The effects of heavy atoms on the exciton diffusion properties in photoactive thin films of tetrakis(4-carbomethoxyphenyl)porphyrins

Abstract

The singlet exciton diffusion properties of solution-cast thin films prepared from mixed substituent monoiodophenyl or monobromophenyl derivatives of 5,10,15,20-tetrakis(4-carbomethoxyphenyl)porphyrin (TCM₄PP) were investigated for use in solar energy conversion applications. The photoluminescent singlet decay lifetime PL(t) of pristine porphyrin films and films lightly doped (0.1–0.6% wt) with [6,6]-phenyl-C61-butryic acid methyl ester (PCBM) were used to obtain relative quenching efficiencies (Q). The exciton diffusion coefficient (D) and exciton diffusion length (L_D) for each derivative were obtained by modeling the quenching efficiency and PL lifetime decay data using a 3D exciton Monte Carlo diffusion simulation. Although the three TCMPP derivatives showed nearly identical absorbances and electrochemical properties, the monobromophenyl or monoiodophenyl substituted porphyrins exhibited significantly lower steady-state emission intensities and fluorescent lifetimes in solution. Amorphous thin films of the halogenated derivatives also exhibited a decrease in the PL decay lifetimes, relative quenching efficiencies, and reduced singlet exciton diffusion lengths. The singlet exciton diffusion length for TCM₄PP was calculated to be 15 nm and decreased by 71% to 4.4 nm for TCM₃IPP with the addition of a single iodo substituent. Photocurrent–voltage measurements of the derivatives in a PCBM bulk heterojunction device suggest that lowered exciton diffusion and enhanced singlet to triplet exciton conversion, due to the heavy atom effect, decreases photoconversion efficiency.