Boosting Up Performance of Inverted Photovoltaic Cells from Bis(alkylthien-2-yl)dithieno[2,3-d:2',3'-d']benzo[1,2-b:4',5'-b']di thiophene-Based Copolymers by Advantageous Vertical Phase Separation

Efficient Organic Solar Cells Enabled by Simple Non-Fused Electron Donors with Low Synthetic Complexity

Fused-ring electron donors boost the efficiency of organic solar cells (OSCs), but they suffer from high cost and low yield for their large synthetic complexity (SC > 30%). Herein, the authors develop a series of simple non-fused-ring electron donors, PF1 and PF2, which alternately consist of furan-3-carboxylate and 2,2'-bithiophene. Note that PF1 and PF2 present very small SC of 9.7% for their inexpensive raw materials, facile synthesis, and high synthetic yield. Compared to their all-thiophene-backbone counterpart PT-E, two new polymers feature larger conjugated plane, resulting in higher hole mobility for them, especially a value up to ≈10^-4 cm² V^-1 ·s for PF2 with longer alkyl side chain. Meanwhile, PF1 and PF2 exhibit larger dielectric constant and deeper electronic energy level versus PT-E. Benefiting from the better physicochemical properties, the efficiencies of PF1- and PF2-based devices are improved by ≈16.7% and ≈71.3% relative to that PT-E-based devices, respectively. Furthermore, the optimized PF2-based devices with introducing PC₇₁ BM as the third component deliver a higher efficiency of 12.40%. The work not only indicates that furan-3-carboxylate is a simple yet efficient building block for constructing non-fused-ring polymers but also provides a promising electron donor PF2 for the low-cost production of OSCs.

Elevated Photovoltaic Performance in Medium Bandgap Copolymers Composed of Indacenodi-thieno[3,2-b]thiophene and Benzothiadiazole Subunits by Modulating the π-Bridge

Two random conjugated polymers (CPs), namely, PIDTT-TBT and PIDTT-TFBT, in which indacenodithieno[3,2-b]thiophene (IDTT), 3-octylthiophene, and benzothiadiazole (BT) were in turn utilized as electron-donor (D), π-bridge, and electron-acceptor (A) units, were synthesized to comprehensively analyze the impact of reducing thiophene π-bridge and further fluorination on photostability and photovoltaic performance. Meanwhile, the control polymer PIDTT-DTBT with alternating structure was also prepared for comparison. The broadened and enhanced absorption, down-shifted highest occupied molecular orbital energy level (E_HOMO), more planar molecular geometry thus enhanced the aggregation in the film state, but insignificant impact on aggregation in solution and photostability were found after both reducing thiophene π-bridge in PIDTT-TBT and further fluorination in PIDTT-TFBT. Consequently, PIDTT-TBT-based device showed 185% increased PCE of 5.84% profited by synergistically elevated V_OC, J_SC, and FF than those of its counterpart PIDTT-DTBT, and this improvement was chiefly ascribed to the improved absorption, deepened E_HOMO, raised μ_h and more balanced μ_h/μ_e, and optimized morphology of photoactive layer. However, the dropped PCE was observed after further fluorination in PIDTT-TFBT, which was mainly restricted by undesired morphology for photoactive layer as a result of strong aggregation even if in the condition of the upshifted V_OC. Our preliminary results can demonstrate that modulating the π-bridge in polymer backbone was an effective method with the aim to enhance the performance for solar cell.

Dual Imide-Functionalized Unit-Based Regioregular D-A1-D-A2 Polymers for Efficient Unipolar n-Channel Organic Transistors and All-Polymer Solar Cells

The demand for the development of more promising n-type semiconducting polymers with excellent electron mobilities and air stabilities is growing fast. In this study, we designed and synthesized a series of new dual imide-functionalized derivative-based regioregular D-A₁-D-A₂ copolymers with different side chains (namely, PNT-R, R = 2-decyltetradecyl (DT), 2-octadecyldodecyl (OD), and 2-hexyldecyl (HD)). These new polymers PNT-R showed strong electron affinities with deep lowest unoccupied molecular orbital (LUMO) levels down to -4.01 eV, indicating that they are promising electron-transporting materials. To optimize the electron mobility, side-chain engineering was adopted. Thus, the effects of the side-chain length on their optoelectronic and charge-transport properties as well as the performances of all-polymer solar cells (all-PSCs) were systematically investigated. Shortening the side-chain length significantly expanded the absorption range, deepened the LUMO energy level, strengthened the molecular packing properties, and developed more crystalline microstructures in the solid state, as evidenced by the ultraviolet-visible absorption spectra, cyclic voltammetry, synchrotron two-dimensional grazing-incidence wide-angle X-ray scattering, and atomic force microscopy measurements. Consequently, the highest electron mobility of 1.05 cm² V^-1 s^-1 was achieved in PNT-HD-based organic thin-film transistors (OTFTs). Also, PNT-R polymers were successfully applied as electron acceptors in all-PSCs. In good agreement with the OTFT results, the highest power conversion efficiency of 6.62% was obtained for the PNT-HD-blend film due to its excellent short-circuit current ( J_sc) value (12.07 mA cm^-2), which was much higher than that of the PNT-DT- and PNT-OD-based all-PSCs (7.67 and 10.19 mA cm^-2, respectively). By further investigating the dependence of the J_sc and open-circuit voltage ( V_oc) on the illuminated light intensity ( P), the high J_sc value of the PNT-HD-based device was found to originate from its highly suppressed mono- and bimolecular recombination as well as efficient exciton dissociation and charge transfer at the donor-acceptor interfaces. Overall, this study provides insights into the naphthalenediimide-based regioregular D-A₁-D-A₂ copolymers used in all-PSCs and offers important design guidelines for future development of n-type semiconducting polymers.

Isomeric Pyrenodithiophenediones and Their Derivatives: Synthesis, Reactivity, and Device Performance

Two isomeric pyrenodithiophenediones and their triisopropylsilyl(TIPS)-ethynyl-functionalized derivatives were synthesized and characterized. With treatment of excess lithium TIPS-acetylide, the isomeric diones could undergo Michael and carbonyl additions to form three derivatives with four, three, and two TIPS-ethynyl groups, respectively. The isomeric TIPS-ethynyl-functionalized derivatives show different physicochemical properties. The single-crystalline transistors based on TIPS-ethynyl-functionalized derivatives exhibit a promising p-type transport behavior, with hole mobilities up to 0.51 cm² V^-1 s^-1.

Systematically investigating the influence of inserting alkylthiophene spacers on the aggregation, photo-stability and optoelectronic properties of copolymers from dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene and benzothiadiazole derivatives

PDTBDT-SBT and PDTBDT-SFBT presented a superior trade-off between band gaps and photo-stabilities.

Enhanced Photovoltaic Performance in D-π-A Copolymers Containing Triisopropylsilylethynyl-Substituted Dithienobenzodithiophene by Modulating the Electron-Deficient Units

Three alternated D-π-A type 5,10-bis(triisopropylsilylethynyl)dithieno[2,3-d:2',3'-d']-benzo[1,2-b:4,5-b']dithiophene (DTBDT-TIPS)-based semiconducting conjugated copolymers (CPs), PDTBDT-TIPS-DTBT-OD, PDTBDT-TIPS-DTFBT-OD, and PDTBDT-TIPS-DTNT-OD, bearing different A units, including benzothiadiazole (BT), 5,6-difluorinated-BT (FBT) and naphtho[1,2-c:5,6-c']-bis[1,2,5]thiadiazole (NT), were designed and synthesized to investigate the impact of the variation in electron-deficient units on the properties of these photovoltaic polymers. It was exhibited that the down-shifted highest occupied molecular orbital energy level (EHOMO), the enhanced aggregation in both the chlorobenzene solution and the solid film, as well as the better molecular planarity, were achieved using methods involving fluorination and the replacement of BT with NT on the polymer backbone. The absorption profile was little changed upon fluorination; however, it was greatly broadened during replacement of BT with NT. Consequently, the optimized photovoltaic device based on the PDTBDT-TIPS-DTNT-OD exhibited synchronous enhancements in the open-circuit voltage (VOC) of 0.88 V, the short-circuit current density (JSC) of 7.21 mA cm-2, and the fill factor (FF) of 52.99%, resulting in a drastic elevation in the PCE by 129% to 3.37% compared to that of the PDTBDT-TIPS-DTBT-OD. This was triggered by PDTBDT-TIPS-DTNT-OD's broadened absorption, deepened EHOMO, improved coplanarity, and enhanced SCLC mobility (which increased 3.9 times), as well as a favorable morphology of the active layer. Unfortunately, the corresponding PCE deteriorated after incorporating fluorine into the BT, due to the oversized aggregation and large phase separation morphology in the blend films, severely impairing its JSC. Our preliminary results demonstrated that the replacement of BT with NT in a D-π-A type polymer backbone was an effective strategy of tuning the molecular structure to achieve highly efficient polymer solar cells (PSCs).

Understanding the Effects of a High Surface Area Nanostructured Indium Tin Oxide Electrode on Organic Solar Cell Performance

Organic solar cells (OSCs) are a complex assembly of disparate materials, each with a precise function within the device. Typically, the electrodes are flat, and the device is fabricated through a layering approach of the interfacial layers and photoactive materials. This work explores the integration of high surface area transparent electrodes to investigate the possible role(s) a three-dimensional electrode could take within an OSC, with a BHJ composed of a donor-acceptor combination with a high degree of electron and hole mobility mismatch. Nanotree indium tin oxide (ITO) electrodes were prepared via glancing angle deposition, structures that were previously demonstrated to be single-crystalline. A thin layer of zinc oxide was deposited on the ITO nanotrees via atomic layer deposition, followed by a self-assembled monolayer of C₆₀-based molecules that was bound to the zinc oxide surface through a carboxylic acid group. Infiltration of these functionalized ITO nanotrees with the photoactive layer, the bulk heterojunction comprising PC₇₁BM and a high hole mobility low band gap polymer (PDPPTT-T-TT), led to families of devices that were analyzed for the effect of nanotree height. When the height was varied from 0 to 50, 75, 100, and 120 nm, statistically significant differences in device performance were noted with the maximum device efficiencies observed with a nanotree height of 75 nm. From analysis of these results, it was found that the intrinsic mobility mismatch between the donor and acceptor phases could be compensated for when the electron collection length was reduced relative to the hole collection length, resulting in more balanced charge extraction and reduced recombination, leading to improved efficiencies. However, as the ITO nanotrees increased in height and branching, the decrease in electron collection length was offset by an increase in hole collection length and potential deleterious electric field redistribution effects, resulting in decreased efficiency.

Self-Assembly of 1-Pyrenemethanol on ZnO Surface toward Combined Cathode Buffer Layers for Inverted Polymer Solar Cells

Solid alcohol 1-pyrenemethanol (PyM) was first introduced to modify the zinc oxide (ZnO) layer which is used in the inverted polymer solar cells (PSCs) as a cathode buffer layer (CBL). As a low-cost industrial product, the PyM can modify the surface defects and improve the electron mobility of ZnO CBL, which can be attributed to the self-assembly of PyM on the ZnO surface due to the hydrogen bonds and the conjugated structure in PyM. With a blend of PTB7:PC₇₁BM as active layer, the device with ZnO/PyM CBL exhibited a notable power conversion efficiency (PCE) of 8.26%, which is better than that of control devices based on bare ZnO CBL (7.26%). With the addition of PyM, the device based on PTB7-Th:PC₇₁BM showed a higher PCE of 9.10%, an obvious improvement from the 7.79% of control devices. There was no obvious change in device performance with the increase of PyM solution concentration, indicating that the device fabrications are thickness-insensitive. These results could be particularly useful in solution processing of buffer layer materials to high-efficiency organic solar cells.