Studies On The Structures Of Metal/Polymer Interfaces And Their Correlation With The Performance Of Polymer Solar Cells

Polymer solar cells (PSCs) have attracted considerable attention, because PSCs offer the potential to become a large-scale, flexible, low-cost renewable energy source. In recent years, the development of PSCs is very promising. However, the power conversion efficiency (PCE) is only around10%for lab-scale devices and the lifetime is not sufficient compared to inorganic solar cells. In these devices, metal electrodes are generally utilized to collect charge from the active layer. The vapor deposition of metal atoms onto the active layer can not only lead to diffusion of metal atoms into the polymer film but also result in interfacial interaction with the polymer. It is worth noting that the interfaces between metals and polymers can strongly affect charge separation, transport, collection and recombination, ultimately influencing the performance and stability of PSCs. Therefore, both a thorough understanding of the nature of the metal/polymer interfaces and the precise control of micro structure and interaction of metal/polymer interfaces can provide important guidance for device fabrication.This thesis thus focuses on studying the strcture of metal/polymer interfaces and the correlation between interfacial structures and the performance of PSCs by using various surface-sensitive analytical techniques. The interfacial interaction and diffusion between metals and polymers were investigated in detail by photoemission spectroscopy (PES) including x-ray photoelectron spectroscopy (XPS)、ultraviolet photoelectron spectroscopy (UPS) as well as synchrotron radiation photoemission spectroscopy (SRPES), near-edge x-ray absorption fine structure (NEXAFS) and adsorption microcalorimetry in combination with sticking probability measurements. The interaction and diffusion of metal/polymer interface are tuned by low-temperature deposition. The charge carrier lifetimes and density were measured using transient photovoltage (TPV) and charge extraction (CE).All these studies help us to better understand the relationship between the interfaces and the device performance and will eventually help to improve the performance of PSCs. The following results have been achieved in this dissertation:(1) The interface between Ca and poly(methyl methacrylate)(PMMA) was studied as a prototype system to investigate chemical reactions occurring at room temperature by XPS and NEXAFS. The changes of the O1s, C1s, and Ca2p core level XP spectra upon Ca deposition directly indicate that Ca atoms preferentially react with the ester groups of PMMA at very low coverages, leading to the formation of polymeric Ca carboxylate and loss of methyl groups. The O K-edge NEXAFS spectra further confirm that the ester groups of the PMMA are the primary active sites for reaction with Ca atoms. The results of XPS and NEXAFS measurements are fully consistent with those from adsorption microcalorimetry studies reported in our previous work. Therefore, the reaction mechanism between Ca and PMMA has confirmed that Ca atoms react with the ester groups in PMMA to form polymeric Ca carboxylate in the early stages of Ca adsorption.(2) The low-temperature deposition was used to control interaction and diffusion of metal/polymer interface. The vapor deposition of Ca atoms on PMMA was studied to investigate the effects of substrate temperature on metal/polymer interfacial structure, using adsorption microcalorimetry, sticking probability measurements and XPS. By deposition of metal vapor onto the polymer substrate at90K, the subsurface reaction is suppressed and a sharp Ca(s)/PMMA interface is instead obtained, essentially eliminating the intermediate layer of polymeric Ca carboxylate that forms at300K. The thickness of this intermediate layer is estimated to be<0.3nm (0.6layers of reacted PMMA) at90K, in contrast to2.5nm (5.4layers of reacted PMMA) at300K. Thus, a cleaner and more abrupt Ca/PMMA interface is obtained by metal deposition at low substrate temperature, due to less competition from subsurface diffusion of the Ca atoms. It seems to be a general strategy for improving electrode/polymer interfaces in PSCs by the low-temperature deposition.(3) The PSCs were fabricated with different cathodes (Ca/Al and Al) as the electron-collection layers and with PCDTBT (poly[N-9"-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)]) and PC70BM ([6,6]-phenyl-C71-butyric acid methyl ester) as the active layers. The insertion of Ca between Al and active layers significantly improves the open circuit voltage (Voc), short circuit current (Jsc), fill factor (FF) so as to improve the PCE in comparison with Al as the cathode. In order to understand how the electrodes affect the device performance, the Ca/PCDTBT and Al/PCDTBT interfaces were investigated by TPV, CE and SRPEPS. The TPV and CE measurements were used to determine the charge carrier lifetime and density. Charge carrier recombination rate constant was found to be much smaller in the device with Ca/Al cathode as compared to that with Al cathode. Energy band diagrams and mterfacial chemical reactions were characterized using high-resolution SRPES. The results indicate that the Ca interlayer can induce the stronger dipole moment, which facilitates electrons collection and drives holes away at the cathode/polymer interface. The device performance was improved because of the lower recombination.