15 Search Results

Reducing recombination in polycrystalline solar cells by orders of magnitude is currently one of the greatest challenges for increasing thin-film solar cell efficiency to theoretical limits. The question of how to do this has been a challenge for the thin-film community for decades. This work indicates that effective interface passivation is critical. Here, polycrystalline Al₂O₃/CdSeTe/Al₂O₃/glass heterostructures are grown, and a combination of spectroscopic, microscopic, and time-resolved electro-optical measurements demonstrates that the interface recombination velocity at alumina/thin-film interfaces can be less than 100 cm/s. This is three orders of magnitude less than typical CdTe interfaces without passivation, commensurate with single-crystal epitaxialmore » CdMgSeTe/CdSeTe/CdMgSeTe double heterostructures, and enables minority-carrier lifetimes in polycrystalline CdSeTe well above 100 ns. Microscopic interfacial electric-field measurements identify the field effect as a potential mechanism for polycrystalline Al₂O₃/CdSeTe interface passivation. Furthermore, the results provide guidance for modeling and interface passivation in devices and indicate future paths to realize highly efficient thin-film solar cells.« less

Postdeposition CdCl₂ treatment of polycrystalline CdTe is known to increase the photovoltaic device efficiency. However, the precise chemical, structural, and electronic changes that underpin this improvement are still debated. In this work, spectroscopic photoemission electron microscopy was used to spatially map the vacuum level and ionization energy of CdTe films, enabling the identification of electronic structure variations between grains and grain boundaries (GBs). In vacuo preparation and inert transfer of oxide-free CdTe surfaces isolated the separate effects of CdCl₂ treatment and ambient oxygen exposure. Qualitatively, grain boundaries displayed lower work function and downward band bending relative to grain interiors, butmore » only after air exposure of CdCl₂-treated CdTe. Analysis of numerous space charge regions at grain boundaries showed an average depletion width of 290 nm and an average band bending magnitude of 70 meV, corresponding to a GB trap density of 10¹¹ cm^–2 and a net carrier density of 10¹⁵ cm^–3. Finally, these results suggest that both CdCl₂ treatment and oxygen exposure may be independently tuned to enhance the CdTe photovoltaic performance by engineering the interface and bulk electronic structure.« less

Resistive oxide materials play an important role in the front contact of CdTe solar cells. The high-resistance transparent (HRT) or “buffer” layer has been used extensively in CdTe thin-film photovoltaics to enable a reduction in CdS thickness while maintaining near-maximum device voltage and fill factor. SnO₂- and ZnO-based alloys were tested as HRT layers on a fluorine-doped tin oxide transparent conducting oxide. SnO₂-based alloy HRT layers were deposited via atmospheric pressure chemical vapor deposition (APCVD). Alloying ZnO with MgO to create MgxZn_1–xO (MZO) via radio-frequency sputter deposition was explored as a way to reduce the electron affinity of ZnO HRTmore » layers. Additionally, to fully understand the behavior of these materials, many devices were fabricated with either no CdS layer, a sublimated CdS layer, or a sputtered, oxygenated CdS layer. MZO layers resulted in high open-circuit voltage and device efficiency even with the complete elimination of the CdS layer. In both HRT systems, controlling electron affinity to optimize front contact band alignment is an important consideration. Band measurements using photoelectron spectroscopy and synchrotron techniques correlate band alignment measurements with efficiency parameters in the design of HRT and CdS layers.« less