Ternary D1–D2–A–D2 Structured Conjugated Polymer: Efficient “Green” Solvent-Processed Polymer/Neat‑C₇₀ Solar Cells

In contrast to the great efforts on developing novel donor (D)–acceptor (A) copolymers, research on investigating the backbone composition of conjugated polymer is rare. In this contribution, we disclose the design and synthesis of a ternary D1–D2–A–D2 structured conjugated polymer PBSF. Compared to the typical D–A polymer with fixed D/A moiety number, the ternary structure can tune the optical and electrical properties more comprehensively and delicately. Precisely control of the ternary fragments relative to the backbone vector was achieved, further promoting sufficient planar structure, strong intermolecular packing, and excellent charge transport. Finally, the additive and annealing-free polymer solar cells based on PBSF and phenyl-C₇₁-butyric acid methyl ester ([70]­PCBM; PCE = 7.4%) or cheap, nonfunctionalized C₇₀ (PCE = 5.3%) demonstrate excellent performance using either chlorinated or nonhalogenated “green” solvent. We believe that this novel and efficient ternary structure may spark future polymer design to achieve sustainable-processed photovoltaic devices for practical mass production

Ultrafast Spectroscopic Identification of Hole Transfer in All-Polymer Blend Films of Poly(1-{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]-benzo[1,2‑<i>b</i>:4,5‑<i>b</i>′]dithiophen-2-yl}-3-methyl-5-(4-octylphenyl)‑4<i>H</i>‑thieno[3,4‑<i>c</i>]pyrrole-4,6(5<i>H</i>)‑dione) and Poly[1,8-bis(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)]

All-polymer solar cells composed of wide-band-gap polymer poly­(1-{4,8-bis­[5-(2-ethylhexyl)­thiophen-2-yl]-benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophen-2-yl}-3-methyl-5-(4-octylphenyl)-4<i>H</i>-thieno­[3,4-<i>c</i>]­pyrrole-4,6­(5<i>H</i>)-dione) (PTP8) as the donor and poly­[1,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)] [P­(NDI2OD-T2), also known as Activink N2200] as the acceptor exhibit a broad absorbance in the range 300–900 nm, thanks to complementary absorption of near-infrared light by N2200. Although N2200 shows reasonably high electron mobility, the contribution of the photogenerated excitons in N2200 to the power conversion of the PTP8/N2200 solar cell is insignificant. Here, the hole transfer from N2200 to PTP8 in PTP8/N2200 blend films was investigated by utilizing ultrafast transient absorption spectroscopy. The spectral fingerprints of ground-state bleaching and hole polaron-induced absorption of PTP8 are identified under selective excitation of the N2200 component and unambiguously indicate hole transfer from N2200 to PTP8. The hole transfer is slow (∼100 ps), comparable to the geminate exciton recombination rate, consequently limiting the transfer efficiency and carrier generation. The hole-transfer efficiency depends on the PTP8/N2200 weight ratio, showing a highest value of ∼14.1% in the 3:2 film

Naphthalene Diimide-Based n‑Type Polymers: Efficient Rear Interlayers for High-Performance Silicon–Organic Heterojunction Solar Cells

Silicon–organic heterojunction solar cells suffer from a noticeable weakness of inefficient rear contact. To improve this rear contact quality, here, two solution-processed organic n-type donor–acceptor naphthalene diimide (NDI)-based conjugated polymers of N2200 and fluorinated analogue F-N2200 are explored to reduce the contact resistance as well as to passivate the Si surface. Both N2200 and F-N2200 exhibit high electron mobility due to their planar structure and strong intermolecular stacking, thus allowing them to act as excellent transporting layers. Preferential orientation of the polymers leads to reduce contact resistance between Si and cathode aluminum, which can enhance electron extraction. More importantly, the substitution of fluorine atoms for hydrogen atoms within the conjugated polymer can strengthen the intermolecular stacking and improve the polymer–Si electronic contact due to the existence of F···H interactions. The power conversion efficiencies of Si-PEDOT:PSS solar cells increased from 12.6 to 14.5% as a consequence of incorporating the F-N2200 polymer interlayers. Subsequently, in-depth density functional theory simulations confirm that the polymer orientation plays a critical role on the polymer–Si contact quality. The success of NDI-based polymers indicates that planar conjugated polymer with a preferred orientation could be useful in developing high-performance solution-processed Si–organic heterojunction photovoltaic devices

Inverted Planar Heterojunction Perovskite Solar Cells Employing Polymer as the Electron Conductor

Inverted planar heterojunction perovskite solar cells employing different polymers, poly­{[<i>N</i>,<i>N</i>′-bis­(2-octyldodecyl)-1,4,5,8-naphthalene diimide-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)} (N2200), poly­{[<i>N</i>,<i>N</i>′-bis­(alkyl)-1,4,5,8-naphthalene diimide-2,6-diyl-<i>alt</i>-5,5′-di­(thiophen-2-yl)-2,2′-(E)-2-(2-(thiophen-2-yl)­vinyl)­thiophene]} (PNVT-8), and PNDI2OD-TT as electron-transporting material (ETM) have been investigated for the first time. The best device performance was obtained when N2200 was applied as the ETM, with <i>J</i>_SC of 14.70 mA/cm2, <i>V</i>_OC of 0.84 V, and fill factor (FF) of 66%, corresponding to a decent power conversion efficiency (PCE) of ∼8.15%. Which is very competitive to the parameters (<i>J</i>_SC 14.65 mA/cm2, <i>V</i>_OC 0.83 V, FF 70%, and PCE 8.51%) of the reference device employing conventional PCBM as the ETM. The slightly lower FF could be mainly accounted for by the increased recombination in the polymer contained devices. This work demonstrated that polymeric materials can be used as efficient ETM in perovskite solar cells, and we believe this class of polymeric ETMs will further promote the performance of perovskite photovoltaic cells after extended investigation

Widely Applicable n‑Type Molecular Doping for Enhanced Photovoltaic Performance of All-Polymer Solar Cells

A widely applicable doping design for emerging nonfullerene solar cells would be an efficient strategy in order to further improve device photovoltaic performance. Herein, a family of compound TBAX (TBA= tetrabutylammonium, X = F, Cl, Br, or I, containing Lewis base anions are considered as efficient n-dopants for improving polymer–polymer solar cells (all-PSCs) performance. In all cases, significantly increased fill factor (FF) and slightly increased short-circuit current density (<i>J</i>_sc) are observed, leading to a best PCE of 7.0% for all-PSCs compared to that of 5.8% in undoped devices. The improvement may be attributed to interaction between different anions X^– (X = F, Cl, Br, and I) in TBAX with the polymer acceptor. We reveal that adding TBAX at relatively low content does not have a significantly impact on blend morphology, while it can reduce the work function (WF) of the electron acceptor. We find this simple and solution processable n-type doping can efficiently restrain charge recombination in all-polymer solar cell devices, resulting in improved FF and <i>J</i>_sc. More importantly, our findings may provide new protocles and insights using n-type molecular dopants in improving the performance of current polymer–polymer solar cells

Toward Thermal Stable and High Photovoltaic Efficiency Ternary Conjugated Copolymers: Influence of Backbone Fluorination and Regioselectivity

Narrow band gap conjugated polymers with a D–A–D′–A repeat unit architecture, namely, PF-0, PF-1a, PF-1b, and PF-2, were designed and synthesized. By precisely controlling the orientation of the asymmetric fluorobenzo­[<i>c</i>]­[1,2,5]­thiadiazole (FBT) fragments as well as incorporating different electron acceptors benzothiadiazole (BT) and difluorobenzo­[<i>c</i>]­[1,2,5]­thiadiazole (DFBT), regioregularity and graded fluorination have been achieved over the polymer backbone. There are evident differences between the properties of PF-1a and PF-1b due to different regioselectivity within the polymer backbone. In addition, the fluorinated analogues can exhibit increased light absorbance, higher electron density in the solid state, a lower-lying valence band, and more ordered solid film structure. The monofluorinated polymer PF-1b with the optimal regioselectivity and bis-fluorinated polymer PF-2 demonstrated improved charge transport as well as thermally resistant film structure (up to 300 °C) in organic field-effect transistors. Moreover, the fluorinated polymers exhibit dramatically increased efficiency from 5.58% to 8.42% in solar cells with lower amount of processing additive, indicating the important role of fluorination and regioselectivity in determining polymer properties. Thus, our systematical study on fluorination may provide an effective approach to precisely control the polymer regioselectivity and improve device performance as well as long-term durability under various environmental stresses

Combinative Effect of Additive and Thermal Annealing Processes Delivers High Efficiency All-Polymer Solar Cells

The combinative effects of thermal annealing and additive processes on the performance of all-polymer bulk heterojunction (BHJ) solar cells with composites of different donor polymers (PTQ1, P3HT, PTB7-Th) and poly­[1,8-bis­(dicarboximide)-2,6-diyl]-<i>alt</i>-5,5′-(2,2′-bithiophene)­P­(NDI2OD-T2) [PolyeraActivInk N2200] were investigated. We found that devices treated with both processes show significant improved performance compared with those treated with either process alone. To reveal the mechanism of this enhancement in device performance, the optical and electrical properties of all-polymer blends were carefully investigated in the PTQ1/N2200 system. The synergetic effect of both processes can largely enhance the polymer aggregation, especially for N2200, leading to improved absorbance, improved charge mobility, and thus higher device performance. In addition, the device efficiency can be further enhanced by postannealing which can improve the interface between the active layer and Al cathode, as revealed by atomic force microscopy investigations. Moreover, the approaches reported here provide a simple and versatile method to optimize all-polymer solar cells and may help pave the route for this emerging system to overtake the state-of-the-art polymer/fullerene solar cells

Room-Temperature Processed Nb₂O₅ as the Electron-Transporting Layer for Efficient Planar Perovskite Solar Cells

In this work, we demonstrate high-efficiency planar perovskite solar cells (PSCs), using room-temperature sputtered niobium oxide (Nb₂O₅) as the electron-transporting layer (ETL). Widely spread ETL-like TiO₂ often requires high-temperature (>450 °C) sintering, which is not desired for the fabrication of flexible devices. The amorphous Nb₂O₅ (labeled as a-Nb₂O₅) ETL, without any heat treatment, can give a best power conversion efficiency (PCE) of 17.1% for planar PSCs. Interestingly, the crystalline Nb₂O₅ (labeled as c-Nb₂O₅), with high-temperature (500 °C) annealing, results in a very similar PCE of 17.2%, indicating the great advantage of a-Nb₂O₅ in energy saving. We thus carried out a systematical investigation on the properties of the a-Nb₂O₅ film. The Hall effect measurements indicate both high mobility and conductivity of the a-Nb₂O₅ film. Kelvin probe force microscopy measurements define the Fermi levels of a-Nb₂O₅ and c-Nb₂O₅ as −4.31 and −4.02 eV, respectively, which allow efficient electron extraction at the Nb₂O₅/perovskite interface, regardless of the additional heat treatment on Nb₂O₅ film. Benefitting from the low-temperature process, we further demonstrated flexible PSCs based on a-Nb₂O₅, with a considerable PCE of 12.1%. The room-temperature processing and relatively high device performance of a-Nb₂O₅ suggest a great potential for its application in optoelectrical devices