New thiophene-based conjugated macrocycles for optoelectronic applications † Materials Chemistry C

Thiophene-based semiconductors are amongst the most successful materials in organic electronics. In this contribution, we present the synthesis and characterisation of two thiophene-based macrocycles as well as their evaluation in organic-electronic devices. McT-1 is composed of ten thiophene moieties, whereas in McT-2 , four additional electron-deficient benzothiadiazole moieties are incorporated to form a donor–acceptor (D–A) p -system. Red-shifted and broadened absorption spectra as well as more positive redox potentials are observed in McT-2 , whereas McT-1 displays a sharper absorption band with a higher extinction coefficient. Macrocycle McT-1 shows emission in the yellow region whereas McT-2 displays emission in the red wavelength region. DFT calculations predict the macrocycles to comprise of mainly the E , E isomers with a near-planar structure, which is further supported by the single crystal X-ray structure for McT-1 . Their charge transporting properties are determined by fabricating thin-film OFETs. The photovoltaic properties of McT-1 and McT-2 are also investigated by fabricating bulk heterojunction (BHJ) devices and their potential as photodetectors has been evaluated.


Introduction
Small molecules and polymers used as active components of organic electronics have received a very high level of attention in recent decades owing to their wide range of attractive features including solution processability, tunable absorption and energy levels, high carrier mobility, and photochemical and thermal stability. [1][2][3][4][5][6][7][8][9] Several devices have evolved through the use of these materials, such as field-effect transistors (FETs) for microelectronics, 10,11 light-emitting diodes (LEDs) for lighting and displays, [12][13][14][15] sensors, [16][17][18] lasers, [19][20][21] molecular switches [22][23][24] and photovoltaics. [25][26][27][28][29][30][31] Despite the tremendous achievements in recent years that have led to the commercialisation of these devices, significant efforts are still devoted to enhance their performance, improve their processability and stability and understand their functionality. Hence, it is of great interest to investigate different or novel molecular systems to allow the discovery of crucial structure-activity relationships that will allow the development of newer materials with enhanced processability and performance. [32][33][34][35][36][37] In both polymer and small molecule based bulk-heterojunction (BHJ) organic photovoltaics (OPVs), the self-assembly of photoactive materials is crucial to generate efficient devices. 38,39 Efficient interaction between donor and acceptor materials is needed to facilitate charge separation, 40 and appropriate intermolecular organisation favours charge transport. 41 Fibre-like domains of active materials with desirable phase-separation or nanowires are often described as efficient morphologies for BHJ OPVs. 36,42 Molecular organisation and orientation are also important for other technologies such as organic light-emitting diodes (OLEDs) 43,44 and organic field-effect transistors (OFETs). [45][46][47][48][49] However, despite being an important and effective concept, the control of the construction of such assembles is challenging. 50,51 In this regard, molecular recognition is promising to manipulate molecular organisation for effective self-assembly of photoactive materials. 52 Conjugated macrocycles appear in this scenario as an emerging family of materials with great potential for optoelectronic applications. [53][54][55][56] These materials possess unique features including their controlled structure that can facilitate intermolecular arrangement, 57 and their defined cavity that can be used to host electronically active molecules such as fullerene derivatives. 58,59 Furthermore, by virtue of their cyclic structure, they do not feature end-group functionalisation, which often result in charge traps in linear molecules. 53 In addition, their extended conjugation with highly aligned molecular frontier orbitals often leads to desirable electronic properties such as high intramolecular charge transfer (ICT). [60][61][62] These positive features can be exploited to deliver optimum morphologies in optoelectronic devices and pave the way to more efficient charge transfer/transport. A great number of reports have made available protocols for the synthesis of macrocycles composed of a wide variety of aromatic moieties such as: benzene, carbazole, thiophene, perylene, porphyrin, BODIPY, furan, acetylene and so forth. 35,54,56,[63][64][65][66][67][68][69][70][71][72][73] Previous work reports the synthesis of cycles of different molecular lengths and cavity sizes. 74,75 Some also report the formation of stable supramolecular complexes with different substrates. [76][77][78][79][80][81] However, in spite of such interesting discoveries, examples of direct application of these interesting molecules in optoelectronic devices are still limited. 58,60,[82][83][84][85][86][87][88][89][90] Some of the pioneering reports include, for instance, a triphenylamine-based compound (namely 3B2A) reported by Zhang et al., 86 and a perylene diimide (PDI)based compound (namely cPBPB) reported by Ball et al. (see Fig. S1 in ESI †). 55,58 The former was applied as a donor material in a BHJ device showing a power conversion efficiency (PCE) of 2.66% in a blend with a fullerene derivative (PC 71 BM), whereas the latter was built to mimic fullerenes and take advantage of their useful properties such as three dimensional shape and fully delocalised p-space with a low lying LUMO. Interestingly, the BHJ cell incorporating cPBPB as the acceptor (with PTB7-Th as the donor) reached a PCE of 3.5% that outperformed an unfolded oligomer and a polymer of the same kind. These recent results emphasise the potential of conjugated macrocycles for optoelectronic applications.
In this contribution, we report two new thiophene-based conjugated macrocycles. Compound McT-1 possesses a conjugated p-system composed of only thiophene units, whereas the formation of a donor-acceptor (D-A) system is explored in compound McT-2 with the insertion of two benzothiadiazole (BT) moieties in each semicircular unit (Fig. 1). Thiophene moieties are found in many of the high efficiency small molecule and polymer-based OPVs 27,91-96 and OFET materials, 10,11,46 due to their excellent charge transport, high stability and polarisability, fluorescence properties and ability to form highly electron-conductive composites. Also, in comparison with benzene, thiophene shows weaker aromaticity and lower steric hindrance between neighbouring units; which leads to smaller band-gaps due to increased quinoidal character upon p-electron delocalisation and improved planarity. 25,75,[97][98][99][100][101] Therefore, it is of great importance to investigate macrocyclic systems of this kind. Utilising the well-established McMurry reaction, alkynespaced building blocks were used in order to suppress steric hindrance and facilitate the cyclisation step, as is common when constructing such molecular systems. 57,74 However, considering results discussed in our previous report which pointed that alkyne spacers are detrimental to charge transfer, charge generation and ultimate photovoltaic performance in thiophene-based molecules, 102 the macrocycles were designed to minimise the number of alkyne groups compared to other reported systems, with only four present in each macrocycle. The synthesis of the new macrocycles was accomplished via dimerisation of two oligothiophene units, in which the concentration of the reacting species was key to yield the desired products. The UV-vis absorption and emission profiles are compared, while DFT calculations are used to investigate the nature of the electronic transitions and probe both their frontier molecular orbitals and molecular conformation. A single crystal X-ray structure was obtained for McT-1, from which it was possible to confirm its molecular structure and arrangement. Their potential as semiconductors is unlocked by fabricating bottom-contact OFETs and BHJ photovoltaic devices (with the macrocycles as the donor and PC 71 BM as the acceptor). Their relatively narrow spectral bandwidth allowed us to investigate their use in photodetectors without using filters in colour image sensing.

Synthesis and molecular design
The synthetic pathway for the target molecules involved the use of Sonogashira reactions to produce the oligothiophene building blocks (see ESI †) and a McMurry coupling for the cyclisation steps (see Scheme 1). The Sonogashira cross-coupling reaction has been used extensively in the construction of these systems, providing the appropriate spacing of adjacent thiophene moieties and suppressing twisted dihedral angles. 103,104 However, in this work the number of alkynes were minimised, contrasting with the most common thiophene-based macrocycles reported. 57,71,74 This leads to a challenging molecular arrangement of the parallel thiophenes since they usually tend to arrange in an anti-conformation when in a linear system. 92,101 In order to achieve an optimal yield by facilitating the cyclisation and suppressing polymerisation, the total concentration of the reacting species (compounds 1 and 2) was optimised as summarised in Tables S1 and S2 (provided in the ESI †). For McT-1, the yield improved significantly on going from a 1.8 mM (2%) to a 3.6 mM (22%) concentration of compound 1. For McT-2, only 7% yield was achieved using a 1.0 mM concentration of compound 2, while at a 1.3 mM concentration the yield improved to 17%. In both cases, further increasing the concentration of the starting material decreased the yield considerably, possibly due to polymerisation now being more favoured.

Thermal properties
The thermal properties of McT-1 and McT-2 were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA was studied to compare the thermal stability of the two macrocycles with the plots shown in Fig. S12 (ESI †). The temperature of 5% mass loss for McT-1 was 401 1C, showing that this macrocycle has good thermal stability. However McT-2 showed a much-reduced temperature of 5% mass loss at 228 1C and suggests that the increased strain caused by increasing the size of the macrocycle leads to a reduction in its thermal stability.
DSC plots are shown in Fig. S13 (ESI †). The heating and cooling scans for McT-1 show no obvious phase transitions between 25 1C and 300 1C, which is an advantage for device processing as the material can be annealed without crystallisation and subsequent formation of grain boundaries. On heating McT-2, there is a melt peak at 50.1 1C and in the reverse scan the peak at 45.5 1C indicates crystallisation upon cooling. This phase change at relatively low temperature may be detrimental to device performance.

Optical and electrochemical properties
The absorption spectra of both macrocycles (5 Â 10 À6 M in CH 2 Cl 2 ) are shown in Fig. 2. McT-1 showed a weak absorbance band in a shorter wavelength region, ca. 300 nm, and a sharp, strong absorbance band at 438 nm (maximum absorbance, l max ) ( Fig. 2(a)). McT-2, however, presented two strong absorbance peaks at shorter wavelengths (ca. 325 nm and 375 nm) and a l max at 476 nm ( Fig. 2(b)). The bands at maximum absorbance are often a result of S 0 -S 1 and S 0 -S 2 type transitions. 72 Notably, this band redshifts for the BT-containing macrocycle McT-2 (ca. 30 nm) with respect to McT-1, with the former also showing a broader spectrum and a high absorbance throughout most of the region covered. This phenomenon is ascribed to a pronounced ICT effect since the electron-withdrawing BT moieties induce a push-pull effect in this D-A system. Furthermore, new bands observed at around 320-400 nm are likely originating from the BT unit's p-p* internal transition. 105 On the other hand, the simplest macrocyclic compound (McT-1) presented a higher value of extinction coefficient (e max , 98 890 M À1 cm À1 ) than its BTcontaining analogue (56 560 M À1 cm À1 ). Values relating to the onset of absorbance (l onset ) were estimated as 561 nm for McT-1 and 612 nm for McT-2, and correspond to optical band-gap (E opt ) values of 2.21 eV and 2.02 eV, respectively. In thin films ( Fig. 2(c)), the two macrocycles present bathochromic shifts in l max (ca. 30 nm), typically as a result of enhanced p-p stacking interactions. Furthermore, the main absorbance band is broadened and extends to longer wavelength for both compounds, while for McT-1 the insurgence of a shoulder at ca. 545 nm is seen which is perhaps a vibronic level or due to p-p stacking. The optical data are summarised in Table 1.
The emission spectra of the two macrocycles were recorded in solution ( Fig. 2(a and b)). McT-1 shows an emission peak at 560 nm (with a vibronic shoulder around 600 nm) and McT-2 presents a peak at 642 nm with reasonably sharp full-width at half-maximum (FWHM) (ca. 70 nm for McT-1 and 82 nm for McT-2). It is also noteworthy that both compounds have large Stokes shifts despite the circularly locked structure. The Stokes shift was higher for McT-2 (ca. 166 nm) than for McT-1 (122 nm), possibly due to rotation of the BT units after excitation. 106-108 A larger Stokes shift has also been observed in a BT-containing [10]cycloparaphenylene (BT [10]CPP) compared to its parent [10]CPP. 109 To study the possible interactions between the macrocycles and fullerenes, fluorescence titration was performed with McT-1 and McT-2 in toluene solution, since fluorescence quenching can be indicative of intermolecular interactions. 110,111 This was done through the addition of aliquots of a solution of C 60 in toluene. When analysing the fluorescence intensity upon addition of small amounts of C 60 (see Fig. S14, ESI †), one can notice a substantial quenching happening only after a considerable increase of C 60 concentration in both cases. In fact, ca. 65% of the intensity was only quenched after an addition of ca. 300 equivalents of C 60 to 1 equivalent of McT-1 and ca. 125 equivalents of C 60 to 1 equivalent of McT-2. According to these McT results, we hypothesised that limited cavity filling or complexation between the McT series and C 60 took place in these solutions. Cyclic voltammetry (CV) and square wave voltammetry (SWV) were used to study the electronic properties of the two molecules. The CV measurements revealed one clear oxidation wave with non-reversible character for McT-1, whereas for McT-2, three low intensity irreversible peaks at ca. 0.0 V, 0.4 V and 0.6 V and a quasi-reversible reduction wave was seen [ Fig. 3(a and b)]. The SWV measurements revealed in turn a clearer peak of reduction for McT-1 and two clear reduction peaks for McT-2 (Fig. S15, ESI †). Utilising the SWV values to estimate the energy levels, the ascribed first reduction peak (E 1/2red ) of McT-2 is more positive than for its counterpart, resulting in a lower-lying electron affinity (EA, À3.2 eV) than for McT-1 (À2.9 eV). The first oxidation peak (E 1/2ox ) of McT-1 was found at 0.17 V, resulting in a high-lying ionisation energy (IE) of À5.0 eV. These low values of E 1/2ox and shallow IE are typical of these types of thiophene-based macrocycles. 57,72,76 Surprisingly, a higher IE was observed for electrochemical measurements performed on McT-2 (À4.8 eV). Hence, based on these data, both McT-1 and McT-2 are considered donor type materials, with a fundamental gap (E fund ) of 2.1 eV and 1.6 eV, respectively (in accordance with the E opt obtained from solution), and can be used with PCBMs as acceptors in BHJ cells (see energy level diagram in Fig. 3(c)). 112,113 The electrochemical data are given in Table 1.

Theoretical calculations
Compounds McT-1 and McT-2 were studied using density functional theory (DFT) calculations (B3LYP/6-311G(d,p) level of theory in the gas-phase) to investigate their dihedral angles, conformation and frontier orbital distribution. To gain insight into whether these specific macrocycles would be generated as E,E or Z,Z isomers, McT-2 was first calculated in both forms (E,E and Z,Z) and the total energies of both analogues were compared upon geometry optimisation (see Fig. S16 in ESI †). A considerably strained conformation was observed for the Z,Z isomer of McT-2, whereas the E,E isomer is more planar (Fig. S16, ESI †). For the E,E isomer, the central thiophene moiety (T1) is allocated syn to its two peripheral neighbours (T1 and T10), whereas for the Z,Z isomer it is allocated anti [see Fig. 4(a) for atom and moiety labels]. A considerably higher bond angle of the vinyl group (bond angle C24-C25-C26 1371) was observed for the Z,Z isomer of McT-2 than for E,E isomer of McT-2 (1261), which implies that the Z,Z isomer has a more strained circular structure. A difference of 22 kJ mol À1 was found between the E,E and the Z,Z isomers ground state total energy, with the E,E-isomer being the lowest in energy. In the literature, similar compounds with a more regular ratio of alkyne spacers are shown to cyclise in both forms. 76,77 However, due to such strain observed for the Z,Z isomer, it was expected that both macrocycles are present as mainly their E,E isomers since the transition state conformation in the final synthetic step is likely to affect the activation energy to form the product. In such forms, both compounds show near-planar structure. For McT-1 only the central thiophene moiety (T1) shows a moderate dihedral angle with its neighbouring moieties (T2 and T10) (i.e. torsion angle S1-C2-C5-S4 36.51) [ Fig. 4(b)], whereas McT-2 presents a notable dihedral angle of 41.11 also between the moieties T3 and BT1 (C16-C17-C21-S20) due to the alkyl chain.
The ring size is naturally wider/bigger for McT-2 since it contains 4 more aromatic moieties than McT-1. According to the calculations, the cavity size varies between 12.9 Å (S11. . .S42) to 16.2 Å (S1. . .S30) for McT-1 and between 20.1 Å (S20. . .S69) to 22.8 Å (S1. . .S48) for McT-2. Therefore, it is reasonable to assume that both compounds possess sufficiently large cavities to accommodate guest molecules such as fullerenes. C 60 for instance, has a mean ball diameter of 7.1 Å, whereas C 70 has a short axis of 7.1 Å and a long axis of 7.9 Å. 115 However, due to the absence of alkyne spacers between several thiophene moieties in the macrocycles, both adopt elliptical-like structures. This could potentially discourage fullerene molecules filling the cavity due to steric reasons. This will be especially true for McT-1, as the cavity has a considerably shorter axis than the other macrocycle. The frontier orbitals, depicted in Fig. 4(c), are completely delocalised in the case of McT-1 with a total overlap of HOMO and LUMO. The distribution is different for McT-2, where the HOMO appears delocalised along the whole molecule but the LUMO is mostly confined over the BT moieties. This is mainly due to the pushpull system generated with the introduction of the BT moieties, which also causes the red-shift in the UV-vis spectrum. The calculated HOMO (À4.83 eV for McT-1 and À5.09 eV for McT-2) and the LUMO (À2.57 eV for McT-1 and À3.01 eV for McT-2) energy levels (Fig. S17, ESI †) are in good agreement with the IE and EA values obtained experimentally for McT-1, whereas for McT-2 the HOMO is lower than the experimentally derived value (see Table S3, ESI †).
Time-dependent DFT calculations were performed to gain more insight into the optical properties of the materials. These were conducted using the same method (B3LYP/6-311G(d,p) in the gas-phase). The calculations provided a reasonable estimation of the absorption spectrum similar to the experimental data presented in Fig. 2 and Table 1 for both McT-1 and McT-2 (Fig. S18, ESI †). Upon analysing the electronic transitions responsible for the main bands of absorption observed for the two molecules (Tables S4 and S5, ESI

X-Ray structures
Single crystals of McT-1 were obtained from slow diffusion of heptane in a benzene solution, and its structure was determined by X-ray analysis. Unfortunately, attempts to obtain single crystals of McT-2 were unsuccessful. The X-ray crystal structure of McT-1 (Fig. 5(a)) shows all the thiophene sulfur atoms face into the centre of the macrocycle and is consistent with the calculated prediction that the vinyl groups would be E-isomers. McT-1 crystallises with half the molecule in the asymmetric unit and the other half is related by inversion symmetry. The two crystallographically independent alkyne bonds have different conformations, with one having two almost exactly coplanar thiophene rings (angle between mean planes 0.991) and the other with the two rings tilted at 27.751 to one another. The three linked thiophene rings have two rings essentially coplanar (torsion angles S4-C5-C2-S1 À2.0(8)1) and the other two tilted at B301 (torsion angle S25-C29-C30-S1iÀ31.3(8)1). Apart from this tilted thiophene ring the macrocycle is essentially planar. The alkyl chain substituents on the central S1 thiophene both lie out of the plane on the same side for a given thiophene ring (also the same side for adjacent ring S18) and on the opposite side to the symmetry equivalent S1 thiophene on the other side of the macrocycle. The S1. . .S1(i) distance is 16.406(2) Å and S18. . .S11(i) 12.423(2) Å. Examination of the packing in McT-1 (Fig. 5(b) and Fig. S21, ESI †) shows that the cavities are filled by the alkyl chains from adjacent molecules. Each cavity contains four chains in total with two each from adjacent S1 two pointing downwards and two upwards forming zig-zag chains. These chains are then 'cross-linked' by the S. . .S contacts between the two S atoms across the other pair of alkyne bonds (S4 and S11) with an adjacent chain at approximately the sum of the van der Waals radii at just under 3.6 Å. The adjacent chain is shown in green in Fig. 5(b).

Organic field-effect transistors
The charge transporting properties of the macrocycles were studied in bottom-gate, bottom-contact OFETs. McT-1 was deposited in two different solutions, chloroform and toluene, to determine whether this influenced the performance of the OFETs. Devices containing McT-2 did not show any field-effect in either p-type or n-type transistors. This may be explained by the increased twisting in the macrocycle as a result of introducing the benzothiadiazole unit, as shown by theoretical calculations in Fig. 4b. DSC data also showed that there is a relatively low melt temperature which suggests McT-2 may undergo a phase change when a voltage is applied across the channel, particularly if there is a high resistance. Moreover, McT-2, showed weak oxidation peaks in cyclic and square-wave voltammetry experiments, suggesting that it may be a poor p-type material.
Examples of the OFET characteristics for McT-1 containing OFETs are shown in Fig. 6 and a summary of the averaged data is presented in Table 2. There are small differences in performance of the OFETs when the solvent is varied. The p-type mobilities are similar at 1.92 AE 0.8 Â 10 À4 cm 2 V À1 s À1 when deposited from toluene and 1.66 AE 0.6 Â 10 À4 cm 2 V À1 s À1 when chloroform is used. The on/off ratios are relatively low at 10 2 which is mainly caused by the relatively low ON current, due to the limited hole mobility, but also partly due to a higher OFF current due to relatively short channel lengths (5, 10 and 20 mm). The main difference is in the threshold voltage which is slightly reduced for OFETs fabricated using toluene. These differences may be influenced by the slower drying of toluene compared to chloroform. Atomic force microscopy was used to study the difference in the topographies of the films. Images with scan area 10 Â 10 mm 2 are presented in Fig. 7 as well at 5 Â 5 mm 2 and 20 Â 20 mm 2 in Fig. S22 and S23 (ESI †), respectively. The general topographies show similarities, with holes emerging in both films. However, the film deposited from chloroform shows smaller, more isolated domains whilst the films formed from toluene show larger domains although there are still pinholes present. The difference in topographies may result from the lower boiling point of chloroform causing quicker drying of the film. The gaps observed in the morphology of both films could help to explain the low mobilities measured, since charge transport will be limited with so many disconnections in the bulk. However, the closeness of     Fig. 8(a and b). With the addition of  Table 3 and the J-V characteristics are shown in Fig. 9(a and b). For McT-1 with the increase of PC 71 BM content, the power conversion efficiency increases and at the optimised blend ratio of 1 : 4, the corresponding solar cells show an efficiency of B1.1%. Photovoltaic devices with higher blend ratios were not fabricated since the power conversion efficiencies for the 1 : 3 and 1 : 4 blends are very similar. For McT-2:PC 71 BM blends, a similar trend is seen and the highest efficiency of 0.63% is obtained for 1 : 3 donor : acceptor ratio.
For both McT-1 and McT-2, with the increase in PC 71 BM content, the short circuit current density ( J sc ) increased and a corresponding increase in power conversion efficiency is obtained (shown in Table 3). This increase in photovoltaic properties with increase in fullerene content has been previously reported for oligothiophene: fullerene BHJ blends. 99,116,117 With increase in fullerene content, crystallisation of the macrocycles is prevented by the fullerenes, resulting in a BHJ morphology with enhanced mixing, favouring the exciton dissociation and photocurrent. Moreover, increased fullerene content has also been   Fig. 9(c and d).
For both donors, EQE increases with increase in PC 71 BM content and this agrees with the increase in J sc shown in Table 3.   bandwidth allows them to be used in photodetector applications without the use of filters in colour image sensing. 1,118 The corresponding spectral responsivity curves for the optimised blend ratios are shown in Fig. 9

Conclusions
In conclusion, we show that incorporating an electron-deficient benzothiadiazole unit within a thiophene-based macrocycle red-shifts and broadens the main band of absorption while improving the absorbance in higher energy wavelengths, whereas the simplest macrocycle McT-1 shows a higher e max value. DFT calculations indicated that the macrocycles would possess a high level of planarity with E,E-isomers being preferred, and in the case of McT-1 this prediction was further indicated by crystallographic data. The theoretical calculations also revealed the ICT character of McT-2, which has a striking impact on the absorption profile. Organic field-effect transistors were fabricated, with McT-1 exhibiting a hole mobility of 1.92 Â 10 À4 cm 2 V À1 s À1 . Efficient charge transfer between the macrocycles and PC 71 BM acceptor was revealed by photoluminescence quenching studies and led us to investigate them in photovoltaic devices. For both macrocycles, the photovoltaic cells showed an increase in J sc with concomitant increase of power conversion efficiency with the increase in PC 71 BM content. The cells displayed an optimal efficiency of B1.1% for the 1 : 4 McT-1 : PC 71 BM blend and 0.63% 1 : 3 for the McT-2 : PC 71 BM blend. The improvement in spectral absorption did not produce higher J sc in the case of McT-2 presumably due to unfavourable nanoscale morphology preventing efficient charge collection, as indicated by the OPV data. This is also likely due to the presence of alkyne residues, which tend to affect charge generation and OPV performance. For future studies of related macrocycles in OPVs, we encourage the total removal of alkyne residues and further investigation of the self-assembly of these molecules. In photodetectors, the McT-1:PC 71 BM and McT-2:PC 71 BM blends showed a maximum responsivity of 0.16 A W À1 at 478 nm, and 0.10 A W À1 at 478 nm, respectively, under zero external bias. This is a relevant result for the field of organic photodetectors without colour filters for imaging applications.

Conflicts of interest
The authors declare that they have no conflict of interest.