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Insight into the efficiency enhancement of polymer solar cells by incorporating gold nanoparticles
Solar Energy Materials & Solar Cells 111 (2013) 1–8
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Insight into the efficiency enhancement of polymer solar cells by incorporating gold nanoparticles Xiaoqiang Chen, Lijian Zuo, Weifei Fu, Quanxiang Yan, Congcheng Fan, Hongzheng Chen n State Key Lab of Silicon Materials, MOE Key Laboratory of Macromolecular Synthesis and Functionalization, & Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 1 October 2012 Received in revised form 11 December 2012 Accepted 14 December 2012
Photovoltaic performances of polymer bulk-heterojunction solar cells (PSCs) with various sized (20, 35, 50, and 75 nm) Au nanoparticles (NPs) incorporated on indium tin oxide (ITO)-coated glass substrates are investigated in detail, wherein poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend serves as active layer and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as anode buffer layer on ITO. The optical and electrical properties of these devices incorporated with Au NPs with different space distributions in the interface of PEDOT:PSS buffer layer and P3HT:PCBM active layer are investigated. We find that, the optical property is improved as the Au NPs are large enough to penetrate into the active layer, while the performance of PSCs with small Au NPs can only benefit from the improved hole collection efficiency. Meanwhile, the exciton dissociation efficiency reduces remarkably as increasing the size of Au NPs. Finally, we demonstrated a maximum power conversion efficiency (PCE) improvement of 23% in the PSCs by incorporating 35 nm Au NPs. & 2012 Elsevier B.V. All rights reserved.
Keywords: Au nanoparticles P3HT:PCBM Polymer solar cells Optical property Electrical property
1. Introduction Polymer-fullerene-based bulk heterojunction (BHJ) solar cells are the highly promising candidates for photovoltaic devices because of their properties including simple fabrication procedure, low-cost materials, physical flexibility and semi-transparency [1–7]. Currently the power conversion efficiency (PCE) of these BHJ polymer solar cells (PSCs) has reached as high as 7–9% [8–10]. However, compared with inorganic solar cells, PSC has one important hindrance for further efficiency improvement: the useful thin active layer for efficient light absorption is limited by the short exciton diffusion length and the low carrier mobility [11,12]. As a result, enormous efforts are being made in the development impactful light-trapping techniques [13–15]. Among the various light-trapping techniques, one promising method is the incorporation of localized surface plasmon resonant (LSPR) metallic (e.g., Au, Ag, Cu) nanoparticles (NPs) into the BHJ PSCs. So far, many studies have shown the PCE improvement of PSCs by incorporating metallic NPs in either the buffer layer such as poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) or the active layer [16–25]. Recently, a high PCE of 8.79% (improved from 7.59%) was reported by utilizing both a metallic nanograting electrode as the back reflector and metallic NPs embedded in the active layer [9].
n
Corresponding author. E-mail address: [email protected] (H. Chen).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.12.016
Although many reports on the metallic NPs enhanced PSCs the exact understanding on the photovoltaic performance changes remains inconclusive. On one hand, several studies have shown that the absorption of photoactive conjugated polymer in the active layer can be enhanced due to the plasmonic scattering or near-field enhancement caused by LSPR effect of the metallic NPs (mainly enhance the optical property) [26,27]. On the other hand, Choy et al. indicated that the strong near field around Au NPs due to LSPR effect mainly distributes laterally along the PEDOT:PSS layer, leading to minimal enhancement of light absorption in the active layer. Thus they contributed to the improvement of PCE to the enlarged interfacial area between active layer and PEDOT:PSS buffer layer as well as the improved PEDOT:PSS conductivity (mainly improve the electrical property) [28]. In these studies, they all investigated the effect of metallic NPs with a fixed size, however, there are distinct optical properties and space distributions in the PEDOT:PSS layer for various sized metallic NPs. Consequently, the investigation of the performance changes of PSCs with various sized metallic NPs is highly important and desirable to better understand the physics within the ‘plasmonic’ PSCs. Herein we study the comparative performances of poly (3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) BHJ PSCs incorporated various sized Au NPs (20 nm, 35 nm, 50 nm and 75 nm) on ITO-coated glass substrates. We investigate how the optical and electrical properties of these devices perform when Au NPs with different space distributions in the interface of PEDOT:PSS buffer layer and P3HT:PCBM active
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layer. At an optimized Au NPs size of 35 nm, a maximum PCE improvement of 23% (from 2.37% to 2.91%) was demonstrated.
2. Experimental 2.1. Synthesis of Au NPs The various sized Au NPs were prepared by a hydroquinonereduction seed-growth method reported previously [29]. Briefly, a 15 nm diameter Au seeds solution was prepared according to the protocol by Frens [30], resulting in a colloidal suspension containing [Au]¼1.5 mM. Then a 99 mL ‘‘growth solution’’ containing 3 mL 10 mM centrifuged HAuCl4 solution, 250 mL 1% w/w sodium citrate solution and a suitable amount of Au seeds solution (ranging from 11.5 mL to 0.16 mL, which yield particle sizes of 20 nm up to 75 nm) was prepared in a 250 mL flask. At last, 1 mL 30 mM hydroquinone was injected into the growth solution under vigorous stirring. These reactions completed within 1 h. In order to cap the Au NPs with poly-(vinylpyrrolidone) (PVP, MW¼10,000, Aldrich), we added 3 mL 100 mM PVP solution to the reaction solution, and stirring for 24 h. Finally, all solutions were centrifuged to eliminate the excess ligands and redissolved in 1 mL of deionized water to avoid the aggregation of Au NPs. 2.2. Device fabrication The ITO-coated glass substrates were ultrasonicated in detergent, acetone and isopropyl alcohol, dried with nitrogen, and treated by ozone–ultraviolet cleaner for 15 min. The same volume of various sized Au NPs aqueous solutions were spin-coated at 2000 rpm onto ITO-coated glass substrates, subsequently annealed at 140 1C for 10 min in air, and treated by ozone–ultraviolet cleaner for another 10 min (which can promote the spread of PEDOT:PSS solution). For the control device under the same conditions, we spin-coated the same volume of pure deionized water on ITO, and denoted as 0 nm Au NPs. The PEDOT:PSS solution (Baytron P 4083) were next spincoated at 4000 rpm on the top of the layer of Au NPs and were subsequently annealed at 140 1C for 15 min in air, resulting in a thickness of 30 nm layer. The mixed solutions consisting of P3HT (10 mg mL 1, Aldrich) and PCBM (9 mg mL 1, Aldrich) in 1,2dichlorobenzene were then spin-coated at 500 rpm on the PEDOT:PSS layer in air. The thickness of the active layer is 150 nm. After spin-coating photoactive layer, Al (100 nm) was thermal evaporated under high vacuum of 4 10 4 Pa with a rate of 0.2 nm/s onto the polymer layer as a cathode to create a device with an active area of 9 mm2 defined by a shadow mask. The final device structure is ITO/Au NPs/PEDOT:PSS/P3HT:PCBM/Al, as shown in Fig. 1a. Finally, thermal annealing was performed by directly placing the device on a hotplate at 150 1C for 10 min in an inert glove-box. It should be noted that the device fabrication conditions were optimized prior to this investigation. 2.3. Device characterization The transmission electron microscope (TEM) images of Au NPs were measured using a JEM-1230EX TEM. The ultraviolet–visible (UV–vis) absorption spectra of solutions and films on ITO substrates were measured on a CARY 100 Bio UV–vis spectrophotometer. For the absorption tests of P3HT:PCBM films on ITO/Au NPs/ PEDOT:PSS, we used the corresponding ITO/Au NPs/PEDOT:PSS sample as the reference, which can eliminate the absorption of ITO/ Au NPs/PEDOT:PSS. The photoluminescent (PL) spectra of P3HT/PCBM films were measured on a Hitachi F-4500 spectrophotometer (excited at 470 nm). The current density–voltage (J–V) characteristics of devices under dark and illumination (100 mW/cm2) were measured
with a Keithley 2400 measurement source unit at room temperature in air. The light intensity for the solar simulator was calibrated with a standard photovoltaic (PV) reference cell. Incident photon to electron conversion efficiency (IPCE) curves were measured with a Stanford lock-in amplifier 8300 unit. Atomic force microscopy (AFM) image was measured by Veeco MultiMode scanning probe microscope operated in tapping mode in air. The film thickness was measured by an AMBIOS XP-1 high-resolution surface profiler.
3. Results and discussion 3.1. Photovoltaic characteristics of devices with various sized Au NPs The current density–voltage (J–V) characteristics of OPV devices incorporated with various sized Au NPs (20 nm, 35 nm, 50 nm and 75 nm) in the PEDOT:PSS anode buffer layer are shown in Fig. 1b. It should be noted that all devices with all sized Au NPs were the ones with optimized Au NPs coverage density. The device photovoltaic performances are summarized in Table 1. It is observed that PCE of our PSCs is improved by 23% from 2.37% (0 nm) to 2.91% (35 nm), as increasing the size of Au NPs from 0 to 35 nm. Further increasing the size of Au NPs, the PCE values drop to 2.49% (50 nm) and 2.04% (75 nm). The improvement of PCE in the devices with small Au NPs is mainly attributed to the increase in fill factor (FF) and short-circuit current (JSC), from 0.5 to 0.52 and 7.59 mA cm 2 to 9.05 mA cm 2, respectively. From the dark J–V curves in Fig. 1c, we can observe that the series resistance (Rs) of the devices, obtained from the inverse slope of the dark J–V curves at a voltage of 0.5 V, slightly decreases from 5.2 O cm2 (0 nm) to 4.94 O cm2 (20 nm), and then increase to 6.76 O cm2 (75 nm) as increasing the size of Au NPs. Fig. 1d shows the IPCE characteristics of these devices, we can obtain the highest IPCE of 70% at 500 nm in the device with 35 nm Au NPs, with a 19% improvement to the control device (0 nm). Further increasing the size of Au NPs, the peak IPCE detrimentally decreases to 60% at 500 nm in the device with 75 nm Au NPs. This is in good agreement with the trend of JSC. We also do some preliminary investigation on the stability of the OPVs with and without Au NPs, and no obvious changes of the stability are observed after incorporating Au NPs. In the following sections, we will study the optical and electrical properties of our PSCs, which describes the underlying physics of the ‘plasmonic’ PSCs in a better way. 3.2. LSPR effect of various sized Au NPs on photovoltaic performance Fig. 2a–d shows TEM images of various sized Au NPs used in our PSCs, it is observed that the average size of these Au NPs is approximately 20 nm, 35 nm, 50 nm, and 75 nm, respectively. The absorption spectra of these Au NPs in water are shown in Fig. 2e. Corresponding to the excitation of LSPR, the absorption peaks for these Au NPs are at 523 nm, 530 nm, 536 nm, and 549 nm respectively, which all match well with the absorption of the P3HT:PCBM film. In order to verify the degree of contributions of the LSPR effects, we measured the absorption spectra of P3HT:PCBM active layers on top of ITO/Au NPs/PEDOT:PSS with various sized Au NPs, as shown in Fig. 3a. During the measurement of light absorption, we have eliminated the absorption of ITO/Au NPs/PEDOT:PSS as it absorbs light. So the absorption spectra represent light absorbance within the active layer only. Interestingly, the devices with smaller Au NPs (20 nm and 35 nm) have no significant difference in absorption compared with the control device (0 nm) while there are obvious absorption enhancements in the devices with larger Au NPs (50 nm and 75 nm). It has been reported that when Au NPs are incorporated into the PEDOT:PSS layer, the strong
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Fig. 1. (a) Device architecture of the OPV incorporating various sized Au NPs at the ITO/PEDOT:PSS interface. (b) J–V characteristics of the OPV devices with various sized Au NPs (0 nm, 20 nm, 35 nm, 50 nm and 75 nm) at the ITO/PEDOT:PSS interface, recorded under illumination at 100 mW/cm2 (AM 1.5 G). (c) Dark J–V characteristics of these OPV devices. (d) Corresponding IPCE curves of these OPV devices.
Table 1 Photovoltaic performances of polymer solar cells (ITO/Au NPs/PEDOT:PSS/P3HT– PCBM/Al) with various sized Au NPs under AM 1.5 G illumination (100 mW/cm2). NP size (nm)
VOC (V)
Jsc (mA cm 2)
FF (%)
PCE (%)
Rsa (O cm2)
0 20 35 50 75
0.62 7 0.01 0.63 7 0.01 0.62 7 0.01 0.62 7 0.02 0.62 7 0.02
7.59 7 0.13 8.23 7 0.15 9.057 0.19 8.78 7 0.25 8.027 0.19
50.17 0.8 53.1 7 1.2 51.8 7 1.0 45.7 7 1.1 41.2 7 1.7
2.37 70.06 2.75 70.11 2.91 70.15 2.49 70.14 2.04 70.17
5.20 7 0.15 4.94 7 0.16 5.16 7 0.19 5.95 7 0.26 6.76 7 0.33
a Device series resistance (Rs) of the OPV obtained from the inverse slope of the dark J–V curve at a voltage of 0.5 V.
LSPR near-field around Au NPs mainly distributes laterally along the PEDOT:PSS layer rather than vertically into the adjacent active layer, leading to minimal optical enhancement of active layer [28]. In our devices, according to the space distribution of Au NPs, the change of light absorption can be divided into three cases: (1) when the size of Au NPs is smaller than the thickness of PEDOT:PSS layer (the devices with 20 nm Au NPs), the Au NPs would be completely covered by PEDOT:PSS and only small bumps (lower than 1 nm) can be left on the surface of PEDOT:PSS, as shown in the AFM 3D height images (Fig. 4g). In this situation, no absorption changes are expected; (2) as the size of Au NPs is almost equal to the thickness of PEDOT:PSS layer (the devices with 35 nm Au NPs), larger bumps ( 5 nm) exhibit on the surface of PEDOT:PSS (see Fig. 4h), indicating that Au NPs will penetrate into the active layer, thus the laterally distributed LSPR of Au NPs can affect the light absorption of the active layer, leading to a slight absorption improvement of the P3HT:PCBM active layer; (3) once the size of Au NPs is much larger than the thickness of
PEDOT:PSS layer (the devices with 50 nm and 75 nm Au NPs), Au NPs would protrude the PEDOT:PSS surface (as shown in Fig. 4i and j), and then penetrate into adjacent P3HT/PCBM active layer, resulting in obvious absorption enhancement of the active layer of P3HT:PCBM. Even though there are no significant absorption changes of the active layer in the devices with 20 nm and 35 nm Au NPs, IPCE of these devices enhances dramatically. Meanwhile, the absorption enhancements of the active layer in the devices with 50 nm and 75 nm Au NPs also mismatch the IPCE improvements of these devices. The evident discrepancy between optical properties and IPCE can be interpreted by the fact that the magnitudes of IPCE not only depend on the optical properties, but also by electrical properties such as exciton dissociation rates, charge transport rates and charge collection efficiencies [31]. Therefore, apart from LSPR effects, the effects of various sized Au NPs on electrical properties of our PSCs must be investigated to understand the origin of performance improvement. 3.3. Effects of various sized Au NPs on PEDOT:PSS layer The distributions of various sized Au NPs in PEDOT:PSS layer are investigated by atomic force microscopy (AFM) 2D height images on the ITO/Au NPs/PEDOT:PSS films with various sized Au NPs, as shown in Fig. 4a–e. It is clear that the Au NPs with various sizes are all well dispersed in the PEDOT:PSS layer, and there are no obvious aggregation in the films. The good dispersibility can be attributed to the PVP capping layer around the surface of Au NPs [32]. Meanwhile, AFM 3D images (Fig. 4f–j) clearly exhibit the morphology changes of PEDOT:PSS film caused by various sized Au NPs. We observe a dramatic roughness increase as the size of Au NPs increases. The mean roughness (RMS) for these devices are 1.25 nm (0 nm),
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Absorbance (a.u.)
0.6 0 20nm 35nm 50nm 75nm
0.4
0.2
0.0
400
500
600
700
800
Wavelength (nm)
PL intensity (a.u.)
4000 0 20nm 35nm 50nm 75nm
3000 2000 1000 0 550
600
650 700 750 Wavelength (nm)
800
850
Fig. 3. (a) Optical absorption spectra and (b) PL spectral of P3HT/PCBM thin film for ITO/Au NPs/PEDOT:PSS/P3HT:PCBM structures with various sized Au NPs.
Fig. 2. (a–d) TEM images of various for (a)–(d) are about 20 nm, 35 nm, absorption spectra of these Au NPs absorption maxima, which are respectively.
sized Au NPs. The average nanoparticle sizes 50 nm and 75 nm. (e) Corresponding UV–vis in water, all spectra are normalized at their 523 nm, 530 nm, 536 nm, and 549 nm,
1.78 nm (20 nm), 2.18 nm (35 nm), 4.89 nm (50 nm) and 9.44 nm (75 nm), respectively. It has been investigated that the increase of anode surface roughness can enlarge the interface area between the anode and the active layer, allowing a relatively short route for holes to reach the anode and hence increasing the holes collection efficiency [33,34]. Thus, in our PSCs, it is expected that the incorporation of large Au NPs will induce an increase of interfacial area between PEDOT:PSS and P3HT:PCBM, leading to an improvement of holes collection efficiency at the anode and, thus, the JSC of our devices. It is the reason why our devices with 20 nm and 35 nm Au NPs can achieve improved JSC (8.23 mA cm 2 and 9.05 mA cm 2) despite there are no significant absorption enhancement in these devices. In the devices with 50 nm and 75 nm Au NPs, where a dual mechanisms combining enhancement of both hole collection and light absorbance of the active layer by the LSPR effects, further enhancement in the JSC should be expected. Unfortunately, the decreased JSC of 8.78 mA cm 2 (50 nm) and 8.02 mA cm 2 (75 nm) is obtained compared to the maximum JSC (9.05 mA cm 2 for 35 nm Au NPs). The possible reason is on other electrical properties degradation due to morphology changes of the active layer, which we will investigate in the next section. Besides, the increased interfacial area between PEDOT:PSS and P3HT:PCBM also reduces the mean distance between generated
holes and PEDOT:PSS surface, which can diminish the dependence of holes on the external electric field for collection at the anode [33,34]. It is a positive effect to improve FF in our devices. Meanwhile, the series resistances of the devices with 20 nm and 35 nm Au NPs is smaller than the one of the control device, which are favorable to the FF; while the series resistances of the devices with 50 nm and 75 nm Au NPs is much larger than the one of the control device, resulting in the reduce of FF [35]. Therefore, the combination of these effects determines the trend in FF variation as shown in Table 1: the magnitudes of FF in our devices increase from 0.50 (control, 0 nm) to 0.53 (20 nm) and 0.52 (35 nm), then decrease to 0.46 (50 nm) and 0.41 (75 nm) as the size of Au NPs increases. These phenomena indicate that the negative effect of increased series resistance outweigh the positive effect of reduced mean distance between generated holes and PEODT:PSS surface, resulting in the reduction of FF as further increasing the size of Au NPs from 35 nm to 75 nm. 3.4. Effects of various sized Au NPs on exciton quenching As described in the above section, an unexpected degradation of JSC and FF in our devices occurs when further increasing the size of Au NPs to 50 nm and 75 nm. To investigate this result, we measured the photoluminescence (PL) spectra of ITO/Au NPs/ PEDOT:PSS/P3HT:PCBM films with various sized Au NPs using a 470 nm excitation source (see Fig. 3b). At the same time, the morphology changes of P3HT:PCBM films on ITO/Au NPs/PEDOT:PSS layer with various sized Au NPs are also investigated through AFM height images, as shown in Fig. 4. In PL spectra, we observe the PL intensity enhancement upon increasing the size of Au NPs, with a maximum increase by 39% at 646 nm. According to the previous studies, PL intensity changes can be attributed to three main reasons: changes in light absorption, affecting the light excitation rate [26,36]; exciton quenching at
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Fig. 4. AFM height images for ITO/Au NPs/PEDOT:PSS films with various sized Au NPs: (a, f) 0 nm; (b, g) 20 nm; (c, h) 35 nm; (d, i) 50 nm; and (e, j) 75 nm. The left column is 2D height images while the right one is corresponding 3D height images. The corresponding mean roughnesses (RMS) of these films are 1.25 nm, 1.78 nm, 2.18 nm, 4.89 nm, and 9.44 nm, respectively.
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metal/organic interfaces [37,38]; and exciton quenching at donor/ acceptor (D/A) interfaces [39,40]. For the first reason, the PL intensity in the devices with 50 nm and 75 nm Au NPs can be improved while the ones in the devices with 20 nm and 35 nm Au NPs should keep the same, as we have previously shown experimentally that only the light absorption in our devices with 50 nm and 75 nm Au NPs increase significantly. But the extent of absorption improvements (5% for 50 nm and 7% for 75 nm) are much less than the one of PL intensity enhancements (25% for 50 nm and 39% for 75 nm) in the devices with 50 nm and 75 nm Au NPs. Therefore, it makes minor contribution to the PL intensity changes in the devices with 50 nm and 75 nm Au NPs. For the second reason, the possibility of exciton quenching at metal/organic interfaces can be eliminated, as previous studies show that Au NPs with an insulating protecting layer can prevent direct contact from organic layer, thus avoiding exciton quenching [41]. In this work, Au NPs are protected by a layer of PVP, which can prevent Au NPs from direct contact with the organic layer. To make sure that the PVP itself does not have any effect on device performance, we compared the performance of the devices with and without PVP (2 mg mL 1) dissolved into the PEDOT:PSS layer and observed no obvious difference in the performance. If there is exciton quenching at Au NPs/organic interface in our devices, a decrease trend of PL intensity in the device as increasing the size of Au NPs should be expected, due to the increased contact between Au NPs and organic layer (see Fig. 4). However, experimental results show an increase trend of PL intensity in these devices. So the effect of exciton quenching by Au NPs can be eliminated. Therefore, the third reason, reduced exciton quenching at D/A junctions, might be the main contribution to the increased PL intensity of the P3HT:PCBM films. The exciton quenching at D/A junctions is mainly influenced by the D/A interface area which determined by the morphology of the active layer [42,43]. In the devices with smaller Au NPs (20 nm and 35 nm), the incorporated Au NPs are covered by PEDOT:PSS layer, only the internal networking of the subsequently spin-coated P3HT:PCBM films near PEDOT:PSS side could have some change, while no
significant change can be observed in the AFM images of the top surface morphology of P3HT:PCBM films (comparing Fig. 5b and c to a). So the PL intensity of these P3HT:PCBM films just increase by about 10%. However, in the devices with larger Au NPs, the Au NPs penetrate into the P3HT:PCBM films, resulting in the change of the whole morphology of P3HT:PCBM films (the domains become larger), as shown in Fig. 5d and f. These changes of the nanoscale morphology, combined with the penetrated Au NPs, lead to a remarkable reduce in the D/A interface area and, thus, the reduced exciton quenching at D/A junctions. As a result, the PL intensity of these P3HT:PCBM films increases by 25% (50 nm) and 39% (75 nm). Therefore, we conclude that the reduced D/A interface area by incorporating Au NPs mainly affects the PL intensity, while the improved light absorption in the larger Au NPs devices can also make minor contribution to the increase of PL intensity. For the device performances, the reduced D/A junction area implies a reduction in exciton dissociation efficiency, leading to a decrease trend of JSC in our devices as the size of Au NPs increases. On the other hand, the previous investigated hole collection efficiency improvement and light absorption enhancement can result in an increase trend of JSC in our devices as the size of Au NPs increases. The JSC change depends on the competition between these two trends. The increase of the size of Au NPs causes the increase of the surface roughness of PEDOT:PSS and the PEDOT:PSS/P3HT:PCBM interfacial areas which can collect more holes at the anode, and also improves the contact of Au NPs to P3HT:PCBM layer which increase the light absorption of the P3HT:PCBM layer, leading to the improvement in JSC. Meanwhile, the morphology of P3HT:PCBM films also changed, resulting in reduced D/A interface area which decrease the exciton dissociation efficiency. Increasing the size of Au NPs from 0 nm to 35 nm, the positive effect play the dominating role, resulting in an increase of JSC; while further increasing towards 75 nm, the negative effect outweighs the positive effect, resulting in a reduction trend of JSC, as shown in Table 1. Combining the trend of JSC and previously investigated trend of FF, we obtain a peak
Fig. 5. AFM 3D height images for P3HT/PCBM active layer on ITO/Au NPs/PEDOT:PSS with various sized Au NPs: (a) 0 nm; (b) 20 nm; (c) 35 nm; (d) 50 nm; and (e) 75 nm. The corresponding mean roughnesses (RMS) of these films are 0.72 nm, 0.79 nm, 0.95 nm, 1.27 nm, and 1.86 nm, respectively.
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PCE of 2.91% for the device with 35 nm Au NPs, with a maximum improvement of 23%.
4. Conclusions In this study, we incorporated various sized Au NPs (20 nm, 35 nm, 50 nm and 75 nm) in the PEDOT:PSS anode buffer layer to enhance the performance of PSCs. The optical and electrical properties of PSCs with Au NPs have been investigated. On one hand, due to the lateral distribution of strong LSPR near-field, the smaller sized Au NPs do not enhance the light absorption, while the larger Au NPs can penetrate into P3HT:PCBM active layer and obviously enhance the light absorption. On the other hand, as the size of Au NPs increased, the roughness of PEDOT:PSS increased, enlarging the interface areas between PEDOT:PSS and P3HT:PCBM, and resulting in the collection of more holes at the anode; meanwhile, the morphology of active layer would change to reduce the D/A interface area, leading to reduction of exciton quenching. The above dual size effects of the Au NPs compete to determine the trend of JSC in our devices. Besides, the trend of FF in our device was originated from competition between the negative effect of increased series resistance and the positive effect of reduced mean distance between generated holes and PEODT:PSS surface as increasing the size of Au NPs. Considering the trend of JSC and FF together, we achieved a maximum improvement of 23% at the device with 35 nm Au NPs. This work contributes to better understanding of the use of Au NPs for enhancing PSC performances.
Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) (No. 50990063) and the Danish National Research Foundation and the NSFC (Grant no. 51011130028) for the Danish–Chinese Center for Organic based Photovoltaic Cells.
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Contents lists available at SciVerse ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Insight into the efficiency enhancement of polymer solar cells by incorporating gold nanoparticles Xiaoqiang Chen, Lijian Zuo, Weifei Fu, Quanxiang Yan, Congcheng Fan, Hongzheng Chen n State Key Lab of Silicon Materials, MOE Key Laboratory of Macromolecular Synthesis and Functionalization, & Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
a r t i c l e i n f o
abstract
Article history: Received 1 October 2012 Received in revised form 11 December 2012 Accepted 14 December 2012
Photovoltaic performances of polymer bulk-heterojunction solar cells (PSCs) with various sized (20, 35, 50, and 75 nm) Au nanoparticles (NPs) incorporated on indium tin oxide (ITO)-coated glass substrates are investigated in detail, wherein poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend serves as active layer and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as anode buffer layer on ITO. The optical and electrical properties of these devices incorporated with Au NPs with different space distributions in the interface of PEDOT:PSS buffer layer and P3HT:PCBM active layer are investigated. We find that, the optical property is improved as the Au NPs are large enough to penetrate into the active layer, while the performance of PSCs with small Au NPs can only benefit from the improved hole collection efficiency. Meanwhile, the exciton dissociation efficiency reduces remarkably as increasing the size of Au NPs. Finally, we demonstrated a maximum power conversion efficiency (PCE) improvement of 23% in the PSCs by incorporating 35 nm Au NPs. & 2012 Elsevier B.V. All rights reserved.
Keywords: Au nanoparticles P3HT:PCBM Polymer solar cells Optical property Electrical property
1. Introduction Polymer-fullerene-based bulk heterojunction (BHJ) solar cells are the highly promising candidates for photovoltaic devices because of their properties including simple fabrication procedure, low-cost materials, physical flexibility and semi-transparency [1–7]. Currently the power conversion efficiency (PCE) of these BHJ polymer solar cells (PSCs) has reached as high as 7–9% [8–10]. However, compared with inorganic solar cells, PSC has one important hindrance for further efficiency improvement: the useful thin active layer for efficient light absorption is limited by the short exciton diffusion length and the low carrier mobility [11,12]. As a result, enormous efforts are being made in the development impactful light-trapping techniques [13–15]. Among the various light-trapping techniques, one promising method is the incorporation of localized surface plasmon resonant (LSPR) metallic (e.g., Au, Ag, Cu) nanoparticles (NPs) into the BHJ PSCs. So far, many studies have shown the PCE improvement of PSCs by incorporating metallic NPs in either the buffer layer such as poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) or the active layer [16–25]. Recently, a high PCE of 8.79% (improved from 7.59%) was reported by utilizing both a metallic nanograting electrode as the back reflector and metallic NPs embedded in the active layer [9].
n
Corresponding author. E-mail address: [email protected] (H. Chen).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.12.016
Although many reports on the metallic NPs enhanced PSCs the exact understanding on the photovoltaic performance changes remains inconclusive. On one hand, several studies have shown that the absorption of photoactive conjugated polymer in the active layer can be enhanced due to the plasmonic scattering or near-field enhancement caused by LSPR effect of the metallic NPs (mainly enhance the optical property) [26,27]. On the other hand, Choy et al. indicated that the strong near field around Au NPs due to LSPR effect mainly distributes laterally along the PEDOT:PSS layer, leading to minimal enhancement of light absorption in the active layer. Thus they contributed to the improvement of PCE to the enlarged interfacial area between active layer and PEDOT:PSS buffer layer as well as the improved PEDOT:PSS conductivity (mainly improve the electrical property) [28]. In these studies, they all investigated the effect of metallic NPs with a fixed size, however, there are distinct optical properties and space distributions in the PEDOT:PSS layer for various sized metallic NPs. Consequently, the investigation of the performance changes of PSCs with various sized metallic NPs is highly important and desirable to better understand the physics within the ‘plasmonic’ PSCs. Herein we study the comparative performances of poly (3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) BHJ PSCs incorporated various sized Au NPs (20 nm, 35 nm, 50 nm and 75 nm) on ITO-coated glass substrates. We investigate how the optical and electrical properties of these devices perform when Au NPs with different space distributions in the interface of PEDOT:PSS buffer layer and P3HT:PCBM active
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layer. At an optimized Au NPs size of 35 nm, a maximum PCE improvement of 23% (from 2.37% to 2.91%) was demonstrated.
2. Experimental 2.1. Synthesis of Au NPs The various sized Au NPs were prepared by a hydroquinonereduction seed-growth method reported previously [29]. Briefly, a 15 nm diameter Au seeds solution was prepared according to the protocol by Frens [30], resulting in a colloidal suspension containing [Au]¼1.5 mM. Then a 99 mL ‘‘growth solution’’ containing 3 mL 10 mM centrifuged HAuCl4 solution, 250 mL 1% w/w sodium citrate solution and a suitable amount of Au seeds solution (ranging from 11.5 mL to 0.16 mL, which yield particle sizes of 20 nm up to 75 nm) was prepared in a 250 mL flask. At last, 1 mL 30 mM hydroquinone was injected into the growth solution under vigorous stirring. These reactions completed within 1 h. In order to cap the Au NPs with poly-(vinylpyrrolidone) (PVP, MW¼10,000, Aldrich), we added 3 mL 100 mM PVP solution to the reaction solution, and stirring for 24 h. Finally, all solutions were centrifuged to eliminate the excess ligands and redissolved in 1 mL of deionized water to avoid the aggregation of Au NPs. 2.2. Device fabrication The ITO-coated glass substrates were ultrasonicated in detergent, acetone and isopropyl alcohol, dried with nitrogen, and treated by ozone–ultraviolet cleaner for 15 min. The same volume of various sized Au NPs aqueous solutions were spin-coated at 2000 rpm onto ITO-coated glass substrates, subsequently annealed at 140 1C for 10 min in air, and treated by ozone–ultraviolet cleaner for another 10 min (which can promote the spread of PEDOT:PSS solution). For the control device under the same conditions, we spin-coated the same volume of pure deionized water on ITO, and denoted as 0 nm Au NPs. The PEDOT:PSS solution (Baytron P 4083) were next spincoated at 4000 rpm on the top of the layer of Au NPs and were subsequently annealed at 140 1C for 15 min in air, resulting in a thickness of 30 nm layer. The mixed solutions consisting of P3HT (10 mg mL 1, Aldrich) and PCBM (9 mg mL 1, Aldrich) in 1,2dichlorobenzene were then spin-coated at 500 rpm on the PEDOT:PSS layer in air. The thickness of the active layer is 150 nm. After spin-coating photoactive layer, Al (100 nm) was thermal evaporated under high vacuum of 4 10 4 Pa with a rate of 0.2 nm/s onto the polymer layer as a cathode to create a device with an active area of 9 mm2 defined by a shadow mask. The final device structure is ITO/Au NPs/PEDOT:PSS/P3HT:PCBM/Al, as shown in Fig. 1a. Finally, thermal annealing was performed by directly placing the device on a hotplate at 150 1C for 10 min in an inert glove-box. It should be noted that the device fabrication conditions were optimized prior to this investigation. 2.3. Device characterization The transmission electron microscope (TEM) images of Au NPs were measured using a JEM-1230EX TEM. The ultraviolet–visible (UV–vis) absorption spectra of solutions and films on ITO substrates were measured on a CARY 100 Bio UV–vis spectrophotometer. For the absorption tests of P3HT:PCBM films on ITO/Au NPs/ PEDOT:PSS, we used the corresponding ITO/Au NPs/PEDOT:PSS sample as the reference, which can eliminate the absorption of ITO/ Au NPs/PEDOT:PSS. The photoluminescent (PL) spectra of P3HT/PCBM films were measured on a Hitachi F-4500 spectrophotometer (excited at 470 nm). The current density–voltage (J–V) characteristics of devices under dark and illumination (100 mW/cm2) were measured
with a Keithley 2400 measurement source unit at room temperature in air. The light intensity for the solar simulator was calibrated with a standard photovoltaic (PV) reference cell. Incident photon to electron conversion efficiency (IPCE) curves were measured with a Stanford lock-in amplifier 8300 unit. Atomic force microscopy (AFM) image was measured by Veeco MultiMode scanning probe microscope operated in tapping mode in air. The film thickness was measured by an AMBIOS XP-1 high-resolution surface profiler.
3. Results and discussion 3.1. Photovoltaic characteristics of devices with various sized Au NPs The current density–voltage (J–V) characteristics of OPV devices incorporated with various sized Au NPs (20 nm, 35 nm, 50 nm and 75 nm) in the PEDOT:PSS anode buffer layer are shown in Fig. 1b. It should be noted that all devices with all sized Au NPs were the ones with optimized Au NPs coverage density. The device photovoltaic performances are summarized in Table 1. It is observed that PCE of our PSCs is improved by 23% from 2.37% (0 nm) to 2.91% (35 nm), as increasing the size of Au NPs from 0 to 35 nm. Further increasing the size of Au NPs, the PCE values drop to 2.49% (50 nm) and 2.04% (75 nm). The improvement of PCE in the devices with small Au NPs is mainly attributed to the increase in fill factor (FF) and short-circuit current (JSC), from 0.5 to 0.52 and 7.59 mA cm 2 to 9.05 mA cm 2, respectively. From the dark J–V curves in Fig. 1c, we can observe that the series resistance (Rs) of the devices, obtained from the inverse slope of the dark J–V curves at a voltage of 0.5 V, slightly decreases from 5.2 O cm2 (0 nm) to 4.94 O cm2 (20 nm), and then increase to 6.76 O cm2 (75 nm) as increasing the size of Au NPs. Fig. 1d shows the IPCE characteristics of these devices, we can obtain the highest IPCE of 70% at 500 nm in the device with 35 nm Au NPs, with a 19% improvement to the control device (0 nm). Further increasing the size of Au NPs, the peak IPCE detrimentally decreases to 60% at 500 nm in the device with 75 nm Au NPs. This is in good agreement with the trend of JSC. We also do some preliminary investigation on the stability of the OPVs with and without Au NPs, and no obvious changes of the stability are observed after incorporating Au NPs. In the following sections, we will study the optical and electrical properties of our PSCs, which describes the underlying physics of the ‘plasmonic’ PSCs in a better way. 3.2. LSPR effect of various sized Au NPs on photovoltaic performance Fig. 2a–d shows TEM images of various sized Au NPs used in our PSCs, it is observed that the average size of these Au NPs is approximately 20 nm, 35 nm, 50 nm, and 75 nm, respectively. The absorption spectra of these Au NPs in water are shown in Fig. 2e. Corresponding to the excitation of LSPR, the absorption peaks for these Au NPs are at 523 nm, 530 nm, 536 nm, and 549 nm respectively, which all match well with the absorption of the P3HT:PCBM film. In order to verify the degree of contributions of the LSPR effects, we measured the absorption spectra of P3HT:PCBM active layers on top of ITO/Au NPs/PEDOT:PSS with various sized Au NPs, as shown in Fig. 3a. During the measurement of light absorption, we have eliminated the absorption of ITO/Au NPs/PEDOT:PSS as it absorbs light. So the absorption spectra represent light absorbance within the active layer only. Interestingly, the devices with smaller Au NPs (20 nm and 35 nm) have no significant difference in absorption compared with the control device (0 nm) while there are obvious absorption enhancements in the devices with larger Au NPs (50 nm and 75 nm). It has been reported that when Au NPs are incorporated into the PEDOT:PSS layer, the strong
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Fig. 1. (a) Device architecture of the OPV incorporating various sized Au NPs at the ITO/PEDOT:PSS interface. (b) J–V characteristics of the OPV devices with various sized Au NPs (0 nm, 20 nm, 35 nm, 50 nm and 75 nm) at the ITO/PEDOT:PSS interface, recorded under illumination at 100 mW/cm2 (AM 1.5 G). (c) Dark J–V characteristics of these OPV devices. (d) Corresponding IPCE curves of these OPV devices.
Table 1 Photovoltaic performances of polymer solar cells (ITO/Au NPs/PEDOT:PSS/P3HT– PCBM/Al) with various sized Au NPs under AM 1.5 G illumination (100 mW/cm2). NP size (nm)
VOC (V)
Jsc (mA cm 2)
FF (%)
PCE (%)
Rsa (O cm2)
0 20 35 50 75
0.62 7 0.01 0.63 7 0.01 0.62 7 0.01 0.62 7 0.02 0.62 7 0.02
7.59 7 0.13 8.23 7 0.15 9.057 0.19 8.78 7 0.25 8.027 0.19
50.17 0.8 53.1 7 1.2 51.8 7 1.0 45.7 7 1.1 41.2 7 1.7
2.37 70.06 2.75 70.11 2.91 70.15 2.49 70.14 2.04 70.17
5.20 7 0.15 4.94 7 0.16 5.16 7 0.19 5.95 7 0.26 6.76 7 0.33
a Device series resistance (Rs) of the OPV obtained from the inverse slope of the dark J–V curve at a voltage of 0.5 V.
LSPR near-field around Au NPs mainly distributes laterally along the PEDOT:PSS layer rather than vertically into the adjacent active layer, leading to minimal optical enhancement of active layer [28]. In our devices, according to the space distribution of Au NPs, the change of light absorption can be divided into three cases: (1) when the size of Au NPs is smaller than the thickness of PEDOT:PSS layer (the devices with 20 nm Au NPs), the Au NPs would be completely covered by PEDOT:PSS and only small bumps (lower than 1 nm) can be left on the surface of PEDOT:PSS, as shown in the AFM 3D height images (Fig. 4g). In this situation, no absorption changes are expected; (2) as the size of Au NPs is almost equal to the thickness of PEDOT:PSS layer (the devices with 35 nm Au NPs), larger bumps ( 5 nm) exhibit on the surface of PEDOT:PSS (see Fig. 4h), indicating that Au NPs will penetrate into the active layer, thus the laterally distributed LSPR of Au NPs can affect the light absorption of the active layer, leading to a slight absorption improvement of the P3HT:PCBM active layer; (3) once the size of Au NPs is much larger than the thickness of
PEDOT:PSS layer (the devices with 50 nm and 75 nm Au NPs), Au NPs would protrude the PEDOT:PSS surface (as shown in Fig. 4i and j), and then penetrate into adjacent P3HT/PCBM active layer, resulting in obvious absorption enhancement of the active layer of P3HT:PCBM. Even though there are no significant absorption changes of the active layer in the devices with 20 nm and 35 nm Au NPs, IPCE of these devices enhances dramatically. Meanwhile, the absorption enhancements of the active layer in the devices with 50 nm and 75 nm Au NPs also mismatch the IPCE improvements of these devices. The evident discrepancy between optical properties and IPCE can be interpreted by the fact that the magnitudes of IPCE not only depend on the optical properties, but also by electrical properties such as exciton dissociation rates, charge transport rates and charge collection efficiencies [31]. Therefore, apart from LSPR effects, the effects of various sized Au NPs on electrical properties of our PSCs must be investigated to understand the origin of performance improvement. 3.3. Effects of various sized Au NPs on PEDOT:PSS layer The distributions of various sized Au NPs in PEDOT:PSS layer are investigated by atomic force microscopy (AFM) 2D height images on the ITO/Au NPs/PEDOT:PSS films with various sized Au NPs, as shown in Fig. 4a–e. It is clear that the Au NPs with various sizes are all well dispersed in the PEDOT:PSS layer, and there are no obvious aggregation in the films. The good dispersibility can be attributed to the PVP capping layer around the surface of Au NPs [32]. Meanwhile, AFM 3D images (Fig. 4f–j) clearly exhibit the morphology changes of PEDOT:PSS film caused by various sized Au NPs. We observe a dramatic roughness increase as the size of Au NPs increases. The mean roughness (RMS) for these devices are 1.25 nm (0 nm),
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Absorbance (a.u.)
0.6 0 20nm 35nm 50nm 75nm
0.4
0.2
0.0
400
500
600
700
800
Wavelength (nm)
PL intensity (a.u.)
4000 0 20nm 35nm 50nm 75nm
3000 2000 1000 0 550
600
650 700 750 Wavelength (nm)
800
850
Fig. 3. (a) Optical absorption spectra and (b) PL spectral of P3HT/PCBM thin film for ITO/Au NPs/PEDOT:PSS/P3HT:PCBM structures with various sized Au NPs.
Fig. 2. (a–d) TEM images of various for (a)–(d) are about 20 nm, 35 nm, absorption spectra of these Au NPs absorption maxima, which are respectively.
sized Au NPs. The average nanoparticle sizes 50 nm and 75 nm. (e) Corresponding UV–vis in water, all spectra are normalized at their 523 nm, 530 nm, 536 nm, and 549 nm,
1.78 nm (20 nm), 2.18 nm (35 nm), 4.89 nm (50 nm) and 9.44 nm (75 nm), respectively. It has been investigated that the increase of anode surface roughness can enlarge the interface area between the anode and the active layer, allowing a relatively short route for holes to reach the anode and hence increasing the holes collection efficiency [33,34]. Thus, in our PSCs, it is expected that the incorporation of large Au NPs will induce an increase of interfacial area between PEDOT:PSS and P3HT:PCBM, leading to an improvement of holes collection efficiency at the anode and, thus, the JSC of our devices. It is the reason why our devices with 20 nm and 35 nm Au NPs can achieve improved JSC (8.23 mA cm 2 and 9.05 mA cm 2) despite there are no significant absorption enhancement in these devices. In the devices with 50 nm and 75 nm Au NPs, where a dual mechanisms combining enhancement of both hole collection and light absorbance of the active layer by the LSPR effects, further enhancement in the JSC should be expected. Unfortunately, the decreased JSC of 8.78 mA cm 2 (50 nm) and 8.02 mA cm 2 (75 nm) is obtained compared to the maximum JSC (9.05 mA cm 2 for 35 nm Au NPs). The possible reason is on other electrical properties degradation due to morphology changes of the active layer, which we will investigate in the next section. Besides, the increased interfacial area between PEDOT:PSS and P3HT:PCBM also reduces the mean distance between generated
holes and PEDOT:PSS surface, which can diminish the dependence of holes on the external electric field for collection at the anode [33,34]. It is a positive effect to improve FF in our devices. Meanwhile, the series resistances of the devices with 20 nm and 35 nm Au NPs is smaller than the one of the control device, which are favorable to the FF; while the series resistances of the devices with 50 nm and 75 nm Au NPs is much larger than the one of the control device, resulting in the reduce of FF [35]. Therefore, the combination of these effects determines the trend in FF variation as shown in Table 1: the magnitudes of FF in our devices increase from 0.50 (control, 0 nm) to 0.53 (20 nm) and 0.52 (35 nm), then decrease to 0.46 (50 nm) and 0.41 (75 nm) as the size of Au NPs increases. These phenomena indicate that the negative effect of increased series resistance outweigh the positive effect of reduced mean distance between generated holes and PEODT:PSS surface, resulting in the reduction of FF as further increasing the size of Au NPs from 35 nm to 75 nm. 3.4. Effects of various sized Au NPs on exciton quenching As described in the above section, an unexpected degradation of JSC and FF in our devices occurs when further increasing the size of Au NPs to 50 nm and 75 nm. To investigate this result, we measured the photoluminescence (PL) spectra of ITO/Au NPs/ PEDOT:PSS/P3HT:PCBM films with various sized Au NPs using a 470 nm excitation source (see Fig. 3b). At the same time, the morphology changes of P3HT:PCBM films on ITO/Au NPs/PEDOT:PSS layer with various sized Au NPs are also investigated through AFM height images, as shown in Fig. 4. In PL spectra, we observe the PL intensity enhancement upon increasing the size of Au NPs, with a maximum increase by 39% at 646 nm. According to the previous studies, PL intensity changes can be attributed to three main reasons: changes in light absorption, affecting the light excitation rate [26,36]; exciton quenching at
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Fig. 4. AFM height images for ITO/Au NPs/PEDOT:PSS films with various sized Au NPs: (a, f) 0 nm; (b, g) 20 nm; (c, h) 35 nm; (d, i) 50 nm; and (e, j) 75 nm. The left column is 2D height images while the right one is corresponding 3D height images. The corresponding mean roughnesses (RMS) of these films are 1.25 nm, 1.78 nm, 2.18 nm, 4.89 nm, and 9.44 nm, respectively.
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metal/organic interfaces [37,38]; and exciton quenching at donor/ acceptor (D/A) interfaces [39,40]. For the first reason, the PL intensity in the devices with 50 nm and 75 nm Au NPs can be improved while the ones in the devices with 20 nm and 35 nm Au NPs should keep the same, as we have previously shown experimentally that only the light absorption in our devices with 50 nm and 75 nm Au NPs increase significantly. But the extent of absorption improvements (5% for 50 nm and 7% for 75 nm) are much less than the one of PL intensity enhancements (25% for 50 nm and 39% for 75 nm) in the devices with 50 nm and 75 nm Au NPs. Therefore, it makes minor contribution to the PL intensity changes in the devices with 50 nm and 75 nm Au NPs. For the second reason, the possibility of exciton quenching at metal/organic interfaces can be eliminated, as previous studies show that Au NPs with an insulating protecting layer can prevent direct contact from organic layer, thus avoiding exciton quenching [41]. In this work, Au NPs are protected by a layer of PVP, which can prevent Au NPs from direct contact with the organic layer. To make sure that the PVP itself does not have any effect on device performance, we compared the performance of the devices with and without PVP (2 mg mL 1) dissolved into the PEDOT:PSS layer and observed no obvious difference in the performance. If there is exciton quenching at Au NPs/organic interface in our devices, a decrease trend of PL intensity in the device as increasing the size of Au NPs should be expected, due to the increased contact between Au NPs and organic layer (see Fig. 4). However, experimental results show an increase trend of PL intensity in these devices. So the effect of exciton quenching by Au NPs can be eliminated. Therefore, the third reason, reduced exciton quenching at D/A junctions, might be the main contribution to the increased PL intensity of the P3HT:PCBM films. The exciton quenching at D/A junctions is mainly influenced by the D/A interface area which determined by the morphology of the active layer [42,43]. In the devices with smaller Au NPs (20 nm and 35 nm), the incorporated Au NPs are covered by PEDOT:PSS layer, only the internal networking of the subsequently spin-coated P3HT:PCBM films near PEDOT:PSS side could have some change, while no
significant change can be observed in the AFM images of the top surface morphology of P3HT:PCBM films (comparing Fig. 5b and c to a). So the PL intensity of these P3HT:PCBM films just increase by about 10%. However, in the devices with larger Au NPs, the Au NPs penetrate into the P3HT:PCBM films, resulting in the change of the whole morphology of P3HT:PCBM films (the domains become larger), as shown in Fig. 5d and f. These changes of the nanoscale morphology, combined with the penetrated Au NPs, lead to a remarkable reduce in the D/A interface area and, thus, the reduced exciton quenching at D/A junctions. As a result, the PL intensity of these P3HT:PCBM films increases by 25% (50 nm) and 39% (75 nm). Therefore, we conclude that the reduced D/A interface area by incorporating Au NPs mainly affects the PL intensity, while the improved light absorption in the larger Au NPs devices can also make minor contribution to the increase of PL intensity. For the device performances, the reduced D/A junction area implies a reduction in exciton dissociation efficiency, leading to a decrease trend of JSC in our devices as the size of Au NPs increases. On the other hand, the previous investigated hole collection efficiency improvement and light absorption enhancement can result in an increase trend of JSC in our devices as the size of Au NPs increases. The JSC change depends on the competition between these two trends. The increase of the size of Au NPs causes the increase of the surface roughness of PEDOT:PSS and the PEDOT:PSS/P3HT:PCBM interfacial areas which can collect more holes at the anode, and also improves the contact of Au NPs to P3HT:PCBM layer which increase the light absorption of the P3HT:PCBM layer, leading to the improvement in JSC. Meanwhile, the morphology of P3HT:PCBM films also changed, resulting in reduced D/A interface area which decrease the exciton dissociation efficiency. Increasing the size of Au NPs from 0 nm to 35 nm, the positive effect play the dominating role, resulting in an increase of JSC; while further increasing towards 75 nm, the negative effect outweighs the positive effect, resulting in a reduction trend of JSC, as shown in Table 1. Combining the trend of JSC and previously investigated trend of FF, we obtain a peak
Fig. 5. AFM 3D height images for P3HT/PCBM active layer on ITO/Au NPs/PEDOT:PSS with various sized Au NPs: (a) 0 nm; (b) 20 nm; (c) 35 nm; (d) 50 nm; and (e) 75 nm. The corresponding mean roughnesses (RMS) of these films are 0.72 nm, 0.79 nm, 0.95 nm, 1.27 nm, and 1.86 nm, respectively.
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PCE of 2.91% for the device with 35 nm Au NPs, with a maximum improvement of 23%.
4. Conclusions In this study, we incorporated various sized Au NPs (20 nm, 35 nm, 50 nm and 75 nm) in the PEDOT:PSS anode buffer layer to enhance the performance of PSCs. The optical and electrical properties of PSCs with Au NPs have been investigated. On one hand, due to the lateral distribution of strong LSPR near-field, the smaller sized Au NPs do not enhance the light absorption, while the larger Au NPs can penetrate into P3HT:PCBM active layer and obviously enhance the light absorption. On the other hand, as the size of Au NPs increased, the roughness of PEDOT:PSS increased, enlarging the interface areas between PEDOT:PSS and P3HT:PCBM, and resulting in the collection of more holes at the anode; meanwhile, the morphology of active layer would change to reduce the D/A interface area, leading to reduction of exciton quenching. The above dual size effects of the Au NPs compete to determine the trend of JSC in our devices. Besides, the trend of FF in our device was originated from competition between the negative effect of increased series resistance and the positive effect of reduced mean distance between generated holes and PEODT:PSS surface as increasing the size of Au NPs. Considering the trend of JSC and FF together, we achieved a maximum improvement of 23% at the device with 35 nm Au NPs. This work contributes to better understanding of the use of Au NPs for enhancing PSC performances.
Acknowledgments This work was supported by the National Natural Science Foundation of China (NSFC) (No. 50990063) and the Danish National Research Foundation and the NSFC (Grant no. 51011130028) for the Danish–Chinese Center for Organic based Photovoltaic Cells.
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