Highly Efficient Organic Solar Cells Consisting of Double Bulk ...

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Jiang Huang, Hanyu Wang, Kangrong Yan, Xiaohua Zhang, Hongzheng Chen, Chang-Zhi Li,* and Junsheng Yu* yielding relatively low short-circuit current density (Jsc) and overall performance of single junction devices. On the other hand, OSCs with thick BHJ layers have advantages not only in rising up efficiency ceilings due to intense absorption but also in gaining good compatibility for upscaled processing.[13–15] Despite devices with 300 nm or thicker BHJs demonstrated, the number of success examples is rare and limited to few absorbers with high-mobility and strongstacking properties.[9,16–18] Encouragingly, the efficient cases exhibited high short-circuit current density (Jsc) over 20 mA cm−2,[6,9] upon a remarkable improvement in the light absorption rate higher than 80%.[6,9,19,20] However, the usual observation in thick-layer-based devices is that the gains in the light absorption and photocurrents often accompany with the loss of fill factor (FF), resulting in the decrease of overall efficiencies. The loss of fill factor can be ascribed to the decreased charge carrier mobilities and unfavored phase distribution in the thick BHJ layer.[6,19] To solve this challenge, tandem devices with multiple stacks of BHJs have been employed, for example, mediate bandgap absorbers (i.e., PTB7, PTB7-Th, and PffBT4T-2OD)[20,21] with narrow bandgap absorbers (i.e., PDPP3T,[22] PMDPP3T,[23] and PBDTT-SeDPP[24]) are employed. In these devices, photons with high and low energy can be selectively harvested by the front and rear cells, which are electrically collected through a transparent internal electrode for recombining charges.[8] However, the difficulties for tandem devices are the complicated fabrication of multilayers and the internal connecting electrode as well as the delicate optimization to ensure the current match between subcells. The ternary blend in single junction device is another approach that can harvest more photons than binary BHJ,[16,25–27] due to the combination of three absorbers into a consolidated BHJ. It requires careful choice of components with processing compatibility and complementary optoelectronic properties, to ensure proper film morphology and charge transport channels.[16,25–27] Beside of that, the phase separation of different components in the ternary blend is difficult to be controlled, leading to the nonideal film morphology, which are drawbacks added to OSCs. Therefore, it still lacks a simple and generally applicable method that can combine multiple absorbers for making thick and intense light-taking OSCs with overall high efficiency. Herein, we report a novel and simple approach to access highly efficient single junction OSCs that sandwich two

An organic solar cell (OSCs) containing double bulk heterojunction (BHJ) layers, namely, double-BHJ OSCs is constructed via stamp transferring of low bandgap BHJ atop of mediate bandgap active layers. Such devices allow a large gain in photocurrent to be obtained due to enhanced photoharvest, without suffering much from the fill factor drop usually seen in thick-layerbased devices. Overall, double-BHJ OSC with optimal ≈50 nm near-infrared PDPP3T:PC71BM layer atop of ≈200 nm PTB7-Th:PC71BM BHJ results in high power conversion efficiencies over 12%.

Organic solar cells (OSCs) are attractive clean solar-to-electricity conversion technologies with the compatibility of high-through solution manufacturability.[1,2] In the past decades, vigorous efforts have been devoted to the innovation of organic semiconductors[2–5] and their related photovoltaic devices,[2,6] resulting in the rapid improvement of power conversion efficiencies (PCEs) to 10%–12% for single junction OSCs.[7–11] Though exciting, one long existing challenge in OSCs remains that the majority of organic absorbers only afford using thin (i.e., ≈100 nm) bulkheterojunction (BHJ) layers in devices. It is because of the intrinsically low carrier mobilities (ranging 10−5–10−3 cm2 V−1 s−1 for electron and hole) and nonideal vertical phase distribution of organic BHJs, which set prerequisites for efficient OSCs employing thin layer to fulfill charge extraction and mitigate recombination. However, the usage of thin BHJ layer inevitably results in the insufficient light absorption,[2,12] i.e., less than 80% light absorption rate in ≈100 nm organic BHJs, thus

Prof. J. Huang, Dr. H. Wang, Dr. X. Zhang, Prof. J. S. Yu State Key Laboratory of Electronic Thin Films and Integrated Devices School of Optoelectronic Information University of Electronic Science and Technology of China (UESTC) Chengdu 610054, P. R. China E-mail: [email protected] Dr. K. Yan, Prof. H. Chen, Prof. C.-Z. Li MOE Key Laboratory of Macromolecular Synthesis and Functionalization State Key Laboratory of Silicon Materials Department of Polymer Science and Engineering Zhejiang University Hangzhou 310027, P. R. China E-mail: [email protected]

DOI: 10.1002/adma.201606729

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Highly Efficient Organic Solar Cells Consisting of Double Bulk Heterojunction Layers


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Figure 1. Structures, energy levels, and light absorption of active materials in this work. a) Chemical structures of polymer donors PTB7-Th, PDPP3T, and fullerene acceptor PC71BM. b) Inverted device configuration and diagrams of energy levels. c) Extinction coefficient (k) of bulk heterojunctions including PTB7-Th:PC71BM and PDPP3T:PC71BM.

vertically connected BHJs with varied bandgaps (thereafter as double-BHJ) between cathode and anode for building thicklayer-based architecture. The low bandgap polymer PDPP3Tbased BHJ is stamp transferred atop of bottom PTB7-Th-based thick BHJ layer, which effectively broadens the light absorption into the near infrared region (up to 925 nm). Interestingly, such double-BHJ configuration not only ensures efficient utilization of both high and low energy photons but also maintains the effective charge transport channels due to the presence of intermixed region between BHJs. New device architecture allows obtaining a large gain in photocurrent, while without suffering much from the energy loss and fill factor drop usually seen in thick-layer-based devices. The optimal ≈50 nm near-infrared PDPP3T BHJ atop of ≈200 nm PTB7-Th BHJ results in 22% gain in the overall photocurrent (to that of only PTB7-Th BHJbased devices) as high as 23.75 mA cm−2. Overall, double BHJbased devices lead to the state-of-art PCE over 12% with a Jsc of 23.75 mA cm−2, a Voc of 0.77 V, and an FF of 0.67. During this independent study, note that Ghasemi et al. recently reported that the sequential cast of two BHJs can improve the PCE of ternary device, from 5.09% of ternary device to 6.73% of sequentially casted devices.[28] Together this, the double-BHJ OSC may represent an effective strategy toward the high-efficiency devices. Figure 1 shows the chemical structures, energy band diagrams, and extinction coefficient of PTB7-Th and PDPP3T 1606729 (2 of 9)


polymer donors and PC71BM acceptor employed in this study. The OSC devices containing 100 nm PTB7-Th:PC71BM (BHJ1) show absorption edge of 780 nm and deliver a typical Voc of 0.81 V with PCE ≈ 10%.[6,14] Our previous work demonstrated an improved PCE near 11% can be obtained with 200 nm PTB7-Th:PC71BM BHJs that are fabricated through off-center spin method.[6] However, the photocurrent reaches the saturate value of 20 mA cm−2 when the PTB7-Th:PC71BM BHJ is thicker than 250 nm.[6] In this study, we incorporate the off-center spun PTB7-Th:PC71BM BHJ1 with a near-infrared PDPP3T:PC71BM BHJ2, to construct thick double-BHJ OSCs. As shown in Figure 1b,c, the PDPP3T has a smaller bandgap of 1.3 eV than that of PTB7-Th (1.5 eV). And the PDPP3T:PC71BM (BHJ2) has a complementary light absorption spectra in the near-infrared range up to 925 nm. It is expected the enhanced light absorption of double-BHJs should rise up the maxim attainable photocurrent in OSCs. Note that the BHJ2-based OSC shows a relatively low Voc of 0.65 V and a PCE near 6% with less than 100 nm active layer in the PEDOT:PSS-based devices.[29,30] To make the above two BHJs into an effective combination, we first explore the ternary blend strategy in single junction devices. Figure S1 (Supporting Information) is the photovoltaic performance of PTB7-Th:PDPP3T:PC71BM (1:0.5:2, by weight ratio)based ternary OSC. It shows that the ternary device decreased PCE by a factor of 15% to that of single BHJ1-based devices, due to the reduced Voc (from 0.81 to 0.74 V) and FF (from 0.71


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Figure 2. Fabrication of double-BHJ OSC. a) Solution fabrication of BHJ1 of PTB7-Th:PC71BM on Glass/Indium tin oxide (ITO)/ZnOx substrate using inverted off-center spinning method.[7] b) Stamp transfer of PDPP3T:PC71BM on PUA/PC onto BHJ1 layer. c) Solvent treatment of double BHJ1/BHJ2. d) Final device configuration of OSC.

to 0.60). The cause of lowered performance for ternary devices might be ascribed to the nonideal phase morphology and complicate donor/acceptor interfacial contacts.[27–29] To avoid these disadvantages, we propose to vertically connect the two BHJs in constructing thick-layer-based OSCs, as shown in Figure 2. We first fabricate the BHJ1 layer on top of ZnOx electron transport layer (ETL) with a thickness of 200 and 250 nm based on the previously reported method (upside-down off-center spin coating).[6] The BHJ2 layer is separately fabricated on the molds composed of polyurethane acrylate (PUA)-coated polycarbonate (PC) films.[31] The BHJ2 is then stamped atop of BHJ1, and the completion of film transfer is via peeling off PUA/PC substrate.[31,32] It has been known that the residue solvent additive 1,8-diiodooctane (DIO) in BHJ layer is detrimental to BHJ morphology and performance stability.[33] Therefore, methanol treatment to the stacked two BHJ layers is performed for purposes of removing DIO residue. We finally deposit MoO3 layer and shaped Ag electrode to complete the device fabrication.

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Figure 3. Photovoltaic characteristics of OSCs. a) Current density versus voltage (J–V) characteristics of OSCs with various BHJs without [5,6]open azafulleroid (OPCBM) under the certified solar simulator. b) J–V curve of 3.14 and 12 mm2 OSCs with OPCBM modified ZnOx ETL, the inset: the histogram of the PCE distribution of 3.14 mm2 devices. c) IQE and EQE quantum efficiencies of OSC with OPCBM. The Jsc obtained by integrating the product of the EQE spectrum with the air mass (AM) 1.5G solar spectrum is 23.56 ± 0.1 mA cm−2 which agreed with the measured Jsc to within 5%.

Figure 3a shows the current density versus voltage (J–V) curves of OSCs based on various BHJs. Their photovoltaic performances are summarized in Table 1. The OSC with solely



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Table 1. Photovoltaic parameters of OSCs using various BHJs, where the BHJ1 and BHJ2 refer to PTB7-Th:PC71BM and PDPP3T:PC71BM bulk heterojunction layers, respectively. Thicknessa) [nm]

Voc [V]

Jsc [mA cm−2]

FF

PCEb) [%]

BHJ1 (100 nm)

0.80 (0.80 ± 0.007)

16.23 (16.15 ± 0.18)

0.73 (0.73 ± 0.018)

9.48 (9.40 ± 0.13)

BHJ1 (200 nm)

0.81 (0.81 ± 0.004)

18.52 (18.43 ± 0.25)

0.70 (0.69 ± 0.020)

10.50 (10.31 ± 0.22)

BHJ1 (250 nm)

0.80 (0.80 ± 0.008)

19.53 (19.35 ± 0.31)

0.68 (0.67 ± 0.016)

10.62 (10.41 ± 0.26)

BHJ2 (50 nm)

0.69 (0.69 ± 0.003)

7.36 (7.22 ± 0.18)

0.71 (0.70 ± 0.014)

3.61 (3.47 ± 0.21)

BHJ1 (200 nm)/BHJ2(50 nm)

0.77 (0.76 ± 0.01)

23.19 (22.91 ± 0.32)

0.65 (0.65 ± 0.016)

11.61 (11.31 ± 0.35)

BHJ1 (200 nm)/BHJ2(50 nm) (OPCBM on ZnOx, 3.14 mm2)

0.77 (0.77 ± 0.004)

23.75 (23.50 ± 0.33)

0.67 (0.66 ± 0.013)

12.25 (11.94 ± 0.33)

BHJ1 (200 nm)/BHJ2(50 nm) (OPCBM on ZnOx, 12 mm2)

0.77 (0.77 ± 0.005)

23.42 (23.22 ± 0.29)

0.64 (0.63 ± 0.015)

11.54 (11.26 ± 0.33)

a)The

standard deviation of BHJ thickness is ±3 nm; b)Average PCE in brackets represent the standard deviation over 20 devices.

250 nm BHJ1 has a Voc of 0.81 V, Jsc of 19.53 mA cm−2, FF of 0.68, and PCE of 10.62%. And the single junction devices with 50 nm BHJ2 exhibits a Voc of 0.69 V, Jsc of 7.36 mA cm−2, FF of 0.71, and PCE of 3.61%. Figure S2 (Supporting Information) summarizes the thickness-dependent photovoltaic performance of double-BHJ OSCs with varied thicknesses for BHJ1 and BHJ2. Encouragingly, it shows that the photocurrent generally keeps increasing by ranging BHJ2 layer from several nano meters to 75 nm atop of BHJ1 (from 175 to 250 nm). The Voc shows a slight decrease from 0.81 to 0.74 V, and the FF is from 0.70 to below 0.62. As a result of fine-tuning of the thickness of each BHJ layer, the best PCE reaches 11.6% based on the double-BHJ configuration of ITO/ZnOx/BHJ1 (200 nm)/BHJ2 (50 nm)/MoO3/Ag. Besides of the active layer engineering, the interfacial properties of organic–inorganic metal oxide contact are also critical to determine overall performance of OSCs.[34] We have previously demonstrated a fullerene interfacial modifier, the [5,6]open azafulleroid (OPCBM) can improve organic BHJ-ZnOx heterojunction to achieve high FF of ≈77% and over 11% PCE in PTB7-Th:PC71BM-based inverted device. The OPCBM treatment not only passivates the charge-trapping hydroxide dangling bands of ZnOx but also promotes the electron extraction from BHJ to ZnOx.[35] We applied OPCBM treatment to the double-BHJ OSCs, and the resultant J–V characteristics are clearly improved to 23.75 mA cm−2 (Jsc) and 0.67 (FF) with a steady Voc of 0.77 V (Figure 3b). It is interesting to note that the highest PCE can reach 12.25% with an averaged value of 11.94% of OSCs with device area of 3.14 mm2. The inset histogram shows that more than half of devices have PCEs higher than 12%. In addition, larger area ≈12 mm2 OSCs were also fabricated and the photovoltaic performance were summarized in Table 1. It shows slight decrease in Jsc and FF due to higher sheet resistance of electrodes.[12] However, the PCE still remains at a high value of 11.54%. Figure 3c shows that the internal quantum efficiency (IQE) of double-BHJ OSCs reaches 90% in the range of 400 to 750 nm and 80% from 750 to 950 nm. The EQE spectrum is beyond 75% from 400 to 750 nm, and the peak EQE at the 850 nm reaches 50%. The overall EQE gives the integral photocurrent of 23.56 mA cm−2, which matches well with the experimental value of 23.75 mA cm−2. As a result, 1606729 (4 of 9)


such thick double-BHJ-based OSCs exhibited both the state-ofart photovoltaic performance and quantum efficiency. Interesting to point out that the double-BHJ device Voc of 0.77 V has a slight voltage loss of 5% to that of PTB7Th:PC71BM BHJ1-based device (0.81 V), whereas the independent PDPP3T: PC71BM BHJ2-based devices show a relatively low Voc of 0.69 V. The maximum Voc of OSC has an empirical relationship with the energy difference (ΔEg) between the highest occupied molecular orbital (HOMO) of polymer donor and lowest unoccupied molecular orbital (LUMO) of acceptor that Voc ≈ |HOMOD − LUMOA|/q − 0.3.[36] In the double-BHJ device, the ΔEg ≈ 0.85 eV of BHJ2 is smaller than that ≈1.01 eV of BHJ1, but the overall device Voc is not solely pinned to that of lower one, i.e., BHJ2. It might be an advantage when comparing with those single BHJ devices with low bandgap polymers. For instance, the low bandgap polymer can absorb both high and low energy photons. However, the device Voc has been predetermined by smallest energy difference (ΔEg). The energetics form high energy photon therefore is averaged out by small ΔEg related to the low energy photon (in the other word, wasted). On the other hand, the double-BHJ single junction OSCs can selectively harvest high and low energy photons in the bottom and top region of devices and yield adjustable Voc by varying the BHJ thickness, which likely exhibit combined electronic and optical advantages from those of ternary and tandem OSCs.[16,25,37,38] To understand the reason leading to large improvement of Jsc in double-BHJ devices, we conduct optical modeling based on transfer matrix method (TMM)[39] to investigate the optical distribution of incident light in the studied devices. Figure S3 (Supporting Information) shows the thickness dependence of separated and combined photocurrent on the BHJ1 and BHJ2 layers. It shows that the lossless photocurrent of the double BHJ OSCs can reach above 25 mA cm−2 when IQE is ≈100%. Figure 4a presents the light absorption rate of the studied device. It shows that the 200 nm BHJ1 absorbs ≈80% of visible light and 50 nm BHJ2 has a contribution ≈50% in the near infrared region. Figure S4 (Supporting Information) presents the normalized electric field intensity |E|2 in the whole layers of studied OSC. From that, the simulated exciton generation rate (G) in the BHJ1 and BHJ2 layers can be calculated which


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Another important reason for PCE improvement is due to maintaining reasonably high FF after inserting a 50 nm BHJ2 layer. The high FF ≈0.7 can be obtained from individual OSC containing either BHJ1 or BHJ2 (50 nm), which indicates either BHJ1 or BHJ2 itself has efficient and balanced charge transport in single BHJ device. The double-BHJ OSC exhibit FF ≈ 0.65 in 300 nm thick device, which indicates the double-BHJ stacks maintain effective and balanced hole/ electron transport. To better understand this, we probe the depth profile of double-BHJ films through the secondary ion mass spectroscopy (SIMS) measurement.[39–41] Fluorine and nitrogen ions were used to track the depth distribution of PTB7-Th and PDPP3T, respectively. Zn ion was used to track ZnOx as a reference range of whole BHJ layer. Figure 5a shows that the ion counts of fluorine increase from the sputtering time of 75 s to the maximum value at about 200 s and remain until sputtering time of 600 s. While the ion counts of nitrogen begin at the top surface and gradually end at 200 s. The signals from 100 to 200 s represent a rough 25 nm of intermixed region between BHJ1 and BHJ2, indicating PTB7-Th, PDPP3T, and PC71BM are cross-stacked. Figure 5b gives the profile of the molecular distribution of double-BHJ device based on the SIMS analysis. Figure 5c presents the SIMS profile of double BHJs without methanol soaking treatment. It shows that the ion counts of fluorine increase at the sputtering time of 140 s to the top within 60 s. Meanwhile, the ion counts of nitrogen ends at the 175 s within 50 s causing a little overlap of two BHJs and ultrathin mixing layer in between leading to poorer photovoltaic performance in both Jsc and FF of corresponding OSCs, as shown in Table S1 (Supporting Information). Accordingly, the vertical molecular distribution of double-BHJ device without solvent treatment can be depicted in Figure 5d. By combining with above analyses, we propose a schematic diagram of the morphology evolution of polymer donor and fullerene acceptor phases, as shown in Figure 6. The treatment of addiFigure 4. Light absorption spectra and corresponding exciton generation rate of OSC. a) Light absorption fraction of BHJ1 and BHJ2 layers. b,c) Exciton generation rate (G) and integrated tional solvent on top of BHJ2 gradually soaks into the active layers and promotes to form lossless Jsc in BHJ1 and BHJ2 layers, respectively. the continuous hole and electron transport paths in the mixed region. Besides the effect of methanol soaking, other reasons such as different glass agree well with the trend of light absorption spectra as shown transition temperature of materials[42] and high boiling point in Figure 4b,c. Accordingly, we obtain the lossless Jsc of 20 and −2 5.64 mA cm for BHJ1 and BHJ2-based OSCs, respectively. solvent residues[43] may not be excluded for the formation of Double-BHJ OSCs with BHJ1 and BHJ2 not only broaden the formation of intermixed region between two BHJs. the solar spectrum coverage of devices but also really enhance To figure out charge transfer process between the double device Jsc to 23.19 mA cm−2. BHJs, we first investigated the electronic structures of PTB7-Th


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Figure 5. a,c) Time of flight secondary ion mass spectrometry of active layers on ITO/ZnOx with and w/o methanol soaking treatment, where the fluorine, nitrogen, and Zn are attributed to PTB7-Th, PDPP3T, and ZnOx, respectively. b,d) Schematic illustration representing polymer donors:fullerene composition gradient in the BHJ1/BHJ2 layers with and w/o methnanol soaking treatment.

and PDPP3T in the polymer phase. The molecular orbitals (MOs) and density-of-states (DOS) of PTB7-Th and PDPP3T were calculated using the Gaussian program using the B3LYP exchange-correlation functional and 6-31G(d) basis set and shown in Figure S5 (Supporting Information). It shows that the HOMO level of PTB7-Th is lower than that of PDPP3T agreeing with the measured energy levels using cyclic voltammetry (CV) method, which presents the efficient hole transfer from PTB7-Th to PDPP3T during hole collecting process. Also, the lower Voc of double BHJ OSC is attributed to this. This result indicates that the mixed region between PTB7-Th and PDPP3T serves as the hole transporting zone and also ensure a high hole mobility. To evaluate the effect of double BHJs on the charge transport, the charge mobilities of various BHJ layers are measured. Figure 7a shows J–V curves of hole-only devices with various BHJ layers. By fitting the J–V curves using the space charge limited current (SCLC) model, their hole mobility and fitting parameters are obtained and listed in Table S2 (Supporting Information). It shows that the hole mobility of 200 nm BHJ1 layer is around 3.0 × 10−3 cm2 V−1 s−1, and the mobility of 50 nm BHJ2 layer is also around 3.6 × 10−3 cm2 V−1 s−1. The 1606729 (6 of 9)


double BHJ layers with and without (w/o) solvent treatment show mobilities with the same order of magnitude, 1.3 × 10−3 and 0.2 × 10−3 cm2 V−1 s−1, respectively, indicating that the hole transport from PTB7-Th to PDPP3T in double-BHJ layers is reasonably good. In the aspect of electron transport, we measured electron mobilities of various BHJ layers via electron-only devices. As shown in Figure 7b, the electron mobility of BHJ1 and BHJ2 are 8.2 × 10−3 and 5.4 × 10−3 cm2 V−1 s−1, respectively. The electron mobilities for double-BHJs with and w/o solvent treatment are 5.4 × 10−3 and 2.1 × 10−3 cm2 V−1 s−1, respectively. Note that the double-BHJ without solvent treatment exhibits lower charge mobility than those of solvent treated ones, which indicates the solvent treatment could be beneficial for the formation of the continuous hole and electron transport channels, thus responsible for high FF and IQE of OSCs. In conclusion, we demonstrate that highly efficient OSCs with an averaged PCE of 11.98% can be realized via vertically stacking two BHJs into cathode and anode, namely, doubleBHJ OSCs. The employed PDPP3T:PC71BM uplayer shows complemental light absorption to that of PTB7-Th:PC71BM bottom layer, resulting in broadening spectra coverage into the near infrared range of 925 nm. More importantly, such


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double-BHJ configuration not only ensures the efficient harvest of both high and low energy photons but also realizes the effective photon-to-electron conversion and charge extraction due to the presence of optimal BHJ phase morphology and interface. As a result, double-BHJ OSCs allow obtaining a large gain in photocurrent, while without suffering much from the energy loss and fill factor drop usually seen in thick-layerbased devices. The optimal ≈50 nm near-infrared PDPP3T BHJ atop of ≈200 nm PTB7-Th BHJ results in the state-of-art PCE over 12% with a Jsc of 23.75 mA cm−2, a Voc of 0.77 V, and a FF of 0.67. We believe that the double-BHJ OSC represents an effective strategy toward the high-efficiency devices with good solar spectrum coverage and thick active layer that are rivalry to amorphous Si solar cells.

Experimental Section Fabrication of OSCs: For the double BHJ OSC fabrication, a 30 nm sol-gel ZnOx layer was spin coated onto the ITO electrode layer on the glass from ZnOx precursor solution and then annealed at 200 °C for 1 h. The precursor solution of ZnOx was prepared using zinc acetate dehydrate and monoethanolamine dissolved in 2-methoxyethanol and stirred overnight for hydrolysis reaction. The OPCBM was dissolved in tetrahydrofuran (THF)/chlorobenzene (1/1, v/v) with the concentration of 1 mg mL−1 and spin coated on the ZnOx forming a very thin layer and kept in the rough vacuum chamber for solvent evaporation. Then, the ZnOx/OPCBM hybrid film was treated under UV light for 3 min. The PTB7-Th:PC71BM BHJ1 film was prepared by dissolving the PTB7-Th (1-material Inc.) and PC71BM (Aldrich, used as received) with a weight ratio of 1:1.5 in a mixed solvent of o-dichlorobenzene

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(o-DCB):1,8-diiodoctane (97%:3% by volume) solution (with a total concentration of 25–38 mg mL−1). The solution was stirred overnight and filtered through a polytetrafluoroethylene filter (0.2 µm). Subsequently, the active layer was inverted off-center spin coated on the ZnOx/OPCBM layer forming 100–250 nm BHJ1 layer.[7] The PDPP3T:PC71BM solution was prepared by dissolving the PDPP3T (Solarmer Materials, Inc.) and PC71BM with a weight ratio of 1:1.2 in a mixed solvent of o-DCB:1,8diiodoctane (3% by volume) solution (with a total concentration of 20 mg ml−1). UV-curable resin (PUA) was dropped onto the precleaned silicon wafer surface and spread to the whole surface area. Then the PUA film was covered with a flexible PC film and flattened with a roller. The precoated PUA/PC films were cured by UV light of 365 nm for 2 min and then detached from the Si wafer to obtain a stamp mold of PUA/PC film with a smooth surface. The PDPP3T:PC71BM BHJ2 was spin coated on the prepared PUA/PC. Then, the 50 nm BHJ2 layer was uniformly stuck onto the as-coated BHJ1 layer and flattened with a roller under heating at over 75 °C for 2 min. After that the PUA/PC film could be peeled off and the BHJ2 layer was stamp transferred onto BHJ1. Then, the double BHJ layers were treated via soaking in the methanol solvent for 20 s and spun at 600 round per minute (RPM) for 1 min. The resulting double BHJ layer was slow dried in the glovebox and then put into the vacuum for 1 h before electrode deposition. A 10 nm thick MoO3 film was deposited on BHJ layer and a 100 nm Ag layer was deposited through shadow masks to define the active area (3.14 × 10−2 cm2) and form the top anode. Characterization and Measurements: The current density versus voltage (J–V) characteristics of OSC devices were measured under N2 condition using a Keithley 2400 source meter, which could measure ultralow current of 10−12 A, which ensured the electrical accuracy for Jsc. A 300 W xenon arc solar simulator (Oriel) with an air mass (AM) 1.5 global filter operated at 100 mW cm−2 was used to simulate the AM 1.5G solar irradiation. The illumination intensity of solar simulator was corrected by using a silicon photodiode with a protective KG5



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Morphological Characterization of SIMS: SIMS of different ions, i.e., fluorine ions for tracking PTB7-Th, nitrogen ions for tracking PDPP3T, Zn ions for tracking ZnOx were measured using a time of flight (TOF) secondary ion mass spectrometry (SIMS) spectrometer. To collect time-dependent ion counts data, the double BHJ film was probed and sputtered with a 10 kV Cs+ source. Sputter times can be converted to depth in the measured film by comparing the sputtering time to the thickness of BHJ film as measured with atomic force microscope (AFM). Optical Simulations: The simulations of light distribution, light absorption efficiency, and thickness dependence of photocurrent of the individual layer within the devices were obtained using TMM. The TMM simulation of OSCs have been widely used to model active layer absorption, accounting for optical interference effects as well as parasitic absorption.[39,44] The optical properties of each layer are represented by the index of refraction (ñ = n + ik), which were measured using a variable angle spectroscopic ellipsometer, as shown in Figure S6 (Supporting Information). All the theoretical simulations of optical properties are based on the assumptions of planar interfaces, isotropy, and practical film thickness for all layers within the device. To calculate the exciton generation rate and theoretical achievable photocurrent, 100% IQE and the AM 1.5 intensity spectrum (ASTM G173-03) were assumed. Moreover, the absorption rate of active layers and IQE were corrected using the method reported by Burkhard et al.[45] SCLC Mobility Measurements: Space charge-limited currents were tested in electron-only devices with a configuration of glass/Al/PTB7Th:PC71BM/PDPP3T:PC71BM/Ca/Al and hole-only devices with a configuration of ITO/PEDOT:PSS/PTB7-Th:PC71BM/PDPP3T:PC71BM/ MoO3/Ag. The mobilities were determined by fitting the current to the model of a single carrier SCLC current with field-dependent mobility, which was described as

V2 9 J = εε 0 µ 0 3 exp( β V /L ) 8 L

(1)

where J is the current density, μ0 is the zero-field mobility, ε0 is the permittivity of free space, εr is the relative permittivity of the material, L is the thickness of the active layer, and V is the effective voltage and equals to Va − Vbi − Vr. The bias (Va) is corrected by the built-in voltage (Vbi) and the voltage drop (Vr) due to substrate series resistance. Computational Modeling: The calculations of MOs and DOS of PTB7-Th and PDPP3T polymers were performed in the density functional theory framework as implemented in the development version of the GAUSSIAN 09 software.[46] The ground state geometries, energies, and electronic structures of PTB7-Th and PDPP3T were obtained using the B3LYP exchange correlation functional and 6-31G(d) basis set.

Figure 7. a,b) J–V characteristics for hole-only and electron-only devices consisting of various BHJs with fitting curves based on SCLC modeling. The bias (Va) is corrected for built-in voltage (Vbi) and the voltage drop due to substrate series resistance (Vr). filter calibrated by the National Renewable Energy Laboratory (NREL). The spectrum of incident light is certified by Chengdu branch of China Academy of Sciences, as shown in the revised Figure S7 (Supporting Information), which shows perfect match with the real solar light. Masks with a well-defined area of 3.14 mm2 were attached to define the effective area for accurate measurement. The photovoltaic performances of OSCs under masked and unmasked tests were consistent with relative errors within 5%. The EQE measuring system uses a lock-in amplifier to record the short circuit current under chopped monochromatic light from 350 to 950 nm. The lock-in amplifier was certified by Chengdu branch of China Academy of Sciences. The standard silicon photodiode in the EQE measuring system was certified by the NREL. Theoretical Jsc values obtained by integrating the product of the EQE spectrum with the AM 1.5G solar spectrum agreed with the measured Jsc to within 5%. The IQE spectra were calculated by the division of EQE to the light absorption rate of the active layer.

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This research was funded by the Foundation for Innovation Research Groups of the National Natural Science Foundation of China (NSFC) (Grant No. 61421002), the NSFC (Grant Nos. 61675041, 21674093, 61177032, and 21372168), the Project of Science and Technology of Sichuan Province (Grant No. 2016HH0027). C.-Z. Li thanks the financial support from the International Science and Technology Cooperation Program of China (ISTCP) (Grant No. 2016YFE0102900) and the Young 1000 Talents Global Recruitment Program of China. J. Huang also thanks the financial support of the UESTC Excellent Young Scholar Project.


Received: December 13, 2016 Revised: February 9, 2017 Published online:

Adv. Mater. 2017, 1606729

www.advmat.de


Adv. Mater. 2017, 1606729

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