Youyu Jiang

Reduction and oxidation of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) induced by methylamine (CH3NH2)-containing atmosphere for perovskite solar cells

Perovskite solar cells have been attracting a lot of attention because of their high power conversion efficiency and low-cost processing. However, device reproducibility has been a problem. The fabrication atmosphere is regarded as one of the possible reasons. So far, there has been a lack of direct evidence to prove which kind of atmosphere and how the atmosphere affects the device performance. Here, we report that the methylamine (MA, boiling point: −6 °C) that is used to synthesize the methylammonium iodide (MAI) could chemically reduce the PEDOT:PSS hole-transporting layer. After the reduction, a strong absorbance band appears at 400–1100 nm and the conductivity and work function simultaneously decrease. Furthermore, the MA-reduced PEDOT:PSS films are also found to be easily oxidized in air. The reduced work function of the PEDOT:PSS layer leads to poor hole collection and yields low open-circuit voltage, short-circuit current and power conversion efficiency of the perovskite solar cells. Therefore, though the MA vapor-containing fabrication atmosphere is beneficial to the performance of TiO2-based perovskite solar cells in which the bottom electrode collects the electrons, it is detrimental to that of PEDOT:PSS-based solar cells.

Improved Performance of Printable Perovskite Solar Cells with Bifunctional Conjugated Organic Molecule

A bifunctional conjugated organic molecule 4-(aminomethyl) benzoic acid hydroiodide (AB) is designed and employed as an organic cation in organic-inorganic halide perovskite materials. Compared with the monofunctional cation benzylamine hydroiodide (BA) and the nonconjugated bifunctional organic molecule 5-ammonium valeric acid, devices based on AB-MAPbI3 show a good stability and a superior power conversion efficiency of 15.6% with a short-circuit current of 23.4 mA cm-2 , an open-circuit voltage of 0.94 V, and a fill factor of 0.71. The bifunctional conjugated cation not only benefits the growth of perovskite crystals in the mesoporous network, but also facilitates the charge transport. This investigation helps explore new approaches to rational design of novel organic cations for perovskite materials.

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.

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%.

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.