Diketopyrrolopyrrole-Polymer Meets Thiol-Ene Click Chemistry: A

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Diketopyrrolopyrrole-Polymer Meets Thiol-Ene Click Chemistry: A Crosslinked Acceptor for Thermally Stable Near-Infrared Photodetectors Manuela Casutt, Marta Ruscello, Noah Strobel, Silke Koser, Uwe H.F. Bunz, Daniel Jänsch, Jan Freudenberg, Gerardo Hernandez-Sosa, and Klaus Müllen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02530 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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Chemistry of Materials

Diketopyrrolopyrrole-Polymer Meets Thiol-Ene Click Chemistry: A Crosslinked Acceptor for Thermally Stable NearInfrared Photodetectors Manuela Casutt‡,†,⊥, Δ, Marta Ruscello†, Δ, Noah Strobel†,§, Silke Koser‡, Uwe H. F. Bunz‡,∥,*, Daniel Jänsch‡,†, Jan Freudenberg‡,†, Gerardo Hernandez-Sosa†,§ and Klaus Müllen⊥,* ‡Organisch-Chemisches

Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany. †InnovationLab, Speyerer Straße 4, 69115 Heidelberg, Germany. ⊥Max-Planck §Light

Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany.

Technology Institute, Karlsruhe Institute of Technology, Engesserstr. 13, 76131 Karlsruhe, Germany.

∥Centre

for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany. *corresponding author: [email protected]

ABSTRACT: A problem of bulk-heterojunction (BHJ) organic photodetectors (OPDs) is the morphological instability arising from segregation and demixing of its donor and acceptor components. To stabilize the morphology of the blended active layer, we synthesized a crosslinkable low-bandgap 1,4-diketopyrrolo[3,4-c]pyrrole (DPP)-based donor-acceptor (DA) co-polymer with ωalkenyl side-chains. Due to its absorption cut-off above 1000 nm, we employed it as an acceptor material in a solution processed BHJ near-infrared-OPD with P3HT as a donor. Photochemical in-situ thiol-ene click chemistry using a tetrathiol as an additive crosslinker renders the BHJ insoluble to organic solvents and stable under accelerated thermal ageing.

Introduction The detection of infrared (IR) light is a critical technology in current consumer, technological and industrial applications, e.g. in contactless heating systems and optical fiber communication. Particularly, near-infrared light (760 nm – 1100 nm)1 is critical for optoelectronic applications such as video imaging, night vision, motion monitoring, medical diagnostics or anti-forgery measures.1-7 Thus, development of photodetectors (PDs), efficiently converting NIR photons into an electrical signal,1, 3 is crucial for these technologies. Current inorganic PDs for visible light are well advanced and in industrial use, however the most common detectors with uniform response between 900 – 1200 nm still present some drawbacks. For instance, avalanche photodiodes (APDs), preferred for this wavelength range due to their internal gain and high speed, are not capable of a fine spatial resolution.8-9 Furthermore, silicon-based APDs can efficiently detect wavelengths up to 900 nm, however, their response drops rapidly when approaching the Si band gap (~1100 nm). In contrast, InGaAs detectors are highly sensitive above 900 nm. Their application is limited by the high processing costs and the necessity to be cooled to low temperatures during operation to reduce dark noise and achieve a high detectivity.10-11 Organic photodetectors (OPDs) exhibit powerful advantages that complement their inorganic counterparts in terms of low production costs, large active area fabrication and room

temperature operation. Above all, flexible and stretchable substrates12-14 and the detection of different colors at room temperature by using suitable absorbers without color filters dramatically adds to the merits of OPDs.13, 15-16 Although an increasing number of NIR-OPDs are reported, only a few offer a continuously high detectivity independent of the irradiation wavelength.17-26 Regarding active materials, these devices are composed of conjugated polymers in combination with fullerenes (PDPP3T:PCBM, 21 PDDTT:PC60BM20, 27 or PDPPT:PC70BM13) as well as small molecules like phthalocyanines,22-23 porphyrins17 or squaraines.26 Due to their long-wavelength absorption up to 1100 nm (band gap of ~1.2 eV) and high absorbance, DPP-based conjugated polymers are investigated in organic solar cells (OPVs).28 Depending on the nature of the connecting (oligo)arylene donor moiety, they either exhibit n- or p-type charge transport capabilities,29-32 the former of which is capable to replace costly fullerene derivatives with low extinction coefficients. These optoelectronic characteristics have motivated the first reports of DPP-based polymers as NIR-OPD materials with promising performance.13, 18, 29, 33 A stable bulk heterojunction morphology is the key in OPV and OPD devices; both perform best with a blend of acceptor and donor material in their active layer.34 Device performance decreases as the devices age due to demixing, phase segregation and the growth of crystalline domains and crystallites,35-38 the latter frequently observed when fullerenes or fulleroids are used

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as acceptors.39-41 Upon demixing, charge separation of the excitons is hampered as a result of their limited diffusion length of 10-20 nm. Crosslinking of the active layer hinders the growth of domains and stabilizes the morphology of the blend, as has been demonstrated for OPVs,34, 42 but, to the best of our knowledge, never for OPDs. Thermal43, photochemical44, ionic45 or hydrogen bond-based46 crosslinking increases long term thermal stability. Among these methods, UV-light induced thiol-ene click chemistry is out-standing. Highly selective bond formation, short reaction times, small doses of irradiation, and a low activation temperature, just above the glass transition (to allow mobility and flexibility of the reaction partners) are key features. Tolerance towards most functional groups and initiator-free reactions holds promise for applications in high throughput processing techniques.47-50 In this contribution, we designed a cross-linkable thiopheneDPP (Th2DPP) -based polymer (P1) with ω-alkenyl sidechains. Due to its excellent absorption properties in the NIR range, we utilize it as the acceptor material in a solution processed fullerene-free BHJ NIR OPD in combination with P3HT as a donor. Processing with pentaerythritol tetrakis (3mercaptopropionate) (X-SH) as a ternary blend and subsequent thiol-ene crosslinking stabilizes the OPD BHJ layer against aggressive solvent washing with chloroform and an artificial thermal ageing without deterioration of the device performance.

Experimental Section Synthesis. All reactions requiring exclusion of oxygen and moisture were carried out in dry glassware under an inert argon atmosphere/Schlenk conditions. For the addition of solvents or reagents, argon flushed stainless steel cannulas and plastic syringes were employed. 3,6-Di(thiophen-2-yl)-pyrrolo[3,4c]pyrrole-1,4(2H,5H)-dione (Th2DPP)51 was synthesized according to literature procedure. 3,6-Bis(5-bromothiophen-2-yl)-2,5-di(hex-5-en-1-yl)-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1): Under an argon atmosphere, Th2DPP-ene (1.30 g, 2.80 mmol, 1.00 eq) was dissolved in chloroform (80.0 mL) and stirred at 0 °C. Under exclusion of light, N-bromosuccineimide (1.24 g, 6.99 mmol, 2.50 eq) was added and the reaction was stirred at room temperature over night. After evaporation of the solvent under reduced pressure, the crude product was subjected to column chromatography on silica gel, eluting with DCM/PE (2/1), to yield the title compound 1 (754 mg, 1.21 mmol, 44%) as a dark purple solid. M.p.: 196 °C. 1H NMR (600 MHz, CDCl3) [ppm]: δ = 8.66 (d, J = 4.26 Hz, 2H), 7.25 (d, J = 4.21 Hz, 2H), 5.865.73 (m, 2H), 5.05-4.95 (m, 4H), 4.01 (m, 4H), 2.12 (q, J = 7.02 Hz, 4H), 1.77-1.69 (m, 4H), 1.57-1.49 (m, 4H). 13C NMR (126 MHz, CDCl3) [ppm]: δ = 161.2, 139.2, 138.4, 135.5, 131.9, 131.4, 119.5, 199.3, 115.3, 108.2, 42.4, 33.5, 29.7, 26.4. IR (ATR): 𝑣 [cm-1] = 3087 (w), 2925 (w), 1706 (w), 1630 (s), 1556 (m), 1504 (w), 1459 (w), 1413 (m), 1398 (s), 1369 (m), 1261 (w), 1099 (m), 1068 (m). HRMS (DART (+)) [M+H]+: Calcd: m/z = 622.9853, found: m/z = 622.9855. Optics λmax,abs,CHCl3 = 563 nm, ε = 3.24∙104 M-1 cm-1, λmax,em,CHCl3 = 580 nm. EA Calcd for C26H26Br2N2O2S2: C, 50.17; H, 4.21; N, 4.50. Found: C, 49.67; H, 4.27; N, 4.26. Poly-3-(5'-(2,5-bis(2-decyltetradecyl)-3,6-dioxo-4(thiophen-2-yl)-2,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)[2,2'-bithiophen]-5-yl)-2,5-di(hex-5-en-1-yl)-6-(thiophen-2yl)-2,5-dihydropyrrolo-[3,4-c]pyrrole-1,4-dione (P1):

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Under an argon atmosphere, 1 (800 mg, 653 µmol, 1.00 eq) and 2b (406 mg, 653 µmol, 1.00 eq) were charged in a Schlenk tube in the presence of Pd2dba3 (41.8 mg, 45.7 µmol, 7 mol%), P(o-tol)3 (19.9 mg, 65.3 µmol, 10 mol%) and K3PO4 (896 mg, 4.22 mmol, 6.47 eq). Degassed dry toluene (12.0 mL) and degassed deionized water (2.00 mL) were added and the reaction mixture was stirred for 24 h at 95 °C before 1-bromo4-(trimethylsilyl)benzene (384 µL, 448 mg, 1.96 mmol, 3.00 eq) was added and the mixture was stirred for additional 6 h. 4-Trimethylsilylphenylboronic acid pinacol ester (541 mg, 1.96 mmol, 3.00 eq) was added via syringe and the mixture was stirred for additional 12 h at 95 °C. The reaction mixture was allowed to cool to room temperature and was added dropwise to methanol (300 mL) under stirring. A dark blue to green solid precipitated which was collected by filtration. Low molecular weight oligomers were removed by Soxhlet extraction with MeOH, hexanes and acetone. The chloroform fraction was dried in vacuo to yield P1 (765 mg, 522 µmol, 82%) as a green solid. 1H NMR (600 MHz, C2D2Cl4) [ppm]: δ = 9.2-8.8 (bs, 4H), 7.5-7.2 (bs, 4H), 5.87-5.74 (bs, 2H), 5.53-5.21 (bs, 4H), 4.12-3.37 (bs, 4H), 2.13-0.95 (m, 98H), 0.79-0.74 (m, 12H). IR (ATR): 𝑣 [cm-1] = 3071 (w), 2919 (s), 2850 (s), 1659 (s), 1541 (s), 1434 (m), 1399 (m), 1363 (w), 1320 (w), 1220 (w), 1063 (w), 1024 (m), 807 (s), 772 (s), 705 (m). Optics λmax,abs,CHCl3 = 756 nm. λmax,abs,solid = 809 nm. GPC Mn: 34.2 kg/mol, Mw: 53.8 kg/mol, Ð: 1.6. TGA 6.9% weight loss: 312 °C, onset: 235 °C; 52.4% weightloss: 430 °C, onset: 379 °C. Film Formation and Characterization. Films were processed from solutions in chloroform (10 mg/mL). The solution was spin-coated with a Spin 150 from S.P.S. at a rotational speed of 800 rpm for 30 s followed by 1500 rpm for 3 s. Absorption spectra were recorded on a Jasco UV-VIS V660 or Jasco UV-Vis-V-670 spectrophotometer. Fluorescence spectra were recorded on a Jasco FP-6500 spectrofluorometer. Atomic force microscopy images were taken with a DME DualScope atomic force microscope, while optical microscope images were recorded with a Nikon Eclipse 80i. In order to fabricate the BHJ based active layers we added dichlorobenzene due to its higher boiling point. OPD Fabrication and Characterization. Active-layer solutions (20 g/L in a mixture dichlorobenzene and chloroform 50:50 v:v) consisting of 1:1 ratio of P3HT (Rieke metals, Mw = 72.800) and P1, or P3HT:P1:X-SH (1:1:0.5) were prepared 48 h prior to deposition in a nitrogen-filled glovebox and processed therein. The preparation of ZnO (Avantama N10) and the PEDOT:PSS (Hereaus VPAI-4083) layers was carried out in a clean room environment under ambient conditions. Glass/ITO substrates were first cleaned in detergent, water, acetone and 2-propanol under sonication for 15 min, respectively, and treated by an oxygen plasma for 5 min. The spin-coating parameters of the PEDOT:PSS were v = 3800 rpm, a = 1500 rpm/s and t = 40 s and delivered a thickness of 30 nm. The spin-coating parameters of the ZnO were v = 4000 rpm, a = 1000 rpm/s and t = 40 s and delivered a thickness of 16 nm. The spin-coating parameters of the active layer were v = 1000 rpm, a = 500 rpm/s and t = 120 s. Subsequently, the samples were annealed in the glove-box at 120°C for 10 min, or crosslinked by illumination with a UV lamp (254 nm) for 10 minutes at 120°C. For the final electrode, 10 nm of Ca or MoO3 followed by 100 nm of Ag were evaporated in a vacuum system with a base pressure of 1·10-7 mbar. For the J-V curves, a sun simulator (with spectral distribution AM 1.5G) was used as a light source. The intensity of the lamp


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was calibrated with a silicon reference cell (RERA Systems RR 106 O) to be 100 mW/cm2. The current was measured with a source meter unit (Keithley 2636A). Series resistances were calculated from linear fitting of the current measured under illumination at open circuit voltage bias. Spectral responsivity (SR), as definition of the wavelength selectivity in terms of light to current conversion, was measured by illumination of the OPDs with monochromatic light created by a combination of a Xenon discharge lamp (450 W Osram XBO), a monochromator (Acton, SP-2150i), and a chopper wheel. The intensity was calibrated with a reference photodiode (Thorlabs, FDS100). The generated current was amplified (Femto DHPCA-100) and recorded with a lock-in amplifier (SR830, Stanford Research Systems). Bias voltages were applied with the aforementioned SMU. The other figures of merit determined were external quantum efficiency (EQE) was and specific detectivity (D*) as previously reported as a measure for sensitivity, according to the formula:52 D* = (√(Δf∙A))/NEP ≈ SR/√(2∙q∙JDark), where Δf is the bandwidth; A is the device area; NEP is the noiseequivalent power; SR is the spectral responsivity, q is the electron charge and JDark is the current in the dark of the device at the selected bias.

S

O HN

NH O

S

Th2DPP -bromohexene K2CO3 DMF, 120 °C, 16 h 32%

2-decyl-1-tetradecylbromide K2CO3 DMF, 120 °C, 16 h 23%

Th2DPP-ene

Th2DPP-C10C12

n-BuLi, DIPA, Me3SnCl

NBS

n-BuLi, DIPA, i-PrOBpin THF, 0 °C - rt, 2h

THF, 0 °C, -78°C - rt, 18 h 87%

CHCl3, 0 °C - rt, 16 h

52%

44% Br

R C12H25

S

O

C10H21

N

S

O

N

N

N C10H21

S

O C12H25

R

Results and Discussion

O

S Br

1

2a R = SnMe3 2b R = Bpin

Synthesis P1 is the target structure for an active-layer crosslinked BHJ NIR-OPD (cf. Scheme 2): The copolymerization of two similar 2,6-thienylated DPP monomers (1 and 2), which simultaneously allows introduction of a terminal alkene for click chemistry as well as a β-branched C10C12-chain providing solubility, should lead to an absorption maximum of the target polymer in the NIR regime. Such homopolymers exhibit n-type behavior in OFETs compared to polymers with even more thiophenylene donor moieties bridging the DPP cores.53-54 To access P1, monomer 1, functionalized with a terminal alkenyl side chain, and monomers 2a,b, functionalized with a β-branched alkyl side chain, were synthesized starting from 3,6di(thiophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (Th2DPP, Scheme 1), as shown in Scheme 1. Nucleophilic substitution of Th2DPP with 6-bromo-1-hexene under basic conditions furnished Th2DPP-ene in 32% yield.55 Bromination with N-bromosuccinimide (NBS) in chloroform afforded monomer 1 in 44% yield. Homopolymerization of 1 under Yamamoto conditions only gave short oligomers precipitating from the reaction mixture, emphasizing the need of solubilitymediating groups in the final polymer. Nucleophilic substitution of Th2DPP with 2-decyl-1-tetradecylbromide under basic conditions furnished Th2DPP-C10C12 in 23% yield, which is comparable to literature.56 Stannylated or borylated 2a,b were synthesized thorough deprotonation of the 5positions of the thiophenes with n-butyllithium and subsequent quenching utilizing Me3SnCl (2a)57 or i-PrOBpin (2b),58 respectively.

Scheme 1. Synthesis of the monomers 1, 2a and 2b starting from Th2DPP.

Various polymerization conditions were tested: A Stille polymerization employing monomers 1 and 2a only led to a molecular weight (Mn) of 14.2 kg/mol (n = 10) with a high dispersity Ð of 2.4 after Soxhlet extraction. A Suzuki polymerization of 1 and 2b with the use of Pd(dppf)Cl2 and Na2CO3 in toluene/ethanol/water (2:1:0.5) yielded P1 in 60% and with a high dispersity Ð of 2.8. By changing the catalytic system to Pd2(dba)3/P(o-tol)3, yields increased to 82% after purification by Soxhlet extraction. To reduce traps and to ensure a good quantum efficiency in the NIR-OPDs, P1 was a) endcapped after polymerization by 4-trimethylsilyl (TMS) –phenyl end groups, allowing for the determination of the polymer chain length via proton NMR, and b) stirred with Basolite® A 100 in chloroform for 12 h to remove trace catalyst residues.59-60 We did not observe any palladium species in the polymer P1 by Xray photoelectron spectroscopy (XPS) (detection limit: 0.1 atom%) of neat films (Figure S1, Supporting Information). P1 is highly soluble in common organic solvents (e.g. toluene, tetrahydrofuran and chloroform, >15 mg/mL). The molecular weight distribution was evaluated by analytical gel permeation chromatography (GPC) versus a polystyrene standard (Mn = 34.2 kg/mol, Pn = 24, Ð of 1.6). The average chain length was determined more precisely with respect to the TMS-end-group resonance via the relative intensities of 1H NMR signals, resulting in an estimated degree of polymerization of nNMR = 22 (equal to 44 monomer units), above the conjugation length of similar Th2DPP-based polymers.29 NMR analysis is consistent with the values obtained by GPC analysis, which, for rigid rod-like polymers, is known to overestimate the molecular weight in some cases.61


Chemistry of Materials Br

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

R C12H25

S

O

Page 4 of 11

S

O

H21C10 N

N

N

N C10H21

O

S

O H25C12

S

Br

R

2a R = SnMe3 2b R = Bpin

1 [Pd] X

TMS

X = Br B(OH)2 SnMe3

H21C10 O

N S

N TMS

C12H25 O

S

S

n

TMS

N

S N

O H25C12

O

C10H21

P1

Scheme 2. Synthesis of P1: Reaction conditions for Stille polymerization: Pd2(dba)3, P(o-tol)3, toluene, 110 °C, 16 h, 65%. Reaction conditions for Suzuki polymerization: Pd2(dba)3, P(otol)3, K3PO4, toluene/H2O, 95 °C, 12 h, 82%.

Polymer P1 exhibits an absorption maximum of 756 nm in chloroform (Figure S2, Supporting Information). Its thin-film absorption spectrum showed a red shift by 53 nm accompanied by slight peak broadening. The shoulder at 925 nm gains increases compared to the solution spectrum. The absorption onset was determined as 1024 nm in thin films (~1.21 eV band gap), relevant for NIR light detection. P1 is theoretically able to absorb ca. 75% of the solar irradiation, rendering it also attractive for photovoltaics.33 Its highest occupied molecular orbital (HOMO, XPS, Figure S2, Supporting Information) corresponds to ~4.6 eV. By taking the optical gap into consideration, the lowest unoccupied molecular orbital (LUMO) is positioned at ~3.39 eV. P1 decomposes at 376 °C (TGA, Figure S3, Supporting Information). Thiol-Ene Click Chemistry To render thin films insoluble, P1 was subjected to thiol-ene click chemistry utilizing tetrafunctional (3mercaptopropionate) X-SH as crosslinker. Desolubilization was achieved by interconnecting two or more polymer strands due to highly selective radical addition of thiol radicals, generated after UV light exposure, to the ω-alkenyl polymer side chains and desolublize the polymer films, the washing step removes excess of unreacted oligothiol (Figure 1).

Figure 1. Schematic illustration of the concept for thiol-ene click chemistry: a) Excerpt of the chemical reaction of thiol ene click chemistry through a radical addition, b) macroscopic visualization and c) microscopic illustration of the concept.

Optimum crosslinking parameters were explored with respect to temperature, irradiation time and stoichiometry (Figure 2a for dependence on reaction time; for detailed information see section S4, Supporting Information).The best crosslinking conditions are a ratio of P1:X-SH (1:2), above or equal to 100 °C reaction temperature and 10 minutes of reaction time. Solvent resistivity after desolubilization was not limited to chloroform, but also to other common organic solvents such as THF, dichloromethane, toluene etc. NIR-OPD We fabricated blends comprising P1 and P3HT in a ratio 1:1 by weight. For crosslinking X-SH was added in a ratio 1:1 (relative to the P1 content) and the film was treated with UV light for 10 minutes at 120 °C. Higher weight ratios of X-SH caused excessive disruption of the active layer morphology, making it inapplicable in devices. In Figure 2b and 2c, we present the absorbance spectra of the different P3HT:P1:X-SH layers, as well as the photographs of the layers used for device fabrication. In the absorption spectra, the contributions from P3HT (450 – 650 nm) and from P1 (650 – 1000 nm) are distinguishable. The final film thickness and morphology, impacting absorption spectra, are affected by the addition of XSH, the UV treatment and the successive solvent washing. After addition of X-SH, its characteristic absorption band below 400 nm can be distinguished, as can be observed in Figure 2b and in the normalized absorption spectra presented in the Supporting Information (Figure S5a). Light scattering, evidenced by a homogenous absorption background at all wavelengths was also present, supported by the observed increased roughness and layer inhomogeneity (see optical micrographs, Figure S5b). Furthermore, the addition of X-SH led to a change in the


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spectral distribution and subsequent color change (figure 2c). These effects decreased after UV treatment, and after solvent rinsing.

Figure 3. AFM images before and after rinsing of thin films of P3HT:P1 (top), P3HT:P1:X-SH after UV irradiation (middle) and P3HT:P1:X-SH after crosslinking and solvent rinsing (bottom).

Figure 2. a) Vis-NIR absorption spectra before and after rinsing of thin films of P1 and X-SH spin-cast from chloroform depending on the irradiation time, (b) Vis-NIR absorption spectra of P3HT:P1 layers with and without X-SH, (c) photographs of devices (cf. Figure 4b for device architecture). The substrates are 2.5 cm2.



The AFM images in Figure 3 depict the topography of films with and without crosslinking agent, and after the solvent rinse. The crosslinking agent X-SH disrupts the morphology of the active layer, responsible for the scattering effect. The rootmean-square roughness values of the layers increased from 19.5 nm to 179.4 nm with the addition of X-SH, only to return to 20.6 nm after rinsing with chloroform. We infer that the bubblelike structures observed in the P3HT:P1:X-SH case are segregated residual domains of X-SH that were removed after crosslinking. The average thicknesses of the samples were 230 nm for pristine P3HT:P1, 1400 nm for P3HT:P1:X-SH and 150 nm for P3HT:P1:X-SH crosslinked and after solvent rinsing (profilometry), in line with the previous observations. P3HT:P1-based NIR OPDs were fabricated (ITO/PEDOT:PSS/Active Layer/Ca/Ag). The device architecture and the energy level diagram of the materials are presented in Figure 4. The low lying HOMO of P1 does not easily allow a good energy level matching with many donor polymers producing a high energy barrier for holes to be transferred to the donor material. Our choice of P3HT, although offering a less favorable type I band alignment, was based on its complementary absorption in the visible range and its wellcharacterized optoelectronic properties. In the presented devices we utilize an optimized ratio of 1:1 of P3HT:P1, which gave the best performance and best film quality. From the device performances, we can conclude that in the photodiode regime, i.e. when a bias is applied, the energy barrier for holes to be extracted from P1 to P3HT is overcome (as it can be seen as well in Figure S13). We also investigated the performance of devices with P1 as acceptor and donor in combination with DPP-TTT and PCBM, respectively (Supporting Information, Sections S6 and S7). In both cases, although type II band alignment was achieved in the PCBM case, the devices did not exhibit any P1-related photo-response in the NIR (see SI for more details).

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pristine and cross-linked devices. Devices with inverted architectures (see Supporting Information, section S8) were inferior to those with a regular architecture. The optical absorption of the blend covered the entire visible spectrum and a good part of the NIR (Figure 2b). This was reflected in the normalized SR curves obtained at -1 V (Figure 6), where both crosslinked and non-crosslinked samples exhibited a clear photocurrent response over the whole absorption range up to 1000 nm. The spectral distribution of the P3HT:P1 and that of the cross-linked/rinsed P3HT:P1:X-SH were identical. Desolublization did not disturb the fundamental device functionality. We observed a slight reduction in the P3HT part of the spectrum (550 nm) from 3.26 mA/W to 2.92 mA/W and a decrease of 0.04 mA/W for the P1 regime (810 nm). In Figure S8 we present the calculated specific detectivity (D*), which confirmed the same trend observed for the spectral responsivity, with maximum values in the NIR (810 nm) of 1.01·108 Jones after crosslinking. Table 1 summarizes the parameters obtained for all devices, which are comparable to current literature reports for polymer-based NIR OPDs.62-64

Figure 4. a) Schematic energy levels and b) device architecture of the NIR-OPD.

Figure 5 and Figure 6 present the IV curves and the spectral responsivity (SR) for the P3HT:P1 device closest to average of OPDs with and without crosslinking treatment and thermal stability test. We focused on the reverse bias section of the IVcurves (i.e. photodiode regime) to characterize dark current and on-off rations of the devices. The dark currents for the P3HT:P1 device and the cross-linked/rinsed P3HT:P1:X-SH are in the range of 10 µA/cm2. At a reverse bias of -1 V and under illumination with a solar simulator (AM 1.5G; 100 mW/cm2), a photocurrent of ~1.8 mA/cm2 was observed for P3HT:P1 while the cross-linked/rinsed P3HT:P1:X-SH showed 0.09 mA/cm2 with on-off ratios of 103 and 101 for the

Figure 5. JV curve under illumination and in the dark of devices before and after a thermal treatment of 180°C for 90 minutes. a) P3HT:P1 and b) P3HT:P1:X-SH crosslinked, after chloroform rinsing. All measurements refer to the pixel closest to the average from four devices.


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Chemistry of Materials the respective SR of the artificially aged devices with P1 as acceptor. Figure 5a documents that in the case of P3HT:P1 devices the thermal ageing partially degraded the device producing a decrease in the photocurrent from 1.86 to 0.12 mA/cm2 at -1V. The degradation effect was particularly severe in the absorption region of the P3HT in the normalized SR (400 – 600 nm), with a relative decrease at 550 nm of 39%. For the crosslinked P3HT:P1:X-SH devices, thermal treatment only caused a minor deterioration of the device performance (Figure 5b, Table 1). The SR in the P3HT region (550 nm) revealed a smaller decrease of 17%, remaining comparable to the pristine reference. Crosslinking of P1 protects the donor polymer from aggressive solvent treatment and increases the overall thermal stability of the BHJ system, rendering the devices more suitable for situations where long term operability is required.

Figure 6. Normalized SR of the different devices P3HT:P1 and P3HT:P1:X-SH (regular architecture) crosslinked and rinsed before and after a thermal treatment of 180°C for 90 minutes. The presented data refers to the pixel closest to the average from four devices.

Both acceptor properties of P1 and the favorable BHJ morphology of the active layer remained after the treatment; also P3HT was not washed away by the chloroform rinsing despite it being highly soluble in this solvent. Crosslinking of P1 creates a fixed network, which retains P3HT after aggressive solvent wash. P3HT itself is not crosslinked under those conditions as films containing solely P3HT and X-SH, treated under cross-linking conditions, easily dissolved in chloroform (Figure S4a, Supporting Information). This is a promising approach for the future fabrication of printed multilayer devices, where the deposition of contacts or interlayers should not dissolve or affect the underlying active layer.65 In order to investigate device stability, we performed an artificial thermal ageing test (180 °C; 90 min) on crosslinked and not crosslinked devices, well below the threshold of decomposition for P1 (see Supporting Information, section S9). Table 1. Dark current (JD), spectral responsivity (SR) and specific detectivity (D*) of the devices shown in Figure 6, at the wavelength of maximum absorption of P3HT (550 nm) and P1 (810 nm). Values were obtained at -1 V.

Conclusion We report P1 as a low-bandgap absorber in NIR photodetectors. The combination of P1 as an acceptor with the donor P3HT, absorbing in the visible regime, furnished fullerene-free OPDs responsive up to 1000 nm. Blends of P1 and P3HT were desolubilized utilizing tetrathiol-crosslinker XSH in combination with benign UV-induced thiol-ene click chemistry. Crosslinking desolubilized the BHJ layer and retained P3HT within the network and simultaneously stabilized the device against thermal ageing without deterioration. Both NIR absorption and crosslinkability of P1 can be generalized to other photovoltaic and/or photodetecting BHJ systems with improved processability and lifetime due to avoidance of demixing and/or crystallite growth. In further studies, we shall extend the time of thermal ageing, while investigating the morphology changes in the BHJ layer with cryo-electron microscopy.

ASSOCIATED CONTENT Supporting Information X-ray photoelectron spectra, optical characterization, thermogravimmetric data, conditions for thiol-ene click chemistry, optical micrographs, further OPD characterizations: combination of P1 with different donor or acceptor/inverted device architecture/performance after thermal ageing, general experimental methods, synthetic procedures, NMR spectra.

AUTHOR INFORMATION Corresponding Author * E-Mail: [email protected]

Author Contributions Δ

M.C. and M.R. contributed equally.

Notes BHJ-based devices undergo deterioration of its performance arising from thermally activated changes in morphological constitution of the donor-acceptor domains resulting in lower charge separation and carrier extraction efficiency. 66-67 Figure S12 shows the degradation of a reference P3HT:PCBM device after the thermal aging. Figure 5 and 6 show the IV curves and

The authors declare no competing financial interest.

ACKNOWLEDGMENT



We acknowledge the support of the German Federal Ministry of Education and Research through Grant No. FKZ 13N13695 and 13N13691 (POESIE). This work was partially supported by the EC through the Horizon 2020 Marie Skłodowska-Curie ITN project INFORM (Grant Agreement 675867).

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