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 (J 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.

Enhancement of near-infrared absorption with high fill factor in lead phthalocyanine-based organic solar cells

“…The exciton diffusion length also increased with increasing the thickness of the PbPc layer in the PbPc-C 60 based planar heterojunction solar cells without CuI. 9 However, the devices showed lower FFs than the devices in this study, probably due to the existence of the thin monoclinic or amorphous layer near the ITO substrate. The enhanced FF can be explained by the increase of J SC and the low series resistance (R S ).…”

“…This indicates that only the 0.5 nm thick CuI affects the formation of the PbPc crystal structures regardless of the PbPc thickness. 9 This result is originated from the low R S of 2.4 U cm 2 , indicating that good electric contact between the ITO and the PbPc layer is formed. Under the condition of AM 1.5G illumination, J SC is dramatically enhanced from 4.6 mA cm À2 to 8.6 mA cm À2 using CuI as the templating layer.…”

“…This is due to the increase of the triclinic phase which is dependent on the thickness. 9,11 In the CuI inserted film, however, the absorption shoulder at a wavelength of around 900 nm is dramatically enhanced more than twice compared to the film in the absence of CuI, although the thickness of the PbPc is only 20 nm. This indicates that only the 0.5 nm thick CuI affects the formation of the PbPc crystal structures regardless of the PbPc thickness.…”

“…This is consistent with the previously reported result that the portion of triclinic increases with increasing thickness in the PbPc films. 9 However, the 20 nm thick PbPc film grown on the CuI layer shows no peak at around 16.4 . All of the films have a diffraction peak at around 12.45 , representing the triclinic phase with the reflection plane of (11 1) or the monoclinic phase with the reflection plane of (320).…”

Effects of humidity on the electrical properties of thermal inkjet-printed films of copper tetrasulfonated phthalocyanine (CuTsPc) onto paper substrates

“…Metallophthalocyanines (MPcs) are organic macrocyclic semiconductors that have been used in several electronic devices such as light-emitting diodes [1,2], photovoltaic cells [3][4][5][6][7], and field-effect transistors [8][9][10]. In these applications, MPcs are usually deposited as thin films by thermal evaporation with very precise control since molecular organization plays an important role on the film electrical properties [11,12].…”