Article pubs.acs.org/JPCC
Tuning Contact Recombination and Open-Circuit Voltage in Polymer Solar Cells via Self-Assembled Monolayer Adsorption at the Organic−Metal Oxide Interface He Wang,†,‡ Enrique D. Gomez,† Zelei Guan,‡ Cherno Jaye,§ Michael F. Toney,∥ Daniel A. Fischer,§ Antoine Kahn,‡ and Yueh-Lin Loo*,† †
Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, United States Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, United States § Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ∥ Stanford Synchrotron Radiation Lightsource, Menlo Park, California 94025, United States ‡
S Supporting Information *
ABSTRACT: We adsorbed fluorinated-alkyl and hydrogenated-alkyl phosphonic acid derivatives onto indium tin oxide (ITO) to form self-assembled monolayers (SAMs). Polymer solar cells having these treated ITOs as anodes display opencircuit voltages (Vocs) that are higher than those with bare ITO as anodes. Although the work function of ITO can be significantly tuned by SAM adsorption, the position of the Fermi level of the anode with respect to the hole transport level in the polymer active layer is essentially the same in all of the devices, suggesting that changes in the work function of the anode are not responsible for the Voc variation. Rather, the barrier for minority carrier transport to ITO is altered through SAM adsorption. The adsorption of fluorinated-alkyl phosphonic acid on ITO, in particular, induces a barrier of 2.4 eV for minority carrier transport, which effectively increases carrier selectivity at the anode and increases the Voc in polymer solar cells comprising such treated ITO as anodes compared to those with untreated anodes.
1. INTRODUCTION
level difference between the electron donor and the electron acceptor. This difference correlates with the energy level difference between the highest occupied molecular orbital (HOMO) of the electron donor and the lowest unoccupied molecular orbital (LUMO) of the electron acceptor, with a larger energy level difference resulting in a higher Voc.2 Accordingly, extensive research has focused on the design and synthesis of new polymer donors that exhibit larger ionization potentials.5,6 When paired with the quintessential electron acceptor of [6,6]-phenyl-C61-butyric acid methyl ester, PCBM, this increase in ionization potential increases the quasiFermi level difference in the active layer, effectively enhancing the Voc of polymer solar cells.5,6 In bulk-heterojunction polymer solar cells, in particular, the Voc can also be limited by the work function difference between the anode and the cathode, especially when this difference is smaller than the quasi-Fermi level difference provided by the active layer.4 As such, many
Organic solar cells have attracted a lot of attention because they promise to be lightweight and low-cost energy harvesting devices. To date, efficiencies as high as 12.0% have been demonstrated.1 Attempts to further improve the efficiencies of polymer and small-molecule organic solar cells have focused on increasing the fill factor, the short-circuit current density (Jsc), and the open-circuit voltage (Voc). While optimizing the fill factor remains complicated, many parameters including those intrinsic to the organic semiconductors and processing conditions have demonstrated to be reliable routes to increase Jsc.2,3 Postdeposition processing including thermal annealing and solvent-vapor annealing can increase the crystallinity of the organic semiconductor constituents, effectively increasing carrier mobility.2,3 Postdeposition annealing has also been reported to moderate the extent of phase separation between the constituents, affecting exciton dissociation and charge recombination in the active layers of solar cells.2,3 The Voc of bulk-heterojunction polymer solar cells is thought to be largely dictated by the active layer and electrode materials selection.2,4 In particular, the Voc depends on the quasi-Fermi © XXXX American Chemical Society
Received: July 4, 2013 Revised: September 18, 2013
A
dx.doi.org/10.1021/jp406625e | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
contact recombination in devices constructed in the conventional and inverted architectures. These works, among other work,25 thus highlight that practically influencing the Voc of polymer solar cells can be challenging and implicates the need to better control the extent of carrier recombination in polymer solar cells in order to obtain robust device characteristics. We conducted experiments with a series of fluorinated-alkyl and hydrogenated-alkyl phosphonic acid derivatives adsorbed on ITO. By keeping all other processing parameters constant, our experiments uniquely examines how the active layer−anode interface affects Voc of bulk-heterojunction polymer solar cells. Our experiments suggest that SAM adsorption can effectively passivate ITO; its adsorption also retards minority carrier transport to the treated anodes, the magnitude of which is dependent on the chemistry of the derivative employed. This retardation in turn reduces contact recombination and increases carrier selectivity to the anode, ultimately increasing the Voc of polymer solar cells.
research groups have also worked to tune the work function of electrodes, with the intention to increase the work function difference between the electrodes for such devices.4,7−12 One common way to alter the work function of metals or metal oxides is to chemically adsorb self-assembled monolayers (SAMs) on these surfaces.10,11,13−15 When molecules containing electron-withdrawing or electron-donating moieties are adsorbed on these surfaces in an organized fashion, the surface dipoles that are generated can in turn increase or decrease the work function of these substrates, depending on the net direction of the induced surface dipole. This route has thus commonly been proposed for tuning the work function difference between the electrodes of bulk-heterojunction polymer solar cells in the hope of increasing Voc. In reality, however, predictively tuning the Voc of bulkheterojunction polymer cells is much more challenging as altering processing parameters can alter the extent of recombination, and carrier recombination during device operation16−19be it at the electrodes16,17 or in the bulk of the active layer19is generally thought to also influence Voc. For this reason, experiments that have employed SAM adsorption on electrodes to adjust the V oc of bulkheterojunction solar cells have remained inconclusive to date. In particular, while SAM adsorption has been widely found to alter the work function of metals and metal oxides that are frequently used as electrodes in polymer solar cells, this induced change in work function does not necessarily translate to the expected increase in the Voc of devices. For example, Kim et al. adsorbed silane-based SAMs having different terminal groups, including −NH2, −CH3, and −CF3, onto indium tin oxide (ITO), thereby inducing surface dipoles on ITO.20 Because the −CF3-terminated derivative is electron withdrawing in nature, its adsorption should effectively increase the work function of ITO and, accordingly, the work function difference between the two electrodes of polymer solar cells constructed in the conventional architecture. As expected, the Voc of these devices increases as a consequence. Interestingly, devices having −NH2- and −CH3-terminated SAMs adsorbed on ITO electrodes exhibit Vocs that are either comparable to or higher than those of devices having bare ITO as electrodes, even though the SAMs that are utilized in these cases are electron donating in nature and should thus decrease the work function difference between the electrodes. This observation is suggestive that contact recombination rate at the electrodes is suppressed with SAM treatment. Whereas monolayers are usually formed by immersing the substrates intended to be treated into solutions comprising molecules of interest, Jen’s group spin-coated carboxylic acidbased molecules atop zinc oxide; these molecules supposedly facilitate electron transport between the active layer and the cathode for polymer solar cells constructed in the conventional architecture. While it is exceedingly difficult to form wellorganized SAMs from spin coating, the authors reported the ability to tune device Voc by varying the directionality of the dipole induced at the electron transport interface through the introduction of carboxylic acid species.21,22 This report seems to suggest that the work function difference between the electrodes, relative to contact recombination, dominates Voc. Yet, when these same species are deposited on titania or zinc oxide nanoparticles that serve to transport electrons in inverted bulk-heterojunction polymer solar cells, the Voc appears comparable across all carboxylic acid derivatives sampled.23,24 This seemingly contradictory observation implies differences in
2. EXPERIMENTAL SECTION 2.1. Phosphonic Acid Derivatives and Self-Assembled Monolayer Formation. Fluoro-alkyl and hydrogenated-alkyl phosphonic acid derivatives (FmSAM and HmSAM) are utilized to form SAMs on ITO in this work. The chemical structures of these compounds are displayed in Scheme 1. Scheme 1. Chemical Structures of the Phosphonic Acid Derivativesa
a Here, m represents the number of methylene units in fluorinatedalkyl and hydrogenated-alkyl phosphonic acid derivatives.
3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,14-Pentacosaflu o r o tet r a de c y l ph o s ph o ni c a c id ( F 1 4 SAM ), 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecylphosphonic acid (F10SAM), 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid (F8SAM), 1-octadecylphosphonic acid (H18SAM), 1-tetradecylphosphonic acid (H14SAM), and 1decylphosphonic acid (H10SAM) were purchased from Specific Polymers. 1-Octylphosphonic acid (H8SAM), 1-hexylphosphonic acid (H 6 SAM), and 1-butanephosphonic acid (H4SAM) were purchased from Alfa Aesar. These derivatives were used as-received without further purification. Precut and prepatterned ITO-coated glass substrates (15 Ω/ sq; Colorado Concept Coatings) were sequentially sonicated in acetone and isopropanol for 10 min each. After the ITO-coated glass substrates were exposed to ultraviolet/ozone for 10 min, they were immediately immersed in phosphonic acid derivative solutions, each having a concentration of 0.3 mmol/L in anhydrous tetrahydrofuran (THF). The ITO-coated glass substrates were kept in the solutions for 24 h and then rinsed with THF, followed by methanol. Finally, the substrates were baked in an oven at 130 °C for 24 h and rinsed again with THF and then with methanol to remove any residual physisorbed phosphonic acid derivatives. B
dx.doi.org/10.1021/jp406625e | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 1. (a) J−V characteristics acquired under illumination of representative solar cells with F8SAM-modified ITO, H8SAM-modified ITO, and bare ITO as anodes. J−V characteristics of a representative solar cell with PEDOT:PSS-coated ITO as anode is also shown in dotted line for comparison. (b) Voc of polymer solar cells having different anodes including PEDOT:PSS-coated ITO, ITO adsorbed with FmSAMs and HmSAMs of variable lengths, and bare ITO. (c) J−V characteristics acquired in the dark of the same devices having SAM-modified ITO and bare ITO as anodes (symbols) shown in (a) along with fits (solid lines) to the data with the equivalent circuit model.
2.2. Fabrication and Characterization of Polymer Solar Cells. Bare ITO and SAM-modified ITO that were prepared as described above were used as anodes in bulkheterojunction polymer solar cells constructed in the conventional architecture. Reference devices were also fabricated with commercially available poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS (Clevios P; HC Starck, an aqueous dispersion having 1.2−1.4% solids content), as the hole transport layer. As-purchased PEDOT:PSS was diluted with H2O at 1:1 volume ratio before the solution was spincoated onto precleaned ITO-coated glass substrates at 3000 rpm for 60 s to form 20 nm thick films. The PEDOT:PSS layers were annealed at 150 °C for 20 min to remove residual water. The PEDOT:PSS-coated as well as SAM-treated substrates were then brought into a nitrogen-filled glovebox (
