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Transparent and Highly Responsive Phototransistors Based on a Solution-Processed, Nanometers-Thick Active Layer, Embedding a High-Mobility Electron-Transporting Polymer and a Hole-Trapping Molecule Lorenzo Caranzi,†,∥ Giuseppina Pace,† Mauro Sassi,‡ Luca Beverina,‡ and Mario Caironi*,† †
Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milano, Italy ‡ Dipartimento di Scienza dei Materiali e INSTM, Università di Milano-Bicocca, Via Roberto Cozzi 55, 20125 Milano, Italy ∥
S Supporting Information *
ABSTRACT: Organic materials are suitable for light sensing devices showing unique features such as low cost, large area, and flexibility. Moreover, transparent photodetectors are interesting for smart interfaces, windows, and displayintegrated electronics. The ease of depositing ultrathin organic films with simple techniques enables low light absorbing active layers, resulting in the realization of transparent devices. Here, we demonstrate a strategy to obtain high efficiency organic photodetectors and phototransistors based on transparent active layers with a visible transmittance higher than 90%. The photoactive layer is composed of two phases, each a few nanometers thick. First, an acceptor polymer, which is a good electrontransporting material, on top of which a small molecule donor material is deposited, forming noncontinuous domains. The small molecule phase acts as a trap for holes, thus inducing a high photoconductive gain, resulting in a high photoresponsivity. The organic transparent detectors proposed here can reach very high external quantum efficiency and responsivity values, which in the case of the phototransistors can be as high as ∼74000% and 340 A W−1 at 570 nm respectively, despite an absorber total thickness below 10 nm. Moreover, frequency dependent 2D photocurrent mapping allows discrimination between the contribution of a fast but inefficient and highly spatially localized photoinduced injection mechanism at the electrodes, and the onset of a slower and spatially extended photoconductive process, leading to high responsivity. KEYWORDS: organic photodetectors, transparent photodetectors, phototransistors, photoconductors, photocurrent mapping, donor−acceptor organic semiconductor
1. INTRODUCTION Light sensing concerns a wide range of applications in industry, home automation, security, logistics, and research. Photodetectors (PDs) need to combine a high level of transparency in the visible spectrum, i.e., a transmittance >90%, and a high response to be appealing for applications such as smart glasses, windows, displays and touchless interactive surfaces.1 The challenge in the development of such PDs consists in (i) an obvious trade-off between transmission losses and external quantum efficiency (EQE) and (ii) the ease of integration of such highly transparent devices in large-area surfaces which are not necessarily rigid and flat. The latter issue can be effectively overcome by adopting solution processable absorbers, such as organic semiconductors/quantum dots heterojunctions,2,3 graphene-based heterostructures,4,5 and hybrid perovskites semiconductors.6,7 This in turn affords low temperature processing, a high level of flexibility in the design and geometry, and compatibility with a vast range of glass or plastic substrates.8−10 Organic materials offer a wide variety of © XXXX American Chemical Society
benefits, such as high absorption coefficients, facile chemical tunability,11 easy device integration and both panchromatic and selective detection from UV to near-infrared (NIR).12−18 Although high EQE in transparent organic photodetectors (OPD) can be obtained for detection in the NIR region,19,20 it is not trivial to devise a transparent device that detects visible light signals.21 Here we propose a strategy to obtain both high transparency and high photoresponsivity in an organic photoactive medium. We combine a very thin ( 200 Hz, photogeneration in the channel center is almost not contributing to the collected charges in both device configurations, highly limiting the overall device efficiency. Instead, the localization of the active area next to the electron injecting electrode suggests that a photoinduced injection mechanism is controlling the device performance. A photoinduced injection mechanism was observed in organic photo-
Figure 7. Schematic drawing illustrating the potential drop at the electrodes for the device based on (a) the pristine polymer and (b−d) the bicomponent device. Schemes b−d show the electron injection and collection at the opposite electrodes in the planar photodetector (b) before and (c, d) after illumination. (b) In dark conditions, the injected holes are trapped in the squaraine phase in proximity of the holes injecting electrode on the right, resulting in the local flattening of the potential profile. Voltage drop occurs mainly on the electrons injecting electrode on the left. (c, d) When light impinges on the two electrodes, further accumulation of positive charges emphasizes (c) the potential drop on the left electrode and (d) the flattening on the right electrode.
injected from the positively biased electrode are either injected directly into the squaraine phase or into the P(NDI2OD-T2) phase and then immediately transferred to the squaraine, resulting in both cases in a positive space charge due to trapping. This leads to a flattening of the potential profile next to this electrode, while the voltage drop is localized on the remaining part of the channel (Figure 7b). Under local illumination conditions, if light is impinging on the electron injecting electrode (i.e., the grounded electrode, Figure 7c) the photoinduced positive space charge trapped in the donor phase enhances the local electric field, thus shortening the tunneling barrier and increasing the overall current flowing in the device per each absorbed photon. On the opposite electrode, the light exposure would similarly induce an enhanced accumulation of positive traps: nevertheless, at this electrode the positive space charge induced reduction of the electric field has no significant effect on charge collection (Figure 7d). In view of the mechanism described above, we can conclude that the highest probability of charge dissociation and collection, in the higher frequencies range (200−1000 Hz), is due for both devices to the photoinduced increase of current injection caused by the space charge accumulation next to the grounded electrode. The low signal detected when light is G
DOI: 10.1021/acsami.7b05259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
the trapping process, responsible for the activation of the photoconductivity, involves a larger area in the phototransistors than in the planar detector. As a consequence of such larger area distribution of photogenerated traps, the phototransistor temporal dynamic will be slower with respect to the planar photodetectors, as the released charges have to migrate for a longer distance to reach the electrodes.
impinging within the channel is related to the impossibility to activate the photoinduced injection mechanism, since holes are predominantly trapped where generated. At high frequencies the photoconductive mechanism is not active, because there is not enough time to collect and reinject an electron many times to obtain the gain, which can be observed only on much slower time scales. At lower frequencies (1 Hz), the photocurrent signal is observed to arise from a wider area, extending toward the center of the channel in both devices, and to the area on top of the source/drain electrodes in the phototransistor case. This is consistent with an increased photocurrent signal with frequency dependent light exposure time, moving from a purely photoinjecting mechanism to the onset of photoconduction. The increased electron mobility in the accumulated channel of the phototransistor is at the origin of the much stronger increase in photocurrent from 200 to 1 Hz and to the less steep profile along the channel, with respect to the two-terminal detector. In the planar photodetector and at lower frequencies ( 200 Hz): building up of trapped, positive space charge lowers the injection barrier for electrons, giving rise to an enhanced photocurrent only in proximity of the electron injecting electrode. No photoconduction mechanism is involved in this case, since mobile carriers do not have time to recirculate in the device. The onset of photoconduction takes place at lower frequencies (1 Hz), with an extension of the active area from the electron injection electrode well into the channel region, and an overall increased photocurrent profile. As a result of the photoconductive mechanism, the dynamic response is slow, on the order of tens of seconds, indicating possible uses of these detectors for applications involving slow light signals, such as human-machine interfaces, environmental monitoring, position sensors. Although the bandwidth may not be yet sufficient for some of these applications, a strong photosignal would be measured even for shorter pulses than the actual response time given the very high regime photoreponsivity. Overall, our findings describe a general strategy to obtain transparent, solution-processed photodetectors with high efficiency in the visible range and to analyze the main mechanisms occurring in these devices. Our findings constitute a solid basis for the development of transparent solutionprocessable detectors suitable for seamless integration of H
DOI: 10.1021/acsami.7b05259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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interactive and narrow bandwidth optical sensing functionalities in rigid and flexible, distributed, and large-area applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05259. UV−vis absorption spectra of the bicomponent layer, EQE spectrum of the phototransistor, forward and backward scan of the transfer characteristics of the phototransistor, responsivity dependence on the temporal width of a light pulse, photographs of the device, specific detectivity calculations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-Mail:
[email protected]. ORCID
Mauro Sassi: 0000-0002-5529-6449 Luca Beverina: 0000-0002-6450-545X Mario Caironi: 0000-0002-0442-4439 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank D. Natali for useful discussions and G. Calisesi for his important contribution in the first part of the research. L.B. gratefully acknowledges financial support from the European Union Seventh Framework Programme under grant agreement 607232 [THINFACE].
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DOI: 10.1021/acsami.7b05259 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
