Simple Approach to the Highly Efficient and Cost-Effective Inverted Perovskite Solar Cells via Solvent-Engineered Electron-Transporting Layers of Fullerene

With dramatic growth in the photovoltaic (PV) market, perovskite solar cells (PSCs) have achieved remarkable performance and demonstrated enormous potential for use in next-generation PVs. In particular, inverted PSCs have advantages in lowering manufacturing costs because they are suitable for low-temperature printable processability and compatibility with existing silicon PVs for tandem cells. However, for the successful commercialization of PSC-based modules, material cost, a crucial factor, has not been considered in depth. Most inverted PSCs with the highest performance usually consist of fullerene derivatives, which are expensive but difficult to replace with other materials. Therefore, a rational method is needed to solve this problem, and we propose a simple idea to reduce material costs. We systematically investigated the correlation between the properties of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) films and various solvents with different boiling points. We discovered that the highly volatile dichloromethane (DCM) solvent forms a thick and uniform PCBM layer even at quarter concentrations. The DCM–PCBM layer improves the interfacial properties of the PCBM–perovskite film, leading to a device with superior performance compared to that of a device prepared with the PCBM layers from the other solvents. Finally, we successfully demonstrated a high-efficiency inverted PSC based on DCM–PCBM with a maximum power conversion efficiency (PCE) of 22.4%

Control of Crystallinity in PbPc:C₆₀ Blend Film and Application for Inverted Near-Infrared Organic Photodetector

Inverted near-infrared (NIR) organic photodetectors (OPDs) are required to combine the OPDs with an n-channel silicon-based integrated circuit. NIR absorption in the 930–960 nm range is important because the intensity of solar irradiation is low in this wavelength regime. Here, we controlled the crystallinity of lead­(II) phthalocyanine (PbPc) in a PbPc:C₆₀ blend film to obtain NIR absorption. To form a triclinic phase responsible for NIR light absorption, a substrate was heated during fabrication and C₆₀ was used as a templating layer, as well as an electron extraction layer, for an inverted structure. NIR absorption near 950 nm was enhanced, and the structural properties of the film changed dramatically. The OPD with enhanced NIR absorption exhibited a responsivity of 244 mA/W and an external quantum efficiency of 31.1% at a reverse bias of −3 V and 970 nm. The OPD detectivity also increased to 9.01 × 10¹² and 1.36 × 10¹¹ cm Hz^1/2/W under a zero bias and a reverse bias of −3 V, respectively

Multilayer Epitaxial Growth of Lead Phthalocyanine and C₇₀ Using CuBr as a Templating Layer for Enhancing the Efficiency of Organic Photovoltaic Cells

The molecular orientation and crystallinity of donor and acceptor molecules are important for high-efficiency organic photovoltaic cells (OPVs) because they significantly influence both the absorption of light and charge-transport characteristics. We report that the templating effect extends to multilayers to increase the crystallinity and to modify the orientation of the crystals of lead phthalocyanine (PbPc) and C₇₀ layers at the same time by adopting CuBr as a new templating layer on indium tin oxide (ITO). The formation of a monoclinic phase with a preferred orientation of (320) for PbPc and a fcc phase with a preferred orientation of (220) for C₇₀ on the PbPc layer is revealed by X-ray diffraction (XRD) patterns. The multilayer epitaxy results in an increase of the exciton diffusion lengths from 5.6 to 8.8 nm for PbPc and from 6.9 to 13.8 nm for C₇₀ to enhance the power conversion efficiency (PCE) of the planar heterojunction OPVs composed of PbPc and C₇₀ from 1.4 to 2.3%. The quasi-epitaxy model is proposed to explain the multilayer epitaxy

Enhancement of the Fill Factor through an Increase of the Crystallinity in Fullerene-Based Small-Molecule Organic Photovoltaic Cells

We report that the crystallinity of C₇₀ is improved significantly if CuI is used as a templating layer, leading to remarkable enhancement of hole mobilities from 8.32 × 10^–6 to 3.26 × 10^–5 cm²/(V s). As a result, the use of the templating layer in C₇₀-based solar cells with low donor concentration resulted in significant improvement of the fill factor from 0.51 to 0.57 and the power conversion efficiency from 5.56% to 6.23% under simulated AM 1.5G, 1 sun irradiation. This result demonstrates that the CuI templating layer is effective at improving the crystallinity of the fullerene derivatives as well as the donor materials

KLF8 directly binds to C/EBPα and PPARγ2 promoter.

<p>(A) EMSA experiment using KLF binding sites of β-globin (positive control), C/EBPα, or PPARγ2 promoter as probes. A FLAG-tagged KLF8 protein was in vitro translated using TNT T7 quick master mix. (B) EMSA experiment using wild type or mutated probe of the C/EBPα promoter. FLAG-tagged C/EBPβ-LAP or KLF8 protein was in vitro translated using TNT T7 quick master mix. (C) Western blot analysis of the in vitro translated proteins. n.s, non-specific. (D) ChIP was performed on 3T3-L1 cell chromatin at the indicated time points after induction of differentiation, using control IgG, anti-C/EBPβ, or anti-KLF8 antibody. The immunoprecipitated DNA was used as a PCR template to detect the PPARγ or C/EBPα promoter regions.</p

KLF8 regulates C/EBPα and PPARγ2 promoters to induce transcription.

<p>(A) A series of C/EBPα promoter constructs cloned into the pGL3-Basic vector was used in this study. NIH3T3 cells were transfected with the luciferase constructs using Lipofectamine, and the luciferase activities were determined after 2 days. The plasmids pcDNA3.0, pCMV-C/EBPβ, or pCMV-KLF8-FLAG were cotransfected. (B) Similarly, the PPARγ2 promoter (−350 counted from transcription initiation site) was investigated by luciferase assay. (C) Mutation analysis of the C/EBPα promoter was performed. The −205 construct was used for the site directed mutagenesis in order to introduce KLF and/or C/EBP regulatory element between −191 and −178 region. The KLF8 binding site was indicated in blue, whereas the C/EBP site was marked in red. The mutated region was underlined. NIH3T3 cells were transfected with these constructs along with KLF8 and/or C/EBPβ overexpression plasmids. (D) Mutation analysis of the PPARγ2 promoter was performed. The KLF site was mutated by site-specific mutagenesis, and the resulting construct was analysed by luciferase assay. Data represent the mean ± SD. **<i>P</i><0.01.</p

KLF8 expression during 3T3-L1 adipocyte differentiation and in mouse adipose tissue.

<p>(A) Total RNA was extracted from 3T3-L1 cells at the indicated times before and after induction of differentiation. The expression levels of KLF5, KLF8, KLF12, and KLF17 were determined by RT-PCR. (B) Anti-rabbit polyclonal KLF8 antibody was generated and tested on 293T cells that were transfected with either pcDNA3.0 or pcDNA3.0-KLF8-FLAG. Whole-cell lysates were immunoblotted (IB) using anti-KLF8 or anti-FLAG, which verified the specific antigen-antibody interaction. (C) Western blot analysis of KLF8 expression was performed at the indicated time points during 3T3-L1 cell differentiation. (D) KLF5, KLF8, and fatty acid synthase (FASN) mRNA levels were measured in the stromal vascular fraction (SVF) or the fat fraction of mouse epididymal adipose tissue using real-time qPCR. Data represent the mean ± SD.</p

Overexpression of KLF8 results in enhanced differentiation.

<p>Preadipocyte 3T3-L1 cells were transiently transfected using electroporation with pcDNA3.0-KLF8-FLAG or empty vector and differentiated with standard hormonal cocktail as described in Materials and Methods. (A) After 5 days of differentiation, lipid accumulation was detected by oil red-O staining. (B) Western blot analysis of whole-cell extracts prepared at the indicated times during differentiation. LAP, liver-activating protein, LIP, liver-inhibitory protein, PPARγ1, PPARγ2, and 43 or 30 kDa of C/EBPα proteins were indicated.</p

Gene expression change of KLF family members during 3T3-L1 differentiation.

*<p>Microarray was performed using total RNA samples from 3T3-L1 cells after 0, 2, 4, and 7 days of differentiation. Gene expression was analyzed by Agilent Mouse Genome 4×44 k oligo chip. The preparation for hybridization and the scanning of mouse chips were performed according to the manufacturer’s protocols (Genocheck). More than 2-fold changes of expression are indicated in bold.</p

KLF8 knockdown blocks 3T3-L1 adipocyte differentiation.

<p>Preadipocyte 3T3-L1 cells were treated with KLF8 siRNA at about 70% confluence using Lipofectamine RNAi/MAX reagent. After 24 h, the cells were trypsinized and replated at confluent cell density. After an additional 24 h, the cells were induced to differentiate. (A) Twenty hours after induction, cell extracts were prepared, and the effect on the expression of KLF8 was analyzed by real-time RT-PCR. (B) Oil red-O staining on day 8. The low panel represents spectrophotometric count of staining from 3 independent experiments. (C) Cells were harvested at the indicated times, and cell lysates were separated by SDS-PAGE and immunoblotted with antibody against C/EBPβ, PPARγ, and C/EBPα. LAP, liver-activating protein, LIP, liver-inhibitory protein, PPARγ1, PPARγ2, and 43 or 30 kDa of C/EBPα proteins were indicated. (D) To investigate the effect of siRNA on mitotic clonal expansion, cell numbers were determined at day 0 or day 2 after induction. Fold increases compared to the cell number in D0 were marked. Data in (A), (B), and (D) represent the mean ± SD. **<i>P</i><0.01.</p