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Efficiency enhancement of organic solar cells enabled by interface engineering of sol-gel zinc oxide with an oxadiazole-based material
Organic Electronics 76 (2020) 105483
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Organic Electronics journal homepage: http://www.elsevier.com/locate/orgel
Efficiency enhancement of organic solar cells enabled by interface engineering of sol-gel zinc oxide with an oxadiazole-based material Billy Fanady a, c, Wei Song c, Ruixiang Peng b, c, Tao Wu a, **, Ziyi Ge b, c, * a
Ningbo New Materials Institute, The University of Nottingham, Ningbo, 315042, PR China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, PR China c Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Organic solar cell Interface engineering Sol-gel ZnO PBD Oxadiazole-based material
Organic solar cells (OSCs) have acquired much attentions owing to their advantages in terms of solutionprocessability, low-cost, lightweight and compatibility for large-scale roll-to-roll processing. Aside from mate rials design, studies on interface engineering are also crucial to enhance photovoltaic performance for the realization of high-performing OSCs. In this study, interface engineering on sol-gel zinc oxide (ZnO) electrontransporting layer (ETL) was conducted by introducing additional oxadiazole-based electron-transporting ma terials, PBD between ZnO ETL and photoactive layer. The significance of incorporating PBD on ZnO was demonstrated by investigating the change in optical, electrical and morphological properties of pristine ZnO ETL. Herein, the utilization of PBD could enhance ZnO film’s conductivity, which was favorable for better charge transport ability. As compared to ZnO ETL, ZnO/PBD ETL had lower work function to facilitate more efficient electron extraction from the photoactive layer. Moreover, PBD could smoothen the ZnO film’s morphology and improve hydrophobicity of the surface to provide uniform and intimate interfacial contact between ETL and the photoactive layer. As a result, through this hybrid bilayer strategy, inverted OSCs based on PBDB-T:IT-M pho toactive layer system exhibited ~7% enhancement in the power conversion efficiency from 10.8% (ZnO-based device) to 11.6% (optimized ZnO/PBD-based device).
1. Introduction Organic solar cells (OSCs) have acquired significant public’s awareness over the years owing to their advantages in terms of me chanical flexibility, low-cost, lightweight and solution-processability making it compatible for large-scale manufacturing through roll-to-roll printing [1–4]. Since its first discovery in 1995, bulk-heterojunction (BHJ) photoactive layer system consisting of two compatible materials as electron donor and acceptor has been dominating in the OSCs field [5, 6]. In fact, considerable amount of attentions have been directed to wards the synthesis of novel electron donor and acceptor materials as the BHJ photoactive layer for the past few years [7–9]. During early stages, fullerene derivatives were studied extensively as electron acceptor [10–12], where a power conversion efficiency (PCE) of over 10% could be achieved [13,14]. However, it was later realized that fullerene acceptor was associated with several drawbacks in terms of i) limited energy level and bandgap tunability, ii) poor absorption
property in the visible and near-infrared (NIR) region, and iii) highly complicated and expensive synthetic procedure of fullerene [15–17]. Both limited tunability and poor light-harvesting property were the key factors that restrained the PCE of fullerene-based OSCs to a much higher value. Driven to overcome such limitations, non-fullerene acceptors (NFAs) were then designed and synthesized as an alternative to the state-of-the-art fullerene acceptors [18–20]. Along with emerging NFAs materials, donor materials were also designed and developed to cope with the growth [21,22]. To date, immense efforts directed towards materials design (e.g. synthesis of novel donor and acceptor materials) have been fruitful, which remarkably enhanced the PCE of single-junction devices to over 15% [23,24]. Generally, NFAs often exhibited a lowest unoccupied molecular orbital (LUMO) energy level of ~3.7–4.0 eV [25,26] when compared to PC71BM (~4.3 eV) [11]. It also suffered from low electron mobility (in the order of 10 4 cm2 V 1 s 1) [26,27]. Interface engineering may serve as a powerful strategy to crucially enhance photovoltaic performance by
* Corresponding author. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China. ** Corresponding author. Ningbo New Materials Institute, The University of Nottingham, Ningbo, 315042, PR China. E-mail addresses: [email protected] (T. Wu), [email protected] (Z. Ge). https://doi.org/10.1016/j.orgel.2019.105483 Received 16 July 2019; Received in revised form 1 October 2019; Accepted 1 October 2019 Available online 2 October 2019 1566-1199/© 2019 Published by Elsevier B.V.
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solving these issues [28–30]. For instance, the incorporation of electron-transporting interfacial layer (ETL) between electrode and photoactive layer can be an effective way to tune the work function (WF) of the electrode, such that it will be close but lower than the LUMO energy level of NFAs [29]. This allows the formation of Ohmic contact at the interface which is beneficial for improving electron extraction, transport and collection, as well as the provision of hole-blocking effect. Therefore, aside from materials design, interface engineering also plays a pivotal role to effectively improve photovoltaic performance, partic ularly when a careful selection of interfacial layers including electron-transporting layer (ETL) and hole-transporting layer (HTL) is made. This selection is typically made by considering materials prop erties and suitability with the different OSCs’ device architecture. In inverted OSCs, MoO3 is mostly utilized as the HTL to promote Ohmic contact between top metal electrodes (e.g. Ag or Au) and highest occupied molecular orbital (HOMO) energy level of donor materials, allowing abundant and efficient hole extraction in inverted device. On the contrary, the role of ETL in inverted OSCs is more prominent to minimize the large contact barrier existing between WF of ITO bottom electrode and LUMO energy level of acceptor materials, so that efficient electron extraction can be achieved. Hence, studies on interfacial layer of inverted OSCs are mostly centered on the development of ETL. So far, several classes of ETL including n-type metal oxides [31], conjugated polyelectrolytes (CPEs) [32], alcohol-/water-soluble materials [33–36], and self-assembled dipole monolayers (SADMs) [37] have been explored; each with varying mechanisms/functions such as by tuning WF of the electrode, altering charge extraction efficiency and selectivity, altering the morphology of the active layer, modulating light absorption in the active layer, and etc [29]. Among them, sol-gel zinc oxide (ZnO) as n-type metal oxide demonstrated a promising optical transparency in the visible light region and capability to extract and transport electrons while effectively block holes, making it to be greatly suitable to be used as ETL in inverted OSCs [28–30]. Once deposited as thin-film, conven tional sol-gel ZnO is often post-treated with high-temperature annealing (~150–200 � C) to promote the formation of crystalline ZnO and reduce high-density defects that may deteriorate device performance. Despite this effort, sol-gel ZnO still suffers from high surface roughness and its hydrophilic nature can limit its interfacial contact with the organic photoactive layer [38,39]. These subsequently lead to poor electron extraction and device performance in inverted OSCs. On that account, interface engineering on sol-gel ZnO is conducted to effectively subdue the limitations of ZnO films. To date, interface engineering or interfacial modification on sol-gel ZnO has been studied through the doping of organic materials into ZnO [38–40] or the deposition of organic/inorganic materials as second interfacial layer on top of ZnO film [39,41]. Both methods can impede surface traps and defects in sol-gel ZnO, provide better interfacial con tact with the photoactive layer, alter the WF of the electrode, improve the conductivity of sol-gel ZnO and suppress recombination losses which are all beneficial for the improvement of photovoltaic performance. For examples, Li et al. [40] used sol-gel ZnO doped with EDTA organic chelating agent to improve the PCE of inverted OSCs from 11.1% (ZnO) to 12.1% (hybrid EDTA-ZnO). Borse et al. [39] introduced Ba(OH)2 layer deposited on top of the ZnO film to smoothen its morphology and pro mote better interfacial contact with the photoactive layer. As a result, PCE was escalated from 7.12% (ZnO) to 8.54% (bilayer ZnO/Ba(OH)2). Herein, 2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4-oxadiazole or abbreviated as PBD was introduced on top of the previously deposited ZnO film to modify its properties. This oxadiazole-based small-molecu lar (SM) material was first used by Adachi [42] in 1989 as ETL in bilayer organic light emitting diodes (OLED). And even up to now, it is still considered as one of the most commonly used ETLs in OLED mainly due to its good electron affinity and mobility as well as outstanding charge injection ability [43]. Even though PBD is frequently used in OLEDs as ETL materials [44–46], its application in other areas of optoelectronic devices is still limited. Recently, PBD was realized to function effectively
as a buffer layer in thin-film transistors (TFTs) as it could tune the energy level alignment for improving charge extraction in TFTs [47]. Yet, no studies were reported regarding the use of PBD in OSCs. In light of its advantages, PBD was incorporated in this work for the first time in OSCs as second interfacial layer on top of ZnO to modify ZnO film’s properties. The introduction of PBD on ZnO film was a promising strategy as it could improve the electrical and morphological properties of sol-gel ZnO film. Furthermore, PBD layer could further tune the WF of the electrode to better match the LUMO energy level of acceptor materials. All those features allow ZnO/PBD interlayer to function effectively as ETL in inverted OSCs. As a result, inverted OSCs based on poly[(2,6-(4,8-bis (5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophe ne))-alt-(5,5-(10 ,30 -di-2-thienyl-50 ,70 -bis(2-ethylhexyl)benzo[10 ,20 -c:40 , 50 -c’]dithiophene-4,8-dione)] (PBDB-T donor) [48] and 3,9-bis (2-methylene-((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5, 11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:20 ,30 -d’]-s-indaceno[1, 2-b:5,6-b’]dithiophene (IT-M acceptor) [49] displayed ~7% PCE enhancement from 10.8% (ZnO-based device) to 11.6% (optimized ZnO/PBD-based device). 2. Materials and methods 2.1. Materials preparation Indium tin oxide (ITO)-coated glass substrates (1.1 mm thick, � 15 Ω/square) were purchased from Wuhu Token Sciences Co., Ltd. Zinc acetate (Zn(CH3COO)2, 97.5%), ethanolamine (99.5%) and 2-methoxye thanol were received from J&K Scientific, Ltd. ZnO precursor solution was prepared by dissolving 100.9 mg of zinc acetate in 33.5 μL of ethanolamine and 1 mL of 2-methoxyethanol. PBD (>99%) was ob tained from Xi’an Polymer Light Technology Corp. PBDB-T and IT-M were purchased from Solarmer Materials Inc., whereas 1,8-diiodoctane (DIO), chlorobenzene and methanol were received from SigmaAldrich, Inc. Unless stated, all chemicals were used without any further purifications. 2.2. Device fabrication OSCs were fabricated with the configuration of glass/ITO/ZnO/ PBD/PBDB-T:IT-M/MoO3/Ag. ITO-coated glass substrate was first cleaned by sequential sonification in detergent, deionized water, acetone and isopropanol for 20 min in each step. Prior to device fabri cation, the pre-cleaned ITO was dried with the flow of N2 and was further treated with ultraviolet ozone for 25 min. The ZnO precursor solution was deposited onto the ITO-coated glass substrate at 4000 rpm for 60s and annealed at 150 � C for 30 min. PBD was dissolved in meth anol with optimized concentration of 1.0 mg/mL. The corresponding PBD solution was stirred overnight without heat before usage. Under optimal conditions, PBD solution was spin-coated on top of the ZnO film at 1000 rpm for 60s and was then dried under vacuum for 15 min. Photoactive blend layer solution was prepared by dissolving PBDB-T donor and IT-M acceptor (D:A ¼ 1:1 by weight) in 20 mg/mL chloro benzene (total solid concentration) with 0.7% by volume addition of DIO additives. The photoactive layer solution (PBDB-T:IT-M) was spincoated at 1900 rpm for 60s onto the PBD film and then annealed at 120 � C for 10 min. Finally, 8 nm MoO3 and 100 nm Ag as electrode were deposited through a shadow mask (effective area of 3.8 mm2) by vac uum evaporation with base pressure of ~5 � 10 6 mbar. 2.3. Device characterization X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelec tron spectroscopy (UPS) were measured using Kratos AXIS Ultra DLD XPS/UPS spectrometer. Transmittance and absorption spectra were collected using PerkinElmer Lambda 950 UV/Vis spectrometer. Atomic force microscopy (AFM) height and phase images were acquired using 2
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Veeco Dimension 3100 instrumentation. Dektak 150 surface profiler was used to measure the film thickness. Current density–voltage (J–V) characteristics of unencapsulated device were measured in N2-filled glove box using Keithley 2440 source meter with AM 1.5G solar simu lator (Newport-Oriel® Sol3A 450W). The light intensity was calibrated at 100 mW cm 2 using a certified reference Si standard cell (SRC-2020 with KG5 filter) obtained from Enli Technology Co., Ltd and the cali bration report was traceable to NREL. External quantum efficiency (EQE) spectra were obtained using Enlitech QE-R solar cell quantum efficiency measurement system with a 75W Xenon lamp source, whose light intensity was calibrated by a reference Si probe (RC-S103011-G) obtained from Enli Technology Co., Ltd. EQE was measured in the N2filled glove box at room temperature. Contact angle was measured using the multifunctional dynamic contact angle measuring device and tensiometer (DCAT21).
In this case, three constituent peaks were found at 284.8, 285.8 and – C, C–H, 288.7 eV. Those peaks were ascribed to the presence of C–C, C– – N bond [50,51], which were all present in PBD molecules. C–O and C– The results confirmed that PBD was successfully deposited on top of the ZnO film to function as a hybrid electron-transporting bilayer. The changes in elemental states upon PBD deposition were investi gated by comparing the positions and shapes of Zn 2p and O 1s peaks of ZnO and ZnO/PBD films. The high-resolution Zn 2p and O 1s XPS spectra are shown in Fig. S1 of Supporting Information, respectively. Two major peaks of 2p3/2 at lower binding energy and 2p1/2 at higher binding en ergy were obtained, confirming the presence of ZnO phase formation in ZnO and ZnO/PBD films [39]. Both 2p1/2 and 2p3/2 peaks looked identical with a slight shift in 2p3/2 peak from 1021.6 to 1021.5 eV upon deposition of PBD on ZnO film. This indicated that the incorporation of PBD had minimal influence on the Zn–O bond formation in ZnO film. This was further supported from O 1s XPS survey, where it showed similar peaks position before and after the deposition of PBD. In here, the constant peak position at higher binding energy was attributed to O atoms in ZnO matrix, which was in agreement to the previous findings [31,38]. Meanwhile, the peak at lower binding energy corresponding to oxygen-deficient components was slightly shifted [31,38]. The relative magnitude for this oxygen-deficient peak was calculated to be 51.28% for pristine ZnO film and 51.22% for ZnO/PBD film. These implied that the number of oxygen-deficient components remained relatively con stant even after PBD deposition.
3. Results and discussions 3.1. Elemental states XPS measurement was performed in order to analyze the elemental states of the modified ZnO/PBD film (chemical structures of PBD depicted in Fig. 1a) and compare it to the pristine ZnO film. The overall XPS survey spectra of those two films are presented in Fig. 1b. Both films showed similar XPS survey spectra with notable peaks of Zn 2p, O 1s, N 1s and C 1s. Among those peaks, the most distinct change could be seen from the enhancement and broadening of N peak due to the introduction of PBD layer (Fig. 1b, inset). To support this, high-resolution N 1s peak of ZnO/PBD film was deconvoluted into its constituents shown in Fig. 1c. A slight increment of N peak at 400.0 eV was attributed to the presence of N–C double bond found in PBD molecules [41], whereas the broadening of N peak was caused by the occurrence of additional peak at 398.5 eV. The peak at lower binding energy was ascribed to the inter action between N atoms of PBD and Zn atoms of ZnO found on the ZnO/PBD interfaces [41]. Fig. 1d illustrates the C 1s survey spectra of ZnO/PBD film to further testify the presence of PBD on top of ZnO film.
3.2. Work function UPS measurements on ZnO- and ZnO/PBD-modified ITO surfaces were conducted to understand the role of PBD in tuning the WF of ITO cathode in inverted OSCs. The UPS spectra of ZnO- and ZnO/PBDmodified ITO cathodes are portrayed in Fig. S3 of Supporting Informa tion. Typically, work function can be calculated based on the following equation [39]: � ϕ ¼ hv Ecutoff ðBEÞ Eonset (1)
Fig. 1. (a) Chemical structures of PBD electron-transporting materials. (b) XPS survey spectra of ZnO and ZnO/PBD on ITO/glass substrates; inset shows the enlarged N 1s spectra. The deconvoluted high-resolution (c) N 1s and (d) C 1s peaks of ZnO/PBD deposited on ITO/glass substrates. 3
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3.5. Morphological properties
where ϕ is the work function, hv is the incident photon energy of He I (21.22 eV), Ecutoff (BE) is the high binding energy (BE) determined by linear extrapolation to zero at the yield of secondary electrons and Eonset is the onset relative to the Fermi level (EFermi) of Au (0 eV). Work function can also be represented in terms of kinetic energy (KE), resulting in the simplification of equation to ϕ ¼ Ecutoff (KE) - Eonset. In this case, Fermi level of Au has been set to 0 eV prior to measurement, thus, the Ecutoff values obtained from KE UPS curve in Fig. S3 directly corre sponded to the work functions of cathode. Due to this reason, the WF of ZnO- and ZnO/PBD-modified ITO cathodes were determined to be 4.12 and 4.03 eV, while the WF of bare ITO cathode was 4.90 eV. It was obvious that the use of interfacial layers (ZnO and ZnO/PBD) could effectively modify the WF of bare ITO cathode. In comparison to ZnOmodified ITO cathode (4.12 eV), ZnO/PBD-modified ITO cathode showed a lower WF of 4.03 eV, which was relatively closer to LUMO energy level of IT-M acceptor (3.98 eV) [49]. This highly suggested that additional PBD layer in ZnO/PBD interlayer could further tune the WF of ITO cathode by creating a better energy level alignment with the LUMO energy level of IT-M acceptor. On one hand, this could promote Ohmic contact for efficient electron extraction and on the other hand, such WF modification might strengthen cell’s built-in field beneficial for boosting photovoltaic performance.
The morphological features of pristine ZnO and ZnO/PBD film were explored through AFM height and phase images shown in Fig. 2. It was observed from the height (Fig. 2a) and phase images (Fig. 2d) that the pristine ZnO film exhibited a rough surface morphology with root-meansquare (RMS) roughness of 5.57 nm. The RMS roughness of ZnO film was toned down significantly to 2.78 nm when PBD was deposited on top of the ZnO film (Fig. 2b), proving the ability of PBD to smoothen the ZnO film’s morphology significantly. Chlorobenzene (CB) as the processing solvent of photoactive layer was deposited on top of the ZnO/PBD film to simulate the actual inverted OSCs when photoactive blend layer of PBDB-T:IT-M in CB was deposited on top of the ETL. As shown in Fig. 2c and f, CB deposition could definitely alter the surface morphology of ZnO/PBD film by partially removing PBD molecules from ZnO/PBD film. Partial removal of PBD was first speculated as the CB deposition on ZnO/PBD film did not raise the surface roughness back to 5.57 nm, which was the RMS value for pristine ZnO film. This was later confirmed by the overall XPS survey spectra presented in Fig. S2 which compared ZnO/PBD/CB film with ZnO/PBD film. In addition, high-resolution N 1s peak of ZnO/PBD/ CB film displayed in Fig. S2 further testified the presence of PBD even after CB deposition. Due to this unique characteristic, the surface roughness of ZnO/PBD film upon CB deposition was increased to 4.52 nm. This explained that CB solvent of the photoactive layer could become the key in optimizing the morphology of ZnO/PBD film by regulating its surface roughness via partial removal of PBD molecules. Such optimized morphology of ZnO/PBD film could provide uniform contact with the PBDB-T:IT-M photoactive blend layer. The change in hydrophobicity of ZnO film upon PBD deposition was also investigated through contact angle measurement with deionized (DI) water shown in Fig. S6. Similar to previous studies [38,39], the pristine ZnO film showed a small contact angle of 27� , indicating its hydrophilic nature. Interestingly, the contact angle was improved to 47� when PBD was deposited on top of the ZnO film, suggesting a more hydrophobic surface of the ZnO/PBD ETL. This hydrophobic surface was essential to promote a more intimate contact with the organic photo active layer for better charge extraction and transport in OSCs.
3.3. Optical properties Considering that these films functioned as ETL in inverted OSCs, the transmittance performance of these films will be a crucial factor to consider for evaluating optical properties. On this account, ultra violet–visible (UV–Vis) transmittance spectra of ZnO/PBD film was examined and compared to the pristine ZnO film. Based on Fig. S4, the transmittance spectra for both films showed comparable performance with a slight red-shifting in peak from ~382 nm (ZnO) to ~394 nm (ZnO/PBD), which could be due to the optical effect induced by addi tional PBD layer. It was also noticed that the ZnO/PBD film had lower transmissivity in the higher wavelength region (λ > 440 nm), most likely due to the absorption originated from PBD layer. Regardless of this, both films still displayed excellent transmissivity above 80% on a broad wavelength range from 360 to 850 nm and even higher than that of ITO. This implied that both films could function effectively as ETL in inverted OSCs due to their good transparency properties.
3.6. Photovoltaic performances To evaluate the usage of ZnO/PBD hybrid electron-transporting bilayer in organic photovoltaics, inverted devices with configuration of ITO/ZnO/PBD/PBDB-T:IT-M/MoO3/Ag were fabricated as illustrated in Fig. 3a. Meanwhile, the corresponding energy level of these materials are shown in Fig. 3b. Inverted devices with pristine ZnO interlayer were fabricated as well for comparison study. The current density–voltage (J–V) characteristics of inverted devices incorporating ZnO and opti mized ZnO/PBD interlayers as depicted in Fig. 3c, were measured under standard AM 1.5G illumination with 100 mW cm 2 light intensity in N2filled glovebox without encapsulation. The detailed photovoltaic pa rameters of OSCs with ZnO and ZnO/PBD interlayers are listed in Table 1. The PBD layer had been optimized previously in terms of its processing condition to achieve the optimal photovoltaic performance. The optimization of PBD layer on ZnO film is summarized in Tables S1–S3 of Supporting Information, where the effects of concen tration, thickness and post-processing treatment on photovoltaic per formance are being explored. As compared to pristine ZnO interlayer, the devices based on ZnO/ PBD interlayer exhibited slightly higher PCE of 11.6% with short circuit current (JSC) of 16.7 mA cm 2, open circuit voltage (VOC) of 0.937 V and fill factor (FF) of 74.1%. In contrast, reference devices based on ZnO interlayer only displayed a PCE of 10.8% with JSC of 15.9 mA cm 2, VOC of 0.935 V and FF of 72.8%. The results indicated that the interfacial modification of ZnO film using PBD could improve the device perfor mance through the simultaneous enhancement in JSC and FF. The major
3.4. Electrical properties The changes in electrical properties of ZnO film upon PBD deposition were studied by analyzing the electrical conductivity of the film. Devices with configuration of ITO/ZnO/Al and ITO/ZnO/PBD/Al were fabri cated to compare the conductivity of the two films and the results were plotted into the J–V curve shown in Fig. S5. The trend on the J–V curve for both films revealed a linear fitted relationship between current and voltage, indicating that both interfacial layers were able to form Ohmic contact with the electrodes. Meanwhile, slopes in the J–V curve were correlated to the electrical conductivity. The electrical conductivity is quantified based on the following equation [13]: δ ¼ Go ⋅ðdo = SÞ
(2)
where δ is the electrical conductivity, Go is the conductance obtained from the slope in J–V curve, do is the thickness of ZnO (~30 nm) and ZnO/PBD (~40 nm) film and S is the device area (0.038 cm2). Based on this relationship, δ of ZnO film was determined to be 2.45 � 10 4 S/m and was raised by ~76% to 4.32 � 10 4 S/m through utilization of PBD on ZnO. The higher δ of ZnO/PBD film suggested that ZnO/PBD had a lower contact resistance favorable for promoting charge transport.
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Fig. 2. AFM (5 � 5 μm) height and phase images of (a, d) ZnO, (b, e) ZnO/PBD and (c, f) ZnO/PBD/CB deposited on ITO/glass substrates.
reasons for such enhancement in ZnO/PBD-based device were mostly attributed to the WF tuning ability, desirable film morphology and higher film conductivity of the bilayer ZnO/PBD ETL. EQE spectra of devices based on ZnO and ZnO/PBD interlayers were analyzed to confirm the increment in JSC. The EQE spectra shown in Fig. 3d displayed a wide photo-response range from 300 to 800 nm. The photo-response peak found in 550–650 nm was ascribed to the absorp tion of PBDB-T donor, while the peak found in 650–750 nm was contributed by the absorption of IT-M acceptor. The absorption profiles of PBDB-T donor, IT-M acceptor and PBDB-T:IT-M blend film are shown in Fig. S7 of Supporting Information. Devices based on ZnO/PBD interlayer had higher EQE performance in the wavelength region above 480 nm as compared to those based on ZnO interlayer, supporting the fact that ZnO/PBD-based device had a higher JSC than ZnO-based de vice. Moreover, the integrated JSC values measured from EQE curves were calculated to be 15.5 and 15.9 mA cm 2 for devices based on ZnO and ZnO/PBD interlayers, agreeing well with the JSC values measured from J–V curves (within 5% deviation range). Exciton dissociation probability, P(E,T) can be estimated using the following equation [52]: � PðE; TÞ ¼ Jph Jsat (3)
expressed as JSC ∝ Pα, where α is the exponential factor denoting the degree of bimolecular recombination [27]. When α is close to 1, there is a negligible bimolecular recombination [27]. As shown in the JSC versus P plot in Fig. 3f, the slopes (α) of 1.09 and 0.97 were obtained for devices based on ZnO and ZnO/PBD interlayers, respectively. The slope closer to 1 indicated that ZnO/PBD-based device had a relatively negligible bimolecular recombination. Non-geminate recombination mechanisms were further studied in terms of trap-assisted recombination by analyzing the dependence of VOC on P. Theoretically, the slope of VOC versus P curve in Fig. 3f is equal to nkT/q, where n is the ideality factor, k is the Boltzmann’s constant, T is the temperature and q is the elementary charge [53]. In normal cir cumstances, the slope is found between 1 kT/q and 2 kT/q for trap-assisted recombination to occur. Slope closer to 1 kT/q indicates pure domination of bimolecular recombination, whereas slope closer to 2 kT/q stipulates domination of trap-assisted recombination [53]. De vices based on ZnO interlayer showed a slope of 1.40 kT/q, indicating the occurrence of trap-assisted recombination. In contrast, devices based on ZnO/PBD interlayer showed a smaller slope of 1.32 kT/q, implying the suppression of trap-assisted recombination via the use of ZnO/PBD interlayer. Overall, studies on charge recombination proved that ZnO/PBD-based device had negligible bimolecular recombination and suppressed trap-assisted recombination, which led to the higher JSC and FF values. Charge transport properties of devices based on ZnO and ZnO/PBD interlayers were estimated using space-charge limited current (SCLC) technique. Electron-only devices with structure of ITO/ZnO/with or without PBD/PBDB-T:IT-M/Ca/Al were fabricated to compare the charge transport in ZnO and ZnO/PBD interlayers in terms of electron mobilities. The mobility can be calculated by fitting dark current density (J) and voltage (V) into the following equation [52]: � � J ¼ ð9 = 8Þεo εr μ V2 L3 (4)
where Jph is the photocurrent density equal to Jsc at short circuit con dition and Jsat is the saturated photocurrent density or the values of Jph when Veff � 2 V. Jph is obtained from JL - JD, where JL is the current density under light irradiation and JD is the current density under dark condition. Meanwhile, Veff is defined by Vo - V, where Vo is the value of voltage when Jph ¼ 0 and V is the applied voltage. A plot of Jph versus Veff is shown in Fig. 3e. Based on Jph versus Veff curve, the Jsat values were determined to be 16.3 mA cm 2 for ZnO-based device and 17.1 mA cm 2 for ZnO/PBD-based device. Under short circuit condi tions, the Jph/Jsat ratio of devices based on ZnO and ZnO/PBD in terlayers were calculated to be 97.5% and 97.7%, indicating that both devices had comparable exciton dissociation efficiency and that the performance improvement in ZnO/PBD-based device was not governed by exciton dissociation processes in the photoactive layer. Non-geminate recombination mechanisms of devices based on ZnO and ZnO/PBD interlayers were investigated to understand the reasons behind improved photovoltaic performance. Bimolecular recombination was first analyzed by investigating the dependence of JSC on light in tensity (P). In principle, the relationship between JSC and P can be
where εo is the permittivity of free space, εr is the relative dielectric constant of the photoactive layer, μ in this case is the electron mobility, L is the thickness of the photoactive layer (~100 nm) and V is the differ ence between applied and built-in voltage. From the typical J1/2–V curves shown in Fig. S8, the electron mobility was calculated to be 1.67 � 10 4 cm2 V 1 s 1 for ZnO-based device and 2.14 � 10 4 cm2 V 1 s 1 for ZnO/PBD-based device. The improvement in electron mobility depicted a better charge transport ability in ZnO/ 5
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Fig. 3. (a) Device architecture of inverted OSCs with ZnO/PBD ETL; inset shows the chemical structures of PBDB-T donor and IT-M acceptor. (b) Energy levels of materials used in the OSCs. (c) J–V characteristics under standard AM 1.5G illumination with 100 mW cm 2 light intensity, (d) EQE spectra, (e) Jph versus Veff curve and (f) JSC and VOC light-intensity dependence curve of devices with or without optimized PBD interlayer.
successfully by introducing additional oxadiazole-based electron-trans porting materials known as PBD on top of the ZnO film to function as a hybrid electron-transporting bilayer. Through this bilayer strategy, inverted devices based on PBDB-T:IT-M photoactive layer demonstrated ~7% increment in the photovoltaic performance from 10.8% (ZnObased device) to 11.6% (ZnO/PBD-based device). The enhancement in efficiency of ZnO/PBD-based device was largely attributed to the simultaneous increase in JSC and FF values, mainly governed by the improvement in charge transport ability and suppression of charge recombination processes. Herein, the engineered ZnO/PBD interlayer could also provide more efficient electron extraction as evidenced from its better WF reduction to match LUMO of IT-M acceptor while at the same time, able to facilitate uniform and intimate contact with the photoactive layer. Moreover, ZnO/PBD interlayer had better interlayer conductivity compared to pristine ZnO interlayer. All these superior features of ZnO/PBD hybrid interlayer have crucially influenced the charge extraction, transport and recombination processes in OSCs,
Table 1 Detailed photovoltaic parameters of inverted OSCs with ZnO and optimized ZnO/PBD interlayer. Devices
VOC [V]
JSC [mA cm
with ZnO with ZnO/PBD
0.935 0.937
15.9 16.7
a
2
]
FF [%]
PCEmax (avg.)a [%]
72.8 74.1
10.8 (10.7) 11.6 (11.3)
Average PCE was obtained from 8 independent devices.
PBD interlayer, mainly caused by the higher conductivity of the hybrid interlayer. Along with suppressed recombination, the good charge transport ability in ZnO/PBD interlayer governed the enhancement in photovoltaic performance (JSC and FF) of ZnO/PBD-based device. 4. Conclusion In conclusion, interfacial modification of ZnO film was conducted 6
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which are conducive for the improvement in photovoltaic performance of ZnO/PBD-based device. Overall, this study highlights the potential of incorporating oxadiazole-based OLED electron-transporting materials into organic photovoltaic devices, and further demonstrates that PBD can be effectively used for interfacial modification of ZnO to boost photovoltaic performance of OSCs by forming hybrid interlayer that can circumvent the inherent weaknesses of sol-gel ZnO (e.g. poor interfacial contacts).
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Efficiency enhancement of organic solar cells enabled by interface engineering of sol-gel zinc oxide with an oxadiazole-based material Billy Fanady a, c, Wei Song c, Ruixiang Peng b, c, Tao Wu a, **, Ziyi Ge b, c, * a
Ningbo New Materials Institute, The University of Nottingham, Ningbo, 315042, PR China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, PR China c Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Organic solar cell Interface engineering Sol-gel ZnO PBD Oxadiazole-based material
Organic solar cells (OSCs) have acquired much attentions owing to their advantages in terms of solutionprocessability, low-cost, lightweight and compatibility for large-scale roll-to-roll processing. Aside from mate rials design, studies on interface engineering are also crucial to enhance photovoltaic performance for the realization of high-performing OSCs. In this study, interface engineering on sol-gel zinc oxide (ZnO) electrontransporting layer (ETL) was conducted by introducing additional oxadiazole-based electron-transporting ma terials, PBD between ZnO ETL and photoactive layer. The significance of incorporating PBD on ZnO was demonstrated by investigating the change in optical, electrical and morphological properties of pristine ZnO ETL. Herein, the utilization of PBD could enhance ZnO film’s conductivity, which was favorable for better charge transport ability. As compared to ZnO ETL, ZnO/PBD ETL had lower work function to facilitate more efficient electron extraction from the photoactive layer. Moreover, PBD could smoothen the ZnO film’s morphology and improve hydrophobicity of the surface to provide uniform and intimate interfacial contact between ETL and the photoactive layer. As a result, through this hybrid bilayer strategy, inverted OSCs based on PBDB-T:IT-M pho toactive layer system exhibited ~7% enhancement in the power conversion efficiency from 10.8% (ZnO-based device) to 11.6% (optimized ZnO/PBD-based device).
1. Introduction Organic solar cells (OSCs) have acquired significant public’s awareness over the years owing to their advantages in terms of me chanical flexibility, low-cost, lightweight and solution-processability making it compatible for large-scale manufacturing through roll-to-roll printing [1–4]. Since its first discovery in 1995, bulk-heterojunction (BHJ) photoactive layer system consisting of two compatible materials as electron donor and acceptor has been dominating in the OSCs field [5, 6]. In fact, considerable amount of attentions have been directed to wards the synthesis of novel electron donor and acceptor materials as the BHJ photoactive layer for the past few years [7–9]. During early stages, fullerene derivatives were studied extensively as electron acceptor [10–12], where a power conversion efficiency (PCE) of over 10% could be achieved [13,14]. However, it was later realized that fullerene acceptor was associated with several drawbacks in terms of i) limited energy level and bandgap tunability, ii) poor absorption
property in the visible and near-infrared (NIR) region, and iii) highly complicated and expensive synthetic procedure of fullerene [15–17]. Both limited tunability and poor light-harvesting property were the key factors that restrained the PCE of fullerene-based OSCs to a much higher value. Driven to overcome such limitations, non-fullerene acceptors (NFAs) were then designed and synthesized as an alternative to the state-of-the-art fullerene acceptors [18–20]. Along with emerging NFAs materials, donor materials were also designed and developed to cope with the growth [21,22]. To date, immense efforts directed towards materials design (e.g. synthesis of novel donor and acceptor materials) have been fruitful, which remarkably enhanced the PCE of single-junction devices to over 15% [23,24]. Generally, NFAs often exhibited a lowest unoccupied molecular orbital (LUMO) energy level of ~3.7–4.0 eV [25,26] when compared to PC71BM (~4.3 eV) [11]. It also suffered from low electron mobility (in the order of 10 4 cm2 V 1 s 1) [26,27]. Interface engineering may serve as a powerful strategy to crucially enhance photovoltaic performance by
* Corresponding author. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, PR China. ** Corresponding author. Ningbo New Materials Institute, The University of Nottingham, Ningbo, 315042, PR China. E-mail addresses: [email protected] (T. Wu), [email protected] (Z. Ge). https://doi.org/10.1016/j.orgel.2019.105483 Received 16 July 2019; Received in revised form 1 October 2019; Accepted 1 October 2019 Available online 2 October 2019 1566-1199/© 2019 Published by Elsevier B.V.
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solving these issues [28–30]. For instance, the incorporation of electron-transporting interfacial layer (ETL) between electrode and photoactive layer can be an effective way to tune the work function (WF) of the electrode, such that it will be close but lower than the LUMO energy level of NFAs [29]. This allows the formation of Ohmic contact at the interface which is beneficial for improving electron extraction, transport and collection, as well as the provision of hole-blocking effect. Therefore, aside from materials design, interface engineering also plays a pivotal role to effectively improve photovoltaic performance, partic ularly when a careful selection of interfacial layers including electron-transporting layer (ETL) and hole-transporting layer (HTL) is made. This selection is typically made by considering materials prop erties and suitability with the different OSCs’ device architecture. In inverted OSCs, MoO3 is mostly utilized as the HTL to promote Ohmic contact between top metal electrodes (e.g. Ag or Au) and highest occupied molecular orbital (HOMO) energy level of donor materials, allowing abundant and efficient hole extraction in inverted device. On the contrary, the role of ETL in inverted OSCs is more prominent to minimize the large contact barrier existing between WF of ITO bottom electrode and LUMO energy level of acceptor materials, so that efficient electron extraction can be achieved. Hence, studies on interfacial layer of inverted OSCs are mostly centered on the development of ETL. So far, several classes of ETL including n-type metal oxides [31], conjugated polyelectrolytes (CPEs) [32], alcohol-/water-soluble materials [33–36], and self-assembled dipole monolayers (SADMs) [37] have been explored; each with varying mechanisms/functions such as by tuning WF of the electrode, altering charge extraction efficiency and selectivity, altering the morphology of the active layer, modulating light absorption in the active layer, and etc [29]. Among them, sol-gel zinc oxide (ZnO) as n-type metal oxide demonstrated a promising optical transparency in the visible light region and capability to extract and transport electrons while effectively block holes, making it to be greatly suitable to be used as ETL in inverted OSCs [28–30]. Once deposited as thin-film, conven tional sol-gel ZnO is often post-treated with high-temperature annealing (~150–200 � C) to promote the formation of crystalline ZnO and reduce high-density defects that may deteriorate device performance. Despite this effort, sol-gel ZnO still suffers from high surface roughness and its hydrophilic nature can limit its interfacial contact with the organic photoactive layer [38,39]. These subsequently lead to poor electron extraction and device performance in inverted OSCs. On that account, interface engineering on sol-gel ZnO is conducted to effectively subdue the limitations of ZnO films. To date, interface engineering or interfacial modification on sol-gel ZnO has been studied through the doping of organic materials into ZnO [38–40] or the deposition of organic/inorganic materials as second interfacial layer on top of ZnO film [39,41]. Both methods can impede surface traps and defects in sol-gel ZnO, provide better interfacial con tact with the photoactive layer, alter the WF of the electrode, improve the conductivity of sol-gel ZnO and suppress recombination losses which are all beneficial for the improvement of photovoltaic performance. For examples, Li et al. [40] used sol-gel ZnO doped with EDTA organic chelating agent to improve the PCE of inverted OSCs from 11.1% (ZnO) to 12.1% (hybrid EDTA-ZnO). Borse et al. [39] introduced Ba(OH)2 layer deposited on top of the ZnO film to smoothen its morphology and pro mote better interfacial contact with the photoactive layer. As a result, PCE was escalated from 7.12% (ZnO) to 8.54% (bilayer ZnO/Ba(OH)2). Herein, 2-(4-tert-butylphenyl)-5-(4-biphenyl)-1,3,4-oxadiazole or abbreviated as PBD was introduced on top of the previously deposited ZnO film to modify its properties. This oxadiazole-based small-molecu lar (SM) material was first used by Adachi [42] in 1989 as ETL in bilayer organic light emitting diodes (OLED). And even up to now, it is still considered as one of the most commonly used ETLs in OLED mainly due to its good electron affinity and mobility as well as outstanding charge injection ability [43]. Even though PBD is frequently used in OLEDs as ETL materials [44–46], its application in other areas of optoelectronic devices is still limited. Recently, PBD was realized to function effectively
as a buffer layer in thin-film transistors (TFTs) as it could tune the energy level alignment for improving charge extraction in TFTs [47]. Yet, no studies were reported regarding the use of PBD in OSCs. In light of its advantages, PBD was incorporated in this work for the first time in OSCs as second interfacial layer on top of ZnO to modify ZnO film’s properties. The introduction of PBD on ZnO film was a promising strategy as it could improve the electrical and morphological properties of sol-gel ZnO film. Furthermore, PBD layer could further tune the WF of the electrode to better match the LUMO energy level of acceptor materials. All those features allow ZnO/PBD interlayer to function effectively as ETL in inverted OSCs. As a result, inverted OSCs based on poly[(2,6-(4,8-bis (5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophe ne))-alt-(5,5-(10 ,30 -di-2-thienyl-50 ,70 -bis(2-ethylhexyl)benzo[10 ,20 -c:40 , 50 -c’]dithiophene-4,8-dione)] (PBDB-T donor) [48] and 3,9-bis (2-methylene-((3-(1,1-dicyanomethylene)-6/7-methyl)-indanone))-5,5, 11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:20 ,30 -d’]-s-indaceno[1, 2-b:5,6-b’]dithiophene (IT-M acceptor) [49] displayed ~7% PCE enhancement from 10.8% (ZnO-based device) to 11.6% (optimized ZnO/PBD-based device). 2. Materials and methods 2.1. Materials preparation Indium tin oxide (ITO)-coated glass substrates (1.1 mm thick, � 15 Ω/square) were purchased from Wuhu Token Sciences Co., Ltd. Zinc acetate (Zn(CH3COO)2, 97.5%), ethanolamine (99.5%) and 2-methoxye thanol were received from J&K Scientific, Ltd. ZnO precursor solution was prepared by dissolving 100.9 mg of zinc acetate in 33.5 μL of ethanolamine and 1 mL of 2-methoxyethanol. PBD (>99%) was ob tained from Xi’an Polymer Light Technology Corp. PBDB-T and IT-M were purchased from Solarmer Materials Inc., whereas 1,8-diiodoctane (DIO), chlorobenzene and methanol were received from SigmaAldrich, Inc. Unless stated, all chemicals were used without any further purifications. 2.2. Device fabrication OSCs were fabricated with the configuration of glass/ITO/ZnO/ PBD/PBDB-T:IT-M/MoO3/Ag. ITO-coated glass substrate was first cleaned by sequential sonification in detergent, deionized water, acetone and isopropanol for 20 min in each step. Prior to device fabri cation, the pre-cleaned ITO was dried with the flow of N2 and was further treated with ultraviolet ozone for 25 min. The ZnO precursor solution was deposited onto the ITO-coated glass substrate at 4000 rpm for 60s and annealed at 150 � C for 30 min. PBD was dissolved in meth anol with optimized concentration of 1.0 mg/mL. The corresponding PBD solution was stirred overnight without heat before usage. Under optimal conditions, PBD solution was spin-coated on top of the ZnO film at 1000 rpm for 60s and was then dried under vacuum for 15 min. Photoactive blend layer solution was prepared by dissolving PBDB-T donor and IT-M acceptor (D:A ¼ 1:1 by weight) in 20 mg/mL chloro benzene (total solid concentration) with 0.7% by volume addition of DIO additives. The photoactive layer solution (PBDB-T:IT-M) was spincoated at 1900 rpm for 60s onto the PBD film and then annealed at 120 � C for 10 min. Finally, 8 nm MoO3 and 100 nm Ag as electrode were deposited through a shadow mask (effective area of 3.8 mm2) by vac uum evaporation with base pressure of ~5 � 10 6 mbar. 2.3. Device characterization X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelec tron spectroscopy (UPS) were measured using Kratos AXIS Ultra DLD XPS/UPS spectrometer. Transmittance and absorption spectra were collected using PerkinElmer Lambda 950 UV/Vis spectrometer. Atomic force microscopy (AFM) height and phase images were acquired using 2
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Veeco Dimension 3100 instrumentation. Dektak 150 surface profiler was used to measure the film thickness. Current density–voltage (J–V) characteristics of unencapsulated device were measured in N2-filled glove box using Keithley 2440 source meter with AM 1.5G solar simu lator (Newport-Oriel® Sol3A 450W). The light intensity was calibrated at 100 mW cm 2 using a certified reference Si standard cell (SRC-2020 with KG5 filter) obtained from Enli Technology Co., Ltd and the cali bration report was traceable to NREL. External quantum efficiency (EQE) spectra were obtained using Enlitech QE-R solar cell quantum efficiency measurement system with a 75W Xenon lamp source, whose light intensity was calibrated by a reference Si probe (RC-S103011-G) obtained from Enli Technology Co., Ltd. EQE was measured in the N2filled glove box at room temperature. Contact angle was measured using the multifunctional dynamic contact angle measuring device and tensiometer (DCAT21).
In this case, three constituent peaks were found at 284.8, 285.8 and – C, C–H, 288.7 eV. Those peaks were ascribed to the presence of C–C, C– – N bond [50,51], which were all present in PBD molecules. C–O and C– The results confirmed that PBD was successfully deposited on top of the ZnO film to function as a hybrid electron-transporting bilayer. The changes in elemental states upon PBD deposition were investi gated by comparing the positions and shapes of Zn 2p and O 1s peaks of ZnO and ZnO/PBD films. The high-resolution Zn 2p and O 1s XPS spectra are shown in Fig. S1 of Supporting Information, respectively. Two major peaks of 2p3/2 at lower binding energy and 2p1/2 at higher binding en ergy were obtained, confirming the presence of ZnO phase formation in ZnO and ZnO/PBD films [39]. Both 2p1/2 and 2p3/2 peaks looked identical with a slight shift in 2p3/2 peak from 1021.6 to 1021.5 eV upon deposition of PBD on ZnO film. This indicated that the incorporation of PBD had minimal influence on the Zn–O bond formation in ZnO film. This was further supported from O 1s XPS survey, where it showed similar peaks position before and after the deposition of PBD. In here, the constant peak position at higher binding energy was attributed to O atoms in ZnO matrix, which was in agreement to the previous findings [31,38]. Meanwhile, the peak at lower binding energy corresponding to oxygen-deficient components was slightly shifted [31,38]. The relative magnitude for this oxygen-deficient peak was calculated to be 51.28% for pristine ZnO film and 51.22% for ZnO/PBD film. These implied that the number of oxygen-deficient components remained relatively con stant even after PBD deposition.
3. Results and discussions 3.1. Elemental states XPS measurement was performed in order to analyze the elemental states of the modified ZnO/PBD film (chemical structures of PBD depicted in Fig. 1a) and compare it to the pristine ZnO film. The overall XPS survey spectra of those two films are presented in Fig. 1b. Both films showed similar XPS survey spectra with notable peaks of Zn 2p, O 1s, N 1s and C 1s. Among those peaks, the most distinct change could be seen from the enhancement and broadening of N peak due to the introduction of PBD layer (Fig. 1b, inset). To support this, high-resolution N 1s peak of ZnO/PBD film was deconvoluted into its constituents shown in Fig. 1c. A slight increment of N peak at 400.0 eV was attributed to the presence of N–C double bond found in PBD molecules [41], whereas the broadening of N peak was caused by the occurrence of additional peak at 398.5 eV. The peak at lower binding energy was ascribed to the inter action between N atoms of PBD and Zn atoms of ZnO found on the ZnO/PBD interfaces [41]. Fig. 1d illustrates the C 1s survey spectra of ZnO/PBD film to further testify the presence of PBD on top of ZnO film.
3.2. Work function UPS measurements on ZnO- and ZnO/PBD-modified ITO surfaces were conducted to understand the role of PBD in tuning the WF of ITO cathode in inverted OSCs. The UPS spectra of ZnO- and ZnO/PBDmodified ITO cathodes are portrayed in Fig. S3 of Supporting Informa tion. Typically, work function can be calculated based on the following equation [39]: � ϕ ¼ hv Ecutoff ðBEÞ Eonset (1)
Fig. 1. (a) Chemical structures of PBD electron-transporting materials. (b) XPS survey spectra of ZnO and ZnO/PBD on ITO/glass substrates; inset shows the enlarged N 1s spectra. The deconvoluted high-resolution (c) N 1s and (d) C 1s peaks of ZnO/PBD deposited on ITO/glass substrates. 3
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3.5. Morphological properties
where ϕ is the work function, hv is the incident photon energy of He I (21.22 eV), Ecutoff (BE) is the high binding energy (BE) determined by linear extrapolation to zero at the yield of secondary electrons and Eonset is the onset relative to the Fermi level (EFermi) of Au (0 eV). Work function can also be represented in terms of kinetic energy (KE), resulting in the simplification of equation to ϕ ¼ Ecutoff (KE) - Eonset. In this case, Fermi level of Au has been set to 0 eV prior to measurement, thus, the Ecutoff values obtained from KE UPS curve in Fig. S3 directly corre sponded to the work functions of cathode. Due to this reason, the WF of ZnO- and ZnO/PBD-modified ITO cathodes were determined to be 4.12 and 4.03 eV, while the WF of bare ITO cathode was 4.90 eV. It was obvious that the use of interfacial layers (ZnO and ZnO/PBD) could effectively modify the WF of bare ITO cathode. In comparison to ZnOmodified ITO cathode (4.12 eV), ZnO/PBD-modified ITO cathode showed a lower WF of 4.03 eV, which was relatively closer to LUMO energy level of IT-M acceptor (3.98 eV) [49]. This highly suggested that additional PBD layer in ZnO/PBD interlayer could further tune the WF of ITO cathode by creating a better energy level alignment with the LUMO energy level of IT-M acceptor. On one hand, this could promote Ohmic contact for efficient electron extraction and on the other hand, such WF modification might strengthen cell’s built-in field beneficial for boosting photovoltaic performance.
The morphological features of pristine ZnO and ZnO/PBD film were explored through AFM height and phase images shown in Fig. 2. It was observed from the height (Fig. 2a) and phase images (Fig. 2d) that the pristine ZnO film exhibited a rough surface morphology with root-meansquare (RMS) roughness of 5.57 nm. The RMS roughness of ZnO film was toned down significantly to 2.78 nm when PBD was deposited on top of the ZnO film (Fig. 2b), proving the ability of PBD to smoothen the ZnO film’s morphology significantly. Chlorobenzene (CB) as the processing solvent of photoactive layer was deposited on top of the ZnO/PBD film to simulate the actual inverted OSCs when photoactive blend layer of PBDB-T:IT-M in CB was deposited on top of the ETL. As shown in Fig. 2c and f, CB deposition could definitely alter the surface morphology of ZnO/PBD film by partially removing PBD molecules from ZnO/PBD film. Partial removal of PBD was first speculated as the CB deposition on ZnO/PBD film did not raise the surface roughness back to 5.57 nm, which was the RMS value for pristine ZnO film. This was later confirmed by the overall XPS survey spectra presented in Fig. S2 which compared ZnO/PBD/CB film with ZnO/PBD film. In addition, high-resolution N 1s peak of ZnO/PBD/ CB film displayed in Fig. S2 further testified the presence of PBD even after CB deposition. Due to this unique characteristic, the surface roughness of ZnO/PBD film upon CB deposition was increased to 4.52 nm. This explained that CB solvent of the photoactive layer could become the key in optimizing the morphology of ZnO/PBD film by regulating its surface roughness via partial removal of PBD molecules. Such optimized morphology of ZnO/PBD film could provide uniform contact with the PBDB-T:IT-M photoactive blend layer. The change in hydrophobicity of ZnO film upon PBD deposition was also investigated through contact angle measurement with deionized (DI) water shown in Fig. S6. Similar to previous studies [38,39], the pristine ZnO film showed a small contact angle of 27� , indicating its hydrophilic nature. Interestingly, the contact angle was improved to 47� when PBD was deposited on top of the ZnO film, suggesting a more hydrophobic surface of the ZnO/PBD ETL. This hydrophobic surface was essential to promote a more intimate contact with the organic photo active layer for better charge extraction and transport in OSCs.
3.3. Optical properties Considering that these films functioned as ETL in inverted OSCs, the transmittance performance of these films will be a crucial factor to consider for evaluating optical properties. On this account, ultra violet–visible (UV–Vis) transmittance spectra of ZnO/PBD film was examined and compared to the pristine ZnO film. Based on Fig. S4, the transmittance spectra for both films showed comparable performance with a slight red-shifting in peak from ~382 nm (ZnO) to ~394 nm (ZnO/PBD), which could be due to the optical effect induced by addi tional PBD layer. It was also noticed that the ZnO/PBD film had lower transmissivity in the higher wavelength region (λ > 440 nm), most likely due to the absorption originated from PBD layer. Regardless of this, both films still displayed excellent transmissivity above 80% on a broad wavelength range from 360 to 850 nm and even higher than that of ITO. This implied that both films could function effectively as ETL in inverted OSCs due to their good transparency properties.
3.6. Photovoltaic performances To evaluate the usage of ZnO/PBD hybrid electron-transporting bilayer in organic photovoltaics, inverted devices with configuration of ITO/ZnO/PBD/PBDB-T:IT-M/MoO3/Ag were fabricated as illustrated in Fig. 3a. Meanwhile, the corresponding energy level of these materials are shown in Fig. 3b. Inverted devices with pristine ZnO interlayer were fabricated as well for comparison study. The current density–voltage (J–V) characteristics of inverted devices incorporating ZnO and opti mized ZnO/PBD interlayers as depicted in Fig. 3c, were measured under standard AM 1.5G illumination with 100 mW cm 2 light intensity in N2filled glovebox without encapsulation. The detailed photovoltaic pa rameters of OSCs with ZnO and ZnO/PBD interlayers are listed in Table 1. The PBD layer had been optimized previously in terms of its processing condition to achieve the optimal photovoltaic performance. The optimization of PBD layer on ZnO film is summarized in Tables S1–S3 of Supporting Information, where the effects of concen tration, thickness and post-processing treatment on photovoltaic per formance are being explored. As compared to pristine ZnO interlayer, the devices based on ZnO/ PBD interlayer exhibited slightly higher PCE of 11.6% with short circuit current (JSC) of 16.7 mA cm 2, open circuit voltage (VOC) of 0.937 V and fill factor (FF) of 74.1%. In contrast, reference devices based on ZnO interlayer only displayed a PCE of 10.8% with JSC of 15.9 mA cm 2, VOC of 0.935 V and FF of 72.8%. The results indicated that the interfacial modification of ZnO film using PBD could improve the device perfor mance through the simultaneous enhancement in JSC and FF. The major
3.4. Electrical properties The changes in electrical properties of ZnO film upon PBD deposition were studied by analyzing the electrical conductivity of the film. Devices with configuration of ITO/ZnO/Al and ITO/ZnO/PBD/Al were fabri cated to compare the conductivity of the two films and the results were plotted into the J–V curve shown in Fig. S5. The trend on the J–V curve for both films revealed a linear fitted relationship between current and voltage, indicating that both interfacial layers were able to form Ohmic contact with the electrodes. Meanwhile, slopes in the J–V curve were correlated to the electrical conductivity. The electrical conductivity is quantified based on the following equation [13]: δ ¼ Go ⋅ðdo = SÞ
(2)
where δ is the electrical conductivity, Go is the conductance obtained from the slope in J–V curve, do is the thickness of ZnO (~30 nm) and ZnO/PBD (~40 nm) film and S is the device area (0.038 cm2). Based on this relationship, δ of ZnO film was determined to be 2.45 � 10 4 S/m and was raised by ~76% to 4.32 � 10 4 S/m through utilization of PBD on ZnO. The higher δ of ZnO/PBD film suggested that ZnO/PBD had a lower contact resistance favorable for promoting charge transport.
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Fig. 2. AFM (5 � 5 μm) height and phase images of (a, d) ZnO, (b, e) ZnO/PBD and (c, f) ZnO/PBD/CB deposited on ITO/glass substrates.
reasons for such enhancement in ZnO/PBD-based device were mostly attributed to the WF tuning ability, desirable film morphology and higher film conductivity of the bilayer ZnO/PBD ETL. EQE spectra of devices based on ZnO and ZnO/PBD interlayers were analyzed to confirm the increment in JSC. The EQE spectra shown in Fig. 3d displayed a wide photo-response range from 300 to 800 nm. The photo-response peak found in 550–650 nm was ascribed to the absorp tion of PBDB-T donor, while the peak found in 650–750 nm was contributed by the absorption of IT-M acceptor. The absorption profiles of PBDB-T donor, IT-M acceptor and PBDB-T:IT-M blend film are shown in Fig. S7 of Supporting Information. Devices based on ZnO/PBD interlayer had higher EQE performance in the wavelength region above 480 nm as compared to those based on ZnO interlayer, supporting the fact that ZnO/PBD-based device had a higher JSC than ZnO-based de vice. Moreover, the integrated JSC values measured from EQE curves were calculated to be 15.5 and 15.9 mA cm 2 for devices based on ZnO and ZnO/PBD interlayers, agreeing well with the JSC values measured from J–V curves (within 5% deviation range). Exciton dissociation probability, P(E,T) can be estimated using the following equation [52]: � PðE; TÞ ¼ Jph Jsat (3)
expressed as JSC ∝ Pα, where α is the exponential factor denoting the degree of bimolecular recombination [27]. When α is close to 1, there is a negligible bimolecular recombination [27]. As shown in the JSC versus P plot in Fig. 3f, the slopes (α) of 1.09 and 0.97 were obtained for devices based on ZnO and ZnO/PBD interlayers, respectively. The slope closer to 1 indicated that ZnO/PBD-based device had a relatively negligible bimolecular recombination. Non-geminate recombination mechanisms were further studied in terms of trap-assisted recombination by analyzing the dependence of VOC on P. Theoretically, the slope of VOC versus P curve in Fig. 3f is equal to nkT/q, where n is the ideality factor, k is the Boltzmann’s constant, T is the temperature and q is the elementary charge [53]. In normal cir cumstances, the slope is found between 1 kT/q and 2 kT/q for trap-assisted recombination to occur. Slope closer to 1 kT/q indicates pure domination of bimolecular recombination, whereas slope closer to 2 kT/q stipulates domination of trap-assisted recombination [53]. De vices based on ZnO interlayer showed a slope of 1.40 kT/q, indicating the occurrence of trap-assisted recombination. In contrast, devices based on ZnO/PBD interlayer showed a smaller slope of 1.32 kT/q, implying the suppression of trap-assisted recombination via the use of ZnO/PBD interlayer. Overall, studies on charge recombination proved that ZnO/PBD-based device had negligible bimolecular recombination and suppressed trap-assisted recombination, which led to the higher JSC and FF values. Charge transport properties of devices based on ZnO and ZnO/PBD interlayers were estimated using space-charge limited current (SCLC) technique. Electron-only devices with structure of ITO/ZnO/with or without PBD/PBDB-T:IT-M/Ca/Al were fabricated to compare the charge transport in ZnO and ZnO/PBD interlayers in terms of electron mobilities. The mobility can be calculated by fitting dark current density (J) and voltage (V) into the following equation [52]: � � J ¼ ð9 = 8Þεo εr μ V2 L3 (4)
where Jph is the photocurrent density equal to Jsc at short circuit con dition and Jsat is the saturated photocurrent density or the values of Jph when Veff � 2 V. Jph is obtained from JL - JD, where JL is the current density under light irradiation and JD is the current density under dark condition. Meanwhile, Veff is defined by Vo - V, where Vo is the value of voltage when Jph ¼ 0 and V is the applied voltage. A plot of Jph versus Veff is shown in Fig. 3e. Based on Jph versus Veff curve, the Jsat values were determined to be 16.3 mA cm 2 for ZnO-based device and 17.1 mA cm 2 for ZnO/PBD-based device. Under short circuit condi tions, the Jph/Jsat ratio of devices based on ZnO and ZnO/PBD in terlayers were calculated to be 97.5% and 97.7%, indicating that both devices had comparable exciton dissociation efficiency and that the performance improvement in ZnO/PBD-based device was not governed by exciton dissociation processes in the photoactive layer. Non-geminate recombination mechanisms of devices based on ZnO and ZnO/PBD interlayers were investigated to understand the reasons behind improved photovoltaic performance. Bimolecular recombination was first analyzed by investigating the dependence of JSC on light in tensity (P). In principle, the relationship between JSC and P can be
where εo is the permittivity of free space, εr is the relative dielectric constant of the photoactive layer, μ in this case is the electron mobility, L is the thickness of the photoactive layer (~100 nm) and V is the differ ence between applied and built-in voltage. From the typical J1/2–V curves shown in Fig. S8, the electron mobility was calculated to be 1.67 � 10 4 cm2 V 1 s 1 for ZnO-based device and 2.14 � 10 4 cm2 V 1 s 1 for ZnO/PBD-based device. The improvement in electron mobility depicted a better charge transport ability in ZnO/ 5
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Fig. 3. (a) Device architecture of inverted OSCs with ZnO/PBD ETL; inset shows the chemical structures of PBDB-T donor and IT-M acceptor. (b) Energy levels of materials used in the OSCs. (c) J–V characteristics under standard AM 1.5G illumination with 100 mW cm 2 light intensity, (d) EQE spectra, (e) Jph versus Veff curve and (f) JSC and VOC light-intensity dependence curve of devices with or without optimized PBD interlayer.
successfully by introducing additional oxadiazole-based electron-trans porting materials known as PBD on top of the ZnO film to function as a hybrid electron-transporting bilayer. Through this bilayer strategy, inverted devices based on PBDB-T:IT-M photoactive layer demonstrated ~7% increment in the photovoltaic performance from 10.8% (ZnObased device) to 11.6% (ZnO/PBD-based device). The enhancement in efficiency of ZnO/PBD-based device was largely attributed to the simultaneous increase in JSC and FF values, mainly governed by the improvement in charge transport ability and suppression of charge recombination processes. Herein, the engineered ZnO/PBD interlayer could also provide more efficient electron extraction as evidenced from its better WF reduction to match LUMO of IT-M acceptor while at the same time, able to facilitate uniform and intimate contact with the photoactive layer. Moreover, ZnO/PBD interlayer had better interlayer conductivity compared to pristine ZnO interlayer. All these superior features of ZnO/PBD hybrid interlayer have crucially influenced the charge extraction, transport and recombination processes in OSCs,
Table 1 Detailed photovoltaic parameters of inverted OSCs with ZnO and optimized ZnO/PBD interlayer. Devices
VOC [V]
JSC [mA cm
with ZnO with ZnO/PBD
0.935 0.937
15.9 16.7
a
2
]
FF [%]
PCEmax (avg.)a [%]
72.8 74.1
10.8 (10.7) 11.6 (11.3)
Average PCE was obtained from 8 independent devices.
PBD interlayer, mainly caused by the higher conductivity of the hybrid interlayer. Along with suppressed recombination, the good charge transport ability in ZnO/PBD interlayer governed the enhancement in photovoltaic performance (JSC and FF) of ZnO/PBD-based device. 4. Conclusion In conclusion, interfacial modification of ZnO film was conducted 6
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which are conducive for the improvement in photovoltaic performance of ZnO/PBD-based device. Overall, this study highlights the potential of incorporating oxadiazole-based OLED electron-transporting materials into organic photovoltaic devices, and further demonstrates that PBD can be effectively used for interfacial modification of ZnO to boost photovoltaic performance of OSCs by forming hybrid interlayer that can circumvent the inherent weaknesses of sol-gel ZnO (e.g. poor interfacial contacts).
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