Versatile Sequential Casting Processing for Highly Efficient and Stable Binary Organic Photovoltaics

Layer-by-Layer Organic Solar Cells Enabled by 1,3,4-Selenadiazole-Containing Crystalline Small Molecule with Double-Fibril Network Morphology

A double-fibril network of the photoactive layer morphology is recognized as an ideal structure facilitating exciton diffusion and charge carrier transport for high-performance organic solar cells (OSCs). However, in the layer-by-layer processed OSCs (LbL-OSCs), polymer donors and small molecule acceptors (SMAs) are separately deposited, and it is challenging to realize a fibril network of pure SMAs with the absence of tight interchain entanglement as polymers. In this work, crystalline small molecule donors (SMDs), named TDZ-3TR and SeDZ-3TR, were designed and introduced into the L8-BO acceptor solution, forcing the phase separation and molecular fibrilization. SeDZ-3TR showed higher crystallinity and lower miscibility with L8-BO acceptor than TDZ-3TR, enabling more driving force to favor the phase separation and better molecular fibrilization of L8-BO. On the other hand, two donor polymers of PM6 and D18 with different fibril widths and lengths were put together to optimize the fibril network of the donor layer. The simultaneously optimization of the acceptor and donor layers resulted in a more ideal double-fibril network of the photoactive layer and an impressive power conversion efficiency (PCE) of 19.38 % in LbL-OSCs.

Impact of Electrostatic Interaction on Vertical Morphology and Energy Loss in Efficient Pseudo-Planar Heterojunction Organic Solar Cells

Although a suitable vertical phase separation (VPS) morphology is essential for improving charge transport efficiency, reducing charge recombination, and ultimately boosting the efficiency of organic solar cells (OSCs), there is a lack of theoretical guidance on how to achieve the ideal morphology. Herein, a relationship between the molecular structure and the VPS morphology of pseudo-planar heterojunction (PPHJ) OSCs is established by using molecular surface electrostatic potential (ESP) as a bridge. The morphological evolution mechanism is revealed by studying four binary systems with vary electrostatic potential difference (∆ESP) between donors (Ds) and acceptors (As). The findings manifest that as ∆ESP increases, the active layer is more likely to form a well-mixed phase, while a smaller ∆ESP favors VPS morphology. Interestingly, it is also observed that a larger ∆ESP leads to enhanced miscibility between Ds and As, resulting in higher non-radiative energy losses (ΔE3). Based on these discoveries, a ternary PPHJ device is meticulously designed with an appropriate ∆ESP to obtain better VPS morphology and lower ΔE3, and an impressive efficiency of 19.09% is achieved. This work demonstrates that by optimizing the ΔESP, not only the formation of VPS morphology can be controlled, but also energy losses can be reduced, paving the way to further boost OSC performance.

High-Efficiency Ternary Polymer Solar Cells with a Gradient-Blended Structure Fabricated by Sequential Deposition

Acquiring the ideal blend morphology of the active layer to optimize charge separation and collection is a constant goal of polymer solar cells (PSCs). In this paper, the ternary strategy and the sequential deposition process were combined to make sufficient use of the solar spectrum, optimize the energy-level structure, regulate the vertical phase separation morphology, and ultimately enhance the power conversion efficiency (PCE) and stability of the PSCs. Specifically, the donor and acceptor illustrated a gradient-blended distribution in the sequential deposition-processed films, thus resulting in facilitated carrier characteristics in the gradient-blended devices. Consequently, the PSCs based on D18-Cl/Y6:ZY-4Cl have achieved a device efficiency of over 18% with the synergetic improvement of open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF). Therefore, this work reveals a facile approach to fabricating PSCs with improved performance and stability.

Solution Sequential Deposition Pseudo-Planar Heterojunction: An Efficient Strategy for State-of-Art Organic Solar Cells

Organic solar cells (OSCs) are considered as a promising new generation of clean energy. Bulk heterojunction (BHJ) structure has been widely employed in the active layer of efficient OSCs. However, precise regulation of morphology in BHJ is still challenging due to the competitive coupling between crystallization and phase separation. Recently, a novel pseudo-planar heterojunction (PPHJ) structure, prepared through solution sequential deposition, has attracted much attention. It is an easy-to-prepare structure in which the phase separation structures, interfaces, and molecular packing can be separately controlled. Employing PPHJ structure, the properties of OSCs, such as power conversion efficiency, stability, transparency, flexibility, and so on, are usually better than its BHJ counterpart. Hence, a comprehensive understanding of the film-forming process, morphology control, and device performance of PPHJ structure should be considered. In terms of the representative works about PPHJ, this review first introduces the fabrication process of active layers based on PPHJ structure. Second, the widely applied morphology control methods in PPHJ structure are summarized. Then, the influences of PPHJ structure on device performance and other property are reviewed, which largely expand its application. Finally, a brief prospect and development tendency of PPHJ devices are discussed with the consideration of their challenges.

Manipulating Vertical Phase Separation Enables Pseudoplanar Heterojunction Organic Solar Cells Over 19% Efficiency via Ternary Polymerization

Controlling vertical phase separation of the active layer to enable efficient exciton dissociation and charge carrier transport is crucial to boost power conversion efficiencies (PCEs) of pseudoplanar heterojunction (PPHJ) organic solar cells (OSCs). However, how to optimize the vertical phase separation of PPHJ OSCs via molecule design is rarely reported yet. Herein, ternary polymerization strategy is employed to develop a series of polymer donors, DL1-DL4, and regulate their solubility, molecular aggregation, molecular orientation, and miscibility, thus efficiently manipulating vertical phase separation in PPHJ OSCs. Among them, DL1 not only has enhanced solubility, inhibited molecular aggregation and partial edge-on orientation to facilitate acceptor molecules, Y6, to permeate into polymer layer and increase donor/acceptor interfaces, but also sustains high crystallinity and appropriate miscibility with Y6 to acquire ordered molecular packing, thus achieving optimized vertical phase separation to well juggle exciton dissociation and charge transport in PPHJ devices. Therefore, DL1/Y6 based PPHJ OSCs gain the best exciton dissociation probability, highest charge carrier mobilities and weakest charge recombination, and thus afford an impressive PCE of 19.10%, which is the record value for terpolymer donors. It demonstrates that ternary polymerization is an efficient method to optimize vertical phase separation in PPHJ OSCs for high PCEs.

Electrostatic force promoted intermolecular stacking of polymer donors toward 19.4% efficiency binary organic solar cells

Conjugated polymers are generally featured with low structural order due to their aromatic and irregular structural units, which limits their light absorption and charge mobility in organic solar cells. In this work, we report a conjugated molecule INMB-F that can act as a molecular bridge via electrostatic force to enhance the intermolecular stacking of BDT-based polymer donors toward efficient and stable organic solar cells. Molecular dynamics simulations and synchrotron X-ray measurements reveal that the electronegative INMB-F adsorb on the electropositive main chain of polymer donors to increase the donor-donor interactions, leading to enhanced structural order with shortened π-π stacking distance and consequently enhanced charge transport ability. Casting the non-fullerene acceptor layer on top of the INMB-F modified donor layer to fabricate solar cells via layer-by-layer deposition evidences significant power conversion efficiency boosts in a range of photovoltaic systems. A power conversion efficiency of 19.4% (certified 18.96%) is realized in PM6/L8-BO binary devices, which is one of the highest reported efficiencies of this material system. The enhanced structural order of polymer donors by INMB-F also leads to a six-fold enhancement of the operational stability of PM6/L8-BO organic solar cells.

Flexible Solution-Processed Electron-Transport-Layer-Free Organic Photovoltaics for Indoor Application

Organic photovoltaics (OPVs) have unique advantages of low weight, mechanical flexibility, and solution processability, which make them exceptionally suitable for integrating low-power Internet of Things devices. However, achieving improved operational stability together with solution processes that are applicable to large-scale fabrication remains challenging. Their major limitation arises due to the instable factors that occur both inside the thick active film and from the ambient environment, which cannot be completely resolved via the current encapsulation techniques used for flexible OPVs. Additionally, thin active layers are highly vulnerable to point defects, which result in low yield rates and impede the laboratory-to-industry translation. In this study, flexible fully solution-processed OPVs with improved indoor efficiency and long-term operational stability than that of conventional OPVs with evaporated electrodes are achieved. Benefiting from the oxygen and water vapor permeation barrier of the spontaneously formed gallium oxide layers on the exposed eutectic gallium-indium surface, fast degradation of the OPVs with thick active layers is prevented, maintaining 93% of its initial P_max after 5000 min of indoor operation under 1000 lx light-emitting diode (LED) illumination. Additionally, by using the thick active layer, spin-coated silver nanowires could be directly used as bottom electrodes without complicated flattening processes, thereby substantially simplifying the fabrication process and proposing a promising manufacturing technique for devices with high-throughput energy demands.

Solid Additive-Assisted Layer-by-Layer Processing for 19% Efficiency Binary Organic Solar Cells

Morphology is of great significance to the performance of organic solar cells (OSCs), since appropriate morphology could not only promote the exciton dissociation, but also reduce the charge recombination. In this work, we have developed a solid additive-assisted layer-by-layer (SAA-LBL) processing to fabricate high-efficiency OSCs. By adding the solid additive of fatty acid (FA) into polymer donor PM6 solution, controllable pre-phase separation forms between PM6 and FA. This intermixed morphology facilitates the diffusion of acceptor Y6 into the donor PM6 during the LBL processing, due to the good miscibility and fast-solvation of the FA with chloroform solution dripping. Interestingly, this results in the desired morphology with refined phase-separated domain and vertical phase-separation structure to better balance the charge transport /collection and exciton dissociation. Consequently, the binary single junction OSCs based on PM6:Y6 blend reach champion power conversion efficiency (PCE) of 18.16% with SAA-LBL processing, which can be generally applicable to diverse systems, e.g., the PM6:L8-BO-based devices and thick-film devices. The efficacy of SAA-LBL is confirmed in binary OSCs based on PM6:L8-BO, where record PCEs of 19.02% and 16.44% are realized for devices with 100 and 250 nm active layers, respectively. The work provides a simple but effective way to control the morphology for high-efficiency OSCs and demonstrates the SAA-LBL processing a promising methodology for boosting the industrial manufacturing of OSCs.

Sequential Deposition of Multicomponent Bulk Heterojunctions Increases Efficiency of Organic Solar Cells

Constructing tandem and multi-blend organic solar cells (OSCs) is an effective way to overcome the absorption limitations of conventional single-junction devices. However, these methods inevitably require tedious multilayer deposition or complicated morphology-optimization procedures. Herein, sequential deposition is utilized as an effective and simple method to fabricate multicomponent OSCs with a double-bulk heterojunction (BHJ) structure of the active layer to further improve photovoltaic performance. Two efficient donor-acceptor pairs, D18-Cl:BTP-eC9 and PM6:L8-BO, are sequentially deposited to form the D18-Cl:BTP-eC9/PM6:L8-BO double-BHJ active layer. In these double-BHJ OSCs, light absorption is significantly improved, and optimal morphology is also retained without requiring a more complicated morphology optimization involved in quaternary blends. Compared to the quaternary blend devices, energy loss (E_loss ) is also reduced by rationally matching each donor with an appropriate acceptor. Consequently, the power conversion efficiency (PCE) is improved from 18.25% for D18-Cl:BTP-eC9 and 18.69% for PM6:L8-BO based binary blend OSCs to 19.61% for the double-BHJ OSCs. In contrast, a D18-Cl:PM6:L8-BO:BTP-eC9 quaternary blend of OSCs exhibited a dramatically reduced PCE of 15.83%. These results demonstrate that a double-BHJ strategy, with a relatively simple processing procedure, can potentially enhance the device performance of OSCs and lead to more widespread use.

Over 18.1% Efficiency of Layer-by-Layer Polymer Solar Cells by Enhancing Exciton Utilization near the ITO Electrode

In this work, layer-by-layer (LbL) polymer solar cells (PSCs) are constructed without/with the incorporation of a dissociation strengthening layer (DSL) on the basis of the wide-bandgap donor D18-Cl, as well as the narrow-bandgap nonfullerene acceptor Y6. The efficiency of LbL PSCs is enhanced from 17.62 to 18.15% through introducing a DSL, originating from the enhanced dissociation of D18-Cl excitons near the ITO electrode. Meanwhile, the interfacial energy between D18-Cl and Y6 layers is decreased by incorporating a DSL, which should facilitate molecular interdiffusion for more adequate exciton dissociation in LbL active layers. This work offers a simple and resultful way for realizing power conversion efficiency (PCE) improvement of LbL PSCs with maximized exciton utilization in LbL active layers. The universality of the DSL incorporation strategy on performance improvement can be further confirmed with a boosted PCE from 17.39 to 18.03% or from 17.13 to 17.61% for D18-Cl/L8-BO- or D18-Cl/N3-based LbL PSCs by incorporating a DSL.

End Group Effect of Asymmetric Benzodithiophene-Based Donor with Liquid-Crystal State for Small-Molecule Binary Solar Cell

Liquid-crystal small molecule donor (LC-SMD) is a new type organic semiconductor, which is attractive not only for the easy synthesis and purification, well-defined chemical structures, etc., but also for the LC state that makes the crystallinity and aggregation state of molecules adjustable. Here, one new LC-SMD (a-BTR-H4) is synthesized with 1D alkoxyl and 2D thiophene-alkylthiol side-chained benzo[1,2-b:4,5-b']dithiophene core, trithiophene π-bridge, and 3-(2-ethylhexyl) rhodanine end group. a-BTR-H4 shows low LC transition temperature, 117 °C, however, counterpart material (a-BTR-H5) with the same main structure but 3-ethyl rhodanine terminal group does not show LC properties. Although a-BTR-H4/H5 show similar Ultraviolet-visible absorption spectrum and energy levels, a-BTR-H4 affords relatively high photovoltaic performances due to favorable blend morphology produced by the consistent annealing temperature of Y6-based accepters and liquid crystal temperature of donors. Preliminary results indicate that a-BTR-H4 gains a power conversion efficiency (PCE) of 11.36% for Y6-based devices, which is ascribed to better light harvest as well as balanced carrier generation and transport, while a-BTR-H5 obtains 7.57% PCE. Therefore, some materials with unique nematic LC phase have great application potential in organic electronics, and further work to utilize a-BTR-H4 for high-performance device is underway.

A Versatile and Low-Cost Polymer Donor Based on 4-Chlorothiazole for Highly Efficient Polymer Solar Cells

Benefiting from the emergence of narrow-band-gap small-molecule acceptors (SMAs), especially "Y" series, the power conversion efficiency (PCE) of polymer solar cells (PSCs) is rapidly improved. However, polymer donors with high efficiency, easy synthesis, and good universality are relatively scarce except PBDB-TF and D18. Herein, two polymer donors are designed and synthesized based on 4-chlorothiazole derivatives with simple structures, namely PTz3Cl and PBTTz3Cl. The OSCs based on PBTTz3Cl with slightly weaker intermolecular forces in comparison to PTz3Cl exhibits a decent PCE of 18.38% in blending with SMA L8-BO, owing to its strong donor/acceptor interaction with L8-BO, which shapes suitable phase separation morphology. Further research finds that PBTTz3Cl can exhibit excellent photovoltaic performances with various SMA materials, highlighting its universality. Based on this, ternary PSCs are designed where BTP-eC9 is introduced as a guest into the PBTTz3Cl:L8-BO host system. Thanks to further optimal blend morphology and more balanced charge transport, the PCE is improved up to 19.12%, which is among the highest values for PSCs. This work provides a new design of low-cost electron-deficient units for constructing highly versatile, high-performance polymer donors.

19.10% Efficiency and 80.5% Fill Factor Layer-by-Layer Organic Solar Cells Realized by 4-Bis(2-Thienyl)Pyrrole-2,5-Dione Based Polymer Additives for Inducing Vertical Segregation Morphology

The morphology plays a key role in determining the charge generation and collection process, thus impacting the performances of organic solar cells (OSCs). The limited selection pool of additives to optimize the morphology of OSCs, especially for the emerging layer-by-layer (LbL) OSCs, impeding the improvements of photovoltaic performances. Herein, a new method of using conjugated polymers as the additives to optimize the morphology for improving the photovoltaic performances of LbL-OSCs is reported. Four polymers of PH, PS, PF, and PCl are developed with different side chains. These polymers exhibit poor performances as donor materials and additives in the BHJ devices, due to the unsuitable energy level alignment and unfavorable molecular interactions. By contrast, they can be served as efficient additives to optimize the PM6 fibril matrix for facilitating the penetration of BTP-eC9 and forming an intertwined D/A bicontinuous network with a vertical segregation. Such morphology is optimized by side chain engineering, which enables the progressive improvement of the charge separation and collection. As a result, adding a small amount of PCl as the additive, the optimized morphology contributes to a champion PCE of 19.10% with a high FF of 80.5%.

Ternary All-Polymer Solar Cells with Efficiency up to 18.14% Employing a Two-Step Sequential Deposition

Achieving a finely tuned active layer morphology with a suitable vertical phase to facilitate both charge generation and charge transport has long been the main goal for pursuing the highly efficient bulk heterojunction all-polymer solar cells (all-PSCs). Herein, a solution to address the above challenge via synergistically combining the ternary blend strategy and the layer-by-layer (LbL) procedure is proposed. By introducing a synthesized polymer acceptor (P_A ), PY-Cl, with higher crystallinity into the designed host acceptor PY-SSe-V, vertical phase distribution and molecular ordering of the LbL-type ternary all-PSCs can be improved in comparison to the LbL-type PM6/PY-SSe-V binary all-PSCs. The formation of the superior bulk microstructure can not only promote charge transport and extraction properties but also reduce energetic disorder and non-radiative recombination loss, thus improving all three photovoltaic parameters simultaneously. Consequently, the PM6/(PY-SSe-V:PY-Cl) ternary all-PSCs show the best efficiency of 18.14%, which is among the highest values reported to date for all-PSCs. This work provides a facile and effective LbL-type ternary strategy for obtaining high-efficiency all-PSCs.