Organic/inorganic semiconductor heterojunctions are being explored in hybrid solar cells that take advantage of unique properties of both material systems. A key question concerns the mechanism of charge separation across the localized/delocalized semiconductor interface. Here we probe photoinduced charge transfer at a model interface between copper phthalocyanine (CuPc) and gallium arsenide (GaAs) by tracking the electric field on the femtosecond time scale using timeresolved second harmonic generation. For above bandgap excitation of GaAs, we resolve distinction channels of charge separation on 10 2 fs time scales: charge carrier separation in GaAs due to the delocalized space charge field and hole injection from photoexcited GaAs to localized CuPc molecular orbitals. At sufficiently high excitation density (>10 17 /cm 3 ), charge separation by the space charge field leads to band flattening, which accelerates resonant hole transfer from GaAs to CuPc. We discuss implications of these findings to the design of organic/inorganic hybrid solar cells.
Plastic photovoltaic devices (PVs)
were fabricated by spray-depositing
copper indium diselenide (CuInSe2) nanocrystals into micrometer-scale
groove features patterned into polyethylene terephthalate (PET) substrates.
Each groove has sidewall coatings of Al/CdS and Au and performs as
an individual solar cell. These PV groove features can be linked electrically
in series to achieve high voltages. For example, cascades of up to
15 grooves have been made with open-circuit voltages of up to 5.8
V. On the basis of the groove geometry, the power conversion efficiencies
(PCEs) of the devices reached as high as 2.2%. Using the active area
and photovoltaic response of devices determined from light-beam-induced
current (LBIC) and photoreflectivity measurements gave PCE values
as high as 4.4%.
Solar cells with bulk heterojunction active layers containing donor-acceptor copolymer PDTSiTzTz exhibit persistent high fill factors with thicknesses up to 400 nm. Transport and recombination in a blend of PDTSiTzTz and fullerene derivative PC 71 BM is studied using lateral organic photovoltaic structures. This material system is characterized by carrier-concentrationdependent charge carrier mobilities, a strongly reduced bimolecular recombination factor, and a negative Poole-Frenkel coefficient. The analysis provides an explanation for the relatively thickness-independent fill factor behaviour seen in solar cells using the copolymer PDTSiTzTz. Cumulative insights from this copolymer can be employed for future organic photovoltaic material development, study of existing high performance bulk heterojunciton blends, and improved solar cell design.
A B S T R A C TLateral bulk heterojunctions (LBHJ) provide a tool to directly probe the active area of photovoltaic devices using microscopy techniques. Here, we use scanning photocurrent microscopy (SPCM) to probe an organic photovoltaic (OPV) device with poly [{4,8-bis[(2-ethylhexyl)(PTB7):[6,6] phenyl-C61-butyric acid methyl ester (PC 61 BM). The effects of the additive 1,8-diiodooctane (DIO) on the recombination dynamics and morphology are probed in real space in a LBHJ structure. By using SPCM, we can see a larger increase in the space charge region for samples with DIO when compared to those without DIO. This indicates that the additive improves film morphology leading to increased charge extraction efficiency and decreased recombination.
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