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.

We demonstrate that the power conversion efficiency can be significantly improved in solution-processed small-molecule solar cells by tuning the thickness of the active layer and inserting an optical spacer (ZnO) between the active layer and the Al electrode. The enhancement in light absorption in the cell was measured with UV–vis absorption spectroscopy and by measurements of the photoinduced carriers generation rate. The ZnO layer used to improve the light-harvesting increases the charge collection efficiency, serves as a blocking layer for holes, and reduces the recombination rate. The combined optical and electrical improvements raise the power conversion efficiency of solution-processed small-molecule solar cells to 8.9%, that is, comparable to that of polymer counterparts.

Solution processable semiconducting polymers with excellent film forming capacity and mechanical flexibility are considered among the most progressive alternatives to conventional inorganic semiconductors. However, the random packing of polymer chains and the disorder of the polymer matrix typically result in low charge transport mobilities (10(-5)-10(-2) cm(2) V(-1) s(-1)). These low mobilities compromise their performance and development. Here, we present a strategy, by utilizing capillary action, to mediate polymer chain self-assembly and unidirectional alignment on nanogrooved substrates. We designed a sandwich tunnel system separated by functionalized glass spacers to induce capillary action for controlling the polymer nanostructure, crystallinity, and charge transport. Using capillary action, we demonstrate saturation mobilities with average values of 21.3 and 18.5 cm(2) V(-1 )s(-1) on two different semiconducting polymers at a transistor channel length of 80 μm. These values are limited by the source-drain contact resistance, Rc. Using a longer channel length of 140 μm where the contact resistance is less important, we measured μh = 36.3 cm(2) v(-1) s(-1). Extrapolating to infinite channel length where Rc is unimportant, the intrinsic mobility for poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo[3,4-c]pyridine] (Mn = 140 kDa) at this degree of chain alignment and structural order is μh ≈ 47 cm(2 )v(-1) s(-1). Our results create a promising pathway toward high performance, solution processable, and low-cost organic electronics.

We successfully demonstrate inverted structure small-molecule (SM) solar cells with an efficiency of 7.88% using ZnO and PEIE as an interfacial layer. Modification of ZnO with a cost-effective PEIE thin layer increases the efficiency of the inverted cell as a result of reducing the work function of the cathode and suppressing the trap-assisted recombination. In addition to the high efficiency, the inverted SM solar cells are relatively stable in air compared to conventional cells.

We report Barium (Ba) cathode layer for bulk-heterojunction solar cells which enhanced the fill factor (FF) of p-DTS(FBTTh2)2/PC71BM BHJ solar cell up to 75.1%, one of the highest value reported for an organic solar cell. The external quantum efficiency exceeds 80%. Analysis of recombination mechanisms using the current-voltage (J–V) characteristics at various light intensities in the BHJ solar cell layer reveals that Ba prevents trap assisted Shockley-Read-Hall (SRH) recombination at the interface and with different thicknesses of the Ba, the recombination shifts towards bimolecular from monomolecular. Moreover, Ba increases shunt resistance and decreases the series resistance significantly. This results in an increase in the charge collection probability leading to high FF. This work identifies a new cathode interlayer which outclasses the all the reported interlayers in increasing FF leading to high power conversion efficiency and have significant implications in improving the performance of BHJ solar cells.

We reported an efficient inverted bulk-heterojunction ͓regioregular of poly͑3-hexylthiophene͒: ͑6,6͒-phenyl C 61 butyric acid methyl ester͔ solar cell with a highly transparent sol-gel derived ZnO film as electron selective layer and MoO 3 as hole selective layer. By modifying the precursor concentration of sol from 0.75 to 0.1M, the optical transmittance of ZnO film increases from 75% to 95%. This improvement in transmittance increases the short-circuit density of inverted solar cell from 5.986 to 8.858 mA/ cm 2 without sacrificing the open-circuit voltage and fill factor of the device. We also demonstrated that the device incorporated with MoO 3 has a larger open-circuit voltage and fill factor than the device without MoO 3. Power conversion efficiency of 3.09% was achieved under simulated AM 1.5G illumination of 100 mW/ cm 2 .

There is little question that the 'electronic revolution' of the 20 th century has impacted every aspect of humanity. However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges that the management of electronic waste (e-waste) will remain through the 21 st century. In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the W. Li and Q. Liu contributed equally to this work. A. K. K. Kyaw is grateful to Guangdong University Key Laboratory for Advanced Quantum Dot Displays and Lighting (No. 2017KSYS007); Shenzhen Science, Technology and Innovation Commission (No. JCYJ20180305180645221); Guangdong Basic and Applied Basic Research Foundation (No. 2020A1515010916); and High-level University Fund (G02236004). Q. Liu wishes to thank Queensland University of Technology (QUT) for offering a scholarship through the QUT Postgraduate Research Award (QUTPRA) to conduct his research. P. S.wishes to thank QUT for the financial support from the Australian Research Council (ARC) for the Future Fellowship (FT130101337) and QUT core funding (QUT/322150-0301/07).

The effects of solvent additive (1,8‐diiodooctane (DIO)) on the morphology, charge generation, transport, and recombination in solution‐processed small‐molecule solar cells are studied and these parameters are correlated with device performance. In the optimum nanoscale morphology, which is processed with 0.4% DIO, the phase separation is large enough to create a percolating pathway for carrier transport, yet still small enough to form large interfacial area for efficient charge separation. Complete phase separation in this film reduces the interfacial defects, which occurs without DIO, and hence suppresses the monomolecular recombination. Moreover, balanced charge transport and weak bimolecular recombination lead to a high fill factor (72%). On the other hand, an excess amount of DIO (0.8%) in the solvent results in the over‐aggregation of the donor phase, which disturbs the percolating pathway of the acceptor phase and reduces the electron mobility. The over‐aggregation of the donor phase also shrinks the interfacial area for charge separation and consequently reduces the photocurrent generation.