Yanping Liab,
Huangzhong Yu*a,
Xinxin Huangab,
Zuping Wuab and
Mingdong Chena
aSchool of Physics and Optoelectronics, South China University of Technology, Guangzhou 510640, China. E-mail: hzhyu@scut.edu.cn; Tel: +86-20-87110768
bSchool of Materials Science & Engineering, South China University of Technology, Guangzhou 510640, China
First published on 23rd January 2017
We report a simple synthetic method to prepare amorphous molybdenum oxide (p-MoO3) using a favorably stable peroxomolybdic acid organosol as the precursor solution prepared by an ultrasonic reaction for the first time. The favorably smooth and dense surface morphology of the p-MoO3 layers are obtained under 150 °C thermal treatment with good optical properties and a high work function (WF) of 5.26 eV. During the annealing treatment two different oxidation states of Mo ions are observed with increasing the annealing temperature to 150 °C and 200 °C. The best performance of the P3HT:PC71BM devices with p-MoO3 anode buffer layers has been achieved under 150 °C treatment with a power conversion efficiency (PCE) of 4.02%, a VOC of 0.59 V, a JSC of 10.70 mA cm−2, and a FF of 63.7%, superior to the corresponding PEDOT:PSS modified devices. Furthermore, the performance of the PTB7:PC71BM devices with the annealed p-MoO3 buffer layers has also been dramatically improved with the best performance parameters of a PCE of 8.46%, a VOC of 0.73, a JSC of 17.02 mA cm−2, a FF of 68.1% for 150 °C. The improved performance of the devices originates from the following factors; (i) the favorable and compact surface morphology of the annealed p-MoO3 films leading to a higher rectification ratio and lower leakage current. (ii) The formation of oxygen vacancies and the growing Mo5+ cation leading to the change of WF under the annealing treatment. The highest WF of 5.26 eV for 150 °C treatment influences the built-in electric field of the devices with the photocurrent being extracted efficiently at a short-circuit.
Nowadays the aqueous poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is typically used as the anode buffer layer in PSCs. However, PEDOT: PSS has the hygroscopic and acidic nature with an associated reduction in device stability.13,14 Therefore, developing a straightforward method to prepare more moderate, stable and effective anode interlayers is highly desirable for the low-cost, high throughput manufacturing of efficient PSC modules. In this regard, transition metal oxide semiconductors including NiO,15,16 MoO3,17,18 V2O5 (ref. 19) and WO3 (ref. 20) are emerging as a promising candidates of substitutes for PEDOT:PSS as the hole-transporting layer (HTL), which possess the demonstrable advantages of the wide range of energy level aligning capabilities, high transmittance in the visible region, and excellent ambient stability. Moreover, the developed solution-processed metal oxides semiconductors are compatible with low-cost all-solution high-throughput manufacturing of PSCs. Among these semiconductors, solution-processed MoO3 thin films have been prepared from different precursors such as ammonium molybdate,21–23 MoO2(acac)2,24,25 Mo(CO)3(EtCN)3 (ref. 26) and dispersed MoO3 nanoparticles27,28 and proved to be a promising candidate as efficient hole extraction layers. However, many weak points can be discerned in the above solution-based synthetic methods for MoO3 films, which may ultimately decrease their hole-transporting efficiency when they become part of an organic optoelectronic devices. For instance, the inadequately homogeneous surface of MoO3 films that are usually formed, and the high temperature of thermal treatment along with additional process steps (such as oxygen plasma treatment) are generally needed after spin-coating the precursor solution. In addition, the sol–gel method involving dissolution of MoO3 in hydrogen peroxide, and then dispersion with polyethylene glycol and 2-methoxyethanol, resulting in smooth MoO3 films after the time-consuming reflux and high temperature (275 °C) thermal treatment.29,30
In this paper, we report a simply synthetic method to prepare the favorably stable dark blue peroxomolybdic acid organosol by the ultrasonic reaction for the first time. Then the molybdenum oxide (marked as p-MoO3) films with varying reduction degrees are prepared using the peroxomolybdic acid organosol as the precursor solution by spin-coating method. The optical characteristic, surface topography and electrical characteristics of p-MoO3 films annealed in a broad temperature range are compared and discussed in depth. In order to validate the effect of our prepared p-MoO3 films annealed under various temperature on the OPV devices, the OPV devices with the structures of ITO/HTL/P3HT (or PTB7):PC71BM/LiF/Al are fabricated. Finally, the mechanism of the efficiency enhancement upon incorporation of p-MoO3 films as anode buffer layers under various annealing temperatures is proposed.
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Fig. 1 (a) The molecular structures of P3HT, PTB7, PC71BM, p-MoO3 and PEDOT:PSS. (b) The typical device structure of P3HT:PC71BM or PTB7:PC71BM polymer solar cell. (c) Energy level diagram of the component materials used.2,31 |
The UV-vis absorption and transmittance spectra of the p-MoO3 thin films under various annealing temperature and PEDOT:PSS thin film were measured with a HP 8453 UV-vis spectrophotometer.
XPS analysis was carried out on an XPS/ESCA (electron spectroscopy for chemical analysis) instrument (Axis Ultra DLD from Kratos Inc.) utilizing a monochromatized Al Kα X-ray source. Spectra were referenced to the residual hydrocarbon contamination at 284.6 ± 0.1 eV. The thin films (ca. 40 nm) were deposited on the ITO substrate for XPS analysis.
The work functions of the p-MoO3 thin film samples under different annealing temperatures were investigated using Kelvin Probe measurements, carried out on a KP Technology SKP5050 in a glove-box filled in a conditioned chamber where the O2 level is <0.1 ppm.
Scanning electron microscope (SEM) and Tapping-mode atomic force microscope (AFM) images using a NanoScope NS3A system (Digital Instruments) were obtained to observe the surface morphology of the annealed films.
The current density–voltage (J–V) characteristics of the devices were performed using a computer controlled Keithley 2400 source-meter under illumination with calibrated AM1.5G (100 mW cm−2) at room temperature.
In accordance with the condition of device fabrication, we further investigated the transmission spectra of the thin films including PEDOT:PSS film to evaluate the transparency of these films in devices (Fig. 2(b)). These films are spin-coated on ITO glasses. It is obvious that transmission values of all the films show good light transmittance except in the wavelength near 500 nm and 750 nm. This phenomenon is attributed to the transparency of ITO glass and small broad absorption mentioned above.33,34 Nevertheless, the transmission values are above 80% for all films regardless of the annealing temperature, implying that p-MoO3 thin films are qualified to be good interfacial later without significant absorption losses.
To understand the surface morphology of the annealed films, SEM and AFM were performed to observe the morphology characteristics of the p-MoO3 films. Fig. 4 shows SEM images of the annealed p-MoO3 thin films with different annealing temperatures. It is clear that the surface under the condition of room temperature drying is quite uneven and not uniform with some obvious small MoO3 gains. However, it can also be noted that the surface become much flatter with the increase of heating temperature. We attribute this phenomenon to the evaporation of the solvent and progress of p-MoO3 thin film formation. When the annealing temperature is increased to 150 °C, surface topography of the p-MoO3 thin film is the most flat and uniform without any small gains. At this point, the heat treatment makes the film more compact, which prevents the current leakage of the solar cells, and is beneficial to the improvement of the devices. But further raising the annealing temperature to 200 °C, thin film appears critically gathered aggregation phenomenon and less uniform. This can be due to enhanced surface diffusion of the species leading to smaller grains joining together to grow into larger grains.
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Fig. 4 SEM images of the thin films morphology of p-MoO3 thin films morphology on ITO treated with room temperature drying (a), 100 °C (b), 150 °C (c), 200 °C (d). |
Fig. 5 shows AFM images of the p-MoO3 thin films with different annealing temperatures. The surface roughness (in Root Mean Square, RMS) of the p-MoO3 thin film treated with different annealing temperatures is 2.36 nm, 2.12 nm, 1.46 nm and 1.51 nm for room temperature drying, 100 °C, 150 °C and 200 °C, respectively. The p-MoO3 thin films treated with heat treatment of 150 °C possess the lowest surface roughness, which is quite well consistent with the result of SEM images. Low surface roughness of the p-MoO3 thin film suggests that they can smoothen the ITO substrate surface effectively to avoid local shorts and at the same time be dense enough to prevent the current leakage, which is prerequisites to guarantee their implementation as effective buffer layer for organic electronics.
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Fig. 5 AFM morphology images of p-MoO3 thin films on ITO treated with room temperature drying (a), 100 °C (b), 150 °C (c), 200 °C (d). |
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Fig. 6 X-ray photoelectron spectroscopy of p-MoO3 thin films. (a) Full scanned spectra, (b) Mo 3d core level of thin films treated with various annealing temperature. |
However, two different oxidation states of Mo ions are observed with further increasing the annealing temperature to 150 °C and 200 °C, the positions of newly formed Mo5+ peaks in the 150 °C and 200 °C annealed films fitted by two 3d5/2 and 3d3/2 doublets in the form of a Gaussian function located 231.7 eV and 234.9 eV, respectively, which is identified as Mo5+.38,39 While the positions of Mo6+ peaks annealed films has not changed with further improve the annealing temperature. The calculated Mo5+/Mo6+ ratios for the 150 °C and 200 °C annealed films are 0.030 and 0.071, respectively. By using the Mo5+/Mo6+ obtained from the Mo 3d core level spectra, the estimated Mo:O stoichiometries of p-MoO3 films annealed in the 150 °C and 200 °C are 2.99 and 2.96, respectively. It can be ascribed that the concentration of the Mo5+ component in the p-MoO3 films is greatly connected with thin film processing conditions, which coincides with the fact that preparation of p-MoO3 in the oxygen deficient environment (in glovebox) and annealing temperatures led to present Mo5+ states, corresponding to the electron transfer from O 2p to Mo 4d and generation of oxygen vacancies.39–42
To further evaluate the energetics of the solution-processed p-MoO3 thin films under annealing conditions, Kelvin probe (KP) analysis was performed to obtain the work function (WF) of annealed films in glovebox referring to literature.26 Kelvin probe analysis of the PEDOT:PSS coated on ITO as a reference gives a WF of 5.20 eV, which is in agreement with the literature.2 The WF of annealed p-MoO3 thin films shows a gradual shift towards the higher values from 4.90 eV to 5.26 eV with increasing annealing temperatures from RT to 150 °C, reaching the highest WF value of 5.26 eV with 150 °C. We attribute this phenomenon to the desorption of adsorbates and evaporation of the solvent.43,44 With further increasing the annealing temperature to 200 °C, the WF value of the film decreases slightly to 5.22 eV. The main reason may be linked with the distinctly reduced cation oxidation shown in Fig. 6(b) and occupied oxygen vacancy defect states within the band that played the part of donor levels. The WF values are shown in Table 1.
Anode interface | T (°C) | WF (eV) |
---|---|---|
PEDOT:PSS | 150 | 5.20 |
p-MoO3 | RT | 4.90 |
p-MoO3 | 100 | 5.10 |
p-MoO3 | 150 | 5.26 |
p-MoO3 | 200 | 5.22 |
(A) ITO/PEDOT:PSS (150 °C)/P3HT:PC71BM/LiF/Al.
(B) ITO/p-MoO3 (RT)/P3HT:PC71BM/LiF/Al.
(C) ITO/p-MoO3 (100 °C)/P3HT:PC71BM/LiF/Al.
(D) ITO/p-MoO3/P3HT:PC71BM/LiF/Al.
(E) ITO/p-MoO3 (200 °C)/P3HT:PC71BM/LiF/Al.
Devices A is the control sample with a PEDOT:PSS buffer layer, devices B, C, D and E are samples with the annealed p-MoO3 layer. The device performance parameters are summarized in Table 2.
Device | T (°C) | JSC (mA cm−2) | VOC (V) | FF (%) | Rs (Ω cm2) | Rsh (Ω cm2) | PCE (%) |
---|---|---|---|---|---|---|---|
A | 150 | 10.50 | 0.59 | 63.1 | 8.15 | 968.57 | 3.91 |
B | RT | 8.49 | 0.57 | 56.9 | 10.50 | 639.03 | 2.75 |
C | 100 | 9.42 | 0.58 | 62.0 | 8.49 | 743.94 | 3.38 |
D | 150 | 10.70 | 0.59 | 63.7 | 8.08 | 1145.46 | 4.02 |
E | 200 | 10.36 | 0.58 | 62.9 | 8.20 | 921.46 | 3.78 |
J–V curves of devices A–E under the illumination of AM1.5G, 100 mW cm−2 are depicted in Fig. 7(a). For device A with PEDOT:PSS buffer layer, it shows a PCE of 3.91% with a short-circuit current density (JSC) of 10.50 mA cm−2, an open-circuit voltage (VOC) of 0.59 V, and a fill factor (FF) of 63.1%. In contrast, the four parameters (VOC, JSC, FF, and PCE) of devices with the p-MoO3 buffer layer exhibit different degrees of growth under the annealing temperature increased from RT to 150 °C. The device with the p-MoO3 anode buffer layer annealed at 150 °C shows the best PCE of 4.02, with a VOC of 0.59 V, a JSC of 10.70 mA cm−2, and a FF of 63.7%, which is better than that using the PEDOT:PSS buffer layer (PCE = 3.91%). Further increasing the annealing temperature to 200 °C, the device performance is decreased apparently with a PCE of 3.78%, a VOC of 0.58 V, a JSC of 10.36 mA cm−2, and a FF of 62.9%.
A closer look at the J–V curves are shown in Fig. 7(b), we can find that the both reverse current density and leakage current of the devices are reduced with the increase of heat treatment p-MoO3 buffer layer, and the devices for 150 °C possess the lowest reverse current density and leakage current. This is because the 150 °C heat treatment makes the film more compact, which has fewer defects, reduces charge recombination, and is beneficial to the improvement of the devices. Moreover, the rectification ratio of the devices calculated based on the J–V curves at ±1.0 V is also calculated. For devices A, B, C, D and E, the rectification ratio is 2.2 × 103, 2.0 × 102, 5.1 × 102, 3.1 × 103 and 1.1 × 103, respectively. Clearly the rectification ratio values of devices D and E with the p-MoO3 buffer layer annealed under 150 °C and 200 °C are simultaneously higher than that of device B and C with the p-MoO3 buffer layer under temperature drying and annealed under 100 °C. Series resistance Rs and shunt resistance Rsh are clear as shown in Table 2. Rsh of P3HT:PC71BM based devices with p-MoO3 buffer layer annealed at 150 °C is 1145.46 Ω cm2, which is much greater than the devices with p-MoO3 buffer layer without annealing (639.03 Ω cm2), annealed at 100 °C (743.96 Ω cm2) and at 200 °C (921.46 Ω cm2). This is presumably due to the increase of compact density of the film surface (see Fig. 4(c) and 5(c)), leading to the decrease of charge recombination and leakage current, which is also in agreement with the Fig. 7(b). Moreover, the Rs of the devices decrease with the increase of heat treatment p-MoO3 buffer layer, and the value of 8.08 Ω cm2 for the device with p-MoO3 layer annealed 150 °C is the lowest among those devices with p-MoO3 layer other heat treatment temperature. Increase of heat treatment temperature usually causes the reduction of molybdenum oxide, which leads to rise in conductivity of molybdenum oxide. In general, reduced transition metal oxides (TMOs) not only shows an increase in conductivity, but also causes stronger optical absorption.45 We attribute this interesting phenomenon to a result of the forming process of p-MoO3 and a slightly non-stoichiometric composition with some oxygen deficiency leading to reduced oxides.
The improved PCE of devices with annealed p-MoO3 hole transporting layers (HTL) may origin from the following factors; (i) the favorable surface morphology of the annealed p-MoO3 HTL. As shown in SEM images (Fig. 4) and AFM images (Fig. 5), the p-MoO3 HTL spin-coated on ITO substrates without the annealing is quite not smooth and dense compared to substrates, leading to the poor device performance. By increasing the annealing temperature, the p-MoO3 surface is growing smoother and compacter with the optimum surface and smallest roughness (RMS) of 1.46 nm for 150 °C, This decreased roughness accounts for the different dark J–V characteristics (Fig. 7(b)) of the devices with p-MoO3 layer annealed various temperatures, in which the devices exhibit increasingly excellent diode characteristics with higher rectification ratio and lower leakage current under reverse bias with improved annealing temperature. It means that the annealed p-MoO3 layer is capable of lowering the leakage current caused by the disagreeable roughness of ITO substrate. Further improving the annealing temperature leads to the slight aggregation and rising back of roughness. Therefore, the contact between the p-MoO3 layer and the active layer becomes less favorable to the hole extraction.
(ii) The formation of oxygen vacancies and the growing Mo5+ cation leading to the change of WF under the annealing. UV-vis absorption spectra analysis (Fig. 2(a)) indicates the formation of oxygen vacancies of the annealed p-MoO3 layer started with the annealing temperature 150 °C and the amount of oxygen vacancies further increased with improved temperature to 200 °C, which is all consistent with the XPS analysis as presented in Fig. 6. During annealing treatment, small amount of oxygen vacancies generated owing to the process of dehydration and desorption is capable of causing effective n-type doping for 150 °C, which is greatly conducive to the improved performance of the devices D with a PCE of 4.02%, the further increasing amount of oxygen vacancies with improved temperature to 200 °C leads to a slightly declined PCE of 3.78%. Furthermore, the oxygen ion vacancies of the substoichiometric p-MoO3 are occupied by two electrons and acted as donor centres with the donor level at 0.27 eV below the conduction band, the charge transfer from oxygen vacancies to Mo ions creates reduce d oxidation states of Mo5+. It is reported that a general correlation between cation oxidation state and an oxide's WF is that metal oxides in their more oxidized forms tend to have higher work functions than their reduced forms.43 Therefore, the increasing change of the WF from 4.90 eV to 5.22 eV under the annealing from RT to 200 °C couples with a fluctuation of the highest WF 5.26 eV for 150 °C, which influences the built-in electric field of the devices and the photocurrent being extracted efficiently at short-circuit.
In order to further testify the possibility of general usage of the p-MoO3 layer as a hole-transporting layer under the mild thermal-annealing in polymer solar cells, we also prepare the devices with the structures of PTB7:PC71BM solar cells fabricated with PEDOT:PSS and p-MoO3 thin films treated with various annealing temperatures as following:
(F) ITO/PEDOT:PSS (150 °C)/PTB7:PC71BM/LiF/Al.
(G) ITO/p-MoO3 (RT)/PTB7:PC71BM/LiF/Al.
(H) ITO/p-MoO3 (100 °C)/PTB7:PC71BM/LiF/Al.
(I) ITO/p-MoO3 (150 °C)/PTB7:PC71BM/LiF/Al.
(J) ITO/p-MoO3 (200 °C)/PTB7:PC71BM/LiF/Al.
In Fig. 7(c), the J–V characteristics of the PTB7:PC71BM devices under AM1.5G illumination are presented. It is evident that the annealing to p-MoO3 layer induces large improvement in JSC from 14.78 mA cm−2 to 16.43 mA cm−2 and FF from 54.9% to 65.1% with the annealing temperature from RT to 200 °C. The best JSC, FF and PCE of the p-MoO3-based devices is 17.02 mA cm−2, 68.1% and 8.46% for 150 °C, respectively, which apparently surpass the PEDOT-based devices with a JSC of 16.17 mA cm−2, FF of 67.2% and PCE of 7.93%. The performance parameters are summarized in Table 3.
Device | T (°C) | JSC (mA cm−2) | VOC (V) | FF (%) | Rs (Ω cm2) | Rsh (Ω cm2) | PCE (%) |
---|---|---|---|---|---|---|---|
F | 150 | 16.17 | 0.73 | 67.2 | 5.05 | 1174.06 | 7.93 |
G | RT | 14.78 | 0.72 | 54.9 | 13.35 | 705.19 | 5.85 |
H | 100 | 15.90 | 0.72 | 66.2 | 5.21 | 827.53 | 7.57 |
I | 150 | 17.02 | 0.73 | 68.1 | 4.49 | 1275.09 | 8.46 |
J | 200 | 16.43 | 0.73 | 65.1 | 4.82 | 976.58 | 7.81 |
As a consequence, the devices with the p-MoO3 films annealed at 150 °C exhibit the largest increase in performance with the best PCE of 8.46%, further confirming that our prepared p-MoO3 thin films annealed at moderate temperature are fit for the PTB7:PC71BM devices due to the favorable surface morphology and electrical properties as discussed above. Meanwhile, according to the leakage dominated region (<0.5 V) as shown in Fig. 7(d), the dark current of the PTB7:PC71BM devices with the annealed p-MoO3 thin films present the trend toward miniaturization with the minimum at 150 °C, which is much less than that of the device F with PEDOT:PSS. This also indicates that p-MoO3 thin film is more efficient in hole-collection and electron-blocking when served as a HTL in PTB7:PC71BM BHJ devices.
Apart from device efficiency, device stability is another important aspect affecting the realization of commercial polymer solar. In this study, we find that the devices with p-MoO3 thin film exhibit enhanced stability. All of these devices constructed as described in Section 2.2 are not encapsulated and stored in the glovebox filled with nitrogen atmosphere (O2 < 0.1 ppm, H2O < 0.1 ppm). We choose the devices with p-MoO3 and PEDOT:PSS thin films annealed at optimal annealing temperature 150 °C as tested objects. Fig. 8 displays the PCEs and normalized PCEs as a function of aging time, of P3HT:PC71BM and PTB7:PC71BM devices incorporated p-MoO3 thin films. For P3HT:PC71BM devices as shown in Fig. 8(a), the PCEs decrease dramatically for the devices fabricated with PEDOT:PSS and reach from 3.91% to 2.40% after 60 days, only keeping about 60% of their initial efficiencies. In comparison, the p-MoO3-based P3HT:PC71BM devices get PCEs of 3.40% after storage for 60 days, remaining about 85% of their initial efficiencies (from 4.02% to 3.40%). On the other hand, Fig. 8(b) presents the stability of PTB7:PC71BM devices with different HTL. The PEDOT-based devices degrade rapidly after 60 days with about 40% decay of PCEs (from 7.93% to 4.83%), whereas the devices incorporating the p-MoO3 HTLs get only 13.3% decay of PCEs (from 8.46% to 7.33%) after 60 days, revealing the high stability of the p-MoO3 interfaces. Clearly, it is indicated that the devices incorporating the p-MoO3 thin films exhibit higher stability.
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Fig. 8 PCEs and normalized PCEs as functions of aging time of P3HT:PC71BM (a) and PTB7:PC71BM (b) devices incorporating p-MoO3 thin films. |
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