Efficient Cathode Buffer Material Based on Dibenzothiophene-S,S-dioxide for Both Conventional and Inverted Organic Solar Cells

A novel conjugated molecule (PBSON) based on a main chain composed of bis(dibenzothiophene-S,S-dioxide) fused cyclopentadiene and side chains containing amino groups is presented as an efficient cathode buffer material (CBM) for organic solar cells (OSCs). PBSON showed a deep highest occupied molecular orbital (HOMO) energy level of −6.01 eV, which was beneficial for building hole-blocking layers at the cathodes of OSCs. The energy bandgap of PBSON reached 3.17 eV, implying high transmittance to visible and near-infrared light, which meant PBSON should be suitable for the applications to most inverted OSCs. The scanning Kelvin probe microscopy measurement and theoretical calculation on the PBSON/cathode interfacial interaction proved the excellent work function-regulating abilities of PBSON for various cathodes, suggesting that PBSON could promote the formation of Ohmic contacts at the cathodes and thus improve the transport and collection of electron carriers for OSCs. The characterization of electron-only devices demonstrated the good electron-transporting performance of PBSON at the cathodes. In the conventional OSCs, it was hinted that PBSON might restrain the infiltrations of evaporated cathode atoms into the active films, consequently reducing the reverse leakage currents. As a result, PBSON was able to boost the power conversion efficiencies (PCEs) by 58.2 and 56.4% for both conventional and inverted OSCs of the typical PTB7:PC71BM system, respectively, as compared to the unadorned devices. In terms of the classical PTB7-Th:PC71BM system, substantial increases in PCEs could also be found with PBSON interlayers, which were 54.7 and 59.8% for the conventional device and inverted device, respectively. Therefore, PBSON is a kind of promising CBM for realizing both conventional and inverted OSCs of high performance.


INTRODUCTION
Organic solar cells (OSCs) show outstanding advantages, including light weight, low cost, solution processability, and mechanical flexibility, and are becoming a kind of promising technology for the light-to-electricity conversion. In the few past decades, great efforts have been made in the engineering of active layers, 1,2 interfaces, 3 and device structures, 4 leading to the rapid improvement in power conversion efficiency (PCE), which has exceeded 18% 1−4 until now.
In conventional and inverted OSCs, the most frequently used cathodes are some metals and metal oxides with high work functions (WFs) showing excellent ambient stability, such as Al, Ag, and indium tin oxide (ITO). However, these materials are not able to form Ohmic contacts with all of the high-performance acceptors, consequently decreasing the builtin potentials (V bi s) across OSCs and increasing the energy barriers of charge transportation and collection. To address the above problem, it is extremely necessary to introduce cathode buffer layers (CBLs) between the active layers and the cathodes. Efficient CBLs need to simultaneously possess many fundamental functions, such as reducing cathodes' WFs, efficiently transporting electron carriers, blocking hole carriers, reducing interfacial trap states, improving interfacial contacts, and so on. 5,6 The existing cathode buffer materials (CBMs) can be classified mainly as inorganic and organic species. The inorganic CBMs, including metal compounds (e.g., ZnO, 7 LiF, 8 TiO 2 9 ), have been employed to improve the interfacial properties of cathodes at first. However, these materials exhibit some disadvantages, such as incompatibility with organic active films, high sensitivities toward moisture, and photocatalytic peculiarities for the degradations of active films, 10 which impede further improvements in the performance of OSCs.
In recent years, water/alcohol-soluble organic molecules (WSOMs) containing high-polarity functional groups (pyridinium salt, 5,6,11 amino, 12−14 quaternary ammonium salt, 15,16 pyridyl, 17,18 etc.) have been widely used as efficient CBMs for fabricating OSCs of high performance. With regard to these WSOMs, the general optoelectronic properties such as ultraviolet−visible (UV−vis) absorption spectra, electronic energy levels, and charge mobility usually originate from their conjugated structures. On the other hand, the cathodemodifying capabilities and water/alcohol solubility are mainly endowed by the functional groups. The water/alcohol solubility of these CBMs enables the fabrication of multilayer OSCs without interfacial corrosion via low-cost orthometric solution processing methods. The other advantages of these WSOM-based CBMs include excellent interfacial compatibility with active films, light weight, flexibility, and so on. Therefore, WSOMs are a class of indispensable materials for organic photovoltaic technologies.
Dibenzothiophene-S,S-dioxide (SO) is an efficient electrontransporting unit for photoelectric materials, which owns a relatively broad conjugated structure, high electron affinity, and excellent chemical stability. 19 Previously, some WSOMs based on SO and pyridinium salt were synthesized and used as CBMs to enhance the PCEs for OSCs successfully by us. 5,6 Moreover, we have designed and prepared a class of amino-functionalized conjugated copolymers (PBSON-P and PBSON-FEO) based on the main chains composed of bis-SO fused cyclopentadiene (BSO) and benzene or fluorene. 20 The novel aromatic systems offered these WSOMs with interesting properties such as efficient electron-transporting and hole-blocking performance when PBSON-P and PBSON-FEO were employed as CBMs to OSCs. As a result, simultaneous improvements in the opencircuit voltage (V oc ), fill factor (FF), and PCE could be realized, suggesting the great potential of BSO units to develop high-efficiency CBMs.
In this work, to further investigate the intrinsic cathodebuffering properties of the BSO building block, a novel conjugated molecule named PBSON based on a repeating unit of amino-modified BSO was presented. It was found that PBSON showed a deeper highest occupied molecular orbital (HOMO) energy level (E HOMO ) of −6.01 eV as compared to the corresponding copolymers, 20 offering efficient hole-blocking performance for the cathodes. The energy bandgap (E g ) of PBSON was enlarged to 3.17 eV, indicating the high transmittance to visible and near-infrared light of CBLs, which should be able to maintain the light absorption of active layers in the inverted devices. The scanning Kelvin probe microscopy (SKPM) measurement and theoretical calculation on an interfacial model verified the prominent WF-regulating abilities of PBSON for cathodes endowed by the amino groups, signifying that PBSON could help form Ohmic contacts at the cathodes and thus improve the electrontransporting and -collecting performance for OSCs. Besides, the characterization of electron-only devices further demon-strated the good electron-transporting performance of PBSON at the cathodes. When PBSON was used as a CBM to the classical OSCs of poly [4,8-bis(2-ethylhexyloxyl)benzo [1,2b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3fluorothieno[3,4-b]thiophene-2-carboxylate-2,6-diyl)] (PTB7): [6,6]-phenyl C 71 -butyric acid methyl ester (PC 71 BM), it was found to enhance PCEs by 58.2 and 56.4% for the conventional and inverted devices, respectively, in comparison to OSCs with unmodified cathodes. In the matter of poly [4,8- 20 Then, the common Pd-catalyzed Stille coupling reaction between monomer 1 and hexabutylditin (2) generated PBSON successfully. PBSON showed a number average molecular weight (M n ) of about 6900 and a polydispersity of 1.3, estimated on gel permeation chromatography (GPC). The relatively small M n might be attributed to the relatively low solubility of the polymeric intermediate in the reactive solvent system composed of toluene and N,N-dimethylformamide, as some precipitate could be observed during the polymerization. PBSON was soluble in the common moderately polar organic solvents such as tetrahydrofuran (THF) and CHCl 3 , and also could be dissolved in the mixed solvent composed of MeOH and trace trifluoroacetic acid (TFA) (with a volume fraction of 0.5%) due to the ionized reaction between the amino groups and TFA. It was found that the solvent system of MeOH containing trace HOAc, which is often used to process the amino-functionalized CBLs, 12 was unable to solubilize PBSON, probably because a stronger ionized conversion of amino groups was required for PBSON with a big conjugated structure to be dissolved in MeOH. In a word, multilayer OSCs can be fabricated via the low-temperature orthometric solution processing technology when employing this new molecule as a CBM.

Photophysical Properties.
The UV−vis absorption and photoluminescence (PL) spectra of PBSON in the dilute CH 2 Cl 2 solution and in the film are shown in Figure 1a, and the corresponding parameters are summarized in absorption peaks at 358 and 380 nm. These correlative signals exhibited bathochromic shifts to 365 and 388 nm in the film state due to the intermolecular π−π aggregation effect. The optical E g (E g opt ) could be calculated as 3.05 eV from the absorption edge in the film. As for the PL properties, the emitting peaks were located at 413 and 424 nm in the solution and in the film, respectively, suggesting blue light emission for PBSON.
The optical transmission spectrum of PBSON in the film is presented in Figure 1b. The processing condition of the PBSON film here was the same as that in the inverted OSCs, as discussed later. The ITO substrate showed extremely high transmittance in the wavelength range higher than 300 nm. For the ITO/PBSON substrate, the transmittance profile in the wavelength range of 300−410 nm matched well with the corresponding absorption curve in the film. The transmittance values exceeded 90% in the wavelength range higher than 425 nm, signifying excellent visible and near-infrared light transparency of PBSON.

Electrochemical Properties and Theoretical
Calculations. The electrochemical properties of PBSON were investigated, and the resulting cyclic voltammetry (CV) curve is exhibited in Figure 1c. In the CV profile of PBSON, the onset potentials of oxidation (E ox ) and reduction (E red ) were 1.66 and −1.51 V, respectively. Besides, the oxidation potential of ferrocene/ferrocenium (Fc/Fc + ) against the reference electrode of saturated calomel was measured as 0.45 V. Therefore, the E HOMO and lowest unoccupied molecular orbital (LUMO) energy level (E LUMO ) of PBSON could be evaluated from the empirical equations of E HOMO = −(4.80 + E ox − 0.45) eV and E LUMO = −(4.80 + E red − 0.45) eV, 21 and the values were −6.01 and −2.84 eV, respectively. Consequently, the electrochemical E g (E g e ) was calculated to be 3.17 eV, as shown in Table 1.
To get an insightful understanding of the optimal molecular geometry and electronic properties of PBSON, a density functional theory (DFT) calculation at the B3LYP level with the 6-31G* basis set was performed for the dimer of the repeating unit, during which the side chains were replaced with  methoxy groups to simplify the computation. 22 The optimized molecular geometry is shown in Figure S1 (see the Supporting Information), and the calculated frontier molecular orbitals (FMOs) are displayed in Figure 1d. The conjugated main chain of this molecule exhibited nonplanar conformation at the lower state due to torsional strains between those neighboring aromatic rings, and both the HOMO and LUMO were delocalized mainly over the conjugated main chain barring the phenyl side chains. The calculated E HOMO and E LUMO were −5.97 and −2.55 eV, respectively.

Photovoltaic Performance.
To study the cathodebuffering performance of PBSON for OSCs, both the conventional and inverted devices based on the PTB7:PC 71 BM system were fabricated and measured. The energy level diagrams, current density-voltage (J−V) curves under AM 1.5G irradiation, and external quantum efficiency (EQE) spectra of these OSCs are shown in Figure 2, and the extracted photovoltaic parameters are summarized in Table 2. As for the conventional device without any cathode treatment, the V oc , short-circuit current density (J sc ), FF, and PCE were 0.57 V, 15.39 mA cm −2 , 51.51 and 4.52%, respectively. When MeOH containing 0.5% TFA was used to wash the active film, the aforementioned parameters were changed to 0.65 V, 15.41 mA cm −2 , 57.44 and 5.75%, respectively. It is worth noting that when a thin layer of PBSON was inserted between the active layer and cathode, the PCE was increased to 7.15%, accompanied by the V oc at 0.73 V, J sc at 15.44 mA cm −2 , and FF at 63.43%. It was found that the significant improvement in PCE by PBSON was mainly attributed to the enhancements in the V oc and FF. On the other hand, it can be seen that the modified device showed slightly lower EQE values than the pristine one, as shown in Figure 2c, which might be due to the higher light transmittance of the cathode substrate for the pristine device. However, these OSCs exhibited close J sc s, probably resulting from that the untreated device owned nonideal energy level alignment (as discussed in   the following WFs test) and thus poor charge carriertransporting ability at the cathode. Therefore, although the EQEs were higher (due to the higher light absorption), the light current could not be converted efficiently for the unmodified device. This could also be proven by the lower FF and electron mobility (see the following electron-only devices test) for the pristine device. In regard to the inverted unadorned OSC, the photovoltaic parameters were 0.57 V, 15.65 mA cm −2 , 52.25 and 4.66% for the V oc , J sc , FF, and PCE, respectively. However, the PBSONmodified device showed evident performance improvement as well, with the V oc at 0.73 V, J sc at 15.50 mA cm −2 , FF at 64.41%, and PCE at 7.29%. Apparently, the V oc and FF increases resulted in the PCE improvement, which was similar to the conventional devices. The EQE spectra are shown in Figure 2f, in which the PBSON-modified device displayed higher EQEs in the short-wavelength region and lower EQEs in the long-wavelength region as compared to the pristine device. This change in the EQE profile might be attributed to the changes in optical parameters (refractive index, extinction coefficient, etc.) of the functional layers, leading to the variation in optical field distribution when inserting PBSON into the device. 23 Moreover, the conventional OSCs based on an ITO/ PEDOT:PSS/PTB7-Th:PC 71 BM/CBL/Al structure and inverted OSCs based on an ITO/CBL/PTB7-Th:PC 71 BM/ MoO 3 /Al structure were also prepared and tested. The J−V curves under AM 1.5G irradiation of these OSCs are shown in Figure S2, and the corresponding photovoltaic parameters are displayed in Table S1. The PCE of the conventional device was increased from 5.23% (V oc = 0.58 V, J sc = 16.46 mA cm −2 , FF = 54.77%) to 6.38% (V oc = 0.66 V, J sc = 16.47 mA cm −2 , FF = 58.70%) when the solvent was used to wash the active layer. However, the PCE could be enhanced to 8.09% (V oc = 0.77 V, J sc = 16.51 mA cm −2 , FF = 63.64%) for the device subjected to PBSON modification. On the other hand, the inverted device was able to realize PCE improvement from 5.10% (V oc = 0.58 V, J sc = 16.29 mA cm −2 , FF = 54.01%) to 8.15% (V oc = 0.77 V, J sc = 16.38 mA cm −2 , FF = 64.59%) when inserting a PBSON interlayer. Hence, PBSON is a kind of high-efficiency CBM which can boost the performance of different OSCs significantly.

WF Properties of Cathodes.
To explore the working mechanisms of PBSON, the regulating abilities of PBSON for the WFs of cathodes were tested on a SKPM, as shown in Table 3. As for the pure Ag electrode, the WF was measured to be 4.73 eV; however, when the surface of Ag was coated with a PBSON thin film, the WF was decreased to 4.18 eV by a large margin. A similar result could be observed with the ITO samples, which showed a WF reduction from 4.70 to 4.58 eV when spin-coating a PBSON thin film on the ITO substrate. The mechanism for changes in WF was speculated to be the formation of directed dipoles at the active film/cathode interfaces induced by PBSON. 12 The decreased WFs of cathodes could enlarge the V bi s across the OSCs, which enhanced the V oc s finally.
To further study the interfacial interactions between PBSON and cathodes, we established a molecular model of PBSON on the surface of ITO. 24 The cathode-modifying effects of organic CBLs containing high-polarity functional groups for OSCs have been proposed to originate from the influences of functional groups, such as the amino group and phosphonic acid group. 25 As a typical example, the amino groups of PFN interlayers dominate the interfacial interactions and formations of interfacial dipoles in OSCs. 24 Therefore, it should be reasonable to examine the interfacial interaction dynamic between the amino-inlayed side chain of the PBSON and cathode such as ITO. In the theoretical calculation, when PBSON was adsorbed on the surface of ITO, the WF of ITO was decreased from 4.9 to 4.0 eV, as shown in Figure 3a,b. The variation of WFs was equivalent to an interfacial dipole of 3.87 D, and the direction of the dipole was from ITO to PBSON, as shown in Figure 3c. Moreover, the vertical component of the dipole of this molecule itself was calculated as 0.6 D, as presented in Figure 3c. It could be inferred that the dipole induced by the charge transfer from the amino group to ITO was 3.27 D, and the direction was also from ITO to PBSON. Hence, these two dipoles could be superposed when PBSON was adsorbed on the surface of ITO, leading to a decrease in the ITO WF. In addition, the electron density differences of PBSON on the surface of ITO were also calculated, as shown in Figure 3d. The result suggested that there was an obvious electron transfer from PBSON to ITO, which further proved the formation of the interfacial dipole, and the direction was from ITO to PBSON.
2.6. J−V Curves in the Dark. The J−V characteristic curves in the dark of conventional and inverted PTB7:PC 71 BM OSCs were recorded, as shown in Figure 4a,b. As for the conventional device, it could be seen that the reverse dark J values were obviously restrained when introducing a PBSON layer. It has been reported that the J values in this region depend mainly on the qualities and thicknesses of active films; hence, the decreases in the reverse leakage J might benefit from that PBSON generated a protective layer to prevent the evaporated cathode atoms from infiltrating into the active film. 26 Moreover, the J values of the PBSON-treated conventional device were distinctly higher than those of the unadorned one in the forward high-bias area, which could be observed in the inverted OSCs as well, meaning the improved electron injections and transports of cathodes when inserting PBSON CBLs. 26 This might be attributed to the formation of Ohmic contacts between the acceptors and cathodes induced  by the WF-adjusting abilities of PBSON for cathodes and to the good electron-transporting capabilities of PBSON at the cathodes, as discussed below.
The series resistance (R s ) calculated from the forward highbias area of the J−V curve in the dark 27 was reduced from 36.9 to 15.4 Ω cm 2 when employing a PBSON interlayer into the conventional device. Besides, as compared to the inverted device with an unmodified cathode showing a R s at 11.7 Ω cm 2 , a smaller R s at 8.81 Ω cm 2 could also be found in the modified one. This implied that PBSON interlayers could improve the interfacial contacts and charge carrier-transporting performance of cathodes, which was consistent with the enhanced FFs of both conventional and inverted OSCs with PBSON interlayers.

Electron-Only Devices.
To investigate the electrontransporting performance of PBSON thoroughly, electron-only devices with a structure of ITO/ZnO/PTB7:PC 71 BM/CBL/ Ag were fabricated and tested, and the results were fitted with the space−charge−limited current (SCLC) model, 28 as shown in Figure 4c,d. The active layers and CBL were processed in the same way as the conventional OSCs. The device without any cathode treatment showed an apparent electron mobility of 6.09 × 10 −6 cm 2 V −1 s −1 . Interestingly, the electron mobility was enhanced to 1.60 × 10 −5 cm 2 V −1 s −1 when introducing a PBSON layer into the cathode. The good charge carriertransporting performance of this molecule, suggested by the increased mobility, could be responsible for the improvements in the FF and PCE of OSCs.
2.8. Topographies. Atomic force microscopy (AFM) was used to study the topographies of PTB7:PC 71 BM active films subjected to different treatments, and the three-dimensional (3D) images are displayed in Figure 5. The unadorned PTB7:PC 71 BM film exhibited a smooth surface with a rootmean-square roughness (R q ) at 2.45 nm. The active film became slightly rougher with a R q of 2.91 nm after washing with MeOH (containing 0.5% TFA). However, the topography turned into a lumpy surface showing a R q at 5.28 nm when a PBSON thin film was deposited on top, which might be due to the rigid molecular structure and mediocre film-forming property of PBSON. The rough CBL could generate large contact areas with the top metal cathodes, which might be helpful to the electron extraction to the cathodes and thus FF improvements, agreeing with the previous reports. 29,30

CONCLUSIONS
In summary, a novel conjugated molecule named PBSON based on a repeating unit of amino-functionalized BSO was designed and synthesized. It was proved that the PBSON CBM could boost the PCEs by 58.2 and 56.4% for the conventional and inverted OSCs of PTB7:PC 71 BM, respectively. As for the PTB7-Th:PC 71 BM-based OSCs, PBSON was able to increase the PCEs by 54.7% for the conventional device and 59.8% for the inverted one. The improvements in PCE for the above OSCs resulted mainly from the synchronous enhancements in the V oc and FF. PBSON exhibited a deep E HOMO of −6.01 eV and a large E g e of 3.17 eV, implying effective hole-blocking properties and favorable applications to inverted devices, respectively. The SKPM measurement and theoretical calculation on an interfacial model verified that PBSON showed excellent WF-regulating abilities for cathodes, which promoted the formation of Ohmic contacts at the cathodes and the V bi and V oc enhancements of OSCs. In the conventional OSCs, PBSON might prevent the evaporated cathode atoms from diffusing into the active films, consequently reducing the reverse leakage currents, which could be hinted from the J−V curves of OSCs in the dark. Moreover, it was suggested from the measurement of electron-only devices that PBSON displayed good electron-transporting capabilities and was able to improve the charge carrier transport of cathodes, giving rise to enlargements in the FF. Therefore, PBSON is a promising CBM which can improve the performance of both conventional and inverted OSCs efficiently.

EXPERIMENTAL SECTION
All of the relevant materials, reagents, and solvents were purchased from Energy Chemical Co., Aladdin Chemical Co., and Sigma Aldrich Chemical Co. and were used directly without further purification. The solvents used in the moisturesensitive reactions and tests were further dried by the general procedures and distilled before use. The synthetic route and procedures, chemical characterizations, and fabrications and characterizations of OSCs are presented in the Supporting Information.