Universal and Versatile MoO3-Based Hole Transport

Apr 28, 2014 - The first reported solution-processed MoO3 was obtained from an acidic precursor with 160 °C annealing.17 To avoid the high-temperatur...
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Universal and Versatile MoO3‑Based Hole Transport Layers for Efficient and Stable Polymer Solar Cells Xiaotian Hu,† Lie Chen,†,‡ and Yiwang Chen*,†,‡ †

Institute of Polymers/Department of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China Jiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

‡

S Supporting Information *

ABSTRACT: Two solution-processed and highly dispersed MoO 3 called d-(MoO3)120 and d-(MoO3)15 with sizes of 120 nm and extremely smaller 15 nm, respectively, are applied into polymer solar cells, and the evaporated MoO3 as hole transport layers (HTLs) in devices is also compared. It is the first time it has been found that the different size of MoO3 can induce the quite different morphologies of the HTLs and their upper active layers due to the unexpectedly caused difference in the surface energy levels. It is worthy to note that the performance of the device with solution-processed d-(MoO3)15 is higher than that of the device with poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTLs and even comparable to that of the device with optimized evaporated-MoO3. Simulated by the transfer matrix method, the light intensity and the exciton generation rate in the active layer are found to be greatly enhanced by incorporation of an ultrathin MoO3 combined with PEDOT:PSS. As a result, by inserting a layer of evaporated MoO3 (e-MoO3) between the ITO and PEDOT:PSS, power conversion efficiency (PCE) can be dramatically improved to 7.10% for PBDTTT-C-T:PC71BM. Moreover, the e-MoO3/PEDOT:PSS bilayer also ensures good stability for the devices, due to the MoO3 preventing moisture and oxygen attack and protecting ITO from corrosion caused by the acid PEDOT:PSS.

1. INTRODUCTION In the past two decades, polymer solar cells (PSCs) have attracted more and more attention due to the great potential for the fabrication of flexible, large-area, lightweight, and simple structure devices through roll to roll technology.1−8 Recently, significant improvement in the power conversion efficiency (PCE) of polymer solar cells has been achieved with a value of ∼10%.9,10 However, there are still some challenges to satisfy the demands of commercial production, such as the further improvement in the PCE (>10%) and lifetime of PSCs. Exploiting the intimate interfacial layers with improved energy level alignment is an effective method to achieve the above goals.9,11−13 Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is most often used as the hole transport layer (HTL) because of its advantages of high transparency, high work function, smooth surface, high conductivity, and easy solution process.14 However, the acidic (pH ≈ 1) and hygroscopic properties of PEDOT:PSS15,16 always result in a dramatic degradation of the PSCs in ambient conditions. Transition metal oxides, such as MoO3, V2O5, NiO, WO3, ReO3, and RuO2, have been successfully demonstrated as alternatives to PEDOT:PSS,17−22 based on their good stability in polymer solar cells. Among these transition metal oxides, MoO3 has been paid great attention for its high transmittance, nontoxicity, and high work function. Up to now, the deposition of MoO3 on the electrode occurred mainly through thermal evaporation and sputtering,23,24 which are not compatible with large-area, flexible, low-temperature, and low-cost processing. © XXXX American Chemical Society

Therefore, the solution-processed MoO3 interfacial layer has been widely mentioned. The first reported solution-processed MoO3 was obtained from an acidic precursor with 160 °C annealing.17 To avoid the high-temperature process, hydrolysis of the precursor at low temperature was employed to prepare the MoO3 interfacial layer.18,25,26 Recently, the MoO3 anode interfacial layer deposited from a nanoparticle suspension at low temperatures enabled the PSC with comparable performance to the reference device with a PEDOT:PSS, but the mentioned dispersant agent was undisclosed.27,28 Furthermore, a combination of advantages of MoO3 and PEDOT:PSS as a composite HTL can also be found to promote the device performance. Despite the successful application in the PSCs, the mechanism of such an interface contact layer in improving the device performance has been seldom studied so far. To improve device efficiency and stability simultaneously, here, we develop a facile method to fabricate the solutionprocessed MoO3 monolayer and e-MoO3/PEDOT:PSS bilayer HTL for polymer solar cells based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methyl ester (PC61BM). The effects of MoO3-based HTLs on the device performance are found to be greatly influenced by the surface energy which is caused by the MoO3 crystal size, the light intensity, and the exciton generation rate which is related to the composition and thickness of the HTL. Unexpectedly, MoO3 particles with the Received: February 25, 2014 Revised: April 23, 2014

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Figure 1. (A) SEM images of MoO3 nanoparticles before plasma treatment, (B) with dispersant, and (C) without dispersant. (Insetted images are camera pictures of dispersing MoO3.)

75% for the O atom. Therefore, it can be confirmed that the chemical formula of the nanoparticles is MoO3. Since the size of the prepared MoO3 nanoparticles is so small (15 nm), estimated by scanning electron microscopy (SEM), the novel dispersant poly(acrylic acid)-b-poly(butyl acrylate) (PAA-b-PBA, Figure 1A) was added into MoO3 p-xylene solution to avoid the aggregation of the MoO3 nanoparticles. The inner picture of Figure 1B and C is MoO3 nanoparticle suspension with and without (defined as (MoO3)15) dispersant. It obviously shows that PAA-b-PBA dispersant can well disperse MoO3 nanoparticles in p-xylene solvent, compared with the MoO3 nanoparticle suspension without dispersant displaying a large amount of precipitation at the bottom of the bottle. The dramatically improved dispersity of d-(MoO3)15 film spin-coated on the ITO substrate can be verified by SEM. As shown in Figure 1B, the PAA-b-PBA dispersant brought a significantly morphological change on the interfacial layers. With respect to (MoO3)15 thin film with some large-scale aggregations, the d-(MoO3)15 film is much smoother and more uniform (Figure 1C). To give a deeper insight into the mechanism of the MoO3 interfacial layer, such as nanoparticle size, surface energy, and dispersity of MoO3, for the device performance, the bigger crystal size (∼120 nm, MoO3 nanoparticle is the commercial product) MoO3 suspensions with (defined as d-(MoO3)120) and without ((MoO3)120) PAA-b-PBA were also prepared for comparison. The MoO3 nanoparticles were empolyed as the anode interfacial HTL in conventional P3HT:PC61BM polymer solar cells on glass-ITO substrates. Figure 2 shows the device structure of the solution-processed polymer solar cells, and the current density−voltage (J−V) curves of the PSCs with the MoO3 nanoparticle layer are plotted in Figure 3, under the illumination of AM 1.5G, 100 mW cm−2. The related performance parameters are provided in Table 1. The two devices with (MoO3)120 and (MoO3)15 as the HTLs only give a low PCE of 0.91% and 1.5%, respectively, due to the big aggregations of MoO3. With addition of dispersant into the HTLs, the device performance can be dramatically improved to 2.06% for d-(MoO3)120 and 3.35% for d-(MoO3)15, owing to the more homogeneous morphology from the highly dispersed nanoparticles. The improvement of PCE

particularly small size of 15 nm (defined as d-(MoO3)15) develop a more condensed and uniform film with smaller surface energy with respect to the ones with larger size of 120 nm. As a result, the reduced surface energy of solution-processed MoO3/dispersant suspensions with the particularly small size of 15 nm (defined as d-(MoO3)15) HTL forms an intimate contact and strong adhesion with the active layer, which improves the PCE of polymer solar cells based on P3HT:PC61BM from 2.06% (with 120 nm size of MoO3/dispersant suspensions defined as d-(MoO3)120) to 3.35%, even comparable to that of the evaporated-MoO3-based device. Furthermore, through the rational combination of the advantages of versatile MoO3 interlayers with the guide of the above-obtained results, the device performance was further optimized, and PCE has been improved to 3.98% by incorporation of a layer of evaporated MoO3 (e-MoO3) between the ITO and PEDOT:PSS. The versatile MoO3-based HTLs show a good universality on the device based on poly[4,8-bis(2-ethyl-hexyl-thiophene-5-yl)benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-(2-ethyl-hexanoyl)thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T):29(6,6)-phenyl-C71 butyric acid methyl ester (PC71BM) blends, realizing an improved PCE of 7.10%. Moreover, these HTLs also demonstrate a good stability for the devices at the same time.

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RESULTS AND DISCUSSION The MoO3 nanoparticles were synthesized from the ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) precursor in the presence of sodium dodecylbenzenesulfonate, followed by a thorough cleaning by nitric acid and anhydrous ethanol under ultrasonic treatment. The detailed synthesis procedure is presented in the Supporting Information. Figure S1 (Supporting Information) displays X-ray diffraction (XRD) patterns of the prepared MoO3 nanoparticles. Some characteristic diffraction peaks at 13°, 23°, and 39° suggest the successful synthesis of MoO3 nanoparticles, which also can be supported by the energydispersive spectrometer (EDS) in Figure S2 and Table S1 (Supporting Information). In Table S1 (Supporting Information), the weight percentage of Mo and O is 66.65% and 33.35%, respectively, and atom percentage is 25% for the Mo atom and B

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Figure 2. Device structure for the PSCs with versatile MoO3 HTLs.

and Jsc is well consistent with the incident photon-to-electron conversion efficiency spectrum (IPCE) of PSCs (Figure 3). It is worthy to note that the performance (a PCE of 3.35% with a Voc of 0.604 V, Jsc of 8.45 mA·cm−2, and FF of 65.7%) of a solutionprocessed device with d-(MoO3)15 is higher than that of the PEDOT:PSS modified device (3.04%), and even comparable to that of the optimized evaporated-MoO3 (e-MoO3 of 9 nm thickness) based device (3.48%, Table S2 and Figure S3, Supporting Information). In Table 1, the device with d-(MoO3)15 shows a slightly higher series resistance (Rs) and lower Jsc than the e-MoO3based device, which should be ascribed to the thickness difference between the HTLs from solution process and evaporation (∼40 vs ∼9 nm). Intriguingly, the d-(MoO3)15 greatly enhances the shunt resistance (Rsh) of the device up to 8111.75 Ω cm2, 1 order of magnitude higher than the value for the evaporated MoO3 (243.27 Ω cm2). The high Rsh suggests a good interfacial modification to reduce the charge recombination,28 consequently leading to a remarkably enhanced FF, as revealed by Table 1. Therefore, the improved Rsh and FF can compensate for the loss in Rs and Jsc, resulting in the satisfying device performance. From the device parameters we can see that the MoO3 nanoparticle size has greatly influenced the device performance. Compared with the devices based on MoO3 with the size of ∼120 nm, the ones with smaller size of MoO3 achieve much better performance together with the improved Jsc and FF. Figure S4 (Supporting Information) exhibits the transmission of the two MoO3 thin films. In the range of the solar spectrum, d-(MoO3)15 and d-(MoO3)120 interfacial layers are transparent with relatively high transmission (>80%), and the transmission of d-(MoO3)15 is slightly higher than the d-(MoO3)120 one, which may contribute to the enhanced Jsc. Beyond this, the morphology of the solution-processed HTLs is also quite different. Figure 4 shows AFM height images (1 μm × 1 μm) of MoO3 nanoparticle thin films after plasma treatment. It can be found that the

Figure 3. (A) Current (J)−voltage (V) characteristics of cells based on devices with PEDOT:PSS (ITO/PEDOT:PSS/P3HT:PC61BM/LiF/Al) and with different size of MoO3 HTLs (ITO/MoO3/P3HT:PC61BM/ LiF/Al) and (B) incident photon-to-current efficiency (IPCE) of photovoltaic cells based on cells of PEDOT:PSS, d-(MoO3)120, and d-(MoO3)15.

removal of PAA-b-PBA does not alter the size, shape, and distribution of the MoO3 nanoparticles in the films. The d-(MoO3)15 film reveals a most smooth surface with a root-mean-square (Rms) roughness of 1.44 nm, even as smooth as the e-MoO3 film with Rms of 1.45 nm (Figure S5, Supporting Information). However, the d-(MoO3)120 film becomes much coarser with a Rms of 2.60 nm. It is also observed that compared with MoO3 with the size of 120 nm the highly dispersed 15 nm MoO3 was closely arranged on the ITO substrates to develop a more condensed and uniform film, which can efficiently avoid the current leakage and decrease bias-dependent carrier recombination, subsequently yielding the increased Jsc and FF. It is well-known that a high quality of buffer layer can not only improve the charge injection, transport, and collection but also favor the upper active layer with more homogeneous interpenetrating networks. Before deposition of the active layer, the interfacial contact between the MoO3 layer and the active layer was clarified by the hydrophilic property of the MoO3 layer, which was measured by contact angle measurement, as shown in Figure S6 (Supporting Information). The d-(MoO3)15, d-(MoO3)120, and e-MoO3 (∼100 nm particle size) films show contact angles of 81°, 69°, and 74° with corresponding interfacial surface energy of 33, 39, and 37 mN/m, respectively. Generally, the smaller size nanoparticles could have a higher surface energy than the bigger ones. However, an opposite result is observed in this case. The MoO3 film with the smallest size of 15 nm possesses the lowest surface energy, while the biggest size of MoO3 (120 nm) has the C

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Table 1. Performance of Solar Cell Devices (P3HT:PC61BM) with Different MoO3 Thin Filma device

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

PEDOT:PSS e-MoO3 (9 nm)b (MoO3)120 (MoO3)15 d-(MoO3)120 d-(MoO3)15

8.40 10.17 3.81 5.38 6.97 8.45

0.604 0.610 0.606 0.606 0.604 0.604

59.9 56.1 39.5 45.5 48.8 65.7

3.04 ± 0.11 3.48 ± 0.08 0.91 ± 0.22 1.50 ± 0.15 2.06 ± 0.13 3.35 ± 0.08

3.19 5.33 43.89 30.23 56.16 16.61

310.27 243.27 586.19 365.68 468.37 8,111.75

a

All the devices of this work: all values represent averages from six 6 mm2 devices on a single chip measuring under AM 1.5 with 100 mW/cm2 irradiation. b(e-MoO3) with 9 nm thickness.

Figure 4. Tapping-mode atomic force microscopy (AFM) images of (A) d-(MoO3)15 height image, (B) d-(MoO3)120 height image, (C) d-(MoO3)15 phase image, and (D) d-(MoO3)120 phase image after plasma treatment (scan range: 1 μm × 1 μm).

Figure 5. Tapping-mode atomic force microscopy (AFM) images of P3HT:PC61BM active layers with (A) d-(MoO3)15 height image, (B) d-(MoO3)120 height image, (C) d-(MoO3)15 phase image, and (D) d-(MoO3)120 phase image (scan range: 3 μm × 3 μm).

highest surface energy. This is because the d-(MoO3)15 and 100 nm size e-MoO3 create much smoother and uniform film surface than d-(MoO3)120. For 15 nm MoO3 and 100 nm e-MoO3 films with the same smooth surface revealed by AFM, the smaller size MoO3 tends to form a tighter stacking of nanoparticles on the film surface, resulting in the reduced surface energy. Therefore, a relatively hydrophobic film surface of d-(MoO3)15 originating from the reduced surface energy can favor a good compatibility with the active layer, which can be proposed as schematically sketched in Figure 2. For d-(MoO3)15, the active layer can be readily deposited on the smooth surface of MoO3 to form an intimate contact with strong adhesion at the interface, and a notably high shunt resistance (Rsh) thus has been achieved. If the HTL surface is rough and hydrophilic, a large amount of voids and defects will be located at the MoO3/active layer interface, leading to the catastrophe of PCE. Figure 5 shows AFM height images (3 μm × 3 μm) of P3HT:PC61BM active layers spin-coated on the MoO3 interfacial layers with different nanoparticle size. It is obviously seen that the active layer with the d-(MoO3)120 film represents a severe phase separation, while the active layer with d-(MoO3)15 film performs the smoother morphology (Rms: 1.12 vs 2.77), which also should be partly responsible for the enhanced Jsc and FF. Moreover, the work function was determined via Kelvin probe. It can be obviously discovered that d-(MoO3)15 exhibits a bit lower work function (5.17 eV) than the d-(MoO3)120 (5.35 eV), coming from the tight arrangement of MoO3 without defects

reducing the bandgap between the Fermi level and vacuum level (Figure S7 and Figure S8, Supporting Information). The lower work function is very suitable for its application as a HTL on P3HT:PC61BM polymer solar cells. It is because the n-type characteristics can avoid the charge from capturing and gathering at the interface to form a recombination center, and the hole transport through the metal oxides becomes more open and is realized by extracting electrons through their conduction bands. The charge mobility was also investigated by the measurements of hole-only devices using the Mott−Gurney space charge limited current (SCLC) model (Figure S9, Supporting Information). As expected, the hole mobility of d-(MoO3)15 is 3.19 × 10−4 cm2/(V s), 1 order of magnitude higher over the d-(MoO3)120 (7.46 × 10−5 cm2/(V s)). Similarly, the conductivities of the films calculated from the slopes of the curve are 8.6 × 10−6 S/cm for d-(MoO3)15 and 3.0 × 10−6 S/cm for d-(MoO3)120. The d-(MoO3)15 film is more condensed and uniform than the d-(MoO3)120 film, providing more efficient pathways for hole transportation. As a result, the improved conductivity and hole mobility again prove the reasons behind the increased efficiency. It is well-known that MoO3 film can effectively prevent the diffusion of humidity compared with a hygroscopic and diffuse PEDOT:PSS film.30 The air stability of P3HT:PC61BM solar cells fabricated with d-(MoO3)15 and PEDOT:PSS HTLs under ambient conditions or in a glovebox was investigated, as shown in Figure 6. No matter what the measure condition is, the D

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a Jsc of 14.79 mA cm−2, and a FF of 59.1%, which obviously surpasses the PCE of the control device with the PEDOT:PSS buffer layer (6.21%). Similarly to the results from the P3HT:PC61BM devices, the improvements in the PCE are mainly attributed to the improved FF and Rsh. The size of the MoO3 nanoparticles has a great impact on the performance, and the bigger size of MoO3 nanoparticles leads to an inferior performance compared to the smaller one. The optimized performance is correlated with the most smooth of the active layer (Rms: 1.74) induced by the d-(MoO3)15 (Figure S10, Supporting Information). The smooth interfacial contact also provides the great air stability for the PBDTTT-C-T:PC71BM PSCs, as shown in Figure S11 (Supporting Information). Combining the advantages of PEDOT:PSS and MoO3 is found to be an effective approach to improve the device efficiency.31,32 Therefore, for further optimization, a bilayer of MoO3/PEDOT:PSS was employed into PSCs as the anode buffer layer. To obtain a self-standing MoO3/PEDOT:PSS film without intermixing and improve the device stability by avoiding the corrosion of the ITO from the acid PEDOT:PSS, a solutionprocessed d-(MoO3)15 film was inserted between the ITO and PEDOT:PSS. The structure of the device (ITO/MoO3/ PEDOT:PSS/P3HT:PC61BM/LiF/Al) is shown in Figure 2. Disappointingly, all the devices deliver very poor performance with a remarkably reduced PCE of 2.7% (Figure S12 and Table S3, Supporting Information). To find out the reason for poor efficiency of PSCs caused by the interfacial bilayer, the simulation of the spatial distribution of the squared optical electric field |E|2 (normalized to the incoming plane wave) for the devices with MoO3, PEDOT:PSS, or MoO3/ PEDOT:PSS HTLs is shown in Figure 8, and the exciton generation rate for the devices is also simulated (Figure S13, Supporting Information). The light intensity redistribution within the device was simulated by the transfer matrix method.33 Since the wavelength absorbance of the P3HT:PC61BM blend is from 400 to 600 nm (Figure S14, Supporting Information), the light intensity redistribution of 400, 500, and 600 nm was thus investigated. From Figure 8, it is clearly discovered that there is a dramatic improvement of the optical electric field intensity of all wavelengths in the active layer of the devices with MoO3 nanoparticles with respect to the devices with PEDOT:PSS. A great increase of excition generation rate can also been found in MoO3-modified devices. However, incorporation of the MoO3/ PEDOT:PSS bilayer with the total thickness of 60 nm (MoO3/ PEDOT:PSS, 30 nm/30 nm) obviously decreases optical electric field intensity and exciton generation rate in the P3HT:PC61BM, displayed in Figure 8 and Figure S14 (Supporting Information). It can be confirmed that too thick an interfacial layer prepared by the solution process prevents light from penetrating into the active layer, which should be responsible for the decreased efficiency. Interestingly, when the thickness of MoO3 in the MoO3/PEDOT:PSS bilayer reduces to ∼10 nm, but with the thickness of PEDOT:PSS unchanged, the optical electric field intensity and exciton generation rate in the P3HT:PC61BM can be greatly promoted and an improved PCE of the devices can be thus anticipated (Figure 8 and Figure S14, Supporting Information).

Figure 6. Normalized efficiency decay of P3HT:PC61BM PSCs with PEDOT:PSS and d-(MoO3)15 as HTLs at ambient atmosphere or in glovebox conditions.

unencapsulated PSCs based on the d-(MoO3)15 film exhibit good stability. About 90% of the original PCE remains after 9 days stored in inert or ambient conditions, whereas under the same conditions, the PCE with PEDOT:PSS film device degraded sharply and even to zero. The improved stability of the device may also be related to the fact that the absence of porosity and nanoscopic voids of the d-(MoO3)15 HTL can provide an intimate contact with the active layers to prevent moisture and oxygen attack.18 To further testify the universality of the MoO3 nanoparticle suspension interfacial layer in the PSCs, we selected a highperformance low-bandgap polymer PBDTTT-C-T as the donor in PSCs. The J−V curves of the PSCs based on the PBDTTT-CT:PC71BM blend are shown in Figure 7, and the device

Figure 7. Current (J)−voltage (V) characteristics of cells based on devices with PEDOT:PSS (ITO/PEDOT:PSS/PBDTTT-CT:PC71BM/LiF/Al) and with different MoO3 HTLs (ITO/MoO3/ PBDTTT-C-T:PC71BM/LiF/Al).

parameters are listed in Table 2. Among these devices with PEDOT:PSS, d-(MoO3)120, or d-(MoO3)15, the device with d-(MoO3)15 shows the best PCE of 6.52% with a Voc of 0.745 V,

Table 2. Performance of Solar Cell Devices (PBDTTT-C-T:PC71BM) with Different MoO3 Thin Film device

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

PEDOT:PSS d-(MoO3)120 d-(MoO3)15

14.17 11.75 14.79

0.745 0.745 0.745

58.8 46.3 59.1

6.21 ± 0.24 4.06 ± 0.22 6.52 ± 0.15

3.03 9.34 9.29

342.22 489.87 782.1

E

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Figure 8. Simulated spatial distribution of the squared optical electric field |E|2 (normalized to the incoming plane wave) for the devices with (A) MoO3, (B) PEDOT:PSS, (C) MoO3 (a thickness of 30 nm)/PEDOT:PSS anode interfacial layers, and (D) MoO3 (a thickness of 10 nm)/PEDOT:PSS anode interfacial layers for 400, 500, and 600 nm with the same y-axes scale. (All the simulations were calculated via the Matlab program, and the Matlab files were provided from McGehee group, Center for Advanced Molecular Photovoltaics, Stanford University.)

The results obtained from the simulations are in good agreement with the Jsc and IPCE of these devices. Inspired by the results from simulation of the spatial distribution of the squared optical electric field, the devices with the thinner MoO3/PEDOT:PSS bilayer as the HTL were fabricated, where the thickness of e-MoO3 varied from 3, 6, 9, and 12 nm and the thickness of PEDOT:PSS was kept constant (∼40 nm). Because the ultrathin MoO3 film is difficult to obtain by common solution spin-coating, the MoO3 layer has to be deposited by thermal evaporation (referred to as e-MoO3). Each layer in the device with self-standing film can be clearly detected by the cross-sectional SEM image, as shown in Figure S15 (Supporting Information), and the component of the interfacial bilayers has also been confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Figure S16 (Supporting Information) shows XPS spectra of Mo 3d levels for the 6 nm e-MoO3, 6 nm e-MoO3/PEDOT:PSS, 9 nm e-MoO3/PEDOT:PSS, and pristine PEDOT:PSS films. The Mo 3d core level of pure e-MoO3 film exhibits Mo 3d3/2 (235.8 eV) and Mo 3d5/2 (232.7 eV).34 In contrast, levels of Mo 3d of both e-MoO3/PEDOT:PSS films almost completely disappear, meaning that the e-MoO3 film has been fully covered by the PEDOT:PSS layer. Figure S17

Figure 9. Current (J)−voltage (V) characteristics of cells based on devices with PEDOT:PSS (ITO/PEDOT:PSS/P3HT:PC61BM/LiF/ Al) and with different thickness e-MoO3 of bilayer HTLs (ITO/MoO3/ PEDOT:PSS/P3HT:PC61BM/LiF/Al).

(Supporting Information) is the ultraviolet photoelectron spectroscopy (UPS) of e-MoO3, e-MoO3/PEDOT:PSS, and PEDOT:PSS. By the calculation, the schematic energy diagram of the F

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Table 3. Performance of Solar Cell Devices (P3HT:PC61BM) with Different Thickness e-MoO3 Thin Film device

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

∼40 nm PEDOT:PSS 3 nm e-MoO3/40 nm PEDOT:PSS 6 nm e-MoO3/40 nm PEDOT:PSS 9 nm e-MoO3/40 nm PEDOT:PSS 12 nm e-MoO3/40 nm PEDOT:PSS

8.40 10.18 10.15 9.82 9.49

0.604 0.606 0.611 0.610 0.613

59.9 60.5 63.5 52.0 52.1

3.04 ± 0.11 3.72 ± 0.08 3.93 ± 0.08 3.13 ± 0.10 3.04 ± 0.15

3.19 15.67 12.90 30.41 41.97

310.27 465.09 2028.71 230.69 276.42

Figure 10. Tapping-mode atomic force microscopy (AFM) images of (A) PEDOT:PSS height image, (B) 6 nm e-MoO3 height image, (C) 6 nm e-MoO3/40 nm PEDOT:PSS height image, (D) PEDOT:PSS three-dimensional image, (E) 6 nm e-MoO3 three-dimensional image, and (F) 6 nm e-MoO3/40 nm PEDOT:PSS three-dimensional image (scan range: 1 μm × 1 μm).

PEDOT:PSS, e-MoO3/PEDOT:PSS, and e-MoO3 involved in the PSCs was shown in Figure S18 (Supporting Information). The work functions of e-MoO3, e-MoO3/PEDOT:PSS, and PEDOT:PSS are 5.37, 5.11, and 4.97 eV, respectively. The moderate work function of the e-MoO3/PEDOT:PSS layer located between e-MoO3 and PEDOT:PSS indicates a better matched energy alignment existing in the device. Figure 9 displays the illuminated current−voltage (J−V) characteristics of P3HT:PC61BM solar cells using the e-MoO3/ PEDOT:PSS anode interfacial layer with different thicknesses. The e-MoO3/PEDOT:PSS bilayers with the ultrathin e-MoO3 film demonstrate enhanced performance compared to the MoO3 and PEDOT:PSS monolayers, and the device with the 6 nm e-MoO3/40 nm PEDOT:PSS bilayer shows the best PCE of 3.93%, with a Voc of 0.611 V, a Jsc of 10.15 mA cm−2, and a FF of 63.5% (Table 3). The improved PCE of e-MoO3/PEDOT:PSS with the 6 nm thickness of e-MoO3 is ascribed to the notable Jsc and FF. Besides the increased optical electric field intensity and exciton generation rate as mentioned above, the well-developed morphology of the HTL is also attributed to the Jsc and FF enhancement. As revealed by AFM height and three-dimensional images (Figure 10), the 6 nm MoO3/40 nm PEDOT:PSS interfacial bilayer shows a much more smooth and homogeneous morphology (Rms: 0.82) than PEDOT:PSS and e-MoO3 layers (Rms: 1.62 and 1.45, respectively), which can enhance the interfacial contact with the active layer and favor the better

Figure 11. Current (J)−voltage (V) characteristics of cells based on devices with different thickness PEDOT:PSS (ITO/e-MoO3/PEDOT:PSS/PBDTTT-C-T:PC71BM/LiF/Al).

morphology of the active layer, resulting in improved PCE for the PCSs. The thickness of the e-MoO3 layer also exerts great influence on the device performance. A too thin or too thick e-MoO3 layer could result in a fluctuation for PCE through the decrease in the values of Jsc, FF, and Rsh (Table 3). Also, the thickness of the PEDOT:PSS layer in the 6 nm e-MoO3/ PEDOT:PSS on the device performance is checked, and the 6 nm G

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Article

Table 4. Performance of Solar Cell Devices (PBDTTT-C-T:PC71BM) with Different Thickness PEDOT:PSS Thin Film 6 nm e-MoO3/25 nm PEDOT:PSS 6 nm e-MoO3/40 nm PEDOT:PSS 6 nm e-MoO3/65 nm PEDOT:PSS

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

15.23 15.79 15.15

0.745 0.749 0.745

57.9 60.0 58.3

6.57 ± 0.22 7.10 ± 0.18 6.58 ± 0.21

11.27 9.06 12.71

439.92 460.05 421.8

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CONCLUSIONS In summary, the solution-processed MoO3 films from the dispersed suspension with different crystal sizes and the e-MoO3/ PEDOT:PSS bilayer with different thickness of evaporated MoO3 were demonstrated as the HTLs for efficient and air-stable PSCs. The proper block polymer dispersant can promote a highly uniform dispersion of the MoO3 with different crystal sizes in the suspension and realize a smooth and homogeneous film, especially for the MoO3 with the extremely small size (15 nm). Amazingly, such extremely small size MoO3 afforded the lower surface energy of the MoO3 film compared to the MoO3 with bigger size, due to the smooth film surface from the tight crystal stacking, which provided an intimate contact with the active layer and dramatically improved the PCE of polymer solar cells. In addition, an ultrathin evaporated e-MoO3 combined with PEDOT:PSS bilayer enhanced the light intensity and the exciton generation rate in the active layer, consequently leading to a further optimized PCE of 3.98% for P3HT:PC61BM and 7.10% for PBDTTT-C-T:PC71BM. Moreover, the e-MoO3/PEDOT:PSS also improved the stability for the devices because the MoO3 not only can prevent the moisture and oxygen attacking but also protect ITO from corrosion caused by the acid PEDOT:PSS. These diversified MoO3 films provide a bright opportunity for understanding the mechanism of this interfacial layer and also imply a promising candidate for fabricating high performance PSCs.

e-MoO3/40 nm PEDOT:PSS is outstanding among these HTLs when performed in PSCs (Figure S19 and Table S4, Supporting Information). Figure S20 (Supporting Information) shows the incident photon conversion efficiency spectra of the devices. An obvious increase of IPCE at wavelengths between 400 and 600 nm was observed for the devices with the 6 nm e-MoO3/40 nm PEDOT:PSS layers in comparison to the control device. The maximum IPCE is over 70%, indicative of efficient photonto-electron conversion. The optimized e-MoO3/PEDOT:PSS was also employed in the PBDTTT-C-T:PC71BM devices (Figure 11 and Table 4). Similarly, among the PSCs based on the PBDTTTC-T:PC71BM blend, the device with 6 nm MoO3/40 nm PEDOT:PSS buffer layer presents a best PCE of 7.10% with a Voc of 0.749 V, a Jsc of 15.79 mA cm−2, and a FF of 60.0%, which is distinctly higher than the PCE of 6.52% for the device with the MoO3 monolayer. For the e-MoO3/PEDOT:PSS devices, of course, the condensed and uniform MoO3 can improve the stability by preventing moisture and oxygen attack. However, at the same time, since e-MoO3 was evaporated on the ITO substrate before deposition of PEDOT:PSS, the device stability is also expected to be improved by avoiding the corrosion of the ITO from the acid PEDOT:PSS.23 Figure 12 demonstrated the stability of the devices

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ASSOCIATED CONTENT

S Supporting Information *

The detailed experimental sections and the other characterization of devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 12. Normalized efficiency decay of P3HT:PC61BM PSCs with different PEDOT:PSS and e-MoO3 HTLs in glovebox conditions.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51273088 and 51263016).

based on e-MoO3/PEDOT:PSS layers in glovebox conditions. The PEDOT:PSS-based control device (P3HT:PC61BM) degrades to an average value below 50% of the initial efficiency within 10 days. In contrast, e-MoO3/PEDOT:PSS-modified devices exhibit significant stability. The devices based on P3HT:PC61BM with 6 and 9 nm e-MoO3/PEDOT:PSS show 90% value of the initial efficiency after 10 days, and the one with 3 nm e-MoO3 shows a smaller value of nearly 70% of the initial efficiency. This is because the devices with the thicker 6 nm e-MoO3 can completely separate the contact between ITO and PEDOT:PSS, as revealed by the cross-sectional SEM image shown in Figure S15 (Supporting Information). The same happens for the P3HT:PC61BM devices: the 6 nm e-MoO3/40 nm PEDOT:PSS improves the stability of the PBDTTT-C-T:PC71BM device while maintaining approximately 90% of the initial PCE (Figure S21, Supporting Information).

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dx.doi.org/10.1021/jp501995t | J. Phys. Chem. C XXXX, XXX, XXX−XXX