Photophysics, morphology and device performances correlation on non-fullerene acceptor based binary and ternary solar cells

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Photophysics, morphology and device performances correlation on non-fullerene acceptor based binary and ternary solar cells Guanqing Zhou , Hong Ding , Lei Zhu , Chaoqun Qiu , Ming Zhang , Tianyu Hao , Wei Feng , Yongming Zhang , Haiming Zhu , Feng Liu PII: DOI: Reference:

S2095-4956(19)30928-3 https://doi.org/10.1016/j.jechem.2019.12.007 JECHEM 1033

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Journal of Energy Chemistry

Received date: Revised date: Accepted date:

11 November 2019 5 December 2019 9 December 2019

Please cite this article as: Guanqing Zhou , Hong Ding , Lei Zhu , Chaoqun Qiu , Ming Zhang , Tianyu Hao , Wei Feng , Yongming Zhang , Haiming Zhu , Feng Liu , Photophysics, morphology and device performances correlation on non-fullerene acceptor based binary and ternary solar cells, Journal of Energy Chemistry (2019), doi: https://doi.org/10.1016/j.jechem.2019.12.007

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Photophysics, morphology and device performances correlation on non-fullerene acceptor based binary and ternary solar cells Guanqing Zhoua,b,1, Hong Dinga,1, Lei Zhub,c, Chaoqun Qiub, Ming Zhanga, Tianyu Haoa, Wei Fenge, Yongming Zhangb, Haiming Zhud,*, Feng Liua,b,*

a

School of Physics and Astronomy, and Collaborative Innovation Center of IFSA (CICIFSA),

Shanghai Jiao Tong University, Shanghai 200240, China b

School of Chemistry and Chemical Engineering, and Center for Advanced Electronic Materials

and Devices, Shanghai Jiao Tong University, Shanghai 200240, China c

State Key Laboratory of Luminescent Materials and Devices, South China University of

Technology, Guangzhou 510640, Guangdong, China d

Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang, China

e

State Key Laboratory of Fluorinated Materials, Zibo City 256401, Shandong, China

1

These authors contributed equally to this work.

* Corresponding authors. Email addresses: [email protected] (H. Zhu), [email protected] (F. Liu). Abstract Non-fullerene acceptor (NFA) based organic solar cells (OSCs) are of high efficiency and low energy loss and low recombination features, which is owing to the advantage of non-fullerene acceptors. The photophysics investigation of non-fullerene solar cells, in comparing to fullerene based analogue as well as mixed acceptor ternary blends could help to understand the working mechanism of NFA functioning mechanism. We choose PBDB-T donor, the fullerene derivative PC71BM acceptor, and the non-fullerene acceptor ITIC as the model system, to construct binary and ternary solar cells, which then subject to ultrafast spectroscopy investigation. The charge transfer pathway in binary and ternary blends is revealed. And it is seen that ITIC leads to a faster exciton separation and exciton diffusion. ITIC in blends suppresses the geminate recombination and shows smaller amount of charge transfer states, which is beneficial for the device performance. And the addition of ITIC enhances the crystallinity for both donor and acceptor leads to a morphology change of forming bicontinuous crystalline networks and phase separation. In a consequence, fill factor and JSC, increase dramatically for the related OSC.

Keywords: Non-fullerene electron acceptors; Charge transfer; Recombination; Crystallinity 1. Introduction Bulk-heterojunction organic solar cells have seen a rapid improvement in device performance in the last few years, with power conversion efficiencies (PCE) rising to 17% in single junction solar cells [1]. However, the state-of-the-art PCEs is still much lower than the c-Si and perovskite counterparts [2,3]. Organic semiconductors have inherently low dielectric properties, such that the incident light generates tightly bounded electron-hole pairs, also known as excitons, with strong Coulombic binding energy [4–6]. Therefore, excess energy is needed to drive its separation into form free carriers, which involves the formation and dissociation of the charge transfer (CT) state. This results in energy loss, that is, the voltage deficiency from the bandgap offset to open-circuit voltage (VOC) is large, and hence limits the PCEs [7]. A high CT energy (ECT) may improve the VOC, yet a lower driving force in CT states generation leads to reduce charge carrier generation, and thus results in low external quantum efficiencies (EQE) [8]. The early stage OPV knowledge based on fullerene acceptors highlights the importance of driving force for free charge generation that the electron affinity (EA) and ionization potential (IP) of the donor material should be higher than that of the fullerene acceptors by at least 0.3 eV [5,9]. And the CT state energy is usually below the offset bandgap that leads to the additional energy loss [11,12]. In most recent development, the non-fullerene acceptors (NFAs) show different behavior that high external quantum efficiencies (EQEs) can be obtained even if the energy offsets of EAs or IPs between the donors and acceptors are small [9,12–17]. Such feature delivers high VOC for non-fullerene solar cells, which, however, triggers a critical question on the mechanism of charge generation. Emerging theories such as the energetic disorder, donor/acceptor orientation/morphology, and entropic gain seems intuitive, which yet can only explain the exciton dissociation in specific cases [18–22]. The spectroscopic methods such as time-resolved photoluminescence and transient absorption are the prominent tools to investigate the dynamics of exciton generation and transformation, which can readily disentangle the driving force and photoelectronic processes but need to be coupled with chemical structure and thin film morphology to look further into the underlying solar cell functioning mechanism [9,18,23,24].

In general, new choice of BHJ material recipes and interfacial layer modifications could improve the short-circuit current (JSC) and open-circuit voltage (VOC) in device performance, which is correlated with the donor/acceptor energy level offsets and related driving forces [18,25,26]. Fill factor is more related with a balanced carrier transport that of heritage of improved thin film morphology [10,27,28]. Thus in developing high efficiency solar cells, these intrinsic properties need to be considered. Apart from the static solar cell performance parameters, the photophysical properties such as exciton diffusion and charge generation kinetics are the fundamental aspects in solar cell functioning [29–32], whose optimization addresses the basic question on how and why device performance is improved. In the current manuscript, we reported a systematical research on the exciton dynamics, morphology, and their correlation with device performances. Efforts were devoted to understand the difference in photopysics of fullerene and non-fullerene BHJ solar cells. Ternary BHJ blends summating donor polymer, fullerence acceptor, and non-fullerene acceptor were also constructed to fine-tune the morphology and carrier dynamics. The optimized ternary devices (1:0.4:1 weight ratio PBDB-T:PC71BM:ITIC) yield an average PCE of 10.39% with a FF of 72.06%, VOC of 0.90 V, and JSC of 16.03 mA cm−2, comparable to PBDB-T:PC71BM binary system that has much improved absorption features. femto-second transient absorption (TA) spectroscopy that was used to selectively excite single component in blends revealed the detailed excitonic processes and the charge transfer pathway of each component. Such results and thin film morphology investigation in collection help to explain the working principle and fundamental advantage of non-fullerene acceptors based BHJ solar cells, and the extended ternary blends and thereof. 2.

Experimental

2.1. Materials/synthesis PBDB-T and ITIC were purchased from Solarmer Materials, Inc. PC71BM were purchased from Nano-C materials, Inc. All of the materials were used as received without further purification. All other materials were commercially available and were unaltered before using. 2.2. Device fabrication The devices were fabricated with an inverted structure of ITO/PEDOT:PSS/ PBDB-T:PC71BM:ITIC/PDINO/Al. The PEDOT:PSS layer was spin-coated on top of a

pre-cleaned, UV-Ozone-treated ITO substrate, and then annealed at 150 °C for 10 min. Subsequently, PBDB-T:PC71BM:ITIC (1:a:b weight ratio) in a 24 mg mL−1 chlorobenzene : DIO (99.5:0.5 volume ratio) solution were spin-coated at 1800 rpm for 60 s to obtain a film thickness of approximately 100 nm. The film thickness data were obtained via a surface profilometer (Dektak XT, Bruker). To control the morphology of the blend film, the active layers were annealed at 140 °C for 10 min. Then the device fabrication was completed by spin-coating PDINO and thermally evaporating 100 nm thick aluminium under vacuum at a pressure of 1.5×10−4 Pa. The effective area of each device is 0.05 mm2 defined by a metal mask. 2.3. Device measurement The J-V curves of the devices were measured under AM 1.5G illumination at 100 mW cm−2 using an AAA solar simulator (XES-40S3, SAN-EI Electric Co., Ltd.) and a keithley 2400 source-measure unit. Light intensity is calibrated with a standard silicon cell (Crystalline Silicon Reference PV Cell AK-200, KONICA MINOLTA, INC). The EQE data were obtained using a solar cell spectral response measurement system (QTEST HIFINITY 5). 2.4. Fluorescence measurements The pumping light source used to excite the samples was a green laser (532 nm), with a power of 10 mW. The photoluminescene spectra were recorded with an Andor spectrometer (Shamrock sr-303i-B), which was coupled with a Newton electron multiplying CCD detector. 2.5. Time-resolved photoluminescence measurements The blend samples were excited with a femto-second laser and TRPL decay kinetics were collected using a TCSPC module (PicoHarp 300) and a SPAD detector (IDQ, id100) with an instrument response function ~100 ps. Here we collect the emission light from donor to determine electron transfer process, which we use 515 nm pump light. And we select 700 nm excitation laser to collect emission light to study hole transfer process. 2.6. Transient absorption spectroscopy For femto-second transient absorption spectroscopy, the fundamental output from Yb:KGW laser (1030 nm, 220 fs Gaussian fit, 100 kHz, Light Conversion Ltd.) was separated to two light beam. One was introduced to NOPA (ORPHEUS-N, Light Conversion Ltd.) to produce a certain wavelength for pump beam (here we use 520 and 750 nm, 30 fs pulse duration), the other was focused onto a YAG plate to generate white light continuum as probe beam. The pump and probe

overlapped on the sample at a small angle less than 10°. The transmitted probe light from sample was collected by a linear CCD array. 3. Results and discussion 3.1. Device performance Fig. 1(a, b) shows the chemical structure and energy levels of the materials used in solar cells. The binary blends PBDB-T:ITIC and PBDB-T:PC71BM show suitable energy levels alignment to split photon-generated excitons. PC71BM has a deeper highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, thus it can better host electrons and form electron transport channels. The absorption spectra of PBDB-T and ITIC is complimentary, and PBDB-T:ITIC blends cover the whole visible to NIR region (350-850 nm, Fig. 1c), from which a broad external quantum efficiency (EQE) response is expected. Binary and ternary solar cells are fabricated using a conventional device structure and detailed performance data are summarized in Fig. 1(d) and Tables 1 and S1. The PBDB-T:PC71BM binary blends showed a moderate efficiency of 7.28%±0.18%, with a VOC of 0.84 V, a JSC of 13.23 mA cm−2 and a FF of 66%. The PDBD-T:ITIC binary blends showed a high efficiency of 10.49%±0.15%, with a VOC of 0.90 V, a JSC of 15.91 mA cm−2, and a FF of 74%. The improved JSC in PBDB-T:ITIC blends is ascribed to the improved light absorption in near IR region due to the contribution from ITIC component. Ternary blends of varied compositions were made, and the detailed device performance is summarized in Table S1. Two model ternary blends PDBD-T:PC71BM:ITIC=1:1:0.4, and 1:0.4:1 are selected in subsequent photophysical investigations due to the similar VOC and JSC, which showed PCE of 9.16% and 10.39% respectively. Although the two BHJ blends showed different ITIC concentration by a large amount, the contribution in near IR light harvesting is close, as seen from the EQE responses. And thus PC71BM and ITIC work collectively in driving the photon harvesting in BHJ blends, giving rise to the high composition tolerance in ternary BHJ blends.

Fig. 1. Properties and photovoltaic performance. (a) Chemical structures of PBDB-T, ITIC and PC71BM in the ternary device. (b) Energy levels of PBDB-T, ITIC and PC71BM. (c) Normalized thin-film absorption of pristine PBDB-T, ITIC and PC71BM film. (d) Current-voltage characteristics of the binary and ternary devices under constant incident light intensity (AM 1.5G, 100 mW cm−2). (e) EQE of the binary and ternary devices.

Table 1. Summary of photovoltaic parameters in binary and ternary solar cells PBDB-T:PC71BM:ITIC 1:1:0 1:1:0.4 1:0.4:1 1:0:1

VOC (V) 0.84 0.88 0.90 0.90

JSC (mA cm−2) 13.23±0.19 15.13±0.31 16.03±0.46 15.91±0.53

FF (%) 66.31±0.28 68.43±0.32 72.06±0.39 74.22±0.56

PCE (%) 7.49（7.28±0.18） 9.16（8.82±0.25） 10.39 (9.93±0.13) 10.63(10.49±0.15)

Fig. 2. (a) Photoluminscence spectra of the pristine donor, NFA and blended films. The intensities are corrected by their absorptions at the excitation wavelength (532 nm). Time resolved

photoluminscence (TRPL) spectra for pristine and blended films excited at 515 nm (b) and 700 nm (c). 3.2. Charge generation investigation via TRPL To investigate the charge generation in BHJ thin films, the steady state photoluminescence (PL) of ternary films was recorded. Reference films of pristine PBDB-T, ITIC and three binary blends were also measured and shown in Fig. 2(a). The photoluminescence of pristine PBDB-T had a broad emission with a peak at 700 nm. Pristine ITIC showed an emission peak at 780 nm. ITIC:PC71BM showed 40% PL intensity comparing to ITIC neat film, indicating the weak electron transfer from ITIC to PC71BM. Quite low PL intensity was seen in PBDB-T:PC71BM and PBDB-T:ITIC blends, thus both blends have strong exciton quenching to ensure high flux carrier generation. Thus efficient electron transfer from PDBD-T to PC71BM and ITIC, and efficient hole transfer from ITIC to PDBD-T are ensured. Ternary blends showed similar quenching efficiency for both PDBD-T and ITIC components, which indicates a suitable morphology avoiding ITIC:PC71BM agglomeration region to efficiently dissociate excitons. Thus inside ternary BHJ blends, there are close contact interfaces between PBDB-T and ITIC to ensure a high JSC. The solar cell efficiency is synergistically determined by multiple processes, including light absorption, charge transfer at donor-acceptor interface, carrier transport and collection at electrodes [33–35]. The electron transfer process was firstly probed by TRPL spectrum. As illustrated in Fig. 2(b), PBDB-T neat film shows fluorescence lifetime (τ) of 1379.4 ps. After blending ITIC, the fluorescence lifetime is significantly decreased with a time constant of 89.9 ps, indicating a highly efficient electron transfer at the PBDB-T:ITIC heterojunction. To avoid the influence of ITIC fluorescence quenching and study electron transfer directly, we only probed 600-700 nm and pump at 515 nm in TRPL measurement. As for PBDB-T:PC71BM binary blend film, it shows a little faster quenching with a lifetime of 76.89 ps. And the PL decaying time in ternary film (1:1:0.4) and (1:0.4:1) are 78.05 ps and 86.26 ps, respectively. The more efficient electron transfer in PC71BM dominant blend film is the result of deeper LUMO energy level and better mixed morphology. Similarly, ITIC neat film possesses long fluorescence lifetime (τ) of 442.7 ps, while ITIC:PC71BM blend film presents PL decaying with shorter fluorescence lifetime (τ=246.2 ps) shown in Fig. 2(c). This result indicates the weak electron transfer pathway between ITIC and PC71BM. The quiet short fluorescence lifetime (τ) of PBDB-T:ITIC probed at 850-950

nm is 74.55 ps shown in Fig. 2(c), indicating of efficient hole transfer between ITIC and PBDB-T.

Fig. 3. Charge transfer dynamics. (a) Colorplot of TA spectrum of PBDB-T:ITIC blend film under 700 nm excitation with a fluence below 10 μJ cm−2. (b) Representative TA spectra at indicated delay time. Gray circles: Pristine ITIC film excited at 700 nm. (c) The kinetics of hole transfer in different blend films which extracts from 580 nm (GSB signal raising of PBDB-T). (d) The kinetics of electron transfer in different blend films which extracts from 1180 nm (Singlet exciton absorption signal decaying of PBDB-T). 3.3. Charge transfer rate enhancement The detailed charge transfer kinetics was investigated using femto-second transient absorption (TA) spectroscopy. The steady-state absorption peaks for PBDB-T and ITIC are well separated in spectral domain, therefore both spectral and temporal characteristics of charge transfer dynamics can be extracted. Hence the excitation wavelength of 750 nm was selected here to excite only acceptors without exciting donors while testing hole transfer process. Shown in Fig. 3(a) showed the color map of TA spectra of PBDB-T:ITIC film. Representative one-dimensional TA profiles with different delay times were summarized in Fig. 3(b). The bleach peaks at 650 nm and 730 nm appeared in neat ITIC and PBDB-T:ITIC blend film, corresponding to stimulated

emission (SE) and ground-state bleach (GSB) of the absorption transition in ITIC due to photoexcitation. In together with the decay of ITIC bleach peak at 650-800 nm, weak bleach peaks at 550-650 nm appeared in the TA spectrum of PBDB-T:ITIC film, which matched well with the absorption feature of PBDB-T. As shown in Fig. S3, the bleach decay kinetics of photoexcited ITIC agrees well with the rising process of the PBDB-T GSB, indicating photoexcited hole transfer from ITIC to PBDB-T. The GSB in PBDB-T rises bi-exponentially with time constant of 0.530±0.025 and 2.648±0.150 ps. The former is assigned to ultrafast exciton dissociation at the interface of PBDB-T and ITIC and the latter is ascribed to exciton diffusion time in ITIC towards interfaces before the hole transfer process. The ultrafast and efficient hole-transfer between ITIC and PBDB-T is due to the large HOMO energy level offset. For PBDB-T:PC71BM:ITIC ternary blends, the TA spectrum was used to study the hole transfer process of different composition of ITIC and PC71BM. From The kinetic features of 580 nm 1D profiles that arise from PDBD-T GSB signals were shown in Fig. 3(c). We fit the hole transfer process in ternary blend films (1:0.4:1 and 1:1:04) using bi-eponential formula, which yielded time constants of 0.684±0.047 ps, 3.123±0.437 ps (1:0.4:1), and 0.963±0.525 ps, 5.746±0.628 ps (1:1:0.4). Thus, the hole transfer rate between ITIC and PBDB-T is much faster, and with rising the proportion of PC71BM, the hole transfer rate gets slower. The diffusion of hole in ITIC to PBDB-T is also slowed down by adding PC71BM, which can be due to a slightly difference in thin film morphology or the thin film electronic structure difference that PC71BM has a deep HOMO energy level to barricade hole jumping from ITIC to PDBD-T. TA measurement was also applied to investigate electron transfer process. The near-infrared TA

spectrum

of

neat

PBDB-T,

neat

ITIC,

PBDB-T:PC71BM,

PBDB-T:ITIC,

PBDB-T:PC71BM:ITIC ternary blend films were recorded. As shown in Fig. 4 the peaks of singlet exciton absorption of PBDB-T and ITIC are 1150 and 950 nm, respectively. As shown in Fig. S5, the peaks of polaron absorption of PBDB-T (in PBDB-T:PC71BM blends) is at 950 nm, which overlapped with the singlet exciton absorption of ITIC and thus difficult for us to obtain the charge transfer information from the kinetics of polaron signals. Thus we select the decaying kinetics of singlet exciton absorption of PBDB-T to probe the electron transfer from PBDB-T to ITIC and to PC71BM. As illustrated in Fig. 3(d), the electron transfer processes in binary and ternary system are quite efficient, and the electron transfer from PBDB-T to PC71BM is faster,

agreeing well with TRPL result. The detailed electron transfer time constants by monoexponential fitting yields value of 0.26 ps (1:1:0), 0.31 ps (1:1:0.4), 0.35 ps (1:0.4:1) and 0.4 ps (1:0:1), respectively. Slightly faster electron transfer rates were seen comparing to hole transfer rates, which is correlated with the energy level offsets that provides driving force. However, both kinetic value in ITIC is comparable to PC71BM, which is quite astonishing and addresses the fundamental issues of the photophysical process and advantage of using ITIC non-fullerene acceptor.

Fig. 4. Colorplot of TA spectrum of pristine (a) PBDB-T and (c) ITIC film under 520 and 750 nm excitation with a fluence below 10 μJ cm−2 probed between 535 nm and 1200 nm. Representative TA spectra of pristine (b) PBDB-T and (d) ITIC film at indicated delay times.

Table 2. The proportion of nongeminate and geminate recombination in different blend films Parameter 1-f (geminate recombination) f (non-geminate recombination)

PDBT: ITIC 0.01 0.99

PBDB-T: PC71BM 0.60 0.40

PBDB-T:PC71B M:ITIC (1:0.4:1) 0.1 0.90

PBDB-T:PC71BM :ITIC (1:1:0.4) 0.25 0.75

Fig. 5. Recombination dynamics. Normalized decay dynamics of the charge-induced absorption between 890 and 930 nm for different pump fluences of (a) PBDB-T:PC71BM, (b) PBDB-T:ITIC, (c) PBDB-T:PB71BM:ITIC (1:1:0.4) and (d) PBDB-T:PC71BM:ITIC (1:0.4:1) blend film. 3.4. Geminate recombination versus nongeminate recombination Recombination process is an important issue in determining the device performance [36–38]. Geminate recombination loss is an important factor that determines the feasibility of charge separation process, which needs to be suppressed to improve JSC. We analyzed the decay dynamics of the TA signals of blend films to evaluate the branching ratio of geminate versus nongeminate recombination. Since the GSB is obscured by spectral relaxation, the dynamics are strongly wavelength dependent. Therefore, we use the photoinduced absorption (PA) signal averaged from 890 to 930 nm to study the recombination processes. In Fig. 5(a) we present the decay traces of the PA signal of PBDB-T:PC71BM obtained from the long delay TA measurements at pump energies between 3 μJ cm−2 and 30 μJ cm−2. This result shows that 60% component of the recombination does not depend on flux, indicating a geminate recombination. Therefore, a portion of the quenched excitons (60%) in the PBDB-T:PC71BM blend form charge-transfer states that decay within 100 ps. And the rest of 40% component of recombination follows the nongeminate

behavior. PBDB-T:ITIC binary blends show high flux denpendence in decaying traces (Fig. 5b), indicating negligible geminate recombination. Comparing four binary and ternary blend films (see Table 2 and Fig. 5), we conclude that with the rising of ITIC concentration, BHJ thin films show less geminate recombination and thus easier charge generation, which highlights another advantage of ITIC non-fullerene acceptors in BHJ solar cells.

Fig.

6.

Morphology

investigations.

(a)

Scattering

profiles

for

different

proportion

PBDB-T:PC71BM:ITIC blend film. (b, c) The peak height and crystal coherence length of different proportion PBDB-T:PC71BM:ITIC blend films. (d) RSoXS scattering profiles of the binary and ternary BHJ thin films using a photon energy of 284.2 eV. q, scattering vector. 3.5. Morphology characterization The crystalline order of nonfullerene acceptors constitutes an important sector in BHJ thin-film morphology that plays a critical role in carrier transport [27,28,39,40,41]. BHJ thin film crystallinity was studied by using grazing incidence wide-angle X-ray Scatering (GIWAXS). Figs. 6(a) and S6–S8 shows the GIWAXS diffraction patterns and line-cut profiles. The PBDB-T in thin film showed quite strong structure order, with (100) diffraction at 0.29 Å−1 with a d-spcing of 21.70 Å in the in-plane (IP) direction and a (010) diffraction peak at 1.71 Å−1 with a d-spacing of 3.67 Å in the out-of-plane (OOP) were recorded. The ITIC shows (001) and (020) peak at 0.333 Å−1 and 0.440 Å−1, respectively, and broad π−π stacking in the out-of-plane direction at 1.698 Å−1. As seen from the PBDB-T:PC71BM blend film, the lamellae diffraction of polymer is observed in both in-plane and out-of plane patterns with a broad azimuthal angle spreading and the crystal coherence length (CCL) is estimated to be 159.5 Å (see Fig. 6b,c and Table S2). With the addition

of ITIC, the crystallization of blend films increased. The in-plane CCL for PBDB-T is enhanced to be 170.8 Å with the pronounced highly ordered reflections (PBDB-T:PC71BM:ITIC=1:1:0.4). And the largest in-plane CCL of PBDB-T is found in PBDB-T:ITIC binary blend film, which is 208.9 Å. This obvious crystallinity enhancement for both donor and acceptor leads to a morphology change of forming bicontinuous crystalline networks, thus charge mobilities undergo an order of magnitude improvement. In a consequence, JSC and FF increase dramatically for the related OSC. BHJ thin film morphology was further studied by transmission electron microscopy (TEM). The results are shown in Fig. S9, ITIC not only promote the formation of bicontinuous interpenetrating BHJ structures with well-sized nanoscopic fibers but also inhibit the aggregation of polymer or acceptors on the film surface. With the addition of ITIC, the fibril structure is more remarkable, and the fiber size is increased, which is more conducive to carrier transport and enhance the current [42]. More detailed statistics on phase separation size were studied via RSoXS by using a high-flux synchrotron X-ray and enhanced carbon k-edge contrast (284.3 eV). As seen from Figs. 6(d) and S10, the PBDB-T:PC71BM blend film shows a sharp scattering peak at 0.01499 Å−1, which corresponds to a domain size of 20.943 nm. When ITIC loading increases, the RSoXS profiles become broadened and shift towards a smaller q region (below 0.01 Å−1) (the detailed domain size data was shown in Table S4), indicating the increased inter highway distance, which agrees well with the TEM characterizations and TA measurements. 4. Conclusions We unraveled for the complex photophysics of the polymer:fullerene:non-fullerene ternary blend PBDB-T:PC71BM:ITIC by transient absorption spectroscopy and revealed how the presence of the non-fullerene molecules improves the overall device performance. Specifically, we observed the electron and hole transfer pathway of each component. The incorporation of ITIC into the blend film improved the blend’s hole mobility. And the attendance of ITIC improve hole transfer rate, which is in accordance with the better current density. Furthermore, we demonstrated that the non-fullerene molecules not only improve charge transfer rate, but suppress the geminate recombination of ternary solar cells. And the addition of ITIC promotes the improvement of crystallinity of BHJ and better phase separation to suit the photophysical process in BHJ device operation.

Conflict of Interest The authors declare no conflict of interest Acknowledgments This work was financially supported by the grant from the National Natural Science Foundation of China (no. 21734009, 11327902, 11574204, 11774224, and 21822505) and National Key Research and Development Program of China (2017YFA0207700). Portions of this research were carried out at beam line 7.3.3 and 11.0.1.2 at the Advanced Light Source, Molecular Foundry, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

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Graphical abstract

Non-fullerene electron acceptor ITIC leads to faster charge transfer, smaller amount of geminate recombination, better crystalinity and phase separation in ternary and binary solar cells.