Structural evolution of evaporated lead phthalocyanine thin films for near-infrared sensitive solar cells
Karolien Vasseur, Barry P. Rand, David Cheyns, Ludo Froyen, and Paul Heremans, Washington (2010)
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 towards 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 nm and 60 nm-thick films differ, this implies an asymmetric layer structure with a more amorphous film close to the substrate, evolving to a more crystalline and mainly 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. PbPc layers exhibiting an improved crystallinity and predominantly triclinic structure by decreasing the evaporation rate result in solar cells with significantly enhanced short-circuit current density (JSC). Overall, an optimized solar cell yields a power conversion effiency of 2.6%, with external quantum efficiencies greater than 11% from λ = 320 – 990 nm, with a peak value of 34% at λ = 900 nm.
Partners : imec
Place of Publication : Washington
Date of Publication : 2010/11/17
Additional Data : Chem. Mater., 2011, 23 (3), pp 886–895