Single-junction Organic Solar Cells Research Articles

Light-induced quinone conformation of polymer donors toward 19.9% efficiency organic solar cells

UV-light illumination converts the aromatic conformation of polymer donors into a rigid quinone structure, resulting in compact fibrillar aggregation of the active layer to achieve a maximum efficiency of 19.9% of single-junction organic solar cells.

Recent Advances of Benzodithiophene-Based Donor Materials for Organic Solar Cells.

Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has increased dramatically, making a big step toward the industrial application of OSCs. Among numerous OSCs, benzodithiophene (BDT)-based OSCs stand out in achieving efficient PCE. Notably, single-junction OSCs using BDT-based polymers as donor materials have completed a PCE of over 19%, indicating a dramatic potential for preparing high-performance large-scale OSCs. This paper reviews the recent progress of OSCs based on BDT polymer donor materials (PDMs). The development of BDT-based OSCs is concisely summarized. Meanwhile, the relationship between the structure of PDMs and the performance of OSCs is further described in this review. Besides, the development and prospect of single junction OSCs are also discussed.

Interface Engineering for Highly Efficient Organic Solar Cells.

Organic solar cells (OSCs) have made dramatic advancements during the past decades owing to the innovative material design and device structure optimization, with power conversion efficiencies surpassing 19% and 20% for single-junction and tandem devices, respectively. Interface engineering, by modifying interface properties between different layers for OSCs, has become a vital part to promote the device efficiency. It is essential to elucidate the intrinsic working mechanism of interface layers, as well as the related physical and chemical processes that manipulate device performance and long-term stability. In this article, the advances in interface engineering aimed to pursue high-performance OSCs are reviewed. The specific functions and corresponding design principles of interface layers are summarized first. Then, the anode interface layer, cathode interface layer in single-junction OSCs, and interconnecting layer of tandem devices are discussed in separate categories, and the interface engineering-related improvements on device efficiency and stability are analyzed. Finally, the challenges and prospects associated with application of interface engineering are discussed with the emphasis on large-area, high-performance, and low-cost device manufacturing.

Selenium‐Substitution Asymmetric Acceptor Enables Efficient Binary Organic Solar Cells over 18.3% via Regulating Molecular Stacking and Phase Separation

AbstractSubstantial efforts of A–DA′D–A type non‐fullerene acceptors (NFAs) molecular design have impelled power conversion efficiency (PCE) of single junction organic solar cells (OSCs) to exceed 19%. Asymmetric geometry strategy, selenium‐substitution, and end‐group engineering are proven to be effective modification methods. Here, two novel selenium substitution asymmetric NFAs, AsymSSe‐2F, and AsymSSe‐2Cl, are synthesized to investigate the synergistic modification effects on device performance compared with symmetric Y6. When blending AsymSSe‐2F with the wide‐bandgap and high crystallinity polymer D18, a remarkable PCE of 18.31% is yielded, and an excellent fill factor of 79.46% is achieved, which is attributed to the broadened absorption, enhanced π–π stacking, balanced carrier mobilities, and fine phase‐separation morphology. Notably, among the reported selenium‐substituted asymmetric NFAs based OSCs, especially combined with the seldom‐reported D18, this PCE is top‐ranked in binary bulk heterojunction organic solar cells. This work indicates that the combined modification of asymmetric geometry and selenium substitution in NFAs is a promising strategy for fabricating high performance OSCs.

Facile direct arylation polycondensation of random semiconducting terpolymers for single-junction non-fullerene organic solar cells

Herein, we designed and synthesized two novel donor-acceptor1-acceptor2-acceptor1 (D-A1-A2-A1) type random conjugated terpolymers, DTP-TPD-DPP and DTP-TPD-BT, as a donor component for single-junction non-fullerene organic solar cells. The terpolymers were randomly combined from N-aryldithienopyrrole (DTP) donor unit and N-alkylthienopyrroledione (TPD) acceptor unit with electron-accepting dibrominated acceptor units including diketopyrrolopyrrole (DPP) or bithiophenebenzothiadiazole (TBT) by direct arylation polycondensation. Compared to two different acceptor units in the polymer backbone, DTP-TPD-DPP containing DPP exhibits stronger intramolecular charge transfer. Thus, the single-layer organic solar cells based on DTP-TPD-DPP:ITIC (1:1) demonstrated a power conversion efficiency (PCE) of 2.92 % and a high open-circuit voltage Voc of 1.0 V while devices using DTP-TPD-BT obtained a PCE of 2.64 % under the same condition fabrication in the ambient condition without any additives. The results showed that random conjugated terpolymer is an efficient and straightforward synthetic strategy to explore promising donor materials for high-performance single-junction organic solar cells.

Latest Updates of Single-Junction Organic Solar Cells up to 20% Efficiency

Single-junction organic solar cells have reached a power conversion efficiency of 20% with narrow bandgap non-fullerene electron acceptor materials such as Y6, as well as with large band gap electron donor materials and their derivatives. The power conversion efficiency improvement of single-junction organic solar cells is a result of highly efficient light harvesting in the near-infrared light range and reduced energy losses with the most promising active layer layout currently available, Bulk-Heterojunction. Ternary blending is known to be the most advanced strategy to construct Bulk-Heterojunction structures in organic solar cells at present. In this review, we examine different devices based on Bulk-Heterojunction structures with efficient electron donors and acceptors. Then, we review the performance of binary and ternary organic solar cells with high power conversion efficiency, in conjunction with different anode and cathode interfaces used in recent studies of high-power conversion efficiency. Finally, we present perspectives on the future development of single-junction organic solar cells.

New Method for Preparing ZnO Layer for Efficient and Stable Organic Solar Cells.

Owing to outstanding optoelectronic properties and simple preparation, zinc oxide (ZnO) has widely been used in organic solar cells (OSCs). Although versatile cathode interface materials have been designed in past, ZnO remains indispensable owing to its excellent overall performance. Therefore, solving the persistent problem of residual amine reacting with non-fullerene acceptors will make ZnO superior over other materials, and thus improve the performance and energy budget of OSCs. Herein, a simple, effective, and economical method for removing residual amine in ZnO without distorting ZnO is reported. By accurately comparing the alkalinities of ZnO and residual amine, boric acid (BA) is selected as the amine-removing agent because of its suitable acidic dissociation constant. Moreover, the high water solubility of BA ensures that the post-cleaning process can be easily performed. The work function, electron extraction, and stability of cathode interface layer are optimized through rinsing them with BA. Consequently, the power conversion efficiency (PCE) and stability of OSCs under long-term illumination are significantly improved. The optimal 0.04and 1.00cm2 single-junction OSCs are based on PBDB-TF:HDO-4Cl:BTP-eC9bulk heterojunction output 18.40% and 17.42% efficiencies, respectively. Furthermore, tandem OSCs based on the BA-treated ZnO exhibit a 19.56% PCE, demonstrating the reliability of this method.

Recent Advances in Single‐Junction Organic Solar Cells

Single-junction organic solar cells (OSCs) have made significant progress in recent years. Innovations in material design and device optimization have improved the power conversion efficiencies to over 19%. In this review, based on recent advances, we discuss molecular design strategies to tune the absorption spectrum, energy level, and intermolecular aggregation as well as highlight the role of molecular electrostatic potential in decreasing energy loss. Then, we introduce the latest progress in four types of OSCs composed of different donor:acceptor combinations: polymer donor:small-molecule acceptor, all-polymer, all-small-molecule, and small-molecule donor:polymer acceptor OSCs. Finally, the challenges of OSCs in practical applications, including material cost, stability, and multi-function integration, are discussed.

Single-junction organic solar cell smashes performance record

Single-junction organic solar cell smashes performance record

High-efficiency single-junction organic solar cells enabled by double-fibril network morphology

High-efficiency single-junction organic solar cells enabled by double-fibril network morphology

Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology.

In organic photovoltaics, morphological control of donor and acceptor domains on the nanoscale is the key for enabling efficient exciton diffusion and dissociation, carrier transport and suppression of recombination losses. To realize this, here, we demonstrated a double-fibril network based on a ternary donor-acceptor morphology with multi-length scales constructed by combining ancillary conjugated polymer crystallizers and a non-fullerene acceptor filament assembly. Using this approach, we achieved an average power conversion efficiency of 19.3% (certified 19.2%). The success lies in the good match between the photoelectric parameters and the morphological characteristic lengths, which utilizes the excitons and free charges efficiently. This strategy leads to an enhanced exciton diffusion length and a reduced recombination rate, hence minimizing photon-to-electron losses in the ternary devices as compared to their binary counterparts. The double-fibril network morphology strategy minimizes losses and maximizes the power output, offering the possibility of 20% power conversion efficiencies in single-junction organic photovoltaics.

Carrier Generation Engineering toward 18% Efficiency Organic Solar Cells by Controlling Film Microstructure

AbstractThe single bulk‐heterojunction active layer based on non‐fullerene acceptors (NFAs) has dominated the power conversional efficiencies above 18% in state‐of‐the‐art organic solar cells (OSCs). However, a deep understanding of the relationship between charge carrier process and film microstructure remains unclear for emerging NFA OSCs. Herein, with the superstar PM6:Y6 blend as a model, the charge generation process in active layers is successfully manipulated by designing three different film microstructures, and they are correlated with the final photovoltaic performance in OSC devices. The amount of intermediate intra‐moiety excited states from the nanoscale Y6 aggregates can be effectively enhanced by controlling the phase separation domains and film crystallinity in the bicontinuous PM6:Y6 networks. This robustly improves the hole transfer, and thus promotes charge generation. As a result, the optimal films show superior device performance, that is, the high efficiencies of 16.53% and 17.98% for PM6:Y6‐ and D18:Y6‐based single junction OSCs, respectively. The results presented here give a rational guide for optimizing the charge carrier process through controlling morphological microstructures toward high‐performance NFA OSCs.

Conjugated Mesopolymer Achieving 15% Efficiency Single-Junction Organic Solar Cells.

The high‐performance organic solar cells (OSCs) tend to choose the polymers with high molecular weight as donors, which easily produce good crystallinity to facilitate intermolecular charge transfer. However, these polymers usually accompanied by the low solubility and synthetic difficulty, increasing batch‐to‐batch variations. The proposal of conjugated mesopolymers (molar mass (M n) in 1–10 kDa) can overcome these problems. Herein, a new mesopolymer, MePBDFCl H as donor material is designed and synthesized, and firstly applied in OSCs. As a comparison, other lower molecular weight mesopolymer of MePBDFCl L and higher molecular weight polymer of PBDFCl with same structure are also prepared and investigated. Because of its appropriate phase separation and miscibility in the blend film, the MePBDFCl H exhibits the highest power conversion efficiency (PCE) of 15.06% among the three materials. Meanwhile, the champion PCE is a new record for benzo[1,2‐b:4,5‐b′]difuran‐based photovoltaic materials. Importantly, comparing to the pronounced PCE decrease of polymer PBDFCl by about 12%, a slightly PCE difference for mespolymer MePBDFCl L is only less than 5%, reducing the batch‐to‐batch variation. This work not only suggests that the benzo[1,2‐b:4,5‐b′]difuran unit is a promising electron‐donating core but also shows that the mesopolymers have great potentials to produce the low‐differentiated and high‐performance organic photovoltaic materials.

Insights into the Structural and Morphological Properties of Layer-by-Layer Processed Organic Photovoltaics.

Recently, with the development of figure-of-merit non-fullerene acceptor materials combined with a ternary strategy and layer-by-layer (LbL) processing, the efficiency of single-junction organic solar cells has exceeded 18%. However, the structural properties of LbL-processed films have not been sufficiently elucidated. Herein, we systematically investigate films fabricated via LbL processing of three different systems, including a ternary system. In particular, we focus on the structural and morphological transitions associated with the diffusion process controlled by thermal annealing and an additive solvent. Different diffusion and crystal formation mechanisms were clearly identified, which were observed to be dependent on the characteristics of the upper layer formed during the LbL process. Based on this insight, the photovoltaic properties associated with various LbL conditions are elucidated, and an ideal path toward a better device is suggested.

Fused-ring acceptors based on quinoxaline unit for highly efficient single-junction organic solar cells with low charge recombination

Two non-fullerene small molecule acceptors (TFQ-F and TFQ-Cl) based on quinoxaline unit were designed and synthesized for efficient organic solar cells (OSCs). These two acceptors showed intense absorption up to 900 nm and high thermal stabilities with decomposition temperatures over 360 °C due to their fused-ring skeletons. TFQ-F and TFQ-Cl are the A-D-A′-D-A type acceptors (A/A′ for acceptor unit and D for donor unit). TFQ-F and TFQ-Cl have the same D-A′-D fragment, which was flanked with different ending groups. The effect of different ending groups on their photophysical properties, electrochemical behaviors, micro-structures and charge recombination properties of active layers, and device performance were investigated systematically. PM6 with the complementary absorption to the two acceptors was used as the donor material. The pristine PM6:TFQ-F blend films displayed the optimal morphologies as revealed by AFM and TEM measurement. Organic solar cells based on PM6:TFQ-Cl blend film showed high JSC of 25.19 mA/cm2 and PCE of 13.2%. The Voc, JSC and PCE for PM6:TFQ-F film based device were 0.857 V, 23.70 mA/cm2 and 13.51%, respectively. The dependence of VOC/JSC on various light intensities indicated that PM6:TFQ-F/Cl based device had low charge recombination.

(Invited) Vibronic Effect of Donor-Acceptor Interaction Determines Fate of Mutiexciton Spins Generated By Singlet Fission

Singlet fission (SF) is expected to exceed the Shockley–Queisser theoretical limit with power conversion efficiencies (PCE) ~33 % of single junction organic solar cells (OSC) because two separated triplet excitons (T+T) can be produced from one excited singlet state (S1S0) sharing its excitation energy with a neighboring ground-state chromophore. The intramolecular SF (iSF) dynamics have widely been studied using the transient absorption spectroscopic methods together with theoretical modeling taking into account the vibronic effects in the ultrafast regimes. The correlated intermolecular triplet pair 1(TT) generated with the singlet character is known to be converted to the quintet (Q) state as 5(TT). These TT pairs separate into individual triplets as the T+T state. In the previous study, we reported the iSF dynamics and subsequent separations to T+T in the pentacene dimers (PcD) bridged by a phenylene at ortho and meta positions by a combination of the ultrafast transient absorption spectroscopy and the time-resolved electron paramagnetic resonance (TREPR) method.1 Before the spin-conversions to the 5(TT) state, the initially populated 1(TT) is strongly coupled in the four unpaired spins of the two triplet excitons, i.e. the entangled spin-state by large spin-spin exchange coupling (J) with the large orbital overlap produced at femtoseconds regions. The modulations of the J-couplings are thought to be essential for the 1(TT) -> 5(TT) conversions. In an iSF system, we interpreted that the quintet ESPs were caused by the anisotropic spin conversions due to singlet-quintet spin-relaxations to the five sublevels at m S = +2, +1, 0, -1 and -2 in accordance with the anisotropy of the ZFS coupling which is highly dependent on the relative orientation of one triplet exciton (TB) with respect to the other triplet (TA) in the TATB multiexciton.2 Internal fluctuations in the J-coupling were concluded to be crucial on the quintet generations. Moreover, we reported the treatment of the EPR transitions in the weakly coupled T+T states in the framework of the spin-correlated triplet pair (SCTP) for the very weak exchange coupling including the superpositions of the nine basis functions of 5(TT) - 3(TT) - 1(TT), as treated in the spin-correlated radical pair model. 3,4 The above pioneering interpretations of the ESP mechanisms may pave a new avenue to clarify how the molecular conformations and their motions drive the spin-conversions and the ultimate T+T generations, both of which are essential but unknown for the photon-to-energy conversion process. In particular, because the J-coupling is anticipated to be sensitive to the TATB conformations, it is expected that the specific molecular motions or phonon modes may play roles for the ultimate T+T de-coupling, as the vibronic effects. So far, details of the TATB geometries, the vibration modes and frequencies, and the electronic couplings are unresolved in the iSF systems for generating the quintet multiexcitons and the subsequent T+T dissociations.We have characterized the molecular geometries, conformation dynamics and J-couplings of the intramolecular multiexcitons after the photoinduced iSF with the proposal of the density matrix analysis of ESP observed by the TREPR method. It is concluded that the J-coupling is determined by the donor-acceptor electronic coupling in the multiexciton and that terahertz TA-TB conformation dynamics plays a significant role for generating the quintet multiexcitons and for the individual T+T states caused by the J-fluctuation in the covalently linked systems.4 Sakai, H.; Inaya, R.; Nagashima, H.; Nakamura, S.; Kobori, Y.; Tkachenko, N. V.; Hasobe, T., J. Phys. Chem. Lett. 2018, 9, 3354-3360.Matsui, Y.; Kawaoka, S.; Nagashima, H.; Nakagawa, T.; Okamura, N.; Ogaki, T.; Ohta, E.; Akimoto, S.; Sato-Tomita, A.; Yagi, S., et al., J. Phys. Chem. C 2019, 123, 18813-18823.Matsuda, S.; Oyama, S.; Kobori, Y., Chem. Sci. 2020, 11, 2934-2942.Kobori, Y.; Fuki, M.; Nakamura,; S. Hasobe, T. J. Phys. Chem. B 2020, 124, 9411–9419.

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%.

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.

Ternary solvent-processed efficient organic solar cells based on a new A-DA′D-A acceptor derivative employing the 3rd-position branching side chains on pyrroles

Recent advances in non-fullerene acceptors (NFAs), typically Y6, have driven power conversion efficiency (PCE) of single-junction organic solar cells (OSCs) over 16%. Meanwhile, it becomes essential to know how to adopt simple strategies to further improve device performance. In this work, a new A-DA′D-A acceptor derivative, Y19-N3 employing 3-ethylheptyl branched at the 3rd-position instead of 2-ethylhexyl on the pyrroles of Y19 is reported. The selection of an appropriate solvent in casting device is implemented to maximize the photovoltaic performance. PBDB-T:Y19-N3-based OSCs treated with a ternary solvent of CF/CB (1:3, v/v) and 0.8% DIO exhibit the optimal PCE of 13.77% here, with the significantly improved Voc (0.78 V) and FF (0.72) as well as the high Jsc (24.46 mA/cm2). Further characterizations indicate that this ternary solvent-treated PBDB-T/Y19-N3 film exhibits the more appropriate morphological features with the highly efficient charge generation and collection as well as the more balanced electron and hole mobilities. This work combines molecular design and device engineering to improve the photovoltaic properties, which is important to the development of OSCs.

Cathode engineering with perylene-diimide interlayer enabling over 17% efficiency single-junction organic solar cells

In organic solar cells (OSCs), cathode interfacial materials are generally designed with highly polar groups to increase the capability of lowering the work function of cathode. However, the strong polar group could result in a high surface energy and poor physical contact at the active layer surface, posing a challenge for interlayer engineering to address the trade-off between device stability and efficiency. Herein, we report a hydrogen-bonding interfacial material, aliphatic amine-functionalized perylene-diimide (PDINN), which simultaneously down-shifts the work function of the air stable cathodes (silver and copper), and maintains good interfacial contact with the active layer. The OSCs based on PDINN engineered silver-cathode demonstrate a high power conversion efficiency of 17.23% (certified value 16.77% by NREL) and high stability. Our results indicate that PDINN is an effective cathode interfacial material and interlayer engineering via suitable intermolecular interactions is a feasible approach to improve device performance of OSCs.