Paramagnetic defects produced by phosphorus ion implantation of C-60 films and the influence of annealing on dangling bonds defect center have been studied by electron spin resonance (ESR) to improve the properties of P+-implanted C-60 films for solar cell applications. The films implanted at room temperature reveal the presence of a paramagnetic defect center, which originated from the carbon dangling bonds, with g = 2.0036 +/- 0.0004. The volume concentration of the defect center increases with the implantation dose, from approximately 4.5 x 10(21) to 4.6 x 10(22) cm(-3) for the lowest implantation dose of 1 x 10(13)p(+) ions cm(-2) and the highest implantation dose of 5 x 10(15) P+ ions cm(-2), respectively. The increase in dose also causes a narrowing of the line width (from 2.6 to 2.15 G) although the g-value of the single Lorentzian line remains unchanged. The narrowing is attributed to exchange (motional) interaction. The ESR results imply that no significant graphitization has occurred as a result of implantation. Furthermore, upon annealing at a temperature of 773 K the spin defect density is reduced by a factor of 3.5, while the g-value remains centered at the same magnetic field. This reduction is due to a reconstruction of the less stable spin defect sites. The originality of this paper is that it is the first ESR study of defects produced by phosphorus implantation of C-60 films. (C) 2002 Elsevier Science B.V. All rights reserved.