We improved the efficiency of ultra‐thin (0.49‐μm‐thick) Cu(In,Ga)Se2 solar cells to 15.2% (officially measured). To achieve these results, we modified growth conditions from the 3‐stage process but did not add post‐deposition treatments or additional material layers. The increase in device efficiency is attributed to a steeper Ga gradient in the CIGS with higher Ga content near the Mo back contact, which can hinder electron‐hole recombination at the interface. We discuss device measurements and film characterization for ultra‐thin CIGS. Modeling is presented that shows the route to even higher efficiencies for devices with CIGS thicknesses of 0.5 μm.

No abstract

Crystal silicon (c-Si) film photovoltaics (PV) fabricated on inexpensive substrates could retain the desirable qualities of silicon wafer PV-including high efficiency and abundant environmentallybenign raw materials-at a fraction of the cost. We report two related advances toward film c-Si PV on inexpensive metal foils. First, we grow heteroepitaxial silicon solar cells on 2 kinds of singlecrystal Al 2 O 3 layers from silane gas, using the rapid and scalable hot-wire chemical vapor deposition technique. Second, we fabricate heteroepitaxial c-Si layers on large-grained, cube-textured NiW metal foils coated with Al 2 O 3 . In both experiments, the deposition temperature is held below 840 C, compatible with low fabrication costs. The film c-Si solar cells are fabricated on both single-crystal sapphire wafer substrates and single-crystal g-Al 2 O 3 -buffered SrTiO 3 wafer substrates. We achieve $400 mV of open-circuit voltage despite crystallographic defects caused by lattice mismatch between the silicon and underlying substrate. With improved epitaxy and defect passivation, it is likely that the voltages can be improved further. On the inexpensive NiW metal foils, we grow MgO and g-Al 2 O 3 buffer layers before depositing silicon. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirm that the silicon layers are epitaxial and retain the $50 mm grain size and biaxial orientation of the foil substrate. With the addition of lighttrapping, >15% film c-Si PV on metal foils is achievable.Crystal silicon semiconductors dominate the existing photovoltaic (PV) industry because the Si wafer is a proven and well-understood industrial commodity: Si is abundant, environmentally benign, and capable of high solar conversion efficiencies. However, the energyintensive, inefficient and expensive processes that turn sand into a crystal silicon (c-Si) wafer hamper efforts to dramatically reduce PV costs. To circumvent the costly wafer fabrication step, it would be ideal to grow a film of PV-quality silicon, perhaps 2 to 20 microns thick, directly from silane gas onto an inexpensive substrate. With excellent light trapping, solar cell efficiencies above 15% are possible. 1 For high conversion efficiency, the silicon layer must have crystal quality high enough for photogenerated minority carriers to diffuse to the collecting contacts before recombining, 2 although the films are likely to be polycrystalline to reduce costs. These polycrystalline Si films will require grain sizes considerably larger than the film

The problem of obtaining ohmic contacts for p-type ZnSe is related to the deep valence band of ZnSe. We have addressed this problem by employing an epitaxial layer of the semimetal HgSe to decrease the interfacial energy barrier, or valence band offset, to about 0.6 eV. This has resulted in improved ohmic contacts for p-type ZnSe films and related diode structures.

We report the successful fabrication of ZnSe p-n junction light-emitting diodes in which Li and Cl are used as p-type and n-type dopants, respectively.