Weifei Fu

Enhanced Photovoltaic Performance of CH3NH3PbI3 Perovskite Solar Cells through Interfacial Engineering Using Self-Assembling Monolayer

Morphology control is critical to achieve high efficiency CH3NH3PbI3 perovskite solar cells (PSC). The surface properties of the substrates on which crystalline perovskite thin films form are expected to affect greatly the crystallization and, thus, the resulting morphology. However, this topic is seldom examined in PSC. Here we developed a facile but efficient method of modifying the ZnO-coated substrates with 3-aminopropanioc acid (C3-SAM) to direct the crystalline evolution and achieve the optimal morphology of CH3NH3PbI3 perovskite film. With incorporation of the C3-SAM, highly crystalline CH3NH3PbI3 films were formed with reduced pin-holes and trap states density. In addition, the work function of the cathode was better aligned with the conduction band minimum of perovskite for efficient charge extraction and electronic coupling. As a result, the PSC performance remarkably increased from 9.81(±0.99)% (best 11.96%) to 14.25(±0.61)% (best 15.67%). We stress the importance of morphology control through substrate surface modification to obtain the optimal morphology and device performance of PSC, which should generate an impact on developing highly efficient PSC and future commercialization.

Dopant-Free Hole-Transporting Material with a C3h Symmetrical Truxene Core for Highly Efficient Perovskite Solar Cells.

Herein we present a new structural design of hole-transporting material, Trux-OMeTAD, which consists of a C3h Truxene-core with arylamine terminals and hexyl side-chains. This planar, rigid, and fully conjugated molecule exhibits excellent hole mobility and desired surface energy to the perovskite uplayer. Perovskite solar cells fabricated using the p-i-n architecture with Trux-OMeTAD as the p-layer, show a high PCE of 18.6% with minimal hysteresis.

Recent advances in perovskite solar cells: efficiency, stability and lead-free perovskite

With the rapid growth of efficiency from 3.8% to 22.1% in recent years, perovskite solar cells (PVSCs) have drawn significant attention of researchers from both academia and industry. However, significant barriers remain standing in the pathway of PVSC advancement. To develop high-efficiency and stable devices as well as environmentally benign perovskites is critical, yet challenging aspects remain in PVSC research. In this review article, we focused on the recent advances in related subjects. The approaches for high-efficiency PVSCs have been introduced and then the instability issues and lead-free perovskite have been discussed. Finally, the conclusion along with brief perspectives has been provided on further advancing PVSCs towards use in efficient and stable solar-to-electricity technologies.

Engineering crystalline structures of two-dimensional MoS2 sheets for high-performance organic solar cells

The effects of different local crystalline structures on two-dimensional (2D) MoS2 sheets are studied to provide new insights into how the local characteristics affect the performance of organic solar cells (OSCs) and how to tailor the local characteristics towards high-performance devices. UV–ozone post-treatment of 2D MoS2 sheets led to incorporation of oxygen atoms into the lattice of the sheets. The incorporated oxygen in 2D MoS2 sheets significantly improved the performance of OSCs, where 2D MoS2 sheets were used as hole transport layers.

An ester-functionalized diketopyrrolopyrrole molecule with appropriate energy levels for application in solution-processed organic solar cells

For highly efficient organic solar cells (OSCs), the electron donor should possess not only a narrow band gap (Eg) but also a low highest occupied molecular orbital (HOMO) energy level. To achieve it, in this paper, we designed and synthesized a diketopyrrolopyrrole (DPP) derivative end capped with an ethyl thiophene-2-carboxylate moiety, 3,6-bis{5-[(ethyl thiophene-2-carboxylate)-2-yl]thiophene-2-yl}-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(CT)2). Through UV-vis absorption and cyclic voltammetry (CV) measurements, we demonstrated that the resulting molecule exhibits both a low optical Eg of 1.65 eV and a lower-lying HOMO energy level of −5.33 eV owing to the electronegativity of the ester group and the conjugation effect of the thiophene ring. Therefore, when DPP(CT)2 is used as the electron donor to blend with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) for solution processable OSCs, a power conversion efficiency (PCE) of 4.02% combined with an open-circuit voltage (VOC) as high as 0.94 V and a broad photovoltaic response range extending to around 750 nm is obtained.

Star-Shaped D–A Small Molecules Based on Diketopyrrolopyrrole and Triphenylamine for Efficient Solution-Processed Organic Solar Cells

Three star-shaped D-A small molecules, (P-DPP)(3)TPA, (4-FP-DPP)(3)TPA, and (4-BuP-DPP)(3)TPA were designed and synthesized with triphenylamine (TPA) as the core, diketopyrrolopyrrole (DPP) as the arm, and unsubstituted or substituted benzene rings (phenyl, P; 4-fluoro-phenyl, 4-FP; 4-n-butyl-phenyl, 4-BuP) as the end-group. All the three small molecules show relatively narrow optical band gaps (1.68-1.72 eV) and low-lying highest occupied molecular orbital (HOMO) energy levels (-5.09∼-5.13 eV), implying that they are potentially good electron donors for organic solar cells (OSCs). Then, photovoltaic properties of the small molecules blended with [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(61)BM) as electron acceptor were investigated. Among three small molecules, the OSC based on (P-DPP)(3)TPA:PCBM blend exhibits a best power conversion efficiency (PCE) of 2.98% with an open-circuit voltage (V(oc)) of 0.72 V, a short-circuit current density (J(sc)) of 7.94 mA/cm(2), and a fill factor (FF) of 52.2%, which may be ascribed to the highest hole mobility of (P-DPP)(3)TPA.

Low temperature solution processed planar heterojunction perovskite solar cells with a CdSe nanocrystal as an electron transport/extraction layer

A cadmium selenide (CdSe) nanocrystal was used as an electron transport/extraction layer for perovskite solar cells due to its high electron mobility and solution-processability at low temperatures. Power conversion efficiency (PCE) up to 11.7% was achieved under standard AM1.5G conditions in air.

Solution-grown organic single-crystalline p-n junctions with ambipolar charge transport.

Organic single-crystalline p-n junctions are grown from mixed solutions. First, C60 crystals (n-type) form and, subsequently, C8-BTBT crystals (p-type) nucleate heterogeneously on the C60 crystals. Both crystals continue to grow simultaneously into single-crystalline p-n junctions that exhibit ambipolar charge transport characteristics. This work provides a platform to study organic single-crystalline p-n junctions.

A solution-processable bipolar diketopyrrolopyrrole molecule used as both electron donor and acceptor for efficient organic solar cells

A new diketopyrrolopyrrole derivative with appropriate energy levels and bipolar charge-transport properties is designed and synthesized. When this molecule is used as either electron donor or acceptor, the resulting organic solar cells both give the power conversion efficiencies over 3%.

Pyrene and Diketopyrrolopyrrole-Based Oligomers Synthesized via Direct Arylation for OSC Applications

In this report, an atom efficient and facile synthetic strategy for accessing multi-diketopyrrolopyrrole (DPP)-based oligomers used in solution-processed organic field effect transistors (OFETs) and organic solar cells (OSCs) has been developed. The DPP units were successfully installed onto benzene and pyrene cores via palladium-catalyzed dehydrohalogenative coupling of mono-capped DPPs with multi-bromo-benzene or -pyrene (direct arylation), affording four oligomer small molecules (SMs 1-4) containing bis-, tri-, tri-, and tetra-DPP, respectively, in high yields of 78-96%. All the designed linear or branched DPP-based oligomers exhibit broad light absorptions, narrow band-gaps (1.60-1.73 eV), deep highest occupied molecular orbital (HOMO) levels (-5.26∼-5.18 eV), and good thermal stability (Td=390-401 °C). OFETs based on SMs 1-4 showed hole mobilities of 0.0033, 0.0056, 0.0005, and 0.0026 cm2 V(-1) s(-1), respectively. OSCs based on SMs 1-4 under one sun achieved power conversion efficiencies of 3.00%, 3.71%, 2.47%, and 1.86% accordingly, along with high open-circuit voltages of 0.86-0.94 V. For OSC devices of SM 1, SM 3, and SM 4, the solvent CHCl3 was solely employed to the formation of active layers; neither high boiling point additives nor annealing post-treatment was needed. Such a simple process benefits the large-scale production of OSCs via roll to roll technology.