Structural Evolution of Evaporated Lead Phthalocyanine Thin Films for Near-Infrared Sensitive Solar Cells

Abstract

In this work, we focus on the deposition conditions as a means to control the structural evolution of lead phthalocyanine (PbPc) films in order to promote the triclinic structure, thereby inducing a shift in the absorption spectrum toward the near-infrared (NIR). Absorption spectra of PbPc films exhibit an enhanced NIR absorption peak at a wavelength of λ = 900 nm upon (i) increasing film thickness, (ii) increasing substrate temperature, or (iii) decreasing evaporation rate. X-ray diffraction measurements correlate the enhancement of the NIR absorption peak with an improved crystallinity and increased average volume of triclinic domains in the mixed monoclinic−triclinic films. As the surface structure of 10 and 60 nm thick films differ, this implies an asymmetric layer structure with a semicrystalline monoclinic film close to the substrate, evolving to a predominantly triclinic structure in the upper part of the film. We have demonstrated the use of structural control of the PbPc layer in a planar heterojunction solar cell with NIR-sensitivity. Decreasing the evaporation rate results in solar cells with significantly enhanced short-circuit current density (<i>J</i>_SC), because of the change in absorption in combination with the longer exciton diffusion length that was estimated for donor layers exhibiting a predominantly triclinic structure. Overall, an optimized solar cell yields a power conversion efficiency of 2.6% (2.1% when correcting for the solar spectrum mismatch), and has external quantum efficiencies above 11% from λ = 320−990 nm with a peak value of 34% at λ = 900 nm.