# Presentation: 542581

## Structure-Function-Property Relationships in Charged Conjugated Polymers (CCPs) Thuc-Quyen Nguyen		                              DMR0547639

|  The aim of this project is to understand structure-function-property relationships in charged conjugated polymers. 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 use various counteranion (CA) sizes to control the interchain packing in films to control the optical and charge transport properties. Local current/voltage measurements of spun cast films at the nanoscale using conducting atomic force microscopy show that the mobility of holes and the barriers to charge injection can be substantially modified by the choice of CA. Smaller CA allows intimate chain contact that leads to low solid state photoluminescence (PL) quantum yield, high charge mobility, and lower charge injection barrier than large CA.
 The result is significant because CCPs have not found widespread application as emitters in devices such as LEDs or light emitting electrochemical cells because of their typically low quantum yield values.  
 Future plan: we will investigate the type of charged functional group (cationic versus anionic) influencing the optical and charge transport properties of charged conjugated polymers.
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-  (PFN+Br-) = 6.1  10-5 cm2/V.s.
-  (PFN+BArF-4) = 9.4  10-6 cm2/V.s.

- PL= 5%

- PL= 41%

**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.  
Simple procedures for exchanging charge-compensating ions in conjugated polyelectrolytes by progressive dilution of the original species and for determining the degree of exchange by using X-ray photoelectron spectroscopy are provided. By using these procedures, the bromide ions in poly[(9,9-bis(6'-N,N,N-trimethylammoniumbromide)hexyl)fluorene-alt-4,7-(2,1,3-benzothiadiazole)] were exchanged with BF4-, CF3SO3-, PF6-, BPh4- and B(3,5-(CF3)2C6H3)4- (BArF4-). Examination of these trends and band shapes suggests that increasing the size of the counteranion (CA) decreases interchain contacts and aggregation.  As a result, the highest solid state photoluminescence quantum yields were obtained with the largest CA (BArF-). Size analysis of polymers containing Br- and BArF4- anions in water by dynamic light scattering techniques indicates suppression of aggregation in solution by the larger species.  Local current/voltage measurements of spun cast films at the nanoscale using conducting atomic force microscopy show that the mobility of holes and the barriers to charge injection can be substantially modified by the choice of CA. 
Experimental techniques:
Scanning probe measurements. All measurements were done under ambient conditions and in the dark using a commercial scanning probe microscope (MultiMode equipped with CAFM module and the Nanoscope Controller IIIa, Veeco Inc.).  Platinum-coated Si tips with spring constant of 0.2 N/m and the tip radius of 25 nm were used (Budget Sensors).  In these measurements, the conducting probe makes contact with the sample (the tip acts as a nanoelectrode) and measures current as a function of applied voltage either at certain points on a surface (I-V curve) or map out a current image at a fixed bias. The bias was applied to the conducting substrate and the current was measured by a preamplifier. For each sample, the I-V curves and topographic images were collected on multiple locations to examine the film uniformity.
The solid state PL measurements involved examination of films spun from methanol, optical excitation at 364 nm and collection of emission using an integrating sphere.
Results:
Examination of the effect of CAs on the optical, charge transport, and aggregation properties of PFBT shows that the solid-state  values and the charge mobilities can be varied by close to an order of magnitude. The most plausible explanation is that the size of the CA modulates the separation between chains, thereby reducing the extent of emission self-quenching. Consistent with this picture is that the absorption and emission of PFBT-BArF4 are less sensitive to the environment, relative to PFBT-Br. Furthermore, the nature of the CA also changes the degree of aggregation in water, thereby providing a way to control interchain contacts in solution. Improvements in emission output in the solid-state could impact the types of technological applications of conjugated polyelectrolytes, for example their use as the emissive layers in organic LEDs and the fabrication of more efficient single component LECs. The efficiency of these devices will depend on the electronic properties of the CPs as a function of CA. Larger counteranions act as spacers that separate polymer chains resulting in lower charge mobilities. Thus, for applications requiring high charge mobility such as ETL or HTL in LEDs, field effect transistors or photovoltaic devices, PFBT-Br should prove more useful than PFBT-BArF4. We have also demonstrated that the nature of the CA also influences the barrier to charge injection, although the exact mechanism of action at this stage of our studies remains undetermined.

## Structure-Function-Property Relationships in Charged Conjugated Polymers (CCPs) Thuc-Quyen Nguyen		                              DMR0547639

| Education and Outreach:
 This summer, an undergraduate from Stanford University (Alissa Walker)  and a chemistry high school teacher from Venture High School (Woody Maxwell) do research in her laboratory. The goal is for the teacher to incorporate the research and learning experience into the high-school curriculum to engage students to science. Mr. Maxwell works on nanoscale characterization of CCPs and Ms. Walker focuses on processing conditions and optical properties of CCPs. The research experience is a key factor in students’ decisions to pursue advanced degrees in science and engineering.
 The PI participates in “Let's Explore Applied Physical Science” (LEAPS) Program. This program engages UCSB graduate 	and undergraduate Fellows as instructors and mentors for inquiry-based science in Grade 8 and Grade 9 classrooms. 	She serves as a faculty mentor for LEAPS fellows, does classroom observation, and evaluate LEAPS fellows (bottom picture).
 She helped organize and raised funds for the Science and Technology Mathematics Engineering Science Achievement Day. This annual event at UCSB is designed to increase student diversity, to encourage students to go to college and to consider careers in science and engineering. She also developed and gave workshops (top picture). |  |  |
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- The PI explained to K-12 students and parents how fiber optics work and the materials used to make fiber optics.

- The PI observed a LEAPS fellow run an after school science club at Santa Barbara Junior High School.