# Quo Vadis Solar Energy Research?
## 3 Results
### Solar Energy Research Evolution between 1900-2020  
Solar energy research evolution is analyzed in terms of all indexed publications retrieved by the WOS Core Collection for the question query \"solar energy\" between 1900 and 2020. As was mentioned before, the WOS database includes more indexed publications than Scopus database before 1990, and therefore it was preferred instead. Question query was searched through publications containing \"solar energy\" string whether as their publication title, author keywords or Keyword Plus(r). Figure 2 shows the publishing evolution in terms of the number of publications per year (records) and the accumulated number of publications (in percentage). Between 1900 and 2020, a total amount of 48,738 publications were published on solar energy research topics with a clear growing trend during the last decade, which confirms the increasing interest of academia in these topics. For the sake of clarity, a close-up figure for the time period 1900-1970 has been included in the graph for the better observation of early solar energy publications. It can also be observed on the secondary vertical axis of the figure that 90% of the total number of solar energy publications were published between 2000 and 2020 and 50% of publications were only published during the time period between 2015 and 2020, which confirms the very high and recent interest in solar energy research. Figure 1: VOSviewer options for bibliometric and networking analysis [26]. It can also be observed that 95% of total publications were released after 1991 and 99% of publications after 1976. Before 1970, the number of indexed publications on solar energy research according to WOS Core Collection was marginal with an accumulated number of 150 publications (of the total 48,738 publications of the corpus data); those numbers represent 0.31% of total publications. It is worth mentioning that before 1950, only 12 publications were indexed about solar energy and the oldest one was recovered is from 1903 regarding the recent discovery of radium and its guess as a source of energy in the sun [35]. Attending to the document types, indexed publications on solar energy research can be classified as they are represented in Figure 3.  
As can be observed, the most common publishing sources in solar energy research were articles (accounting for 63% of total) followed by proceedings papers (26%), both together accounted for 89% of total publications. To a lesser extent, review articles (8%) together with book chapters, meeting abstract and editorial material covered 99% of total  
Figure 3: Publishing sources on solar energy research (between 1900 and 2020). Figure 2: Number of publications evolution on Solar Energy research (between 1900 and 2020). publications, while notes, letters, new items or retracted publications had a marginal impact in publishing distribution. Figure 4 shows worldwide publishing distribution in solar energy research for the whole time period (between 1900 and 2020). Regions without publications have been omitted for representation purposes. In that figure, no distinction is made among the different publishing sources sorted in Figure 3, as all documents' contributions have been counted equally. As it can be observed, solar energy research is dominated by the United States and China in terms of scientific publications, as both of them account for 39.7% of the total indexed documents. Table 1 ranks the 10 most productive countries, both in terms of documents number and their global contribution. As can be noted, publications from these countries (35,855 documents) account for 73.6% of the total indexed documents. Figure 5 highlights publishing source distribution for the most productive countries gathered in Table 1. As can be observed, China and Turkey have the highest ratio of article publications--over 72% of their total documents, while India shows the lowest with less than 55%. On the contrary it has the highest contribution on proceedings papers with more than 31% of their publications.