Boosting Efficiency and Stability of Organic Solar Cells Using Ultralow

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Surfaces, Interfaces, and Applications

Boosting Efficiency and Stability of Organic Solar Cells Using Ultralow-Cost BiOCl Nanoplates as a Hole Transporting Layer Bin Liu, Yang Wang, Peng Chen, Xianhe Zhang, Huiliang Sun, Yumin Tang, Qiaogan Liao, Jiachen Huang, Hang Wang, Hong Meng, and Xugang Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12583 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

Boosting Efficiency and Stability of Organic Solar Cells Using Ultralow-Cost BiOCl Nanoplates as a Hole Transporting Layer Bin Liu, †,‡ Yang Wang, ‡ Peng Chen, ‡ Xianhe Zhang, ‡ Huiliang Sun, ‡ Yumin Tang, ‡ Qiaogan Liao, ‡ Jiachen Huang, ‡ Hang Wang, ‡ Hong Meng,† and Xugang Guo*‡

†School

of Advanced Materials, Peking University Shenzhen Graduate School, Peking

University, Shenzhen 518055, China ‡Department

of Materials Science and Engineering, Southern University of Science and

Technology (SUSTech), No. 1088, Xueyuan Road, Nanshan, Shenzhen, Guangdong 518055, China

KEYWORDS: organic solar cells, BiOCl nanoplates, hole transporting layer, high efficiency, device stability

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ABSTRACT: A novel nanomaterial, bismuth oxychloride nanoplates (BiOCl NPs), was firstly applied in organic solar cells (OSCs) as a hole transporting layer (HTL). It is worth noting that the BiOCl NPs can be facilely synthesized at ~1/200 cost of the commercial PEDOT:PSS and well dissolved in green solvents. Different from PEDOT:PSS interlayer, the deposition of BiOCl HTL is free of post-treatment at elevated temperature, which reduces device fabrication complexity. To demonstrate the universality of BiOCl in improving photovoltaic performance, OSCs containing various representative active layers were investigated. The power conversion efficiencies (PCEs) of the P3HT:PC61BM, PTB7-Th:PC71BM, and PM6:Y6-based OSCs with the BiOCl HTL were boosted from 3.62%, 8.78%, and 15.63% to 4.24%, 9.92%, and 16.11%, respectively, compared to the PEDOT:PSS based ones. It was found that the superior performances of the BiOCl-based OSCs are mainly attributed to the sufficient oxygen vacancies and improved interfacial contact. Moreover, the BiOCl-based OSCs show a much better stability than the cells with the PEDOT:PSS interfacial layer.

1. INTRODUCTION As one of the highly promising photovoltaic techniques, organic solar cells (OSCs) with a bulk heterojunction (BHJ) structure have been attracting wide attention with the record power conversion efficiency (PCE) for single junction cells exceeding 16% achieved under the joint research efforts.1-5 In quest of high-efficiency OSCs, a variety of strategies have been developed, such as material innovation,6-13 morphology optimization,14-18 and device engineering.19-22 It is well known that inserting an appropriate interfacial layer between active layer and electrode is an effective approach to collect the desired charge carriers and block the opposite ones, which can largely suppress the unwanted recombination of the photon-generated charge carriers.23 Recently, extensive researches on developing new hole transporting layers (HTLs) have provided deeper understandings on the materials design and yielded more 2


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alternative interfacial layers, which result in enhanced device performance and long-term stability of OSCs.24,25 Poly(styrenesulfonate)-doped poly(ethylenedioxythiophene) (PEDOT:PSS) is the most used HTL in OSCs to date. However, the strong acidity of PEDOT:PSS leads to severe corrosion of the underlying indium tin oxide (ITO) electrodes and results in performance degradation.26-28 In addition, the PEDOT:PSS HTL is energy-consuming in terms of both synthesis and film post-treatments at elevated temperature. Consequently, a great number of organic conjugated polymers, graphene oxides (GOs), inorganic metal oxides/sulfides, and other alternatives29 have been introduced as the attractive substitutes for PEDOT:PSS. Among them, many solution-processable polymer interlayers have been synthesized to avoid the postprocessing, such as thermal annealing, and improve their compatibility with BHJ active layers. In addition, the optoelectronic properties and energy levels can be readily tuned via molecular engineering, thereby effectively modifying the work function (WF) of electrodes and improving the performances of OSCs.30-32 Nevertheless, the complicated synthesis and time-consuming purification of most polymers hindered their usage in OSCs. In contrast, GOs have emerged as new interfacial materials with simplified synthesis and low synthetic cost, but they suffered from poor device performances due to their insulating nature and inappropriate energy level alignments.33-35 Unlike the above interlayers, inorganic nanomaterials such as NiOx,36 WO3,37,38 MoOx,39 VOx,40 and BiI,41 have been developed as highly promising HTLs due to their higher optical transparency, better environmental stability, and easier synthesis than the conjugated polyelectrolyte

poly(styrenesulfonate)

(PSS)

and

conducting

polymer

poly(ethylenedioxythiophene) (PEDOT).42 Among various inorganic HTL materials, bismuth halide oxide nanomaterial can be envisioned as a novel semiconductor having an anisotropic ternary layered structure, high chemical stability as well as excellent optoelectronic and magnetic properties. As a quasi-two3



dimensional material, surface-exposed atoms on bismuth oxychloride (BiOCl) are prone to escape from lattices to form vacancies, which can greatly affect their physicochemical properties. BiOCl has a layered structure containing [Bi2O2]2+ layers sandwiched between two slabs of halogen ions.43 Previous studies have shown that BiOCl nanoplates (NPs) with facet exposed have high oxygen vacancy density, which is generally considered to be responsible for enhancing photoactivity and photoelectricity.44 Furthermore, BiOCl NPs with large content of oxygen vacancies and interfacial contact can enhance the adsorption capacity and increase the conductivity of buffer layers, hence effectively helping facilitate the hole transfer and separate the electron-hole pairs in the ultrathin nanoplates.45 Based on the above considerations, BiOCl NPs were introduced into OSCs as an HTL to substitute the widely used PEDOT:PSS for the first time. Specifically, ultrathin BiOCl NPs (40~50 nm) were synthesized at ultra-low cost by the mannitol-assisted hydrothermal process and utilized as anode interlayer in OSCs. The synthesis of BiOCl NPs was very facile and environmentally friendly, which could be evenly dispersed in the green solvent (ethanol).45-46 Please note that the deposition of BiOCl film is free of post-treatment at elevated temperature in comparison with the traditional PEDOT:PSS buffer layer. To demonstrate the effectiveness and universality of the BiOCl NPs on improving device performance, OSCs with three representative bulk heterojunction (BHJ) active layers, namely PTB7-Th:PC71BM blend47 and P3HT:PC61BM blend48 as the fullerene-based system and PM6:Y6 blend49 as the non-fullerene system, were fabricated. OSCs with the PEDOT:PSS HTL were also fabricated in parallel for control. To our delight, the improved PCEs of BiOCl NP based devices with PTB7-Th:PC71BM, P3HT:PC61BM and PM6:Y6 active layers are increased by 13%, 17% and 3% in comparison to the cells with the traditional PEDOT:PSS interlayer, respectively. To the best of our knowledge, these PCE enhancements are the record values for the three OSCs using inorganic HTLs as the substitutes for the conventional PEDOT:PSS (Figure S1 and Table S1). Besides, 4


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detailed characterizations of photodiode behavior, film morphology, and charge carrier recombination kinetics have been performed to further study the generation, transport, and collection of photogenerated charge carriers in the devices. This work, for the first time, demonstrates the remarkable potential of BiOCl NPs as an HTL for enabling highly efficient OSCs with reduced cost and improved stability. 2. EXPERIMENTAL SECTION Device fabrication: The ITO-coated glasses (12 Ω sq-1) were cleaned according to the established procedure by sequential ultrasonic cleaning in detergent water (3 times), deionized water (3 times), acetone and isopropyl alcohol for 15 min each. The PEDOT:PSS (Clevios P VP A1 4083) film was spin-coated on the top of UV-ozone (UVO) treated ITO substrates at 3000 rpm for 30 s and annealed at 150 °C for 15 min in air, while the BiOCl ethanol solution was spin-coated on the above treated ITO glasses at 2500 rpm followed by 60 C baking (~ 5 min) to remove the residual solvent and UVO treatment (~ 10 min) to modulate the work function. The PEDOT:PSS or BiOCl coated ITO substrates were then transferred to a N2-filled glove box for subsequent processing. The PTB7-Th:PC71BM blend solution (1:1.5 w/w; total concentration: 20 mg mL-1) was prepared in chlorobenzene (CB) with 3% DIO additive, and the P3HT:PC61BM solution (1:1 w/w; polymer concentration: 20 mg mL-1) was prepared in dichlorobenzene (DCB) without any additives. The PM6:Y6 solution (1:1.2 w/w; polymer concentration: 7.5 mg mL-1) was prepared in chloroform (CF) with 0.5% chloronaphthalene (CN) as the additive. All these solutions were stirred for 8 h to reach complete dissolution. After the deposition, the PTB7-Th:PC71BM films were dried at room temperature for 30 min, whereas P3HT:PC61BM and PM6:Y6 active layers were annealed at 150 ℃ and 110 C for 10 min, respectively. Finally, a perylene diimide functionalized with amino N-oxide (PDINO)50 was

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spin-coated at 3000 rpm for 30 s atop followed by the deposition of 110 nm Al top electrode in a high vacuum (3E−6 Torr) using thermal evaporation. Characterization and Measurements: The X-ray photoelectron spectroscopy (XPS) results were acquired with a Thermo esca lab 250Xi electron spectrometer using 150 W Al Ka (hv =1486.6 eV) radiation. The WF was characterized with a Kelvin Probe system (KP020, KP Technology) in the atmosphere. The transmission spectra of the BiOCl coated ITO glass were recorded on a UV-vis spectrophotometer (Perkin Elmer Lambda 950) at room temperature in the wavelength from 200 to 800 nm. The J-V curves of ITO glass, PEDOT:PSS/ITO and BiOCl/ITO film were measured, and the series resistance can be derived from these curves based on the slopes of each film. The contact angle measurement was implemented on the VCA Optima XE (American Stress Technologies Inc.). To be specific, the contact angles of deionized water were characterized on the ITO glass, ITO/PEDOT:PSS and ITO/BiOCl substrates, respectively. The droplet diameter and droplet height were set as 10.0 μL and 3.0 cm, respectively, and then the solutions were dripped onto the substrates. The surface morphology of the films was characterized by an atomic force microscope (Asylum Research MFP-3DStand Alone). 3. RESULTS AND DISCUSSIONS The BiOCl NPs were synthesized via a modified method reported by Xie et al.45-46 Bi(NO3)3·5H2O and NaCl solutions were reacted via a simple mannitol-assisted solvothermal route, and the complete synthesis details are provided in the Supporting Information (SI). Please note that the facile synthesis of the NPs under mild conditions dramatically reduced the cost. Without considering the product profit, the synthetic cost of 1 L BiOCl NPs solution was 51 RMB (~7.4 US dollar) based on the prices of the commercially available chemicals (Table S2), which is roughly less than 1/200 of the cost of the widely used PEDOT:PSS (Table S3), showing a distinctive advantage of this inorganic nanomaterial for practical applications. 6


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The morphological characteristics of as-prepared ultrathin BiOCl NPs were first probed by transmission electron microscopy (TEM). As shown in the inset of Figure 1a, these nanoparticles feature square-like appearance with areal dimensions of 40-50 nm, which should be favorable for the subsequent HTL formation. Scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDS) elemental mapping was carried out to determine the corresponding elemental composition and distribution (inset of Figure 1b-d), which indicates that the nanoplates primarily consist of Cl, O and Bi with a uniform spatial distribution of Cl and Bi over the flakes.

Figure 1. XPS spectra of the BiOCl NPs: (a) survey scan, (b) Bi 4f spectra, (c) Cl 2p spectra and (d) O spectra. Inset: TEM images for the BiOCl NPs and the corresponding STEM-EDS elemental maps, indicating a homogeneous distribution of (b) bismuth (in blue), (c) chlorine (in red) and (d) oxygen (in green). X-ray photoemission spectroscopy (XPS) was further employed to characterize the chemical composition and analyze the chemical state of elements on the BiOCl surface. As displayed in Figure 1a, the survey XPS spectra of the specimen show the Bi, O, and Cl peaks. 7



Note that the C peak mainly originates from the incidental hydrocarbon of the XPS apparatus. The high-resolution Bi 4f spectrum features two distinctive peaks at 158.7 and 163.9 eV (Figure 1b), which are assigned to the binding energies of Bi 4f7/2 and Bi 4f5/2, respectively.51,52 The peaks located at 197.6 and 199.2 eV (Figure 1c) can be ascribed to the binding energies of Cl 2p3/2 and Cl 2p1/2, respectively, indicating the existence of Cl element in BiOCl NPs. Additionally, the O 1s peak (Figure 1d) at ~529.7 eV is a characteristic feature of O species in the Bi-O bond of BiOCl,53 and the O 1s spectrum fitted with the peak at 530.9 eV can be ascribed to the O species in surface hydroxyl groups absorbed in ambient,54 which should be conducive to the photoelectron transfer process. It is well known that the light absorption characteristics of materials play a critical role in the photovoltaic effect. Thus, the ultraviolet-visible (UV-vis) absorption of BiOCl NPs was measured in the range from 200 to 800 nm (Figure S2). A strong absorption in the UV range below 350 nm is recorded for BiOCl NPs, a distinctive absorption feature for BiOCl materials with oxygen defects55, which can be assigned to the intrinsic band gap absorption of BiOCl NPs. Based on the XPS results, the UV light absorption indicates the presence of oxygen vacancies in the BiOCl NPs. Furthermore, the optical transmittance was characterized (Figure S3a). The light transmittance for the ITO/PEDOT:PSS in the range of 320~430 nm is higher than that of bare ITO and ITO covered with BiOCl, which is due to the fact that the PEDOT:PSS film diminishes the surface roughness and consequently decreases the scattering of the glasses.56,57 Nevertheless, the ITO:BiOCl substrate shows improved transmission in the range of 430~550 nm and 750~1000 nm than the ITO/PEDOT:PSS one, basically equal to that of the bare ITO substrate. Hence, the ITO coated with BiOCl NPs is highly transparent in the range of 200~1000 nm and shows minimal effects on the transmission in comparison with the bare ITO, which indicates that the introduction of BiOCl does not result in photon loss, a primary premise for 8


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replacing PEDOT:PSS. Another key parameter related to the charge carrier transport ability and collection efficiency is the resistance of the interlayer. As shown in Figure S3b, a resistance of 0.79 Ω cm2 was found for the bare ITO substrate as calculated from the J−V curve, while the lowest slope is recorded for the PEDOT:PSS-coated ITO, suggesting a significant increase of the series resistance (1.43 Ω cm2). In contrast, the ITO substrate coated with the BiOCl NPs shows a relatively higher slope, yielding a lower resistance of 0.83 Ω cm2, which is almost comparable to that of the bare ITO glasses.58 As a HTL material, the BiOCl film with such a high conductivity is beneficial for the charge transport, which can facilitate the hole collection from the active layer to anode electrode. The surface energies of bare ITO, ITO coated with PEDOT:PSS or BiOCl HTL were probed by measuring contact angle. As shown in Figure S4, the contact angle of the bare ITO is 77.6°, while the angle of the PEDOT:PSS coated ITO drops to 16.7°. To our delight, for the BiOCl coated ITO, the angle increases to 50.9°, which greatly differs from other nanomaterials.59 The hydrophobic property of the BiOCl interlayer can lead to increased moisture resistance for the devices, which should be beneficial to device stability. Moreover, the increase of contact angle enabled by the BiOCl film indicates a lowered surface free energy, giving rise to more effective interfacial contacts with photo-active materials.60 To assess the potential of BiOCl NPs as HTL material, OSCs with a conventional device structure of ITO/HTL/active layer/PDINO50/Al were fabricated (Figure 2a), where PDINO is a perylene diimide derivative functioning as an electron transporting layer. The molecular structures of the materials that make the BHJ active layers in this work are presented in Figure 2b, wherein the well-studied p-type polymers PTB7-Th, P3HT, and PM6 were used as the donor materials, and the n-type fullerene derivatives PC71BM and PC61BM together with the state-ofthe-art non-fullerene acceptor Y6 were employed as the acceptor materials. The energy level 9



diagram of the materials utilized in the OSC devices is illustrated in Figure 2c. The WF of the BiOCl film was determined by Kelvin Probe, and the corresponding energy levels of the frontier molecular orbitals (FMOs) of PTB7-Th, P3HT, PM6, PC71BM, PC61BM, and Y6 were extracted from the literatures.47-49, 61 It can be observed that the WF of BiOCl is slightly lower than that of PEDOT:PSS and closer to the highest occupied molecular orbital (HOMO) energy level of the donor materials, indicating a smaller energy offset at the active layer/BiOCl HTL interface. Benefiting from the smaller energy offset, BiOCl anode interface will facilitate the hole extraction and collection from the active layer to the anode.62

Figure 2. (a) Photovoltaic device architecture and (b) the molecular structures and (c) energy level alignment of the electrode and hole transport layer together with the donor and acceptor materials used in this study.

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Firstly, device optimization of PTB7-Th:PC71BM OSCs was carried out by varying the BiOCl HTL thickness using different solution concentration from 0.5 to 5.0 mg mL-1. The current density-voltage (J−V) characteristics under the solar illumination of AM 1.5G (100 mW cm-2) are shown in Figure S5 and the device performance parameters are listed in Table S4. With increasing the concentration of BiOCl NPs, a quality interfacial layer is gradually formed to improve the interfacial contact between the ITO substrate and the BHJ layers. An optimal short-circuit current (Jsc) up to 18.42 mA cm-2 is achieved when the concentration reaches 2.0 mg mL-1. Nevertheless, as the concentration further increases to 3.0 and 5.0 mg mL-1, high series resistance resulting from thicker HTL leads to a Jsc reduction from 18.42 to 17.53 mA cm-2 but with no significant change of open-circuit voltage (Voc) and fill factor (FF). To demonstrate the universality of BiOCl NPs on improving the photovoltaic performances of OSCs versus the conventional PEDOT:PSS as a control, the optimal BiOCl NP film prepared with a concentration of 2.0 mg mL-1 is used in OSCs with the BHJ active layers based on the classical materials. The J−V characteristics are shown in Figure 3a-c and device parameters are summarized in Table 1. For the PTB7-Th:PC71BM-based OSCs, the devices with the PEDOT:PSS anode interfacial layer show a PCE of 8.78% with a Voc of 0.79 V, a Jsc of 16.22 mA cm-2, and an FF of 68.17%, which is comparable to the performance parameters reported in the literature.47 To our delight, a distinctive increment of Jsc (18.42 mA cm-2) enabled by utilizing the BiOCl NP interlayer contributes to the enhanced PCE of 9.92%. In the case of P3HT:PC61BM system, the PCE of BiOCl based device is promoted to 4.24% with a Voc of 0.59 V, an FF of 71.12% and an increased Jsc of 10.03 mA cm-2 compared to PEDOT:PSS based cell showing a PCE of 3.62% with a Voc of 0.60 V, an FF of 70.94% and a Jsc of 8.58 mA cm-2. Furthermore, in the PM6:Y6 system, the BiOCl HTL based device exhibits a remarkable PCE of 16.11% with a Jsc of 27.07 mA cm-2, a Voc of 0.84 V, and an FF of 71.70%, while in the PEDOT:PSS-based cell, a relatively lower Jsc of 26.31 mA cm-2 and an FF of 70.87% 11



are recorded, leading to a reduced PCE of 15.63%. As it can be seen, despite different HTL materials used, the Vocs of both solar cells are nearly same. In fact, the Voc of an organic solar cell is not only related to the energy level of hole transport material, but also affected by the charge recombination in active layer.63-65 In our work, there is a small difference (0.05 eV) between the WFs of BiOCl HTL (5.05 eV) and PEDOT:PSS (5.00 eV). Considering such a small WF difference between two HTLs together with the effect of charge recombination on Voc, hence it is reasonable that Voc remains nearly unchanged in the cells with distinct HTL. Apparently, the improvement of PCEs in all these BiOCl-based devices are originated from the enhancement of Jsc and/or FF, which indicates a superior hole transporting capacity and extraction ability of BiOCl NPs compared to the conventional PEDOT:PSS HTL.

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1 2 3 Figure 3. (a-c) J−V characteristics of PTB7-Th:PC71BM, P3HT:PC61BM and PM6:Y6 solar 4 5 cells with PEDOT:PSS or BiOCl NPs as the hole transporting layer and (d-f) the corresponding 6 7 EQE curves of the solar cells. 8 Table 1. Photovoltaic performance parameters of organic solar cells with PEDOT:PSS or 9 10 BiOCl NPs as the hole transporting layer under the Illumination of AM 1.5G, 100 mW cm-2. 11 12 b b c b b Hole Transport Layer 13 Active Layera Voc (V) Jsc (mA cm-2) Cal. Jsc (mA cm-2) FF (%) PCE (%) 14 15 PTB7-Th:PC BM PEDOT:PSS 0.79±0.003 (0.79) 16.10±0.12 (16.22) 16.18 65.27±2.9 (68.17) 8.30±0.48 (8.78) 71 16 17 BiOCl NPs 0.79±0.005 (0.79) 18.29±0.13 (18.42) 17.95 66.06±2.1 (68.16) 9.55±0.37 (9.92) 18 19 P3HT:PC BM PEDOT:PSS 0.59±0.007 (0.60) 8.35±0.23 (8.58) 8.37 69.24±1.7 (70.94) 3.41±0.21 (3.62) 61 20 21 BiOCl NPs 0.59±0.004 (0.59) 9.72±0.31 (10.03) 9.59 69.52±1.6 (71.12) 3.98±0.26 (4.24) 22 PM6:Y6 PEDOT:PSS 0.84±0.004 (0.84) 26.12±0.19 (26.31) 25.47 68.87±2.0 (70.87) 15.11±0.52 (15.63) 23 24 BiOCl NPs 0.83±0.003 (0.83) 26.87±0.20 (27.07) 26.04 69.90±1.8 (71.70) 15.60±0.51 (16.11) 25 a b 26 The device area is 4.5 mm2. Average values with standard deviation obtained from 20 devices; 27 c 28 the values in parentheses are the parameters of the best devices. Jsc integrated from EQE curve 29 30 31 32 The corresponding external quantum efficiency (EQE) spectra of all devices are provided 33 34 35 in Figure 3d-f. As can be seen, the spectra of PTB7-Th:PC71BM-based OSCs with the 36 37 PEDOT:PSS interlayer show the highest EQE of 71% at 640 nm, while the devices based on 38 39 the BiOCl HTL display a further enhanced EQE of 79%, achieving a boosted integrated Jsc 40 41 42 from 16.18 to 17.95 mA cm-2 accordingly. Similarly, for the P3HT:PC61BM-based OSCs with 43 44 a BiOCl interlayer, a higher EQE response from 300 to 700 nm is achieved than that of the 45 46 PEDOT:PSS-based devices, sequentially improving the integrated Jsc from 8.37 to 9.59 mA cm47 48 2. For the PM6:Y6 system, the EQE shape of the devices with the BiOCl HTL is nearly the 49 50 51 same as the control devices, except slightly higher light responses across the entire wavelength 52 53 range for devices with BiOCl layer, leading to a boosted Jsc of 26.04 mA cm-2. The integrated 54 55 56 Jscs agree well with the photocurrents from the J−V curves with a mismatch < 5%, indicating a 57 58 good reliability of our results. 59 13 60



To probe the mechanism of device performance improvement and study the morphologyperformance relationship, tapping-mode atomic force microscopy (AFM) was employed to investigate the surface morphologies of active layers based on PEDOT:PSS or BiOCl NP HTL. As depicted in Figure 4, a coarse phase separation is found for the PTB7-Th, P3HT and PM6based blend films deposited on the BiOCl HTL, especially for the high-efficiency PM6:Y6 OSCs. The rougher surfaces are likely to assure increased contact area and better interaction between the BiOCl interlayer and the active films, thereby achieving more efficient charge carrier extraction from the active layer to anode.66 In addition, a more favorable interpenetrating network with an appropriate phase separation is obtained for the BiOCl HTL-based OSCs, which is conducive to exciton dissociation and charge transport. These features in principle explain the improved OSC performances for the devices containing BiOCl HTL.67

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Figure 4. AFM height and phase images of (a-d) PTB7-Th:PC71BM, (e-h) P3HT:PC61BM and (i-l) PM6:Y6 solar cells with PEDOT:PSS or BiOCl NPs as the hole transporting layer, respectively. The photodiode behavior was subsequently studied and the J−V curves of the fabricated devices under dark were characterized. As shown in Figure 5a-c, lower leakage currents of all OSCs based on BiOCl interlayer at reverse bias from -2 to 0 V are observed in comparison with those of PEDOT:PSS based cells, suggesting that the BiOCl based devices can significantly suppress the reverse currents. The slightly lower current indicates that the BiOCl HTL with favorable electron-blocking property can effectively suppress electron leakage by the anode interfacial modification under reversed bias.68,69 When the bias is greater than 1 V, higher forward current is recorded from the BiOCl-based OSCs. This is in accordance with the improved photovoltaic performance, indicative of an enhanced hole extraction by the anode, balanced charge transport, and suppressed charge recombination in the BiOCl based devices.

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Figure 5. (a-c) J-V characteristics in dark and (d-f) Pdiss versus Veff plots of PTB7-Th:PC71BM, P3HT:PC61BM and PM6:Y6 solar cells with PEDOT:PSS or BiOCl NPs as the hole transporting layer, respectively. To further investigate the dynamics of exciton generation and charge extraction and collection in the optimized devices, photocurrent density (Jph) was analyzed as a function of effective applied voltage (Veff) for these OSCs. Jph refers to the current density difference between Jlight (under the illumination of AM 1.5G, 100 mW cm-2) and Jdark (in dark). Veff is calculated from Veff = V0 −Vapp, where V0 is defined as the voltage when Jph = 0 and Vapp denotes the applied bias. The curves of Jph versus Veff are plotted in Figure 5d-f and the corresponding parameters are included in Table S5. With the increase of Veff in low bias (< 0.3 V), the Jphs of all the optimal devices with different HTLs increase sharply, implying that more excitons dissociate into free charge carriers.70 When the Veff is large enough (> 0.6 V), all the charge carriers in the active layers could be swept out, hence generating a saturated photocurrent (Jsat). Apparently, the Jphs of the BiOCl based OSCs can reach saturation at a lower applied bias than OSCs with the PEDOT:PSS HTL, suggesting a more favorable exciton dissociation and charge extraction capability for the BiOCl based OSCs. Simultaneously, the probability of exciton dissociation (Pdiss) can be calculated based on the Jph/Jsat ratio and used to assess the exciton dissociation ability.71 As exhibited in Table S5, Pdisss over 90% are observed for all the devices with either PEDOT:PSS or BiOCl HTLs. It is worth noting that all the BiOCl based cells exhibit higher Pdisss in comparison to the control devices with the PEDOT:PSS HTL, indicating the stronger exciton dissociation and charge collecting efficiency for the BiOCl based devices. To gain deep insights into the bimolecular recombination in the devices, the light intensity (Plight) dependence of Jsc under various intensities (from 10 to 100 mW cm-2) was measured (Figure S6a-c). The Jsc can be related to the Plight by the power-law equation Jsc ∝ Plightα (α ≤ 1), where the exponential factor α indicates the slope of a logarithmic plot of Jsc vs. Plight.72 In 16


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ideal conditions, the electrodes can effectively collect all dissociated carriers in OSCs before their recombination and α is then equal to 1, suggesting the bimolecular recombination is negligible.73 In the OSCs with PEDOT:PSS and BiOCl interfacial layer, the fitted exponential factors are calculated to be 0.924 and 0.967 for the PTB7-Th:PC71BM system, 0.903 and 0.940 for the P3HT:PC61BM blend, 0.949 and 0.974 for the PM6:Y6 film, respectively, implying that the bimolecular recombination kinetics is efficiently suppressed in the optimized devices. Significantly, all the devices with the BiOCl HTL exhibit higher α values than the PEDOT:PSS based ones, indicating a higher degree of bimolecular recombination in the PEDOT:PSS-based devices. The suppressed bimolecular recombination in the BiOCl NPs based devices can facilitate exciton dissociation and charge carrier transport, leading to the higher Jscs and PCEs for the BiOCl based devices. Additionally, the Voc as a function of Plight was characterized to probe the geminate recombination at open-circuit conditions. The slope of Voc vs. ln(Plight) can be obtained from the equation kT/q, where k, T, and q refer to the Boltzmann’s constant, Kelvin temperature, and elementary charge, respectively. As illustrated in Figure S6d-f, the slopes of PTB7Th:PC71BM, and PM6:Y6 cells with BiOCl HTL are 1.21 and 1.12, respectively, showing a slight decrease compared to those (1.34 and 1.16) of PEDOT:PSS-based cells. For the P3HT based devices, a similar slope is observed. All these observations manifest the suppressed monomolecular and/or trap-assisted recombination in the BiOCl-based OSCs. The dominant bimolecular recombination and negligible unwanted recombination at the open-circuit condition demonstrate efficient hole/electron transport in devices with the BiOCl interlayer. As a result, improved device performances were attained. In addition to the efficiency, the stability of OSCs is also of great importance for practical application. It is generally believed that interfacial layer plays a vital role in device stability, therefore, the stability of PTB7-Th:PC71BM based devices with PEDOT:PSS and BiOCl 17



interfacial layer has been periodically monitored. As shown in Figure S7 and Table S6-S9, the BiOCl based devices can maintain more than 80% of its original efficiencies after 360 h storage, nevertheless the devices with the PEDOT:PSS HTL suffer from ~50% degradation of PCE during the same time frame. As can be found in Figure 6, the Vocs for all the devices show very small changes (~1% degradation versus the initial Vocs) when aging up to 360 h. Previous works exhibited similar phenomena, in which the Voc basically remained unchanged over time.74,75 In fact, the Voc typically is the most stable one among all three parameters in OSCs, while the Jsc and FF are more prone to degrade over time, since Voc is mainly related to the energetics of solar cell components, which typically shows minimal change over time unless the components decompose chemically. However, Jsc and FF decrease sharply in the PEDOT:PSS based devices compared to those of the BiOCl-based ones, leading to significant PCE reduction for the PEDOT:PSS based ones. It was found that the degradation of PEDOT:PSS-based OSCs is mainly through the followings mechanisms: (1) PEDOT:PSS with high acidity and hygroscopic nature etches ITO, leading to indium diffusing into the HTL and active layer;39 (2) PSS can reach the active layer and causes its components to decompose chemically;76 (3) The fullerene in the active layers can penetrate into the PEDOT:PSS interlayer, reducing its electron-blocking ability.77 In contrast, the BiOCl based devices display much better stability due to the stable and hydrophobic surface of BiOCl NPs, which enables good interfacial contact between active layer and anode and yields a kinetic barrier for H2O penetration.

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Figure 6. Evolution of normalized (a) Voc, (b) Jsc, (c) FF, and (d) PCE versus time of the PTB7Th:PC71BM based solar cells with PEDOT:PSS or BiOCl NPs as the hole transporting layer. 4. CONCLUSION In summary, we have successfully developed a novel BiOCl anode interfacial layer for utilization in high-performance OSCs to replace the conventional PEDOT:PSS, which can be facilely synthesized at ultra-low cost. The as-prepared BiOCl NPs with dimensions of 40-50 nm are highly homogeneous in the green solvent, such as ethanol, which is favorable for film deposition and practical application. It is worth noting that the fabrication of BiOCl hole transport layer avoids thermal annealing at elevated temperature and the PCEs of the optimized OSCs with the BiOCl HTL are greatly enhanced in comparison to those of solar cells with the PEDOT:PSS interlayer. In fact, these results represent the record values in terms of PCE enhancements in three typical OSCs using inorganic HTL as the substitute for organic PEDOT:PSS interlayer. Furthermore, OSCs with the novel BiOCl NPs anode interlayer exhibit a much better stability than the PEDOT:PSS-based analogous cells. Through a series of materials and device characterization, it was found that the superior photovoltaic performance 19



and improved stability of the BiOCl-based solar cells are attributed to the favorable transmittance, hydrophobicity, reduced chemical reactivity, more sufficient oxygen vacancies and improved interfacial contact of the novel HTL. These features will considerably increase the interlayer conductivity, promote the electron-hole pair dissociation, and facilitate the hole collection. This work demonstrates the great potentials of the BiOCl NPs for boosting the photovoltaic performances with much reduced cost and increased device stability in solar cells, which will trigger the development of novel interfacial materials and advance the practical applications of OSCs.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX Synthesis details; absorption spectra of BiOCl NPs; optical transmittance; contact angle; device performance; Jph, Jsat and Pdiss parameters, the stability data and curves. AUTHOR INFORMATION Corresponding Author *Email: [email protected] (X.G.). ORCID Yang Wang: 0000-0002-3669-0192 Hong Meng: 0000-0001-5877-359X Xugang Guo: 0000-0001-6193-637X Notes The authors declare no competing financial interest. 20


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ACKNOWLEDGMENTS X.G. is grateful to the Shenzhen Basic Research Fund (JCYJ20170817105905899 and JCYJ20180504165709042). H. M. is grateful to the Shenzhen Science and Technology research grant (JCYJ20180302153406868) and Shenzhen Hong Kong Innovation Circle Joint R & D project (SGLH20161212101631809). Y.W and H.S. thank NSFC (21805128 and 21801124) for the financial support.

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Boosting Efficiency and Stability of Organic Solar Cells Using Ultralow

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