Zhao Chao, Tang Cindy G, Seah Zong-Long, Koh Qi-Mian, Chua Lay-Lay, Png Rui-Qi, Ho Peter K H
Department of Physics, National University of Singapore, Singapore, Singapore.
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, Shaanxi, People's Republic of China.
Nat Commun. 2021 Apr 14;12(1):2250. doi: 10.1038/s41467-021-22358-y.
As electrode work function rises or falls sufficiently, the organic semiconductor/electrode contact reaches Fermi-level pinning, and then, few tenths of an electron-volt later, Ohmic transition. For organic solar cells, the resultant flattening of open-circuit voltage (V) and fill factor (FF) leads to a 'plateau' that maximizes power conversion efficiency (PCE). Here, we demonstrate this plateau in fact tilts slightly upwards. Thus, further driving of the electrode work function can continue to improve V and FF, albeit slowly. The first effect arises from the coercion of Fermi level up the semiconductor density-of-states in the case of 'soft' Fermi pinning, raising cell built-in potential. The second effect arises from the contact-induced enhancement of majority-carrier mobility. We exemplify these using PBDTTPD:PCBM solar cells, where PBDTTPD is a prototypal face-stacked semiconductor, and where work function of the hole collection layer is systematically 'tuned' from onset of Fermi-level pinning, through Ohmic transition, and well into the Ohmic regime.
随着电极功函数充分上升或下降，有机半导体/电极接触达到费米能级钉扎，然后，在几个十分之一电子伏特之后，发生欧姆转变。对于有机太阳能电池，开路电压（V）和填充因子（FF）的这种结果性的扁平化导致一个使功率转换效率（PCE）最大化的“平台”。在此，我们证明这个平台实际上略微向上倾斜。因此，进一步驱动电极功函数可以继续提高V和FF，尽管提升缓慢。第一个效应源于在“软”费米钉扎情况下费米能级对半导体态密度的强制作用，提高了电池的内建电势。第二个效应源于接触诱导的多数载流子迁移率增强。我们使用PBDTTPD:PCBM太阳能电池来举例说明这些，其中PBDTTPD是一种典型的面堆叠半导体，并且空穴收集层的功函数从费米能级钉扎开始，经过欧姆转变，一直到欧姆区都被系统地“调节”。
