Widely Applicable n-Type Molecular Doping for Enhanced

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Widely Applicable n‑Type Molecular Doping for Enhanced Photovoltaic Performance of All-Polymer Solar Cells Yalong Xu,† Jianyu Yuan,*,† Jianxia Sun,† Yannan Zhang,† Xufeng Ling,† Haihua Wu,† Guobing Zhang,‡ Junmei Chen,† Yongjie Wang,† and Wanli Ma*,† †

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China ‡ Key Laboratory of Special Display Technology of the Ministry of Education, National Engineering Laboratory of Special Display Technology, National Key Laboratory of Advanced Display Technology, Academy of Photoelectric Technology, Hefei University of Technology, Hefei 230009, China S Supporting Information *

ABSTRACT: A widely applicable doping design for emerging nonfullerene solar cells would be an efficient strategy in order to further improve device photovoltaic performance. Herein, a family of compound TBAX (TBA= tetrabutylammonium, X = F, Cl, Br, or I, containing Lewis base anions are considered as efficient n-dopants for improving polymer−polymer solar cells (all-PSCs) performance. In all cases, significantly increased fill factor (FF) and slightly increased short-circuit current density (Jsc) are observed, leading to a best PCE of 7.0% for all-PSCs compared to that of 5.8% in undoped devices. The improvement may be attributed to interaction between different anions X− (X = F, Cl, Br, and I) in TBAX with the polymer acceptor. We reveal that adding TBAX at relatively low content does not have a significantly impact on blend morphology, while it can reduce the work function (WF) of the electron acceptor. We find this simple and solution processable n-type doping can efficiently restrain charge recombination in all-polymer solar cell devices, resulting in improved FF and Jsc. More importantly, our findings may provide new protocles and insights using n-type molecular dopants in improving the performance of current polymer−polymer solar cells. KEYWORDS: n-type doping, all-polymer solar cells, charge recombination, tetrabutylammonium iodide (TBAI), morphology, electron transfer small molecule acceptors.6,14−16 Therefore, the polymer− polymer (all-polymer) solar cells require rapid improvement in order to make full use of their unique advantages like low material cost and improved device stability. The predominant challenge for all-polymer photovoltaic device is to minimize losses associated with recombination,17,18 which will overcome the relatively low short-circuit current density (Jsc) and fill factors (FF) in these devices. Generally, the bottlenecks that limit the progress of power conversion efficiency (PCE) for BHJ-OPVs are the exciton migration and dissociation, charge transport, and charge recombination,19−21 and the morphology of BHJ blend films is at the core of these charge carrier process.22,23 In comparison with fullerene-based electron acceptors, the electronic disorder and structural heterogeneity of n-type conjugated polymer acceptors is totally different.17,24 Thus, charge generation process may be hindered, and internal loss ways called geminate and nongeminate recombination may be increased. Hence,

1. INTRODUCTION Bulk heterojunction (BHJ) organic photovoltaics (OPVs) using nonfullerene electron acceptors have demonstrated rapid progress in both efficiency and stability during the past five years,1−4 with the record efficiency for single-junction devices over 13%.5−9 This emerging technology enables low-cost photovoltaic technology for alternative energy applications, with the disadvantages of light weight and high flexibility and processability.10,11 Representative devices utilize a BHJ blend adopting electron donor of conjugated polymer and electron acceptor of either nonfullerene conjugated molecule8 or polymer.1,12 In comparison with the previously studied polymer:fullerene solar cells, the nonfullerene acceptors exhibit higher tunability in molecular structure and electrical properties and show enhanced absorption in visible and near-infrared regions.1,13 With respect to the recently developed highperformance nonfullerene small molecule acceptors, polymer acceptors provide features like mass synthesis, superior charge transport, and more importantly, enhanced thermal and mechanical stability.1,6,12 However, the current development of polymer acceptors is significantly behind in either molecular synthesis or device performance synthesis compared to that of © XXXX American Chemical Society

Received: October 2, 2017 Accepted: December 26, 2017

A

DOI: 10.1021/acsami.7b15000 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Molecular structure of donor polymer PBDB-T, polymer acceptor N2200, and TBAX salt n-type dopants (X = F, Cl, Br, and I). (b) Thin film UV−vis absorption spectra of neat PBDB-T and N2200 film cast from chlorobenzene solutions. (c) Schematic energy levels of the inverted all-polymer solar cell device.

selected to improve the electron transport and photovoltaic performance of all-PSCs. For all dopants, increased FF, Jsc, and device performance are observed, with the best PCE reaching 7.0% compared to that of 5.8% for the pristine devices. We observed that adding TBAX at relatively low content would not significantly change the blend morphology. Instead, it could modify the optoelectrical properties of the active film which were responsible for the FF and Jsc improvements. These findings may become important for the further application of ntype doping in all-PSCs as an efficient and facile method to improve the device performance.

relatively low photocurrent and FF were often observed in allPSCs. One widely adopted strategy to increase the charge generation is optimizing the donor and acceptor orientation at their interfaces to achieve better exciton generation and dissociation.24,25 However, such approach requires great efforts in molecular design or morphology optimization, which are highly empirical and system-dependent. Other methods were also adopted to improve the quality of the active materials by using ternary structure26,27 and doping strategy.28,29 Semiconductor doping has been reported as an effective strategy for achieving ideal device characteristics in organic field-effect transistors (OFETs).30−32 Through adding a small amount of organic or inorganic compounds to pristine organic semiconductors, the conductivity of the semiconductor layer, the charge carrier injection,28 and device stability can be improved.31 In the past few years, p-type molecular doping was reported to be an efficient strategy for enhancing the charge extraction and charge transport in polymer:fullerene solar cells.28,29 As a result, improved photocurrent, FF, and PCEs were obtained due to the increased carrier mobilities which can be enhanced by the doping strategy. Quite recently, a n-dopant tetrabutylammonium iodide (TBAI), which includes Lewis base anions I−, was reported to produce a highly conductive C60 film and n-type polymer film by anion assisted electron transfer33 or chemical reaction.34 However, despite the wide application of n-type molecular doping in organic materials, its impact on the performance and properties of emerging nonfullerene solar cells remain unknown and require in-depth study. Herein, we reported a successful n-type doping method to improve the performance of all-PSCs composed of a donor polymer PBDB-T35 and a widely used polymer acceptor P(NDI2OD-T2), (commercial name N2200).36 A serials of ndopants tetrabutylammonium fluoride (TBAF), chloride (TBACl), bromide (TBABr) (Figure 1a), and TBAI were

2. EXPERIMENTAL DETAILS 2.1. Material Preparation. PBDB-T was synthesized on the basis of previous report by Hou et al.35 and N2200 was synthesized in accordance with the approaches in our recent works.17,26 TBAF, TBACl, TBABr and TBAI were purchased from TCI, processingsolvent-like chlorobenzene was from Sigma-Aldrich. MoO3 and silver were purchased from Alfa Aesar. All chemical products could be commercially available and used without any further treatment. 2.2. Device Fabrication and Characterization. The structure of all-polymer soar cells were ITO/ZnO/blend/MoO3/Ag. Glass substrates with ITO were cleaned sequentially with acetone, deionized water, isopropanol, and acetone for 20 min, respectively, and then with ultraviolet/ozone treatment for 15 min. ZnO layer with a thickness of ∼40 nm was obtained according to previous report.37 PBDB-T:N2200 all-polymer blend solutions in chlorobenzene were kept heating at 40 °C for at least 6 h. The concentration is 8 mg/mL with a blend ratio of 2/1. All-polymer active layer was spin-coated onto the top of ZnO layer at 2500 rpm for 60 s with or without TBAX. TBAX was dissolved in chlorobenzene, which was added by controlling the doping concentration. Then, ITO/ZnO/active layer were thermal annealed at 120 °C for 10 min in glovebox. Finally, MoO3 at a speed of 0.2 A/s (9.0 nm), and anode Ag at a speed of 2 A/s (100 nm) layers were then thermally evaporated according to our previous method.15−17 The protocol for solar cells characterization (current density−voltage (J− V) characteristics, external quantum efficiency (EQE)) was according B


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Figure 2. (a) Schematic illustrations of solar cell device configuration. J−V curves of optimized PBDB-T:N2200 solar cell devices with or without optimal TBAX doping measured under AM 1.5 G solar illumination (b) and corresponding EQE curves (c). (d) Characteristics of photocurrent density (Jph) versus effective voltage (Veff) for PBDB-T:N2200 solar cells with or without addition of TBAI.

Table 1. Device Parameters Based on Active Layers of PBDB-T: N2200 with or without Optimal TBAX (X = F, Cl, Br, and I) Dopinga Voc (V)b none TBAF TBACl TBABr TBAI a

0.86 0.86 0.86 0.86 0.86

(0.86 (0.86 (0.86 (0.86 (0.86

± ± ± ± ±

0.00) 0.01) 0.01) 0.00) 0.00)

Jsc (mA/cm2)b 11.54 11.95 12.21 12.17 12.45

(11.56 (11.86 (12.09 (12.10 (12.21

± ± ± ± ±

0.15) 0.21) 0.17) 0.13) 0.12)

FF (%)b 58.4 61.4 62.2 64.6 65.3

(57.3 (60.3 (61.5 (63.4 (64.8

± ± ± ± ±

PCE (%)b 0.8) 0.7) 0.8) 0.9) 0.3)

5.81 6.33 6.55 6.80 7.00

(5.70 (6.15 (6.40 (6.60 (6.80

± ± ± ± ±

0.09) 0.14) 0.07) 0.11) 0.10)

Rs (Ω cm2)

Rsh (Ω cm2)

rs

rsh

4.13 3.82 2.98 2.65 2.72

570.14 642.22 631.86 697.74 670.86

0.05 0.05 0.04 0.04 0.04

7.65 8.92 8.97 9.87 9.71

Blend weight ratio 2:1, film thickness around 100 nm. bAverage date from 6 devices.

to our previous report.15−17 Solar cell devices were measured in forward scan (−1.0 V → 1.0 V, step 0.0125 V, scan rate: 0.1 V s−1) without an illumination mask in glovebox. The stabilized PCE value was taken for each device.

are achieved by utilizing an inverted structure of ITO/ZnO (40 nm)/Blend (∼100 nm)/MoO3(9 nm)/Ag (100 nm). To realize fully solution process of molecular doping in the fabrication of photoactive blends, the TBAX dopants were first dissolved in chlorobenzene separately. Then, the TABX solutions can be added to the chlorobenzene solutions of PBDB-T:N2200 at varying doping level. As shown in Tables S1−S5, solar cells optimization includes adjusting the donor:acceptor blend ratio, molecular doping ratio, and thermal annealing temperatures. The optimal PBDB-T:N2200 (8 mg/ mL) blend ratios were found to be 2:1, which is similar to the previous report.35 After spin-coating, all devices exhibit similar thickness around 100 nm. We further studied the TBAI doping concentration on the device photovoltaic characteristics. With the addition of TBAI, we observe a significant enhancement of FF from 58.4 up to 65.3% and a slight improvement of Jsc from 11.50 to 12.45 mA/cm2. The change of both FF and Jsc are sensitive to the doping concentration of TBAI in the range from 0.01 to 0.5 wt % (Table S4). Devices employing similar dopants of TBAF, TBACl and TBABr were also fabricated (see Table S5). Figure 2b depicts the J−V curves of PBDB-T:N2200 solar cells adopting TBAX dopants with the optimal concentration, with the detailed Jsc, Voc, FF, and PCE

3. RESULTS AND DISCUSSION The all-PSCs using PBDB-T:N2200 as the active layer was first reported by Hou et al.35 The molecular structures of PBDB-T, N2200, and TBAX are displayed in Figure 1a. The film absorption of the materials is shown in Figure 1b, and the optical band gap of donor polymer PBDB-T can be calculated to be 1.80 eV.35 In addition, the thin film optical absorption of N2200 is greatly extended relative to PCBM, exhibiting complementary absorption with the donor polymer. Since allPSCs using PBDB-T:N2200 has been previously reported by Hou et al.,35 for fair comparison, the values of LUMO and HOMO energy levels were adopted on the basis of their results. As shown in Figure 2a, PBDB-T:N2200 can form a type II heterojunction, which is helpful to facilitate charge separation. A sufficient lowest unoccupied molecular orbital (LUMO) offset can be found between PBDB-T and N2200, which is beneficial for electron transfer from PBDB-T to N2200. As illustrated in Figures 1c and 2a, the optimized all-PSCs devices C


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Figure 3. AFM height images and related stereoscopic images of pristine PBDB-T:N2200 blend film (a) and with optimal TBAI doping (b); 2dGIWAXS patterns of pristine PBDB-T:N2200 blend film (c) and with optimal TBAI doping (d).

PBDB-T:N2200 all-PSCs via n-type doping. Therefore, it should be reasonable to check into the effect of n-type doping about the morphology, charge transport, and recombination to reveal the detailed mechanisms of doping induced enhancement. At first, we examined the impact of TBAX dopants on allpolymer blend morphology. Figure 3a,b shows the topographic images of PBDB-T:N2200 films with or without optimal amount of TBAI dopant (0.04 wt %) investigated by AFM. For the pristine all-polymer blend, the thin film surface displays a relatively smooth and textured polymer structure, which may be due to the strong aggregation of polymer N2200.17,38 The surface of PBDB-T:N2200 film displays a relatively small rootmean-square (RMS) roughness value (0.88 nm). After doping with 0.04 wt % TBAF, TBACl, TBABr, and TBAI, we observe the RMS roughness does not change significantly, with a similar value around 0.85 nm (see Figure S2), and there is not an obvious surface reconstruction. Meanwhile, we also studied the effect of doping concentration of TBAI on the blend morphology (see Figure S3). Again, the surface morphology and the RMS roughness do not change significantly with the doping concentration lower than 0.1 wt %. A slight change in the AFM topographic images is observed when the doping content is further increased to a higher value of 0.5 wt %. Since AFM only probes the film surface morphology, we further conducted grazing incidence wide-angle X-ray scattering (GIWAXS)38−40 to investigate the change of crystallinity and

parameters listed in Table 1. Quite similarly, we observe increased FF after the addition of TBAX with the same optimal doping concentration of 0.04 wt %. A slight increase of the Jsc is also observed, which may arise from the enhanced charge generation and reduced recombination after doping. In contrast, the open-circuit voltage (Voc) remains nearly unchanged after molecular doping. As a result, the best PCE was enhanced from the original 5.81 to 7.00%. To confirm the Jsc improvement, we measured the incident photon-to-electron conversion efficiency (IPCE) of these devices and the results were shown in Figure 2c. A slight IPCE increase upon adding TBAX is found in the area around 750 nm, where only N2200 has absorption. Meanwhile, after adding TBAX dopants, absorption in the area near 400 nm and at a lower energy region between 700 and 800 nm were slightly increased (see Figure S1), which corresponds well with the absorption curve of N2200. These spectroscopic changes indicate that the doping induced changes may mainly occur on the electron acceptor N2200.31 The increased current may stem from the enhanced absorption or improved carrier extraction of N2200 after the use of dopant. Nguyen and Seferos et al.28,29 recently used F4-TCNQ as the p-type dopant, and charge transfer states and trap-filling in polymer:PCBM solar cells can effectively restrain the charge recombination. Consequently, the photocurrent and FF were significantly enhanced via p-type doping. In this work, likewise, we report similar enhancement28,29 on the performance of nonfullerene D


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Figure 4. Surface potential images of (a) PBDB-T, (b) 0.04 wt % TBAI doped PBDB-T, (d) N2200 and (e) 0.04 wt % TBAI doped N2200 obtained from KPFM measurements. (c, f) differences of surface potential between neat polymer film and TBAI doped film.

N2200 and will not affect the p-type donor polymer, confirming the n-type doping in PBDB-T:N2200 blend using TBAI. In short, the addition of TBAI can selectively reduce the work function (WF) of the electron acceptor, as a result of molecular interaction between TBAI and N2200. To understand the effect of n-doping on the charge generation and extraction process, the change of the photocurrent density (Jph) at effective voltage (Veff) were recorded under light irradiation at 100 mW/cm2 (Figure 2d).40 From Jph−Veff characteristics for the best performance devices with or without doping, we find that Jph reaches a plateau when Veff is around 1.0 V, indicating that all the free charges are rapidly extracted. At sufficiently high Veff, Jph values of doped devices achieve a slightly higher value. The results suggest that a little TBAI is beneficial for the process of charge generation. We further studied the charge transport properties17 in these devices (Figure 5a,b). The influence of molecular doping on the charge transport in PBDB-T:N2200 was investigated by holeonly and electron-only diode measurements using a device

molecular stacking when processed with or without TBAI doping. The results are shown in Figure 3c,d, with the line-cuts exhibited in Figure S4. Both blend films exhibit apparent (010) diffraction peaks at ∼1.6 Å−1 in the out-of-plane direction, a (100) and combined (001)/(200) peak at ∼0.25 and ∼0.50 Å−1 in the in-plane direction respectively, which corresponds to the characteristic scattering peaks of N2200.16,17 Neat N2200 film was widely reported to be a high-crystalline film with strong face-on orientation. Since the contrast between the scattering peaks of the blend film with or without doping is very small, the GIWAXS results indicate that there is only very small crystallization behavior change after TBAI doping. In summary, the morphological results demonstrate that the introduction of an optimal TBAX dopant in PBDB-T:N2200 blend does not lead to a significant morphological change. With low doping concentrations (0.04 wt %), slightly increased Jsc and significantly increased FF are observed in all cases, resulting in an enhancement of PCEs from 5.8 to 7.0%. We find that the addition of TBAI can reduce the work function of the polymer acceptor, while the morphological characterizations reveal that small amount of n-dopants do not alter active layer morphology. On the basis of the charge transport and recombination investigations, we attribute the improvement primarily to the improved electron mobility and reduced trap sites in the polymer film. The n-type molecular doping can efficiently restrain charge recombination in the bulk film and at the interface, resulting in improved diode ideality factor, FF, and Jsc. In order to further gain insight into the change of surface potentials with or without TBAI doping, we conducted KPFM measurements measured the of the neat polymer films according to previous method. The 2D surface images together with the detailed analysis corresponding to each image are shown in Figure 4. Appraently, the surface potentials of neat PBDB-T film are found to be similar with or without TABI doping. In contrast, the surface potentials of neat N2200 film processed with TBAI were found to be 150 mV lower than those of the film without doping. Thus, the addition of TBAI mainly lower the surface potential of the electron acceptor

Figure 5. (a) Hole mobilities (μh) of PBDB-T with or without addition of TBAI, electron mobilities (μe) of N2200 with or without addition of TBAI. (b) Hole mobilities (μh) and electron mobilities (μe) of PBDB-T:N2200 blend with or without addition of TBAI. E


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Figure 6. Jsc (a) and Voc (b) as a function of light intensity, (c) Nyquist plots of impedance spectra measured at the Voc condition under 100 mW/ cm2 irradiation, and (d) Mott−Schottky plot of capacitance versus voltage measured in the dark for PBDB-T:N2200 solar cells with or without addition of TBAI.

undoped device (1.65 kT/q), suggesting that the n-type dopant could effectively reduce the monomolecular SRH recombination and accordingly result in the enhancement of PBDBT:N2200 based all-PSCs. To further confirm this phenomenon, we investigated the device equivalent series resistance (R) and the background charge density (p0) of PBDB-T:N2200 all-PSCs. Figure 6c shows Nyquist plots of the impedance spectra for PBDBT:N2200 all-polymer solar cell with or without doping under light irradiation at 100 mW cm−2 and an external bias voltage that equals Voc for frequencies from 500 Hz to 1 MHz. The data was well-fitted by the equivalent circuit model comprising a double RC in parallel connection (inserted in Figure 6c). From the intercept (left part) on Z axis at high frequencies in Nyquist plots,28 we could obtain R. After doping treatment, the R is reduced from 29.49 to 15.24 Ω in PBDB-T:N2200 allpolymer solar cells. Therefore, the lower R of doped devices indeed helps reducing the current and FF losses across the active films. Figure 6d indicates the dependence of dark capacitance on voltage (C−V) of the photovoltaic devices in the dark. All the measured devices have a similar active layer thickness of ∼100 nm. After adding TBAI, the capacitances of the devices become higher. This is a sign that the dopant has been ionized to decrease depletion width of the active layer as a result of excess background charges (p0) induced by doping.28 The magnitude of p0 is 1.08 × 1018 cm−3 in PBDB-T:N2200 without TBAI ascertained by Mott−Schottly analysis,47 and it rises to 4.25 × 1019 cm−3 with a 0.04 wt % TBAI doping. It is evident that the improvement of FF is the most apparent effect induced by doping. We need to further understand the mechanism for the FF change. According to previous study in polymer−fullerene BHJ solar cells,29 FF is an important device parameters, which is decided by lots of factors, including device series resistance (Rs), shunt resistance (Rsh), as well as device

structure of ITO/PEDOT:PSS (40 nm)/blend (120 nm)/ MoO3 (9 nm)/Ag (100 nm) and ITO/ZnO (40 nm)/blend (120 nm)/LiF (0.6 nm)/Al (100 nm),41−44 respectively. From by the J−V curves (Figure S5), the electron mobilities drastically enhanced from 6.8 × 10−4 to 2.5 × 10−3 cm2 V−1 s−1 and 3.5 × 10−3 cm2 V−1 s−1 with a doping concentration of 0.04 and 0.1 wt %, respectively. Under the same conditions, the increase of hole mobilities in either pristine PBDB-T or PBDBT:N2200 blend is less than the enhancement of the electron mobilities. It is worth noting that after doping with the optimal content (0.04 wt %), the charge transport becomes more balanced, which can reduce the space charge accumulation and consequent charge recombination.45,46 The light dependency of the Jsc and Voc for solar cell devices with or without n-doping was measured to gain deeper insight into the kinetics of charge recombination. The Jsc measurements as a function of light intensity, should follow a relationship of J ∼ Iα, where α < 1.0 represents the level of bimolecular recombination.47 As shown in Figure 6a, the pristine PBDB-T:N2200 device yields α = 0.96 (approaching unity), which is close to that of the TBAI doped device (α = 0.97). These values indicate that the bimolecular recombination may not be significantly affected by the TBAI doping. As shown in Figure 6b, the measurements of Voc as a function of light intensity was also recorded to analyze if trap-assisted recombination may occupy an important position. In the ideal case, the slope of the Voc versus light intensity should be equal to kT/q (where q represents the elementary charge and k represents the Boltzmann constant) if bimolecular recombination is dominant. If the Voc relies strongly on light intensity, then the slope would be greater than kT/q as implied by the extra interfacial trap-assisted Shockley−Read−Hall (SRH) recombination theory.48,49 We observe that the slope for the TBAI doped device (1.50 kT/q) is smaller than that of the F


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respectively, n is the diode ideality factor. Thus, FF becomes a function of n, rs, and rsh. By using n obtained in eq 1, we can obtain FF0 = 67.4 and 74.7% for the undoped and 0.04 wt % TBAI doped device, respectively. Meanwhile, the values of rs and rsh are shown in Table 1; we further evaluated the effect of rs and rsh on the FF of solar cell devices. After calculation, we reveal that the variation of the rs and rsh can only lead to a slight change of below 2% in FF. Thus, the improvement of n by doping is the critical factor leading to the increased FF in our work.

diode performance (see Figure S6). In addition, the values of Rs and Rsh can be extracted from the slope of device J−V curve at the Voc and Jsc, respectively, and the device diode performance is dependent on the ideality factor n.50 Thus, we need to calculate ideality factor n for these devices. On the basis of the widely used equivalent circuit model in Figure S6, the current density and voltage (J−V) can be represented by the relationship mathematically: ⎡⎛ e(V − JR ) ⎞ ⎤ V − JR s s J = J0 exp⎢⎜ − Jph ⎟ − 1⎥ + ⎢⎣⎝ nkBT ⎠ R sh ⎦⎥

(1)

4. CONCLUSIONS In conclusion, the effect of n-type molecular doping with TBAX (TBAF, TBACl, TBABr, and TBAI) on the performance of allpolymer solar cells has been studied. Under the condition of relatively low concentration n-doping (0.04 wt %), significantly increased FF and slightly increased Jsc are observed in all cases, resulting in an enhancement of PCEs from 5.8 to over 7.0%. We find that the addition of TBAI can reduce the work function of the polymer acceptor, while the morphological characterizations reveal that small amount of n-dopants do not alter active layer morphology. On the basis of the charge transport and recombination investigations, we attribute the improvement primarily to the improved electron mobility and reduced trap sites in the polymer film. The n-type molecular doping can efficiently suppress charge recombination, resulting in improved diode ideality factor, FF, and Jsc. We propose that the n-dopant like TBAI, which includes Lewis base anions I−, can dope the n-type polymer with extended π-conjugated by anion assisted electron transfer,33 a possible explanation why TBAI outputs the best performance may be that the halogen atom bears a different partial negative charge and polymer acceptor is electron-deficient, which should serve as the reason for their relatively different electron transfer between each other. These findings may become important for the further application of n-type doping in all-PSCs as an efficient and facile method to improve the device performance.

where T represents temperature, Rsh represents the shunt resistance, Rs represents the series resistance, e represents the elementary charge, J0 represents the reverse bias saturation current density, Jph represents the photo current, n represents the diode ideality factor, and kB represents Boltzmann’s constant. For ideal diode, n is equal to 1. When n > 1, it suggests SRH recombination due to defect states at the p−n junction.51−53 As shown in Figure 7, Region I (low voltages),

Figure 7. Current density versus voltage characteristics PBDBT:N2200 solar cells with or without addition of TBAI under 100 mW/cm2 irradiation (bottom) and dark current density versus voltage of according devices (up).

■

S Supporting Information *

Rsh will primarily determine the J−V characteristics; Region II (Intermediate voltages) will determine the diode parameters J0 and n. Region III (high voltages) will determine Rs.53 We then did the curve fitting on the current in Region II. For the undoped device, we obtained n = 4.12; for 0.04 wt % TBAI doped device, we achieved n = 2.49. Therefore, by using eq 1, we can clearly observe the effect of doping on device ideality factor n. Then we need to understand the correlation between n and FF. FF can be obtained with the following expression:29,54 ⎛ v + 0.7 FFs ⎞ FF = FFs⎜1 − oc ⎟ voc rsh ⎠ ⎝

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15000. Absorption spectra, AFM height images and line-cuts of 2d-GIWAXS patterns of PBDB-T:N2200 film cast with or without dopants, device optimization, device performance with various TBAX (X = F, Cl, Br, and I) content, scheme of equivalent circuit of OPVs, SCLC mobility measurements (PDF)

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(2)

AUTHOR INFORMATION

Corresponding Authors

while

*E-mail: [email protected]. *E-mail: [email protected].

r2 FFs = FF0(1 − 1.1rs) + s 5.4

(3)

v − ln(voc + 0.72) FF0 = oc voc + 1

(4)

ORCID

Jianyu Yuan: 0000-0002-5131-1285 Xufeng Ling: 0000-0003-3472-6224 Guobing Zhang: 0000-0001-6053-2015 Wanli Ma: 0000-0002-2001-3234

in which Voc divided by nkBT/q expresses voc, which is the normalized Voc, the normalized resistances rs and rsh are represent the resistances Rs and Rsh divided by Voc/Jsc,

Notes

The authors declare no competing financial interest. G


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ACKNOWLEDGMENTS This work was supported by the National Key Research Projects (Grant No. 2016YFA0202402), the Natural Science Foundation of Jiangsu Province of China (BK20170337), the National Natural Science Foundation of China (Grant No. 51761145013 and 61674111), and “111” projects. We thank the Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University. We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). We thank Pohang Accelerator Laboratory (9A beamlines) for their helpful GIWAXS measurements.

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