Efficient Ternary Organic Solar Cells Research Articles

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.

Nonfullerene Ternary Organic Solar Cell with Effective Charge Transfer between Two Acceptors.

High power conversion efficiency can be realized by using a ternary bulk heterojunction with complementary absorption spectra in organic solar cells. However, as the development of nonfullerene acceptors with a broad absorption spectrum makes the absorption efficiency of the photovoltaic devices close to optimal, such a strategy needs modifying. In particular, charge transfer between the two acceptors is necessary to be considered. Herein, we purposely design a ternary system based on PTB7-Th:COi8DFIC:ITIC-4F. Though the presence of ITIC-4F in PTB7-Th:COi8DFIC could not broaden the absorption spectrum obviously, the formed cascade-energy-level alignment is beneficial for promoting and balancing exciton separation and charge transport between the donor and two acceptors and even between the acceptors. Insights into the charge transport route in the completed system are provided via using the techniques including photoluminescence spectroscopy and pump-probe photoconductivity spectroscopy. This work provides a new idea for designing highly efficient ternary organic solar cells.

Dual non-fullerene acceptors based high efficiency ternary organic solar cells

<sec>Organic photovoltaics (OPVs) have been considered as one of the preliminary candidates for the third-generation solar cells due to their particularly advantages, such as light-weight, low cost, solution processability and mechanical flexibility. In recent years, the power conversion efficiency (PCE) of OPVs has achieved remarkable progress with the development of non-fullerene acceptors (NFAs), which exhibit stronger capability of light absorption and stability than the fullerene acceptors. However, the narrow absorption properties of organic materials still restrict the further breakthrough of the performance of OPVs. It is difficult for the binary heterojunction films to realize the effective wide spectrum utilization of solar energy. Ternary strategy, which consists of one donor and two acceptors or two donors and one acceptor in a bulk-heterojunction, has proven to be an effective and facile way to enhance performances of OPVs. The emergence of new NFAs such as ITIC, IT-4F, Y6 <i>etc.</i> greatly increases the selectivity of acceptors in a ternary system. It is necessary to investigate the compatibility of latest NFA materials and corresponding ternary device performance.</sec> <sec>In this article, we report a new ternary OPV system with dual NFAs and a polymer donor. The excellent NFA material (Y6), which was first reported in 2019, is used as the third component. Different concentrations of Y6 are introduced into the binary system based on PBDB-T-2F:IT-4F. The compatibility between materials and device performances are investigated through absorption capability of blend films, AFM, water contact angle, GISAXS, and corresponding electrical properties of devices. The 12.48% PCE is achieved from ternary OPVs with PBDB-T:IT-4F as the active layers containing 20wt% Y6 in acceptors, resulting from the enhanced <i>J</i><sub>sc</sub> of 22.09 mA/cm<sup>2</sup>, <i>V</i><sub>oc</sub> of 0.83 V and <i>FF</i> of 68.45%. The origin of the improvement of the ternary OPVs is summarized below. Firstly, an apparently complementary absorption spectrum is obtained through the introduction of Y6, which has a stronger photo harvesting capability in the spectral range from 750 to 950 nm than IT-4F, and higher short current density (<i>J</i><sub>sc</sub>) is observed in the ternary devices than in the binary device. Secondly, a rougher surface of the active layer is observed by increasing the Y6 concentration, which may result in an inferior exciton dissociation and charge transport process, the existence of larger-scaled crystal is proved by the GISAXS technology. Thirdly, the introduced Y6 can help to suppress the bimolecular recombination, which is in favor of the incremental device performance. Approximately 17.85% <i>PCE</i> improvement is obtained in comparison with PBDB-T-2F:IT-4F based binary OPVs. </sec>

Over 17% efficiency ternary organic solar cells enabled by two non-fullerene acceptors working in an alloy-like model

An alloy-like model based on Y6 and its derivative BTP-M is constructed to fabricate ternary organic solar cells, leading to a best efficiency of 17.03%.

Cyano-functionalized small-molecule acceptors for high-efficiency wide-bandgap organic solar cells

Wide-bandgap non-fullerene acceptors (NFAs) are in high demand for constructing efficient ternary or tandem organic solar cells (OSCs), yet the scarcity of them remains an important issue that needs exploration and perfection.

Energetic Disorder and Activation Energy in Efficient Ternary Organic Solar Cells with Nonfullerene Acceptor Eh‐IDTBR as the Third Component

Solution‐processed ternary organic solar cells (OSCs) contain a third component in the active layer in addition to the donor/acceptor materials. Two main avenues are considered to fabricate ternary OSCs: 1) to improve the short‐circuit current density by the selected third component that broadens and/or enhances the absorption of the host films and 2) to increase the fill factor by adding materials with diverse crystallinity to tune the film morphology. However, little work is reported for the improvement of open‐circuit voltage (VOC), energetic disorder, charge transfer state energy (ECT), and activation energy in ternary OSCs. Herein, ternary OSCs with active layer composed of PCE10:F8IC:Eh‐IDTBR as the model to examine these parameters in addition to the morphology are used. In the ternary device, the additional Eh‐IDTBR improves the crystallinity of the acceptor phase in the ternary mixture; the VOC is 58 mV higher than that of the reference caused by the reduced energetic disorder; due to the good miscibility of Eh‐IDTBR with both PCE10 and F8IC, only 50 meV in ECT is observed; and the zero‐field activation energy is lower than that for the reference. The findings provide an alternative way to understand the complex ternary OSC structural–electrical properties’ correlations.

Building a planar single and binary blend stack ternary organic solar cells

Higher efficiency ternary organic solar cells (OSCs) can be built by capping the photoactive layer of a binary blend OSC device with a planar single photovoltaic layer, with an absorption spectrum complementary to that for the original blend layer. A ternary active layer material system of two donors poly (3-hexylthiophene) and Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′] dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] and an acceptor [6,6]-Phenyl C61 butyric acid methyl ester is considered in this work. From the various possibilities of building ternary device with a binary blend layer and planar single layer stacks, the low efficiency options are eliminated. Simulation studies of the final structure are carried out to decide the optimum thicknesses of the different photovoltaic layers. The optimised ternary device fabricated shows an improvement in power conversion efficiency of up to 23.5% over the base binary blend solar cells. The primary contribution to efficiency improvement is from the increase in short circuit current from the increased harvest of photons by the planar single capping layer.

Efficient ternary organic solar cells based on a twin spiro-type non-fullerene acceptor

A novel small-molecule (SM) acceptor DTF-IC is designed and synthesized in this work. The power conversion efficiency (PCE) of ternary OSCs increased up to 12.14% from 10.90% by incorporating 10 wt% of DTF-IC as second acceptors into the binary OSCs consisting of PBDB-T as donor and IT-M as acceptor. This was mainly due to the large increase in short-circuit current (Jsc) from 16.18 to 17.95 mA/cm2, without any drop in the open-circuit voltage (Voc) and fill factor (FF). The addition of DTF-IC enabled the donor and acceptor to form a distinct complementary absorption profile in the visible-light region, which boosted the photon harvesting in the range of 730–800 nm and consequently increased the Jsc of the ternary system by 11%. Moreover, there was an energy transfer between the two SM acceptors, favorable for enhancing charge separation and transfer as well as reducing charge recombination at PBDB-T:IT-M and PBDB-T:DTF-IC interface. Simultaneously, HOMO and LUMO energy levels of DTF-IC were lower than those of PBDB-T, but still higher than those of IT-M. Thus, DTF-IC is able to provide a cascading energy level with the host donor and acceptor which are beneficial for efficient charge transfer between the acceptors and facilitating exciton dissociation and carrier transport. Meanwhile, the highly crystalline DTF-IC as a third component can improve the crystallization process of the active layer while maintaining proper phase separation. This work proposes a novel idea for non-fullerene acceptors achieved via twin spiro-type structure modifying by indanone and provides a new direction for the selection of ternary solar cell materials.

Dual Sensitizer and Processing-Aid Behavior of Donor Enables Efficient Ternary Organic Solar Cells

Summary Herein, we report ternary organic solar cells with a power conversion efficiency (PCE) of 14.0%. By incorporating 10 wt % of BIT-4F-T in the PTB7-Th:IEICO-4F blend, we obtain an enhancement of all photovoltaic parameters compared to the binary devices, leading to a 15% performance improvement in ternary blend. The high photocurrent in 10% BIT-4F-T blend results from a complementary absorption profile of donor components and a hole transfer from BIT-4F-T to PTB7-Th. Morphological and device characterizations reveal that the addition of 10% BIT-4F-T acts not only as a sensitizer but also as a solid processing aid, which is beneficial for charge generation and transport. The effect of the third component is observed in different non-fullerene and fullerene OSCs. Our study demonstrates that careful selection of a third component, where dual sensitizing and processing-aid effects are observed, can be a design strategy to achieve a concomitant improvement in all photovoltaic parameters.

Employing structurally similar acceptors as crystalline modulators to construct high efficiency ternary organic solar cells

Structurally similar acceptors could function as crystalline modulators to fine-tune the morphology for constructing efficient ternary organic solar cells.

16.55% efficiency ternary organic solar cells enabled by incorporating a small molecular donor

A 16.55% high efficiency organic solar cell was achieved by employing a PM6:SM1:Y6 system with an effective and simple ternary strategy.

Efficient Ternary Organic Solar Cells Enabled by the Integration of Nonfullerene and Fullerene Acceptors with a Broad Composition Tolerance

AbstractThe ternary structure that combines fullerene and nonfullerene acceptors in a photoactive layer is demonstrated as an effective approach for boosting the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Here, highly efficient ternary OSCs comprising a wide‐bandgap polymer donor (PBT1‐C), a narrow‐bandgap nonfullerene acceptor (IT‐2F), and a typical fullerene derivative (PC71BM) are reported. It is found that the addition of PC71BM into the PBT1‐C:IT‐2F blend not only increases the device efficiency up to 12.2%, but also improves the ambient stability of the OSCs. Detailed investigations indicate that the improvement in photovoltaic performance benefits from synergistic effects of increased photon‐harvesting, enhanced charge separation and transport, suppressed trap‐assisted recombination, and optimized film morphology. Moreover, it is noticed that such a ternary system exhibits excellent tolerance to the PC71BM component, for which PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of PBT1‐C:IT‐2F binary reference device.

Enhanced Charge Transfer between Fullerene and Non-Fullerene Acceptors Enables Highly Efficient Ternary Organic Solar Cells.

Insufficient driving forces defined as the energetic offsets of the frontier molecular orbitals between a donor and an acceptor influence the charge separation in organic solar cells (OSCs), thus restricting the improvement of quantum efficiencies. Herein, we demonstrate that enhancing charge transfer between fullerene and non-fullerene acceptors via ternary strategy is an effective method to address this problem. By introducing an electron acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the third component to the binary blends based on the polymer donor of poly[(2,6-(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)-benzo[1,2- b:4,5- b']dithiophene))- alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'- c:4',5'- c']dithiophene-4,8-dione)] (PBDB-TF) and the small-molecule acceptor of 2,2'-((2 Z,2' Z)-(((2,5-difluoro-1,4-phenylene)bis(4,4-bis(2-ethylhexyl)-4 H-cyclopenta[2,1- b:3,4- b']dithiophene-6,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1 H-indene-2,1-diylidene))dimalononitrile (HF-PCIC) or 2,2'-((2 Z,2' Z)-(((2,5-difluoro-1,4-phenylene)bis(4,4-bis(2-ethylhexyl)-4 H-cyclopenta[2,1- b:3,4- b']dithiophene-6,2-diyl))bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1 H-indene-2,1-diylidene))dimalononitrile (HC-PCIC) with unfused cores, the quantum efficiencies can be boosted from ∼70% for binary blends to over 80% for ternary blends in the longer wavelength ranges. PC71BM shows lower energy levels and higher electron mobility, benefiting the charge transfer and transport in ternary OSCs and resulting in an enhanced quantum efficiency. As a result, ternary OSCs based on PBDB-TF/HF-PCIC/PC71BM and PBDB-TF/HC-PCIC/PC71BM exhibit high power conversion efficiencies (PCEs) of 11.55 and 12.36%, respectively. In addition, excellent thermal stabilities are realized for both ternary OSCs, which retained ∼80% initial PCEs after thermal treatment at 130 °C for 12 h, indicating that the active layer morphology containing fullerene/non-fullerene acceptors is stabilized. This work demonstrates efficient and thermally stable ternary OSCs with enhanced charge transfer between fullerene and non-fullerene acceptors via the modulation of energy levels, which helps to better understand the working mechanism of ternary OSCs.

Efficient Ternary Organic Solar Cells with Two Compatible Non-Fullerene Materials as One Alloyed Acceptor.

Efficient ternary organic solar cells (OSCs) are fabricated by employing a polymer PBT1-C as the donor and two non-fullerene materials, MeIC and MeIC2, as one alloyed acceptor. The optimized ternary OSCs with 30 wt% MeIC2 in acceptors achieve a power conversion efficiency (PCE) of 12.55%, which is much higher than that of 11.47% for MeIC-based binary OSCs and 11.41% for MeIC2-based binary OSCs. The >9.4% improvement in PCE is mainly attributed to the optimized photon harvesting and morphology of ternary active layers, resulting in the simultaneously improved short-circuit current and fill factor. Furthermore, good compatibility and similar lowest unoccupied molecular orbital energy levels of MeIC and MeIC2 are beneficial to form one alloyed acceptor for efficient electron transport in the ternary active layers. This work may provide new insight when selecting the third component for preparing efficient ternary OSCs.

Reduced Energy Offsets and Low Energy Losses Lead to Efficient (∼10% at 1 sun) Ternary Organic Solar Cells

Two organic semiconductor molecules, CS01 and MPU3, an electron donor and electron acceptor, respectively, are described for use in organic bulk heterojunction solar cells. The highest occupied molecular orbital (HOMO) energy offset (ΔEHOMO) between these two molecules is as low as 0.29 eV. Moreover, the lowest unoccupied molecular orbital (LUMO) energy offset (ΔELUMO) is as low as 0.14 eV. Nonetheless, the interfacial charge-transfer process upon light excitation is extremely efficient, and solar cells with efficiencies, under normal conditions, as high as 7.81% have been fabricated. Furthermore, the incorporation of PC71BM (another electron acceptor molecule) to fabricate a triple organic heterojunction led to efficiencies close to 10% at 1 sun irradiation conditions. The improvement is due to a better panchromatic harvesting of the sunlight and a better charge balance, which allows the rapid extraction of carriers before they can recombine.

Efficient ternary organic solar cells with high absorption coefficient DIB-SQ as the third component**Project supported by the National Natural Science Foundation of China (Grant Nos. 61675017, 61377029, and 61705161) and the National Training Program of Innovation and Entrepreneurship for Undergraduates, China (Grant No. 170170010).

A series of organic solar cells (OSCs) are prepared with PTB7:PC71BM as the host materials and DIB-SQ as the third component. The power conversion efficienty (PCE) of OSCs can be improved from 6.79% to 7.92% by incorporating 6 wt% DIB-SQ into donors, resulting from the enhanced short circuit current density (JSC) and fill factor (FF). The increased JSC of the optimized ternary OSCs should be attributed to the enhanced photon harvesting of teranry active layer by incorporating DIB-SQ. Meanwhile, FF of the optimized ternary OSCs should be due to the optimied phase separation. The open circuit voltage (VOC) of tenray OSCs can be maintained at a constant of 0.75 V, indicating that all photogenerated holes willl be transported along the channels formed by PTB7.

A two-dimension-conjugated small molecule for efficient ternary organic solar cells

Ternary organic solar cells (OSCs) are burgeoning as one of the effective strategies to achieve high power conversion efficiencies (PCEs) by incorporating a third component with a complementary absorption into the binary blends. In this study, we presented a new two-dimension-conjugated small molecule denoted by DR3TBDTTVT, which alone gave rise to a best PCE of 5.71% with acceptor PC71BM as active layer. Given the complementary absorption with PTB7-Th, DR3TBDTTVT was doped into (PTB7-Th:PC71BM)-based binary blends, and ternary OSCs were developed. The ternary OSCs with 10 wt% of DR3TBDTTVT displayed improved hole-mobility, reduced device resistance and better phase separation of active layer, thus leading to an impressive PCE of 7.77% with open-circuit voltage of 0.77 V, short-circuit density of 14.52 mA cm−2 and fill factor of 70.3%. Ternary OSCs well make up for the light-harvesting insufficiency of binary OSCs, and this research provides a new material for the improvement of PCEs for single-junction OSCs.

Improving the Compatibility of Donor Polymers in Efficient Ternary Organic Solar Cells via Post-Additive Soaking Treatment.

In dual-donor ternary organic solar cells, the compatibility between the donor polymers plays important roles to control the conformational change and govern the photophysical behavior in the blend films. Here, we apply a post-additive soaking (PAS) approach to reconstruct the morphology in a ternary organic photovoltaic BHJ of PTB7-Th: PCDTBT: PC71BM. The PAS-treated device has a maximum power conversion efficiency (PCE) of about 8.7% in this ternary system. From the analyses of GIWAXS and GISAXS, the superior device performance is attributed to the favorable nanomorphology with optimum crystallinity of PTB7-Th and good intermixing of PCDTBT with PTB7-Th:PC71BM, leading to improved charge transport in the vertical direction. AFM and TRPL measurements clearly demonstrate PAS-treated film envisages a homogeneous distribution of smaller PC71BM aggregates to facilitate the exciton dissociation and carrier extraction at the interface. The increased PCE ascribed to not only the enhancement of absorption and nonradiative Förster resonance energy transfer (FRET) between two donors (PCDTBT and PTB7-Th) but also the formation of a bicontinuous interpenetrating network of PC71BM.

Efficient ternary organic solar cells based on immiscible blends

Organic photovoltaic cells based on ternary blends of materials with complementary properties represent an approach to improve the photon-absorption and/or charge transport within the devices. However, the more complex nature of the ternary system, i.e. in diversity of materials' properties and morphological features, complicates the understanding of the processes behind such optimizations. Here, organic photovoltaic cells with wider absorption spectrum composed of two electron-donor polymers, F8T2, poly(9,9-dioctylfluorene-alt-bithiophene), and PTB7, poly([4,8-bis[(2′-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2′-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]), mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) are investigated. We demonstrate an improvement of 25% in power conversion efficiency in comparison with the most efficient binary blend control devices. The active layers of these ternary cells exhibit gross phase separation, as determined by Atomic Force Microscopy (AFM) and Synchrotron-based Scanning Transmission X-ray Microscopy (STXM).

Utilizing Energy Transfer in Binary and Ternary Bulk Heterojunction Organic Solar Cells.

Energy transfer has been identified as an important process in ternary organic solar cells. Here, we develop kinetic Monte Carlo (KMC) models to assess the impact of energy transfer in ternary and binary bulk heterojunction systems. We used fluorescence and absorption spectroscopy to determine the energy disorder and Förster radii for poly(3-hexylthiophene-2,5-diyl), [6,6]-phenyl-C61-butyric acid methyl ester, 4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (DIBSq), and poly(2,5-thiophene-alt-4,9-bis(2-hexyldecyl)-4,9-dihydrodithieno[3,2-c:3',2'-h][1,5]naphthyridine-5,10-dione). Heterogeneous energy transfer is found to be crucial in the exciton dissociation process of both binary and ternary organic semiconductor systems. Circumstances favoring energy transfer across interfaces allow relaxation of the electronic energy level requirements, meaning that a cascade structure is not required for efficient ternary organic solar cells. We explain how energy transfer can be exploited to eliminate additional energy losses in ternary bulk heterojunction solar cells, thus increasing their open-circuit voltage without loss in short-circuit current. In particular, we show that it is important that the DIBSq is located at the electron donor-acceptor interface; otherwise charge carriers will be trapped in the DIBSq domain or excitons in the DIBSq domains will not be able to dissociate efficiently at an interface. KMC modeling shows that only small amounts of DIBSq (<5% by weight) are needed to achieve substantial performance improvements due to long-range energy transfer.