Impact of Nonfullerene Molecular Architecture on Charge Generation,Transport, and Morphology in PTB7‐Th‐Based Organic Solar Cells

Despite the rapid development of nonfullerene acceptors (NFAs), the fundamental understanding on the relationship between NFA molecular architecture, morphology, and device performance is still lacking. Herein, poly[[4,8‐bis[5‐(2‐ethylhexyl)thiophene‐2‐yl]benzo[1,2‐b:4,5‐b0]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]‐thieno[3,4‐b]thiophenediyl]] (PTB7‐Th) is used as the donor polymer to compare an NFA with a 3D architecture (SF‐PDI4) to a well‐studied NFA with a linear acceptor–donor–acceptor (A–D–A) architecture (ITIC). The data suggest that the NFA ITIC with a linear molecular structure shows a better device performance due to an increase in short‐circuit current ( J_sc) and fill factor (FF) compared to the 3D SF‐PDI4. The charge generation dynamics measured by femtosecond transient absorption spectroscopy (TAS) reveals that the exciton dissociation process in the PTB7‐Th:ITIC films is highly efficient. In addition, the PTB7‐Th:ITIC blend shows a higher electron mobility and lower energetic disorder compared to the PTB7‐Th:SF‐PDI4 blend, leading to higher values of J_sc and FF. The compositional sensitive resonant soft X‐ray scattering (R‐SoXS) results indicate that ITIC molecules form more pure domains with reduced domain spacing, resulting in more efficient charge transport compared with the SF‐PDI4 blend. It is proposed that both the molecular structure and the corresponding morphology of ITIC play a vital role for the good solar cell device performance.     

Efficient Ternary Organic Solar Cells Enabled by the Integration of Nonfullerene and Fullerene Acceptors with a Broad Composition Tolerance

The ternary structure that combines fullerene and nonfullerene acceptors in a photoactive layer is demonstrated as an effective approach for boosting the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Here, highly efficient ternary OSCs comprising a wide‐bandgap polymer donor (PBT1‐C), a narrow‐bandgap nonfullerene acceptor (IT‐2F), and a typical fullerene derivative (PC₇₁BM) are reported. It is found that the addition of PC₇₁BM into the PBT1‐C:IT‐2F blend not only increases the device efficiency up to 12.2%, but also improves the ambient stability of the OSCs. Detailed investigations indicate that the improvement in photovoltaic performance benefits from synergistic effects of increased photon‐harvesting, enhanced charge separation and transport, suppressed trap‐assisted recombination, and optimized film morphology. Moreover, it is noticed that such a ternary system exhibits excellent tolerance to the PC₇₁BM component, for which PCEs over 11.2% can be maintained throughout the whole blend ratios, higher than that (11.0%) of PBT1‐C:IT‐2F binary reference device.     

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.     

Acceptor-rich bulk heterojunction polymer solar cells with balanced charge mobilities

In this work, we reported efficient polymer solar cells with balanced hole/electron mobilities tuned by the acceptor content in bulk heterojunction blend films. The photovoltaic cells were fabricated with two new wide band-gap D-A polymers PBDDIDT and PBDDIDTT as the donor material. The molecular conformations of new polymers are carefully evaluated by theoretical calculations. The results of photovoltaic studies show that two devices reach their optimal conditions with rich PC₇₁BM content up to 80% in blend films, which is uncommon with most of reported PSCs. The as-cast devices based on PBDDIDT and PBDDIDTT reveal good photovoltaic performance with PCE of 7.04% and 6.40%, respectively. The influence of PC₇₁BM content on photovoltaic properties is further detailed studied by photoluminescence emission spectra, charge mobilities and heterojunction morphology. The results exhibit that more efficient charge transport between donor and acceptor occurs in rich PC₇₁BM blend films. Meanwhile, the hole and electron mobilities are simultaneously enhanced and afford a good balance in rich PC₇₁BM blend films (D/A, 1:4) which is critical for the improvement of current density and fill factors.     

Revealing the effect of donor/acceptor intermolecular arrangement on organic solar cells performance based on two-dimensional conjugated small molecule as electron donor

A series of solution processed organic solar cells (OSCs) were fabricated with a two-dimensional conjugated small molecule SMPV1 as electron donor and fullerene derivatives PC₇₁BM or ICBA as electron acceptor. The champion power conversion efficiency (PCE) of OSCs arrives to 7.05% for the cells with PC₇₁BM as electron acceptor. A relatively large open circuit voltage (V_OC) of 1.15 V is obtained from cells using ICBA as electron acceptor with an acceptable PCE of 2.54%. The fill factor (FF) of OSCs is 72% or 61% for the cells with PC₇₁BM or ICBA as electron acceptor, which is relatively high value for small molecule OSCs. The relatively low performance of OSCs with ICBA as electron acceptor indicates that ICBA cannot play positive role in photoelectric conversion processes, which is very similar to the phenomenon observed from the OSCs with high efficient narrow band gap polymers other than P3HT as electron donor, the underlying reason is still in debate. The SMPV1 has strong self-assemble ability to form an ordered two dimensional lamellar structure, which provides an effective platform to investigate the effect of electron acceptor chemical structure on the performance of OSCs. Experimental results exhibit that ICBA molecules may prefer to vertical cross-intercalation among side chains of SMPV1, PC₇₁BM molecules may have better miscibility with SMPV1 in the active layer. The different donor/acceptor (D/A) intermolecular arrangement strongly influences photon harvesting, exciton dissociation and charge carrier transport, which may provide a new sight on performance improvement of OSCs by adjusting D/A intermolecular arrangements.     

Hydrogen Bond Induced Green Solvent Processed High Performance Ternary Organic Solar Cells with Good Tolerance on Film Thickness and Blend Ratios

For comprehensive development of organic solar cells (OSCs), some factors such as environmental stability, low cost, insensitive film thickness, component contents tolerance, and green preparation processes are equally crucial to achieve high power conversion efficiencies (PCEs). In this work, a small molecule 3‐(diethylamino)‐7‐imino‐7H‐benzo[4,5]imidazo[1,2‐a]chromeno[3,2‐c]pyridine‐6‐carbonitrile (DIBC), which is commercially available at low cost, is utilized to realize high‐performance ternary OSCs. Demonstrated via Fourier transform infrared and 2D‐¹HNMR, DIBC can form hydrogen bond interactions with [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) in solid films. Further electrostatic potential (ESP) calculations indicate that the hydrogen bond interaction enhances the ESP of PC₇₁BM and accelerates charge transport between donor and acceptor. As a result, poly(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b0]dithiophene‐2,6‐diylalt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl (PTB7‐Th):DIBC:PC₇₁BM‐based ternary OSC achieves a maximum efficiency of 12.17%, which is the best result of green solvent processed fullerene OSCs at present. It is noteworthy that the ternary OSCs also show great tolerance to film thickness and blend ratios. These unique properties are attributed to the hydrogen‐bond‐linked DIBC and PC₇₁BM, which modulates molecule distribution and improves film morphology with an interpenetrating network structure. Furthermore, the DIBC containing device also exhibits good thermal and light radiation stability. These results illustrate that intermolecular hydrogen bond interaction has great potential for realizing high‐performance OSCs.     

Energy levels in dilute-donor organic solar cell photocurrent generation: A thienothiophene donor molecule study

To investigate photocurrent generation mechanisms in these organic solar cells (OSCs), we design and synthesize four thienothiophene (TT)-based small-molecule donors with the highest occupied molecular orbital (HOMO) levels varying from −6.4 eV to −5.1 eV, which span across the HOMO value of the [6,6]-phenyl-C70-butyric acid methyl ester (PC₇₁BM) acceptor. We measure TT-based donor:PC₇₁BM films’ electronic and optical properties, OSC current density-voltage characteristic, and external quantum efficiency, and perform density functional theory (DFT) calculations. Our results show that photocurrent generation depends strongly on the substitutions of the center TT groups, cyano (-CN) versus hexyloxy (-OHex). With 1 wt% donor, TTOHex:PC₇₁BM devices produce seven times, increasing to twelve times for 5 wt % donor, higher photocurrent than neat PC₇₁BM devices. In contrast, TTCN:PC₇₁BM devices do not generate additional photocurrent even with 10 wt% donor. The photocurrent generation in TT-based donor:PC₇₁BM devices depends critically on the HOMO value of the donor molecule with respect to that of PC₇₁BM, indicating the importance of type II energy level alignment to facilitate exciton dissociation at the donor-acceptor interface. The photovoltage of all TT:PC₇₁BM devices are comparable to neat PC₇₁BM devices, 0.85–0.90 V, with a low voltage loss due to non-radiative recombination. The fill factor of TTOHex:PC₇₁BM devices are low due to the low hole mobility, ~10⁻⁸ cm²/V. Following exciton dissociation, hole transport is analyzed according to three possible mechanisms: tunneling, percolation pathways, and hole back transfer. We find that the hole back transfer mechanism can explain all experimental results and therefore is the primary hole transport mechanism for photocurrent generation in TT-based donor:PC₇₁BM dilute-donor OSCs.     

Origin of Reduced Bimolecular Recombination in Blends of Conjugated Polymers and Fullerenes

Bimolecular charge carrier recombination in blends of a conjugated copolymer based on a thiophene and quinoxaline (TQ1) with a fullerene derivative ((6,6)‐phenyl‐C₇₁‐butyric acidmethyl ester, PC₇₁BM) is studied by two complementary techniques. TRMC (time‐resolved microwave conductance) monitors the conductance of photogenerated mobile charge carriers locally on a timescale of nanoseconds, while using photo‐CELIV (charge extraction by linearly increasing voltage) charge carrier dynamics are monitored on a macroscopic scale and over tens of microseconds. Despite these significant differences in the length and time scales, both techniques show a reduced Langevin recombination with a prefactor ζ close to 0.05. For TQ1:PC₇₁BM blends, the ζ value is independent of temperature. On comparing TRMC data with electroluminescence measurements it is concluded that the encounter complex and the charge transfer state have very similar energetic properties. The ζ value for annealed poly(3‐hexylthiophene) (P3HT):(6,6)‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM) is approximately 10^?4, while for blend systems containing an amorphous polymer ζ values are close to 1. These large differences can be related to the extent of charge delocalization of opposite charges in an encounter complex. Insight is provided into factors governing the bimolecular recombination process, which forms a major loss mechanism limiting the efficiency of polymer solar cells.     

Cubic‐Like Bimolecular Crystal Evolution and over 12% Efficiency in Halogen‐Free Ternary Solar Cells

In this study, the solubility properties of a given ternary blend set, with two donors (poly(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b;4,5‐b′]dithiophene‐2,6‐diyl‐alt‐(4‐(2‐ethylhexyl)‐3‐fluorothieno[3,4‐b]thiophene‐)‐2‐carboxylate‐2‐6‐diyl (PTB7‐Th) and benzo[1,2‐b;4,5‐b′]dithiophene‐based small molecule (DR3TSBDT)) and one acceptor ([6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM)), in a series of solvents are determined, and active material–solvent interactions are used as an aid for finding suitable nonchlorinated solvents to achieve effective ternary organic solar cells (OSCs) based on PTB7‐Th:DR3TSBDT:PC₇₁BM. An exceptional power conversion efficiency (PCE) as high as 12.3% (certified 11.94%) is obtained using the developed nonhalogenated processing system. In‐depth investigations (morphology, charge mobility, recombination dynamics, and OSC characteristics) uncover the underlying structure–property relationships as a function of the chosen nonhalogenated systems. Another intriguing finding of this study is the formation of a cubic bimolecular crystal structure of PTB7‐Th:PC₇₁BM in a nonhalogenated system, which is the first such demonstration in blend films. This sheds light upon the fact that the physical properties of a material applied from different solutions may surpass the variation in the properties between two material having totally different molecular structure. Therefore, this work not only offers important scientific insights into developing highly efficient and eco‐friendly OSCs but also improves our understanding of achievable bimolecular crystals with an intercalated structure.     

Acetylene-bridged D–A–D type small molecule comprising pyrene and diketopyrrolopyrrole for high efficiency organic solar cells

To explore effects of acetylene-incorporation, acetylene-bridged D–A–D type small molecules ((HD/OD)-DPP-A-PY) using pyrene as a donor and diketopyrrolopyrrole as an acceptor were successfully synthesized and characterized. (HD/OD)-DPP-A-PY exhibited planar back-bone, conjugation extension, enhanced light absorption, and low HOMO energy level. Combined with the advanced properties, solution-processed OSCs based on a blend of HD-DPP-A-PY as a donor and [6,6]-phenyl-C₇₁-butyric-acid-methyl-ester (PC₇₀BM) as an acceptor exhibited PCEs as high as 3.15%.     

An insight on oxide interlayer in organic solar cells: From light absorption and charge collection perspectives

A comprehensive study of the effect of oxide interlayer on the performance of bulk-heterojunction organic solar cells (OSCs), based on poly[[4,8-bis[(2-ethylhexyl)oxy] benzo [1,2-b:4,5-b'] dithiophene-2,6- diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno [3,4-b] thiophenediyl]] (PTB7): [6,6]-phenyl C71 butyric acid methyl ester (PC₇₀BM) blend system, is carried out by optical simulation, interfacial exciton dissociation and charge collection analyses. It is found that a PTB7:PC₇₀BM blend layer thickness optimized for maximum light absorption in OSCs does not generally give rise to the highest power conversion efficiency (PCE). OSCs, e.g., based on PTB7:PC₇₀BM blend system, can benefit from the oxide interlayer in two ways, (1) to enhance the built-in potential for reducing recombination loss of the photo-generated charges, and (2) to improve charge collection by removal of unfavorable interfacial exciton dissociation. The combined effects result in ∼20% improvement in PCE over an optimized control cell, having an identical layer configuration without an oxide interlayer.     

Solution-processable tetrazine and oligothiophene based linear A–D–A small molecules: Synthesis,hierarchical structure and photovoltaic properties

A series of high coplanar alternative linear small molecules with acceptor–donor–acceptor (A–D–A) structure containing electron-accepting tetrazine (Tz) moiety and electron-donating oligothiophenes (OTs) moiety, alkylated thiophene attached to both sides of the Tz moiety were designed and synthesized. The influences of varied oligothiophene length on small molecules’ optical and electrochemical properties, crystallization, self assembling morphology in blend film with (6,6)-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), and photovoltaic properties for the application as donor materials in organic solar cells (OSCs) were studied. The optical and electrochemical properties of small molecules showed that the HOMO and LUMO energy levels were determined by the number of OTs moiety and electron-accepting ability of Tz in the alternative small molecules, respectively. Meanwhile, the varied OT moieties can significantly affect the hierarchical structures when mixed with PC₆₁BM. The molecule with intermediate conjugate moity length showed the highest ordering in its crystalline state, as revealed by differential scanning calorimetry (DSC) and X-ray diffraction experiments, and best photovoltaic properties when blended together with PC₆₁BM or (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as active layer in photovoltaic devices. The results indicate that hierarchical structures controlled by adjusting the conjugate moity length of small molecules is an effective way to improve the performance of OSCs. The photovoltaic device based on TT(HTTzHT)₂:PC₇₁BM with 1% DIO additives showed the best performance, with a J_sc of 7.87 mA/cm² and a PCE of 3.24%.     

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.     

A two-dimension-conjugated small molecule for efficient ternary organic solar cells

Ternary organic solar cells (OSCs) are burgeoning as one of the effective strategies to achieve high power conversion efficiencies (PCEs) by incorporating a third component with a complementary absorption into the binary blends. In this study, we presented a new two-dimension-conjugated small molecule denoted by DR3TBDTTVT, which alone gave rise to a best PCE of 5.71% with acceptor PC₇₁BM as active layer. Given the complementary absorption with PTB7-Th, DR3TBDTTVT was doped into (PTB7-Th:PC₇₁BM)-based binary blends, and ternary OSCs were developed. The ternary OSCs with 10 wt% of DR3TBDTTVT displayed improved hole-mobility, reduced device resistance and better phase separation of active layer, thus leading to an impressive PCE of 7.77% with open-circuit voltage of 0.77 V, short-circuit density of 14.52 mA cm⁻² and fill factor of 70.3%. Ternary OSCs well make up for the light-harvesting insufficiency of binary OSCs, and this research provides a new material for the improvement of PCEs for single-junction OSCs.     

Functionalized Graphene as an Electron‐Cascade Acceptor for Air‐Processed Organic Ternary Solar Cells

Functionalized graphene nanoflakes (GNFs) are used as an electron‐cascade acceptor material in air‐processed organic ternary bulk heterojunction solar cells. The functionalization is realized via the attachment of the ethylenedinitrobenzoyl (EDNB) molecule to the GNFs. Simulation and experimental results show that such nanoscale modification greatly influences the density of states near the Fermi level. Consequently, the GNF‐EDNB blend presents favorable highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels to function as a bridge structure between the poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and the [6,6]‐phenyl‐C71‐butyric‐acid‐methyl‐ester (PC₇₁BM). The improved exciton dissociation and charge transport are associated with the better energy level alignment of the ternary blend and the high electrical conductivity of the GNFs, which act as additional electron transport channels within the photoactive layer. The resulting PCDTBT/GNF‐EDNB/PC₇₁BM ternary organic solar cells, fabricated entirely under ambient conditions, exhibit an average power conversion efficiency enhancement of ≈18% as compared with the binary blend PCDTBT/PC₇₁BM.     

Fluorine functionalized asymmetric indo [2,3-b]quinoxaline framework based D-A copolymer for fullerene polymer solar cells

Innovating molecular structure of copolymer donor materials is still one of the prominent approach to obtain high-performance polymer solar cells (PSCs). In this paper, two novel wide bandgap (WBG) copolymers, namely PBDTTS-IQ and PBDTTS-DFIQ, based on asymmetric planar aromatic core indo [( Li et al., 2012; Wang et al., 2020) 2,32,3-b]quinoxaline (IQ) as acceptor unit through tuning side chains with fluorine (F) atom engineering and exemplary alkylthio-thienyl substituted benzodithiophene (BDTTS) donor group, are synthesized and finally employed as the photovoltaic donor materials for fullerene polymer solar cells (PSCs). After blending with PC₇₁BM acceptor, the PBDTTS-DFIQ:PC₇₁BM blend film presented better efficient exciton dissociation and charge extraction, more balanced electron/hole mobility (μ_h/μ_e), and nice morphology in comparison with PBDTTS-IQ:PC₇₁BM blend film. Encouragingly, the PBDTTS-DFIQ:PC₇₁BM based PSCs exhibits a higher power conversion efficiency (PCE) of 7.4% than that of the device based on the PBDTTS-IQ:PC₇₁BM blend with a PCE of 4.96%, which thanks to an enhancement of open-circuit voltage (V_oc) of 0.84 V, short current density (J_sc) of 13.26 mA cm⁻² and fill factor (FF) of 66.00% simultaneously. These results demonstrate that this asymmetric IQ framework is a wonderful acceptor moiety to build light-harvesting copolymers for highly efficient PSCs.     

Molecular Orientation Unified Nonfullerene Acceptor Enabling 14% Efficiency As‐Cast Organic Solar Cells

Molecular orientation and π–π stacking of nonfullerene acceptors (NFAs) determine its domain size and purity in bulk‐heterojunction blends with a polymer donor. Two novel NFAs featuring an indacenobis(dithieno[3,2‐b:2?,3?‐d]pyrrol) core with meta‐ or para‐alkoxyphenyl sidechains are designed and denoted as m‐INPOIC or p‐INPOIC , respectively. The impact of the alkoxyl group positioning on molecular orientation and photovoltaic performance of NFAs is revealed through a comparison study with the counterpart ( INPIC‐4F ) bearing para‐alkylphenyl sidechains. With inward constriction toward the conjugated backbone, m‐INPOIC presents predominant face‐on orientation to promote charge transport. The as‐cast organic solar cells (OSCs) by blending m‐INPOIC and PBDB‐T as active layers exhibit a power conversion efficiency (PCE) of 12.1%. By introducing PC₇₁BM as the solid processing‐aid, the ternary OSCs are further optimized to deliver an impressive PCE of 14.0%, which is among the highest PCEs for as‐cast single‐junction OSCs reported in literature to date. More attractively, PBDB‐T: m‐INPOIC :PC₇₁BM based OSCs exhibit over 11% PCEs even with an active layer thickness over 300 nm. And the devices can retain over 95% of PCE after storage for 20 days. The outstanding tolerance to film thickness and outstanding stability of the as‐cast devices make m‐INPOIC a promising candidate NFA for large‐scale solution‐processable OSCs.     

Enhancing efficiency for additive–free blade–coated small–molecule solar cells by thermal annealing

Blade coating was successfully applied to realise high-efficiency small-molecule organic solar cells (OSCs) with a solution-processed active layer comprising a small organic molecule DR3TBDTT with a benzo[1,2–b:4,5–b′]dithiophene (BDT) unit as the central building block as the donor and [6,6]–phenyl–C71–butyric acid methyl ester (PC₇₁BM) as the acceptor. Using chloroform as the solvent, a DR3TBDTT/PC₇₁BM blend active layer without an additive was effectively formed through blade coating. The power conversion efficiency (PCE) of small organic molecule solar cells was enhanced by 3.7 times through thermal annealing at 100 °C. This method produces OSCs with a high PCE of up to 6.69%, with an open circuit voltage (V_oc) of 0.97 V, a short-circuit current density (J_sc) of 12.60 mA/cm², and a fill factor (FF) of 0.55.     

Enhancing the performance of porphyrin based solar cells by the Bipy coordination

Solution processed organic solar cells (OSCs) often suffer from low charge mobilities partially due to the disordered non-crystalline or amorphous morphology of their films. In this study, 4,4′-bipyridyl (Bipy) is introduced to coordinate a zinc(II) porphyrin to form porphyrin complexes. A significant enhanced hole mobility and solar cell device performance are attained when the ratio of Bipy is 0.25 in blend films with [6, 6]-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM). When [6, 6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) was used instead of PC₆₁BM, the PCE is enhanced to 2.83% in the presence of 0.25 equiv Bipy, which is an increase of 53% compared to that without Bipy.     

Synthesis of organic molecule donor for efficient organic solar cells with low acceptor content

A new planar A-D-A structured organic small molecule semiconductor (O-SMS) with dialkyl-thiophene substituted benzodithiophene (BDT) as central electron-rich core flanked by relatively electron-deficient units of [1,2,5]thiadiazolo[3,4-c]pyridine (PTz) and terminated with alkyl-bithiophene as π-conjugated end-caps, BDTDPTz, was designed and synthesized for the application as donor material in organic solar cells (OSCs). BDTDPTz possesses wider absorption spectra with an optical bandgap of 1.65 eV, lower the highest occupied molecular orbital (HOMO) energy level of −5.42 eV and highly crystalline structures in solid films. The OSCs based on BDTDPTz:PC₇₁BM blend film with a lower PC₇₁BM content of 40% demonstrate a power conversion efficiency (PCE) of 6.28% with a relatively higher open-circuit voltage of 0.868 V and short circuit current density of 12.83 mA cm⁻². These results indicate that highly coplanar and crystalline structure of BDTDPTz can effectively reduce the content of fullerene acceptor in the active layer and then enhance the absorption and PCE of the OSCs.