In article number 2100839, Jafar I. Khan, Frédéric Laquai and co-workers report on the impact of the energetic landscape of non-fullerene acceptors on charge generation and recombination in organic bulk heterojunction solar cells. The acceptor's quadrupole moment plays a key role in determining the device efficiency and thus is an important material parameter in the design of novel non-fullerene acceptor molecules.
Additive-induced synergies of defect passivation and energetic modification in perovskite solar cells are investigated, which boost power conversion efficiency and stability of the devices.
Defect passivation via additive and energetic modification via interface engineering are two effective strategies for achieving high-performance perovskite solar cells (PSCs). Here, the synergies of pentafluorophenyl acrylate when used as additive, in which it not only passivates surface defect states but also simultaneously modifies the energetics at the perovskite/Spiro-OMeTAD interface to promote charge transport, are shown. The additive-induced synergy effect significantly suppresses both defect-assisted recombination and interface carrier recombination, resulting in a device efficiency of 22.42% and an open-circuit voltage of 1.193 V with excellent device stability. The two photovoltaic parameters are among the highest values for polycrystalline CsFormamidinium/Methylammonium (FAMA)/FAMA based n-i-p structural PSCs using low-cost silver electrodes reported to date. The findings provide a promising approach by choosing the dual functional additive to enhance efficiency and stability of PSCs.
The concept of a ternary-cation two-step sequential deposition method by incorporating cesium acetate (CsAc) into a lead iodide precursor is put forward, which generates cesium lead iodide (CsPbI3) crystal nuclei. When an organic amine salts solution spin coats the substate, the acetate moves upward and induces perovskite orientational and uniform crystallization achieving fewer defects and higher photovoltaic efficiency.
State-of-the-art, high-performance formamidinium-lead-iodide-based (FAPbI3-based) perovskite photovoltaics are mainly prepared by one-step antisolvent dripping deposition or two-step sequential fabrication methods. Compared with the one-step deposition, the two-step fabricated perovskite films tend to grow columnar perovskite grains vertically which is easier for carrier extraction and transportation. Herein, the concept of formamidinium methylammonium cesium based ternary-cation two-step sequential deposition method is put forward by incorporating cesium acetate (CsAc) into a lead iodide precursor, which generates CsPbI3 crystal nuclei improving the further perovskite crystallization. When the formamidinium/methylammonium-based organic amine salts solution is spin coated on the PbI2 substrate, the acetate moves upward and induces perovskite orientational and uniform crystallization, which can go a step further for the vertical columnar grains achieving fewer defects and higher photovoltaic efficiency. The champion outdoor power conversion efficiency of the modified device under AM 1.5G reaches 21.50% and its indoor efficiency at 1000 lux reaches 40.99%. This work paves the way for further exploring ternary-cation two-step sequential deposition methods to prepare high-performance perovskite photovoltaics.
Charge dynamics and energy loss (E loss) in ternary organic solar cell (OSCs) with an impressive fill factor (FF) of 80.88% are thoroughly investigated by transient characterization techniques, which is expect to aid development of high-FF and low-E loss ternary OSCs.
Ternary architecture is a promising strategy to enhance power conversion efficiencies (PCEs) of organic solar cells (OSCs). However, among all the photovoltaic parameters that govern the final PCEs, the fill factor (FF) for ternary OSCs is generally below 78%, limiting solar cells’ performance. Here, charge dynamics in the ternary cells PM6:DRTB-T-C4:Y6 with a FF of 80.88% and a PCE of 17.05% are thoroughly investigated by a series of transient characterization technologies, including transient absorption spectroscopy, transient photovoltage, and transient photocurrent measurements. The impressive FF results from effective exciton dissociation, enhanced charge transport and suppressed recombination in ternary cells. Moreover, the correlation between the measured FF and the charge recombination-extraction competition is quantitatively analyzed by using a circuit model. The ternary cells also show small energy loss (E loss). The findings here provide insight into achieving high-FF and low-E loss ternary OSCs.
Molecular hole-transporting materials with different anchoring groups are synthesized. The anchoring groups with a stronger bonding strength enable greatly enhanced compactness of self-assembly monolayer, which benefits hole-extraction and electron-blocking in complete devices. When applied in inverted perovskite solar cells, 1 cm2 devices show a promising power conversion efficiency of over 20% with high stability.
Anchoring-based self-assembly (ASA) has emerged as a material-saving and highly scalable strategy to fabricate charge-transporting monolayers for perovskite solar cells (PSCs). However, the interfacial hole-extraction and electron-blocking performances are highly dependent on the compactness of the ASA monolayers, which has been largely ignored though it is very crucial to the efficiency and stability of PSCs. Here, strategically designed hole-transporting molecules with different anchoring groups are incorporated to investigate the effect of bonding strength on monolayer quality and correlate these with the performance of p-i-n structured PSCs. It is unraveled that the anchoring groups with a stronger bonding strength are advantageous for improving the assembly rate, density, and compactness of ASA monolayer, thus enhancing charge collection and suppressing interfacial recombination. The prototypical PSCs based on optimal ASA monolayer achieve a high power conversion efficiency (PCE) of 21.43% (0.09 cm2). More encouragingly, when enlarging the device area by tenfold, a comparable PCE of 20.09% (1.0 cm2) can be obtained, suggesting that the ASA strategy is practically useful for scaling-up. The robust anchoring of the ASA monolayer also enhances devices stability, retaining 90% of initial PCE after three months. This study provides important insights into the ASA charge-transporting monolayers for efficient and stable PSCs.
This study develops a universal bottom interface modification method with diverse 2D spacers, which significantly enhance the device performance of inverted perovskite solar cells from 20.7% to 21.6%. The lift-off method is used to directly study the change of optoelectronic properties at the bottom interface and unveils the formation of 2D/3D heterojunction as the general mechanism underlying the device performance enhancement.
Although the 2D spacer modification is widely studied in perovskite solar cells (PVSCs), the energy level alignment between the 2D/3D interfaces makes it unfavorable for top surface passivation in the inverted p-i-n device structure. To address this issue, the effect of bottom interface modification is studied with three representative 2D spacers, i.e., the Ruddlesden-Popper 2D spacer, Dion-Jacobson 2D spacer, and strong passivation 2D spacer, in inverted p-i-n PVSCs. After optimization, the PVSCs with these 2D spacer modifications universally exhibit the best efficiencies of ≈21.6%, which constitutes dramatic improvement compared to the control device (20.7%). By lifting off the perovskite layer, the optoelectronic properties of the bottom surface are studied, and the mechanism underlying the improved device performance is unveiled to be uniformly originated from the formation of 2D/3D heterojunction, where the cascade valence band facilitates the hole collection and electron back scattering field suppresses the charge recombination at the anode interface. Besides, the unencapsulated device retains 90% of initial efficiency after 30 days of storage in ambient air with a relative humidity of 30 ± 5%, indicating excellent stability against moisture and oxygen. This study provides insight into the bottom interface modification with diverse 2D spacers for high-performance p-i-n structured PVSC devices.
Replacing one of the chlorinated terminal groups with norbornyl-modified one endows the reported asymmetric acceptors with not only enhanced solubility but also more favorable morphology and higher EQEEL in the blend with PBDB-T. A power conversion efficiency of 16.82% with a J sc over 26.5 mA cm−2 and a ΔV nr of 0.18 V are realized, representing the state-of-the-art in PBDB-T based organic solar cells.
Three asymmetric non-fullerene acceptors (LL2, LL3, and LL4) are designed and synthesized with one norbornyl-modified 1,1-dicyanomethylene-3-indanone (CBIC) terminal group and one chlorinated 1,1-dicyanomethylene-3-indanone (IC-2Cl) terminal group. The three-dimensional shape-persistent CBIC terminal group can effectively enhance the solubility and tune the packing mode of acceptors. Compared with their symmetric counterparts (LL2-2Cl, LL3-2Cl, and LL4-2Cl) bearing two IC-2Cl terminals, the asymmetric acceptors show improved solubilities, giving rise to enhanced crystallinity and favored nanomorphology for charge transport in the blend films with PBDB-T. Asymmetric acceptors based organic solar cells (OSCs) also show much lower voltage loss due to their higher E CT and EQEEL values. Therefore, they exhibit 17−27% higher power conversion efficiency (PCE) than OSCs based on the corresponding symmetric acceptors. Among these six acceptors, LL3 with a central benzotriazole core shows the best PCE of 16.82% with an outstanding J sc of 26.97 mA cm−2 and a low nonradiative voltage loss (ΔV nr) of 0.18 V, the best values for PBDB-T based OSCs. The J sc and ΔV nr also represent the best reported for asymmetric non-fullerene acceptors-based OSCs to date. The results demonstrate that the combination of the unique CBIC terminal group with the asymmetric strategy is a promising way to enhance the performance of OSCs.
A bifunctional SnO2 colloid is developed using small molecular oxalate. The resultant SnO2 films show “no annealing effect”, contributing to stabilized PCEs of 22.40% and 22.37% for high temperature process (HTP) SnO2 planar and mesoporous PSCs, respectively. The high stability of HTP SnO2 PSCs may ascribe to low oxygen vacancy and adsorbed water of HTP SnO2.
SnO2 compact layer (c-SnO2) frequently suffers from degradation in high temperature processes (HTP) such as crack, worse interfacial contact, and electrical properties, that is, annealing effect. To solve this problem, a kind of bifunctional SnO2 colloid is developed by using small molecular oxalate whose organic components can be removed clearly at a low temperature process (LTP). The c-SnO2 and SnO2 mesoporous layer (m-SnO2) derived from the fresh and aged sols with the same colloid show no annealing effect, decreasing oxygen vacancy, and adsorbing water on increasing annealing temperature. The champion devices of LTP and HTP SnO2 planar perovskite solar cells (PSCs) achieve, respectively, stabilized photoelectric conversion efficiencies (PCEs) of 20.74% and 20.70%. In contrast, the performance of champion devices of their mesoporous counterparts is significantly improved, showing nearly hysteresis free character with stabilized PCEs of 22.40% and 22.37%, respectively. The inclusion of m-SnO2 plays a role of an energy bridge, improving electrons collection efficiency, which is supported by photoluminescence and transient photoluminescence characterizations. HTP SnO2 mesoporous PSCs can preserve 97.6% and 80% of their initial PCEs after aging for 25 weeks and 8-h irradiated/16-h dark cycle within 104 h. The high stability of HTP SnO2 PSCs may ascribe to low oxygen vacancy and adsorbed water of HTP SnO2.
Polar branched oligo(ether) side-chain functionalized dioxythiophene-based conjugated polymers provide a pathway to achieve benign solvent processing, aqueous compatibility during redox switching, and high solid-state conductivity (430 ± 60 S cm−1) after oxidative doping. The first organic electrochemical transistor fabricated from an acetone-processed conjugated and electroactive polymer is reported, which exhibits stable switching response up to 500 cycles.
Commercialization of stable conjugated polymers (CPs) with tunable electronic properties will remain a challenge without adequate solution processability due to the importance of techniques such as roll-to-roll manufacturing. Consequently, modifying CP backbones with polar side chains has recently resurged as an attractive structural design approach to improve polymer solubility and to provide CPs with the capability of transporting both electrons and ions, which is crucial for applications such as organic electrochemical transistors (OECTs). Here, a new dioxythiophene copolymer comprised of 2,2'-bis-(3,4-ethylenedioxy)thiophene (biEDOT) and 3,4-propylenedioxythiophene (ProDOT) substituted with branched oligo(ether) side chains (PE2-biOE2OE3) is synthesized using two direct hereto(arylation) polymerization (DHAP) techniques. The typical DHAP technique results in a lower molecular weight polymer (PE2-biOE2OE3(L)), which is soluble in acetone and demonstrated a solid-state conductivity after oxidative doping of 55 ± 3 S cm−1. Alternatively, a unique temperature ramp DHAP methodology results in a higher molecular weight polymer (PE2-biOE2OE3(H)) with an especially high solid-state conductivity of 430 ± 60 S cm−1. Notably, the first OECT fabricated from an acetone-processed polymer is reported, which is stable up to 500 cycles and can provide a pathway for future material design aimed at eliminating the use of toxic chlorinated solvents in OECT active layer processing.
An intrinsically stretchable organic solar cell (OSC) with an efficiency of over 10% is achieved by the transfer printing method. The ductility of bulk heterojunction film is greatly improved to 20% by introducing polydimethylsiloxane additives, and intimated multilayer stacking is realized with the assistance of electrical adhesive D-Sorbitol. The stretchable OSC exhibits ultra-flexibility and superior stretchability without sacrificing the device performance.
Stretchable organic solar cells (OSCs) simultaneously possessing high-efficiency and robust mechanical properties are ideal power generators for the emerging wearable and portable electronics. Herein, after incorporating a low amount of trimethylsiloxy terminated polydimethylsiloxane (PDMS) additive, the intrinsic stretchability of PTB7-Th:IEICO-4F bulk heterojunction (BHJ) film is greatly improved from 5% to 20% strain without sacrificing the photovoltaic performance. The intimate multi-layers stacking of OSCs is also realized with the transfer printing method assisted by electrical adhesive “glue” D-Sorbitol. The resultant devices with 84% electrode transmittance exhibit a remarkable power conversion efficiency (PCE) of 10.1%, which is among the highest efficiency for intrinsically stretchable OSCs to date. The stretchable OSCs also demonstrate the ultra-flexibility, stretchability, and mechanical robustness, which keep the PCE almost unchanged at small bending radium of 2 mm for 300 times bending cycles and retain 86.7% PCE under tensile strain as large as 20% for the devices with 70% electrode transmittance. The results provide a universal method to fabricate highly efficient intrinsically stretchable OSCs.
Materials and device architectures for wavelength-selective organic photodetectors spanning the visible to the near-infrared wavelength range are discussed. Insights in their influence on the main performance parameters such as specific detectivity (D*) and dark current (J d) are provided, together with potential applications and an outlook to extend the employability of these devices.
Spectroscopic sensing combined with optical imaging is crucial with respect to today's ever-growing demand for instant analytical techniques to be incorporated in various handheld and wearable devices. Further miniaturization and integration of such types of sensors is critical and wavelength-selective organic photodetectors (OPDs) may provide the required technology. In this progress report, some early OPD applications and their potential are presented. Crucial device parameters such as the specific detectivity, external quantum efficiency, and dark current density of visible and near-infrared wavelength-selective photodetectors are compared and assayed to theoretical and semi-empirical limits. The different organic detector approaches include the use of inherently narrow-band absorbers as well as internally filtered and microcavity devices. Each of these strategies comes with its own specific material and device design criteria, around which material development and selection should be centered to move beyond the current state of the art. As OPD technology matures, device stability becomes important and is hence also briefly discussed. Via this perspective, it is aimed to provide the reader with critical insights into the device physics and chemistry of wavelength-selective OPDs, hereby providing leverage for new ideas to bring this technology to the market.
Two configurational controlled polymers are reported here. The γ-position based polymer exhibits good solubility, broadened UV absorption, and enhanced charge mobility, while the δ-position based polymer shows excessive aggregation and is difficult to process in solution. When blended with PM6, PBTIC-γ-2F2T achieves excellent device performance with a PCE of 14.32%, but the PBTIC-δ-2F2T delivers a PCE of almost zero (0.02%).
The design of polymer acceptors plays an essential role in the performance of all-polymer solar cells. Recently, the strategy of polymerized small molecules has achieved great success, but most polymers are synthesized from the mixed monomers, which seriously affects batch-to-batch reproducibility. Here, a method to separate γ-Br-IC or δ-Br-IC in gram scale and apply the strategy of monomer configurational control in which two isomeric polymeric acceptors (PBTIC-γ-2F2T and PBTIC-δ-2F2T) are produced is reported. As a comparison, PBTIC-m-2F2T from the mixed monomers is also synthesized. The γ-position based polymer (PBTIC-γ-2F2T) shows good solubility and achieves the best power conversion efficiency of 14.34% with a high open-circuit voltage of 0.95 V when blended with PM6, which is among the highest values recorded to date, while the δ-position based isomer (PBTIC-δ-2F2T) is insoluble and cannot be processed after parallel polymerization. The mixed-isomers based polymer, PBTIC-m-2F2T, shows better processing capability but has a low efficiency of 3.26%. Further investigation shows that precise control of configuration helps to improve the regularity of the polymer chain and reduce the π–π stacking distance. These results demonstrate that the configurational control affords a promising strategy to achieve high-performance polymer acceptors.
![Asymmetric Isomer Effects in Benzo[c ][1,2,5]thiadiazole-Fused Nonacyclic Acceptors: Dielectric Constant and Molecular Crystallinity Control for Significant Photovoltaic Performance Enhancement](https://onlinelibrary.wiley.com/cms/asset/e9cdae76-feb6-40cd-be0d-634e232f473d/adfm202104369-gra-0001-m.png)
Asymmetric isomerization from BP6T-4F to ABP6T-4F not only lowers the exciton bonding energy but also optimizes the crystallization performance, achieving a pronounced isomer effect with 9.4% power conversion efficiency enhancement. Moreover, ternary devices are also fabricated, considering good compatibility between ABP6T-4F and CH1007, to deliver a power conversion efficiency over 17%.
Herein, asymmetric isomer effects are systematically explored by designing and synthesizing two benzo[c][1,2,5]thiadiazole (BT)-fused nonacyclic electron acceptors. By changing from BP6T-4F to asymmetric ABP6T-4F, significantly enhanced dielectric constant and inhibited excessive molecular aggregation and unfavorable edge-on orientation could be achieved. The reduced exciton binding energy also facilitates a more efficient dissociation process in PM6:ABP6T-4F compared to PM6:BP6T-4F with the same energy offset. Moreover, the weaker crystallization behavior enables a significantly enhanced miscibility between PM6 and ABP6T-4F than that between PM6 and BP6T-4F, which leads to an optimized micromorphology with smooth surface, suitable domain size, and ordered π–π stacking. Organic solar cells (OSCs) based on PM6:ABP6T-4F achieve a 15.8% power conversion efficiency (PCE), which is remarkably higher than that of PM6:BP6T-4F-based OSCs (6.4%). Furthermore, ternary devices are also fabricated considering good compatibility between ABP6T-4F and CH1007 to deliver a PCE over 17%. This study reveals the effectiveness and great potential of asymmetric isomerization strategy in regulating molecular properties, which will provide guidance for the future design of non-fullerene acceptors.
Perovskite solar cells (PSCs) are fabricated using a novel organic cation (TTMAI) treatment on a 3D perovskite, which enables higher power conversion efficiency (PCE) and improves stability. The PCE enhancement is explained by the drift-diffusion modeling. In addition, TTMAI-treated 3D perovskite-based semitransparent PSCs are also realized, and a notable increase in PCE and stability is obtained.
Perovskite surface treatment with additives has been reported to improve charge extraction, stability, and/or surface passivation. In this study, treatment of a 3D perovskite ((FAPbI3)1− x (MAPbBr3) x ) layer with a thienothiophene-based organic cation (TTMAI), synthesized in this work, is investigated. Detailed analyses reveal that a 2D (n = 1) or quasi-2D layer does not form on the PbI2-rich surface 3D perovskite. TTMAI-treated 3D perovskite solar cells (PSCs) fabricated in this study show improved fill factors, providing an increase in their power conversion efficiencies (PCEs) from 17% to over 20%. It is demonstrated that the enhancement is due to better hole extraction by drift-diffusion simulations. Furthermore, thanks to the hydrophobic nature of the TTMAI, PSC maintains 82% of its initial PCE under 15% humidity for over 380 h (the reference retains 38%). Additionally, semitransparent cells are demonstrated reaching 17.9% PCE with treated 3D perovskite, which is one of the highest reported efficiencies for double cationic 3D perovskites. Moreover, the semitransparent 3D PSC (TTMAI-treated) maintains 87% of its initial efficiency for six weeks (>1000 h) when kept in the dark at room temperature. These results clearly show that this study fills a critical void in perovskite research where highly efficient and stable semitransparent perovskite solar cells are scarce.
A universal morphology optimization method is developed by applying anthracene as a solid additive to improve the photovoltaic performance of polymer solar cells. Anthracene can restrict the over-aggregation of nonfullerene acceptors during the film-forming process, and then facilitate bicontinuous phase separation during the kinetic process of its removal in the blend under thermal annealing.
Currently, morphology optimization methods for the fused-ring nonfullerene acceptor-based polymer solar cells (PSCs) empirically follow the treatments originally developed in fullerene-based systems, being unable to meet the diverse molecular structures and strong crystallinity of the nonfullerene acceptors. Herein, a new and universal morphology controlling method is developed by applying volatilizable anthracene as solid additive. The strong crystallinity of anthracene offers the possibility to restrict the over aggregation of fused-ring nonfullerene acceptor in the process of film formation. During the kinetic process of anthracene removal in the blend under thermal annealing, donor can imbed into the remaining space of anthracene in the acceptor matrix to form well-developed nanoscale phase separation with bi-continuous interpenetrating networks. Consequently, the treatment of anthracene additive enables the power conversion efficiency (PCE) of PM6:Y6-based devices to 17.02%, which is a significant improvement with regard to the PCE of 15.60% for the reference device using conventional treatments. Moreover, this morphology controlling method exhibits general application in various active layer systems to achieve better photovoltaic performance. Particularly, a remarkable PCE of 17.51% is achieved in the ternary PTQ10:Y6:PC71BM-based PSCs processed by anthracene additive. The morphology optimization strategy established in this work can offer unprecedented opportunities to build state-of-the-art PSCs.
Four tetrathiophene-based fully non-fused acceptors are obtained via simple syntheses. The side chain selection is crucially important to the corresponding solubility, absorption, packing mode etc. The four 2-ethylhexyl functionalized 4T-3 achieves a champion power conversion efficiency of 12.04% with an excellent figure-of-merit of 32.8, which is the highest value among the reported acceptors. Such cost-effective strategy paves new way for future commercial applications.
A series of tetrathiophene-based fully non-fused ring acceptors (4T-1, 4T-2, 4T-3, and 4T-4), which can be paired with the star donor polymer PBDB-T to fabricate highly efficient organic solar cells are developed. Tailoring the size of lateral chains can tune the solubility and packing mode of acceptor molecules in neat and blend films. It is found that the incorporation of 2-ethylhexyl chains can effectively change the compatibility with the donor polymer PBDB-T, and an encouraging power conversion efficiency of 10.15% is accomplished by 4T-3-based organic solar cells. It also presents good compatibility with the other polymer donor and an even higher power conversion efficiency (PCE) of 12.04% is achieved based on D18:4T-3 blend, which is the champion PCE for the fully non-fused acceptors. Importantly, these inexpensive tetrathiophene fully non-fused ring acceptors provide cost-effective photovoltaic performance. The results demonstrate a high photovoltaic performance from synthetically inexpensive materials could be achieved by the rational design of non-fused ring acceptor molecules.
An efficient hybrid quantum dot (QD)/organic film is demonstrated, which involves emerging CsPbI3 perovskite QDs and Y6 series non-fullerene molecules. Consequently, the CsPbI3 QD/Y6 hybrid solar cells (HSCs) deliver a champion power conversion efficiency of 15.05%, which is one of the highest reports among QD/organic HSCs.
Organic-inorganic hybrid film using conjugated materials and quantum dots (QDs) are of great interest for solution-processed optoelectronic devices, including photovoltaics (PVs). However, it is still challenging to fabricate conductive hybrid films to maximize their PV performance. Herein, for the first time, superior PV performance of hybrid solar cells consisting of CsPbI3 perovskite QDs and Y6 series non-fullerene molecules is demonstrated and further highlights their importance on hybrid device design. In specific, a hybrid active layer is developed using CsPbI3 QDs and non-fullerene molecules, enabling a type-II energy alignment for efficient charge transfer and extraction. Additionally, the non-fullerene molecules can well passivate the QDs, reducing surface defects and energetic disorder. The champion CsPbI3 QD/Y6-F hybrid device has a record-high efficiency of 15.05% for QD/organic hybrid PV devices, paving a new way to construct solution-processable hybrid film for efficient optoelectronic devices.
An effective strategy to simultaneously obtain high photocurrent and fill factor in tandem organic solar cells is presented. By increasing the proportion of the non-fullerene acceptor with strong absorption in the front sub-cell, maximum photocurrent can be obtained without significantly increasing the thickness of the front sub-cell, thus ensuring a high fill factor and high photocurrent in device, with a power conversion efficiency over 18%.
The maximum photocurrent in tandem organic solar cells (TOSCs) is often obtained by increasing the thicknesses of sub-cells, which leads to recombination enhancement of such devices and compromises their power conversion efficiency (PCE). In this work, an efficient interconnecting layer (ICL) is developed, with the structure ZnO NPs:PEI/PEI/PEDOT:PSS, which enables TOSCs with very good reproducibility. Then, it is discovered that the optimal thickness of the front sub-cell in such TOSCs can be reduced by increasing the proportion of a non-fullerene acceptor in the active layer. The non-fullerene acceptor used in this work has a much larger absorption coefficient than the donor in the front sub-cell, and the absorption reduction of donor can be well complemented by that of the acceptor when increasing the acceptor proportion, thus leading to a significant overall absorption enhancement even with a thinner film. As a result, the optimal thickness of the front sub-cell is reduced and its charge recombination is suppressed. Ultimately, the use of this ICL combined with fine-turning of the composition in the front sub-cell enables an efficient TOSC with a very high fill factor of 78% and an excellent PCE of 18.71% (certified by an accredited institute to be 18.09%) to be obtained.
A highly crystalline and wide bandgap electron acceptor, IDTT-M, is used to fabricate high efficiency ternary solar cells with PM6 and another narrow bandgap electron acceptor, Y6. Benefiting from efficient Förster resonance energy transfer, a significantly improved power conversion efficiency of up to 16.63% is achieved with simultaneously enhanced device characteristics.
Introducing a third component into organic bulk heterojunction solar cells has become an effective strategy to improve photovoltaic performance. Meanwhile, the rapid development of non-fullerene acceptors (NFAs) has pushed the power conversion efficiency (PCE) of organic solar cells (OSCs) to a higher standard. Herein, a series of fullerene-free ternary solar cells are fabricated based on a wide bandgap acceptor, IDTT-M, together with a wide bandgap donor polymer PM6 and a narrow bandgap NFA Y6. Insights from the morphological and electronic characterizations reveal that IDTT-M has been incorporated into Y6 domains without disrupting its molecular packing and sacrificing its electron mobility and work synergistically with Y6 to regulate the packing pattern of PM6, leading to enhanced hole mobility and suppressed recombination. IDTT-M further functions as an energy-level mediator that increases open-circuit voltage (V OC) in ternary devices. In addition, efficient Förster resonance energy transfer (FRET) between IDTT-M and Y6 provides a non-radiative pathway for facilitating exciton dissociation and charge collection. As a result, the optimized ternary device features a significantly improved PCE up to 16.63% with simultaneously enhanced short-circuit current (J SC), V OC, and fill factor (FF).
Highly efficient organic solar cells are fabricated using a ternary approach, wherein a novel non-fullerene acceptor L8-BO-F is designed and incorporated into the PM6:BTP-eC9 blend. L8-BO-F and BTP-eC9 are found to form a homogeneous mixed phase, which improves the molecular packing of both donor and acceptor materials, and optimizes the ternary blend morphology. A record-high efficiency of 18.66% is consequently achieved.
The ternary strategy, introducing a third component into a binary blend, opens a simple and promising avenue to improve the power conversion efficiency (PCE) of organic solar cells (OSCs). The judicious selection of an appropriate third component, without sacrificing the photocurrent and voltage output of the OSC, is of significant importance in ternary devices. Herein, highly efficient OSCs fabricated using a ternary approach are demonstrated, wherein a novel non-fullerene acceptor L8-BO-F is designed and incorporated into the PM6:BTP-eC9 blend. The three components show complementary absorption spectra and cascade energy alignment. L8-BO-F and BTP-eC9 are found to form a homogeneous mixed phase, which improves the molecular packing of both the donor and acceptor materials, and optimizes the ternary blend morphology. Moreover, the addition of L8-BO-F into the binary blend suppresses the non-radiative recombination, thus leading to a reduced voltage loss. Consequently, concurrent increases in open-circuit voltage, short-circuit current, and fill factor are realized, resulting in an unprecedented PCE of 18.66% (certified value of 18.2%), which represents the highest efficiency values reported for both single-junction and tandem OSCs so far.
A hydrophobic ammonium salt, 4-(trifluoromethyl) benzylamine, is introduced to form a quasi-2D hybrid perovskite by a one-step spin-coating method. Due to the relatively low surface energy of fluorinated molecules, an upper gradient low-dimensional structure is formed spontaneously from top to bottom, and more stable devices are obtained with a power conversion efficiency of 17.07%.
Highly efficient and stable quasi-2D hybrid perovskite solar cells (PSCs) using hydrophobic 4-(trifluoromethyl) benzylamine (4TFBZA) as the spacer cation are successfully demonstrated. It is found that the incorporation of hydrophobic 4TFBZA into MAPbI3 can effectively induce a spontaneous upper gradient 2D (SUG-2D) structure, passivate the trap states, and restrain the ion motion. Meanwhile, the strong hydrogen bonding of F···HN between 4TFBZA ions and methylamine ions can effectively suppress the decomposition of perovskite, which gives the device a better thermal stability. Besides, due to the SUG-2D structure with hydrophobic 4TFBZA, the device also exhibits a better moisture stability. The SUG-2D-structure-based device exhibits a power conversion efficiency of 17.07% with a high open-circuit voltage of 1.10 V and a notable fill factor of 71%. This work provides a new strategy for constructing efficient and stable quasi-2D PSCs, and it is an inspiration for the packaging strategy of perovskites.
Polyaromatic passivator 4-hydroxybiphenyl substituted naphthalene-1,8-dicarboximide provides chemical passivation (protonic/Lewis-base groups system) and energetic passivation (creating benign midgap states) effects. The Lewis-base/polyaromatic conjugation/protonic system reduces defects efficiently and avoids superoxide anions in perovskite solar cells.
As game-changers in the photovoltaic community, perovskite solar cells are making unprecedented progress while still facing grand challenges such as improving lifetime without impairing efficiency. Herein, two structurally alike polyaromatic molecules based on naphthalene-1,8-dicarboximide (NMI) and perylene-3,4-dicarboximide (PMI) with different molecular dipoles are applied to tackle this issue. Contrasting the electronically pull–pull cyanide-substituted PMI (9CN-PMI) with only Lewis-base groups, the push–pull 4-hydroxybiphenyl-substituted NMI (4OH-NMI) with both protonic and Lewis-base groups can provide better chemical passivation for both shallow- and deep-level defects. Moreover, combined theoretical and experimental studies show that the 4OH-NMI can bind more firmly with perovskite and the polyaromatic backbones create benign midgap states in the excited perovskite to suppress the damage by superoxide anions (energetic passivation). The polar and protonic nature of 4OH-NMI facilitates band alignment and regulates the viscosity of the precursor solution for thicker perovskite films with better morphology. Consequently, the 4OH-NMI-passivated perovskite films exhibit reduced grain boundaries and nearly three-times lower defect density, boosting the device efficiency to 23.7%. A more effective design of the passivator for perovskites with multi-passivation mechanisms is provided in this study.
Two narrow-bandgap block copolymers PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6 are designed and synthesized for single-component polymer solar cells, and a record-high efficiency of 8.64% is obtained. Moreover, these block copolymers exhibit relatively small energy loss and improved storage stability under both ambient condition and continued 80 °C thermal stresses for over 1000 h.
Two narrow-bandgap block conjugated polymers with a (D1–A1)–(D2–A2) backbone architecture, namely PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6, are designed and synthesized for single-component organic solar cells (SCOSCs). Both polymers contain same donor polymer, PBDB-T, but different polymerized nonfullerene molecule acceptors. Compared to all previously reported materials for SCOSCs, PBDB-T-b-PIDIC2T and PBDB-T-b-PTY6 exhibit narrower bandgap for better light harvesting. When incorporated into SCOSCs, the short-circuit current density (J sc) is significantly improved to over 15 mA cm−2, together with a record-high power conversion efficiency (PCE) of 8.64%. Moreover, these block copolymers exhibit low energy loss due to high charge transfer (CT) states (E ct) plus small non-radiative loss (0.26 eV), and improved stability under both ambient condition and continuous 80 °C thermal stresses for over 1000 h. Determination of the charge carrier dynamics and film morphology in these SCOSCs reveals increased carrier recombination, relative to binary bulk-heterojunction devices, which is mainly due to reduced ordering of both donor and acceptor fragments. The close structural relationship between block polymers and their binary counterparts also provides an excellent framework to explore further molecular features that impact the photovoltaic performance and boost the state-of-the-art efficiency of SCOSCs.
![Triisopropylsilyl-Substituted Benzo[1,2-b:4,5-c′]dithiophene-4,8-dione-Containing Copolymers with More Than 17% Efficiency in Organic Solar Cells](https://onlinelibrary.wiley.com/cms/asset/7c36539f-b8a2-469f-a541-5d30eb131548/adfm202102371-gra-0001-m.png)
Copolymer series with varying contents of triisopropylsilyl-substituted benzo[1,2-b:4,5-c′]dithiophene-4,8-dione are synthesized and characterized. Using them as donors for bulk-heterojunction organic solar cells, a high power conversion efficiency of 17.01% is achieved from optimal composition of monomers with balanced charge transport, enhanced charge generation/dissociation kinetics, and minimized total energy and recombination losses.
Considering the special functions of fused-ring aromatic building blocks and Si-atom in high-performance donor–acceptor-conjugated materials at the same time, herein the synthesis of a novel fused-ring tricyclic heterocycle, triisopropylsilyl-substituted benzo[1,2-b:4,5-c′]dithiophene-4,8-dione (iBDD-Si), an isomer of well-known benzo[1,2-c:4,5-c′]dithiophene-4,8-dione is presented. The iBDD-Si-based copolymer series (PM6, PM6-5Si, PM6-10Si, and PM6-15Si) is synthesized via Stille polymerization, revealing fine-tuned optical and electrochemical properties, and molecular packing with varying iBDD-Si contents in the backbone. Organic solar cells are fabricated by pairing the copolymer donors with nonfullerene acceptor N3 and characterized. High power conversion efficiency of more than 17% is achieved using the PM6-5Si-based solar cell, which is attributed to the balanced charge transport, enhanced charge generation/dissociation kinetics, and minimized total energy and recombination losses. It is demonstrated that iBDD-Si is a promising backbone toolbox for various high-performance conjugated materials.
A dopant-free small molecule hole-transporting material (HTM), SFDT-TDM, is designed and synthesized through facile routes and applied in perovskite solar cells (PVSCs). Remarkable efficiencies of 21.7% for Methylammonium (MA)-free PVSCs and 17.1% for all-inorganic PVSCs are realized, and a 1 cm2 MA-free device achieves a high efficiency of 20.3%. The intrinsic hydrophobicity and dopant-free design of SFDT-TDM also enables the enhancement of device stability.
Developing low-cost, efficient, and stable dopant-free hole-transporting materials (HTMs) in perovskite solar cells (PVSCs) is essential to their commercial deployment. Herein, the synthesis of a novel spirofluorene-dithiolane based small molecular HTM, SFDT-TDM, through facile and low-cost synthetic routes is reported. The CH…π interactions in adjacent SFDT-TDM are beneficial for high hole mobility and the methylthio groups in SFDT-TDM can serve as Lewis bases to passivate the defects on the surface of perovskite films, leading to suppressed non-radiative recombination and enhanced charge extraction at the perovskite/HTM interface. As a result, Cs x FA1− x PbI3 based PVSCs with SFDT-TDM as the HTM realize champion power conversion efficiencies (PCEs) of 21.7% and 20.3% for small-area (0.04 cm2) and large-area (1.0 cm2) devices with negligible photocurrent hysteresis, respectively. Additionally, all-inorganic CsPbI3− x Br x based PVSCs with SFDT-TDM demonstrate an impressive PCE of 17.1% along with excellent stability. This work highlights the great potential of the spirofluorene core for exploring low-cost and dopant-free HTMs for PVSCs with high efficiency and stability.
Direct electrical property regulation of organic semiconductors by heterojunction molecular doping is applied to improve photovoltaic performance in planar heterojunction model devices. The upward charge-transfer state movement and reduced non-radiative recombination increase V oc. The significant 20% J sc increment is ascribed to the doping modified electrostatic field and the entropy-related activation process at organic heterojunctions.
The electron donor/acceptor (D/A) heterojunction is the core for photocharge generation and recombination in organic photovoltaics (OPVs). Developing practical methods for the D/A heterojunction modification remains challenging and is rarely discussed in OPV research. Herein, the roles of molecular doping at the D/A heterojunction in the charge-transfer exciton dissociation and detailed energy loss are investigated, and new insights are gained into the functions of doping on the OPV performance. Heterojunction doping simultaneously enhances all three OPV parameters, especially the short-circuit current (J sc). It is shown that the J sc improvement is due to the combined effects of strengthened electric field and reduced activation energy, which is regulated via an entropy-related mechanism. The performance enhancement is further demonstrated in homojunction devices showing the great potential of interfacial doping to overcome the intrinsic limitation between high J sc and open-circuit voltage (V oc) in OPVs.
Two new small-molecule acceptors with different bandgaps are designed and synthesized for application in front and rear cells in tandem organic solar cells (OSCs) processed by non-halogenated solvents. When cooperating with appropriate polymer donors, the tandem OSCs processed by non-halogenated solvents demonstrate a power conversion efficiency of 16.67%.
Organic solar cells (OSCs) have recently reached a remarkably high efficiency and become a promising technology for commercial application. However, OSCs with top efficiency are mostly processed by halogenated solvents and with additives that are not environmentally friendly, which hinders large-scale manufacture. In this study, high-performance tandem OSCs, based on polymer donors and two small-molecule acceptors with different bandgaps, are fabricated by solution processing with non-halogenated solvents without additive. Importantly, the two active layers developed from non-halogenated solvents show better phase segregation and charge transport properties, leading to superior performance than halogenated ones. As a result, a tandem OSC with high efficiency of up to 16.67% is obtained, showing unique advantages in future massive production.
A mold-assisted decal-coating process is developed by controlling the surface energy of polymer molds by calculating for adhesion based on a wetting coefficient. The mold-assisted decal-coating contributes to higher performance of both photovoltaic device and photodetector than that of the spin-coated device. The decal-coated device reveals a favorable charge transfer property and suppressed morphological change over time due to morphology inversion and stabilization.
In this study, a promising film formation technique is highlighted, named mold-assisted decal-coating, as a thin film transfer printing process using the polyurethane acrylate-based stamping mold. By optimizing the surface energy of the mold with wetting coefficient theory, the mold-assisted decal-coating process is successfully demonstrated by transferring the photoactive layer composed of the polymer donor, poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] and a narrow bandgap non-fullerene acceptor (NFA), 2,2′-[[4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl]bis[[4-[(2-ethylhexyl)oxy]-5,2-thiophenediyl]methylidyne(5,6-difluoro-3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile]. This process induces a well-ordered morphology of photoactive material, prevents damage to the underlying layer by suppressing the solvent penetration. Both photovoltaic cells and photodetectors prepared by the decal-coated photoactive layers containing fluorinated NFAs showed higher performance (power conversion efficiency = 10.69% and specific detectivity = 1.27 × 1012 A cm Hz1/2 W−1, respectively) than those of cells prepared by the spin-coating method owing to morphology inversion and smoother interface that led to suppressed internal resistance and enhanced charge flow in normal structure. Thus, the reproducible decal-coating process using a customized elastomeric mediator is an important thin film coating technique for efficient next-generation organic optoelectronic materials.
In the PM6:Y6 blend, the strong intermolecular interaction and the formation of the delocalized excited state in acceptor Y6 are favorable for rapid exciton migration and hole transfer at donor/acceptor interfaces, thus resulting in a considerably high hole transfer efficiency of 71.4%. While the transfer efficiency only reaches 13.1% in PM6:3,9-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) blend, due to the weak intermolecular π–π interaction in the ITIC component.
Organic solar cells (OSCs) based on small molecular acceptors (SMAs) have made great development with a power conversion efficiency (PCE) over 16% due to the design of novel materials and advances in device preparation technology. This work fabricates two bulk-heterojunction photovoltaic devices containing the same wide-bandgap donor PM6, respectively, matched with popular Y6 and ITIC SMAs. The PM6:Y6-based device achieves a much higher PCE of 15.21% than the PM6:ITIC-based device of 9.02%. On the basis of comparisons of macroscopic performances in the quasistatic regime, transient absorption spectroscopy (TAS) is further performed to better understand the microscopic dynamics difference in charge separation processes between the two photovoltaic blends. According to the TAS results, the calculated hole transfer efficiency in PM6:Y6 is 71.4%, far greater than the efficiency of 13.1% in PM6:ITIC, demonstrating favorable charge separation at donor/acceptor interfaces via hole transfer channel in PM6:Y6. The favorable hole transfer in PM6:Y6 is accounted for by its better mutual miscibility between the donor and acceptor, and the formation of long-lived delocalized intramoiety excimer state in the acceptor. These results highlight the important role of proper molecular design strategy with strong intermolecular coupling and beneficial film morphology on facilitating charge generation in OSCs adopting SMAs.