Efficient Small-Molecular-Weight Organic Solar Cells through Orientation Control and Nano-structuring

Abstract

nano-structuring and orientation control of active materials. Small molecular weight OSCs have advantages of easy purification, good reproducibility, easy fabrication of multi-layered structure and so on over polymer based solar cells. One drawback of small molecular weight OSCs is located on the difficulties to form interpenetrating network between donor and acceptor molecules in nanometer scale because of the large entropy of mixing in the amorphous structure. A new method to form small-molecular based bulk heterojunction (BHJ) through alternating thermal deposition (ATD) is proposed, which is a simple modification of conventional thermal evaporation. The formation of a BHJ in copper(II) phthalocyanine (CuPc) and fullerene (C60) systems is confirmed by atomic microscopy (AFM), grazing incidence X-ray small angle scattering (GISAXS), and absorption measurements. From the analysis of the data, CuPc

Due to the fast growing energy consumption and the growing environmental concerns over the climate change risks such as global warming, use of solar energy is one of the promising candidates for renewable sources of electricity. Among various solar cells, organic solar cells (OSCs) are considered to have large potential due to the fact that organic materials are cheap and easy to form a film with inexpensive process even in the large area. Unfortunately, OSCs have relatively low power conversion efficiency compared to other kinds of solar cells. Therefore the most important issue in OSCs is how to improve the power conversion efficiency (PCE). This thesis reports a couple of methods to improve the PCE of small molecular weight OSCs

C60 films fabricated by ATD are composed of nanometer sized disk-shape CuPc nano grains and aggregated C60, which explains the phase separation of CuPc

C60. Compared with co-deposited OSCs, the ATD OSCs show significant enhanced performance. However ZnPc, which has the same crystalline structure with CuPc, did not show the improvement by ATD due to the initial growth difference. To understand the mechanism of ATD, the initial growth of CuPc on different substrate condition is monitored using GISAXS. Disk-type nano grains of CuPc were observed in an ultrathin CuPc layer evaporated on a hydrophilic Si surface. The disk type grains consisted of a crystalline part and a non- crystalline part. The disk type grains were smaller in the case of CuPc on hydrophobic Si surface, which showed lower crystallinity with random distribution. Despite regularly distributed CuPc grains the mobility was lower in a thin film transistor device fabricated on a hydrophilic surface than on a hydrophobic surface due to the lower average density of the molecules relating to porous molecular packing between nanograins on a hydrophilic surface. Improvement of the performance of OSCs was also achieved by controlling the molecular orientation. A highly efficient planar heterojunction OSC based on zinc phthalocyanine (ZnPc)/C60 is obtained by controlling the orientation of the ZnPc using copper iodide (CuI) as the interfacial layer. The proportion of face-on ZnPc molecules was increased significantly on the CuI layer compared to the layer without the CuI layer analyzed with wide angle X-ray scattering (WAXS) and optical absorption. The PCE of the orientation controlled planar heterojunction OSC was remarkably enhanced to 3.2% compared with 1.2% without the control of the molecular orientation. The mechanism of formation of the face-on molecular orientation of ZnPc on CuI layer is proposed. Using the two different incidence angles in the glazing incidence wide angle x-ray scattering (GIWAXS), it turns out that the ZnPc layer on the (111) gamma phase CuI is the beta phase ZnPc with (-101) orientation which is transformed to (313) gamma phase ZnPc as the film growth. These phase transitions can be explained with quasi epitaxial growth.