A two-dimensional DNA lattice implanted polymer solar cell

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Nanotechnology 22 (2011) 375202 (6pp) doi:10.1088/0957-4484/22/37/375202

A two-dimensional DNA lattice implantedpolymer solar cellKeun Woo Lee1,2, Kyung Min Kim1, Junwye Lee3, Rashid Amin3,Byeonghoon Kim3, Sung Kye Park2, Seok Kiu Lee2, Sung Ha Park3

and Hyun Jae Kim1

1 School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea2 Research and Development Division, Hynix Semiconductor Incorporated, Icheon 467-701,Korea3 Department of Physics and SKKU Advanced Institute of Nanotechnology (SAINT),Sungkyunkwan University, Suwon 440-746, Korea

E-mail: keunwoo.lee@hynix.com, sunghapark@skku.edu and hjk3@yonsei.ac.kr

Received 2 April 2011, in final form 17 July 2011Published 18 August 2011Online at stacks.iop.org/Nano/22/375202

AbstractA double crossover tile based artificial two-dimensional (2D) DNA lattice was fabricated andthe dry–wet method was introduced to recover an original DNA lattice structure in order todeposit DNA lattices safely on the organic layer without damaging the layer. The DNA latticewas then employed as an electron blocking layer in a polymer solar cell causing an increase ofabout 10% up to 160% in the power conversion efficiency. Consequently, the resulting solar cellwhich had an artificial 2D DNA blocking layer showed a significant enhancement in powerconversion efficiency compared to conventional polymer solar cells. It should be clear that theartificial DNA nanostructure holds unique physical properties that are extremely attractive forvarious energy-related and photonic applications.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Structural DNA nanotechnology [1] using DNA as a buildingmaterial can produce numerous artificial self-assembledstructures with nanometer scale precision [2–4]. Thepredictable hybridization between DNA strands and a goodunderstanding of the B-form DNA double helix (duplexdiameter is 2 nm, one helical full turn contains 10.5 nucleotideswith length 3.4 nm) has made DNA a promising tool for currentand future bottom-up nanofabrication [5–11]. Scientistshave constructed several DNA nanoarchitectures with diversetopologies by programmable base sequences and with distinctperiodicities decorated by functional materials and molecules.Because of its versatility, DNA nanotechnology has not onlybeen recognized for the fabrication of complicated models ofself-assembled systems but has also earned a leading placein the practical demonstration of bottom-up nanotechnology.Structural DNA nanotechnology has matured to a stage wherecompletely addressable nanostructures can now be readilyassembled in all dimensions. This gives us exceptional

opportunities in nanoscale multiplexing for a wide range ofapplications in multidimensional fields.

In order to explore the next frontiers of science andtechnology, researchers are considering organic electronicand photonic devices as functional applications that can befabricated with biological materials such as DNA moleculesand proteins. Due to their large band gap and large dielectricconstant, thin films of DNA–hexadecyltrimethylammoniumchloride (CTMA) have been successfully used in multiple ap-plications such as organic light emitting diodes (OLED) [12],a cladding and host material in nonlinear optical devices [13],and organic field-effect transistors (OFET) [14]. Hagen et al[15] have reported the incorporation of simple duplex DNA–CTMA as an electron blocking layer (EBL) in fluorescenttype OLEDs. They showed that DNA can play an importantrole in enhancing light emission in OLEDs from the factthat the double helix structure of DNA–CTMA has a largeband gap of 4.7 eV for potential use as an EBL. Theremight be certain advantages to using base-sequence designedand properly dimensionalized DNA nanostructures instead of

0957-4484/11/375202+06$33.00 © 2011 IOP Publishing Ltd Printed in the UK & the USA1

Nanotechnology 22 (2011) 375202 K W Lee et al

Figure 1. Schematic drawing of the structure of the DX-tile based 2D DNA lattice being incorporated into a polymer solar cell.

Figure 2. The detailed DNA strand structures and base sequences used in 2D DNA lattice formation. Tiles consist of four strands (DX1#1 toDX1#4 for tile DX1 and DX2#1 to DX2#4 for tile DX2) indicated in the schematics above. Sticky ends, Sn and Sn′ are complementary basesto each other.

simple duplex or short strands of synthetic oligonucleotides.Customized DNA structures might have higher chances ofperforming device functionalities than simple strands or duplexDNA. In order to verify the difference in the power conversionefficiency (PCE) of a polymer solar cell, we have introducedartificial two-dimensional (2D) DNA lattices as an EBL in asolar cell.

2. Experiments

The structure of an artificial 2D DNA lattice being incor-porated into a solar cell is shown in figure 1. The 2DDNA lattice implanted polymer solar cell structure consistsof the following layers: indium tin oxide (ITO) anode;poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PE-DOT:PSS) as a hole transport layer; 2D DNA as an EBL;regioregular poly(3-hexylthiophene) and phenyl-C61-butyricacid methyl ester (P3HT:PCBM) as the exciton generationactive layer; and a LiF/Al cathode. The detailed fabricationmethod is as follows. The ITO coated glass was used as asubstrate and the patterned ITO acts as the bottom electrode.First, the PEDOT:PSS layer is spin-coated and annealed inambient air to form a 30 nm thick film for use as a polymerelectrode. After that annealing, a 2D DNA complex layerwas formed by dropping its solution on the PEDOT:PSS layer.There was a certain possibility of damaging the PEDOT:PSSlayer severely during deposition of the DNA structures due tothe water-based physiological buffer, 1 × TAE/Mg2+ (40 mMTris acetate (pH 8.0), 2 mM EDTA, and 12.5 mM magnesiumacetate) used in common DNA nanostructure fabrication.

In order to avoid this problem, we introduced the dry–wetmethod (DWM): replacement of the water-based DNA buffer

with an appropriate solvent to avoid damaging the PEDOT:PSSlayer. The 2D DNA lattices were fabricated by unit DNAbuilding blocks, in the form of an artificial double crossover(DX) molecule with dimensions of 12 nm × 4 nm [16].The DNA lattice was composed of two side-by-side double-stranded helices linked at two crossover junctions. The DXmolecules have a stiffness which is deficient in conventionallybranched junctions, indicating its appropriateness for use in theassembly of periodic materials. Here we had fabricated a 2DDX nanostructure by programming sticky-ended associations,a design based on Watson–Crick base-paring. The DX unitscan be designed such that they self-assemble together intoa two-dimensional crystalline lattice. The DX moleculeshad consisted of four strands of DNA, each of which hadcontributed to both helices. Each corner of every DX unit hada single-stranded sticky end with a distinctive sequence. Wehad chosen the simplest non-trivial set of tiles to fabricate the2D DNA lattice shown in figure 2.

It was a challenge to keep the DNA structure in its originalshape after conducting the DWM and to conserve its physicaland especially mechanical and electrical properties withoutdamaging the PEDOT:PSS thin film. We had observed thatthe efficiency value decreased dramatically as the DNA waspipetted onto the PEDOT:PSS surface along with the water-based 1 × TAE/Mg2+ buffer. Since the PEDOT:PSS layer isreadily dispersed in the water [17], its electrical conductivitydecreases considerably due to damage. Chlorobenzene isan aromatic organic compound which evaporates quickly andis also known for high efficiency [18] when used in solarcells, and moreover it also does not significantly damage thestructure and functionality of DNA molecules reported in theprevious paper [19]. We took 30 µl of DNA in a 1×TAE/Mg2+

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Figure 3. (a) DNA lattice restoration by the dry–wet method. Annealed DNA lattices can be restored by the following steps: (i)–(iii) theannealed DNA sample can be placed in a centrifuge until dried into a small pellet; (iv) after completely removing water, an appropriate bufferor organic solvent is added into a test tube for restoring the DNA lattices. (b) AFM image of the 2D DNA lattices after annealing iscompleted. Scan size, 1 µm × 1 µm. The inset image shows high resolution AFM data with 100 nm × 100 nm scan size. (c)–(e) AFM imagesof the restored 2D DNA lattices by adding 30 µl of 1 × TAE/Mg2+ buffer, deionized water and chlorobenzene, respectively. Scan size,1 µm × 1 µm. All AFM images were performed under the liquid phase.

buffer with a concentration of 200 nM and centrifuged it for24 h at 25 ◦C until dried into a small pellet. This small pellethad to contain DNA structures in the solid phase with the bufferchemicals because only the water molecules were evaporated.Later we added 30 µl of 1 ×TAE/Mg2 buffer, deionized waterand chlorobenzene in three test tubes. A cartoon of the DWMprocess is depicted in figure 3(a). Figures 3(b)–(e) showedatomic force microscope (AFM) images of 2D DNA latticesafter annealing was completed and after the restoring processwas done by adding 30 µl of 1 × TAE/Mg2, deionized (DI)water and chlorobenzene, respectively. The three samples of2D DNA lattice were transferred to the PEDOT:PSS layer andlight conversion measurements were carried out accordingly.

We have an active layer which is a blend of P3HT andPCBM where light is absorbed, for use as the donor andacceptor, respectively. The P3HT:PCBM 200 nm thick active

layer is spin-coated and annealed in an argon-filled glovebox. Finally the top electrodes, 2 nm thick LiF and 150 nmthick aluminum, were deposited by thermal evaporation.LiF/Al electrodes are widely used for the enhancement ofthe efficiency of polymer light emitting diodes (PLEDs) andpolymer solar cells [20].

3. Results and discussions

The energy level diagram of a properly designed 2D DNAlattice implanted polymer solar cell is shown in figure 4. Thehighest occupied molecular orbital (HOMO) energies and thelowest unoccupied molecular orbital (LUMO) energies of theindividual component materials are shown along with the workfunctions of the metal layers. The PEDOT:PSS has a LUMO

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Figure 4. The energy level diagram showing the HOMO and LUMOenergies of different component materials.

level of 3.5 eV and a HOMO level of 5.2 eV, which doesnot suggest that the material will effectively block electrons.However, artificial 2D DNA lattices might act as an EBLin the device. With a LUMO level of 0.9 eV, electrons inthe active layer (P3HT) will experience an energy barrier of2.4 eV [14, 15]. The 2D DNA HOMO level of 5.6 eVshould not inhibit hole transport. Long-distance hole transferin double-helical DNA has also been reported [21].

We introduce a qualitative model of 2D DNA latticeimplanted polymer solar cell operation. There are fiveprocesses that induce free electrons and holes in the system,which lead to the significant increase in power convergenceefficiency: (1) absorption of a photon to create an exciton–bound carrier pair; (2) dissociation or separation of the exciton;(3) transport of electrons to one contact, and holes to the othercontact, driven by the built-in field; (4) the 2D DNA plays twosignificant roles as the hole transport layer and also as an EBLdue to the high energy barrier formed during the light soaking,which should cause the higher power convergence efficiency ofthe 2D DNA implanted polymer solar cell; (5) collection of thecharges at the electrodes.

The absorption spectrum of the P3HT:PCBM and the2D DNA/P3HT:PCBM composite obtained by UV–visibleabsorption spectroscopy (Shimadzu UV-2401PC) is shownin figure 5. As a result of this investigation, no changein the absorption spectrum can be seen. In the case ofthe P3HT:PCBM mixture, two peaks are distinguished inthe absorption spectrum of the film. The first peak around334 nm stems from PCBM, while the second peak at 500 nmrepresents the contribution from P3HT. As expected, the 2DDNA/P3HT:PCBM composite has a significant decrease of∼29% in its absorption intensity due to additional DNAstructures.

Figure 5. Absorption spectra of the P3HT:PCBM and 2DDNA:P3HT:PCBM films.

Figure 6. J –V characteristics of reference solar cells and 2D DNAlattice implanted solar cells under AM1.5G illumination with acalibrated solar simulator, irradiation intensity of 100 mW cm−2

(about one sun).

The current density versus voltage (J–V ) characteristicsof the reference cells and the 2D DNA implanted cells withwater and chlorobenzene solvents, under Air Mass 1.5 Global(AM1.5G) illumination from a calibrated solar simulator withradiation intensity (Pinc) of 100 mW cm−2 are shown infigure 6. In addition to investigating the reproduction of the2D DNA lattice implanted polymer solar cell, we carried outmultiple experiments over time. We have summarized themeasured data in table 1. Each split group has eight sampleswith the average and standard deviation values of the measureddata per split group. The characteristics of these devices, suchas the open circuit (VOC), the short current (JSC), the fill factor(FF) = (JmaxVmax)100/(JSCVOC), and the power conversionefficiency (PCE) = (JSCVOCFF)/Pinc of polymer solar cells,are studied. In the first experiment, the reference and 2DDNA implanted polymer solar cells (first experiment in table 1)show a typical photovoltaic response with device performancecomparable to that reported in previous studies [22]; theP3HT:PCBM reference cell without DNA (Ref-1) yields

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Table 1. Summary of the electrical characteristics of polymer solar cells (AVG, average; STD, standard deviation).

JSC (mA cm−2) VOC (V) FF PCE (%)

Split items AVG STD AVG STD AVG STD AVG STD

First Ref-1 5.79 0.17 0.57 0.03 53.66 10.09 1.77 0.42DW-1 1.35 0.58 0.08 0.07 28.53 3.69 0.05 0.07DCB-1 5.85 0.18 0.58 0.00 56.93 2.03 1.95 0.10

Second Ref-2 3.66 0.73 0.58 0.01 11.99 3.63 0.26 0.13DCB-2 5.92 0.84 0.56 0.01 19.70 5.92 0.68 0.29

Third Ref-3 7.13 1.35 0.58 0.00 54.83 1.63 2.25 0.46DCB-3 7.42 0.37 0.59 0.00 56.29 1.83 2.48 0.18

JSC = 5.79 mA cm−2, VOC = 0.57 V, FF = 53.66, andPCE = 1.77%. Whereas the 2D DNA/P3HT:PCBM cell (DW-1; here DNA lattice was initially in the water-based buffer andthen deposited on the appropriate layer) has a drastic drop offof its performance compared to the Ref-1 cell, which showedvalues of JSC = 1.35 mA cm−2, VOC = 0.08 V, F = 28.53,and PCE = 0.05%, because the former has a significant effecton the damage to the PEDOT:PSS layer caused by dissolvingin water [23]. However, the 2D DNA/P3HT:PCBM cell(DCB-1; DNA lattice was in the chlorobenzene solvents afterconducting the DWM and then deposited on the appropriatelayer) yields JSC = 5.85 mA cm−2, VOC = 0.58 V, FF =56.93, and PCE = 1.95% (PCE was obtained in a 10.2%increase compared to that of the Ref-1 cell), which may bedue to electron blocking effects at the 2D DNA layer. Theadvantage of using 2D DNA as the EBL for polymer solar cellsis apparent from the data shown in figure 6.

In order to examine the reproducibility of these samples,we carried out two different experiments for both the low andhigh performance cases. First, the low performance of sampleswas obtained in the second experiment as shown in figure 7,the reference cell (Ref-2) yields JSC = 3.66 mA cm−2, VOC =0.58 V, FF = 11.99, and PCE = 0.26%, whereas the 2DDNA/P3HT:PCBM cell (DCB-2) yields JSC = 5.92 mA cm−2,VOC = 0.56 V, FF = 19.70, and PCE = 0.68%. The 2DDNA implanted polymer solar cell has a higher performance(JSC ∼ 61.7%, FF ∼ 64.3%, PCE ∼ 161.5%) than thereference cell (Ref-2, second experiment in table 1). Withthese performance parameters, it is evident that the 2D DNAlattice plays the critical role in the cell and offers enhancedperformance for polymer solar cells in the worst case as well.Contrary to expectation, optimized samples (DCB-3 from thethird experiment as shown in figure 3(e)) has a slightly higherperformance (JSC ∼ 4.1%, VOC ∼ 1.7%, FF ∼ 2.7%,PCE ∼ 10.2%) than the reference cell (Ref-3). Although themechanisms are not yet completely understood, it is believedthat the 2D DNA as an EBL exhibits a significant enhancementin the PCE compared to conventional polymer solar cellswithout a 2D DNA layer. It is clear that the unique structureof DNA has many optical and electrical properties that areextremely attractive for photonic applications.

4. Conclusion

In conclusion, an artificial 2D DNA lattice was fabricatedand the DWM was introduced to construct a polymer solar

Figure 7. The open circuit (VOC), the short current (JSC), the fillfactor (FF) = (JmaxVmax)100/(JSCVOC), and the power conversionefficiency (PCE) = (JSCVOCFF)/Pinc of polymer solar cells plottedas a function of the split items, where Pinc is the intensity of incidentlight. The PCE of the optimized 2D DNA implanted polymer solarcell reaches its maximum PCE = 2.75% at 100 mW cm−2.Ref-1, -2, -3 are the reference polymer solar cells, which arecomposed of only the P3HT:PCBM active layer. DW-1 samplemeans that the 2D DNA lattice was dissolved into deionized (DI)water. DCB-1, -2 samples mean that the 2D DNA lattice wasdissolved in chlorobenzene.

cell with a significant enhancement in the PCE compared toconventional polymer solar cells. Artificial DNA structures asa physical ingredient in certain devices or as scaffolds for theconstruction of devices and sensors will have a great impacton nanotechnology due to their unique physical, chemicaland biological properties. Consequently, various dimensionalartificial DNA nanostructures may have several applicationsin nanoscale device fabrication and applications in the nearfuture.

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Acknowledgments

This work was supported by research funds from the NationalResearch Foundation of Korea (NRF) through the NationalResearch Laboratory Program grant, funded by the KoreanMinistry of Education, Science and Technology (MEST) [no.R0A-2007-000-10044-0(2007)], a Research Grant from theNational Research Foundation of Korea (NRF2008; 331-2008-1-C00126) to SHP, and Hynix semiconductor Inc. to KWL.

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