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Solar Cells Incorporating Water/AlcoholSoluble Electron-Extracting DNA Nanolayers Janardan Dagar, Manuela Scarselli, Maurizio De Crescenzi, and Thomas M. Brown ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00192 • Publication Date (Web): 30 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016

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Solar Cells Incorporating Water/Alcohol-Soluble Electron-Extracting DNA Nanolayers Janardan Dagar,1 Manuela Scarselli,2 Maurizio De Crescenzi,2 Thomas M. Brown1,* 1

CHOSE (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering, University of Rome Tor Vergata, Via del Politecnico 1, 00133 Rome, Italy

2

Department of Physics, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy

AUTHOR INFORMATION Corresponding Author *[email protected]

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ABSTRACT. Deoxyribonucleic acid (DNA) was successfully incorporated as a nano-layer between the bottom indium tin oxide (ITO) transparent electrode and the photoabsorbing film in a polymer solar cell. Upon film optimization and analyses with scanning transmission spectroscopy/currents, we demonstrate that a 1-6 nm thick DNA stratum functions as an effective electron extracting layer, leading to strong improvements in rectifying behaviour (by two orders of magnitude with rectifying ratios reached larger than 103), and in photovoltaic parameters like open circuit voltage (VOC from 0.39V to 0.73V), and power conversion efficiencies (PCE from ∼ 2% to ∼ 5%). The results show that DNA is a very ductile nanomaterial for electronic purposes, paving the way for future exploration in photovoltaic combining its electron-extracting properties with its functional and selfassembly behaviour. Furthermore, the solar cell devices were completely processed at the room temperature and the DNA cast from eco-friendly solvents such as water and alcohol.

TOC GRAPHIC


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Deoxyribonucleic acid (DNA), arguably the most preeminent of biomaterials1, has been researched in a range of different fields that even include nanowires, nano architectures, and electronic devices2-3. Integrated in these, it has been shown to improve performance of organic light emitting diodes (OLEDs)4-5 and thin film transistors (OTFTs)6-7. However, its electron-extraction capabilities in solar cells have not yet been reported. Apart from the two outer conducting electrodes, polymer photovoltaic devices consist of a blend layer of donor and acceptor semiconductors (102 nm thick) in which photon absorption and charge separation occurs, sandwiched between two charge-extracting layers (one for holes and the opposite for electrons). These interlayers, which are typically of the order of 101 nm thick, are critical in determining not only the polarity of the organic solar cell but also the strength of the built-in field, the VOC, and PCE as a result of their greatly differing work function/energylevels and carrier transport properties8-9. In “inverted” architectures with bottom ITO electrodes, several EELs have been employed, including metal oxides (ZnO and TiO2), selfassembled monolayers, cross-linked fullerene, caesium carbonate and polyelectrolytes10. Here, for the first time, we successfully combine 1-6 nm thick DNA stratum with an organic solar cell as one of the main electronically-active individual films showing that DNA functions as an effective extraction layer (EEL). We used DNA in its “natural”3 fibre-like salt with Na, resulting from the isolation

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from

salmon fish, which is soluble in polar solvents12-13 and was employed by us as an EEL by spin coating a thin film from a water: methanol solution. DNA1 consists of spirals of two inter-twined sugar and phosphate macromolecular strands linked by hydrogen-bonded base pairs, the phosphate backbone being negatively charged with balancing Na+ (or H+) for neutrality14. We chose the blend of PTB7 as the donor polymer and PC70BM as the acceptor molecule, since, together with the MoO3 hole transport layer (HTL), they are amongst the


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best-performing layers of their type15-16, also delivering maximum performance in solar cells without the need of an annealing step. This enabled us to carry out all fabrication steps of our ITO/DNA/PTB7:PC70BM/MoO3/Ag architecture (see Fig. 1a), including that of DNA, at room temperature (although as we will demonstrate DNA works even in cells with photoactive polymers that do require an annealing step).

Figure 1. Device architecture and current density-voltage characteristics of polymer solar cells incorporating DNA. a) schematics showing the ITO/DNA/PTB7:PC70BM/MoO3/Ag 4 ACS Paragon Plus Environment

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device structure (left) and insets (right) showing the chemical structure of PTB7, PC70BM and

a

generic

strand

of

DNA,

b)

best

current

density-voltage

curves

of

ITO/DNA/PTB7:PC70BM/MoO3/Ag polymer solar cells with DNA electron extraction layers of different thickness under AM1.5G, 1000 W/m2 irradiation, c) current density-voltage curves in the dark. DNA was deposited by spin coating at different speeds, i.e. 1000 rpm (Orange open hexagon), 2000 rpm (green open square), 3000 rpm (black solid diamond), 4000 rpm (pink open pentagon). Also reported are the JV curves of a cell with no DNA layer (ITO only, blue solid hexagon). The current density (JSC) vs voltage characteristics of the best cells with DNA interlayer deposited at different spin speeds measured under 1 sun illumination are shown in Fig. 1b together with those with no DNA (“ITO only”). Table 1 and Fig. S1 provides instead the average values for the PV parameters, including PCE, JSC, VOC and FF (fill factor). DNA insertion clearly improves performance. The thicker DNA EELs (1000 rpm, 2000 rpm), however, provide detrimental series resistances (> 20-30 Ω/cm2) which decrease monotonically with spin speed (see Fig. 2) as would be expected for an interlayer which is a non-conductor in the bulk. The shunt resistance, RSH, improves with the insertion of DNA on ITO, an indication that its presence- results in improved electron transfer from the organic semiconductor and lower charge carrier recombination10, 17 and is highest when deposited at spin speeds of 3000 rpm (see Fig. 2). Conversely, at the highest spin speed (4000 rpm), the off current (Fig. 1c), RSH and PCE (Table 1) suffer, indicating that at these deposition conditions the DNA layer is too thin and likely to lead to less-complete coverage of the ITO surface. As a result, the insertion of a DNA layer at optimized spin speeds (3000 rpm) between ITO and the photo-absorbing blend layers leads to dramatic enhancement in VOC (from 0.39 V to 0.71 V), JSC (by almost 20%) and PCE (from 2.3% to 4.9% for the best cells of each type). The dark current plots of Figure 1c also show that the same DNA EEL increases the On/Off current ratio (calculated at +1.1 V/-1 V) to a best value of 4.7 x 103. This is 125 times higher than that of ITO-only cells (a low 3.7x101) demonstrating that the 5 ACS Paragon Plus Environment

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ITO/DNA contact has remarkably strong rectifying properties. We confirmed the effectiveness of DNA as an EEL by fabricating the same solar cell architectures incorporating ITO and ITO/DNA on three separate occasions (samples) over 12 different cells for each type obtaining consistent results.

Table 1. Averages of the PV parameters of ITO/DNA/PTB7:PC70BM/MoO3/Ag polymer solar

cells made with different electron extraction layers over four different samples: ITO only, ITO/DNA with DNA deposited at different spin speeds. Electron Extraction Layer

JSC [mA/cm2]

VOC [V]

FF [%]

PCE [%]

None (ITO only)

11.97 ± 0.01

0.391 ± 0.006

49.0 ± 0.2

2.29 ± 0.05

DNA (1000rpm)

13.26 ± 0.31

0.604 ± 0.016

39.8 ± 3.8

3.18 ± 0.33

DNA (2000rpm)

14.01 ± 0.38

0.594 ± 0.017

43.0 ± 1.0

3.58 ± 0.07

DNA (3000rpm)

14.07 ± 0.01

0.710 ± 0.010

48.8 ± 0.9

4.88 ± 0.06

DNA (4000rpm)

13.52 ± 0.01

0.727 ± 0.005

45.4 ± 0.3

4.46 ± 0.05


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Figure 2. Average solar cell resistances of ITO/DNA/PTB7:PC70BM/MoO3/Ag solar cells

with different DNA electron extraction layers. Series resistance (RS, open squares) and shunt resistance (RSH, closed circles) extracted from the J-V curves of four cells each under AM1.5G, 1000 W/m2 irradiation with DNA deposited at different spin speeds. We also carried out Scanning Tunneling Microscopy (STM) and spectroscopy (STS) measurements to investigate the morphological and electronic properties of the DNA/ITO contact for the DNA deposited at 3000 rpm. The clean ITO surface appears corrugated and granular (Figure 3a and inset) with an average diameter of 13 ± 0.3 nm and a very low surface roughness (rms) of 3.0 ± 0.1 nm (see Fig. S2a). The STM images obtained after DNA deposition (Fig. 3b-c) show elongated structures that conform flat on the surface, with an orientation that may be ascribed to its spin-deposition, exceeding 200 nm in length and with a variable height between 1 nm and 6 nm (see Fig. 3c inset for a profile example). The average lateral dimension of these structures is between ∼1-6 nm (Fig. S2b) suggesting that each one consists of more DNA filaments that auto-organize in oriented bundles. This stable and reproducible DNA assembly (confirmed by reproducible STM images even after repetitive 7 ACS Paragon Plus Environment

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scans) was obtained as a result of the balance between attractive capillary forces and bending stress due to the elasticity of the DNA film. In addition, although the ITO substrate surface is structured, the degree of coverage of these very thin DNA films is significant and within the DNA domains the bundles are tightly packed.

Figure 3d reports the acquired STS current-voltage curves averaged over different sample areas of the bare ITO and after DNA deposition, as well as the corresponding derivative (dI/dV vs V) in the inset. Whereas the tunneling current is quite low on the ITO sample, showing the zero-current gap typical of semiconductor behavior seen previously18, we observe a huge increase of one order of magnitude (on average over all data points) of the response after DNA deposition.


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Figure 3. Scanning Tunneling Microscopy (STM) images of the ITO and ITO/DNA samples.

a) Scan of the ITO-only substrate over a 1500 nm × 1500 nm area (Vsample= 1.2 V, Iset-point= 50 pA); the inset is a 110 nm x 110 nm image evidencing the granular structure of the same sample. b) Scan of the ITO surface after DNA deposition over a 1500 nm × 1500 nm area. c) Better-resolved image (300 nm x 300 nm), showing the DNA bundles formed on the surface with the profile obtained along the line reported in the inset. d) Scanning Tunneling Spectroscopy (STS) response on the ITO before and after DNA deposition. Typical tunneling current vs voltage curves measured on bare ITO (empty circles) and after DNA deposition (black squares). Inset: the corresponding dI/dV derivative vs voltage curves.


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The strong changes in the STS spectra are a further proof that DNA affects the samples electronic structure and are also consistent with a significant lowering of the work function (i.e. tunneling barrier) after the deposition of DNA on the electrodes. This lowering has been evaluated using Kelvin probe measurements by Zhang et al. on conducting substrates 6-7 to be as much as 0.3-0.55 eV6-7. It is likely that a similar effect occurs when DNA is deposited on ITO. In fact the STM currents depend not only on the electronic density of states but also on the average barrier height which in first approximation is given by the average work function of sample and tip and is thus lowered with the incorporation of DNA[31]. Transferring this behaviour to our DNA-based solar cells, rather than calling upon enhanced exciton dissociation as in the photoresponse of OTFTs6, we note the improved tunneling and rectification (>103) induced by DNA in our devices, and that, starting from the value of work function of clean ITO of ∼4.7eV

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, an equivalent lowering brings it much closer to the

electron affinity of PC70BM which is ∼4.0-4.3eV20-21 and much further away from that of the opposite hole-collecting MoO3/Ag contact (∼5.1 eV). As demonstrated when employing different conventional EELs8, an equivalent better alignment of this magnitude and large difference between the hole and electron-extracting contacts is able to promote a considerable enhancement in the VOC and PCE of polymer solar cells (as well as rectification). This is supported by the dark current curves of Figure 1c where the device with DNA shows a considerably higher turn-on voltage compared to that with ITO only. This effect is a result of an increased built-in potential due to a larger difference in the work functions of the two opposite contacts8 (the application of a bias must first counter such a built in field for significant injection to occur). Similar dark current behaviour was also seen by other groups when comparing organic solar cells fabricated with or without a low work function metal such as Ca or Ba (instead of our DNA)8,

22-23

. The resulting large built-in field is also

beneficial in the transport/collection of the opposite photogenerated charges to their 10 ACS Paragon Plus Environment

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respective contacts. Furthermore, in OLEDs, DNA has been shown to block holes5 which also represents a beneficial property of an effective EEL, signs of which may be seen in the high rectifying ratios of our solar cells and improvement of RSH with the presence of DNA over the ITO (Fig. 2).

The DNA-induced lowering of the work function on metals has been attributed mainly to the formation of an interfacial dipole at the contact’s interface with hardly any role played by potentially mobile ions5, with the phosphate anions in the DNA at its origin7. One can draw parallels considering the presence of polar groups in DNA with those present on the side chains of effective polymers EELs24 (such as PFN and alcohol-soluble naphthalene diimidebased conjugated polymers) which endow them with excellent interfacial modification as well as charge collection capability25. The DNA we used here is a sodium DNA complex where the Na+ is used for DNA extraction and neutralizes the DNA’s phosphate anions PO3making it also soluble in polar solvents7,

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. The central role of the DNA anion is also

supported by studies by Sun et al. carried out on inorganic sodium compounds used as EELs over ITO in inverted polymer solar cells26. There the inorganic anions had been shown to also form complexes with In and Sn at the ITO surface. The degree of work function lowering was shown26 to depend on the acid dissociation constants, pKa, being more significant for salts with anions whose pKa was greater than 4, i.e. values that have also been reported for DNA27. The operation of an extracting layer is not only related to its work function but also its energy levels (e.g. as occurs in oxide or organic semiconductor transport layers) and how they match those of the active semiconductors28 (e.g. high electron affinities and very high ionization potentials for EELs). Although quantifying the relative role of these is non trivial when considering the very thin nm-scale layers such as the ones implemented here, and further studies, including surface sensitive techniques on ITO/DNA samples, can assist in gaining a deeper understanding of the mechanisms and their importance, the picture 11 ACS Paragon Plus Environment

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described above is also supported by the demonstration that the DNA salt does not work at all when implemented as a thin hole extraction layer (HEL) (as opposed as its successful implementation as an EEL). Clearly, figure 4a shows the very poor IV performance delivered by ITO/DNA/P3HT:PC60BM/Ca/Al direct structure (i.e. where this time it is the Ca that extracts electrons and ITO the holes) compared to that of the same cells where DNA is replaced with the prototypical high work function HEL, i.e. PEDOT:PSS23. As reported in Table S2, the PCE of the DNA as an HEL is < 0. 1%, a factor 25 lower than that cells with PEDOT:PSS. In fact, the reason that previous incorporation of DNA in a polymer PV cell had either been unsuccessfully (i.e. PCE ≤ 10-4 %)29 or only worked in tandem with the common23 and already-effective PEDOT:PSS HTL30 is likely due to the fact that it had been tested as an HTL rather than as an EEL and/or to lack of thickness and film optimization or different functionalization. Figure 4b, also shows that when DNA is employed as an EEL with another widely-used photo-absorbing blend layer, i.e. P3HT:PC60BM, improved performance is also observed (PV parameters are reported in Table S1). The latter results also prove that the DNA can withstand annealing at 130 °C widening its practical applicability as an electron extraction layer.


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Figure

4.

(a)

Current

density-voltage

curves

of

direct

structure

ITO/DNA/P3HT:PC60BM/Ca/Al and ITO/PEDOT:PSS/P3HT:PC60BM/Ca/Al polymer solar cells, where DNA is used as a hole extracting layer (HEL); (b) Current density-voltage curves of inverted ITO/DNA/P3HT:PC60BM/MoO3/Ag polymer solar cells with and without DNA, with DNA working as an electron extracting layer (EEL). Results show the large difference in performance when the thin DNA layer is used either as an HEL or an EEL. Measurements were carried out under AM1.5G, 1000 W/m2 irradiation. 13 ACS Paragon Plus Environment

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Even though the gains in PV performance demonstrated when incorporating DNA in the cells are striking with PCE of 5% there remains room for improvement for the EEL. Working on a better uniformity/coverage of the DNA film over the ITO surface (e.g. some ITO grains are visible amongst the DNA bundles in the highest resolution image of Fig. 3c) whilst keeping the right thickness, on its influence on its electronic states (e.g. by solvent engineering, temperature treatments, use with other materials, surfactants or additives), on the morphology of the photoactive blend over such a layer, and on functionalization of DNA with other cations or functional groups, may be valid approaches for pushing the limits of DNA’s electron-extractive properties even further, to the level of the best PTB7:PC70BM-based cells15,16.

Our successful integration of DNA 1-6 nm thick nano-layers in polymer solar cells, resulting in power conversion efficiencies increasing by ∼110% and rectifying ratios by two orders of magnitude, adds additional important exploitable characteristics to both DNA as biomaterial and to organic semiconductors in electronic devices. It opens up future explorations that can combine DNA’s electron extracting property with that of forming selective bonds with other molecules or with substrates (e.g. self-assembly)31 in order to develop new material layers, assemblies, nanostructures, and concepts in polymer-based optoelectronic devices. It has not escaped our notice that the DNA is a naturally occurring biological material and can be isolated even from plant species, cast as a sodium DNA salt from solvents such as water and alcohols, processed all at room temperature, suggesting a green route for the formation of the EELs. DNA can also be functionalized with other molecular groups, ions, metal nanoparticles32, nanotubes33 or wires34 which can be used to tailor its electronic influence on photovoltaic devices, as well as its solubility in different solvents (e.g. replacing sodium with


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alkyl quaternary ammonium ions)3-4. Finally, our findings expand the arsenal of materials and architectures available to scientists developing bio-hybrid devices based on organic semiconductors, including bio-detectors and medical sensors, an expanding field with great potential for the future.

ASSOCIATED CONTENT Supporting Information Available: Experimental information, device characterization Average curve of the solar cell devices, Tables showing PV parameters of ITO/DNA/P3HTPC60BM/MoO3/Ag inverted structure solar cell devices and ITO/DNA/P3HT-PC60BM/Ca/Al direct structure solar cell devices and thickness measurements of the DNA layer.

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENTS We are grateful to Dario Di Carlo Rasi and Dr.Andrea Zampetti for preliminary experiments with DNA and useful discussions. We thank Dr Luca La Notte, Dr Luigi Salamandra, Dr Giampaolo Susanna, Dr Giuseppina Polino, Enrico LaManna, Dr Francesca Brunetti, Prof Andrea Reale and Prof Aldo Di Carlo for useful discussions. We thank MIUR for PRIN 2012 (2012A4Z2RY) ‘‘AQUASOL’’ (Celle solari polimeriche processabili da mezzi acquosi: dai materiali ai moduli fotovoltaici), and ‘‘Polo Solare Organico’’ Regione Lazio for financial support. MS and MDC acknowledge EC for the RISE Project CoExAN GA644076.


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REFERENCES (1) Watson, J. D.; Crick, F. H. Molecular structure of nucleic acids. Nature 1953, 171, 737738. (2) Steckl, A. J. DNA - a new material for photonics? Nat Photon 2007, 1, 3-5. (3) Kwon, Y. W.; Lee, C. H.; Choi, D. H.; Jin, J. I. Materials science of DNA. J.Mater.Chem. 2009, 19, 1353-1380. (4) Hagen, J. A.; Li, W.; Steckl, A. J.; Grote, J. G. Enhanced emission efficiency in organic light-emitting diodes using deoxyribonucleic acid complex as an electron blocking layer. Appl.Phys.Lett. 2006, 88, 171109-171112. (5) Zalar, P.; Kamkar, D.; Naik, R.; Ouchen, F.; Grote, J. G.; Bazan, G. C.; Nguyen, T. Q. DNA electron injection interlayers for polymer light-emitting diodes. J.Am.Chem. Soc. 2011, 133, 11010-11013. (6) Zhang, Y.; Wang, M.; Collins, S. D.; Zhou, H.; Phan, H.; Proctor, C.; Mikhailovsky, A.; Wudl, F.; Nguyen, T. Q. Enhancement of the Photoresponse in Organic Field‐Effect Transistors by Incorporating Thin DNA Layers. Angew.Chem. 2014, 53, 244-249. (7) Zhang, Y.; Zalar, P.; Kim, C.; Collins, S.; Bazan, G. C.; Nguyen, T. Q. DNA interlayers enhance charge injection in organic field-effect transistors. Adv. mater.2012, 24, 4255-4260. (8) Zampetti, A.; Fallahpour, A. H.; Dianetti, M.; Salamandra, L.; Santoni, F.; Gagliardi, A.; Auf der Maur, M.; Brunetti, F.; Reale, A.; Brown, T. M.; Di Carlo, A. Influence of the interface material layers and semiconductor energetic disorder on the open circuit voltage in polymer solar cells. J. Polym. Sci. B Polym. Phys. 2015, 53, 690-699. (9) Elumalai, N. K.; Uddin, A. Open circuit voltage of organic solar cells: an in-depth review. Energy Environ. Sci. 2016, 9, 391-410. (10) Yip, H.-L.; Jen, A. K.-Y. Recent advances in solution-processed interfacial materials for efficient and stable polymer solar cells. Energy Environ. Sci. 2012, 5, 5994-6011. (11) Zamenhof, S. Preparation and assay of deoxyribonucleic acid from animal tissue. Methods in Enzymology 1957, 3, 696-704. (12) Wang, L.; Yoshida, J.; Ogata, N.; Sasaki, S.; Kajiyama, T. Self-assembled supramolecular films derived from marine deoxyribonucleic acid (DNA)-cationic surfactant complexes: large-scale preparation and optical and thermal properties. Chem. Mater. 2001, 13, 1273-1281. (13) Zhang, G.; Wang, L.; Yoshida, J.; Ogata, N. Optical and optoelectronic materials derived from biopolymer deoxyribonucleic acid (DNA). Asia-Pacific Optical and Wireless Communications Conference and Exhibit 2001, 4580, 337-346. (14) Singh, T. B.; Sariciftci, N. S.; Grote, J. G. Bio-organic optoelectronic devices using DNA. Organic Electronics 2009, 223, 73-112. (15) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nature Photonics 2012, 6, 591-595.


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(16) Susanna, G.; Salamandra, L.; Ciceroni, C.; Mura, F.; Brown, T. M.; Reale, A.; Rossi, M.; Di Carlo, A.; Brunetti, F. 8.7% Power conversion efficiency polymer solar cell realized with non-chlorinated solvents. Solar Energy Materials and Solar Cells 2015, 134, 194-198. (17) Hau, S. K.; Yip, H.-L.; Acton, O.; Baek, N. S.; Ma, H.; Jen, A. K.-Y. Interfacial modification to improve inverted polymer solar cells. J.Mater. Chem. 2008, 18, 5113-5119. (18) Matino, F.; Persano, L.; Arima, V.; Pisignano, D.; Blyth, R. I. R.; Cingolani, R.; Rinaldi, R. Electronic structure of indium-tin-oxide films fabricated by reactive electronbeam deposition. Phys.Rev.B 2005, 72, 085437-085443. (19) Brown, T. M.; Lazzerini, G. M.; Parrott, L. J.; Bodrozic, V.; Bürgi, L.; Cacialli, F. Time dependence and freezing-in of the electrode oxygen plasma-induced work function enhancement in polymer semiconductor heterostructures. Organic Electronics 2011, 12, 623633. (20) Yang, Y.; Chen, W.; Dou, L.; Chang, W.-H.; Duan, H.-S.; Bob, B.; Li, G.; Yang, Y. High-performance multiple-donor bulk heterojunction solar cells. Nat Photon 2015, 9, 190198. (21) Kyaw, A. K.; Wang, D. H.; Gupta, V.; Zhang, J.; Chand, S.; Bazan, G. C.; Heeger, A. J. Efficient solution-processed small-molecule solar cells with inverted structure. Adv. mater. 2013, 25, 2397-2402. (22) Gupta, V.; Kyaw, A. K. K.; Wang, D. H.; Chand, S.; Bazan, G. C.; Heeger, A. J. Barium: an efficient cathode layer for bulk-heterojunction solar cells. Scientific reports 2013, 3, 1965-1971. (23) Brown, T. M.; Kim, J. S.; Friend, R. H.; Cacialli, F.; Daik, R.; Feast, W. J. Built-in field electroabsorption spectroscopy of polymer light-emitting diodes incorporating a doped poly(3,4-ethylene dioxythiophene) hole injection layer. Appl. Phys. Lett. 1999, 75, 16791681. (24) Duan, C.; Zhang, K.; Zhong, C.; Huang, F.; Cao, Y. Recent advances in water/alcoholsoluble [small pi]-conjugated materials: new materials and growing applications in solar cells. Chem. Soc. Rev.2013, 42, 9071-9104. (25) Wu, Z.; Sun, C.; Dong, S.; Jiang, X.-F.; Wu, S.; Wu, H.; Yip, H.-L.; Huang, F.; Cao, Y. n-Type Water/Alcohol-Soluble Naphthalene Diimide-Based Conjugated Polymers for HighPerformance Polymer Solar Cells. J.Am.Chem. Soc. 2016, 138, 2004-2013. (26) Sun, K.; Zhang, H.; Ouyang, J. Indium tin oxide modified with sodium compounds as cathode of inverted polymer solar cells. J. Mater. Chem. 2011, 21, 18339-18346. (27) Plum, G. E.; Breslauer, K. J. Thermodynamics of an intramolecular DNA triple helix: a calorimetric and spectroscopic study of the pH and salt dependence of thermally induced structural transitions. J. mol. Biol. 1995, 248, 679-695. (28) Yin, Z.; Wei, J.; Zheng, Q. Interfacial Materials for Organic Solar Cells: Recent Advances and Perspectives. Adv. Sci. 2016, 20, 1500362-1500399. (29) Kolachure, V.; Jin, M. H. C.Fabrication of P3HT/PCBM bulk heterojunction solar cells with DNA complex layer. Photovoltaic Specialists Conference, 2008. PVSC '08. 33rd IEEE 2008, 1-5. (30) Yengel, E.; Guvenc, A. B.; Guo, S.; Akin, H. E.; Ozkan, M.; Ozkan, C. S. Metalized DNA Electrodes for Improved Hole Collection Efficiency in Polymer Heterojunction Solar Cells. J. Nanoelectron. Optoelectron. 2011, 6, 121-126. 17 ACS Paragon Plus Environment

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