Enhancement in Organic Photovoltaics Controlled by the Interplay between Charge-Transfer Excitons and Surface Plasmons.

In this work, we investigate plasmonic enhancement in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester organic photovoltaics (OPVs) by integrating shape- and size-controlled bimetallic gold core-silver shell nanocrystals (Au-Ag NCs) into the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate hole-transport layer. We observed that the best-performing Au-Ag NC-incorporated OPVs improved the power conversion efficiency by 9% via a broadband increase in photocurrent throughout the visible spectrum. Our experimental and computational results suggest that the observed photocurrent enhancement in plasmonic OPVs originates from both enhanced absorption and improved exciton dissociation and charge collection. This is particularly achieved by placing metal NCs near the interface of the active layer and hole-transport layer. The impedance spectroscopy results suggest that Au-Ag NCs reduce recombination and also increase the internal exciton to carrier efficiency by driving the dissociation of bound charge-transfer states to free carriers.

Introduction

Thin-film organic photovoltaics (OPVs) have emerged as a promising alternative to inorganic PVs due to inexpensive fabrication and processing techniques. Whereas a few low band gap polymers with competitive efficiencies are viable competitors in the solar market,[1−3] the power conversion efficiencies (PCEs) of OPVs remain low relative to market-leading inorganic PVs, such as Si, CdTe, or CuInGaSe2.[4] A number of factors impact the performance of the OPVs, including photon absorption, carrier generation, and carrier collection. The contribution from each of these factors can be understood from the external quantum efficiency (EQE) expression, which is the wavelength-dependent metric for photon to charge carrier efficiency. The expression for EQE is given by EQE(λ) = ηabs × ηgen × ηcoll, where ηabs is the ratio of absorbed light to incident light, ηgen is the ratio of photoexcited excitons that are converted to free carriers, and ηcoll is the carrier collection efficiency at the electrodes.[5−7] In the archetypal OPV, charge separation is facilitated by bulk heterojunction (BHJ) active layers, where the high interfacial area between the continuous veins of electron donor polymers and electron acceptor molecules provides the driving force necessary for the generation of free carriers via the separation of charge-transfer excitons.[5,8] High efficiencies in OPVs are ultimately limited by charge transport and recombination (ηcoll); however, locally improving the light absorption (ηabs) and the charge generation rate near the collection interface has the potential to improve PCE. Recent approaches to increasing PCE in OPVs have emphasized on improving ηgen via the modification of the BHJ architecture and the design of more efficient donor/acceptor systems,[9−14] as well as enhancing ηabs by designing organic–inorganic hybrid devices,[15] tandem junctions,[9,16] and incorporating metal nanocrystals (NCs) that support surface plasmons.[5,17,18] Augmentation of light harvesting in OPVs has been achieved by embedding colloidal metal NCs in the active layer[19−22] and the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) hole-transport layer.[5,23−26] Although metal-NC-enhanced OPVs have attracted significant interest and resulted in 5–50% increase in photocurrents,[26−29] the precise origin of plasmonic enhancement in OPVs has remained elusive and has been primarily correlated with improved absorption.

In this work, we demonstrate that BHJ OPVs integrated with gold core–silver shell NCs (Au–Ag NCs) show both increased absorption via light scattering and enhanced local electromagnetic fields, as well as electric field assisted exciton dissociation in the OPVs. The Au–Ag NC-incorporated OPVs show a 9% increase in efficiency relative to the reference devices from 2.85 to 3.1% for best devices. Our experimental and theoretical analyses support that the observed enhancements in photocurrent are collectively driven by increases in ηabs, ηgen, and ηcoll. The contribution from ηgen and ηcoll can be understood from the origin of Frenkel-type excitons in conjugated polymers. Photoexcitation of poly(3-hexylthiophene-2,5-diyl) (P3HT) gives rise to Frenkel excitons where the Coulomb binding energy restricts the electron–hole separation to ∼2 nm with binding energy in the range of 0.4–1.0 eV, which is above the threshold for dissociation.[30,31] The unique geometry-driven optical properties of Au–Ag NCs give rise to strong electric fields in the vicinity of the NCs, which extend to tens of nanometers. These strong electric fields improve the absorption (ηabs) near the P3HT:PCBM/PEDOT:PSS interface, as well as enhance the conversion of bound Frenkel-type charge-transfer excitons to free carriers, subsequently improving ηgen and the overall photocurrent. Our proposed mechanism is supported by recent studies which describe that nonequilibrium “hot” excitons at the donor/acceptor interface in BHJ architectures contribute strongly to carrier generation.[32,33] This motivates our work where we demonstrate that plasmonic enhancement in BHJ OPVs is catalyzed by the interplay between charge-transfer excitons and surface plasmons and is not simply a result of improved light absorption. We have combined optical and optoelectronic characterization to study and understand the performance Au–Ag NC-incorporated OPVs. Experimental observations are supplemented with a three-dimensional finite-difference time-domain (FDTD) model to support our hypothesis.

Results and Discussion

OPVs were fabricated by adapting a standard procedure from the literature;[18] details are available in the Materials and Methods section. Au–Ag NCs were embedded in the 70 nm thick PEDOT:PSS hole-transport layer of the OPVs. Embedding the NCs in the hole-transport layer minimizes any change in the morphology of the active layer and prevents the introduction of defects that can be detrimental to OPV performance.[35] The 140 nm thick active layer is composed of Poly(3-hexylthiophene-2,5-diyl):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The plasmon resonance of Au–Ag NCs spans the visible spectrum (Figure a), which is ideal for OPVs where a broadband response results in increased light capture. A representative transmission electron microscopy (TEM) image of Au–Ag NCs features two distinct bimetallic geometries (Figure b), nanocubes and nanopyramids. The origin of these shapes lies in the synthesis process and is described in detail in our previous work.[36] Although the synthesis results in two distinct geometries, this is advantageous in this work as the multiple geometries result in a broad spectral resonance. The plasmon mode at 530 nm is attributed to the dipole mode of the nanocubes, and that at 630 nm is contributed by the dipolar resonance of the nanopyramids.[37,38] The sharp corners and edges of Au–Ag NCs generate strong local fields that extend up to 30 nm from the surface of the Au–Ag NCs (see Figure S1). These intense fields are a result of charge accumulation at the corners and edges of the Au–Ag NCs, described as the lightning-rod effect, which is frequently observed in nonspherical nanostructures.[39−41]

Figure 1

(a) Representative extinction spectrum of Au–Ag NCs in aqueous media. (b) Representative TEM image of Au–Ag NCs showing both nanocubes and nanopyramids. (c) PCE as a function of Au–Ag NC concentration. The error bars represent 16 devices. (d) Current density vs potential (J–V) curves for OPVs at different Au–Ag NC particle densities where the numbers refer to the NCs/μm2.

The PCE of OPVs is enhanced by 9% upon integrating Au–Ag NCs in the PEDOT:PSS layer (Figure c). Our average device efficiency of reference OPVs was 2.58% (max 2.85%) and those of the best plasmon-enhanced devices were 2.80% (max 3.10%). The concentration of Au–Ag NCs was varied in the PEDOT:PSS solution before spin coating to determine the effect of NC density on PCE enhancement. The particle density was confirmed by examining the PEDOT:PSS layer by scanning electron microscopy (SEM) and counting the number of Au–Ag NCs per area (Figure S2). The effect of Au–Ag NC concentration on the fill factor, short-circuit current density (Jsc), and open-circuit voltage (Voc) is provided in Figure S3. The improvements in PCE were attributed to the increased current density in the presence of the Au–Ag NCs, which peaks at an area density of 0.25 NCs/μm2, as shown in the J–V curves in Figure d. The trend observed in PCE closely resembles the trend in Jsc (Figure S3b), which likely results from two primary effects: (1) increased overall light harvesting in the active layer (ηabs) and (2) increased exciton separation to collected carrier efficiency (ηgen + ηcoll). At high concentrations of Au–Ag NCs, the PCE is reduced due to the formation of nanoparticle aggregates that increase recombination losses. All device performance parameters are provided in Table S1.

To understand the origin of the photocurrent enhancement, we compared the EQE of a reference OPV (no NCs) to that with optimum Au–Ag NC concentration (Figure a). The average EQE enhancement of 10% agrees with the 9% enhancement found in PCE. The normalized EQE in Figure b suggests that the photocurrent enhancement in the presence of Au–Ag NCs is broadband throughout the visible region and does not show any spectral dependence overlapping with the Au–Ag NC plasmon resonance (Figure a). The broadband enhancement is attributed to the plasmon red-shift of Au–Ag NCs spanning the visible and near-infrared and spectral broadening in the presence of PEDOT:PSS (Figure S4). We measured the extinction of the plasmon-enhanced devices relative to the reference OPVs (Figure c) and observed that the two spectra overlapped to within 0.1% when integrated over the visible spectrum. This indicates that the absorption contribution from the nanoparticles when averaged over the entire cross section of the active layer is minimal; however, we find that in the presence of the Au–Ag NCs, the electric field preferentially increases near the PEDOT:PSS interface where charge collection occurs. Simulations (Figure ) clearly demonstrate that plasmonic enhancement of ∼35% is achieved localized at the P3HT:PCBM/PEDOT:PSS interface averaged over the bottom 10 nm volume of the active layer. The lack of enhancement in the experiments (Figure c) is because experimental extinction is bulk measurements accounting for the entire OPV. The spectrophotometer is unable to measure improved extinction at the interface. However, we note that the strong radiative properties of the Au–Ag NCs, attributed to both the size and the presence of the Ag layer, are the driving force for the observed enhancement in EQE and J–V curves.[37,39] To demonstrate this, we also incorporated Au nanocubes (Figure S5a) without any Ag layer into devices. Au nanocubes have narrow spectral resonance relative to Au–Ag NCs (Figure S5b) and are primarily absorbing rather than scattering (Figure S5c). Due to their low scattering properties, we observed a decrease in EQE relative to the reference (Figure S5d).

Figure 2

(a) EQE of both control and Au–Ag-enhanced OPVs. (b) Ratio of the EQE of the Au–Ag-enhanced OPV to the control OPV. (c) Experimentally observed extinction spectra for control and Au–Ag-enhanced OPVs.

Figure 3

(a) Cross section of the electromagnetic field enhancement in the hole-transport layer (PEDOT:PSS) and active layer (P3HT:PCBM) with an embedded Au–Ag nanocube. (b) Cross sections perpendicular to the k-vector of the top, middle, and bottom sections of the active layer. (c) Calculated absorbance spectra corresponding to the cross sections shown in (b) showing a 13% increase in absorbance at the top of the OPV, a 35% increase near the PEDOT:PSS/P3HT:PCBM interface, and a 37% decrease in absorbance in the middle of the OPVs.

To further understand the origin of plasmonic enhancement, FDTD simulations were performed to measure the spatially dependent electric field enhancement in the OPV (Figure ). To gain an understanding of the plasmonic radiative field strength of the Au–Ag NCs at different locations in the OPV active layer, E-field monitors were placed at the top, middle, and bottom of the P3HT:PCBM layer. These simulations demonstrate three different effects across the OPV. First, the simulations show that Au–Ag NCs have significant forward scattering into the active layer, with spatially dependent average electric field intensity up to 18× the incident field over the wavelength range of 400–700 nm. An ∼35% plasmonic enhancement is achieved in the OPVs localized within 10 nm of the P3HT:PCBM/PEDOT:PSS interface (Figure b,c bottom), which is within the electric field decay length of the Au–Ag NCs. The near field decay length of Au–Ag NCs is within 30 nm from their surface (Figure S1). Second, a 13% increase in absorption is observed at the top of the OPV (140 nm from the PEDOT:PSS interface) closer to the Al top electrode, which we attribute to backscattered light from the Al electrode increasing the light harvested by the OPVs. Third, in the middle of the OPVs (70 nm from the PEDOT:PSS interface), there is an ∼37% decrease in absorbance in the presence of the NCs. This decrease is because in the middle of the OPVs there is no effect from backscattered light from the Al top electrode and the electric fields from the Au–Ag NCs have already decayed. Therefore, light re-radiated from the NCs (or Al electrode) does not reach the middle of the OPV and instead we see parasitic effect that reduces the overall absorbance in the presence of the NCs. Collectively, the three effects correlate well with the experimental enhancement of 9% observed in the PCE and J–V curves (Figure c,d) and EQE (Figure a) from the plasmonic devices. But we anticipate that the increase in absorption at the interface of the active layer and hole-transport layer likely contributes to the increased photocurrent, as the carrier generation profile is strongly skewed toward the illumination side of the device. The simulated OPV with the Au−Ag nanopyramid embedded in the hole transport layer generates similar results and is shown in Figure S6. It has been proposed by Fung et al.[17] that increased roughness at the PEDOT:PSS/P3HT:PCBM interface due to incorporation of nanoparticles could increase the interfacial area for charge separation, thereby increasing PCE. However, atomic force microscopy (AFM) images (Figure S7) indicate that the average roughness at the interface is <1 nm in both reference and NC-incorporated samples, which likely does not contribute to performance enhancement. We note that the roughness measurement from AFM images occurs on a length scale (10’s of nanometer) that is orders of magnitude lower than the spacing between the NCs (microns) even at the highest NC concentration. At the highest concentration (∼0.4 NCs/μm2), with an average NC edge length of 80 nm, the bimetallic NCs cover a negligible fraction of the surface (0.26%). Therefore, the incorporation of Au–Ag NCs into the OPVs will have minimal effect on the overall roughness, interfacial area, and charge transport.

To further understand plasmonic enhancement in OPVs by Au–Ag NCs, we performed electrochemical impedance spectroscopy (EIS). Nyquist plots of reference OPVs and best-performing Au–Ag NC embedded OPVs at a particle concentration of 0.25 NCs/μm2 are shown in Figure a. The Nyquist plots are fit to an equivalent circuit model (Figure b), which has four components: series resistance (Rs), recombination resistance (Rr), contact capacitance (Cc), and Gerischer impedance (G). G represents diffusion of separated charges. The high-frequency region shown in the Nyquist plot is attributed to Rr and Cc owing to charge buildup between the P3HT:PCBM active layer and its adjacent contacts, including the PEDOT:PSS hole-transport layer and the LiF electron-transport layer. According to circuit model fits, Au–Ag NC integration did not significantly alter the Rs value relative to the reference OPVs but lowered Rr (24% decrease), Cc (25% decrease), and Gerischer impedance, G, (82% decrease). The fit parameters are included in Table S2. These results indicate that bimetallic NCs decrease carrier recombination and promote diffusion across the PEDOT:PSS layer, improving the charge generation and collection efficiency (ηgen + ηcoll). Bode phase plots were used to examine the behavior of charged species within the reference and plasmon-enhanced OPVs at different particle concentrations of Au–Ag NCs (Figure c). The Bode phase peak frequency decreases as the Au–Ag NC concentration increases. The phase represents the shift of the input phase; the frequency at the maximum phase shift of these OPVs is proportional to the inverse of the electron lifetime. Equation shows the relationship between the frequency at peak phase shift and carrier lifetime.[42]The effective lifetime of carriers, τeff, decreases from ∼4.0 to ∼2.5 μs with the addition of Au–Ag NCs (Figure d). We attribute the decrease in τeff to two distinct factors resulting from the presence of the Au–Ag NCs and the subsequent enhanced electric field: (1) increased carrier generation and (2) decreased carrier diffusion length. As is shown in Figure , the presence of Au–Ag NCs causes regions of intense electric field, which in turn creates carrier concentrations higher than those in reference cells, leading to increased probability for carrier collision, a phenomenon that has been previously demonstrated in the literature.[43] Additionally, because the electric field is distorted such that charge generation is concentrated near the P3HT:PCBM/PEDOT:PSS interface in the presence of Au–Ag NCs (Figure ), the required diffusion length to carrier collection is shortened, thus the free carrier lifetime decreases. This is in agreement with the decrease in Gerischer impedance in the presence of the NCs, which is representative of diffusion of separated charges across the hole-transport layer.

Figure 4

EIS of the OPVs. (a) Best-performing Au–Ag NC enhanced and reference Nyquist plots fitted to a circuit model shown in (b). (c) Bode phase plots of OPVs with increasing Au–Ag NC particle concentration. The numbers in brackets correspond to the number of NCs/μm2. (d) Carrier lifetimes of reference and Au–Ag NC enhanced OPVs calculated from the frequency at peak phase shift in the Bode plots in (c). Error bars represent the standard deviation of n = 3 measurements per sample.

On the basis of our experimental and simulated results, our hypothesis is that the strong electric fields generated by Au–Ag NCs collectively improve the absorption efficiency (ηabs), carrier generation efficiency (ηgen), and charge collection efficiency (ηcoll) in our system. Strong electric fields in conjugated polymers have been shown to be capable of providing sufficient driving force for exciton dissociation,[44−46] yet this concept has not been emphasized in plasmonic OPV systems. To further understand the origin of the enhanced performance of the Au–Ag NC-incorporated devices, we analyzed the photocurrents of the reference and the best plasmonic device (Figure ) as a function of the effective potential (Veff).[26]Figure shows a plot of the photocurrent density (Jph = Jlight – Jdark) at AM 1.5 illumination versus the effective voltage (Veff = V0 – Va), where V0 is the potential where Jph equals zero, and Va is the applied potential (Figure a). This plot was used to calculate the maximum exciton generation rate, Gmax, which is given by Jsat = qGmaxL, where Jsat is the saturated or maximum Jph that is obtained at higher potential, q is the electronic charge, and L is the active layer thickness. The Jsat values for our reference and plasmonic devices were 72 and 77 A m–2, respectively, and resulted in Gmax values of 3.21 × 1027 and 3.43 × 1027 m–3 s–1, respectively. As q and L are constant in both devices, this 6.4% improvement in Gmax from the reference to plasmonic OPVs indicates a direct enhancement in exciton generation in the active layer. We also calculated the exciton dissociation probability, P(E, T), which simply provides a measure of effective charge separation (Figure b). P(E, T) was calculated under the short-circuit condition (Va = 0), where Jph = qGmaxP(E, T)L. We calculated that the Jph values at Va = 0 were 67.7 and 74.5 mA cm–2 resulting in P(E, T) of 94 and 97%, for the reference and plasmonic devices, respectively, indicating that the presence of the NCs increased the dissociation of excitons into free carriers. We observe that Jph and P(E, T) are both higher for the plasmonic OPVs in the saturation current region (Veff > 0.25 V) where the electrical and thermal carriers play a role. But in the undersaturated region, both Jph and P(E, T) are higher for the reference OPVs. We attribute this phenomenon to decreased series resistance in the plasmonic devices at high bias in the dark (Figure S8), indicating that the NCs likely play a role in charge transport even in the absence of light when the plasmon is not excited. However, further study is required to fully elucidate the cause of this phenomenon. These P(E, T) values correlate well with those observed in the literature and support our initial hypothesis that the observed enhancements in photocurrent are collectively driven by increases in ηabs, ηgen, and ηcoll.[26,27]

Figure 5

(a) Photocurrent density vs effective potential for the reference and best plasmonic device. (b) Probability for exciton dissociation vs effective potential for the reference and best plasmonic device. The open-circuit (Voc) condition is shown in (a).

The presence of Au–Ag NCs in our BHJ OPVs not only improves the absorption (ηabs) near the P3HT:PCBM/PEDOT:PSS interface but also promotes dissociation of Frenkel-type charge-transfer excitons in P3HT. Upon photoexcitation of the P3HT:PCBM active layer, a tightly bound exciton with a diffusion length of ∼10 nm is formed.[7] In the reference OPVs, free carriers are formed when the exciton diffuses to the donor/acceptor interface and sufficient energy is available for charge separation. In the Au–Ag NC-incorporated OPVs, two effects simultaneously impact the performance: (1) Strong radiative enhancement—contributed by the intense near-fields and light scattering from the Au–Ag NCs localized within 10 nm of the interface of the active layer and hole-transport layer, as shown in our simulated results (Figure ). This 35% increase in absorbance increases the absorption efficiency (ηabs) concentrated near the interface of the active layer and hole-transport layer. (2) Enhanced exciton dissociation near a charge collection interface—the strong electric fields of Au–Ag NCs in the vicinity of the NCs increase the probability for the dissociation of charge-transfer excitons in P3HT, improving the photoexcited exciton to carrier generation efficiency (ηgen). These separated electrons and holes are in the proximity of the hole conductor, PEDOT:PSS, which facilitates efficient charge collection efficiency (ηcoll).

In summary, we have fabricated plasmon-enhanced P3HT:PCBM OPVs by embedding bimetallic Au core–Ag shell NCs in the PEDOT:PSS hole-transport layer. PCE improved by 9% relative to the reference at the optimized concentration of Au–Ag NCs. Our experimental and simulated results suggest that Au–Ag NCs result in broadband enhancement in the visible and near-infrared spectrum due to improved absorption and increased charge separation, where strong electric fields of Au–Ag NCs assist in separating bound Frenkel-type excitons. EIS measurements support our hypothesis that plasmonic enhancement in our system also results from increased internal exciton to carrier efficiency and increased charge collection, in addition to increased photon to carrier efficiency. We envision that by carefully controlling the interplay between surface plasmons and charge-transfer excitons, metal nanostructures can be designed to have dual roles—as light harvesters which will improve ηabs, and as a driving force for exciton dissociation and carrier collection which will improve ηgen and ηcoll. Energy up-conversion via plasmon–exciton coupling may play a role in increasing the absorption of low-energy light in conjugated polymers and could be potentially explored in the future. We anticipate that this work will provide design rules for the fabrication of plasmonic OPVs with improved efficiencies, reduced recombination, and better carrier collection efficiency across the charge-transport layers. These enhancements controlled by electric fields of shape- and size-controlled plasmonic nanostructures will ultimately enable thin-film, flexible devices viable for large-scale manufacture and integration.

Materials and Methods

All chemicals were purchased from Sigma-Aldrich unless noted otherwise; all water was nanopure from a Milli-Q Direct-Q 3UV system. All work conducted under inert atmosphere was conducted in a nitrogen environment in an M-Braun LabStar glovebox (>0.5 ppm O2). Thermal evaporation was conducted on an Angstrom Amod system.

NC Synthesis

Bimetallic NCs were synthesized by a modified procedure in our previous work[47] and consisted of three steps: seed growth, Au nanocube growth, and Ag capping layer growth.

Seed Solution

A solution of 2.75 mL water, 7.5 mL 100 mM cetyltrimethylammonium bromide (CTAB), and 250 μL 10 mM HAuCl4 were added to a 10 mL glass vial, mixed by inversion, and placed in a 35 °C water bath for temperature equilibration. After 10 min, 600 μL of ice cold, freshly prepared 10 mM NaBH4 solution was injected with vigorous stirring, followed by 1 min of stirring before being returned to the 35 °C water bath for 1 h undisturbed.

Au Nanocube Growth

The seed solution was removed from the water bath and reduced 10× with water. Next, in a 50 mL vial, 6.4 mL 100 mM CTAB, 800 μL 10 mM HAuCl4, 3.8 mL 100 mM ascorbic acid, and 20 μL diluted seed solution were added and mixed by inversion. The vial was then placed in a 35 °C water bath and allowed to sit undisturbed for 5 h.

Ag Capping Layer Growth

Au nanocubes (7.5 mL) were centrifuged at 1100 rcf for 15 min, the supernatant removed and the pellet redispersed in 3.75 mL of 20 mM hexadecyl trimethylammonium chloride (CTAC) and allowed to sit for 15 min. The centrifugation and redispersal step was repeated two additional times; however, on the final redispersal, the pellet was combined with 500 μL water, rather than a CTAC solution. Next, 20 mL 20 mM CTAC, 800 μL concentrated Au nanocubes from the previous step, and 200 μL 10 mM KBr were combined in a 50 mL vial and placed in a 65 °C water bath. After 10 min, the vial was removed from the water bath and mixed by inversion with 220 μL of 10 mM AgNO3 and 600 μL 100 mM ascorbic acid. Finally, the vial was placed in a 65 °C water bath for 2 h. The colloid was stable for up to 60 days in the dark at 2 °C.

Device Fabrication

ITO glass (150 nm, <10 Å RMS roughness) was purchased from Thin Film Devices Inc. Phenyl-C61-butyric acid (PCBM) was purchased from Rieke Materials, and P3HT was from Sigma-Aldrich (698989). OPV fabrication was adapted from a procedure in the literature.[48] One inch ITO glass squares were cleaned with acetone and isopropanol and dried with nitrogen, followed by air plasma treatment for 10 min to remove any organic residue. Next, a solution of PEDOT:PSS was filtered through a 0.45 μm PVDF filter and concentrated Au–Ag NCs were spin coated onto the cleaned substrates at 1500 rpm for 60 seconds and dried at 120 °C for 10 min, then transferred to an inert environment. Under inert atmosphere, 20 mg P3HT and 16 mg PCBM were combined with 720 μL dichlorobenzene and stirred at 125 °C for 30 min, then allowed to be stirred at room temperature overnight. A 100 μL drop of P3HT:PCBM solution was spin cast onto the PEDOT:PSS coated substrates at 500 rpm for 5 s and 2000 rpm for 13 s, followed by immediate transfer to a dichlorobenzene environment until the film transitioned from bright orange to dark red/purple. This solvent annealing step was essential for the formation of the BHJ and dictates the charge separation and charge-transfer efficiency. The substrate was kept in an inert environment and transferred to a vacuum evaporation chamber where 1 nm LiF and 100 nm Al were deposited in 8 mm diameter circles via shadow masks.

Material Characterization

All optical spectroscopic measurements were carried out with a Varian Cary 5000 ultraviolet–visible spectrophotometer. SEM and TEM were conducted using a Zeiss Merlin and FEI Osiris, respectively. A Veeco Dektak 150 profilometer was used for all layer thickness measurements. AFM was performed on a Bruker Dimension Icon AFM.

Photoelectrical Characterization

All current–potential scans were conducted under AM 1.5G 100 mW/cm2 illumination at a 10 mV/s scan rate using a MetroOhm Autolab potentiostat. EIS was conducted with an amplitude of 10 mV over a frequency range of 103−106 Hz. Quantum efficiency measurements were conducted on a homemade setup using a Fianum supercontinuum laser, a Thorlabs PM100D power meter, and a Keithley 2600 sourcemeter. To block any additional current via lateral charge transport, shadow masks were used.

Electrodynamic Modeling

All electrodynamic modeling was carried out in Lumerical FDTD Solutions software with a simulation time of 500 fs and a spectral range of 400–700 nm. The solar cell stack of ITO/PEDOT:PSS/P3HT:PCBM/Al had thicknesses of 100/70/140/100 nm, respectively. All metal/dielectric interfaces were contained in a 0.7 nm mesh grid. Dielectric parameters for all layers are available in the literature.[49,50]Figure S1 was generated by taking a slice from the center of the nanoparticles in the x, y, and z planes and by averaging the enhancement of field intensity as a function of distance from the edge of the particle.