Small-bandgap polymer solar cells with unprecedented short-circuit current density and high fill factor

High Polymer Molecular Weight Yields Solar Cells with Simultaneously Improved Performance and Thermal Stability

Simple synthetic routes, high active layer thickness tolerance as well as stable organic solar cells are relentlessly pursued as key enabling traits for the upscaling of organic photovoltaics. Here, the potential to address these issues by tuning donor polymer molecular weight is investigated. Specifically, the focus is on PTQ10, a polymer with low synthetic complexity, with number average molecular weights of 2.4, 6.2, 16.8, 52.9, and 54.4 kDa, in combination with three different non-fullerene acceptors, namely Y6, Y12, and IDIC. Molecular weight, indeed, unlocks a threefold increase in power conversion efficiency for these blends. Importantly, efficiencies above 10% for blade coated devices with thicknesses between 200 and 350 nm for blends incorporating high molecular weight donor are shown. Spectroscopic, GIWAXS and charge carrier mobility data suggest that the strong photocurrent improvement with molecular weight is related to both, improved electronic transport and polymer contribution to exciton generation. Moreover, it is demonstrated that solar cells based on high molecular weight PTQ10 are more thermally stable due to a higher glass transition temperature, thus also improving device stability.

Effects of the diphenyl ether additive in halogen-free processed non-fullerene acceptor organic solar cells

The development of an environmentally friendly fabrication process for non-fullerene acceptor organic solar cells is an essential condition for their commercialization. However, devices fabricated by processing the active layer with green solvents still struggle to reach, in terms of efficiency, the same performance as those fabricated with halogenated solvents. The reason behind this is the non-optimal nanostructure of the active layer obtained with green solvents. Additives in solution have been used to fine-tune the nanostructure and improve the performance of organic solar cells. Therefore, the identification of non-halogenated additives and the study of their effects on the device performance and stability are of primary importance. In this work, we propose the use of diphenyl ether (DPE) as additive, in combination with the non-halogenated solvent o-xylene, to fabricate organic solar cells with a completely halogen-free process. Thanks to the addition of DPE, a best efficiency of 11.7% have been obtained for the system TPD-3F:IT-4F, an increase over 15% with respect to the efficiency of devices fabricated without additive. Remarkably, the stability under illumination of the solar cells is also improved when DPE is used. The addition of DPE has effects on the molecular organization in the active layer, with an enhancement in the donor polymer ordering, showing a higher domain purity. The resulting structure improves the charge carrier collection, leading to a superior short-circuit current and fill factor. Furthermore, a reduction of the non-radiative recombination losses and an improved exciton diffusion, are the results of the superior molecular ordering. With a comprehensive insight of the effects of DPE when used in combination with a non-halogenated solvent, our study provides an approach to make the fabrication of organic solar cell environmentally friendlier and more suitable for large scale production.

Robust singlet fission process in strong absorption π-expanded diketopyrrolopyrroles

Singlet fission (SF) has drawn tremendous attention as a multiexciton generation process that could mitigate the thermal loss and boost the efficiency of solar energy conversion. Although a SF-based solar cell with an EQE above 100% has already been fabricated successfully, the practical efficiency of the corresponding devices is plagued by the limited scope of SF materials. Therefore, it is of great importance to design and develop new SF-capable compounds aiming at practical device application. In the current contribution, via a π-expanded strategy, we presented a new series of robust SF chromophores based on polycyclic DPP derivatives, Ex-DPPs. Compared to conventional DPP molecules, Ex-DPPs feature strong absorption with a fivefold extinction coefficient, good molecular rigidity to effectively restrain non-radiative deactivation, and an expanded π-skeleton which endow them with well-suited intermolecular packing geometries for achieving efficient SF process. These results not only provide a new type of high-efficiency SF chromophore but also address some basic guidelines for the design of potential SF materials targeting practical light harvesting applications.

Effect of Solvent Removal Rate and Annealing on the Interface Properties in a Blend of a Diketopyrrolopyrrole-Based Polymer with Fullerene

We study the effect of solvent-free annealing and explicit solvent evaporation protocols in classical molecular dynamics simulations on the interface properties of a blend of a diketopyrrolopyrrole (DPP) polymer with conjugated substituents (DPP2Py2T) and PCBM[60]. We specifically analyze the intramolecular segmental mobility of the different polymer building blocks as well as intermolecular radial and angular distribution functions between donor and acceptor. The annealing simulations reveal an increase of the glass-transition temperature of 45 K in the polymer–fullerene blend compared to that of pure DPP2Py2T. Our results show that the effective solvent evaporation rates at room temperature only have a minor influence on the segmental mobility and intermolecular orientation, characterized in all cases by a preferential arrangement of PCBM[60] close to the electron-donating substituents in DPP2Py2T. In contrast, solvent-free annealing from a liquid yields clustering of the fullerene close to the electron-withdrawing DPP, generally considered to be detrimental for application in organic solar cells. We find that the difference can be attributed to differences in the behavior of 2-hexyldecyl side-chains, which collapse toward DPP when solvent is explicitly removed, thereby blocking access of PCBM[60].

Matching electron transport layers with a non-halogenated and low synthetic complexity polymer:fullerene blend for efficient outdoor and indoor organic photovoltaics

The desired attributes of organic photovoltaics (OPV) as a low cost and sustainable energy harvesting technology demand the use of non-halogenated solvent processing for the photoactive layer (PAL) materials, preferably of low synthetic complexity (SC) and without compromising the power conversion efficiency (PCE). Despite their record PCEs, most donor-acceptor conjugated copolymers in combination with non-fullerene acceptors are still far from upscaling due to their high cost and SC. Here we present a non-halogenated and low SC ink formulation for the PAL of organic solar cells, comprising PTQ10 and PC61BM as donor and acceptor materials, respectively, showing a record PCE of 7.5% in blade coated devices under 1 sun, and 19.9% under indoor LED conditions. We further study the compatibility of the PAL with 5 different electron transport layers (ETLs) in inverted architecture. We identify that commercial ZnO-based formulations together with a methanol-based polyethyleneimine-Zn (PEI-Zn) chelated ETL ink are the most suitable interlayers for outdoor conditions, providing fill factors as high as 74% and excellent thickness tolerance (up to 150 nm for the ETL, and >200 nm for the PAL). In indoor environments, SnO2 shows superior performance as it does not require UV photoactivation. Semi-transparent devices manufactured entirely in air via lamination show indoor PCEs exceeding 10% while retaining more than 80% of the initial performance after 400 and 350 hours of thermal and light stress, respectively. As a result, PTQ10:PC61BM combined with either PEI-Zn or SnO2 is currently positioned as a promising system for industrialisation of low cost, multipurpose OPV modules.

Efficient Medium Bandgap Electron Acceptor Based on Diketopyrrolopyrrole and Furan for Efficient Ternary Organic Solar Cells

We report the design of novel medium bandgap nonfullerene small molecule acceptor NFSMA SPS-TDPP-2CNRh with A₂-π-A₁-π-A₂ architecture, with the molecular engineering of this material comprising a strong electron-accepting backbone unit DPP (A₁) as the acceptor, which is attached to the dicyanomethylene-3-hexylrhodanine (A₂) acceptor via a furan (π-spacer) linker. We systematically studied its structural and optoelectronic properties. The incorporation of dicyanomethylene-3-hexylrhodanine and furan enhance the light absorption and electrochemical properties by extending π-conjugation and is anticipated to improve V_OC by decreasing the LUMO level. The long alkyl chain units were responsible for the better solubility and aggregation of the resultant molecule. Binary BHJ-OSCs constructed with polymer P as the donor and SPS-TDPP-2CNRh as the acceptor resulted in a PCE of 11.49% with improved V_OC = 0.98 V, J_SC = 18.32 mA/cm², and FF = 0.64 for P:SPS-TDPP-2CNRh organic solar cells. A ternary solar cell device was also made using Y18-DMO and SPS-TDPP-2CNRh as acceptors having complementary absorption profiles and polymer P as the donor, resulting in a PCE of 15.50% with improved J_SC = 23.08 mA/cm², FF = 0.73, and V_OC = 0.92 V for the P:SPS-TDPP-2CNRh:Y18-DMO solar cell. The ternary OSCs with SPS-TDPP-2CNRh as the host acceptor in the P:Y18-DMO binary film were shown to have improved PCE values, which is mainly attributed to the effective photoinduced charge transfer through multiple networks and the use of excitons from SPS-TDPP-2CNRh and Y18-DMO. Moreover, in the ternary BHJ active layers, the superior stable charge transport that was observed compared to the binary counterparts may also lead to an increase in the fill factor. These results demonstrate that combining medium bandgap and narrow bandgap NFSMAs with a wide bandgap polymer donor is a successful route to increasing the overall PCE of the OSCs via the ternary BHJ concept.

Photocatalytic Water-Splitting by Organic Conjugated Polymers: Opportunities and Challenges

The future challenges associated with the shortage of fossil fuels and their current environmental impacts intrigued the researchers to look for alternative ways of generating green energy. Solar-driven water splitting into oxygen and hydrogen is one of those advanced strategies. Researchers have studied various semiconductor materials to achieve potential results. However, it encountered multiple challenges such as high cost, low photostability and efficiency, and required multistep modifications. The conjugated polymers (CPs) have emerged as promising alternatives for conventional inorganic semiconductors. The CPs offer low cost, sufficient light absorption efficiency, excellent photo and chemical stability, and molecular optoelectronic tunable characteristics. Furthermore, organic CPs also present higher flexibility to tune the basic framework of the backbone of the polymers, amendments in the sidechain to incorporate desired functionalities, and much-needed porosity to serve better for photocatalytic applications. This review article summarizes the recent advancements made in visible-light-driven water splitting covering the aspects of synthetic strategies and experimental parameters employed for water splitting reactions with special emphasis on conjugated polymers such as linear CPs, planarized CPs, graphitic carbon nitride (g-C₃ N₄ ), conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), and conjugated polymer-based nanocomposites (CPNCs). The current challenges and future prospects have also been described briefly.

Near-Infrared Materials: The Turning Point of Organic Photovoltaics

Near-infrared (NIR)-absorbing organic semiconductors have opened up many exciting opportunities for organic photovoltaic (OPV) research. For example, new chemistries and synthetical methodologies have been developed; especially, the breakthrough Y-series acceptors, originally invented by our group, specifically Y1, Y3, and Y6, have contributed immensely to boosting single-junction solar cell efficiency to around 19%; novel device architectures such as tandem and transparent organic photovoltaics have been realized. The concept of NIR donors/acceptors thus becomes a turning point in the OPV field. Here, the development of NIR-absorbing materials for OPVs is reviewed. According to the low-energy absorption window, here, NIR photovoltaic materials (p-type (polymers) and n-type (fullerene and nonfullerene)) are classified into four categories: 700-800 nm, 800-900 nm, 900-1000 nm, and greater than 1000 nm. Each subsection covers the design, synthesis, and utilization of various types of donor (D) and acceptor (A) units. The structure-property relationship between various kinds of D, A units and absorption window are constructed to satisfy requirements for different applications. Subsequently, a variety of applications realized by NIR materials, including transparent OPVs, tandem OPVs, photodetectors, are presented. Finally, challenges and future development of novel NIR materials for the next-generation organic photovoltaics and beyond are discussed.

Adjusting the photovoltaic performance of big fused ring-based small molecules by tailoring with different modifications

Three novel A-D-A type small-molecule donor materials, namely AAN-DPP2, AAN(T-DPP)2 and AANT(T-DPP)2, with anthanthrene (AAN) as the electron-donating core, diketopyrrolopyrrole (DPP) as the electron-accepting moiety, and thiophene as π-bridge units, have been designed and synthesized for application in bulk-heterojunction (BHJ) organic solar cells (OSCs). Compared to AAN-DPP2, devices based on AAN(T-DPP)2 and AANT(T-DPP)2 show better photovoltaic performance due to broader absorption and better planarity of the molecular backbone. A maximum power conversion efficiency (PCE) of 2.33% with a short-circuit current density (J sc) of 6.82 mA cm-2 and a fill factor (FF) of 39.80 was obtained in the AAN(T-DPP)2/PC71BM-based solar cells. This is resulting from the suitable thickness of the active layer, improving the ability of catching light and decreasing the twist angle of the backbone by inserting a thiophene spacer. The results indicate that strategic substitution of π-bridges and side-chains in A-D-A type SMs is an efficient strategy to improve photovoltaic performance.

Porphyrin Acceptors with Two Perylene Diimide Dimers for Organic Solar Cells

Three small-molecule acceptors (Por-PDI, TEHPor-PDI, and BBOPor-PDI) with different side chains were synthesized by using a porphyrin core as the electron-donating unit and connecting electron-withdrawing perylene diimide dimers via acetylene bridges. The bulk heterojunction organic solar cells based on the three acceptors and a polymer donor provided power conversion efficiencies (PCEs) of 3.68-5.21 % when the active layers were fabricated with pyridine additives. Though the synthesis of Por-PDI is easier with fewer reaction steps and higher yields, the devices based on Por-PDI showed the best performance with a PCE of 5.21 %. The more ordered intermolecular packing due to the reduced steric hindrance at the porphyrin core of Por-PDI could contribute to the more balanced hole/electron mobilities, higher maximum charge generation rate, and less bimolecular recombination in Por-PDI devices, which are beneficial for the higher PCE.

Toward Highly Robust Nonvolatile Multilevel Memory by Fine Tuning of the Nanostructural Crystalline Solid-State Order

Organic resistive memory (ORM) offers great promise for next-generation high-density multilevel-cell (MLC) data storage. However, the fine tuning of crystalline order among its active layer still remains challenging, which largely restricts ORM behavior. Here, an exceptional solid-state transition from disordered orientations to highly-uniform orientation within the ORM layer is facilely triggered via molecular strategic tailoring. Two diketopyrrolopyrrole-based small molecular analogues (NI₁ TDPP and NI₂ TDPP) are demonstrated to display different symmetry. The asymmetric NI₁ TDPP shows an irregular solid-state texture, while the centro-symmetric NI₂ TDPP conforms to an ordered out-of-plane single-crystalline pattern that aligns with the foremost charge transportation along the substrate normal, and exhibits excellent MLC memory characteristics. Moreover, this highly oriented pattern guarantees the large-area film uniformity, leading to the twofold increase in the yield of as-fabricated ORM devices. This study reveals that the solid-state crystalline nanostructural order of organic materials can be controlled by reasonable molecular design to actuate high-performance organic electronic circuits.

Copolymers based on trialkylsilylethynyl-phenyl substituted benzodithiophene building blocks for efficient organic solar cells

Trialkylsilylethynyl-phenyl was explored as a side chain to construct a benzodithiophene-containing polymer that demonstrated a PCE of 7.81% in organic solar cells.

Development and Testing of an All-Atom Force Field for Diketopyrrolopyrrole Polymers with Conjugated Substituents

We develop an all-atom force field for a series of diketopyrrolopyrrole polymers with two aromatic pyridine substituents and a variable number of π-conjugated thiophene units in the backbone (DPP2PymT), used as donor materials in organic photovoltaic devices. Available intrafragment parameterizations of the individual fragment building blocks are combined with interfragment bonded and nonbonded parameters explicitly derived from density functional theory calculations. To validate the force field, we perform classical molecular dynamics simulations of single polymer chains with m = 1, 2, 3 in good and bad solvents and of melts. We observe the expected dependence of the chain conformation on the solvent quality, with the chain collapsing in water, and swelling in chloroform. The glass-transition temperature for the polymer melts is found to be in the range of 340–370 K. Analysis of the mobility of the conjugated segments in the polymer backbone reveals two relaxation processes: a fast one with a characteristic time at room temperature on the order of 10 ps associated with nearly harmonic vibrations and a slow one on the order of 100 ns associated with temperature-activated cis–trans transitions.

Inorganic Molecular Clusters with Facile Preparation and Neutral pH for Efficient Hole Extraction in Organic Solar Cells

The development of electrode interlayers for hole extraction is a great challenge in the field of organic solar cells (OSCs). At present, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is the only solution-processed anode interlayer (AIL) that can be used to achieve power conversion efficiencies (PCEs) over 15% in OSC devices, even though there are several well-known drawbacks in practical applications of PEDOT:PSS. Herein, we use an inorganic molecular cluster (IMC) as the AIL for making highly efficient and large-area OSCs. The IMC possesses several advantages in serving as the AIL, such as neutral pH, excellent optical transmittance, high work function, good film-forming properties, and low cost. OSCs using the IMC can achieve a high PCE of 13.38%, which is superior to the PCE of the PEDOT:PSS device. This is among the few examples of OSC devices with solution-processed and pH neutral AILs showing higher PCE than PEDOT:PSS devices. Ultraviolet photoelectron spectroscopy and electron spin resonance results indicate the formation of inorganic-organic heterojunction, which is crucial for efficient hole extraction. More importantly, the IMC is compatible with printing processing. Using a blade-coated IMC film, we fabricated a large-area OSC of 1 cm² and a high PCE of 9.5% was achieved.

Cosolvent Effects When Blade-Coating a Low-Solubility Conjugated Polymer for Bulk Heterojunction Organic Photovoltaics

The adoption of solution-processed active layers in the production of thin-film photovoltaics is hampered by the transition from research fabrication techniques to scalable processing. We report a detailed study of the role of processing in determining the morphology and performance of organic photovoltaic devices using a commercially available, low-solubility, high-molar mass diketopyrrolopyrrole-based polymer donor. Ambient blade coating of thick layers in an inverted architecture was performed to best model scalable processing. Device performance was strongly dependent on the introduction of either o-dichlorobenzene (DCB), 1,8-diiodooctane, or diphenyl ether cosolvent into the chloroform (CHCl₃) solution, which were all shown to drastically improve the morphology. To understand the origin of these morphological changes as a result of the addition of the cosolvent, in situ studies with grazing-incidence X-ray scattering and optical reflection interferometry were performed. Use of any of the cosolvents decreases the domain size relative to the single solvent system and moved the drying mechanism away from what is likely liquid-liquid phase separation to solid-liquid phase separation driven by polymer aggregation. Comparing the CHCl₃ + DCB cast films to the CHCl₃-only cast films, we observed both the formation of small domains and an increase in crystallinity during the evaporation of DCB due to a high nucleation rate from supersaturation. This resulted in percolated bulk heterojunction networks that performed similarly well with a wide range of film thicknesses from 180 to 440 nm, making this system amenable to continuous roll-to-roll processing methods.

Synthesis and characterization of donor–acceptor semiconducting polymers containing 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole for organic solar cells

D–A polymers containing 4-(4-((2-ethylhexyl)oxy)phenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole and 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione were successfully synthesized and applied for organic solar cells.

Recent developments in the synthesis of regioregular thiophene-based conjugated polymers for electronic and optoelectronic applications using nickel and palladium-based catalytic systems

Thiophene-based conjugated polymers are important conjugated polymers due to their exceptional optical and conductive properties, over the past few decades many researchers have designed novel strategies to reach more efficient materials for electronic applications.

BODIPY bearing alkylthienyl side chains: a new building block to design conjugated polymers with near infrared absorption for organic photovoltaics

A new benzene-fused BODIPY unit for designing polymer donors with near-infrared absorption for organic photovoltaics.

New Antimony-Based Organic-Inorganic Hybrid Material as Electron Extraction Layer for Efficient and Stable Polymer Solar Cells

Hybrid organic-inorganic materials are a new class of materials used as interfacial layers (ILs) in polymer solar cells (PSCs). A hybrid material, composed of antimony as the inorganic part and diaminopyridine as the organic part, is synthesized and described as a new material for application as the electron extraction layer (EEL) in PSCs and compared to the recently demonstrated hybrid materials using bismuth instead of antimony. The hybrid compound is solution-processed onto the photoactive layer based on a classical blend, which is composed of a PTB7-Th low band gap polymer as the donor mixed with PC₇₀BM fullerene as the acceptor material. By using a regular device structure and an aluminum cathode, the solar cells exhibited a power conversion efficiency of 8.42%, equivalent to the reference device using ZnO nanocrystals as the IL, and strongly improved compared to the bismuth-based hybrid material. The processing of extraction layers up to a thickness of 80 nm of such hybrid material reveals that the change from bismuth to antimony has strongly improved the charge extraction and transport properties of the hybrid materials. Interestingly, nanocomposites made of the hybrid material mixed with ZnO nanocrystals in a 1:1 ratio further improved the electronic properties of the extraction layers, leading to a power conversion efficiency of 9.74%. This was addressed to a more closely packed morphology of the hybrid layer, leading to further improved electron extraction. It is important to note that these hybrid EELs, both pure and ZnO-doped, also greatly improved the stability of solar cells, both under dark storage in air and under lighting under an inert atmosphere compared to solar cells treated with ZnO intermediate layers.

Solution-Processed Semitransparent Organic Photovoltaics: From Molecular Design to Device Performance

Recent research efforts on solution-processed semitransparent organic solar cells (OSCs) are presented. Essential properties of organic donor:acceptor bulk heterojunction blends and electrode materials, required for the combination of simultaneous high power conversion efficiency (PCE) and average visible transmittance of photovoltaic devices, are presented from the materials science and device engineering points of view. Aspects of optical perception, charge generation-recombination, and extraction processes relevant for semitransparent OSCs are also discussed in detail. Furthermore, the theoretical limits of PCE for fully transparent OSCs, compared to the performance of the best reported semitransparent OSCs, and options for further optimization are discussed.

Fluoro-Modulated Molecular Geometry in Diketopyrrolopyrrole-Based Low-Bandgap Copolymers for Tuning the Photovoltaic Performance

Fluorination of conjugated polymers is an effective strategy to tune the energy levels for obtaining high power conversion efficiency (PCE) in organic solar cells. In this work, we have developed fluoro-modulated molecular geometries in diketopyrrolopyrrole based low-bandgap copolymers. In these polymers, planar conformation can be locked by intramolecular non-covalent interaction (intramolecular supramolecular interaction) between the sulfur atoms and the introduced F atoms (F···S interaction). By varying the fluorinated moieties, such a planarity can be disturbed and the molecular geometry is tuned. As a result, the polymer' properties can be modulated, including the ultraviolet-visible absorption spectrum to become broaden, charge mobility to be enhanced, open-circuit voltage (V _oc) and short-circuited current (J _sc) to be elevated, and thus photovoltaic performance to be improved. The photovoltaic device based on PCFB, one of the fluorinated terpolymers, exhibited a high PCE near 8.5% with simultaneously enhanced V _oc and J _sc relative to the non-fluorinated one (PCB).

Recent Advances in n-Type Polymers for All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) based on n- and p-type polymers have emerged as promising alternatives to fullerene-based solar cells due to their unique advantages such as good chemical and electronic adjustability, and better thermal and photochemical stabilities. Rapid advances have been made in the development of n-type polymers consisting of various electron acceptor units for all-PSCs. So far, more than 200 n-type polymer acceptors have been reported. In the last seven years, the power conversion efficiency (PCE) of all-PSCs rapidly increased and has now surpassed 10%, meaning they are approaching the performance of state-of-the-art solar cells using fullerene derivatives as acceptors. This review discusses the design criteria, synthesis, and structure-property relationships of n-type polymers that have been used in all-PSCs. Additionally, it highlights the recent progress toward photovoltaic performance enhancement of binary, ternary, and tandem all-PSCs. Finally, the challenges and prospects for further development of all-PSCs are briefly considered.

Preparation and Physical Characterization of Pyrene and Pyrrolo[3,4-c]pyrrole-1,4-dione-Based Copolymers

Two narrow band-gap copolymers consisting of 2,7-bis(5-(trimethylstannyl)thiophen-2-yl)-4,5,9,10-tetrakis(2-ethylhexyloxy)-pyrene (M1) as an electron-rich unit and diketopyrrolopyrrole (DPP) as an electron-deficient unit have been synthesized and characterized for polymer solar cells. The two polymers were prepared by Stille coupling reactions. Two solubilizing alkyl chains (ethylhexyl and octlydodecyl) were attached to the DPP unit in order to evaluate their impact upon the optical and electrochemical characteristics of the two polymers. Poly[4,5,9,10-tetrakis[(2-ethylhexyl)oxy]pyrene-alt-3,6-bis(thiophen-2-yl)-2,5- bis(2-octyldodecyl)pyrrole[3,4-c]pyrrole- 1,4(2H,5H)-dione] (PPEHDT-DPPODo ) and poly[4,5,9,10-tetrakis((2-ethylhexyl)oxy)pyren-alt-3,6-bis(thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrole[3,4-c]pyrrole-1,4(2H,5H)-dione] (PPEHDT-DPPEH ) exhibited high thermal stability with decomposition temperatures over 300 °C. Optical properties showed that PPEHDT-DPPODo and PPEHDT-DPPEH have optical band gaps of around 1.40 eV. It is believed that both polymers adopt high planar structures in the thin film, leading to more electronic conjugation along the backbone of the conjugated polymers. Powder X-ray diffraction revealed that PPEHDT-DPPODo and PPEHDT-DPPEH seem to have an amorphous nature. The HOMO energy levels of the two polymers are clearly affected by changing alkyl chains. The HOMO levels of PPEHDT-DPPODo and PPEHDT-DPPEH were found to be at -5.27 and -5.38 eV, respectively. PPEHDT-DPPODo showed a HOMO energy level approximately 0.11 eV shallower than that of PPEHDT-DPPEH , which is probably a consequence of attaching a larger alkyl chain to the DPP moiety reducing its electron accepting ability.

Morphology-Dependent Hole Transfer under Negligible HOMO Difference in Non-Fullerene Acceptor-Based Ternary Polymer Solar Cells

In the field of organic solar cells, it has been generally accepted until recently that a difference in band energies of at least 0.3 eV between the highest occupied molecular orbital (HOMO) level of the donor and the HOMO of the acceptor is required to provide adequate driving force for efficient photoinduced hole transfer due to the large binding energy of excitons in organic materials. In this work, we investigate polymeric donor:non-fullerene acceptor junctions in binary and ternary blend polymer solar cells, which exhibit efficient photoinduced hole transfer despite negligible HOMO offset and demonstrate that hole transfer in this system is dependent on morphology. The morphology of the organic blend was gradually tuned by controlling the amount of ITIC and PC₇₀BM. High external quantum efficiency was achieved at long wavelengths, despite ITIC-to-PC₇₀BM ratio of 1:9, which indicates efficient photoinduced hole transfer from ITIC to the donor despite an undesirable HOMO energy offset. Transient absorption spectra further confirm that hole transfer from ITIC to the donor becomes more efficient upon optimizing the morphology of the ternary blend compared to that of donor:ITIC binary blend.

Understanding charge carrier dynamics in a P3HT:FLR blend

In organic semiconductors, optical absorption is pivotal for the performance of optoelectronic devices. The absorption by the semiconductors generates excitons which dissociate into free charge carriers, resulting in energy conversion. Although high performance has been achieved in non-fullerene organic solar cells, their charge generation behavior is far from being well understood. Keeping this in view, we have employed optical spectroscopic tools to study the charge generation mechanism in FLR (1,6,7,10-tetramethylfluoranthene) as a non-fullerene electron acceptor blended with P3HT (poly(3-hexylthiophene)) as an electron donor in five different solvents. Through steady state UV-visible and photoluminescence spectroscopy, we provide a basic understanding of charge transport by enlightening the influence of solvents on the aggregation behavior and exciton bandwidth. Furthermore, for the first time, by employing ultrafast vis-NIR transient absorption spectroscopy, we address the ultrafast charge generation and charge separation mechanism with systematic variation in solvent polarity by incorporating the time evolution of the transient species under various pump-probe wavelengths in the range of 450 nm to 1600 nm. For the different excitation wavelengths, the lifetime kinetics have been depicted by their multiexponential fits. The results show a fast decay term at a lifetime of a few picoseconds (ps) (∼1 to 5 ps) and a slow decay term at a lifetime of ∼500 ps. The charge generation in the P3HT:FLR blend proceeds on a ps time scale, which implies good intermixing of the components. It is clearly established that the non-halogenated solvents influence this aggregation behavior and higher conjugation lengths with higher photoluminescence quenching contribute to the higher charge generation. The enhanced polaron population in P3HT with the addition of FLR illustrates the importance of this acceptor material in the blend because a good solvent-material combination is essential to enhance the charge generation. As such, this comprehensive study explicitly shows the role of FLR as an emerging efficient non-fullerene acceptor for further improving the performance of devices.

Morphology Driven by Molecular Structure of Thiazole-Based Polymers for Use in Field-Effect Transistors and Solar Cells

The effects of the molecular structure of thiazole-based polymers on the active layer morphologies and performances of electronic and photovoltaic devices were studied. Thus, thiazole-based conjugated polymers with a novel thiazole-vinylene-thiazole (TzVTz) structure were designed and synthesized. The TzVTz structure was introduced to extend the π conjugation and coplanarity of the polymer chains. By combining alkylthienyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDT) or dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDT) electron-donating units and a TzVTz electron-accepting unit, enhanced intermolecular interactions and charge transport were obtained in the novel polymers BDT-TzVTz and DTBDT-TzVTz. With a view to using the polymers in transistor and photovoltaic applications, the molecular self-assembly in and their nanoscale morphologies of the active layers were controlled by thermal annealing to enhance the molecular packing and by introducing a diphenyl ether solvent additive to improve the miscibility between polymer donors and [6,6]phenyl-C71-butyric acid methyl ester (PC₇₁ BM) acceptors, respectively. The morphological characterization of the photoactive layers showed that a higher degree of π-electron delocalization and more favorable molecular packing in DTBDT-TzVTz compared with in BDT-TzVTz leads to distinctly higher performances in transistor and photovoltaic devices. The superior performance of a photovoltaic device incorporating DTBDT-TzVTz was achieved through the superior miscibility of DTBDT-TzVTz with PC₇₁ BM and the improved crystallinity of DTBDT-TzVTz in the nanofibrillar structure.

π-Expanded diketopyrrolopyrroles as acceptor building blocks for the formation of novel donor–acceptor copolymers

The incorporation of rigid and planar, π-expanded diketopyrrolopyrrole (EDPP) units into alternating donor–acceptor copolymers is reported.

Regio-regular alternating diketopyrrolopyrrole-based D1–A–D2–A terpolymers for the enhanced performance of polymer solar cells

We designed and synthesized regio-regular alternating diketopyrrolopyrrole (DPP)-based D₁–A–D₂–A terpolymers (PDPPF2T2DPP-T2, PDPPF2T2DPP-TVT, and PDPPF2T2DPP-DTT) using a primary donor (D₁) [3,3′-difluoro-2,2′-bithiophene (F2T2)] and a secondary donor (D₂) [2,2′-bithiophene (T2), (E)-1,2-di(thiophen-2-yl)ethene (TVT), or dithieno[3,2-b:2′,3′-d]thiophene (DTT)]. A PDPP2DT-F2T2 D–A polymer was synthesized as well to compare optical, electronic, and photovoltaic properties. The absorption peaks of the terpolymers (PDPPF2T2DPP-T2, PDPPF2T2DPP-TVT, and PDPPF2T2DPP-DTT) were longer (λ_max = 801–810 nm) than the peak of the PDPP2DT-F2T2 polymer (λ_max = 799 nm), which is associated with the high-lying HOMO levels of the terpolymers (−5.08 to −5.13 eV) compared with the level of the PDPP2DT-F2T2 polymer (−5.38 eV). The photovoltaic properties of these DPP-based polymers were investigated under simulated AM 1.5G sunlight (100 mW cm⁻²) with a conventional structure (ITO/PEDOT:PSS/polymer:PC₇₁BM/Al). The open-circuit voltages (V_oc) of photovoltaic devices containing the terpolymers were slightly lower (0.68–0.70 V) than the V_oc of the device containing the PDPP2DT-F2T2 polymer (0.79 V). The short-circuit current (J_sc) of the PDPPF2T2DPP-DTT device was significantly improved (14.14 mA cm⁻²) compared with that of the PDPP2DT-F2T2 device (8.29 mA cm⁻²). As a result, the power conversion efficiency (PCE) of the PDPPF2T2DPP-DTT device (6.35%) was increased by 33% compared with that of the simple D–A-type PDPP2DT-F2T2 device (4.78%). The highest J_sc and PCE values (the PDPPF2T2DPP-DTT device) were attributed to an optimal nanoscopically mixed morphology and strong interchain packing with a high face-on orientation in the blend film state. The study demonstrated that our strategy of using multiple donors in a regio-regular alternating fashion could fine-tune the optical, electronic, and morphological properties of D–A-type polymers, enhancing the performance of polymer solar cells.

We designed and synthesized regio-regular alternating diketopyrrolopyrrole (DPP)-based D₁–A–D₂–A terpolymers (PDPPF2T2DPP-T2, PDPPF2T2DPP-TVT, and PDPPF2T2DPP-DTT) for use in polymer solar cells.

Design and application of volatilizable solid additives in non-fullerene organic solar cells

Most of the high-performance organic solar cells are fabricated with the assistance of high-boiling-point solvent additives to optimize their charge transport properties; this has adverse effects on the OSCs' stability and reproducibility in large-scale production. Here, we design volatilizable solid additives by considering the molecular structure feature of an acceptor-donor-acceptor-type non-fullerene acceptor. The application of solid additives can enhance the intermolecular π-π stacking of the non-fullerene acceptor and thus facilitate the charge transport properties in the active layers, leading to improved efficiencies of OSCs. Importantly, devices fabricated using volatilizable solid additives exhibit higher stability and reproducibility when compared with the OSCs processed with solvent additives. Our results not only demonstrate an approach of applying volatilizable solid additives to benefit the large-scale production of OSCs but also provide a potential direction for designing specific solid additives for different active layers.

Theoretical Calculations for Highly Selective Direct Heteroarylation Polymerization: New Nitrile-Substituted Dithienyl-Diketopyrrolopyrrole-Based Polymers

Direct Heteroarylation Polymerization (DHAP) is becoming a valuable alternative to classical polymerization methods being used to synthesize π-conjugated polymers for organic electronics applications. In previous work, we showed that theoretical calculations on activation energy (E_a) of the C–H bonds were helpful to rationalize and predict the selectivity of the DHAP. For readers’ convenience, we have gathered in this work all our previous theoretical calculations on E_a and performed new ones. Those theoretical calculations cover now most of the widely utilized electron-rich and electron-poor moieties studied in organic electronics like dithienyl-diketopyrrolopyrrole (DT-DPP) derivatives. Theoretical calculations reported herein show strong modulation of the E_a of C–H bond on DT-DPP when a bromine atom or strong electron withdrawing groups (such as fluorine or nitrile) are added to the thienyl moiety. Based on those theoretical calculations, new cyanated dithienyl-diketopyrrolopyrrole (CNDT-DPP) monomers and copolymers were prepared by DHAP and their electro-optical properties were compared with their non-fluorinated and fluorinated analogues.

Quinoxaline-Based Wide Band Gap Polymers for Efficient Nonfullerene Organic Solar Cells with Large Open-Circuit Voltages

We present here a series of wide-band-gap ( E_g: >1.8 eV) polymer donors by incorporating thiophene-flanked phenylene as an electron-donating unit and quinoxaline as an electron-accepting co-unit to attain large open-circuit voltages ( V_ocs) and short-circuit currents ( J_scs) in nonfullerene organic solar cells (OSCs). Fluorination was utilized to fine-tailor the energetics of polymer frontier molecular orbitals (FMOs) by replacing a variable number of H atoms on the phenylene moiety with F. It was found that fluorination can effectively modulate the polymer backbone planarity through intramolecular noncovalent S···F and/or H···F interactions. Polymers (P2-P4) show an improved molecular packing with a favorable face-on orientation compared to their nonfluorinated analogue (P1), which is critical to charge carrier transport and collection. When mixed with IDIC, a nonfullerene acceptor, P3 with two F atoms, achieves a remarkable V_oc of 1.00 V and a large J_sc of 15.99 mA/cm², simultaneously, yielding a power-conversion efficiency (PCE) of 9.7%. Notably, the 1.00 V V_oc is among the largest values in the IDIC-based OSCs, leading to a small energy loss ( E_loss: 0.62 eV) while maintaining a large PCE. The P3:IDIC blend shows an efficient exciton dissociation through hole transfer even under a small energy offset of 0.16 eV. Further fluorination leads to the polymer P4 with increased chain-twisting and mismatched FMO levels with IDIC, showing the lowest PCE of 2.93%. The results demonstrate that quinoxaline-based copolymers are promising donors for efficient OSCs and the fluorination needs to be fine-adjusted to optimize the interchain packing and physicochemical properties of polymers. Additionally, the structure-property correlations from this work provide useful insights for developing wide-band-gap polymers with low-lying highest occupied molecular orbitals to minimize E_loss and maximize V_oc in nonfullerene OSCs for efficient power conversion.

Hybrid PbS Quantum-Dot-in-Perovskite for High-Efficiency Perovskite Solar Cell

In this study, a facile and effective approach to synthesize high-quality perovskite-quantum dots (QDs) hybrid film is demonstrated, which dramatically improves the photovoltaic performance of a perovskite solar cell (PSC). Adding PbS QDs into CH3 NH3 PbI3 (MAPbI3 ) precursor to form a QD-in-perovskite structure is found to be beneficial for the crystallization of perovskite, revealed by enlarged grain size, reduced fragmentized grains, enhanced characteristic peak intensity, and large percentage of (220) plane in X-ray diffraction patterns. The hybrid film also shows higher carrier mobility, as evidenced by Hall Effect measurement. By taking all these advantages, the PSC based on MAPbI3 -PbS hybrid film leads to an improvement in power conversion efficiency by 14% compared to that based on pure perovskite, primarily ascribed to higher current density and fill factor (FF). Ultimately, an efficiency reaching up to 18.6% and a FF of over ≈0.77 are achieved based on the PSC with hybrid film. Such a simple hybridizing technique opens up a promising method to improve the performance of PSCs, and has strong potential to be applied to prepare other hybrid composite materials.

Nanoparticle-Enhanced Silver-Nanowire Plasmonic Electrodes for High-Performance Organic Optoelectronic Devices

Improved performance in plasmonic organic solar cells (OSCs) and organic light-emitting diodes (OLEDs) via strong plasmon-coupling effects generated by aligned silver nanowire (AgNW) transparent electrodes decorated with core-shell silver-silica nanoparticles (Ag@SiO₂ NPs) is demonstrated. NP-enhanced plasmonic AgNW (Ag@SiO₂ NP-AgNW) electrodes enable substantially enhanced radiative emission and light absorption efficiency due to strong hybridized plasmon coupling between localized surface plasmons (LSPs) and propagating surface plasmon polaritons (SPPs) modes, which leads to improved device performance in organic optoelectronic devices (OODs). The discrete dipole approximation (DDA) calculation of the electric field verifies a strongly enhanced plasmon-coupling effect caused by decorating core-shell Ag@SiO₂ NPs onto the AgNWs. Notably, an electroluminescence efficiency of 25.33 cd A^-1 (at 3.2 V) and a power efficiency of 25.14 lm W^-1 (3.0 V) in OLEDs, as well as a power conversion efficiency (PCE) value of 9.19% in OSCs are achieved using hybrid Ag@SiO₂ NP-AgNW films. These are the highest values reported to date for optoelectronic devices based on AgNW electrodes. This work provides a new design platform to fabricate high-performance OODs, which can be further explored in various plasmonic and optoelectronic devices.

Aggregation Strength Tuning in Difluorobenzoxadiazole-Based Polymeric Semiconductors for High-Performance Thick-Film Polymer Solar Cells

High-performance polymer solar cells (PSCs) with thick active layers are essential for large-scale production. Polymer semiconductors exhibiting a temperature-dependent aggregation property offer great advantages toward this purpose. In this study, three difluorobenzoxadiazole (ffBX)-based donor polymers, PffBX-T, PffBX-TT, and PffBX-DTT, were synthesized, which contain thiophene (T), thieno[3,2- b]thiophene (TT), and dithieno[3,2- b:2',3'- d]thiophene (DTT) as the π-spacers, respectively. Temperature-dependent absorption spectra reveal that the aggregation strength increases in the order of PffBX-T, PffBX-TT, and PffBX-DTT as the π-spacer becomes larger. PffBX-TT with the intermediate aggregation strength enables well-controlled disorder-order transition in the casting process of blend film, thus leading to the best film morphology and the highest performance in PSCs. Thick-film PSCs with an average power conversion efficiency (PCE) of 8.91% and the maximum value of 9.10% are achieved using PffBX-TT:PC₇₁BM active layer with a thickness of 250 nm. The neat film of PffBX-TT also shows a high hole mobility of 1.09 cm² V^-1 s^-1 in organic thin-film transistors. When PffBX-DTT and PffBX-T are incorporated into PSCs utilizing PC₇₁BM acceptor, the average PCE decreases to 6.54 and 1.33%, respectively. The performance drop mainly comes from reduced short-circuit current, as a result of nonoptimal blend film morphology caused by a less well-controlled film formation process. A similar trend was also observed in nonfullerene type thick-film PSCs using IT-4F as the electron acceptor. These results show the significance of polymer aggregation strength tuning toward optimal bulk heterojunction film morphology using ffBX-based polymer model system. The study demonstrates that adjusting π-spacer is an effective method, in combination with other important approaches such as alkyl chain optimization, to generate high-performance thick-film PSCs which are critical for practical applications.

Solvent Additives: Key Morphology-Directing Agents for Solution-Processed Organic Solar Cells

Organic photovoltaics (OPV) have the advantage of possible fabrication by energy-efficient and cost-effective deposition methods, such as solution processing. Solvent additives can provide fine control of the active layer morphology of OPVs by influencing film formation during solution processing. As such, solvent additives form a versatile method of experimental control for improving organic solar cell device performance. This review provides a brief history of solution-processed bulk heterojunction OPVs and the advent of solvent additives, putting them into context with other methods available for morphology control. It presents the current understanding of how solvent additives impact various mechanisms of phase separation, enabled by recent advances in in situ morphology characterization. Indeed, understanding solvent additives' effects on film formation has allowed them to be applied and combined effectively and synergistically to boost OPV performance. Their success as a morphology control strategy has also prompted the use of solvent additives in related organic semiconductor technologies. Finally, the role of solvent additives in the development of next-generation OPV active layers is discussed. Despite concerns over their environmental toxicity and role in device instability, solvent additives remain relevant morphological directing agents as research interests evolve toward nonfullerene acceptors, ternary blends, and environmentally sustainable solvents.

Fabrication and Characterization of Hybrid Organic-Inorganic Electron Extraction Layers for Polymer Solar Cells toward Improved Processing Robustness and Air Stability

Organic-inorganic hybrid materials composed of bismuth and diaminopyridine are studied as novel materials for electron extraction layers in polymer solar cells using regular device structures. The hybrid materials are solution processed on top of two different low band gap polymers (PTB7 or PTB7-Th) as donor materials mixed with fullerene PC₇₀BM as the acceptor. The intercalation of the hybrid layer between the photoactive layer and the aluminum cathode leads to solar cells with a power conversion efficiency of 7.8% because of significant improvements in all photovoltaic parameters, that is, short-circuit current density, fill factor, and open-circuit voltage, similar to the reference devices using ZnO as the interfacial layer. However when using thick layers of such hybrid materials for electron extraction, only small losses in photocurrent density are observed in contrast to the reference material ZnO of pronounced losses because of optical spacer effects. Importantly, these hybrid electron extraction layers also strongly improve the device stability in air compared with solar cells processed with ZnO interlayers. Both results underline the high potential of this new class of hybrid materials as electron extraction materials toward robust processing of air stable organic solar cells.

Polymer Solar Cells with 90% External Quantum Efficiency Featuring an Ideal Light- and Charge-Manipulation Layer

Rapid progress in the power conversion efficiency (PCE) of polymer solar cells (PSEs) is beneficial from the factors that match the irradiated solar spectrum, maximize incident light absorption, and reduce photogenerated charge recombination. To optimize the device efficiency, a nanopatterned ZnO:Al₂ O₃ composite film is presented as an efficient light- and charge-manipulation layer (LCML). The Al₂ O₃ shells on the ZnO nanoparticles offer the passivation effect that allows optimal electron collection by suppressing charge-recombination loss. Both the increased refractive index and the patterned deterministic aperiodic nanostructure in the ZnO:Al₂ O₃ LCML cause broadband light harvesting. Highly efficient single-junction PSCs for different binary blends are obtained with a peak external quantum efficiency of up to 90%, showing certified PCEs of 9.69% and 13.03% for a fullerene blend of PTB7:PC₇₁ BM and a nonfullerene blend, FTAZ:IDIC, respectively. Because of the substantial increase in efficiency, this method unlocks the full potential of the ZnO:Al₂ O₃ LCML toward future photovoltaic applications.