Bangwu Luo

A two-terminal perovskite/perovskite tandem solar cell

Building a tandem structure is an effective strategy to enhance the photovoltaic performance of solar cells. In the realization of a two-terminal tandem device, the charge recombination layer (CRL) plays an essential role. In the current study, we demonstrate the first bottom-up solution-processed two-terminal perovskite/perovskite tandem solar cell via developing a novel CRL: spiro-OMeTAD/PEDOT:PSS/PEI/PCBM:PEI. This CRL is efficient to collect electrons and holes at its top and bottom surfaces, and robust enough to protect the bottom perovskite film during the top perovskite film deposition. Moreover, the CRL is prepared by orthogonal solvent processing at low temperature, which is compatible with the pre-deposited perovskite film underneath. The PEI/PCBM:PEI is specially developed for efficient electron collection in both single-junction and tandem perovskite solar cells. With the optimized CRL to bridge the two CH3NH3PbI3 perovskite subcells, the tandem solar cell yields an open-circuit voltage (VOC) of up to 1.89 V that is close to the sum of the two perovskite subcells.

Universal Strategy To Reduce Noise Current for Sensitive Organic Photodetectors

Low noise current is critical for achieving high-detectivity organic photodetectors. Inserting charge-blocking layers is an effective approach to suppress the reverse-biased dark current. However, in solution-processed organic photodetectors, the charge-transport material needs to be dissolved in solvents that do not dissolve the underneath light-absorbing layer, which is not always possible for all kinds of light-absorbing materials developed. Here, we introduce a universal strategy of transfer-printing a conjugated polymer, poly(3-hexylthiophene) (P3HT), as the electron-blocking layer to realize highly sensitive photodetectors. The transfer-printed P3HT layers substantially and universally reduced the reverse-biased dark current by about 3 orders of magnitude for various photodetectors with different active layers. These photodetectors can detect the light signal as weak as several picowatts per square centimeter, and the device detectivity is over 1012 Jones. The results suggest that the strategy of transfer-printing P3HT films as the electron-blocking layer is universal and effective for the fabrication of sensitive organic photodetectors.

Indium tin oxide (ITO)-free, top-illuminated, flexible perovskite solar cells

Flexible and light-weight photovoltaics are desirable for applications that involve their integration with flexible electronics. In this study, we demonstrate efficient flexible perovskite solar cells with a novel device architecture (that is indium-tin-oxide (ITO) free and top-illuminated) using a low-temperature processed doped fullerene as the electron-transporting layer. Silver is used as the bottom electrode, and a transparent conducting polymer electrode is used as the top electrode for light illuminating through to the active layer. Stearyldimethylbenzylammonium chloride (SDBAC)-doped [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) was the electron-transporting layer wherein SDBAC could enhance the conductivity by three orders of magnitude and therefore enhance the solar cell performance. The ITO-free flexible perovskite solar cells display power conversion efficiency of 11.8% on a polyethersulfone substrate and can maintain 84% of the initial PCE after 1000 bending cycles at a bending radius of 6 mm.

Chlorine-Incorporation-Induced Formation of the Layered Phase for Antimony-Based Lead-Free Perovskite Solar Cells

The environmental toxicity of Pb in organic-inorganic hybrid perovskite solar cells remains an issue, which has triggered intense research on seeking alternative Pb-free perovskites for solar applications. Halide perovskites based on group-VA cations of Bi3+ and Sb3+ with the same lone-pair ns2 state as Pb2+ are promising candidates. Herein, through a joint experimental and theoretical study, we demonstrate that Cl-incorporated methylammonium Sb halide perovskites (CH3NH3)3Sb2ClXI9-X show promise as efficient solar absorbers for Pb-free perovskite solar cells. Inclusion of methylammonium chloride into the precursor solutions suppresses the formation of the undesired zero-dimensional dimer phase and leads to the successful synthesis of high-quality perovskite films composed of the two-dimensional layered phase favored for photovoltaics. Solar cells based on the as-obtained (CH3NH3)3Sb2ClXI9-X films reach a record-high power conversion efficiency over 2%. This finding offers a new perspective for the development of nontoxic and low-cost Sb-based perovskite solar cells.

Flexible large-area organic tandem solar cells with high defect tolerance and device yield

The fabrication of thin layers of organic photoactive materials (typically ca. 100–200 nm thick) over large area is needed for the commercial realization of organic solar cells. This is challenging because defects on these thin layers can cause high leakage currents which lead to poor device performance and, ultimately, to poor device yield. Here, we report that organic solar cells with a tandem structure can display an increased tolerance to defects and are found less susceptible to parasitic area scaling-up effects compared to single-junction solar cells. We demonstrate 10.5 cm2 flexible tandem solar cells with a power conversion efficiency of 6.5% with a fabrication yield of over 90% in a laboratory environment. The high fabrication yield and good performance displayed by tandem organic solar cells suggest that despite their increased complexity, they could provide a viable path towards the commercial realization of efficient large-area organic solar cells.

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%.

Colorful flexible polymer tandem solar cells

Fabrication of efficient colorful flexible polymer tandem solar cells remains a challenge. In this work, we report the use of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the top transparent electrode in tandem solar cells that in the meantime acts as an optical coating for light engineering. Modification of the thickness of the transfer-printed PEDOT:PSS allows tailoring the reflectance spectra and yields colorful polymer solar cells with easy processability. Compared to the strategies of adopting optical microcavities and photonic crystals, the use of PEDOT:PSS also offers the important advantage of excellent mechanical flexibility that enables the fabrication of flexible colorful solar cells. The fabricated colorful flexible polymer tandem solar cells with polymer electrodes display power conversion efficiency (PCE) values from 7.23% to 8.34% depending on the yielded color of the cells, which are among the highest values for reported colorful polymer solar cells.

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.

Patching defects in the active layer of large-area organic solar cells

Large area fabrication of organic solar cells is critical to achieve sufficient energy for power applications. However, large-area organic solar cells still suffer from low production yields because their thin active layer (100–300 nm) is prone to form leakage current when film defects exist in the active layer. In this study, we proposed a novel strategy to patch the defects to suppress the leakage current and improve the device yield of large-area organic solar cells. The defects in the active layers have been patched by coating an insulating polymer using a Maobi tool that has the advantage of patterning and ink holding. Insulating polymers including polyethylenimine (PEI), polyvinyl alcohol (PVA), and polymethylmethacrylate (PMMA) have been used to patch the defects. Electrical insulativity, solution processability, solvent orthogonality to the active layer, good wetting on the active layer, and chemical inactivity with the active layer and interfacial layer are important properties for a good candidate used for patching the defects. The patching strategy is applicable to both fullerene-based and non-fullerene-based solar cells and can effectively restore the photovoltaic properties of the active layers with defects. Finally, we have demonstrated the construction of large-area (up to 52 cm2) monolithic none-fullerene organic solar cells with the assistance of the proposed patching strategy.

Stacking Sequence and Acceptor Dependence of Photocurrent Spectra and Photovoltage in Organic Two-Junction Devices

Both single-junction and tandem organic photovoltaic cells have been well developed. A tandem cell contains two junctions with a charge recombination layer (CRL) inserted between the two junctions. So far, there is no detailed report on how the device will perform that contains two junctions but without a CRL in between. In this work, we report the photocurrent spectra and photovoltage output of the devices that contains two bulk-heterojunctions (BHJ) stacked directly on top of each other without a CRL. The top active layer is prepared by transfer printing. The photocurrent response spectra and photovoltage are found to be sensitive to stacking sequence and the selection of electron acceptors. The open-circuit voltage of the devices (up to 1.09 V) can be higher than the devices containing either junction layer. The new phenomenon in the new device architecture increases the versatility of the optoelectronic devices based on organic semiconductors.