Small molecule donors with different conjugated π linking bridges: Synthesis and photovoltaic properties

    Corresponding author: Zeyun Xiao, xiao.z@cigit.ac.cn
    Corresponding author: Shirong Lu, lushirong@cigit.ac.cn
  • 1. Organic Semiconductor Research Center, Chongqing Institute of Green and Intelligent Technology, Chongqing School, University of Chinese Academy of Sciences (UCAS Chongqing), Chinese Academy of Sciences, Chongqing 400714, China
  • 2. University of Chinese Academy of Sciences, Beijing 100049, China
  • 3. University of Manchester, Manchester M13 9PL, UK
  • 4. School of Chemistry and Chemical Engineering, Qiannan Normal University for Nationalities, Duyun 558000, China

Key words: organic solar cellsmall molecule donormolecule energy levelsmorphology

Abstract: Three small molecule (SM) donors, namely B-T-CN, B-TT-CN and B-DTT-CN, with different π conjugated bridges were synthesized in this research. Interestingly, with the conjugated fused rings of the π linking bridge increasing, the SM HOMO levels exhibit a decline tendency with –5.27 eV for B-T-CN, –5.31 eV for B-TT-CN and –5.40 eV for B-DTT-CN. After blending the SM donors with the fullerene acceptor PC71BM, the all SM organic solar cells (OSCs) achieved high Vocs of 0.90 to 0.96 V. However, the phase separation morphology and molecule stacking are also unexpectedly changed together with the enhancement of conjugated degree of π bridges, resulting in a lower power conversion efficiency (PCE) for the B-DTT-CN:PC71BM device. Our results demonstrate and provide a useful way to enhance OSC Voc and the morphology needs to be further optimized.

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1.   Introduction
  • Bulk heterojunction (BHJ) solar cells have attracted extensive attention due to their lightweight, flexibility and potential low cost by solution and roll-to-roll print processes[1-4]. As a result of the development of new materials and device engineering, polymer based organic solar cells (OSCs) have achieved over 17% power conversion efficiencies (PCEs) in single-junction BHJ[5, 6]. However, considering the drawbacks of polymers, such as molecular weight uncertainty, polydispersity and batch-to-batch variation, the OSCs are still far from industrialization[7, 8]. Compared with the polymer, small molecule (SM) donors have advantages such as specific molecular weight, an adjustable chemical structure to easily control the molecular absorption and energy levels, and an easy process to purify[9, 10]. Recently, accompanied with the nonfullerene acceptor (NFA) development (e.g., Y6)[11], the PCEs of small molecular OSCs have progressed thanks to the significant improvement of the short-circuit current (Jsc)[12-16]. Based on the premise of high Jsc value, researchers are paying more attention to enhancing open circuit voltage (Voc) to obtain further improvement of PCEs. The construct of small molecular donors traditionally consists of electron donor core, electron deficiency ending group and the π linking bridge (A-π-D-π-A type). As shown in the molecular molding (Fig. S1), the electron donor parts are mostly distributed on the donor core and π linking bridge while the electron deficient part is located on the acceptor moiety, which determines the molecular energy levels and even the device Voc. Because the conjugated skeleton is limited, the SMs that are usually present insufficient π–π stacking probability and mobility[17, 18]. Many researchers have systematically studied the influence of donor core and ending group on the SM OSCs[19-21]. It should be noted that the π linking bridge also plays an irreplaceable function to adjust molecular aggregation and molecular stacking, and determines molecular energy levels[12, 15, 22].

    Herein, we focus on the π bridge engineering and design three small molecular donors consisting of the benzodithiophene (BDT) core, the cyano-ester ending group with branched alkyl chains to improve the solubility[23], and three types of π linking bridge with different fused ring length to explore their influence on molecule and device properties. The SM donors with the alpha-terthiophene, thieno[3,2-b]thiophene-thiophene and dithieno[2,3-b:2’,3’-D]thiophene π linking bridges are named B-T-CN, B-TT-CN and B-DTT-CN respectively (Fig. 1). The difference among the donor molecules is only the π bridges, while the Voc value of the primary devices shows a distinct difference, 0.90 V for B-T-CN based, 0.94 V for B-TT-CN based and 0.96 V for B-DTT-CN based devices, respectively. We employed the cyclic voltammetry (CV) to estimate the molecular energy levels and found that the highest occupied molecular orbital (HOMO) energy levels decline as the fused rings of π linking bridge increase, while deeper HOMO levels for the donor afford higher Voc for devices. Moreover, we employed atomic force microscopy (AFM) and grazing-incidence wide-angle X-ray scattering (GIWAXS) to explore the influence of three different π bridge-based donors on the blend film morphology and molecular stacking. The B-T-CN:PC71BM shows more optimized donor/acceptor interfaces and molecular aggregation for exciton dissociation and transportation and a majority of molecules tend to take a uniform edge-on π–π stacking for efficient intermolecular carrier transport. We systemically design and synthesize three SM donors with different π linking bridges to adjust the material HOMO levels, which provides a new method of molecular design to promote Voc value for high efficiency systems.

2.   Results and discussion

    2.1.   Synthesis and characterizations

  • The B-T-CN, B-TT-CN and B-DTT-CN were synthesized by n-bromosuccinimide bromination, Vilsmerier-Haack formylation, Suzuki and Stille coupling and Knoevenagel condensation, and the detailed synthetic routes are shown as Scheme 1 and in the Supporting Information. All of the intermediates and final products were fully characterized (Supporting Information). The thermal stabilities of three donors were measured by thermal gravimetric analysis (TGA) and the 5% weight loss decomposition temperatures are 341 °C for B-T-CN, 353 °C for B-TT-CN and 316 °C for B-DTT-CN, respectively (Fig. S2).

  • 2.2.   Optical and electrochemical properties

  • To obtain potential high Jsc values, the absorption of materials should cover from visible light to near infrared region[24]. The UV–Vis spectra of B-T-CN, B-TT-CN and B-DTT-CN in chloroform and neat film are shown in Fig. S3 and Fig. 2(a) and the detailed parameters are summarized in Table 1. In the film condition, three molecules exhibit absorption peaks at 561 nm for B-T-CN, 549 nm for B-TT-CN and 541 nm for B-DTT-CN and another shoulder peaks were found at 609, 596, 588 nm, respectively, which indicates the molecules have intensely intermolecular aggregation[25, 26]. Moreover, the film absorption edge of B-T-CN (666 nm) is larger than the other two SM donors (645 nm for B-TT-CN, 637 nm for B-DTT-CN) which could generate wider external quantum efficiency (EQE) response[27]. The optical band gaps (Egopt) of three molecules are estimated by formula as Egopt = 1240/λonset, where the λonset is the edge of film absorption and defined as 1.86 eV for B-T-CN, 1.92 eV for B-TT-CN, and 1.95 eV for B-DTT-CN, respectively. The molecule HOMO and lowest unoccupied molecular orbital (LUMO) energy levels and electrical band gaps were measured by CV test (Fig. S4), and calculated by formula as HOMO/LUMO = – [Eox/redE(Fc/Fc+) + 4.8] eV, where Eox/red is the potential from the first oxidation and reduction peak and E(Fc/Fc+) is the half-wave potential of the standard. As shown in Fig. 2(b), the HOMO levels are slightly deeper as the conjugated fused rings increasing (–5.27 eV for B-T-CN, –5.31 eV for B-TT-CN, and –5.40 for B-DTT-CN) which could lead to higher Voc value in OSCs[28]. The LUMO levels exhibit little difference attributed to the same ending group on the chemical structures and the huge LUMO difference with the acceptor (PC71BM) could give sufficient driving force for the carrier separation at the D/A interface[29].

  • 2.3.   Photovoltaic performance

  • To explore the potential photovoltaic properties of three small molecular donors, thin film BHJ SM OSCs were fabricated using a conventional device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiopene):poly(styrene sulfonate) (PEDOT:PSS)/B-T-CN or B-TT-CN or B-DTT-CN:PC71BM/Phen-NaDPO/Ag (Fig. 3(a), device area = 0.11 cm2). Three independent devices were primarily optimized through adjusting spin coating rotational speed and using solvent vapor annealing treatment (SVA). The champion devices of three materials were treated by tetrahydrofuran SVA for 10 s. The merits of B-T-CN:PC71BM, B-TT-CN:PC71BM, and B-DTT-CN:PC71BM based devices parameters are summarized in Table 2 which were tested under simulated AM 1.5G irradiation (100 mW/cm2) condition. Fig. 3(b) shows the best devices J–V curves. The Voc values of three devices (0.90 V for B-T-CN:PC71BM, 0.94 V for B-TT-CN:PC71BM, and 0.96 V for B-DTT-CN:PC71BM) are gradually increased as the fused ring expands, which demonstrates our strategy of adjusting π linking bridge to control molecular energy levels. In addition, the enhanced device Voc is reliable. The EQE spectra of three devices are shown in Fig. 3(c). The photoresponse range for three molecule-based devices is located from 350 to 670 nm which is in consistency with the film absorption. The B-T-CN:PC71BM based curve is higher than others in all EQE response region bring a higher Jsc for device (5.29 mA/cm2 for B-T-CN:PC71BM, 4.20 mA/cm2 for B-TT-CN:PC71BM, and 2.61 mA/cm2 for B-DTT-CN:PC71BM). Furthermore, The B-DTT-CN:PC71BM based device exhibits a significantly lower fill factor (FF) (39.8%) than others (55.7% for B-T-CN:PC71BM and 54.8% for B-TT-CN:PC71BM), which could be caused by the vast recombination of these devices[30].

  • 2.4.   Charge recombination

  • Based on the optimal devices, we explored the charge recombination of each material based system to explain the difference of device properties. Firstly, we studied Jsc under different incident light intensities to evaluate the degree suffering from bimolecular recombination of devices. As previous studies, the dependence of current density on incident light intensity (I) obey to the power law equation as JIα, where α represents the power factor. Briefly, fitting α value is between 0 to 1, the value more closes to 1 means the devices are less influenced by bimolecular recombination[31, 32]. From Fig. 4(a), we found that the α values are 0.998 for B-T-CN:PC71BM, 0.995 for B-TT-CN:PC71BM, and 0.992 for B-DTT-CN:PC71BM based devices respectively which indicate three systems have less bimolecular recombination in the devices. A slightly increased α of B-T-CN:PC71BM based devices suggests more efficient charge extraction in devices, which is consistent with the higher FF value. For trap-assistant recombination, we measured Voc under various incident light intensities to estimate it. Similarly, the fitting curves of Voc under different incident light intensities (I) are also complied with another relationship as VocnkT/qln(I), where k is the Boltzmann constant, T is the temperature in Kelvin, and q is the elementary charge. The parameter n represents the degree suffering from trap assistant recombination of devices which is often between at 1 to 2[33, 34]. Generally, the n value more far away from 1 means the devices have more serious carrier traps crossed the active layer or at the interface between the organic semiconductor and the electrode[32, 35]. As the fitted data shown in Fig. 4(b), the n values are 1.64 for B-T-CN:PC71BM, 1.80 for B-TT-CN:PC71BM, and 1.92 for B-DTT-CN:PC71BM based devices, respectively, which indicates that the trap recombination seriously occurred in B-TT-CN:PC71BM and B-DTT-CN:PC71BM based devices.

    Furthermore, we measured the hole and electron carrier mobilities by space charge limited current (SCLC) method (Fig. 5). For hole-only device, the device structure is ITO/MoO3/active layer/MoO3/Ag, and electron-only cell was fabricated with the device architectures as ITO/ZnO/Phen-NaDPO/active layer/Phen-NaDPO/Ag. After blending with PC71BM, the B-T-CN:PC71BM exhibited both higher hole mobility of 2.55 × 10–5 cm2/(V·s) and electron mobility of 3.37 × 10–5 cm2/(V·s) than B-TT-CN:PC71BM (hole mobility of 1.25 × 10–5 cm2/(V·s) and electron mobility of 1.87 × 10–5 cm2/(V·s)) and B-DTT-CN:PC71BM (hole mobility of 9.14 × 10–6 cm2/(V·s) and electron mobility of 1.58 × 10–5 cm2/(V·s)). Even though the three small molecule donors have a similar chemical backbone, the blend mobilities are mainly affected by the film morphology which will be discussed in the next section. The ratio of μe and μh can evaluate the recombination of devices. As the conjugated fused rings of π bridges increase, the devices present a bigger μe/μh ratio as 1.73 for B-DTT-CN:PC71BM, 1.50 for B-TT-CN:PC71BM, and 1.32 for B-T-CN:PC71BM respectively. The more balanced mobility of B-T-CN:PC71BM could bring a lower recombination device, which is consistent with the fill factor result.

  • 2.5.   Blend morphology

  • As mentioned earlier, three devices are mainly affected by trap-assistant recombination. To better understand the charge recombination, we employed AFM and GIWAXS measurements to find the structure–properties relationship, and to analyze the active layers morphology and molecular stacking[36]. Under the same device processing condition, we measured the AFM phase images of active layers (Figs. 6(a)6(c)). It is clear that the B-T-CN based film (Fig. 6(a)) exhibits an obvious “interpenetrating network” with acceptors and well-defined domain areas, which could produce enough interface and moderate pure donor or acceptor domain for carrier dissociation and transportation. Besides, the root mean square (RMS) roughness of blends is 16.1 nm for B-T-CN:PC71BM, 13.8 nm for B-TT-CN:PC71BM, and 7.9 nm for B-DTT-CN:PC71BM based films, respectively. The higher RMS value of B-T-CN:PC71BM based active layer means that the molecules are more likely to form a great crystallization, which could benefit the intermolecular charge transportation[37]. It is known that excellent blend morphology and moderate molecular aggregation are key factors for the less trap recombination. For B-T-CN:PC71BM based blend film, more optimized blend morphology to prevent trap generation afforded better device performances than others. The huge morphological difference in the three donor based blends illustrates that the conjugated π linking bridges with different conjugated lengths play a crucial role to adjust molecule aggregation when blended with the same acceptor.

    We also employed GIWAXS to study the insight molecular stacking of blend films[38]. In organic materials, the intermolecular π–π stacking has two directions as in-plane (IP) and out-of-plane (OOP) which produce diffraction peaks around 1.7 Å–1 (010). As shown in Figs. 7(a)7(c), all of the molecular donors have well-defined edge-on π–π stacking in blend films. Detailed IP and OOP scattering profiles of blend films are shown in Figs. S5(a)–S5(c). Moreover, distribution of azimuthal angle of π–π stacking from 0° to 90° (Fig. 7(d)) proves that the B-T-CN:PC71BM and B-TT-CN:PC71BM based blends prefer an edge-on orientation for efficient intermolecular charge transport, while the B-DTT-CN:PC71BM based blend exhibits isotropy orientation at π–π staking area which could result in abundant recombination[39, 40]. Compared with B-DTT-CN based blend, the uniform edge-on π–π stacking of B-T-CN:PC71BM and B-TT-CN:PC71BM based blends could provide more charge transport tubes, which is consistent with the devices and recombination results[27, 41].

3.   Conclusion
  • In summary, we have designed and synthesized three different type π linking bridge small molecular donors. By changing conjugated length, the small molecules achieved a deeper HOMO level, which result in a higher Voc for all SM OSCs. The Voc increased from 0.90 V enhance to 0.96 V and an overall PCE of 2.65% for B-T-CN:PC71BM based, 2.16% for B-TT-CN:PC71BM based and 1.00% for B-DTT-CN:PC71BM based devices was achieved. However, the blend morphology and molecular stacking are also changed by adjusting π linking bridges. When the π linking bridge from dithieno[2,3-b:2’,3’-D]thiophene (B-DTT-CN) change to alpha-terthiophene (B-T-CN), the blend morphology tends to present more clear interpenetrating network structures and the molecular packing becomes more uniform on the OOP detraction which could provide more moderate D/A interfaces for carrier desolation and more effective intermolecular charge transport tubes to ensure less trap recombination and increased Jsc. Our research provides a method to enhance OSC Voc, which could promote the development of high efficiency OSCs.

Acknowledgements
  • This work was supported by National Natural Science Foundation of China (21801238), National Youth Thousand Program Project (R52A199Z11), CAS Pioneer Hundred Talents Program B (Y92A010Q10) and Organic Semiconductor Center of Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences.

Figure (8)  Table (2) Reference (41) Relative (20)

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