Thermally Durable Nonfullerene Acceptor with Nonplanar Conjugated Backbone for High‐Performance Organic Solar Cells

A nonfullerene acceptor (NFA) with acceptor–donor–acceptor (A–D–A) architecture, i‐IEICO‐2F, based on 4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene as an electron‐donating core and 2‐(6‐fluoro‐2,3‐dihydro‐3‐oxo‐1H‐inden‐1‐ylidene)‐propanedinitrile as electron‐withdrawing end groups, is designed and synthesized. i‐IEICO‐2F has a twist structure in the main conjugated chain, which causes blueshifted absorption and leads to harmonious absorption with a high bandgap donor. The bandgap of i‐IEICO‐2F compliments the bandgap of suitable wide bandgap donor polymers such as J52, leading to complete light absorption throughout the visible spectrum. Devices based on i‐IEICO‐2F exhibit optimized photovoltaic performance including an open‐circuit voltage of 0.93 V, a short‐circuit current density of 16.61 mA cm^?2, and a fill factor of 73%, and result in a power conversion efficiency (PCE) of 11.28%. The i‐IEICO‐2F‐based devices reach PCEs of >11% without using any additives or post‐treatments. Devices are found to be thermally stable and maintain 44% of their initial PCE after 184.5 h of continuous thermal annealing (TA) treatment at 150 °C. Based on UV, atomic force microscopy (AFM), and grazing incidence wide angle X‐ray scattering (GIWAXS) results, i‐IEICO‐2F devices show almost identical morphology and molecular orientation throughout the TA treatment and excellent stability compared to other IEICO derivatives.     

Burn‐in Free Nonfullerene‐Based Organic Solar Cells

Organic solar cells that are free of burn‐in, the commonly observed rapid performance loss under light, are presented. The solar cells are based on poly(3‐hexylthiophene) (P3HT) with varying molecular weights and a nonfullerene acceptor (rhodanine‐benzothiadiazole‐coupled indacenodithiophene, IDTBR) and are fabricated in air. P3HT:IDTBR solar cells light‐soaked over the course of 2000 h lose about 5% of power conversion efficiency (PCE), in stark contrast to [6,6]‐Phenyl C61 butyric acid methyl ester (PCBM)‐based solar cells whose PCE shows a burn‐in that extends over several hundreds of hours and levels off at a loss of ≈34%. Replacing PCBM with IDTBR prevents short‐circuit current losses due to fullerene dimerization and inhibits disorder‐induced open‐circuit voltage losses, indicating a very robust device operation that is insensitive to defect states. Small losses in fill factor over time are proposed to originate from polymer or interface defects. Finally, the combination of enhanced efficiency and stability in P3HT:IDTBR increases the lifetime energy yield by more than a factor of 10 when compared with the same type of devices using a fullerene‐based acceptor instead.     

Spectral Engineering of Semitransparent Polymer Solar Cells for Greenhouse Applications

In this study, a wavelength selective semitransparent polymer solar cell (ST‐PSC) with a proper transmission spectrum for plant growth is proposed for greenhouse applications. A ternary strategy combining a wide bandgap polymer donor with a near‐infrared absorbing nonfullerene acceptor and a high electron mobility fullerene acceptor is introduced to achieve PSCs with power conversion efficiency (PCE) over 10%. The addition of PC₇₁BM into J52:IEICO‐4F binary blend contributes to the suppressed trap‐assisted recombination, enhanced charge extraction, and improved open‐circuit voltage simultaneously. ST‐PSC based on the J52:IEICO‐4F:PC₇₁BM ternary blend shows an optimized performance with PCE of 7.75% and a defined crop growth factor of 24.8%. Such high‐performance ST‐PSC is achieved by carefully engineering the absorption spectrum of the light harvesting materials. As a result, the transmission spectra of the semitransparent devices are well‐matched with the absorption spectra of the photoreceptors, such as chlorophylls, in green plants, which provides adequate lighting conditions for photosynthesis and plant growth, and therefore making it a competitive candidate for photovoltaic greenhouse applications.     

Efficient Ternary Polymer Solar Cells with Two Well‐Compatible Donors and One Ultranarrow Bandgap Nonfullerene Acceptor

Nonfullerene polymer solar cells (PSCs) are fabricated by using one wide bandgap donor PBDB‐T and one ultranarrow bandgap acceptor IEICO‐4F as the active layers. One medium bandgap donor PTB7‐Th is selected as the third component due to the similar highest occupied molecular orbital level compared to that of PBDB‐T and their complementary absorption spectra. The champion power conversion efficiency (PCE) of PSCs is increased from 10.25% to 11.62% via incorporating 20 wt% PTB7‐Th in donors, with enhanced short‐circuit current (J_SC) of 24.14 mA cm^?2 and fill factor (FF) of 65.03%. The 11.62% PCE should be the highest value for ternary nonfullerene PSCs. The main contribution of PTB7‐Th can be summarized as the improved photon harvesting and enhanced exciton utilization of PBDB‐T due to the efficient energy transfer from PBDB‐T to PTB7‐Th. Meanwhile, PTB7‐Th can also act as a regulator to adjust PBDB‐T molecular arrangement for optimizing charge transport, resulting in the enhanced FF of ternary PSCs. This experimental result may provide new insight for developing high‐performance ternary nonfullerene PSCs by selecting two well‐compatible donors with different bandgap and one ultranarrow bandgap acceptor.     

Rational Strategy to Stabilize an Unstable High‐Efficiency Binary Nonfullerene Organic Solar Cells with a Third Component

Long device lifetime is still a missing key requirement in the commercialization of nonfullerene acceptor (NFA) organic solar cell technology. Understanding thermodynamic factors driving morphology degradation or stabilization is correspondingly lacking. In this report, thermodynamics is combined with morphology to elucidate the instability of highly efficient PTB7‐Th:IEICO‐4F binary solar cells and to rationally use PC₇₁BM in ternary solar cells to reduce the loss in the power conversion efficiency from ≈35% to <10% after storage for 90 days and at the same time improve performance. The hypomiscibility observed for IEICO‐4F in PTB7‐Th (below the percolation threshold) leads to overpurification of the mixed domains. By contrast, the hypermiscibility of PC₇₁BM in PTB7‐Th of 48 vol% is well above the percolation threshold. At the same time, PC₇₁BM is partly miscible in IEICO‐4F suppressing crystallization of IEICO‐4F. This work systematically illustrates the origin of the intrinsic degradation of PTB7‐Th:IEICO‐4F binary solar cells, demonstrates the structure–function relations among thermodynamics, morphology, and photovoltaic performance, and finally carries out a rational strategy to suppress the degradation: the third component needs to have a miscibility in the donor polymer at or above the percolation threshold, yet also needs to be partly miscible with the crystallizable acceptor.     

A Halogenation Strategy for over 12% Efficiency Nonfullerene Organic Solar Cells

Three acceptor–donor–acceptor type nonfullerene acceptors (NFAs), namely, F–F, F–Cl, and F–Br, are designed and synthesized through a halogenation strategy on one successful nonfullerene acceptor FDICTF (F–H). The three molecules show red‐shifted absorptions, increased crystallinities, and higher charge mobilities compared with the F–H. After blending with donor polymer PBDB‐T, the F–F‐, F–Cl‐, and F–Br‐based devices exhibit power conversion efficiencies (PCEs) of 10.85%, 11.47%, and 12.05%, respectively, which are higher than that of F–H with PCE of 9.59%. These results indicate that manipulating the absorption range, crystallinity and mobilities of NFAs by introducing different halogen atoms is an effective way to achieve high photovoltaic performance, which will offer valuable insight for the designing of high‐efficiency organic solar cells.     

High‐Performance Tandem Organic Solar Cells Using HSolar as the Interconnecting Layer

Tandem structure provides a practical way to realize high efficiency organic photovoltaic cells, it can be used to extend the wavelength coverage for light harvesting. The interconnecting layer (ICL) between subcells plays a critical role in the reproducibility and performance of tandem solar cells, yet the processability of the ICL has been a challenge. In this work the fabrication of highly reproducible and efficient tandem solar cells by employing a commercially available material, PEDOT:PSS HTL Solar (HSolar), as the hole transporting material used for the ICL is reported. Comparing with the conventional PEDOT:PSS Al 4083 (c‐PEDOT), HSolar offers a better wettability on the underlying nonfullerene photoactive layers, resulting in better charge extraction properties of the ICL. When FTAZ:IT‐M and PTB7‐Th:IEICO‐4F are used as the subcells, a power conversion efficiency (PCE) of 14.7% is achieved in the tandem solar cell. To validate the processability of these tandem solar cells, three other research groups have successfully fabricated tandem devices using the same recipe and the highest PCE obtained is 16.1%. With further development of donor polymers and device optimization, the device simulation results show that a PCE > 22% can be realized in tandem cells in the near future.     

N‐Annulated Perylene as a Coplanar π‐Linker Alternative to Benzene as a Low Energy‐Gap,Metal‐Free Dye in Sensitized Solar Cells

Perylenes are well‐known pigments with excellent chemical, thermal, and photochemical stabilities and have been used in various optical and electronic fields. Although for sensitized mesoscopic solar cells there is rapid progress of metal‐free thiophene dyes, which now reach over 11.5% power conversion efficiency (PCE) at air mass 1.5 global (AM1.5G) conditions, the so far reported highest PCE of a perylene dye is only 6.8%. Here, a new metal‐free organic donor‐acceptor (D‐A) dye ( C261 ) featuring a bisarylamino functionalized N‐annulated perylene electron‐releasing segment and a cyanoacrylic acid electron‐withdrawing unit is synthesized. Combining a mesoporous titania film grafted by this structurally simple perylene dye with a non‐corrosive cobalt redox shuttle, an 8.8% PCE is achieved at an irradiance of the AM1.5G sunlight. By selecting the model dye G221 as a reference, theoretical calculations, steady‐state and time‐resolved spectroscopies, and electrical measurements are used to compare the energy‐levels, light absorptions, and mutichannel charge transfer dynamics that contribute to the photovoltaic behavior.     

High‐Efficiency CsPbI2Br Perovskite Solar Cells with Dopant‐Free Poly(3‐hexylthiophene) Hole Transporting Layers

CsPbI₂Br is emerging as a promising all‐inorganic material for perovskite solar cells (PSCs) due to its more stable lattice structure and moisture resistance compared to CsPbI₃, although its device performance is still much behind this counterpart. Herein, a preannealing process is developed and systematically investigated to achieve high‐quality CsPbI₂Br films by regulating the nucleation and crystallization of perovskite. The preannealing temperature and time are specifically optimized for a dopant‐free poly(3‐hexylthiophene) (P3HT)‐based device to target dopant‐induced drastic performance degradation for spiro‐OMeTAD‐based devices. The resulting P3HT‐based device exhibits comparable power conversion efficiency (PCE) to spiro‐OMeTAD‐based devices but much enhanced ambient stability with over 95% PCE after 1300 h. A diphenylamine derivative is introduced as a buffer layer to improve the energy‐level mismatch between CsPbI₂Br and P3HT. A record‐high PCE of 15.50% for dopant‐free P3HT‐based CsPbI₂Br PSCs is achieved by alleviating the open‐circuit voltage loss with the buffer layer. These results demonstrate that the preannealing processing together with a suitable buffer layer are applicable strategies for developing dopant‐free P3HT PSCs with high efficiency and stability.     

Thermally Stable All‐Polymer Solar Cells with High Tolerance on Blend Ratios

Tuning the blend composition is an essential step to optimize the power conversion efficiency (PCE) of organic bulk heterojunction (BHJ) solar cells. PCEs from devices of unoptimized donor:acceptor (D:A) weight ratio are generally significantly lower than optimized devices. Here, two high‐performance organic nonfullerene BHJ blends PBDB‐T:ITIC and PBDB‐T:N2200 are adopted to investigate the effect of blend ratio on device performance. It is found that the PCEs of polymer‐polymer (PBDB‐T:N2200) blend are more tolerant to composition changes, relative to polymer‐molecule (PBDB‐T:ITIC) devices. In both systems, short‐circuit current density (J_sc) is tracked closely with PCE, indicating that exciton dissociation and transport strongly influence PCEs. With dilute acceptor concentrations, polymer‐polymer blends maintain high electron mobility relative to the polymer‐molecule blends, which explains the dramatic difference in PCEs between them as a function of D:A blend ratio. In addition, polymer‐polymer solar cells, especially at high D:A blend ratio, are stable (less than 5% relative loss) over 70 d under continuous heating at 80 °C in a glovebox without encapsulation. This work demonstrates that all‐polymer solar cells show advantage in operational lifetime under thermal stress and blend‐ratio resilience, which indicates their high potential for designing of stable and scalable solar cells.     

Reduced Energy Loss Enabled by a Chlorinated Thiophene‐Fused Ending‐Group Small Molecular Acceptor for Efficient Nonfullerene Organic Solar Cells with 13.6% Efficiency

Generally, highly efficient organic solar cells require both a high open‐circuit voltage (V_OC) and a high short‐circuit current density (J_SC). Reducing the energy loss (E_loss) is an effective way to achieve a high V_OC without compromising the photocurrent, which is ideal for enhancing the power conversion efficiencies (PCEs). Herein, a new chlorinated nonfullerene acceptor (ITC‐2Cl) with chlorinated thiophene‐fused end groups is developed. In comparison with the unchlorinated counterpart (ITCPTC), the introduction of Cl improves not only the electronic properties by redshifting the absorption spectra and deepening the lowest unoccupied molecular orbital energy levels, but also the molecular packing and thus thin‐film morphology. The PM6:ITC‐2Cl‐based device yields a significantly higher PCE (13.6%) with a lower E_loss (0.67 eV) than the ITCPTC‐based device (PCE of 12.3% with E_loss of 0.70 eV). More importantly, compared to the archetypal nonfullerene acceptors such as IT‐4F (PCE of 12.9% with E_loss of 0.73 eV) and IT‐4Cl (PCE of 12.7% with E_loss of 0.76 eV), the ITC‐2Cl‐based device shows a higher PCE and a lower E_loss. These results demonstrate that the chlorinated thiophene‐fused end group is a promising candidate for a high‐performance nonfullerene acceptors with low energy loss.     

15% Efficiency Tandem Organic Solar Cell Based on a Novel Highly Efficient Wide‐Bandgap Nonfullerene Acceptor with Low Energy Loss

A tandem organic solar cell (OSC) is a valid structure to widen the photon response range and suppress the transmission loss and thermalization loss. In the past few years, the development of low‐bandgap materials with broad absorption in long‐wavelength region for back subcells has attracted considerable attention. However, wide‐bandgap materials for front cells that have both high short‐circuit current density (J_SC) and open‐circuit voltage (V_OC) are scarce. In this work, a new fluorine‐substituted wide‐bandgap small molecule nonfullerene acceptor TfIF‐4FIC is reported, which has an optical bandgap of 1.61 eV. When PBDB‐T‐2F is selected as the donor, the device offers an extremely high V_OC of 0.98 V, a high J_SC of 17.6 mA cm^?2, and a power conversion efficiency of 13.1%. This is the best performing acceptor with such a wide bandgap. More importantly, the energy loss in this combination is 0.63 eV. These properties ensure that PBDB‐T‐2F:TfIF‐4FIC is an ideal candidate for the fabrication of tandem OSCs. When PBDB‐T‐2F:TfIF‐4FIC and PTB7‐Th:PCDTBT:IEICO‐4F are used as the front cell and the back cell to construct tandem solar cells, a PCE of 15% is obtained, which is one of best results reported to date in the field of organic solar cells.     

Side‐Chain Impact on Molecular Orientation of Organic Semiconductor Acceptors: High Performance Nonfullerene Polymer Solar Cells with Thick Active Layer over 400 nm

A new n‐type organic semiconductor (n‐OS) acceptor IDTPC with n‐hexyl side chains is developed. Compared to side chains with 4‐hexylphenyl counterparts (IDTCN), such a design endows the acceptor of IDTPC with higher electron mobility, more ordered face‐on molecular packing, and lower band gap. Therefore, the IDTPC‐based polymer solar cells (PSCs) with a newly developed wide bandgap polymer PTQ10 as donor exhibit the maximum power conversion efficiency (PCE) of 12.2%, a near 65% improvement in PCE relative to the IDTCN‐based control device. Most importantly, the IDTPC‐based device is insensitive to the thickness of the active layer from 70 to 505 nm, which still gives a PCE of 10.0% with the active‐layer thickness of 400 nm. To the best of the authors' knowledge, a PCE of 10.0% is the highest value for the nonfullerene PSCs with an active layer thicker than 400 nm. These results reveal that the blend of PTQ10 and IDTPC exhibits great potential for highly efficient nonfullerene PSCs and large‐area device fabrication.     

The Physics of Small Molecule Acceptors for Efficient and Stable Bulk Heterojunction Solar Cells

Organic bulk heterojunction solar cells based on small molecule acceptors have recently seen a rapid rise in the power conversion efficiency with values exceeding 13%. This impressive achievement has been obtained by simultaneous reduction of voltage and charge recombination losses within this class of materials as compared to fullerene‐based solar cells. In this contribution, the authors review the current understanding of the relevant photophysical processes in highly efficient nonfullerene acceptor (NFA) small molecules. Charge generation, recombination, and charge transport is discussed in comparison to fullerene‐based composites. Finally, the authors review the superior light and thermal stability of nonfullerene small molecule acceptor based solar cells, and highlight the importance of NFA‐based composites that enable devices without early performance loss, thus resembling so‐called burn‐in free devices.     

High‐Performance Multilayer Encapsulation for Perovskite Photovoltaics

An encapsulation system comprising of a UV‐curable epoxy, a solution processed polymer interlayer, and a glass cover‐slip, is used to increase the stability of methylammonium lead triiodide (CH₃NH₃PbI₃) perovskite planar inverted architecture photovoltaic (PV) devices. It is found this encapsulation system acts as an efficient barrier to extrinsic degradation processes (ingress of moisture and oxygen), and that the polymer acts as a barrier that protects the PV device from the epoxy before it is fully cured. This results in devices that maintain 80% of their initial power conversion efficiency after 1000 h of AM1.5 irradiation. Such devices are used as a benchmark and are compared with devices having initially enhanced efficiency as a result of a solvent annealing process. It is found that such solvent‐annealed devices undergo enhanced burn‐in and have a reduced long‐term efficiency, a result demonstrating that initially enhanced device efficiency does not necessarily result in long‐term stability.     

Layer‐by‐Layer Solution‐Processed Low‐Bandgap Polymer‐PC61BM Solar Cells with High Efficiency

Polymer solar cells (PSCs) are fabricated without solvent additives using a low‐bandgap polymer, PBDTTT‐C‐T, as the donor and [6,6]‐phenyl‐C61‐butyric‐acid‐methyl‐ester (PC₆₁BM) as the acceptor. Donor‐acceptor blend and layer‐by‐layer (LL) solution process are used to form active layers. Relative to the blend devices, the LL devices exhibit stronger absorption, better vertical phase separation, higher hole and electron mobilities, and better charge extraction at correct electrodes. As a result, after thermal annealing the LL devices exhibit an average power conversion efficiency (PCE) of 6.86%, which is much higher than that of the blend devices (4.31%). The best PCE of the LL devices is 7.13%, which is the highest reported for LL processed PSCs and among the highest reported for PC₆₁BM‐based single‐junction PSCs.     

A High‐Performance D–A Copolymer Based on Dithieno[3,2‐b:2′,3′‐d]Pyridin‐5(4H)‐One Unit Compatible with Fullerene and Nonfullerene Acceptors in Solar Cells

Development of high‐performance donor–acceptor (D–A) copolymers is vital in the research of polymer solar cells (PSCs). In this work, a low‐bandgap D–A copolymer based on dithieno[3,2‐b:2′,3′‐d]pyridin‐5(4H)‐one unit (DTP), PDTP4TFBT, is developed and used as the donor material for PSCs with PC₇₁BM or ITIC as the acceptor. PDTP4TFBT:PC₇₁BM and PDTP4TFBT:ITIC solar cells give power conversion efficiencies (PCEs) up to 8.75% and 7.58%, respectively. 1,8‐Diiodooctane affects film morphology and device performance for fullerene and nonfullerene solar cells. It inhibits the active materials from forming large domains and improves PCE for PDTP4TFBT:PC₇₁BM cells, while it promotes the aggregation and deteriorates performance for PDTP4TFBT:ITIC cells. The ternary‐blend cells based on PDTP4TFBT:PC₇₁BM:ITIC (1:1.2:0.3) give a decent PCE of 9.20%.     

Asymmetric Nonfullerene Small Molecule Acceptors for Organic Solar Cells

Symmetry breaking provides a new material design strategy for nonfullerene small molecule acceptors (SMAs). The past 10 years have witnessed significant advances in asymmetric nonfullerene SMAs in organic solar cells (OSCs) with power conversion efficiency (PCE) increasing from ≈1% to ≈14%. In this review, the progress of asymmetric nonfullerene SMAs, including early reports of asymmetric nonfullerene SMAs, asymmetric PDI‐based nonfullerene SMAs, and asymmetric acceptor–donor–acceptor (A–D–A)‐type nonfullerene SMAs, is summarized. The structure–property relationships and the perspectives for future development of asymmetric nonfullerene SMAs are also discussed.     

Simple Bithiophene–Rhodanine‐Based Small Molecule Acceptor for Use in Additive‐Free Nonfullerene OPVs with Low Energy Loss of 0.51 eV

The introduction of rigid and extended ladder‐type fused‐ring cores, such as indacenodithiophene, has enabled the synthesis of a variety of nonfullerene small molecules for use as electron acceptors in high‐performance organic photovoltaic cells. Contrasting with recent trends, a very simple‐structured nonfullerene acceptor (NFA), T2‐ORH , consisting of a bithiophene core and octyl‐substituted rhodanine ends, is synthesized in two steps from inexpensive commercially available raw materials. Its relatively short π‐conjugation results in a wide bandgap and a blue‐shifted UV–vis absorption profile complementary to those of poly[4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐3‐fluorothieno[3,4‐b]thiophene‐2‐carboxylate] (PTB7‐Th). Despite a sufficient offset between T2‐ORH and PTB7‐Th, the lowest unoccupied molecular orbital (LUMO) energy level of T2‐ORH is still higher than the LUMOs of other NFAs (e.g., ITIC). Therefore, the PTB7‐Th: T2‐ORH blend film exhibits an efficiency of 9.33% with a high open‐circuit voltage of 1.07 V and a short‐circuit current of 14.72 mA cm^?2 in an additive‐free single‐junction cell. Importantly, the optimized device displays a remarkably low energy loss of 0.51 eV, in which bimolecular and monomolecular charge recombination is effectively suppressed by solvent vapor annealing treatment. The blend film has a very smooth and homogeneous morphology, providing both vertical and parallel charge transport in the devices.     

Liquid‐Crystalline Small Molecules for Nonfullerene Solar Cells with High Fill Factors and Power Conversion Efficiencies

Compared with nonfullerene‐based polymer solar cells, all‐small‐molecule solar cells normally show low power conversion efficiencies (PCEs) due to their low fill factors (FFs). Molecular stacking orientation and phase separation are the main factors affecting the performance of all‐small‐molecule solar cells. In this work, two liquid‐crystalline small‐molecule donors are designed and synthesized and a novel nonfullerene acceptor with good crystallinity developed. Owing to the face‐on orientation of the component molecules and appropriate phase separation in the active layer, a high FF of over 70% and a PCE of 10.7% are obtained from the resulting solar cells; these values are among the best obtained thus far for all‐small‐molecule solar cells. The high FF reported here is significant for a further design of high‐performance all‐small‐molecule solar cells.