17 research outputs found
Organic narrowband near-infrared photodetectors based on intermolecular charge-transfer absorption
Blending organic electron donors and acceptors yields intermolecular charge-transfer states with additional optical transitions below their optical gaps. In organic photovoltaic devices, such states play a crucial role and limit the operating voltage. Due to its extremely weak nature, direct intermolecular charge-transfer absorption often remains undetected and unused for photocurrent generation. Here, we use an optical microcavity to increase the typically negligible external quantum efficiency in the spectral region of charge-transfer absorption by more than 40 times, yielding values over 20%. We demonstrate narrowband detection with spectral widths down to 36 nm and resonance wavelengths between 810 and 1,550 nm, far below the optical gap of both donor and acceptor. The broad spectral tunability via a simple variation of the cavity thickness makes this innovative, flexible and potentially visibly transparent device principle highly suitable for integrated low-cost spectroscopic near-infrared photodetection
Stacked Dual-Wavelength Near-Infrared Organic Photodetectors
Organic near-infrared (NIR) detectors have potential applications in biomedicine, agriculture, and manufacturing industries to identify and quantify materials contactless, in real time and at a low cost. Recently, tunable narrow-band NIR sensors based on charge-transfer state absorption of bulk-heterojunctions embedded into Fabry-Pérot micro-cavities have been demonstrated. In this work, this type of sensor is further miniaturized by stacking two sub-cavities on top of each other. The resulting three-terminal device detects and distinguishes photons at two specific wavelengths. By varying the thickness of each sub-cavity, the detection ranges of the two sub-sensors are tuned independently between 790 and 1180, and 1020 and 1435 nm, respectively, with full-width-at-half-maxima ranging between 35 and 61 nm. Transfer matrix modeling is employed to select and optimize device architectures with a suppressed cross-talk in the coupled resonator system formed by the sub-cavities, and thus to allow for two distinct resonances. These stacked photodetectors pave the way for highly integrated, bi-signal spectroscopy tunable over a broad NIR range. To demonstrate the application potential, the stacked dual sensor is used to determine the ethanol concentration in a water solution
Fast Organic Near-Infrared Photodetectors Based on Charge-Transfer Absorption
We present organic near-infrared photodetectors based on the absorption of charge-transfer (CT) states at the zinc-phthalocyanine–C60 interface. By using a resonant optical cavity device architecture, we achieve a narrowband detection, centered around 1060 nm and well below (>200 nm) the optical gap of the neat materials. We measure transient photocurrent responses at wavelengths of 532 and 1064 nm, exciting dominantly the neat materials or the CT state, respectively, and obtain rise and fall times of a few nanoseconds at short circuit, independent of the excitation wavelength. The current transients are modeled with time-dependent drift-diffusion simulations of electrons and holes which reconstruct the photocurrent signal, including capacitance and series resistance effects. The hole mobility of the donor material is identified as the limiting factor for the high-frequency response. With this knowledge, we demonstrate a new device concept, which balances hole and electron extraction times and achieves a cutoff frequency of 68 MHz upon 1064 nm CT excitation
Organic Cavity Photodetectors Based on Nanometer-Thick Active Layers for Tunable Monochromatic Spectral Response
Hole Transport in Low-Donor-Content Organic Solar Cells
Organic solar cells with an electron donor diluted in a fullerene matrix
have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier
recombination and increased open-circuit voltages. However, the low donor
concentration prevents the formation of percolation pathways for holes. Notwithstanding,
high (>75%) external quantum efficiencies can be reached, suggesting an effective holetransport
mechanism. Here, we perform a systematic study of the hole mobilities of 18
donors, diluted at ∼6 mol % in C60, with varying frontier energy level offsets and
relaxation energies. We find that hole transport between isolated donor molecules occurs
by long-range tunneling through several fullerene molecules, with the hole mobilities
being correlated to the relaxation energy of the donor. The transport mechanism
presented in this study is of general relevance to bulk heterojunction organic solar cells
where mixed phases of fullerene containing a small fraction of a donor material or vice
versa are present as well
Hole transport in low-donor-content organic solar cells
Organic solar cells with an electron donor diluted in a fullerene matrix have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier recombination and increased open-circuit voltages. However, the low donor concentration prevents the formation of percolation pathways for holes. Notwithstanding, high (>75%) external quantum efficiencies can be reached, suggesting an effective hole-transport mechanism. Here, we perform a systematic study of the hole mobilities of 18 donors, diluted at ∼6 mol % in C60, with varying frontier energy level offsets and relaxation energies. We find that hole transport between isolated donor molecules occurs by long-range tunneling through several fullerene molecules, with the hole mobilities being correlated to the relaxation energy of the donor. The transport mechanism presented in this study is of general relevance to bulk heterojunction organic solar cells where mixed phases of fullerene containing a small fraction of a donor material or vice versa are present as well
Hole transport in low-donor-content organic solar cells
Organic solar cells with an electron donor diluted in a fullerene matrix have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier recombination and increased open-circuit voltages. However, the low donor concentration prevents the formation of percolation pathways for holes. Notwithstanding, high (>75%) external quantum efficiencies can be reached, suggesting an effective hole-transport mechanism. Here, we perform a systematic study of the hole mobilities of 18 donors, diluted at ∼6 mol % in C60, with varying frontier energy level offsets and relaxation energies. We find that hole transport between isolated donor molecules occurs by long-range tunneling through several fullerene molecules, with the hole mobilities being correlated to the relaxation energy of the donor. The transport mechanism presented in this study is of general relevance to bulk heterojunction organic solar cells where mixed phases of fullerene containing a small fraction of a donor material or vice versa are present as well
Emissive and charge-generating donor–acceptor interfaces for organic optoelectronics with low voltage losses
Intermolecular charge-transfer states at the interface
between electron donating (D) and accepting (A) materials
are crucial for the operation of organic solar cells but can also
be exploited for organic light-emitting diodes. Non-radiative
charge-transfer state decay is dominant in state-of-the-art
D–A-based organic solar cells and is responsible for large
voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields
in the 0.01–0.0001% range. In contrast, the electroluminescence external quantum yield reaches up to 16% in D–A-based
organic light-emitting diodes. Here, we show that proper
control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum
yield within a single, visible-light-emitting D–A system. This
leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These
results unify the description of the electro-optical properties
of charge-transfer states in organic optoelectronic devices
and foster the use of organic D–A blends in energy conversion
applications involving visible and ultraviolet photons
Alkyl Branching Position in Diketopyrrolopyrrole Polymers Interplay between Fibrillar Morphology and Crystallinity and Their Effect on Photogeneration and Recombination in Bulk Heterojunction Solar Cells
Diketopyrrolopyrrole
(DPP)-based donor–acceptor copolymers
have gained a significant amount of research interest in the organic
electronics community because of their high charge carrier mobilities
in organic field-effect transistors (OFETs) and their ability to harvest
near-infrared (NIR) photons in solar cells. In this study, we have
synthesized four DPP-based donor–acceptor copolymers with variations
in the donor unit and the branching point of the solubilizing alkyl
chains (at the second or sixth carbon position). Grazing incidence
wide-angle X-ray scattering (GIWAXS) results suggest that moving the
branching point further away from the polymer backbone increases the
tendency for aggregation and yields polymer phases with a higher degree
of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of
the energetics of the neat polymer and blend films reveals that the
charge-transfer state energy (ECT) of
the blend films lies exceptionally close to the singlet energy of
the donor (ED*), indicating near zero
electron transfer losses. The difference between the optical gap and
open-circuit voltage (VOC) is therefore
determined to be due to rather high nonradiative (≈ 418 ±
13 mV) and unavoidable radiative voltage losses (≈ 255 ±
8 mV). Even though the four materials have similar optical gaps, the
short-circuit current density (JSC) covers
a vast span from 7 to 18 mA cm–2 for the best performing
system. Using photoluminescence (PL) quenching and transient charge
extraction techniques, we quantify geminate and nongeminate losses
and find that fewer excitons reach the donor–acceptor interface
in polymers with further away branching points due to larger aggregate
sizes. In these material systems, the photogeneration is therefore
mainly limited by exciton harvesting efficiency