Effects of Photo‐oxidation on the Performance of Poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene]:[6,6]‐Phenyl C₆₁‐Butyric Acid Methyl Ester Solar Cells

Abstract

A study of the photo‐oxidation of films of poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV) blended with [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar‐cell performance is degraded primarily through loss in short‐circuit current density, JSC. The effect of the same photodegradation treatment on the optical‐absorption, charge‐recombination, and charge‐transport properties of the active layer is studied. It is concluded that the loss in JSC is primarily due to a reduction in charge‐carrier mobility, owing to the creation of more deep traps in the polymer during photo‐oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo‐bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.

The Critical Role of Processing and Morphology in Determining Degradation Rates in Polymer Solar Cells

“…The subunity value of α has been discussed previously in terms of polaron trapping in a distribution of energetic traps in polymers. [ 28 ] The deviation in values away from one indicates progressively more dispersive (trap-dependent) bimolecular charge recombination. It can be observed that degradation causes the slope of α to decrease; indicating that the energetic tail of trap states is longer after degradation.…”

The effect of oxygen induced degradation on charge carrier dynamics in P3HT:PCBM and Si-PCPDTBT:PCBM thin films and solar cells

“…A decrease of charge carrier mobility would also lead to reduced recombination, as actually observed by transient absorption measurements on degraded MEH-PPV:PCBM devices and by TPV measurements on degraded P3HT:PCBM devices, both cells being of regular architecture. 19,20 However, in the case of bimolecular recombination, the overall effect of reduced mobility on short circuit current would be detrimental by virtue of an increase of n SS . The charge extraction yield can also be reduced by an increased charge recombination rate k rec ("trap-induced recombination"); in the framework of eqn (3), trap-induced recombination would either be caused by reduced mobility m or by an increase of the non-Langevin factor f LV .…”

Carrier traps as transport states in bulk-heterojunction P3HT:PCBM structures for solar photovoltaics

“…The evidence of charge trapping effects in the bulk-heterojunction solar cells fabricated from the blends of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene (MDMO-PPV) and methanofullerene [6,6]-phenyl C 61 -butyric acid methyl ester ([60] PCBM) was demonstrated in [3], depending on the applied bias. Carrier trapping was evidenced not only in material blends and organic devices on their basis [2][3][4][5], but in single layer samples as well (e. g., [6]), proving that the carrier trapping is an internal feature of organic materials. This results in an imbalance of different carrier flows, and therefore, to eliminate the problem, specific technological solutions have to be found.…”

“…Charge carrier mobility is one of the main factors conditioning the carrier transport in organic materials which are naturally highly disordered [1,2]. On the other hand, in organic materials the mobility itself (at least of one type of carriers) is often limited by a severe carrier trapping.…”