Scalable organic semiconductor crystals for electronics and perovskite photovoltaics

Abstract

Organic semiconductors are excellent candidates for scalable electronics and photovoltaics, with the compatibility of low-cost and high-throughput mass production. Single crystals of organic semiconductors are particularly fascinating due to their long-range periodic order, minimal traps, and eliminated grain boundaries (GBs). However, it is still challenging to obtain highly-crystalline and single-crystalline organic films to explore their full potential. The present thesis aims to establish simple yet effective methods for the fabrication of well-aligned organic crystals, study crystal growth mechanisms, improve the functionality of organic crystals, and ultimately promote more efficient use of organic crystals in organic field-effect transistors (OFETs) and perovskite solar cells (PSCs). Primarily, an effective hybrid deposition approach that combines a blade solution shearing and the common thermal evaporation was developed to fabricate an ultrathin C10-DNTT film with high crystallinity and low density of GBs. More specifically, a highly crystallized C10-DNTT monolayer with large-area uniformity was firstly obtained by an ultraslow shearing method (USS), and its growth pattern showed a kinetic Wulff’s construction with preferential crystal orientation. The high-quality monolayer with millimeter-scale monocrystalline domains was then utilized as the template for the growth of the same organic semiconductor by thermal evaporation. The hybrid C10-DNTT film grown by this hybrid approach exhibited an impressively structural coherence with a copied molecular orientation of the monolayer template. The OFETs based on hybrid C10-DNTT ultrathin films showed superior electrical performances with average mobility of 14.7 cm2V−1s−1, which surpassed that of other devices fabricated by either solution shearing or thermal evaporation. Secondly, a simple nucleation seed-controlled shearing (NSCS) method was successfully developed to fabricate inch-scale and grain boundary-free C8-BTBT organic single-crystal thin films via controlling the nucleation seed and the subsequent crystal growth. The critical feature of our NSCS method was the oriented seed crystals developed by the first shearing. These crystal seeds can reduce the nucleation energy barrier, suppress the random and spontaneous nucleation, and regulate the crystal growth direction to form a perfect single crystal after the second shearing process. The formation mechanism of the single crystal achieved by our NSCS method was proposed based on the surface energies calculated by density functional theory (DFT). The OFETs based on the inch-scale single-crystal had a high yield with good mobility uniformity and low trap density and the highest mobility was up to 14.9 cm2V−1s−1. Finally, a functionalized organic semiconductor crystal, DPh-DNTT, was applied as a dopant-free hole transporting material (HTM) for PSCs. By engineering the temperature-dependent molecular orientation, the out-of-plane hole mobility of DPh-DNTT was significantly enhanced. This mobility enhancement was attributed to the dominated face-on molecular orientation with strong intermolecular π-π interactions along the out-of-plane direction. Such orientation-reinforced mobility and the suitable highest occupied molecular orbital (HOMO) of DPh-DNTT contributed to a remarkable power conversion efficiency (PCE) of 20.18% for MAPbI3-based PSCs utilizing the pristine DPh-DNTT films as the HTM. Moreover, the DPh-DNTT-based devices also exhibited superior long-term stability and thermal stability due to the S-Pb coordination formed at the perovskite/DPh-DNTT interface. (490 words)