Photochemical stability of high efficiency PTB7:PC70BM solar cell blends

Abstract

Thieno[3,4 b]thiophene-alt-benzodithiophene (PTB7) is a promising donor–acceptor copolymer that has achieved high efficiencies (7–9%) in organic solar cells but suffers from poor stability and degrades when exposed to light and oxygen. Using resonant Raman spectroscopy to examine the nature of this photo-oxidation, three main changes to the vibrations of the conjugated backbone are observed: (1) shift of the benzodithiophene (BDT) CC stretch peak at ∼1489 cm⁻¹ up to ∼1499 cm⁻¹; (2) increase in the relative intensity of coupled fused thiophene and benzene CC stretch peaks at ∼1535 and ∼1575 cm⁻¹; (3) appearance of a new peak at ∼1650 cm⁻¹; which suggest oxidation takes place on the BDT unit without loss of conjugation. In situ accelerated photo-degradation reveals that the observed oxidation is the initial step of degradation, which is followed by reductions in absorption and Raman scattering intensities that indicate the loss of chromophores by a second, more extensive oxidation step. Blending PTB7 with PC₇₀BM is found to accelerate the polymer's degradation, and further shift the BDT peak to ∼1509 cm⁻¹. Using density functional theory to simulate Raman spectra for several possible oxidised products, the initial oxidation is best described by hydroxylation of 3^rd and 7^th positions on the BDT donor unit.