Qingwu Yin

High-Performance Ternary Organic Solar Cell Enabled by a Thick Active Layer Containing a Liquid Crystalline Small Molecule Donor

Ternary organic solar cells (OSCs) have attracted much research attention in the past few years, as ternary organic blends can broaden the absorption range of OSCs without the use of complicated tandem cell structures. Despite their broadened absorption range, the light harvesting capability of ternary OSCs is still limited because most ternary OSCs use thin active layers of about 100 nm in thickness, which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers. In this paper, we report a highly efficient ternary OSC (11.40%) obtained by incorporating a nematic liquid crystalline small molecule (named benzodithiophene terthiophene rhodanine (BTR)) into a state-of-the-art PTB7-Th:PC71BM binary system. The addition of BTR into PTB7-Th:PC71BM was found to improve the morphology of the blend film with decreased π-π stacking distance, enlarged coherence length, and enhanced domain purity. This resulted in more efficient charge separation, faster charge transport, and less bimolecular recombination, which, when combined, led to better device performance even with thick active layers. Our results show that the introduction of highly crystalline small molecule donors into ternary OSCs is an effective means to enhance the charge transport and thus increase the active layer thickness of ternary OSCs to make them more suitable for roll-to-roll production than previous thinner devices.

Cross-Linkable and Dual Functional Hybrid Polymeric Electron Transporting Layer for High-Performance Inverted Polymer Solar Cells

A cross-linkable dual functional polymer hybrid electron transport layer (ETL) is developed by simply adding an amino-functionalized polymer dopant (PN4N) and a light crosslinker into a commercialized n-type semiconductor (N2200) matrix. It is found that the resulting hybrid ETL not only has a good solvent resistance, facilitating multilayers device fabrication but also exhibits much improved electron transporting/extraction properties due to the doping between PN4N and N2200. As a result, by using PTB7-Th:PC71 BM blend as an active layer, the inverted device based on the hybrid ETL can yield a prominent power conversion efficiency of around 10.07%. More interestingly, photovoltaic property studies of bilayer devices suggest that the absorption of the hybrid ETL contributes to photocurrent and hence the hybrid ETL simultaneously acts as both cathode interlayer material and an electron acceptor. The resulting inverted polymer solar cells function like a novel device architectures with a combination of a bulk heterojunction device and miniature bilayer devices. This work provides new insights on function of ETLs and may be open up a new direction for the design of new ETL materials and novel device architectures to further improve device performance.

Non-fullerene polymer solar cells with VOC > 1 V based on fluorinated quinoxaline unit conjugated polymers

To achieve efficient non-fullerene polymer solar cells (NF-PSCs), it is important to design and synthesize donor materials. To investigate the substituent effects of electron-withdrawing fluorine atoms and different alkyl chains (–C8C12 or –C10C14) on the thiophene π bridge, four novel conjugated polymers composed of dicyanodistyrylbenzene (DCB)-based and quinoxaline-based units were synthesized and applied in NF-PSC photovoltaic devices. It was found that the different alkyl side groups had a minimal influence on the molecular energy level but a moderate effect on the absorption coefficient, whereas the highest occupied molecular orbital (HOMO) of the resulting copolymers could be effectively lowered by introducing highly electronegative fluorine atoms into the quinoxaline moiety. As a result, the NF-PSCs based on the fluorinated quinoxaline-based copolymer exhibited high open-circuit voltages (VOC) of up to 1.043 V, which is the highest value to date for devices based on quinoxaline moiety copolymers. Moreover, fluorination also improved the copolymer carrier mobility and absorption coefficient, leading to enhanced JSC and FF, thus giving rise to higher overall efficiencies. NF-PSCs based on PDCB-DFQ812:ITIC exhibited the best performance, with a power conversion efficiency (PCE) of 8.37%. Our comparative research indicates that fluorinated quinoxaline-based conjugated polymers are promising donor materials for NF-PSCs.

Naphthalene Diimide-Based N-type Conjugated Polymers as Efficient Cathode Interfacial Materials for Polymer and Perovskite Solar Cells

A series of naphthalene diimide (NDI) based n-type conjugated polymers with amino-functionalized side groups and backbones were synthesized and used as cathode interlayers (CILs) in polymer and perovskite solar cells. Because of controllable amine side groups, all the resulting polymers exhibited distinct electronic properties such as oxidation potential of side chains, charge carrier mobilities, self-doping behaviors, and interfacial dipoles. The influences of the chemical variation of amine groups on the cathode interfacial effects were further investigated in both polymer and perovskite solar cells. We found that the decreased electron-donating property and enhanced steric hindrance of amine side groups substantially weaken the capacities of altering the work function of the cathode and trap passivation of the perovskite film, which induced ineffective interfacial modifications and declining device performance. Moreover, with further improvement of the backbone design through the incorporation of a rigid acetylene spacer, the resulting polymers substantially exhibited an enhanced electron-transporting property. Upon use as CILs, high power conversion efficiencies (PCEs) of 10.1% and 15.2% were, respectively, achieved in polymer and perovskite solar cells. Importantly, these newly developed n-type polymers were allowed to be processed over a broad thickness range of CILs in photovoltaic devices, and a prominent PCE of over 8% for polymer solar cells and 13.5% for perovskite solar cells can be achieved with the thick interlayers over 100 nm, which is beneficial for roll-to-roll coating processes. Our findings contribute toward a better understanding of the structure-performance relationship between CIL material design and solar cell performance, and provide important insights and guidelines for the design of high-performance n-type CIL materials for organic and perovskite optoelectronic devices.

Efficient device engineering for inverted non-fullerene organic solar cells with low energy loss

In recent years, the use of non-fullerene acceptors in organic solar cells has rapidly advanced with new acceptor materials, which have enabled devices to achieve a power conversion efficiency greater than 13%. In addition to new acceptor materials’ design, device engineering plays an important role in improving the device performance. In this study, we develop effective device engineering strategies, including thermal annealing and interlayer modification, to improve the device performance from 7.39% to 9.39%. With the use of PTB7-Th as the donor and IDT-BT-R as the non-fullerene acceptor, we achieved an efficient non-fullerene organic solar cell based on an inverted device architecture with a power conversion efficiency as high as 9.39%. It is worthy of note that the energy loss of the optimized device is only around 0.5 eV, which can be attributed to weak recombination and the appropriate high energy level of the charge transfer states within the optimized device.

Electron acceptors with a truxene core and perylene diimide branches for organic solar cells: the effect of ring-fusion

In this work, a star-shaped planar acceptor named FTr-3PDI was synthesized via ring-fusion between truxene core and three bay-linked perylene diimide (PDI) branches. Compared to the unfused non-planar acceptor Tr-3PDI, FTr-3PDI exhibits better structural rigidity and planarity, as well as more effective conjugation between truxene core and PDI branches. As a result, FTr-3PDI shows up-shifted energy levels, enhanced light absorption coefficient, increased electron mobility, and more favorable phase separation morphology in bulk-heterojunction (BHJ) blend films as compared to Tr-3PDI. Consequently, FTr-3PDI afforded higher power conversion efficiency (PCE) in BHJ solar cells when blended with a polymer donor PTB7-Th. This work demonstrates that ring-fusion is a promising molecular design strategy to combine the merits of truxene and PDI for non-fullerene acceptors used in organic solar cells.

Perylene Diimide Based Isomeric Conjugated Polymers as Efficient Electron Acceptors for All-polymer Solar Cells

We present here a series of perylene diimide (PDI) based isomeric conjugated polymers for the application as efficient electron acceptors in all-polymer solar cells (all-PSCs). By copolymerizing PDI monomers with 1,4-diethynylbenzene (para-linkage) and 1,3- diethynylbenzene (meta-linkage), isomeric PDI based conjugated polymers with parallel and non-parallel PDI units inside backbones were obtained. It was found that para-linked conjugated polymer (PA) showed better solubility, stronger π-π stacking, more favorable blend morphology, and better photovoltaic performance than those of meta-linked conjugated polymers (PM) did. Device based on PTB7-Th:PA (PTB7-Th:poly{4,8-bis[5-(2-ethylhexyl)-thiophen-2-yl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)- carbonyl]thieno[3,4-b]thiophene-4,6-diyl}) showed significantly enhanced photovoltaic performance than that of PTB7-Th:MA (3.29% versus 0.92%). Moreover, the photovoltaic performance of these polymeric acceptors could be further improved via a terpolymeric strategy. By copolymerizing a small amount of meta-linkages into PA, the optimized terpolymeric acceptors enabled to enhance photovoltaic performance with improved the short-circuit current density (Jsc) and fill factor (FF), resulting in an improved power conversion efficiency (PCE) of 4.03%.