ARTICLE Received 20 Feb 2013 | Accepted 9 May 2013 | Published 13 Jun 2013 DOI: 10.1038/ncomms3004 Uncovering loss mechanisms in silver nanoparticle-blended plasmonic organic solar cells Bo Wu1, Xiangyang Wu2, Cao Guan1, Kong Fai Tai1, Edwin Kok Lee Yeow2, Hong Jin Fan1,4,5, Nripan Mathews3,4,5 & Tze Chien Sum1,4,5 There has been much controversy over the incorporation of organic-ligand-encapsulated plasmonic nanoparticles in the active layer of bulk heterojunction organic solar cells, where both enhancement and detraction in performance have been reported. Here through comprehensive transient optical spectroscopy and electrical characterization, we demonstrate evidence of traps responsible for performance degradation in plasmonic organic solar cells fabricated with oleylamine-capped silver nanoparticles blended in the poly (3-hexylthiophene):[6,6]-phenyl-C 61-butyric acid methyl ester active layer. Despite an initial increase in exciton generation promoted by the presence of silver nanoparticles, transient absorption spectroscopy reveals no increase in the later free polaron population—attributed to fast trapping of polarons by nearby nanoparticles. The increased trap-assisted recombi- nation is also reconfirmed by light intensity-dependent electrical measurements. These new insights into the photophysics and charge dynamics of plasmonic organic solar cells would resolve the existing controversy and provide clear guidelines for device design and fabrication. 1 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore. 2 Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore. 3 Division of Materials Technology, School of Materials Science and Engineering, Nanyang Technological University, Block N4.1 Nanyang Avenue, Singapore 639798, Singapore. 4 Energy Research Institute @ NTU (ERI@N), 1 CleanTech Loop, #06-04 CleanTech One, Singapore 637141, Singapore. 5 Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE), 1 Create Way, Singapore 138602, Singapore. Correspondence and requests for materials should be addressed to T.C.S. (email: [email protected]) or to N.M. (email: [email protected]). NATURE COMMUNICATIONS | 4:2004 | DOI: 10.1038/ncomms3004 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3004 ulk heterojunction (BHJ) organic solar cells (OSCs) scattering could be achieved14. The as-synthesized AgNPs had an comprising of bi-continuous networks of a semiconducting average diameter of B24.7 nm (measured through transmission Bpolymer/fullerene derivative have been under intense electron microscopy, see Supplementary Fig. S1 inset). Steady- scrutiny in the recent years because of their attractive properties state optical absorption spectra of AgNPs in 1,2-dichlorobenzene such as light weight, high flexibility, cost effectiveness, ease of (DCB) indicated a peak at 417 nm, corresponding to localized fabrication, and so on. The power conversion efficiencies (PCEs) surface plasmon resonance peak of the AgNPs in DCB of such cells have attained the 8 À 10% mark1,2. One of the main (Supplementary Fig. S1). A comparison of the optical bottlenecks in improving efficiencies is ascribed to the trade-off absorption in the bare P3HT:PCBM control film and the between light absorption and charge extraction, which restricts AgNPs-P3HT:PCBM films indicated an enhanced absorption the active layer thickness. Light-trapping strategies utilizing over a broad region for the AgNP-embedded film, which can be plasmonic metallic nanostructures as light concentrators offer a ascribed to be plasmonic in origin (that is, enhanced local highly attractive solution to this predicament. For instance, electromagnetic field and strong far-field scattering)3,15. metallic nanoparticles (NPs) have been commonly employed as Furthermore, the three vibronic peaks/shoulders of P3HT (at subwavelength antennas and scattering centres to enhance the B510 , B560 and B600 nm, corresponding to the 0–2, 0–1 and absorption of BHJ films3. Typically, these metal NPs are either 0–0 vibronic transitions of P3HT) do not exhibit any prominent embedded in the charge transfer layer such as PEDOT:PSS4–7 or shift or relative magnitude variation with increasing volume simply blended within the active BHJ layer8–11. In the latter ratios of AgNPs in the BHJ films, indicating that the crystalline approach, NPs coated with organic ligands are typically used to order of P3HT is not significantly compromised16. disperse the particles within the organic blends. Despite its simplicity, the findings from this approach are highly Charge dynamics in the fs-ns temporal regime 8–11 12,13 .Transient controversial as both enhancement and detraction in absorption were performed on 100-nm-thick P3HT:PCBM organic photovoltaic (OPV) devices performance have been control films and AgNPs-P3HT:PCBM films. Figure 1a shows the reported. Physical insight into the fundamental photophysics and representative differential transmission (DT) spectra spanning charge dynamics in these NP-blended plasmonic OPV cells is 520–1,200 nm for the control samples at different delay times fol- urgently required. Investigating the fundamental photophysical lowing photoexcitation. The vibronic peaks corresponding to 0–0 and dynamical processes governing charge generation, and 0–1 absorption transitions (520–620 nm) manifest as ground recombination and extraction in these hybrid plasmonic films, state bleaching (GSB) peaks16. This photobleaching signal (that is, and correlating them to the electrical properties/device DT/T40orrelativeDT40), which arises from the state-filling of performance of functional devices are the main foci of this work. the excitonic and polaronic states, is proportional to the population Here, we present a comprehensive transient optical spectro- of the excitons and polarons in the photoexcited films17. scopy and electrical characterization study of the carrier dynamics and transport in thin films and devices fabricated using a representative hybrid plasmonic OPV system comprising of 10 GSB 1 ps 1 ns oleylamine-capped silver nanoparticles (AgNPs) blended within 10 ps 2 ns poly (3-hexylthiophene):[6,6]-phenyl-C 61-butyric acid methyl ) ester (P3HT:PCBM) films. Transient optical studies spanning the –3 SE 100 ps temporal regimes of charge generation, recombination, transport (10 0 T / and extraction (that is, over 7 orders of magnitude from 100 fs to T 5 ms) were used to probe the system. Femtosecond transient Δ a P absorption spectroscopy (fs-TAS) revealed that while the initial –10 EX exciton generation density improved with the incorporation of AgNPs, there is also a higher rate of polaron quenching in the GSB 1:1 10 At 1 ps 1:1/16 AgNPs-P3HT:PCBM blends compared with the P3HT:PCBM ) control blends—ascribed to the fast trapping of polarons by the –3 SE 1:0 0 AgNPs. Nanosecond transient absorption spectroscopy (ns-TAS) (10 T uncovered the presence of fast trap-assisted recombination— / T attributed to increased subgap trapping states at the interfacial Δ –10 region of P3HT:PCBM originating from the AgNPs. Intensity- b EX dependent open-circuit voltage (VOC) and dark current measure- –20 ments confirmed the increased trap density (that is, increased 5 GSB 1:1 disorder in the density of states) that leads to higher trap-assisted At 1 ns 1:1/16 recombination. Our findings provide new insights into the ) –3 fundamental photophysics and charge dynamics in plasmonic 1:0 (10 0 OSCs. Importantly, our work sheds light on the present T / controversy regarding the positive and negative influence of T Δ adding AgNPs to the BHJ. On the basis of these findings, we have c also provided suggestions and guidelines for developing high- –5 performance hybrid plasmonic OSC devices. 600 700 800 900 1,000 1,100 1,200 Wavelength (nm) Results Figure 1 | The effect of AgNPs on the exciton and polaron generation Fabrication and physical characterization. Fabrication of observed via fs-TAS. (a) Relative differential transmission (DT/T)of AgNPs using oleylamine as both surfactant and reducing agent is P3HT:PCBM ranged from 520 nm to 1,200 nm. Selected times are 1 ps, a simple, inexpensive, versatile and highly reproducible method. 10 ps, 100 ps, 1 ns and 2 ns after excitation with 500-nm laser beam. A The size of the metallic NPs could be easily tuned using the comparison of the fs-TAS signals of Ag-P3HT:PCBM and P3HT:PCBM BHJ reaction temperature and larger particle sizes for better light films at (b) 1 ps and (c) 1 ns after photoexcitation. 2 NATURE COMMUNICATIONS | 4:2004 | DOI: 10.1038/ncomms3004 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3004 ARTICLE The negative DT/T signal centred at B700 nm is attributed to the 1.0 a 1:1 photo-induced absorption (PIA) of photogenerated delocalized At 1,150 nm polarons in the crystalline P3HT domains. The spectrum at 1 ps 1:1/16 shows a weak positive peak around 740 nm (within the broad 0.5 1:0 negative PIA band) that arises from the stimulated emission of 16,18 Normalized P3HT singlet excitons . This peak disappears within 10 ps due decay (a.u.) 0.0 to the fast quenching of P3HT excitons by the PCBM acceptor. In the near-infared region (NIR), there is a broad PIA peak centred 0 20 40 60 80 100 120 140 160 180 200 around 1,150 nm at 1 ps, which subsequently decays within 100 ps. Time (ps) Following which, a long-lived (ns) PIA peak centred around 1.0 1,000 nm remains. The PIA peak at 1,150 nm is ascribed to singlet b At 1,000 nm 1:1 excitons (EX) while the 1,000 nm peak originates from an overlap 1:1/16 of signals from P3HT singlet excitons, P3HT polarons (P) in 17,18 1:0 disordered domains and PCBM anions . At 1-ps delay, the PIA 0.5 peak at 1,150 nm originates from the singlet exciton population Normalized present in the films, while at 1-ns delay the PIA peak at 1,000 nm decay (a.u.) originates predominantly from the polaron population present.