High-efficiency Non-fullerene Organic Solar Cells Research Articles

Dissociation of singlet excitons dominates photocurrent improvement in high-efficiency non-fullerene organic solar cells

Dissociation of singlet excitons dominates photocurrent improvement in high-efficiency non-fullerene organic solar cells

Isomerization of Benzothiadiazole Yields a Promising Polymer Donor and Organic Solar Cells with Efficiency of 19.0.

The exploration of high-performance and low-cost wide-bandgap polymer donors remains critical to achieve high-efficiency nonfullerene organic solar cells (OSCs) beyond current thresholds. Herein, the 1,2,3-benzothiadiazole (iBT), which is an isomer of 2,1,3-benzothiadiazole (BT), is used to design wide-bandgap polymer donor PiBT. The PiBT-based solar cells reach efficiency of 19.0%, which is one of the highest efficiencies in binary OSCs. Systemic studies show that isomerization of BT to iBT can finely regulate the polymers' photoelectric properties including i) increasing the extinction coefficient and photon harvest, ii) downshifting the highest occupied molecular orbital energy levels, iii) improving the coplanarity of polymer backbones, iv) offering good thermodynamic miscibility with acceptors. Consequently, the PiBT:Y6 bulk heterojunction (BHJ) device simultaneously reaches advantageous nanoscale morphology, efficient exciton generation and dissociation, fast charge transportation, and suppressed charge recombination, leading to larger VOC of 0.87V, higher JSC of 28.2 mA cm-2, greater fill factor of 77.3%, and thus higher efficiency of 19.0%, while the analog-PBT-based OSCs reach efficiency of only 12.9%. Moreover, the key intermediate iBT can be easily afforded from industry chemicals via two-step procedure. Overall, this contribution highlights that iBT is a promising motif for designing high-performance polymer donors.

Ultrafast energy transfer from polymer donors facilitating spectral uniform photocurrent generation and low energy loss in high-efficiency nonfullerene organic solar cells

Ultrafast (∼80 fs) lossless Förster resonance energy transfer process followed by a reverse hole transfer process in high efficiency nonfullerene OSCs, facilitating spectral uniform photocurrent generation and lowering the energy loss.

Mapping the energy level alignment at donor/acceptor interfaces in non-fullerene organic solar cells

Energy level alignment (ELA) at donor (D) -acceptor (A) heterojunctions is essential for understanding the charge generation and recombination process in organic photovoltaic devices. However, the ELA at the D-A interfaces is largely underdetermined, resulting in debates on the fundamental operating mechanisms of high-efficiency non-fullerene organic solar cells. Here, we systematically investigate ELA and its depth-dependent variation of a range of donor/non-fullerene-acceptor interfaces by fabricating and characterizing D-A quasi bilayers and planar bilayers. In contrast to previous assumptions, we observe significant vacuum level (VL) shifts existing at the D-A interfaces, which are demonstrated to be abrupt, extending over only 1–2 layers at the heterojunctions, and are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of donor - non-fullerene-acceptor systems.

Block copolymer compatibilizer for efficient and stable nonfullerene organic solar cells

High-efficiency nonfullerene organic solar cells with long-term stability is crucial for future commercialization in the field of organic photovoltaics. Herein, we designed and synthesized a block copolymer, namely PM6-b-PTY6, containing both donor PM6 and Y6 segments in the polymer backbone. We then first demonstrated that the use of such block copolymer as a compatibilizer in high-efficiency PM6:Y6 based benchmark nonfullerene solar cells, simultaneously enhancing their power conversion efficiency and long-term stability. Detailed experimental results reveal that the addition of PM6-b-PTY6 into PM6:Y6 blend can effectively optimize carrier dynamic process, enhance donor–acceptor miscibility, and stabilize the blend morphology. Additionally, this strategy shows similar function when applied to other system like D18-Cl:Y6. As a result, enhanced efficiencies of 16.48% and 17.55% for PM6:Y6 and D18-Cl:Y6 solar cell are achieved, respectively. More importantly, the inverted PM6:Y6 devices exhibit improved stability, retaining 0.81 and 0.86 of the initial efficiency after 550 h aging under ambient and thermal stress, respectively. These results indicate that the compatible block copolymer could provide a fresh strategy to design more efficient nonfullerene organic solar cells.

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-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.

Burn-In Degradation Mechanism Identified for Small Molecular Acceptor-Based High-Efficiency Nonfullerene Organic Solar Cells.

Organic solar cells (OSCs) have again become a hot research topic in recent years. The record power conversion efficiency (PCE) of OSCs has boosted to over 17% in 2020. Apart from the high PCE, the stability of OSCs is also critical for their future applications and commercialization. Recently, many studies have proposed that burn-in degradation can be considered as an ineluctable barrier to long-term stable OSCs. However, there is still lack of studies to explain the detailed mechanism of this burn-in process. In this work, we first investigated the mechanism of the burn-in process in the high-efficiency PM6:N3-based nonfullerene OSCs. The PM6:N3-based device achieved a profound average PCE of 14.10% but also showed a significant performance loss after the burn-in degradation. Following characterizations such as dark J-V, photoluminescence (PL), time-resolved PL, Urbach energy estimation, and electrochemical impedance spectroscopy reveal that the burn-in degradation observed is closely related to the current extraction, energy transfer, nonradiative recombination, and charge transport process in the PM6:N3-based device. At the same time, it has small effects on the exciton dissociation process and energetic disorder in the PM6:N3-based device. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and grazing incidence X-ray diffraction measurements gratifyingly found that the morphology of the PM6:N3 active layer is relatively stable during the burn-in degradation. Therefore, these observed degradations are suspected results from the instability of interfaces and electrodes. The atoms in carrier transport layers and electrodes may diffuse to the active layer during the degradation, which changes the energy levels of each layer and causes traps at the interface and in the active layer. Conquering the instability of interfaces and electrodes is proposed as the prior task for PM6:N3-based OSCs to achieve long-term stability. Our study provides insights into the mechanism behind the burn-in degradation of the PM6:N3-based OSCs, which takes the first step to conquer this barrier.

High-Efficiency Nonfullerene Organic Solar Cells Enabled by 1000 nm Thick Active Layers with a Low Trap-State Density.

The high-efficiency organic solar cells (OSCs) with thicker active layers are potential candidates for the fabrication of large-area solar panels. The low charge carrier mobility of the photoactive materials has been identified as the major problem hindering the photovoltaic performance of the thick-film OSCs. In this study, high performance of ultra-thick-film OSCs employing a nonfullerene acceptor BTP-4Cl and a polymer donor PBDB-TF is demonstrated. Two blends (PBDB-TF:BTP-4Cl and PBDB-TF:IT-4F) show comparable mobilities and excellent photovoltaic characteristics in thin-film devices, while in the 1000 nm thick devices, although they both exhibit desirable and balanced mobilities, the PBDB-TF:BTP-4Cl-based blend possesses lower trap-state density than the IT-4F-based counterpart, leading to lower trap-assist recombination, longer carrier lifetime, and thus a much higher short-circuit current density in the device. As a result, the BTP-4Cl-based 1000 nm thick OSC achieves a remarkable power conversion efficiency of 12.1%, which greatly outperforms the IT-4F-based devices (4.72%). Furthermore, for a 1000 nm thick device with an active area of 4 cm2, a promising efficiency of 10.1% was obtained, showing its great potential in future large-scale production.

Barrier-Free Charge Separation Enabled by Electronic Polarization in High-Efficiency Non-fullerene Organic Solar Cells.

The separation of charge-transfer states into free charges at the donor/acceptor (D/A) interfaces plays a central role in organic solar cells (OSCs). Because of strong Coulomb attraction, the separation mechanisms are elusive, particularly for the high-efficiency non-fullerene (NF) OSCs with low exciton-dissociation driving forces. Here, we demonstrate that the Coulomb barriers can be substantially overcome by electronic polarization for OSCs based on a series of A-D-A acceptors (ITIC, IT-4F, and Y6). In contrast to fullerene-based D/A heterojunctions, the polarization energies for both donor holes and acceptor electrons are remarkably increased from the interfaces to pure regions in the NF heterojunctions because of strong stabilization on electrons but destabilization on holes by electrostatic interactions in the A-D-A acceptors. In particular, upon incorporation of fluorine substituents and electron-poor cores into ITIC, the increased polarization energies can completely compensate for the Coulomb attraction in the IT-4F- and Y6-based heterojunctions, leading to barrierless charge separation.

Cold Crystallization Temperature Correlated Phase Separation, Performance, and Stability of Polymer Solar Cells

Summary Fabrication of non-fullerene (NF)-based polymer solar cells (PSCs) has been an arduous task involving various approaches to optimize the morphology of blend film for superior performance. In this work, unique exothermic peaks can be observed in differential scanning calorimetry first heating scan of the blend films, which are assigned to the cold crystallization temperature (TCC) of NF acceptors when blended with different degree of crystallinity polymers, and TCC contains the phase information at quenched stage of the film development. A linear correlation between the phase separation parameters and TCC was established, which helps to select the appropriate donor for a specific acceptor. Utilizing multiple polymer-NF blend systems, we report an expectation TCC value between 210°C and 230°C, which would promise balanced domain size, purity, and consequentially superior performance. Further investigation revealed that TCC can also be used to evaluate the thermal stability.

Alkynyl-Functionalized Pyrene-Cored Perylene Diimide Electron Acceptors for Efficient Nonfullerene Organic Solar Cells

Integrating two or more perylene diimide (PDI) monomers through the single bond or ring fusion forming a star-shaped structure is a useful strategy for designing high-efficiency electron acceptors. Herein, we implemented an effectual acetylene-linked method of constructing star-shaped PDI-derivative-based high-efficiency nonfullerene organic solar cells (OSCs). A rigid and conjugated pyrene-cored PDI acceptor, B-4TPDI, has been designed and synthesized. The new acceptor manifests a favorable morphology, effective π-electron delocalization, and complementary absorption when it is paired with the polymer donors PTB7-Th or PBDB-T. It is shown that OSCs based on the PTB7-Th:B-4TPDI blend film achieved a power conversion efficiency (PCE) of 7.71% and an outstanding Jsc of 18.13 mA cm–2. More importantly, after adding a small amount of B-4TPDI into the PTB7-Th:ITIC and PBDB-T:ITIC system, the formed ternary OSCs obtained an optimized PCE of 8.76% and 10.93%, respectively. Our work confirms that acetylene linkage would be a prospective π-spacer for designing conjugated PDI-derivative-based new and excellent nonfullerene acceptors.

Photovoltaics literature survey (no. 149)

Photovoltaics literature survey (no. 149)

Unveiling Excitonic Dynamics in High-Efficiency Nonfullerene Organic Solar Cells to Direct Morphological Optimization for Suppressing Charge Recombination.

Nonfullerene acceptors (NFAs)‐based organic solar cells (OSCs) have recently drawn considerable research interests; however, their excitonic dynamics seems quite different than that of fullerene acceptors‐based devices and remains to be largely explored. A random terpolymer of PBBF11 to pair with a paradigm NFA of 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone)‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (ITIC) such that both complementary optical absorption and very small offsets of both highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels are acquired is designed and synthesized. Despite the small energy offsets, efficient electron/hole transfer between PBBF11 and ITIC is both clearly observed from steady‐state photoluminescence and transient absorption spectra and also supported by the measured low exciton binding energy in ITIC. Consequently, the PBBF11:ITIC‐based OSCs afford an encouraging power conversion efficiency (PCE) of 10.02%. Although the good miscibility of PBBF11 and ITIC induces a homogenous blend film morphology, it causes severe charge recombination. The fullerene acceptor of PC71BM with varying loading ratios is therefore added to modulate film morphology to effectively reduce the charge recombination. As a result, the optimal OSCs based on PBBF11:ITIC:PC71BM yield a better PCE of 11.4% without any additive or annealing treatment.

Revealing the Impact of F4-TCNQ as Additive on Morphology and Performance of High-Efficiency Nonfullerene Organic Solar Cells

Revealing the Impact of F4-TCNQ as Additive on Morphology and Performance of High-Efficiency Nonfullerene Organic Solar Cells

Simple-structured small molecule acceptors constructed by a weakly electron-deficient thiazolothiazole core for high-efficiency non-fullerene organic solar cells

A novel strategy is proposed to construct simple-structured SMAs using a weakly electron-deficient thiazolothiazole (TTz) core. The highest PCE of 8.77% is recorded for TTz1-based OSCs, which is the highest efficiency to date among non-fullerene OSCs with simple-structured SMAs.

Perylene diimide arrays: promising candidates for non-fullerene organic solar cells

Herein, two different perylene diimides trimer arrays have been investigated for high-efficiency non-fullerene organic solar cells (OSCs).

High-efficiency non-fullerene organic solar cells enabled by a difluorobenzothiadiazole-based donor polymer combined with a properly matched small molecule acceptor

Non-fullerene organic solar cells with power conversion efficiencies of up to 6.3% are reported using properly matched donor and acceptor.

Towards high-efficiency non-fullerene organic solar cells: Matching small molecule/polymer donor/acceptor

Two low band-gap donors, small molecule p-DTS(FBTTh2)2 (SD) and polymer PBDTTT-C-T (PD), and two perylene diimide acceptors, small molecule PDI-2DTT (SA) and polymer PPDIDTT (PA), were used to fabricate non-fullerene organic solar cells. The effects of four donor/acceptor combinations, PD/PA, PD/SA, SD/SA and SD/PA, on the morphology, charge transfer, charge transport and photovoltaic performance were investigated. Power conversion efficiencies (PCEs) of PD/PA, PD/SA, SD/SA and SD/PA were 3.04%, 0.28%, 2.52% and 0.29%, respectively. The PD/PA blend and SD/SA blend exhibited relatively uniform and continuous morphology, efficient photoinduced charge transfer, high mobility and balanced charge transport relative to PD/SA and SD/PA blends, leading to much higher PCEs.