Correlating triplet yield, singlet oxygen generation and photochemical stability in polymer/fullerene blend films

Abstract

The photostability of two donor polymers, DPP-TT-T and PTB7, is compared in neat films and blend films with PC(71)BM. In both neat and blend films, PTB7 is shown to be relatively unstable. This observation is shown to correlate with transient optical studies of long lived polymer triplets and with molecular probe studies of singlet oxygen yields.

Reducing burn-in voltage loss in polymer solar cells by increasing the polymer crystallinity

“…A high degree of crystallinity could reduce the impact of lightinduced traps on device performance by a spatial separation of electrons and holes. Moreover, a high degree of crystallinity also seems to reduce photo-oxidation in unencapsulated polymer lms 37 suggesting that crystalline materials may be more stable under various conditions. By reapplying new electrodes to aged devices we distinguished bulk and interface degradation that affect solar cell performance in a different way.…”

Fabricating high performance conventional and inverted polymer solar cells by spray coating in air

“…This loss in efficiency primarily results from a reduction in Jsc, which suggest that the slight loss of performance on spray-coating most probably results from photo-oxidation of the active layer as it was exposed to the atmosphere. [28,29] We note that some high-performing OPV polymers can be very sensitive to exposure to light in the presence of oxygen, [11] whilst others are apparently significantly more stable. [30] We believe that such oxidation could be reduced by performing air-based spray-coating of the active layer under appropriate safe-lights.…”

The significance of fullerene electron acceptors in organic solar cell photo-oxidation

“…20 Figure 6 illustrates the triplet-mediated photo-oxidation pathways which can result in singlet oxygen. The blue solid arrow shows the triplet formation in a neat polymer film via intersystem crossing from the polymer singlet state ( 1 P * ) to the lower energy triplet state ( 3 P * ).…”

“…8). 20 The relative instability of PTB7 is due to the generation of singlet oxygen from triplet excitons, as described above. However, DPP-TT-T is actually unstable when in a device, suggesting that photobleaching may not be a direct indicator of the level of photo-oxidation.…”

“…18 The organic semiconducting materials can also undergo photo-oxidation under ambient conditions. 19,20 There are two pathways for the photodegradation that studies focus on: the generation of singlet oxygen (high energy quantum state of oxygen) or superoxide radical anions (O − 2 ) (a negative ion with an unpaired valence, or outer orbital, electron which makes it highly chemically reactive), both of which change the electrical and optical properties of the active layer materials, creating charge traps and will be described further in the next section. 19 This review will focus on the photochemical stability of the active layer materials but first will be a short summary of other degradation mechanisms within organic solar cells.…”