Boosting Up Performance of Inverted Photovoltaic Cells from Bis

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Boosting up Performance of Inverted Photovoltaic Cells from Bis(alkylthien-2-yl)dithieno[2,3-d:2#,3#-d#]benzo[1,2-b:4#,5#-b#]dithiophenebased Copolymers by Advantageous Vertical Phase Separation Pengzhi Guo, Guoping Luo, Qiang Su, Jianfeng Li, Peng Zhang, Junfeng Tong, Chunyan Yang, Yangjun Xia, and Hongbin Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15436 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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ACS Applied Materials & Interfaces

Boosting up Performance of Inverted Photovoltaic Cells from Bis(alkylthien-2-yl)dithieno[2,3-d:2′,3′d′]benzo[1,2-b:4′,5′-b′]dithiophene-based Copolymers by Advantageous Vertical Phase Separation Pengzhi Guo,† Guoping Luo,‡ Qiang Su,† Jianfeng Li,† Peng Zhang,† Junfeng Tong,† Chunyan Yang,† Yangjun Xia,* †,§ and Hongbin Wu* ‡ †

Key Laboratory of Optoelectronic Technology and Intelligent Control of Ministry

Education, Lanzhou Jiaotong University, Lanzhou, 730070, P. R. China ‡

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P.R. China §

Center for Polymers and Organic Solids, University of California, Santa Barbara, CA

93106-5090 USA Author to whom correspondence should be addressed: [email protected] (Y. Xia), [email protected] (H. Wu)

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KEYWORDS:

Benzothiadiazole,

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Dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene,

Optical modelling calculations, Photovoltaic cells, Vertical phase separation, X-ray photoelectron spectroscopy.

ABSTRACT: The photovoltaic cells (PVCs) from conjugated copolymers of PDTBDT-BT and PDTBDT-FBT with 5,10-bis(4,5-didecylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2b:4,5-b′]dithiophene as electron donor moieties and benzothiadiazole and/or 5,6-difluorobenzothiadiazole as electron acceptor moieties, are optimized by employing alcohol soluble PFN

(poly(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-

fluorene)) as cathode modification interlayer. The power conversion efficiencies (PCEs) of inverted PVCs (i-PVCs) from PDTBDT-BT and PDTBDT-FBT with devices configuration as ITO/PFN/active layer/MoO3/Ag, are increased from 4.97% to 8.54% and 5.92% to 8.74% in contrast to those for the regular PVCs (r-PVCs) with devices configuration as ITO/PEDOT: PSS/active layer/Ca/Al under 100 mW/cm2 AM 1.5 illumination, respectively. The optical modelling calculations and X-ray photoelectron spectroscopy (XPS) investigations reveal that, the r-PVCs and i-PVCs from the copolymers exhibit similar light harvesting characteristics, and the enhancements of the PCEs of the i-PVCs from the copolymers are mainly contributed to the favorable vertical phase separation as the strongly polymersenriched top surface layers and slightly PC71BM (phenyl-C71-butyric acid methyl ester) enriched bottom surface layers are correspondingly connected to the anodes and cathodes of the i-PVCs, while they are opposite in the r-PVCs. As we known, it is the first time to experimentally verify that the i-PVCs with alcohol soluble conjugated polymers cathode modification layers enjoy favorable vertical phase separation.


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1. INTRODUCTION In the past few years, the inverted photovoltaic cells (i-PVCs) with water/alcohol soluble conjugated polymers or polyelectrolytes (W/ACPs) as cathode modification interlayers, had attracted much attention as the approach could not only ameliorate the ambient stability of the PVCs by the avoiding usage of corrosive and hygroscopic poly(3,4-ethyleneoxy-thiophene): poly(styrene sulfonate) (PEDOT:PSS)1−7 and sensitive low work function metals,4,8 but also provide the possibility to achieve the roll-to-roll or printable PVCs in virtue of the unique characteristics of the W/ACPs such as solution processibility and resistance of the common organic solvent used for photoactive materials in organic/polymeric photovoltaic cells.9−14,24-26 Meanwhile, the advantages such as the enhancement of open circuit voltage (VOC) by effectively increasing the built-in potential and/or decreasing the dark current density,14−23,27 improvement of short current density (JSC) and fill factor (FF) by decreasing the series resistance,28−30 increase of the light harvesting,9 and reduction of the non-geminated electronhole recombination etc.31−33 of the i-PVCs with W/ACPs as cathode modifying layers, have been concluded. Moreover, researchers also speculated that the i-PVCs with W/ACPs as cathode modifying interlayers would exploit the favorable vertical phase separation so as demonstrated in i-PVCs with alkali-metal compound or transition metal oxides as cathode modifying interlayers etc.2,34,35 However, it has been clearly demonstrated that the vertical phase separation in the polymer blends are strongly related to the differences between the surface energy or wetting characteristics of each component and the devices substrate interfaces.2,34−37 Regarding this line, it would be expected that the potential differences between the alkali-metal compound or transition metal oxides etc. modified-ITO (FTO) and W/ACPs-modified ITO interfaces might give reliable influence on the composition of the bottom surfaces of the devices.2,34−37 Unfortunately, the vertical phase separation of electron



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donor and electron acceptor materials in the active layers of i-PVCs with W/ACPs as cathode interlayer, were seldom experimentally investigated so far. In this study, the photovoltaic cells (PVCs) from the blend films of PC71BM and conjugated copolymers of PDTBDT-BT38 and/or PDTBDT-FBT39,40 with 5,10-bis(4,5-didecylthien-2yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene as electron donor moieties and benzothiadiazole and/or 5,6-difluorobenzothiadiazole as electron acceptor moieties (Figure 1a, Scheme S1, Supporting Information), are optimized by employing PFN (poly(9,9-bis(3'(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))

as

cathode

modification interlayer in the PVCs (Figure 1a). The PCEs of i-PVCs from PDTBDT-BT and PDTBDT-FBT with devices configuration as ITO/PFN/active layer/MoO3/Ag, are increased about 35.8% to 71.8% (8.54% vs. 6.29 %, and 8.54% vs. 4.97%) and 22.1% to 47.6% (8.74% vs. 7.16%, 8.74% vs. 5.92%) in contrast to those for the r-PVCs with devices configuration as ITO/PEDOT:PSS/active layer/PFN/Al and/or ITO/PEDOT:PSS/active layer/Ca/Al under 100 mW/cm2 AM 1.5 illumination, respectively. The optical modelling calculations and X-ray photoelectron spectroscopy (XPS) investigations reveal that the r-PVCs and i-PVCs from the copolymers exhibit similar light harvesting characteristics, and the enhancements of the PCEs of the i-PVCs from the copolymers and PC71BM blend films are mainly contributed to the favorable vertical phase separation as the strongly copolymers-enriched upper surface layers and slightly PC71BM enriched bottom surface layers are correspondingly connected to the anodes and cathodes of the i-PVCs, while they are opposite in the r-PVCs. 2. RESULTS AND DISCUSSION 2.1. Photovoltaic properties of the regular and inverted photovoltaic devices Firstly, the r-PVCs with device configuration of ITO/ PEDOT: PSS/active layer/Ca/Al, in which the PDTBDT-BT and/or PDTBDT-FBT were used as electron donor materials, and PC71BM was used as electron acceptor materials (Figure 1a and Figure 1b), were 4 ACS Paragon Plus Environment

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implemented. The thickness of the blend films were fixed at 80−90 nm. The optimal weight ratios of the polymers and PC71BM were verified to be 1:2 as the power conversion efficiencies of the r-PVCs from PDTBDT-BT/PC71BM were increased from 2.77%, 3.13% to 4.31% and then drop to 3.84%, and the PCEs of the r-PVCs from PDTBDT-FBT/PC71BM were increased from 4.93%, 5.31% to 5.74% and then drop to 5.01% under 100mW/cm2 1.5 AM illumination, while the weight ratios of the polymers and PC71BM were varied from 1:1, 1:1.5, 1:2 and 1:3. (Table S1, Supporting Information). Meanwhile, the PCEs of the r-PVCs from the PDTBDT-BT/PC71BM and/or PDTBDT-FBT/PC71BM blend films with weight ratios of 1: 2, were increased from 4.31% to 4.97%, and 5.74% to 5.92%, while the 3% (VDIO: VDichlorobenzene; 3:100) of 1,8-diiodooctane (DIO) was used as solvent additives (Table S1, Supporting Information). It found that the improvement of the PCEs of the r-PVCs from the copolymers was related to the modification of the morphologies and uniforms of the active layers by the usage of DIO as solvent additives. As shown in the atomic force microscope (AFM) topography images of the blend films, the isolated domains size and root-mean-square (RMS) of the films from PDTBDT-BT/PC71BM (W: W, 1:2) were decreased clearly, and the isolated domains size and root-mean-square (RMS) of the films from PDTBDT-BT/PC71BM (W: W, 1:2), were slightly improved. (Figure S1 and S2, Table S2, Supporting Information). As a 5-nm PFN interlayer is applied in the r-PVCs from the 80-90 nm copolymers/PC71BM blend films with devices configuration as ITO/PEDOT:PSS/active layer/PFN/Al under the application of the DIO in the fabrication of the active layers (Figure 1b), the PCEs of the rPVCs from PDTBDT-BT and PDTBDT-FBT are correspondingly increased from 4.97 % and 5.92% to 6.29% and 7.16% with the simultaneous enhancement of VOC (0.82 V vs. 0.80 V and 0.92 V vs. 0.88 V), JSC (11.19 mA/cm2 vs. 10.32 mA/cm2 and 11.2 mA/cm2 vs. 10.98 mA/cm2) and FF (68.5% vs. 60.2%, 69.5% vs. 61.3%) (Table 1, Figure 2a and 2b). These enhancements should attribute to the decrease of the dark current density (Figure S3a and 5 ACS Paragon Plus Environment


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S3b, Supporting Information), the series resistance and non-geminated electron-hole recombination etc. for the r-PVCs with PFN layers in contrast to the r-PVCs without PFN layers.41−43 Recently, we have demonstrated that employing of PFN to effectively tune the ITO work function not only could lead to an approach to achieve the enhanced stability of i-PVCs with devices configuration as ITO/PFN/active layer/MoO3/Ag by the avoiding the usage of the corrosive and hygroscopic hole-transporting poly(3,4-ethylenedioxythiophene: poly(styrene sulfonate) (PEDOT:PSS) and/or low-work-function metal cathode, but also produce the further improvement of the PCEs and JSC due to the enhancement of the light harvesting of iPVCs with 5-nm-thickness of PFN-modified ITO as cathode in contract to the r-PVCs with PFN/Al as cathode.9 Following this line, a 5-nm-thickness of PFN interlayer is applied to tune the ITO work function, thus to achieve i-PVCs from the copolymers with devices configuration as ITO/PFN/active layer/MoO3/Ag (Figure 1b), the PCEs of i-PVCs from the PDTBDT-BT and PDTBDT-FBT are correspondingly increased to 6.94% and 8.02% with the further slight enhancement of VOC (0.83 V vs. 0.82 V, 0.93 V vs. 0.92 V), JSC (11.9 mA/cm2 vs. 11.19 mA/cm2 and 12.02 mA/cm2 vs. 11.2 mA/cm2) and FF (70.3% vs. 68.5% and 71.9 % vs. 69.5%) as compared with those of 6.29% and 7.16% for the r-PVCs with PFN interlayers (Table 1, Figure 2a and 2b). It notes that further enhancements of the PCEs of the i-PVCs are mainly attributed to the simultaneous improvement of the VOC, JSC and FF of the i-PVCs in contrast to those for the r-PVCs with PFN interlayers. And the dark current densities under the reverse bias from the i-PVCs with PFN layers are 3−4 times smaller than those from the rPVCs with PFN layers (Figure S3a and S3b, Supporting Information). The lowering dark current density of the i-PVCs should lead to the enhancement of the VOC of the i-PVCs in contract to those for the r-PVCs with PFN layers.27,43 2.2. Optical modelling calculations 6 ACS Paragon Plus Environment

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To get insight on the origin of the enhanced JSC of the i-PVCs with PFN interlayers, the optical modelling calculations, to which a one-dimensional transfer matrix formalism (TMF) has been employed, were implemented to investigate the light harvesting characteristics of the both types PVCs with PFN interlayers,9,44 and experimentally measured refractive indices and extinction coefficients of the blend films from the copolymers and PC71BM were used (Figure S4a and S4b, Supporting Information). Interestingly, the JSC of the i-PVCs and r-PVCs with PFN layer with 80-nm-thick active layer from the copolymers and PC71BM with weight ratios of 1:2, are almost same (calculated JSC of 13.45 mA/cm2 and 13.24 mA/cm2 for PDTBDTBT-based r-PVCs and i-PVCs, 14.82 mA/cm2 vs 14.68 mA/cm2 for PDTBDT-FBT-based iPVCs and r-PVCs) (Figure 3). These results certify that the enhancements of the JSC of the iPVCs from the copolymers might not attribute to the enhancement of the light harvesting as we expected.9 However, the optical modelling calculations guide us that the further optimization of the iPVCs from the copolymers with PFN layers would be practical via increasing the thickness of the active layers to enhance the absorption of the sun light spectra (Figure 3). Moreover, the increased probability of recombination of the photo-generated charge carrier by the increase of the thickness of the bulk heterojunction active layer might be alleviated by the high charge mobility of the bulk heterojunction active layers from the copolymers and PC71BM45−54 (Figure S5, Table S3 and Table S4, Supporting Information). As expected, the maximal PCEs of the i-PVCs from the PDTBDT-BT and PDTBDT-FBT are correspondingly improved to 8.54% and 8.74% with the remarkable enhancement of the JSC (15.02 mA/cm2 vs 11.90 mA/cm2 for i-PVCs from PDTBDT-BT, and 14.89 mA/cm2 vs 12.02 mA/cm2 for i-PVCs from PDTBDT-FBT) and slight decrease of the VOC (0.82 V vs. 0.83 V, 0.91 V vs. 0.93 V) and FF (68.5% vs. 70.3% and 64.6% vs. 71.9%) while the thickness of the active layers were increased from 80 nm to 220 nm (Figure 2a and 2b). The theoretical JSC values obtained by integrating the products of the EQE data of the i-PVCs from PDTBDT-BT and PDTBDT7 ACS Paragon Plus Environment


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FBT under the AM 1.5G solar spectrum are 15.02 mA/cm2 and 14.80 mA/cm2, respectively, which are in good agreement with the values obtained from the J–V characteristics (Figure 2c, Table 1). It noted that the PCEs of 8.54% and 8.74% for the i-PVCs from the PDTBDT-BT and PDTBDT-FBT are among the highest PCEs demonstrated by dithieno[2,3-d:2′,3′d′]benzo[1,2-b:4,5-b′]dithiophene-based conjugated polymers,38−40,49−54 and the calculated JSC of the PVCs from 220 nm-PDTBDT-BT/PC71BM and/or PDTBDT-FBT/PC71BM blend films were also not clearly increased while the devices configurations were transformed from rPVCs to i-PVCs (calculated JSC of 16.93 mA/cm2 and 17.08 mA/cm2 for 220 nm PDTBDTBT/PC71BM blend films based r-PVCs and i-PVCs, 17.13 mA/cm2 vs 17.26 mA/cm2 for 220 nm PDTBDT-BT/PC71BM blend films based i-PVCs and r-PVCs, Figure 3). 2.3. XPS investigation of the top and bottom surfaces of the active layers from copolymer/PC71BM blend films As the vertical phase separation of the bulk heterojunction PVCs from the polymers and fullerene derivatives played a reliable influence on the charge extraction and recombination etc., thus to pay impact on the performance of the PVCs,2,34−37 the vertical phase separation of the i-PVCs and r-PVCs from the copolymers with PFN layers are investigated to get insight into the enhancement of the JSC and FF of the i-PVCs with PFN layers in relative to those for the r-PVCs. Figure 4a and Figure 4b show the XPS spectra of the top and bottom surfaces of PDTBDT-FBT/PC71BM blend films on the substrate of ITO/PEDOT:PSS and ITO/PFN, respectively. As shown in Figure 4a, the top surfaces of the PDTBDT-FBT/PC71BM blend films on both the types substrates exhibit similar XPS peaks at around of 163.76 eV, 284.78 eV, 398.70 eV, 531.86 and 687.59 eV binding energies, respectively assigned to S 2p, C 1s, N 1s, O 1s and F 1s of the top surfaces of PDTBDT-FBT/PC71BM blend films. The PDTBDTFBT to PC71BM weight ratios at the top surfaces valuated by S/C atomic ratios from infinitesimal calculus of the peaks at 163.76 eV and 284.78 eV with the XPS measurement are 8 ACS Paragon Plus Environment

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about 4.12:1 and 4.23:1, respectively (Figure 4c, Table S5 and S6, Supporting Information). It verified that the PDTBDT-FBT was remarkably enriched at the top surface of PDTBDTFBT/PC71BM blend films on both substrates of ITO/PEDO:PSS and ITO/PFN. Since the surfaces of the films might be contaminated by the compounds containing sulfur and carbon elements, the S/F atomic ratios were calculated to evaluate the S contamination. The F/S ratios at the top surface obtained by XPS measurements were to be 2:6.96 − 2:7.04 (Table S6, Supporting Information), which are very close to the S/F stoichiometric ratio of 2:7 in PDTBDT-FBT. Meanwhile, the PDTBDT-FBT to PC71BM weight ratios of 4.03: 1 and 4.19: 1 at the top surfaces evaluated by C/F ratios (Figure 4c and Table S6, Supporting Information), are similar with those estimated by S/C ratios. These results verify that the S and C contamination at the surface are negligible, and the evaluated PDTBDT-FBT to PC71BM weight ratios at top surfaces by S/C atomic ratios are credible. The bottom surfaces of the blend films of PDTBDT-FBT/PC71BM on both types substrates exhibit similar XPS peaks at around of 163.76 eV, 284.78 eV, 398.70 eV, 531.86 and 687.59 eV binding energies with top surfaces of the blend films, except that the 531.86 eV assigned to O 1s are clearly increased (Figure 4b), which might be contributed to the contamination of water during lifting-off process of the blend films. The PDTBDT-FBT to PC71BM weight ratios at the bottom surfaces of PDTBDT-FBT/PC71BM blend films on the substrates of ITO/PEDOT: PSS and ITO/PFN evaluated by S/C atomic ratios from infinitesimal calculus of the peaks at 163.76 eV and 284.78 eV from the XPS measurement, are about 1:2.23 and 1:2.47, respectively. The results indicate that the PC71BM is slightly enriched at the bottom surface of the active layers in the i-PVCs and r-PVCs from PDTBDT-FBT/PC71BM blend films. Meanwhile, the similar respective enrichments of PDTBDT-FBT and PC71BM at the top and bottom surfaces of the PDTBT-FBT/PC71BM blend films on the substrates of ITO/PEDOT: PSS and ITO/PFN, were verified by the XPS investigations of the top and bottom surfaces of PDTBDT-BT/PC71BM blend films (Figure 4d, 4e, 4f). The results 9 ACS Paragon Plus Environment


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confirmed, that the copolymers were strongly enriched at top surface layers of the blend films, and PC71BM was slightly enriched at the bottom surface layers of the blend films. And the iPVCs with PFN as cathode modification interlayer exhibit favorable vertical phase separation in relative to the r-PVCs with PFN layers, as the strongly polymers-enriched upper surface layers and slightly PC71BM enriched bottom surface layer are correspondingly connected to the anode and cathode of the inverted PVCs, while they are opposite in the r-PVCs. 2.4. Wettability of the materials and substrates It was believed that the vertical phase separation in the polymer blend films would be influenced by the differences between the surface energy of each components and the characteristics of the devices substrates interface.33−37 We investigated the wettability of the copolymers, PC71BM, the PEDOT:PSS and/or PFN coated ITO by depositing the droplet of deionized (DI) water on the film surface. The water contact angles were obtained from an automated image analysis system. Figure 5 shows the water contact angles of the PDTBDTBT (103.76°), PDTBDT-FBT, (102.51°), PC71BM (89.86°), 5-nm thick of PFN coated ITO (42.33°), and 40-nm thick of PEDOT: PSS coated ITO (46.46°). As the PDTBDT-BT and PDTBDT-FBT have the lower surface energy than PC71BM (Figure 5a, 5b and 5c), they tend to accumulate at the top (free) surface to reduce the overall energy. This similar surface enrichment phenomenon has been verified in other polymer blend systems such as P3HT/PC71BM2,34 APFO-3/PCBM35−37 etc. Meanwhile, that the enrichments of PC71BM at the bottom surface of the copolymer/PC71BM blend films on both type substrates of ITO/PEDOT: PSS and ITO/PFN, should attribute to the larger surface energy of PC71BM as compared with those for the copolymers. It was clear that the wettability investigations were in favor to the XPS investigation results. Moreover, in spite that the presence of uncertainty of the slight accumulation of the PC71BM were expected as the weight ratios of the copolymers and PC71BM at the bottom surfaces of the active layers would be affected by the XPS 10 ACS Paragon Plus Environment

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investigation errors and potential dissolve of copolymers and PC71BM during the preparation of the lifted-off active layers, it is reasonable that the i-PVCs exhibited favorable vertical phase separation as the enrichments of the copolymers at the top surfaces of the blend films, which have been directly investigated by XPS, are so significant (4.1−4.5:1 vs. 1:2 at the top surfaces and blend films), and the favorable vertical phase separation of the i-PVCs might contribute to the decrease of the bimolecular recombination, facilitation of the electron and hole transportation and extraction at the surface cathode and anode in the i-PVCs, thus resulted in the enhancement of the JSC and FF of the i-PVCs in relative to those for rPVCs.2,35−39 3. CONCLUSION In summary, the photovoltaic cells from conjugated copolymers of PDTBDT-BT and PDTBDT-FBT, were optimized via the employing PFN as cathode modification interlayer, and comparative investigations of the light harvesting characteristics of the PVCs and the distribution of copolymers and PC71BM at the top and bottom surfaces of the active layers, were investigated by optical modelling calculation and X-ray photoelectron spectroscopy. The PCEs of inverted PVCs from PDTBDT-BT and PDTBDT-FBT with devices configuration as ITO/PFN/active layer/MoO3/Ag, were increased about 35.8% to 71.8% (8.54% vs. 6.29%, and 8.54% vs. 4.97%) and 22.1% to 47.6% (8.74% vs. 7.16%, 8.74% vs. 5.92%) in contrast to those for the regular PVCs with devices configuration as ITO/PEDOT:PSS/active layer/PFN/Al and/or ITO/PEDOT:PSS/active layer/Ca/Al under 100 mW/cm2 AM 1.5 illumination, respectively. It has been found that both regular PVCs and inverted PVCs from the copolymers with PFN layers exhibit similar light harvesting characteristics, and the inverted PVCs with PFN as cathode modifying interlayer exhibit favorable vertical phase separation in relative to the regular PVCs with PFN layers, as the strongly polymers-enriched upper surface layers and slightly PC71BM enriched bottom surface layer are correspondingly 11 ACS Paragon Plus Environment


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connected to the anodes and cathodes of the inverted PVCs, while they are opposite in the regular PVCs. In more general terms, we have experimentally verified that the enhanced performance of the inverted PVCs with PFN layers from PDTBDT-BT and PDTBDT-FBT attributed to the favorable vertical phase separation in contrast to that for the regular PVCs with or without PFN layers. 4. EXPERIMENTS SECTIONS 4.1. Materials Poly(5,10-bis(4,5-didecylthien-2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene2,7-diyl-alt-benzothiadiazole-4,7-diyl) (PDTBDT-BT)38 and poly(5,10-bis(4,5-didecylthien2-yl)dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene-2,7-diyl-alt-5,6-difluorobenzothiadiazole-4,7-diyl) (PDTBDT-FBT)39,40 and poly(9,9-bis(3'-(N,N-dimethylamino)propyl)2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) (PFN)55 were synthesized following the reported processes in the references. [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) was purchased from N.-C.. All other materials and reagents were obtained from Sigma-Aldrich Com., and used as received unless otherwise specified. 4.2. Fabrication and characterization of photovoltaic cells The device structures of regular and inverted polymer solar cells are presented in Figure 1b. The patterned indium tin oxide (ITO) coated glass with a sheet resistance of 10–15 Ω/square was cleaned by a surfactant scrub, followed by a wet-cleaning process inside an ultrasonic bath, beginning with de-ionized water, followed by acetone and iso-propanol, then cleaned by oxygen plasma for 5 min. The regular devices with thermally-evaporated Ca (8 nm) and Al (100 nm) as cathode and an ITO substrate coated with a PEDOT: PSS interfacial layer as the anode, were prepared as the follow processes: a 40 nm thick poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) (Bayer Baytron 4083) anode buffer layer was spin-casted onto the ITO substrate and then dried by baking in a vacuum oven at 80 12 ACS Paragon Plus Environment

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ºC overnight. The active layers, with a thickness in the 80 nm- 90nm range, were then deposited on top of the PEDOT: PSS layer by spin-casting from the chlorobenzene solution containing copolymers and PC71BM with weight ratios of 1:1, 1:1.5 and 1:2. Then a 8-nm Ca and 100-nm Al evaporated with a shadow mask under vacuum of (1–5) ×10–5 Pa. The regular photovoltaic cells with PFN interlayers, were prepared following the similar fabrication process of regular photovoltaic cells with Ca/Al as cathode, except that a 5-nm PFN layers were used to instead of the Ca layers. The inverted photovoltaic cells with 5-nm PFN coated ITO as cathodes, were fabricated following the similar preparation processes of regular photovoltaic devices except that the 5-nm PFN coated ITO were used as cathode and thermally-evaporated MoO3 (8 nm) and Ag (100 nm) were used as anodes. The thickness of the active layers was determined by a profilometer (Dektak XT) and the thickness of the evaporated cathode was monitored by a quartz crystal thickness/ratio monitor (SI-TM206, Shenyang Sciens Co.) and the thickness of PFN was monitored following the reported procedure.55 Except for the deposition of the PEDOT:PSS and PFN layers, all the fabrication processes were carried out inside a controlled atmosphere in a nitrogen drybox (Etelux Co.) containing less than 1 ppm oxygen and moisture. The overlapping area between the cathode and anode defined a pixel size of device of 0.1 cm2. The PCEs of the resulting polymer solar cells, were measured under 1 sun, AM 1.5G (Air mass 1.5 global) condition using a solar simulator (XES-70S1, San-EI Electric Co.) with irradiation of 100 mW/cm2. The current density–voltage (J–V) characteristics were recorded with a Keithley 2400 sourcemeasurement unit. The spectral responses of the devices were measured with a commercial EQE/incident photon to charge carrier efficiency (IPCE) setup (7-SCSpecIII, Bejing 7-star Opt. In. Co.). A calibrated silicon detector was used to determine the absolute photosensitivity. 4.3. Instruments and investigation of the surfaces composition of the active layers



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The number-average molecular weights (Mn) and weight-average molecular weights (Mw) of the copolymers, were measured by a Waters GPC 2410 in tetrahydrofuran (THF) using a calibration curve of polystyrene standards. Tapping-mode Atomic Force Microscopy (AFM) measurement was implemented on a DINS4 AFM instrument (Veeco). The refractive indices and extinction coefficients were investigated by a Horiba Jobin Yvon AUTO SE ellipsometer. The XPS was measured on a ESCALAB 250Xi X-ray Photoelectron Spectrometer (XPS) (Thermo Fisher Scientific). The water contact angles of the donor and acceptor materials, PEDOT: PSS modified-ITO and PFN modified-ITO films, were measured on a Drop Shape Analyzer DSA100 (KRUSS), respectively. The compositions of the surfaces of blend films by spin-coating the solution containing the copolymers and PC71BM with weight ratio of 1:2 and 3% DIO as solvent additive, were investigated by the X-ray photoelectron spectroscopy (XPS), and the weight ratios of PDTBDT-FBT/PC71BM and PDTBDT-BT/PC71BM at the top and bottom surfaces are evaluated using C/S atomic ratios obtained from the XPS measurement.56,57 The compositions of the top (free) surface of the blend films on the substrates of ITO/PEDOT: PSS and ITO/PFN were directly determined by the XPS investigation after the films were spin-coated on the substrates and dried in vacuum oven. The compositions of the bottom surfaces of the blend films on the substrates of ITO/PEDOT: PSS and ITO/PFN, were decided by the XPS investigation of the lifted-off blend films on ITO substrates with bottom surfaces facing up. And the lift-off blend films with bottom surface facing up were prepared following the below process: the prepared blend films from the copolymers and PC71BM with 3% DIO solvent additives on the substrates of ITO/PEDOT:PSS were firstly sellotaped, and dipped into the dilute hydrochloric acid (1−2%) for 15−20 h, then the blend films were floated on the surfaces of the solution, and transferred to ITO substrate with the bottom surface facing up, soaked in water 3 times each time 3−4 h in distilled water. After that, the films were slowly dried in vacuum oven for XPS analysis. It noted that the dilute hydrochloric acid and the length of the 14 ACS Paragon Plus Environment

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dipping time are important to relieve the PEDOT:PSS on the bottom surfaces of the films. The blend films on the substrates of ITO/PFN were prepared following the similar process as the above, except that the concentration of dilute hydrochloric acid was replaced with 5% and the lifted-off films on the ITO substrates firstly soaked in the mixture solution of methanol and acetic acid (with acetic acid of 0.5%) for 1−2 h to remove the PFN layers, and then soaked in water for 3−4 times. ACCOSICATED CONTENT Supporting Information The Supporting Information is available free of charge at the ACS publications website at: DOI: Additional figures and tables as mentioned in the text, including general synthetic route of the copolymers, AFM and XPS experiment information. AUTHOR INFORMATION Corresponding Author *[email protected] (Yangjun Xia); [email protected] (Hongbin Wu) Author Contributions The manuscript wrote through contributions of all authors, and all authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGEMENTS

The authors are deeply grateful to National Nature Science Foundation of China (51463011, 61264002, 91333206, 61404067, 61404147), National Key R&D Program of China (2016YFB0400801), Key Technology Research and Development Program of Jiangsu Province (Grant No. BE2014147-1), Open Fund of State Key Laboratory of Infrared Physics 15 ACS Paragon Plus Environment


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(Z201302), the Program for New Century Excellent Talents in University of Ministry of Education of China (Grant No. NCET-13-0840), and China Scholarship Council for financial support.

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Synthetically

Controlling

the

Optoelectronic

Properties

of Dithieno[2,3-d:2ʹ,3ʹ-

dʹ]benzo[1,2-b:4,5-bʹ]dithiophene-alt-diketopyrrolopyrrole Conjugated Polymers for Efficient Solar Cell. J. Mater. Chem. A 2014, 2, 15316–15325. (53) Wu, Y.; Li, Z.; Ma, W.; Huang, Y.; Huo, L.; Guo, X.; Zhang, M.; Ade, H.; Hou, J. PDT-S-T: A New Polymer with Optimized Molecular Conformation for Controlled Aggregation and π–π Stacking and Its Application in Efficient Photovoltaic Devices. Adv. Mater. 2013, 25, 3449–3455. (54) Huo, L.; Liu, T.; Sun, X.; Cai, Y.; Heeger, A. J.; Sun, Y. Single-Junction Organic Solar Cells Based on a Novel Wide-Bandgap Polymer with Efficiency of 9.7%. Adv. Mater. 2015, 27, 2938–2944. 22 ACS Paragon Plus Environment

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Table 1. Parameters of the photovoltaic cells from PDTBDT-BT/PC71BM and PDTBDTFBT/PC71BM with weight ratios of 1:2.

Active layer

PDTBDTBT/PC71BM

PDTBDTFBT/PC71BM

Calculated JSC

J

Thickness of active layer (nm)

VOC (V)

(mA/cm )

(mA/cm )

r-PVCs with /Ca/Al

80−90

0.80

10.32

r-PVCs with PFN/Al

80−90

0.82

i-PVCs with ITO/PFN

80−90

i-PVCs with ITO/PFN

FF

PCEs

(%)

(%)

−

60.20

4.97

11.19

−

68.50

6.29

0.83

11.90

11.43

70.30

6.94

220

0.82

15.20

15.02

68.50

8.54

r-PVCs with Ca/Al

80−90

0.88

10.98

−

61.30

5.92

r-PVCs with PFN/Al

80−90

0.92

11.20

−

69.50

7.16

Devices configuration

i-PVCs with ITO/PFN i-PVCs with ITO/PFN

80−90 220

0.93 0.91

SC 2

12.02 14.89

2

11.82 14.80

71.90 64.50

8.02 8.74


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Caption of the Figures Figure 1. Chemical structure of the PC71BM, copolymers and PFN (a) and devices configurations from the copolymers and PC71BM blend films (b). Figure 2. J-V (a, b) and IPCEs (c) characteristics of the optimal PVCs from the polymers and PC71BM with weight ratio of 1:2. Figure 3. The calculated JSC of regular and inverted PVCs with PFN layers devices from PDTBDT-BT/PC71BM (W:W, 1:2) (a) and PDTBDT-FBT/PC71BM (W:W, 1:2) (b) under assuming the internal quantum efficiency of (IQE) of 100%. Figure 4. Normalized XPS spectra and weight ratios of copolymers to PC71BM at the top and bottom surfaces of PDTBDT-FBT/PC71BM (W:W, 1:2) (a, b and c) and PDTBDTBT/PC71BM (d, e and f) blend films on the substrates of ITO/PEDOT:PSS and ITO/PFN. Figure 5. Distilled water contact on the surfaces of PDTBDT-BT (a), PDTBDT-FBT (b), PC71BM (c), 40-nm-thickness of PEDOT:PSS coated ITO (d) and 5-nm-thickness of PFN coated ITO (e).



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Figure 1. Chemical structure of the PC71BM, copolymers, PFN (a) and devices configurations from the copolymers and PC71BM blend films (b).


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Figure 2. J-V (a and b) and IPCEs (c) characteristics of the optimal PVCs from PDTBDTBT/PC71BM (a) and PDTBDT-FBT/PC71BM (b) with weight ratio of 1:2.



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Figure 3. The calculated Jsc of regular and inverted PVCs with PFN layers devices from PDTBDT-BT/PC71BM (W:W, 1:2) and PDTBDT-FBT/PC71BM (W:W, 1:2) under assuming the internal quantum efficiency of (IQE) of 100%.


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Figure 4. Normalized XPS spectra and weight ratios of copolymers to PC71BM at the top and bottom surfaces of PDTBDT-FBT/PC71BM (W:W, 1:2) (a, b and c) and PDTBDTBT/PC71BM (d, e and f) blend films on the substrates of ITO/PEDOT:PSS and ITO/PFN.



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Figure 5. Distilled water contact on the surfaces of PDTBDT-BT (a), PDTBDT-FBT (b), PC71BM (c), 40-nm-thickness of PEDOT:PSS coated ITO (d) and 5-nm-thickness of PFN coated ITO (e)


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