Solution‐Processed Ternary Organic Photodetectors with Ambipolar Small‐Bandgap Polymer for Near‐Infrared Sensing

Organic photodetectors (OPDs) detecting light in the near‐infrared (NIR) range from 900 to 1200 nm offer numerous applications in biomedical imaging and health monitoring. However, an ultra‐low bandgap of the electron donor compound required to achieve NIR detection poses a unique challenge in selecting a complementary acceptor material with a suitable energy‐level offset. To tackle this, a solution‐processed, fullerene‐dominated, ternary device is engineered by adding an ultra‐low bandgap (0.6–0.8 eV) ambipolar polymer, polybenzobisthiadiazole‐dithienocyclopentane (PBBTCD), into the active layer of visible‐light‐responsive OPDs (bandgap of 1.8 eV) to form a ternary blend. The resulting OPD benefits from the extended absorption beyond 1000 nm. The cascaded energy level alignment within the ternary blend and the applied reverse bias both improve the overall NIR photocurrent responsivity by 2 orders of magnitude, reaching 0.4 mA W−1 at 1050 nm and −2 V for ternary devices. Furthermore, a photovoltage responsivity of 0.3 mV m2 W−1 along with significant open‐circuit voltage (Voc) of 0.12 V allow NIR detection in the Voc mode. Prominently, this ability is accomplished with a minimal presence of PBBTCD. Taken together, this work indicates potential strategies for extending the spectral activity of conventional OPDs through introduction of an ambipolar ultra‐low bandgap polymer as a minor element.


Introduction
4][5] DOI: 10.1002/aelm.[20] Near-infrared (NIR) OPDs, [21,22] that offer additional advantages of reduced light scattering, minimal absorption and high tissue penetration, [23][24][25] have been of capital importance in optical, biomedical and photoacoustic imaging, [23,[26][27][28] as well for food quality inspection, [29] artificial vision, [30,31] night vision [32] and military surveillance. [33]While much progress has been demonstrated in recent years, [24,34] NIR OPDs remain challenging to manufacture, especially in the short-wave infrared region (SWIR, 1-3 μm).Standard approaches of forming donor-acceptor blend would require an appropriate acceptor with ultra-low electron affinity, something that makes existing electron acceptor materials largely incompatible. [24]For instance, detecting light at 1800 nm requires the electron donor to have a bandgap below 0.7 eV and possibly a novel non-fullerene acceptor. [35]iven this as a design rule challenge, it is worth noting that alternative approaches such as the utilisation of NIR sensitive nanocomposites [6,36] or optical microcavities [37] have been reported.Apart from the electron donor bandgap, desired control of the electronic level energy alignment of the entire device is also the key to mitigate charge transport and recombination losses that limit critical figures of merit of OPDs, such as dark current and detectivity. [38]][41] In the ternary blend approach, either two donor materials can be combined with one acceptor material, or one donor material can be combined with two acceptors with appropriate energy levels for energy cascade.Such low-cost blend approach allows to harvest photons from short to long wavelengths without using complicated tandem device engineering.There has been an extensive progress made on ternary organic photovoltaics (OPVs), [42][43][44] in which a narrow-bandgap near-infrared absorber is incorporated as a third component in a donor-acceptor bulk heterojunction (BHJ) blend to improve the power conversion efficiency of organic solar cells.In light of this complementary absorption in the near-infrared spectrum, the ternary strategy bears the right potential to also advance the field of NIR OPDs.Among the noteworthy examples, Jiang et al reported an OPD with a ternary ratio of 1:1:0.2(donor:fullerene:acceptor) featuring a responsivity of 0.21 A W −1 at −1 V and 850 nm. [20]Closely to this wavelength (860 nm), Yu et al reported an OPD with a ternary ratio of 1:1.2:0.5 (donor:acceptor:fullerene) featuring a responsivity of 0.59 A W −1 at −1.5 V. [39] Among the non-fullerene OPDs, the work of Zhang et al demonstrated OPDs with a donor:acceptor(1):acceptor(2) ratio of 1:0.38:0.62 that yielded a responsivity of 0.10 A W −1 at −2 V and 710 nm. [7][41] It is worth noting that although the above devices target high NIR responsivity under low reverse bias, their external quantum efficiency (EQE) is limited by 100% due to inevitable energy losses associated with the photovoltaic conversion process.[47][48] Lan et al demonstrated a bias-switchable dual-mode PM-type OPD to detect either visible or NIR light.Using a binary active layer (donor:acceptor, 100:1 weight ratio) and a ternary configuration (donor(1):donor(2):acceptor, 70:30:1 weight ratio), PM-type OPDs yielded EQE values of 1643% for NIR and 2465% for visible light at −40 and 15 V, respectively. [49]While PM-type OPDs exhibit high sensitivity to weak light signals, they suffer from low response speeds limited by the time required for charge accumulation to obtain large gain. [50,51]n this work, we further exploit the ternary cascade strategy in NIR OPDs by incorporating an ambipolar ultra-low bandgap polymer, polybenzobisthiadiazole-dithienocyclopentane (PBBTCD), that extends the device detection at wavelengths beyond 1000 nm.This material, capable of both hole and electron transport, is introduced in small proportion as a NIR sensitizer into the visible-range active blend of poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT): [6,6]-Phenyl-C61-butyric acid methyl ester (PC 61 BM) (1:3.5 ratio).Compared with control devices that offer an absorption peak at 568 nm, the ternary OPD brings complementary NIR absorption peaking at 1211 nm.Furthermore, benefiting from the cascaded energy level alignment and the applied reverse bias, the ternary blend yields low yet significant photocurrent and photovoltage responsivities of 0.4 mA W −1 and 0.3 mV m 2 W −1 at 1050 nm at −1 V, respectively.The photovoltage mode is particularly noteworthy as it allows to sense near-infrared light without the need for elaborate and expensive materials and device engineering.This work also combines spin-coating and screen-printing techniques to achieve the first fully solution-processed ternary OPD with an active area of 2.25 mm 2 .

Materials and Design for Near-Infrared Ternary Organic Photodetectors
Two sets of solution-processed devices were fabricated and optimized on a glass substrate (Figure 1a).The first set incorporates a control BHJ active layer of PCDTBT and PC 61 BM (Figure 1b).The choice of PCDTBT as electron donor and PC 61 BM as electron acceptor is driven by their universality and performance.[54] In particular, remarkable levels of stability have been reported in OPDs made of this BHJ despite their fabrication being undertaken in the air during screenprinting. [53,55]Although ultra-efficient, the spectral absorption of PCDTBT:PC 61 BM is limited to 300-650 nm (peak at 568 nm) due to a large bandgap of 1.8 eV for PCDTBT (Figure 1c). [56]o enhance the OPD spectral responsivity, in the second set of devices, we added PBBTCD into the control BHJ to form ternary OPDs.PBBTCD's backbone consists of a repeating sequence of monomer units, with each unit alternating between electron-donating cyclopentadithiophene (CDT) and electronaccepting fused-ring benzobisthiadiazole (BBT) (Figure 1b) [57,58] This results in the optical bandgap of ≈0.7 eV (0.6-0.8 eV) for PBBTCD, allowing the absorption of photons with wavelengths up to 1800 nm, (Figure 1d).The peak of NIR absorption was measured at 1211 nm.Apart from the ultra-low bandgap, PBBTCD was also chosen due to its ambipolar nature reflected in high charge mobilities for both electrons (0.076 cm 2 V −1 s −1 ) and holes (0.10 cm 2 V −1 s −1 ).Balanced ambipolar charge carrier mobilities are likely to benefit OPDs through the reduction of charge recombination in the blend films. [59]he fusion of PCDTBT, PBBTCD and PC 61 BM provides routes for charge transfer by forming a cascade energy heterojunction (Figure 1e). [44]Indeed, the lowest unoccupied molecular orbital (LUMO) of PBBTCD (4.0 eV) lies between the LUMO levels of PCDTBT (3.6 eV) and PC 61 BM (4.2 eV).This provides essential pathways for charge transfer and charge dissociation of the exciton.In addition, to achieve optimal charge collection, the work function (WF) of indium tin oxide (ITO), used here as the semi-transparent bottom electrode, is lowered from 5 to 4.25 eV to match the LUMO of PC 61 BM.This is achieved by the means of the ITO modifier, polyethylenimine ethoxylated (PEIE). [60]To collect holes, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), a widely used hole-transport material, [61] is manually screen-printed as the top electrode, and features a WF of 5 eV in close agreement with the highest occupied molecular orbital (HOMO) of PBBTCD reported at 4.8 eV.Although a barrier of 0.2 eV at the PBBTCD/PEDOT:PSS interface would likely be detrimental to the OPV performance, OPDs benefit from the application of an externally applied bias to facilitate the separation of hole/electron pairs and charge transport under an external electric field. [62]

Control Organic Photodetectors based on PCDTBT and PC 61 BM
Control devices featuring a conventional BHJ (Figure 2a) were made as a benchmark against which the ternary devices can be compared.From the current density-voltage (J-V) characteristics under dark, the device yielded a high shunt resistance of 9.3 ± 2.5 MΩ cm 2 along with a low series resistance of 46 ± 23 Ω cm 2 (Figure 2b).High R shunt and low R series are ideal to ensure low dark current and high external quantum efficiency (EQE) in OPDs. [4]urther J-V characteristics under green illumination (528 nm) at 25 mW cm −2 yielded a spectral responsivity of 0.11 A W −1 at −1 V, an on-off ratio in the 10 4 range at −1 V, and a large opencircuit voltage (V oc ) of 0.67 V (Figure 2c).We note that although responsivity is measured to be lower than in the state-of-the-art OPDs based on PCDTBT, [53] the performance remains significant, given the fabrication challenges undertaken to manufacture these NIR OPDs (manual screen-printing in the air).The dark current density of 10 −7 A cm −2 at −1 V strongly depends on the PEIE concentration and is consistent with the previous records. [53]Measured EQE spectrum peaks in the visible region at 568 nm (12%, 23% and 25% at 0, −1 and −2 V, respectively) and vanishes at 800 nm (Figure 2d).The extended EQE measurements emphasizing the near-infrared region (800-1200 nm) yielded no significant signal past 900 nm (Figure 2e).This is consistent with the absorption spectrum of PCDTBT:PC 61 BM (Figure 1c).NIR responsivities at 1050 nm were deducted from the EQE spectra using the following equation: where q is elementary charge, c is speed of light, h is Planck's constant, q is the elementary charge and  is the corresponding wavelength (here 1050 nm).These responsivities are marked by a large overdispersion (Figure 2f), with the standard deviation being consistently larger than the mean.As the signal is con-sistently below the three-sigma (3) rule of confidence (in other words, falls within three standard deviations of the mean), it is reasonable to conclude that control devices do not absorb NIR light, or at least, that the EQE is not sensitive enough to collect these signals (especially for EQE values below 0.01%).
To further investigate the NIR performance, low-noise linearity measurement was carried out using a calibrated highpower 1050 nm LED (Figure 2g).No signal was observed below 10 −4 W cm −2 as it falls under the sensitivity of the recording unit (<1 pA).That being said, a linear trend above noise was still observed for larger irradiance levels, featuring an ultra-low photocurrent responsivity of 217 nA W −1 in the short-circuit current mode (0 V) (Figure 2h).The photovoltage measurements to increasing light intensity also yielded a linear trend (Figure 2i), with a photovoltage responsivity of 5.6 μV m 2 W −1 at 1050 nm.The device also exhibited a maximum V oc of 5.2 mV at 95 mW cm −2 under 1050 nm light.The lack of a logarithmic trend for V oc reflects ultra-low absorption levels at this wavelength.Indeed, for efficient OPDs and OPVs, photogenerated charges in excess typically recombine mono-and bimolecularly before being able to be efficiently collected, which leads to the logarithmic dependence. [63]e note that while the photocurrent and photovoltage signals show NIR linear trends clearly above noise levels, they might not necessarily indicate a "true" 1050 nm absorption as the LED light is not monochromatic, i.e., featuring a single frequency, and presents a rather large spectral emission tail.

Ternary Organic Photodetectors using PBBTCD as a Third Component
To achieve ternary OPDs, a new set of devices was fabricated in which PBBTCD was added to the active layer to form a ternary configuration PCDTBT:PBBTCD:PC 61 BM with an optimized ratio 1:0.35:3.5 (Figure 3a).Although one could increase the amount PBBTCD within the active layer, the resulting performance was unsatisfactory (Figure S1, Supporting Information).With the exception of photomultiplication (PM)-type OPDs, [40] the donor-donor-acceptor configuration has not been utilized in ternary OPDs, these devices are also the first to feature a fullerene-dominated active blend.The addition of PBBTCD resulted in a significant drop in shunt resistance (Figure 3b), with an average R shunt of 74 kΩ cm 2 in comparison to 9.3 MΩ cm 2 for the control devices.This decrease is attributed to a much lower bandgap of the ternary component (0.7 eV) which intrinsically increases dark currents through the charge injection phenomenon. [24,64]The change in series resistance was observed to be not significant.Current density-voltage characteristics of a ternary OPD in dark and under green illumination yielded a spectral responsivity of 0.04 A W −1 at −1 V, which is a reduction by 64% in the visible range when compared with a control OPD.This drop was fairly expected as trap-assisted charge recombination is common in ternary blends, especially when the bandgap of the third component is too narrow. [43,44]The dark current density of 5 × 10 −6 A cm −2 at −1 V yielded an overall on-off ratio in the 10 2 range under green illumination (Figure 3c).Compared to control devices (Figure 2c), dark current greatly increased upon addition of the ultra-low bandgap PBBTCD.A binary PCDTBT:PC 61 BM device benefited from large electron and hole injection barriers at reverse bias (1.4 and 1.3 eV, respectively), which were shown to significantly reduce dark current (here 1.3 × 10 −7 A cm −2 at −1 V). [53] The introduction of PBBTCD reduced these barriers to 1 and 0.3 eV, allowing for parasitic charge injection (mostly holes) at reverse bias.This leakage is reflected in lower values of shunt resistance observed for ternary devices (Figure 3b). [65]Measured EQE spectrum of ternary devices features similar peaks centred at 568 nm, with the maximum values of 4, 10 and 15% measured at 0, −1 and −2 V, respectively (Figure 3d).These results are consistent with the drop in responsivity observed earlier.Importantly, extended EQE measurements revealed NIR absorption in the 900-1200 nm window that is well above noise levels (Figure 3e).Deducted responsivities of 39, 272 and 385 μA W −1 were observed at 0, −1 and −2 V, respectively, under 1050 nm illumination, indicating a significant improvement when compared to control OPDs (Figure 3f).We note the importance of the reverse bias as it boosts the cascaded energy level alignment by minimizing the effect of energy misalignment that is often inevitable in ultra-low bandgap NIR materials such as PBBTCD.The reverse bias helps driving the photogenerated charged out of the ternary blend for fast collection at the electrodes.Responsivity values at 1050 nm from the EQE spectra were further confirmed by the current density-voltage characteristics performed under 1050 nm light (Figure 3g).A photocurrent responsivity of 0.3 mA W −1 at −1 V is in close agreement with a responsivity of 0.27 mA W −1 at −1 V deducted from EQE, and a minor mismatch can be attributed to the LED light not being monochromatic, as discussed earlier.More importantly, ternary OPDs feature a high V oc of 0.12 V under 1050 nm light (Figure 3g) that could be used for photodetection. [63]The linearity measurement revealed a strong signal to 1050 nm light, with a responsivity of 22 μA W −1 measured in the short-circuit current mode (0 V), that is an increase of 2 orders of magnitude when compared to control OPDs.The value is consistent with the EQE data and also highlights the necessity of external bias to boost the performance of ternary OPDs.
Although one could use reverse bias to enhance the NIR photocurrent signal, the photodetection in the V oc mode of OPDs has been reported to offer stronger signals despite the presence of dark current or reduced photocurrent responsivities. [63]As these drawbacks often coexist in low-bandgap NIR OPDs, the open-circuit mode can be considered as a viable, alternative solution for light sensing.The photovoltage measurements to increasing light intensity yielded both linear and logarithmic trends (Figure 3i), with a photovoltage responsivity of 0.3 mV m 2 W −1 recorded at 1050 nm.The OPD is capable of sensing NIR light as low as 4 × 10 −6 W cm −2 .At the other end of the intensity spectrum, the device exhibited a large open-circuit voltage (V oc ) of 104 mV at 95 mW cm −2 (Figure 3i), which represents a significant improvement when compared to control devices featuring a V oc of 5 mV.With strong photovoltage signals, and once the ternary OPD is calibrated, the open-circuit voltage approach can lead to an easy light detection of NIR photons using a simple instrumentation. [63]Here, a ternary device placed under a dim 1050 nm irradiance of 1 mW cm −2 yielded a large photovoltage signal of 5 mV and a low photocurrent signal of 450 pA at 0 V.In contrast to photocurrent, this strong photovoltage signal can be easily detected using a simple multimeter.

Temporal Performance of Ternary OPDs based on PBBTCD
Having demonstrated the optoelectronic performance of ternary OPDs, we further characterised their transient photocurrent ) and series (R series = 98 ± 65 Ω cm 2 ) resistances of ternary OPDs and their comparison with control devices.A significant drop in R shunt (p-value = 0.003) is observed for ternary OPD.The difference in series resistances between control and ternary OPDs is not significant (marked as "ns").c) Current density-voltage characteristics of a ternary OPD in dark and under green illumination (528 nm, 25 mW cm −2 ).The photodetector exhibits a spectral responsivity of 0.04 A W −1 at −1 V, an on-off ratio at −1 V in the 10 2 range, and an open-circuit voltage (V oc ) of 0.2 V. d) Measured external quantum efficiency (EQE) spectrum response of ternary OPDs biased at 0, −1 and −2 V. EQE peaks in the visible region at 568 nm.e) Extended EQE measurements highlighting the near-infrared region (800-1200 nm).Dotted lines represent three times the standard deviation (3SD) of the signal measured in the window of 1150-1200 nm.Within this window, the OPD signal is significant, as it is above the three-sigma rule of confidence.f) Responsivities at 1050 nm deducted from the EQE spectra of ternary OPDs and their comparison with the control devices.Standard deviation is plotted for n = 6.An increase in near-infrared responsivity is significant at −1 and −2 V (p-value < 0.0001) for the ternary devices (272 ± 15 μA W −1 at −1 V, and 385 ± 25 μA W −1 at −2 V). g) Current density-voltage characteristics of a ternary OPD in dark and under near-infrared illumination (1050 nm, 25 and 90 mW cm −2 ).The photodetector exhibits a spectral responsivity of 0.3 mA W −1 at −1 V, and an open-circuit voltage (V oc ) of 0.12 V under near-infrared illumination.h) Current density as a function of increasing near-infrared light intensity highlighting the photodetector responsivity of 22 μW A −1 at 0 V. Standard deviation is plotted for n = 10.i) V oc of a ternary OPD as a function of increasing irradiance of 1050 nm.Standard deviation is plotted for n = 10.Both linear and logarithmic V oc dependence on irradiance are observed.A photovoltage responsivity of 0.3 mV m 2 W −1 is deducted from the linear fit.
response under NIR illumination.When exposed to a 500 μs light pulse at 1050 nm, the device exhibited a rise time (t rise , time taken for the device response to increase from 10% to 90%) of 51 μs, and a fall time (t fall , time taken for the device response to increase from 90% to 10%) of 55 μs (Figure 4a).These values were recorded in the short-circuit current mode (0 V) and could be further improved with an external bias. [8,66]The temporal signal retained its characteristic plateau up to 2 kHz (Figure 4b), after which the response began to fade.0]

Discussion
This work demonstrates a simple method to harness the NIR light in the 900-1200 nm window by using an ultra-low bandgap ambipolar polymer PBBTCD, incorporated as a minor element into an active layer of PCDTBT:PC 61 BM to form a ternary configuration.Apart from the NIR performance described above, this research aimed at reducing the complexity of fabrication methods.The top electrode (PEDOT:PSS) was screen-printed which eliminated the need for costly and time-consuming vacuum techniques required to deposit interlayers (e.g.,: MoO 3 ) [41] and metals (e.g.,: Ag, Al). [20,40]Moreover, as the other organic layers were spin-coated, this ternary OPD can be considered as solutionprocessed.This quality is of capital importance in light of possible future applications and commercial opportunities.
Future work may focus on further optimizing the ratio of all components within the active layer as well as the device structure and fabrication methods.Indeed, the optimized design involving a third component needs to form a cascade energy heterojunction for efficient photon absorption and collection. [43,44]One way of achieving this would be to replace PC 61 BM with a low-bandgap acceptor polymer such as IEICO-4F. [20]This non-fullerene electron acceptor features a very low band-gap of 1.20 eV and would offer a better cascade energy alignment with respect to PBBTCD and PCDTBT.Thus, it could be considered as a better component in the ternary formulation with PBBTCD and PCDTBT to facilitate effective charge transfer and collection.Non-fullerene acceptors have shown encouraging OPD performance in the past. [67]Nevertheless, the choice of PC 61 BM as fullerene acceptor in our work was justified by its ease of use, extensive research available, [68] fairly predictable behavior and also low cost.
Taken together, this work indicates a potential material and device design strategy for extending the spectral responsivity of conventional OPDs into the NIR region past 1000 nm through the introduction of an ambipolar ultra-low bandgap polymer as a minor element.

Experimental Section
Materials: Indium tin oxide (ITO) substrates (10 Ω per square) were purchased from Xinyan Technology Limited.PC 61 BM (purity 99%) was purchased from CNano Inc. Polyethylenimine, 80% ethoxylated solution (PEIE) was purchased from Sigma-Aldrich.Poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), 5.0 wt.%, conductive screen printable ink was purchased from Sigma-Aldrich and used for screen printing the top electrode.Polybenzobisthiadiazoledithienocyclopentane (PBBTCD) was purchased from Ossila.Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-b e nzothiadiazole)] (PCDTBT) was used from a batch synthesised by Pandey et al. [69] OPD Fabrication: ITO-coated glass substrates were cleaned using Alconox cleaning detergent (Sigma-Aldrich) and DI water.Following that, the substrates were cleaned in a sonicator using ethanol, acetone and isopropanol for 15 mins each, then dried using nitrogen gun and shortly annealed in air to remove any trace of solvent.The samples were placed on hotplate for 5 mins at 120 °C.Finally, the samples were treated in UVozone for 5 mins.PEIE solution (37 wt.% in H 2 O) was further diluted to 0.075 wt.% in deionized water, then spin-coated at 5000 rpm for 60 s on the ITO substrates, samples were then dried at 100 °C for 10 min in air.For the ternary device, 3.5 mg of PBBTCD and 10 mg of PCDTBT were mixed with 35 mg of PC 61 BM in 1 mL of 1,2-dichlorobenzene (99.9%, anhydrous, Sigma-Aldrich), the weight ratio was therefore 0.35:1:3.5 for an overall concentration of 48.5 mg mL −1 .The ternary blend solution was stirred overnight in a sealed vial at 80 °C using a hotplate.For the control OPDs, the solution was 10 mg of PCDTBT with 35 mg of PC 61 BM dissolved in 1 mL of 1,2-dichlorobenzene prepared similarly in a sealed vial that was stirred overnight at 80 °C on a hotplate.Both solutions were spincoated on top of the PEIE layer using a spin-coating speed of 800 rpm for 3 min to let the solvent evaporate and leave uniform layers of the control and ternary blends.Following this step, there was no additional annealing.The active layers were prepared and formed in a N 2 -filled glovebox environment (O 2 and H 2 O < 0.1 ppm).For the top electrode, PEDOT:PSS was manually screen-printed in the air using predesigned aluminium masks (thickness 0.15 mm) as stencil.PEDOT:PSS was removed from the fridge 1 h prior to screen-printing.Due to a manual nature of deposition, only devices featuring uniform PEDOT:PSS films with no visible defects were taken into this study.This step was important to ensure that the OPD active area of 2.25 mm 2 remained stable as it is defined as the overlap between ITO and PEDOT:PSS.Resulting screen-printed devices were annealed on a hotplate at 80 °C for 15 min.The entire screen-printing process and annealing were carried out in the air and under cleanroom lighting.
OPD Characterization: A high-power green (528 nm) LED from Intelligent LED Solutions (ILH-ON04-TRGR-SC201) and a high-power nearinfrared (1050 nm) LED from OSRAM (LZ1-10R802) were used to record current density-voltage characteristics and linearity measurements for control and ternary OPDs, respectively.Both LEDs were calibrated using a silicon diode (Osram BPX61).To power the LEDs and collect data (current and voltage), a Tektronix Keithley 2604B dual channel source-measure unit was utilized.An Agilent digital multimeter (34450A) was used to record low voltage measurements.External quantum efficiency (EQE) and responsivity were measured using a Newport QANTAX-300 quantum efficiency measurement system with 300-1800 nm range placed inside N 2filled glovebox.Transient photocurrent measurements were conducted using an Agilent 33500B function generator and a Keysight InfiniiVision DSO-X 2004A 70 MHz oscilloscope.Except for EQE and responsivity, all measurements were carried out in a box that was double-shielded to form a Faraday cage.The measurements were performed under a constant N 2 flow.Data acquisition was performed using customised programs written for this purpose in National Instruments LabVIEW.
Data Analysis of Obtained Results: All data presented here, when relevant, denote mean and standard deviation.Unpaired t-tests were performed to compare the means of different parameters (R shunt , R series , responsivity) between control and ternary OPDs and to establish possible significance.The significance thresholds are as follow: if a p-value was less than 0.05, it was flagged with one star (*); if a p-value was less than 0.01, it was flagged with 2 stars (**); if a p-value was less than 0.001, it was flagged with three stars (***); and for p-values higher than 0.05, the result was flagged as not significant (ns).

Figure 1 .
Figure 1.Ternary organic photodetector with ambipolar small-bandgap polymer for near-infrared sensing.a) Cross-sectional device structure of the near-infrared OPD based on PCDTBT, PBBTCD and PC 61 BM as ternary blend.Control devices retain the bulk-heterojunction of PCDTBT and PC 61 BM and lack in near-infrared responsivity.b) Molecular structures of all organic materials used in this work to manufacture solution-processed OPDs.The top electrode PEDOT:PSS is screen-printed.c) Absorption spectrum of a control blend consisting of PCDTBT:PC 61 BM.Absorption peaks in the visible region at 568 nm and vanishes at 800 nm.Inset shows control devices deposited on a 25 mm square glass substrate.d) Absorption spectrum of a ternary blend PCDTBT:PBBTCD:PC 61 BM.Near-infrared absorption peaks at 1211 nm and vanishes at ≈2100 nm.Inset shows ternary OPDs on a 25 mm square glass substrate.See a slight shift in device colour when compared to control devices lacking PBBTCD.e) Graphical illustration showing the working mechanisms of an inverted structure fullerene-dominated ternary OPD.Near-infrared photosensing is achieved through the cascaded energy level alignment.Filled circles are electrons, empty circles represent holes.PBBTCD represents a minor element in the ternary blend.

Figure 2 .
Figure 2. Optoelectronic performance of control organic photodetectors.a) Current density-voltage characteristics of a control device in dark.Inset shows the OPD structure.b) Average (n = 3) shunt (R shunt = 9.3 ± 2.5 MΩ cm 2 ) and series (R series = 46 ± 23 Ω cm 2 ) resistances of control OPDs.c) Current density-voltage characteristics of a control OPD in dark and under green illumination (528 nm, 25 mW cm −2).The photodetector exhibits a spectral responsivity of 0.11 A W −1 at −1 V, an on-off ratio in the 10 4 range at -1 V, and a large open-circuit voltage (V oc ) of 0.67 V. d) Measured external quantum efficiency (EQE) spectral response of control OPDs biased at 0, −1 and −2 V. EQE peaks in the visible region at 568 nm and vanishes at 800 nm.e) Extended EQE measurements highlighting the near-infrared region (800-1200 nm).Dotted lines represent three times the standard deviation (3SD) of the signal measured in the window of 1000-1200 nm.Within this window, the OPD signal is not significant, as it is below the threesigma rule of confidence.f) Responsivities at 1050 nm deducted from the EQE spectra of the control OPDs.Standard deviation is plotted for n = 6.Large overdispersion of data is observed.g) Photocurrent density as a function of irradiance for an OPD biased at 0 V. Standard deviation is plotted for n = 10.h) OPD responsivity at 1050 nm and 0 V (217 ± 29 nA W −1 ) deducted from the linearity measurement in g. i) V oc of a control OPD as a function of increasing irradiance at 1050 nm.Standard deviation is plotted for n = 10.A photovoltage responsivity of 5.6 μV m 2 W −1 is deducted from the linear fit.

Figure 3 .
Figure 3. Optoelectronic performance ternary organic photodetectors in the near-infrared region.a) Current density-voltage characteristics of a ternary OPD in dark.Inset shows the OPD structure.b) Average (n = 3) shunt (R shunt = 74 ± 27 kΩ cm2 ) and series (R series = 98 ± 65 Ω cm 2 ) resistances of ternary OPDs and their comparison with control devices.A significant drop in R shunt (p-value = 0.003) is observed for ternary OPD.The difference in series resistances between control and ternary OPDs is not significant (marked as "ns").c) Current density-voltage characteristics of a ternary OPD in dark and under green illumination (528 nm, 25 mW cm −2 ).The photodetector exhibits a spectral responsivity of 0.04 A W −1 at −1 V, an on-off ratio at −1 V in the 10 2 range, and an open-circuit voltage (V oc ) of 0.2 V. d) Measured external quantum efficiency (EQE) spectrum response of ternary OPDs biased at 0, −1 and −2 V. EQE peaks in the visible region at 568 nm.e) Extended EQE measurements highlighting the near-infrared region (800-1200 nm).Dotted lines represent three times the standard deviation (3SD) of the signal measured in the window of 1150-1200 nm.Within this window, the OPD signal is significant, as it is above the three-sigma rule of confidence.f) Responsivities at 1050 nm deducted from the EQE spectra of ternary OPDs and their comparison with the control devices.Standard deviation is plotted for n = 6.An increase in near-infrared responsivity is significant at −1 and −2 V (p-value < 0.0001) for the ternary devices (272 ± 15 μA W −1 at −1 V, and 385 ± 25 μA W −1 at −2 V). g) Current density-voltage characteristics of a ternary OPD in dark and under near-infrared illumination (1050 nm, 25 and 90 mW cm −2 ).The photodetector exhibits a spectral responsivity of 0.3 mA W −1 at −1 V, and an open-circuit voltage (V oc ) of 0.12 V under near-infrared illumination.h) Current density as a function of increasing near-infrared light intensity highlighting the photodetector responsivity of 22 μW A −1 at 0 V. Standard deviation is plotted for n = 10.i) V oc of a ternary OPD as a function of increasing irradiance of 1050 nm.Standard deviation is plotted for n = 10.Both linear and logarithmic V oc dependence on irradiance are observed.A photovoltage responsivity of 0.3 mV m 2 W −1 is deducted from the linear fit.

Figure 4 .
Figure 4. Temporal performance of optimized ternary organic photodetectors.a) Normalized near-infrared photocurrent response of the ternary device to a 500 μs light pulse at a frequency of 1 kHz.The photodetector is not biased and exhibits a rise time t rise (time for which the device response rises from 10% to 90%) and a fall time t fall (time for which the sensor response decreases from 90% to 10%) of 51 and 55 μs respectively.b) Transient photocurrent response at pulse frequencies of 2, 5 and 10 kHz.The photodetector is not biased.c) Measured cut-off frequency (f -3 dB ) of the optimized device at 0 V.The −3 dB threshold corresponds to the frequency at which the photocurrent signal is attenuated by 1.414.