Photostability of organic materials used in polymer solar cells

Abstract

To make polymer solar cells (PSCs) a competitive market technology, integrated efforts are required toward the development of highly efficient light harvesting and charge transporting materials with good thermal and photochemical stability, and which can be processed from solution. Nowadays, a critical issue to be solved is enhancing the stability and durability of PSCs. Indeed, the photoactive material used in the active layer dictates the efficiency of the device on the one hand but, on the other hand, it is well known that organic materials are unstable when exposed to light irradiation, which provokes a degradation of their properties. Making long lifetime solar cells with polymers that are susceptible to degradation under light exposure could be an unrealistic challenge. Therefore, elucidating the mechanism of polymer photodegradation is a key point for developing strategies to decrease or prevent the loss of the functional properties of the material. In this paper, the basic concepts of polymer photo-aging are explained first. Then the photodegradation mechanisms of conjugated polymers currently used in PSCs are reported. Finally, as barrier materials able to cut off moisture and oxygen ingress are essential for the stability of PSCs, methods for designing coatings for PSC encapsulation are presented, based on recent publications.

Molecular Packing and Arrangement Govern the Photo-Oxidative Stability of Organic Photovoltaic Materials

“…In an effort to accelerate material ageing, researchers illuminate thin films of OSC materials in air. 18 The degradation of the OSC material is measured by tracking a decrease in the film absorption over time, which usually occurs on the span of hours. The loss of absorption is attributed to a loss of extended π-conjugation in the OSC material through photochemical reactions with oxygen, which generally occur through a free-radical mechanism.…”

“…The role of the side chains in photo-degradation, particularly in photo-oxidation, is well established. 18,29,30 For several polymers investigated in detail, 22,31,32 one of the key steps in a photo-degradation reaction pathway is abstraction of a hydrogen atom on the alpha carbon of the solubilizing side chain, and polymers with thermocleavable side chains are observed to be more stable to photobleaching. 33 Thus, both the molecular structure of the backbone and the solubilizing side chain can have a strong impact on observed stability.…”

Suppressing photooxidation of conjugated polymers and their blends with fullerenes through nickel chelates

“…[1][2][3][4][5] This translates into the need for careful encapsulation strategies, compromising both applicability and cost benefits. 6 Conceptually new strategies are necessary to suppress degradation reactions.…”

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

“…6 Finally, lightinduced degradation is highly important, since solar cells operate under relatively high photon uxes. Recent research has proposed both general mechanisms for light-induced degradation in all polymers 16,17 and more specic reactions 18,19 in various materials, including crosslinking 20 and fullerene dimerization. 21 A conclusive explanation of the mechanisms behind light-induced degradation, however, is still missing.…”