Switchable Crystal Phase and Orientation of Evaporated Zinc Phthalocyanine Films for Efficient Organic Photovoltaics

Abstract

In photovoltaic cells, increasing the light absorption coefficient enhances the photocurrent and quantum efficiency of the cell. Here, we report an efficient approach that enhances light absorption by changing the crystal phase of an evaporated ZnPc film on indium tin oxide (ITO) substrates. Under thermal treatment (>200 °C), the crystal phase of ZnPc changed from metastable α-ZnPc to stable β-ZnPc. We tested and compared the effects of this phase transition on the donor film of organic photovoltaic (OPV) cells. After treating the β-ZnPc film at 220 °C, the short-circuit current of the OPV was improved from 2.58 to 3.40 mA/cm2, and the quantum efficiency was improved from 650 to 750 nm. Moreover, on a copper iodide (CuI)–ITO substrate, the ZnPc molecules were precisely controlled in the lying-down orientation. With their lying-down orientations, the light absorbance capacities of the metastable α-ZnPc phase and stable β-ZnPc phase (annealed at 300 °C) were 1.3- and 1.5-fold higher, respectively, than that of metastable α-ZnPc standing upright on the ITO substrate. This work provides a fundamental understanding of the crystal phases and orientation changes of the ZnPc molecules, enabling maximization of the light absorption coefficient and the design of efficient small-molecule OPV cells.

Vibrationally resolved absorption spectra and ultrafast exciton dynamics in α-phase and β-phase zinc phthalocyanine aggregates

“…The experimental crystal structure of β-phase ZnPc 17,41 is adopted for investigation. As for the α phase, since the crystal structure is currently unavailable in the literature, we use the structure of α-phase CuPc 42 as the initial geometry and then replace the Cu atom with a Zn atom while fixing the positions of the other atoms.…”

“…In ZnPc thin films, the microscopic structures are sensitive to the preparation conditions, e.g. , the deposition or annealing temperatures, 12–15 pressures, 16 substrates, 15,17 and film thickness. 15,18,19 So far, several polymorphs of ZnPc have been found in the thin films, 15,18,19 among which the two most common phases are the α and β phases, which are generally distinguished via their X-ray diffraction patterns.…”

“…For α-phase ZnPc, the intensity of the high-energy peak is larger than the low-energy peak, whereas the opposite situation occurs for the β phase. 7,17,25,28–31 Furthermore, evidence from time-resolved photoluminescence spectra has indicated that the ultrafast exciton dynamics that occur right after photo-excitation in the two phases can also exhibit significant differences. 27 Since the optical properties are crucial for the performance of the optoelectronic devices, it is necessary to establish unambiguous structure–spectra and structure–dynamics relationships for α-phase and β-phase ZnPc, so as to provide possible guidelines for fabrication conditions or material design routes.…”

Metallo‐phthalocyanines Containing 1,3,4‐oxadiazole Substituents: Synthesis, Characterization, Electrochemical and Spectroelectrochemical Properties

“…The electrochemical and spectroelectrochemical properties are important in determining the electronic properties of materials, illuminating the redox mechanism and giving clues about the use of materials in different application areas. It has been reported that MPcs having rich redox and coordination properties can be used as an electrocatalyst in different application areas such as battery, [30] photovoltaic cells, [31] water splitting, [32] reduction of CO 2 , [33] supercapacitors [34] and electrochemical sensors [35] . In addition to their catalytic properties, their color changes during redox reactions depending on the substituted groups of the MPcs allow using in electrochromic applications [36–38] …”

Fabrication and Local Electrical Characterization of p–n Junction Copper Phthalocyanine Nanorods

“…[1][2][3][4] Metal phthalocyanines (MPcs) are promising organic semiconductors with desirable properties including nontoxicity, high chemical and thermal stabilities, and excellent optical properties. They are used as active layers in various devices, such as organic solar cells, [5][6][7][8] organic transistors, [9][10][11] and gas sensors. [12][13][14] Pcs have extended π-conjugation on their molecular planes and aggregate in the direction of stacking molecular planes, which are favorable for charge transport, owing to their π-π interactions.…”