Synergistic Amplification of Short-Circuit Current for Organic Solar Cells via Modulation of P3HT:PCBM Spatial Distribution with Solvent Treatment

Efficient charge transport across proper interfacial contacts is crucial in organic solar cells. In this paper, the surface of the P3HT:PCBM bulk heterojunction (BHJ) layer is treated with ethylene glycol monomethyl ether (EGME) prior to cathode buffer layer and top cathode deposition. Comparing to the nontreated P3HT:PCBM active layers, enhancement of short-circuit current (<i>J</i>_sc) to about 11 mA/cm² is observed in the EGME-treated P3HT:PCBM BHJ layers. On the other hand, two cathode buffer layers, ZnO nanoparticles (ZnO NPs) and LiF, are utilized to block holes and diminish the electron–hole recombination near the cathode, which consequently promote the open-circuit voltage (<i>V</i>_oc) to about 0.6 V. To integrate these two approaches, the EGME-treated P3HT:PCBM active layer and the ZnO NPs or LiF cathode buffer layer are applied concurrently. These two approaches work synergistically to enhance the <i>J</i>_sc to 16.3 mA/cm² or higher. With the distinct <i>J</i>_sc performance, the efficiencies of P3HT:PCBM cells using EGME treatment in combination with the ZnO/Al and LiF/Al cathodes are successfully amplified to 4.68% and 4.32%, respectively. The mechanism behind the synergistic amplification on <i>J</i>_sc will be analyzed based on the superficial morphology of the P3HT:PCBM active layer

Enhanced and Anisotropic Charge Transport in Polymer-Based Thin-Film Transistors by Guiding Polymer Growth

Ideal molecular features and microstructural properties of organic semiconducting thin films are being explored to achieve high-performance organic thin-film transistors (OTFTs). We prepared and processed hexamethylbenzene (HMB)/poly­(3-hexylthiophene) (P3HT) mixtures using a thermal gradient system to fabricate P3HT-based OTFTs. In the thermal gradient system, the HMB separated from the HMB/P3HT mixtures and crystallized along the sample movement direction. The crystallized HMB affected and guided the growth behavior of P3HT at the molecular level. Observations from joint microscopic and spectroscopic analyses revealed that the HMB-processed P3HT (H-P3HT) thin film possessed anisotropic and improved microstructures, particularly in crystalline domains. The improved molecular features and microstructural properties of the H-P3HT thin film enhanced the intramolecular and intermolecular charge transport by extending the π-conjugation, decreasing the reorganization energy, and strengthening the π–π overlaps. The electrical performance of the H-P3HT OTFT was augmented significantly with respect to that of the spin-coated P3HT OTFT. In addition, the H-P3HT OTFT exhibited an anisotropic charge transport property, correlating with microstructure directionality and resulting from the difference in the directions of the π–π overlaps. This effective and simple technique can be applied to other device types and has the potential to achieve high-performance organic electronic/photonic devices

Lithium-Induced Defect Levels in ZnO Nanoparticles To Facilitate Electron Transport in Inverted Organic Photovoltaics

In this work, lithium-doped zinc oxide nanoparticles (LZO NPs) with different Li/Zn molar ratios (Li/Zn = 0, 0.05, 0.2) are successfully prepared to form an electron transporting layer (cathode buffer layer) in the inverted-type P3HT:ICBA organic photovoltaic (OPV) devices. As compared with the undoped ZnO NPs buffer layer, a considerable improvement OPVs from 2.344% to 2.946% is obtained by using 5%-LZO NPs as a buffer layer, which owns <i>J</i>_sc of 7.22 mA/cm², <i>V</i>_oc of 0.86 V, and FF of 47.4%. X-ray absorption near-edge structure (XANES) spectra show the increase of unoccupied O 2p-derived states in 5%-LZO NPs, which leads to better carrier conductance. The energy levels of defects in 5%-LZO NPs analyzed by photoluminescence are found to facilitate electron extraction to the cathode. Impedance measurement results indicate that the carrier lifetime is effectively increased to 2176 μs by applying the 5%-LZO NPs buffer layer, showing the improvement of carrier extraction efficiency and resulting in its progressive performance

Synergistic Effects of Binary-Solvent Annealing for Efficient Polymer–Fullerene Bulk Heterojunction Solar Cells

Conjugated polymer–fullerene-based bulk-heterojunction (BHJ) organic solar cells (OSCs) have attracted tremendous attention over the past two decades because of their potential to develop low-cost and easy methods to produce energy from light. The complicated microstructure and morphology with randomly organized architecture of these polymer–fullerene-based active layers (ALs) is a key factor that limits photovoltaic performance. In this study, a binary-solvent annealing (BSA) approach was established to improve the poly­(3-hexylthiophene):indene-C₆₀ bisadduct-based AL for efficient BHJ-type OSCs by varying the second solvents with different boiling points (BP). Thus, we were able to change the evaporation behavior of cosolvents and consequently obtain the various microstructural properties of the AL. An in-depth study was conducted on the solvent-evaporation driven morphology of the active layer under various cosolvent conditions and its effect on the photovoltaic parameters of OSCs. Under the BSA processes, we found that the specimens with low-BP second solvents allows us to observe a more ideal AL for increasing photon absorption and efficient charge transport and collection at the respective electrodes, resulting in enhanced PCE of the corresponding OSCs. By contrast, the specimens with high-BP second solvents exhibit random microstructures, which are detrimental to charge transport and collection and lead to diminished PCE of the corresponding OSCs. By appropriately selecting the composition of a binary solvent, BSA can be employed as an easy method for the effective manipulation of the microstructures of ALs. BSA is a promising technique for the performance enhancement of not only OSCs but also other organic/polymeric-based electronic devices