High-efficiency organic solar cells prepared using a halogen-free solution process

Although the power conversion efficiency (PCE) of organic photovoltaic (OPV) devices has recently improved to more than 16%, halogenated solution processes are typically employed to obtain optimal performance. However, halogenated processing is harmful to health and the environment, which can be a significant obstacle to commercialization. Therefore, development of active materials processable under halogen-free conditions is of great importance in this field. In this study, a 16.04% OPV device, processed under halogen-free conditions, is developed by employing a new active-blend system, PBDTTPD-HT:BTP-2F-BO. It is highly soluble in halogen-free solvents, forming a preferential bulk-heterojunction morphology. The PBDTTPD-HT:BTP-2F-BO device achieves a PCE comparable with current state-of-the-art devices based on PBDB-TF:BTP-4F (also known as PM6:Y6) using a conventional halogenated process (16.40% versus 16.33%). Furthermore, it demonstrates a significantly higher PCE than the PBDB-TF:BTP-4F device with a halogen-free process (16.04% versus 9.70%)

Improved performance of colloidal quantum dot solar cells using high-electric-dipole self-assembled layers

High performance colloidal quantum dot (CQD) solar cells were developed by modifying ZnO electron accepting layers (EALs) using self-assembled monolayers (SAMs) of highly polar molecules. A high molecular dipole moment of -10.07D was achieved by conjugating a strong electron donor, julolidine, to an electron acceptor, a cyanoacetic acid unit, through a thiophene moiety. The energetic properties of ZnO EALs were manipulated with respect to the dipole moment of the modifying molecules. The built-in potential (V-bi) and internal electric field (E-int) of CQD solar cells could thereby be tuned. The power conversion efficiency (PCE) of the SAM modified devices was improved from 3.7% to 12.9% relative to the unmodified devices as a function of molecular dipole moments (from -5.13D to -10.07D). All figures-of-merit of solar cells were improved simultaneously by SAM modification due to enhanced V-bi, E-int, and charge collection efficiency. The PCE of the highly polar molecule modified devices reached 10.89% with a V-OC of 0.689 V, whereas that of the unmodified devices was 9.65% with a V-OC of 0.659 V. Notably, the remarkably low energy loss of 0.433 eV is achieved in the SAM modified devices

Near-Infrared Harvesting Fullerene-Free All-Small-Molecule Organic Solar Cells Based on Porphyrin Donors

Fullerene-free all-small-molecule organic photovoltaic cells (SM-OPVs) have attracted considerable attention because of well-defined molecular structures with low batch-to batch variation. Porphyrin derivatives have recently emerged as one of the promising conjugated building blocks for the small molecule (SM) donors. Herein, we first report fullerene-free SM-OPVs employing porphyrin-based donors. Three zinc porphyrin (P-zn)-based SM donors, which have strong bimodal absorption in the visible region and near-infrared region, are synthesized. Constructing bulk-heterojunction (BHJ) active layers using the P-zn donors and a SM acceptor, IDIC, which have complementary absorption, achieved panchromatic photon-to-current-conversion from 400 to 900 nm. The manipulation of side chains in the Pin donors considerably influenced the molecular ordering and nanomorphology of the BHJ active layers. P-zn-based fullerene-free SM-OPV devices with promising power conversion efficiency of 6.13% were achieved, which also offers crucial guidance for developing fullerene-free OPVs using porphyrin derivatives

Highly efficient air-stable colloidal quantum dot solar cells by improved surface trap passivation

While the power conversion efficiency (PCE) of colloidal quantum dot (CQD) solar cells can reach > 10%, the major obstacle for charge extraction and energy loss in such devices is the presence of surface trap sites within CQDs. In this work, highly trap-passivated PbS CQDs were developed using a novel iodide based ligand, 1-propyl-2,3-dimethylimidazolium iodide (PDMII). We examined the effects of PDMII on the surface quality of PbS-CQDs and compared them with TBAI, which is the best-selling iodide based ligand. By using PDMII, improved surface passivation with reduced sub-bandgap trap-states compared to TBAI was achieved. The reduced trap density resulted in enhanced charge extraction with diminished energy loss (0.447 eV) in the devices. Solar cell devices using our PDMII based CQDs displayed high PCE and air stability. The certified PCE of our PDMII based devices reached 10.89% and was maintained at 90% after 210 days of air storage

Performance Optimization of Parallel-Like Ternary Organic Solar Cells through Simultaneous Improvement in Charge Generation and Transport

Ternary organic photovoltaic (OPV) devices with multiple light-absorbing active materials have emerged as an efficient strategy for realizing further improvements in the power conversion efficiency (PCE) without building complex multijunction structures. However, the third component often acts as recombination centers and, hence, the optimization of ternary blend morphology poses a major challenge to improving the PCE of these devices. In this work, the performance of OPVs is enhanced through the morphological modification of nonfullerene acceptor (NFA)-containing binary active layers. This modification is achieved by incorporating fullerenes into the layers. The uniformly dispersed fullerenes are sufficiently continuous and successfully mediate the ordering of NFA without charge or energy transfer. Owing to the simultaneous improvement in the charge generation and extraction, the PCE (12.1%) of these parallel-linked ternary devices is considerably higher than those of the corresponding binary devices (9.95% and 7.78%). Moreover, the additional energy loss of the ternary device is minimized, compared with that of the NFA-based binary device, due to the judicious control of the effective donor:acceptor composition of the ternary blends