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.     

Efficient Organic Solar Cells with Extremely High Open‐Circuit Voltages and Low Voltage Losses by Suppressing Nonradiative Recombination Losses

One of the most important factors that limits the efficiencies of bulk‐heterojunction organic solar cells (OSCs) is the modest open‐circuit voltage (V_oc) due to their large voltage loss (V_loss) caused by significant nonradiative recombination loss. To boost the performance of OSCs toward their theoretical limit, developing high‐performance donor: acceptor systems featuring low V_loss with suppressed nonradiative recombination losses (<0.30 V) is desired. Herein, high performance OSCs based on a polymer donor benzodithiophene‐difluorobenzoxadiazole‐2‐decyltetradecyl (BDT‐ffBX‐DT) and perylenediimide‐based acceptors (PDI dimer with spirofluorene linker (SFPDI), PDI4, and PDI6) are reported which offer a high power conversion efficiency (PCE) of 7.5%, 56% external quantum efficiency associated with very high V_oc (>1.10 V) and low V_loss (<0.60 V). A high V_oc up to 1.23 V is achieved, which is among the highest values reported for OSCs with a PCE beyond 6%, to date. These attractive results are benefit from the suppressed nonradiative recombination voltage loss, which is as low as 0.20 V. This value is the lowest value for OSCs so far and is comparable to high performance crystalline silicon and perovskite solar cells. These results show that OSCs have the potential to achieve comparable V_oc and voltage loss as inorganic photovoltaic technologies.     

Structure Evolution of Oligomer Fused‐Ring Electron Acceptors toward High Efficiency of As‐Cast Polymer Solar Cells

The structure evolution of oligomer fused‐ring electron acceptors (FREAs) toward high efficiency of as‐cast polymer solar cells (PSCs) is reported. First, a series of FREAs (IC‐(1‐3)IDT‐IC) based on indacenodithiophene (IDT) oligomers as cores are designed and synthesized, effects of IDT number (1–3) on their basic optical and electronic properties are investigated, and more importantly, the relationship between device performance of as‐cast PSCs and donor(D)/acceptor(A) matching (absorption, energy level, morphology, and charge transport) of IC‐(1‐3)IDT‐IC acceptors and two representative polymer donors, PTB7‐Th and PDBT‐T1 is surveyed. Then, the most promising D/A system (PDBT‐T1/IC‐1IDT‐IC) with the best D/A harmony among the six D/A combinations, which yields a power conversion efficiency (PCE) of 7.39%, is found. Finally, changing the side‐chains in IC‐1IDT‐IC from alkylphenyl to alkyl enhances the PCE from 7.39% to 9.20%.     

Simultaneous Open‐Circuit Voltage Enhancement and Short‐Circuit Current Loss in Polymer: Fullerene Solar Cells Correlated by Reduced Quantum Efficiency for Photoinduced Electron Transfer

The limits of maximizing the open‐circuit voltage V_oc in solar cells based on poly[2,7‐(9,9‐didecylfluorene)‐alt‐5,5‐(4,7‐di‐2‐thienyl‐2,1,3‐benzothiadiazole)] (PF10TBT) as a donor using different fullerene derivatives as acceptor are investigated. Bulk heterojunction solar cells with PF10TBT and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) give a V_oc over 1 V and a power conversion efficiency of 4.2%. Devices in which PF10TBT is blended with fullerene bisadduct derivatives give an even higher V_oc, but also a strong decrease in short circuit current (J_sc). The higher V_oc is attributed to the higher LUMO of the acceptors in comparison to PCBM. By investigating the photophysics of PF10TBT:fullerene blends using near‐IR photo‐ and electroluminescence, time‐resolved photoluminescence, and photoinduced absorption we find that the charge transfer (CT) state is not formed efficiently when using fullerene bisadducts. Hence, engineering acceptor materials with a LUMO level that is as high as possible can increase V_oc, but will only provide a higher power conversion efficiency, when the quantum efficiency for charge transfer is preserved. To quantify this, we determine the CT energy (E_CT) and optical band gap (E_g), defined as the lowest first singlet state energy E_S1 of either the donor or acceptor, for each of the blends and find a clear correlation between the free energy for photoinduced electron transfer and J_sc. We find that E_g ? qV_oc > 0.6 eV is a simple, but general criterion for efficient charge generation in donor‐acceptor blends.     

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.     

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.     

Reduced Nonradiative Energy Loss Caused by Aggregation of Nonfullerene Acceptor in Organic Solar Cells

Reducing energy loss (E_loss) is of critical importance to improving the photovoltaic performance of organic solar cells (OSCs). Although nonradiative recombination ( $E_{\mathrm{loss}}^{\mathrm{nonrad}}$) is investigated in quite a few works, the method for modulating $E_{\mathrm{loss}}^{\mathrm{nonrad}}$ is seldom reported. Here, a new method of depressing E_loss is reported for nonfullerene OSCs. In addition to ternary‐blend bulk heterojunction (BHJ) solar cells, it is proved that a small molecular material (NRM‐1) can be selectively dispersed into the acceptor phase in the PBDB‐T:IT‐4F‐based OSC, resulting in lower $E_{\mathrm{loss}}^{\mathrm{rad}}$ and $E_{\mathrm{loss}}^{\mathrm{nonrad}}$, and hence a significant improvement in the open‐circuit voltage (V_OC); under an optimal feed ratio of NRM‐1, an enhanced power conversion efficiency can also be gained. Moreover, the role of NRM‐1 in the method is illustrated and its applicability for several other representative OSCs is validated. This work paves a new pathway to reduce the E_loss for nonfullerene OSCs.     

Modulation of End Groups for Low‐Bandgap Nonfullerene Acceptors Enabling High‐Performance Organic Solar Cells

The field of nonfullerene organic solar cells (OSCs) has seen an impressive progress, largely due to advances in high‐performance small molecule acceptors (SMAs). As a large portion of the solar energy is located in the near‐infrared region, it is important to develop ultralow‐bandgap SMAs that have extended absorption in the spectral range of 800–1000 nm to maximize light absorption and efficiencies. In this work, three low‐bandgap SMAs, namely, IXIC, IXIC‐2Cl, and IXIC‐4Cl, are designed and synthesized with same fused terthieno[3,2‐b]thiophene donor unit and different end groups (EGs). The three SMAs all have low optical bandgap (E_g) of 1.35, 1.30, and 1.25 eV, respectively. The chlorination on EGs can lower the energy level and broaden absorption range of the SMAs. As a result, the V_oc of the devices is reduced but the J_sc is significantly increased. In addition, the addition of chlorine atoms can enhance π–π stacking and crystallinity of the SMAs, which result in high fill factors. Overall, the optimum EGs are monochlorine‐substituted IC and OSCs based on PBDB‐T:IXIC‐2Cl that can achieve remarkable power conversion efficiencies (PCEs) of 12.2%, which is one of the highest PCEs for nonfullerene organic solar cells based on low‐bandgap SMAs.     

Achieving Both Enhanced Voltage and Current through Fine‐Tuning Molecular Backbone and Morphology Control in Organic Solar Cells

It is a great challenge to simultaneously improve the two tangled parameters, open circuit voltage (V_oc) and short circuit current density (J_sc) for organic solar cells (OSCs). Herein, such a challenge is addressed by a synergistic approach using fine‐tuning molecular backbone and morphology control simultaneously by a simple yet effective side chain modulation on the backbone of an acceptor–donor–acceptor (A–D–A)‐type acceptor. With this, two terthieno[3,2‐b]thiophene (3TT) based A–D–A‐type acceptors, 3TT‐OCIC with backbone modulation and 3TT‐CIC without such modification, are designed and synthesized. Compared with the controlled molecule 3TT‐CIC, 3TT‐OCIC shows power conversion efficiency (PCE) of 13.13% with improved V_oc of 0.69 V and J_sc of 27.58 mA cm^?2, corresponding to PCE of 12.15% with V_oc of 0.65 V and J_sc of 27.04 mA cm^?2 for 3TT‐CIC–based device. Furthermore, with effective near infrared absorption, 3TT‐OCIC is used as the rear subcell acceptor in a tandem device and gave an excellent PCE of 15.72%.     

A Small Molecule Non‐fullerene Electron Acceptor for Organic Solar Cells

Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.     

3,4‐Dicyanothiophene—a Versatile Building Block for Efficient Nonfullerene Polymer Solar Cells

In this contribution, a versatile building block, 3,4‐dicyanothiophene (DCT), which possesses structural simplicity and synthetic accessibility for constructing high‐performance, low‐cost, wide‐bandgap conjugated polymers for use as donors in polymer solar cells (PSCs), is reported. A prototype polymer, PB3TCN‐C66, and its cyano‐free analogue polymer PB3T‐C66, are synthesized to evaluate the potential of using DCT in nonfullerene PSCs. A stronger aggregation property in solution, higher thermal transition temperatures with higher enthalpies, a larger dipole moment, higher relative dielectric constant, and more linear conformation are exhibited by PB3TCN‐C66. Solar cells employing IT‐4F as the electron acceptor offer power conversion efficiencies (PCEs) of 11.2% and 2.3% for PB3TCN‐C66 and PB3T‐C66, respectively. Morphological characterizations reveal that the PB3TCN‐C66:IT‐4F blend exhibits better π–π paracrystallinity, a contracted domain size, and higher phase purity, consistent with its higher molecular interaction parameter, derived from thermodynamic calculations. Moreover, PB3TCN‐C66 offers a higher open‐circuit voltage and reduced energy loss than most state‐of‐the‐art wide‐bandgap polymers, without the need of additional electron‐withdrawing substituents. Two additional polymers derived from DCT also demonstrate promising performance with a higher PCE of 13.4% being achieved. Thus, DCT represents a versatile and promising building block for constructing high‐performance, low‐cost, conjugated polymers for application in PSCs.     

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.     

Benzotriazole-Based Polymer Acceptor for High-Efficiency All-Polymer Solar Cells with High Photocurrent and Low Voltage Loss

The power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) have already exceeded 17%. However, the limited absorption range of an all-polymer system results in significantly reduced short-circuit current density (J_sc), which eventually influences the PCE improvement. To broaden the light absorption of polymer acceptors, herein, benzotriazole is introduced in the core unit of small molecule acceptors and thus two narrow-bandgap polymer acceptors named PTz-BO and PTz-C11 featuring the same molecular backbone and different side-chain length are synthesized. Compared with PTz-C11, the PTz-BO based-all PSCs deliver a slightly reduced J_sc, a large open-circuit voltage (V_oc) and a low voltage loss below 0.50 V. Moreover, ternary all-PSCs are constructed by introducing PTz-C11 as a guest component. Benefiting from the reduced recombination, improved exciton generation and dissociation, and balanced charge transport, a high efficiency of 16.58% is obtained for the ternary all-PSCs, with a high J_sc over 25 mA cm⁻² without sacrificing the V_oc. Such result represents the highest efficiency reported for benzotriazole-based all-PSCs in the literature thus far. This work demonstrates the great potential of benzotriazole for the synthesis of efficient narrow-bandgap polymer acceptors.     

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.     

H‐Bonds‐Assisted Molecular Order Manipulation of Nonfullerene Acceptors for Efficient Nonannealed Organic Solar Cells

Various substituents have been incorporated into nonfullerene acceptors (NFAs) to modulate absorption scopes and energy levels for boosting efficiencies of organic solar cells (OSCs). The manipulation of the NFAs' molecular order and crystallinity via those substitutions is equally crucial to OSC performances, which yet remains interesting and challenging. The hydroxyl group, which can potentially form strong intermolecular hydrogen bonds (H‐bonds) for improving molecular arrangements, has, however, never been considered. Herein, two hydroxyl‐functionalized NFAs, IT‐OH with one hydroxyl and IT‐DOH with two hydroxyls, are synthesized to tune the molecular packing and crystallinity. The ordered molecular arrangement and higher crystallinity are observed for the IT‐OH and IT‐DOH than the parent ITIC. This is assigned to the formation of intermolecular H‐bonds induced by the hydroxyls, which elongates molecular conjugated planes leading to long‐range‐ordered structures via π–π stacking. By the appropriate crystallinity and miscibility with donor polymer, an IT‐DOH‐based nonannealed OSC affords an efficiency of 12.5% with good device stability. This work provides a promising strategy to tune the molecular packing and crystallinity to design NFAs by introducing hydroxyl groups.     

Understanding Open‐Circuit Voltage Loss through the Density of States in Organic Bulk Heterojunction Solar Cells

The field of organic photovoltaics has recently produced highly efficient single‐junction cells with power conversion efficiency >10%, yet the open‐circuit voltage (V_OC) remains relatively low in many high performing systems. An accurate picture of the density of states (DOS) in working solar cells is crucial to understanding the sources of voltage loss, but remains difficult to obtain experimentally. Here, the tail of the DOS is characterized in a number of small molecule bulk heterojunction solar cells from the charge density dependence of V_OC, and is directly compared to the disorder present within donor and acceptor components as measured by Kelvin probe. Using these DOS distributions, the total energy loss relative to the charge transfer state energy (E_CT)—ranging from ≈0.5 to 0.7 eV—is divided into contributions from energetic disorder and from charge recombination, and the extent to which these factors limit the V_OC is assessed.     

Record Open‐Circuit Voltage Wide‐Bandgap Perovskite Solar Cells Utilizing 2D/3D Perovskite Heterostructure

In this work, the authors realize stable and highly efficient wide‐bandgap perovskite solar cells that promise high power conversion efficiencies (PCE) and are likely to play a key role in next generation multi‐junction photovoltaics (PV). This work reports on wide‐bandgap (≈1.72 eV) perovskite solar cells exhibiting stable PCEs of up to 19.4% and a remarkably high open‐circuit voltage (V_OC) of 1.31 V. The V_OC‐to‐bandgap ratio is the highest reported for wide‐bandgap organic?inorganic hybrid perovskite solar cells and the V_OC also exceeds 90% of the theoretical maximum, defined by the Shockley–Queisser limit. This advance is based on creating a hybrid 2D/3D perovskite heterostructure. By spin coating n‐butylammonium bromide on the double‐cation perovskite absorber layer, a thin 2D Ruddlesden–Popper perovskite layer of intermediate phases is formed, which mitigates nonradiative recombination in the perovskite absorber layer. As a result, V_OC is enhanced by 80 mV.     

Organic Salts as a Route to Energy Level Control in Low Bandgap,High Open‐Circuit Voltage Organic and Transparent Solar Cells that Approach the Excitonic Voltage Limit

A new series of organic salts with selective near‐infrared (NIR) harvesting to 950 nm is reported, and anion selection and blending is demonstrated to allow for fine tuning of the open‐circuit voltage. Extending photoresponse deeper into the NIR is a significant challenge facing small molecule organic photovoltaics, and recent demonstrations have been limited by open‐circuit voltages much lower than the theoretical and practical limits. This work presents molecular design strategies that enable facile tuning of energy level alignment and open‐circuit voltages in organic salt‐based photovoltaics. Anions are also shown to have a strong influence on exciton diffusion length. These insights provide a clear route toward achieving high efficiency transparent and panchromatic photovoltaics, and open up design opportunities to rapidly tailor molecules for new donor–acceptor systems.     

Nonfullerene n‐Type Organic Semiconductors for Perovskite Solar Cells

Compared to inorganic semiconductors and/or fullerene derivatives, nonfullerene n‐type organic semiconductors present some advantages, such as low‐temperature processing, flexibility, and molecule structure diversity, and have been widely used in perovskite solar cells (PSCs). In this research news article, the recent advances in nonfullerene n‐type organic semiconductors which function as electron‐transporting, interface‐modifying, additive, and light‐harvesting materials in PSCs are summarized. The remaining challenges and promising future directions of nonfullerene‐based PSCs are also discussed.