High‐Throughput Multiparametric Screening of Solution Processed Bulk Heterojunction Solar Cells

Abstract

(MDMO-PPV) or poly(3-hexylthiophene) (P3HT) combined with soluble fullerenes, [6,7] up to current PCE values exceeding 10% for several systems. [9][10][11][12][13] There is no specific fundamental limitation to organic materials that indicates that much higher values are not possible, and the number of potential candidates is nearly infinite. Indeed, the organic nature of the photoactive materials offers a myriad of possibilities to modify their chemical structure; for the case of conjugated polymers, there exists a vast assortment of combinations depending on the choice of moieties, the bridging atoms, the length and branching points of the alkyl side chains, and the molecular weight, to name but a few. Using search engines to inspect the literature, we estimate that in the last ten years ≈5000 organic conjugated materials have been tested in BHJ solar cells, albeit only a few tenths have been studied and optimized in depth. While applied quantum theory can help to select promising candidates, [14][15][16] the final performance often depends on a number of issues difficult to predict a priori, such as solubility, miscibility of compounds, tendency to crystallize, exact energy levels in the blend, etc. In practice, this means that for a given promising backbone, a family of systems need to be tested, including different side chains, molecular weights, donor:acceptor combinations, etc. [17] Within this large and uncharted spectrum of materials and processing variables, combinatorial screening methodologies are highly on demand to speed up their exploration while helping the technology to approach the theoretical Shockley-Queisser limit of >20% PCE. [18,19] From the engineering point of view, three main aspects must be addressed for the evaluation of a material system for BHJ solar cells, namely the active layer thickness, the donor-acceptor (D:A) blending ratio and the nanoscale morphology (typically controlled by deposition conditions, thermal annealing, and use of additives). Ideally, those three variables can be optimized separately and to some extent it is usually an acceptable approximation; however, the full potential of a novel active layer material requires for fine tuning of the preparation conditions that take into consideration the subtle interplay between them. [20] For instance, the optimum thickness is often found close to the first interference maximum, which is governed by the refractive index of the active layer; this, on the other hand, will be a function of the D:A ratio (fullerenes typically have higher refractive index than polymers) and the degree of crystallinity One of the major bottlenecks in the development of organic photovoltaics is the time needed to evaluate each material system. This time ranges from weeks to months if different variables such as blend composition, thickness, annealing, and additives are to be explored. In this study, the use of lateral gradients is proposed in order to evaluate the photovoltaic potential of a material system up to 50 times faster. A platform that c...

Blade coated P3HT:non-fullerene acceptor solar cells: a high-throughput parameter study with a focus on up-scalability

“…In order to rapidly screen the processing conditions, we have employed samples with gradients in two parameters of interest, thickness and thermal annealing temperature. 35 For the rst one, a home-made speed controller was coupled to the blade coater. Photoactive layers were deposited by decelerating the blade from 90 mm s À1 to 5 mm s À1 during solution spreading across the 7 cm length of the substrate.…”

“…This approach enables rapid evaluation of the thickness dependence of each combination. 35,36 A second type of gradedsample consists of applying a post-deposition thermal gradient annealing on samples exhibiting a homogeneous thickness. This is carried out by using a Koer bench, which is a hot plate with a position-dependent temperature.…”

“…This high-throughput approach enables saving both time and resources when evaluating an OPV system. 35 In this approach, the blade is gradually decelerated during coating resulting in a sample with a thickness gradient. 35 In order to ensure the reproducibility of this approach, we prepared three P3HT:O-IDTBR solar cells under the same processing conditions which yielded comparable PCE results and thickness trends for the thickness sweep explored (from 50 nm to 180 nm; see Fig.…”

Beyond Ternary OPV: High‐Throughput Experimentation and Self‐Driving Laboratories Optimize Multicomponent Systems

“…One of the main challenges of combinatorial research in OPV is the scalable fabrication of high‐quality individual films. Recent approaches are based on gradient coating or ink‐jet printing, which are usually limited by the number of mixable components, the number of samples, or the need for specific ink properties 10–12. An HTE coating method for organic semiconductors with a scalable number of mixable components and scalable number of films is not yet established.…”

Microfluidic‐Assisted Blade Coating of Compositional Libraries for Combinatorial Applications: The Case of Organic Photovoltaics

“…feedback between theoretical screening studies [8][9][10][11] and highthroughput experimentation [12][13][14] has accelerated the discovery of new high-performing organic conjugated materials. In the case of organic photovoltaics (OPV), nonfullerene acceptors have recently emerged as promising substitutes for traditional fullerenes as n-type semiconductors.…”