Novel thieno[3,2-b]thiophene-embedded small-molecule donors for highly efficient and photostable vacuum-processed organic photovoltaics

Highlights

• Four thieno[3,2-b]thiophene-based donor-π-acceptor–type small-molecule donors were synthesized and characterized.

• The influences of introducing cyano groups onto molecules on the physical and crystal stacking properties are elucidated.

• DTDCPTT:C₇₀ binary photovoltaic device shows prominent power conversion efficiency under both outdoor and indoor conditions.

• Vacuum-processed DTDCPTT-based devices exhibit high stability under continuous 1 sun illumination for more than 300 h.

Abstract

Four thieno[3,2-b]thiophene (TT)-based donor-π-acceptor–type small-molecule donors DTCPTT, DTCPTT-2CN, DTDCPTT, and DTDCPTT-2CN were synthesized and characterized. A clear structure-property relationship correlating the effects of introducing cyano groups onto the TT and the end-capping groups has been successfully established. DTDCPTT and DTDCPTT-2CN were adopted as electron donors for organic photovoltaics (OPVs) mainly due to the strong intramolecular charge transfer absorption in the visible light region, sufficiently high thermal stability, and appropriate energy level alignment with C₇₀. Moreover, the analyses of crystal structures reveal that DTDCPTT and DTDCPTT-2CN form antiparallel dimer pairs through strong dipolar and non-covalent intermolecular interactions. X-ray structures also indicate that introducing cyano groups onto the TT moiety does not affect the backbone planarity significantly. Compared with DTDCPTT-2CN, the more compact and uniform crystal packing renders DTDCPTT advantageous for exciton dissociation and charge carrier transport. DTDCPTT-based device achieves power conversion efficiency as high as 7.81% and 16.89%, respectively, under simulated AM 1.5G illumination and 500 lux TLD-840 fluorescent lamp illumination, which could be attributed to stronger absorption and better photocurrent extraction. In addition, both devices exhibit high stability under continuous simulated 1 sun illumination for more than 300 h. As far as we know, this is the best performance for vacuum-processed small-molecule indoor OPVs to date.

Introduction

Organic photovoltaics (OPVs), which use clean, inexpensive, and inexhaustible solar energy, have drawn lots of attention in recent decades because of exacerbating energy depletion [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. In addition, compared with silicon-based solar cells, OPVs possess many attractive advantages, such as tunable energy levels of active materials, capability of making flexible devices, and lower cost and contamination for mass production. To date, the highest power conversion efficiency (PCE) of a laboratory-scale OPV with the active layer comprising a polymer donor D18 and a non-fullerene acceptor (NFA) Y6 was reported up to 18.22% under simulated AM 1.5G illumination by Liu et al. [10]. Recent studies in the field of OPVs mainly focus on the improvement of inferior PCE in comparison with commercial silicon-based solar cells, along with addressing the challenging issues on large-area fabrication and device lifetime [11,12], which are essential for commercialization. Apart from device structure optimization, it is crucial to develop new active layer materials, which are responsible for photovoltage and photocurrent generation upon illumination. In a conventional binary bulk heterojunction (BHJ)–type active layer, p-type electron donor and n-type electron acceptor are blended to create more donor/acceptor interfaces for exciton dissociation, as well as charge carrier transportation. Therefore, to boost the PCE of OPV devices, different kinds of strategies for designing either electron donors or electron acceptors were developed to enhance the open-circuit voltage (V_OC), short-circuit current (J_SC), and fill factor (FF) without undermining other parameters simultaneously [13]. According to different kinds of organic molecules used in the active layer, OPVs could be roughly divided into two major classes: polymer solar cells (PSCs) and small-molecule organic solar cells (SMOSCs). The most successful OPVs are based on solution-processed active layers composed of polymer donors and tailor-made small-molecule NFAs [[14], [15], [16], [17]]. Although suffering from inferior PCEs in comparison with that of PSCs so far, SMOSCs are still advantageous for the synthetic reproducibility of active materials with well-defined structures, thus being promising candidates for mass production [6]. Recently, SMOSCs showing PCEs more than 14% have been reported [[18], [19], [20], [21]], demonstrating the feasibility of well-designed small-molecule donors and acceptors for high-efficiency photovoltaics.

Although spin-coated and roll-to-roll–coated OPV devices composed of polymer donors and NFA materials have shown the highest PCEs to date, the inferior device stability has been an unsolved issue which remains to be conquered. In contrast, vacuum-deposited OPVs are preferable for their precisely controlled fabrication process for the accurate thickness of each layer and, most importantly, for the high device stability. For example, Burlingame et al. [22] have shown a vacuum-deposited OPV cell composed of tetraphenyldibenzoperiflanthene (DBP) as an electron donor and C₇₀ as an electron acceptor exhibits extraordinary photostability under high-intensity illumination. The extrapolated lifetime T₈₀, which is defined as the time required for the device to reach 80% of its initial PCE, is estimated to be more than 4.9 × 10⁷ h. The exceptional device stability is attributed to not only the inherent photostability of both hydrocarbons DBP and C₇₀ but also the near-equilibrium morphology throughout the vacuum evaporation process. Therefore, developing small-molecule electron donors and acceptors with sufficient thermal stability for the vacuum deposition process is highly desirable for satisfying the industrial criteria of light harvesting applications. However, the small molecule materials suitable for vacuum-processed OPVs are still limited [[23], [24], [25]]. Among these reported small-molecule donors, an acceptor (A)-donor (D)-acceptor (A)–configured dicyanovinylene-capped oligothiophene derivative (DCV5T) reported by Fitzner et al. [26] gave the device a PCE of 6.9% with C₆₀ as an electron acceptor. In addition, Steinmann et al. [27] had demonstrated that an optimized OPV device with the blend comprising merocyanine-based donor HB194 and C₆₀ exhibited PCE up to 6.1%. Along with these excellent examples, a vacuum-deposited OPV device with the active blend composed of a D-A-A–type small-molecule donor DTDCPB and C₇₀ was reported previously by our group [28], achieving 6.8% PCE. The device based on DTDCPB/C₇₀ blend was further optimized to give PCE up to 9.8% [29]. With the asymmetric structure and strong push-pull character by introducing two electron-withdrawing acceptor units, the enhanced molecular dipole moment of DTDCPB results in not only strong intramolecular charge transfer (ICT) absorption but also the formation of antiparallel centrosymmetric dimer pairs in solid state, which can minimize the net dipole moment and thus reduce the detrimental energetic disorder for charge carrier transportation. These results still largely lag behind the state-of-the-art OPVs realized by polymer donors, indicating a tremendous demand of developing new small-molecule donors for boosting the efficiency of SMOSCs.

Among various aromatic structures, organic conjugated materials of thienothiophene derivatives have been proved to be powerful candidates for organic optoelectronic applications owing to their high planarity and high charge carrier mobility [30]. Among the four isomers of TT, thieno[3,2-b]thiophene possesses a small optical energy gap due to the enhanced propensity of forming the quinoidal structure, and the π-conjugation along the molecular skeleton is beneficial for designing molecules with red-shifted absorption, which is important for light harvesting. As a result, thieno[3,2-b]thiophene has been widely used as electron-donating building blocks for OPVs. For example, Zhang et al. [31] have demonstrated that an OPV device based on thieno[3,2-b]thiophene-embedded DRCN8TT exhibits a favorable morphology as compared with that of bithiophene-based analog DRCN8T, leading to improved charge transport and thus better device performance. Moreover, the viability of thieno[3,2-b]thiophene-based small molecules for vacuum-processed OPVs has also been reported by Kim et al [32] with D-π-A–type small-molecule donors DTTh and DTTz. The OPV devices based on DTTh and DTTz blended with C₇₀ as electron acceptor could achieve PCE as high as 6.2%. Thereafter, Shim et al. [33] reported the vacuum-deposited ternary cells comprising two donor molecules DTTz and DTDCTB, as well as C₇₀ as the electron acceptor, showing a more decent PCE up to 8.02%.

Motivated by our previous success on D-A-A–configured small-molecule donors, in this work, four new D-π-A–type molecules DTCPTT, DTCPTT-2CN, DTDCPTT, and DTDCPTT-2CN (Fig. 1) were designed, synthesized, and characterized. By replacing the 2,1,3-benzothiadiazole (BT) unit of D-A-A–type donor with planar but less electron-withdrawing thieno[3,2-b]thiophene (TT), a suitable spectral response to the visible light region and well-ordered crystal packing motif could be achieved. In addition, adjusting the electron-withdrawing feature of the central and terminal moieties can elaborately control the absorption wavelength and highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels. Notably, this work clearly rationales the effects of cyanosubstitution on the thieno[3,2-b]thiophene unit on the photophysical, electrochemical, and crystal packing behaviors. While DTCPTT and DTCPTT-2CN were not subjected to device fabrication because of the unsatisfactory reaction yield and less overlap between absorption spectra and the solar radiation spectrum, vacuum-deposited binary cells based on DTDCPTT and DTDCPTT-2CN with C₇₀ as an electron acceptor achieved PCE as high as 7.81% and 6.00%, respectively, under simulated AM 1.5 G illumination.

Other than traditional photovoltaics aiming at solar energy harvesting, there has been a rising demand for indoor photovoltaics (IPVs) owing to the emergence of small, low-power consumption electronic devices related to the Internet of Things technology [34]. These electronic devices could be driven under ambient lighting (such as light-emitting diodes or fluorescent lamps) with emitting wavelength ranging from 400 to 700 nm [34], hence being desirable for OPVs with strong absorption in the visible light region. For example, Lee et al. [35] have shown that a record high 28.1% PCE could be achieved by an OPV device based on small-molecule donor BTR and fullerene-derivative acceptor PC₇₁BM under 1,000 lux of fluorescent lamp illumination. Our group has also demonstrated that a highly stable vacuum-deposited OPV device, which comprises small-molecule donor DTCPB and fullerene acceptor C₇₀, could show a superb PCE up to 15.78% under 800 lux ambient lighting condition [36]. Herein, owing to the highly overlapped absorption spectra with visible light region, the OPV devices based on donors DTDCPTT and DTDCPTT-2CN show outstanding PCEs under ambient light illumination. Particularly, the DTDCPTT-based device can achieve PCE up to 16.89% under 500 lux TLD-840 fluorescent lamp illumination. Combined with trivial efficiency loss after more than 300 h of continuous light soaking, the DTDCPTT- and DTDCPTT-2CN-based OPV devices have been proved to become one of the most competitive candidates for highly efficient and highly stable vacuum-deposited OPVs under both outdoor and indoor conditions.