2021 Adv. Energy Mater. (IF 29.368): How Mathematical Equations Predict the Molecular Weights Required for Photovoltaic Polymers
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Advance Energy Materials (IF 29.368) published a study by Changduk Yang and Oh-Hoon Kwon of Ulsan National Institute of Science and Technology (UNIST) and others in October 2021. Bulk-heterojunction (BHJ) polymer solar cells (PSCs) with p-type conjugated polymers as donors and n-type organic semiconductors as acceptors. Because of their simple device structure, lightweight, flexibility, and low manufacturing cost using printing technologies, they have aroused wide interest. The rapid development of BHJ PSC community has made a breakthrough of more than 18% in power conversion efficiency (PCE). However, batch-to-batch differences in molecular weight of a conjugated polymer still exists in one of the active layer components within a BHJ active layer, resulting in inconsistency in macromolecular ordering, optoelectronic, charge transport characteristics and ultimately device performance. In order to obtain a method for polymer photovoltaic materials with controllable/predictable molecular weights, efficient mathematical equations were developed to combine as-synthesized ones with different molecular weights to precisely reproduce the desired molecular weight polymer batches.
Schematic illustration of the physical mixing method.
Enlitech’s QE-R Quantum Efficiency Measurement system, Fourier Transform Photocurrent System (FTPS), REPS photovoltaic Voc-loss analyzer and other instruments were used in this study to assist in the measurement. The authors investigated the properties of different molecular weight PM6 polymer series prepared using as-synthesized and mixed methods to determine the effect of molecular weight on relevant the performance of PSCs. It was found that the PCEs of the devices increased steadily with Mw reached 120 kDa, but decreased significantly in devices with Mw above 120 kDa. This trend in PCE correlates with charge-separation efficiency as a function of the balance between the counteracting domain size and packing in the blends. Furthermore, reducing the polydispersity index in the optimal molecular weight PM6 sample enhanced the PCE to about 16.5%. This simple mixing methodology should help overcome the semiconducting polymer’s inevitable batch-to-batch difficulty, thereby accelerating the commercial viability of PSCs.
a) FTPS-EQE spectrum of the devices based on S- and M-batches. b) EQEEL spectra of the polymer solar cells (PSCs) at various injection current densities.
Power conversion efficiencies (PCEs) of PBDB-T:ITIC (grey star symbol) and PTQ10:Y6 (red star symbol) devices as a function of Mw .