•Li and Ag diffusion has been carried out in OLED device based on NPB|Alq3|Bphen.•Ag diffusion is prevented by a thin layer of Li or Bphen deposited on top of Alq3.•Li appears to diffuse into Bphen irrespective of Li and Bphen deposition order.A study of metal (Li, Ag) diffusion has been carried out in an archetypal OLED device based on N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine|tris(8-hydroxyquinolinato) aluminum|4,7-diphenyl-1,10-phenanthroline (NPB|Alq3|Bphen). Using single-stack and two-stack tandem OLED structures with variations of layer thicknesses and metal layer placements, we have found that Ag vapor-deposited on Alq3 layer can diffuse or penetrate deep into Alq3, up to ∼2,000 Å, causing luminescence quenching. This diffusion can be substantially prevented by a thin layer of Li or Bphen deposited on Alq3 prior to the deposition of Ag. In contrast, Li diffusion in either Alq3 or Bphen is limited to about 50–100 Å. Li appears to be able to diffuse into Bphen irrespective of the order of Li and Bphen depositions.

•An effective method for making an ohmic back contact to CdTe without wet etching.•The Cu to Te ratio is precisely controlled by the Te and Cu film thickness.•Enhanced efficiency and stability were got using optimized Te/Cu buffer.•Enhanced device stability can be attributed to the mediation of Cu diffusion by Te.•11.1% efficiency was achieved using 0.9 µm CdTe and Te/Cu bi-layer buffer.A buffer layer based on a Te/Cu bi-layer useful for forming ohmic contact to p-CdTe has been developed for application in CdS/CdTe solar cells. The bi-layer buffer was prepared by vapor deposition and a thermal annealing (~200 °C) was required for activation. Enhanced efficiency and stability were obtained by optimizing the Cu/Te compositions and the thermal activation conditions. Characterization by XRD, XPS, and PL indicates that under the thermal activation conditions Cu diffuses rapidly in the Te without forming CuxTe compounds. The enhanced stability can be attributed to the mediation of Cu diffusion into CdTe by the Te layer. The Te/Cu buffers are particularly useful for the fabrication of ultra-thin CdS/CdTe solar cells.A contact buffer based on a Te/Cu bi-layer for forming ohmic contact to p-CdTe has been developed for application in CdS/CdTe solar cells. The bi-layer structure enables a better control of Cu diffusion in the back contact Te-rich region as well as Cu penetration into bulk CdTe film, providing the necessary balance in Cu concentrations in these regions. With optimized Cu/Te compositions and thermal activation conditions, much improved efficiency and stability have been obtained. The Te/Cu buffers are particularly useful for the fabrication of ultra-thin CdS/CdTe solar cells.

•Self-assembled monolayers used as buffer layer in organic photovoltaic devices.•Device efficiency and lifetime upon exposure to air have been studied.•Moisture diffusion from the top electrode as the cause of degradation.Self-assembled monolayers (SAMs) based on n-octylphosphonic acid (C8PA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA) were investigated for application as an anode buffer layer in C60-based organic photovoltaic (OPV) devices. We found that the degradation of the OPV efficiency with respect to air exposure was significantly reduced with the perfluorinated PFOPA compared to the aliphatic C8PA. We attribute the OPV degradation to moisture diffusion from the top aluminum electrode and the lowering of the anode work function as a result of hydrolysis of the SAM buffer layer.

We demonstrate that enhanced efficiency can be achieved in organic tandem photovoltaic cells using identical bulk heterojunction subcells based on 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane doped C60 in series. Power conversion efficiencies greater than 4% have been achieved in 2- and 3-stack tandem cells, an improvement of at least 30% over the single-stack cell.Graphical abstractHighlights► We demonstrate organic tandem photovoltaic cells using identical subcells. ► C60 is the sole absorber in the subcells. ► Power conversion efficiencies greater than 4% have been achieved.