties for the three compounds with known crystal structure have been calculated. In line with the experimental result that PbPc can also work as an n-type semiconductor in addition to a p-type one, theoretical calculations reveal that PbPc has relatively large electron affinity to ensure effective electron injection from Au electrode. Introducing four ethoxy groups on the nonperipheral positions of PbPc decreases both the hole and electron injection barrier relative to Au electrode, and the hole and electron reorganization energy becomes very balanced, making PbPc(α-OC₂H₅)₄ a better ambipolar semiconductor material than PbPc. However, nonperipheral pentyloxy substitution lifts the energy level of both HOMO and LUMO and thus decreases both the IP and EA value of PbPc, resulting in improved hole injection ability but worsened electron injection process. The transfer mobility for electron is revealed to be as large as 0.39 cm² V^-1 s^-1 for PbPc and 0.16 cm² V^-1 s^-1 for PbPc(α-OC₅H₁₁)₄. The present work will be helpful to understand the electronic nature for PbPc to work as ambipolar semiconductor and to rationally design novel semiconductor materials for OFET usage.