We present a series of new dopants based on a bicyclcic guanidine-type structure, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), for organic semiconductors. A series of TBD derivatives that were alkylated at the 7-position were synthesized and their physical properties were determined. These stable dopants were shown to be effective n-type dopants for [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), poly{[N,N′-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene (ITIC). Films of PC61BM doped with 10 mol% of a dimeric derivative of TBD had electrical conductivities of 0.065 S cm−1. The utility of the dopants was further shown by doping electron transport layers of PC61BM with 2TBD-C10 for methyl ammonium lead iodide (MAPbI3) solar cells leading to improved fill factors and PCEs relative to undoped ETLs.