Lin Hu

Polyfluorene Electrolytes Interfacial Layer for Efficient Polymer Solar Cells: Controllably Interfacial Dipoles by Regulation of Polar Groups

The polar groups in the conjugated polyelectrolytes (CPEs) can create the favorable dipoles at the electrode/active layer interface, which is critical for the CPEs to minimize the interfacial energy barrier in polymer solar cells (PSCs). Herein, a series of CPEs based on poly [(9,9-bis(3-(N,N-dimethylamino)propyl)-2,7-fluorene)-co-2,7-(9,9-dioctylfluorene)] derivates (PFNs) (PFN30, PFN50, PFN70, and PFN100) with different mole ratio of polar groups (-N(C2H5)2) were designed and synthesized to investigate the effect of the numbers of polar groups on the interfacial dipoles. Controllably interfacial dipoles could be readily achieved by only tuning the numbers of -N(C2H5)2 in PFNs, as revealed by the work function of the PFNs modified ITO gradually reduced as the loadings of the -N(C2H5)2 increased. In addition, increasing the numbers of -N(C2H5)2 in PFNs were also favorable for developing the smooth and homogeneous morphology of the active layer. As a result, the content of the polar amine in the PFNs exerted great influence on the performance of polymer solar cells. Increasing the numbers of the pendent -N(C2H5)2 could effectively improve the power conversion efficiency (PCE) of the devices. Among these PFNs, PFN100 with the highest content of -N(C2H5)2 polar groups delivered the device with the best PCE of 3.27%. It indicates tailoring the content of the polar groups in the CPEs interlayer is a facial and promising approach for interfacial engineering to developing high performance PSCs.

Dual functions of interface passivation and n-doping using 2,6-dimethoxypyridine for enhanced reproducibility and performance of planar perovskite solar cells

The reproducibility of high-performance perovskite solar cells (PVSCs) remains a major obstacle. Herein, for the first time, we report the use of 2,6-dimethoxypyridine (2,6-Py) for interface chemistry engineering to fabricate reproducible high-efficiency planar perovskite solar cells. The 2,6-Py serves dual functions: (1) as a Lewis base enabling surface passivation of Lewis acid traps (e.g., under-coordinated Pb ions) without corroding the perovskite; (2) as a chemical dopant for [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) to improve its conductivity and mobility for efficient electron extraction and transport. Thus, through both the surface passivation of the perovskite layer and the doping of the electron transport layer with 2,6-Py, the resultant MAPbI3-based planar solar cells outperform the untreated devices with power conversion efficiency (PCE) significantly improved from 15.53% to 19.41%. The devices with the dual-function treatment yield effectively improved reproducibility with a narrow PCE distribution – that is, around 90% of the devices afford a PCE of over 17.50% (about 90% of the champion PCE), and also display enhanced air stability – that is, they maintain nearly 80% of their initial PCEs after 200 h in ambient air without any encapsulation.

Writable and patternable organic solar cells and modules inspired by an old Chinese calligraphy tradition

Scalable, patternable and affordable thin-film fabrication techniques for solution-processed organic photovoltaics are highly desirable. In this work, we report a new fabrication technique inspired by an old Chinese calligraphy tradition to fabricate organic solar cells and modules. The fabrication tool of “Maobi”, also called “Chinese ink brush”, has over 2000 years of history and has been widely used for writing and painting in old Chinese history. Using Maobi coating, the thickness of the active layers and the polymer electrodes could be tuned by optimizing the coating speed and substrate temperature. Solar cells based on Maobi-coated active layers (P3HT:ICBA, PTB7-Th:PC71BM and PBDB-T:ITIC) display comparable performance to the devices with spin-coated active layers. Among them, the cells with the Maobi-coated PBDB-T:ITIC active layer exhibit high power conversion efficiencies of 10.1%. Furthermore, based on the inherent advantage of the easy patterning of Maobi, we demonstrated Maobi-coated solar modules containing 8 sub-cells that exhibit a high open-circuit voltage (VOC) of 6.3 V and a high fill factor of 0.71. At the end, large-area solar modules (18 cm2) were demonstrated via a motor-driven computer-controlled automatic Maobi coating. The module displays VOC of up to 11.6 V and a power conversion efficiency of 6.3%.

Enhanced Thermochemical Stability of CH3NH3PbI3 Perovskite Films on Zinc Oxides via New Precursors and Surface Engineering

Hydroxyl groups on the surface of ZnO films lead to the chemical decomposition of CH3NH3PbI3 perovskite films during thermal annealing, which limits the application of ZnO as a facile electron-transporting layer (ETL) in perovskite solar cells. In this work, we report a new recipe that leads to substantially reduced hydroxyl groups on the surface of the resulting ZnO films by employing polyethylenimine (PEI) to replace generally used ethanolamine in the precursor solutions. Films derived from the PEI-containing precursors are denoted as P-ZnO and those from the ethanolamine-containing precursors as E-ZnO. Besides the fewer hydroxyl groups that alleviate the thermochemical decomposition of CH3NH3PbI3 perovskite films, P-ZnO also provides a template for the fixation of fullerene ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM) owing to its nitrogen-rich surface that can interact with PCBM. The fullerene was used to block the direct contact between P-ZnO and CH3NH3PbI3 films and therefore further enhance the thermochemical stability of perovskite films. As a result, perovskite solar cells based on the P-ZnO/PCBM ETL yield an optimal power conversion efficiency (PCE) of 15.38%. We also adopt P-ZnO as the ETL for organic solar cells that yield a remarkable PCE of 10.5% based on the PBDB-T:ITIC photoactive layer.

Enhancing Photovoltaic Performance of Inverted Planar Perovskite Solar Cells by Cobalt-Doped Nickel Oxide Hole Transport Layer

Electron and hole transport layers have critical impacts on the overall performance of perovskite solar cells (PSCs). Herein, for the first time, a solution-processed cobalt (Co)-doped NiO X film was fabricated as the hole transport layer in inverted planar PSCs, and the solar cells exhibit 18.6% power conversion efficiency. It has been found that an appropriate Co-doping can significantly adjust the work function and enhance electrical conductivity of the NiO X film. Capacitance-voltage ( C- V) spectra and time-resolved photoluminescence spectra indicate clearly that the charge accumulation becomes more pronounced in the Co-doped NiO X-based photovoltaic devices; it, as a consequence, prevents the nonradiative recombination at the interface between the Co-doped NiO X and the photoactive perovskite layers. Moreover, field-dependent photoluminescence measurements indicate that Co-doped NiO X-based devices can also effectively inhibit the radiative recombination process in the perovskite layer and finally facilitate the generation of photocurrent. Our work indicates that Co-doped NiO X film is an excellent candidate for high-performance inverted planar PSCs.

Chemical reaction between an ITIC electron acceptor and an amine-containing interfacial layer in non-fullerene solar cells

The development of non-fullerene acceptors keeps pushing forward the efficiency record of organic solar cells and resulting in new solar cell physics and chemistry to research. ITIC-based materials are an important type of non-fullerene acceptors. Herein, we report a new finding that the ITIC acceptor can react with the widely used low work-function interfacial layer polyethylenimine (PEI or PEIE) in non-fullerene organic solar cells. The amine interfacial material reacts as a nucleophile with the CO moiety of the ITIC, that destroys the original electronic structure and the intramolecular charge transfer of the ITIC molecules. Inverted non-fullerene solar cells with a PBDB-T:ITIC active layer deposited on PEI or PEIE display much lower PCE values compared with those of reference cells with the active layers deposited on ZnO electron-collecting interlayers. This chemical reaction between the ITIC acceptor and PEI or PEIE becomes more severe under thermal annealing and further deteriorates the solar cell performance. This original finding pays important attention to the design and development of the interfaces required for efficient non-fullerene organic solar cells.

Fluorine-induced self-doping and spatial conformation in alcohol-soluble interlayers for highly-efficient polymer solar cells

The molecular design strategy for high-performance photoelectric materials emphasizes the intrinsic charge transfer/transport as well as the role of the polymer chemical structure and chain conformation. Here, we report a new interface engineering strategy for non-fullerene polymer solar cells (PSCs) by employing highly conductive polyelectrolyte interface layers with a fluorinated conjugated backbone. The fluorine atom-induced strong n-type self-doping effect and optimized expanded conformation were observed to substantially improve their intrinsic charge mobility. An outstanding power conversion efficiency of 11.51% was obtained when applying the new polyelectrolyte interlayer in PSCs based on a PBDB-T:ITIC active layer.