Design,Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra‐Narrow Band Gap

The design of narrow band gap (NBG) donors or acceptors and their application in organic solar cells (OSCs) are of great importance in the conversion of solar photons to electrons. Limited by the inevitable energy loss from the optical band gap of the photovoltaic material to the open‐circuit voltage of the OSC device, the improvement of the power conversion efficiency (PCE) of NBG‐based OSCs faces great challenges. A novel acceptor–donor–acceptor structured non‐fullerene acceptor is reported with an ultra‐narrow band gap of 1.24 eV, which was achieved by an enhanced intramolecular charge transfer (ICT) effect. In the OSC device, despite a low energy loss of 0.509 eV, an impressive short‐circuit current density of 25.3 mA cm⁻² is still recorded, which is the highest value for all OSC devices. The high 10.9 % PCE of the NBG‐based OSC demonstrates that the design and application of ultra‐narrow materials have the potential to further improve the PCE of OSC devices.     

A chlorinated low-bandgap small-molecule acceptor for organic solar cells with 14.1% efficiency and low energy loss

A new acceptor-donor-acceptor(A-D-A) type small-molecule acceptor NCBDT-4 Cl using chlorinated end groups is reported.This new-designed molecule demonstrates wide and efficient absorption ability in the range of 600–900 nm with a narrow optical bandgap of 1.40 eV. The device based on PBDB-T-SF:NCBDT-4 Cl shows a power conversion efficiency(PCE) of 13.1%without any post-treatment, which represents the best result for all as-cast organic solar cells(OSCs) to date. After device optimizations, the PCE was further enhanced to over 14% with a high short-circuit current density(Jsc) of 22.35 m A cm-2 and a fill-factor(FF) of 74.3%. The improved performance was attributed to the more efficient photo-electron conversion process in the optimal device. To our knowledge, this outstanding efficiency of 14.1% with an energy loss as low as 0.55 eV is among the best results for all single-junction OSCs.     

The Synthesis of a Multiple D–A Conjugated Macrocycle and Its Application in Organic Photovoltaic

As a novel class of materials, D–A conjugated macrocycles hold significant promise for chemical science. However, their potential in photovoltaic remains largely untapped due to the complexity of introducing multiple donor and acceptor moieties into the design and synthesis of cyclic π-conjugated molecules. Here, we report a multiple D–A ring-like conjugated molecule ( RCM ) via the coupling of dimer molecule DBTP-C3 as a template and thiophenes in high yields. RCM exhibits a narrow optical gap (1.33 eV) and excellent thermal stability, and shows a remarkable photoluminescence yield (Φ_PL) of 11.1 % in solution, much higher than non-cyclic analogues. Organic solar cell (OSC) constructed with RCM as electron acceptor shows efficient charge separation at donor-acceptor band offsets and achieves a power conversion efficiency (PCE) of 14.2 %-approximately fourfold higher than macrocycle-based OSCs reported so far. This is partly due to low non-radiative voltage loss down to 0.20 eV and a high electroluminescence yield (Φ_EL) of 4×10⁻⁴. Our findings emphasize the potential of D–A cyclic conjugated molecules in advancing organic photovoltaic technology.     

Potential of Nonfullerene Small Molecules with High Photovoltaic Performance

Over the past decades, fullerene derivatives have become the most successful electron acceptors in organic solar cells (OSCs) and have achieved great progress, with power conversion efficiencies (PCEs) of over 11 %. However, fullerenes have some drawbacks, such as weak absorption, limited energy‐level tunability, and morphological instability. In addition, fullerene‐based OSCs usually suffer from large energy losses of over 0.7 eV, which limits further improvements in the PCE. Recently, nonfullerene small molecules have emerged as promising electron acceptors in OSCs. Their highly tunable absorption spectra and molecular energy levels have enabled fine optimization of the resulting devices, and the highest PCE has surpassed 12 %. Furthermore, several studies have shown that OSCs based on small‐molecule acceptors (SMA) have very efficient charge generation and transport efficiency at relatively low energy losses of below 0.6 eV, which suggests great potential for the further improvement of OSCs. In this focus review, we analyze the challenges and potential of SMA‐based OSCs and discuss molecular design strategies for highly efficient SMAs.     

Efficient Organic Solar Cells with a High Open‐Circuit Voltage of 1.34 V

One of the most important challenges that hinders the power conversion efficiencies (PCEs) of organic solar cells (OSCs) is the modest open‐circuit voltages (V_OC) due to large energy losses. The large driving force during for charge generation and the non‐radiative recombination are the main causes of energy losses. To maximize the V_OC of OSCs, herein, we modulate the end‐groups and design a non‐fullerene acceptor ITCCM‐O, which shows a bandgap of 2.0 eV. By blending a polymer donor named J52, the device demonstrates a PCE of 5.5% with an outstanding V_OC of 1.34 V, which is the highest value for single‐junction OSCs over 5% PCEs. The high V_OC is benefited from 1) the negligible driving force for charge transfer, and 2) the suppressed non‐radiative recombination loss, as low as 0.22 V.     

Constructing a Strongly Absorbing Low‐Bandgap Polymer Acceptor for High‐Performance All‐Polymer Solar Cells

All‐polymer solar cells (all‐PSCs) offer unique morphology stability for the application as flexible devices, but the lack of high‐performance polymer acceptors limits their power conversion efficiency (PCE) to a value lower than those of the PSCs based on fullerene derivative or organic small molecule acceptors. We herein demonstrate a strategy to synthesize a high‐performance polymer acceptor PZ1 by embedding an acceptor–donor–acceptor building block into the polymer main chain. PZ1 possesses broad absorption with a low band gap of 1.55 eV and high absorption coefficient (1.3×10⁵ cm⁻¹). The all‐PSCs with the wide‐band‐gap polymer PBDB‐T as donor and PZ1 as acceptor showed a record‐high PCE of 9.19 % for the all‐PSCs. The success of our polymerization strategy can provide a new way to develop efficient polymer acceptors for all‐PSCs.     

A non-fullerene acceptor enables efficient P3HT-based organic solar cells with small voltage loss and thickness insensitivity

The large D core of DFPCBR results in efficient P3HT-based OSCs with a high V_OC and thickness insensitivity.     

13.34 % Efficiency Non‐Fullerene All‐Small‐Molecule Organic Solar Cells Enabled by Modulating the Crystallinity of Donors via a Fluorination Strategy

Non‐fullerene all‐small‐molecule organic solar cells (NFSM‐OSCs) have shown potential as OSCs, owing to their high purity, easy synthesis and good reproducibility. However, challenges in the modulation of phase separation morphology have limited their development. Herein, two novel small molecular donors, BTEC‐1F and BTEC‐2F, derived from the small molecule DCAO3TBDTT, are synthesized. Using Y6 as the acceptor, devices based on non‐fluorinated DCAO3TBDTT showed an open circuit voltage (V_oc) of 0.804 V and a power conversion efficiency (PCE) of 10.64 %. Mono‐fluorinated BTEC‐1F showed an increased V_oc of 0.870 V and a PCE of 11.33 %. The fill factor (FF) of di‐fluorinated BTEC‐2F‐based NFSM‐OSC was improved to 72.35 % resulting in a PCE of 13.34 %, which is higher than that of BTEC‐1F (61.35 %) and DCAO3TBDTT (60.95 %). To our knowledge, this is the highest PCE for NFSM‐OSCs. BTEC‐2F had a more compact molecular stacking and a lower crystallinity which enhanced phase separation and carrier transport.     

Rylene diimide and dithienocyanovinylene copolymers for polymer solar cells

Two polymers containing(E)-2,3-bis(thiophen-2-yl)acrylonitrile(CNTVT) as a donor unit, perylene diimide(PDI) or naphthalene diimide(NDI) as an acceptor unit, are synthesized by the Stille coupling copolymerization, and used as the electron acceptors in the solution-processed organic solar cells(OSCs). Both polymers exhibit broad absorption in the region of 300–850 nm. The LUMO energy levels of the resulted polymers are ca. –3.93 eV and the HOMO energy levels are –5.97 and –5.83 eV. In the binary blend OSCs with PTB7-Th as a donor, PDI polymer yields the power conversion efficiency(PCE) of up to 1.74%, while NDI polymer yields PCE of up to 3.80%.     

Designing Nonfullerene Acceptors with Oligo(Ethylene Glycol) Side Chains: Unraveling the Origin of Increased Open-Circuit Voltage and Balanced Charge Carrier Mobilities

Despite the recent rapid development of organic solar cells (OSCs), the low dielectric constant (ϵ_r=3–4) of organic semiconducting materials limits their performance lower than inorganic and perovskite solar cells. In this work, we introduce oligo(ethylene glycol) (OEG) side chains into the dicyanodistyrylbenzene-based non-fullerene acceptors (NIDCS) to increase its ϵ_r up to 5.4. In particular, a NIDCS acceptor bearing two triethylene glycol chains (NIDCS-EO3) shows V_OC as high as 1.12 V in an OSC device with a polymer donor PTB7, which is attributed to reduced exciton binding energy of the blend film. Also, the larger size grain formation with well-ordered stacking structure of the NIDCS-EO3 blend film leads to the increased charge mobility and thus to the improved charge mobility balance, resulting in higher J_SC, FF, and PCE in the OSC device compared to those of a device using the hexyl chain-based NIDCS acceptor (NIDCS-HO). Finally, we fabricate NIDCS-EO3 devices with various commercial donors including P3HT, DTS-F, and PCE11 to show higher photovoltaic performance than the NIDCS-HO devices, suggesting versatility of NIDCS-EO3.     

Soluble narrow‐band‐gap copolymers containing novel cyclopentadithiophene units for organic photovoltaic cell applications

Five novel conjugated copolymers ( P1 – P5 ) containing coplanar cyclopentadithiophene (CPDT) units (incorporated with arylcyanovinyl and keto groups in different molar ratios) were synthesized and developed for the applications of polymer solar cells (PSCs). Polymers P1 – P5 covered broad absorption ranges from UV to near infrared (400–900 nm) with narrow optical band gaps of 1.38–1.70 eV, which are compatible with the maximum solar photon reflux. Partially reversible p‐ and n‐doping processes of P1 – P5 in electrochemical experiments were observed, and the proper molecular design for highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of P1 – P5 induced the highest photovoltaic open‐circuit voltage in the PSC devices, compared with those previously reported CPDT‐based narrow‐band‐gap polymers. Powder X‐ray diffraction (XRD) analyses suggested that these copolymers formed self‐assembled π‐π stacking and pseudobilayered structures. Under 100 mW/cm² of AM 1.5 white‐light illumination, bulk heterojunction PSC devices containing an active layer of electron donor polymers P1 – P5 mixed with electron acceptor [6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) in the weight ratio of 1:4 were investigated. The PSC device containing P1 gave the best preliminary result with an open‐circuit voltage of 0.84 V, a short‐circuit current of 2.36 mA/cm², and a fill factor of 0.38, offering an overall power conversion efficiency (PCE) of 0.77% as well as a maximal quantum efficiency of 23% from the external quantum efficiency (EQE) measurements. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2073–2092, 2009     

Novel narrow‐band‐gap conjugated copolymers containing phenothiazine‐arylcyanovinyl units for organic photovoltaic cell applications

A series of novel narrow‐band‐gap copolymers ( P1 ‐ P12 ) composed of alkyl‐substituted fluorene (FO) units and six analogous mono‐ and bis(2‐aryl‐2‐cyanovinyl)‐10‐hexylphenothiazine monomers ( M1 ‐ M6 ) were synthesized by a palladium‐catalyzed Suzuki coupling reaction with two different feed in ratios of FO to M1 ‐ M6 (molar ratio = 3:1 and 1:1). The absorption spectra of polymers P1 ‐ P12 exhibited broad peaks located in the UV and visible regions from 400 to 800 nm with optical band gaps at 1.55–2.10 eV, which fit near the wavelength of the maximum solar photon reflux. Electrochemical experiments displayed that the reversible p‐ and n‐doping processes of copolymers were partially reversible, and the proper HOMO/LUMO levels enabled a high photovoltaic open‐circuit voltage. As blended with [6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) as an electron acceptor in bulk heterojunction photovoltaic devices, narrow‐band‐gap polymers P1 ‐ P12 as electron donors showed significant photovoltaic performance which varied with the intramolecular donor‐acceptor interaction and their mixing ratios to PCBM. Under 100 mW/cm² of AM 1.5 white‐light illumination, the device of copolymer P12 produced the highest preliminary result having an open‐circuit voltage of 0.64 V, a short‐circuit current of 2.70 mA/cm², a fill factor of 0.29, and an energy conversion efficiency of 0.51%. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4285–4304, 2008     

Bandgap engineering of indenofluorene‐based conjugated copolymers with pendant donor‐π‐acceptor chromophores for photovoltaic applications

Three narrow‐band‐gap conjugated copolymers based on indenofluorene and triphenylamine with pendant donor‐π‐acceptor chromophores were successfully synthesized by post‐functionalization approach. All the polymers have good solubility in common solvents and excellent thermal stability. The photophysical properties, energy levels and band gaps of the polymers were well manipulated by introducing different acceptor groups onto the end of their conjugated side chains. By using different acceptor groups, the band gaps of the polymers were narrowed from 1.86 to 1.53 eV by lowering their lowest unoccupied molecular orbital levels, whereas their relatively deep highest occupied molecular orbital levels of approximately ?5.35 eV were maintained. Bulk‐heterojunction solar cells with these polymers as electron donors and (6,6)‐phenyl‐C₇₁‐butyric acid methyl ester as acceptor showed power conversion efficiencies as high as 3.1% and high open circuit voltages more than 0.88 eV. The relationships between the performance and film morphology, energy levels, charge mobilities were discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Low band‐gap modulation of isoindigo‐based copolymers toward high open‐circuit voltage of polymer solar cells

Designing low band‐gap‐conjugated polymers coupled with low HOMO levels attracts great attention in the field of polymer solar cells (PSCs). By using donor–acceptor (D‐A) copolymerization strategy, we designed and synthesized a series of low band‐gap copolymers with deep HOMO levels via introducing an isoindigo (IID) acceptor unit in the copolymers with the donor unit of fluorene (F) (PIID‐F), carbazole (Cz) (PIID‐Cz), thiophene (Th) (PIID‐Th), dithiophene (DTh) (PIID‐DTh), or dithienosilole (DTS) (PIID‐DTS). The HOMO level of the copolymers, measured by electrochemical cyclic voltammetry, varies from ?5.3 eV to ?5.8 eV, depending on different donor units in the copolymers. However, the LUMO levels of all the copolymers are fixed at about ?3.6 eV, which is mainly determined by IID acceptor unit due to its strong electron‐withdrawing ability. The new results will provide an effect help in designing IID based molecular structures. Among the copolymers, PIID‐DTS has a low band gap of 1.58 eV and possesses a low‐lying HOMO energy level of ?5.33 eV. The PSCs based on PIID‐DTS as donor and PC₇₀BM as acceptor exhibited a high open‐circuit voltage (V_oc) of 0.93 V and a primary power conversion efficiency of 2.45%. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3477–3485     

Supramolecular assembly of H‐bonded side‐chain polymers containing conjugated pyridyl H‐acceptor pendants and various low‐band‐gap H‐donor dyes bearing cyanoacrylic acid groups for organic solar cell applications

Novel supramolecular side‐chain polymers were constructed by complexation of proton acceptor (H‐acceptor) polymers, i.e., side‐chain conjugated polymers P1–P2 containing pyridyl pendants, with low‐band‐gap proton donor (H‐donor) dyes S1–S4 (bearing terminal cyanoacrylic acids) in a proper molar ratio. Besides unique mesomorphic properties confirmed by DSC and XRD results, the H‐bonds of supramolecular side‐chain structures formed by pyridyl H‐acceptors and cyanoacrylic acid H‐donors were also confirmed by FTIR measurements. H‐donor dyes S1–S4 in solid films exhibited broad absorption peaks located in the range of 471–490 nm with optical band‐gaps of 1.99–2.14 eV. Furthermore, H‐bonded polymer complexes P1/S1–P1/S4 and P2/S1–P2/S4 exhibited broad absorption peaks in the range of 440–462 nm with optical band‐gaps of 2.11–2.25 eV. Under 100 mW/cm² of AM 1.5 white‐light illumination, the bulk heterojunction polymer solar cell (PSC) devices containing an active layer of H‐bonded polymer complexes P1/S1–P1/S4 and P2/S1–P2/S4 (as electron donors) mixed with [6,6]‐phenyl C₆₁ butyric acid methyl ester (i.e., PCBM, as an electron acceptor) in the weight ratio of 1:1 were investigated. The PSC device containing H‐bonded polymer complex P1/S3 mixed with PCBM (1:1 w/w) gave the best preliminary result with an overall power conversion efficiency (PCE) of 0.50%, a short‐circuit current of 3.17 mA/cm², an open‐circuit voltage of 0.47 V, and a fill factor of 34%. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5998–6013, 2009     

High electron mobility fluorinated indacenodithiophene small molecule acceptors for organic solar cells

Indacenodithiophene (IDT) derivatives are kinds of the most representative and widely used cores of small molecule acceptors (SMAs) in organic solar cells (OSCs). Here we systematically investigate the influence of end-group fluorination density and position on the photovoltaic properties of the IDT-based SMAs IDIC-nF (n = 0, 2, 4). The absorption edge of IDIC-nF red-shifts with the π-π stacking and crystallinity improvement, and their electronic energy levels downshift with increasing n. Due to the advantages of J_sc and FF as well as acceptable V_oc, the difluorinated IDIC-2F acceptor based OSCs achieve the highest power conversion efficiency (PCE) of 13%, better than the OSC devices based on IDIC and IDIC-4F as acceptors. And the photovoltaic performance of the PTQ10: IDIC-2F OSCs is insensitive to the active layer thickness: PCE still keep high values of 12.00% and 11.46% for the devices with active layer thickness of 80 and 354 nm, respectively. This work verifies that fine and delicate modulation of the SMAs molecular structure could optimize photovoltaic performance of the corresponding OSCs. Meanwhile, the thickness-insensitivity property of the OSCs has potential for large-scale and printable fabrication technology.     

Manipulating active layer morphology of molecular donor/polymer acceptor based organic solar cells through ternary blends

The development of molecular donor/polymer acceptor blend(MD/PA)-type organic solar cells(OSCs) lags far behind other type OSCs. It is due to the large-size phase separation morphology of MD/PAblend, which results from the high crystallinity of molecular donors. In this article, to suppress the crystallinity of molecular donors, we use ternary blends to develop OSCs based on one polymer acceptor(P-BNBP-f BT) and two molecular donors(DR3 TBDTT and BTR) with similar chemical structures.The ternary OSC exhibits a power conversion efficiency(PCE) of 4.85%, which is higher than those of the binary OSCs(PCE=3.60% or 3.86%). To our best knowledge, it is the first report of ternary MD/PA-type OSCs and this PCE is among the highest for MD/PA-type OSCs reported so far. Compared with the binary blends, the ternary blend exhibits decreased crystalline size and improved face-on orientation of the donors. As a result, the ternary blend exhibits improved and balanced charge mobilities, suppressed charge recombination and increased donor/acceptor interfacial areas, which leads to the higher shortcircuit current density. These results suggest that using ternary blend is an effective strategy to manipulate active layer morphology and enhance photovoltaic performance of MD/PA-type OSCs.     

Dimer Acceptor Adopting a Flexible Linker for Efficient and Durable Organic Solar Cells

Organic solar cells (OSCs) have advanced rapidly due to the development of new photovoltaic materials. However, the long-term stability of OSCs still poses a severe challenge for their commercial deployment. To address this issue, a dimer acceptor (dT9TBO) with flexible linker is developed for incorporation into small-molecule acceptors to form molecular alloy with enhanced intermolecular packing and suppressed molecular diffusion to stabilize active layer morphology. Consequently, the PM6 : Y6 : dT9TBO-based device displays an improved power conversion efficiency (PCE) of 18.41 % with excellent thermal stability and negligible decay after being aged at 65 °C for 1800 h. Moreover, the PM6 : Y6 : dT9TBO-based flexible OSC also exhibits excellent mechanical durability, maintaining 95 % of its initial PCE after being bended repetitively for 1500 cycles. This work provides a simple and effective way to fine-tune the molecular packing with stabilized morphology to overcome the trade-off between OSC efficiency and stability.     

Non-fullerene acceptor fibrils enable efficient ternary organic solar cells with 16.6% efficiency

Optimizing the components and morphology within the photoactive layer of organic solar cells(OSCs) can significantly enhance their power conversion efficiency(PCE). A new A-D-A type non-fullerene acceptor IDMIC-4F is designed and synthesized in this work, and is employed as the third component to prepare high performance ternary solar cells. IDMIC-4F can form fibrils after solution casting, and the presence of this fibrillar structure in the PBDB-T-2F:BTP-4F host confines the growth of donors and acceptors into fine domains, as well as acting as transport channels to enhance electron mobility. Single junction ternary devices incorporating 10 wt% IDMIC-4F exhibit enhanced light absorption and balanced carrier mobility, and achieve a maximum PCE of 16.6% compared to 15.7% for the binary device, which is a remarkable efficiency for OSCs reported in literature. This non-fullerene acceptor fibril network strategy is a promising method to improve the photovoltaic performance of ternary OSCs.     

Facile,Versatile and Stepwise Synthesis of High-Performance Oligomer Acceptors for Stable Organic Solar Cells

Oligomer acceptors have recently emerged as promising photovoltaic materials for achieving high power conversion efficiency (PCE) and long-term stability in organic solar cells (OSCs). However, the limited availability of diverse acceptors, resulting from the sole synthetic approach, has hindered their potential for future industrialization. In this study, we present a facile and effective stepwise approach that utilizes two consecutive Stille coupling reactions for the synthesis of oligomer acceptors. To demonstrate the feasibility of the novel approach, we successfully synthesize a trimer acceptor, Tri-Y6-OD, and further systematically investigate the impact of oligomerization on device performance and stability. The results reveal that this approach has significant advantages compared to the conventional method, including reduced formation of unwanted by-products and lower difficulties in purification. Remarkably, the OSC based on PM6 : Tri-Y6-OD achieves an impressive PCE of 18.03 % and maintains 80 % of the initial PCE (T₈₀) for 1523 h under illumination, surpassing the performance of the corresponding small molecule acceptor Y6-OD-based device. Furthermore, the versatility of the synthetic strategy in obtaining diverse acceptors is further demonstrated. Overall, our findings provide a facile, versatile and stepwise way for synthesizing oligomer acceptors, thereby facilitating the development of stable and efficient OSCs.