Wei Lin Leong

Solution-processed small-molecule solar cells with 6.7% efficiency

Organic photovoltaic devices that can be fabricated by simple processing techniques are under intense investigation in academic and industrial laboratories because of their potential to enable mass production of flexible and cost-effective devices. Most of the attention has been focused on solution-processed polymer bulk-heterojunction (BHJ) solar cells. A combination of polymer design, morphology control, structural insight and device engineering has led to power conversion efficiencies (PCEs) reaching the 6-8% range for conjugated polymer/fullerene blends. Solution-processed small-molecule BHJ (SM BHJ) solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts. Here, we report efficient solution-processed SM BHJ solar cells based on a new molecular donor, DTS(PTTh(2))(2). A record PCE of 6.7% under AM 1.5 G irradiation (100 mW cm(-2)) is achieved for small-molecule BHJ devices from DTS(PTTh(2))(2):PC(70)BM (donor to acceptor ratio of 7:3). This high efficiency was obtained by using remarkably small percentages of solvent additive (0.25% v/v of 1,8-diiodooctane, DIO) during the film-forming process, which leads to reduced domain sizes in the BHJ layer. These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts.

Intensity Dependence of Current–Voltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells

Solution-processed small-molecule p-DTS(FBTTh2)2:PC71BM bulk heterojunction (BHJ) solar cells with power conversion efficiency of 8.01% are demonstrated. The fill factor (FF) is sensitive to the thickness of a calcium layer between the BHJ layer and the Al cathode; for 20 nm Ca thickness, the FF is 73%, the highest value reported for an organic solar cell. The maximum external quantum efficiency exceeds 80%. After correcting for the total absorption in the cell through normal incidence reflectance measurements, the internal quantum efficiency approaches 100% in the spectral range of 600-650 nm and well over 80% across the entire spectral range from 400 to 700 nm. Analysis of the current-voltage (J-V) characteristics at various light intensities provides information on the different recombination mechanisms in the BHJ solar cells with different thicknesses of the Ca layer. Our analysis reveals that the J-V curves are dominated by first-order recombination from the short-circuit condition to the maximum power point and evolve to bimolecular recombination in the range of voltage from the maximum power point to the open-circuit condition in the optimized device with a Ca thickness of 20 nm. In addition, the normalized photocurrent density curves reveal that the charge collection probability remains high; about 90% of charges are collected even at the maximum power point. The dominance of bimolecular recombination only when approaching open circuit, the lack of Shockley-Read-Hall recombination at open circuit, and the high charge collection probability (97.6% at the short circuit and constant over wide range of applied voltage) lead to the high fill factor.

Lead-Free Halide Perovskite Solar Cells with High Photocurrents Realized Through Vacancy Modulation

Lead free perovskite solar cells based on a CsSnI3 light absorber with a spectral response from 950 nm is demonstrated. The high photocurrents noted in the system are a consequence of SnF2 addition which reduces defect concentrations and hence the background charge carrier density.

Lead-free germanium iodide perovskite materials for photovoltaic applications

Computational screening based on density-functional-theory calculations reveals Ge as a candidate element for replacing Pb in halide perovskite compounds suitable for light harvesting. Experimentally, three AGeI3 (A = Cs, CH3NH3 or HC(NH2)2) halide perovskite materials have been synthesized. These compounds are stable up to 150 °C, and have bandgaps correlated with the A-site cation size. CsGeI3-based solar cells display higher photocurrents, of about 6 mA cm−2, but are limited by poor film forming abilities and oxidising tendencies. The present results demonstrate the utility of combining computational screening and experimental efforts to develop lead-free halide perovskite compounds for photovoltaic applications.

Charging phenomena in pentacene-gold nanoparticle memory device

The authors demonstrate a new organic memory system, using pentacene as the active semiconductor layer and citrate-stabilized gold (Au) nanoparticles as charge storage elements. A pronounced clockwise capacitance-voltage (C-V) hysteresis is observed with a memory window of 1.25–2.05V achievable under 5–10V programing range. Similar clockwise C-V hysteresis window and an almost constant full width at half maximum of the conductance peaks in conductance-voltage (G-V) characteristics, obtained in the frequency range of 50kHz–1MHz, indicated that positive charge trapping/detrapping originated mainly from the Au nanoparticles.

A swivel-cruciform thiophene based hole-transporting material for efficient perovskite solar cells

A novel swivel-cruciform 3,3′-bithiophene based hole-transporting material (HTM) with a low lying highest occupied molecular orbital (HOMO) level was synthesized. This new HTM (KTM3) in CH3NH3PbI3 perovskite solar cells showed a higher Voc (1.08 V) and fill factor (78.3%) compared to solar cells fabricated using the widely used spiro-OMeTAD.

Formamidinium tin-based perovskite with low Eg for photovoltaic applications

A lead-free low bandgap organic–inorganic hybrid perovskite, formamidinium tin iodide, is utilized as a light absorbing layer in photovoltaics. This material has a bandgap of 1.41 eV which allows light harvesting from the near infrared region, making high photocurrents achievable. A power conversion efficiency of 2.10% was accomplished upon incorporating SnF2.

Towards printable organic thin film transistor based flash memory devices

The implementation of plastic electronic solutions to large area displays, disposable sensor arrays, radio-frequency identification tags (RFIDs), and various smart packaging devices necessitate the development of organic memories that are solution-processable and readily integrated with the transistors for digital logic. This article highlights recent research progress made towards organic memory transistors based on charge trapping and focuses on the principles and materials (namely, nanoparticles and polymer electrets) for these devices. The challenges and prospects of charge trapping memories are also discussed.

Micellar poly(styrene-b-4-vinylpyridine)-nanoparticle hybrid system for non-volatile organic transistor memory

We present organic field-effect transistor (OFET) memories where in-situ synthesized gold (Au) nanoparticles in self-assembled polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) block copolymer nanodomains successfully functioned as charge storage elements. Both p-type (pentacene) and n-type (perfluorinated copper phthalocyanine) OFET based memories are reported, which have stable large charge capacity and programmable-erasable properties due to charge confinement in the embedded Au nanoparticles. Optical excitation has been utilized to demonstrate photogenerated minority carrier trapping in the Au nanoparticles for efficient erasing operations. The memory devices can hence be written and read electrically and erased optically, resulting in large memory windows (∼9–11 V), high on/off ratio between memory states (103–105) and long retention times (>1000 s). The results clearly indicate the utility of the block copolymer-nanoparticle approach for OFET based memories.

Role of trace impurities in the photovoltaic performance of solution processed small-molecule bulk heterojunction solar cells

The final step in the preparation of ppp-DTS(PTTh222)222 involves end capping of the PT-DTS-PT core with 2-hexylbithiophene units via a microwave assisted Stille cross coupling reaction. Methyl transfer (instead of 2-hexylbithiophene transfer) can occur leading to the formation of (MePT)DTS(PTTh22). Although (MePT)DTS(PTTh22) is difficult to separate from the ppp-DTS(PTTh222)222 product via column chromatography, it is readily extracted using hexanes solvent to give absolute ppp-DTS(PTTh222)222. Trace impurities of (MePT)DTS(PTTh22) in BHJ solar cells fabricated from synthesis batches of ppp-DTS(PTTh222)222 significantly influence the photovoltaic properties, causing a ∼50% reduction in efficiency and affecting all of the relevant device parameters (Jsc, Voc and FF). From a broader perspective, despite molecular design, the suitability of a material for efficient devices is often only determined by trial and error in the device processing laboratory. As shown by the data presented in this publication, promising materials found to be unsuitable for device applications may suffer from highly dilute impurities that act to increase carrier recombination.