Terry McAfee

Mobility-Controlled Performance of Thick Solar Cells Based on Fluorinated Copolymers

Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure-property relationships has been very challenging. Here we report that a single parameter, hole mobility, determines the fill factor of several hundred nanometer thick bulk heterojunction photovoltaic devices based on a series of copolymers with varying amount of fluorine substitution. We attribute the steady increase of hole mobility with fluorine content to changes in polymer molecular ordering. Importantly, all other parameters, including the efficiency of free charge generation and the coefficient of nongeminate recombination, are nearly identical. Our work emphasizes the need to achieve high mobility in combination with strongly suppressed charge recombination for the thick devices required by mass production technologies.

High-Performance Non-Fullerene Polymer Solar Cells Based on a Pair of Donor-Acceptor Materials with Complementary Absorption Properties.

A 7.3% efficiency non-fullerene polymer solar cell is realized by combining a large-bandgap polymer PffT2-FTAZ-2DT with a small-bandgap acceptor IEIC. The complementary absorption of donor polymer and small-molecule acceptor is responsible for the high-performance of the solar-cell device. This work provides important guidance to improve the performance of non-fullerene polymer solar cells.

Tuning Open-Circuit Voltage in Organic Solar Cells with Molecular Orientation.

The role of molecular orientation of a polar conjugated polymer in polymer-fullerene organic photovoltaic (OPV) cells is investigated. A planar heterojunction (PHJ) OPV cell composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) is used as a model system to isolate the effect of the interfacial orientation on the photovoltaic properties. The molecular orientation of the aggregate P3HT relative to the PCBM layer is varied from highly edge-on (conjugated ring plane perpendicular to the interface plane) to appreciably face-on (ring plane parallel to the interface). It is found that as the P3HT stacking becomes more face-on there is a positive correlation to the OPV open-circuit voltage (V(OC)), attributed to a shift in the highest occupied molecular orbital (HOMO) energy level of P3HT. In addition, the PHJ OPV cell with a broad P3HT stacking orientation distribution has a V(OC) comparable to an archetypal bulk heterojunction (BHJ) device. These results suggest that, in the BHJ OPV cell, the hole energy level in the charge transfer state is defined in part by the orientation distribution of the P3HT at the interface with PCBM. Finally, the photoresponses of the devices are also shown to have a dependence on P3HT stacking orientation.

Quantitative relations between interaction parameter, miscibility and function in organic solar cells

Although it is known that molecular interactions govern morphology formation and purity of mixed domains of conjugated polymer donors and small-molecule acceptors, and thus largely control the achievable performance of organic solar cells, quantifying interaction–function relations has remained elusive. Here, we first determine the temperature-dependent effective amorphous–amorphous interaction parameter, χaa(T), by mapping out the phase diagram of a model amorphous polymer:fullerene material system. We then establish a quantitative ‘constant-kink-saturation’ relation between χaa and the fill factor in organic solar cells that is verified in detail in a model system and delineated across numerous high- and low-performing materials systems, including fullerene and non-fullerene acceptors. Our experimental and computational data reveal that a high fill factor is obtained only when χaa is large enough to lead to strong phase separation. Our work outlines a basis for using various miscibility tests and future simulation methods that will significantly reduce or eliminate trial-and-error approaches to material synthesis and device fabrication of functional semiconducting blends and organic blends in general.This work reports a quantitative investigation of the interaction parameter and miscibility of donor and acceptor organic molecules and their relationship with the fill factor and photovoltaic performance of bulk-heterojunction organic solar cells.

Lowest energy Frenkel and charge transfer exciton intermixing in one-dimensional copper phthalocyanine molecular lattice

We report the results of the combined experimental and theoretical studies of the low-lying exciton states in crystalline copper phthalocyanine. We derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer exciton state and compare it with temperature dependent optical absorption spectra measured experimentally, to obtain the parameters of the Frenkel-charge-transfer exciton intermixing. The two Frenkel exciton states are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the charge transfer exciton, showing the coupling constant 0.17 eV which agrees with earlier experimental measurements. These results can be used for the proper interpretation of the physical properties of crystalline phthalocyanines.

Monitoring Charge Separation Processes in Quasi-One-Dimensional Organic Crystalline Structures

We perform the transient absorption spectroscopy experiments to investigate the dynamics of the low-energy collective electron-hole excitations in α-copper phthalocyanine thin films. The results are interpreted in terms of the third-order nonlinear polarization response function. It is found that, initially excited in the molecular plane, the intramolecular Frenkel exciton polarization reorients with time to align along the molecular chain direction to form coupled Frenkel-charge-transfer exciton states, the eigenstates of the one-dimensional periodic molecular lattice. The process pinpoints the direction of the charge separation in α-copper phthalocyanine and similar organic molecular structures. Being able to observe and monitor such processes is important both for understanding the physical principles of organic thin film solar energy conversion device operation and for the development of organic optoelectronics in general.

Intrinsic Charge Trapping Observed as Surface Potential Variations in diF-TES-ADT Films

Spatial variations in surface potential are measured with Kelvin probe force microscopy for thin films of 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophenes (diF-TES-ADT) grown on SiO2 and silane-treated SiO2 substrates by organic molecular beam deposition. The variations are observed both between and within grains of the polycrystalline organic film and are quantitatively different than electrostatic variations on the substrate surfaces. The skewness of surface potential distributions is larger on SiO2 than on HMDS-treated substrates. This observation is attributed to the impact of substrate functionalization on minimizing intrinsic crystallographic defects in the organic film that can trap charge.

Growth of thermally stable crystalline C60 films on flat-lying copper phthalocyanine

We observe thermally stable growth of fcc(111) films of fullerene-C60 on top of crystalline, flat-lying, CuPc film structures on graphite using combined grazing incidence wide-angle X-ray scattering and atomic force microscopy. Such a morphology is nearly ideal for bilayer films of C60/CuPc in solar cells. Very similar crystallinity and morphology is observed when varying the film thickness of either material from 5 nm up to more than 100 nm, suggesting that this advantageous solar cell morphology is not only robust, but may actually be preferred compared to the typical mounded fullerene morphology often seen on standing, edge-on aromatic film substrates. The large, ordered domains observed in both materials and the cofacial interface should greatly increase charge carrier mobility leading to increased fill factor. We envision that bilayer films, such as described here, grown on very high quality graphene as a transparent conducting electrode would simultaneously optimize crystallinity and molecular orientation for peak solar cell performance.