- Email: [email protected]
High-performance all-polymer nonfullerene solar cells by employing an efficient polymer-small molecule acceptor alloy strategy
Nano Energy 36 (2017) 356–365
Contents lists available at ScienceDirect
Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Full paper
High-performance all-polymer nonfullerene solar cells by employing an efficient polymer-small molecule acceptor alloy strategy
MARK
⁎
Guanqun Dinga, Jianyu Yuana, , Feng Jinb, Yannan Zhanga, Lu Hana, Xufeng Linga, ⁎ Haibin Zhaob, Wanli Maa, a Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren’ai Road, Suzhou 215123, Jiangsu, PR China b Shanghai Ultra-precision Optical Manufacturing Engineering Research Center, and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Optical Science and Engineering, Fudan University, Shanghai 200433, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: All-polymer solar cells Acceptor alloy Non-fullerene Morphology
In the current research field of non-fullerene solar cells, the device performance of all polymer solar cells (allPSCs) lags significantly behind those based on polymer donor and molecule acceptor. To further improve the device performance of all-PSCs, the small molecule electron acceptor ITIC was introduced for the first time to the all-PSCs based on PTP8/P(NDI2HD-T). The combinative polymer/molecule acceptor P(NDI2HD-T)/ITIC with only a small amount of ITIC, 15 wt% in acceptors and 6 wt% in total, can significantly improve the device PCE from 6% to over 7%, which is among the highest-efficiencies for reported all-PSCs. The improved device performance can be attributed to the broadened absorption and optimized blend morphology, which can further enhance charge carrier generation and balance charge carrier mobility in the ternary blend. More importantly, we discovered that this strategy can be successfully applied to all-PSCs based on other polymer donors like widely spread P3HT or PTB7. Therefore, our work not only reveals the detailed effect of polymer-small molecule acceptor alloy in all-PSCs, but also suggest that the ternary cell strategy of polymer donor/polymer:molecule acceptor may become a general and facile approach to further boost the performance of current all-PSCs.
1. Introduction
alternative to the fullerene system and power conversion efficiencies (PCE) now are approaching 12% [19–22]. Among them, all-polymer solar cells (all-PSCs), consisting of polymer-donor and polymeracceptor materials, which possess extraordinary mechanical properties over other non-fullerene counterparts, exhibiting great potential for the future flexible device engineering [23]. Compared to the efficient non-fullerene molecule acceptors, the development of non-fullerene polymer acceptors has lagged significantly behind in both synthesis and device performance [24–26]. Naphthalene diimide (NDI)-based copolymers, for example, Poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophe-ne)} [P(NDI2OD-T2); Polyera ActiveInkN2200] [27], is the most widely used polymer acceptor so far. These NDI analogues can simultaneously maintain highly ordered structure, excellent electron mobility and strong π-π intermolecular interaction [28,29]. However, only a few reported all-PSCs exhibited PCEs above 6% [30–35], with the very recent two ones reaching 7–9% [36–38]. Apparently, the typical values for Jsc and fill factor (FF) are relatively lower than their fullerene-containing counterparts. The lower
Due to the superior advantages like lightweight, semi-transparency, flexibility and low-temperature fabrication process [1,2], organic solar cells (OSCs) are currently viewed as promising power generation technologies that can be integrated into future flexible and wearable devices. To date, nearly all the highest efficient OSCs devices [3–7] utilize a hetero-junction consisting of either a conjugated polymer or molecule as the donor material and a fullerene derivative as the electron acceptor [8,9]. However, fullerenes are not ideal acceptor materials due to many intrinsic issues, such as weak light absorption, almost fixed chemical structure and energy levels [10,11], further limiting the open-circuit voltage (Voc) and short-circuit current density (Jsc) of these solar cell devices. Most importantly, fullerene-based OSCs have low flexibility and stretchability due to the brittle crystalline features of the fullerenes [12,13]. Quite recently, solution-processed OSCs using non-fullerene electron acceptors [14–16], which possess advantages including tunable chemical and electronic properties as well as enhanced stabilities [17,18], have been proven to be a viable
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Yuan), [email protected] (W. Ma).
http://dx.doi.org/10.1016/j.nanoen.2017.04.061 Received 6 April 2017; Received in revised form 28 April 2017; Accepted 30 April 2017 Available online 01 May 2017 2211-2855/ © 2017 Elsevier Ltd. All rights reserved.
Nano Energy 36 (2017) 356–365
G. Ding et al.
ITIC can form a type II energy level alignment, while P(NDI2HD-T) and ITIC form a type I structure. The lowest unoccupied molecular orbital (LUMO) energy level of P(NDI2HD-T) and ITIC is at around −3.9 eV, which provides a ~0.3 eV offset from the LUMO of PTP8 for electron transfer from donor to acceptor. In contrast, the hole transfer from PTP8 to P(NDI2HD-T) should be more efficient than that to ITIC due to the larger offset between the highest occupied molecular orbital (HOMO) of donor and acceptor. The film absorption spectra of the adopted materials are shown in Fig. 1(d); the donor polymer PTP8 exhibits a wide band-gap of 1.80 eV. Compared to the fullerene acceptors, the absorption of P(NDI2HD-T) is greatly improved, which, however, is overlapped with the absorption of PTP8. Herein, the competitive absorption in the PTP8/P(NDI2HD-T) all-polymer blend is not desired to realize the merits of non-fullerene solar cells. Extending the absorption into the red and near-infrared regions of the solar spectrum should be ideal to maximize the device photocurrent. Fortunately, the popular small molecule acceptor, ITIC with a high absorption coefficient in the range from 700 nm to 800 nm, can be used as a solid additive in the PTP8/P(NDI2HD-T) binary system to extend the absorption range without increase the active film thickness.
device performance of all-PSCs is considered to be the large polymer/ polymer phase separation, relatively low electron mobility of polymer acceptors and the inefficient charge dissociation at the donor/acceptor (D/A) interface [36,37,39]. In recent years, one of the promising strategies for utilizing full potential of photons in polymer/fullerene BHJ solar cells is to introduce a third component to the host binary D/A blend [40–43]. This is important for obtaining improved device performance with similar film thickness. It has been generally pointed out that the ternary organic solar cells can be divided into two types according to the content of the third component. One is called the “parallel” model proposed firstly by You et. Al [44], in which the two donors or acceptors co-exist at a ratio around 1/1. The other one is called “cascade” or “alloy” model, in which the weight ratio of the third component content was typical at lower value around 10–15%. In addition, the “cascade” [45,46] and “alloy” [47] type, were divided according to the energy level position of the third component. The introduction of the third component as a cascade material leads to improved short circuit current density (Jsc) and PCEs due to the complementary absorption. The “alloy” model ternary solar cells were proposed quite recently by Wei et al. [47], in which the third component (normally conjugated molecules) can form binary alloy for better morphology and exciton dissociation. The ternary solar cells of either “parallel” or “solid additive” model have been demonstrated in multiple cases in polymer/fullerene and polymer/molecule systems [48–50]. However, in all-PSCs based on polymer acceptors, the ternary solar cells have rarely been reported so far and the resultant PCEs are relatively low. In these very limited cases of all-PSC ternary cells, only polymer acceptors as additives were investigated. To the best of our knowledge, the addition of small molecule acceptor as “solid additive” in efficient all-PSCs has not been reported. The interesting interactions between the molecule acceptor and the polymer materials remain unknown and the consequent effect of combinative polymer/molecule acceptor on the properties and performance of the all-PSCs requires in-depth study. In this contribution, inspired by the successful results in ternary polymer/fullerene and polymer/molecule solar cells, we report for the first time the introduction of molecule acceptor as additive in ternary all-PSCs, which is designed by incorporating highly efficient small molecule acceptor ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno-[2,3d:2’,3’-d’]-s-indaceno[1,2-b:5,6b’]-dithiophen-e) [14] into a host binary all-polymer blend system PTP8/ P(NDI2OD-T). PTP8 [51] was reported as an efficient donor for all-PSCs in our previous work, with a PCE of 4.3% when using together with the popular polymer acceptor P(NDI2OD-T2) [52]. And the PCE can be improved to 6% by using acceptor P(NDI2HD-T) with more matched crystallinity. After the addition of ITIC (6 wt% in total), the PCE of the ternary all polymer system was further increased to over 7%, which is among the highest efficiencies in all-PSCs. However, adding more ITIC will actually reduce the performance of our all-PSCs. We show that optimal amount of ITIC as additive can significantly increase light absorption, optimize blend morphology, enhance charge carrier generation and balance charge carrier transport in the binary blend. More importantly, we discovered that this strategy can be successfully applied to all-PSCs systems based on other polymer donors like widely spread P3HT or PTB7. Therefore, our work not only reveals the detailed effect of small molecule “solid additive” in all-PSCs, but also suggest that the ternary cell strategy may become a general and facile approach to further boost the performance of current all polymer solar cell.
2.2. All-polymer solar cells performance The best solar cell devices were achieved using the conventional architecture of ITO/PEDOT: PSS (40 nm)/Active layer (~120 nm)/LiF (0.6 nm)/Al (80 nm). As show in Table S1 and Table S2, we at first optimized PTP8/P(NDI2HD-T) and PTP8/ITIC binary system independently. Device optimization included adjustments of solvents additives, donor-acceptor blend ratio and active layer thickness. Both PTP8/P(NDI2HD-T) and PTP8/ITIC exhibit quite similar optimal fabrication conditions, which makes it easier to manipulate the corresponding ternary solar cells. The binary all-PSCs based on PTP8/P(NDI2HD-T) blend exhibit the best PCE of 6.00%, with a Voc of 0.976 V, a Jsc of 10.71 mA/cm2 and a FF of 0.57. Meanwhile, the PTP8/ITIC binary system delivers an optimal PCE of 5.11%, with a slight higher Voc of 1.003 V, a Jsc of 8.76 mA/cm2 and a FF of 0.58. Both systems have an optimal blend ratio of 1.5:1. Note that even with complementary absorption between the donor and acceptor, the PTP8/ ITIC system still achieves a lower photocurrent than that of PTP8/ P(NDI2HD-T). Meanwhile, the P(NDI2HD-T)/ITIC binary system was also fabricated to reveal the dynamic process between the two acceptors P(NDI2HD-T) and ITIC, with a Jsc of 0.026 mA/cm2, a Voc of 0.513 V, a FF of 0.29 and PCE of 0.004% (Fig. S2), which indicates very poor charge transfer at the interface of these two materials [53]. As shown in Fig. 2 and Table 1, we systematically investigated the ternary solar cells PTP8/P(NDI2HD-T)/ITIC with a wide range of blend ratios: 1.5/0.9/0.1, 1.5/0.85/0.15, 1.5/0.8/0.2, 1.5/0.7/0.3, 1.5/ 0.5/0.5 and 1.5/0.3/0.7. Note that the blend ratio between the donor PTP8 and combinative polymer/molecule acceptors P(NDI2HD-T)/ ITIC is fixed to 1.5:1. The amount of the ITIC is varied to invest its effect on the device performance. It is clear that the PCE increases with the addition of acceptor ITIC. At the amount of 15 wt% (1.5/0.85/ 0.15), calculated only based on the total content of acceptors, the PCE of the optimized ternary systems reaches 7.01% with a Voc of 0.976 V, a Jsc of 12.60 mA/cm2 and a FF of 0.57. According to the previous report [45–47], we hypostasized the enhancement after adding small amount of ITIC may attributed to the improved blend morphology, further strengthen the charge generation, separation and transportation. After that, the PCE gradually decreases with further increased amount of ITIC. The optimal PCE of this ternary system is ~20% higher than that of the PTP8/P(NDI2HD-T) binary system and 37% higher than that of the PTP8/ITIC system. The Voc of the ternary systems is around 0.976 V constantly, the same with that of PTP8/P(NDI2HD-T), indicating that acceptor P(NDI2HD-T) stills plays the most important role in exciton dissociation and suggesting the formation of “alloy” acceptors according to the previous work [18]. The FF of ternary
2. Results and discussion 2.1. Molecular structure and properties The chemical structures and energy levels of PTP8, P(NDI2HD-T) and ITIC are shown in Fig. 1. Both PTP8/P(NDI2HD-T) and PTP8/ 357
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 1. (a) Chemical structure of PTP8, P(NDI2HD-T) and ITIC; (b) device structure of devices; (c) energy level diagram and (d) normalized thin film UV–vis absorption spectra of donor and acceptor materials.
well agreement with the Jsc changes in ternary solar cell devices as shown in Fig. 3(a). It is very important to notice that fullerene-based organic ternary solar cells are highly empirical and system dependent. A more general approach to construct efficient nonfullerene ternary cells should be of high significance. Thus we further tried our approach on other two allpolymer systems adopting classical polymer P3HT or high-efficiency PTB7 as the donor and P(NDI2HD-T) as the acceptor. As shown in Fig. S5 and Table S3, by adding 15 wt% ITIC in both P3HT/P(NDI2HD-T) and PTB7/P(NDI2HD-T) all-polymer systems, the PCEs of the ternary solar cells exhibit 24% and 17% enhancement relative to the binary systems, respectively, which indicates the advantage and high compatibility of polymer/molecule combinative acceptors.
systems shows nearly no apparent changes, which may be due to the similar FF values for PTP8/P(NDI2HD-T) and PTP8/ITIC binary systems. However, we do observe slight drop of FF after the addition of more than 15 wt% ITIC in acceptors. In contrast, the Jsc makes the most contributions to the performance enhancement. We observe that the Jsc and PCE are highly correlated with the amount of ITIC in the combinative acceptors. When adding 15 wt% ITIC, the Jsc approaches 12.60 mA/cm2, nearly 20% higher compared with the Jsc of the PTP8/ P(NDI2HD-T) binary system, which may largely result from the enhanced light absorption. Nonetheless, further increasing the loading of ITIC to 20% and 30%, the Jsc gradually decreases to 10.27 mA/cm2. When the content of ITIC is over 50%, the Jsc decreases to a similar value of that for the PTP8/ITIC binary systems, indicating that ITIC starts to play the dominate roll in the acceptors. To explicate the effect of ITIC on the improved performance in ternary systems, UV–vis absorption spectra (Fig. 3(b)) and EQE spectra (Fig. 3(c)) were further measured (the detailed spectra are shown in Fig. S4). As shown in Fig. 3(b) and Fig. S4(a), the absorption of the ternary blend films is gradually enhanced in the range from 650 nm to 800 nm with increased ITIC amount. The change of blend absorption is directly reflected in the corresponding EQE spectra. The ternary cell with optimal ITIC shows red-shifted EQE spectrum compared to the control devices, attributed to the addition of ITIC. Meanwhile, the absorption of the ternary blend film slowly decreases in the range from 550 nm to 650 nm owing to the decreased P(NDI2HDT), while the EQE is actually increased in this region, which may be due to other factors like morphology. The ternary cell with 50 wt% ITIC in acceptors, however, has much lower EQE value in the visible region despite the increased response in the 700–800 nm region, likely due to the lower efficiency of PTP8/ITIC system as demonstrated by its much smaller EQE value and device current. Note that all these results are in
2.3. Charge generation, transport and recombination To understand the effect of added ITIC on the charge generation and extraction process, the dependence of the photocurrent density (Jph) on the effective voltage (Veff) were recorded under illumination of 100 mW/cm2 (Fig. 3(d)). Jph is equal to JL –JD, where JL and JD are the experimental current under illumination and in dark, respectively. Veff is equal to Vo-Va where Vo is the voltage when Jph is zero and Va is the applied voltage [54]. From Jph-Veff curves for the optimal ternary devices, we observe the Jph reaching a plateau at a low Veff around 0.5 V, suggesting that free charges are quickly swept out. In contrast, the curve for the 50 wt% ternary devices shows a smaller slope, indicating less efficient charge extraction. At sufficiently high electric fields, the Jph of PTP8/P(NDI2HD-T)/ITIC (with 15 wt% ITIC in acceptor) devices show notably higher value. For PTP8/ITIC and PTP8/P(NDI2HD-T)/ITIC (with 50 wt% ITIC in acceptor) devices, the photocurrent saturate at higher bias with lower values. The results 358
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 2. Photovoltaic parameters (Jsc, Voc, FF, and PCE) of PTP8/P(NDI2HD-T) binary, PTP8/ITIC binary and PTP8/P(NDI2HD-T)/ITIC ternary solar cells with different amounts of ITIC.
dence of Voc on light intensity to examine if trap-assisted recombination may also significantly limit the Jsc. In Voc-light intensity measurements, the Voc should follow a power law relationship as Voc ~ sI, where s > 1.0 kT/q is indicative of significant trap-assisted recombination [56]. Quite similarly, the Voc-light intensity measurements exhibit similar trend compared to that of Jsc-light intensity measurements, which confirms that increasing the ITIC content in the blend leads to increased recombination in solar cell devices. However, for the all-PSCs without ITIC or with low ITIC content exhibit serious monomolecular recombination and consequently lead to the restrained improvement of performance. In short, the charge recombination is closely related to the concentration of ITIC in these ternary blends. We speculate that the mismatched LUMO and HOMO levels between the two acceptors my hinder the transport of carrier between them. In addition, ITIC molecules may has less favorable morphology than P(NDI2HD-T) in this system, resulting in more recombination.. The charge carrier mobility is another important factor that affects solar cell device performance. Both the electron and hole mobilities are important because if they are too low or imbalanced, charge carriers
reveal that optimal amount of ITIC is beneficial for the charge generation process, partially due to the broadened absorption, while more ITIC will actually hinder the generation process in these ternary blends. The dependence of Jsc on light intensity was also measured to examine if non-geminate recombination may significantly limit the device Jsc. In Jsc-light intensity measurements, the Jsc should follow a power law relationship as J ~ I α where α < 1.0 is indicative of significant bimolecular recombination [55]. As shown in Fig. 4(a), for all the devices, the Jsc scales quite differently with light intensity (α=0.993 for 0% ITIC, α=0.964 for 15% ITIC, α=0.928 for 50% ITIC and α=0.892 for 100% ITIC), which indicates increased bimolecular recombination in the solar cell devices with higher ITIC loading.[30] In addition, these values indicate that the charge collection efficiency is weakly dependent on the light intensity at low ITIC loading. This further implies that the charge carrier losses at low ITIC loading are dominated by monomolecular recombination captured in the defect sites, while bimolecular exciton annihilation may cause only rather minor losses. As shown in Fig. 4(b), we further measure the depen-
Table 1 Photovoltaic parameters of the PTP8/P(NDI2HD-T), PTP8/ ITIC binary systems and PTP8/P(NDI2HD-T)/ITIC ternary systems with different ratios of P(NDI2HD-T) and ITIC. PTP8/P (NDI2HD-T)/ITIC
Voc (V)
Jsc (mA/cm2)
FF
PCE (Avg)a (%)
μe (10−4cmV−1s−2)
μh (10−4cmV−1s−2)
1.5/1.0/0.0 1.5/0.9/0.1 1.5/0.85/0.15 1.5/0.8/0.2 1.5/0.7/0.3 1.5/0.5/0.5 1.5/0.3/0.7 1.5/0.0/1.0
0.976 0.976 0.976 0.976 0.976 0.976 0.976 1.003
10.71 11.83 12.60 11.61 10.47 9.23 9.24 8.76
0.57 0.56 0.57 0.56 0.56 0.57 0.54 0.58
6.00 6.47 7.01 6.35 5.72 5.14 4.87 5.10
3.69 2.99 1.85 1.83 2.65 2.95 3.38 3.78
1.04 1.25 1.95 2.49 3.34 3.79 5.53 9.47
a
(5.79) (6.25) (6.82) (6.09) (5.54) (5.03) (4.77) (5.01)
The values are the photovoltaic parameters for the optimal devices. The average values are calculated over 10 devices.
359
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 3. (a) J-V curves of PTP8/P(NDI2HD-T), PTP8/ITIC and PTP8/P(NDI2HD-T) /ITIC with 15 wt% ITIC and 50 wt% ITIC blend systems; b) UV–vis absorption spectra of corresponding systems; (c) EQEs of corresponding systems and d) Photocurrent versus effective voltage of corresponding systems.
MEH-PPV/PCBM blend film, the addition of small molecule improved the formation of donor polymer fiber structure, further leading to enhanced hole mobility [61,62], which may interpret the improved hole mobility in our ternary devices. Meanwhile, the µe decreases from 3.69×10−4 cm2 V−1s−1 to 1.83×10−4 cm2 V−1 s−1 at first with the amount of ITIC increased from 0 wt% to 20 wt% in acceptors, and then gradually goes up to 3.78×10−4 cm2 V−1 s−1 with the amount of ITIC further increased to 100 wt% in acceptors. The initially decreased electron mobility may be attributed to the trap effect of isolated ITIC clusters in P(NDI2HD-T) matrix. With the addition of more ITIC,
may recombine before they are collected at the electrodes [30–35,57]. To gauge the charge transport properties of these all-polymer blends, hole only and electron-only diodes were fabricated and measured (see Supporting Information) [58]. Fig. 5 and Table 1 show the hole mobility (µh) and electron mobility (µe) along with the ratio (in weight) changes of two acceptors, P(NDI2HD-T) and ITIC. Both µh and µe were measured by the space-charge-limited current (SCLC) method with the Mott-Gurney equation [59,60]. The µh increases from 1.04×10−4 cm2 V−1 s−1 to 2.71×10−3 cm2 V−1 s−1 by varying the amount of ITIC in the blends. According to the early investigation of neat MEH-PPV film and
Fig. 4. (a) Jsc and (b) Voc dependence on illumination intensity for PTP8/P(NDI2HD-T), PTP8/ITIC and PTP8/P(NDI2HD-T)/ITIC with 15 wt% ITIC and 50 wt% ITIC blend systems in symbols, together with linear fits in lines.
360
Nano Energy 36 (2017) 356–365
G. Ding et al.
of low ITIC content. However, the RMS value for film with higher ITIC amount (over 30 wt%) is drastically increased. We hypothesize that with high ITIC loading the highly ordered packing and crystallinity of ITIC molecules may result in large ITIC domains, leading to enhanced polymer-molecule de-mixing and a rougher surface, which may result in inefficient exciton dissociation and charge transport. In order to confirm the results, we also investigated the film morphology by using transmission electron microscopy (TEM), as shown in Fig. 7. For PTP8/P(NDI2HD-T) all-polymer blend film, we observe the “river” like morphology, indicating the phase separation of the donor acceptor polymers. According to the previous study by Marks et al. [70], the NDI-based polymers acceptor have a strong tendency to aggregate in the all-polymer BHJ blend. After the incorporation of ITIC (15 wt%) into the blend film, we observe the film phase separation is suppressed, suggesting that the ITIC can help reducing the aggregation of the polymer materials and resulting in slightly better intermixing and finer phase-separated domains. Further increasing ITIC (50%) in the ternary blend film, the D/A phase separation is slightly enhanced, likely due to the ITIC aggregation. These observations are consistent with the AFM results. For 100% ITIC acceptor without P(NDI2HD-T), the PTP8/ITIC film morphology appears differently. The large amount of ITIC in the blend film should result in strong aggregation due to its large tendency to crystallization. We observe that the black “river” like domains are much less distinct, while more structures appear in the white domains, indicating enhanced aggregation as expected. Overall, the ternary blend film with optimal ITIC content (15 wt%) exhibits optimal phase separation extent and thus delivers superior solar cell performance. Finally, grazing incidence wide-angle X-ray scattering (GIWAXS) is used to probe the molecular stacking in the neat and blend films [71]. To quantify the scattering data, the corresponding out-of-plane and inplane cuts are given in Fig. 8. A broad peak at q =~1.7 Å−1 and q =~1.6 Å−1 arises in the out-of-plane direction in neat PTP8 and P(NDI2HD-T) films respectively, which corresponds to the π-π stacking peak. This result indicates that both PTP8 and P(NDI2HD-T) have a preferred face-on orientation with respect to substrate. In contrast, the 2D GIWAXS pattern of ITIC thin film exhibits a high degree of molecular ordering in both in plane and out-of-plane direction, as evidenced by the strong lamellar (100), (200), (300) and π-π (010) diffraction peaks. Compared with the neat film, the intensity of the π-π stacking peak (010) of PTP8 in the ternary blend film becomes weaker with the addition of acceptors (see Fig. 8), which suggests that the acceptor ITIC suppressed the crystallization of PTP8 after mixing,
Fig. 5. The hole mobility (µh) and electron mobility (µe) of PTP8/P(NDI2HD-T), PTP8/ ITIC binary systems and PTP8/P(NDI2HD-T)/ITIC ternary systems with different amount of ITIC.
percolation pathway can forms and increased electron mobility can be achieved. When the amount of ITIC in acceptors is at 15 wt%, we found the blend exhibits the most balanced hole mobility (µh=1.95×10 −4 cm2 V−1 s−1) and electron mobility (µe=1.85×10 −4 cm2 V−1 s−1), with an ideal ratio (µh/µe) value of 1.05. This balanced carrier transport may be one of the important reasons for the optimal device performance at this ratio. 2.4. All-polymer blend morphology Lots of previous work [63–65] have demonstrated that the exciton dissociation and charge transport process are strongly affected by the polymer/PCBM blend morphology. Therefore, a better understanding of the key factors driving blend morphology is crucial to further improve the morphology of all-polymer solar cells. Herein, the morphology of optimal all-polymer and ternary blend films were examined by using atomic force microscopy (AFM) (Fig. 6(a–h)). By incorporating ITIC into the host all-polymer system, the high crystallinity of rigid conjugated molecule ITIC should prevent the entanglement and aggregation of donor and acceptor polymers [66–69]. Therefore, when the content of ITIC is lower than 20 wt%, the ternary blend films display smooth surface similar to that of PTP8/P(NDI2HDT) binary blend film. In fact, the surface roughness (RMS) value is even slightly lower, indicating reduced polymer domain sizes by the addition
Fig. 6. AFM height image for blend films with 0 wt% (a); 15 wt% (b); 50 wt% (c) and 100 wt% ITIC (d) in acceptors.
361
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 7. TEM for blend films with (a) 0 wt%; (b) 15 wt%; (c) 50 wt% and (d) 100 wt% ITIC in acceptors.
excitation in these films. The long-lived components which centered at 900 nm in blend films is attributed to the absorption of hole polarons residing in PTP8. As shown in Fig. S9, the lifetime of excitons in P(NDI2HD-T) and ITIC films is merely about tens of picosecond, which is much faster than that in neat PTP8 and the blend films. Fig. 9(b) displays the comparison of normalized ΔT/T dynamics at 900 nm for these binary and ternary blend films. We also note that the decay becomes slower and the lifetime grows longer when decreasing the content of ITIC in the blend. Considering the evidence of charge transfer (CT) at the interface of PTP8-ITIC binary blend is not distinct, the combination of ITIC in this work does not effectively facilitate interfacial CT between donor and acceptor materials. In addition, the dynamics at 900 nm in PTP8, ITIC and PTP8/ITIC, as shown in Fig. S9, also supports the inefficient interfacial CT between PTP8 and ITIC. Compared to the recently reported high-efficiency non-fullerene solar cells composed of a wide gap polymer donor and ITIC acceptor [14– 16], our PTP8/ITIC system exhibits relatively smaller HOMO-HOMO offset, which may limit the hole transfer from acceptor to the donor. Thus, the enhanced PCE of ternary solar cell is more likely due to the broadened absorption and improved morphology, rather than “cascade” model for efficient charge transfer.
3. Conclusion
Fig. 8. In-plane and out-of-plane line cuts from the 2D GIWAXS patterns of neat (a) PTP8, (b) P(NDI2HD-T), (c) ITIC thin films cast from chloroform and (d) PTP8/ P(NDI2HD-T)/ITIC (1.5/0.85/0.15) ternary blend film cast from optimal conditions.
The small molecule electron acceptor (ITIC) was introduced for the first time to the all polymer solar cells based on PTP8/P(NDI2HD-T), aiming to further tune the device photovoltaic properties. The combinative polymer/molecule acceptor P(NDI2HD-T)/ITIC with only a small amount of ITIC, 15 wt% in acceptors and 6 wt% in total, can significantly improve the device PCE from 6% to over 7%, which is among the highest-efficiencies for reported all polymer solar cells. The improved device performance can be attributed to the broadened absorption and optimized blend morphology, which can further enhance charge carrier generation and balance charge carrier mobility in the ternary blend. More importantly, we discovered that this strategy can be successfully applied to all polymer solar cells based on other polymer donors like widely spread P3HT or PTB7. Therefore, our work not only reveals the detailed effect of small molecule “solid additive” in all-PSCs, but also suggest that the ternary cell strategy of polymer donor/polymer: molecule acceptor is a general and facile approach to further boost the performance of current all polymer solar cells.
consistent with the TEM results. In contrast, the π-π stacking peak (010) of P(NDI2HD-T) are almost absent in the ternary blend while a weak (200) peak can still be observed. For ITIC in the blend film, its strong π-π stacking (010), lamellar (100) and (200) diffraction peaks totally disappear at the optimal weight ratio of ~15%, which indicates that ITIC may be well mixed into the polymer matrix. According to the latest report by Wei et al. [48], our observations may indicate the formation of “alloy” between the polymers and the small amount of molecular acceptor ITIC.
2.5. Transient absorption spectroscopy measurements We utilized the ultrafast Transient Absorption (TA) spectroscopy to characterize charge transfer dynamics in ternary blend films composed of PTP8, P(NDI2HD-T) and ITIC [72]. Fig. 9(a) shows the measured ΔT/T spectra, which was probed at 830 ps after the pump pulse 362
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 9. (a) Differential transmission spectra of blend films with 0 wt%, 15 wt%, 50 wt% and 100 wt% ITIC probed at 830 ps after the pump pulse excitation; (b) The decay dynamics of transient absorption spectra which pumped at 400 nm and probed at 900 nm in blend films with 0 wt%, 15 wt%, 50 wt% and 100% ITIC. The measurement was carried out at pump fluence of ~70 μJ/cm2.
scopy, the laser source was a Ti: Sapphire amplifier laser whose center wavelength is 800 nm, the pulse width and the repetition rate are 120 fs and 1 kHz respectively. The pump pulses used in our measurement is 400 nm, which were obtained through doubling the amplifier's fundamental output with a beta barium borate (BBO) crystal. The pump beam was modulated by a mechanical chopper at the frequency of ~325 Hz and focused on the sample with a spot diameter of ~3 mm. We focused the fundamental 800 nm light onto a sapphire plate to generate the supercontinuum white light, which passed though the different color filters with a 10 nm bandwidth, serving as the probe pulses in the range of 500–1000 nm. The pump and probe beam were spatially overlapped on the sample, and the probe light has much smaller energy density than the pump beam. The transmitted probe beam was detected by a silicon photodiode connected to a lock-in amplifier.
3.1. Experimental section 3.1.1. Materials PTP8 and P(NDI2HD-T) was synthesized following the methods in our previous work [34,48], respectively. ITIC were purchased from Solarmer Materials Inc. Unless otherwise stated, all chemicals were commonly commercially available products and used as received. 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were performed at the Advanced Light Source (ALS)-Lawrence Berkeley National Laboratory on Beamline 7.3.3. 3.1.2. Characterization UPS were carried out on a Kratos AXIS UltraDLD ultrahigh vacuum (UHV) surface analysis system under a base pressure of 5×10−10 Torr with an unfiltered Hel (21.21 eV) gas discharge lamp. UV–vis–NIR spectra were recorded on a Perkin Elmer model Lambda 750. Tappingmode AFM images were obtained with a Veeco Multimode V instrument. TEM images were performed on Tecnai G2 F20 S-Twin Transmission Electron Microscope.
Acknowledgments The author thanks Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University. GIWAXS measurements were taken at Advanced Light Source (ALS)-Lawrence Berkeley National Laboratory on Beamline 7.3.3. This work was supported by “111 project”, the National Key Research Projects (Grant No. 2016YFA0202402), the National Natural Science Foundation of China (Grant No. 61222401, No. 61674111). And we also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
3.1.3. Solar cells device fabrication and characterization The indium tin oxide (ITO) coated glass substrates were cleaned by ultrasonic in acetone, detergent, isopropyl alcohol and acetone sequentially, each for 20 min. The dried substrates were further treated with UV-ozone for 30 min to prove its work function. Next, PEDOT: PSS (Al 4083) was spin-coated at a rate of 4500 rpm for 40 s on the top of ITO substrates and then annealed at 140 °C for 10 min. The active layer materials, PTP8, P(NDI2HD-T) and ITIC were dissolved in chloroform with 0.5% DIO. The total concentration of blended solution was maintained at 10 mg/ml and weight ratio of donor material (PTP8) to acceptor material (P(NDI2HD-T) and ITIC) maintained 1.5:1. The weight ratios of ITIC in acceptors were 0 wt%, 10w%, 15 wt%, 20 wt%, 30 wt%, 50 wt%, 70 wt% and 100 w%. The blends were then spincoated on PEDOT: PSS coated ITO with a rate of 2000 rpm for 40 s in a nitrogen filled glove box. At last, 0.6 nm thick lithium fluoride (LiF) with 0.1 A/s and 80 nm thick aluminum (Al) electrode with 2 A/s were thermal evaporated at a pressure nearly 1.0×10−6 mbar upon the active layer through a shadow mask. The active area is 7.25 mm2. The performance of all cells were tested under AM 1.5 G light illumination with an intensity of 100 mW/cm2 (Newport, Class AAA solar simulator, 94023A-U). The EQE was determined using a certified IPCE equipment (Zolix Instruments, Inc, SolarCellScan100).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2017.04.061. References [1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. [2] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. [3] R.C. Coffin, J. Peet, J. Rogers, G.C. Bazan, Nat. Chem. 1 (2009) 657. [4] R. Po, C. Carbonera, A. Bernardi, N. Camaioni, Energy Environ. Sci. 4 (2011) 286. [5] Z.C. He, C.M. Z, S.J. Su, M. Xu, H.B. Wu, Y. Cao, Nat. Photonics 6 (2012) 591. [6] Z.C. He, B. Xiao, F. Liu, H.B. Wu, Y.L. Yang, S. Xiao, C. Wang, T.P. Russell, Y. Cao, Nat. Photonics 9 (2015) 175. [7] C. Liu, C. Yi, K. Wang, Y.L. Yang, R.S. Bhatta, M. Tsige, S.Y. Xiao, X. Gong, ACS Appl. Mater. Interfaces 7 (2015) 4933. [8] A. Mishra, P. Bauerle, Angew. Chem. Int. Ed. 51 (2012) 2020. [9] Y. Lin, Y. Li, X. Zhan, Chem. Soc. Rev. 41 (2012) 4245. [10] J.U. Lee, J.W. Jung, J.W. Jo, W.H. Jo, J. Mater. Chem. 22 (2012) 24265. [11] A. Eftaiha, J. Sun, I.G. Hill, G.C. Welch, J. Mater. Chem. A 2 (2014) 1201. [12] D.J. Lipomi, H. Chong, M. Vosgueritchian, J. Mei, Z. Bao, Sol. Energy Mater. Sol.
3.1.4. Transient Transmission Spectroscopy (TAS) measurements The TAS measurements were performed in an optical cryostat under vacuum at room temperature. For transient absorption spectro363
Nano Energy 36 (2017) 356–365
G. Ding et al. [59] [60] [61] [62]
Cells 107 (2012) 355. [13] S. Savagatrup, A.D. Printz, T.F. O’Connor, A.V. Zaretski, D. Rodriquez, E.J. Sawyer, K.M. Rajan, R.I. Acosta, S.E. Root, D.J. Lipomi, Energy Environ. Sci. 8 (2015) 55. [14] Y. Lin, J. Wang, Z.G. Zhang, H. Bai, Y. Li, D. Zhu, X. Zhan, Adv. Mater. 27 (2015) 1170. [15] W. Zhao, D. Qian, S. Zhang, S. Li, O. Inganas, F. Gao, J. Hou, Adv. Mater. 28 (2016) 4734. [16] L. Gao, Z.G. Zhang, H. Bin, L. Xue, Y. Yang, C. Wang, F. Liu, T.P. Russell, Y. Li, Adv. Mater. 28 (2016) 8288. [17] W.R. Mateker, J.D. Douglas, C. Cabanetos, I.T. Sachs-Quintana, J.A. Bartelt, E.T. Hoke, A. El Labban, P.M. Beaujuge, J.M.J. Fréchet, M.D. McGehee, Energy Environ. Sci. 6 (2013) 2529. [18] J. Kong, S. Song, M. Yoo, G.Y. Lee, O. Kwon, J.K. Park, H. Back, G. Kim, S.H. Lee, H. Suh, K. Lee, Nat. Commun. 5 (2014) 5688. [19] L. Gao, Z.G. Zhang, H. Bin, L. Xue, Y. Yang, C. Wang, F. Liu, T.P. Russell, Y. Li, Adv. Mater. 28 (2016) 8288. [20] H. Yao, Y. Chen, Y. Qin, R. Yu, Y. Cui, B. Yang, S. Li, K. Zhang, J. Hou, Adv. Mater. 28 (2016) 8283. [21] W. Zhao, D. Qian, S. Zhang, S. Li, O. Inganas, F. Gao, J. Hou, Adv. Mater. 28 (2016) 4734. [22] B. Fan, K. Zhang, X. Jiang, L. Ying, F. Huang, Y. Cao, Adv. Mater. (2017). http:// dx.doi.org/10.1002/adma.201606396. [23] S. Wu, Polymer 28 (1987) 1144. [24] X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen, S.R. Marder, J. Am. Chem. Soc. 129 (2007) 7246. [25] D. Mori, H. Benten, I. Okada, H. Ohkita, S. Ito, Energy Environ. Sci. 7 (2014) 2939. [26] C. Lee, H. Kang, W. Lee, T. Kim, K.H. Kim, H.Y. Woo, C. Wang, B.J. Kim, Adv. Mater. 27 (2015) 2466. [27] H. Yan, Z. Chen, Y. Zheng, C. Newman, J.R. Quinn, F. Dotz, M. Kastler, A. Facchetti, Nature 457 (2009) 679. [28] R. Steyrleuthner, M. Schubert, F. Jaiser, J.C. Blakesley, Z. Chen, A. Facchetti, D. Neher, Adv. Mater. 22 (2010) 2799. [29] T. Earmme, Y.J. Hwang, S. Subramaniyan, S.A. Jenekhe, Adv. Mater. 26 (2014) 6080. [30] L. Ye, X. Jiao, M. Zhou, S. Zhang, H. Yao, W. Zhao, A. Xia, H. Ade, J. Hou, Adv. Mater. 27 (2015) 6046. [31] W. Lee, C. Lee, H. Yu, D.J. Kim, C. Wang, H.Y. Woo, J.H. Oh, B.J. Kim, Adv. Funct. Mater. 26 (2016) 1543. [32] Y.J. Hwang, T. Earmme, B.A. Courtright, F.N. Eberle, S.A. Jenekhe, J. Am. Chem. Soc. 137 (2015) 4424. [33] J.W. Jung, J.W. Jo, C.C. Chueh, F. Liu, W.H. Jo, T.P. Russell, A.K. Jen, Adv. Mater. 27 (2015) 3310. [34] T. Kim, J.H. Kim, T.E. Kang, C. Lee, H. Kang, M. Shin, C. Wang, B. Ma, U. Jeong, T.S. Kim, B.J. Kim, Nat. Commun. 6 (2015) 8547. [35] S. Shi, J. Yuan, G. Ding, M. Ford, K. Lu, G. Shi, J. Sun, X. Ling, Y. Li, W. Ma, Adv. Funct. Mater. 26 (2016) 5669. [36] Y.J. Hwang, B.A. Courtright, A.S. Ferreira, S.H. Tolbert, S.A. Jenekhe, Adv. Mater. 27 (2015) 4578. [37] L. Gao, Z.G. Zhang, L. Xue, J. Min, J. Zhang, Z. Wei, Y. Li, Adv. Mater. 28 (2015) 1884. [38] B. Fan, Lei Ying, Z. Wang, B. He, X. Jiang, F. Huang, Y. Cao, Energy Environ. Sci. (2017). http://dx.doi.org/10.1039/C7EE00619E. [39] J. Yuan, W. Guo, Y. Xia, M.J. Ford, F. Jin, D. Liu, H. Zhao, O. Inganäsc, G.C. Bazand, Wanli Ma, Nano Energy 35 (2017) 251. [40] L. Lu, M.A. Kelly, W. You, L. Yu, Nat. Photonics 9 (2015) 491. [41] Y. Yang, W. Chen, L. Dou, W.H. Chang, H.S. Duan, B. Bob, G. Li, Y. Yang, Nat. Photonics 9 (2015) 190. [42] V. Gupta, V. Bharti, M. Kumar, S. Chand, A.J. Heeger, Adv. Mater. 27 (2015) 4398. [43] G. Zhang, K. Zhang, Q. Yin, X. Jiang, Z. Wang, J. Xin, W. Ma, H. Yan, F. Huang, Y. Cao, J. Am. Chem. Soc. 139 (2017) 2387. [44] L. Yang, H. Zhou, S.C. Price, W. You, J. Am. Chem. Soc. 134 (2012) 5432. [45] L.Y. Lu, T. Xu, W. Chen, E.S. Landry, L.P. Yu, Nat. Photonics 8 (2014) 716. [46] P. Cheng, Y.F. Li, X.W. Zhan, Energy Environ. Sci. 7 (2014) 2005. [47] J. Zhang, Y. Zhang, J. Fang, K. Lu, Z. Wang, W. Ma, Z. Wei, J. Am. Chem. Soc. 137 (2015) 8176. [48] H. Lu, J.C. Zhang, J.Y. Chen, Q. Liu, X. Gong, S.Y. Feng, X.J. Xu, W. Ma, Z.S. Bo, Adv. Mater. 28 (2016) 9559. [49] T. Liu, Y. Guo, Y. Yi, L. Huo, X. Xue, X. Sun, H. Fu, W. Xiong, D. Meng, Z. Wang, F. Liu, T.P. Russell, Y. Sun, Adv. Mater. 28 (2016) 10008. [50] H. Yao, Y. Cui, R. Yu, B. Gao, H. Zhang, J. Hou, Angew. Chem. Int. Ed. 56 (2017) 1. [51] J.Y. Yuan, H.L. Dong, M. Li, X.D. Huang, J. Zhong, Y.Y. Li, W.L. Ma, Adv. Mater. 26 (2014) 7077. [52] J.Y. Yuan, W.L. Ma, J. Mater. Chem. A 3 (2015) 7077. [53] H. Benten, T. Nishida, D. Mori, H. Xu, H. Ohkita, S. Ito, Energy Environ. Sci. 9 (2016) 135. [54] P.W.M. Blom, V.D. Mihailetchi, L.J.A. Koster, D.E. Markov, Adv. Mater. 19 (2007) 1551. [55] S.R. Cowan, A. Roy, A.J. Heeger, Phys. Rev. B 82 (2010) 245207. [56] L.J.A. Koster, V.D. Mihailetchi, R. Ramaker, P.W.M. Blom, Appl. Phys. Lett. 86 (2005) 123509. [57] A. Facchetti, Mater. Today 16 (2013) 123. [58] L.M. Wang, X. Hu, P. Liu, W. Li, X. Gong, F. Huang, Y. Cao, J. Am. Chem. Soc. 133 (2011) 9638.
[63] [64] [65] [66] [67] [68] [69] [70]
[71] [72]
G.G. Malliaras, J.R. Salem, P.J. Brock, C. Scott, Phys. Rev. B 58 (1998) 13411. Y. Shen, A.R. Hosseini, M.H. Wong, G.G. Malliaras, ChemPhysChem 5 (2004) 16. V.D. Mihailetchi, J. Wildeman, P.W. Blom, Phys. Rev. Lett. 94 (2005) 126602. X.D. Liu, Z. Xu, F.J. Zhang, S.L. Zhao, T.H. Zhang, W. Gong, J.L. Song, C. Kong, G. Yan, X.R. Xu, Chin. Phys. B 19 (2010) 118601. J.M. Frost, F. Cheynis, S.M. Tuladhar, J. Nelson, Nano. Lett. 6 (2006) 1674. M.A. Loi, S. Toffanin, M. Muccini, M. Forster, U. Scherf, M. Scharber, Adv. Funct. Mater. 17 (2007) 2111. R.R. Lunt, N.C. Giebink, A.A. Belak, J.B. Benziger, S.R. Forrest, J. Appl. Phys. 105 (2009) 053711. R.R. Lunt, J.B. Benziger, S.R. Forrest, Adv. Mater. 22 (2010) 1233. K.H. Lam, T.R.B. Foong, Z.E. Ooi, J. Zhang, A.C. Grimsdale, Y.M. Lam, ACS Appl. Mater. Interfaces 5 (2013) 13265. M. Abdelsamie, N.D. Treat, K. Zhao, C. McDowell, M.A. Burgers, R. Li, D.M. Smilgies, N. Stingelin, G.C. Bazan, A. Amassian, Adv. Mater. 27 (2015) 7285. D. Meng, D. Sun, C. Zhong, T. Liu, B. Fan, L. Huo, Y. Li, W. Jiang, H. Choi, T. Kim, J.Y. Kim, Y. Sun, Z. Wang, A.J. Heeger, J. Am. Chem. Soc. 138 (2016) 375. N. Zhou, H. Lin, S.J. Lou, X. Yu, P. Guo, E.F. Manley, S. Loser, P. Hartnett, H. Huang, M.R. Wasielewski, L.X. Chen, R.P.H. Chang, A. Facchetti, T.J. Marks, Adv. Energy Mater. 4 (2014) 1300785. J. Rivnay, S.C.B. Mannsfeld, C.E. Miller, A. Salleo, M.F. Toney, Chem. Rev. 112 (2012) 5488. W. Guo, J. Yuan, H. Yuan, F. Jin, L. Han, C. Sheng, W. Ma, H. Zhao, Adv. Funct. Mater. 26 (2016) 713.
Guanqun Ding received his Bachelor’s degree in 2014 and now as a master student under Prof. Wanli Ma in Institute of Functional Nano & Soft Materials (FUNSOM) of Soochow University. His research is mainly focused on the research of organic solar cells and hybrid solar cells.
Jianyu Yuan obtained his Ph.D. degree from Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University in 2016 under the supervision of Prof. Wanli Ma. After that, he became an Associate Professor in FUNSOM, Soochow University. He has more than 50 peer-reviewed publications in prestigious chemistry and materials SCI journals, with an H-index of 15 and total citation approaching 1000 times. His research interests focus on design and synthesis of functional materials, fabrication and characterization of solution-processed photovoltaic devices.
Feng Jin is pursuing his PhD in Optics at Fudan University under the supervision of Prof. Haibin Zhao. He has been actively involved in the research on the dynamics of charge generation, separation, and transportation in new-type solar cells based on the ultrafast transient and quasi-steady state absorption spectra.
Yannan Zhang received her Bachelor's degree in Soochow University in 2015. She is a master student under the supervision of Prof. Wanli Ma at the Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University. Her research interest focus on developing high-performance solution-processed solar cells using organic materials, nanocrystals, and perovskites.
364
Nano Energy 36 (2017) 356–365
G. Ding et al. Lu Han is a Ph.D candidate student in Prof. Wanli Ma's group at Soochow University. In 2008, she graduated from Soochow University with a BS in Materials Chemistry. Currently, she is in Prof. M. A. Loi's group at University of Groningen as a visiting student. Her research focused on the morphology control of semiconducting nanocrystals and their application for photovoltaic and field-effect transistor.
Haibin Zhao is a professor and the associate chair in Department of Optical Science and Engineering, Fudan University. He received his Ph.D. from College of William and Mary, and then he engaged in postdoctoral research in College of William and Mary and University of California at Riverside. His current research is focused mainly on (i) the spin-dynamics in magnetic metal and semiconductor; (ii) ultrafast spin regulation and control which is driven by photo, magnetism, and electricity by utilizing time-resolved magneto-optical kerr and Faraday spectroscopy; and (iii) the dynamics of charge generation, separation, and transportation in new-type solar cells.
Xufeng Ling received his Bachelor's degree in College of Nano Science & Technology (CNST) of Soochow University (P. R. China). He is currently a master student under the supervision of Prof. Wanli Ma in Institute of Functional Nano & Soft Materials (FUNSOM) of Soochow University. His research is mainly focused on perovskite solar cells.
Wanli Ma is currently a professor in Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University. He received PhD degree in 2006 from the University of California at Santa Barbara under the supervision of Prof. Alan J. Heeger. Before he joined Soochow University in 2010, he worked as a Postdoctoral Scholar in Prof. Paul Alivisatos’ group at Lawrence Berkeley national laboratory. He received the Chinese Young 1000 program award in 2011. His publications were cited over 10000 times. His current research interest focus on developing solution-processed solar cells, including organic materials, nanocrystals, and perovskites.
365
Contents lists available at ScienceDirect
Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Full paper
High-performance all-polymer nonfullerene solar cells by employing an efficient polymer-small molecule acceptor alloy strategy
MARK
⁎
Guanqun Dinga, Jianyu Yuana, , Feng Jinb, Yannan Zhanga, Lu Hana, Xufeng Linga, ⁎ Haibin Zhaob, Wanli Maa, a Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren’ai Road, Suzhou 215123, Jiangsu, PR China b Shanghai Ultra-precision Optical Manufacturing Engineering Research Center, and Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Optical Science and Engineering, Fudan University, Shanghai 200433, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: All-polymer solar cells Acceptor alloy Non-fullerene Morphology
In the current research field of non-fullerene solar cells, the device performance of all polymer solar cells (allPSCs) lags significantly behind those based on polymer donor and molecule acceptor. To further improve the device performance of all-PSCs, the small molecule electron acceptor ITIC was introduced for the first time to the all-PSCs based on PTP8/P(NDI2HD-T). The combinative polymer/molecule acceptor P(NDI2HD-T)/ITIC with only a small amount of ITIC, 15 wt% in acceptors and 6 wt% in total, can significantly improve the device PCE from 6% to over 7%, which is among the highest-efficiencies for reported all-PSCs. The improved device performance can be attributed to the broadened absorption and optimized blend morphology, which can further enhance charge carrier generation and balance charge carrier mobility in the ternary blend. More importantly, we discovered that this strategy can be successfully applied to all-PSCs based on other polymer donors like widely spread P3HT or PTB7. Therefore, our work not only reveals the detailed effect of polymer-small molecule acceptor alloy in all-PSCs, but also suggest that the ternary cell strategy of polymer donor/polymer:molecule acceptor may become a general and facile approach to further boost the performance of current all-PSCs.
1. Introduction
alternative to the fullerene system and power conversion efficiencies (PCE) now are approaching 12% [19–22]. Among them, all-polymer solar cells (all-PSCs), consisting of polymer-donor and polymeracceptor materials, which possess extraordinary mechanical properties over other non-fullerene counterparts, exhibiting great potential for the future flexible device engineering [23]. Compared to the efficient non-fullerene molecule acceptors, the development of non-fullerene polymer acceptors has lagged significantly behind in both synthesis and device performance [24–26]. Naphthalene diimide (NDI)-based copolymers, for example, Poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophe-ne)} [P(NDI2OD-T2); Polyera ActiveInkN2200] [27], is the most widely used polymer acceptor so far. These NDI analogues can simultaneously maintain highly ordered structure, excellent electron mobility and strong π-π intermolecular interaction [28,29]. However, only a few reported all-PSCs exhibited PCEs above 6% [30–35], with the very recent two ones reaching 7–9% [36–38]. Apparently, the typical values for Jsc and fill factor (FF) are relatively lower than their fullerene-containing counterparts. The lower
Due to the superior advantages like lightweight, semi-transparency, flexibility and low-temperature fabrication process [1,2], organic solar cells (OSCs) are currently viewed as promising power generation technologies that can be integrated into future flexible and wearable devices. To date, nearly all the highest efficient OSCs devices [3–7] utilize a hetero-junction consisting of either a conjugated polymer or molecule as the donor material and a fullerene derivative as the electron acceptor [8,9]. However, fullerenes are not ideal acceptor materials due to many intrinsic issues, such as weak light absorption, almost fixed chemical structure and energy levels [10,11], further limiting the open-circuit voltage (Voc) and short-circuit current density (Jsc) of these solar cell devices. Most importantly, fullerene-based OSCs have low flexibility and stretchability due to the brittle crystalline features of the fullerenes [12,13]. Quite recently, solution-processed OSCs using non-fullerene electron acceptors [14–16], which possess advantages including tunable chemical and electronic properties as well as enhanced stabilities [17,18], have been proven to be a viable
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Yuan), [email protected] (W. Ma).
http://dx.doi.org/10.1016/j.nanoen.2017.04.061 Received 6 April 2017; Received in revised form 28 April 2017; Accepted 30 April 2017 Available online 01 May 2017 2211-2855/ © 2017 Elsevier Ltd. All rights reserved.
Nano Energy 36 (2017) 356–365
G. Ding et al.
ITIC can form a type II energy level alignment, while P(NDI2HD-T) and ITIC form a type I structure. The lowest unoccupied molecular orbital (LUMO) energy level of P(NDI2HD-T) and ITIC is at around −3.9 eV, which provides a ~0.3 eV offset from the LUMO of PTP8 for electron transfer from donor to acceptor. In contrast, the hole transfer from PTP8 to P(NDI2HD-T) should be more efficient than that to ITIC due to the larger offset between the highest occupied molecular orbital (HOMO) of donor and acceptor. The film absorption spectra of the adopted materials are shown in Fig. 1(d); the donor polymer PTP8 exhibits a wide band-gap of 1.80 eV. Compared to the fullerene acceptors, the absorption of P(NDI2HD-T) is greatly improved, which, however, is overlapped with the absorption of PTP8. Herein, the competitive absorption in the PTP8/P(NDI2HD-T) all-polymer blend is not desired to realize the merits of non-fullerene solar cells. Extending the absorption into the red and near-infrared regions of the solar spectrum should be ideal to maximize the device photocurrent. Fortunately, the popular small molecule acceptor, ITIC with a high absorption coefficient in the range from 700 nm to 800 nm, can be used as a solid additive in the PTP8/P(NDI2HD-T) binary system to extend the absorption range without increase the active film thickness.
device performance of all-PSCs is considered to be the large polymer/ polymer phase separation, relatively low electron mobility of polymer acceptors and the inefficient charge dissociation at the donor/acceptor (D/A) interface [36,37,39]. In recent years, one of the promising strategies for utilizing full potential of photons in polymer/fullerene BHJ solar cells is to introduce a third component to the host binary D/A blend [40–43]. This is important for obtaining improved device performance with similar film thickness. It has been generally pointed out that the ternary organic solar cells can be divided into two types according to the content of the third component. One is called the “parallel” model proposed firstly by You et. Al [44], in which the two donors or acceptors co-exist at a ratio around 1/1. The other one is called “cascade” or “alloy” model, in which the weight ratio of the third component content was typical at lower value around 10–15%. In addition, the “cascade” [45,46] and “alloy” [47] type, were divided according to the energy level position of the third component. The introduction of the third component as a cascade material leads to improved short circuit current density (Jsc) and PCEs due to the complementary absorption. The “alloy” model ternary solar cells were proposed quite recently by Wei et al. [47], in which the third component (normally conjugated molecules) can form binary alloy for better morphology and exciton dissociation. The ternary solar cells of either “parallel” or “solid additive” model have been demonstrated in multiple cases in polymer/fullerene and polymer/molecule systems [48–50]. However, in all-PSCs based on polymer acceptors, the ternary solar cells have rarely been reported so far and the resultant PCEs are relatively low. In these very limited cases of all-PSC ternary cells, only polymer acceptors as additives were investigated. To the best of our knowledge, the addition of small molecule acceptor as “solid additive” in efficient all-PSCs has not been reported. The interesting interactions between the molecule acceptor and the polymer materials remain unknown and the consequent effect of combinative polymer/molecule acceptor on the properties and performance of the all-PSCs requires in-depth study. In this contribution, inspired by the successful results in ternary polymer/fullerene and polymer/molecule solar cells, we report for the first time the introduction of molecule acceptor as additive in ternary all-PSCs, which is designed by incorporating highly efficient small molecule acceptor ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno-[2,3d:2’,3’-d’]-s-indaceno[1,2-b:5,6b’]-dithiophen-e) [14] into a host binary all-polymer blend system PTP8/ P(NDI2OD-T). PTP8 [51] was reported as an efficient donor for all-PSCs in our previous work, with a PCE of 4.3% when using together with the popular polymer acceptor P(NDI2OD-T2) [52]. And the PCE can be improved to 6% by using acceptor P(NDI2HD-T) with more matched crystallinity. After the addition of ITIC (6 wt% in total), the PCE of the ternary all polymer system was further increased to over 7%, which is among the highest efficiencies in all-PSCs. However, adding more ITIC will actually reduce the performance of our all-PSCs. We show that optimal amount of ITIC as additive can significantly increase light absorption, optimize blend morphology, enhance charge carrier generation and balance charge carrier transport in the binary blend. More importantly, we discovered that this strategy can be successfully applied to all-PSCs systems based on other polymer donors like widely spread P3HT or PTB7. Therefore, our work not only reveals the detailed effect of small molecule “solid additive” in all-PSCs, but also suggest that the ternary cell strategy may become a general and facile approach to further boost the performance of current all polymer solar cell.
2.2. All-polymer solar cells performance The best solar cell devices were achieved using the conventional architecture of ITO/PEDOT: PSS (40 nm)/Active layer (~120 nm)/LiF (0.6 nm)/Al (80 nm). As show in Table S1 and Table S2, we at first optimized PTP8/P(NDI2HD-T) and PTP8/ITIC binary system independently. Device optimization included adjustments of solvents additives, donor-acceptor blend ratio and active layer thickness. Both PTP8/P(NDI2HD-T) and PTP8/ITIC exhibit quite similar optimal fabrication conditions, which makes it easier to manipulate the corresponding ternary solar cells. The binary all-PSCs based on PTP8/P(NDI2HD-T) blend exhibit the best PCE of 6.00%, with a Voc of 0.976 V, a Jsc of 10.71 mA/cm2 and a FF of 0.57. Meanwhile, the PTP8/ITIC binary system delivers an optimal PCE of 5.11%, with a slight higher Voc of 1.003 V, a Jsc of 8.76 mA/cm2 and a FF of 0.58. Both systems have an optimal blend ratio of 1.5:1. Note that even with complementary absorption between the donor and acceptor, the PTP8/ ITIC system still achieves a lower photocurrent than that of PTP8/ P(NDI2HD-T). Meanwhile, the P(NDI2HD-T)/ITIC binary system was also fabricated to reveal the dynamic process between the two acceptors P(NDI2HD-T) and ITIC, with a Jsc of 0.026 mA/cm2, a Voc of 0.513 V, a FF of 0.29 and PCE of 0.004% (Fig. S2), which indicates very poor charge transfer at the interface of these two materials [53]. As shown in Fig. 2 and Table 1, we systematically investigated the ternary solar cells PTP8/P(NDI2HD-T)/ITIC with a wide range of blend ratios: 1.5/0.9/0.1, 1.5/0.85/0.15, 1.5/0.8/0.2, 1.5/0.7/0.3, 1.5/ 0.5/0.5 and 1.5/0.3/0.7. Note that the blend ratio between the donor PTP8 and combinative polymer/molecule acceptors P(NDI2HD-T)/ ITIC is fixed to 1.5:1. The amount of the ITIC is varied to invest its effect on the device performance. It is clear that the PCE increases with the addition of acceptor ITIC. At the amount of 15 wt% (1.5/0.85/ 0.15), calculated only based on the total content of acceptors, the PCE of the optimized ternary systems reaches 7.01% with a Voc of 0.976 V, a Jsc of 12.60 mA/cm2 and a FF of 0.57. According to the previous report [45–47], we hypostasized the enhancement after adding small amount of ITIC may attributed to the improved blend morphology, further strengthen the charge generation, separation and transportation. After that, the PCE gradually decreases with further increased amount of ITIC. The optimal PCE of this ternary system is ~20% higher than that of the PTP8/P(NDI2HD-T) binary system and 37% higher than that of the PTP8/ITIC system. The Voc of the ternary systems is around 0.976 V constantly, the same with that of PTP8/P(NDI2HD-T), indicating that acceptor P(NDI2HD-T) stills plays the most important role in exciton dissociation and suggesting the formation of “alloy” acceptors according to the previous work [18]. The FF of ternary
2. Results and discussion 2.1. Molecular structure and properties The chemical structures and energy levels of PTP8, P(NDI2HD-T) and ITIC are shown in Fig. 1. Both PTP8/P(NDI2HD-T) and PTP8/ 357
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 1. (a) Chemical structure of PTP8, P(NDI2HD-T) and ITIC; (b) device structure of devices; (c) energy level diagram and (d) normalized thin film UV–vis absorption spectra of donor and acceptor materials.
well agreement with the Jsc changes in ternary solar cell devices as shown in Fig. 3(a). It is very important to notice that fullerene-based organic ternary solar cells are highly empirical and system dependent. A more general approach to construct efficient nonfullerene ternary cells should be of high significance. Thus we further tried our approach on other two allpolymer systems adopting classical polymer P3HT or high-efficiency PTB7 as the donor and P(NDI2HD-T) as the acceptor. As shown in Fig. S5 and Table S3, by adding 15 wt% ITIC in both P3HT/P(NDI2HD-T) and PTB7/P(NDI2HD-T) all-polymer systems, the PCEs of the ternary solar cells exhibit 24% and 17% enhancement relative to the binary systems, respectively, which indicates the advantage and high compatibility of polymer/molecule combinative acceptors.
systems shows nearly no apparent changes, which may be due to the similar FF values for PTP8/P(NDI2HD-T) and PTP8/ITIC binary systems. However, we do observe slight drop of FF after the addition of more than 15 wt% ITIC in acceptors. In contrast, the Jsc makes the most contributions to the performance enhancement. We observe that the Jsc and PCE are highly correlated with the amount of ITIC in the combinative acceptors. When adding 15 wt% ITIC, the Jsc approaches 12.60 mA/cm2, nearly 20% higher compared with the Jsc of the PTP8/ P(NDI2HD-T) binary system, which may largely result from the enhanced light absorption. Nonetheless, further increasing the loading of ITIC to 20% and 30%, the Jsc gradually decreases to 10.27 mA/cm2. When the content of ITIC is over 50%, the Jsc decreases to a similar value of that for the PTP8/ITIC binary systems, indicating that ITIC starts to play the dominate roll in the acceptors. To explicate the effect of ITIC on the improved performance in ternary systems, UV–vis absorption spectra (Fig. 3(b)) and EQE spectra (Fig. 3(c)) were further measured (the detailed spectra are shown in Fig. S4). As shown in Fig. 3(b) and Fig. S4(a), the absorption of the ternary blend films is gradually enhanced in the range from 650 nm to 800 nm with increased ITIC amount. The change of blend absorption is directly reflected in the corresponding EQE spectra. The ternary cell with optimal ITIC shows red-shifted EQE spectrum compared to the control devices, attributed to the addition of ITIC. Meanwhile, the absorption of the ternary blend film slowly decreases in the range from 550 nm to 650 nm owing to the decreased P(NDI2HDT), while the EQE is actually increased in this region, which may be due to other factors like morphology. The ternary cell with 50 wt% ITIC in acceptors, however, has much lower EQE value in the visible region despite the increased response in the 700–800 nm region, likely due to the lower efficiency of PTP8/ITIC system as demonstrated by its much smaller EQE value and device current. Note that all these results are in
2.3. Charge generation, transport and recombination To understand the effect of added ITIC on the charge generation and extraction process, the dependence of the photocurrent density (Jph) on the effective voltage (Veff) were recorded under illumination of 100 mW/cm2 (Fig. 3(d)). Jph is equal to JL –JD, where JL and JD are the experimental current under illumination and in dark, respectively. Veff is equal to Vo-Va where Vo is the voltage when Jph is zero and Va is the applied voltage [54]. From Jph-Veff curves for the optimal ternary devices, we observe the Jph reaching a plateau at a low Veff around 0.5 V, suggesting that free charges are quickly swept out. In contrast, the curve for the 50 wt% ternary devices shows a smaller slope, indicating less efficient charge extraction. At sufficiently high electric fields, the Jph of PTP8/P(NDI2HD-T)/ITIC (with 15 wt% ITIC in acceptor) devices show notably higher value. For PTP8/ITIC and PTP8/P(NDI2HD-T)/ITIC (with 50 wt% ITIC in acceptor) devices, the photocurrent saturate at higher bias with lower values. The results 358
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 2. Photovoltaic parameters (Jsc, Voc, FF, and PCE) of PTP8/P(NDI2HD-T) binary, PTP8/ITIC binary and PTP8/P(NDI2HD-T)/ITIC ternary solar cells with different amounts of ITIC.
dence of Voc on light intensity to examine if trap-assisted recombination may also significantly limit the Jsc. In Voc-light intensity measurements, the Voc should follow a power law relationship as Voc ~ sI, where s > 1.0 kT/q is indicative of significant trap-assisted recombination [56]. Quite similarly, the Voc-light intensity measurements exhibit similar trend compared to that of Jsc-light intensity measurements, which confirms that increasing the ITIC content in the blend leads to increased recombination in solar cell devices. However, for the all-PSCs without ITIC or with low ITIC content exhibit serious monomolecular recombination and consequently lead to the restrained improvement of performance. In short, the charge recombination is closely related to the concentration of ITIC in these ternary blends. We speculate that the mismatched LUMO and HOMO levels between the two acceptors my hinder the transport of carrier between them. In addition, ITIC molecules may has less favorable morphology than P(NDI2HD-T) in this system, resulting in more recombination.. The charge carrier mobility is another important factor that affects solar cell device performance. Both the electron and hole mobilities are important because if they are too low or imbalanced, charge carriers
reveal that optimal amount of ITIC is beneficial for the charge generation process, partially due to the broadened absorption, while more ITIC will actually hinder the generation process in these ternary blends. The dependence of Jsc on light intensity was also measured to examine if non-geminate recombination may significantly limit the device Jsc. In Jsc-light intensity measurements, the Jsc should follow a power law relationship as J ~ I α where α < 1.0 is indicative of significant bimolecular recombination [55]. As shown in Fig. 4(a), for all the devices, the Jsc scales quite differently with light intensity (α=0.993 for 0% ITIC, α=0.964 for 15% ITIC, α=0.928 for 50% ITIC and α=0.892 for 100% ITIC), which indicates increased bimolecular recombination in the solar cell devices with higher ITIC loading.[30] In addition, these values indicate that the charge collection efficiency is weakly dependent on the light intensity at low ITIC loading. This further implies that the charge carrier losses at low ITIC loading are dominated by monomolecular recombination captured in the defect sites, while bimolecular exciton annihilation may cause only rather minor losses. As shown in Fig. 4(b), we further measure the depen-
Table 1 Photovoltaic parameters of the PTP8/P(NDI2HD-T), PTP8/ ITIC binary systems and PTP8/P(NDI2HD-T)/ITIC ternary systems with different ratios of P(NDI2HD-T) and ITIC. PTP8/P (NDI2HD-T)/ITIC
Voc (V)
Jsc (mA/cm2)
FF
PCE (Avg)a (%)
μe (10−4cmV−1s−2)
μh (10−4cmV−1s−2)
1.5/1.0/0.0 1.5/0.9/0.1 1.5/0.85/0.15 1.5/0.8/0.2 1.5/0.7/0.3 1.5/0.5/0.5 1.5/0.3/0.7 1.5/0.0/1.0
0.976 0.976 0.976 0.976 0.976 0.976 0.976 1.003
10.71 11.83 12.60 11.61 10.47 9.23 9.24 8.76
0.57 0.56 0.57 0.56 0.56 0.57 0.54 0.58
6.00 6.47 7.01 6.35 5.72 5.14 4.87 5.10
3.69 2.99 1.85 1.83 2.65 2.95 3.38 3.78
1.04 1.25 1.95 2.49 3.34 3.79 5.53 9.47
a
(5.79) (6.25) (6.82) (6.09) (5.54) (5.03) (4.77) (5.01)
The values are the photovoltaic parameters for the optimal devices. The average values are calculated over 10 devices.
359
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 3. (a) J-V curves of PTP8/P(NDI2HD-T), PTP8/ITIC and PTP8/P(NDI2HD-T) /ITIC with 15 wt% ITIC and 50 wt% ITIC blend systems; b) UV–vis absorption spectra of corresponding systems; (c) EQEs of corresponding systems and d) Photocurrent versus effective voltage of corresponding systems.
MEH-PPV/PCBM blend film, the addition of small molecule improved the formation of donor polymer fiber structure, further leading to enhanced hole mobility [61,62], which may interpret the improved hole mobility in our ternary devices. Meanwhile, the µe decreases from 3.69×10−4 cm2 V−1s−1 to 1.83×10−4 cm2 V−1 s−1 at first with the amount of ITIC increased from 0 wt% to 20 wt% in acceptors, and then gradually goes up to 3.78×10−4 cm2 V−1 s−1 with the amount of ITIC further increased to 100 wt% in acceptors. The initially decreased electron mobility may be attributed to the trap effect of isolated ITIC clusters in P(NDI2HD-T) matrix. With the addition of more ITIC,
may recombine before they are collected at the electrodes [30–35,57]. To gauge the charge transport properties of these all-polymer blends, hole only and electron-only diodes were fabricated and measured (see Supporting Information) [58]. Fig. 5 and Table 1 show the hole mobility (µh) and electron mobility (µe) along with the ratio (in weight) changes of two acceptors, P(NDI2HD-T) and ITIC. Both µh and µe were measured by the space-charge-limited current (SCLC) method with the Mott-Gurney equation [59,60]. The µh increases from 1.04×10−4 cm2 V−1 s−1 to 2.71×10−3 cm2 V−1 s−1 by varying the amount of ITIC in the blends. According to the early investigation of neat MEH-PPV film and
Fig. 4. (a) Jsc and (b) Voc dependence on illumination intensity for PTP8/P(NDI2HD-T), PTP8/ITIC and PTP8/P(NDI2HD-T)/ITIC with 15 wt% ITIC and 50 wt% ITIC blend systems in symbols, together with linear fits in lines.
360
Nano Energy 36 (2017) 356–365
G. Ding et al.
of low ITIC content. However, the RMS value for film with higher ITIC amount (over 30 wt%) is drastically increased. We hypothesize that with high ITIC loading the highly ordered packing and crystallinity of ITIC molecules may result in large ITIC domains, leading to enhanced polymer-molecule de-mixing and a rougher surface, which may result in inefficient exciton dissociation and charge transport. In order to confirm the results, we also investigated the film morphology by using transmission electron microscopy (TEM), as shown in Fig. 7. For PTP8/P(NDI2HD-T) all-polymer blend film, we observe the “river” like morphology, indicating the phase separation of the donor acceptor polymers. According to the previous study by Marks et al. [70], the NDI-based polymers acceptor have a strong tendency to aggregate in the all-polymer BHJ blend. After the incorporation of ITIC (15 wt%) into the blend film, we observe the film phase separation is suppressed, suggesting that the ITIC can help reducing the aggregation of the polymer materials and resulting in slightly better intermixing and finer phase-separated domains. Further increasing ITIC (50%) in the ternary blend film, the D/A phase separation is slightly enhanced, likely due to the ITIC aggregation. These observations are consistent with the AFM results. For 100% ITIC acceptor without P(NDI2HD-T), the PTP8/ITIC film morphology appears differently. The large amount of ITIC in the blend film should result in strong aggregation due to its large tendency to crystallization. We observe that the black “river” like domains are much less distinct, while more structures appear in the white domains, indicating enhanced aggregation as expected. Overall, the ternary blend film with optimal ITIC content (15 wt%) exhibits optimal phase separation extent and thus delivers superior solar cell performance. Finally, grazing incidence wide-angle X-ray scattering (GIWAXS) is used to probe the molecular stacking in the neat and blend films [71]. To quantify the scattering data, the corresponding out-of-plane and inplane cuts are given in Fig. 8. A broad peak at q =~1.7 Å−1 and q =~1.6 Å−1 arises in the out-of-plane direction in neat PTP8 and P(NDI2HD-T) films respectively, which corresponds to the π-π stacking peak. This result indicates that both PTP8 and P(NDI2HD-T) have a preferred face-on orientation with respect to substrate. In contrast, the 2D GIWAXS pattern of ITIC thin film exhibits a high degree of molecular ordering in both in plane and out-of-plane direction, as evidenced by the strong lamellar (100), (200), (300) and π-π (010) diffraction peaks. Compared with the neat film, the intensity of the π-π stacking peak (010) of PTP8 in the ternary blend film becomes weaker with the addition of acceptors (see Fig. 8), which suggests that the acceptor ITIC suppressed the crystallization of PTP8 after mixing,
Fig. 5. The hole mobility (µh) and electron mobility (µe) of PTP8/P(NDI2HD-T), PTP8/ ITIC binary systems and PTP8/P(NDI2HD-T)/ITIC ternary systems with different amount of ITIC.
percolation pathway can forms and increased electron mobility can be achieved. When the amount of ITIC in acceptors is at 15 wt%, we found the blend exhibits the most balanced hole mobility (µh=1.95×10 −4 cm2 V−1 s−1) and electron mobility (µe=1.85×10 −4 cm2 V−1 s−1), with an ideal ratio (µh/µe) value of 1.05. This balanced carrier transport may be one of the important reasons for the optimal device performance at this ratio. 2.4. All-polymer blend morphology Lots of previous work [63–65] have demonstrated that the exciton dissociation and charge transport process are strongly affected by the polymer/PCBM blend morphology. Therefore, a better understanding of the key factors driving blend morphology is crucial to further improve the morphology of all-polymer solar cells. Herein, the morphology of optimal all-polymer and ternary blend films were examined by using atomic force microscopy (AFM) (Fig. 6(a–h)). By incorporating ITIC into the host all-polymer system, the high crystallinity of rigid conjugated molecule ITIC should prevent the entanglement and aggregation of donor and acceptor polymers [66–69]. Therefore, when the content of ITIC is lower than 20 wt%, the ternary blend films display smooth surface similar to that of PTP8/P(NDI2HDT) binary blend film. In fact, the surface roughness (RMS) value is even slightly lower, indicating reduced polymer domain sizes by the addition
Fig. 6. AFM height image for blend films with 0 wt% (a); 15 wt% (b); 50 wt% (c) and 100 wt% ITIC (d) in acceptors.
361
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 7. TEM for blend films with (a) 0 wt%; (b) 15 wt%; (c) 50 wt% and (d) 100 wt% ITIC in acceptors.
excitation in these films. The long-lived components which centered at 900 nm in blend films is attributed to the absorption of hole polarons residing in PTP8. As shown in Fig. S9, the lifetime of excitons in P(NDI2HD-T) and ITIC films is merely about tens of picosecond, which is much faster than that in neat PTP8 and the blend films. Fig. 9(b) displays the comparison of normalized ΔT/T dynamics at 900 nm for these binary and ternary blend films. We also note that the decay becomes slower and the lifetime grows longer when decreasing the content of ITIC in the blend. Considering the evidence of charge transfer (CT) at the interface of PTP8-ITIC binary blend is not distinct, the combination of ITIC in this work does not effectively facilitate interfacial CT between donor and acceptor materials. In addition, the dynamics at 900 nm in PTP8, ITIC and PTP8/ITIC, as shown in Fig. S9, also supports the inefficient interfacial CT between PTP8 and ITIC. Compared to the recently reported high-efficiency non-fullerene solar cells composed of a wide gap polymer donor and ITIC acceptor [14– 16], our PTP8/ITIC system exhibits relatively smaller HOMO-HOMO offset, which may limit the hole transfer from acceptor to the donor. Thus, the enhanced PCE of ternary solar cell is more likely due to the broadened absorption and improved morphology, rather than “cascade” model for efficient charge transfer.
3. Conclusion
Fig. 8. In-plane and out-of-plane line cuts from the 2D GIWAXS patterns of neat (a) PTP8, (b) P(NDI2HD-T), (c) ITIC thin films cast from chloroform and (d) PTP8/ P(NDI2HD-T)/ITIC (1.5/0.85/0.15) ternary blend film cast from optimal conditions.
The small molecule electron acceptor (ITIC) was introduced for the first time to the all polymer solar cells based on PTP8/P(NDI2HD-T), aiming to further tune the device photovoltaic properties. The combinative polymer/molecule acceptor P(NDI2HD-T)/ITIC with only a small amount of ITIC, 15 wt% in acceptors and 6 wt% in total, can significantly improve the device PCE from 6% to over 7%, which is among the highest-efficiencies for reported all polymer solar cells. The improved device performance can be attributed to the broadened absorption and optimized blend morphology, which can further enhance charge carrier generation and balance charge carrier mobility in the ternary blend. More importantly, we discovered that this strategy can be successfully applied to all polymer solar cells based on other polymer donors like widely spread P3HT or PTB7. Therefore, our work not only reveals the detailed effect of small molecule “solid additive” in all-PSCs, but also suggest that the ternary cell strategy of polymer donor/polymer: molecule acceptor is a general and facile approach to further boost the performance of current all polymer solar cells.
consistent with the TEM results. In contrast, the π-π stacking peak (010) of P(NDI2HD-T) are almost absent in the ternary blend while a weak (200) peak can still be observed. For ITIC in the blend film, its strong π-π stacking (010), lamellar (100) and (200) diffraction peaks totally disappear at the optimal weight ratio of ~15%, which indicates that ITIC may be well mixed into the polymer matrix. According to the latest report by Wei et al. [48], our observations may indicate the formation of “alloy” between the polymers and the small amount of molecular acceptor ITIC.
2.5. Transient absorption spectroscopy measurements We utilized the ultrafast Transient Absorption (TA) spectroscopy to characterize charge transfer dynamics in ternary blend films composed of PTP8, P(NDI2HD-T) and ITIC [72]. Fig. 9(a) shows the measured ΔT/T spectra, which was probed at 830 ps after the pump pulse 362
Nano Energy 36 (2017) 356–365
G. Ding et al.
Fig. 9. (a) Differential transmission spectra of blend films with 0 wt%, 15 wt%, 50 wt% and 100 wt% ITIC probed at 830 ps after the pump pulse excitation; (b) The decay dynamics of transient absorption spectra which pumped at 400 nm and probed at 900 nm in blend films with 0 wt%, 15 wt%, 50 wt% and 100% ITIC. The measurement was carried out at pump fluence of ~70 μJ/cm2.
scopy, the laser source was a Ti: Sapphire amplifier laser whose center wavelength is 800 nm, the pulse width and the repetition rate are 120 fs and 1 kHz respectively. The pump pulses used in our measurement is 400 nm, which were obtained through doubling the amplifier's fundamental output with a beta barium borate (BBO) crystal. The pump beam was modulated by a mechanical chopper at the frequency of ~325 Hz and focused on the sample with a spot diameter of ~3 mm. We focused the fundamental 800 nm light onto a sapphire plate to generate the supercontinuum white light, which passed though the different color filters with a 10 nm bandwidth, serving as the probe pulses in the range of 500–1000 nm. The pump and probe beam were spatially overlapped on the sample, and the probe light has much smaller energy density than the pump beam. The transmitted probe beam was detected by a silicon photodiode connected to a lock-in amplifier.
3.1. Experimental section 3.1.1. Materials PTP8 and P(NDI2HD-T) was synthesized following the methods in our previous work [34,48], respectively. ITIC were purchased from Solarmer Materials Inc. Unless otherwise stated, all chemicals were commonly commercially available products and used as received. 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were performed at the Advanced Light Source (ALS)-Lawrence Berkeley National Laboratory on Beamline 7.3.3. 3.1.2. Characterization UPS were carried out on a Kratos AXIS UltraDLD ultrahigh vacuum (UHV) surface analysis system under a base pressure of 5×10−10 Torr with an unfiltered Hel (21.21 eV) gas discharge lamp. UV–vis–NIR spectra were recorded on a Perkin Elmer model Lambda 750. Tappingmode AFM images were obtained with a Veeco Multimode V instrument. TEM images were performed on Tecnai G2 F20 S-Twin Transmission Electron Microscope.
Acknowledgments The author thanks Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University. GIWAXS measurements were taken at Advanced Light Source (ALS)-Lawrence Berkeley National Laboratory on Beamline 7.3.3. This work was supported by “111 project”, the National Key Research Projects (Grant No. 2016YFA0202402), the National Natural Science Foundation of China (Grant No. 61222401, No. 61674111). And we also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
3.1.3. Solar cells device fabrication and characterization The indium tin oxide (ITO) coated glass substrates were cleaned by ultrasonic in acetone, detergent, isopropyl alcohol and acetone sequentially, each for 20 min. The dried substrates were further treated with UV-ozone for 30 min to prove its work function. Next, PEDOT: PSS (Al 4083) was spin-coated at a rate of 4500 rpm for 40 s on the top of ITO substrates and then annealed at 140 °C for 10 min. The active layer materials, PTP8, P(NDI2HD-T) and ITIC were dissolved in chloroform with 0.5% DIO. The total concentration of blended solution was maintained at 10 mg/ml and weight ratio of donor material (PTP8) to acceptor material (P(NDI2HD-T) and ITIC) maintained 1.5:1. The weight ratios of ITIC in acceptors were 0 wt%, 10w%, 15 wt%, 20 wt%, 30 wt%, 50 wt%, 70 wt% and 100 w%. The blends were then spincoated on PEDOT: PSS coated ITO with a rate of 2000 rpm for 40 s in a nitrogen filled glove box. At last, 0.6 nm thick lithium fluoride (LiF) with 0.1 A/s and 80 nm thick aluminum (Al) electrode with 2 A/s were thermal evaporated at a pressure nearly 1.0×10−6 mbar upon the active layer through a shadow mask. The active area is 7.25 mm2. The performance of all cells were tested under AM 1.5 G light illumination with an intensity of 100 mW/cm2 (Newport, Class AAA solar simulator, 94023A-U). The EQE was determined using a certified IPCE equipment (Zolix Instruments, Inc, SolarCellScan100).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2017.04.061. References [1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. [2] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. [3] R.C. Coffin, J. Peet, J. Rogers, G.C. Bazan, Nat. Chem. 1 (2009) 657. [4] R. Po, C. Carbonera, A. Bernardi, N. Camaioni, Energy Environ. Sci. 4 (2011) 286. [5] Z.C. He, C.M. Z, S.J. Su, M. Xu, H.B. Wu, Y. Cao, Nat. Photonics 6 (2012) 591. [6] Z.C. He, B. Xiao, F. Liu, H.B. Wu, Y.L. Yang, S. Xiao, C. Wang, T.P. Russell, Y. Cao, Nat. Photonics 9 (2015) 175. [7] C. Liu, C. Yi, K. Wang, Y.L. Yang, R.S. Bhatta, M. Tsige, S.Y. Xiao, X. Gong, ACS Appl. Mater. Interfaces 7 (2015) 4933. [8] A. Mishra, P. Bauerle, Angew. Chem. Int. Ed. 51 (2012) 2020. [9] Y. Lin, Y. Li, X. Zhan, Chem. Soc. Rev. 41 (2012) 4245. [10] J.U. Lee, J.W. Jung, J.W. Jo, W.H. Jo, J. Mater. Chem. 22 (2012) 24265. [11] A. Eftaiha, J. Sun, I.G. Hill, G.C. Welch, J. Mater. Chem. A 2 (2014) 1201. [12] D.J. Lipomi, H. Chong, M. Vosgueritchian, J. Mei, Z. Bao, Sol. Energy Mater. Sol.
3.1.4. Transient Transmission Spectroscopy (TAS) measurements The TAS measurements were performed in an optical cryostat under vacuum at room temperature. For transient absorption spectro363
Nano Energy 36 (2017) 356–365
G. Ding et al. [59] [60] [61] [62]
Cells 107 (2012) 355. [13] S. Savagatrup, A.D. Printz, T.F. O’Connor, A.V. Zaretski, D. Rodriquez, E.J. Sawyer, K.M. Rajan, R.I. Acosta, S.E. Root, D.J. Lipomi, Energy Environ. Sci. 8 (2015) 55. [14] Y. Lin, J. Wang, Z.G. Zhang, H. Bai, Y. Li, D. Zhu, X. Zhan, Adv. Mater. 27 (2015) 1170. [15] W. Zhao, D. Qian, S. Zhang, S. Li, O. Inganas, F. Gao, J. Hou, Adv. Mater. 28 (2016) 4734. [16] L. Gao, Z.G. Zhang, H. Bin, L. Xue, Y. Yang, C. Wang, F. Liu, T.P. Russell, Y. Li, Adv. Mater. 28 (2016) 8288. [17] W.R. Mateker, J.D. Douglas, C. Cabanetos, I.T. Sachs-Quintana, J.A. Bartelt, E.T. Hoke, A. El Labban, P.M. Beaujuge, J.M.J. Fréchet, M.D. McGehee, Energy Environ. Sci. 6 (2013) 2529. [18] J. Kong, S. Song, M. Yoo, G.Y. Lee, O. Kwon, J.K. Park, H. Back, G. Kim, S.H. Lee, H. Suh, K. Lee, Nat. Commun. 5 (2014) 5688. [19] L. Gao, Z.G. Zhang, H. Bin, L. Xue, Y. Yang, C. Wang, F. Liu, T.P. Russell, Y. Li, Adv. Mater. 28 (2016) 8288. [20] H. Yao, Y. Chen, Y. Qin, R. Yu, Y. Cui, B. Yang, S. Li, K. Zhang, J. Hou, Adv. Mater. 28 (2016) 8283. [21] W. Zhao, D. Qian, S. Zhang, S. Li, O. Inganas, F. Gao, J. Hou, Adv. Mater. 28 (2016) 4734. [22] B. Fan, K. Zhang, X. Jiang, L. Ying, F. Huang, Y. Cao, Adv. Mater. (2017). http:// dx.doi.org/10.1002/adma.201606396. [23] S. Wu, Polymer 28 (1987) 1144. [24] X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen, S.R. Marder, J. Am. Chem. Soc. 129 (2007) 7246. [25] D. Mori, H. Benten, I. Okada, H. Ohkita, S. Ito, Energy Environ. Sci. 7 (2014) 2939. [26] C. Lee, H. Kang, W. Lee, T. Kim, K.H. Kim, H.Y. Woo, C. Wang, B.J. Kim, Adv. Mater. 27 (2015) 2466. [27] H. Yan, Z. Chen, Y. Zheng, C. Newman, J.R. Quinn, F. Dotz, M. Kastler, A. Facchetti, Nature 457 (2009) 679. [28] R. Steyrleuthner, M. Schubert, F. Jaiser, J.C. Blakesley, Z. Chen, A. Facchetti, D. Neher, Adv. Mater. 22 (2010) 2799. [29] T. Earmme, Y.J. Hwang, S. Subramaniyan, S.A. Jenekhe, Adv. Mater. 26 (2014) 6080. [30] L. Ye, X. Jiao, M. Zhou, S. Zhang, H. Yao, W. Zhao, A. Xia, H. Ade, J. Hou, Adv. Mater. 27 (2015) 6046. [31] W. Lee, C. Lee, H. Yu, D.J. Kim, C. Wang, H.Y. Woo, J.H. Oh, B.J. Kim, Adv. Funct. Mater. 26 (2016) 1543. [32] Y.J. Hwang, T. Earmme, B.A. Courtright, F.N. Eberle, S.A. Jenekhe, J. Am. Chem. Soc. 137 (2015) 4424. [33] J.W. Jung, J.W. Jo, C.C. Chueh, F. Liu, W.H. Jo, T.P. Russell, A.K. Jen, Adv. Mater. 27 (2015) 3310. [34] T. Kim, J.H. Kim, T.E. Kang, C. Lee, H. Kang, M. Shin, C. Wang, B. Ma, U. Jeong, T.S. Kim, B.J. Kim, Nat. Commun. 6 (2015) 8547. [35] S. Shi, J. Yuan, G. Ding, M. Ford, K. Lu, G. Shi, J. Sun, X. Ling, Y. Li, W. Ma, Adv. Funct. Mater. 26 (2016) 5669. [36] Y.J. Hwang, B.A. Courtright, A.S. Ferreira, S.H. Tolbert, S.A. Jenekhe, Adv. Mater. 27 (2015) 4578. [37] L. Gao, Z.G. Zhang, L. Xue, J. Min, J. Zhang, Z. Wei, Y. Li, Adv. Mater. 28 (2015) 1884. [38] B. Fan, Lei Ying, Z. Wang, B. He, X. Jiang, F. Huang, Y. Cao, Energy Environ. Sci. (2017). http://dx.doi.org/10.1039/C7EE00619E. [39] J. Yuan, W. Guo, Y. Xia, M.J. Ford, F. Jin, D. Liu, H. Zhao, O. Inganäsc, G.C. Bazand, Wanli Ma, Nano Energy 35 (2017) 251. [40] L. Lu, M.A. Kelly, W. You, L. Yu, Nat. Photonics 9 (2015) 491. [41] Y. Yang, W. Chen, L. Dou, W.H. Chang, H.S. Duan, B. Bob, G. Li, Y. Yang, Nat. Photonics 9 (2015) 190. [42] V. Gupta, V. Bharti, M. Kumar, S. Chand, A.J. Heeger, Adv. Mater. 27 (2015) 4398. [43] G. Zhang, K. Zhang, Q. Yin, X. Jiang, Z. Wang, J. Xin, W. Ma, H. Yan, F. Huang, Y. Cao, J. Am. Chem. Soc. 139 (2017) 2387. [44] L. Yang, H. Zhou, S.C. Price, W. You, J. Am. Chem. Soc. 134 (2012) 5432. [45] L.Y. Lu, T. Xu, W. Chen, E.S. Landry, L.P. Yu, Nat. Photonics 8 (2014) 716. [46] P. Cheng, Y.F. Li, X.W. Zhan, Energy Environ. Sci. 7 (2014) 2005. [47] J. Zhang, Y. Zhang, J. Fang, K. Lu, Z. Wang, W. Ma, Z. Wei, J. Am. Chem. Soc. 137 (2015) 8176. [48] H. Lu, J.C. Zhang, J.Y. Chen, Q. Liu, X. Gong, S.Y. Feng, X.J. Xu, W. Ma, Z.S. Bo, Adv. Mater. 28 (2016) 9559. [49] T. Liu, Y. Guo, Y. Yi, L. Huo, X. Xue, X. Sun, H. Fu, W. Xiong, D. Meng, Z. Wang, F. Liu, T.P. Russell, Y. Sun, Adv. Mater. 28 (2016) 10008. [50] H. Yao, Y. Cui, R. Yu, B. Gao, H. Zhang, J. Hou, Angew. Chem. Int. Ed. 56 (2017) 1. [51] J.Y. Yuan, H.L. Dong, M. Li, X.D. Huang, J. Zhong, Y.Y. Li, W.L. Ma, Adv. Mater. 26 (2014) 7077. [52] J.Y. Yuan, W.L. Ma, J. Mater. Chem. A 3 (2015) 7077. [53] H. Benten, T. Nishida, D. Mori, H. Xu, H. Ohkita, S. Ito, Energy Environ. Sci. 9 (2016) 135. [54] P.W.M. Blom, V.D. Mihailetchi, L.J.A. Koster, D.E. Markov, Adv. Mater. 19 (2007) 1551. [55] S.R. Cowan, A. Roy, A.J. Heeger, Phys. Rev. B 82 (2010) 245207. [56] L.J.A. Koster, V.D. Mihailetchi, R. Ramaker, P.W.M. Blom, Appl. Phys. Lett. 86 (2005) 123509. [57] A. Facchetti, Mater. Today 16 (2013) 123. [58] L.M. Wang, X. Hu, P. Liu, W. Li, X. Gong, F. Huang, Y. Cao, J. Am. Chem. Soc. 133 (2011) 9638.
[63] [64] [65] [66] [67] [68] [69] [70]
[71] [72]
G.G. Malliaras, J.R. Salem, P.J. Brock, C. Scott, Phys. Rev. B 58 (1998) 13411. Y. Shen, A.R. Hosseini, M.H. Wong, G.G. Malliaras, ChemPhysChem 5 (2004) 16. V.D. Mihailetchi, J. Wildeman, P.W. Blom, Phys. Rev. Lett. 94 (2005) 126602. X.D. Liu, Z. Xu, F.J. Zhang, S.L. Zhao, T.H. Zhang, W. Gong, J.L. Song, C. Kong, G. Yan, X.R. Xu, Chin. Phys. B 19 (2010) 118601. J.M. Frost, F. Cheynis, S.M. Tuladhar, J. Nelson, Nano. Lett. 6 (2006) 1674. M.A. Loi, S. Toffanin, M. Muccini, M. Forster, U. Scherf, M. Scharber, Adv. Funct. Mater. 17 (2007) 2111. R.R. Lunt, N.C. Giebink, A.A. Belak, J.B. Benziger, S.R. Forrest, J. Appl. Phys. 105 (2009) 053711. R.R. Lunt, J.B. Benziger, S.R. Forrest, Adv. Mater. 22 (2010) 1233. K.H. Lam, T.R.B. Foong, Z.E. Ooi, J. Zhang, A.C. Grimsdale, Y.M. Lam, ACS Appl. Mater. Interfaces 5 (2013) 13265. M. Abdelsamie, N.D. Treat, K. Zhao, C. McDowell, M.A. Burgers, R. Li, D.M. Smilgies, N. Stingelin, G.C. Bazan, A. Amassian, Adv. Mater. 27 (2015) 7285. D. Meng, D. Sun, C. Zhong, T. Liu, B. Fan, L. Huo, Y. Li, W. Jiang, H. Choi, T. Kim, J.Y. Kim, Y. Sun, Z. Wang, A.J. Heeger, J. Am. Chem. Soc. 138 (2016) 375. N. Zhou, H. Lin, S.J. Lou, X. Yu, P. Guo, E.F. Manley, S. Loser, P. Hartnett, H. Huang, M.R. Wasielewski, L.X. Chen, R.P.H. Chang, A. Facchetti, T.J. Marks, Adv. Energy Mater. 4 (2014) 1300785. J. Rivnay, S.C.B. Mannsfeld, C.E. Miller, A. Salleo, M.F. Toney, Chem. Rev. 112 (2012) 5488. W. Guo, J. Yuan, H. Yuan, F. Jin, L. Han, C. Sheng, W. Ma, H. Zhao, Adv. Funct. Mater. 26 (2016) 713.
Guanqun Ding received his Bachelor’s degree in 2014 and now as a master student under Prof. Wanli Ma in Institute of Functional Nano & Soft Materials (FUNSOM) of Soochow University. His research is mainly focused on the research of organic solar cells and hybrid solar cells.
Jianyu Yuan obtained his Ph.D. degree from Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University in 2016 under the supervision of Prof. Wanli Ma. After that, he became an Associate Professor in FUNSOM, Soochow University. He has more than 50 peer-reviewed publications in prestigious chemistry and materials SCI journals, with an H-index of 15 and total citation approaching 1000 times. His research interests focus on design and synthesis of functional materials, fabrication and characterization of solution-processed photovoltaic devices.
Feng Jin is pursuing his PhD in Optics at Fudan University under the supervision of Prof. Haibin Zhao. He has been actively involved in the research on the dynamics of charge generation, separation, and transportation in new-type solar cells based on the ultrafast transient and quasi-steady state absorption spectra.
Yannan Zhang received her Bachelor's degree in Soochow University in 2015. She is a master student under the supervision of Prof. Wanli Ma at the Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University. Her research interest focus on developing high-performance solution-processed solar cells using organic materials, nanocrystals, and perovskites.
364
Nano Energy 36 (2017) 356–365
G. Ding et al. Lu Han is a Ph.D candidate student in Prof. Wanli Ma's group at Soochow University. In 2008, she graduated from Soochow University with a BS in Materials Chemistry. Currently, she is in Prof. M. A. Loi's group at University of Groningen as a visiting student. Her research focused on the morphology control of semiconducting nanocrystals and their application for photovoltaic and field-effect transistor.
Haibin Zhao is a professor and the associate chair in Department of Optical Science and Engineering, Fudan University. He received his Ph.D. from College of William and Mary, and then he engaged in postdoctoral research in College of William and Mary and University of California at Riverside. His current research is focused mainly on (i) the spin-dynamics in magnetic metal and semiconductor; (ii) ultrafast spin regulation and control which is driven by photo, magnetism, and electricity by utilizing time-resolved magneto-optical kerr and Faraday spectroscopy; and (iii) the dynamics of charge generation, separation, and transportation in new-type solar cells.
Xufeng Ling received his Bachelor's degree in College of Nano Science & Technology (CNST) of Soochow University (P. R. China). He is currently a master student under the supervision of Prof. Wanli Ma in Institute of Functional Nano & Soft Materials (FUNSOM) of Soochow University. His research is mainly focused on perovskite solar cells.
Wanli Ma is currently a professor in Institute of Functional Nano & Soft Materials (FUNSOM) at Soochow University. He received PhD degree in 2006 from the University of California at Santa Barbara under the supervision of Prof. Alan J. Heeger. Before he joined Soochow University in 2010, he worked as a Postdoctoral Scholar in Prof. Paul Alivisatos’ group at Lawrence Berkeley national laboratory. He received the Chinese Young 1000 program award in 2011. His publications were cited over 10000 times. His current research interest focus on developing solution-processed solar cells, including organic materials, nanocrystals, and perovskites.
365