Transparent and Highly Responsive Phototransistors Based on a

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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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05259 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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ACS Applied Materials & Interfaces

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, P.za 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

KEYWORDS: organic photodetectors, transparent photodetectors, phototransistors, photoconductors, photocurrent mapping, donor-acceptor organic semiconductor

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 display-integrated electronics. The ease of depositing ultra-

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thin 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. Firstly, an acceptor polymer, which is a good electron transporting material, on top of which a small molecule donor material is deposited, forming non-continuous 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 photo-induced injection mechanism at the electrodes, and the onset of a slower and spatially extended photoconductive process, leading to high responsivity.

1. INTRODUCTION Light sensing concerns a wide range of applications in industry, home automation, security, logistic 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


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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 heterostructures4,5 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 benefits, such as high absorption coefficients, facile chemical tunability11, easy device integration and both panchromatic and selective detection from UV to near-infrared (NIR). 12–18 While high EQE in transparent organic photodetectors (OPD) can be fabricated 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 (< 10 nm) nanostructured layer of a good electron transporting polymer with a non-continuous, molecularly thick, top-layer composed of an electron donor squaraine dye. In such donor-acceptor heterojunction,22,23 showing a light transmission higher than 90% from 400 nm to 800 nm, the dye acts as a trap for holes and triggers a photoconductive gain mechanism.24–27 In a planar photodetector configuration a responsivity of 0.6 A W-1 at 570 nm is achieved, corresponding to an EQE of 130 %. In order to further improve the photoresponsivity, we exploit the same active layer in a fieldeffect transistor geometry, which allows to electrostatically enhance electron conductivity in the accumulated channel, thus introducing a gain boosting mechanism. The fabricated phototransistor 28–42 achieves a very high responsivity of 340 A W-1 at 570 nm, corresponding to an EQE of ∼74000 %. Importantly, we also investigate the photogeneration mechanism of both architectures by sub-micrometer photocurrent mapping, highlighting the most efficient


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photoactive areas along the active channels as a function of the modulation frequency of the impinging light. An inefficient photoinduced charge injection mechanism dominates at higher frequencies and is strongly localized at the electron injecting electrode. The onset of a photoconduction mechanism, which is recorded at lower frequencies (< 200 Hz), extends the active area into the active channel and strongly increases the photoresponse. 2. EXPERIMENTAL SECTION 2.1. Materials: the hydrazonic Squaraine47 and P(NDI2OD-T2)43 were synthesized according to the published procedures. HPLC grade ethanol and chloroform were purchased from Sigma Aldrich and used directly without further purification. 2.2. Samples Preparations: coplanar, 30 nm thick, Au interdigitated electrodes were patterned on a Corning 1737F glass substrate with a lift-off photolithographic process. A layer of P(NDI2OD-T2) was deposited on top by spin coating a 1.5 g l-1 mesitylene solution at 1000 rpm for 30 s. The films were annealed at 180 °C for 40 min in a controlled nitrogen atmosphere. Then, an ethanol solution (2.5 g l-1) of squaraine was deposited by dynamic spin coating, i.e. with the substrates already rotating, at 4000 rpm for 1 min. Subsequently, the samples were again annealed at 180 °C for 40 min in a controlled nitrogen atmosphere. For the phototransistors, the dielectric layer (CYTOP CTL-809M dielectric from Asahi Glass) was spin coated at 5000 rpm for 1 min in order to obtain a 600 nm thick layer. Finally, a 12 nm semi-transparent gate electrode was thermally evaporated in two steps: firstly an 8 nm thick layer of aluminum, followed by a 4 nm thick layer of gold. All solvents were purchased from Sigma Aldrich at the highest purity and employed without further purification. Active area of devices goes from 0.005 cm2 for the 20 µm channel length case to 0.035 cm2 for the 5 µm case.


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2.3. UV-Vis absorption and transmission: a Varian Cary 50 Spectrophotometer has been used to evaluate thin film transmittance and absorbance after deposition of the organic layer on the same Corning glass substrates as used for device fabrication. 2.4. Optoelectronic Measurements: current-voltage (IV) measurements in dark and under white light (6400 K, 10.9 mW cm−2) were performed in a nitrogen glovebox and acquired with an Agilent B1500A Semiconductor Parameter Analyzer. All the time dependent photo-current, IV and EQE measurements were acquired with the same Semiconductor parameter analyzer. EQE spectra were acquired by illuminating the devices with a set of LEDs covering the spectral range from 370 and 710 nm, each showing ∼ 1 mW cm−2. EQE measurements, dynamics and IV characteristics were all obtained in a controlled nitrogen atmosphere. 2.5. Photocurrent maps: Maps were acquired with a homemade confocal microscope. The light source was generated by a supercontinuum laser (SuperK Extreme, NKT Photonics). Monochromatic light (475-1100 nm, 2-5 nm line widths) was obtained from and acousto-optic modulator (SuperK Select, NKT Photonics) coupled to the laser beam. The monochromatic and polarized incident light beam was focused on the sample with a 0.7 N.A. objective (Nikon S Plan Fluor 60×) with sub-micrometer resolution (spot size ≈ 500 nm). The monochromatic beam (λ = 690 nm), was modulated at the frequency indicated in the text and the laser power was kept constant at 4.2 µW. The sample was mounted on a home-made chamber, provided with electrical feedthroughs for connection to the sample electrodes and kept in an inert atmosphere under continuous N2 ﬂow for the entire duration of measurement. The photocurrent was measured by connecting the device to a voltage source and by monitoring the current with a lock-in ampliﬁer (SR830, Stanford Research), with the laser light modulated at 3.3 kHz. The raster scan of the sample is operated by a 3D piezoelectric stage (P-517, Physik Instrumente). A dual channel


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source-measure unit (Agilent B2912A) is used to apply voltage to the gate and drain electrodes. The photocurrent signal was collected during the raster scan from the source electrode and amplified with a transimpedance amplifier (Femto DHPCA-100) before entering in a lock-in amplifier (SR830 DSP, Stanford Research Systems). Prior to each measurement, the phase of the lock-in amplifier is synchronized with the light frequency (f) pulse allowing detecting only the light frequency dependent differential photocurrent signal. The lock-in time constant is selected to be 3 times 1/f. The pixel size selected in the raster scan of the channel length is equal to 333 nm. A custom LabView program is used for all data acquisition. 3. RESULTS AND DISCUSSION 3.1. Thin film Optical and morphological characterization We first fabricated and characterized the bi-component thin films based on the polymer acceptor and the small molecule donor, with the purpose to characterize their transparency (Figure 1). The bottom electron-transporting semiconductor layer is formed by the naphthalenediimide (NDI)-based co-polymer poly{[N,N’-bis(2-octyldodecyl)-naphthalene1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} (P(NDI2OD-T2) (scheme


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Figure 1), which was spin coated from a diluted mesitylene solution.

Figure 1. (a) Schematic of the planar photodetector: a bottom layer of P(NDI2OD-T2) and a non-homogeneous layer of the squaraine dye. (b) Optical transmission of the active layer (grey line) and of the active layer with glass (black line). (c) The energy level diagram shows the two


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different photogeneration mechanisms, one following the squaraine dye light absorption and the other following the P(NDI2OD-T2) light absorption. The pristine P(NDI2OD-T2) thin film, which is only a few nanometers thick, is characterized by a compact layer of fibril-like microstructures, typically obtained owing to chain preaggregation in solution in the solvent used, as we extensively reported in previous works.43 Atomic force microscopy (AFM) images (Figure 2a) further confirm the presence of such a mesoscale fibrillar network. We have already demonstrated this to be mostly responsible for the optimal charge transport achieved in this semiconductor.44 The line profile reported below Figure 2a shows the surface cross section extracted from the AFM pictures in correspondence of a bundle of contiguous P(NDI2OD-T2) fibrils, highlighting their mesoscopic nature and typical roughness. Such profile is representative of the fibrils homogeneous distribution and relative height across all of the films surface.


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Figure 2. AFM maps of the P(NDI2OD-T2) layer, characterized by a fibrillary topography, before (a) and after (b) the deposition of the second layer of the squaraine derivative. The r.m.s. surface roughness value is 0.55 nm for P(NDI2OD-T2) only and 0.64 nm after the deposition of the squaraine. Below each image we reported the line profile acquired along the image section indicated by the black line, showing the local roughness of the film. The rationale behind the design of a photoactive phase foresees the proper selection of a donor molecule fulfilling the following conditions: i) being processable from an orthogonal solvent with respect to the bottom layer, in order to preserve the microstructure and electronic properties of the acceptor phase; ii) having the suitable frontier energy levels to generate a photoactive heterojunction in combination with the P(NDI2OD-T2) acceptor. For these reasons we adopted the diphenylhydrazone end-capped squaraine molecule45–47 which is soluble in ethanol and whose molecular structure is reported in Fig. 1a. The HOMO-LUMO energy diagram showed in Figure 1c47,48 schematically reports the photogeneration mechanism following the light excitation of the squaraine molecule and of the polymer. The donor molecule was spin-coated from an ethanol solution on top of the electron transporting polymer. The low concentration used for both materials results in a < 10 nm thick layer with a very low optical density (Figure S6). As shown by the transmission spectrum of the bi-component active layer (Figure 1b), more than 90 % of photons are transmitted from 400 nm to 800 nm, and even when the glass substrate is considered, transmittance is higher than 85 % in the same range. UV-Vis absorption spectra of the pristine materials (Figure S1) show the visible absorption band maxima of squaraine and P(NDI2OD-T2) appearing at around 600 and 700 nm, respectively. In particular, the P(NDI2OD-T2) band at 700 nm is typically observed in films where the polymer chains aggregate in a fibril-like structure.49 The same fibril-like topography of


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the bottom layer, already seen in Figure 2a, is still largely visible in the AFM image of the bicomponent layer (Figure 2b): this indicates that the morphology of the acceptor bottom layer is not significantly altered by the subsequent deposition of the squaraine dye and that there are areas of the photoactive film where small molecules are not present. The latter point is confirmed by comparing the line profile acquired along the bi-component surface image (bottom Figure 2b) and that of the pristine polymer fibrillar film (bottom Figure 2a).We can observe that indeed the surface roughness of the fibrillar bundles is not affected by the squaraine layer deposition. We can therefore exclude the presence of large squaraine aggregates, and, since the polymer fibers topography profile is not altered upon deposition of the squaraine dye, the formation of a continuous layer of the small molecule component can also be excluded. Nevertheless, the UV-Vis spectra acquired on the bi-component layer demonstrates the presence of both materials in the film (Figure 1c). All the above experimental evidences lead to the conclusion that the squaraine deposition results in a non-homogenous molecularly thin layer of the small molecule, which creates a discontinuous, charge trapping phase for holes. 3.2. Planar photodetector and Field-Effect phototransistor devices characterization In Figure 3a the IV characteristics of the planar photodetector with a channel length of 20 µm are reported. We also fabricated a reference device containing only a P(NDI2OD-T2) layer which shows a low dark current (1 nA at 100 V). When illuminated with a polychromatic light (10.9 mW cm-2), the reference device produces a very modest photocurrent signal (2 nA at 100 V). Instead, in presence of the photoactive donor-acceptor film we found a dark current of 330 nA, when the photodetector is polarized at 100 V, and a photocurrent up to 7.2 μA under the


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same white light illumination. The enhancement of dark currents with respect to the pristine P(NDI2OD-T2) device is likely due to positive charge trapping in the active area close to the hole injecting electrode. In fact, holes are injected and then subsequently trapped in the squaraine, owing to its discontinuous morphology, giving rise to a space charge area of positive charges that induces a reduction of the electric field in this region. Therefore, the potential drop is confined on the grounded electrode, where electrons injection takes place. A higher electric field fosters negative charge injection, thus increasing the dark currents. Figure 3b shows the wavelength dependent responsivity and External Quantum Efficiency (EQE) of the planar photodetector polarized at 100 V. The broad responsivity spectrum retraces the absorption features of both active materials. The maximum responsivity value is 0.6 A W-1 at 570 nm. Notably, the EQE value at 570 nm exceeds 100%, therefore indicating the presence of a photoconductive gain in the photogeneration mechanism. At this wavelength, excitons are photogenerated in the squaraine, which then dissociate at the small molecule-polymer interface via electron transfer to the polymer phase, as predicted by the energy diagram, reported in Figure 1c, showing that the process is energetically strongly favored. The complementary process of hole transfer to the squaraine, upon light absorption from the polymer phase, might appear less likely because of the small offset between the HOMO levels. Since we can clearly distinguish the absorbing peaks of both phases in our EQE spectra, this process must be as efficient as the former electron transfer. Owing to the discontinuity of the squaraine phase, holes are trapped, while electrons, which are transferred to the polymer, can drift to the collecting electrodes under the effect of the applied voltage. Such strong transport unbalance triggers a photoconductive gain, justifying the reported EQE values, which are exceptionally high considering that at 570 nm less than 5 % of the photons are absorbed. Efficient photocurrent generation is also observed


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in the P(NDI2OD-T2) absorption region, meaning that the photons absorbed by the polymer also lead to charge generation and dissociation.

Figure 3. (a) IV curves in dark (solid line) and white light conditions (dashed lines, 10.9 mW cm2

) of the device based on P(NDI2OD-T2)/Squaraine bilayer (in blue) and the one based on the

pristine P(NDI2OD-T2) (black). (b) EQE and Responsivity spectra of the planar photodetector. Dashed line refers to the absorption spectra of the bi-component layer on solid substrate. The efficiency peaks at 570 nm and at 710 nm fall respectively in the squaraine and P(NDI2OD-T2)


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absorption regions. (c) Transient photocurrent of the planar detector under 570 nm LED illumination (0.29 mW cm-2). In this photoconductive regime a long response time is expected, since the gain is proportional to the number of times a single electron can be transported through the channel and re-injected while maintaining charge neutrality, before recombining with a hole.50 As a consequence, the rise and fall times of the planar and transparent photodetector are in the order of several seconds as reported in Figure 3c, which shows the time dependent photocurrent recorded on a planar detector exposed for 30 s to a 570 nm LED light (0.29 mW cm-2). The device requires around 4.5 s to pass from 10 % to 90 % of the photocurrent saturation value in the rising edge and 32 seconds to pass from 90 % to 10 % in the falling edge. Such long response times are typically observed in planar photodetectors, and are associated to the dispersive diffusion which the photogenerated charges undergo after the light signal is switched off.41,51 Following the successful realization of the two-terminal transparent photodetector we aimed at further improving the device responsivity. The strategy we adopted was to increase electron mobility, since the photoconductive gain is proportional to the mobility of the mobile/faster carrier.50 To enhance electron mobility, we introduced a third semi-transparent aluminum/gold top gate electrode, electrically isolated from the active phase by a transparent polymer dielectric (CYTOP), in order to realize a field-effect transistor (FET) structure (see Figure 4a). Thanks to the FET we enable electrostatic n-type doping of a part of the active phase, and mobility


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increases as a function of charge density.52

Figure 4. (a) Device structure of the phototransistor. (b) Trans-characteristic curves in saturation regime of the reference FET based on the P(NDI2OD-T2) polymer only (grey lines) and of the phototransistor in dark and under the exposure of polychromatic light. (c) Responsivity spectra for devices with different channel lengths. (d) Transient photocurrent of the phototransistor when exposed to a 570 nm LED illumination (0.29 mW cm-2). (e) Photocurrent as a function of the incident light intensity in the OFF-state (open circles) and in the ON-state (closed circles), whereas blue solid lines refer to the fitting curves of the experimental data.


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A direct comparison, between the trans-characteristic curves of a reference FET based on the pristine P(NDI2OD-T2) and of the phototransistor based on P(NDI2OD-T2)/squaraine film, is reported in Figure 4b (channel length L = 20 µm, channel width W = 10 mm). The current of the bi-component phototransistor can be modulated as a function of the gate voltage both in dark and under white light irradiation, showing in both conditions n-type saturation mobility of 0.17 cm2 V-1 s-1. The bi-component FET displays an electron accumulation current which is almost identical to the one measured in the reference FET, which shows no sensitivity to light exposure and an electron mobility of 0.27 cm2 V-1 s-1. Therefore, electron transport is not substantially disturbed by the presence of the squaraine, indicating that: i) the deposition of the top squaraine layer is not affecting the electronic properties of the electron acceptor P(NDI2OD-T2) phase; ii) the electron transfer from the polymer to the dye is prevented by the energetic barrier formed by the offset of their LUMO levels (around 0.5 eV). On the contrary, positive charge transfer from the polymer to the non-continuous dye is possible (energy diagram Figure 1c), as seen from the active photodissociation process in the planar detector. This determines a strong suppression of the hole current for VGS < 0 V, with respect to the device based on the pristine P(NDI2OD-T2) (Figure 4b) owing to holes being trapped in the squaraine phase. Consequently, in the OFF state the P(NDI2OD-T2)/squaraine device shows very low dark currents (between 10 and 100 pA for -20 V < VGS < 0 V, L = 20 µm). The device reaches a light current to dark current ratio of up to 104 (VGS = - 3 V) when illuminated under polychromatic light. Figure 4c shows the responsivity spectrum of the device in transdiode regime (VGS = VDS = 80 V) as a function of the channel length, from 20 µm down to 5 µm. As in the case of the planar photodetector, the maxima found in the responsivity spectra correspond to the two absorption bands of both materials. For the device with a channel length L = 20 µm, a maximum


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responsivity value of 48 A W-1 is reached at 570 nm, showing that the introduction of the electrical gating results in an increase of almost two orders of magnitude in efficiency with respect to the planar detector. It is reasonable to suggest that this is associated with the improved electron mobility, which is in turn due to the accumulation of the n-type channel at positive VGS. The devices fabricated with channel lengths of 10 and 5 µm show higher responsivity, up to 125 and 340 A W-1 respectively at 570 nm. The same lateral voltage used for the 20 µm channel (VDS = 80 V) is also applied for shorter channels, resulting in an increase of the electric field which consequently affects both dark current and photocurrent. Performances achieved in our phototransistor represent a remarkable boost with respect to previous reports on organic and hybrid detectors which are both solution-processed and semi-transparent in the UV-Vis range.19, 42

Figure 4d reports the photocurrent transient acquired under 570 nm LED illumination for the phototransistor with a 20 µm channel. The device requires around 5 s to pass from 10 % to 90 % of the saturation value in the rising edge, while in the falling edge the dynamics are slower with respect to what was observed in the planar detector. The device requires around 115 s to pass from 90 % to 10 % of the photocurrent. Such a difference in the relaxation time of the photogenerated carriers in the two kinds of device structures will be better elucidated from the photocurrent mapping presented in the next section. In Figure 4e we report the photocurrent dependence on the incident light intensity, both in the ON state (VGS = VDS = 30 V) and in the OFF state (VGS = -10 V, VDS = 30 V). When the phototransistor is in the ON state, the photocurrent variation can be fitted with a logarithmic function, while in the OFF state the photocurrent shows a linear dependence with respect to the incident optical power (slope of 1.43 nA cm2 mW-1). Therefore, as predicted by the analytical


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model proposed by Romero et al.53, the phototransistor operates in the so-called photovoltaic regime when biased at VGS > VTH and in the photoconductive regime when VGS < VTH, the latter corresponding to the working condition of the planar detector. The cited model has been derived from studies made on inorganic phototransistors.54,55 Although the same behaviors can be identified in previously reported organic phototransistors,56–58 the overall picture of these phenomena is still not entirely clear, especially due to the lack of direct experimental observation of the photogenerated charge density distribution within the active channel at different biasing conditions. The observed current enhancement and threshold voltage shift obtained upon illumination were instead attributed to the lowering of the contact resistance either for injection or extraction of charges.59 Moreover, as suggested by Yu et al.,30 an enhanced injection assisted by trapped charges can play a fundamental role in the photogeneration and collection in highly efficient organic phototransistors. 3.3 Photocurrent mapping Aiming at clarifying in more detail the working mechanism(s) of the two and three terminal detectors proposed here, we performed 2D photocurrent mapping.60,61 We focused a laser beam with a wavelength of 690 nm on the active layer in a confocal microscope geometry, enabling a submicron spatial resolution limited by diffraction (~ 500 nm, 4.2 μW). We modulated the laser intensity at different frequencies and measured the photocurrent signal with a lock-in amplifier. This technique allows us to detect a site-specific photocurrent, which is directly correlated with the local photocurrent generation efficiency. The maps are therefore showing regions of higher and lower photocurrent generation efficiency, a fundamental aspect to be known for discriminating between different working mechanisms. Figure 5 reports the photocurrent maps acquired at different light modulation frequencies on a planar photodetector biased at 80 V (5a,


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b, c) and on a phototransistor operating in transdiode regime at VGS = VDS = 40 V (5d, e, f). The photocurrent maps obtained by biasing first the left electrode and then the right electrode, while keeping the same scan direction, show the same profiles, indicating that the influence of secondary effects due to scanning direction of the laser are negligible.


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Figure 5. Photocurrent maps acquired by raster scanning a 690 nm laser beam modulated at 1 Hz (a), 200 Hz (b) and 1000 Hz (c), for a planar photodetector polarized at 80 V. Photocurrent maps acquired on the phototransistor (VGS = VDS = 40 V) at 1 Hz (d), 200 Hz (e) and 1000 Hz (f). Dashed areas indicate the presence of the electrodes. In both devices, the channel is 20 µm long and the right electrode is positively biased with respect to the left electrode.

In Figure 6 we report the photocurrent profiles acquired along the channel at different light modulation frequencies for the planar photodetector (Figure 6a) and phototransistor (Figure 6b). In both device configurations, we could measure a maximum in the photocurrent signal located next to the grounded electrode edge, i.e. the electron injecting electrode, which is the source electrode for the phototransistor. The site-specific photocurrent increases by reducing the light modulation frequency (f), and its maximum value along the channel goes from 0.4 nA (f = 10 kHz) to 3 nA (f = 1 Hz) for the photodetector and from ∼1 nA (f = 10 kHz) to 38 nA (f = 1 Hz) for the phototransistor.

Figure 6. Photocurrent line profiles measured at different light modulation frequencies for (a) the planar photodetector at 80 V and (b) the phototransistor at VGS = VDS = 40 V.


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For both devices, by decreasing the light modulation frequency from 10 kHz to 1 Hz, we observe a broadening of the photocurrent signal extending farther into the channel. Besides showing a higher local photocurrent, in agreement with global photoresponse of the whole device, phototransistors are characterized by a higher photocurrent intensity on top of the electrodes, consistent with the formation of an accumulated channel in that area. Importantly, the same experiment was performed on reference devices made with the pristine polymer, in absence of the squaraine dye. These devices showed no significant photocurrent signal, further indicating that the photocurrent yield is strictly dependent on the formation of the polymer dye heterojunction. The photocurrent maps indicate that at higher frequencies, f > 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 photo-induced injection mechanism is controlling the device performance. A photo-induced injection mechanism was observed in organic photodetectors with a vertical diode configuration by Li et al. for a P3HT:PC71BM blend 26 and by Hiramoto et al. for perylene pigment films.27 This effect consists in an enhanced injection due to the space charge field generated next to the electrode and is caused by photo-induced charges trapped in its proximity. In our particular case, the photoinduced trapped holes can build up an additional electric field in the proximity of the electrode interface that further favors the electrons injection in the P(NDI2OD-T2) transporting layer. A simple scheme representing the processes occurring when a positive space charge accumulates at the electrodes, in dark and under light exposure, is depicted in Figure 7. In dark conditions, holes injected from the positively biased electrode are either injected directly into the squaraine phase or into the P(NDI2OD-T2) phase


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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).

Figure 7. Schematic drawing illustrating the potential drop at the electrodes for the device based on the pristine polymer (a) and on the bi-component device (b, c, d). Schemes b, c, d show the


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electron injection and collection at the opposite electrodes in the planar photodetector before (b) and after (c,d) 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) When light impinges on the two electrodes, further accumulation of positive charges emphasizes the potential drop on the left electrode (c) and the flattening on the right electrode (d).

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 Hz - 1000 Hz), is due for both devices to the photo-induced increase of current injection caused by the space charge accumulation next to the grounded electrode. The low signal detected when light is impinging within the channel is related to the impossibility to activate the photo-induced injection mechanism, since holes are predominantly trapped where generated. At high frequencies the photoconductive mechanism is not active, since there is not enough time to collect and re-inject 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 towards 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


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photocurrent from 200 Hz 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 (< 1 kHz), we observe that a second photocurrent maximum appears within the channel at a relative distance of ~ 5 µm from the grounded electrode. At 1 Hz, when spatial extension spanned by transported electrons starts to be significant, this feature becomes dominant and broadens into the channel, rising in intensity and eventually hiding the contribution of the photo-induced injection at the electrode edge. The presence of the maximum at 5 µm can be due to two factors: on the one hand the farther the photoexcited area is from the electron injecting electrode, the less the influence of the photoinduced trapped holes on the injection of negative charges becomes. On the other, the larger the distance is between the photoexcited region and the electron collecting electrode, the higher is the probability for photogenerated electrons to undergo recombination. We speculate that the trade-off between these two factors gives rise to a maximum in the inner part of the channel. Photo-current maps in phototransistors are comparable in photocurrent distribution and intensity to the case of the photodetector until 200 Hz, where similar considerations apply. The gating effect is much more evident at longer timescales, i.e. at 1 Hz. The formation of the electron accumulated channel enhances the electrons mobility and reduces their transit time and recombination probability, thus leading to easier carrier extraction along the whole channel and eventually achieving an onset of the photoconductive mechanism. This explains why at 1 Hz the photocurrent profile still presents a maximum at the edge of the source electrode and at the same time the photocurrent does not quickly decrease along the channel, while it remains considerably high along the full active area. In fact, despite the lower lateral voltage applied (30 V), the


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amount of photocurrent collected at 1 Hz is higher than in the simple planar photodetector configuration polarized at 80 V. This is in agreement with the almost two orders of magnitude higher responsivity obtained in the phototransistor. An important outcome linked to mapping the photocurrent density distribution along the channel of the devices is the possibility to provide an explanation for the observed slower phototransistor fall time in the decay dynamics in spite of its higher responsivity. In fact, in the case of the planar photodetector, even at that low frequency (1 Hz), we observe that the photocurrent mostly localizes next to the electrode. In the phototransistor there is an almost homogeneous photocurrent distribution all along the channel. This also implies that 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. 4. CONCLUSIONS We have proposed a strategy for the fabrication of solution processed organic detectors combining high transparency and high responsivity. Our approach is based on the use of a thin, nanostructered layer of a high mobility acceptor polymer, interfaced with a non-homogeneous and molecularly distributed trapping phase of a small molecule donor. By exploiting orthogonal solvent deposition of the squaraine dye, the donor phase fulfils its hole trapping functionality without interfering with the electron transport properties of the selected high mobility polymer. We compared two and three terminal devices: the introduction of a gate electrode in order to


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obtain a phototransistor proved successful in enhancing the photoresponse by two orders of magnitude with respect to the two terminal device. This enhancement derives from a higher photoconductive gain owing to an improved electronic transport in the accumulated channel. Thanks to the proposed strategy, we have demonstrated highly responsive photodetectors based on a solution-processed organic active layer that transmits more than 90 % of light from 400 nm to 800 nm. We achieved a photoresponsivity of 0.6 A W-1 at 570 nm (EQE ≈ 130 %) in simple two terminal devices, and 340 A W-1 at 570 nm (EQE ≈ 74000 %) in phototransistor configuration. A simple estimation of the specific detectivity, assuming that the shot noise of the dark currents is the dominant factor, returns a value higher than 1012 cm Hz0.5 W-1 for all devices (see Supporting Information for details). The 2D photocurrent mapping rationalizes the success of the proposed strategy, revealing the presence of two concomitant mechanisms operating in the device at different frequency regimes over different active areas. A poorly efficient, spatially localized photo-injection effect dominates at high frequency (f > 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, in 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. While the bandwidth may not be yet sufficient for


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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 solution-processable detectors suitable for seamless integration of interactive and narrow bandwidth optical sensing functionalities in rigid and flexible, distributed and large-area applications.

ASSOCIATED CONTENT Supporting Information. UV-Vis absorption spectra of the bi-component 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. AUTHOR INFORMATION Corresponding Author *(M.C). E-Mail: [email protected] ACKNOWLEDGMENT


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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 n° 607232 [THINFACE]. REFERENCES (1)

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Graphical TOC 83x33mm (300 x 300 DPI)


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Transparent and Highly Responsive Phototransistors Based on a

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