Biodegradable Materials and Green Processing for Green Electronics

There is little question that the "electronic revolution" of the 20th century has impacted almost every aspect of human life. However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges such as the management of electronic waste (e-waste) that will remain through the 21st century. In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the use of toxic materials and solvents in electronics processing, and lowering energy usage during fabrication methods. In response to these issues, the construction of electronic devices from renewable or biodegradable materials that decompose to harmless by-products is becoming a topic of great interest. Such "green" electronic devices need to be fabricated on industrial scale through low-energy and low-cost methods that involve low/non-toxic functional materials or solvents. This review highlights recent advances in the development of biodegradable materials and processing strategies for electronics with an emphasis on areas where green electronic devices show the greatest promise, including solar cells, organic field-effect transistors, light-emitting diodes, and other electronic devices.

Near-IR Absorbing D-A-D Zn-Porphyrin-Based Small-Molecule Donors for Organic Solar Cells with Low-Voltage Loss

Two D-A-D small molecules with a DPP acceptor core and Zn-porphyrin donor with different electron-donating substituents, namely, 2,6-bis(dodecyloxy)phenyl and 5-hexylthieno[3,2- b]thiophen-2-yl at mesopositions, VC4 and VC5, were synthesized, and their optical and electrochemical properties were investigated. The results reveal that both molecules are suitable as donors for organic solar cells (OSCs) in which PC₇₁BM is employed as the acceptor. Overall power conversion efficiencies of 8.05% ( J_sc = 13.83 mA/cm², V_oc = 0.91 V, and FF = 0.64) and 8.89% ( J_sc = 16.98 mA/cm², V_oc = 0.79 V, and FF = 0.663) were obtained, respectively. The high V_oc value for the VC4-based OSC correlates with the deeper HOMO, whereas the high J_sc value for VC5 may be attributed to the extended absorption spectrum toward the longer wavelength region. Moreover, the relatively high FF value for VC5-based OSCs as compared to the VC4 counterparts may be related to the more balanced charge transport in the active layer, reduced charge recombination, and efficient charge collection. The energy loss for VC5 is smaller (0.52 eV) than that for VC4 (0.56 eV).

Panchromatic ternary organic solar cells with 9.44% efficiency incorporating porphyrin-based donors

In an effort to improve the short-circuit current and fill factor, organic solar cells have been developed with ternary blending in a single bulk heterojunction active layer. We report here several all small molecule organic solar cells based on ternary bulk heterojunction active layers. These layers consist of two small molecule porphyrin donors (MV71 and MV72), which have the same backbone but different end-capping acceptor units, and PC71BM as the acceptor. The organic solar cells showed overall power conversion efficiencies of 3.21% and 4.03% for the as-cast MV71:PC71BM and MV72:PC71BM binary active layers, respectively. However, the power conversion efficiency of the ternary active layer, i.e., MV71:MV72:PC71BM (0.2 : 0.8 : 2), was 6.72% and this is higher than the two binary active layer counterparts. The enhancement in the PCE of the ternary active layer is mainly related to the improvement in both the short-circuit current and fill factor and is related to the synergistic effect of the good miscibility of the two donors and improved hole transportation due to the slightly deeper highest occupied molecular orbital energy level of MV72 than MV71. The PCE was further improved to 9.44% with an enhanced short-circuit current and fill factor when the ternary active layer was subjected to solvent vapour annealing for 40 seconds. The ternary organic solar cells showed higher values of the incident photon to current conversion efficiency across the entire wavelength region when compared to the binary counterparts. The same donor backbone facilitates miscibility at the molecular level and the different HOMO and LUMO energy levels of the donors enable charge transport in the devices based on the ternary active layers. The increase in the power conversion efficiency after SVA treatment may be attributed to the migration of MV71 from the mixed region to the donor-acceptor (D-A) interfaces, which in turn affects the charge transfer and recombination processes and is confirmed by the impedance spectroscopy and dark current-voltage measurements.

Ternary Solar Cells Based on Two Small Molecule Donors with Same Conjugated Backbone: The Role of Good Miscibility and Hole Relay Process

Ternary organic solar cells (OSCs) are very attractive for further enhancing the power conversion efficiencies (PCEs) of binary ones but still with a single active layer. However, improving the PCEs is still challenging because a ternary cell with one more component is more complicated on phase separation behavior. If the two donors or two acceptors have similar chemical structures, good miscibility can be expected to reduce the try-and-error work. Herein, we report ternary devices based on two small molecule donors with the same backbone but different substituents. Whereas both binary devices show PCEs about 9%, the PCE of the ternary cells is enhanced to 10.17% with improved fill factor and short-circuit current values and external quantum efficiencies almost in the whole absorption wavelength region from 440 to 850 nm. The same backbone enables the donors miscible at molecular level, and the donor with a higher HOMO level plays hole relay process to facilitate the charge transportation in the ternary devices. Since side-chain engineering has been well performed to tune the active materials' energy levels in OSCs, our results suggest that their ternary systems are promising for further improving the binary cells' performance although their absorptions are not complementary.

Enhancing Performance of Large-Area Organic Solar Cells with Thick Film via Ternary Strategy

Large-scale fabrication of organic solar cells requires an active layer with high thickness tolerability and the use of environment-friendly solvents. Thick films with high-performance can be achieved via a ternary strategy studied herein. The ternary system consists of one polymer donor, one small molecule donor, and one fullerene acceptor. The small molecule enhances the crystallinity and face-on orientation of the active layer, leading to improved thickness tolerability compared with that of a polymer-fullerene binary system. An active layer with 270 nm thickness exhibits an average power conversion efficiency (PCE) of 10.78%, while the PCE is less than 8% with such thick film for binary system. Furthermore, large-area devices are successfully fabricated using polyethylene terephthalate (PET)/Silver gride or indium tin oxide (ITO)-based transparent flexible substrates. The product shows a high PCE of 8.28% with an area of 1.25 cm² for a single cell and 5.18% for a 20 cm² module. This study demonstrates that ternary organic solar cells exhibit great potential for large-scale fabrication and future applications.

New Terthiophene-Conjugated Porphyrin Donors for Highly Efficient Organic Solar Cells

To mimic the natural photosynthetic systems utilizing chlorophylls to absorb light and store light energy, two new porphyrin-based small molecules of PTTR and PTTCNR have been developed for photovoltaic applications. The highest power conversion efficiency of 8.21% is achieved, corresponding to a short-circuit current of 14.30 mA cm^-2, open-circuit voltage of 0.82 V, and fill factor of 70.01%. The excellent device performances can be ascribed to the engineering of molecule structure and film morphology. The horizontal conjugation of 3,3″-dihexyl-terthiophene to porphyrin-core with the vertical aliphatic 2-octylundecyl peripheral substitutions, can not only effectively increase the solar flux coverage between the conventional Soret and Q bands of porphyrin unit, but also optimize molecular packing through polymorphism associated with side-chains and the linear π-conjugated backbones. And the additive of 1,8-diiodooctane and subsequent chloroform solvent vapor annealing facilitate the formation of the blend films with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) characteristics of bicontinuous, interpenetrating networks required for efficient charge separation and transportation.