Photovoltaic Effect in Phthalocyanine-Based Organic Solar Cells: 1. Thermal Ionization of Molecular Excitons
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Photovoltaic Effect in Phthalocyanine-Based Organic Solar Cells: 1. Thermal Ionization of Molecular Excitons V. A. Benderskiia, * and I. P. Kima aInstitute
of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia *e-mail: [email protected] Received April 6, 2020; revised April 6, 2020; accepted April 20, 2020
Abstract—Based on the previously obtained data [1, 2], according to which the photoelectric quantum yield (Y) in crystals of the β-form of metal-free phthalocyanine and copper phthalocyanine is the same for electrons and holes and has an Arrhenius temperature dependence with an activation energy EY equal to the difference between the band gap and the molecular exciton (ME) energy, a three-dimensional stochastic model of thermal ionization of MEs is proposed, which is considered as a sequence of reversible transitions from the ME to the lowest charge-transfer (CT) state and then to a CT state with an increasing charge-transfer distance. A relationship has been found between EY and the radial density of CT states at which the probability of charge separation is greater than that of the decay of these states. Keywords: phthalocyanines, molecular excitons, charge transfer states, quantum yield of charge carriers DOI: 10.1134/S0018143920050033
I. INTRODUCTION Long-term studies of the photovoltaic effect in organic materials have recently been marked by the creation of laboratory samples of organic solar cells (OSCs) with a high (up to 17%) power conversion efficiency [3–14]. The main difference between OCSs and devices based on inorganic semiconductors is that the absorption of light is due to intramolecular optical transitions that do not form charge carriers, but produce molecular excitons (MEs), which are capable of transferring energy but not generating a photocurrent. Progress in creating effective OSCs has been achieved for a large number of substances with different composition and structure of molecules, which nevertheless have a number of common properties that determine their effectiveness in the OSC composition. To establish these properties and determine their quantitative characteristics is an obvious but yet unsolved problem. One of the possible ways to solve it is to choose one of the sufficiently well-studied substances used in organic solar cells and, using its example, to trace the supposed mechanisms of the formation and transport of molecular excitons and charge carriers. Among other well-documented substances useful for the purpose, there are phthalocyanines (Pc) of various metals, which are synthetic dyes that are close in structure to porphyrin, the main component of light-harvesting natural aggregates [15]. Phthalocyanines have high thermal stability (there are no chemical bonds in the molecule with an energy below 70 kcal/mol), they are easy to purify to an impurity
content of ~1 ppm, and are available both in the form of crystals and micron and submicron films used in OSCs. We list the main properties of phthalocyanines that to justi
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