Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells

Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106, USA.
Journal of the American Chemical Society (Impact Factor: 12.11). 04/2008; 130(11):3619-23. DOI: 10.1021/ja710079w
Source: PubMed


Two criteria for processing additives introduced to control the morphology of bulk heterojunction (BHJ) materials for use in solar cells have been identified: (i) selective (differential) solubility of the fullerene component and (ii) higher boiling point than the host solvent. Using these criteria, we have investigated the class of 1,8-di(R)octanes with various functional groups (R) as processing additives for BHJ solar cells. Control of the BHJ morphology by selective solubility of the fullerene component is demonstrated using these high boiling point processing additives. The best results are obtained with R = Iodine (I). Using 1,8-diiodooctane as the processing additive, the efficiency of the BHJ solar cells was improved from 3.4% (for the reference device) to 5.1%.

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    • "It does this by modifying the morphology and controlling the phase separation between the donor and acceptor components[6]. Reports indicate that ODT leads to enhanced connectivity of PC 71 BM networks[4], a small increase in polymer crystallinity[7], reduced charge recombination and reduced charge carrier loss[8]. However, recent data indicates that inclusion of additives has a detrimental effect on the stability of the solar cell. "
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    ABSTRACT: The use of processing additives is known to accelerate the degradation of organic photovoltaics (OPVs) and therefore, this paper studies the impact of selecting alternative processing additives for PCPDTBT:PC71BM solar cells in order to improve the stability. The use of naphthalene-based processing additives has been undertaken, which is shown to reduce the initial power conversion efficiency by 23%–42%, primarily due to a decrease in the short-circuit current density, but also fill factor. However, the stability is greatly enhanced by using such additives, with the long term stability (T50%) enhanced by a factor of four. The results show that there is a trade-off between initial performance and stability to consider when selecting the initial process additives. XPS studies have provided some insight into the decreased degradation and show that using 1-chloronaphthalene (ClN) leads to reduced morphology changes and reduced oxidation of the thiophene-ring within the PCPDTBT backbone.
    Full-text · Article · Jan 2016
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    • "To date, extensive efforts have been directed to improve the PCE of the PSCs and the addition of a small fraction of solvent additive has been widely used [2] [3] [4]. These solvent additives include high boiling point solvents such as 1-chlor- onaphthalene (CN) [5], N-methyl pyrrolidone [6], 1,8-octanedithiol (ODT) [7], 1,8-diiodooctane [8], and low boiling point solvents such as tetrahydrofuran [9] and so on. Recently, binary additives are also found effective in improving the photovoltaic property. "
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    ABSTRACT: Two high boiling point solvents namely diphenylether (DPE) and dimethylsulfoxide (DMSO) were employed as co-additives in fabricating poly [N-9″]-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3-benzothiadiazole) (PCDTBT): [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) based polymer solar cells (PSCs). It was found that the power conversion efficiency (PCE) can be improved to 7.13% for the PSCs processed with co-additives in comparison with ~6.5% for the PSCs processed with either DPE or DMSO additives. The enhanced PCE is benefited from simultaneous increase in Voc, Jsc and FF due to the co-additives fabricating process and the two additives are found to improve the photovoltaic performance with different mechanisms.
    Full-text · Article · Dec 2015
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    • "Different fabrication/processing procedures can lead to substantially different film morphologies even when the same active materials are used. These will be detailed in Section 3. Blends of solvents, the use of additives , manual manipulation of the deposition temperature and the creation of a solvent saturated atmosphere are some of the strategies most commonly used in order to obtain a better control of the film drying process and manipulate the resulting bulk-in morphology [11] [12] [13] [14] [15] [16] [17] [18]. In this way, it is possible to achieve ideal spatial distributions of connected electron and hole favorable domains with sizes in the order of the exciton diffusion length that guarantee efficient charge separation and transport. "
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    ABSTRACT: Organic photovoltaics will become 30 years old relatively soon. In spite of the impressive development achieved throughout these years, especially in terms of reported power conversion efficiencies, there are still important technological and fundamental obstacles to circumvent before they can be implemented into reliable and long-lasting applications. Regarding device processing, the synthesis of highly soluble polymeric semiconductors first, and fullerene derivatives then, was initially considered as an important breakthrough that would definitely change the fabrication of photovoltaics once for all. Nowadays, the promise of printing solar cells by low-cost and high throughput mass production techniques still stands. However, the potential and the expectation raised by this technology is such that it is considerably difficult to keep track of the most significant progresses being now published in different and even monographic journals. There is therefore the need to compile the most remarkable advances in well-documented reviews than can be used as a reference for future ideas and works. In this letter, we review the development of polymeric solar cells from its origin to the most efficient devices published to date. After analyzing their fundamental limits, we separate these achievements into three different categories traditionally followed by the scientific community to push devices over 10% power conversion efficiency: Active materials, strategies -fabrication/processing procedures- that can mainly modify the active film morphology and result in improved efficiencies for the same starting materials, and all the different cell layout/architectures that have been used in order to extract as high photocurrent as possible from the Sun. The synthesis of new donors and acceptors, the use of additives and post-processing techniques, buffer interlayers, inverted and tandem designs are some of the most important aspects that are in detailed reviewed in this letter. All have equally contributed to develop this technology and leave it at doors of commercialization.
    Full-text · Article · Apr 2015 · Organic Electronics
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