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

Large-Scale Flexible and Highly Conductive Carbon Transparent Electrodes via Roll-to-Roll Process and Its High Performance Lab-Scale Indium Tin Oxide-Free Polymer Solar Cells

A scalable and highly conductive PEDOT:PSS:CNTs transparent electrode (TE) is demonstrated for high performance optoelectronics. The aligned and uniform dispersion of electron conduction favored CNTs in the PEDOT:PSS matrix can achieve the rearrangement of the PEDOT chains with more expended conformation via the π–π interaction between CNTs and PEDOT. As a result, PEDOT:PSS:CNTs electrode presents a high conductivity of 3264.27 S cm^–1 with a high transmittance over 85%, and ITO-free PSCs based on PEDOT:PSS:CNTs electrode achieves a PCE of 7.47% with high stability. Furthermore, a large-scale flexible electrode was obtained by a roll-to-roll technique, which demonstrates an excellent property with a sheet resistance of 17 Ω sq^–1 and 80.7% optical transmittance. Combining the flexible and conductive PEDOT:PSS:CNTs film with the scalable roll-to-roll process, we anticipate that the commercial production of a large-scale transparent electrode, replacing ITO, will be realized in the near future

Large-Scale Flexible and Highly Conductive Carbon Transparent Electrodes via Roll-to-Roll Process and Its High Performance Lab-Scale Indium Tin Oxide-Free Polymer Solar Cells

A scalable and highly conductive PEDOT:PSS:CNTs transparent electrode (TE) is demonstrated for high performance optoelectronics. The aligned and uniform dispersion of electron conduction favored CNTs in the PEDOT:PSS matrix can achieve the rearrangement of the PEDOT chains with more expended conformation via the π–π interaction between CNTs and PEDOT. As a result, PEDOT:PSS:CNTs electrode presents a high conductivity of 3264.27 S cm^–1 with a high transmittance over 85%, and ITO-free PSCs based on PEDOT:PSS:CNTs electrode achieves a PCE of 7.47% with high stability. Furthermore, a large-scale flexible electrode was obtained by a roll-to-roll technique, which demonstrates an excellent property with a sheet resistance of 17 Ω sq^–1 and 80.7% optical transmittance. Combining the flexible and conductive PEDOT:PSS:CNTs film with the scalable roll-to-roll process, we anticipate that the commercial production of a large-scale transparent electrode, replacing ITO, will be realized in the near future

Alcohol-Soluble n‑Type Conjugated Polyelectrolyte as Electron Transport Layer for Polymer Solar Cells

A novel alcohol-soluble n-type conjugated polyelectrolyte (n-CPE) poly-2,5-bis­(2-octyldodecyl)-3,6-bis­(thiophen-2-yl)-pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-dione-<i>alt</i>-2,5-bis­[6-(<i>N</i>,<i>N</i>,<i>N</i>-trimethyl­ammonium)­hexyl]-3,6-bis­(thiophen-2-yl)-pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-dione (PDPPNBr) is synthesized for applications as an electron transport layer (ETL) in an inverted polymer solar cells (PSCs) device. Because of the electron-deficient nature of diketopyrrolopyrrole (DPP) backbone and its planar structure, PDPPNBr is endowed with high conductivity and electron mobility. The interfacial dipole moment created by n-CPE PDPPNBr can substantially reduce the work function of ITO and induce a better energy alignment in the device, facilitating electron extraction and decreasing exctions recombination at active layer/cathode interface. As a result, the power conversion efficiency (PCE) of the inverted devices based poly­(3-hexylthiophene) (P3HT):(6,6)-phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM) active layer with PDPPNBr as ETL achieves a value of 4.03%, with 25% improvement than that of the control device with ZnO ETL. Moreover, the universal PDPPNBr ETL also delivers a notable PCE of 8.02% in the devices based on polythieno­[3,4-<i>b</i>]-thiophene-<i>co</i>-benzo­dithiophene (PTB7):(6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). To our best knowledge, this is the first time that n-type conjugated polyelectrolyte-based cathode interlayer is reported. Quite different from the traditional p-type conjugated and nonconjugated polyelectrolytes ETLs, n-CPE PDPPNBr as ETL could function efficiently with a thickness approximate 30 nm because of the high conductivity and electron mobility. Furthermore, the PDPPNBr interlayer also can ensure the device with the improved long-term stability. The successful application of this alcohol solution processed n-type conjugated polyelectrolyte indicates that the electron-deficient planar structure with high electron mobility could be very promising in developing high performance and environmentally friendly polymer solar cells

In Situ Formation of ZnO in Graphene: A Facile Way To Produce a Smooth and Highly Conductive Electron Transport Layer for Polymer Solar Cells

A novel electron transport layer (ETL) based on zinc oxide@graphene:ethyl cellulose (ZnO@G:EC) nanocomposite is prepared by in situ formation of zinc oxide (ZnO) nanocrystals in a graphene matrix to improve the performance of polymer solar cells. Liquid ultrasound exfoliation by ethyl cellulose as stabilizer not only allows for uniform dispersion of graphene solution but also maintains an original structure of graphene gaining a high conductivity. The ZnO@G:EC ETL displays a quite smooth morphology and develops the energy-level alignment for the electron extraction and transportation. Subsequently, the device based on poly­(3-hexylthiophene) (P3HT):(6,6)-phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM) with the ZnO@G:EC as ETL obtains a power conversion efficiency (PCE) of 3.9%, exhibiting a ∼20% improvement compared to the familiar device with bare ZnO nanocrystals as ETL. Replacing the active layer with polythieno­[3,4-<i>b</i>]­thiophene/benzodithiophene (PTB7): (6,6)-phenyl-C₇₁ butyric acid methyl ester (PC₇₁BM), the PCE can be dramatically improved to 8.4%. This facile and fascinating method to produce a smooth and highly conductive electron transport layer provides an anticipated approach to obtain high performance polymer solar cells

A Facile Approach To Fabricate High-Performance Polymer Solar Cells with an Annealing-Free and Simple Device of Three Layers

With the rapid development of polymer solar cell research, an “annealing-free” method and simplifying the device structure become the main problems of commercialization of polymer solar cells (PSCs). To resolve these challenges, a novel, facile approach to develop favorable vertical separation in a poly­(3-hexylthiophene):(6,6)-phenyl-C61 butyric acid methyl ester:2,3,5,6-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (P3HT:PC₆₁BM:F4TCNQ) ternary blend through the interaction between P3HT and F4TCNQ has been demonstrated; consequently, highly efficient PSCs with only three layers have been realized. Driven by the low surface energy of F4TCNQ, a spontaneous P3HT–F4TCNQ layer was enriched on the surface of the active layer. The device can escape the annealing treatment and interfacial modification due to the well-defined vertical separation and favorable work function gradient in the active layer. As a result, without thermal annealing and an additional interlayer, PSCs with only three layers based on the ternary blend attain a power conversion efficiency of up to 4.1%. This also demonstrates good adaptation for all solution-processed and flexible methods. This simple device structure and “annealing-free” method of efficient polymer solar cells provide an opportunity for large-scale commercial production in the near future

Grain Boundary Modification via F4TCNQ To Reduce Defects of Perovskite Solar Cells with Excellent Device Performance

Solar cells based on hybrid organic–inorganic metal halide perovskites are being developed to achieve high efficiency and stability. However, inevitably, there are defects in perovskite films, leading to poor device performance. Here, we employ an additive-engineering strategy to modify the grain boundary (GB) defects and crystal lattice defects by introducing a strong electron acceptor of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) into perovskite functional layer. Importantly, it has been found that F4TCNQ is filled in GBs and there is a significant reduction of metallic lead defects and iodide vacancies in the perovskite crystal lattice. The bulk heterojunction perovskite–F4TCNQ film exhibits superior electronic quality with improved charge separation and transfer, enhanced and balanced charge mobility, as well as suppressed recombination. As a result, the F4TCNQ doped perovskite device shows excellent device performance, especially the reproducible high fill factor (up to 80%) and negligible hysteresis effect

Large-Scale Stretchable Semiembedded Copper Nanowire Transparent Conductive Films by an Electrospinning Template

With recent emergence of wearable electronic devices, flexible and stretchable transparent electrodes are the core components to realize innovative devices. The copper nanowire (CuNW) network is commonly chosen because of its high conductivity and transparency. However, the junction resistances and low aspect ratios still limit its further stretchable performance. Herein, a large-scale stretchable semiembedded CuNW transparent conductive film (TCF) was fabricated by electrolessly depositing Cu on the electrospun poly­(4-vinylpyridine) polymer template semiembedded in polydimethylsiloxane. Compared with traditional CuNWs, which are as-coated on the flexible substrate, the semiembedded CuNW TCFs showed low sheet resistance (15.6 Ω·sq^–1 at ∼82% transmittance) as well as outstanding stretchability and mechanical stability. The light-emitting diode connected the stretchable semiembedded CuNW TCFs in the electric circuit still lighted up even after stretching with 25% strain. Moreover, this semiembedded CuNW TCF was successfully applied in polymer solar cells as a stretchable conductive electrode, which yielded a power conversion efficiency of 4.6% with 0.1 cm² effective area. The large-scale stretchable CuNW TCFs show potential for the development of wearable electronic devices

Butanedithiol Solvent Additive Extracting Fullerenes from Donor Phase To Improve Performance and Photostability in Polymer Solar Cells

In this work, we demonstrated that the excited poly­[4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2-<i>b</i>;4,5-<i>b</i>′]­dithiophene-2,6-diyl-<i>alt</i>-(4-(2-ethylhexyl)-3-fluorothieno­[3,4-<i>b</i>]­thiophene-)-2-carboxylate-2,6-diyl)] (PTB7-Th) will be degraded by [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) or photolysis fragment of 1,8-diiodooctane (DIO) in the presence of oxygen and under irradiation of red light. From the previous reports, the fragment of DIO may be involved in the reaction directly. Our work indicates the PC₇₁BM is not directly involved in the reaction, but is acting as a catalyst to promote the reaction of excited donors with oxygen. Thus, PTB7-Th urgently needs a kind of nonresidual iodine-free additive to replace DIO and remove the fullerene from the donor phase at the same time. Taking into consideration PC₇₁BM solubility and boiling point difference between solvent additives and host solvents, 1,4-butanedithiol solvent was selected to fabricate PTB7-Th:PC₇₁BM-based solar cells achieving a best power conversion efficiency (PCE) of 10.2% (8.5% for PTB7:PC₇₁BM). Iodine-free butanedithiol can not only avoid excited polymer reacting with the photolysis fragment of DIO but also suppress the degradation of the excited PTB7-Th caused by synergistic effect between the fullerene and oxygen via extracting the free/trapped PC₇₁BM from the donor phase. Eventually, the film prepared with 1,4-butanedithiol shows higher stability than the film prepared without any additives and much better than the film with DIO in macro-/micromorphology, light absorption, and device performance

Roll-To-Roll Printing of Meter-Scale Composite Transparent Electrodes with Optimized Mechanical and Optical Properties for Photoelectronics

Flexible transparent electrodes are an indispensable component for flexible optoelectronic devices. In this work, the meter-scale composite transparent electrodes (CTEs) composed of poly­(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and Ag grid/polyethylene terephthalate (PET) with optimized mechanical and optical properties are demonstrated by slot–die roll-to-roll technique with solution printing method under a low cost ($15–20 per square meter), via control of the viscosity and surface energy of PEDOT:PSS ink as well as the printing parameters. The CTEs show excellent flexibility remaining 98% of the pristine value after bending 2000 times under various bending situations, and the square resistance (<i>R</i>_s) of CTEs can be reduced to 4.5–5.0 Ω/sq with an appropriate transmittance. Moreover, the optical performances, such as haze, extinction coefficient, and refractive index, are investigated, as compared with indium tin oxide/PET, which are potential for the inexpensive optoelectronic flexible devices. The CTEs could be successfully employed in polymer solar cells with different areas, showing a maximal power conversion efficiency of 8.08%