Relative Photon-to-Carrier Efficiencies of Alternating Nanolayers of Zinc Phthalocyanine and C₆₀ Films Assessed by Time-Resolved Terahertz Spectroscopy

Abstract

Multilayer and 1:1 blended films of zinc phthalocyanine (ZnPc) and buckminsterfullerene (C 60 ) were investigated as model active layers for solar cells by time-resolved terahertz spectroscopy (TRTS). Relative photon-to-carrier efficiencies were determined from ultrafast decay dynamics of photogenerated carriers using 400 and 800 nm excitation for delay times up to 0.5 ns. The findings are in good agreement with reported solar-cell device measurements, and the results exhibit a near linear increase of the relative efficiencies with the interface number of multilayer films. The relative photon-to-carrier efficiencies of films composed of alternating layers with an individual layer thickness of less than 20 nm were higher than that of a 1:1 blended film. In contrast, 400 nm excitation of a C 60 only film initially yields a relatively strong THz signal that is followed by a rapid (picosecond) decay almost to its base value and results in a very low carrier density beyond a few picoseconds. For a given film thickness and optical density, our data suggest that the relative photon-to-carrier efficiency of multilayer films increases with increasing total interfacial area, emphasizing the importance of close proximity between the fullerene and phythalocyanine. These findings suggest that the highest photon-to-free-carrier efficiencies can be achieved by designing ultrathin films (having layers a few nanometers thick) with alternating multilayer structures to achieve high photon harvesting and charge separation to opposite layers.

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…It is clear that, given our current state of understanding of THz spectroscopy to these types of systems, future applications of time-resolved terahertz spectroscopy can focus on systems of increasing complexity, such as those encountered in photovoltaic devices where different semiconductor phases are intermixed on nanometer lengths scales (Esenturk et al, 2009). Such complex systems may include, for instance, semiconductor nanostructures (quantum dots or nanorods) embedded in an organic polymer semiconductor phase and quantum dot or dye-sensitized transition metal oxide systems (Tiwana et al, 2009;Nemec, Rochford et al, 2010;Pijpers et al, 2010Pijpers et al, , 2011.…”

Ultrafast charge-transfer processes at an oriented phthalocyanine/C60interface

“…Ultrafast optical and terahertz studies of polymer blend and multilayer photovoltaic systems have probed charge separation processes into the subpicosecond regime, [3][4][5] producing much useful insight. For instance, recent work has highlighted the potential impact of bound interfacial chargetransfer ͑ICT͒ states on charge separation.…”

Photo-carrier dynamics of blended and multi-layered films of zinc phthalocyanine and C<inf>60</inf> measured by time-resolved terahertz spectroscopy

“…In each case these curves can be fit to a bi-exponential decay (see Table 2). Our TRTS measurements have shown that the long-lived component decays on a time scale that approaches nanoseconds [4], and is assigned to quasi-free photocarriers. These carriers are of technological interest since they represent photoconversion efficiency.…”

“…Using TRTS, the electrical properties of the organic thin films are determined using spectroscopy, which eliminates the need to make "ohmic" contacts with the organic material. TRTS has been applied recently to a variety of organic materials [4,5]. background pressure of 10 -6 Torr at a rate of 0.5 -1 Å /second.…”