Mechanical & Materials Engineering, Department of


Date of this Version

Summer 7-31-2015


A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Engineering (Materials Engineering), Under the Supervision of Professor Jinsong Huang. Lincoln, Nebraska: July, 2015

Copyright (c) 2015 Zhenggo Xiao


Solution processed thin film photovoltaic devices are one of the most promising renewable energy sources. Organic solar cells have been intensively studied due to their advantages of light-weight, flexibility and low-cost materials and manufacturing. The organic-inorganic hybrid perovskite materials have recently shown great potential application in solar cells. The PCE increased dramatically from 3.8% in 2009 to a certified efficiency of 20.1% in 2014. In this dissertation, we focus on the efficiency enhancement for solution processed organic and organic-inorganic solar cells.

In Chapter 2, I demonstrated that the crystallinity of the ferroelectric polymer P(VDF-TrFE) at the organic active layer/ electrode interface plays a critical role in the efficiency enhancement of organic solar cells. Then, The ferroelectric P(VDF-TrFE) nanocrystals was synthesized and successfully applied in the low band gap polymers. A high efficiency of 6.8% was achieved in the PCDTBT:PCBM system. Another small polar molecule, TPACA, was also applied to increase the efficiency of organic solar cells.

In Chapter 3, I developed a universal approach of solvent fluxing to fabricate graded bulk heterojunction (BHJ) polymer:fullerene films to increase the device efficiency. The solvent fluxing process can extract part of the fullerene inside the BHJ film to the top surface to form graded BHJ. The PCE of the devices after solvent fluxing is increased by 15%-50% compared with the control devices without solvent fluxing.

In Chapter 5, a two-step spin coating approach was developed to fabricate the continuous and compact organolead trihalide perovskite (OTP) films. The average PCE of methylammonium lead iodide (MAPbI3) perovskite devices reached 14.5% and 85% of the devices had efficiency above 14%. In Chapter 6, I discovered that the solvent annealing can be used to increase the grain size and crystallinity of the perovskite films. The highest device efficiency reached 15.6%, and device efficiency stayed above 14.5% when the perovskite thickness increased to 1,015 nm.

In chapter 7, a novel phenomenon, giant switchable photovoltaic, was discovered in the perovskite solar cells. The origin of the switchable photovoltaic effect was figured out: ion motion induced switchable p-i-n structure.

Adviser: Jinsong Huang,