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Finding a needle in a haystack: efficient identification of high performing organic energy materials

Periodic Reporting for period 3 - FOREMAT (Finding a needle in a haystack: efficient identification of high performing organic energy materials)

Reporting period: 2018-10-01 to 2020-03-31

Organic materials have a huge potential as the main ingredient in cost-effective and clean energy technologies based on abundant and non-toxic materials. In particular, these compounds are being thoroughly investigated as the active layer in devices that convert light or waste heat into electricity, i.e. photovoltaic and thermoelectric technologies, respectively. Interestingly, no fundamental limitation has yet been discovered that says that organics cannot deliver devices with high efficiencies. Indeed, the library of potential candidates is literally infinite, as there is an infinite way in which carbon atoms can form conjugated systems through alternating single and double bonds. The issue is how to find such amazing materials. While theory points towards some directions, the performance of the synthetized compounds strongly depends on specific properties of the materials made such as molecular weight, sidechains, solubility or packing tendency, thus one should be talking about material families. One of the major bottlenecks is the time needed to evaluate each compound, let alone a full system family. For instance, at lab scale, the fabrication of an organic solar cell takes from days to a few hours (if processed in parallel), while measuring its efficiency takes just minutes.

In order to identify very efficient organic energy systems in a time effective manner, we propose the use of combinatorial processing in the form of samples with controlled gradients in the parameters of interest to make 100 faster the photovoltaic evaluation and optimization of a system. For this, we aim to introduce a fabrication platform that combines several solution processing methods in order to produce films exhibiting 2D gradients, which are equivalent to more than 100 conventional (homogeneous) samples. These samples are then analyzed using photocurrent and Raman imaging in order to correlate one to one the device performance and structural information (thickness, composition, doping level, nanostructure). FOREMAT will, thus, enable the identification of organic materials suitable for highly efficient photovoltaics and thermoelectric applications, thus bringing a step closer the change in energy paradigm towards fully sustainable renewable energy technologies.
During the first part of the project, we have placed special emphasis on developing the combinatorial platform for evaluation of photovoltaic materials. Specific results obtained in this respect during the first half of FOREMAT are:
1) Development of the mathematical framework to quantify composition and film thickness from Raman scattering measurements and its experimental demonstration (J. Mat. Chem. 2017). Interestingly, the applicability of our approach broadly expands beyond photovoltaics to include the composition quantification using Raman of other thin film and even bulk multicomponent systems. Thanks to this work, Raman spectroscopy passes from the very useful qualitative technique that most physico-chemial labs have in acedemia and industry, to a powerful tool that provides quantitative chemical identification and imaging.
2) Development of the first series of methods to produce samples with gradients in parameters of interest. These parameters include film thickness and composition (Adv. Elect. Mater. 2018), molecular orientation (Adv. Opt. Mater. 2017) and degree of doping (unpublished). Besides its use in the highthrouput methodology, we have also explored the use of these samples in other applications, for instance, we have shown that can be used to fabricate a minituarized spectrometer (Adv. Mater. 2018) as well as a polarization detector with no moving parts (Adv. Opt. Materi. 2017).
3) Proof of concept demonstration of the combinatorial evaluation and optimization of organic photovoltaic cells with evaporated active layer (Org. Elect. 2018) and solution processed bulk heterojunction active layer (Adv. Elect. Mater. 2018). We show that our novel methodology is between 10 and 50 times faster than conventional methods, requiring less than 10 mg of the chemical under study in order to evaluate its suitability in photovoltaics.
4) Putting all the above together, the photovoltaic efficiency at ICMAB has increased by an order of magnitude since the beginning of the project up to state-of-the-art-values (see Figure 1).

Moreover, we have made the largest study to date of the optical properties of organic photovoltaic materials evaluating more than 40 different systems. This allowed to determine design rules for the strength of absorption in polymer semiconductors. We showed that, while the band gap is controlled by conjugated length, the strength of absorption is governed by the polymers persistence length, which is related to the average curvature of the polymer chain within the film. We used theoretical tools to predict materials with enhanced absorption and then synthesized three different polymers with an absorption about 50% stronger than most state-of-the-art materials. These results were published in the prestigious scientific journal Nature Materials (2016).

We have also started the combinatorial screening of organics for thermoelectric applications focusing on composites of polymer matrices and conductive carbon nanotube fillers. In this respect, we have demonstrated relatively stable n-type composites (elusive until now, Adv. Mater. 2016), improvement of mechanical properties by ternary mixing (Adv. Scien. 2017) and photoinduced switch of the majority carriers in several composites (Adv. Mater. 2016 and Synth. Metals 2017), which will strongly facilitate and speed up the fabrication of thermoelectric legs as they can be deposited from a single solution. A summary of thermoelectric performance from FOREMATs materials can be seen in Figure 2.
"We have demonstrated a rational to predict the absorption of novel compounds and demonstrated an increase of 50% with respect to state-of-the-art in the absorption of low band gap polymers using our design rules. This means that for a given film thickness of the active layer, the material will absorb significantly more generating more charges. Together with the additional electronic improvement, the enhanced absorption has a very large impact on organic photovoltaics built with these systems, with power conversion efficiencies going from about 5% to more than 8%. Our discoveries will help to bring photovoltaic efficiencies higher than ever. This work, published in Nature Materials in 2016, has been catalogued as a ""Highly Cited Paper"" by ISI Web of Knowledge and has recieved attention from different public media.

We have successfully demonstratated a combinatorial evaluation of organic photovoltaic cells method that is 50 times faster and saves about 90% of the material compared to the conventional evaluation protocols.

On the other hand, once the combinatorial platform is complete, we will be able to screen large libraries of materials and combinations thereof, potentially discovering systems with photovoltaic efficiencies going towards 20% and organic thermoelectric figures of merit in the range between 1 and 2. As these technologies are based on abundant, non-toxic materials processed using very little amounts of energy, they are amongst the most sustainable material candidates to employ in one of the most important societal challenges: the need to swift the energy paradigm towards ever cleaner energy sources and harvesting technologies.
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High throughput screening of organic thermoelectrics
High throughput screening of organic photovoltaics