Synthesis And Photovoltaic Properties Of Electron Donors Based On Acceptor-Acceptor Conjugated Materials

In the past decades,organic solar cells(OSCs)have attracted great attention because of its potential advantages of low-cost fabrication,promising flexibility,and capability for large-dvices.Significant progress has been achieved due to the development of novel photovoltaic materials and device structures,but the practical application is still a long way to go.As the photocurrent generation related to the conversion efficiency of the OSCs is the result of the absorption of light and charge generation of the active layer,materials of active layers have been one of the key factors of the OSC study.In this dissertation,two electron-deficient units are incorporated into conjugated polymer or small molecule.Several conjugated polymers and small molecules as electron donor have been developed and their photovoltaic properties were carefully studied.The main results are as follows:1.A semi-crystalline conjugated polymer named PDPP2TPCL based on two electron-deficient units,pentacyclic lactam(PCL)and diketopyrrolopyrrole(DPP),was designed and synthesized for application in field-effect transistors(FETs)and polymer solar cells(PSCs).The polymer has a high molecular weight,near-infrared absorption up to 900 um and good solubility in toluene.When the polymer thin films were solution-processed from toluene with diphenyl ether as an additive,the FET devices achieved a high hole mobility of 0.81 cm2 V-1 s-1.With the same solution-processing solvents,bulk-heterojunction solar cells based on this polymer as an electron donor provided a power conversion efficiency(PCE)of 4.7%with an optimal energy loss of 0.65 eV due to its deep lowest unoccupied molecular orbital level.This work demonstrates that conjugated polymers incorporating several electron-deficient units can be potentially used in high performance FETs and PSCs.2.Three conjugated polymers incorporating based on PDPP2TPCL were designed and synthesized,in which the aromatic linkers between PCL and DPP varied from thiophene to methylthiophene.Methylated polymers were found to show slightly blue-shift absorption spectra and high-lying energy levels compared to non-methylated polymers.The three polymers also exhibit good crystalline properties and hole mobilities up to 0.57 cm2 V-1 s-1 in field-effect transistors.Non-methylated polymer as electron donor in solar cells shows a PCE of 4.2%with a relatively low short circuit current density(Jsc)of 8.3 mA cm-2,while methylated polymers exhibit dramastically enhanced Jsc of 12.8 mA cm-2 and PCEs up to 6.1%.Micro-phase separation in bulk-heterojunction thin films based on these polymers were sysmatically investigated,in which methylated polymers in blended thin films were found to provide better micro-phase separation with small crystal doamin.The observation can explain their increased photocurrent in solar cells.These studies demonstrate that by intentionally structural modification,conjugated polymers incorporating several electron-deficient units can have the great potential application in high performance solar cells.3.A small molecular donor incorporating binary electron-deficient units,DPP and PCL,was synthesized to provide low band gap of 1.65 eV and low-lying energy levels.Three molecues,from a fullerene derivative to non-fullerene perylene bisimide-based acceptors,were selected as electron acceptors to construct organic solar cells.The results showed that fullerene-based solar cells provided the PCEs up to 4.8%,while the non-fullerene solar cells also exhibit promising PCEs of 2.4%and 3.5%,with the photoresponse up to 750 nm.Further analysis of the bulk-heterojunction systems between donor and acceptors reveals that the relatively low carrier mobilities of non-fullerene acceptors and the large phase separation are mainly responsible for the less efficient solar cells.Our results demonestrate that the molecules containing several electron-deficient units can effectively reduce the band gap of small molecules,and thus have great potential for realizing high performance fullerene and non-fullerene solar cells.