Conjugated Block Copolymers as Model Systems to Examine Mechanisms of Charge Generation in Donor–Acceptor Materials

Fully conjugated donor–acceptor block copolymers are established as model systems to elucidate fundamental mechanisms of photocurrent generation in organic photovoltaics. Using analysis of steady‐state photoluminescence quenching, exciton dissociation to a charge transfer state within individual block copolymer chains is quantified. By making a small adjustment to the conjugated backbone, the electronic properties are altered enough to disrupt charge transfer almost entirely. Strong intermolecular coupling of the electron donor is introduced by synthesizing block copolymer nanoparticles. Transient absorption spectroscopy is used to monitor charge generation in block copolymer isolated chains and nanoparticles. While efficient charge transfer is observed in isolated chains, there is no indication of complete charge separation. In the nanoparticles, long‐lived polarons are observed as early as ≈15 ns. Thus, aggregation of electron donors can facilitate efficient charge generation.     

Photoexcitation dynamics in polythiophene/fullerene blends for photovoltaic applications

We used the transient and steady state photomodulation spectroscopies for studying the photoexcitations dynamics in blends of regio-regular poly(3-hexylthiophene) (RR-P3HT) and fullerene in a broad spectral range from 0.13 to 2.25 eV. We found that both localized polarons and singlet excitons are instantaneously photogenerated in the blends. However the photogeneration process of delocalized polarons which contribute to the photocurrent proceeds in two steps: first, within a couple of ps the excitons generated in the polymer domains populate the charge transfer complex states at the RR-P3HT/fullerene interfaces; this is followed by the charge transfer ionization into delocalized charge polarons in the polymer and fullerene constituents within ～20 ps. In contrast, the localized polaron dynamics are unrelated with the excitons and delocalized polarons dynamics. We also report on the occurrence of ultrafast quantum interference anti-resonances between photoinduced infrared-active vibrations and the delocalized polaron band in the blends, which shows the delocalization character of the photogenerated charges that contribute to the photocurrent.     

Nature of Charge Carriers in a High Electron Mobility Naphthalenediimide Based Semiconducting Copolymer

The nature of charge carriers in recently developed high mobility semiconducting donor‐acceptor polymers is debated. Here, localization due to charge relaxation is investigated in a prototypal system, a good electron transporting naphthalenediimide based copolymer, by means of current‐voltage I‐V electrical characteristics and charge modulation spectroscopy (CMS) in top‐gate field‐effect transistors (FETs), combined with density functional theory (DFT) and time dependent DFT (TDDFT) calculations. In particular, pristine copolymer films are compared with films that underwent a melt‐annealing process, the latter leading to a drastic change of the microstructure. Despite the packing modification, which involves also the channel region, both the electron mobility and the energy of polaronic transitions are substantially unchanged upon melt‐annealing. The polaron absorption features can be rationalized and reproduced by TDDFT calculations for isolated charged oligomers. Therefore, it is concluded that in such a high electron mobility copolymer the charge transport process involves polaronic species which are intramolecular in nature and, from a more general point of view, that interchain delocalization of the polaron is not necessary to sustain charge mobilities in the 0.1 to 1 cm² V^{– 1} s^–1 range. These findings contribute to the rationalization of the charge transport process in the recently developed class of donor‐acceptor π‐conjugated copolymers featuring high charge mobilities and complex morphologies.     

Cover Picture: Excitonic and Vibrational Properties of Single‐Walled Semiconducting Carbon Nanotubes (Adv. Funct. Mater. 17/2007)

The cover picture illustrates excited state dynamics of semiconducting single‐walled carbon nanotubes studied theoretically. Sergai Tretiak and Svetlana Kilina report on p. 3405 that absorption of the light quantum leads to spatially delocalized photoexcitation, which can be described as tightly bound excitons and characterized by 2D plots of transition density. The photoexcitation is coupled to the vibrational degrees of freedom, leading to complex exciton‐phonon dynamics, which can be monitored experimentally using ultrafast spectroscopic probes. We review quantum‐chemical studies of the excited‐state electronic structure of finite‐size semiconducting single‐walled carbon nanotubes (SWCNTs) using methodologies previously successfully applied to describe conjugated polymers and other organic molecular materials. The results of our simulations are in quantitative agreement with available spectroscopic data and show intricate details of excited‐state properties and photoinduced vibrational dynamics in carbon nanotubes. We analyze in detail the nature of strongly bound first and second excitons in SWCNTs for a number of different tubes, emphasizing emerging size‐scaling laws. Characteristic delocalization properties of excited states are identified by the underlying photoinduced changes in charge densities and bond orders. Due to the rigid structure, exciton–phonon coupling is much weaker in SWCNTs compared to typical molecular materials. Yet we find that, in the ground state, a SWCNT's surface experiences the corrugation associated with electron–phonon interactions. Vibrational relaxation following photoexcitation reduces this corrugation, leading to a local distortion of the tube surface, which is similar to the formation of self‐trapped excitons in conjugated polymers. The calculated associated Stokes shift increases with enlargement of the tube diameters. Such exciton vibrational phenomena are possible to detect experimentally, allowing for better understanding of photoinduced electronic dynamics in nanotube materials.     

Emission Color Trajectory and White Electroluminescence Through Supramolecular Control of Energy Transfer and Exciplex Formation in Binary Blends of Conjugated Polyrotaxanes

White electroluminescence and fine‐tuning of the emission color from binary blends of a blue‐emitting polymer and a green/yellow‐emitting threaded molecular wire consisting of a conjugated polymer supramolecularly encapsulated by functionalized cyclodextrins are demonstrated. Encapsulation controls the minimum intermolecular distance on the nanoscale, resulting in suppressed energy‐transfer between the blend constituents and reduced formation of interchain charge‐transfer complexes. The use of a green‐emitting polyrotaxane significantly improves the electrical properties with respect to blends of a blue electroluminescent polyrotaxane and leads to a significant reduction in the turn‐on voltage required for achieving white electroluminescence (V_ON = 3 V), with only 20% by weight of the encapsulated material.     

Spectroscopic Studies of Photoexcitations in Regioregular and Regiorandom Polythiophene Films

Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly‐3‐alkyl thiophene with regioregular order, which forms self‐assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain‐like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long‐lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long‐lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano‐type interference mechanism, using the amplitude mode model.     

Open‐Shell Donor–Acceptor Conjugated Polymers with High Electrical Conductivity

Conductive polymers largely derive their electronic functionality from chemical doping, processes by which redox and charge‐transfer reactions form mobile carriers. While decades of research have demonstrated fundamentally new technologies that merge the unique functionality of these materials with the chemical versatility of macromolecules, doping and the resultant material properties are not ideal for many applications. Here, it is demonstrated that open‐shell conjugated polymers comprised of alternating cyclopentadithiophene and thiadiazoloquinoxaline units can achieve high electrical conductivities in their native “undoped” form. Spectroscopic, electrochemical, electron paramagnetic resonance, and magnetic susceptibility measurements demonstrate that this donor–acceptor architecture promotes very narrow bandgaps, strong electronic correlations, high‐spin ground states, and long‐range π‐delocalization. A comparative study of structural variants and processing methodologies demonstrates that the conductivity can be tuned up to 8.18 S cm^?1. This exceeds other neutral narrow bandgap conjugated polymers, many doped polymers, radical conductors, and is comparable to commercial grades of poly(styrene‐sulfonate)‐doped poly(3,4‐ethylenedioxythiophene). X‐ray and morphological studies trace the high conductivity to rigid backbone conformations emanating from strong π‐interactions and long‐range ordered structures formed through self‐organization that lead to a network of delocalized open‐shell sites in electronic communication. The results offer a new platform for the transport of charge in molecular systems.     

Enhancing Field‐Effect Mobility of Conjugated Polymers Through Rational Design of Branched Side Chains

The design of polymer semiconductors possessing effective π–π intermolecular interactions coupled with good solution processability remains a challenge. Structure‐property relationships associated with side chain structure, π–π intermolecular interactions, polymer solubility, and charge carrier transport are reported for a donor–acceptor(1)‐donor–acceptor(2) polymer: 5‐Decylheptadecyl (5‐DH), 2‐tetradecyl (2‐DT), and linear n‐octadecyl (OD) chains are substituted onto a polymer backbone consisting of terthiophene units (T) between two different electron acceptors, benzothiadiazole (B), and diketopyrrolopyrrole (D), pTBTD, to afford pTBTD‐5DH, pTBTD‐2DT, and pTBTD‐OD, respectively. In the 5‐DH side chain, the branching position is remote from the polymer backbone, whereas it is proximal in 2‐DT. This study demonstrates that incorporation of branched side chains where the branching position is remote from the polymer backbone merges the advantages of improved solubility from branched units with effective π–π intermolecular interactions normally associated with linear chains on conjugated polymers. pTBTD‐5DH exhibits superior qualities with respect to the degree of polymerization, solution processability, π–π interchain stacking, and charge carrier transport relative to the other analogs. pTBTD‐5DH exhibits a field‐effect hole mobility of up to 2.95 cm² V^–1 s^–1, a factor of 3–7 times that achieved with pBDT6‐DT and pBDT6‐OD.     

Linking Group Influences Charge Separation and Recombination in All‐Conjugated Block Copolymer Photovoltaics

All‐conjugated block copolymers bring together hole‐ and electron‐conductive polymers and can be used as the active layer of solution‐processed photovoltaic devices, but it remains unclear how molecular structure, morphology, and electronic properties influence performance. Here, the role of the chemical linker is investigated through analysis of two donor–linker–acceptor block copolymers that differ in the chemistry of the linking group. Device studies show that power conversion efficiencies differ by a factor of 40 between the two polymers, and ultrafast transient absorption measurements reveal charge separation only in block copolymers that contain a wide bandgap monomer at the donor–acceptor interface. Optical measurements reveal the formation of a low‐energy excited state when donor and acceptor blocks are directly linked without this wide bandgap monomer. For both samples studied, it is found that the rate of charge recombination in these systems is faster than in poly­mer–polymer and polymer–fullerene blends. This work demonstrates that the linking group chemistry influences charge separation in all‐conjugated block copolymer systems, and further improvement of photovoltaic performance may be possible through optimization of the linking group. These results also suggest that all‐conjugated block copolymers can be used as model systems for the donor–acceptor interface in bulk heterojunction blends.     

Tunable Dopants with Intrinsic Counterion Separation Reveal the Effects of Electron Affinity on Dopant Intercalation and Free Carrier Production in Sequentially Doped Conjugated Polymer Films

Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)‐based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox‐potential‐driven. Remarkably, X‐ray scattering shows that despite their large 2‐nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.     

Photovoltaic Function and Exciton/Charge Transfer Dynamics in a Highly Efficient Semiconducting Copolymer

Exciton dissociation is a key step for the light energy conversion to electricity in organic photovoltaic (OPV) devices. Here, excitonic dissociation pathways in the high‐performance, low bandgap “in‐chain donor–acceptor” polymer PTB7 by transient optical absorption (TA) spectroscopy in solutions, neat films, and bulk heterojunction (BHJ) PTB7:PC₇₁BM (phenyl‐C₇₁‐butyric acid methyl ester) films are investigated. The dynamics and energetics of the exciton and intra‐/intermolecular charge separated states are characterized. A distinct, dynamic, spectral red‐shift of the polymer cation is observed in the BHJ films in TA spectra following electron transfer from the polymer to PC₇₁BM, which can be attributed to the time evolution of the hole–electron spatial separation after exciton splitting. Effects of film morphology are also investigated and compared to those of conjugated homopolymers. The enhanced charge separation along the PTB7 alternating donor–acceptor backbone is understood by intramolecular charge separation through polarized, delocalized excitons that lower the exciton binding energy. Consequently, ultrafast charge separation and transport along these polymer backbones reduce carrier recombination in these largely amorphous films. This charge separation mechanism explains why higher degrees of PCBM intercalation within BHJ matrices enhances exciton splitting and charge transport, and thus increase OPV performance. This study proposes new guidelines for OPV materials development.     

Systematic Investigation of Side‐Chain Branching Position Effect on Electron Carrier Mobility in Conjugated Polymers

Recently, polymer field‐effect transistors have gone through rapid development. Nevertheless, charge transport mechanism and structure‐property relationship are less understood. Here we use strong electron‐deficient benzodifurandione‐based poly(p‐phenylene vinylene) ( BDPPV ) as polymer backbone and develop six BDPPV ‐based polymers ( BDPPV‐C1 to C6 ) with various side‐chain branching positions to systematically study the side‐chain effect on device performance. All the polymers exhibited ambient‐stable n‐type transporting behaviors with the highest electron mobility of up to 1.40 cm² V^?1 s^?1. The film morphologies and microstructures of all the six polymers were systematically investigated. Our results demonstrate that the interchain π–π stacking distance decreases as moving the branching position away from polymer backbones, and an unprecedentedly close π–π stacking distance down to 3.38 Å is obtained for BDPPV‐C4 to C6 . Nonetheless, closer π–π stacking distance does not always correlate with higher electron mobility. Polymer crystallinity, thin film disorder, and polymer packing conformation, which all influenced by side‐chain branching position, are proved to show significant influence on device performance. Our study not only reveals that π–π stacking distance is not the decisive factor on carrier mobility in conjugated polymers but also demonstrates that side‐chain branching position engineering is a powerful strategy to modulate and balance these factors in conjugated polymers.     

Counterion Control and the Spectral Signatures of Polarons,Coupled Polarons,and Bipolarons in Doped P3HT Films

When an electron is removed from a conjugated polymer, such as poly(3-hexylthiophene-2,5-diyl) (P3HT), the remaining hole and associated change in the polymer backbone structure from aromatic to quinoidal are referred to as a polaron. Bipolarons are created by removing the unpaired electron from an already-oxidized polymer segment. In electrochemically-doped P3HT films, polarons, and bipolarons are readily observed, but in chemically-doped P3HT films, bipolarons rarely form. This is explained by studying the effects of counterion position on the formation of polarons, strongly coupled polarons, and bipolarons using both spectroscopic and X-ray diffraction experiments and time-dependent density functional theory calculations. The counterion positions control whether two polarons spin-pair to form a bipolaron or whether they strongly couple without spin-pairing are found. When two counterions lie close to the same polymer segment, bipolarons can form, with an absorption spectrum that is blueshifted from that of a single polaron. Otherwise, polarons at high concentrations do not spin-pair, but instead J-couple, leading to a redshifted absorption spectrum. The counterion location needed for bipolaron formation is accompanied by a loss of polymer crystallinity. These results explain the observed formation order of single polarons, coupled single polarons, and singlet bipolarons in electrochemically- and chemically-doped conjugated polymers.     

Energy Level Modulation of HOMO,LUMO, and Band‐Gap in Conjugated Polymers for Organic Photovoltaic Applications

To devise a reliable strategy for achieving specific HOMO and LUMO energy level modulation via alternating donor‐acceptor monomer units, we investigate a series of conjugated polymers (CPs) in which the electron withdrawing power of the acceptor group is varied, while maintaining the same donor group and the same conjugated chain conformation. Through experiment and DFT calculations, good correlation is identified between the withdrawing strength of the acceptor group, the HOMO and LUMO levels, and the degree of orbital localization, which allows reliable design principles for CPs. Increasing the acceptor strength results in an enhanced charge transfer upon combination with a donor monomer and a more pronounced decrease of the LUMO level. Moreover, while HOMO states remain delocalized along the polymer chain, LUMO states are strongly localized at specific bonds within the acceptor group. The degree of LUMO localization increases with increasing polymer length, which results in a further drop of the LUMO level and converges to its final value when the number of repeat units reaches the characteristic conjugation length. Based on these insights we designed PBT8PT, which exhibits 6.78% power conversion efficiency after device optimization via the additive assisted annealing, demonstrating the effectiveness of our predictive design approach.     

Optical,Electrical, and Magnetic Studies of Organic Solar Cells Based on Low Bandgap Copolymer with Spin ½ Radical Additives

The charge photogeneration and recombination processes in organic photovoltaic solar cells based on blend of the low bandgap copolymer, PTB7 (fluorinated poly‐thienothiophene‐benzodithiophene) with C60‐PCBM using optical, electrical, and magnetic measurements in thin films and devices is studied. A variety of steady state optical and magneto‐optical techniques were employed, such as photoinduced absorption (PA), magneto‐PA, doping‐induced absorption, and PA‐detected magnetic resonance (PADMR); as well as picosecond time‐resolved PA. The charge polarons and triplet exciton dynamics in films of pristine PTB7, PTB7/fullerene donor–acceptor (D–A) blend is followed. It is found that a major loss mechanism that limits the power conversion efficiency (PCE) of PTB7‐based solar cell devices is the “back reaction” that leads to triplet exciton formation in the polymer donor from the photogenerated charge‐transfer excitons at the D–A interfaces. A method of suppressing this “back reaction” by adding spin½ radicals Galvinoxyl to the D–A blend is presented; this enhances the cell PCE by ≈30%. The same method is not effective for cells based on PTB7/C70‐PCBM blend, where high PCE is reached even without Galvinoxyl radical additives.     

Plastic Solar Cells

Recent developments in conjugated‐polymer‐based photovoltaic elements are reviewed. The photophysics of such photoactive devices is based on the photo‐induced charge transfer from donor‐type semiconducting conjugated polymers to acceptor‐type conjugated polymers or acceptor molecules such as Buckminsterfullerene, C₆₀. This photo‐induced charge transfer is reversible, ultrafast (within 100 fs) with a quantum efficiency approaching unity, and the charge‐separated state is metastable (up to milliseconds at 80 K). Being similar to the first steps in natural photosynthesis, this photo‐induced electron transfer leads to a number of potentially interesting applications, which include sensitization of the photoconductivity and photovoltaic phenomena. Examples of photovoltaic architectures are presented and their potential in terrestrial solar energy conversion discussed. Recent progress in the realization of improved photovoltaic elements with 3 % power conversion efficiency is reported.     

Photoinduced Hole Transfer Becomes Suppressed with Diminished Driving Force in Polymer‐Fullerene Solar Cells While Electron Transfer Remains Active

Device performance and photoinduced charge transfer are studied in donor/acceptor blends of the oxidation‐resistant conjugated polymer poly[(4,8‐bis(2‐hexyldecyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(2,5‐bis(3‐dodecylthiophen‐2‐yl)benzo[1,2‐d;4,5‐d′]bisthiazole)] (PBTHDDT) with the following fullerene acceptors: [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM); [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM); and the indene‐C₆₀ bis‐adduct IC₆₀BA). Power conversion efficiency improves from 1.52% in IC₆₀BA‐based solar cells to 3.75% in PC₇₁BM‐based devices. Photoinduced absorption (PIA) of the PBTHDDT:fullerene blends suggests that exciting the donor polymer leads to long‐lived positive polarons on the polymer and negative polarons on the fullerene in all three polymer fullerene blends. Selective excitation of the fullerene in PC₇₁BM or PC₆₁BM blends also generates long‐lived polarons. In contrast, no discernible PIA features are observed when selectively exciting the fullerene in a PBTHDDT/IC₆₀BA blend. A relatively small driving force of ca. 70 meV appears to sustain charge separation via photoinduced hole transfer from photoexcited PC₆₁BM to the polymer. The decreased driving force for photoinduced hole transfer in the IC₆₀BA blend effectively turns off hole transfer from IC₆₀BA excitons to the host polymer, even while electron transfer from the polymer to the IC₆₀BA remains active. Suppressed hole transfer from fullerene excitons is a potentially important consideration for materials design and device engineering of organic solar cells.     

Dielectric–Semiconductor Interface Limits Charge Carrier Motion at Elevated Temperatures and Large Carrier Densities in a High‐Mobility Organic Semiconductor

The fundamental nature of charge transport in highly ordered organic semiconductors is under constant debate. At cryogenic temperatures, effects within the semiconductor such as traps or the interaction of charge carriers with the insulating substrate (dipolar disorder or Fröhlich polarons) are known to limit carrier motion. In comparison, at elevated temperatures, where charge carrier mobility often also decreases as function of temperature, phonon scattering or dynamic disorder are frequently discussed mechanisms, but the exact microscopic cause that limits carrier motion is debated. Here, the mobility in the temperature range between 200 and 420 K as function of carrier density is explored in highly ordered perylene‐diimide from 3 to 9 nm thin films. It is observed that above room temperature increasing the gate electric field or decreasing the semiconducting film thickness leads to a suppression of the charge carrier mobility. Via X‐ray diffraction measurements at various temperatures and electric fields, changes of the thin film structure are excluded as cause for the observed mobility decrease. The experimental findings point toward scattering sites or traps at the semiconductor–dielectric interface, or in the dielectric as limiting factor for carrier mobility, whose role is usually neglected at elevated temperatures.     

Enhanced Charge Separation Efficiency in DNA Templated Polymer Solar Cells

The insertion of a DNA nanolayer into polymer based solar cells, between the electron transport layer (ETL) and the active material, is proposed to improve the charge separation efficiency. Complete bulk heterojunction donor–acceptor solar cells of the layered type glass/electrode (indium tin oxide)/ETL/P3HT:PC₇₀BM/hole transport layer/electrode (Ag) are investigated using femtosecond transient absorption spectroscopy both in the NIR and the UV–vis regions of the spectrum. The transient spectral changes indicate that when the DNA is deposited on the ZnO nanoparticles (ZnO‐NPs) it can imprint a different long range order on the poly(3‐hexylthiophene) (P3HT) polymer with respect to the non‐ZnO‐NPs/DNA containing cells. This leads to a larger delocalization of the initially formed exciton and its faster quenching which is attributed to more efficient exciton dissociation. Finally, the temporal response of the NIR absorption shows that the DNA promotes more efficient production of charge transfer states and free polarons in the P3HT cation indicating that the increased exciton dissociation correlates with increased charge separation.     

Difluorobenzothiadiazole‐Based Small‐Molecule Organic Solar Cells with 8.7% Efficiency by Tuning of π‐Conjugated Spacers and Solvent Vapor Annealing

The synthesis of a series of tetrafluorine‐substituted, wide‐bandgap, small molecules consisting of various π‐conjugated spacers (furan, thiophene, selenophene) between indacenodithiophene as the electron‐donating core and the electron‐deficient difluorobenzothiadiazole unit is reported and the effect of the π‐conjugated spacers on the photovoltaic properties is investigated. The alteration of the π‐conjugated spacer enables fine‐tuning of the photophysical properties and energy levels of the small molecules, and allows the adjustment of the charge‐transport properties, the morphology of the photoactive films, as well as their photovoltaic properties. Moreover, most of these devices exhibit superior device performances after CH₂Cl₂ solvent annealing than without annealing, with a high fill factor (0.70–0.75 for all cases). Notably, the devices based on the new molecule BIT4FTh (with thiophene as the spacer) show an outstanding PCE of 8.7% (with an impressive FF of 0.75), considering its wide‐bandgap (1.81 eV), which is among the highest efficiencies reported so far for small‐molecules‐based solar cells. The morphologies of the photoactive layers with/without CH₂Cl₂ solvent annealing are characterized by atomic force microscopy, transmission electron microscopy and two‐dimensional grazing incidence X‐ray diffraction analysis. The results reported here clearly indicate that highly efficient small‐molecules‐based solar cells can be achieved through rational design of their molecular structure and optimization of the phase‐separated morphology via an adapted solvent–vapor annealing process.