Modeling the temperature induced degradation kinetics of the short circuit current in organic bulk heterojunction solar cells

Abstract

In organic bulk heterojunction solar cells, the nanoscale morphology of interpenetrating donor-acceptor materials and the resulting photovoltaic parameters alter as a consequence of prolonged operation at temperatures above the glass transition temperature. Thermal annealing induces clustering of the acceptor material and a corresponding decrease in the short circuit current. A model based on the kinetics of Ostwald ripening is proposed to describe the thermally accelerated degradation of the short circuit current of solar cells with poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) as donor and (6,6)-phenyl C61-butyric acid methyl ester (PCBM) as acceptor. The activation energy for the degradation is determined by an Arrhenius model, allowing to perform shelf life prediction.

An Effective Area Approach to Model Lateral Degradation in Organic Solar Cells

“…The resulting decay curves of the short-circuit current for four samples with different hole transport materials are shown in Figure 1 . It is obvious that the degradation is accelerated by higher humidity as well as enhanced temperature as already studied by Conings et al [ 22 ] More importantly, the replacement of PEDOT:PSS in these devices by solution-based metal oxides signifi cantly increases the stability. The decay curves can be fi tted nicely with a square root power law…”

Stability of Polymer Solar Cells

“…The activation energy for the phase segregation of MDMO-PPV and PCBM blends have been modeled from the short circuit current decay of devices based on this active material and found to be rather low (0.85 eV) corresponding to the rapid degradation observed. [116] Ray and Alam developed a mathematical model for the short circuit current degradation as a function of how the domain sizes evolve with time at different temperatures. [117] The influence of different processing parameters on the morphological stability have been studied by Kumar et al who found that P3HT:PCBM films spin-coated with added 1,8-octanedithiol degraded much faster than films made from other solvents.…”

Predicting thermal stability of organic solar cells through an easy and fast capacitance measurement

“…The test runs for 20 hours and the solar cells were illuminated to monitor the OPV performance for a total period shorter than 30 minutes, allowing to neglect the initial burn-in light induced degradation [20]. The following degradation effects on the photovoltaic parameters are expected: i) a reduction of the short circuit current density (J sc ) due to morphological changes of the BHJ blend [27,35,36] that can affect the charge mobility and the generation of a charge transfer complex; 38 ii) a variation of the open circuit voltage (V oc ) as a consequence of a different D:A phase segregation [56] and increase of defect states which controls the recombination [37]; iii) those phenomena can affect also the fill factor (FF), by a limitation of the charge transport and increase in the series resistance [38]. Fig 2. Fitting of the PCE data (Fig 2a) was carried out using an exponential decay: PCE = PCE f + (1-PCE f ) exp (-t/τ), where t represents the experiment time, PCE f is the final value attained by the efficiency and accounts for the percentage of the initial efficiency preserved after thermal degradation, while the time constant τ is the lifetime at which the efficiency value is decreased by a factor e, and allows to compare the kinetic of the PCE decay of each samples.…”

“…These effects can be discarded if a device is properly encapsulated. However, degradation pathways due to light soaking and high temperature cannot be eliminated, and, in general, they induce morphology evolution of the active layer [27], interlayer and electrode diffusion [28], and electrode interaction with the organic materials [29]. The behavior of BHJ OPVs with thermal degradation is generally correlated to morphological changes occurring in the active layer that can affect: i) charge separation process by formation of fullerene aggregates in polymer:fullerene blends, which leads to a PCE loss due to the reduction of the donor:acceptor (D:A) interfacial area [30][31][32], ii) charge extraction by a migration of a skinlayer of either polymer [33] or fullerene [34] adhering to the top contact, generating barriers or selective transport regions as a function of the device architecture; iii) transport properties by modification of the polymer packing in the blend [35,36], iv) recombination by an increase of the number of defect states at the bulk of the active layer [37], v) optical properties by generation of a charge transfer complex between donor and acceptor molecules which also acts as an exciton quencher [38].…”