The present invention relates to phase change materials (PCMs) which may be used with photovoltaic (PV) modules. More specifically, solid/solid PCMs having a polyolefin backbone polymer and a crystallizable side chain are provided. Preferably, the solid/solid PCM has a copolymer backbone having a polyolefin repeating unit, and the crystallizable side chains are grafted onto the copolymer backbone. Most preferably, the copolymer is poly(ethylene-co-glycidyl methacrylate) (PE-co-GMA), and polyethylene glycol (PEG) is the crystallizable side chain. Photovoltaic modules and backsheets for photovoltaic modules having solid/solid PCMs are also provided.