Self-Assembled Conjugated Polyelectrolyte–Ionic Liquid Crystal Complex as an Interlayer for Polymer Solar Cells: Achieving Performance Enhancement via Rapid Liquid Crystal-Induced Dipole Orientation

A simple approach was demonstrated to manipulate dipole moment of interlayer in polymer solar cells (PSCs). The ionic liquid crystals (ILCs) 3-((2′-(4″-cyano­biphenyl-4-yloxy)­ethyl)­dimethyl­ammonio)­propane­sulfonate (CbpNSO) with zwitterionic charges were blended with cationic conjugated polyelectrolyte (CPE) poly­[3-(6-trimethylammoniumhexyl)­thiophene] (PTNBr) to afford a novel CPE–ILC complex. The water/alcohol solubility of the CPE–ILC complex enables it to be green solvent processable. The spontaneous orientation of liquid crystal (LC) favors more ordered structural arrangement in CPE–ILC complexes. More importantly, LC-assistant assembly improves the orientation of dipole at cathode and significantly reduces the work function of ITO. The power conversion efficiency (PCE) of P3HT:PC₆₀BM-based inverted PSCs with the layer of PTNBr–CbpNSO is increased by 37% with respect to that of the device with pure PTNBr. Incorporation of PTNBr–CbpNSO into the devices based on PBDTTT-C-T and PC₇₁BM affords a notable PCE of 7.49%. It should be noted that mesogens reduce the activation energy of molecular reorganization and accelerate dipole orientation in CPE–ILC interlayer under external electric field, which enables the dipole of this interlayer can be readily manipulated. Because of the rapid orientation of the dipole, PTNBr–CbpNSO shows reversible dipole at the active layer/ITO interface during the reversible bias process

Electrostatic Self-Assembled Metal Oxide/Conjugated Polyelectrolytes as Electron-Transporting Layers for Inverted Solar Cells with High Efficiency

Three conjugated polyelectrolytes (CPEs) based on polythiophenes bearing anionic (poly­[(3-(4′-sulfonatobutyl)­oxymethyl-2,5-thiophene)-<i>alt</i>-2,5-thiophene] sodium salt, PTSO-Na), neutral (HT-poly­[3-(6′-diethanolamino)-hexylthiophene], PTNOH) and cationic (HT-poly­[3-(6′-<i>N</i>,<i>N</i>,<i>N</i>-trimethylammonium)-hexylthiophene], PTN-Br) pendant groups were synthesized to improve the power conversion efficiency (PCE) of inverted polymer solar cells (I-PSCs) by deposition on the surface of ZnO to form a ZnO/CPE electron-transporting layer (ETL). Insertion of CPE to ZnO–active layer interfaces effectively lowered the energy barrier for electron transport and reduced the inherent incompatibility between the hydrophilic metal oxide and hydrophobic active layers. The I-PSCs (ITO/ZnO/CPE/P3HT:PCBM/PEDOT:PSS/Ag) incorporating anionic PTSO-Na achieved a 16% efficiency enhancement (PCE = 3.47%) over the standard device with a ZnO monolayer ETL (PCE = 2.99%). For the deposition of neutral PTNOH and cationic PTN-Br on top of ZnO, we observed strong electrostatic interaction between cationic quaternary amines of the CPE and anionic oxygen ions of the ZnO surfaces, which obtained a uniform formation of strong dipoles across the interfaces and an intimate interfacial contact. The self-assembly formed by partial protonation in neutral PTNOH increased the PCE of I-PSC to 3.98%, whereas the stronger electrostatic self-assembly produced in ZnO/PTN-Br bilayers not only delivered the device with the best PCE (4.08%) among the three CPEs but also yielded an exceptional device lifetime without encapsulation. It is worth noting that the performance of the I-PSC with PTN-Br already surpassed that of conventional ones (ITO/PEDOT:PSS/P3HT:PCBM/PTN-Br/LiF/Al). Moreover, the PCE of the device based on a ZnO/PTN-Br ETL was further improved to 4.45% after UV treatment with a 43% enhancement compared with the monolayer ZnO device, which is due to improved electrostatic self-assembly. These findings on electrostatic self-assembled metal oxide/CPE bilayer ETL provide a simple and easy strategy for fabrication of high-performance and long-term stable I-PSCs

Versatile Electron-Collecting Interfacial Layer by in Situ Growth of Silver Nanoparticles in Nonconjugated Polyelectrolyte Aqueous Solution for Polymer Solar Cells

Novel PEIE-Ag composites by in situ growth of silver nanoparticles in poly­(ethylenimine)-ethoxylated (PEIE) aqueous solution are explored as an efficient interfacial layer for improving inverted polymer solar cells (PSCs) performance. The hybrid PEIE-Ag interfacial material is simple to fabricate only via ultraviolet irradiation with good water-solubility and unique film formation. The generated Ag nanoparticles can anchor in the PEIE polymer chains to form a conductive continuous interpenetrating network structure. Combining of the advantages of PEIE and Ag nanoparticles, the PEIE-Ag shows enhanced charge transport, electron selective and collection, and improved light-harvesting, mainly due to the surface plasmon resonance effect, better energy alignment induced by the formation of ideal dipole layer, as well as the improved conductivity. These distinguished interfacial properties result in the power conversion efficiency of inverted PSCs based on poly­[4,8-bis­(2-ethyl-hexyl-thiophene-5-yl)-benzo­[1,2-b:4,5-b]­dithiophene-2,6-diyl]-<i>alt</i>-[2-(2-ethyl-hexanoyl)-thieno­[3,4-<i>b</i>]­thiophen-4,6-diyl] (PBDTTT-C-T) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) photoactive layer substantially improved up to 7.66% from 6.11%. Moreover, the device performance is insensitively dependent on the thickness of the PEIE-Ag interfacial layer, broadening the thicknesses selection window for interfacial materials. These results demonstrate that PEIE-Ag is a potential interfacial material compatible with roll-to-roll techniques and suitable for printed electronic devices

Optical Engineering of Uniformly Decorated Graphene Oxide Nanoflakes via in Situ Growth of Silver Nanoparticles with Enhanced Plasmonic Resonance

A nanocomposite of silver-nanoparticle-decorated graphene oxide (GO–Ag NPs), enhanced by the surface plasmon resonance (SPR) effect, improved the performance of polymer solar cells (PSCs). The GO–Ag NPs were fabricated in situ via ultraviolet (UV) irradiation (254 nm) of GO and an aqueous solution of AgNO₃. The photoexcited GO accelerated reduction of Ag⁺ ions into silver nanoparticles (Ag NPs) upon UV irradiation, and the Ag NPs spontaneously deposited on the GO nanoflakes because the numerous functional groups on GO enable efficient adsorption of Ag⁺ ions and Ag NPs via electrostatic interactions. The strong coupling between the SPR effect of GO–Ag NPs and incident light offers the probability of improved light absorption and corresponding exciton generation rate with enhanced charge collection, resulting in significant enhancement in short-circuit current density and power conversion efficiency (PCE). Therefore, the PCE of PSCs based on poly­[4,8-bis­(2-ethylhexylthiophene-5-yl)-benzo­[1,2-<i>b</i>:4,5-<i>b</i>]­dithiophene-2,6-diyl]-<i>alt</i>-[2-(2-ethylhexanoyl)­thieno­[3,4-<i>b</i>]­thiophen-4,6-diyl] and [6,6]-phenyl C₇₁-butyric acid methyl ester has been substantially elevated to 7.54% from 6.58% by introducing GO–Ag NPs at the indium tin oxide/poly­(3,4-ethylenedioxythiophene):polystyrene sulfonic acid interface. In addition, the excellent properties of GO–Ag NPs, including its simple preparation, processability in aqueous solution, cost-effectiveness, and sustainability, make it suitable for the roll-to-roll manufacturing of PSCs

Universal and Versatile MoO₃‑Based Hole Transport Layers for Efficient and Stable Polymer Solar Cells

Two solution-processed and highly dispersed MoO₃ called d-(MoO₃)₁₂₀ and d-(MoO₃)₁₅ with sizes of 120 nm and extremely smaller 15 nm, respectively, are applied into polymer solar cells, and the evaporated MoO₃ as hole transport layers (HTLs) in devices is also compared. It is the first time it has been found that the different size of MoO₃ can induce the quite different morphologies of the HTLs and their upper active layers due to the unexpectedly caused difference in the surface energy levels. It is worthy to note that the performance of the device with solution-processed d-(MoO₃)₁₅ is higher than that of the device with poly­(3,4-ethylenedioxythiophene):­poly­(styrenesulfonate) (PEDOT:PSS) HTLs and even comparable to that of the device with optimized evaporated-MoO₃. Simulated by the transfer matrix method, the light intensity and the exciton generation rate in the active layer are found to be greatly enhanced by incorporation of an ultrathin MoO₃ combined with PEDOT:PSS. As a result, by inserting a layer of evaporated MoO₃ (e-MoO₃) between the ITO and PEDOT:PSS, power conversion efficiency (PCE) can be dramatically improved to 7.10% for PBDTTT-C-T:PC₇₁BM. Moreover, the e-MoO₃/PEDOT:PSS bilayer also ensures good stability for the devices, due to the MoO₃ preventing moisture and oxygen attack and protecting ITO from corrosion caused by the acid PEDOT:PSS

Cooperative Assembly of Pyrene-Functionalized Donor/Acceptor Blend for Ordered Nanomorphology by Intermolecular Noncovalent π–π Interactions

A facile approach to develop the stable and well-defined bulk heterojunction (BHJ) nanomorphology has been demonstrated. Novel pyrene (Py)-functionalized diblock copolymers poly­(3-hexylthiophene)-<i>block</i>-poly­[3-(10-(pyren-1-yloxy)­decyloxy)­thiophene] (P3HT-<i>b</i>-P3TPy), and pyrene-functionalized fullerene [6,6]-phenyl-C₆₁-butyric acid 1-pyrene butyl ester (PCBPy), were successfully synthesized. The π–π interactions of Py mesogens interdigitated between the functionalized fullerene and P3TPy segment can allow for the cooperative assembly of P3HT-<i>b</i>-P3TPy and PCBPy. The orientation of the Py mesogens also can further enhance the molecular arrangement. Compared with the as-cast and thermal annealing, solvent annealing can promote cooperative assembly of P3HT-<i>b</i>-P3TPy:PCBPy undergoing the slow film growth. Note that the assembly microstructure strongly depends on the molar ratio of P3HT and P3TPy with Py mesogens. Low loading of P3TPy block in the copolymers blends keeps the same behavior to the P3HT, whereas relatively high loading of Py mesogens favors the better intermolecular π–π stacking interactions between P3HT-<i>b</i>-P3TPy and PCBPy. As a result, the P3HT-<i>b</i>-P3TPy­(3/1) forms the orientated nanowires with PCBPy in bulk heterojunction, and the average domain size is estimated to be 10–20 nm, which is desirable for enlarge surface area for donor/acceptor interfaces and give a bicontinuous pathway for efficient electron transfer. Furthermore, the cooperative assembly between P3HT-<i>b</i>-P3TPy and PCBPy is found to effectively suppress the PCBPy macrophase separation, and stabilize the blend morphology

Self-Organized Hole Transport Layers Based on Polythiophene Diblock Copolymers for Inverted Organic Solar Cells with High Efficiency

Novel fluoroalkyl side-chain diblock copolymers, poly­(3-hexylthiophene)-block-poly­[3-(4-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxy)­phenyl)­decyloxy)­thiophene] (P3HT-b-P3FAT), were successfully synthesized by Grignard metathesis (GRIM) polymerization. Driven by the low surface energy of fluoroalkyl side chains, the fluorinated polymers can spontaneously segregate on the surface of poly-(3-hexylthiophene) (P3HT) during spin-coating processes. As the P3HT block increases in the copolymer, higher concentrations of fluoropolymers are required to form the self-assembled monolayer on the surface. The fluorinated part forms an interfacial dipole that shifts the work function of the anode metal, while the P3HT block can interact with the P3HT donor for hole transport. With this self-assembly hole transport layer to align the energy levels, P3HT:PCBM photovoltaic devices are easily fabricated to achieve improved performance. Overall, devices prepared with 1.5 mg mL^–1 copolymer PFT-3HT with a 3:1 ratio of P3HT to P3FAT block in the active layer solution displayed PCE values of up to 4.6% (50% PCE increase over a PEDOT:PSS control device) and showed a significant long-term stability in excess of 300 h in air

Self-Assembly of Diblock Polythiophenes with Discotic Liquid Crystals on Side Chains for the Formation of a Highly Ordered Nanowire Morphology

Diblock copolymers bearing a triphenylene (TP) discotic liquid crystals moiety, poly­(3-hexylthiophene)-block-poly­[3-(10-(2,3,6,7,10-pentakis­(hexyloxy)­triphenylen)-decyloxy)­thiophene] (P3HT-b-P3TPT), was successfully synthesized by Grignard metathesis polymerization. The self-assembled nanowire structures of these diblock copolymers have been investigated by atomic force microscopy and transmission electron microscopy. The domain size and crystallinity of the nanostructures can be easily controlled by tuning the P3HT/P3TPT block ratio and by employing different annealing processes such as thermal and solvent annealing. The results of X-ray diffraction indicate that both intermolecular interactions and mesogen packing are essential for the formation of nanostructures in the diblock copolymers. Although the block ratio of P3HT and P3TPT comes to 9:1 and the copolymer undergoes solvent annealing followed by thermal treatment, an optimal crystalline nanowire with a size of 16.9 nm is formed. In addition, solar cells based on these copolymers as electron donors in combination with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) or <i>N</i>,<i>N</i>′-di­(2-ethylhexyl)­perylene-3,4,9,10-tetracarboxylbisimide (PDI) as electron acceptors have been constructed, and the effect of the nanomorphology on device performance has been investigated

In Situ Fabricating One-Dimensional Donor–Acceptor Core–Shell Hybrid Nanobeams Network Driven by Self-Assembly of Diblock Copolythiophenes

In situ growth of cadmium sulfide (CdS) quantum dots (QDs) was achieved directly through solvent-assisted grafting in the self-assembled templates of amphiphilic all conjugated diblock copolythiophene, poly­(3-hexylthiophene)-<i>b</i>-poly­(3-(2-(2-(2-methoxyethoxy)­ethoxy)­ethoxy)­methylthiophene) (P3HT-<i>b</i>-P3TEGT) and gas–solid reaction. Such diblock polymer templates allowed a desired amount of cadmium sulfide salt (Cd­(Ac)₂) to easily accomplish dispersion and self-assembly via controlled assembling block copolymers in selective solvents. After P3HT-<i>b</i>-P3TEGT polymer templates grafted with Cd²⁺ precursor (P3HT-<i>b</i>-P3TEGT/Cd²⁺) reacting in hydrogen sulfide (H₂S) gas, one-dimensional core–shell nanobeams network P3HT-<i>b</i>-P3TEG/CdS (donor–acceptor) was formed with excellent phase separation between P3HT-<i>b</i>-P3TEGT crystalline domains and inorganic CdS QDs domains at nanoscales, which was driven by the interaction between oxygen atoms of ethylene oxide side chains and Cd²⁺ ions, and the thermodynamic equilibrium between polymer chains deformation. The one-dimensional wire-like nanostructure were highly desirable for the active layers in photovoltaic devices as providing high carrier mobility, large interfacial area between electron donor and acceptor, and highly efficient transport pathways to improve the power conversion efficiency (PCE) of hybrid bulk heterojunction solar cells

Experimental Investigation and Theoretical Calculation of Molecular Architectures on Carbazole for Photovoltaics

A new conjugated polymer containing 6<i>H</i>-phenanthro­[1,10,9,8-cdefg]­carbazole (PC) and 4,7-dithien-2-yl-2,1,3-benzo­thiadiazole (DTBT) units, so-called PPCDTBT, is synthesized based on the further modification of carbazole moieties for poly­[<i>N</i>-9″-hepta-decanyl-2,7-carbazole-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzo­thiadiazole)] (PCDTBT). The resulting polymer exhibits a narrow band gap with 1.77 eV, resulting from the broader conjugation, while maintaining a low-lying HOMO energy level. The polymer geometry is severely transformed by the large fused block phenanthrocarbazole (PC). Through the density functional theory and time-dependent density functional theory calculations at the B3LYP/6-31G­(d,p) level on the polymer dimer models, a big torsion angle is the main reason for breaking the backbone coplanarity and, consequently, the conjugation and organization. Moreover, a different transition from the HOMO-2 orbital is responsible for the absorption shoulder at a short wavelength. After ordinary optimization, the best power conversion efficiency of 2.3% is achieved with a preferable <i>V</i>_oc of 0.80 V and <i>J</i>_sc of 7.9 mA/cm². Additionally, for holding extended conjugation from the fused carbazole-like unit and suppressing the strong torsion, naphthocarbazole (NC) and the counterpart alternative polymer of NC and DTBT (PNCDTBT) are proposed and simulated, which would be more planar for better intra- and intermolecular interactions