Highly Efficient and Stable Organic Solar Cells Enabled by a Commercialized Simple Thieno[3,2-b]thiophene Additive.

Delicately manipulating nanomorphology is recognized as a vital and effective approach to enhancing the performance and stability of organic solar cells (OSCs). However, the complete removal of solvent additives with high boiling points is typically necessary to maintain the operational stability of the device. In this study, two commercially available organic intermediates, namely thieno[3,2-b]thiophene (TT) and 3,6-dibromothieno[3,2-b]thiophene (TTB) are introduced, as solid additives in OSCs. The theoretical simulations and experimental results indicate that TT and TTB may exhibit stronger intermolecular interactions with the acceptor Y6 and donor PM6, respectively. This suggests that the solid additives (SAs) can selectively intercalate between Y6 and PM6 molecules, thereby improving the packing order and crystallinity. As a result, the TT-treated PM6:Y6 system exhibits a favorable morphology, improved charge carrier mobility, and minimal charge recombination loss. These characteristics contribute to an impressive efficiency of 17.75%. Furthermore, the system demonstrates exceptional thermal stability (T80 > 2800 h at 65 °C) and outstanding photostability. The universal applicability of TT treatment is confirmed in OSCs employing D18:L8-BO, achieving a significantly higher PCE of 18.3%. These findings underscore the importance of using appropriate solid additives to optimize the blend morphology of OSCs, thereby improving photovoltaic performance and thermal stability.

Eliminating the Burn-in Loss of Efficiency in Organic Solar Cells by Applying Dimer Acceptors as Supramolecular Stabilizers.

The meta-stable active layer morphology of organic solar cells (OSCs) is identified as the main cause of the rapid burn-in loss of power conversion efficiency (PCE) during long-term device operation. However, effective strategies to eliminate the associated loss mechanisms from the initial stage of device operation are still lacking, especially for high-efficiency material systems. Herein, the introduction of molecularly engineered dimer acceptors with adjustable thermal transition properties into the active layer of OSCs to serve as supramolecular stabilizers for regulating the thermal transitions and optimizing the crystallization of the absorber composites is reported. By establishing intimate π-π interactions with small-molecule acceptors, these stabilizers can effectively reduce the trap-state density (Nt) in the devices to achieve excellent PCEs over 19%. More importantly, the low Nt associated with an initially optimized morphology can be maintained under external stresses to significantly reduce the PCE burn-in loss in devices. This research reveals a judicious approach to improving OPV stability by establishing a comprehensive correlation between material properties, active-layer morphology, and device performance, for developing burn-in-free OSCs.

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications.

Interfacial engineering has long been a vital means of improving thin-film device performance, especially for organic electronics, perovskites, and hybrid devices. It greatly facilitates the fabrication and performance of solution-processed thin-film devices, including organic field effect transistors (OFETs), organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light-emitting diodes (OLEDs). However, due to the limitation of traditional interfacial materials, further progress of these thin-film devices is hampered particularly in terms of stability, flexibility, and sensitivity. The deadlock has gradually been broken through the development of self-assembled monolayers (SAMs), which possess distinct benefits in transparency, diversity, stability, sensitivity, selectivity, and surface passivation ability. In this review, we first showed the evolution of SAMs, elucidating their working mechanisms and structure-property relationships by assessing a wide range of SAM materials reported to date. A comprehensive comparison of various SAM growth, fabrication, and characterization methods was presented to help readers interested in applying SAM to their works. Moreover, the recent progress of the SAM design and applications in mainstream thin-film electronic devices, including OFETs, OSCs, PVSCs and OLEDs, was summarized. Finally, an outlook and prospects section summarizes the major challenges for the further development of SAMs used in thin-film devices.

Rational molecular design of multifunctional self-assembled monolayers for efficient hole selection and buried interface passivation in inverted perovskite solar cells.

Self-assembled monolayers (SAMs) have been widely employed as the bottom-contact hole-selective layer (HSL) in inverted perovskite solar cells (PSCs). Besides manipulating the electrical properties, molecularly engineering the SAM provides an opportunity to modulate the perovskite buried interface. Here, we successfully introduced Lewis-basic oxygen and sulfur heteroatoms through rational molecular design of asymmetric SAMs to obtain two novel multifunctional SAMs, CbzBF and CbzBT. Detailed characterization of single-crystal structures and device interfaces shows that enhanced packing, more effective ITO work function adjustment, and buried interface passivation were successfully achieved. Consequently, the champion PSC employing CbzBT showed an excellent power conversion efficiency (PCE) of 24.0% with a high fill factor of 84.41% and improved stability. This work demonstrates the feasibility of introducing defect-passivating heterocyclic groups into SAM molecules to help passivate the interfacial defects in PSCs. The insights gained from this molecular design strategy will accelerate the development of new multifunctional SAM HSLs for efficient PSCs.

A Hole-Selective Self-Assembled Monolayer for Both Efficient Perovskite and Organic Solar Cells.

Self-assembled monolayers (SAMs) emerging as promising hole-selective layers (HSLs) are advantageous for facile processability, low cost, and minimal material consumption in the fabrication of both perovskite solar cells (PSCs) and organic solar cells (OSCs). However, owing to the different nature between perovskites and organic semiconductors, few SAMs were reported to effectively accommodate both PSCs and OSCs at the same time. In this regard, a universally applicable SAM that can accommodate both perovskites and organic semiconductors could be desirable for simplifying cell manufacturing, especially from an industrial perspective. In this work, we designed a SAM, TDPA-Cl by introducing chlorinated phenothiazine as the headgroup and linking with anchor phosphonic acid through a butyl chain. The resulting dense SAM was carefully characterized in terms of molecular bonding, surface morphology, and packing density, and its functions in OSCs and PSCs were discussed from the aspects of interactions with the absorber layer, energy level alignment, and charge-selective dipoles. The PM6:Y6-based OSCs with TDPA-Cl SAM as the HSL showed a superior performance to those with PEDOT:PSS. Furthermore, the universality was proved with an efficiency of 17.4% in the D18:Y6 system. In PSCs, the TDPA-Cl-based devices delivered a better performance of 22.4% than the PTAA-based devices (20.8%) with improved processability and reproducibility. This work represents a SAM with reasonably good compromise between the differing requirements of OSCs and PSCs.

Recent Progress of Oligomeric Non-Fullerene Acceptors for Efficient and Stable Organic Solar Cells.

Organic solar cells (OSCs) have achieved remarkable power conversion efficiencies (PCEs) of over 19 % in the past few years due to the rapid development of non-fullerene acceptors (NFAs). However, the operational stability remains a great challenge that inhibits their commercialization. Recently, oligomeric NFAs (ONFAs) have attracted great attention, which not only can deliver excellent device performance, but also improve the thermal-/photo- stability of OSCs. This is attributed to the suppressed molecular diffusion of ONFAs associated with their high glass-transition temperature (Tg ) and improved thermodynamic properties of ONFAs. Herein, we focus on investigating the correction between the ONFA chemical structure, material properties, device performance, and stability. In addition, we also try to point out the challenges in synthesizing ONFAs and provide potential directions for future ONFA designs.

Current State and Future Perspectives of Printable Organic and Perovskite Solar Cells.

Photovoltaic technology presents a sustainable solution to address the escalating global energy consumption and a reliable strategy for achieving net-zero carbon emissions by 2050. Emerging photovoltaic technologies, especially the printable organic and perovskite solar cells, have attracted extensive attention due to their rapidly transcending power conversion efficiencies and facile processability, providing great potential to revolutionize the global photovoltaic market. To accelerate these technologies to translate from the laboratory scale to the industrial level, it is critical to develop well-defined and scalable protocols to deposit high-quality thin films of photoactive and charge-transporting materials. Herein, the current state of printable organic and perovskite solar cells is summarized and the view regarding the challenges and prospects toward their commercialization is shared. Different printing techniques are first introduced to provide a correlation between material properties and printing mechanisms, and the optimization of ink formulation and film-formation during large-area deposition of different functional layers in devices are then discussed. Engineering perspectives are also discussed to analyze the criteria for module design. Finally, perspectives are provided regarding the future development of these solar cells toward practical commercialization. It is believed that this perspective will provide insight into the development of printable solar cells and other electronic devices.

Correlation of Broad Absorption Band with Small Singlet-Triplet Energy Gap in Organic Photovoltaics.

Organic photovoltaics (OPV) are one of the most effective ways to harvest renewable solar energy, with the power conversion efficiency (PCE) of the devices soaring above 19 % when processed with halogenated solvents. The superior photocurrent of OPV over other emerging photovoltaics offers more opportunities to further improve the efficiency. Tailoring the absorption band of photoactive materials is an effective way to further enhance OPV photocurrent. However, the field has mostly been focusing on improving the near-infrared region photo-response, with the absorption shoulders in short-wavelength region (SWR) usually being neglected. Herein, by developing a series of non-fullerene acceptors (NFAs) with varied side-group conjugations, we observe an enhanced SWR absorption band with increased side-group conjugation length. The underpinning factors of how molecular structures and geometries improve SWR absorption are clearly elucidated through theoretical modelling and crystallography. Moreover, a clear relationship between the enhanced SWR absorption and reduced singlet-triplet energy gap is established, both of which are favorable for the OPV performance and can be tailored by rational structure design of NFAs. Finally, the rationally designed NFA, BO-TTBr, affords a decent PCE of 18.5 % when processed with a non-halogenated green solvent.

Optimizing Crystallization in Wide-Bandgap Mixed Halide Perovskites for High-Efficiency Solar Cells.

Wide-bandgap (WBG) perovskites have attracted considerable attention due to their adjustable bandgap properties, making them ideal candidates for top subcells in tandem solar cells (TSCs). However, WBG perovskites often face challenges such as inhomogeneous crystallization and severe nonradiative recombination loss, leading to high open-circuit voltage (VOC ) deficits and poor stability. To address these issues, a multifunctional phenylethylammonium acetate (PEAAc) additive that enhances uniform halide phase distribution and reduces defect density in perovskite films by regulating the mixed-halide crystallization rate, is introduced. This approach successfully develops efficient WBG perovskite solar cells (PSCs) with reduced VOC loss and enhanced stability. By applying this universal strategy to the FAMACsPb(I1- x Brx )3 system with a range of bandgaps of 1.73, 1.79, 1.85, and 1.92 eV, power conversion efficiencies (PCE) of 21.3%, 19.5%, 18.1%, and 16.2%, respectively, are attained. These results represent some of the highest PCEs reported for the corresponding bandgaps. Furthermore, integrating WBG perovskite with organic photovoltaics, an impressive PCE of over 24% for two-terminal perovskite/organic TSCs, with a record VOC of ≈ 2.2 V is achieved. This work establishes a foundation for addressing phase separation and inhomogeneous crystallization in Br-rich perovskite components, paving the way for the development of high-performance WBG PSCs and TSCs.

Compact Hole-Selective Self-Assembled Monolayers Enabled by Disassembling Micelles in Solution for Efficient Perovskite Solar Cells.

Self-assembled monolayers (SAMs) are widely employed as effective hole-selective layers (HSLs) in inverted perovskite solar cells (PSCs). However, most SAM molecules are amphiphilic in nature and tend to form micelles in the commonly used alcoholic processing solvents. This introduces an extra energetic barrier to disassemble the micelles during the binding of SAM molecules on the substrate surface, limiting the formation of a compact SAM. To alleviate this problem for achieving optimal SAM growth, a co-solvent strategy to disassemble the micelles of carbazole-based SAM molecules in the processing solution is developed. This effectively increases the critical micelle concentration to be above the processing concentration and enhances the reactivity of the phosphonic acid anchoring group to allow densely packed SAMs to be formed on indium tin oxide. Consequently, the PSCs derived from using MeO-2PACz, 2PACz, and CbzNaph SAM HSLs show universally improved performance, with the CbzNaph SAM-derived device achieving a champion efficiency of 24.98% and improved stability.

Target Therapy for Buried Interface Enables Stable Perovskite Solar Cells with 25.05% Efficiency.

The buried interface in perovskite solar cells (PSCs) is pivotal for achieving high efficiency and stability. However, it is challenging to study and optimize the buried interface due to its non-exposed feature. Here, a facile and effective strategy is developed to modify the SnO2 /perovskite buried interface by passivating the buried defects in perovskite and modulating carrier dynamics via incorporating formamidine oxalate (FOA) in SnO2 nanoparticles. Both formamidinium and oxalate ions show a longitudinal gradient distribution in the SnO2 layer, mainly accumulating at the SnO2 /perovskite buried interface, which enables high-quality upper perovskite films, minimized defects, superior interface contacts, and matched energy levels between perovskite and SnO2 . Significantly, FOA can simultaneously reduce the oxygen vacancies and tin interstitial defects on the SnO2 surface and the FA+ /Pb2+ associated defects at the perovskite buried interface. Consequently, the FOA treatment significantly improves the efficiency of the PSCs from 22.40% to 25.05% and their storage- and photo-stability. This method provides an effective target therapy of buried interface in PSCs to achieve very high efficiency and stability.

Understanding the Role of Removable Solid Additives: Selective Interaction Contributes to Vertical Component Distributions.

Sequentially deposited organic solar cells (SD-OSCs) have attracted great attention owing to their ability in achieving a more favorable, vertically phase-separated morphology to avoid the accumulation of counter charges at absorber/transporting layer interfaces. However, the processing of SD-OSCs is still quite challenging in preventing the penetration of small-molecule acceptors into the polymer donor layer via erosion or swelling. Herein, solid additives (SAs) with varied electrostatic potential distributions and steric hinderance are introduced into SD-OSCs to investigate the effect of evaporation dynamics and selective interaction on vertical component distribution. Multiple modelings indicate that the π-π interaction dominates the interactions between aromatic SAs and active layer components. Among them, p-dibromobenzene shows a stronger interaction with the donor while 2-chloronaphthalene (2-CN) interacts more preferably with acceptor. Combining the depth-dependent morphological study aided by multiple X-ray scattering methods, it is concluded that the evaporation of SAs can drive the stronger-interaction component upward to the surface, while having minor impact on the overall molecular packing. Ultimately, the 2-CN-treated devices with reduced acceptor concentration at the bottom surface deliver a high power conversion efficiency of 19.2%, demonstrating the effectiveness of applying selective interactions to improve the vertical morphology of OSCs by using SAs with proper structure.

Correlation of Local Isomerization Induced Lateral and Terminal Torsions with Performance and Stability of Organic Photovoltaics.

Organic photovoltaics (OPVs) have achieved great progress in recent years due to delicately designed non-fullerene acceptors (NFAs). Compared with tailoring of the aromatic heterocycles on the NFA backbone, the incorporation of conjugated side-groups is a cost-effective way to improve the photoelectrical properties of NFAs. However, the modifications of side-groups also need to consider their effects on device stability since the molecular planarity changes induced by side-groups are related to the NFA aggregation and the evolution of the blend morphology under stresses. Herein, a new class of NFAs with local-isomerized conjugated side-groups are developed and the impact of local isomerization on their geometries and device performance/stability are systematically investigated. The device based on one of the isomers with balanced side- and terminal-group torsion angles can deliver an impressive power conversion efficiency (PCE) of 18.5%, with a low energy loss (0.528 V) and an excellent photo- and thermal stability. A similar approach can also be applied to another polymer donor to achieve an even higher PCE of 18.8%, which is among the highest efficiencies obtained for binary OPVs. This work demonstrates the effectiveness of applying local isomerization to fine-tune the side-group steric effect and non-covalent interactions between side-group and backbone, therefore improving both photovoltaic performance and stability of fused ring NFA-based OPVs.

Deciphering the Roles of MA-Based Volatile Additives for α-FAPbI3 to Enable Efficient Inverted Perovskite Solar Cells.

Functional additives that can interact with the perovskite precursors to form the intermediate phase have been proven essential in obtaining uniform and stable α-FAPbI3 films. Among them, Cl-based volatile additives are the most prevalent in the literature. However, their exact role is still unclear, especially in inverted perovskite solar cells (PSCs). In this work, we have systematically studied the functions of Cl-based volatile additives and MA-based additives in formamidinium lead iodide (FAPbI3)-based inverted PSCs. Using in situ photoluminescence, we provide clear evidence to unravel the different roles of volatile additives (NH4Cl, FACl, and MACl) and MA-based additives (MACl, MABr, and MAI) in the nucleation, crystallization, and phase transition of FAPbI3. Three different kinds of crystallization routes are proposed based on the above additives. The non-MA volatile additives (NH4Cl and FACl) were found to promote crystallization and lower the phase-transition temperatures. The MA-based additives could quickly induce MA-rich nuclei to form pure α-phase FAPbI3 and dramatically reduce phase-transition temperatures. Furthermore, volatile MACl provides a unique effect on promoting the growth of secondary crystallization during annealing. The optimized solar cells with MACl can achieve an efficiency of 23.1%, which is the highest in inverted FAPbI3-based PSCs.

Dimer Acceptor Adopting a Flexible Linker for Efficient and Durable Organic Solar Cells.

Organic solar cells (OSCs) have advanced rapidly due to the development of new photovoltaic materials. However, the long-term stability of OSCs still poses a severe challenge for their commercial deployment. To address this issue, a dimer acceptor (dT9TBO) with flexible linker is developed for incorporation into small-molecule acceptors to form molecular alloy with enhanced intermolecular packing and suppressed molecular diffusion to stabilize active layer morphology. Consequently, the PM6 : Y6 : dT9TBO-based device displays an improved power conversion efficiency (PCE) of 18.41 % with excellent thermal stability and negligible decay after being aged at 65 °C for 1800 h. Moreover, the PM6 : Y6 : dT9TBO-based flexible OSC also exhibits excellent mechanical durability, maintaining 95 % of its initial PCE after being bended repetitively for 1500 cycles. This work provides a simple and effective way to fine-tune the molecular packing with stabilized morphology to overcome the trade-off between OSC efficiency and stability.

π-Expanded Carbazoles as Hole-Selective Self-Assembled Monolayers for High-Performance Perovskite Solar Cells.

Carbazole-derived self-assembled monolayers (SAMs) are promising hole-selective materials for inverted perovskite solar cells (PSCs). However, they often possess small dipoles which prohibit them from effectively modulating the workfunction of ITO substrate, limiting the PSC photovoltage. Moreover, their properties can be drastically affected by even subtle structural modifications, undermining the final PSC performance. Here, we designed two carbazole-derived SAMs, CbzPh and CbzNaph through asymmetric or helical π-expansion for improved molecular dipole moment and strengthened π-π interaction. The helical π-expanded CbzNaph has the largest dipole, forming densely packed and ordered monolayer, facilitated by the highly ordered assembly observed in its π-scaffold's single crystal. These synergistically modulate the perovskite crystallization atop and tune the ITO workfunction. Consequently, the champion PSC employing CbzNaph showed an excellent 24.1 % efficiency and improved stability.

Importance of structural hinderance in performance-stability equilibrium of organic photovoltaics.

Power conversion efficiency and long-term stability are two critical metrics for evaluating the commercial potential of organic photovoltaics. Although the field has witnessed a rapid progress of efficiency towards 19%, the intrinsic trade-off between efficiency and stability is still a challenging issue for bulk-heterojunction cells due to the very delicate crystallization dynamics of organic species. Herein, we developed a class of non-fullerene acceptors with varied side groups as an alternative to aliphatic chains. Among them, the acceptors with conjugated side groups show larger side-group torsion and more twisted backbone, however, they can deliver an efficiency as high as 18.3% in xylene-processed cells, which is among the highest values reported for non-halogenated solvent processed cells. Meanwhile, decent thermal/photo stability is realized for these acceptors containing conjugated side groups. Through the investigation of the geometry-performance-stability relationship, we highlight the importance of side-group steric hinderance of acceptors in achieving combined high-performance, stable, and eco-friendly organic photovoltaics.

Renewed Prospects for Organic Photovoltaics.

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (Eloss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (Y6 = 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]-thiadiazolo[3,4-e]-thieno[2″,3″:4',5']thieno-[2',3':4,5]pyrrolo-[3,2-g]thieno-[2',3':4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller Eloss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

Intramolecular Chloro-Sulfur Interaction and Asymmetric Side-Chain Isomerization to Balance Crystallinity and Miscibility in All-Small-Molecule Solar Cells.

Intramolecular Cl-S non-covalent interaction is introduced to modify molecular backbone of a benzodithiophene terthiophene rhodamine (BTR) benchmark structure, helping planarize and rigidify the molecular framework for improving charge transport. Theoretical simulations and temperature-variable NMR experiments clearly validate the existence of Cl-S non-covalent interaction in two designed chlorinated donors and explain its important role in enhancing planarity and rigidity of the molecules for enhancing their crystallinity. The asymmetric isomerization of side-chains further optimizes the molecular orientation and surface energy to strike a balance between its crystallinity and miscibility. This carefully manipulated molecular design helps result in increased carrier mobility and suppressed charge recombination to obtain simultaneously enhanced short-circuit current (Jsc ) and fill factor (FF) and a very high efficiency of 15.73 % in binary all-small-molecule organic solar cells.