Layering on solar cell power and stability: Solar cells made from carbon-based polymers are helpfully flexible. However, there's been a frustrating tradeoff between cell stability and efficiency when converting solar power to electrical power. Page et al. offer a strategy to partially resolve this dilemma by inserting a layer of polar organic compound (a fullerene derivative) between the cathode (the positive pole in the circuit) and the rest of the cell. Aluminum is an efficient cathode material but is prone to oxidative degradation. The easily applied polar layer enables the use of more stable metals, such as silver and copper, for the cathode, while counteracting their tendencies to diminish power conversion efficiency. Science , this issue p. 441

Transient and spectral photovoltage (PV) is investigated in nanocrystalline porous ${\mathrm{TiO}}_2,$ which belongs to a class of materials with a very low electrical conductivity, i.e., with a huge Maxwell relaxation time. The PV in such materials is caused by diffusion of excess charge carriers with different diffusion coefficients. Usually, the diffusion coefficient for electrons is larger than that for holes in porous ${\mathrm{TiO}}_2.$ The PV transients are strongly retarded in time with respect to the exciting light pulse. The retardation of the photovoltage transients becomes stronger with decreasing size of the interconnected ${\mathrm{TiO}}_2$ nanoparticles and generation of defect states. The band gap of porous ${\mathrm{TiO}}_2$ (rutile, anatase) and the preferential formation of electron traps below the band gap are analyzed.