Dynamic PCBM:Dimer Population in Solar Cells under Light and Temperature Fluctuations

Abstract

polymers and device architectures are steadily increasing the performance of polymer:fullerene OSCs. [3][4][5] Evidently, the competitiveness of this technology is predicated on the achievement of longterm stability, required for practical utilization. The loss of PCE over time can be attributed to numerous stress factors. Environmental elements such as oxygen and humidity exposure can adversely affect performance, as well as mechanical failure, and robust encapsulation, mechanical, and interfacial design are required to mitigate their effect. Intrinsic factors such as thermal stress, illumination, and interlayer stability are, however, always present during operation. [6][7][8] OSCs often experience a pronounced performance degradation during the initial stages of operation under illumination, which is termed "burn-in," and attributed to several loss mechanisms currently under investigation. [9][10][11] One key mechanism has been related to be the photochemical dimerization of ubiquitous electron transporting material, phenyl-C61-butyric acid methyl ester (PCBM), and ensuing reduced charge mobility and the degradation in several polymer:fullerene systems. [12,13] However, other studies have found that photodimerization improves morphological stability and, in turn, device stability under thermal stress, [14][15][16] while yet others have suggested neutral impact. [17] Recently, we reported a neutron reflectivity study on the competitive effects of light and temperature on the morphological stability of a polymer:fullerene blend. [18] These results found PCBM dimers to be effectively immobile compared to PCBM monomers in the blend matrix. The impact of PCBM dimerization on OSC performance appears thus nontrivial, and quantifying the PCBM dimer population throughout the processing steps, at both short-and long-term operation conditions, is important to predict and improve OSC stability.The photochemical dimerization (and polymerization) of C 60 fullerenes was reported two decades ago by Eklund and co-workers, [19] who found crosslinking of up to 20 molecules upon photoirradiation of neat fullerene films. The reaction was found to be quenched by the presence of oxygen. Since oxygen is known to quench triplets in fullerenes to form singlet oxygen, dimerization was thus concluded to proceed via a triplet state. Using Raman spectroscopy, the reaction rate was found to be linear with irradiance (light intensity). [20] From these two observations it was suggested the fullerene 2 + 2 cycloaddition reaction proceeds via a triplet-ground state mechanism. The Photoinduced dimerization of phenyl-C61-butyric acid methyl ester (PCBM) has a significant impact on the stability of polymer:PCBM organic solar cells (OSCs). This reaction is reversible, as dimers can be thermally decomposed at sufficiently elevated temperatures and both photodimerization and decomposition are temperature dependent. In operando conditions of OSCs evidently involve exposure to both light and heat, following periodic diurnal and seasonal profiles. In th...

From fullerene acceptors to non-fullerene acceptors: prospects and challenges in the stability of organic solar cells

“…5(d). 8,79,99 While fullerene dimerisation generally dominates under environmentally relevant operating conditions, significant decrease of the fullerene dimer population can occur with increasing temperature, where the morphological stabilisation effect of fullerene dimerisation can be reversed.…”

“…(c) The PC 61 BM monomer and PC 61 BM dimer populations in OSC systems with different polymer:PC 61 BM (1:2) blend systems are shown with (left) increasing radiant exposure and (right) with increasing annealing temperature for prior irradiated PCDTBT:PC 61 BM (1:2) blend films. The ''2+2'' cycloaddition dimerisation process (see schematic in (c)) is reversible, significant dimer population decrease is observed above 80 • C. 8,79 (d) A minimal model built to predict the dynamic PC 61 BM monomer:dimer populations under simultaneous and fluctuating light and thermal stress exposure associated with in operando conditions 79. Reproduced with permission [79].…”

“…The ''2+2'' cycloaddition dimerisation process (see schematic in (c)) is reversible, significant dimer population decrease is observed above 80 • C. 8,79 (d) A minimal model built to predict the dynamic PC 61 BM monomer:dimer populations under simultaneous and fluctuating light and thermal stress exposure associated with in operando conditions 79. Reproduced with permission [79]. Copyright 2019, Wiley-VCH.…”

Progress in Materials, Solution Processes, and Long‐Term Stability for Large‐Area Organic Photovoltaics

“…The kinetics of dimer formation and decomposition depend on temperature, light irradiation dose, and blend composition. [ 127 ] The activation energy for photodimerization is estimated to be 0.021 eV, which is considerably smaller than that for decomposition (0.96 eV), consistent with decomposition occurring at elevated temperatures. PCBM dimerization results in widening of the bandgap, which in turn results in increased disorder of both the HOMO and the LUMO level of PCBM.…”

The role of spin in the degradation of organic photovoltaics

“…These topo-chemical considerations explain the smaller chain lengths obtained for these fullerenes as well as the reaction's suppression at lower temperatures 12 . Heumüller et al 17 and Pont et al 18 have investigated the role of polymers in determining the dimerization rate and fraction of PC 61 BM undergoing dimerization in BHJs. They concluded that PC 61 BM crystallinity and PC 61 BM exciton quenching time dictate the reaction kinetics.…”