Non-fullerene System Research Articles

TBP precursor agent passivated ZnO electron transport layer for highly efficient polymer solar cells

Abstract Defects passivation in electron transport layer (ETL) is a key issue to optimize the performance of polymer solar cells (PSCs). In this work, a novel strategy is developed to form defects passivated ZnO ETL by introducing 4-tert-butylpyridine (TBP) agent into precursor. While the power conversion efficiency (PCE) of the inverted PSCs based poly{4,8-bis [(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno [3,4-b]thiophene-4,6-diyl}:[6,6]-phenyl C71-butyric acid methyl ester (PTB7:PC71BM) with the pure ZnO ETL is 8.02%, that of the device with modified ZnO ETL is dramatically improved to 10.26%, with TBP accounting for ~28% efficiency improvement. Our study demonstrates that the precursor agent significantly affect the surface morphology and size of ZnO in ETL. Furthermore, it proves that the ZnO ETL with TBP (T-ZnO) is beneficial to polish interfacial contact between ETL and active layer and depress exciton quenching loss, resulting in enhanced exciton dissociation, efficient carrier collection and reduced charge recombination, thus improving the device performance. To verify the universality of T-ZnO ETL, the champion photovoltaic performance with a PCE of 11.74% (10% improvement) is obtained in the PBDB-T-2F:IT-4F based nonfullerene PSCs using T-ZnO as ETL. Our work developed a new, universal and facile strategy for designing highly efficient PSCs based on fullerene and nonfullerene blend systems.

Improved charge transfer, mobility and morphology for high performance panchromatic organic photodetectors by adding PC71BM in P3HT:IEICO-4F

Abstract High performance of panchromatic organic photodetectors (OPDs) with a wide spectral response ranged from 300 to 1000 nm were realized through adding [6,6]-phenyl-C71-butyric acid methylester (PC71BM) fullerene as an electron acceptor into the polymer donor of poly (3-hexylthiophene) (P3HT) and the small molecular non-fullerene acceptor of 2, 2′-((2Z,2′Z)-(((4, 4, 9, 9-tetrakis(4-hexylphenyl)-4, 9-dihydro-sindaceno[1, 2-b:5, 6-b′]dithiophene-2, 7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-5, 2-diyl))bis-(methanylylidene))bis(5, 6-difluoro-3-oxo-2, 3-dihydro-1H-indene-2, 1-diylidene))dimalononitrile (IEICO-4F) host system. The fabricated OPD exhibited a high detectivity (D*) of 1.35 × 1012 Jones at 805 nm by adding 10 wt% PC71BM, which is 1.5 folds higher than it from the control system. The improved performance was mainly attributed to the increased light absorption in the short wavelength range and cascade energy level alignment, which is responsible for the efficient light harvesting and exciton utilization. Furthermore, the active layer morphology was optimized by adjusting the ratio of PC71BM acceptor, which efficiently enhances charge transport and mobility of the device as well as suppress bimolecular recombination. This work indicates that adding fullerene into non-fullerene system plays a positive effect on the device performance of panchromatic OPDs.

A General Approach for Lab-to-Manufacturing Translation on Flexible Organic Solar Cells.

The blossoming of organic solar cells (OSCs) has triggered enormous commercial applications, due to their high-efficiency, light weight, and flexibility. However, the lab-to-manufacturing translation of the praisable performance from lab-scale devices to industrial-scale modules is still the Achilles' heel of OSCs. In fact, it is urgent to explore the mechanism of morphological evolution in the bulk heterojunction (BHJ) with different coating/printing methods. Here, a general approach to upscale flexible organic photovoltaics to module scale without obvious efficiency loss is demonstrated. The shear impulse during the coating/printing process is first applied to control the morphology evolution of the BHJ layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot-die printing and spin-coating is detected. Compelling results of morphological evolution, molecular stacking, and coarse-grained molecular simulation verify the validity of the impulse translation. Accordingly, the efficiency of flexible devices via slot-die printing achieves 9.10% for PTB7-Th:PC71 BM and 9.77% for PBDB-T:ITIC based on 1.04 cm2 . Furthermore, 15 cm2 flexible modules with effective efficiency up to 7.58% (PTB7-Th:PC71 BM) and 8.90% (PBDB-T:ITIC) are demonstrated with satisfying mechanical flexibility and operating stability. More importantly, this work outlines the shear impulse translation for organic printing electronics.

Novel 2D material from AMQS-based defect engineering for efficient and stable organic solar cells

In this work, soluble two-dimensional (2D) material of antimonene quantum sheets (AMQSs) is used to form a bilayer hole extraction layer (HEL) with CuSCN. It proves that the application of AMQSs helps to passivate surface defects of CuSCN, resulting in diminished recombination loss and depressed exciton quenching effect, and thereby achieving improved photovoltaic performance. In OPVs based on poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]: [6,6]-phenyl C71-butyric acid methyl ester (PTB7-Th:PC71BM), over 12% enhancement of power conversion efficiency (PCE) is observed compared to that of the reference cell fabricated with pure CuSCN as an HEL. The advantage of the bilayer CuSCN/AMQS HEL is also confirmed in non-fullerene systems of PBDB-T-2F:IT-4F and PTB7-Th:ITIC. In a cell based on PBDB-T-2F:IT-4F, a PCE of 10.14% was obtained after application of AMQSs, which improved by about 10% compared to that of the reference cell using pure CuSCN as an HEL. Furthermore, cells based on CuSCN and CuSCN/AMQS HEL exhibit superior air stability. The use of a bilayer CuSCN/AMQS HEL provides a promising approach to obtain efficient and stable organic solar cells.

Slot-die printed non-fullerene organic solar cells with the highest efficiency of 12.9% for low-cost PV-driven water splitting

Solution processed organic solar cells (OSCs), as one promising solar energy conversion technology, have attracted much attention for their potential use in photovoltaic market, but their promise in the economic conversion of solar energy into chemical energy (solar fuels) has only recently been recognized. In this study, we apply a non-fullerene system (PBDB-T-SF:IT-4F) for closing the efficiency gap between lab-produced OSCs and large-scale printed modules via a slot-die coating system. By finely balancing the flowing rate and coating speed combination as a printing strategy, we obtain high-quality printed blend microstructure. The printed solar cells not only exhibit a power conversion efficiency (PCE) of 12.90%, which is comparable to that of state-of-the-art devices fabricated by spin coating and doctor-blading, but also present the impressive photovoltaic performance with the PCE of over 12% in the flexible devices and the modules with two cells connected in series. Coupling a large-scale printed module to an existing water splitting system would yield a solar-to-hydrogen (STH) efficiency of 6.15%. Furthermore, design principles for combining solar cells, electrolysis cell and new catalysts are provided for minimizing their power loss during the conversion of electrical energy into fuel.

Introduction of co-additives to form well dispersed photoactive layer to improve performance and stability of organic solar cells

We fabricated organic solar cells (OSCs) based on bulk-heterojunction photoactive layers with fullerene (PTB7:PC71BM) and non-fullerene (PBDB-T:ITIC) systems by introducing 1,8-diiodooctane (DIO) and 4-fluorobenzaldehyde (4-FBA) as co-additives, respectively. Introduction of the co-additives led to a uniform surface morphology of photoactive layer and formed well distributed interpenetrating networks between donors and acceptors. Carrier recombination were reduced due to the favorable structure for charge transport. Consequently, the devices with co-additives achieved an enhanced performance with a PCE of 8.5% (JSC = 16.4 mA/cm2 and FF = 68.3%) in the fullerene system, and a PCE of 10.1% (JSC = 16.9 mA/cm2 and FF = 67.8%) in the non-fullerene system. In addition, the devices with co-additives showed improved stabilities compared to those with single additives. Correspondingly, the reduction ratio of PCE at ambient atmosphere conditions decreased from 17.07% to 10.59% in the fullerene system, and 34.02% to 24.75% in the non-fullerene system.

Manipulate Micrometer Surface and Nanometer Bulk Phase Separation Structures in the Active Layer of Organic Solar Cells via Synergy of Ultrasonic and High-Pressure Gas Spraying.

For organic solar cells, the vertical and lateral micro-/nanometer-scale structure in the active layer largely determines the device performance. In this work, the surface and bulk domain size of the photoactive layer are successfully manipulated with a facile two-step spraying method, that is, an ultrathin active layer by high-pressure spraying is deliberately stacked on top of the thick active layer by ultrasonic spraying. Thus, the morphology is effectively optimized with the comprehensive study of optical and electrical characteristics, such as photon absorption, exciton dissociation efficiency, and bimolecular recombination. Moreover, the novel method can be used not only in the fullerene system but also in the nonfullerene system, demonstrating the remarkable universality through this synergy method. This work provides an easy and reliable strategy to improve photovoltaic device performance in the industrial large-area spray-coating process.

Flexible ITO-free organic solar cells over 10% by employing drop-coated conductive PEDOT:PSS transparent anodes

Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonic acid) (PEDOT:PSS) has been explored to fabricate flexible and stretchable conductors. Generally, PEDOT:PSS transparent anodes are prepared by spin-coating method. In this article, we adopt a method by dropping PEDOT:PSS aqueous solution on the PET plastic substrate to fabricate flexible electrodes. Compared with spin coating, drop-coating is simple and cost-effective with large-area fabrications. Through this method, we fabricated highly transparent conductive electrodes and systematically studied their electrical, optical, morphological and mechanical properties. With dimethyl sulfoxide/methanesulfonic acid (DMSO/MSA) treated PEDOT:PSS electrode, bendable devices based on non-fullerene system displayed an open-circuit voltage of 0.925 V, a fill factor of 70.74%, and a high power conversion efficiency (PCE) of 10.23% under 100 mW cm−2 illumination, which retained over 80% of the initial PCE value after 1000 bending cycles. Based on the findings, drop-coated PEDOT:PSS electrodes exhibited high suitability for the development of large-area and high-efficiency printed solar cell modules in the future.

Thermal-assisted Voc increase in an indenoindene-based non-fullerene solar system

An indenoindene-cored small molecule acceptor of 3-ethylrodanine terminals, IBR, was synthesized to achieve high open-circuit voltage (Voc) solar cells. Adopting PBDB-T as the donor and IBR as the acceptor, the as-cast bulk-heterojunction non-fullerene solar cells show a promising Voc of 0.88 V. After a thermal annealing process at 160 °C for 10 min, the PBDB-T:IBR solar cells exhibit an enhanced Voc up to 1.16 V, among the highest Voc value reported for single junction organic solar cells, with obtaining a power conversion efficiency of 6.66%. Detailed investigation indicates that this thermal-activated Voc increase is caused by the microstructural rearrangement within polymer:IBR domains, which diminishes morphological defects, contributes to reduced bimolecular recombination and results in small saturation dark current density of the device. The work demonstrates the potential of IBR as efficient molecular acceptor for high Voc devices of special applications on tandem or ternary solar cells.

Vertical Phase Separation for Highly Efficient Organic Solar Cells Incorporating Conjugated‐Polyelectrolytes

AbstractHybrid organic solar cells are made through a simple one‐pot coating process with conjugated polyelectrolytes (CPEs), poly[9,9‐bis(4′‐sulfonatobutyl)fluorenealt‐thiophene‐doped (PFT‐D). The hybrid active layer incorporated with PFT‐D shows vertical phase separation by self‐assembled properties of PFT‐D, which result from a molecular dipole between the conjugated backbone and the side chain. The hybrid active layer with PFT‐D shows that homogeneous morphology, surface potential properties, and hydrophobic surface properties favor for enhancing photovoltaic performance. These results are identified by contact angle characteristics, X‐ray photoelectron spectroscopy (XPS) profiling, and X‐ray diffraction (XRD), atomic force measurement (AFM), and electrostatic force measurement (EFM) analyses. With fullerene based hybrid active layer, the power conversion efficiency (PCE) reaches to 8.7% with the enhanced short‐circuit current density (J SC), open‐circuit voltage (V OC), and fill factor (FF). The hybrid device with PFT‐D has a higher stability with a lower reduction ratio of 5.74% compared with only bulk‐heterojunction device (with reduction rate of 30.15%) and incorporating poly(3,4‐ethylene dioxythiophene)‐poly(styrene sulfonate) (PEDOT:PSS) device (with reduction rate of 10.84%). These results are also found in nonfullerene based system (PCE of 10.8%) and other conjugated polyelectrolytes system (PCE of 8.4%). These results have the potential of significantly contributing to the upsizing and commercialization of organic solar cells.

Modulation of the Molecular Orientation at the Bulk Heterojunction Interface via Tuning the Small Molecular Donor-Nonfullerene Acceptor Interactions.

The interfacial molecular packing orientation of the nonfullerene systems at the donor-acceptor interface is considered as one of the key parameters in fabricating high-performance devices because of the anisotropic molecular characteristics of conjugated donors (D) and nonfullerene acceptors (A). However, regulating the interfacial orientation for the nonfullerene systems is still scarcely studied. Herein, modulation of the interfacial molecular packing orientation of bulk heterojunction layer is successfully realized via tuning the D-A interactions. The results indicate that the molecule with relatively shorter alkyl side chain (2F-C4C6) because of weak D-A interactions is unable to influence the molecular orientation of the active layer, as compared to their longer alkyl side-chain counterpart (2F-C6C8), which demonstrates strong D-A interactions and thus efficiently modulates the overall packing orientation. The power conversion efficiencies of 6.41 and 8.23% are obtained for the relatively short and long alkyl side-chain donors with IDIC acceptor, respectively. Hence strong D-A interactions because of long enough alkyl side chain on a donor small molecule can modify the interfacial molecular packing orientation of the system, leading to a better performing device.

High-efficiency quaternary polymer solar cells enabled with binary fullerene additives to reduce nonfullerene acceptor optical band gap and improve carriers transport

The polymer/small-molecule electron donor and nonfullerene organic electron acceptor are of structural similarity with both donor and acceptor molecules consisting of polycyclic fused-ring backbone and being decorated with alkyl-chains. In this study, we report that the introduction of binary fullerenes (C60-/C70-PCBM and C60-/C70-ICBA) into a nonfullerene binary system PBDB-T:ITIC reduces the polymer-nonfullerene acceptor intermixing, obtaining higher crystallinity with (100) crystal coherence length from 28 to 29–33 nm for the ITIC, and from 14 to 20–24 nm for the PBDB-T, and improved electron and hole mobilities both. Unprecedentedly, such a protocol reduces the ITIC optical band gap from 1.59 to 1.55 eV. As consequences, higher short-circuit current-density (17.8–18.4 vs. 15.8 mA/cm2), open-circuit voltage (0.92 vs. 0.90 V) and fill-factor (0.72–0.73 vs. 0.68) are simultaneously obtained, which ultimately afford higher efficient quaternary polymer solar cells with power conversion efficiencies (PCEs) up to 12.0%–12.8% comparing to the host binary device with 9.9% efficiency. For the polymer, ITIC, and ICBA/PCBM ternary blends, 11% PCEs were recorded. The use of PCBM leads to larger red-shifting in thin film absorption and external quantum efficiency (EQE) response. Such effect is more pronounced when ICBA:PCBM mixture is used. These results indicate the size and shape of C60 and C70 as well as the substituent position of the second indene unit on C60-/C70-ICBA affect not only the blend morphology but also the electronic coupling in BHJ mixtures: the quaternary device performance increased in sequences of C70-PCBM:C70-ICBA ® C70-PCBM:C60-ICBA ® C60-PCBM:C70-ICBA ® C60-PCBM:C60-ICBA. The resonant soft X-ray scattering (RSoXS) data indicated the most refined phase separation in the C60-PCBM:C60-ICBA based blend, corresponding to its best device function among the quaternary devices. These results indicate that the using of binary fullerenes as the acceptor additives allows for tuning nonfullerene blended film’s optical properties and film-morphologies, shedding light on the designing high-performance multi-acceptor polymer solar cells.

High Mobility Indium Oxide Electron Transport Layer for an Efficient Charge Extraction and Optimized Nanomorphology in Organic Photovoltaics.

The electron transport layer (ETL) plays an important role in determining the device efficiency of organic solar cells (OSCs). A rational design of an ETL for OSCs targets high charge extraction and induction of an optimized active layer morphology. In this Letter, a high mobility In2O3 synthesized via a solution-processed combustion reaction is successfully used as a universal ETL in an organic photovoltaic device. With the modification of a thin layer of polyethylenimine ethoxylated (PEIE), a device based on crystalline In2O3 outperforms its counterpart, ZnO, in both PBDTTT-EFT-based fullerene and nonfullerene systems. As ZnO is replaced by In2O3, the average efficiency increases from 9.5% to 10.5% for PBDTTT-EFT-PC71BM fullerene-based organic solar cells and also increases from 10.8% to 11.5% for PBDTTT-EFT-IEICO-4F nonfullerene-based organic solar cells, respectively. Morphological studies have unraveled the fact that the crystallineIn2O3 ETL with highly aligned nanocrystallites has induced the crystallization of polymer into a preferential molecular packing that favors the charge transport across an active layer. From the photophysical study, it is found that charge extraction in the crystalline In2O3 device is significantly faster than in the ZnO device due to the higher mobility of In2O3 and optimized nanomorphology of the active layer.

Improve the Performance of the All‐Small‐Molecule Nonfullerene Organic Solar Cells through Enhancing the Crystallinity of Acceptors

AbstractSignificant development has been achieved in nonfullerene organic solar cells. However, most of the high‐efficiency nonfullerene systems are composed of polymer donors and fused‐ring acceptors, and only a few small molecule donors can work well. Herein, a new A–D–A small molecule donor named NDTSR with naphtho[1,2‐b:5,6‐b′]dithiophene (NDT) as building blocks is synthesized. Two energy levels well‐matched fused‐ring acceptors ITIC and IDIC are chosen to construct all‐small‐molecule solar cells with NDTSR, respectively. When mixed with IDIC, a high power conversion efficiency (PCE) of 8.05% is achieved, which is the highest efficiency for NDT‐based small molecule donor. However, the NDTSR:ITIC system only exhibits a low PCE of 1.77%. The big difference in the performance of these two systems should be attributed to the different morphology and phase separation resulting from the crystallinity and aggregation ability of the acceptors. The results demonstrate that NDT‐based small molecule is a promising candidate donor for all‐small‐molecule systems, while the crystallinity of fused‐ring acceptors is a critical factor for optimizing the phase separation in the active layer.

A novel small molecule based on naphtho[1,2-b:5,6-b′]dithiophene benefits both fullerene and non-fullerene solar cells

The all-round small molecule donor NDTR exhibited good photovoltaic performance in both fullerene and non-fullerene systems.

Improved Domain Size and Purity Enables Efficient All-Small-Molecule Ternary Solar Cells.

An all-small-molecule ternary solar cell is achieved with a power conversion efficiency of 10.48% by incorporating phenyl-C71 -butyric-acid-methyl ester (PC71 BM) into a nonfullerene binary system. The addition of PC71 BM is found to modulate the film morphology by improving the domain purity and decreasing the domain size. This modulation facilitates charge generation and suppresses charge recombination, as manifested by the significantly enhanced short-circuit current density and fill factor. The results correlate the domain characteristics with the device performance and offer new insight from the perspective of morphology modulation for constructing efficient ternary devices.

Charge Generation in Non-Fullerene Donor–Acceptor Blends for Organic Solar Cells

The power conversion efficiencies of solar cells incorporating non-fullerene donor–acceptor blends now approach those of fullerene-based devices in the best performing examples. However, the lack of clear structure–property relationships means that the poor device performance of many novel non-fullerene systems cannot be readily explained. We report a series of non-fullerene acceptors, comprising essentially the same chromophore, that differ in terms of their shape and number of chromophores. To understand the impact of these structural differences on charge generation, we have employed transient absorption spectroscopy to investigate the photophysical properties of the acceptors in blends with the conjugated polymer PTB7. To minimize the impact of morphology, we employed a broad range of acceptor concentrations and compared the results with those of blends containing the fullerene derivative PC70BM. In terms of singlet exciton harvesting, the non-fullerene acceptors exhibited similar performance to PC70B...

Time-resolved photoemission studies of exciton dissociation in organic photovoltaics

In the drive to improve efficiencies to market-penetration levels, organic photovoltaics would benefit from the introduction of new organic electronic materials in the form of nonfullerene acceptors, allowing the greater tunability of electronic levels and improved optical absorption. Efforts to incorporate new acceptors have produced limited results although the improvements have recently shown steady progress. Possible barriers to the use of nonfullerene acceptors include the formation of unfavorable morphologies in nonfullerene systems and/or favorable excitation/carrier delocalization in fullerenes, both factors that may be overcome through advanced synthesis. More problematic are suggestions that the molecular excited states of fullerenes lead to greatly enhanced dissociation of the initial S1 exciton in the donor material. This would suggest a fundamental limitation on the use of many nonfullerene acceptors. The authors tested this suggestion using time-resolved two-photon photoemission to directly compare exciton dissociation at the interfaces between phthalocyanine donors and a nonfullerene acceptor, perylene tetracarboxylic dianhydride, versus the analogous interface with the fullerene, C60. The authors conclude that for the systems studied, exciton dissociation rates are comparable with both the acceptors, leaving open the possibility of further improvements through the introduction of advanced acceptor materials.

Semitransparent, non-fullerene and flexible all-plastic solar cells

Semitransparent, non-fullerene and flexible all-plastic organic solar cell (OSC) based on a blend of poly(3-hexylthiophene) (P3HT) and non-fullerene acceptor IDT-2BR is fabricated using conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as bottom electrode and film-transfer laminated PEDOT:PSS as top electrode. The solar cell shows average visible transmittance of ca. 50%, which can be potentially used for electricity-generating windows. The all-plastic device shows lower power conversion efficiency (PCE) of 2.88% than the traditional inverted device (4.2%), owing to much higher transmittance and lower conductivity of PEDOT:PSS. Our non-fullerene system P3HT:IDT-2BR shows better performance than fullerene system P3HT:PC61BM (PCE = 2.2%) in all-plastic OSCs. Furthermore, the non-fullerene OSC shows better bending stability than the fullerene counterpart.

Effect of Alkyl Side Chains of Conjugated Polymer Donors on the Device Performance of Non-Fullerene Solar Cells

The influence of the chemical structure of conjugated polymers on the nanophase separation and device performance in fullerene-based solar cells has been widely studied, while this is less investigated in non-fullerene solar cells. In this work, we design three conjugated polymers with different length of side chains, and we find that the length of side chains has little influence on the quantum efficiencies of non-fullerene solar cells. As a comparison, the length of side chains has a significant effect on the quantum efficiencies of fullerene-based solar cells. This indicates that morphology of the blended thin films in non-fullerene solar cells is rather independent of the length of the donor side chains, and the mechanism for morphology evolution in the non-fullerene system is completely different from that in the fullerene system. Our conclusion is confirmed by a variety of advanced characterization techniques. The studies reveal that in blended thin films based on the non-fullerene material the dono...