DNA Based Hybrid Material for Interface Engineering in Polymer Solar

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Organic Electronic Devices

DNA Based Hybrid Material for Interface Engineering in Polymer Solar Cells Anders Fredrik Elfwing, Wanzhu Cai, Liangqi Ouyang, Xianjie Liu, Yuxin Xia, Zheng Tang, and Olle Inganäs ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17807 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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ACS Applied Materials & Interfaces

DNA Based Hybrid Material for Interface Engineering in Polymer Solar Cells Anders Elfwing†,‡, Wanzhu Cai†,‡,*, Liangqi Ouyang†,*, Xianjie Liu#, Yuxin Xia†, Zheng Tang¢, Olle Inganäs†,* †

Biomolecular and Organic Electronics, Department of Physics, Chemistry and Biology,

Linköping University, SE-581 83 Linköping, Sweden. #

Surface Physics and Chemistry Division, Department of Physics, Chemistry and Biology,

Linköping University, SE-581 83 Linköping, Sweden. ¢

Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and Institute

for Applied Physics, Technische Universität Dresden, Nöthnitzer Str. 61, 01187 Dresden, Germany. Corresponding Author: Wanzhu Cai: [email protected]; Liangqi Ouyang: [email protected]; Olle Inganäs: [email protected]. Keywords: (organic photovoltaic device, electron transport material, DNA based hybrid material, selfdoped polyelectrolyte, high optical transmittance) 1 ACS Paragon Plus Environment

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ABSTRACT

A new solution processable electron transport material (ETM) is introduced for use in photovoltaic devices, which consists of a metallic conjugated polyelectrolyte, poly(4-(2,3dihydrothieno[3,4-b][1,4]dioxin-2-yl-methoxy)-1-butanesulfonic

acid

(PEDOT-S),

and

surfactant-functionalized deoxyribonucleic acid (DNA) (named DNA: CTMA: PEDOT-S). This ETM is demonstrated to effectively work for bulk-heterojunction organic photovoltaic devices (OPV) based on different electron acceptor materials. The fill factor, open circuit voltage and the overall power conversion efficiency of the solar cells with a DNA: CTMA: PEDOT-S modified cathode are comparable to that of devices with a traditional Lithium fluoride/Aluminum cathode. The new electron transport layer has high optical transmittance, desired work function and selective electron transport. A dipole effect induced by the use of the surfactant, CTMA, is responsible for lowering the electrode work function. The DNA: CTMA complex works as an optical absorption dilutor, while PEDOT-S provides the conducting pathway for electron transport, and allows thicker layer to be used, enabling printing. This materials design opens a new pathway to harness and optimize the electronic and optical properties of printable interface materials.

1. Introduction Interfacial engineering aims to minimize both the electrical and optical loss for optoelectronic devices.1–3 To meets these demands, materials classed as charge carrier transport materials, which not only should enable an Ohmic and selective contact 2, but also have proper conductivity and high transparency, are needed. This is because adjustable thickness may be used to offset the inefficient light absorption in OPV active layer by optical engineering.4-15 In addition, tolerance of thickness variations of the interface layer can ease the technological difficulty in processing, which will benefit the up-scaling of OPV printing. Considerable 2 ACS Paragon Plus Environment

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research efforts have been put into materials development.7 Here we focus on electron transport materials (ETM), which are used on the cathode side. Existing ETM systems include inorganic materials,8 inorganic/organic hybrid materials9,10 and side chain functionalized conjugated polymer/small-molecules.11-15,16 A development and synthesis rule of new organic transport materials is to couple strong electron-withdrawing groups with extended πconjugation structures, and excellent materials are already emerging.11,14,17 However, the transmittance of these ETMs is in general low due to their strong absorption in the visible light range. Biopolymers have also been used in interface materials due to their interesting molecular structure18,19 and self-assembly properties.20 Here we present a new strategy to develop easy processing ETM with more freedom for tuning optical transmittance, conductivity and work function at the same time. This solutionprocessable material system is based on a self-doped polyelectrolyte PEDOT-S, deoxyribonucleic acid (DNA) and an ammonium surfactant, cetyltrimethylammonium chloride (CTMA). This ETM effectively modified the work function of the cathode. Although the thickness of the ETM still has influence on the device performance, the hybrid material shows a larger tolerance to thickness variation than insulating interface materials. Meanwhile, the high optical transmittance, which exceeds ~98% when the thickness is 35nm, outperforms other typical interface layers. Devices based on this ETM with optimized thickness show high FF, VOC and PCE values that are comparable to that of the device based on a widely used Lithium fluoride/Aluminum (LiF /Al) cathode. Finally, we propose a working mechanism for this ETM. 2. Results and discussion 2.1. Materials fabrication The hybrid material system consists of three functional components: DNA, CTMA and the self-doped conducting polymer PEDOT-S. Their corresponding molecular structures are 3 ACS Paragon Plus Environment


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shown in Figure 1(a). The DNA molecular structure is composed of a sugar-phosphate backbone, on which nucleobase sidechains are attached. As shown in Figure 1(b), by forming a complex with the surfactant CTMA through phosphate-ammonium interactions, DNA can be precipitated out from aqueous solution and re-dissolved in a number of alcohols. This method allows processing DNA materials from low-water-content solutions with improved thermal stability. PEDOT-S is a sulfonic acid side-chain functionalized PEDOT with high water solubility and good conductivity.21, 22 When PEDOT-S is blended into DNA: CTMA and processed from the corresponding ethanol solution, they form an ETM.

Figure 1. (a) Molecular structures of materials involved in the ETM: DNA, CTMA and PEDOT-S. Interaction groups are highlighted with color. (b) Schematic diagram of the materials fabrication process. (c) UV-VIS absorption of DNA: CTMA (in ethanol ) , DNA (in water) and DNA:CTMA:PEDOT-S (in ethanol ) (d) Transmittance of hybrid material of 35 nm thick DNA:CTMA:PEDOT-S film in comparison with PEDOT-S and PEDOT:PSS Al 4083 at the same thickness. 4 ACS Paragon Plus Environment

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2.2. Optical characterization The optical absorption spectra of the materials in solvent are shown in Figure 1(c). DNA: CTMA is a very low optical loss material over a broad range of wavelengths.23 DNA: CTMA and native DNA films show negligible absorption from 300 to 800 nm. The UV-vis spectrum of DNA: CTMA: PEDOT-S blend in ethanol solvent shows a faint shoulder at 500-600 nm, which can be assigned to the π-π* transition of the thiophene backbone from PEDOT-S. This implies slight de-doping of the conjugated backbone of PEDOT-S22. Here the slight de-doping is believed due to the interaction between the amine functionality of CTMA and the sulfonate sidechains of PEDOT-S. We also observe an increasing absorption in the long wavelength range after 700 nm from both the hybrid material solution and corresponding films (Figure S1), which is related to the polaron absorption in doped PEDOT structure22. Its transmittance is up to ~98% when the thickness is ~35 nm and presents a flat profile over the entire visible light range over 1000 nm. At the same thickness, PEDOT-S shows obviously lower transmittance. We also compared it with a commercial hole transport material, Clevios™ AI 4083. Both materials have comparable transmittance. Therefore, DNA: CTMA can be used to decrease absorption in the ETM, like a light absorption ability “diluter”. 2.3. Photovoltaic properties. The schematic device architecture of ITO/PEDOT: PSS/Active layer/Interface layer/Al was used to evaluate the electron extraction and transport ability of DNA: CTMA: PEDOT-S, which is shown in Figure S2. We used commercial materials of polythieno[3,4-b]-thiopheneco-benzodithiophene: (6,6)-phenyl-C71-butyric acid methyl ester (PTB7:PC71BM) as the representative active layer here. The current density (J)-Voltage (V) characteristics with different thickness DNA: CTMA: PEDOT-S, and reference devices with Al only electrode and LiF/Al electrode in light and under dark are plotted in Figure 2(a) and 2(b), respectively; the corresponding external quantum efficiency (EQE) of the device with ~10nm DNA: 5 ACS Paragon Plus Environment


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CTMA: PEDOT-S film compared with that of the reference devices is plotted in Figure 2(c). The performance parameters are summarized in Table 1.

Figure 2. J–V characteristics of devices interfaced with DNA: CTMA: PEDOT-S films of different thickness, the reference device without cathode interface layer (Al only) and LiF /Al electrode. (a) In light and (b) in dark. (c) Corresponding EQE. (d) ELEQE of device with DNA: CTMA: PEDOT-S and reference device with Al and LiF /Al. With DNA: CTMA: PEDOT-S as the interface layer, devices with a FF of 70%, VOC of 0.76 V, and JSC of 14.29 mA/cm2 are achieved. These devices perform comparably well to that of devices with LiF/Al cathodes, which presents a FF of 73%, VOC of 0.75 V and JSC of 14.21 mA/cm2. Increasing the thickness of DNA: CTMA: PEDOT-S leads to a slight decrease of VOC. Without the ETM at the interface between the active layer and the Al electrode, VOC

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decreases to 0.59V. This large VOC loss at the Al-only electrode confirms that the thermally evaporated Al does not provide Ohmic contact with the active layer. The distinct improvement of the VOC and FF due to the DNA: CTMA: PEDOT-S interlayer demonstrates its excellent work function modification effect. The DNA: CTMA: PEDOT-S layer is also used with other active material systems, with acceptors such as ITIC24 and N2200.25 The corresponding results are shown in Table S1. In both cases, VOC of the devices based on DNA: CTMA: PEDOT-S is as good as that obtained from LiF/Al. Table 1. Performance parameters of PTB7:PC71BM devices with different thickness of DNA: CTMA: PEDOT-S and reference layers. (Average performance parameters of six devices with the peak value in the brackets.) DNA:CTMA:PEDOS JSC(mA/cm2)

VOC(V)

FF(%)

PCE(%)

10nm

13.52 (14.29) 0.76 (0.76) 70 (70) 7.20 (7.6)

16nm

11.76 (11.97) 0.74 (0.74) 61 (63) 5.38 (5.6)

22nm

12.23 (12.45) 0.73 (0.73) 52 (53) 4.64 (4.8)

Other Interface layer

Al only

12.35 (12.88) 0.59 (0.59) 62 (62) 4.55 (4.7)

LiF/Al

14.25 (14.21) 0.75 (0.75) 70 (73) 7.51 (7.8)

In Figure 2(b), the dark J-V curves of devices with different thicknesses of DNA: CTMA: PEDOT-S are shown. For the Al only device, the lower value of Voc and the lower voltage onset for forward injection are due to a lower built-in potential associated with a higher injection barrier.2,26 The device based on a DNA: CTMA: PEDOT-S interlayer shows diode characteristic with a large rectification ratio of 105, similar to that of the LiF/Al device, demonstrating the work function of Al is effectively lowered and better aligned with the electron transport level by DNA: CTMA: PEDOT-S. The J-V response of a device based on a 7 ACS Paragon Plus Environment


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DNA: CTMA: PEDOT-S layer acting as the active layer was measured by sandwiching DNA: CTMA: PEDOT-S between ITO and Al (Figure S3). With a 15 nm thicker layer, a linear J-V curve was recorded from the device. The corresponding conductivity is estimated to be 4.6×10-6 S/cm using Ohm’s law. For a 22nm thick DNA: CTMA: PEDOT-S film, the JV curve slightly deviates from linear behavior, which may be due to accumulation of free insulating DNA: CTMA at the surface. It is possibly responsible for the slightly decreased VOC in the corresponding device. Compared to a LiF/Al and an Al based device, the EQE of the DNA: CTMA: PEDOT-S device show a small increased in the long wavelength range (Figure 2(c)), which is an optical interference effect due to the thicker interface layer. The electroluminescence quantum efficiency (EQEEL) of the device with a thin DNA: CTMA: PEDOT-S layer is compared with that of the LiF/Al and Al only devices (Figure 2(d)). We found that both devices with LiF/Al and DNA: CTMA: PEDOT-S have similar ELEQE of 10-4 %, which indicates an identical nonradiative recombination loss27 of ~0.3 eV. For the Al-based device, ELEQE of 10-5 %, corresponding to a non-radiative recombination loss of ~0.4 eV, was obtained. Therefore, we estimate that a large part of the VOC loss (0.06 V/ (0.76 - 0.59 V) ≈35%) saved by inserting DNA: CTMA: PEDOT-S layer is due to the suppression of non-radiative recombination at the interface.


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2.4. UPS Characterization and work function

Figure 3. Secondary-electron cut-off region in UPS spectra of PEDOT-S, DNA: CTMA, PEDOT-S: CTMA and Hybrid material DNA: CTMA: PEDOT-S. Sample structure: glass/ITO/sample Ultraviolet photoemission spectroscopy (UPS) was used to illustrate the different electronic structure of all components, as shown in Figure 3. The work function (WF) of a thin film was determined from the secondary electron cut-off. It is calculated to be 4.80 eV, 4.32 eV and 4.20 eV for PEDOT-S, DNA: CTMA, and the hybrid material DNA: CTMA: PEDOT-S, respectively. All films were coated on ITO substrate, which intrinsically has a work function of 4.8 eV. First, we find the hybrid material shifts WF from 4.8 eV to 4.2 eV. Second, we find DNA: CTMA can also decrease the WF. The low WF of the hybrid material may therefore originate from the DNA: CTMA part. The high work function of PEDOT-S makes this a hole transport material more than an electron transport material.18 The small drop of WF when PEDOT-S added into DNA: CTMA is possibly due to the de-doping of PEDOT-S. As reported, the doping level of conjugated polymer (PEDOT is the one of the mostly studied 9 ACS Paragon Plus Environment


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model) strongly influences its work function.28,29 As PEDOT-S is a P-doped material, dedoping will cause a decrease in its work function.30 Correspondingly, we found evidence of de-doping of PEDOT-S from its absorption spectrum (section 2.2). We also studied the work function modification of DNA: CTMA: PEDOT-S on a variety of substrates (summarized in Table S2). The results indicated that depositing DNA: CTMA: PEDOT-S onto the surfaces of two commonly used electron acceptors in polymer solar cells, PCBM and ITIC, also gives lowered WFs. The WFs of pristine PCBM and ITIC on ITO were found to be 4.7 eV and 4.8 eV, respectively. When coated with the hybrid materials, WFs are shifted to 4.42 eV and 4.26 eV, respectively. It is well-known that amine-containing compounds can form a dipole layer at an interface3 in organic PV devices. The decreased work function of the hybrid material is therefore attributed to the dipole effect due to CTMA. In another experiment we add equal amount of CTMA and PEDOT-S into ethanol, without DNA. The PEDOT-S: CTMA blend shows a work function of 4.3 eV, a decrease of 0.5 eV from the pristine PEDOT-S. These observation strongly suggest that the decreases in WF follows a mechanism where the amine based surfactant forms a dipole on the PEDOT-S surface. It is also noted that PEDOT-S is a metallic material with conductivity up to 10 S/cm.22 The metallic behavior of PEDOT-S can be proved by the UPS valence band spectrum (Figure S4), in which the valence band tail at the lower binding energy approach to the Fermi level which is referred at the binding energy of 0 eV. Similar with the high conductive PEDOT: PSS,31 it is not surprising that PEDOT-S can be used for electron transport in metallic state(doping state). Considering the absorption spectrum of DNA: CTMA: PEDOT-S, which clearly shows the doping feature of PEDOT-S, we consider that PEDOT-S provide the electron transport in the hybrid material. On the other hand, DNA: CTMA is a material with very low conductivity, due to the insulating peripheral part consisting of sugars, phosphates and 10 ACS Paragon Plus Environment

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CTMA.32 The corresponding valence band spectrum shows a broad band at the high binding energy centered around 9.5 eV, indicate its large bandgap. (Figure S4). 2.5. Topography Study The DNA: CTMA: PEDOT-S layer has a smooth, uniform and condensed topography from atomic force microscope (AFM) imaging. We also found that the DNA hybrid solution has excellent wetting on the active material, allowing a fully uniform coverage of the interlayer on the active layer. As shown in Figure 4(a) & (b), the top of PTB7: PC71BM surface is very smooth, exhibits a roughness Root Mean Square (RMS) of around 1 nm. With a 15 nm thick DNA: CTMA: PEDOT-S layer, the roughness of the surface increases to 5 nm. A distinct granule surface topography is found on DNA: CTMA: PEDOT-S film (Figure 4(b)). In the corresponding phase image, grains with clear boundaries are shown (Figure 4(c)). The size of grain is measured to be ~40 nm. DNA: CTMA: PEDOT-S on top of Silicon wafer also show a similar granular feature.



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Figure 4. AFM height images (5 µm × 5 µm) for (a) the blend film of PTB7:PC71BM and (b) DNA: CTMA: PEDOT-S on PTB7:PC71BM film. (c) Phase image of DNA: CTMA: PEDOTS on PTB7:PC71BM film. (d) DNA: CTMA: PEDOT-S on silicon wafer 2.6. Temperature dependent and light intensity dependent JV characteristic The influence of the DNA to PEDOT-S stoichiometry in the hybrid material is shown in Figure 5. The PEDOT-S to DNA: CTMA weight ratio was varied from 1:1, 1:2 and 1:4, while the thickness of interface layer is kept constant (around 10 nm). In the JV curve, we observe an S-shape. We notice a slightly decreased Voc with an increased DNA:CTMA concentration, which suggests a deviation from Ohmic contact at the interface2. The device with 1:1 DNA to PEDOT-S ratio shows a JV curve that almost overlaps with that of the optimized device based on LiF/Al. When we vary the light intensity, a power law fit shows 12 ACS Paragon Plus Environment

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the dependence of Jsc upon light intensity, and we found the slope to be respectively 1.03, 0.95 and 0.78 for LiF/Al, DNA: CTMA: PEDOT-S (1:1) and DNA: CTMA: PEDOT-S(4:1). For the case with DNA: CTMA: PEDOT-S 4:1, the Jsc largely deviates from the linear relationship with a power dependence of close to 3/4. This suggests space charge limited photocurrents induced by an interface barrier.27,33,34 With more PEDOT-S, even in the very low light intensity, the device still has a good linear light intensity dependence comparable to that of device with LiF/Al, which indicate the efficient charge transport and extraction in the devices.

Figure 5. (a) J-V characteristics of devices with different hybrid stoichiometry of DNA: CTMA to PEDOT-S. (b) Light intensity dependence of the devices’ short circuit current. (c) Temperature dependence of the J-V characteristics for device with DNA: CTMA to PEDOTS 1:1, and (d) LiF/Al. 13 ACS Paragon Plus Environment


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In semiconductor devices, energy barrier crossing is temperature activated. We measured the temperature dependence of the JV measurements on devices with different interface layers in order to understand the working mechanism of this new hybrid electron transport layer. For the device with DNA:CTMA:PEDOT-S 1:1, Voc increases linearly as temperature decreases down to 100 K (see Figure S5 ). This trend is similar to that with the LiF/Al reference device. It can be explained by the diode equation

= −

(1)

where is the effective band gap of the blend, is the Boltzmann constant, and is the effective density states of electrons and holes, and n and p is free electron and hole concentrations.35,36 This predicts a linear increase of Voc with decreasing temperature when losses at the contact are negligible. The linear fitting on VOC (T) for the devices based on LiF/Al and DNA: CTMA: PEDOT-S/Al indicated that both of the contacts are Ohmic. The intersection at 0 K of the linear fitting for the VOC (T) gives Eg, which is found to be 1.15 eV in our case. Although the Voc dependence on temperature is the same for both interface layers, the J-V curve of the DNA: CTMA: PEDOT-S 1:1 device presents a distingushable S-shape under 100 K, which is not observed in the LiF/Al reference device. For the DNA: CTMA: PEDOT-S 2:1 based-devices (Figure S6), the J-V curve show an S-shape under room temperaure. When the temperature decreases, a trend of stronger S kinks appears. VOC is saturated at 0.8 V when the temperature decreased to 250 K. All these features strongly indicates the existence of an extraction barrier.2 The barrier crossing probability would decrease when the temperature becomes lower.37–39 If we assume that the Eg of the device is the same as that of the optimized devices (the bulk of the active layer doesn’t change), we estimate an extraction barrier of about 0.35 eV (1.15-0.8 eV).40 In our case, with more DNA:CTMA introduced, the extraction barrier induced S-shape becomes more significant. 14 ACS Paragon Plus Environment

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This indicates that the barrier originates from DNA:CTMA. It is consistent with energy level matching between DNA:CTMA and the electron acceptor in the active layer. DNA:CTMA complex is reported as a electron blocking layer with very shallow LUMO and large bandgap.41,42 For the device with DNA:CTMA to PEDOT-S 1:1 in the interface layer, this extraction barrier is only observed under 100 K and disappeared when the temperature is increased, which means the extraction barrier is not a problem under normal operation condition, and it can be easily overcome by thermal activation. 2.7. Mechanism considerations From the device analysis and UPS measurements, we learned that PEDOT-S is the component that provides the electron transport ability, while the dipole effect due to CTMA would be the reason for the work function modification. DNA: CTMA dilutes PEDOT-S to realize a high optical transmission. We constructed reference devices with DNA: CTMA, CTMA-only and PEDOT-S: CTMA as interface layers. The corresponding JV-curves are plotted in Figure S7. The complex of DNA: CTMA did not work as an ETL layer even when it is very thin (Figure S7 (a)). The strong S kink on the JV curve indicates that it forms a non-Ohmic contact at the interface. In fact, due to its energy level mismatch, there is an extraction barrier at the interface between DNA: CTMA and PC71BM . Pure CTMA did not function as ETL as shown by the low VOC of 0.5V compared to the optimized one at 0.72V. On the other hand, PEDOT-S: CTMA blend processed from ethanol solution gives a sufficiently high VOC of 0.75 V in devices, consistent with its observed low work function, as discussed in the UPS section. These observations suggests that CTMA plays an important role in forming a dipole layer in the hybrid material matrix. The DNA: CTMA may work as a dipole (CTMA) carrier, or the metallic polymer promotes the formation of CTMA dipole on the interface, with certain direction.



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3. Conclusion We demonstrated a new, easy-processing electron transport material for organic solar cells. We successfully combined a high conductivity self-doped polyelectrolyte and optically transparent structural DNA-surfactant complex. A possible working mechanism of this hybrid material has been explored: PEDOT-S contributes the conducting pathway for electron transport, CTMA help lowering the work function, while DNA: CTMA dilutes PEDOT-S to reduce optical absorption. This work not only presents a new promising ETM candidate, but also open up a new interface materials design strategy in order to meet the materials development challenges, aiming for higher optical transmittance, low work function and proper conductivity. 4. Experiment: Materials fabrication DNA (low molecular weight from salmon sperm, Sigma 31149-10G-F) was dissolved in DI water and filtrated through 0.2um polyethersulfone filter (WVR) to remove undissolved particles. The final concentration was set to 1 mg/mL, corresponding to 3 µM monomeric DNA using 330 g/mol as an average DNA unit molecular weight. Cetyltrimethylammonium chloride (CTMA) (25% aqueous solution, Aldrich) was diluted in water to a final concentration of 25 mg/mL. 25 µL CTMA solution was added dropwise to DNA solution to form white precipitates to a final concentration of 2.7 µM. The precipitated complex was collected by centrifugation at 15000 rpm for 10 minutes and the pellet was dried under N2 flow for 1h. The dry pellet was re-dissolved in absolute ethanol (LiChrosolv, Merck). This resulting material can be easily dissolved in many alcohol solvents. PEDOT-S was prepared as previously described. It was dissolved in Milli-Q water to a final concentration of 10 mg/mL. In a typical procedure, the PEDOT-S solution was added to the 0.8 mg/mL DNA: CTMA ethanol solution to a final concentration of 1 mg/mL (3 µM of EDOT units). 16 ACS Paragon Plus Environment

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Therefore, in 1:1 DNA: CTMA: PEDOT-S solutions, the weight ratio of DNA to PEDOT-S is 0.8:1. It is interesting to notice that upon addition of PEDOT-S into the DNA: CTMA solution the resulting solution turned slightly purple. Solar cell fabrication PTB7 were purchased from 1-Material Inc. PC71BM was purchased from Solenne. Firstly, ~40nm thick PEDOT:PSS (Baytron PVP, Al 4083) was spin-coated (3000rpm) on to precleaned ITO substrate, then annealed in air at 120°C for 20 mins. An layer of (PTB7:PC71BM) was spin-coated on PEDOT: PSS with spin speed of 1000rpm. The ratio of PTB7:PC71BM is 1:1.5, concentration of polymer in chlorobenzene is 7 mg/ml, and 0.5% of 1,8-diiodooctane

was

used

as

processing

additive.

The

interface

layer

of

DNA:CTMA:PEDOT-S was deposited from ethanol: water(10:1) on top of active layer. Thickness of DNA: CTMA: PEDOT-S is tuned by using different spin coating rates and measured by AFM and Ellipsometer. Then a 90nm layer of aluminum was deposited on the interface layer to finish the structure. AFM The measurements were carried out in air using a Dimension 3100/NanoScope IV system. We used commercial silicon probes with a spring constant of 40 N/m in the tapping mode. UPS UPS (ultraviolet photoemission spectroscopy) experiments were carried out using a Scienta ESCA 200 spectrometer with a standard He-discharge lamp as the excitation source (21.22 eV) in ultrahigh vacuum with a base pressure of 1x10-10 mbar. The total energy resolution in UPS measurement is about 80 meV as extracted from the width of the Fermi edge of a clean gold foil. All spectra were recorded at a photoelectron takeoff angle of 0° (normal emission). The work functions of the films were extracted from the determination of the high bindingenergy cutoff of the UPS spectra by applying a bias of -3 V to the sample. 17 ACS Paragon Plus Environment


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Supporting Information Figure S1. Absorption spectrum of 35nm thick DNA: CTMA: PEDOT-S film on glass; Figure S2. The schematic device architecture; Table S1. Performance parameters of devices with different electron accepter; Figure S3. JV response of device ITO/DNA: CTMA: PEDOTS/Al; Table S2. The work function modification effect of DNA: CTMA: PEDOT-S; Figure S4. UPS valence band spectra of PEDOT-S, DMA: CTMA and DNA: CTMA: PEDOT-S; Figure S5. VOC VS Temperature trends measured under AM1.5G (100 mW/cm2) illumination; Figure S6. Temperature dependence of the JV characteristics for device with DNA: CTMA: PEDOT-S 2:1; Figure S7. J-V characteristic of device (a) with reference interface layer (b) with different thickness of PEDOT-S: CTMA interface layer. Author Contributions O. I. gave the initial idea and supervised the project. A.E. designed and fabricated materials DNA: CTMA: PEDOT-S with L.O. L.O. measured the optical absorption of materials in solvents. W. C. carried out the device fabrication and characterized devices and materials including AFM, optical transmittance and ELEQE. W. C. and A. E. wrote the manuscript. X.L carried out the UPS measurement. Z. T. and Y. X. help the discussion of data. All authors discussed the results and revise the manuscript. ‡ these authors contributed equally to this work Acknowledgement Research has been funded by Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University SFO-Mat-LiU 2009-00971, the Strategic Research Foundation of Sweden through the project SiOS, and the Knut and Alice Wallenberg foundation through a Wallenberg Scholar grant to O.I. We thank Dr. Roger Gabrielsson for synthesizing the conducting polymer. We thank Nikolas Felekidis assistance


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DNA Based Hybrid Material for Interface Engineering in Polymer Solar

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