PCBM is present as a single isomer. An interesting feature of PCBM which may correlate with its performance is that it preserves to a high degree the electronic and physical properties of C60.
Single crystal structure analysis shows that intermolecular spacing is essentially identical to C60, with the shortest ball-to-ball spacing curiously being slightly smaller in PCBM than C60. It has been consistently demonstrated that deviation to too great of a degree from the compact structure of PCBM (and thus from the parent fullerene) leads to diminished performance. Please see Table 1 for a list of PCBM products available from Aldrich Materials Science.
PCBM in Organic Photovoltaics (OPV)
PCBM is still a very commonly used n-type component in organic photovoltaics. Over at least the last 6 years, the published world record power conversion efficiency (η) for a bulk heterojunction (BHJ) organic photovoltaic device to our knowledge has been continuously held by devices incorporating PCBM, save for a period in which a device containing PCBM held the record. MDMO-PPV:PCBM devices were thoroughly studied and characterized, eventually leading to η=3.0% when PCBM was substituted for PCBM, which earlier had given η=2.5%. The increase was due to the higher optical absorption of PCBM in visible wavelengths compared to PCBM. PCBM has an even stronger absorption in the visible wavelengths, though the better electron accepting ability led to a diminished performance in OPV, because it was used in combination with a relatively strongly electron donating donor polymer. More recently, researchers and developers have transitioned to polythiophene/PCBM systems, and η’s of 4.4%–6% have been published by several groups. Careful control of morphology, either by annealing or slow evaporation, provides a significant improvement in performance.
The state of the art roadmap for research and development to achieve η of 10% focuses on improving morphology , composite materials and polymer characteristics leading to an inference that PCBM is largely adequate as the n-type semiconductor for improved devices. However, improvements in morphology control with polythiophenes, where more extensive demixing of the PCBM and polymer phase has been observed, is also desired. This has led to the design and testing of a new molecule, ThCBM, which in preliminary results does appear to give a slightly more advantageous morphology with P3HT. ThCBM also preserves the electronic properties (LUMO and mobility) of PCBM. Increases in LUMO level of the n-type have also been long sought by OPV developers and a recently synthesized molecule, 2,3,4-OMe-PCBM, shows a modest though significant increase in LUMO. This molecule has been shown to give a higher open circuit voltage (VOC) in combination with MDMO-PPV but has not yet been fully characterized in OPV devices.
PCBM in Organic Field Effect Transistors (OFETs)
Relatively high mobilities for an organic semiconductor have been demonstrated for PCBM devices (1 x 10–2–2 x 10–1 cm2/Vs), as well as ambipolar transport which allowed for the construction of inverters. Stability has been an issue, though efficient passivation has been reported. PCBM thus far has shown about an order of magnitude lower electron mobilities but allows for shorter annealing times and higher stability. PCBM has shown very good stability, in combination with an electron mobility up to 3 x 10–3 and a hole mobility of 10–5–10–4 cm2/Vs. Blends of conjugated polymers with PCBMs can also be used for ambipolar OFETs. Less work has been done with OFET devices using PCBMs compared to OPV, and it can be expected that mobility improvements can be obtained applying similar control of film morphology (optimal solvents and evaporation/annealing) as has been demonstrated with OPV.
PCBM in Organic Photodetectors
Concurrent with the early development of OPV devices, bulk heterojunction organic photodetectors based on similar photodiodes were also developed. Performance adequate for commercial application was realized, with low dark currents, high external quantum efficiencies (80%), and fast transient behavior. Significantly, large area applications are envisioned due to the cost advantages of organic thin films over siliconbased devices.
This information has been sourced, reviewed and adapted from materials provided by Sigma Aldrich.
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