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Package - free flexible organic solar cells were fabricated with multilayer graphene as top transparent electrodes , which show the highest power conversion efficiency of about 3 . 2 % and excellent flexibility and bending stability .
 Chemical: graphene
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: graphene