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Accelerated testing

Morten Vesterager Madsen infinityPV solar concentrator - ISOSun100

Doing accelerated testing of polymer solar cells have become an almost near common practice. The main acceleration parameter used for accelerated studies is temperature, however, many other parameters affect the lifetime of the solar cell. Therefore, a wide variety of accelerated test conditions can be imagined. Time compression is one in which the solar cell is tested by rapidly cycling test parameters. Hereby daily cycles of environmental parameters are compressed to within a few hours. Compressed cycles generate mechanical stress of thermal origin due to the fast change in temperature.

High temperature

Using temperature as an acceleration parameter is attractive since the temperature is easily controlled and easily reported. The rationale behind using the temperature as an acceleration parameter is that the decay process, which may be chemical in nature follows an Arrhenius-type model. Assuming this model the temperature dependence of the reaction can be described by an exponential function $$k_{deg} = A e^{\frac{E_A}{RT}}$$ where $R$ is the gas constant, $E_A$ is the activation energy, $T$ is the temperature, and $A$ is the reaction dependent pre-factor. Alternatively the rate constant can be expressed in terms of the Boltzmann constant. Hereby the energy will be expressed directly instead of energy per mol as $k_B=R/N_A$. From the exponential behavior the reaction rate is clearly extremely temperature dependent. An acceleration factor can be defined as the ratio of two reaction rates at different temperature $$K = \frac{A e^{\frac{E_A}{RT'}}} {A e^{\frac{E_A}{RT}}} = e^{\frac{E_A}{R}\left( \frac{1}{T} - \frac{1}{T'} \right)}$$ The prefactor is the same for both equations and is therefore eliminated in the equation. Accelerated testing was applied to MDMO-PPV/PCBM solar cells by Schuller et al. determining the acceleration factor in the temperature range of 40–105 °C.DOI:10.1007/s00339-003-2499-4 They observed a roughly linear behavior of $\log(K)$ versus $1/T$ with a more than ten-fold increase in the rate of degradation from 40 to 105 °C. They concluded that the activation energy was in the range of 300–350 meV. Using temperature as an acceleration factor seems simple with the Arrhenius formula and the model has been used to predict lifetimes of solar cells.DOI:10.1016/j.synthmet.2005.06.016 The main problem with the model is that it assumes that only the activation energy of one decay mechanism is needed to describe the entire system. As demonstrated by Gevorgyan et al. the acceleration factor may change during the lifetime of the device.DOI:10.1016/j.solmat.2008.02.008 In addition temperature independent processes can take place, further invalidating the model. UV light at 400 nm has an energy of 3 eV, which is a high energy compared even with the thermal energy at room temperature. Heating the sample by an additional 50–60 °C does not necessarily affect processes much. Degradation limited by diffusion depends directly on the diffusion coefficient of the chemical species responsible for the decay. In this case temperature can play an important role since this diffusion coefficient is temperature dependent according to an Arrhenius-type exponential equation. So in theory, at low temperature, the UV degradation processes may dominate, while the diffusion process could take over at higher temperatures. If this is the case, accelerated testing would give false temperature dependence for the stability. If increasing the temperature causes thermo-oxidative processes to become the main degradation pathway and altering the chemical evolution observed without acceleration, the test is invalid. Therefore it is important to understand the degradation mechanisms involved when designing acceleration conditions.

High light intensity

As an alternative to the above described acceleration methodologies the use of concentrated light is perhaps the conceptually simplest type of accelerated studies. Sunlight concentration setups within the field of inorganic photovoltaics, has been developed for high performance solar cells with increased power output as the main goal. The goal is to effectively increase the active area of high price multijunction solar cells by relatively cheap concentrator systems. Within the field of polymer solar cells concentrated light has been scarcely used to study degradation. It is, however clear that with the multitude of degradation mechanisms that are accelerated by concentrated light, the polymer solar cell response is complex, and even effects such as reversible degradation have been observed.DOI:10.1088/0957-4484/22/22/225401 Conventional stability assessments of organic solar cells are performed by studying the decrease of power conversion efficiency during the degradation time. However, a multitude of parameters including the polymer, the electron and hole transport layers, the electrodes, and the interfaces influence the device performance. As a result the interplay between many different parameters is probed making single parameters such as the stability of the polymer itself rather inconclusive. To focus on the actual stability of the polymer, degradations under 1 sun illumination of the polymer themselves have demonstrated the intrinsic stability of polymers directly. The exact same approach have been used to expose pure polymers to concentrated light. Tromholt et al. performed such accelerated degradations of conjugated polymers up to 200 suns.DOI:10.1063/1.3298742 For both MEH-PPV and P3HT the acceleration factors were found to increase linearly with solar intensity and at 200 suns a complete degradation of MEH-PPV took place within 80 seconds. This study demonstrated that degradation of polymers can be highly accelerated by concentrated light, and that the approach has the potential to serve as a standard tool for rapid polymer stability evaluation. As the field of concentrated light is still new, a rigorous analysis of degradation rates observed at 1 sun and concentrated light has been needed.



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