“ dark –axis”

1. Undergraduate Research on Complex Materials Dennis P. Clougherty, University of Vermont & State Agricultural College, DMR 1062966 0.77 0.62 In recent years organic photovoltaics (OPVs) have gained attention as devices that can be used with alternative energy sources. Although much research has shifted to polymer-based OPVs, some of the highest efficiencies to date have been recorded in cells based on small-molecule semiconductors. We investigated the electronic properties and potential for PV applications of Cu- and metal-free (H2)- phthalocyanine (Pc) poly-crystalline thin films. Erik Horak imaged the correlations between long range ordering and the nature of electron–hole pair exciton states using linear dichroism (LD) and photoluminescence spectroscopy (PL). He found that a low -temperature rotation of the molecular plane with respect to the crystalline c-axis is responsible for the observation of a long-lived (dark) exciton recombination, labeled as feature (2) in Figure 1(a). Roy Anderson used a simple 1D molecular crystal model first proposed by Davydov to estimate the dark-bright exciton splitting. His calculations, shown in Figure 2(c) indicate the exciton ground state is optically –dark if the angle between the molecular plane and the c-axis is larger than 54.7o. Finally, Michael Durant fabricated a working inverted OPV device (Figure 1(b)) and characterized the effects of different parameters such as film thickness, annealing, and a modified ZnO layer on cell performance. He found the addition of the ZnO layer increases the power conversion efficiency in the inverted cell in comparison to previous geometries. 0.47 (a) (b) 0.32 0.17 0.02 -0.13 -0.28 -0.43 10g/l ob-H2 Pc Davydov Splitting (1)- “bright” exciton (2)- “dark” exciton 89 T = 5K- ObPc LD 80 70 Pen Writing E =E(1)-E(2) 60 50 Y(mm) 40 2c (Å) P 30 20 A 10 0 0 10 20 30 40 50 60 70 80 89 Au (Electrode) X(mm) Michael Durant Erik Horak CuPc (Electron Donor) d d Figure 1. (a) Linear Dichroism (LD) and Photoluminescence (PL) microscopy of solution-deposited polycrystalline H2Pc films with mm-sized grains. The LD contrast at the grain boundary accounts for the different orientation of adjacent grains. Low temperature PL spectra indicates the presence of an optically-forbidden (dark) exciton ground state. (b) Schematics and current-voltage characteristic of the inverted organic photovoltaic cell (IOPV). The use of the ZnO layer allows the use of the higher work function gold electrode as a replacement for the traditional aluminum which greatly increases cell durability and lifespan. (c) Calculation of the dark-bright exciton (Davydov) splitting in an H2Pc quasi 1D molecular chain predicts the singlet ground state is forbidden (“dark”). (c) dark axis PTCDA (Electron Acceptor) c “dark –axis” ZnO (Hole Blocking Layer)  ITO (Transparent Electrode) Eexp= 0.09 eV Roy Anderson

2. Undergraduate Research on Complex Materials Dennis P. Clougherty, University of Vermont & State Agricultural College, DMR 1062966 The UVM REU students give presentations at UVM physics and chemistry seminars attended by undergraduates, graduate students and faculty. Erik Horak: “Electronic Properties of Organic Discotic Molecules” Danica Kennedy: “Characterization and Synthesis of New Zirconium-Arsinidene Complexes” The UVM REU program includes weekly cohort meetings with faculty to discuss the nature of science and research, written and oral presentation skills, NSF funding process and proposal writing, and legal and ethical issues in research.