# Quo Vadis Solar Energy Research?
## 3 Results
### Solar Energy Research during 2019-2020  
Attending to solar energy publications published in 2019-2020, Figure 7 shows their geographical distribution, where China was the most productive country with more than 3500 publications, followed by the United States with 1270 documents.  
Table 2 gathers publishing information from the 10 most productive countries in solar energy research during 2019-2020 and data comparison of those countries between 1900 and 2018. As can be noticed, China has become the main publishing country in solar energy research with their publications accounting for 34.2% of the total number in the last couple of years, compared to 18.5% (between 1900 and 2018) what represented the second position of the ranking. On the contrary, the United States, Germany and Italy research contribution during 2019-2020 was reduced in terms of percentage of publications compared to the period 1900-2018. As can be observed, Iran, Australia and Turkey escalated several positions from global rankings 16 th, 10 th and 12 th positions up to 5 th, 6 th and 9 th global positions during the last couple of years.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**2019–2020**} & \multicolumn{3}{c}{**1900–2018**} \\ \hline \multirow{2}{*}{**Rank**} & \multirow{2}{*}{**Country**} & **Number of** & **\% of** & \multirow{2}{*}{**Rank**} & **\% of** \\  & & **Publications** & **Publications** & & **Publications** \\ \hline
1 & China & 3517 & 34.2 & 2 & 18.5 \\
2 & United States & 1270 & 12.4 & 1 & 19.4 \\
3 & India & 1062 & 10.3 & 3 & 8.7 \\
4 & United Kingdom & 541 & 5.3 & 4 & 4.3 \\
5 & Iran & 405 & 3.9 & 16 & 1.8 \\
6 & Australia & 373 & 3.6 & 10 & 2.9 \\
7 & Spain & 367 & 3.6 & 8 & 3.4 \\
8 & Germany & 348 & 3.4 & 5 & 4.2 \\
9 & Turkey & 324 & 3.2 & 12 & 2.6 \\
10 & Italy & 322 & 3.1 & 7 & 3.5 \\ - & _accumulated_ & 8529 & 83.0 & - & 69.2 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Most productive countries in Solar Energy research comparison.  
Figure 7: Most productive countries in terms of Solar Energy research in 2019–2020 (regions without publications have been omitted).  
The same publication trend can be observed in Figure 8, where publishing evolution during the last three decades has been shown for the most productive countries. As it was stated in Figure 2, this time period covers more than 95% of total research documents. The publishing decline of the United States and the growing dominance of China from 2014 can be appreciated. The decline of Japanese solar energy research contribution from 2010 compared to other countries and the fast growing of Iranian publications over the last couple of years can also be noted. It can also be appreciated that the 12 countries represented in Figure 8 accounted for 90% of publications in year 2020 compared to 79.6% contribution in 2016 or a modest 36.8% back in 1992. It is also remarkable that back in 1990, Chinese contribution in solar energy research accounted for 0.95% of total publications, while in 2020, it was the dominant country, with 36.2% of the total documents.  
Regarding the most common research categories for solar energy publications during 2019-2020, it can be observed in Figure 9 that Energy Fuels (3973 documents), Materials Science Multidisciplinary (2001) and Chemistry Physical were the preferred categories. Meanwhile, Green Sustainable Science Technology that had been reported as the 6 th most common research topic in Figure 6 escalat  
Figure 8: Publishing evolution of the most productive countries (from 1990–2020).  
Figure 9: Publications according to WOS research categories for solar energy research in 2019–2020.