Nanocontacts give efficient hole injection in organic electronics

Short Communication

Nanocontacts give efficient hole injection in organic electronics

Qingzhen Bian

, Chiara Musumeci

, Chuanfei Wang

, Andreas Skallberg

, Yongzhen Chen

,

Zhangjun Hu

, E. Peter Münger

, Kajsa Uvdal

, Mats Fahlman

, Olle Inganäs

a

Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden

b

Laboratory of Organic Electronics, Department of Science and Technology, Linköping University, 60174 Norrköping, Sweden

c

Molecular Surface Physics and Nano Science, Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden

d

Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden

a r t i c l e i n f o

Article history:

Received 31 October 2020

Received in revised form 3 December 2020 Accepted 4 December 2020

Available online 16 December 2020

Ó 2020 Science China Press. Published by Elsevier B.V. and Science China Press. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

A key requirement for well performing devices based on organic semiconductors is to ensure ohmic contacts, where an efficient hole and electron injection and extraction can be achieved at the electrode/semiconductor interfaces. The usual way to obtain ohmic contacts involves fine tuning of the energet-ics at the interfaces by using dopants (p-type and n-type) to con-trol the Fermi level of the semiconductor[1,2], or using suitable metals or metal oxides as electrodes [3]. These strategies are not often compatible with large area printing on flexible sub-strates. Similar difficulties are found with printing of molecular layers of dipoles to allow tunnelling to the electrode in organic solar cells. The insulating dipole layers[4]used to prevent recom-bination of electrons and holes at the contacts may compatible with nanocontacts formed by a metallic or semimetallic elec-tronic conductor, covering a small fraction of the interface area. This type of nanocontact has recently been demonstrated by Hat-ton and co-workers [5,6]. The nanoparticles of gold were immersed in an insulating polymer layer as contact to organic solar cells (OPV) materials. It was demonstrated that it is suffi-cient to cover a few percent of the electrode area with gold nano-electrodes in order to collect the full generated photocurrent. Combining a dipole layer with a point contact, a nanosized con-tact, is one possible way out of the problem of combining a dense dipole layer at the interface to the semiconductor, with an elec-tronic contact from the electrode to the semiconductor. We note the use of many small contacts at the rear of silicon solar cells in devices with passivated emitter and rear contact (PERC)[7].

In this study, we mix the insulator deoxyribonucleic acid-cetyltrimethylammonium chloride complex (DNA:CTMA) with the

ambipolar polymer PCDTFBT, {poly[(5-fluoro-2,1,3-benzothiadia-zole-4,7-diyl) (4,4-dihexadecyl4Hcyclopenta[2,1-b:3,4-b’]dithio- phene-2,6-diyl)(6-fluoro-2,1,3-benzothiadiazole-4,7-diyl)(4,4-dihexadecyl-4H-cyclopenta[2,1-b:3,4-b’]dithiophene-2,6-diyl)]}. The conjugated polymer is sufficiently different from the polymeric insulator with a separate phase when forming a solid film upon removal of the solvent. The domains of the minority polymer often below micrometres, and therefore point contacts are formed. The electronic polymer is hole doped when interacting with the insula-tor, and therefore can carry current. It is possible that charge carriers transport through the low bandgap semiconductor, over longer dis-tances and through thicker layers. Detailed morphology images indicate that the polymer demonstrate a separated phase and form a great number of nano and micro sized electronic contacts to the semiconductor in the active layer in organic semiconductor devices. The hybrid material can be used as an interlayer enabling efficient hole extraction/injection in high-performance solution-processed OPV, transistor (OFET) and light-emitting diodes (OLEDs), generat-ing submicron OLEDs. This material also works well for large area solution processing on flexible substrates.

Fig. 1a shows the molecular structures and a schematic of the DNA:CTMA complex. Sample preparation details are found in the

Supplementary materials (online). Several DNA:CTMA/PCDTFBT weight ratios were characterized, and in order to optimize device performance, the optimal stoichiometry for this hole transport layer (HTL) was 8:1 (Fig. S1online). The pure polymer film has a morphology featuring oriented nanofibrils as observed by atomic force microscopy (Fig. S1online), and these oriented features are to a certain extent preserved after dilution with the insulating DNA:CTMA complex in the HTL.

We sandwiched several conjugated polymers between indium tin oxide (ITO) and Ag electrodes. The MoO3(high work function

6.9 eV) here used as a reference material for the hole contact. https://doi.org/10.1016/j.scib.2020.12.023

2095-9273/Ó 2020 Science China Press. Published by Elsevier B.V. and Science China Press. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

⇑Corresponding authors.

E-mail addresses: qingzhen.bian@liu.se (Q. Bian), olle.inganas@liu.se (O. Inganäs).

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The material (poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo [2,1,3]thiadiazol-4,8-diyl)]) (F8BT) demonstrate a deep highest occupied molecular orbital (HOMO) energy (5.8 eV)[8], and the current density–voltage (J-V) characteristics for the devices exhibit

the ideal form for space-charge-limited current (SCLC) conduction (J

a

a significant enhancement of injection current when using the hybrid HTL at the contact. Furthermore, the space-charge-limited

Fig. 1. (Color online) Efficient hole transport Layer (HTL). (a) Schematic of the DNA:CTMA complex and chemical structures. (b) Hole only performance of F8BT device, the device structure is ITO/Interlayer/F8BT/MoO3/Ag. (c) Optical microscopy images of nano-LEDs (~500 nm). The HTL was in a highly diluted ratio (DNA/PCDTFBT) = 30:1).

Exposure time: 1 min. (d) Solar cell performance with different HTL layer. (e) Measured photocurrent (Jsc) against light intensity on a logarithmic scale. (f) Kelvin probe force

microscopy results. To the left is the topographic image, to the right is the KPFM image. Vcpd(inset value) is the contact potential difference between the AFM probe and the

current is increased with decreasing thickness of F8BT and HTL (Fig. S2online). The HTL based device demonstrates a weakly tem-perature dependent injection current at temtem-peratures from 150– 300 K (Fig. S2 online), indicating that charge transfer between HTL and F8BT is efficient and almost barrier free. These results are different from PEDOT:PSS. Moreover, the hybrid HTL also works well for ambipolar materials, such as N2200 (HOMO _{~5.7 eV,}

Fig. S3online). The injection current shows space-charge-limited feature and is comparable to the injection current from MoO3side.

These results are different from PEDOT:PSS contact layer, where the holes are difficult to inject from the PEDOT:PSS layer.

To characterize electronic transport in this hybrid material, we also evaluate the HTL material by measurements of transport in OFETs (Figs. S4–S7 online), where electronic conduction is only expected to occur in the polymer semiconductor. The HTL demon-strates hole transport (Fig. S5online). Due to the insulating DNA-CTMA, the hybrid materials demonstrate two domains ((1) poly-mer; (2) DNA-CTMA), and charge transport between polymer domains occurs by percolation. In the low voltage regime, the cur-rent demonstrates a linear dependence on bias at diffecur-rent temper-ature (Fig. S6 online), indicating an ohmic transport process occurred at the electrode interface[9]. These results are consistent with the results from hole only devices, and confirm that an ohmic contact was formed between the HTL and metal electrode.

If the HTL is effectively a collection of nanocontacts in a DNA: CTMA blend, we would expect that both charge collection in OPVs and charge injection in LEDs reflect the size and density of the nanocontacts. To test this hypothesis, we fabricated OLEDs using our HTL. We have previously demonstrated the construction of nano LEDs by nanoelectrodes [10], or self-assembled blends of insulating and luminescent polymers in the active layer[11].

The OLEDs structure was ITO/HTL/MDMO-PPV/LiF/Al. PEDOT: PSS was chosen as a reference material. A sensitive optical micro-scope (Axiovert 200 M) was used to image the light distribution. The nanocontacts from the DNA:CTMA blend was investigated in a highly diluted HTL film, with a weight ratio (DNA:PCDTFBT) of 30:1. As shown inFig. 1c, individual light spots with a submicron size (~500 nm) were visible. Due to the optical microscopy limits, we cannot determine the exact size and shape of the light source. These results indicate that reduced contact area is found in the HTL based device. This submicron LED is strong experimental evidence of the HTL operating with nanocontacts between light emitting polymer and electrode. Compared to the reference device, having PEDOT:PSS as interlayer, OLED devices based on the HTL (DNA: CTMA/PCDTFBT = 8:1)/MDMO-PPV show an enhanced electrolumi-nescence (Fig. S8online). This result indicates that holes are effi-ciently injected into MDMO-PPV from the hybrid HTL. As a result, compared to PEDOT:PSS (~0.4%), the electroluminescence quantum yield value (_{~4%) demonstrate a ten-fold enhancement} in the HTL based OLED (Fig. S8online) consistent with the electro-luminescence results (Fig. S8b online).

A standard solar cell was also investigated (Fig. S9online) to further study the extraction capability of the HTL. The device struc-ture is ITO/HTL/PBDB-T:PC71BM/PFN-Br/Al. When the polymer

fraction in the DNA:CTMA matrix increased from 10% to 50% (Fig. 1d), all characterized device parameters (photocurrent, fill factor, and open circuit voltage) decreased. This demonstrates that the fraction of electronic polymer in the HTL influence the extrac-tion capability. If the HTL is an insulator with a minority fracextrac-tion of doped electronic polymer, we are effectively contacting the pho-toactive materials through a myriad of nanocontacts. If these are dense enough, the diffusion length of photogenerated charge carri-ers will allow transport to the nanocontacts, to effectively collect all photocurrent. Technical implementation of a similar geometry is found in the recent work of Hatton and co-workers[5].

We have prepared series of devices with varying thickness of active layer, and find the collected photocurrent is significantly decreased when the thickness of active layer increased to 250 nm (Fig. S9online).Fig. 1e demonstrates the photocurrent as a function of light intensity. Regardless the thickness of active layer, an almost similar slope (~1) was obtained, indicating that there is weak monomolecular recombination. While nonselective contacts would allow surface recombination, which is reported as monomolecular recombination, this observation fulfils one of the requirements for selective contacts.

To evaluate the general applicability of our novel hybrid hole transport material, we used it as interlayer to fabricate large-area flexible printed devices. We combined the HTL with a polymer– polymer blend (PBDTS-TPD/PNDI-T)[12]by lamination technique

[13], where PEDOT:PSS (PH 1000) works as electrode and pro-cessed on a large area flexible polymer substrate PET (sample preparation details are found in the Supplementary materials online). The donor polymer PBDTS-TPD has a deep HOMO energy (~6.0 eV). A set of laminated devices were fabricated (Fig. S10

online). Compared to the reference devices, an efficient hole injec-tion/extraction can be achieved in the HTL based device.

Energy filtered photoemission electron microscopy (PEEM) was used to investigate the homogeneity of thin films of HTL, DNA, DNA:CTMA and PCDTFBT deposited by spin-coating on silicon sub-strates (Fig. S11online). Images and spectroscopic information of the low energy emitted electrons were measured over the photoe-mission threshold, to map out the samples over an area of 15
10 mm2_{. This enables us to estimate the work function}

varia-tion of the thin films. In themm range, the narrow energy distribu-tion (0.10 eV) of the work funcdistribu-tion values for these images (FoV = 29mm) indicates that these films are uniform.

To assess the surface-dipole effects, the HTL was also investi-gated by Kelvin probe force microscopy (KPFM) measurements. The potential maps were measured at several locations on the sam-ples (Fig. 1f). The contact potential differences (VCPD) between the

KPFM probe and bare gold (Au) electrode, Au/HTL, Au/PCDTFBT, and Au/DNA:CTMA can be determined. Compared to pure Au, there were no significant changes for the Au/PCDTFBT and Au/DNA: CTMA samples. In contrast, the DVCPD between Au and Au/HTL

was found to be 0.24 V (Fig. 1g).

Furthermore, two different regions with diameter in the submi-cron range are resolved by KPFM measurement (Fig. S11online), and demonstrates a very non-uniform surface potential distribution.

The selective hole transport found in this hybrid HTL material come from hole doping. This conclusion can be further confirmed by XPS (Fig. S12online) and FTIR (Fig. S13online) results. Evidence for this doping process is also found in the OFET data (Fig. S14

online).

We note that the electric field distribution will be non-planar at a nanocontact, this is different from plane electrodes, which demonstrates a standard planar field distribution. We have simu-lated the field distribution at a nanocontact with 10–20 nm diam-eter, and the simulation also included a dipole layer covering the area between nanocontacts (Fig. 1g, details are found in the Sup-plementary materials online). The results demonstrate that the electric field and field gradients, are higher at the junction of active material to a nanocontact, and this causes a higher field assisted charge injection. Filamentary conduction in organic solids has been extensively discussed in the past[14]and more recently in studies of three-dimensional current distribution with master-equation methods [15]. It is to be expected that transport through thin HTL and thin disordered organic solids is filamentary, with a very non-uniform current distribution. As proposed here, these proper-ties harmonized with filamentary injection.

Field emission into an insulator has been described with Fow-ler-Nordheim theory, which can be used to study the non-uniform local field enhancement in nanostructured materials[16–18]. Fow-ler-Nordheim plots are obtained from the FowFow-ler-Nordheim equa-tion: I / AV2_/1_exp(B/3/2_{/V), where A and B are material and}

structural parameters,/ is the work function of the solid and V is the voltage. For forward current in the HTL based nanocontact LED, linearity in the Fowler-Nordheim plots of current at high elec-tric field can be resolved (Fig. S15online). This is the experimental evidence of the enhanced electric field from the nanocontacts allowing efficient charge injection by field emission, which results in efficient electroluminescence emission. The fitting slope value could be used to determine the barrier height, if a number of unknown parameters were available. We note that a weaker field dependence obtained in the low fields, which could be attributed to the field dependence of the mobility in the organic semiconduc-tor, often associated with Poole-Frenkel models.

At short circuit and the maximum power point, due to a HTL nanocontact and a dipole layer in between nanocontacts, the diffu-sion of photogenerated charge carriers contribute a large fraction of the photocurrent in the OPV devices. Neglecting the drift compo-nent of the photocurrent, we note that the diffusion of particles to a distribution of nanosized sinks can be almost complete, as also found in the case of nanoelectrodes in electrochemistry[19].

In summary, a highly diluted semiconducting polymer blended into an insulating DNA complex was investigated, and this print-able hybrid material suitprint-able as an efficient hole contact in organic semiconductor devices. Hole doping process contributes to a selec-tive hole transport, as determined in single charge carrier devices and field effect transistors. Efficient charge injection can be obtained at the interface between the hybrid HTL and materials with deep HOMOs (from 5.4 to 5.8 eV), resulting in ohmic contacts and significantly enhanced injection current. The effective nanocontacts of the HTL material results in a nanosized LEDs. Fur-thermore, this hybrid HTL material can be used in large area flex-ible devices. Our study potentially could inspire the study of doping and charge transfer physics at the interface of optoelectron-ics and biomaterial electronoptoelectron-ics [20], and could inspire interface physics understanding and efficient interface materials design. We hope that the ease and wide applicability of this solution-based method will inspire other organic and hybrid printed electronic applications.

Conflict of interest

Olle Inganäs has ownership in Epishine AB, developing printed organic photovoltaic devices. The other authors declared that they have no conflict of interest.

Acknowledgments

This work was supported by the Knut and Alice Wallenberg Foundation (KAW) through a Wallenberg Scholar grant to Olle Inganäs. Helpful discussions with Prof. Ingemar Lundström are acknowledged.

Author contributions

Olle Inganäs and Qingzhen Bian designed the project. Qingzhen Bian performed the device-based tests, micrometer optical micro-scopy and participated in all experiments. Chiara Musumeci

car-ried out the atomic force microscopy and Kelvin probe force microscopy measurements. Andreas Skallberg carried out the elec-trostatic photoemission electron measurements. Chuanfei Wang and Yongzhen Chen carried out the X-ray photoelectron spec-troscopy measurements. Zhangjun Hu carried out the Fourier transform infrared spectroscopy measurements. Peter Münger car-ried out the simulation of field distribution. Kajsa Uvdal and Mats Fahlman participated in data interpretation. Qingzhen Bian ana-lyzed the data and wrote the manuscript together with Olle Inganäs. All authors discussed the results and commented on the final manuscript.

Appendix A. Supplementary materials

Supplementary materials to this article can be found online at

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Qingzhen Bian obtained his Ph.D. degree in Physics from Linköping University (Sweden). His Ph.D. work was supervised by Prof. Olle Inganäs, and he studied the exciton physics of bioelectronics and organic electron-ics. Now his research interest is focused on the excitonic and charge (energy) transport in organic electronics and interface physics.

Olle Inganäs is an emeritus professor of Linköping University (Sweden). He has focused on studies of the class of conjugated polymers throughout areas of poly-mer physics, electrochemistry, electronics and optics. The use of biopolymers as organisers of electronic polymers and as media for charge storage and organic photovoltaics are the topics of his research.