A star-shaped oligothiophene with triphenylamine as core and octyl cyanoacetate as end groups for solution-processed organic solar cells

A new star-shaped D–π–A molecule with triphenylamine (TPA) as core and donor unit, octyl cyanoacetate (CA) as end group and acceptor unit, and 2,2′-bithiophene vinylene (bTV) as π bridge, S(TPA-bTV-CA) was designed and synthesized for the application as donor materials in solution-processed bulk-heterojunction organic solar cells (OSCs). The compound is soluble in common organic solvents. The thermal, optical and electrochemical properties of the star molecule were studied. The OSC devices were fabricated by spin-coating the blend solution of the molecule as donor and PC₇₀BM as acceptor (1:3, w/w). The OSC device based on S(TPA-bTV-CA)/PC₇₀BM demonstrated a high open circuit voltage of 0.91 V, a short circuit current density of 4.64 mA/cm², a fill factor (FF) of 50%, corresponding to a power conversion efficiency of 2.1%, under the illumination of AM 1.5, 100 mW/cm².     

Dicyano-functionalized chlorophyll derivatives with ambipolar characteristic for organic photovoltaics

Two ambipolar chlorophyll derivatives, namely, 3²,3²-dicyano-pyropheophorbide-a (Chl-1) and methyl 13¹-deoxo-13¹-(dicyanomethylene) pyropheophorbide-a (Chl-2), were synthesized for use as either the electron acceptor or the electron donor in organic planar-heterojunction solar cells. Despite the higher electron mobilities of these chlorophyll derivatives compared with their hole mobilities, devices using them as the electron donor with fullerene C₇₀ give much better photovoltaic performance than when they are used as the electron acceptor with copper phthalocyanine. In these Chl-based solar cells, the energy gap between the LUMO levels of the donor and acceptor molecules substantially affects the charge separation and resultant photocurrent and photovoltaic performance. The highest solar energy-to-electricity conversion efficiency of up to 2.3% has been achieved using the Chl-2/C₇₀ solar cell, under AM1.5 solar illumination (100 mW/cm²) after thermal annealing of the device. It was also confirmed that the electron mobility of blend films containing Chls and fullerene derivative PC₇₀BM was determined not only by the electron mobility of PC₇₀BM but also by that of Chls.     

Synthesis and characterization of the conjugated polymers tethered with dipolar side chains containing a benzothiadiazole entity for bulk heterojunction solar cells

New conjugated copolymers (P1?P3) containing dipolar side chains connected to the main chain via triphenylamine donors have been synthesized and characterized. The side chains of these polymers have an electron deficient benzothiadiazole moiety in the spacer, but with different acceptors at the end. By changing the acceptor moieties of the side chain, the absorption spectra and HOMO/LUMO gaps of the polymers can be fine-tuned, ranging from 1.86 to 1.59 eV. Solution processed bulk heterojunction (BHJ) solar cells using these polymers as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor were fabricated and measured under 100 mW cm^?2 of AM 1.5 illumination. The cell based on the blend of P1/PCBM (1:1, w/w) exhibited the highest power conversion efficiency of 1.78%, with open circuit voltage (V_oc) = 0.79 V, short circuit current (J_sc) = 6.63 mA cm^?2 and fill factor (FF) = 0.34, respectively.     

Solvent–vapor induced morphology reconstruction for efficient PCDTBT based polymer solar cells

This paper reports polymer solar cells with a 7% power conversion efficiency (PCE) based on bulk heterojunction (BHJ) composites of the alternating co-polymer, poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT), and the fullerene derivative [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM). As confirmed by transmission electron microscopy, solvent–vapor annealing (SVA) of the thin (70 nm) BHJ photoactive layer by exposure to chloroform vapor, for a short period of time (30 s) after deposition, leads to reconstructed nanoscale morphology of donor/acceptor domains, well-dispersed fullerene phase and effective photo-absorption of BHJ. Consequently, SVA-reconstructed devices with a PCDTBT:PC₇₁BM blend ratio of 1:5 (wt%) exhibit ～50% improvement in PCE, with short-circuit current J_sc = 15.65 mA/cm², open-circuit voltage V_oc = 0.87 V, and PCE = 7.03%, in comparison to those of the 1:4 (wt%) blends with SVA treatment.     

Synthesis and photovoltaic properties of new small molecules with rhodanine derivative as the end-capped blocks

Two new acceptor–donor–acceptor (A–D–A) type small molecules DCAO3TIDT and DCNR3TIDT, with 4,4,9,9-tetrakis(4-(dodecyloxy)phenyl)-4,9-dihydro-s-indaceno-[1,2-b:5,6-b′]dithiophene (IDT) as the core group and 2-ethylhexyl cyanoacetate (CAO) and 2-(1,1-dicyanomethylene)-3-octyl rhodanine (CNR) as different end-capped blocks, have been designed and synthesized. Both of them have been employed as donor for solution-processed bulk hetero-junction (BHJ) organic solar cells (OSCs). The two compounds showed deep highest occupied molecular orbital (HOMO) energy levels (∼−5.30 eV) and strong absorption. The DCAO3TIDT and DCNR3TIDT with PC₇₁BM as acceptor based BHJ solar cell devices showed short circuit current density (J_sc) of 6.93 mA/cm² and 8.59 mA/cm², power conversion efficiency (PCE) of 3.34% and 4.27%, respectively, and with almost same open-circuit voltage (∼0.93 V), under the illumination of AM 1.5 G, 100 mW/cm². The high J_sc for DCNR3TIDT could result from its wider and red-shifted absorption than that of DCAO3TIDT, which was probably induced by the end-capped block rhodanine derivative. The results demonstrate that the end group would be taken into full account when designing new solution-processed small molecules, which is an important factor to determine their photovoltaic properties.     

Tin naphthalocyanine complexes for infrared absorption in organic photovoltaic cells

In this work, we examine the optical properties of tin naphthalocyanine dichloride (SnNcCl₂), and its performance as an electron donor material in organic photovoltaic cells (OPVs). As an active material, SnNcCl₂ is attractive for its narrow energy gap which facilitates optical absorption past a wavelength of λ = 1100 nm. We demonstrate a power conversion efficiency of η_P = (1.2 ± 0.1)% under simulated AM1.5G solar illumination at 100 mW/cm² using the electron donor–acceptor pairing of SnNcCl₂ and C₆₀ in a bilayer device architecture. While some phthalocyanines have been previously used to improve infrared absorption, this is often realized through the formation of molecular dimers. In SnNcCl₂, the infrared absorption is intrinsic to the molecule, arising as a result of the extended conjugation. Consequently, it is expected that SnNcCl₂ could be utilized in bulk heterojunction OPVs without sacrificing infrared absorption.     

Development of a new diindenopyrazine–benzotriazole copolymer for multifunctional application in organic field-effect transistors,polymer solar cells and light-emitting diodes

A new donor–acceptor (D?A) copolymer (PIPY–DTBTA) containing 6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine donor and benzotriazole acceptor was synthesized and characterized for multifunctional applications in organic field-effect transistors (OFETs), polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs). The polymer exhibits high molecular weights, excellent film-forming ability, a deep HOMO energy level, and good solution processability. Solution-processed thin film OFETs based on this polymer revealed good p-type characteristic with a high hole mobility up to 0.0521 cm² V^?1 s^?1. Bulk-heterojunction PSCs comprising this polymer and PC₆₁BM gave a power conversion efficiency (PCE) of 0.77%. The single-layer PLEDs based on PIPY–DTBTA emitted a yellow–red light with a maximum brightness of 385 cd m^?2 at the turn-on voltage of 6 V.     

High-performance n-type organic field-effect transistors fabricated by ink-jet printing using a C60 derivative

We report on the performance of ink-jet-printed n-type organic thin-film transistors (OTFTs) based on a C₆₀ derivative, namely, C₆₀-fused N-methyl-2-(3-hexylthiophen-2-yl)pyrrolidine (C₆₀TH-Hx). The new devices exhibit excellent n-channel performance, with a highest mobility of 2.8 × 10^?2 cm² V^?1 s^?1, an I_On/I_Off ratio of about 1 × 10⁶, and a threshold voltage of 7 V. The C₆₀TH-Hx films show large crystalline domains that result from the influence of an evaporation-induced flow, thus leading to high electron mobility in the ink-jet-printed devices.     

Novel dithieno[3,2-b:2′,3′-d]pyrrole-based organic dyes with high molar extinction coefficient for dye-sensitized solar cells

Three new metal-free organic dyes FD1–3 with a planar dithieno[3,2-b:2′,3′-d]pyrrole unit as linker were synthesized and used for dye-sensitized solar cells with high molar extinction coefficients. In this work, dithieno[3,2-b:2′,3′-d]pyrrole was employed as π-conjugated bridge to construct A–π–d–π–A organic dyes, where 9,9-dihexyl-9H-fluorene was used as a donor, and cyanoacrylic acid as an electron acceptor. For a typical device, a solar energy conversion efficiency (η) of 6.36% based on FD2 was achieved under simulated AM 1.5 solar irradiation (100 mW cm^?2) with a short-circuit photocurrent density (J_sc) of 13.76 mA cm^?2, an open-circuit voltage (V_oc) of 669 mV, a fill factor (ff) of 0.691. The results suggest that the organic dye with a functionalized dithienopyrrole unit is a promising candidate for DSSCs due to its high molar extinction coefficients.     

Solution-processed small molecules based on indacenodithiophene for high performance thin-film transistors and organic solar cells

Solution-processed indacenodithiophene (IDT)-based small molecules with 1,3-indanedione (ID) as terminal acceptor units and 3,3′-hexyl-terthiophene (IDT-3Th-ID(I)) or 4,4′-hexyl-terthiophene (IDT-3Th-ID(II)) as π-bridges, have been designed and synthesized for the application in organic field-effect transistors (OFETs) and organic solar cells (OSCs). These molecules exhibited excellent solubility in common organic solvents, good film-forming ability, reasonable thermal stability, and low HOMO energy levels. For the OFETs devices, high hole motilities of 0.52 cm² V⁻¹ s⁻¹ for IDT-3Th-ID(I) and 0.61 cm² V⁻¹ s⁻¹ for IDT-3Th-ID(II) were achieved, with corresponding high I_ON/I_OFF of ca. 10⁷ and ∼10⁹ respectively. The OSCs based on IDT-3Th-ID(I)/PC₇₀BM (2:1, w/w) and IDT-3Th-ID(II)/PC₇₀BM (2:1, w/w) without using any treatment of solvent additive or thermal annealing, showed power conversion efficiencies (PCEs) of 3.07% for IDT-3Th-ID(I) and 2.83% for IDT-3Th-ID(II), under the illumination of AM 1.5G, 100 mW/cm². The results demonstrate that the small molecules constructed with the highly π-conjugated IDT as donor unit, 3Th as π-bridges and ID as acceptor units, could be promising organic semiconductors for high-performance OFETs and OSCs applications.     

Novel pyrene-based donor–acceptor organic dyes for solar cell application

Pyrene is an alternant polycyclic aromatic hydrocarbon consisting of four fused benzene rings with a large, flat aromatic system, showing high thermal stability, extensive electron delocalization and electron accepted nature. In this work, pyrene was firstly employed as π-conjugated bridge to construct electron donor–π–electron acceptor (D–π–A) organic dyes, where diarylamine or indoline was used as donor, and cyanoacrylic acid as electron acceptor. The peryne-based dyes were employed as sensitizers in dye-sensitized solar cells, and give a short circuit photocurrent density (J_sc) of 12.1 mA/cm², an open circuit voltage (V_oc) of 0.71 V, and a fill factor (FF) of 0.71, corresponding to an overall conversion efficiency (η) of 6.1% under AM 1.5 conditions.     

High open-circuit voltage polymer solar cells based on D–A copolymer of indacenodithiophene and fluorine-substituted benzotriazole

A medium band gap D–A copolymer of indacenodithiophene (IDT) and fluorinated dithienylbenzotriazole (FBTA), PIDT-FBTA, was synthesized for the application as donor material in polymer solar cells (PSCs). PIDT-FBTA showed deeper highest occupied molecular orbital (HOMO) energy level due to the strong electron-withdrawing difluorine substitution on benzotriazole acceptor unit in the D–A copolymer. The PSCs based on PIDT-FBTA:PC₇₀BM (1:3) exhibited a high V_oc of 0.90 V and a power conversion efficiency (PCE) of 3.62% under the illumination of AM 1.5G, 100 mW cm⁻². The device performance was further improved by methanol treatment with PCE increased to 4.90% and V_oc increased to 0.92 V.     

D–A copolymer with high ambipolar mobilities based on dithienothiophene and diketopyrrolopyrrole for polymer solar cells and organic field-effect transistors

Donor–acceptor (D–A) type conjugated polymers have been developed to absorb longer wavelength light in polymer solar cells (PSCs) and to achieve a high charge carrier mobility in organic field-effect transistors (OFETs). PDTDP, containing dithienothiophene (DTT) as the electron donor and diketopyrrolopyrrole (DPP) as the electron acceptor, was synthesized by stille polycondensation in order to achieve the advantages of D–A type conjugated polymers. The polymer showed optical band gaps of 1.44 and 1.42 eV in solution and in film, respectively, and a HOMO level of 5.09 eV. PDTDP and PC₇₁BM blends with 1,8-diiodooctane (DIO) exhibited improved performance in PSCs with a power conversion efficiency (PCE) of 4.45% under AM 1.5G irradiation. By investigating transmission electron microscopy (TEM), atomic force microscopy (AFM), and the light intensity dependence of J_SC and V_OC, we conclude that DIO acts as a processing additive that helps to form a nanoscale phase separation between donor and acceptor, resulting in an enhancement of μ_h and μ_e, which affects the J_SC, EQE, and PCE of PSCs. The charge carrier mobilities of PDTDP in OFETs were also investigated at various annealing temperatures and the polymer exhibited the highest hole and electron mobilities of 2.53 cm² V⁻¹ s⁻¹ at 250 °C and 0.36 cm² V⁻¹ s⁻¹ at 310 °C, respectively. XRD and AFM results demonstrated that the thermal annealing temperature had a critical effect on the changes in the crystallinity and morphology of the polymer. The low-voltage device was fabricated using high-k dielectric, P(VDF-TrFE) and P(VDF-TrFE-CTFE), and the carrier mobility of PDTDP was reached 0.1 cm² V⁻¹ s⁻¹ at V_d = −5 V. PDTDP complementary inverters were fabricated, and the high ambipolar characteristics of the polymer resulted in an output voltage gain of more than 25.     

2,6-Conjugated Bodipy sensitizers for high-performance dye-sensitized solar cells

A series of novel 2,6-conjugated Bodipy metal-free organic dyes (UY5–8) with phenothiazine (PTZ) moiety as electron donor for the dye-sensitized solar cells (DSSCs) have been designed and synthesized. The optical, electrochemical properties and photovoltaic performances are extensively investigated. The structure–property relationship shows that the introduction of various auxiliary conjugated spacers and anchoring groups are favorable to changing the efficiency of DSSCs. Among these dyes, UY7 comprised of furan with lower resonance energy as linker and cyanoacetic acid unit as electron acceptor possesses a flatter structure and longer electron recombination lifetime. Hence, a DSSCs using UY7 showing best photovoltaic performance with a short-circuit photocurrent density (J_sc) of 13.64 mA cm⁻², an open-circuit photovoltage (V_oc) of 590 mV and a fill factor (ff) of 0.66, corresponding to an overall conversion efficiency (η) of 5.31% under 100 mW cm⁻² simulated AM 1.5 G solar irradiation. This is the best reported result in the solar cell with a Bodipy dye as photosensitizer.     

Efficiency enhancement in organic solar cells by configuring hybrid interfaces with narrow bandgap PbSSe nanocrystals

We hereby present an incorporation technique for inorganic nanocrystals (NCs) in organic solar cells (OSCs) for the improvement of power conversion efficiency (PCE). Ternary PbSSe NCs constitute stable conformations with regular poly(3-hexylthiophene):phenyl-C₇₀ butyric acid methyl ester (P3HT:PCBM) organic composites under two heterojunction systems, and significant solar performance modification was obtained, depending on the incorporation type. Bilayer heterojunction (Bi-HJ) SCs, in which a pristine NC layer is sandwiched between the organic composite and cathode, showed significantly broadened photon-harvesting resulting from combination of both layers and energetic carrier transport as a result of reduced recombination losses. In contrast, bulk heterojunction (BHJ) SCs comprising combined composites of P3HT:PCBM:NCs in a single layer suffered from inefficient charge transport as a result of ubiquitous charge traps. Use of Bi-HJ cells with an NC layer of optimal thickness greatly enhanced the short-circuit current (J_SC) to 10.54 mA cm^?2 and a PCE of 3.12% was achieved; this is a 31% improvement over the conversion efficiency of purely organic cells without NCs. The separate PbSSe NC layer coupled well with the organic composite to provide a broad-range photon-harvesting ability and vertically efficient interfacial junctions for systematic charge transport; this greatly enhances the photovoltaic performances of the OSCs.     

Evidences of photocurrent generation by hole–exciton interaction at organic semiconductor interfaces

The charge–exciton interaction at the donor/acceptor interface plays a significant role in the exciton dissociation processes, and thus influences the performance of organic solar cells. In this work, the evidences of photocurrent generation via hole–exciton interaction (HEI) at the organic semiconductor interface in organic solar cells, which is the counterpart of photocurrent generated by electron–exciton interaction, is demonstrated. A heterojunction, composed of copper phthalocyanine (CuPc) and fullerene (C₆₀), is used to provide free holes that interact with the excitons supplied by perfluorinated hexadecafluorophthalo-cyaninatozinc (F16ZnPc). The fact that photocurrent generation via HEI is well evidenced by: (1) a short circuit current of 0.38 mA cm⁻²; (2) the jump of an external quantum efficiency (EQE) around 800 nm after adding a bias light; (3) the EQE variations under bias light of different wavelengths and light intensities; and (4) the superlinear dependence of the photocurrent on the light intensity.     

Fluorene-centered perylene monoimides as potential non-fullerene acceptor in organic solar cells

A fluorene-centered perylene monoimide dimer, PMI-F-PMI with a partly non-coplanar configuration has been developed as a potential non-fullerene acceptor for organic solar cells (OSCs). The optimum power conversion efficiency (PCE) of the OSC based on PMI-F-PMI as acceptor and poly (3-hexyl thiophene) (P3HT) as donor is up to 2.30% after annealing at 150 °C. The PCE of 2.30% is the highest value for the OSCs based on P3HT donor and non-fullerene acceptor lies in that PMI-F-PMI’s lowest unoccupied molecular orbital (LUMO) level around −3.50 eV matches well with the donor P3HT to produce higher open-circuit voltage (V_oc) of 0.98 V. Meanwhile, PMI-F-PMI makes remarkable contribution to devices’ light absorption as the maximum EQE (30%) of the devices is at 512 nm, same to the maximum absorption wavelength of PMI-F-PMI. The other favorable characteristics of PMI-F-PMI in bulk heterojunction (BHJ) active layers is proved through the photo current density measures, the relatively balanced electron–hole transport, and the smooth morphology with root mean square (RMS) value of 1.86 nm. For these advantages, PMI-F-PMI overwhelms its sister PMI-F and parent PMI as an acceptor in BHJ solar cells.     

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.     

Synthesis and photovoltaic properties of organic dyes containing N-fluoren-2-yl dithieno[3,2-b:2′,3′-d]pyrrole and different donors

New organic dyes containing fluorene appended dithienopyrrole as electron rich linker, different arylamine/heterocyclic units as conjugating donors and cyanoacrylic acid as acceptor have been synthesized and characterized as sensitizers for dye-sensitized solar cells. The effect of different conjugated donors such as triarylamine, carbazole and phenothiazine on the photophysical, electrochemical and photovoltaic properties is investigated. The optical and electrochemical properties of the dyes are strongly influenced by conjugating donors. The dye containing phenothiazine donor exhibited longer wavelength absorption and low oxidation potential. The time dependent density functional calculations performed on the dye models reveal charge transfer character for the longer wavelength absorption. The dye-sensitized solar cells fabricated using a dye containing fluorenyldiphenylamine donor displayed highest power conversion efficiency (6.81%) in the series originating from the high short circuit current density (J_SC = 14.01 mA cm⁻²) and high open circuit voltage (V_OC = 738 mV).     

Charge transport properties and microstructure of polythiophene/polyfluorene blends

We present a combined charge transport and X-ray diffraction study of blends based on regioregular poly(3-hexylthiophene) (P3HT) and the polyfluorene co-polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (F8TBT) that are used in efficient all-polymer solar cells. Hole mobility is observed to increase by nearly two orders of magnitude from less than 10^?7 cm² V^?1 s^?1 for as spin-coated blends to 6 × 10^?6 cm² V^?1 s^?1 for blends annealed at 453 K at a field of 2.7 × 10⁵ V/cm, but still significantly below the time-of-flight mobility of unblended P3HT of 1.7 × 10^?4 cm² V^?1 s^?1. The hole mobility of the blends also show a strong negative electric-field dependence, compared with a relatively flat electric-field dependence of unblended P3HT, suggestive of increased spatial disorder in the blends. X-ray diffraction measurements reveal that P3HT/F8TBT blends show a phase separation of the two components with a crystalline part attributed to P3HT and an amorphous part attributed to F8TBT. In as-spun and mildly annealed blends, the measured d-values and relative intensities of the 100, 200 and 300 P3HT peaks are noticeably different to unblended P3HT indicating an incorporation of F8TBT in P3HT crystallites that distorts the crystal structure. At higher anneal temperatures the blend d-values approach that of unblended P3HT suggesting a well separated blend with pure P3HT crystallites. P3HT crystallite size in the blend is also observed to increase with annealing from 3.3 to 6.1 nm, however similar changes in crystallite size are observed in unblended P3HT films with annealing. The lower mobility of P3HT/F8TBT blends is attributed not only to increased P3HT structural disorder in the blend, but also due to the blend morphology (increased spatial disorder). Changes in hole mobility with annealing are interpreted in terms of the need to form percolation networks of P3HT crystallites within an F8TBT matrix, with a possible contribution due to the intercalation of F8TBT in P3HT crystallites acting as defects in the as-prepared state.