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).
Optical and electrochemical properties
To obtain potential high Jsc values, the absorption of materials should cover from visible light to near infrared region. 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. 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/red – E(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. 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.
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.
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 J ∝ Iα, 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 Voc ∝ nkT/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.
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. 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. 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. 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].