High Performance Organic Photodetectors from a High Bandgap

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Organic Electronic Devices

High Performance Organic Photodetectors from a High Bandgap Indacenodithiophene-Based #-Conjugated D–A Polymer Cindy Montenegro Benavides, Petri Murto, Christos Chochos, Vasilis Gregoriou, Apostolos Avgeropoulos, Xiaofeng Xu, Kim Bini, Anirudh Sharma, Mats Andersson, Oliver Schmidt, Christoph J. Brabec, Ergang Wang, and Sandro Francesco Tedde ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03824 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Applied Materials & Interfaces

High Performance Organic Photodetectors from a High Bandgap Indacenodithiophene-Based πConjugated D–A Polymer Cindy Montenegro Benavides†,£,∆, Petri Murto‡,ϕ,∆, Christos L. Chochos§,ℓ, Vasilis G. Gregoriou§, Apostolos Avgeropoulos,ℓ, Xiaofeng Xu‡, Kim Bini‡, Anirudh Sharma ϕ, Mats R. Anderssonϕ, Oliver Schmidt†, Christoph J. Brabec£, Ergang Wang‡,*, and Sandro F. Tedde†,* †

Siemens Healthcare GmbH, Günther-Scharowsky-Str. 1, 91058 Erlangen, Germany

£

Department für Material Science, i-MEET, Friedrich-Alexander Universität Erlangen-Nürnberg,

Martensstr. 7, 91058 Erlangen, Germany ‡

Department of Chemistry and Chemical Engineering/Applied Chemistry, Chalmers University

of Technology, SE-412 96 Gothenburg, Sweden ϕ

Flinders Centre for Nanoscale Science and Technology, Flinders University, Sturt Road,

Bedford Park, Adelaide, SA, 5042, Australia §

Advent Technologies SA, Patras Science Park, Stadiou Street, Platani-Rio, 26504, Patra, Greece

ℓ

Department of Materials Science Engineering, University of Ioannina, Ioannina 45110, Greece

ACS Paragon Plus Environment

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KEYWORDS: concentration ratio, dark current density, frequency response, high-speed, molecular weight, red organic photodetectors

ABSTRACT:

A

conjugated

donor–acceptor

(D–A)

polymer,

poly[4,4,9,9-tetrakis(4-

hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl-alt-5-(2-ethylhexyl)-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione-1,3-diyl] (PIDT-TPD), is blended with the fullerene derivative [6,6]phenyl-C61-butyric acid methyl ester (PC61BM) for the fabrication of thin and solution-processed organic photodetectors (OPDs). Systematic screening of the concentration ratio of the blend and the molecular weight of the polymer is performed in order to optimize the active layer morphology and the OPDs performance. The device comprising a medium molecular weight polymer (27.0 kg/mol) in a PIDT-TPD:PC61BM 1:1 ratio exhibits an external quantum efficiency (EQE) of 52% at 610nm, a dark current density of 1 nA/cm2, a detectivity of 1.44 × 1013 Jones, and a maximum 3 dB cut-off frequency of 100 kHz at −5 V bias. These results are remarkable among the state-of-the-art red photodetectors based on conjugated polymers. As such, this work presents a functional organic active material for high-speed OPDs with linear photoresponse at different light intensities.

INTRODUCTION One of the main advantages of organic photodetectors (OPDs) is the possibility to target different applications, such as light communication, night vision, environmental monitoring, photo and video imaging, and biosensing, by tuning their spectral response with the chemical structure of the

active

layer.1-8

Photodetectors

based

on

nanostructured

inorganic

and

hybrid

inorganic/organic materials have shown strong photoresponse in the short-wavelength


2

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UV/visible region.9-13 However, conjugated polymers represent an alternative in that they are structurally adjustable and mechanically conformable, and therefore useful for the solution-based fabrication of low-cost, thin, flexible, and large surface area OPDs for multi-color detection.2,6,1415

The two major methodologies to tune the bandgap and the frontier orbital energy level of the conjugated polymers are either by using the so-called donor–acceptor (D–A) design strategy or by stabilizing the quinoid structure of the polymer.16-18 The former has been the most common approach thanks to the plethora of functional electron-rich (donor) and electron-deficient (acceptor) building blocks. Conjugated D–A polymers comprising indacenodithiophene (IDT) as the donor unit have shown strong light absorption and high charge mobilities and power conversion efficiencies in organic photovoltaic (OPV) and organic field-effect transistor (OFET) applications.19-26 Their rigid and planar backbone ensured strong intermolecular interactions in the solid state, which accordingly facilitated high charge mobilities.19-20,23-24 Therefore, IDTbased polymers are promising candidates for the fabrication of fast-response OPDs as well. Previous works1,3,27 have reported solution-processed OPDs with the 3 dB bandwidth cut-off frequency ranging from 1MHz up to 50MHz in the visible and near-infrared spectral regions. However, the dark current density (Jd) values at high reverse bias were not lower than few µA/cm2. Later studies15,28-29 presented Jd values in the order of few nA/cm2, yet the EQE responses were not higher than 30%. The Jd and the EQE are the main factors that limit the sensitivity of the OPD devices. Therefore, it is of crucial importance the development of new conjugated polymers that exhibit simultaneously high performance by keeping a low Jd, high external quantum efficiency (EQE), and high speed even at low irradiance power, which are consider ideal for imaging and communication applications. Moreover, these conditions should


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be satisfied under high reverse bias (–5 V) in order to integrate the photodetector as the lightsensitive component into real life systems/applications, while increasing charge collection efficiency.2 The morphology of the bulk heterojunction (BHJ) plays an important role in the OPDs performance. An interpenetrating network of the donor and acceptor material should ensure efficient dissociation of excitons and successful charge extraction. The donor:acceptor blend ratio is among the factors affecting the BHJ morphology. The optimal ratio of polymer and fullerene derivative (PCBM) usually depends on the concentration, solubility, and the interaction between the BHJ components as well as the processing solvent.30-31 In blends with amorphous polymers, high PCBM contents are believed to maximize the efficiency. This has been explained by the poor exclusion of fullerene from the polymers, which requires higher amount of the fullerene to build up a continuous and conducting PCBM network, in addition to the intermixing of the polymer chains for balanced charge mobilities.30 However, early studies32-33 of the blends of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV) and PCBM have shown that large concentration of PCBM may produce films with substantial phase separation and fullerene aggregates up to sizes of hundreds of nanometers. Moreover, Ma et al.34 demonstrated that the optimal weight ratio for the poly(3-hexylthiophene) (P3HT):PCBM system in chlorobenzene was between 1:0.8 and 1:1, respectively, with an overall concentration of around 1 wt-% of P3HT (i.e. 10 mg/mL). They showed that small variations in the blend ratio resulted in significant changes of the device efficiency. Thus, the evaluation of the blend ratio is an important step in the optimization of a new polymer for OPD applications. Furthermore, the molecular weight of the polymer has been shown to affect significantly the BHJ morphology, and eventually the device performance.20-22,30,35-38 For semi-crystalline


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polymers like P3HT, it has been showed35,38 that low molecular weights would lead to poor device performance due to the reduction of hole mobility. It was proposed that although polymers with high molecular weight exhibit low solubility, the long conjugation length of the polymer chains allowed better π-π stacking within the bi-continuous network, which enhanced the hole mobility. However, Ballantyne et al.37 demonstrated that molecular weights higher than 34.4 kg/mol led to an entanglement of the P3HT chains, which limited the polymer crystallinity in the film and reduced the hole mobility. In contrast, Ma et al.36 suggested that the best efficiency for P3HT:PCBM solar cells is achieved with an average medium molecular weight polymer of 55.0 kg/mol. It was proposed that the 55.0 kg/mol P3HT encompassed the good hole mobility of the higher molecular weight (62.5 kg/mol) fraction and the good solubility of the lower molecular weight (13.0 kg/mol) fraction. They further affirmed that the high molecular weight P3HT limited the performance of P3HT:PCBM BHJ photoresponse devices by reducing the diffusion of PCBM. Even if conjugated polymers based on IDT have shown a lack of observable crystallinity,20,22 their molecular weight still plays an important role on the BHJ morphology. McCulloch et al.20 proposed that molecular weights of above 25 kg/mol are optimal for IDT-based polymers. They argued that very high molecular weight fractions would not only present low quality films but also low probability for the desired thin film crystallization due to their high solution viscosities. However, further work of Intemann et al.21 on different molecular weights of an IDT-based polymer (PIDSe-DFBT) showed that the highest molecular weight fraction (61.8 kg/mol) provided the best photovoltaic performance thanks to its high absorptivity and charge carrier mobilities. Similarly, Gasparini et al.24 studied different molecular weight fractions of an indacenodithienothiophene-based polymer (PIDTTQ) in blends with PCBM, and obtained


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improved charge carrier transport and less non-radiative recombination losses with increasing molecular weight of up to 58 kg/mol. Even though the influence of the molecular weight and the BHJ morphology on the performance of OPVs has been broadly screened, studies on the impact of these factors on the performance of OPDs are very limited or scarce. Therefore, evaluation of the donor:acceptor blend ratios and the molecular weight variations in the newly established polymers are required. In this work, we report on the synthesis and application of poly[4,4,9,9-tetrakis(4hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl-alt-5-(2-ethylhexyl)-4Hthieno[3,4-c]pyrrole-4,6(5H)-dione-1,3-diyl] (PIDT-TPD) for solution-processed thin-film OPDs. Three different batches of PIDT-TPD were synthesized, and the number average molecular weight (Mn) was varied from 20.9 kg/mol to 27.0 kg/mol, and finally to 40.7 kg/mol. We conducted a systematic study on an ideal polymer:PCBM blend ratio and then screened the different Mn fractions and deposition conditions to optimize the morphology of the active layer. Significantly, OPDs fabricated from the three batches of PIDT-TPD all exhibited high cut-off frequencies (>50 kHz), dark current densities in the order of 10–9 A/cm (i.e. nA/cm2), and specific detectivities in the order of 1012–1013 Jones under –5 V bias. The optimal performance was recorded for the medium Mn polymer (27.0 kg/mol) in a PIDT-TPD:PC61BM 1:1 ratio, with the 3 dB cut-off frequency at 100 kHz and a maximum EQE of 52% at 610 nm, as measured at – 5V.

RESULTS AND DISCUSSION


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Figure 1. (a) OPD device architecture and chemical structures of PIDT-TPD and PC61BM. (b) Absorption coefficients of PIDT-TPD (green line) and PIDT-TPD:PC61BM blends in 1:1 (black squares), 1:2 (blue circles), and 2:1 (red triangles) ratio, as measured for thin films. The polymer was medium Mn = 27.0 kg/mol. (c) Energy levels and working mechanism of the OPD in dark at 0 V (left panel), in dark at reverse bias (middle panel), and under illumination at reverse bias (right panel).

Material Design. Figure 1a presents the chemical structures of PC61BM and PIDT-TPD, the latter incorporating 4-hexylphenyl-substituted IDT as the electron donating (indicated by blue color) and 5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione (TPD) as the electron withdrawing (indicated by pink color). The bulky 4-hexylphenyl substituents on IDT and the


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branched 2-ethylhexyl side chains on TPD were introduced for good miscibility with PCBM, which should ensure sufficient intermixing of the donor and acceptor material and thereby large interfacial area between the two regions in the BHJ.39 The three different Mn batches of PIDTTPD were synthesized via Pd-catalyzed Stille polycondensation polymerization, as described in detail in the Supporting Information (Scheme S1 and Figure S1).

Physical Properties. Figure 1b shows the absorption coefficient versus wavelength curves of the neat polymer and the PIDT-TPD:PC61BM blends in 1:1, 1:2, and 2:1 ratios. The polymer used was medium Mn = 27.0 kg/mol. The thin-film absorption spectrum of PIDT-TPD is peaked at 580 nm and the optical bandgap, as estimated from the low-energy onset of absorption, was 2.0 eV. Introduction of PC61BM to the blend revealed another absorption band peaking at 332 nm. All blends exhibited a strong complementary absorption ranging from 300 to 650 nm, but the relative contribution of PIDT-TPD and PC61BM to the absorption spectrum varied according to their respective concentration. The photoluminescence (PL) measurements of the corresponding polymer and blend films showed that the emission from PIDT-TPD was completely quenched in all three blends (Figure S2, Supporting Information). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymer and PC61BM were obtained by cyclic voltammetry (CV) measurements (Figure S3, Supporting Information). As shown in Figure 1c (left panel), the HOMO–HOMO and LUMO–LUMO offsets between PIDT-TPD and PC61BM were both 0.6 eV. This facilitates an efficient exciton separation in the BHJ, as demonstrated by the PL quenching, and indicates the suitability of the PIDT-TPD:PC61BM blends as the photoactive layer.


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Device Architecture. Figure 1a,c presents the architecture and energy level diagram of the devices used in this study. The OPDs were built on an indium tin oxide (ITO) coated glass substrate and the BHJ, comprising PIDT-TPD:PC61BM blend, was sandwiched between the interlayer and the Al contact. Moreover, Figure 1c illustrates the device performance in dark conditions at 0 V (left panel), in dark at reverse bias (middle panel), and under illumination at reverse bias (right panel). As observed, the 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) interlayer exhibits a high LUMO level (–3.1 eV) and acts as an electron blocking layer, which is crucial for reducing the Jd in the OPD at reverse bias.40 Under illumination, the photogenerated electrons and holes are driven by the external negative bias and drift to the respective electrodes. Concentration Ratio of the Blend. In order to determine the optimal concentration ratio, the medium Mn batch of PIDT-TPD (27.0 kg/mol) was blended with PC61BM in chlorobenzene (CB). The PIDT-TPD:PC61BM ratio was varied between 2:1, 1:1, and 1:2, by keeping the polymer concentration constant (15 mg/mL) for all blends. Table 1 summarizes the experimental parameters and the device performance, while the optoelectronic characterization is illustrated in Figure 2a,b. Larger number of devices were fabricated for the best performing blend as a proof of reproducibility of the findings. The ideal blend ratio was 1:1, approaching a Jd of 1 nA/cm2 at −5 V reverse bias and displaying the highest EQE value of 52% at 610 nm. Similar EQE values has been reported for PIDT-TPD:PCBM blends in photovoltaic applications.25 The OPD device with a blend ratio of 2:1 exhibited much higher Jd of 290 µA/cm2 and only a negligible photocurrent at reverse bias, while the device with a blend ratio of 1:2 led to electrical short circuits. The EQE values obtained for the 2:1 and 1:2 blends are considerably low in comparison to the 1:1 blend. Figure 2c presents the frequency response of the OPD using the blend ratio of


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1:1. The 3 dB cut-off frequency at −5 V bias was recorded at 100 kHz. This result is consistent with the high charge mobilities reported for IDT-based conjugated polymers.19-20,23-24 Moreover, our space-charge-limited current (SCLC) mobility measurements indicated that the 1:1 blend exhibited the highest and most balanced electron and hole mobility, as compared to the 2:1 and 1:2 blends (Figure S4, Supporting Information), and further supports its superior OPD device performance.

Table 1. BHJ domain sizes and device characteristics of different blend ratios Polymer:PC61BM Domain Mn a ratio (kg/mol) size (nm)b

Jd at –5 V (mA/cm2)c

EQE at –5 V, 610 nm (%)c

Cut-off D* at –5 V frequency c (Jones) (kHz)c

1:1

27.0

High Performance Organic Photodetectors from a High Bandgap

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