Fast Organic Near-Infrared Photodetectors Based on Charge-Transfer Absorption

Abstract

We present organic near-infrared photodetectors based on the absorption of charge-transfer (CT) states at the zinc-phthalocyanine-C 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.

Comment on “Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities”

“…the orange line in Figure b, but is composed of a very steep part at short times (below ≈1 µs) which cannot be fully resolved, followed by a slower response at longer times. This suggests that the transients could be biexponential, with electrons and holes each giving a (roughly) exponential contribution, in line with recent transient photocurrent measurements with high (≈ns) temporal resolution by Ullbrich et al The response of PTB7:PC 71 BM on the other hand seems to be fitted adequately by a monoexponential decay which we attribute to the balanced electron and hole mobilities and, possibly, to the limited temporal resolution of the experimental setup (biexponential decay cannot be adequately resolved). This highlights the main drawback of electrical transient photocurrent measurements—limited temporal resolution.…”

Organic Photodetectors and their Application in Large Area and Flexible Image Sensors: The Role of Dark Current

“…An alternative approach employs a thick ( ≈ 2 μm) BHJ layer to induce charge collection narrowing, in which only weakly absorbed photons produce electrons sufficiently close to the EEL to be collected . Further increase of the EQE in specific wavelength ranges can be obtained by exploiting microcavity effects . Since heterojunction OPDs behave as thin film optical cavities, interference phenomena derived from multiple reflections between two metallic semitransparent electrodes can enhance absorption within the photoactive layer, leading to broadening or narrowing of certain spectral features.…”

Efficiency and spectral performance of narrowband organic and perovskite photodetectors: a cross-sectional review

“…Within the realm of internally filtered devices, the highest EQE of IntF-1 devices is at 13.6% [73]; IOEF|PCL generally perform below 30% [48,67,68,73,84,114,135] (with the exception of the work of Higashi et al which reaches 51% with an optimised architecture) [69]; CCN works typically give EQEs in the 1%-20% range [33,34,85,86,98] (the work of Arca et al reaching an EQE of 47%, is an important exception) [73]. The EQEs of most microcavity-based photodetectors are less or much less than 25% [42,65,[136][137][138][139] (the only exception being found in the work of Tang et al [42] who achieved EQEs up to 50% through detailed modelling and design). This is reflective of the fact that internally filtered and microcavity-based strategies are typically prone to greater photoconversion losses.…”

“…Microcavity-based (organic) photodetectors fare well in terms of spectral width, with most implementations delivering ultranarrowband behaviour (FWHM EQE ≈15-50 nm, see figure 6(a)) [42,65,[136][137][138][139]. It is important to note, however, that microcavity-based devices also provide significant photoresponse in the spectral range where the photoactive material absorbs appreciably.…”