High-efficiency Organic Solar Cells Research Articles

Non-fullerene acceptors with branched side chains and improved molecular packing to exceed 18% efficiency in organic solar cells

Molecular design of non-fullerene acceptors is of vital importance for high-efficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach, as it may introduce an undesirable steric hindrance that reduces charge transport in non-fullerene acceptors. Here we show the design and synthesis of a highly efficient non-fullerene acceptor family by substituting the beta position of the thiophene unit on a Y6-based dithienothiophen[3,2-b]-pyrrolobenzothiadiazole core with branched alkyl chains. It was found that such a modification to a different alkyl chain length could completely change the molecular packing behaviour of non-fullerene acceptors, leading to improved structural order and charge transport in thin films. An unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of the branched alkyl chain topology in tuning the molecular packing and blend morphology, which leads to improved organic photovoltaic performance. Molecular design of acceptor and donor molecules has enabled major progress in organic photovoltaics. Li et al. show that branched alkyl chains in non-fullerene acceptors allow favourable morphology in the active layer, enabling a certified device efficiency of 17.9%.

Triplet exciton formation for non-radiative voltage loss in high-efficiency nonfullerene organic solar cells

<h2>Summary</h2> Nonfullerene acceptor (NFA) organic solar cells (OSCs) with power conversion efficiency (PCE) reaching up to 18% has shown tremendous potential toward practical applications. However, the non-radiative recombination loss in high-efficiency NFA OSCs is evidently large, and the origin remains unclear. Herein, by combining transient absorption spectroscopy, photovoltaic characterization, and detailed energy loss analysis, we unambiguously show the formation of NFA triplet exciton from free carrier recombination as a main non-radiative recombination and energy loss channel. More importantly, in contrast to the previous energetics point of view, we show side-chain fluorination in high-efficiency NFA OSCs has little effect on charge transfer energy landscapes but efficiently inhibits charge recombination to NFA triplet excitons, leading to higher <mml:math><mml:mrow><mml:mi>V</mml:mi>OC</mml:mrow></mml:math> and PCE. Our findings provide a new view about energy loss in NFA OSCs and pave the way toward OSCs with lower <mml:math><mml:mrow><mml:mi>V</mml:mi>OC</mml:mrow></mml:math> loss and efficiency exceeding 20%.

High-Efficiency and Stable Organic Solar Cells with Stacked LiF and Organic Electrolytes as Cathode Interface Layers

The cathode interfacial layer plays a key role in enhancing the efficiency and stability of organic solar cells. Constructing efficient two-layer interfaces is one of the important strategies to im...

Intrachain and Interchain Exciton–Exciton Annihilation in Donor–Acceptor Copolymers

PM6 is a donor-acceptor copolymer widely used in high-efficiency organic solar cells (OSC). Here, we report the observation of exciton-exciton annihilation (EEA) in both PM6 copolymer solution and film. We find that the EEA mechanisms are distinct in these two systems. In the PM6 solution, EEA occurs via the long-range dipole-dipole interaction between intrachain excitons, while in the PM6 film it takes place between the interchain polaron pairs (spatially indirect excitons) via the short-range Coulomb interaction. The quantum yield of free polaron formation in the film is also quantified, which indicates that EEA leads to extra interchain polaron-pair dissociation, accounting for the enhanced free polaron formation.

Side‐Chain Engineering on Y‐Series Acceptors with Chlorinated End Groups Enables High‐Performance Organic Solar Cells

AbstractChemical modifications of non‐fullerene acceptors (NFAs) play vital roles in the development of high efficiency organic solar cells (OSCs). In this work, on the basis of the previously reported molecule named Y6‐1O, chlorination and inner side‐chain engineering are adopted to endow the corresponding devices with higher open‐circuit voltage (VOC) and short‐circuit current density (JSC) as well as good morphology for high fill factor (FF). As a result, the molecule named BTP1O‐4Cl‐C12 can help achieve a higher power conversion efficiency (PCE) of 17.1% than that of Y6‐1O (16.1%). Furthermore, the following comparisons between BTP1O‐4Cl‐C12 and the two symmetric acceptors named BTP2O‐4Cl‐C12 and BTP‐4Cl‐C12 demonstrate the effect of asymmetric alkoxy substitution on the outer side chains, which not only achieves a balance between VOC and JSC, but also help obtain appropriate morphology for efficient charge dissociation and suppressed charge recombination. Therefore, the asymmetric BTP1O‐4Cl‐C12 can achieve a higher PCE compared to the symmetric BTP2O‐4Cl‐C12 and BTP‐4Cl‐C12. The work not only reports an excellent NFA for high‐performance OSCs, but also puts forward a series of methods for consecutive chemical modifications on Y‐series acceptors, which can be further applied to boost the PCE of OSCs to a higher level.

Efficient Solar Cells Based on a Polymer Donor with β-Branching in Trialkylsilyl Side Chains

Side-chain engineering is an important strategy in designing novel polymer semiconductor materials for high-efficient organic solar cells. The use of trialkylsilyl side chains can improve the photovoltaic efficiency by decreasing the energy of the HOMO of the polymer and improving its crystallinity and hole mobility. Compared to simple linear derivatives, α-branching in the alkyl groups of trialkylsilyl side chains causes strong aggregation and excessive phase separation in the photoactive layer, leading to poor device performance. β-Branching of the alkyl groups has not yet been used in trialkylsilyl side chains. Herein, we describe a new polymer (J77) with triisobutylsilyl side chains to investigate the effect of β-branching on the molecular aggregation, optical properties, energy levels, and photovoltaic properties. We find that compared to α-branching, β-branching of alkyl groups in trialkylsilyl side chains significantly reduces aggregation. This enables J77 to form blend morphologies in films that provide high-efficient solar cells in combination with different non-fullerene acceptors. Moreover β-branching of the alkyl groups in trialkylsilyl side chains lowers the HOMO energy level of J77 and increases the open-circuit voltage of J77-based solar cells without sacrificing short-circuit current density or fill factor.

Interconnecting layers for tandem organic solar cells

The tandem structure provides a practical way to realize high-efficiency organic solar cells (OSCs). It overcomes several bottlenecks in single-junction OSCs such as thermalization loss, transmission loss, and inefficient light absorption. In tandem OSCs, the interconnecting layer (ICL) electrically connects two subcells and it plays a critical role in determining the device performance and reproducibility. In this review paper, we first give a brief introduction to the working mechanism and the requirements for ICLs. We then compare three classes of ICLs based on their optical, mechanical, and chemical properties. Finally, the possible paths forward for tandem OSCs are discussed.

Quantum chemical design of near‐infrared sensitive fused ring electron acceptors containing selenophene as π‐bridge for high‐performance organic solar cells

AbstractEnd‐capped acceptors modification of fused ring electron acceptors (FREAs) is an attractive strategy to boost the optoelectronic and photovoltaic properties of the materials. FREAs are also proven beneficial due to their tremendous applications in organic solar cells (OSCs). Among fused‐ring electronic species, small fullerene‐free FREAs have already been drawn more attention due to their near‐infrared sensitivity and constantly increasing efficiencies. Therefore, we have designed six new FREAs (K1–K6) having selenophene as π‐bridge in between the central alkylated indaceno[1,2‐b:5,6b]dithiophene (IDT) unit after the incorporations of various end‐capped acceptors on to the recently synthesized IDT2SeC2C4‐4F molecule. Structural–property relationship and photophysical and photovoltaic properties of newly designed molecules are studied with the help of density functional theory (DFT) and time‐dependent DFT (TD‐DFT). Certain critical specifications like frontier molecular orbitals (FMOs) alignment, density of states (DOS), absorption maxima, excitation energy, binding energy (BE) along with transition density matrix (TDM), and the specifically estimated reorganizational energy values of electron and hole and the open circuit voltages of newly designed molecules are computed and compared with reference molecule. Generally, a red‐shifting absorption behavior of FREAs is considered the most important reason for their high efficiencies in OSCs. Our novel designed molecules exhibit red shift in absorption spectrum. Similarly, low excitation and binding energies of designed molecules offer improved power conversion efficiencies (PCEs) with highest possible charge photocurrent density (Jsc) in OSCs devices. Furthermore, study of PTB7‐Th/K1 complex is also done in order to examine charge transfer between within complex. By introducing the efficient end‐capped acceptor moieties in reference molecule, enhancement in charge mobilities is noted. The large open circuit voltage, low reorganizational energies, narrower highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energy gap, lower binding and excitation energies, and highly red shifting in absorption phenomenon indicate an efficient designing of molecules, which could be best fitted for high efficiency OSCs. Finally, theorized molecules are much superior related to their photovoltaic and electronic properties and thus are recommended to experimentalist for their synthesis and out‐looking future developments of highly efficient solar cells devices.

A Facile Synthesized Polymer Featuring B‐N Covalent Bond and Small Singlet‐Triplet Gap for High‐Performance Organic Solar Cells

High-efficiency organic solar cells (OSCs) largely rely on polymer donors. Herein, we report a new building block BNT and a relevant polymer PBNT-BDD featuring B-N covalent bond for application in OSCs. The BNT unit is synthesized in only 3 steps, leading to the facile synthesis of PBNT-BDD. When blended with a nonfullerene acceptor Y6-BO, PBNT-BDD afforded a power conversion efficiency (PCE) of 16.1 % in an OSC, comparable to the benzo[1,2-b:4,5-b']dithiophene (BDT)-based counterpart. The nonradiative recombination energy loss of 0.19 eV was afforded by PBNT-BDD. PBNT-BDD also exhibited weak crystallinity and appropriate miscibility with Y6-BO, benefitting of morphological stability. The singlet-triplet gap (ΔEST ) of PBNT-BDD is as low as 0.15 eV, which is much lower than those of common organic semiconductors (≥0.6 eV). As a result, the triplet state of PBNT-BDD is higher than the charge transfer (CT) state, which would suppress the recombination via triplet state effectively.

Cross-Linkable and Alcohol-Soluble Pyridine-Incorporated Polyfluorene Derivative as a Cathode Interface Layer for High-Efficiency and Stable Organic Solar Cells.

Device performance and commercialization of organic solar cells (OSCs) are strongly influenced by the characteristics of the interface layers. Cross-linked polymer interface layers with solvent-resistant properties are very compatible with large-area solution-processing methods of OSCs and may be beneficial to the environmental stability of OSCs due to the viscoelastic and cross-linked characteristics of the cross-linked polymer. In this work, a novel cross-linkable and alcohol-soluble pyridine-incorporated polyfluorene derivative, denoted as PFOPy, is synthesized and used as a cathode interface layer (CIL) in OSCs. For PFOPy, the pendant epoxy group can be effectively cross linked through cationic polymerization under thermal treatment and the pendant pyridine group can offer good alcohol solubility. Optical absorption tests of PFOPy films before/after washing by chloroform demonstrate the excellent solvent-resistance property for the cross-linked PFOPy film. Compared with the typical ZnO CIL, the cross-linked PFOPy CIL can also substantially reduce ITO's work function and form a better interface contact with the active layer. Utilizing an inverted device structure and a typical active layer of PM6:Y6, ZnO-based OSCs display an optimal power conversion efficiency (PCE) of 15.83% while PFOPy-based OSCs exhibit superior photovoltaic performance with an optimal PCE of 16.20%. Moreover, ZnO-based and PFOPy-based OSCs separately maintain 89% and 90% of the corresponding initial PCE after 12 h of illumination, indicating similarly excellent photostability. More importantly, after 26 complete thermal cycles, ZnO-based OSCs only maintain 81% of the initial PCE while PFOPy-based OSCs retain 92% of the initial PCE and exhibit obviously better thermal cycling stability, indicating that the cross-linked PFOPy CIL should offer stronger interface robustness against thermal cycling stress due to the viscoelastic and cross-linked characteristics of PFOPy. The impressive results indicate that the cross-linked PFOPy CIL would be a very promising CIL in OSCs.

High-Efficiency Thickness-Insensitive Organic Solar Cells with an Insulating Polymer.

Achieving high-efficiency thick-film bulk heterojunction (BHJ) organic solar cells (OSCs) with thickness-independent power conversion efficiencies (PCEs) in a wide thickness range is still a challenge for the roll-to-roll printing techniques. The concept of diluting the transport sites within BHJ films with insulating polymers can effectively eliminate charge trapping states and optimize the charge transport. Herein, we first adopted the concept with insulating polypropylene (PP) in the efficient non-fullerene system (PM6:Y6) and demonstrated its potential to fabricate thick-film OSCs. The PP can form an insulating matrix prior to PM6 and Y6 within the BHJ film, resulting in an enhanced molecular interaction and isolated charge transport by expelling Y6 molecules. We thus observed reduced trap state density and improved charge transport properties in the PP-blended device. At around 300 nm, the PM6:Y6:PP device enjoys a high PCE of 15.5% and achieves over 100% of the efficiency of the optimal thin-film device, which is significantly improved compared to the binary PM6:Y6 counterpart. This research promotes an effective strategy with insulating polymers and provides knowledge of commercial production with response to the roll-to-roll technique demands.

Probing the properties of polymer/non-fullerene/fullerene bulk heterojunction ternary blend solar cells, study of varied blend ratios of PBDB-T:ITIC-Th:PC71BM

A major noble way to realizing high-efficiency organic solar cell device is the use of ternary blends with non-fullerene acceptors. Although offset in the band gap (E-gap) of the donor/acceptor material is negligible, it still exhibits ultra-fast and efficient charge separation. Regardless of the progress made, device optimization and fundamental understanding of the role of structural configurations in charge transport mechanisms is still on the way. The performances and stability of such devices largely depend on the nanoscale solid-state interpenetrating morphology of their donor/acceptor components within the photoactive region. Here, we analyze the effects of the second acceptor material PC71BM which is fullerene derivative on the nano-morphology within the bulk-heterojunction active layer made of PBDB-T (as donor) and non-fullerene derivative ITIC-Th (as acceptor) as well as the photovoltaic performance. Upon varying the blend ratio, 2:1:1 ratio showed a remarkable performance with more photo-generated current resulting from the favorable nano-scaled interpenetrating network with less traps as can be visualized in Fig. 1.

Asymmetric simple unfused acceptor enabling over 12% efficiency organic solar cells

High photovoltaic performance with cost-effective material synthesis and device fabrication are the prerequisite for commercialization of organic solar cells (OSCs). Herein, we conduct a systematic structure–property correlation study on unfused acceptors adopting different frameworks. Dithieno[3,2-b:2′,3′-d]pyrrole (DTP) based A-D1-D-D2-A asymmetric acceptor (DBT-4F) has been screened as a promising acceptor for high-efficient OSCs. Three DTP-based unfused acceptors display strong near-infrared absorption with narrow optical band gap (Egopt < 1.45 eV). Blends of DBT-4F with PBDB-T contribute a champion power conversion efficiency (PCE) of 12.14% due to better miscibility to form homogenous fibrous phase separation. To the best of our knowledge, the PCE is among the highest values for unfused acceptor based binary OSCs in the literature. The excellent performance of DBT-4F is revealed with more efficient photon-harvesting, higher and more balanced charge transport, and less charge recombination in the blends. Our work manifests the great potential of DTP based asymmetric unfused acceptor for developing high-efficient OSCs via rational molecular design.

A universal method for constructing high efficiency organic solar cells with stacked structures

Quantitative relationship between the protective factor (δ) and PCE of stacked structures of OSC with a record PCE of 17.52% is proposed to understand the mechanism and provide a guideline for solvent choices of eco-friendly solvent protection method.

High-quality WS2 film as a hole transport layer in high-efficiency non-fullerene organic solar cells.

Liquid-exfoliated 2D transition metal disulfides (TMDs) are potential substitutes for poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole transport layers (HTLs) in Organic Solar Cells (OSCs). Herein, high-yield and high-quality WS2 flake layers are prepared by comprehensively controlling the initial concentration, sonication processing time and centrifugal speed. The WS2 layers deposited on in situ transparent indium tin oxide (ITO) without plasma treatment show higher uniformity and conductivity than that formed on ITO after plasma treatment. With a significant increase in the short-circuit current density (JSC), the power conversion efficiency (PCE) of PM6:Y6-based non-fullerene OSCs using optimized WS2 as the HTL is higher than that using PEDOT:PSS as the HTL(15.75% vs. 15.31%). Combining the morphology characteristics with carrier recombination characteristics, the higher quality of the ITO/WS2 composite substrate leads to better charge transport and a lower bimolecular recombination rate in OSCs, thereby improving the device performance.

Modified fullerenes as acceptors in bulk heterojunction organic solar cells - a theoretical study.

In the present study, electronic structure calculations were used to provide strategies for designing poly(3-hexylthiophene) (P3HT)-fullerene-derivative-based donor-acceptor materials for use in high-efficiency bulk heterojunction organic solar cells (BHJ OSCs). The work systematically analyses the impact of electron-donating and -withdrawing substituents on the opto-electronic properties of the fullerene structures. Parameters relating to the absorption spectra, orbital distributions, and energy ordering of the frontier molecular orbitals (FMO), the interactions between P3HT and the fullerene derivatives, and charge transfer across the interface were investigated. We found that substitution with the electron-withdrawing group NO2 enhances the electronic coupling between the fullerene and P3HT; however, it reduces the open-circuit voltage (VOC) of the OSC through lowering the LUMO energy level. Furthermore, the results show that substitution with an electron-withdrawing group (NO2) and electron-donating group (OCH3) can improve the power conversion efficiency (PCE) of the OSC, since this slightly improves the photon absorption abilities and charge transfer coupling at the interface without overly compromising VOC relative to PC61BM. Our study shows that alkyl chain modification in the PC61BM acceptor is a promising strategy for improving the performances of OSCs.

Surface passivation of organometal halide perovskites by atomic layer deposition: an investigation of the mechanism of efficient inverted planar solar cells.

Interface passivation plays a pivotal role in achieving high-efficiency organic metal halide perovskite solar cells (PSCs). It has been recently revealed that atomic layer deposition (ALD) of wide-band gap oxides shows great potential to effectively passivate defects at the interface, and ALD is also of great technological promise for industrial upscaling. However, the conflicting observations of ALD passivation are often reported in the literature, even with very similar ALD conditions. To unveil the involved crucial mechanism, this work carefully investigates the evolution of a representative MAPbI3 perovskite surface during the ALD of Al2O3, by employing the technique of in situ X-ray photoelectron spectroscopy. The ALD at 125 °C was found to cause significant degradation of the perovskite; lowering the deposition temperature can largely minimize the degradation, and 75 °C was found to be the best ALD temperature. Following this conclusion, inverted planar perovskite solar cells were prepared in ambient conditions with ALD Al2O3 interlayers. Indeed, cells with the interlayer deposited at 75 °C exhibited a significantly enhanced power conversion efficiency from 18.8% (champion 19.2%) to 20.0% (champion 20.4%). Photoluminescence measurements further evidence that the ALD layer can effectively passivate defect states at the perovskite surface. Considering the great representativeness and broad applicability of MAPbI3 and ALD Al2O3, the mechanism and strategy reported herein should be of significant value for the perovskite interface engineering in general.

Revealing the photo-degradation mechanism of PM6:Y6 based high-efficiency organic solar cells

The photo-degradation mechanism of OSCs based on PM6:Y6 or its derivatives is studied. Ultraviolet photons cause more severe photo-degradation in OSCs than others, and the photo-degradation of PM6 can dominate the photo-degradation in devices.

A donor polymer based on 3-cyanothiophene with superior batch-to-batch reproducibility for high-efficiency organic solar cells

A donor polymer based on 3-cyanothiophene, a structurally simple unit, is synthesized for organic solar cells, which exhibited prominent power conversion efficiency and excellent batch-to-batch reproducibility in a wide molecular weight range.

Non-fullerene electron acceptors with benzotrithiophene with π-extension terminal groups for the development of high-efficiency organic solar cells

In this work, two non-fullerene electron acceptors were synthesized by combining BTT as a central unit with two terminal groups. Consequently, organic solar cells based on BTTBo-4F and BTTBo-4FN achieved 11.60% and 8.27% power conversion efficiencies, respectively.