High-efficiency Organic Solar Cells Research Articles

High-efficiency organic solar cells with low non-radiative recombination loss and low energetic disorder

Energy loss within organic solar cells (OSCs) is undesirable as it reduces cell efficiency1–4. In particular, non-radiative recombination loss3 and energetic disorder5, which are closely related to the tail states below the band edge and the overall photon energy loss, need to be minimized to improve cell performance. Here, we report how the use of a small-molecule acceptor with torsion-free molecular conformation can achieve a very low degree of energetic disorder and mitigate energy loss in OSCs. The resulting single-junction OSC has an energy loss due to non-radiative recombination of just 0.17 eV and a high power conversion efficiency of up to 16.54% (certified as 15.89% by the National Renewable Energy Laboratory). The findings take studies of organic photovoltaics deeper into a new regime, beyond the limits of energetic disorder and large energy offset for charge generation. An organic solar cell designed with minimal energetic disorder exhibits very low energy loss due to non-radiative recombination and highly efficient operation.

Stability of Lead and Tin Halide Perovskites: The Link between Defects and Degradation.

The field of photovoltaic research has been lately dominated by the rapid evolution of low-cost and high-efficiency hybrid organic lead halide perovskite solar cells. Despite the considerable progress made in the efficiency of such devices, the achievement of long-term material and device stability remains a challenge. In this Perspective, insights into the role structural defects play in the stability of these perovskite absorbers are examined, highlighting the critical importance of vacancy type defects as the initiation sites for moisture-, oxygen-, and light-induced degradation and the approaches that are emerging to help overcome these issues. In the second part of the Perspective we consider the stability of tin-based perovskites. Here, the Sn4+ defects that arise upon material degradation are described along with the strategies being developed to enhance stability and decrease their formation. Finally, the discussion is extended to innately more stable layered tin-based perovskites, identifying them as a route to the development of efficient lead-free perovskite solar cells.

An oxygen heterocycle-fused fluorene based non-fullerene acceptor for high efficiency organic solar cells

A new acceptor–donor–acceptor (A–D–A) small molecule acceptor, named FCO-2F, is designed and synthesized based on the previous acceptor F-H.

The role of connectivity in significant bandgap narrowing for fused-pyrene based non-fullerene acceptors toward high-efficiency organic solar cells

Isomers of non-fullerene acceptors with pyrene as cores but fused at different positions were studied. FPIC6 possessed ∼119 nm of red-shift absorption and much higher power conversion efficiency of 11.55% as compared to its structural isomer FPIC5.

High-efficiency ternary nonfullerene organic solar cells with record long-term thermal stability

ITC6-2F as a third component in OSCs helps “freeze” the film morphology, leading to significantly improved long-term thermal stability.

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

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

2-Dimensional cross-shaped tetrathienonaphthalene-based ladder-type acceptor for high-efficiency organic solar cells

Development of 2-dimensional cross-shaped ladder-type TC is promising for achieving high-performance n-type materials and the device using PBDB-T:TC-FIC:PC71BM ternary blend achieves a high efficiency of 13.5%.

Isomeric effect of fluorene-based fused-ring electron acceptors to achieve high-efficiency organic solar cells

A TT-terminal ladder-type donor is generally a better molecular design than the corresponding T-terminal ladder-type isomer for the development of new A–D–A NFEAs.

High-efficiency organic solar cells enabled by halogenation of polymers based on 2D conjugated benzobis(thiazole)

A novel polymer based on 2D conjugated benzobis(thiazole) exhibits a high power conversion efficiency of 14.8% in an organic solar cell with IT-4F as the acceptor, with short circuit density and open circuit voltage well-balanced therein.

Optimizing the component ratio of PEDOT:PSS by water rinse for high efficiency organic solar cells over 16.7%

For the state-of-the-art organic solar cells (OSCs), PEDOT:PSS is the most popularly used hole transport material for the conventional structure. However, it still suffers from several disadvantages, such as low conductivity and harm to ITO due to the acidic PSS. Herein, a simple method is introduced to enhance the conductivity and remove the additional PSS by water rinsing the PEDOT:PSS films. The photovoltaic devices based on the water rinsed PEDOT:PSS present a dramatic improvement in efficiency from 15.98% to 16.75% in comparison to that of the untreated counterparts. Systematic characterization and analysis reveal that although part of the PEDOT:PSS is washed away, it still leaves a smoother film and the ratio of PEDOT to PSS is higher than before in the remaining films. It can greatly improve the conductivity and reduce the damage to substrates. This study demonstrates that finely modifying the charge transport materials to improve conductivity and reduce defeats has great potential for boosting the efficiency of OSCs.

High-efficiency organic solar cells with low voltage-loss of 0.46 V

The recent evolution of active components yielded brilliant progresses for organic solar cells (OSCs), yet the mechanism is needed to be clearly understood. In this work, two electron acceptors, a linear SN6-2Br and a V-shaped BTP-2Br, are developed with nitrogen atoms introduced to replace the traditional sp3-hybridized carbon in the fused ring. BTP-2Br possesses an electron-deficient central core, which exhibits slightly blue-shifted absorption as well as deepened HOMO-level compared with SN6-2Br. The corresponding photovoltaic performance from V-shaped BTP-2Br based devices exhibit superior performance especially in short-circuit current (Jsc), despite an enhanced absorption and charge carrier mobilities for SN6-2Br. The primary reason for the higher Jsc from BTP-2Br is faster exciton diffusion and dissociation in blends, than those of SN6-2Br. As a result, PBDB-TF:BTP-2Br based devices achieve a power conversion efficiency (PCE) of 13.84% with an voltage-loss of only 0.46 V, which is one of the lowest values ever reported. Moreover, we fabricated semitransparent OSCs that exhibit an excellent PCE of 9.62% with average visible transparency of 20.1%.

Organic Solar Cells Based on Non-fullerene Small-Molecule Acceptors: Impact of Substituent Position

Summary Molecular engineering of non-fullerene small-molecule acceptors (SMAs) plays a key role in enhancing the performance of organic solar cells. An effective strategy is to introduce functional groups into SMA end groups to tune the electronic and morphological properties of polymer/SMA blends. Here, molecular dynamics simulations and long-range corrected density functional theory calculations are combined to examine the impact of the position of methoxy substitution in the SMA end groups. As representative systems, blends of the IT-OM small-molecule acceptor with the PBDB-T polymer donor are explored; three different positions of the methoxy substitution of the IT-OM end groups are examined. By considering intermolecular mixing and packing, the energetic distribution of the charge-transfer electronic states, the exciton-dissociation and non-radiative recombination processes, and the electron-transfer rates among adjacent acceptors, we provide a comprehensive molecular-scale rationalization of the significant experimental variations in device performance for PBDB-T/IT-OM-based solar cells as a function of methoxy position.

A facile strategy for enhanced performance of inverted organic solar cells based on low-temperature solution-processed SnO2 electron transport layer

High-efficiency organic solar cells (OSCs) based on low-temperature (LT) processed SnO2 electron transport layer (ETL) are promising for their commercial use. However, high density of traps and large contact barrier for carriers at the interface between LT SnO2 and the active layer has been reported. To solve the problem, various interface modifying layer materials, such as PFN, has been introduced. Currently, the fabrication process of such interface modifying layer materials is complex and expensive. Herein, a facile strategy involved a polar solvent ethanolamine (EA) is introduced to modify LT SnO2 surface. By soaking the SnO2 film into EA solution in 2-Methoxyethanol (2-ME), EA can easily anchor into SnO2 film surface and forms a continuous monomolecular layer via dehydration reaction. The whole process is green and highly compatible with a roll-to-roll process. Further study suggests that the deep trap centers on SnO2 surface are substantially reduced and the built-in potential in OSCs is reinforced. Finally, OSCs based on EA-modified SnO2 demonstrated an enhanced power conversion efficiency from 10.71% to 12.45% which was comparable to those based on ZnO (12.26%) under the same experiment parameters. Our work boosts the development of the inverted OSCs with easy fabrication and compatibility with roll-to-roll process.

Disorder vs Delocalization: Which Is More Advantageous for High-Efficiency Organic Solar Cells?

We investigate the combined influence of energetic disorder and delocalization on electron-hole charge-transfer state separation efficiency in donor-acceptor organic photovoltaic systems using an analytical hopping model and Monte Carlo calculations, coupled with an effective mass model. Whereas energetic disorder increases the separation yield at intermediate and low electric fields for low-efficiency blends with strongly localized carriers, we find that it reduces dramatically the fill factors and power conversion efficiencies in high-efficiency solar cells that require high carrier delocalization within the conjugated segment and high mobility-lifetime product. We further demonstrate that the initial electron-hole distance and thermalization processes play only a minor role in the separation dynamics.

Effect of Solvent Additives on the Morphology and Device Performance of Printed Nonfullerene Acceptor Based Organic Solar Cells.

Printing of active layers of high-efficiency organic solar cells and morphology control by processing with varying solvent additive concentrations are important to realize real-world use of bulk-heterojunction photovoltaics as it enables both up-scaling and optimization of the device performance. In this work, active layers of the conjugated polymer with benzodithiophene units PBDB-T-SF and the nonfullerene small molecule acceptor IT-4F are printed using meniscus guided slot-die coating. 1,8-Diiodooctane (DIO) is added to optimize the power conversion efficiency (PCE). The effect on the inner nanostructure and surface morphology of the material is studied for different solvent additive concentrations with grazing incidence small-angle X-ray scattering (GISAXS), grazing incidence wide-angle X-ray scattering (GIWAXS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Optical properties are studied with photoluminescence (PL), UV/vis absorption spectroscopy, and external quantum efficiency (EQE) measurements and correlated to the corresponding PCEs. The addition of 0.25 vol % DIO enhances the average PCE from 3.5 to 7.9%, whereas at higher concentrations the positive effect is less pronounced. A solar cell performance of 8.95% is obtained for the best printed device processed with an optimum solvent additive concentration. Thus, with the large-scale preparation method printing similarly well working solar cells can be realized as with the spin-coating method.

Individual nanostructure optimization in donor and acceptor phases to achieve efficient quaternary organic solar cells

Fullerene derivative (PC71BM) and high crystallinity molecule (DR3TBDTT) are employed into PTB7-Th:FOIC based organic solar cells (OSCs) to cooperate an individual nanostructure optimized quaternary blend. PC71BM functions as molecular adjuster and phase modifier promoting FOIC forming “head-to-head” molecular packing and neutralizing the excessive FOIC crystallites. A multi-scale modified morphology is present thanks to the mixture of FOIC and PC71BM while DR3TBDTT disperses into PTB7-Th matrix to reinforce donor's crystallinity and enhance domain purity. Morphology characterization highlights the importance of individually optimizated nanostructures for donor and acceptor, which contributes to efficient hole and electron transport toward improved carrier mobilities and suppressed non-geminated recombination. Therefore, a power conversion efficiency of 13.51% is realized for a quaternary device which is 16% higher than the binary device (PTB7-Th:FOIC). This work demonstrates that utilizing quaternary strategy for simultaneous optimization of donor and acceptor phases is a feasible way to realize high efficient OSCs.

High-Efficiency Organic Solar Cells With Solution Processable Non-Fullerene Acceptor as an Interlayer

Interlayers and interface engineering plays a pivotal role in achieving efficient charge extraction necessary for realizing high power conversion efficiency (PCE) in bulk heterojunction organic solar cells. In this study, we report a novel strategy of combining advantages of both fullerene derivative and non-fullerene acceptor materials by employing solution processable non-fullerene acceptor 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indan-one))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (ITIC) to modify the interface between ZnO-based electron transport layer and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo [1,2-b;4,5-b'] dithiophe-ne-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carb-oxylate-2-6-diyl)] (PTB7-Th): phenyl-C71-butyric acid methyl ester-based photoactive layer. An improvement in PCE from ∼ 7.5% to ∼ 9% has been achieved for the devices with ITIC modified interlayer. Time resolved photoluminescence (PL) via time correlated single photon counting and photo-electrochemical impedance spectroscopy (photo-EIS) measurements were carried out to investigate the causes for improved PCE for the devices with ITIC modified interlayer.

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.

Low Energetic Disorder in Small-Molecule Non-Fullerene Electron Acceptors

The energetic disorder in representative small-molecule non-fullerene electron acceptors (SM-NFAs) exploited in high-efficiency organic solar cells, is quantified via the combination of molecular d...

Simple and Versatile Non-Fullerene Acceptor Based on Benzothiadiazole and Rhodanine for Organic Solar Cells.

Most non-fullerene acceptors (NFAs) are designed in a complex planar molecular conformation containing fused aromatic rings in high-efficiency organic solar cells (OSCs). To obtain the final molecules, however, numerous synthetic steps are necessary. In this work, a novel simple-structured NFA containing alkoxy-substituted benzothiadiazole and a rhodanine end group (BTDT2R) is designed and synthesized. We also investigate the photovoltaic properties of BTDT2R-based OSCs employing representative polymer donors (wide band gap and high-crystalline P3HT, medium band gap and semicrystalline PPDT2FBT, and narrow band gap and low-crystalline PTB7-Th) to compare the performance capabilities of fullerene acceptor-based OSCs, which are well matched with various polymer donors. OSCs based on P3HT:BTDT2R, PPDT2FBT:BTDT2R, and PTB7-Th:BTDT2R achieved efficiency as high as 5.09, 6.90, and 8.19%, respectively. Importantly, photoactive films incorporating different forms of optical and molecular ordering characteristics exhibit favorable morphologies by means of solvent vapor annealing. This work suggests that the new n-type organic semiconductor developed here is highly promising as a universal NFA that can be paired with various polymer donors with different optical and crystalline properties.