A new generation of anode interlayers (AILs) has been introduced in recent years for improving the efficiency and stability of organic solar cell (OSC) devices. Electrode interlayer modification is a simple and effective way of enhancing OSC device performance. We used poly(vinyl pyrrolidone) (PVP) as an AIL modifier to alter molybdenum trioxide (MoO3) and vanadium pentoxide (V2O5) AILs in OSC devices and compared them with pure metal oxide AILs. Using this modification, average power conversion efficiencies were raised from 5.2 % ± 0.4 % to 6.0 % ± 0.3 % for OSCs with MoO3-based AILs, and from 6.2 % ± 0.1 % to 6.8 % ± 0.3 % for OSCs with V2O5-based AILs. Moreover, the PVP-metal oxide AILs also improved the overall device quality, producing a nanotextured morphology with good optical properties and favorable chemical composition. Beneficial wetting properties for interfacial adhesion between anode and active layer are observed using contact angle measurements. Overall, devices with PVP-modified metal oxide AILs showed promising results with greater device stability compared to pure metal oxide AIL-based OSC devices.
Organic solar cells (OSCs) have recently reached power conversion efficiencies (PCEs) of up to 16%.1 A particularly attractive feature of OSCs is the possibility of developing cost-effective large-scale production methods, as devices can be fabricated using solution-processable materials. The typical structure of an OSC (Fig. 1) includes a bulk heterojunction active layer, consisting of a mixture of donor and acceptor materials. Apart from the active layer, interlayers between the active layer and the electrodes play an important role in the extraction of photogenerated charges, affecting the efficiency of the overall OSC device.
In terms of anode interlayers (AILs), poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) has been almost universally used over the past decade2,3 on top of an indium tin oxide (ITO) anode. However, the negative effects of PEDOT:PSS on active layer morphology have been well documented and can affect device longevity and overall efficiency.4–6 These problems have been associated with low charge carrier mobilities and nonradiative recombination losses within the device.7 These results suggest that even though PEDOT:PSS has served well as a solution-processable AIL, it is unlikely to be the ultimate AIL material in a future OSC technology.
In recent years, a new generation of AILs has been introduced using materials such as metal oxides,8–10 conducting polymers,11 graphene-based AILs,12 conductive polyelectrolytes,13 and cross-linkable materials.14 Semiconductor metal oxide interlayers can have consistent long-term stability in high-efficiency OSCs.15,16 Among these metal oxides, molybdenum trioxide () and vanadium pentoxide () have been widely used to produce AILs processed at low temperatures. The combination of low temperature processing conditions with high PCE motivates the use of metal oxides as AILs in OSC devices.17
Electrode interlayer modification using polymers is known to be a simple and effective way to improve OSC device performance.18,19 For example, metal oxides modified with polymers, such as poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene] (PFN) and poly(vinyl pyrrolidone) (PVP), have produced reduced surface defects when the polymer was directly mixed with the metal oxide rather than coated on top as a bilayer.20,21 PVP is a widely known nonconjugated polymer, which has been used both as a stand-alone cathode interlayer (CIL) in OSCs and as a CIL in combination with metal oxides.21,22 Most of the interlayer modifications reported so far have involved CILs, especially the use of various polymer composites with zinc oxide (ZnO)23,24 and titanium oxide ().25 In contrast, there are very few reports on metal oxide AIL modifications with polymers in OSC devices, and those reports have mostly used a metal-oxide/polymer bilayer structure.26 Nanocomposite-based AILs are slowly gaining attention in OSCs due to their structural and optical advantages over pure metal oxide AILs.27,28 Similar to pure metal oxides,29,30 the work function of these nanocomposite AILs can also be tuned to align energy levels for a wide range of donor materials.31 In addition, compared to the complex functionalization methods reportedly required to modify interfacial layers,32 a simple mixture of alcohol-soluble modifier like PVP and the metal oxide precursors would be more beneficial toward cost-effective large-scale printing of next-generation solar cells.33 Most of the polymer-based composites for metal oxide AIL modifications have used only PEDOT:PSS.32,34,35 However, the acidic nature of PEDOT:PSS mixed with water-based metal oxide precursors limits the long-term air stability. Therefore, there is a need for water-free and nonacidic AIL precursors to enhance the air stability along with the efficiency of the OSC devices.
In this work, PVP nanocomposite was used for modification of () and () AILs. The alcohol-soluble PVP mixed metal oxide interlayers are compared with pure metal oxide interlayers. The influence of the nanocomposite on the physical and chemical properties of the AIL was analyzed using various characterization techniques, and the effect on OSC device performance has been measured. In all devices, the active layer that was used is poly (4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl):[6,6]-phenyl--butyric acid methyl ester (). PTB7 was chosen due to its expanded absorption spectrum up to the near-infrared wavelength region.36
Materials and Solutions
Prepatterned ITO glass slides with sheet resistance of were purchased from Kintec Company. Bis(2,4-pentanedionato)molybdenum(VI) dioxide was purchased from Sigma-Aldrich. Vanadium triisopropoxide (VTIPO) was purchased from Alfa Aesar. PTB7 was purchased from 1-Material Inc. was purchased from Nano-C. PVP (Mw by light-scattering); isopropanol, chlorobenzene, and 1,8-diiodooctane (DIO) were purchased from Sigma-Aldrich. All materials were used without further purification.
A precursor solution was freshly prepared by dissolving bis(2,4-pentanedionato)molybdenum(VI) dioxide in isopropanol with a 1:150 w/v ratio. A 0.05 M solution precursor of VTIPO was freshly prepared by dissolving of VTIPO in 8 mL of isopropanol. For interlayers, PVP () was initially dissolved in isopropanol solvent and ultrasonicated for 10 min until it formed a clear solution. Bis(2,4-pentanedionato)molybdenum(VI) dioxide or VTIPO was then added to the PVP/isopropanol solution at the same concentration as for the pure and solutions. An active layer solution was prepared by dissolving PTB7 (10 mg): (13 mg) in chlorobenzene (1 mL) and stirring inside the glovebox overnight at room temperature. DIO (3 vol%) was added to the solution and stirred at 60°C for 3 h before the deposition process.
Thin-Film Deposition and Device Fabrication
Prepatterned ITO glass substrates were sequentially cleaned by ultrasonication with acetone and isopropanol for 10 min each. The surface of the cleaned substrates was treated with a UV–ozone cleaner (Novascan Technologies, Inc.). , , and AIL precursor solutions were deposited on the cleaned ITO substrates by spin coating at 5000 rpm for 30 s in air. was spin-coated at 6000 rpm for 30 s. The resultant thicknesses were 30 nm for , and AILs, and 28 nm for . The spin-coated films were annealed at 150°C, and AILs were further treated with a UV–ozone cleaner (Novascan) for 15 min.
After depositing AILs, blended solutions were spin coated at 1000 rpm for 40 s under nitrogen atmosphere inside a glove box to form a 100-nm thin film without a postdeposition annealing process. The substrates were then transferred to a thermal evaporator to deposit a 1-nm lithium fluoride (LiF) interlayer followed by a 100-nm aluminum (Al) electrode under a base pressure of . A mask with a designated active area of was used during the evaporation process. In total, at least six devices were made for each architecture. A schematic for the OSC devices is shown in Fig. 1.
The surface morphology of AILs and active layers was analyzed using an MFP-3D Origin atomic force microscopy (AFM) in tapping mode under atmospheric conditions. A UV–visible spectrometer (V650, Jasco) was used to measure the transmittance spectra of films. Work function measurements were carried out in air using AC2 photoelectron spectroscopy (RKI Instruments). The surface energy of the films was estimated using contact angle measurements (KSV Instruments Ltd.). The chemical environment and oxidation states were analyzed using X-ray photoelectron spectroscopy (XPS, The Kratos Axis DLD). The photovoltaic device performance was characterized using a 1-kW Oriel solar simulator with an AM 1.5 G filter as the light source in conjunction with a Keithley 2400 source measurement unit. The best performed devices were kept in a dark room in a nitrogen environment to study their stability for 96 h.
Results and Discussion
Physical and Chemical Characterization
AIL surface morphology
Tapping mode AFM was performed for morphological analysis of metal oxide and PVP-metal oxide films. The root mean square roughness () for was 2.60 nm, increasing to 8.75 nm for untreated AILs and decreasing to 6.91 nm after UV–ozone treatment [Figs. 2(a)–2(c)]. For and AILs, was 1.90 and 6.90 nm, respectively [Figs. 2(d) and 2(e)]. As shown in Fig. 2, there is an obvious structural change from a flat surface for a thin layer of metal oxide on top of ITO to a nanotextured surface with the addition of PVP, consistent with previous reports.37,38
The presence of nanoscopic voids appearing in between nanoclusters due to rapid solvent evaporation could be a reason for the higher of PVP-AILs.39 PVP has a well-known influence on synthesized metal oxide morphologies.40 The hydrophobic vinyl groups and hydrophilic carbonyl groups present in PVP can induce shape effects by forming polarized micelles in precursor solutions, leading to the formation of metal oxide nanoclusters.41 The size and self-assembly of these nanoclusters can be controlled by changing the molecular weight of PVP.42 Hence, by choosing PVP with the highest available average molecular weight (), 5–15 nm metal oxide nanoclusters were obtained in Fig. 2.
The agglomeration of clusters can leave parts of the surface uncovered [Fig. 2(b)]. These surfaces would be PVP-rich, which would be detrimental for OSC device performance. Therefore, after postdeposition annealing, we further treated the AILs with UV–ozone for 15 min to remove the upper layer of PVP and form a more uniform top surface, as shown in Fig. 2(c).21 Unlike untreated AILs, AIL-based devices worked well without performing any PVP surface removal techniques.
Although higher surface roughness can cause local shorts in thin-film devices, it is not a limiting factor for overall device performance. AILs with as high as 25 nm have been successfully used as hole injection layers for organic electronic devices.43 In fact, nanostructured hole-transporting layers can be beneficial for thin-film solar devices by avoiding electronic losses at the electrode/polymer interface, resulting in an increased fill factor (FF)44,45 and device stability.46 The surface roughness of active layers deposited on different AILs was also analyzed (see Sec. 5.1). The increment in surface roughness which is required to improve internal reflection in the active layer.47
AIL optical properties
To understand changes in the device performance, the optical transmittance has been analyzed for pure metal oxide AILs with and without the addition of PVP (Fig. 3). A high transmission rate in the visible region could be beneficial for AILs in OSC devices. Figure 3(a) shows that the pure layer has slightly less transmittance compared to the composite layers through the visible region. The transmittance of AILs increased after exposure to UV–ozone for 15 min, possibly due to slightly decreased thickness of the layer following removal of the organic layer by UV–ozone treatment. However, the slightly decreased optical transmittance of films below could result in the reduction of short-circuit current density () in OSC devices.48 There is less difference between the optical transmittance in the case of and AILs, as shown in Fig. 3(b). In both cases, the highest transmission rate is observed around 500 nm and above, covering most of the solar spectrum. The antireflective effect observed from 330 to 410 nm can be attributed to the formation of smooth surfaces,49,50 which could decrease the extinction and dispersion of light.
The photoelectron spectroscopy in air technique was used to determine the work function of the AILs presented in Figs. 4(a) and 4(b). These figures plot the square-root of the counting rate as a function of the photon energy in order to find the photoemission threshold energy. The crossing point of the background and the yield line gives the work function of the material.51,52 The work function for the film was determined as 5.7 eV and was decreased to 5.6 eV in the case of AIL [Fig. 4(a)]. The work function was unchanged with and without UV–ozone treatment of AILs. The work function of AILs was 5.5 eV [Fig. 4(b)], only 0.17 eV was lower than the HOMO of the donor PTB7 [Fig. 4(c)]. Hence, holes can more easily travel from the donor’s HOMO level to and AILs and then pass on to the ITO electrode.53,54 The energy level diagrams shown in Fig. 4(c) were obtained with the values from Figs. 4(a) and 4(b) similar to the previous reports.52,55,56
X-ray photoelectron spectroscopy
XPS core level scans of the Mo 3d region provided an insight into the chemistry of pure and modified films. For pure [Fig. 5(a)], the intensity is split into two regions that are identified with and orbitals (the spin-orbit splitting is 3.3 eV) with an area ratio of 1:0.6, respectively. There are no traces of Mo(V) species. These values are consistent with the Mo(VI) state being present in AILs.57
For with and without UV–ozone treatment [Fig. 5(b)], additional peaks of Mo(V) appear at lower binding energy (by ) than the Mo(VI) peaks. Similar spectra were previously reported for different ratios of Mo(V) and Mo(VI) at higher annealing temperatures, from 150°C to 250°C.58,59 Both of these studies attributed the presence of Mo(V) to distortion of the molybdenum lattice due to gradual dehydration and organic thermalization at elevated temperatures. Since both and AILs were annealed at 150°C, the addition of PVP could be the reason behind the observation of Mo(V) states, indicating that some lattice distortion had occurred. With the UV–ozone treatment, these peaks attenuated noticeably, suggesting that the reduced Mo(V) species are forming at the very surface (top molecular layers) of the layer and that the UV–ozone treatment effectively suppresses Mo(V) species in AILs. Since these particular AIL samples only possess Mo(V) and Mo(VI) species, the minimum Mo:O stoichiometry can be assessed directly from the known Mo(V):Mo(VI) ratios.58,59 Using ratios obtained from the high-resolution core-level spectra in Fig. 5, the Mo:O stoichiometries of the samples coated on ITO substrates were determined and recorded in Table 1.
XPS compositional analysis for MoO3 and PVP-MoO3 AILs. Mo(V):Mo(VI) and Mo:O were calculated using high resolution of Mo 3d and O 1s core level scans.
|AILs||Oxygen (%)||Carbon (%)||Molybdenum (%)||Mo(V):Mo(VI)||Mo:O|
A quantitative analysis of the composition of the samples is also listed in Table 1. The relative content of oxygen, carbon, and molybdenum was estimated from O1s, C1s, and Mo 3d core level scans, respectively. The AIL has a relatively high oxygen content and almost equal carbon and molybdenum content, with stable Mo:O lattice stoichiometry. without UV–ozone treatment contained less oxygen, with relatively high carbon and low molybdenum content. After UV treatment, the sample has relatively low carbon and molybdenum content. As pointed out before, the atomic ratio of Mo(V):Mo(VI) has decreased from 0.2 to 0.1 after UV–ozone treatment. The resulting calculated stoichiometries are before UV–ozone treatment and after UV–ozone treatment. Previous reports have obtained similar values for solution-processed AILs and have shown that low annealing temperatures are more suitable for achieving stable Mo:O stoichiometries with less oxygen deficiencies.60
From the XPS fits in Fig. 6, we can deduce the presence of two vanadium oxidation states in -based AILs by deconvoluting two peaks from the vanadium peak. Namely, V(IV) generates a peak between 517 and 516 eV, and V(V) produces a peak at around 518 eV.61 V(IV) occurs as a result of the partial reduction of V(V) after exposure to air for 10 min. Interestingly, the fraction of V(IV) species slightly increased for the films, which indicates that more oxygen vacancies are generated during hydrolysis. This could lead to enhanced electrical conductivity of films.62
The O1s peak is also fitted using two contributing peaks, which are the peaks for O atoms in between 530.7 and 530.8 eV,61,63,64 and the undecomposed precursor peak between 531.3 and 531.9 eV.65 The effective oxidation state of vanadium () can be calculated using the splitting between the O1s and transition centroids, so that .66 The values of listed in Table 2 are in agreement with oxidation states from previous reports.67
XPS data obtained for different V2O5-based AILs from high resolution scans.
|AIL||V(V) position (eV)||V(IV) position (eV)||V(V):V(IV)||O1s position in V2O5 (eV)||O1s position in undecomposed precursor (eV)||VOX|
Wettability of AILs
Wettability measurements can provide information on interfacial interactions between the anode and the active layer in an OSC device.68 From Figs. 7(a) and 7(b), the surfaces of the -based AILs clearly become more hydrophilic after the addition of PVP. There was a corresponding small increase in the contact angle for nonpolar chlorobenzene [Figs. 7(c) and 7(d)], which could produce diminished in an OSC device.69 In contrast, Figs. 8(a) and 8(b) show that water contact angles increased for AILs when PVP was added. This suggests that is less hydrophobic than , which is consistent with the slightly lower contact angles for chlorobenzene on pure [Figs. 8(c) and 8(d)].
In previous work on hole transporting layers, Manders et al. and Cho et al. have demonstrated that hydrophobic AILs result in favorable OSC device characteristics compared to hydrophilic surfaces.69,70 Moreover, hydrophilic surfaces coated beneath polymer: active layers have resulted in adverse aggregation of , which, in turn, resulted in lower PCE.71 The decrease in chlorobenzene contact angle on AILs could therefore lead to strong interfacial cohesion between the anode and the active layer, low series resistance in the OSC device, and consequently enhanced photovoltaic parameters.
Photovoltaic Performance and Stability
The photovoltaic performance of OSC devices with PVP composite metal oxide AILs was tested and compared with OSCs, which have pure metal oxide AILs. The four OSC device structures for these experiments are shown, as indicated in Fig. 1. Based on topographical measurements (above), oxide encapsulation of PVP22 seems to be high for AILs, and OSC devices showed diminished performance () without performing the UV–ozone treatment described previously to remove PVP. Therefore, device analysis for the high efficiency of low band-gap polymer donor PTB7 was carried out only for the best performed AILs; that is, with UV–ozone treatment was studied.
Photovoltaic performance and stability data are presented in Fig. 9 and Table 3. The best efficiency for a single device was obtained with a AIL, with , , , and a PCE of 7.10%. The slight dip in the current-density in devices with AILs of following UV–ozone treatment, in comparison to devices with interlayers, can be explained by the increasing surface roughness (from 2.60 to 6.91 nm), which can lead to the formation of local shorts in the interlayer.18,72 On the other hand, the increment in FF from 0.53 (for ) to 0.60 (for ) shows improvement in the OSC device quality with the addition of PVP. Also, is enhanced for AILs compared with AILs, from 0.70 to 0.72 V. These improvements in FF and can be attributed to efficient hole transportation because of the reduced work function of the AILs. The hydrophilic nature of the AIL surface is also consistent with the decrease in relative to pure , but this did not offset the increased FF measurement.
Photovoltaic parameters for PTB7:PC71BM OSCs with various AILs. Mean and standard deviation values are presented for six devices of each type. Shunt (RSH) and series (RS) resistances were calculated from the J–V curves [Fig. 9(a)].
|AIL||VOC (V)||JSC (mA/cm2)||FF||PCEave (%)||PCEbest (%)||RSH (Ω.cm2)||RS (Ω.cm2)|
Similar enhancement in FF was seen in compared to pure -based devices. The enhancement in to 0.72 V for and compared to -based devices can be explained by more efficient charge transfer because of the reduced work function of layers. Wettability of the AILs seems to have no large effect on the overall device performance although the hydrophobic AIL, which could have stronger adhesion with the active layer showed slightly higher .
The devices were also measured in reverse bias conditions to better understand their performance with respect to the addition of PVP nanocomposite. As shown in Fig. 9(b), the photocurrent density () increases with effective voltage ()73 and reaches saturation at , suggesting that most of the electron–hole pairs are dissociated into free carriers at this potential. Since the photocurrent generation mechanism is dependent on the quality of charge carrier injection and extraction rates, at any given point in the device only a certain number of electron–hole (e–h) pairs can dissociate and transfer to appropriate electrodes. For the maximum power output condition (), is 0.85, 0.90, 0.87, and 0.96 for the best-performed , , , and AIL OSC devices, respectively. The high values of and FF observed for AIL-based OSC devices can be partly attributed to efficient charge extraction at the AIL/active layer interface, with suppressed nongeminate recombination leading to an improved collection of charge carriers at the respective electrodes.74
In Fig. 9(c), the lowest leakage current of was obtained for a device. This is one order of magnitude less than the leakage current for the best-performed pure and AIL-based devices. This result is consistent with the high shunt resistance () of and the moderately low series resistance () of recorded in Table 3 for AIL devices.
Stability was also studied for solar cell devices with the various AILs [Fig. 9(d)]. After 96 h, and AIL devices had decreases in of 20% and 17%, and in FF of 5.6% and 4.9%, respectively. This led to efficiency retention of 81% and 91% for and OSC devices, respectively. The AIL-based device showed a 9.5% reduction in and a 4.5% reduction in FF, retaining 79% of its initial PCE. For the device, decreased 10.5% and only 1.8% in FF, with 91% retention of PCE. Overall, the PVP nanocomposite metal oxide AIL devices had better efficiency retention than pure metal oxide AIL devices. This enhanced stability is likely to be due to the nonacidic nature of the PVP-modified metal AILs.
In summary, the results have demonstrated that solution-processible PVP nanocomposite metal oxide AILs showed distinctive nanotextured morphology with higher transmittance compared to pure metal oxide AILs. Both PVP mixed metal oxides also showed favorable chemical composition, thereby producing improved photovoltaic performance compared to pure metal oxide AILs. They exhibited greater stability (higher efficiency retention) and better shunt resistance () with lower leakage currents. The results suggest that charge extraction within OSC devices was more efficient for the PVP nanocomposites. This photovoltaic performance in conjunction with the optical, chemical, and interfacial properties suggest that the PVP nanocomposite metal oxide AILs could give not only unperturbed diode characteristics but could also improve the photovoltaic parameters of OSC devices. Indeed, the clear increase in FF for PVP-based AILs indicates an enhancement in the overall device quality. Thus, PVP-metal oxide mixtures provide promising AILs. One of the reasons to extend this study to PVP- was to avoid UV–ozone treatment, which was necessary for removal of a surface organic layer on AILs, and therefore to achieve enhanced photovoltaic performance without this additional fabrication procedure. These results demonstrate the potential for AILs processed at moderate annealing temperatures with simple fabrication procedures to improve the overall quality and stability of OSC devices.
Surface Morphology of Active Layers
The differences in surface roughness for the active layer were comparatively small for devices with AILs [Figs. 10(a)–10(c)]. for coated on pure was 2.40 nm. This increased to 2.85 nm for coating on untreated and increased slightly further to 2.90 nm for coating on UV–ozone treated . An increase in the surface roughness of active layers was consistently observed with the addition of PVP to the AILs. For active layers deposited on AIL, the value was 2.84 nm and this increased to 4.85 nm for layers [Figs. 10(d) and 10(e)]. Rough surfaces of layers can improve the internal reflection in the active layer, which can lead to increased device efficiency.47
This work was supported by New Zealand’s Ministry of Business, Innovation and Employment (Contract UOAX0911), the Faculty of Science at the University of Auckland, and a Universitas 21 Graduate Research Project grant. The authors thank Dr. Yiran An for providing technical support with AFM and Dr. Krishnachary Salikolimi for providing photoelectron spectroscopy measurements. The authors wish to thank Dr. Jegadesan Subbiah and Dr. David Jones at the University of Melbourne for providing facilities and expert advice for some parts of this research.