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 (J_sc)^[12-16]. Based on the premise of high J_sc value, researchers are paying more attention to enhancing open circuit voltage (V_oc) 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 V_oc. 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 V_oc 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 V_oc 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 V_oc value for high efficiency systems.

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).

To obtain potential high J_sc 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 (E_g^opt) of three molecules are estimated by formula as E_g^opt = 1240/λ_onset, where the λ_onset 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 = – [E^ox/red – E(Fc/Fc⁺) + 4.8] eV, where E^ox/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 V_oc 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].

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 V_oc 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 V_oc 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 J_sc 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].

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 J_sc 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 ∝ I^α, 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 V_oc under various incident light intensities to estimate it. Similarly, the fitting curves of V_oc under different incident light intensities (I) are also complied with another relationship as V_oc ∝ 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 μ_e and μ_h can evaluate the recombination of devices. As the conjugated fused rings of π bridges increase, the devices present a bigger μ_e/μ_h 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^[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]}.

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 V_oc for all SM OSCs. The V_oc 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 increasedJ_sc. Our research provides a method to enhance OSC V_oc, which could promote the development of high efficiency OSCs.

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.