Ultrasensitive organic phototransistors with multispectral response based on thin-film/single-crystal bilayer structures

Field-effect transistors based on organic single-crystals (SCs) have been often used as tools to investigate the intrinsic properties of organic semiconductors, due to the long-range order of the active medium.^1,21. E. Menard, V. Podzorov, S.-H. Hur, A. Gaur, M. E. Gershenson, and J. A. Rogers, Adv. Mater. 16, 2097 (2004). https://doi.org/10.1002/adma.2004010172. I. N. Hulea, S. Russo, A. Molinari, and A. F. Morpurgo, Appl. Phys. Lett. 88, 113512 (2006). https://doi.org/10.1063/1.2185632 They can serve as light-sensing optoelectronic devices termed phototransistors (OPTs), if the semiconductor comprising the active channel is photosensitive.³3. M. Mas-Torrent, P. Hadley, N. Crivillers, J. Veciana, and C. Rovira, Chem. Phys. Chem. 7, 86 (2006). https://doi.org/10.1002/cphc.200500325 In recent years, research on OPTs has been active on bringing the best of organic materials, application-tuned functionality, to an increasing number of applications, e.g., light-induced switches,⁴4. N. Marjanović, T. B. Singh, G. Dennler, S. Günes, H. Neugebauer, N. Sariciftci, R. Schwödiauer, and S. Bauer, Org. Electron. 7, 188 (2006). https://doi.org/10.1016/j.orgel.2006.01.002 inverters,⁵5. J. Labram, P. Wöbkenberg, D. Bradley, and T. Anthopoulos, Org. Electron. 11, 1250 (2010). https://doi.org/10.1016/j.orgel.2010.04.024 memory circuits,⁶6. K. H. Kim, S. Y. Bae, Y. S. Kim, J. A. Hur, M. H. Hoang, T. W. Lee, M. J. Cho, Y. Kim, M. Kim, J.-I. Jin, S.-J. Kim, K. Lee, S. J. Lee, and D. H. Choi, Adv. Mater. 23, 3095 (2011). https://doi.org/10.1002/adma.201100944 and highly sensitive image sensors.⁷7. K.-J. Baeg, M. Binda, D. Natali, M. Caironi, and Y.-Y. Noh, Adv. Mater. 25, 4267 (2013). https://doi.org/10.1002/adma.201204979 Typically, OPTs are more sensitive than photodiodes, with lower dark-levels, due to their built-in capacity of providing large signal amplification.⁷7. K.-J. Baeg, M. Binda, D. Natali, M. Caironi, and Y.-Y. Noh, Adv. Mater. 25, 4267 (2013). https://doi.org/10.1002/adma.201204979 Their responsivity (R_ph) can be tuned by the voltage applied to source/drain/gate (S/D/G) electrodes.⁶6. K. H. Kim, S. Y. Bae, Y. S. Kim, J. A. Hur, M. H. Hoang, T. W. Lee, M. J. Cho, Y. Kim, M. Kim, J.-I. Jin, S.-J. Kim, K. Lee, S. J. Lee, and D. H. Choi, Adv. Mater. 23, 3095 (2011). https://doi.org/10.1002/adma.201100944 Unlike photodiodes,⁸8. J. D. Zimmerman, V. V. Diev, K. Hanson, R. R. Lunt, E. K. Yu, M. E. Thompson, and S. R. Forrest, Adv. Mater. 22, 2780 (2010). https://doi.org/10.1002/adma.200904341 OPTs can reach external quantum efficiencies (EQEs) in excess⁹9. H. Yu, Z. Bao, and J. H. Oh, Adv. Funct. Mater. 23, 629 (2013). https://doi.org/10.1002/adfm.201201848 of 100%, with spectral coverage depending on the materials used.

To broaden the spectral response and increase charge-separation, heterojunctions (HJs) of donor and acceptor materials have been in used in OPTs.^4,54. N. Marjanović, T. B. Singh, G. Dennler, S. Günes, H. Neugebauer, N. Sariciftci, R. Schwödiauer, and S. Bauer, Org. Electron. 7, 188 (2006). https://doi.org/10.1016/j.orgel.2006.01.0025. J. Labram, P. Wöbkenberg, D. Bradley, and T. Anthopoulos, Org. Electron. 11, 1250 (2010). https://doi.org/10.1016/j.orgel.2010.04.024 While this strategy is common practice in organic solar cells,^10,1110. F. Yang, M. Shtein, and S. R. Forrest, Nat. Mater. 4, 37 (2005). https://doi.org/10.1038/nmat128511. W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger, Adv. Funct. Mater. 15, 1617 (2005). https://doi.org/10.1002/adfm.200500211 its application to OPTs is still limited.⁷7. K.-J. Baeg, M. Binda, D. Natali, M. Caironi, and Y.-Y. Noh, Adv. Mater. 25, 4267 (2013). https://doi.org/10.1002/adma.201204979 Devices based on solution-processed blends exhibit low charge-mobilities (10⁻² cm² V⁻¹ s⁻¹), which limit the use of OPTs as regular transistors.⁴4. N. Marjanović, T. B. Singh, G. Dennler, S. Günes, H. Neugebauer, N. Sariciftci, R. Schwödiauer, and S. Bauer, Org. Electron. 7, 188 (2006). https://doi.org/10.1016/j.orgel.2006.01.002 By using single-crystals as active channels, this problem diminishes.³3. M. Mas-Torrent, P. Hadley, N. Crivillers, J. Veciana, and C. Rovira, Chem. Phys. Chem. 7, 86 (2006). https://doi.org/10.1002/cphc.200500325 Yet, until now, SC-based OPTs have been restricted to single-layer architectures, where spectral coverage is limited by the absorption range of the one single material used.^3,6,73. M. Mas-Torrent, P. Hadley, N. Crivillers, J. Veciana, and C. Rovira, Chem. Phys. Chem. 7, 86 (2006). https://doi.org/10.1002/cphc.2005003256. K. H. Kim, S. Y. Bae, Y. S. Kim, J. A. Hur, M. H. Hoang, T. W. Lee, M. J. Cho, Y. Kim, M. Kim, J.-I. Jin, S.-J. Kim, K. Lee, S. J. Lee, and D. H. Choi, Adv. Mater. 23, 3095 (2011). https://doi.org/10.1002/adma.2011009447. K.-J. Baeg, M. Binda, D. Natali, M. Caironi, and Y.-Y. Noh, Adv. Mater. 25, 4267 (2013). https://doi.org/10.1002/adma.201204979 To this end, further research on OPTs based on SC interfaces and bilayers is of primary importance to the development of high-performance optoelectronic devices.

In this letter, we present OPTs with an active layer comprised of SC rubrene on top of a PC₆₁BM thin-film [Fig. 1]. Such OPTs operate at low-voltage (≤5 V), exhibit an average field-effect mobility (${\mu }_{\text{FE}}$) of 4–5 cm² V⁻¹ s⁻¹, a $I_{\text{ON}/\text{OFF}}$ ratio of 10⁴, and a photosensitivity ($\mathrm{P}=I_{\text{ph}}/I_{\mathrm{d}}$) of 10⁴. They also show an extended responsivity over the entire visible region (400–750 nm) with EQE reaching 52 000%.

The fabrication of the interfaces is similar to that reported in our previous work,¹²12. R. M. Pinto, E. M. Maçôas, and H. Alves, J. Mater. Chem. C 2, 3639 (2014). https://doi.org/10.1039/c4tc00051j except that here we use a Si/SiO₂ substrate and a thinner PC₆₁BM layer. Prior to spin-coating a PC₆₁BM solution on top of the substrate, the SiO₂ surface is cleaned by reactive ion etching (RIE) in an oxygen plasma. PC₆₁BM:chlorobenzene solution (20 mg ml⁻¹) is sonicated overnight (∼12 h, 50 °C), filtered (0.2 μm PTFE), and spin-casted on top of heavily doped n-type Si substrate (5 × 20 mm) with a thermally grown 200 nm thick SiO₂ layer. The latter two act as gate electrode and gate dielectric, respectively. The substrate was held at room temperature during coating, resulting in a smooth PC₆₁BM film (r.m.s. roughness = 0.7 nm), as shown in the atomic force microscopy (AFM) image in Fig. 1(a), and in the supplementary material (Fig. S1, Ref. 1313. See supplementary material at http://dx.doi.org/10.1063/1.4937005 for details on S1: AFM measurements, S2: contact resistance extraction, and S3: EQE behavior with channel length.) The thickness of the films was 60–90 nm, measured with a contact surface profilometer. The films were left air-dry in a laminar flow hood for ∼12 h before the lamination step, to minimize solvent inclusion.

Stripe-like rubrene SCs are grown by physical vapor transport¹⁴14. R. Laudise, C. Kloc, P. Simpkins, and T. Siegrist, J. Cryst. Growth 187, 449 (1998). https://doi.org/10.1016/S0022-0248(98)00034-7 (PVT), under a stream of high-purity Ar, as reported before.^12,1512. R. M. Pinto, E. M. Maçôas, and H. Alves, J. Mater. Chem. C 2, 3639 (2014). https://doi.org/10.1039/c4tc00051j15. H. Alves, R. M. Pinto, and E. S. Maçôas, Nature Commun. 4, 1842 (2013). https://doi.org/10.1038/ncomms2890 The selected rubrene SCs with length(L)/width(W) ratio >1 and thickness t < 500 nm were carefully laminated on top of the PC₆₁BM layer. If channel ($R_{\text{ch}}$) and contact ($R_{\mathrm{c}}$) resistances are comparable, then opting for L/W > 1 minimizes the negative effect of contact resistance on charge-extraction, since $R_{\mathrm{T}}=R_{\text{ch}}+R_{\mathrm{c}}=R_{\mathrm{s}}(\mathrm{L}/\mathrm{W})+R_{\mathrm{c}}$, where $R_{\mathrm{T}}$ and $R_{\mathrm{s}}$ are the total and sheet resistances, respectively. The crystals completely adhere to the surface of the film, guaranteeing the formation of a nanoscale interface [Figs. 1(d) and 1(e)]. The structural integrity of rubrene is preserved with lamination, and this results in a hybrid-phase bilayer junction of a crystalline electron-donor layer (rubrene) and an amorphous acceptor layer (PC₆₁BM). S/D contacts are formed using a water-based carbon solution, deposited at the far edges of the interface across the long axis of crystal growth (b-axis). This is the axis of closest π-stacking and highest-mobility in organic field-effect transistors (OFETs) of single-crystal rubrene.¹1. E. Menard, V. Podzorov, S.-H. Hur, A. Gaur, M. E. Gershenson, and J. A. Rogers, Adv. Mater. 16, 2097 (2004). https://doi.org/10.1002/adma.200401017 The resulting devices have the same bottom-gate/middle-contact (BG/MC) three-terminal configuration also found in, e.g., C₆₀/pentacene OFETs.¹⁶16. E. Kuwahara, Y. Kubozono, T. Hosokawa, T. Nagano, K. Masunari, and A. Fujiwara, Appl. Phys. Lett. 85, 4765 (2004). https://doi.org/10.1063/1.1818336 In our devices, the channel conductance can also be controlled by light irradiation [Fig. 1(c)].

Figures 2(a) and 2(b) present the transfer ($I_{\text{DS}}-V_{\text{GS}}$) and output ($I_{\text{DS}}-V_{\text{DS}}$) characteristics of a representative PC₆₁BM/rubrene OPT (L = 260 μm, L/W ∼ 1.5) measured in the dark, under ambient conditions. The current increases with increasing negative gate voltage, typical of field-effect-induced hole conduction. This is the expected behavior for rubrene, meaning that the interface conduction is dominated by unipolar transport in the p-type layer. Unlike PC₆₁BM/pentacene thin-film based OPTs,⁵5. J. Labram, P. Wöbkenberg, D. Bradley, and T. Anthopoulos, Org. Electron. 11, 1250 (2010). https://doi.org/10.1016/j.orgel.2010.04.024 we did not observe ambipolar behavior in the PC₆₁BM/rubrene devices. We attribute this to the large mobility unbalance (${\mu }_h>100{\mu }_e$) arising from the long-range order of the rubrene SC layer, which allows mobilities as high as 10 cm² V⁻¹ s⁻¹, in contrast to the low electron mobility observed in PC₆₁BM amorphous thin-film transistors (10⁻² cm² V⁻¹ s⁻¹).^1,171. E. Menard, V. Podzorov, S.-H. Hur, A. Gaur, M. E. Gershenson, and J. A. Rogers, Adv. Mater. 16, 2097 (2004). https://doi.org/10.1002/adma.20040101717. M. Chikamatsu, S. Nagamatsu, Y. Yoshida, K. Saito, K. Yase, and K. Kikuchi, Appl. Phys. Lett. 87, 203504 (2005). https://doi.org/10.1063/1.2130712 Note that ambipolar operation in monolithic or bilayer/heterojunction OFETs often requires lower-work function electrodes,^18,1918. S. Z. Bisri, T. Takenobu, T. Takahashi, and Y. Iwasa, Appl. Phys. Lett. 96, 183304 (2010). https://doi.org/10.1063/1.341989919. T. Uemura, M. Yamagishi, Y. Okada, K. Nakayama, M. Yoshizumi, M. Uno, and J. Takeya, Adv. Mater. 22, 3938 (2010). https://doi.org/10.1002/adma.201000480 trap passivating layers,⁵5. J. Labram, P. Wöbkenberg, D. Bradley, and T. Anthopoulos, Org. Electron. 11, 1250 (2010). https://doi.org/10.1016/j.orgel.2010.04.024 and higher operating voltages—none of which were used herein.

The mobility and threshold voltage ($V_{\text{th}}$) in the saturation regime ($V_{\text{DS}}>V_{\text{GS}}$) are 4.9 cm² V⁻¹ s⁻¹ and 0.59 V, respectively, calculated according to $I_{\text{DS}}=(\mu C_{i}W/2L){(V_{\text{GS}}-V_{\text{th}})}^2$ and considering only the capacitance of the SiO₂ layer (C_i = 17.3 nF cm⁻²). The OPTs exhibit low pinch-off voltages, as estimated by V_DS above which $\partial I_{\text{DS}}/\partial V_{\text{DS}}$ becomes constant [Fig. 2(b)].²⁰20. M. Imakawa, K. Sawabe, Y. Yomogida, Y. Iwasa, and T. Takenobu, Appl. Phys. Lett. 99, 233301 (2011). https://doi.org/10.1063/1.3666236 The hole mobility is 5 cm² V⁻¹ s⁻¹, measured in the saturation regime, with average $V_{\text{th}}$ of 0.67 V. The 14 devices present some dispersion due to differences in crystal quality [Fig. 2(c)]. If the capacitance of the PC₆₁BM layer is added (ϵ_r = 3, t = 90 nm), the average hole mobility drops to 40% of the original value (5 → 2 cm² V⁻¹ s⁻¹), still in line with data reported for rubrene single-crystal FETs. The non-zero threshold voltage could be related to the existence of a built-in channel, formed from partial charge-transfer between PC₆₁BM and rubrene,¹²12. R. M. Pinto, E. M. Maçôas, and H. Alves, J. Mater. Chem. C 2, 3639 (2014). https://doi.org/10.1039/c4tc00051j or to a non-negligible density of charge traps at the active channel/dielectric interface.²¹21. R. de Boer, M. Gershenson, A. Morpurgo, and V. Podzorov, Phys. Status Solidi A 201, 1302 (2004). https://doi.org/10.1002/pssa.200404336 Overall, the performance of these devices as standard OFETs is in line with other systems based on p-type organic SCs, such as acenes and tetrathiafulvalene derivatives.^3,213. M. Mas-Torrent, P. Hadley, N. Crivillers, J. Veciana, and C. Rovira, Chem. Phys. Chem. 7, 86 (2006). https://doi.org/10.1002/cphc.20050032521. R. de Boer, M. Gershenson, A. Morpurgo, and V. Podzorov, Phys. Status Solidi A 201, 1302 (2004). https://doi.org/10.1002/pssa.200404336 $R_{\mathrm{c}}$ is 20 kΩ cm, extracted by the transmission line method²²22. Y. Xu, R. Gwoziecki, I. Chartier, R. Coppard, F. Balestra, and G. Ghibaudo, Appl. Phys. Lett. 97, 063302 (2010). https://doi.org/10.1063/1.3479476 (TLM) in the linear regime, and gate dependent as presented in Fig. S2 (see Ref. 1313. See supplementary material at http://dx.doi.org/10.1063/1.4937005 for details on S1: AFM measurements, S2: contact resistance extraction, and S3: EQE behavior with channel length.). This value is close to 5 kΩ cm measured in bottom-gate/top-contact rubrene SC-FETs.²2. I. N. Hulea, S. Russo, A. Molinari, and A. F. Morpurgo, Appl. Phys. Lett. 88, 113512 (2006). https://doi.org/10.1063/1.2185632

OPT performance is analysed based on three figures-of-merit: light responsivity ($R_{\text{ph}}$), photosensitivity (P), and EQE. These parameters enable a normalized comparison between devices. $R_{\text{ph}}$, in AW⁻¹, can be defined by the following equation:⁴4. N. Marjanović, T. B. Singh, G. Dennler, S. Günes, H. Neugebauer, N. Sariciftci, R. Schwödiauer, and S. Bauer, Org. Electron. 7, 188 (2006). https://doi.org/10.1016/j.orgel.2006.01.002

$$R_{\text{ph}}=\frac{I_{\text{ph}}{S}_{\text{ch}}^{-1}}{E_{\text{light}}}=\frac{(I_{\mathrm{l}}-I_{\mathrm{d}}){\left(\text{LxW}\right)}^{-1}}{P_{\text{opt}}{S}_{\mathrm{b}}^{-1}},$$

(1)

where $I_{\text{ph}}$ is the source-drain photocurrent, $I_{\mathrm{l}}$ and $I_{\mathrm{d}}$ are the source-drain current at fixed drain and gate voltages, under light illumination and in the dark, respectively. $E_{\text{light}}=P_{\text{opt}}/S_{\mathrm{b}}$ is the irradiance of the excitation source, where P_opt is the optical power and $S_{\mathrm{b}}$ the excitation beam spot size (typically, 1 mm²). To enable a comparison among devices with crystals of different sizes, $I_{\text{ph}}$ is normalized by the active (interface) channel area $S_{\text{ch}}$.

Figure 3(a) shows $V_{\text{th}}$-normalized transfer curves of a PC₆₁BM/rubrene OPT, in the dark and under illumination with a monochromatic green light (λ = 500 nm, $E_{\text{light}}\approx 16.8$ μW mm⁻²). The measurement setup used for optoelectronic characterization of the OPTs is described elsewhere.^12,2312. R. M. Pinto, E. M. Maçôas, and H. Alves, J. Mater. Chem. C 2, 3639 (2014). https://doi.org/10.1039/c4tc00051j23. R. M. Pinto, E. M. S. Macoas, A. I. Neves, S. Raja, C. M. Baleizão, I. C. Santos, and H. Alves, J. Am. Chem. Soc. 137, 7104 (2015). https://doi.org/10.1021/ja512886h Light at 500 nm matches the maximum absorption peak of rubrene SC in the visible range (see Fig. 1(a) and Ref. 1212. R. M. Pinto, E. M. Maçôas, and H. Alves, J. Mater. Chem. C 2, 3639 (2014). https://doi.org/10.1039/c4tc00051j). Hence, photocurrent build-up in the active channel should originate from rubrene's excitons that split at the interface with PC₆₁BM. The large increase in current upon illumination indicates that light can act as an additional terminal that controls device operation, along with the standard S/D/G electrodes. The effect is also observed at other wavelengths in the visible range, as can be seen on Fig. 3(b) with illumination at 680 nm. However, at on-state, the difference between dark and light is less pronounced, and in the saturation regime, $I_{\text{DS}}$ light levels rise above the dark current. The $I_{\text{DS}}$ increase at wavelengths higher than 550 nm is due to PC₆₁BM excitons evolving into free-charges via a hole transfer (HT) mechanism.¹²12. R. M. Pinto, E. M. Maçôas, and H. Alves, J. Mater. Chem. C 2, 3639 (2014). https://doi.org/10.1039/c4tc00051j

When the transistor operates in accumulation mode (on-state, i.e., $V_{\text{GS}}–V_{\text{th}}<0$ for p-type device), it presents the maximum responsivity, $\approx 20$ AW⁻¹, at lowest optical power [Fig. 3(a)]. A possible explanation is a photovoltaic (PV) effect, causing a P_opt-dependent photocurrent that can be expressed as²⁴24. C. Choi, H. Kang, W.-Y. Choi, H. Kim, W. Choi, D. Kim, K. Jang, and K. Seo, IEEE Photonics Technol. Lett. 15, 846 (2003). https://doi.org/10.1109/LPT.2003.811339

$$I_{\text{ph},\text{pv}}=\frac{\mathit{A}\mathit{k}\mathit{T}}{\mathrm{q}}\text{ln}\left(1+\frac{B\eta \mathrm{q}\lambda P_{\text{opt}}}{I_{\mathrm{d}}hc}\right),$$

(2)

where A and B are fitting parameters, $hc/\lambda$ is the photon energy, $I_{\mathrm{d}}$ the dark current for electrons, and η the photogeneration quantum efficiency. Fittings to measured data using Eq. (2) show that $I_{\text{ph}}$ saturates at high P_opt [Fig. 3(c)] values. These results indicate that PC₆₁BM/rubrene OPTs follow the PV effect in the turn-on state, at spectral regions (500 nm vs. 680 nm) where excitons from either p- (rubrene) or n-type (PC₆₁BM) materials contribute to photocurrent. In the PV effect, photogenerated holes flow to the drain electrode, while negative charges accumulate under the source electrode, reducing the barrier height for hole injection and, thus, the contact resistance.²⁴24. C. Choi, H. Kang, W.-Y. Choi, H. Kim, W. Choi, D. Kim, K. Jang, and K. Seo, IEEE Photonics Technol. Lett. 15, 846 (2003). https://doi.org/10.1109/LPT.2003.811339 This leads to a positive shift in $V_{\text{th}}$.

At 680 nm, there is a sublinear dependence of $R_{\text{ph}}$ on P_opt, i.e., $R_{\text{ph}}\propto P^{-0.9}$, which likely comes from enhanced singlet-singlet exciton annihilation, due to higher density of photogenerated excitons at increasing optical power [Fig. 3(d)]. The photosensitivity (or photoswitching ratio) for a typical PC₆₁BM/rubrene OPT, defined as $P=I_{\text{ph}}/I_{\mathrm{d}}$, peaks at 4 × 10⁴ under 500 nm light illumination, near $V_{\text{GS}}-V_{\text{th}}=0$ in the off-state of the transistor, as displayed in Fig. 3(e). Similarly to other p-type OPTs, photosensitivity decreases with more negative V_GS, owing to the large drain current already flowing through the channel without illumination. Increasing P_opt leads to negligible changes in photosensitivity; therefore, P is almost independent of light power.

Also, in Fig. 3(e), EQE is presented, which takes only into account the electronic processes in the device and is related to $R_{\text{ph}}$ as $\text{EQE}=(hc/\lambda \mathrm{q})R_{\text{ph}}$. At low irradiance values ($E_{\text{light}}=27$ μW cm⁻², 500 nm), EQE reaches 52 900%. This value is almost 20× higher than the gain observed for high-quality n⁺p photodiodes (>3000%).²⁵25. A. J. Said, D. Recht, J. T. Sullivan, J. M. Warrender, T. Buonassisi, P. D. Persans, and M. J. Aziz, Appl. Phys. Lett. 99, 073503 (2011). https://doi.org/10.1063/1.3609871 It can be attributed to the existence of a photomultiplication (PM) mechanism, and to the low charge-recombination due to the defect-free nature of rubrene SCs. Under stronger irradiance ($E_{\text{light}}=0.9$ mW cm⁻²), EQE is 900% at 400 nm and follows the absorption profile of the interface up to ca. 700 nm, and then steeply goes below 100%, signaling the absence of a PM mechanism [Fig. 3(f)]. The onset of the EQE spectrum occurs right at the onset of absorbance for the film/single-crystal interface shown in Fig. 1(b), as observed before.¹²12. R. M. Pinto, E. M. Maçôas, and H. Alves, J. Mater. Chem. C 2, 3639 (2014). https://doi.org/10.1039/c4tc00051j Specific detectivity ($D^{*}$) is 1–2 × 10¹¹ Jones, calculated using $D^{*}=R_{\text{ph}}/\sqrt{2e{I}_{\mathrm{d}}/S_{\text{ch}}}$, where e is the electron charge, $R_{\text{ph}}$ is responsivity, $I_{\mathrm{d}}$ the dark current for electrons, and $S_{\text{ch}}$ is the device area. Shot noise from dark current was assumed as the dominant contribution over Johnson, dielectric or flicker noise.²⁶26. G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond, M. Bernechea, F. P. G. de Arquer, F. Gatti, and F. H. Koppens, Nat. Nanotechnol. 7, 363 (2012). https://doi.org/10.1038/nnano.2012.60

In OPTs, PM-based EQEs in excess of 100% can have multiple origins, e.g., (i) singlet-fission, (ii) impact ionization by hot carriers, or (iii) enhanced injection via trap-assisted tunneling (TAT).^9,27,289. H. Yu, Z. Bao, and J. H. Oh, Adv. Funct. Mater. 23, 629 (2013). https://doi.org/10.1002/adfm.20120184827. M. Hiramoto, K. Nakayama, I. Sato, H. Kumaoka, and M. Yokoyama, Thin Solid Films 331, 71 (1998). https://doi.org/10.1016/S0040-6090(98)00900-628. W. Wang, F. Zhang, L. Li, M. Zhang, Q. An, J. Wang, and Q. Sun, J. Mater. Chem. C 3, 7386 (2015). https://doi.org/10.1039/C5TC01383F Singlet-fission implies very energetic photons, while impact ionization is hindered by large exciton binding energies of organic materials. However, PM via TAT injection of charges has been reported for polymer:fullerene diodes²⁸28. W. Wang, F. Zhang, L. Li, M. Zhang, Q. An, J. Wang, and Q. Sun, J. Mater. Chem. C 3, 7386 (2015). https://doi.org/10.1039/C5TC01383F and nanowire OPTs⁹9. H. Yu, Z. Bao, and J. H. Oh, Adv. Funct. Mater. 23, 629 (2013). https://doi.org/10.1002/adfm.201201848 with EQEs well above 100%. A possible explanation for the high EQE in our devices can be described as follows. Illumination in the off-state leads to photogenerated excitons in rubrene SC that diffuse towards the organic/organic (O/O) interface. There they split driven by an electrical field due to intrinsic interfacial band bending,¹²12. R. M. Pinto, E. M. Maçôas, and H. Alves, J. Mater. Chem. C 2, 3639 (2014). https://doi.org/10.1039/c4tc00051j aided by the transversal gate-field. Holes drift to the drain, while remnant electrons fill interfacial trap states, creating a Coulomb field²⁷27. M. Hiramoto, K. Nakayama, I. Sato, H. Kumaoka, and M. Yokoyama, Thin Solid Films 331, 71 (1998). https://doi.org/10.1016/S0040-6090(98)00900-6 that could further enhance exciton splitting. Ohmic contacts ensure electrical neutrality; thus, more holes will be injected for each collection-trapping event until recombination occurs. The OPT operates as a photoconductor under the effect of a transversal field, and the PM gain scales with the ratio between recombination and hole transit time.⁷7. K.-J. Baeg, M. Binda, D. Natali, M. Caironi, and Y.-Y. Noh, Adv. Mater. 25, 4267 (2013). https://doi.org/10.1002/adma.201204979 Further support for the transversal field effect on the gain mechanism is given in Fig. S3 (see Ref. 1313. See supplementary material at http://dx.doi.org/10.1063/1.4937005 for details on S1: AFM measurements, S2: contact resistance extraction, and S3: EQE behavior with channel length.), showing an increase in EQE with decreasing channel length. In the on-state, photogenerated carriers in rubrene will add to the current already being injected into the channel. The LUMO offset¹²12. R. M. Pinto, E. M. Maçôas, and H. Alves, J. Mater. Chem. C 2, 3639 (2014). https://doi.org/10.1039/c4tc00051j between rubrene (−2.7 eV) and PC₆₁BM (−3.7 eV) provides deep electron traps ($\approx 1$ eV) at the O/O interface.²⁸28. W. Wang, F. Zhang, L. Li, M. Zhang, Q. An, J. Wang, and Q. Sun, J. Mater. Chem. C 3, 7386 (2015). https://doi.org/10.1039/C5TC01383F Trapped electrons will also accumulate under the source, reducing the hole-tunneling injection barrier to rubrene. This TAT mechanism adds to the PV effect to further enhance charge injection. TAT should also occur for PC₆₁BM excitons only (>550 nm), but their lower diffusion length (${\mathrm{L}}_{\mathrm{D}}\approx 5\hspace{0.17em}\text{nm}$) decreases the splitting efficiency and the EQE. While these mechanisms already define a route to achieve very high photomultiplication gain, control experiments on devices with different configurations (e.g., top-gate) should be included in future studies.

The dynamic response of an average mobility OPT (L × W = 468 × 239 μm², μ_FE = 4.5 cm² V⁻¹ s⁻¹) is displayed in Fig. 4(a), showing multispectral photoresponse from 450 to 750 nm, with P as high as 3.1 × 10⁴ when a gate-reset pulse is used. This broad spectral response, which covers the entire visible range and extends into the NIR, is a consequence of enhanced electron (ET) and HT in the active layer. While this strategy is frequently used to exploit excitons from both organic materials in donor-acceptor (p-n) junctions in organic solar cells,²⁹29. J. D. Douglas, M. S. Chen, J. R. Niskala, O. P. Lee, A. T. Yiu, E. P. Young, and J. M. Fréchet, Adv. Mater. 26, 4313 (2014). https://doi.org/10.1002/adma.201305444 here we show that it can also be applied to OPTs to achieve multispectral response. A closer look at the photocurrent dynamics reveals fast rise times, ${\tau }_{\mathrm{r}}<0.5\hspace{0.17em}\mathrm{s}$, and slow single exponential decays, ${\tau }_{\mathrm{d}}\approx \hspace{0.17em}$ 4.0–5.5 s [Figs. 4(b) and 4(c)]. These values are similar to the corresponding ${\tau }_{\mathrm{r}}$ and ${\tau }_{\mathrm{d}}$ reported for hybrid graphene-quantum dots photodetectors,²⁶26. G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond, M. Bernechea, F. P. G. de Arquer, F. Gatti, and F. H. Koppens, Nat. Nanotechnol. 7, 363 (2012). https://doi.org/10.1038/nnano.2012.60 while ${\tau }_{\mathrm{r}}$ is 10× faster than that of recently developed MoS₂ light sensors.³⁰30. O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, Nat. Nanotechnol. 8, 497 (2013). https://doi.org/10.1038/nnano.2013.100 They also represent a large improvement over OPTs based on amorphous oxide semiconductors, where persistent photoconductivity (PPC) can last for several hours or days.³¹31. S. Jeon, S.-E. Ahn, I. Song, C. J. Kim, U.-I. Chung, E. Lee, I. Yoo, A. Nathan, S. Lee, J. Robertson et al., Nat. Mater. 11, 301 (2012). https://doi.org/10.1038/nmat3256 As reported for other OPTs, ${\tau }_{\mathrm{d}}$ can be improved to less than 0.5 s (i.e., the temporal resolution of our setup) by applying a short gate pulse (2s, V_GS = −10 V), which causes a full release of trapped charge carriers.^3,26,303. M. Mas-Torrent, P. Hadley, N. Crivillers, J. Veciana, and C. Rovira, Chem. Phys. Chem. 7, 86 (2006). https://doi.org/10.1002/cphc.20050032526. G. Konstantatos, M. Badioli, L. Gaudreau, J. Osmond, M. Bernechea, F. P. G. de Arquer, F. Gatti, and F. H. Koppens, Nat. Nanotechnol. 7, 363 (2012). https://doi.org/10.1038/nnano.2012.6030. O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, Nat. Nanotechnol. 8, 497 (2013). https://doi.org/10.1038/nnano.2013.100 This mode of operation yields lower dark currents, allowing higher detectivity (7–9 × 10¹² Jones) than that from on-state operation.

In conclusion, OPTs based on single-crystal rubrene laminated onto PC₆₁BM films show an average hole mobility in the dark of 4–5 cm² V⁻¹ s⁻¹, and an EQE that reaches 52 000% under low power light irradiation (500 nm, 27 μW cm⁻²). Response over a wide spectral range (vis-NIR) with photosensitivity P as high as 4 × 10⁴ is achieved by grasping the potential of both p- and n-type materials, whose primary excitons contribute to photocurrent build-up via electron and hole-transfer mechanisms, respectively. These characteristics show the potential of bilayer organic interfaces based on materials with contrasting structural phases (single-crystal vs. amorphous) to be used in high-quality optoelectronic applications.