Cover Picture: phys. stat. sol. (RRL) 4/2008

Research on photovoltaic materials and devices is currently one of the most urgent and future oriented areas of solid state physics. The search for highly efficient and durable solar cells will surely trigger new technological developments and thus bears a real chance to have a huge impact on our future energy supply. A strong and steady growth of research activities has been observed during the last years. On pp. 145–180 of the present issue of pss – Rapid Research Letters recent advances in photovoltaic materials, theory and simulation, new developments and improvements of technologically relevant devices are reported by authors from the leading European institutions. The cover picture relates to the Rapid Research Letter by Carsten Deibel et al. on page 175ff. The authors performed a macroscopic simulation to explain the dependence of the charge carrier mobility on the illuminated current–voltage characteristics of organic bulk heterojunction solar cells. In the coloured contour plot the current density in A/m 2 is given. The authors find the optimum power conversion efficiency at a mobility of about 10 –6 m 2 /Vs.