Photoinduced Charge‐Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?

Abstract

For inorganic semiconductors crystalline order leads to a band structure which gives rise to drastic differences to the disordered material. An example is the presence of an indirect band gap. For organic semiconductors such effects are typically not considered, since the bands are normally flat, and the band-gap therefore is direct. Herein we show results from electronic structure calculations demonstrating that ordered arrays of porphyrins reveal a small dispersion of occupied and unoccupied bands leading to the formation of a small indirect band gap. We demonstrate herein that such ordered structures can be fabricated by liquid-phase epitaxy and that the corresponding crystalline organic semiconductors exhibit superior photophysical properties, including large charge-carrier mobility and an unusually large charge-carrier generation efficiency. We have fabricated a prototype organic photovoltaic device based on this novel material exhibiting a remarkable efficiency.

van der Waals Epitaxial Growth of 2D Metal–Porphyrin Framework Derived Thin Films for Dye‐Sensitized Solar Cells

“…XRD data recorded after thin film preparation in in-plane and out-of-plane geometry show well-defined diffraction peaks, revealing the formation of a highly oriented, crystalline 2D porphyrin framework (referred to as ZnTCPP SURMOF) thin film. [14,36] The out-of-plane XRD pattern recorded for the ZnTCPP SUR-MOFs ( Figure 2) exhibited sharp diffraction peaks at 2θ = 11.3°, 17°, and 22.6° which are attributed to (002), (003), and (004) planes, respectively. [14,36] The out-of-plane XRD pattern recorded for the ZnTCPP SUR-MOFs ( Figure 2) exhibited sharp diffraction peaks at 2θ = 11.3°, 17°, and 22.6° which are attributed to (002), (003), and (004) planes, respectively.…”

“…[8,9] Metal-organic frameworks (MOFs) represent a class of porous coordination polymers that consist of organic ligands linker by metal ions to form crystalline assemblies, [10,11] which have been widely investigated for their attractive physical and chemical properties. [14,15] In particular, porphyrin-based MOFs (also called metal-porphyrin frameworks) [16] have been demonstrated to exhibit tunable optical, electrical, and photophysical properties, [17][18][19] which in particular suggest applications in DSSCs. [14,15] In particular, porphyrin-based MOFs (also called metal-porphyrin frameworks) [16] have been demonstrated to exhibit tunable optical, electrical, and photophysical properties, [17][18][19] which in particular suggest applications in DSSCs.…”

Crystalline, Highly Oriented MOF Thin Film: the Fabrication and Application

“…Liu and coworkers fabricated the prototype organic photovoltaic device (Figure ) based on the oriented crystalline porphyrin film fabricated by liquid‐phase epitaxy method . Since the thin film is highly porous, ordered and oriented, it reveals a small dispersion of occupied and unoccupied bands leading to the formation of a small indirect band gap which can substantially improve device performance (Indirect band gaps support fast and highly efficient charge separation and strongly suppress charge‐carrier recombination).…”

Photoleitfähigkeit in Dünnfilmen Metall‐organischer Gerüste

“…C 60 -Fullerene sind vielversprechende Ladungsakzeptoren, weil die delokalisierten p-Systeme zu einer großen Elektronenaffinitätund hohen Stabilitätf ühren. [11] Des Weiteren ermçglichen MOFs mit Porphyrin als lichtsammelndem Linkermolekül Anwendungen in der Photovoltaik [12] und Photokatalyse. [10] Fürd ie Einlagerung von C 60 in die regelmäßigen MOF-Poren wurde eine interessante Bandstruktur vorhergesagt.…”

“…[13] Bei diesen Anwendungen wird die Tatsache genutzt, dass Porphyrine ausgezeichnete Elektronendonoren mit delokalisierten p-Systemen sind. [12,16] Darüber hinaus kçnnen die Eigenschaften des Porphyrins durch verschiedene Methoden der organischen Chemie angepasst werden, z. [15] Zu diesem Zweck werden Porphyrine oft als aktive Komponenten in verschiedene optoelektronische Vorrichtungen integriert.…”