High-Reproducibility Layer-by-Layer Non-Fullerene Organic Photovoltaics with 19.18% Efficiency Enabled by Vacuum-Assisted Molecular Drift Treatment

The thin film deposition engineering of layer-by-layer (LbL) non-fullerene organic solar cells (OSCs) favors vertical phase distributions of donor:acceptor (D:A), effectively boosting the power conversion efficiency (PCE). However, previous deposition strategies mainly aimed at optimizing the morphology of LbL films, and paid limited attention to the reproducibility of device performance. To achieve high device performance and maintain reproducibility, a strategy for hierarchical morphology manipulation in LbL OSCs is developed. A series of LbL devices are fabricated by introducing vacuum-assisted molecular drift treatment (VMDT) to the donor or acceptor layer individually or simultaneously to elucidate the functionalities of this treatment. Essentially, the VMDT provides an extended drift driving force to manipulate the donor and acceptor molecules, resulting in a well-defined vertical phase distribution and ordered molecular packing. These enhancements facilitate improvement in the D:A interface area and charge transport channel, ultimately contributing to impressive PCEs of 19.18% from 18.27% in the LbL devices. More importantly, using VMDT overcomes the notorious batch-dependent and heat treatment degradation issues of OSCs, leading to excellent batch-to-batch reproducibility and enhanced stability of the devices. This reported method provides a promising strategy available for industrial and laboratory use to controllably manipulate the morphology of LbL OSCs.     

Additive‐Free Organic Solar Cells with Power Conversion Efficiency over 10%

Nowadays, solvent additives are widely used in organic solar cells (OSCs) to tune the nano‐morphology of the active blend film and enhance the device performance. With their help, power conversion efficiencies (PCEs) of OSCs have recently stepped over 10%. However, residual additive in the device can induce undesirable morphological change and also accelerate photo‐oxidation degradation of the active blend film. Thereby, their involvements are actually unfavorable for practical applications. Here, a donor material PThBDTP is employed, and PThBDTP:PC71BM based OSCs are fabricated. A PCE of over 10% is achieved without using any additives and film post‐treatments. The device displays a high open‐circuit voltage of 0.977 V, a large short‐circuit current density of 13.49 mA cm^‐2, and a high fill factor of 76.3%. These results represent an important step towards developing high‐efficiency additive‐free OSCs.     

Efficient Organic Solar Cell with 16.88% Efficiency Enabled by Refined Acceptor Crystallization and Morphology with Improved Charge Transfer and Transport Properties

Single‐layered organic solar cells (OSCs) using nonfullerene acceptors have reached 16% efficiency. Such a breakthrough has inspired new sparks for the development of the next generation of OSC materials. In addition to the optimization of electronic structure, it is important to investigate the essential solid‐state structure that guides the high efficiency of bulk heterojunction blends, which provides insight in understanding how to pair an efficient donor–acceptor mixture and refine film morphology. In this study, a thorough analysis is executed to reveal morphology details, and the results demonstrate that Y6 can form a unique 2D packing with a polymer‐like conjugated backbone oriented normal to the substrate, controlled by the processing solvent and thermal annealing conditions. Such morphology provides improved carrier transport and ultrafast hole and electron transfer, leading to improved device performance, and the best optimized device shows a power conversion efficiency of 16.88% (16.4% certified). This work reveals the importance of film morphology and the mechanism by which it affects device performance. A full set of analytical methods and processing conditions are executed to achieve high efficiency solar cells from materials design to device optimization, which will be useful in future OSC technology development.     

Guiding the Selection of Processing Additives for Increasing the Efficiency of Bulk Heterojunction Polymeric Solar Cells

In bulk heterojunction (BHJ) polymeric organic solar cells (OSCs), the use of processing additives in the material formulation has emerged as a promising, cost‐effective, and widely applicable method for optimizing the phase separation between the donor (D) and acceptor (A) materials, thus increasing their efficiency. So far, however, there has been no systematic approach for identifying suitable processing additives for a given D:A system. A method based on the Hansen solubility parameters (HSPs) is proposed for guiding the selection of processing additives for a given D:A combination. The method is applied to the archetypical poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) system. The HSPs of these materials are determined and used to define a set of numerical criteria that need to be satisfied by a processing additive in order for it to be effective in realizing a higher efficiency OSC. Applying the selection criteria results in the identification of three novel processing additives. OSCs made of these formulations demonstrate an increase in their short‐circuit current density (J_SC) and power conversion efficiency (PCE). These results demonstrate the efficiency of these novel processing additives and show that the HSPs represent a useful tool to determine and explore new types of processing additives for BHJ‐OSCs.     

Delicate Morphology Control Triggers 14.7% Efficiency All‐Small‐Molecule Organic Solar Cells

Morphology is a critical factor to determine the photovoltaic performance of organic solar cells (OSCs). However, delicately fine‐tuning the morphology involving only small molecules is an extremely challenging task. Herein, a simple, generic, and effective concentration‐induced morphology manipulation approach is demonstrated to prompt both the state‐of‐the‐art thin‐film BTR‐Cl:Y6 and thick‐film BTR:PC₇₁BM all‐small‐molecule (ASM) OSCs to a record level. The morphology is delicately controlled by subtly altering the prepared solution concentration but maintaining the identical active layer thickness. The remarkable performance enhancement achieved by this approach mainly results from the enhanced absorption, reduced trap‐assistant recombination, increased crystallinity, and optimized phase‐separated network. These findings demonstrate that a concentration‐induced morphology manipulation strategy can further propel the reported best‐performing ASM OSCs to a brand‐new level, and provide a promising way to delicately control the morphology towards high‐performance ASM OSCs.     

Printed Nonfullerene Organic Solar Cells with the Highest Efficiency of 9.5%

The current work reports a high power conversion efficiency (PCE) of 9.54% achieved with nonfullerene organic solar cells (OSCs) based on PTB7‐Th donor and 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) acceptor fabricated by doctor‐blade printing, which has the highest efficiency ever reported in printed nonfullerene OSCs. Furthermore, a high PCE of 7.6% is realized in flexible large‐area (2.03 cm²) indium tin oxide (ITO)‐free doctor‐bladed nonfullerene OSCs, which is higher than that (5.86%) of the spin‐coated counterpart. To understand the mechanism of the performance enhancement with doctor‐blade printing, the morphology, crystallinity, charge recombination, and transport of the active layers are investigated. These results suggest that the good performance of the doctor‐blade OSCs is attributed to a favorable nanoscale phase separation by incorporating 0.6 vol% of 1,8‐diiodooctane that prolongs the dynamic drying time of the doctor‐bladed active layer and contributes to the migration of ITIC molecules in the drying process. High PCE obtained in the flexible large‐area ITO‐free doctor‐bladed nonfullerene OSCs indicates the feasibility of doctor‐blade printing in large‐scale fullerene‐free OSC manufacturing. For the first time, the open‐circuit voltage is increased by 0.1 V when 1 vol% solvent additive is added, due to the vertical segregation of ITIC molecules during solvent evaporation.     

Interface Modification of Tin Oxide Electron-Transport Layer for the Efficiency and Stability Enhancement of Organic Solar Cells

A small molecular phenanthroline derivative Phen-NaDPO (3-[6-(diphenylphosphinyl)-2-naphthalenyl]-1,10-Phenanthroline) to modify tin oxide (SnO₂) electron-transport layer (ETL) in organic solar cells is employed. Quantum chemistry calculations and experimental results show that Phen-NaDPO can interact with SnO₂, thereby effectively passivating the surface defects, reducing the work function and improving the electrical conductivity of SnO₂, leading to more efficient electron extraction and transport in the organic solar cells (OSCs). Moreover, upon the Phen-NaDPO modification, the decreased surface energy of SnO₂ ETL accounts for enhanced exciton dissociation and charge transport, due to the more ordered molecular organizations of the active layers. Consequently, the inverted OSCs involving Phen-NaDPO/SnO₂ ETLs exhibit an enhanced power conversion efficiency of 17.06% (PM6:Y6) and 18.31% (PM6:L8-BO), which is the highest efficiency for SnO₂ ETL-based binary solar cells to date. Furthermore, the devices based on Phen-NaDPO/SnO₂ ETL show better device stability (storage stability, photostability and humid stability), with T₈₀ exceeding 200 h encapsulated under light irradiation and 400 h without encapsulation in high-humidity ambient condition. These results demonstrate that the modification of SnO₂ using wide-band highly stable conjugated small molecules is very promising for simultaneously improve the efficiencies and device stability of OSCs.     

High Performance Thick‐Film Nonfullerene Organic Solar Cells with Efficiency over 10% and Active Layer Thickness of 600 nm

Developing efficient organic solar cells (OSCs) with relatively thick active layer compatible with the roll to roll large area printing process is an inevitable requirement for the commercialization of this field. However, typical laboratory OSCs generally exhibit active layers with optimized thickness around 100 nm and very low thickness tolerance, which cannot be suitable for roll to roll process. In this work, high performance of thick‐film organic solar cells employing a nonfullerene acceptor F–2Cl and a polymer donor PM6 is demonstrated. High power conversion efficiencies (PCEs) of 13.80% in the inverted structure device and 12.83% in the conventional structure device are achieved under optimized conditions. PCE of 9.03% is obtained for the inverted device with active layer thickness of 500 nm. It is worth noting that the conventional structure device still maintains the PCE of over 10% when the film thickness of the active layer is 600 nm, which is the highest value for the NF‐OSCs with such a large active layer thickness. It is found that the performance difference between the thick active layer films based conventional and inverted devices is attributed to their different vertical phase separation in the active layers.     

Constructing High‐Performance All‐Small‐Molecule Ternary Solar Cells with the Same Third Component but Different Mechanisms for Fullerene and Non‐fullerene Systems

Two types of all‐small‐molecule ternary solar cells consisting of two small‐molecule donors and one acceptor (fullerene/non‐fullerene) are developed. Interestingly, both these devices have a common component: a carefully designed medium bandgap small molecule, which possesses appropriate energy levels and displays good compatibility with the host donor. In the fullerene system, the charge‐relaying role of the additive donor is confirmed by the improved charge transportation and suppressed charge recombination. While in the non‐fullerene system, the mixed face‐on and edge‐on orientation of the ternary film induced by the additive donor dominates the promotion of charge transportation. Accordingly, both ternary devices deliver higher short‐circuit current density, fill factor, and power conversion efficiencies of over 10% compared to binary ones. This work offers a promising guideline on the construction of high‐performance all‐small‐molecule ternary solar cells by incorporating a miscible small‐molecule donor.     

Dopamine Semiquinone Radical Doped PEDOT:PSS: Enhanced Conductivity,Work Function and Performance in Organic Solar Cells

Poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been is applied as hole transport material in organic electronic devices for more than 20 years. However, the redundant sulfonic acid group of PEDOT:PSS has often been overlooked. Herein, PEDOT:PSS‐DA is prepared via a facile doping of PEDOT:PSS with dopamine hydrochloride (DA·HCl) which reacts with the redundant sulfonic acid of PSS. The PEDOT:PSS‐DA film exhibits enhanced work function and conductivity compared to those of PEDOT:PSS. PEDOT:PSS‐DA‐based devices show a power conversion efficiency of 16.55% which is the highest in organic solar cells (OSCs) with (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)‐4‐fluorothiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithio‐phene))‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)‐benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione))] (PM6):(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) (Y6) as the active layer. Furthermore, PEDOT:PSS‐DA also exhibits enhanced performance in three other donor/acceptor systems, exhibiting high compatibility in OSCs. This work demonstrates that doping PEDOT:PSS with various amino derivatives is a potentially efficient strategy to enhance the performance of PEDOT:PSS in organic electronic devices.     

Design of Nonfullerene Acceptors with Near‐Infrared Light Absorption Capabilities

A series of narrow bandgap electron acceptors is designed and synthesized for efficient near‐infrared (NIR) organic solar cells. Extending π‐conjugation of donor frameworks leads to an intense intramolecular charge transfer, resulting in broad absorption profiles with band edge reaching 950 nm. When blended with an electron donor polymer PTB7‐Th, IOTIC‐2F exhibits efficient charge transfer even with a small energetic offset, so as to achieve a large photogenerated current over 22 mA cm^?2 with small energy losses (≈0.49 eV) in solar cell devices. With an intense NIR absorbance, PTB7‐Th:IOTIC‐2F‐based cells achieve a power conversion efficiency of 12.1% with good visible transparency (52% transmittance from 370 to 740 nm). Analysis of film morphology reveals that processing with solvent additives enhances crystalline features of acceptor components, while keeping an appropriate level of donor:acceptor intermixing in the binary blends. The incorporation of the third component, ITIC‐2F, into the PTB7‐Th:IOTIC‐2F blends increases the device efficiency up to 12.9%. The improvement is assigned to the cascaded energy‐level structure and desirable nanoscale phase separation of the ternary blends, which is beneficial to the photocurrent generation. This work provides an efficient molecular design strategy to optimize nonfullerene acceptor properties for efficient NIR organic photovoltaics.     

Design of Star-Shaped Trimer Acceptors for High-Performance (Efficiency > 19%), Photostable,and Mechanically Robust Organic Solar Cells

High power conversion efficiency (PCE), long-term stability, and mechanical robustness are prerequisites for the commercial applications of organic solar cells (OSCs). In this study, a new star-shaped trimer acceptor (TYT-S) is developed and high-performance OSCs with a PCE of 19.0%, high photo-stability (t_80% lifetime = 2600 h under 1-sun illumination), and mechanical robustness with a crack-onset strain (COS) of 21.6% are achieved. The isotropic molecular structure of TYT-S affords efficient multi-directional charge transport and high electron mobility. Furthermore, its amorphous structure prevents the formation of brittle crystal-to-crystal interfaces, significantly enhancing the mechanical properties of the OSC. As a result, the TYT-S-based OSCs demonstrate a significantly higher PCE (19.0%) and stretchability (COS = 21.6%) than the linear-shaped trimer acceptor (TYT-L)-based OSCs (PCE = 17.5% and COS = 6.4%) and the small-molecule acceptor (MYT)-based OSCs (PCE = 16.5% and COS = 1.3%). In addition, the increased molecular size of TYT-S, relative to that of MYT and dimer (DYT), suppresses the diffusion kinetics of the acceptor molecules, substantially improving the photostability of the OSCs. Finally, to effectively demonstrate the potential of TYT-S, intrinsically stretchable (IS)-OSCs are constructed. The TYT-S-based IS-OSCs exhibit high device stretchability (strain at PCE_80% = 31%) and PCE of 14.4%.     

High‐Efficiency Nonfullerene Organic Solar Cells: Critical Factors that Affect Complex Multi‐Length Scale Morphology and Device Performance

Organic solar cells (OSCs) made of donor/acceptor bulk‐heterojunction active layers have been of widespread interest in converting sunlight to electricity. Characterizing of the complex morphology at multiple length scales of polymer:nonfullerene small molecular acceptor (SMA) systems remains largely unexplored. Through detailed characterizations (hard/soft X‐ray scattering) of the record‐efficiency polymer:SMA system with a close analog, quantitative morphological parameters are related to the device performance parameters and fundamental morphology–performance relationships that explain why additive use and thermal annealing are needed for optimized performance are established. A linear correlation between the average purity variations at small length scale (≈10 nm) and photovoltaic device characteristics across all processing protocols is observed in ≈12%‐efficiency polymer:SMA systems. In addition, molecular interactions as reflected by the estimated Flory–Huggins interaction parameters are used to provide context of the room temperature morphology results. Comparison with results from annealed devices suggests that the two SMA systems compared show upper and lower critical solution temperature behavior, respectively. The in‐depth understanding of the complex multilength scale nonfullerene OSC morphology may guide the device optimization and new materials development and indicates that thermodynamic properties of materials systems should be studied in more detail to aid in designing optimized protocols efficiently.     

Precise Control of Phase Separation Enables 12% Efficiency in All Small Molecule Solar Cells

Compared to conjugated polymers, small‐molecule organic semiconductors present negligible batch‐to‐batch variations, but presently provide comparatively low power conversion efficiencies (PCEs) in small‐molecular organic solar cells (SM‐OSCs), mainly due to suboptimal nanomorphology. Achieving precise control of the nanomorphology remains challenging. Here, two new small‐molecular donors H13 and H14 , created by fluorine and chlorine substitution of the original donor molecule H11 , are presented that exhibit a similar or higher degree of crystallinity/aggregation and improved open‐circuit voltage with IDIC‐4F as acceptor. Due to kinetic and thermodynamic reasons, H13 ‐based blend films possess relatively unfavorable molecular packing and morphology. In contrast, annealed H14 ‐based blends exhibit favorable characteristics, i.e., the highest degree of aggregation with the smallest paracrystalline π–π distortions and a nanomorphology with relatively pure domains, all of which enable generating and collecting charges more efficiently. As a result, blends with H13 give a similar PCE (10.3%) as those made with H11 (10.4%), while annealed H14 ‐based SM‐OSCs have a significantly higher PCE (12.1%). Presently this represents the highest efficiency for SM‐OSCs using IDIC‐4F as acceptor. The results demonstrate that precise control of phase separation can be achieved by fine‐tuning the molecular structure and film formation conditions, improving PCE and providing guidance for morphology design.     

Facile Synthesis of Polycyclic Aromatic Hydrocarbon (PAH)–Based Acceptors with Fine‐Tuned Optoelectronic Properties: Toward Efficient Additive‐Free Nonfullerene Organic Solar Cells

A series of polycyclic aromatic hydrocarbons (PAHs) with extended π‐conjugated cores (from naphthalene, anthracene, pyrene, to perylene) are incorporated into nonfullerene acceptors for the first time. Four different fused‐ring electron acceptors (FREAs), i.e., DTN‐IC‐2Ph , DTA‐IC‐3Ph , DTP‐IC‐4Ph , and DTPy‐IC‐5Ph , are prepared via simple and facile synthetic procedures, yielding a remarkable platform to study the structure–property relationship for nonfullerene solar cells. With the PAH core being extended systematically, the gradually redshifted absorption with enhanced molar extinction coefficient (ε) is realized, the energy level of the highest occupied molecular orbital is up‐shifted, and the electron mobility is greatly enhanced. Meanwhile, the solubility decreases and the molecular packing becomes strengthened. As a result, with an optimized combination of these characteristics, DTP‐IC‐4Ph attains good solubility, high molar extinction coefficient, complementary absorption, suitable morphology, well‐matched energy levels, as well as efficient charge dissociation and transport in blend film. Consequently, the DTP‐IC‐4Ph ‐based solar cells with a donor polymer, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBDB‐T) exhibit a promising power conversion efficiency of 10.37% without any additives, which is close to the best performance achieved in additive‐free nonfullerene solar cells (NFSCs). The results demonstrate that the PAH building blocks have great potential for the construction of novel FREAs for efficient additive‐free NFSCs.     

Efficiency Enhancement of Polymer Solar Cells Based on Poly(3‐hexylthiophene)/Indene‐C70 Bisadduct via Methylthiophene Additive

Photovoltaic performance of polymer solar cells based on poly(3‐hexylthiophene) (P3HT) as the donor and indene‐C₇₀ bisadduct (IC₇₀BA) as the acceptor is improved by adding 3 vol% 3‐methylthiophene (MT) or 3‐hexylthiophene (HT) as processing additives. The results of UV‐vis absorption spectroscopy, X‐ray diffraction analysis and atomic force microscopy indicate that with the MT or HT processing additive, the active layer of the blend of P3HT/IC₇₀BA showed strengthened absorbance, enhanced crystallinity and improved film morphology. The power conversion efficiency (PCE) of the PSCs was improved from 5.80% for the device without the additive to 6.35% for the device with HT additive and to 6.69% with MT additive. The PCE of 6.69% is the top value reported so far for the PSCs based on P3HT.     

Extended Conjugation Length of Nonfullerene Acceptors with Improved Planarity via Noncovalent Interactions for High‐Performance Organic Solar Cells

Three low‐bandgap nonfullerene acceptors (NFAs) IDTO‐T‐4F, IDTO‐Se‐4F, and IDTO‐TT‐4F with extended conjugation length are designed and synthesized. Various π‐spacers, thiophene, selenophene, and thieno[3,2‐b]thiophene are incorporated to extend the conjugated length and enhance the backbone planarity via noncovalent O···S or O···Se interactions. These NFAs exhibit strong light absorption in the range of 600–900 nm with narrow bandgaps between 1.38 and 1.45 eV. By blending with a wide‐bandgap donor material PBDB‐T, organic solar cells (OSCs) based on these NFAs all yield high efficiency over 10% with low energy losses ranging from 0.52 to 0.59 eV. Importantly, as a result of relatively high lowest unoccupied molecular orbital level, large hole and electron mobility in blend film, and low charge carrier recombination loss, optimized devices based on IDTO‐T‐4F exhibit a large open‐circuit voltage of 0.864 V, a high short‐circuit current density of 20.12 mA cm^?2, and a notable fill factor of 72.7%, leading to an impressive efficiency of 12.62%, which represents the best performance for NFA OSCs using noncovalent interactions in acceptor molecule design. The results indicate that optimizing the conjugation length and backbone planarity via intramolecular noncovalent O···S or O···Se interactions is a useful strategy for NFA materials invention toward high‐performance solar cells.     

Foldable Semitransparent Organic Solar Cells for Photovoltaic and Photosynthesis

Semitransparent organic solar cells (ST‐OSCs) have attracted extensive attention for their potential greenhouse applications. Conventional ST‐OSCs are typically based on indium tin oxide (ITO) electrodes which suffer from mechanical brittleness. Therefore, alternatives for ITO are required for realization of foldable‐flexible ST‐OSCs (FST‐OSCs). Herein, flexible poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) electrodes are prepared as ITO alternatives via polyhydroxy compound (xylitol) microdoping and acid treatment. As a result, flexible opaque OSCs based on PBDB‐T‐2F:Y6 photoactive system yield a high efficiency of 14.20%. The desirable optical properties of modified PEDOT:PSS electrodes in the visible light region and PBDB‐T‐2F:Y6 photoactive layer in the near‐infrared region facilitate the fabrication of FST‐OSCs with over 10% efficiency and 21% average visible light transmittance. Those FST‐OSCs also display excellent mechanical stability against bending and folding due to the xylitol doping, where over 80% of the initial efficiency can still be maintained even after 1000 folding cycles. Meanwhile, parallel comparisons between plants grown under direct sunlight with a FST‐OSCs roof and those under direct sunlight yield very similar results in terms of branch sturdiness and hypertrophic leaves. The results pave the way for realizing high‐performing FST‐OSCs based on PEDOT:PSS electrodes that could utilize visible light for plant growth and infrared light for power generation.     

Effects of Solvent Additives on Morphology,Charge Generation,Transport, and Recombination in Solution‐Processed Small‐Molecule Solar Cells

The effects of solvent additive (1,8‐diiodooctane (DIO)) on the morphology, charge generation, transport, and recombination in solution‐processed small‐molecule solar cells are studied and these parameters are correlated with device performance. In the optimum nanoscale morphology, which is processed with 0.4% DIO, the phase separation is large enough to create a percolating pathway for carrier transport, yet still small enough to form large interfacial area for efficient charge separation. Complete phase separation in this film reduces the interfacial defects, which occurs without DIO, and hence suppresses the monomolecular recombination. Moreover, balanced charge transport and weak bimolecular recombination lead to a high fill factor (72%). On the other hand, an excess amount of DIO (0.8%) in the solvent results in the over‐aggregation of the donor phase, which disturbs the percolating pathway of the acceptor phase and reduces the electron mobility. The over‐aggregation of the donor phase also shrinks the interfacial area for charge separation and consequently reduces the photocurrent generation.     

An Efficient Solution‐Processed Intermediate Layer for Facilitating Fabrication of Organic Multi‐Junction Solar Cells

Photovoltaic tandem technology has the potential to boost the power conversion efficiency of organic photovoltaic devices. Here, a reliable and efficient fully solution‐processed intermediate layer (IML) consisting of ZnO and neutralized poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is demonstrated for series‐connected multi‐junction organic solar cells (OSCs). Drying at 80 °C in air is sufficient for this solution‐processed IML to obtain excellent functionality and reliability, which allow the use of most of high performance donor materials in the tandem structure. An open circuit voltage (V_oc) of 0.56 V is obtained for single‐junction OSCs based on a low band‐gap polymer, while multi‐junction OSCs based on the same absorber material deliver promising fill factor values along with fully additive V_oc as the number of junctions increase. Optical and electrical simulations, which are reliable and promising guidelines for the design and investigation of multi‐junction OSCs, are discussed. The outcome of optical and electrical simulations is in excellent agreement with the experimental data, indicating the outstanding efficiency and functionality of this solution‐processed IML. The demonstration of this efficient, solution‐processed IML represents a convenient way for facilitating fabrication of multi‐junction OSCs to achieve high power conversion efficiency.