Bifunctional Polymer Nanocomposites as Hole-Transport Layers for

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Bi-Functional Polymer Nanocomposites as Hole Transport Layers for Efficient Light Harvesting: Application to Perovskite Solar Cells Jhong-Yao Wang, Fang-Chi Hsu, Jeng-Yeh Huang , Leeyih Wang, and Yang-Fang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08157 • Publication Date (Web): 25 Nov 2015 Downloaded from http://pubs.acs.org on November 25, 2015

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ACS Applied Materials & Interfaces

Bi-Functional Polymer Nanocomposites as Hole Transport Layers for Efficient Light Harvesting: Application to Perovskite Solar Cells Jhong-Yao Wanga, Fang-Chi Hsub,*, Jeng-Yeh Huangb, Leeyih Wangc,*, Yang-Fang Chena,* a

Department of Physics, National Taiwan University, Taipei 106, Taiwan

b

Department of Materials Science and Engineering, National United University, Miaoli 360, Taiwan c

Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan

KEYWORDS. polymer nanocomposite; perovskite; hole transport material; solar cell; light harvesting; nanoparticle

ABSTRACT. A new approach to largely enhance light harvesting of solar cells by employing bi-functional polymer nanocomposites as hole transport layers (HTLs) is proposed. To illustrate our working principle, CH3NH3PbI(3-x)Clx perovskite solar cells are used as examples. Gold-nanoparticles (Au-NPs) are added into a conjugated poly(3-hexylthiophene-2,5-diyl) (P3HT) matrix, resulting in ~ 4-fold enhancement in electrical conductivity and carrier mobility of the native P3HT film. The improved electrical properties are attributed to enhanced polymer chain ordering caused by

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Au-NPs. By integrating those P3HT:Au-NPs films with an optimum loading concentration of 20% into perovskite solar cells as HTLs, it leads to a more than 25% enhancement in power conversion efficiency (PCE) compared with the nanoparticle-free one. In addition to the modulated electrical properties of the HTL, the improved performance can also be attributed to the scattering effect from the incorporated Au-NPs, which effectively extends the optical pathway to amplify the photon absorption of the photoactive layer. The design principle shown here can be generalized to other organic materials as well, which should be very useful for the further development of high performance optoelectronic devices.

INTRODUCTION. Polymer nanocomposites, composed of nano-sized structures of at least one dimension less than 50 nm dispersed in a polymer matrix, are of interest to both scientists and engineers alike because they can be prepared with a range of electrical and optical properties similar to inorganic semiconductors or even metals.1 – 4 Additional advantages such as low-cost, easy processing, and tunable electrical properties over intrinsic polymers make polymer nanocomposites a potential candidate for the applications, such as anti-static materials, electromagnetic interference (EMI) shielding, sensors, and conductors.5 – 8 Those reported unique features of nanocomposites so far are mostly based on insulating polymers. Nanocomposites based on conjugated polymers are relatively limited due to the immature development of host polymers. Poly(3-hexylthiophene2,5-diyl) (P3HT) is a well known conjugated polymer, of relatively lower synthesis cost, well studied electrical properties, and has a great potential for a variety of applications, such as field effect transistors,9, 10 organic solar cells,11, 12 and a model hole transport layer (HTL) material in


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perovskite solar cells.13–15 It has been reported that the conductivity of P3HT can be tailored by blending with multi-walled carbon nanotubes, leading to the composite’s conductivity of ~ 10–2 Scm

–1

as compared to native P3HT of ~10–7 Scm–1 under the same preparation condition.16

Another well known conjugated polymer is poly(3,4-ethylenedioxythiphene) (PEDOT) chemically doped with poly(styrene sulfonate) (PSS) to increase its conductivity to serve as a HTL material in organic solar cells.17 In the past few years, scientists have tried to incorporate gold-nanoparticles (Au-NPs) into PEDOT:PSS to further boost up the power conversion efficiency (PCE) of organic solar cells by taking the advantage of the localized surface plasmon resonance (LSPR) effect of Au-NPs.18–20 Using the PEDOT:PSS:Au-NPs nanocomposite as the HTL in the organic solar cells enables to largely enhance the light absorption efficiency of the device due to the LSPR induced light trapping behavior. Organometal halide perovskite-type semiconductors have become a promising alternative for light harvesting because of their strong and broad light absorption characteristics over visible spectrum.21,22 This new class of material delivers high open circuit voltages in photovoltaics showing a remarkable PCE up to 19.3% in five years,23 an encouraging breakthrough that could possibly overpass commercial silicon photovoltaics. Generally, the organometal halide perovskites have a formula of CH3NH3PbX3, where X = Cl, Br or I, with a wide range of band gaps through tuning the halide content and possibly the organic part.22,24,25 Typically, CH3NH3PbX3 functions as the primary light absorber sandwiched between electron and hole transport layers (ETLs & HTLs) resulting in a planar-heterojunction perovskite solar cell structure. Much success has been achieved by introducing TiO2, ZnO, and TiO2-graphene composites26–28 as ETL materials. To date, there are various kinds of hole transporting materials used in fabricating perovskite cells including organic and inorganic hole conductors. The


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incorporated organic hole conductors can be categorized into three groups, small molecules, conjugated polymers, and organometallic complexes. 13,25,29–31 Devices with small molecules such as triphenylamine-based compounds, oligothiophene derivatives, carbazole derivatives and so on have been shown to achieve cell efficiencies between 10–19%.31–35 Thiophene-based conjugated polymers including P3HT, poly(triarylamie) (PTAA), etc. are also promising with the highest reported efficiency of 18.4%.36 An initial attempt at using organometallic compound, copper phthalocyanine (CuPc), as a HTL material demonstrates a cell efficiency of 5%.37 Moreover, a maximum efficiency of 12.4% is achieved based on inorganic hole conductor, copper thiocyanate (CuSCN).38 Among the HTL materials used, it has been demonstrated that 2,2ˊ,7,7ˊ-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9ˊ-spirobifluorene (spiro-OMeTAD) is the most effective one for fabricating high efficiency organic-inorganic perovskite solar cells. Though spiro-OMeTAD shows its promising potential for commercializing perovskite cells, the long-term stability of spiro-OMeTAD cells are inferior than those using P3HT as HTLs. Acording to Kwon et al.,39 after 1000 hours of aging, the PCE value of spiro-OMeTAD cells showed a 28% reduction relative to the initial PCE whereas devices consisting of P3HT exhibited a 6.5% decrease. The better stability of P3HT cells is attributed to the high hydrophobicity of P3HT, which prevents water molecules penetrating into the underneath perovskite surface to damage the layer. From the stability perspective, it is of great importance to develop high hydrophobicity HTL materials with acceptable cell performance. Herein, we report the application of bi-functional polymer nanocomposites based on a hydrophobic polymer as HTLs in CH3NH3PbI(3-x)Clx perovskite solar cells. To the best of our knowledge, this is the first study of using bi-functional polymer nanocomposites as HTLs in solar cell fabrication, including perovskite one. We physically tailor both the conductivity and


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optical absorbance of a P3HT matrix by introducing Au-NPs. The resulting polymer nanocomposite (P3HT:Au-NPs) films exhibit a loading dependency on both electrical and optical properties. The polymer chains become more oriented as suggested by X-ray diffraction (XRD) and UV-vis absorbance measurements. Thus, the conformational change of polymer in the presence of highly conductive Au-NPs drives the enhancement of the conductivity and the carrier mobility of the nanocomposite films. By integrating P3HT:Au-NPs films with 20% loading into perovskite solar cells as HTLs, the PCE of devices can be improved from 7.81% to 10.71%. The enhancement of performance can be ascribed to improved electrical properties of the HTL and the light absorption efficiency of the photoactive layer based on the scattering effect caused by Au-NPs.

EXPERIMENTAL SECTION. Material Preparation. Fluorine-doped tin oxide (FTO) glass substrates (~7 Ω sq –1, Ruilong) were cleaned by successively ultrasonicating in detergent, deionized water, acetone, and isopropyl alcohol for 20 min for each step and then dried in nitrogen gas flow. Methylammonium iodide (CH3NH3I, MAI) was synthesized according to Ref. 40. The CH3NH3PbI(3-x)Clx perovskite precursor solution (40 wt%) was prepared by mixing 399 mg of MAI and 232 mg of lead(II) chloride (PbCl2, Sigma-Aldrich) in 1 mL of N,N-dimethylmethanamide (DMF) solvent. Poly(3hexylthiophene)(P3HT) and Au-NP suspension were purchased commercially from Aldrich and used as received. The concentration for Au-NP suspension was ~3.5×1010 particles mL–1 in 0.1 mM citrate buffer. The average particle size is approximately 50 nm. P3HT of 10 mg was dissolved in 1 mL of 1,2-dichlorobenzene (ODCB) solvent forming a P3HT solution. The AuNPs purchased from commercial source were dispersed in citrate buffer containing water.


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Considering the damage of moisture on the perovskite layer, we prepared P3HT:Au-NPs solutions in the following way to minimize the water content. The Au-NP suspensions of 0, 100, 200, 300 µL were vacuumed dry to remove most of the solvent and the resulting slightly wet powders were then added into 1 mL of P3HT solutions resulting in Au-NP concentrations of 0, 10, 20, and 30%, respectively. Those P3HT:Au-NPs solutions were then stirred at 60 °C for 1 day before use. Sample Fabrication. For solar cell fabrication, initially, a compact-TiO2 (c-TiO2) layer of ~ 80 nm thick was deposited onto the cleaned FTO-coated glass substrate by spin-coating the TiO2 sol-gel solution (FrontMaterials, AA) at 1000 rpm followed by sintering the film at 600 °C for 6 h. Perovskite precursor solution was spin-coated on the annealed c-TiO2 films at 2000 rpm. The yellowish films were subsequently annealed at 100 °C for 1 h. After annealing, the dried perovskites film (~250 nm) turned into black. Then, a layer of P3HT or P3HT:Au-NPs (~ 80 nm) was deposited on top of the perovskite film by the spin-coating method with a spin rate of 800 rpm for 60 s to serve as the HTL. Finally, a 100 nm thick gold metal was thermally deposited on top of the HTL through a shadow mask to complete the device fabrication. The area of the cell was 0.07 cm2. Samples for characterizing XRD and UV-vis absorption of P3HT:Au-NPs films were prepared on cleaned indium-tin-oxide(ITO)-coated and bare glass substrates, respectively. Devices for measuring conductivity and mobility of P3HT:Au-NPs films were prepared as follows. The P3HT:Au-NPs films were sandwiched between two gold electrodes to complete the structures for conductivity measurements. For mobility measurements, mixture of deionized water, ethanol, and PEDOT:PSS (Clevios™ P VP AI 4083) in a volume ratio of 1:1:10 was spincoated on ITO-coated glass substrates followed by P3HT:Au-NPs deposition before evaporating


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gold electrode to finish device preparation. The deposition of all P3HT:Au-NPs films followed the same procedures as those for HTL preparation. The resulting thicknessnes of the P3HT:AuNPs films were ~80, ~120, and ~100 nm for structures for XRD, conductivity, and mobility measurements, respectively. All the fabrication procedures were carried out in an argon-filled glove box. Characterization. The top-view of the deposited perovskite layer and the cross-scetional image of the device configuration were obtained by scanning electron microscopy (SEM). The UV-visible absorption spectra were measured by using a Shimazu Model UV-2600 spectrophotometer. The X-ray diffraction spectra were collected by an X-ray diffractometer (Rigaku, Ultima IV, Japan) with Cu Kα radiation and a Ni filter at a scanning rate (2θ) of 1 °/min. The current – voltage (J – V) characteristics of the finished photovoltaic devices were evaluated in vacuum of 60 torr by using a Keithley Model 2400 source meter under irradiation intensity of 100 mW/cm2 from a calibrated solar simulator (Newport Inc.) with AM 1.5G filter. The voltage was scanned from + 0.8 to 0 V with a constant scanning velocity of －0.05V/s for all devices. The calibration was done by using a standard Si photodiode. The incident-photon-conversionefficiency (IPCE) spectra were performed using a setup consisting of a lamp system, a chopper, a monochromator, a lock-in amplifier, and a standard silicon photodetector (ENLI Technology). The reflection spectra were collected by using a Lambda Model 850 UV-Vis spectrophotometer. The thicknesses of the films were measured by a Veeco dektak 6M surface profiler. All the measurements were conducted in the air.

RESULTS AND DISCUSSION.


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Figure 1 presents the optical absorbance of Au-NPs in citric buffer measured by UV-vis spectrometer, showing a plasmonic resonance peak centered at 530 nm. A photograph of Au-NPs solution is displayed in the inset and the average diameter is ~50 nm determined from the SEM image for Au-NPs deposited on a silicon wafer as shown in the inset.

Absorbance (a.u.)

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300

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Wavelength (nm)

Figure 1. UV-vis absorbance for Au-NPs in adequate solution. The inset shows the photograph of Au-NP solution with the corresponding SEM image of Au-NPs deposited on a silicon wafer.

Figure 2(a) depicts the top-view SEM image of the deposited CH3NH3PbI(3-x)Clx perovskite film on the c-TiO2 layer, exhibiting a connection of irregular grains and an incomplete coverage of the surface. The grain sizes range from a few hundred nanometers to a few micrometers. The poor surface coverage is commonly seen in a single solution processed photoactive layer.41The crystallography of the perovskite film deposited on c-TiO2 is presented in Figure 2(b). The XRD spectra were measured from 2θ = 10 – 60° and the characteristic diffraction peaks for the FTO/cTiO2 structure are as marked on the spectrum. The main characteristic diffraction peaks of the perovskite observed at 14.1°, 28.4°, 43.2°, and 58.9° are assigned to the (110), (220), (330), and (440) orthorhombic crystal structure peaks for CH3NH3PbI342 while the peak at 31.5° is the cubic phase trichloride perovskite, CH3NH3PbCl3.43


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(a)

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(110)

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(220)

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Figure 2. (a) Top-view SEM image of the perovskite film. (b) XRD spectra for FTO/cTiO2/perovskite structure.

Instead of commonly used spiro-MeOTAD, we choose a more hydrophobic conjugated P3HT polymer doped with various amounts of Au-NPs as HTL materials. We note here that perovskite is moisture sensitive, the commonly used water-based PEDOT:PSS hole conductor is not appropriate to serve as a HTL here. After depositing the P3HT nanocomposite films on the perovskite layers, the poor surface coverage of the perovskite can be smoothed out before anode deposition. The cross-sectional SEM image of the finished device is shown in the inset in Figure 3(a) and the performances of those devices were evaluated in the laboratory environment under the same condition. Figure 3(a) displays the typical J – V characteristics for those devices and the average values for the performance parameters are summarized in Figure 4 and Table 1 for comparison. Device with native P3HT layer (0% Au-NPs, standard device) shows the average short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) of (19.06±0.85) mA/cm2, (0.70±0.04) V, and (56.11±6.04)%, respectively, leading to an average power


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conversion efficiency (PCE) of (7.891±0.92)%. The performance of our standard device agrees well with those reported in the literatures.15,44 After doping, the average PCEs of devices are improved due to the enhancement in Jsc and FF values. With the optimum doping concentration of 20%, the average value of Jsc, Voc, FF and PCE is (21.53±0.48) mA/cm2, (0.72±0.03) V, (60.72±3.86)%, and (9.82±0.89)%, respectively. For the best device, a PCE of 10.71% has been achieved and the values for the associated parameters are listed in Table 1. Figure 3 (b) illustrates the IPCE curves obtained from another set of devices. The spectral response of NPsfree device spans the whole visible light spectrum. After doping, IPCE values increase in the whole wavelength range (370 – 800 nm) and the order of increment follows that obtained in Jsc. Based on Figure 3(b), we further calculate the increase in IPCE (△IPCE) with doping concentration, which is shown in the inset in Figure 3(b). Although there are some particular features including a maximum at ~ 530 nm in the spectra, the enhancement is broadly distributed over the whole spectral range and does not specifically follow the plasmonic mode profile. Therefore, the LSPR induced light trapping behavior may be involved, but is not the dominant mechanism for the device improvement.


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Figure 3. Evaluation of cell performance. (a) Current density (J) – voltage (V) characteristics, and (b) the incident-photon-conversion-efficiency (IPCE) curves for devices with and without doping Au-NPs in the HTLs. The Cross-sectional SEM image of the device configuration using native P3HT as the HTL is shown in the inset in (a).

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Figure 4. Dependence of device performance on the concentration of Au-NPs in the HTL measured over 20 devices. (a) Jsc, (b)Voc, (c) FF, and (d) PCE.

Table 1 Performance parameters of solar cells with and without doping the HTL with Au-NPs under AM 1.5G illumination at 100 mW/cm2. The values in the parenthesis denote the values for the best device. Au (%)

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω/cm2)

0 10

19.06±0.85 20.02±0.11 21.53±0.48 (22.05) 20.87±0.39

0.70±0.04 0.71±0.02 0.72±0.03 (0.75) 0.68±0.04

56.11±6.04 61.03±3.04 60.72±3.86 (64.79) 58.81±4.22

7.81±0.92 8.76±0.61 9.82±0.89 (10.71) 8.13±0.98

12.57±2.74 11.41±2.62 10.52±2.17 (8.35) 11.13±2.76

20 30

To elucidate the improvement of Jsc under the incorporation of Au-NPs into the P3HT matrix, we compare the photon absorption properties of the perovskite after depositing a HTL. Figure 5(a) depicts the UV-vis absorption spectra for perovskite/P3HT:Au-NPs films with various NP concentrations deposited on glass substrates. The perovskite/P3HT absorbs mostly the photons in the visible regime with a lower energy bound of ~800 nm. Adding Au-NPs into the P3HT layer enhances the optical absorption intensity with the increase in loading. Because the absorption ranges of P3HT and Au-NPs overlap with that of the perovskite, the enhanced intensity can possibly originate from the absorption of P3HT due to conformational change in the presence of Au-NPs, in-part absorption of or the scattering effect from Au-NPs. Among them, the in-part absorption of Au-NPs, which can excite the plasmon of NPs, is not the dominant mechanism for Jsc improvement as revealed in the broadly distributed profile of △IPCE. For the


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other two cases, the role of Au-NPs in P3HT and its effect on the whole device are important for the largely improved Jsc, and hence PCE. Concerning the effect of Au-NPs on a native P3HT matrix, we conducted both optical and electrical measurements to understand the fundamental properties of those nanocomposite films. Figure 5(b) illustrates the UV-vis spectra for P3HT:Au-NPs films (~80 nm) deposited on glass substrates and the spectra are normalized at 515 nm for comparison. The native P3HT film absorbs photons in the wavelength range of 400– 700 nm with shoulders (peaks) at 515, 550, and 600 nm. After introducing Au-NPs, there is a slightly red shift of those spectra and those shoulders (peaks) become pronounced and intensified, suggesting a more ordered polymer chain stacking due to stronger π-π interaction.45 Similar effect has also been seen when adding a small amount of multi-wall carbon nanotubes into P3HT.46 The enhanced absorption intensity of P3HT in more ordered states can contribute to the intensity enhancement in Figure 5(a) in the spectra range of 400 – 700 nm. To further distinguish the conformation differences of P3HT, we measure the crystallography of those films deposited on indium-doped tin oxide (ITO)-coated glass substrates. As can be seen is Figure 5(c), the broad background for all spectra comes from the amorphous glass substrate. The XRD peak centered at 2θ = 21.4° is originated from the ITO glass and its intensity serves as the reference for comparison. The main diffraction peak at 2θ = 5.6° occurs for all films is attributed to the diffraction of the crystallographic (100) plane of P3HT.47,48 The intensity of the (110) peak increases and the full-width-maximum-height of the peak slightly decreases with increasing Au-NP loading (inset in Figure 5(b)), suggesting an improved chain ordering due to enlarged ordered region in the presence of Au-NPs. Thus, the result of analyzing


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XRD spectra supports the finding of UV-vis measurements in Figure 5(b), demonstrating an improvement in the chain ordering of P3HT in the nanocomposite.

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0.2

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Figure 5. UV-vis spectra of (a) the perovskite/P3HT:Au-NPs and (b) P3HT:Au-NPs films of various Au-NP concentrations. The spectra in (b) are normalized at 515 nm. (c) The XRD spectra for P3HT:Au-NPs films on ITO-coated glass substrates measured from 2θ= 4 – 23°. The zoom-in chart for the (100) plane of P3HT is shown in the inset. (d) J1/2 –V measured from the hole only devices based on the structure of ITO/PEDOT:PSS/P3HT:Au-NPs/Au for various concentrations of Au-NPs. The solid lines are the fits.


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It is known that both conductivity and mobility of a polymer vary with the ordering of chain stacking.49 Because carrier transport of a P3HT film is anisotropic,50 it is more appropriate to compare the transport properties of those four films in the direction parallel to charge traveling inside a solar cell. We thus measured the conductivity of these four films using the standard twoprobe approach by applying bias and measuring the current response simultaneously across the Ag/nanocomposite (~ 120 nm)/Ag structure. The calculated conductivities for P3HT doped with 0, 10, 20, and 30% Au-NPs are (2.2±0.1)×10–5, (3.8±0.3)×10–5, (5.2±0.4)×10–5, and (6.3±0.4)×10–5 Scm–1, respectively. The obtained conductivity for a pristine P3HT film agrees well with the value reported in the literature using ITO and Au as electrodes.51After adding AuNPs, the conductivity values of the films improve with the increase in loading. We also evaluated the hole mobility of those nanocomposite films by the space-charge-limited-current (SCLC) method adopting the hole only structure of ITO/PEDOT:PSS (30 nm)/nanocomposite (~100 nm)/Au. The mobility values calculated from the obtained curves using Mott-Gurney square law52 for 0, 10, 20, and 30% Au-NPs yield (1.08±0.05)×10–4, (2.65±0.10)×10–4, (4.14±0.21)×10–4, and (6.62±0.28)×10–4 cm2V–1s–1(see Figure 5(d) for example), respectively. The obtained mobility value for a pristine P3HT film is in good agreement with that obtained by employing the time-of-flight approach.53Apparently, the hole mobility increases with the increased concentration of Au-NPs in P3HT. It is well known that carriers hop along polymer chains to conduct electricity in polymer materials and the energy barriers among hops become lower as the polymer chains are more oriented. Since both XRD and UV-vis absorption results suggest an enhanced chain ordering of P3HT after the addition of Au-NPs, carriers are thus more delocalized in the nanocomposite resulting in the enhancement of conductivity and mobility. Additionally, it is also likely for carriers to partially hop through highly conducting Au-NPs to


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conduct electricity as that reported in P3HT:carbon-nanotubes composite system.54 This intersystem hopping mechanism is expected to be more efficient at higher density of NPs and can contribute to conductivity and mobility improvement. It is noted that Au-NPs are dispersed in citrate buffer, the insulating citrate ions absorbing on the surface of NPs could possibly hinder charges hopping onto NPs. Fortunately, those citrate ions are of at most 0.6 nm long and charges can transfer to NPs through tunneling.

As for the optical effect of Au-NPs on the entire cell, we measured the reflectance spectra of the real devices with light incident from the FTO side. As shown in Figure 6(a), the reflectance decreases with the concentration of Au-NPs. We then calculated absorption enhancement (△α) from those reflectance spectra based on the equation55 △α=－ln(R0/R) to quantify the difference of absorption in the measured range, where R0 and R stand for the percentage of the reflected light from standard and Au-NP doped devices, respectively. Apparently, △α increases in the visible range of 400 – 800 nm, which indicates an additional absorption of the devices (see Figure 6(b)). This region of △α enhancement coincides with the observed intensity enhancement of pervoskite/P3HT:Au-NP films (see Figure 5(a)). In a sense, those Au-NPs can serve as scatters to redirect the optical pathway of incoming photons, which in turn enhances the probability of light absorption. Approximately, an additional 15% charges is generated at a loading concentration of 20% due to the scattering of Au-NPs as revealed by △IPCE spectra.


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(a)

(b) 0% 10% 20% 30%

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8

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6 400

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Figure 6. (a) The reflectance spectra of devices with and with incorporated Au-NPs Insect: schematic illustration of the incident light path in a completed device. (b) The additional absorption (△α) spectrum resulting from the difference of reflectance spectra between Au-NPs doped and pristine P3HT as HTLs.

Based on the above results, the role of Au-NPs in the P3HT matrix and in the whole perovskite cell can be understood as follows. In the P3HT matrix, Au-NPs function as an additive to tune the polymer morphology, which directly modulates the optical absorption as well as both conductivity and mobility of the nanocomposites. In the whole cell, Au-NPs in HTL can scatter incident photons to lengthen the traveling pathway, which in turn enhances the probability of light absorption in the photoactive layer. In addition to the perovskite absorber, P3HT can also harvest photons and generate excitons. According to the energy band alignment shown in Figure 7, those excitons generated in P3HT are likely to be dissociated at the perovskite/P3HT interfaces. Due to the short exciton diffusion length of polymer materials, only the excitons generated less than 20 nm away from the interface can possibly be dissociated. This effective


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volume is less than one-twelfth the photoactive volume of perovskite. Therefore, the role of P3HT layer in the studied structure may primary serve as the HTL although the generation of excitons does occur after absorbing photons. Therefore, from the optical perspective, the scattering effect from Au-NPs in P3HT, which actually extends the optical path throughout the whole perovskite film, contributes to promote the exciton generation leading to the improved Jsc. From the electrical viewpoint, conductivity and mobility of a HTL are also important in determining the device performance. The conductivity (mobility) of the HTL increases three (four) times after doping 20% of Au-NPs and this property is reflected in the reduction of the series resistance (Rs) of devices. As Rs calculated from the J – V curves, by taking the reciprocal of the slope at Voc, Rs reduces from (12.57±2.74) to (10.52±2.17) Ωcm 2 after Au-NP doping, an almost one-third dropping of Rs for the best device (8.35 Ωcm2). The reduced Rs is beneficial for charge transport because carriers can be extracted out of the photoactive layer to the anode electrode through a higher conducting and efficient pathway, which reduces the unnecessary ohmic losses, a non-ideal space charge distribution within the device, and carrier recombination at perovskite/HTL interface. All those improved electrical properties of the HTL are also beneficial for Jsc and FF enhancement.

Figure 7. Energy band diagram for the fabricated perovskite solar cells.


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We stress here that the Voc values obtained in this study (~0.7 V) is slightly lower than most recent literature reports based on the similar device structure ( > 0.8 V). It is known that the Voc of a hybrid solar cell is closely related to the differences between the quasi-Fermi level of the n-type semiconductor and the highest-occupied-molecular-level (HOMO) level of the p-type HTL under illumination. Based on the energy band alignment of the studied device (see Figure 7), the ideal Voc value is close to the difference between the conduction band of TiO2 and the HOMO level of P3HT, which yields 1.0 eV. However, several studies show Voc values in the range of 0.6–0.9 V for the similar device structure.14,30,56,57 One of the possible reasons responsible for the difference among those devices is the preparation of the TiO2 layer. The quality of TiO2 film can greatly affect its charge transport, especially for sol-gel derived TiO2 film with more defects. In such case, the quasi-Fermi level of the prepared TiO2 film will lie deeper in the bandgap as compared to a highly crystallized one, which in-turn lowering the Voc. This result implies that based on our approach the cell efficiency can be further improved by a high quality TiO2 film.

We note that both electrical and optical properties exhibit the best condition for the devices doped with 30% Au-NPs; however, the performance of the devices shows a lower PCE due to the dropping of Jsc, Voc, and FF as well as slightly increased Rs. We suspect that at high concentration (~ 107 particles/cm2), it is highly probable for Au-NPs to be in contact with the perovskite surface resulting in severe carrier recombination through Au-NPs.


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Here, we compare our study with previous reports regarding to the properties of HTL materials and their influences on cell performance. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (tBP) are commonly used ionic additives to enhance the conductivity of polymers and small molecules. However, Guo et al.57 have shown that devices using P3HT doped with LiTFSI and tBP exhibit a 34.8% increment in PCE relative to those using native P3HT. In our study, there is a 25% (or 37% for the best device) improvement in PCE value for Au-NP incorporated devices as compared to the standard cell. Since Au-NPdoped and ionic additive-doped P3HT devices show similar enhancement factor in PCE, the performance for both types of devices should be similar. The advantages of incorporating AuNPs into a conjugated polymer as a HTL material are not only to improve the conductivity of the HTL due to their synergetic properties arising from the high conductivity in contrast to other insulting or semiconducting nanoparticles, but also enhance the light trapping of a device. Thus, the reported P3HT:Au-NPs HTL material is bi-functional and serves as both electrical and optical manipulator in devices. Unlike the common approach of improving conductivity through chemically doping spiro-MeOTAD and other conjugated polymers,13, 58 our method provides an additional benefit in manipulating optical properties of devices, which is unique in this study. On the other hand, it is reported that dopant-free-HTL cells are more stable than those with ionic additive-doped spiro-OMeTAD and oligomers due to the deliquenscent behavior of lithium salt, which tends to enhance the affinity of water molecules to diffuse into the underneath perovskite layer.59,60 Concerning the stability issue, using non-ionic Au-NPs as dopants should be able to avoid the penetration of water molecules though P3HT.


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Zhang et al.61 have tried to incorporate core-shell Au@SiO2 NPs into the perovskite layer through a mesoporous Al2O3 scaffold and deliver a ~ 10% increase in PCE under similar values of Voc and FF. In their case, the enhanced electric field around the Au-NPs can directly act on the perovskite, resulting in lowering the exciton binding energy. In contrast, we directly introduce Au-NPs into the rear HTL to improve both the electrical and optical properties of devices simultaneously, and leading to a more than 25% increment in PCE based on largely enhanced Jsc and FF. Though Zhang et al.’s and our approaches present different underlying mechanisms in adopting LSPR concept, it seems that our design targeting on both optical and electrical manipulation demonstrates a higher outcome in boosting up the PCE of perovskite solar cells. Fortunately, it is likely that both approaches can be implemented into one perovskite solar cell to further boost up the performance of the device, which should be an interesting topic awaiting for future investigation.

SUMMARY In summary, we present an effective method to enhance both electrical and optical properties of perovskite solar cells by replacing the hydroscopic spiro-MeOTAD with a hydrophobic conjugated polymer based nanocomposite, P3HT:Au-NPs, as the HTL. By varying the concentration of Au-NPs in the polymer matrix, both the electrical conductivity and the carrier mobility of the nanocomposite film are improved due to tuning the polymer chain ordering and carrier hopping motion in the presence of Au-NPs. Integrating this polymer nanocomposite with an optimum loading of 20% into a perovskite solar cell as the HTL, the device exhibits a significantly enhanced Jsc, FF, and PCE as compared to the standard cell. In addition to the improved electrical properties, the light absorption efficiency of the new device is also enhanced


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based on the scattering effect from Au-NPs as revealed in the reduced reflectance of the device. As a result, the highest PCE of 10.71% has been achieved, which is a more than 25% increment. The presented approach can also be generalized to other conjugated polymers or small molecules to further advance the development of organic optoelectronic devices with high performance.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (F. C. Hsu) * E-mail: [email protected] (L. Wang) * E-mail: [email protected] (Y. F. Chen)

ACKNOWLEDGMENT This work is supported by the Ministry of Science and Technology, Taiwan (Project Nos. MOST 102-2112-M-239-001-MY3, and MOST 104-3113-E-002-010) and Academia Sinica of Taiwan (Project no. AS-103-SS-A02).

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(59) Kwon, S. Y.; Lim, J.; Yun, H.-J.; Kim, Y. H.; Park, T. Kwon, S. Y.; Lim, J.; Yun, H.-J.; Kim, Y. H.; Park, T. A Diketopyrrolopyrrole-Containing Hole Transporting Conjugated Polymer for Use in Efficient Stable Organic–Inorganic Hybrid Solar Cells Based on a Perovskite. Energy Environ. Sci. 2014,7, 1454 –1460. (60) Liu, J.; Wu, Y.; Qin, C.; Yang, X.; Yasuda, T.; Islam, A; Zhang, K.; Peng, W.; Chen, W.; Han, L. A Dopant-Free Hole-Transporting Material for Efficient and Stable Perovskite Solar Cells. Energy Environ. Sci. 2014,7, 2953–2967. (61) Zhang, W.; Saliba, M.; Stranks, S. D.; Sun, Y.; Shi, X.; Wiesner, U.; Snaith, H.J. Enhancement of Perovskite-Based Solar Cells Employing Core-Shell Metal Nanoparticles. Nano Lett. 2013, 13, 4505 – 4510 .


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Bifunctional Polymer Nanocomposites as Hole-Transport Layers for

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