​Hanlin Hu gave an excellent presentation and defended his PhD thesis yesterday.

Congratulations to Dr. Hu. Everyone at KSC wishes him all the best for the future.

Aggregation of Conjugated Polymers in the Solution State and Its Influence on Carrier Transport and Solar Cell Performance

Hanlin Hu

Ph.D. Candidate supervised by Prof. Aram Amassian

ABSTRACT: Photovoltaic technology based on solution-processable organic solar cells (OSCs) provides a promising route towards a low-cost strategy to address the sharply increasing energy demands worldwide. With the contribution from novel materials development and device engineering, the power conversion efficiency (PCE) of single junction bulk heterojunction (BHJ) OSCs has been continuously increased in the past decade and has recently surpassed 12%, showing a bright future for the application of OSCs in portable and/or wearable consumer goods, as well as building or automotive integration. However, up to date, the vast majority of solar cell reports have been based on spin-cast BHJ layers. Spin coating is not compatible with high speed and scalable coating processes, such as blade-coating and slot-die coating, which require the nanoscale morphology to be reproduced in scalable coating methods. Another important challenge is the vast majority of optimal BHJ layers are too thin (80-120 nm) to absorb all the sunlight above the bandgap of the absorber. This has been widely ascribed to low carrier mobility in one or both components of the BHJ, a challenge which must be overcome. Tolerance for thicker BHJ films would also facilitate high speed scalable coating.

This thesis investigates the above-mentioned aspects of transport, processing and scalable coating. We begin by investigating transport in the archetypal conjugated polymer poly-3(hexylthiophene) (P3HT) and how it is link to ink pre-processing to control the aggregation behavior of the polymer. We then move on to scalable coating of modern BHJ layers with the aim of matching the performance of spin-cast BHJ OSCs. Finally, we investigate scalable coating of BHJs which can outperform spin-cast BHJs in the thick solar cell regime. 

In the first part of this thesis, we investigate how pre-aggregating the conjugated polymer in solution impacts the charge transport in polymer films. We use P3HT in a wide range of molecular weights in different solvents of common use in organic electronics to investigate how they impact the aggregation behavior in the ink and in the solid state. By deliberately disentangling polymer chains via sonication of the solution in the presence of solvophobic driving forces, we show a remarkable ability to tune aggregation, which directly impacts charge transport, as measured in the context of field effect transistors. ​

The second part of this thesis looks at the impact of the solution-coating method and the photovoltaic performance gap when applying modern BHJ inks developed for spin coating to scalable coating methods, namely blade coating. We do so for an important class of modern polymers exhibiting no long range crystalline order. We find performance differences even when thickness parity is achieved from identical formulations, indicating that other process differences play a role in BHJ formation. We ascribe this to significant differences in the drying kinetics between the processes. We propose an approach which benchmarks the film drying kinetics measured in situ against those of the champion laboratory devices prepared by spin-coating, by adjusting the process temperature. When the solution concentration needed to be changed, we found it crucial to maintain the additive-to-solute volume ratio as in the optimized ink formulation. Emulating the drying kinetics of spin-coating was found to result in performance parity as well as morphological parity across several systems, resulting in demonstration of PTB7:PC71BM solar cells with efficiency of 9% and 6.5% PCEs on glass and flexible PET substrates, respectively. ​

The last part of this thesis looks into going beyond performance parity by leveraging the differences of the scalable coating method to enable highly efficient thick solar cells which surpass the performance of spin-cast devices. High-speed wire-bar coating (up to 0.25 m/s) was used to produce OPV devices with power conversion efficiency (PCE) >10% and significantly outperforming devices prepared by spin-coating the BHJ layer for thicknesses >100 nm by maintaining a higher fill factor. This enhancement is due to enhanced electron mobility in the BHJ. Morphological studies reveal improved fullerene aggregation which we link to the significantly slower solvent evaporation rate in wire-bar coating as compared to spin coating.