Over the last 5 years, research on the synthesis, device engineering, and device physics of solution-processed small molecule solar cells (SMSCs) has rapidly expanded. Improvements in molecular design and emergent device processing techniques have helped solution-processed SMSCs overcome earlier difficulties in controlling active layer morphology, such that many systems are now at—or approaching—10% power conversion efficiency. In this review, details of the highest performing blend systems are presented in order to identify key trends and provide perspective on current progress in the field. Among the best systems, a planarized molecular structure is prevalent, which can be achieved using large fused-ring moieties, intermolecular non-bonding interactions, and side chain engineering. To obtain efficient devices, the highest performing systems have been optimized through the careful combination of thermal and solvent annealing procedures. Even without additional processing, some systems have been able to obtain interconnected morphologies and efficient charge generation and charge transport. Ultimately, the design of more efficient materials also requires additional understanding of the device physics and loss mechanisms. After highlighting what is known to date on processes limiting device efficiency, an outlook on the most important challenges remaining to the field is provided.

Small is Powerful: Recent Progress in Solution‐Processed Small Molecule Solar Cells

Discovering and optimizing commercially viable materials for clean energy applications typically takes more than a decade. Self-driving laboratories that iteratively design, execute, and learn from materials science experiments in a fully autonomous loop present an opportunity to accelerate this research process. We report here a modular robotic platform driven by a model-based optimization algorithm capable of autonomously optimizing the optical and electronic properties of thin-film materials by modifying the film composition and processing conditions. We demonstrate the power of this platform by using it to maximize the hole mobility of organic hole transport materials commonly used in perovskite solar cells and consumer electronics. This demonstration highlights the possibilities of using autonomous laboratories to discover organic and inorganic materials relevant to materials sciences and clean energy technologies.

Self-driving laboratory for accelerated discovery of thin-film materials.

The external quantum efficiencies of state-of-the-art colloidal quantum dot light-emitting diodes (QLEDs) are now approaching the limit set by the out-coupling efficiency. However, the brightness of these devices is constrained by the use of poorly conducting emitting layers, a consequence of the present-day reliance on long-chain organic capping ligands. Here, we report how conductive and passivating halides can be implemented in Zn chalcogenide-shelled colloidal quantum dots to enable high-brightness green QLEDs. We use a surface management reagent, thionyl chloride (SOCl2), to chlorinate the carboxylic group of oleic acid and graft the surfaces of the colloidal quantum dots with passivating chloride anions. This results in devices with an improved mobility that retain high external quantum efficiencies in the high-injection-current region and also feature a reduced turn-on voltage of 2.5 V. The treated QLEDs operate with a brightness of 460,000 cd m−2, significantly exceeding that of all previously reported solution-processed LEDs. Green light-emitting diodes with a brightness of 460,000 cd m–2 and a low turn-on voltage of 2.5 V are enabled by the use of a chlorination treatment to provide conductive passivation of the devices.

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Bright colloidal quantum dot light-emitting diodes enabled by efficient chlorination

Impedance spectroscopy has been widely applied over the last decades to study electrochemical systems and solid-state devices. However, performing impedance spectroscopy on emerging photovoltaics p...

Impedance Spectroscopy for Emerging Photovoltaics

Analysis of measured charge-carrier mobilities and fill factors in solution-processable small-molecule bulk-heterojunction solar cells reveals that in order to achieve a high FF, the hole and electron mobilities must be >10(-4) cm 2 V(-1) s(-1) . Neat-film mobility measurements are also found to be a useful predictor of the maximum blend film mobility and FF obtained in blend film solar cells.

Caroline Arantes

Papers

Towards reliable charge-mobility benchmark measurements for organic semiconductors

Efficient terbium complex based on a novel pyrazolone derivative ligand used in solution-processed OLEDs