23605
We find that the open - circuit voltage and efficiency of PbS QD / ZnO devices can be improved by 50 % upon doping of the ZnO with nitrogen to reduce its carrier concentration .
 Chemical: PbS; Chemical: ZnO; Chemical: ZnO; Chemical: nitrogen
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
 Entities: PbS, ZnO, ZnO, nitrogen 
Answer:
 Chemical: PbS; Chemical: ZnO; Chemical: ZnO; Chemical: nitrogen