Bibliometric maps are one way of visualizing academic output in terms of co-authorship, co-occurrence, citation, bibliographic coupling, and co-citation. Different units of analysis can be used such as all keywords, author keywords or KeyWords Plus. KeyWords Plus are words or phrases that frequently appear in the titles of an article’s references.

For the construction of the maps in this blog and their graphical representation, VOSviewer¹ was used. Counting the co-occurrence of only author keywords (items) and imposing a minimum of 4 for the number of occurrences of a keyword, Figure 1 was created.

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Figure 1. Co-occurrence bibliometric map of research and knowledge domain of slot-die coating.

Each circle represents an item, and the size of the circle indicates the occurrences of the item or keyword in different publications. The shorter the distance between two circles, the higher the number of co-occurrences. The line connecting the circles indicates the relation between two items. Each link has a strength, represented by a positive numerical value. The higher this value, the stronger the link and the thicker the line connecting the items.

One very visually attractive feature of bibliometric maps is that items are grouped into clusters represented with different colors. A cluster is a set of items included in a map. Clusters are non-overlapping (an item may belong to only one cluster).

Using a Topic search (searches title, abstract, author keywords and keywords plus) under the slot-die and slot die coating thereof, 541 results were found. The search found 483 articles, 54 proceedings papers, 27 review articles, and more. Materials Science Multidisciplinary, Physics Applied and Energy Fuels are the most popular Web of Science Categories in this search.

When mapping author keywords 11 clusters are formed, and the clusters are presented below ranked by their total link strength shown in parenthesis.

1. Slot-die coating (295)
2. Organic solar cells (120)
3. Roll-to-roll (106)
4. Perovskite solar cells (69)
5. Polymer solar cells (59)
6. Coating (37)
7. Flexible (36)
8. Viscosity (24)
9. Perylene diimide (22)
10. Films (22)
11. Non-fullerene acceptors (21)

At first glance, it becomes clear that the 4 clusters in the center of the Figure 1 are closely related, they are slot-die coating, organic solar cells, roll-to-roll and polymer solar cells. Over the years, organic or polymer solar cells have been fabricated by slot-die coating and frequently in roll or roll-to-roll platforms. The search on Web of Science to collect the data presented in this section shows that the most popular affiliation is the Technical University of Denmark (47 entries) and Helmholtz Association (41 entries). Here, it is interesting to mention that FOM Technologies spinout from the Technical University of Denmark in 2012 and possesses a great background knowledge for slot-die coating of functional materials in sheet-to-sheet or roll-to-roll. Non-fullerene acceptors cluster is strongly related to organic solar cells and stability, morphology, flexible, large area, and ito-free items.

Interestingly perovskite solar cells form their own cluster not too far from the center of the map where slot-die coating is. The thickness of the link between slot-die coating and perovskite solar cells shows how strongly related they are in scientific publications (link strength: 33). The link strength between slot-die coating and organic solar cells or roll-to-roll are 37 and 34, respectively. It shows how fast the research in perovskite solar cells is moving and how fast it is relating to scalable fabrication techniques such as slot-die coating and roll-to-roll. The same trend is expected for R&D and production of batteries, at the date of writing this blog, only lithium-ion batteries appear to be related to slot-die coating. Lithium-ion batteries are nowadays widely used in portable electronics and electric vehicles. A sustainable future for batteries requires that batteries move away from rare elements and sodium-ion batteries and zinc batteries now mainly in R&D stage will need a cost-efficient coating platform, such as slot-die coating, to cope with the exponential market growth.

Some of the research clusters that form around slot-die are purely related to the fundamental properties of the coating materials. For example, the film’s cluster contains fluid mechanics, laminar flow, and elastohydrodynamic. The viscosity cluster relates to surface tension, drying, and coating window.

The coating cluster is related to other deposition techniques such as blade coating, printing, ribbing and processing in general.

In the following section, we will present a bibliometric map of the relation of different deposition, coating, and fabrication techniques.

Searching by topic (by title, abstract, author keywords and Keywords Plus) slot-die coating or blade-coating or spin-coating, without any other refinement 37650 results are found in Web of Science (May 2022). Counting the co-occurrence of only author keywords and imposing a minimum of 12 for the number of occurrences of a keyword, figure 2 was created.

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Figure 2. Co-occurrence bibliometric map of research and knowledge domain of slot-die coating, blade-coating, spin-coating

Figure 2 is a co-occurrence bibliometric map showing 9 clusters of research and knowledge domains from the year of 1954 (with one occurrence) to 2022 (with 655 occurrences in May). Formed mainly from 31 524 Articles and Proceedings Papers (8 486 entries), review articles, and book chapters.

The clusters in order of total link strength (in parenthesis) are:

1. Thin-film (5297)
2. Spin coating (4818)
3. Sol-gel (3497)
4. Optical properties (1891)
5. Coating (1264)
6. Solar cells (1078)
7. Photoluminescence (1005)
8. AFM (591)
9. Chalcopyrite (37)

For the terms searched transformed into a co-occurrence map, spin-coating is the predominant cluster shown in the center of the figure because it has a higher number of co-occurrences (2339) than thin-film (2280) however, thin-film is a keyword used in many scientific publications dealing with materials’ properties, characterization, and several kinds of devices. Thin-film can be considered a general term applied to many research fields resulting in a cluster with a higher total link strength (5297) than spin-coating (4818).

Sol-gel keyword with 1560 co-occurrences is the third item with the higher total link strength (3497). It is strongly related (co-occurrence) with other items such as optical properties, ZnO, TiO₂, SnO₂, that have been historically fabricated by sol-gel.

The cluster coating naturally relates to deposition techniques such as slot-die coating, blade-coating, spin-coating, and electrospinning. Polymers, graphene, and photovoltaic cells keywords are linked to the coting cluster.

Perovskite solar cells, organic solar cells, and slot-die coating belong to the solar cells cluster. It shows that historically slot-die coating has been an attractive deposition technique for organic solar cells, used and reported more frequently than blade-coating (link strength 37) or spin-coating (link strength 10) for the fabrication of organic solar cells. This picture about slot-die coating could change if, in the search, other terms such as OLEDs, batteries and Power-to-X were included.

It is very interesting to find that there is a cluster for AFM, a single characterization technique. Inside that cluster, we can find other characterization techniques such as XPS, GISAXS, X-ray reflectivity, and many others.

Another cluster that comes up is the chalcopyrite cluster. It evidences all the research in CIGS, CIS, and other chalcogen-based materials during the last decades. It is far from spin-coating since the most common deposition techniques for chalcopyrite materials include vacuum steps.

FOM is playing a competitive role in technology innovation, helping scientists and engineers with the industrial implementations they require for their manufacturing needs. In future blogs, in-depth bibliometric maps will be presented around specific technologies and industrial applications.

¹ van Eck NJ, Waltman L. Software survey: VOSviewer, a computer program for bibliometric mapping. Scientometrics. 2010;84:523-538. doi:10.1007/s11192- 009-0146-3.