A unified picture of aggregate formation in a model polymer semiconductor during solution processing

One grand challenge for printed organic electronics is the development of a knowledge platform that describes how polymer semiconductors assemble from solution, which requires a unified picture of the complex interplay of polymer solubility, mass transport, nucleation and, e.g., vitrification. One crucial aspect, thereby, is aggregate formation, i.e., the development of electronic coupling between adjacent chain segments. Here, it is shown that the critical aggregation temperatures in solution (no solvent evaporation allowed) and during film formation (solvent evaporation occurring) are excellent pointers to i) establish reliable criteria for polymer assembly into desired aggregates, and ii) advance mechanistic understanding of the overall polymer assembly. Indeed, important insights are provided on why aggregation occurs via a 1- or 2-step process depending on polymer solubility, deposition temperature and solvent evaporation rate; and the selection of deposition temperatures for specific scenarios (e.g., good vs bad solvent) is demystified. Collectively, it is demonstrated that relatively straightforward, concurrent in situ time-resolved absorbance and photoluminescence spectroscopies to monitor aggregate formation lead to highly useful and broadly applicable criteria for processing functional plastics. In turn, improved control over their properties and device performance can be obtained toward manufacturing sensors, energy-harvesting devices and, e.g., bioelectronics systems at high yield

Investigating the influence of charge transport on the performance of PTB7:PC71BM based organic solar cells

A key challenge for researchers in the field of organic solar cells (OSCs) is to develop a physical model for a device that correctly describes the charge carrier transport phenomenon. In this article, an analytical study on the charge carrier transport phenomenon in an OSC is reported, which expresses a balance between free charge carrier generation and recombination in low mobility PTB7:PC71BM blend layers. First, the current density-voltage (J-V) data for the fabricated OSC were extracted from experiments by varying the incident power light intensity (IPL) and then analysis through theoretical simulation was used to quantify the dominant interface recombination parameters limiting the device's performance. It was found that although the generation of free charge carriers increased at higher IPL values, the performance of the device remained low due to poor electrical transport properties which resulted in a considerable accumulation of generated charge carriers in the active layer. Therefore, it has become important to work out the complex relation between charge carrier mobility, exciton-recombination dynamics and the overall electrical performance parameters in a single framework. This article explains the influence of incident power light intensity and charge carrier mobility on performance parameters, which limits the power conversion efficiency (PCE) of the OSC. The presented analysis could be helpful in optimizing the architecture of future devices to increase the PCE of OSCs

Revealing the correlation between charge carrier recombination and extraction in an organic solar cell under varying illumination intensity

The design and fabrication of better excitonic solar cells are the need of the hour for futuristic energy solutions. This designing needs a better understanding of the charge transport properties of excitonic solar cells. One of the popular methods of understanding the charge transport properties is the analysis of the J-V characteristics of a device through theoretical simulation at varied illumination intensity. Herein, a J-V characteristic of a polymer: fullerene based bulk heterojunction (BHJ) organic solar cells (OSCs) of structure ITO/PEDOT: PSS (similar to 40 nm)/PTB7: PC71BM (similar to 100 nm)/Al (similar to 120 nm) is analyzed using one-and two-diode models at varied illumination intensity in the range of 0.1-2.33 Sun. It was found that the double diode model is better with respect to the single diode model and can explain the J-V characteristics of the OSCs correctly. Further, the recombination mechanism is investigated thoroughly and it was observed that fill factor (FF) is in the range of 62.5%-41.4% for the corresponding values of the recombination-to-extraction ratio (theta) varying from 0.001 to 0.023. These findings are attributed to the change in charge transport mechanism from trap-assisted to bimolecular recombination with the variation of illumination intensity

Photo-induced characteristic study of the smallest fullerene fragment, 1,6,7,10-tetramethylfluoranthene as an acceptor

In the present study, a novel solution-processable 1,6,7,10-tetramethylfluoranthene (1), as the smallest fragment of fullerene, is synthesized and systematically studied for use as a non-fullerene acceptor in an organic solar cell (OSC). Molecule 1 has the potential to be synthesized on a large scale. Optical and electrochemical studies revealed that 1 has a high absorption range and an appropriate band gap, which is compatible with poly(3-hexylthiophene-2,5-diyl) (P3HT) as a donor. The morphology showed high intermixed blend with uniform distribution of donor-acceptor molecules. Steady state and time resolved photoluminescence studies showed efficient charge separation from the donor and acceptor. These studies can be confirmed by the conventional OSC device structure employing a blend of P3HT:1, which gives a power conversion efficiency (PCE) of up to 0.71% with an open circuit voltage (V-OC) of 0.52 V and a fill factor (FF) of 44.89%. Despite this, the significant stability of the device, even after 60 h in open air, demonstrates that this molecule shows promising output and the result can be enhanced in the future with further optimization