Nanoscale Phase Separation in P3HT:PCBM Solar Cells Studied by Ultrasonic Force Microscopy
Cheng-En Cheng1,2*, Franco Dinelli3, Chen-Te Yu2, Hwa-Wei Shih2, Zingway Pei4, Chen-Shiung Chang1, Forest Shih-Sen Chien2
1Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan
2Department of Applied Physics, Tunghai University, Taichung, Taiwan
3Consiglio Nazionale delle Ricerche, Istituto Nazionale di Ottica, Pisa, Italy
4Graduate Institute of Optoelectronic Engineering, Department of Electrical Engineering, National Chung Hsin University, Taichung, Taiwan
* presenting author:鄭承恩, email:s922143@gmail.com
Understanding the nanoscale morphology in the polymer active layer of bulk-heterojunction solar cells is an important issue in order to achieve efficient charge separation of electron-hole pairs. In this study, the thermal-induced nanoscale lateral and vertical phase separation between donor (P3HT) and acceptor (PCBM) in polymer solar cells (PSCs) is studied by ultrasonic force microscopy (UFM). UFM shows the regions with low/high stiffness values, corresponding to the P3HT/PCBM phases. The fiber-like P3HT configuration is found after solvent annealing, indicating the self-organization of P3HT. It results a noticeable power conversion efficiency (PCE) of PSC. Moreover, the coverage of stiffer region (PCBM) increases as the annealing temperature increase, implying the PCBM is diffused out to the surface of active layers. It evidences that the 3D nanoscale phase separation of P3HT and PCBM is achieved during post-thermal annealing. Accordingly, the PCE of PSC is improved. Based on this approach, the nanoscale phase separation of polymer films can be easily probed, and a PSC with an optimal PCE can be also obtained.


Keywords: polymer solar cells, phase separation, ultrasonic force microscopy