Polymer photodetectors (PPDs) have attracted considerable attention because organic materials have easily adjustable bandgaps and high extinction coefficients.^[1–5] However, most PPDs are based on the photovoltaic (PV) effect with the photocurrent originating from exciton dissociation. The external quantum efficiency (EQE) of PV-type photodetectors is less than 100% owing to the limited photon harvesting efficiency, exciton dissociation efficiency, charge carrier transport, and collection efficiency.^[6–8] In addition to the EQE values, low dark current ( is also considered a key parameter for evaluating the performance of PPDs because photodetectors should be in a turn-off state in dark conditions.^[9–11] For practical application of PV-type photodetectors, a preamplifier is needed to increase the photodetector detectivity (D*) for weak light, which makes the design and application more complex. Photomultiplication-type (PM–type) photodetectors may be potential candidates to realise weak light detection without preamplifier. Recently, we reported PM-type PPDs based on poly(3-hexylthiophene) (P3HT):[6,6]phenyl-C71-butyric acid methyl ester (PC BM) ( wt/wt) as the active layer.^{[12, 13]} The working mechanism is attributed to interfacial trap-assisted hole tunnelling injection from the external circuit.

For inorganic photodetectors or photomultiplier tubes, EQE can be easily realized based on the photoelectron emission effect, impact ionization triggered by hot carriers, or the secondary emission of electrons under high bias ( V).^{[14, 15]} However, it is impossible to generate hot carriers in organic films owing to their disordered structures and large binding energy.^{[16, 17]} Hiramoto et al. first reported PM-type PPDs in perylene pigment film sandwiched by Au electrodes, which is ascribed to electron injection from the Au electrode to the perylene film assisted by photogenerated hole accumulation near the Au/perylene interface.^[18] Yang et al. also successfully achieved PM-type organic/inorganic hybrid photodetectors (OIPDs) with inorganic nanoparticles (CdTe) placed into P3HT:[6,6]-phenyl-C61-butyric acid methyl ester (PC BM) (1:1 wt/wt) as active layers.^[19] Huang's group successfully fabricated PM-type UV or UV–visible OIPDs based on ZnO nanoparticles blended with P3HT or poly N-vinylcarbazole as an active layer.^[20] These results demonstrate that the spectral response range of PPDs can be broadened to the near-infrared (NIR) or UV region by doping different bandgap materials (so-called ternary PPDs). In this work, the narrow-bandgap polymer 2,6-bis(trimethylstannyl)-4,8-bis(5-(octylthio)-thiophen-2-yl)benzo[1,2-b:4,5-b’]-dithiophene (PBDT-TS1) was selected as the second donor material to harvest low-energy photons in NIR range. A series of PPDs was fabricated with P3HT :PBDT-TS1 _x :PC BM as the active layers, where x represents the PBDT-TS1 doping weight ratio in donors. The EQE values and spectral shape of PPDs could be easily adjusted by changing the PBDT-TS1 doping ratio. For ternary PPDs with P3HT :PBDT-TS1 :PC BM as active layers, the spectral response range covers from 350 to 800 nm. The best EQE values are , and under 390-, 625-, and 760-nm light illumination and −10 V bias, respectively.

The glass substrates coated with indium tin oxide (ITO) (purchased from Shenzhen Jinghua Display Co., Ltd) with a sheet resistance of 15 Ω/square were consecutively cleaned in ultrasonic baths containing detergent, deionised water and ethanol. The cleaned ITO glass substrates were then dried with high-purity nitrogen gas and treated with oxygen plasma for 1 min to improve their work function and clearance. Poly-(3,4-ethylenedioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS) was spin-coated onto the cleaned ITO glass substrates to prepare an ultrathin anode buffer layer at a spin speed of 5000 rounds per minute (rpm) for 40 s. Then the PEDOT:PSScoated ITO substrates were baked at 120 for 10 min under ambient conditions. Polymer P3HT (purchased from Luminescence Technology Corp.), PBDT-TS1 (purchased from Organtec Materials, Inc) and PC BM (purchased from Luminescence Technology Corp.) were dissolved in 1,2-dichlorobenzene (o-DCB) to prepare 40, 25 and 10 mg/mL solutions, respectively. The solutions were continuously stirred at 80 for about 12 h. The mixed solutions were prepared with different doping weight ratios of P3HT:PBDT-TS1:PCBM from 100:0:1, 90:10:1, 70:30:1, 50:50:1, 30:70:1, 10:90:1 to 0:100:1, respectively. The mixed solutions were spin-coated onto the PEDOT:PSS layers at 800 rpm for 30 s, forming ∼230-nm-thick active layers. The 100 nm thick aluminium (Al) was thermally evaporated on the active layers with a shadow mask in a highvacuum ( Pa) chamber. The active area of PPDs is ∼3.8 mm which is defined by the vertical overlap of the Al cathode and the ITO anode.

The current density versus voltage (J–V) curves of all PDDs were measured with a Keithley 2400 source meter under 625- and 760-nm light illumination with light intensity of 8.07 and 5.94 W/cm , respectively. The monochromatic light was provided by a 150-W xenon lamp coupled with a monochromator. The monochromatic light intensity and wavelength were monitored and calibrated by using a Thorlabs S120VC power meter. The absorption spectra of the active layers were measured with a SHINADZU UV-3101 PC spectrophotometer. The optical constants (refractive index (n) and extinction coefficients) of the materials used were measured with a SE200BM-M100 spectroscopic ellipsometer (Angstrom Sun Technologies Inc.). The thickness of the active layers was measured with a AMBIOS Technology XP-2 stylus profilometer. All the measurements were conducted at room temperature in ambient conditions. The chemical structure of materials used and a schematic diagram of the device structure are shown in Fig. 1.

Fig. 1.

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	Fig. 1. (color online) (a) Chemical structures of the materials used: P3HT, PC BM, and PBDT-TS1. (b) Schematic diagram of the device structure.

The EQE spectra of PPDs with P3HT :PBDT-TS1 _x :PC BM as active layers were measured under −10 V bias and are shown in Fig. 2(a). It is apparent that the EQE spectral range of PPDs is markedly extended to the NIR by doping PBDT-TS1 in the active layer. Meanwhile, EQE values of PPDs also decreased along with the increase of PBDT-TS1 doping ratio in donors. For PPDs with PBDT-TS1 doping ratios wt% in donors, EQE values are between 350 and 700 nm. For PPDs with 50 wt% PBDT-TS1 doping ratio in donors, the EQE spectral shape is almost flat and the response range can be extended to 800 nm. The EQE spectral shape of PPDs becomes flatter along with increasing PBDT-TS1 doping ratio. For PPDs with P3HT:PC BM ( wt/wt) as the active layer, the EQE spectra exhibit a distinct dip between 490 and 570 nm, in accordance with the strong absorption range of P3HT:PC BM ( wt/wt) blend film, as exhibited in Fig. 2(b). The underlying reason can be attributed to the weakened hole tunnelling injection induced by the less trapped electrons in PC BM near the Al cathode.^{[12, 13]} The peak EQE values of PPDs with P3HT:PC BM ( wt/wt) as the active layer are 6700% and 4750% at −10 V bias under 390- and 625-nm light, respectively. The EQE values of PPDs markedly decrease with the increase of PBDT-TS1 doping ratio in donors under the same reverse bias, which may be attributed to the disrupted hole transport channels and the shallower electron trap of PC BM surrounded by PBDT-TS1. Meanwhile, the dip in the EQE spectra of PPDs becomes shallower with decreasing P3HT doping ratio in donors. This means that more photons with wavelengths between 490 and 570 nm can be harvested by the P3HT near the Al electrode in the active layer, resulting in more trapped electrons in PC BM close to the Al electrode for better hole tunnelling injection from the external circuit. The EQE values of PPDs are significantly enhanced in the long-wavelength range for PBDT-TS1 doping ratios in donors up to 50 wt%, but then decrease for higher PBDT-TS1 doping ratios. The best EQE values reach 830%, 720%, and 330% under 390-, 625-, and 760-nm light illumination, respectively, for optimised ternary PPDs with P3HT:PBDT-TS1:PC BM (50:50:1 wt/wt/wt) as the active layer at −10 V bias, indicating that the ternary strategy may have great potential for obtaining high-performance NIR PPDs.^[21] The extended spectral response range of the optimised PPDs accords well with the absorption spectra of the corresponding blend films, as exhibited in Fig. 2(b).

Fig. 2.

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	Fig. 2. (color online) (a) EQE spectra of all PPDs with P3HT :PBDT-TS1 x :PC BM as an active layer (x = 0, 10, 30, 50, 70, 90, and 100) under −10 V bias. (b) Absorption spectra of the corresponding P3HT :PBDT-TS1 x :PC BM films.

Clarifying the underlying reason for why PBDT-TS1 doping ratio in donors affects the performance of PPDs entails a discussion on electron trap depth and hole mobility in the active layers. Three kinds of electron traps (P3HT/PC BM/P3HT, P3HT/PC BM/PBDT-TS1 and PBDT-TS1/PC BM/PBDT-TS1) can be formed in the active layers with different PBDT-TS1 doping ratios in donors. The corresponding energy levels of the materials used and the electron trap depth are depicted in Figs. 3(a)–3(c). For the P3HT/PC BM/ P3HT system, the deeper electron traps form as a result of the relatively large energy barrier of 1.4 eV between the lowest unoccupied molecular orbital (LUMO) levels of P3HT and PC BM. For the PBDT-TS1/PC BM/PBDT-TS1 system, the shallower electron traps form as a result of the relatively small energy barrier of 0.9 eV between the LUMO levels of PBDT-TS1 and PC BM. The asymmetric electron traps are formed as a result of PC BM being surrounded by P3HT and PBDT-TS1. The PBDT-TS1 doping ratio influences the number of electron traps in the active layers. It is apparent that the deeper electron trap formed by P3HT/PC BM/P3HT can trap more electrons, which is beneficial to interfacial band bending owing to the strong Coulomb force induced by the additional trapped electrons. Therefore, holes can be easily injected from the external circuit as a result of the interfacial band bending. Figure 3(d) depicts the process of trap-assisted hole tunnelling injection from the external circuit in PM-type PPDs. The dynamic PM process can be summarised as follows: (i) photon harvesting and exciton dissociation; (ii) photogenerated electrons accumulation in PC BM near the Al electrode under reverse bias; (iii) interfacial band bending induced by trapped electrons in PC BM near the Al electrode; and (iv) interfacial trap-assisted hole tunnelling injection from the Al electrode. It is apparent that the EQE values of PPDs with P3HT:PC BM ( wt/wt) as the active layer are much larger than those of PPDs with PBDT-TS1:PC BM ( wt/wt) as the active layer under the same reverse bias. This can be well explained by the hole injection barrier and electron trap depth in the active layer. The highest occupied molecular orbital (HOMO) levels of P3HT and PBDT-TS1 are ∼5.1 and ∼5.3 eV, respectively. The hole injection barrier from the Al electrode is 1.8 eV onto the HOMO level of P3HT or 2.0 eV onto the HOMO level of PBDT-TS1 in dark conditions. Meanwhile, because there the fewer electrons in the shallower traps formed by PBDT-TS1/PC BM /PBDT-TS1, it is difficult to induce more interfacial band bending for hole tunnelling injection from the Al electrode onto the HOMO of PBDT-TS1.

Fig. 3.

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	Fig. 3. (color online) Three kinds of electron traps for PC BM surrounded by (a) P3HT, (b) P3HT/PBDT-TS1, and (c) PBDT-TS1; (d) process of trap-assisted hole tunnelling injection from the external circuit.

To further confirm our speculation, the optical field distribution and photogenerated electron volume density in the active layer were simulated by using the transfer matrix method.^[22–24] The corresponding optical field distribution in the typical blend films are shown in Figs. 4(a), 4(c), and 4(e). In the optical field distribution images, the red region represents a strong optical field caused by interference between incident and reflected light from the Al electrode. The photogenerated electron volume density in the active layer can be estimated according to

(1)

(2)

Fig. 4.

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	Fig. 4. (color online) Optical field distribution and photogenerated electron volume density in the active layers of PPDs with ((a), (b)) P3HT:PC BM (100:1), ((c), (d)) P3HT:PBDT-TS1:PC BM (50:50:1), and ((e), (f)) PBDT-TS1:PC BM (100:1) as the active layers.

If the distributions of donors and acceptors in the blend films are homogeneous, then is kept constant in the whole active layer for the same incident light. Therefore, the photogenerated electron distribution can be estimated according to the optical field distribution and the absorption coefficients of the blend films, as exhibited in Figs. 4(b), 4(d), and 4(f). For the photogenerated electron distribution images, the black region represents the large photogenerated electron volume density. According to the above analysis, the photogenerated electron volume density near the Al electrode plays a crucial role in determining hole injection from the Al electrode under light illumination. The obvious dip in the EQE spectra can be well explained by the relatively low photogenerated electron volume density in this spectral range. Meanwhile, the EQE spectral shape of PPDs becomes flatter as the PBDT-TS1 doping ratio is increased, which can be well explained by the more continuous photogenerated electron distribution near the Al electrode.

Based on the above analysis, the major charge carriers in the PPDs should be the holes injected from the Al electrode under light illumination. Therefore, hole transport in the active layers should play a key role in determining the EQE values. It is known that the EQE values can also be calculated according to the number of charge carriers flowing across the photodetector per incident photon, as expressed by^{[20, 25]}

(3)

Fig. 5.

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	Fig. 5. (color online) –V curves of hole-only devices with ITO/PEDOT:PSS/active layers/MoO /Ag with different PBDT-TS1 doping ratios.

To further clarify the working mechanism of PPDs containing different PBDT-TS1 doping ratios in donors, the current density versus voltage (J–V) curves under 625-nm light illumination with an intensity of 8.07 W/cm and 760-nm light illumination with an intensity of 5.94 W/cm and in the dark were measured, respectively. The selected 625- or 760-nm illumination light is based on the best EQE values of PPDs under light illumination. The photocurrent densities ( were calculated according to the J–V curves in the dark and under 625-nm light illumination, as shown in Fig. 6(a). It is apparent that the value of PPDs continuously decreased with increasing PBDT-TS1 doping ratios up to 90 wt% in donors, which is attributed to the increased hole injection barrier and disrupted hole transport channel in the ternary active layers. The PPDs with PBDT-TS1:PC BM ( wt/wt) as active layers exhibit a relatively large owing to the enhanced hole transport along with PDBT-TS1 channels in the binary active layer. The –V curves for PPDs under 760-nm light illumination are shown in Fig. 6(b). The PPDs with P3HT:PC BM ( wt/wt) as the active layer exhibit a rather low owing to the fewer trapped electrons in PC BM near the Al electrode, resulting from the rather low photon harvesting of P3HT in this spectral range. It is apparent that the value of PPDs markedly increases by doping PBDT-TS1 in the active layer under 760-nm light illumination, which is mainly attributed to the contribution of PBDT-TS1 photon harvesting of the trapped electrons in PC BM near the Al electrodes. The variation of PPDs is in accordance with the observed EQE spectra of PPDs with different PBDT-TS1 doping ratios in donors.

Fig. 6.

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	Fig. 6. (color online) J–V curves of PPDs with different PBDT-TS1 doping ratios in donors under different light illumination: (a) 625-nm light illumination with an intensity of 8.07 W/cm (b) 760-nm light illumination with an intensity of 5.94 W/cm .

In addition to the EQE values, response range, and speed of PPDs, the responsivity (R) and specific detectivity (D*) are also key parameters for evaluating PPD performance. R and D* values of typical PPDs with P3HT :PBDT-TS1 _x :PC BM as active layers (x = 0, 50, 100) were calculated under −10 V bias according to

(4)

The R and spectra of typical PPDs with P3HT :PBDT-TS1 _x :PC BM as active layers (x = 0, 50, and 100) under −10 V bias were calculated and are shown in Fig. 7. It is apparent that P3HT:PC BM-based PPDs exhibit the highest EQE, R, and D* values in the short-wavelength range; the corresponding values are rather low in the NIR owing to the absence of photon harvesting. The three key parameters in the short-wavelength range all markedly decrease with increasing PBDT-TS1 doping ratio in donors, which can be well explained by the decreased hole transport ability and weakened hole injection from the external circuit. The EQE, R, and values of PPDs with P3HT :PBDT-TS1 :PC BM as active layers obviously improved in the NIR, which can be attributed to photon harvesting of PBDT-TS1. For PBDT-TS1:PC BM-based PPDs, R and values were lower than those of the optimised ternary PPDs in the whole spectral range. The low R and values can be attributed to the limited hole tunnelling injection from Al onto the HOMO of PBDT-TS1. Experimental results indicate that the energy levels, absorption coefficients, and charge mobility of the materials used are the key parameters for obtaining highly sensitive PM-type NIR PPDs.

Fig. 7.

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	Fig. 7. (color online) (a) Responsivity and (b) specific detectivity spectra of PPDs with P3HT :PBDT-TS1 x :PC BM as active layers (x = 0, 50, 100) under −10 V bias.

A series of high-performance PM-type PPDs was fabricated with P3HT :PBDT-TS1 _x :PC BM as the active layers. The spectral response range of PM-type PPDs can be extended to the NIR by adjusting the PBDT-TS1 doping ratio in donors. The EQE, R, and values of PPDs strongly depend on the interfacial band bending and hole mobility in the active layers. The best EQE values of PPDs with P3HT :PBDT-TS1 :PC BM as the active layers reach 830%, 720%, and 330% under 390-, 625- and 760-nm light illumination and −10 V bias, respectively. We proposed a simple method to prepare high-performance NIR PPDs by doping narrow-bandgap material in the active layers.