Improved Efficiency in All-Small-Molecule Organic Solar Cells with Ternary Blend of Nonfullerene Acceptor and Chlorinated and Nonchlorinated Donors

Optimization of Charge Management and Energy Loss in All-Small-Molecule Organic Solar Cells

All-small-molecule organic solar cells (ASM-OSCs) have received tremendous attention in recent decades because of their advantages over their polymer counterparts. These advantages include well-defined chemical structures, easy purification, and negligible batch-to-batch variation. Remarkable progress with a power conversion efficiency (PCE) of over 17% has recently been achieved with improved charge management (FF × JSC) and reduced energy loss (Eloss). Morphology control is the key factor in the progress of ASM-OSCs, which remains a significant challenge because of the similarities in the molecular structures of the donors and acceptors. In this review, the effective strategies for charge management and/or Eloss reduction from the perspective of effective morphology control are summarized. The aim is to provide practical insights and guidance for material design and device optimization to promote further development of ASM-OSCs to a level where they can compete with or even surpass the efficiency of polymer solar cells.

Designing Benzodithiophene-Based Small Molecule Donors for Organic Solar Cells by Regulation of Halogenation Effects

The donors are key components of organic solar cells (OSCs) and play crucial roles in their photovoltaic performance. Herein, we designed two new donors (BTR-γ-Cl and BTR-γ-F) by finely optimizing small molecule donors (BTR-Cl and BTR-F) with a high performance. The optoelectronic properties of the four donors and their interfacial properties with the well-known acceptor Y6 were studied by density functional theory and time-dependent density functional theory. Our calculations show that the studied four donors have large hole mobility and strong interactions with Y6, where the BTR-γ-Cl/Y6 has the largest binding energy. Importantly, the proportion of charge transfer (CT) states increases at the BTR-γ-Cl/Y6 (50%) and BTR-γ-F/Y6 (45%) interfaces. The newly designed donors are more likely to achieve CT states through intermolecular electric field (IEF) and hot exciton mechanisms than the parent molecules; meanwhile, donors containing Cl atoms are more inclined to produce CT states through the direct excitation mechanism than those containing F atoms. Our results not only provided two promising donors but also shed light on the halogenation effects on donors in OSCs, which might be important to design efficient photovoltaic materials.

Simultaneously enhancing the photovoltaic parameters of ternary organic solar cells by incorporating a fused ring electron acceptor

The ternary strategy has been recognized as an effective method to improve the photovoltaic performance of organic solar cells (OSCs). In ternary OSCs, the complementary or broadened absorption spectrum, optimized morphology, and enhanced photovoltaic performance could be obtained by selecting a third rational component for the host system. In this work, a fused ring electron acceptor named BTMe-C8-2F, which possesses a high-lying lowest unoccupied molecular orbital (LUMO) energy level and a complementary absorption spectrum to PM6:Y6, was introduced to a PM6:Y6 binary system. The ternary blend film PM6:Y6:BTMe-C8-2F showed high and more balanced charge mobilities, and low charge recombination. Therefore, the OSC based on the PM6:Y6:BTMe-C8-2F (1 : 1.2 : 0.3, w/w/w) blend film achieved the highest power conversion efficiency (PCE) of 17.68%, with an open-circuit voltage (V_OC) of 0.87 V, a short-circuit current (J_SC) of 27.32 mA cm^-2, and a fill factor (FF) of 74.05%, which are much higher than the binary devices of PM6:Y6 (PCE = 15.86%) and PM6:BTMe-C8-2F (PCE = 11.98%). This work provides more insight into the role of introducing a fused ring electron acceptor with a high-lying LUMO energy level and complementary spectrum for simultaneously enhancing the V_OC and J_SC to promote the performance of ternary OSCs.

Progress and Future Potential of All-Small-Molecule Organic Solar Cells Based on the Benzodithiophene Donor Material

Organic solar cells have obtained a prodigious amount of attention in photovoltaic research due to their unique features of light weight, low cost, eco-friendliness, and semitransparency. A rising trend in this field is the development of all-small-molecules organic solar cells (ASM-OSCs) due to their merits of excellent batch-to-batch reproducibility, well-defined structures, and simple purification. Among the numerous organic photovoltaic (OPV) materials, benzodithiophene (BDT)-based small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking the 17% efficiency barrier in single-junction OPV devices, indicating the significant potential of this class of materials in commercial photovoltaic applications. This review specially focuses on up-to-date information about improvements in BDT-based ASM-OSCs since 2011 and provides an outlook on the most significant challenges that remain in the field. We believe there will be more exciting BDT-based photovoltaic materials and devices developed in the near future.

Recent Progress in All-Small-Molecule Organic Solar Cells

Active layer material plays a critical role in promoting the performance of an organic solar cell (OSC). Small-molecule (SM) materials have the merits of well-defined chemical structures, few batch-to-batch variations, facile synthesis and purification procedures, and easily tuned properties. SM-donor and non-fullerene acceptor (NFA) innovations have recently produced all-small-molecule (ASM) devices with power conversion efficiencies that exceed 17% and approach those of their polymer-based counterparts, thereby demonstrating their great future commercialization potential. In this review, recent progress in both SM donors and NFAs to illustrate structure-property relationships and various morphology-regulation strategies are summarized. Finally, ASM-OSC challenges and outlook are discussed.

Indacenodithiophene-based small-molecule donor with strong crystallinity for efficient organic solar cells

Two indacenodithiophene (IDT)-based small-molecule analogues (IDBT and IDBT-Cl) are designed as donor materials for organic solar cells. Relative to the amorphous IDBT-Cl, the IDBT with strong crystallinity shows overall better photovoltaic performance when blended with a Y6 acceptor. The results demonstrate the great potential of IDT units in designing efficient small-molecule donors.

Fine-Tuning the Dipole Moment of Asymmetric Non-Fullerene Acceptors Enabling Efficient and Stable Organic Solar Cells

Modifying molecular conjugation has been demonstrated as an effective strategy to enhance the photovoltaic performance of the non-fullerene small molecule acceptors (SMAs), which would regulate the molecular packing and nanoscale morphology in the active layer of organic solar cells (OSCs). Here, two novel SMAs PTIC-4Cl and PT2IC-4Cl are designed and synthesized by expanding the core unit of TB-4Cl in one or two directions. The effects of how to expand the conjugation length on the absorption property, energy levels, dipole moment, and solubility are studied via theoretical calculation and experiments. Compared to PT2IC-4Cl, PTIC-4Cl with a more asymmetric structure exhibits the larger dipole moment and enhanced intermolecular packing. The PTIC-4Cl-based OSCs exhibit a favorable morphology and balanced charge transport, thereby leading to the highest power conversion efficiencies. In addition, PTIC-4Cl-based devices show outstanding thermal and air stability. These results reveal that fine-tuning the dipole moment via rationally expanding the conjugation in asymmetric A-D₁A'D₂-A-type non-fullerene acceptors is critical to achieve high-performance OSCs.

Backbone Engineering with Asymmetric Core to Finely Tune Phase Separation for High-Performance All-Small-Molecule Organic Solar Cells

In order to obtain high-performance all-small-molecule organic solar cells (ASM-OSCs), it is crucial to exploit the available strategy for molecular design and to further understand key structure-property relationship that can rationally control the blend nanomorphology and influence the physical process. In this work, we design two small molecule donors FBD-S1 and TBD-S2 with identical electron-withdrawing units but various asymmetric central cores, which exhibit differing phase separation in Y6-based blend films. It is found that TBD-S2 with increased phase separation between donor and acceptor can lead to more favorable interpenetrating networks, effective exciton dissociation, and enhanced and more balanced charge transport. Importantly, a remarkable PCE of 13.1% is obtained for TBD-S2:Y6 based ASM-OSCs, which is an attractive photovoltaic performance for ASM-OSCs. This result demonstrates that the central core modification at the atomic level for small molecule donors can delicately control the phase separation and optimize photophysical processes, and refines device performance, which facilitate development in the ASM-OSC research field.

Fullerene/Non-fullerene Alloy for High-Performance All-Small-Molecule Organic Solar Cells

Organic solar cells (OSCs) that contain small molecules only were prepared with FG1 as the donor, a narrow band gap non-fullerene acceptor MPU4, and a wide band gap PC₇₁BM. The OSCs based on optimized FG1:MPU4 (1:1.2) and FG1:PC₇₁BM (1:1.5) active layers, respectively, gave power conversion efficiencies (PCEs) of 11.18% with a short circuit current (J_SC) of 19.54 mA/cm², open circuit voltage (V_OC) of 0.97 V, and fill factor (FF) of 0.59, and 6.62% with a J_SC of 12.50 mA/cm², V_OC of 0.84 V, and FF of 0.63%, respectively. A PCE of 13.26% was obtained from the optimized ternary FG1:PC₇₁BM:MPU4 (1:0.3:0.9) OSCs and this arises because of the boost in a J_SC of 21.91 mA/cm² and FF of 0.68. The V_OC of the ternary OSCs (0.89 V) lies between those for the OSCs based on FG1:MPU4 and FG1:PC₇₁BM, which indicates the formation of an alloy of the two acceptors. The increase in J_SC and FF in the ternary OSCs may result from the efficient energy transfer from PC₇₁BM to MPU4 as well as more charge-transfer donor/acceptor interfaces, enhanced charge carrier mobilities resulting in better adjusted charge transport, and lower bimolecular and trap-assisted recombination. The appropriate phase separation, increased crystallinity, and reduced π-π stacking distance in the ternary active layer are consistent with the enhancement in the FF for OSCs based on a ternary active layer. The results of this work suggest the merging of the fullerene acceptor into the non-fullerene acceptor to form a fullerene/non-fullerene acceptor alloy, and this may be a viable approach to obtain high-performance OSCs.

High-Efficiency Thermal-Annealing-Free Organic Solar Cells Based on an Asymmetric Acceptor with Improved Thermal and Air Stability

The stability of organic solar cells (OSCs) is an urgent problem for commercialization. In this work, a novel asymmetric molecule TB-4Cl was designed and synthesized. Quantum chemical computations revealed that TB-4Cl has a larger dipole moment of 1.98 Debye than that of Y6, which can induce a stronger intermolecular interaction. Without thermal annealing, devices based on PM6:TB-4Cl achieved a higher efficiency of 14.67%. Impressively, all of the devices showed a negligible difference in power conversion efficiency (PCE) before and after thermal-annealing treatment. Compared to the unencapsulated PM6:Y6-based devices, PM6:TB-4Cl-based devices exhibited improved thermal and air stability, evidenced by retaining around 75% (TB-4Cl) and 60% (Y6) after being heated at 100 °C in nitrogen for 110 h and 65% (TB-4Cl) and 50% (Y6) in air for 92 h. This work indicates that an A-D₁A'D₂-A asymmetric molecule can be a promising candidate for achieving stable OSCs with high efficiency.

Optimization of the Bulk Heterojunction of All-Small-Molecule Organic Photovoltaics Using Design of Experiment and Machine Learning Approaches

All-small-molecule organic photovoltaic (OPV) cells based upon the small-molecule donor, DRCN5T, and nonfullerene acceptors, ITIC, IT-M, and IT-4F, were optimized using Design of Experiments (DOE) and machine learning (ML) approaches. This combination enables rational sampling of large parameter spaces in a sparse but mathematically deliberate fashion and promises economies of precious resources and time. This work focused upon the optimization of the core layer of the OPV device, the bulk heterojunction (BHJ). Many experimental processing parameters play critical roles in the overall efficiency of a given device and are often correlated and thus are difficult to parse individually. DOE was applied to the (i) solution concentration of the donor and acceptor ink used for spin-coating, (ii) the donor fraction, (iii) the temperature, and (iv) duration of the annealing of these films. The ML-based approach was then used to derive maps of the power conversion efficiencies (PCE) landscape for the first and second rounds of optimization to be used as guides to determine the optimal values of experimental processing parameters with respect to PCE. This work shows that with little knowledge of a potential combination of components for a given BHJ, a large parameter space can be effectively screened and investigated to rapidly determine its potential for high-efficiency OPVs.

Over 15% Efficiency in Ternary Organic Solar Cells by Enhanced Charge Transport and Reduced Energy Loss

In this study, an efficient ternary bulk-heterojunction (BHJ) organic solar cell (OSC) is demonstrated by incorporating two acceptors, PC₆₁BM and ITC6-4F, with a polymer donor (PM6). This reveals that the addition of PC₆₁BM not only enhances the electron mobility of the derived BHJ blend but also facilitates exciton dissociation, resulting in a more balanced charge transport alongside with reduced trap-assisted charge recombination. Consequently, as compared to the pristine PM6/ITC6-4F device, the optimal ternary OSC is revealed to deliver an improved power conversion efficiency (PCE) of 15.11% with a boosted J_SC, V_OC, and fill factor (FF) simultaneously. The resultant V_OC and FF are among the highest values recorded in the literature for the ternary OSCs with a PCE exceeding 15%. This result thus suggests that besides improving the charge transport characteristics in devices, incorporating a fullerene derivative as part of the acceptor can also improve the resultant V_OC, which can reduce the energy loss to realize efficient organic photovoltaics.

Novel Nitrogen-Containing Heterocyclic Non-Fullerene Acceptors for Organic PhotovoltaicCells: Different End-Capping Groups Leading to a Big Difference of Power Conversion Efficiencies

Novel cores for high performance nonfullerene acceptors (NFAs) remain to be developed. In this work, two new n-type nitrogen-containing organic heterocyclic NFAs, namely, BDTN-BF and BDTN-Th, were designed and synthesized based on a new seven fused-ring core (BDTN) with two different end-capping groups. As a result, BDTN-BF possessed similar absorption spectra in solution and solid state to BDTN-Th, but a slightly higher maximum molar extinction coefficient. Manufacturing the polymer solar cells with PM6 as the donor, the photovoltaic performance of BDTN-BF and BDTN-Th was investigated. The PM6:BDTN-BF-based device achieved the highest power conversion efficiency (PCE) of 11.54% with a high J_sc of 20.20 mA cm^-2, a fill factor (FF) of 61.46%, and a large V_oc of 0.93 V, and the energy loss (E_loss) was calculated to be 0.48 eV. Comparatively, the PM6:BDTN-Th-based device achieved the maximum PCE value of only 3.53% because of inadequate J_sc and FF. The higher J_sc and FF for the PM6:BDTN-BF-based device was mainly due to the effective electron transfer from PM6 to BDTN-BF, more balanced μ_h/μ_e, higher electron mobility of the neat film, better charge collection and dissociation efficiency, and more favorable morphology. These results demonstrate that the acceptors with nearly identical absorption spectra could result in a significant difference in photovoltaic performance, which stress the importance of end-capping units. Furthermore, few NFA-based devices achieve large V_oc and high J_sc simultaneously as one based on PM6:BDTN-BF, indicating that nitrogen hybridization of NFAs may be an efficient strategy to realize high and balanced V_oc and J_sc.