Article pubs.acs.org/JPCC
Energy Level Alignment and Morphology of Ag and Au Nanoparticle Recombination Contacts in Tandem Planar Heterojunction Solar Cells K. Xerxes Steirer,† Gordon A. MacDonald,† Selina Olthof,‡ Jeremy Gantz,† Erin L. Ratcliff,† Antoine Kahn,‡ and Neal R. Armstrong*,† †
Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85745, United States Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, United States
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ABSTRACT: We present the interface characterization of vacuum-deposited metal nanoparticle recombination layers (Ag, Au; 1 nm equivalent thickness) at donor/ acceptor heterojunctions comprising copper phthalocyanine (CuPc) and C60 as model interfaces for tandem planar heterojunction organic photovoltaics (TOPVs). We compare the extent to which voltage doubling occurs using these two metal recombination contacts (RC) in TOPVs (ITO/CuPc/C60/(Ag,Au)/CuPc/C60/ BCP/Al) and correlate the differences with energetic dissimilarities revealed by UVphotoemission (UPS) and inverse photoemission (IPES) spectroscopies, and morphology as revealed by atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). Ag interlayer RCs produce the expected voltage doubling in the open-circuit voltage (VOC) for the TOPV, whereas Au RCs showed poor voltage addition. Significant shifts in ionization potential and electron affinity and shifts in local work function were observed for C60/metal heterojunctions and for heterojunctions based on C60/ metal/C60 and for C60/metal/CuPc, with clear evidence for partial charge redistribution between C60 and Ag nanoparticles. AFM and FE-SEM images revealed discrete Ag nanoparticles at the C60 interface, whereas Au/C60 heterojunctions consisted of more uniform Au thin films that wet the C60 surface and penetrated below the surface. These studies point to the need for careful control of both electronic and morphological properties of thin RCs in emerging tandem organic solar cell technologies.
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INTRODUCTION Tandem organic photovoltaic cells (TOPV) with two or more subcells in series have the potential to increase power conversion efficiency by 20−50% beyond the maximum efficiencies of single heterojunction OPV cells1−4 but can be challenging to create with full control over their electronic properties. Optimized TOPVs include current-matched subcells comprising complementary absorbers to maximize spectral overlap with solar irradiance.5,6 The recombination contact (RC) between subcells must provide for (i) alignment of charge carrier energy levels between subcells (quasi-Fermi levels for holes in one layer and electrons in the adjacent layer);7 (ii) optimization of recombination rates at the RC interface(s) joining the two subcells to ensure complete annihilation of charges from adjacent cells; and (iii) control of the optical field distribution.8 Since the energetics of RC/ subcell interfaces ((i) above) cannot be identified a priori, they must be assessed for each organic and inorganic material combination.3,9−11 This report focuses on electronic interactions and morphologies of two prototypical vacuum-deposited metal nanoparticle layers (Ag and Au) on C60 thin films in TOPVs created from copper phthalocyanine (CuPc)/C60 subcells. We observe variations in the electronic interactions between C60 and these metal nanoparticle layers using ultraviolet photoemission (UPS) and inverse photoemission (IPES) spectros© 2013 American Chemical Society
copies. We also note important differences in Ag or Au nanoparticle film morphology when deposited on C60 films, differences that appear to be associated with differences in voltage addition of the subcells using Ag or Au RCs in TOPVs. Figure 1 provides a simplified framework to estimate the effects of energy level offsets in an ideal TOPV (a) and a nonideal TOPV (b), where we show effects of voltage loss at the RC (VRC) on the open circuit voltage of the TOPV under illumination (VTOPV).7 The highest occupied molecular orbital (hole transport) level of the donor (HOMOD) and lowest unoccupied molecular orbital (electron transport) levels of the acceptor (LUMOA) in each subcell are shown as solid lines. The LUMO and HOMO levels for the donor and acceptor, LUMOD and HOMOA, respectively, are not shown for clarity. For the TOPV under illumination, photogenerated holes and electrons split the Fermi energies in each subcell into hole and electron quasi-Fermi levels (EF′), depicted as dashed lines. The RC ideally equilibrates electronically with the photogenerated majority carriers in each subcell, leading to their efficient annihilation at the RC (Figure 1a) and maximization of VTOPV.12 RCs have been created from single materials such as Special Issue: Ron Naaman Festschrift Received: March 17, 2013 Revised: July 1, 2013 Published: July 3, 2013 22331
dx.doi.org/10.1021/jp402672j | J. Phys. Chem. C 2013, 117, 22331−22340
The Journal of Physical Chemistry C
Article
Figure 1. Energy level offsets in an ideal TOPV (a) and a nonideal TOPV (b) showing the possible origin of voltage loss (VRC) originating at the recombination contact (RC). The solid lines for HOMO and LUMO positions are offset from the dotted quasi-EF levels achieved under illumination.
evaporated ultrathin metal films,12−14 TiO2 and ZnO nanoparticles,15−19 self-assembled monolayers (SAMs),20 doped and undoped transition metal oxides,21,23,24 conducting polymers,19,25 doped and undoped organic molecular semiconductors,10,26,27 graphene oxide,28,29 carbon nanotubes,30 and multiple electron-/hole-harvesting combinations of these materials, with variable levels of success in voltage addition between subcells.31−33 When good recombination efficiencies are achieved in the RC, voltage addition between subcells can be additive over a number of subcells. For example, a 10-fold heterojunction TOPV has been recently demonstrated, using a series of identical donor/acceptor subcells based on ClAlPc/ C60 heterojunctions and Ag nanoparticle/MoOx RCs between the subcells.34 RC structures of BCP/Ag/MoO x have demonstrated high voltage summation and mitigate deleterious interactions of Ag with C60 as well as improving the energy level alignment via strong dipole formation between MoOx and adjacent donor materials.21,35,36 The rules for energy level tuning of interfaces between active layers and RCs in general, however, are poorly understood. We hypothesize that offsets in these energy levels, VRC, may appear for some RC/donor or RC/acceptor heterojunctions and/or that significant lowering of recombination efficiency occurs in the RC as a result of unoptimized positioning of the RC between the adjacent active layers (e.g., mixing of the RC and one of the active layer components), both of which greatly impact on the final VTOPV.26,37 Ultrathin metal films such as Ag and Au have been used as the sole component of RCs, or as one of the components of multilayer RCs, and are easily added by evaporation during formation of small molecule or polymer TOPVs.5,6,12,14,34 These ultrathin metal layers are believed to exist as nanoparticles at many donor/acceptor (D/A) interfaces.38−40 The density of states near the Fermi energy in Ag or Au nanoparticles suggest that electronic equilibration with adjacent subcells and high recombination efficiencies at the RC ought to be easily achievable. Some chemical or physical interactions may produce unwanted energy barriers at the RC/donor or RC/acceptor interface, however, depending upon the reactivity of the metal NP and active layer components and the processing conditions, which will lower the efficiency of charge recombination and the overall energy conversion efficiency.22,41,42 For the studies reported here, we used prototypical D/A heterojunction TOPVs comprising vacuum deposited C60 and copper phthalocyanine (CuPc) with identical subcell thick-
nesses and either Ag or Au nanoparticle RCs. We have chosen metal interlayer thicknesses that have been shown effective at adding VOC in tandem OPV architectures.12,43 We did not use spectrally complementary donor layers or current matching of the two subcells since such steps are not necessary to study the processes underpinning voltage doubling in the TOPV.5,6,34,44,45 Ag and Au RC interlayers have been shown to provide voltage addition between several D/A heterojunction subcells, such as PTCBI/Ag/CuPc,12 C60/Ag/ pentacene,13 and C60/Au/N,N,N,N-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD);7 however, the degree of voltage addition and the overall quality of TOPV performance varies substantially between experiments. For this work, we focus on spectroscopic characterization, thin film morphology, and device performance for C60/CuPc, C60/Ag/CuPc, and C60/ Au/CuPc heterojunctions and TOPVs based on a series addition of these heterojunctions. Further studies are performed on C60/metal nanoparticle heterojunctions (metal on C60 or C60 on metal). AFM and field emission scanning electron microscopy (FE-SEM) studies suggest nanoscale heterogeneity of these thin metal interlayers deposited on C60 and nanoparticle formation at the C60 surface and subsurface for Ag and Au, respectively. UPS/IPES measurements suggest charge redistribution at the Ag/C60 heterojunction but not substantially at the Au/C60 heterojunction. These findings are consistent with recent reports using both photoemission spectroscopies and scanning tunneling spectroscopies, that highlighted the differences in charge redistribution that occur for Ag/C60 versus Au/C60 heterojunctions.38,46−51 Charge redistribution at the Ag nanoparticle/C60 interface appears to be correlated with the electronic coupling needed to ensure minimal voltage losses at the C60/RC/CuPc interface and a VTOPV close to the sum of the voltages of the two subcells. Charge redistribution is smaller for the Au/C60 when compared to the Ag/C60 interfaces, and there are sizable differences in morphology revealed by AFM. Observed differences correlate with near ideal behavior for Ag RCs and significant voltage loss, VRC, for Au RCs. There is a clear need for characterization of energetics and morphologies of RC/active layer heterojunctions as a component of creating of optimized TOPVs.
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EXPERIMENTAL SECTION Materials. Organic semiconductors C60 and CuPc were obtained from Aldrich and purified by three successive thermal train sublimations to obtain 99.999% pure materials verified by mass spectrometry. Au and Ag pellets were obtained from Kurt 22332
dx.doi.org/10.1021/jp402672j | J. Phys. Chem. C 2013, 117, 22331−22340
The Journal of Physical Chemistry C
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RESULTS AND DISCUSSION TOPV Performance. The TOPV architectures studied here were based on well-established CuPc/C60 single heterojunction OPVs: ITO/CuPc/C60/RC/CuPc/C60/BCP/Al where the RC is nominally 1 nm of vapor deposited Au or Ag (on the C60 layer of the lower subcell) and BCP is bathocuproine. An ITO/ CuPc/C60/CuPc/C60/BCP/Al TOPV with no metal RC was included as a control. The logarithmic and linear J/V responses for these cells are shown in Figure 2. Solar cell performance
J. Lesker quoting purity of 99.99% and used as received. Organic and metal depositions were performed using custom effusion cells at base pressures
