# Presentation: 469005

## Structure-Function-Property Relationships inConjugated Polyelectrolytes (CPEs)  Thuc-Quyen Nguyen, University of California-Santa Barbara, DMR 0547639 	

|  The aim of this project is to understand structure-function-property relationships in CPEs. Our goal is to optimize the materials function and properties by a systematic study that incorporates new materials design, a range of processing options and characterization techniques. 
 We studied the charge transport properties of CPEs as a function of the conjugated backbone and charge reversal (cationic versus anionic). These materials are used as an electron transporting layer in polymer light-emitting diodes (PLEDs). More efficient electron injection from Al and better device performance are attained with the CPE bearing negative charges. For cationic CPEs having the same counterion but different conjugated structure, one observes better device efficiency using the material with higher electron mobility. Thus, both charge and backbone are important for optimizing device performance.
 The result is significant because now the fabrication of organic LEDs with excellent performance does not need to rely on unstable metals, such as calcium or barium. 
 Future plan: We will investigate the chemical structures of the charge and the side group and the length of the side group influencing the optical and charge transport properties of CPEs.
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**PFBTN****+****Br****-**** 1.0 ******** 10****-7**** cm****2****/V.s**
**PFN****+****Br****- ****1.2 ******** 10****-5**** cm****2****/V.s**
**PFSO****3****-****Na****+ ****1.2 ******** 10****-8**** cm****2****/V.s**

**Mobility, ******

**Notes:**
Introduction:
Conjugated polyelectrolytes (CPs) are polymers characterized by a pi-conjugated backbone and functional groups that ionize in high dielectric media and therefore make the material solvent soluble in water and polar organic solvents. CPs embody the properties of polyelectrolytes, which are modulated by complex long-range electrostatic interactions, with the useful optical and electronic functions of organic semiconductors, which are determined to a significant extent by the chain conformation and the interchain arrangements in the bulk. Optical and electronic properties in solution and in the solid state of these materials are therefore difficult to predict a priori from simple molecular structure considerations. Despite their complex structure/property relationships, CPs have been used as the optical amplification of fluorescent biosensors, as an emitter and charge compensating ions in light-emitting electrochemical cells (LECs), and as electron transporting layer (ETL) or hole transporting layer (HTL) in LEDs because their solid-state emission quantum yields are typically low and are not likely to function well as the emitting layer. 
We examine how charge reversal for a given backbone influences the performance as an electron injection layer (EIL) in test bed PLEDs containing poly(2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) as the emissive layer and the effect of the charge carrier mobility of CPs. We do so by using sodium poly[9,9-bis(4'-sulfonatobutyl)fluorene-alt-co-1,4-phenylene] (PFSO3-Na+) and poly[(9,9-bis(6'-N,N,N-trimethylammonium)hexyl)fluorene-alt-co-1,4-phenylene] bromide (PFN+Br-), as shown in the Slide. Additionally, we alter the chemical structure of the backbone while keeping the same charge/compensating ions constant by including poly[(9,9-bis(6'-N,N,N-trimethylammonium)hexyl)fluorene-alt-co-4,7-(2,1,3-benzothiadiazole)] bromide (PFBTN+Br-). We find that both the type of charges and counterions accumulated at the Al-cathode and the conjugated backbone, which transport electrons, are crucial in determining the device efficiency. 

Experimental techniques:
Charge mobility measurements. To measure the electron mobility, we fabricated electron-only devices. PFN+Br- and PFBTN+Br- were dissolved in methanol. A solution of PFSO3-Na+ was prepared in a mixture of 95% methanol and 5% water (v/v). A solution of CPE was spun at ~1000 rpm onto an Al-coated glass substrate, followed by a 10 nm-thick Ba capped with a 100 nm-thick Al. Ba and Al were used because of their low work functions, 2.7 eV and 4.3 eV, respectively. Application of a bias under normal conditions, i.e. a 0.5 V/sec, led to light emission. Remarkably, this observation implies hole injection from the Al anode despite 1.5 eV difference between the work function of the metal and the HOMO of the CPE (5.8 eV). Thus, the single-layer CPE device functions as a light-emitting electrochemical cell, where ion motion and accumulation at the electrodes redistribute the electric fields, and hence, reduce the charge injection barriers. Ion motion occurs in the order of seconds. By applying a short pulsed bias (250 nanoseconds on and off times and at 0 V during off times), it is possible to suppress the effect of ion motion; therefore, electrons are the majority carriers when the CPE is sandwiched between Al and Ba electrodes. Under these conditions no light emission was observed. 
LED Fabrication. A solution of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) (Baytron P 4083, Bayer AG.) was spun onto a clean indium-tin oxide (ITO)-coated glass substrate to yield ~100 nm thick film. PEDOT layer was used as a hole injection layer. Subsequently, a solution of MEH-PPV in toluene was spun on top of the ITO/PEDOT as the emissive layer (~100 nm), followed by a ca. 25 nm thick CPE as EIL. A 100 nm-thick Al cathode was thermally evaporated at pressure of 10-8 Torr using a shadow mask. Two reference devices without the EIL, ITO/PEDOT/MEH-PPV/Al and ITO/PEDOT/MEH-PPV/Ba/Al, were also prepared for a comparison. The device fabrication and testing were done in an inert atmosphere of a glovebox. 

Results:
The performance of the device with ITO/PEDOT/MEH-PPV/PFSO3-Na+/Al is comparable to those with the Ba-cathode; therefore, the PFSO3-Na+ EIL works effectively in modifying the work function of the Al-cathode to improve electron injection. PFSO3-Na+ thus provides, among the CPEs tested here, the PLEDs with the highest efficiencies, despite the fact that it displays the lowest electron mobility. It is possible that electron injection is more efficient in the ITO/PEDOT/MEH-PPV/PFSO3-Na+/Al device. Within the framework of field redistribution by ion migration to the electrodes, the rate and efficiency of ion motion is important. The time responses of the current density and the luminance (i.e. the time required to achieve 50 % of the maximal value) are faster with PFSO3-Na+ (< 0.5 sec) as the EIL than with PFN+Br- and PFBTN+Br- (> 5 sec). These data are consistent with sodium ion motion being more facile than for bromide. In the case of the PFN+Br- and PFBTN+Br-, the bromide moves away from the cathode and the ammonium ions localize near the cathode. It is important also to note that there is little known on whether the chemical structure of the cation (i.e. ammonium vs. sodium) influences the interaction with the aluminum surface and the extent to which such interactions could modify the work function of the electrode. Additionally, the positive Clarification of these mechanistic uncertainties is part of ongoing studies. In summary, the composite chemical nature of CPEs, i.e. backbone, charge and compensating counterions, need to be considered in future optimization of EILs based on CPEs. It is relevant to emphasize here that such layers in PLEDs enable the use of high work function metals such as Al, Ag, or Au as the cathode, thus reducing the fabrication cost by simplifying the device encapsulation.

## Mentoring Young Students and Outreach  Thuc-Quyen Nguyen, UCSB, DMR-0547639 

|  This summer, the PI had four undergraduate students working in her laboratory: Jennifer Sift (UCSB), Jason Lin (UCLA), Manuel Schnabel (Oxford University), and Victoria Crockett (Jackson State University). Each focused on different aspects of materials from materials processing to characterization at the nanoscale and in bulk to device fabrication. The research experience is a key factor in students’ decisions to pursue advanced degrees in science and engineering. Jason Lin, Jennifer Sift, and Victoria Crockett plan to attend graduate schools.
 The PI led the “Ask a Scientist” section for the NanoQuest Day at UCSB. This event was organized by the The National Nanotechnology Infrastructure Network (NNIN) Education Program. There were 34 middle school students and 3 teachers from Tehachapi, CA participating in the event to learn various aspects of nanoscience and nanotechnology.
 The PI participated in “Science Night” at local middle schools. She introduced various types of materials and their applications to young students, parents, and teachers (Bottom picture).
 The PI is a guest speaker and a member of the planning board for the Women in Science and Engineering (WiSE) at UCSB. WiSE promotes equal opportunity for women and girls in science and engineering and supports their scientific and career advancement. |  |  |
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- The PI discussed organic semiconducting materials with her summer intern students: Jason Lin, Manuel Schnabel, and Victoria Crockett.

**Notes:**
WiSE promotes equal opportunity for women and girls in science and engineering, supports their scientific and career advancement, and encourages the participation of all science/engineering undergraduates, graduates, researchers, faculty, and professionals in the Santa Barbara area. It also helps to raise awareness and educates women about seeking science careers. I talked about graduate school, how to choose graduate program, and how to prepare the application at the "Applying to Graduate School" workshops on November 8, 2005 and October 20, 2006. Since August 2006, I serve on a WiSE planning board committee to help plan future events. On May 23, 2007, I was a guest speaker to discuss about “Life as a female assistant professor.” Topics discussed were teaching, mentoring, getting funding and students, setting up a lab, collaborating, publishing, etc. Basically, I shared my experience as an assistant professor, challenges that I encountered, and how to overcome these challenges.