The Effect of PCBM Dimerization on the Performance of Bulk Heterojunction Solar Cells

Abstract

Increasing the lifetime of polymer based organic solar cells is still a major challenge. Here, the photostability of bulk heterojunction solar cells based on the polymer poly[4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt -[2,5-bis(3-tetradecylthiophen-2-yl)thiazole[5,4-d]thiazole)-1,8-diyl] (PDTSTzTz) and the fullerene [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 60 BM) under inert atmosphere is investigated. Correlation of electrical measurements on complete devices and UV-vis absorption measurements as well as highperformance liquid chromatography (HPLC) analysis on the active materials reveals that photodimerization of PC 60 BM is responsible for the observed degradation. Simulation of the electrical device parameters shows that this dimerization results in a signifi cant reduction of the charge carrier mobility. Both the dimerization and the associated device performance loss turn out to be reversible upon annealing. BisPC 60 BM, the bis-substituted analog of PC 60 BM, is shown to be resistant towards light exposure, which in turn enables the manufacture of photostable PDTSTzTz:bisPC 60 BM solar cells.

Increased thermal stabilization of polymer photovoltaic cells with oligomeric PCBM

“…Fullerenes tend to diffuse through layers to form large mm-scale crystals or to accumulate at active layer-electrode interfaces, 12 both occurring notably when heated above the glass transition temperature and potentially leading to reductions in device efficiencies. 11,13,14 While photo-induced reactions between PCBM molecules may lead to morphological stability, 15,16 they can also give rise to dimers that act as charge traps and reduce short circuit current ( J sc ) values. 17 PCBM is also difficult to cast as a thin film, resulting in rough surfaces of large aggregates.…”

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

“…Another degradation mechanism we exclude in this work is fast initial short-circuit current loss in KP115:PC 60 BM solar cells 9 that was proposed to be caused by light-induced fullerene dimerization. 21 By using PC 70 BM in our KP115 solar cells, we inhibit this loss mechanism because C 70 cages have a lower susceptibility to dimerization.…”

“…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.…”

Role of Stress Factors on the Adhesion of Interfaces in R2R Fabricated Organic Photovoltaics

“…At the same time, it was recently discovered that UV light also results in dimerization of PCBM molecules. [ 20 ] The two effects may eventually lead to more amorphous structures with reduced cluster dimensions. Further experiments will however be required for better understanding the nature of the observed phenomenon.…”