(Invited) Probing Charge Carrier Dynamics in Porphyrin-Based Bulk Heterojunction Thin Films with Time-Resolved Terahertz Spectroscopy
Authors: Ohta Kaoru , Suzuki Mitsuharu , Yamada Hiroko , Tominaga Keisuke 
Published: July 2022

Organic photovoltaic (OPV) solar cells have attracted much attention in recent years because of the low production cost and ease of device fabrication. However, the power conversion efficiency is still low compared to the silicon and perovskite solar cells so that further research is necessary to improve their performance. In particular, understanding of the detailed mechanisms of charge carrier generation, recombination and carrier transport process is very important for developing design principles of new compounds for OPVs. Until now, polymer-based compounds have been widely used for p-type semiconductors. However, it is not easy to obtain information on how the microscopic structure of bulk heterojunction (BHJ) blend of donor and acceptor materials affect the charge carrier dynamics because one cannot easily control the molecular weight and conformational distributions of polymer compounds. On the other hand, small molecule-based semiconductors have several unique advantages such well-defined chemical structures, ease of purification and small batch-to-batch variations.

Tetrabenzoporphyrin (BP) has been frequently used as a small-molecule organic semiconductor. Recently, Yamada and coworkers synthesized diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) conjugates that have linear alkyl groups on the DPP moieties whose chain length is defined as n, namely, Cn-DPP-BP (n=4, 6, 8, or 10). For DPP-BP based solar cells, they demonstrated that alkyl chain length affects the power conversion efficiency. Therefore, it is very interesting to see how the local BHJ structures of DPP-BP based thin films affect the initial step of the charge carrier dynamics because such studies are difficult to perform for polymer-based systems. We can take advantage of small molecule-based systems to study the relationship between the local structure and the charge carrier dynamics.

In this work, we used time-resolved terahertz (THz) spectroscopy to investigate the charge carrier dynamics of DPP-BP BHJ thin films blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Time-resolved THz spectroscopy is a unique method to quantify charge carrier mobility with very fast time resolution (less than picosecond). This spectroscopy reflects the charge carrier dynamics in the local spatial region because of the high-frequency nature of the THz electric field. We chose C4-DPP-BP:PC61BM and C10-DPP-BP:PC61BM thin films for the THz measurements. In the previous study, it was found that C4-DPP-BP molecule prefers to take a face-on geometry on the substrate. A grain size of the aggregates was around 100 nm or smaller which were measured by atomic force microscopy. In such a structure, one can expect that charge transport takes place very efficiently to the electrode. Actually, power conversion efficiency is 5.2 % which is highest among a series of DPP-BP chromophores. On the other hand, C10-DPP-BP adopts an edge-on geometry on the substrate and forms micrometer-sized aggregates. In this structure, molecular stacking is oriented perpendicular to the electrode so that power conversion efficiency decreases to 0.19%.

In contrast to the other measurements, the THz spectroscopy has sensitivity to in-plane motion of the charge carriers with respect to the substrate, we expect to observe higher in-plane mobility of the charge carriers in the C10-DPP-BP:PC61BM BHJ films compared to C4-DPP-BP based films. In contrast to our intuitive expectation, we found that the product of the carrier mobility and the yield of the photo-generated charge carriers is similar to each other. We consider that the similarity of the charge carrier dynamics results from the high-frequency nature of the local mobility of the polarons because most of the polarons are confined in less than a 10-nm length scale. Furthermore, the lowest limit of the carrier mobility obtained from our results is about 1.0 cm2V-1s-1 when the quantum yield of the charge carrier generation is assumed to be unity. This value is much higher than those obtained by space-charge-limited current method. We expect that charge separation occurs very efficiently when the local mobility of the charge carrier is high.

In this contribution, we will present importance of the local mobility of the charge carriers and difference of charge transport between short and long length scales in DPP-BP based solar cells.