A new noncentrosymmetric (NCS) and polar material containing two lone-pair cations, Bi(3+) and I(5+), and exhibiting an Aurivillius-type (Bi(2)O(2))(2+) layer has been synthesized and structurally characterized. The material, BiO(IO(3)), exhibits strong second-harmonic generation (SHG), ∼12.5 × KDP (or ∼500 × α-SiO(2)), using 1064 nm radiation, and is found in the NCS polar orthorhombic space group Pca2(1) (No. 29). The structure consists of (Bi(2)O(2))(2+) cationic layers that are connected to (IO(3))(-) anions. The macroscopic polarity, observed along the c-axis direction, may be attributed to the alignment of the IO(3) polyhedra. In addition to the crystal structure and SHG measurements, polarization and piezoelectric measurements were performed, as well as electronic structure analysis.

The oxidative degradation rate, kdeg, of the solar cell dye (Bu4N+)2[Ru(dcbpyH)2(NCS)2]2-, referred to as N719 or [RuL2(NCS)2], was obtained by applying a simple model system. Colloidal solutions of N719-dyed TiO2 particles in acetonitrile were irradiated with 532-nm monochromatic light, and the sum of the quantum yields for the oxidative degradation products [RuL2(CN)2], [RuL2(NCS)(CN)], and [RuL2(NCS)(ACN)], Phideg, was obtained at eight different light intensities in the range of 0.1-16.30 mW/cm2 by LC-UV-MS. The Phideg values decreased from 3.3 x 10-3 to 2.0 x 10-4 in the applied intensity range. By using the relation kdeg = Phidegkback and back electron-transfer reaction rates, kback, obtained with photoinduced absorption spectroscopy, it was possible to calculate an average value for the oxidative degradation rate of N719 dye attached to TiO2 particles, kdeg = 4.0 x 10-2 s-1. The stability of N719 dye during solar cell operation was discussed based on this number, and on values of the electron-transfer rate between [Ru(III)L2(NCS)2] and iodide ion that are available in the literature.

Implementation of an optically active material on silicon has been a persistent technological challenge. For tandem photovoltaics using a Si bottom cell, as well as for other optoelectronic applications, there has been a longstanding need for optically active, wide band gap materials that can be integrated with Si. ZnSiP 2 is a stable, wide band gap (2.1 eV) material that is lattice matched with silicon and comprised of inexpensive elements. As we show in this paper, it is also a defect-tolerant material. Here, we report the first ZnSiP 2 photovoltaic device. We show that ZnSiP 2 has excellent photoresponse and high open circuit voltage of 1.3 V, as measured in a photoelectrochemical configuration. The high voltage and low band gap-voltage offset are on par with much more mature wide band gap III-V materials. Photoluminescence data combined with theoretical defect calculations illuminate the defect physics underlying this high voltage, showing that the intrinsic defects in ZnSiP 2 are shallow and the minority carrier lifetime is 7 ns. These favorable results encourage the development of ZnSiP 2 and related materials as photovoltaic absorber materials. Broader ContextOf all the renewable energy technologies, solar photovoltaic electricity has one of the highest resource potentials; there is enough energy in the sunlight incident on the surface of the earth to meet the world's energy demands many times over (∼10,000:1). However, significant market penetration requires photovoltaics to be cost competitive with fossil fuels, even when unsubsidized. Currently, balance of system costs, rather than module costs, represent the majority of the total installed cost. Thus, increasing module efficiency is attractive as high efficiency cells can reduce installation size and therefore cost. Tandem photovoltaic architectures can provide a transformative boost in module efficiency over the single junction alternative due to reduced thermalization losses. Silicon photovoltaics is a well established (>90% market share), high efficiency, low cost technology that provides a crystalline template to grow top cells upon. However, the top cell material must satisfy strict requirements, including high efficiency and long reliability, or its presence will simply reduce the performance of the silicon bottom cell. The primary top cell materials considered to date include III-V materials, but the cost of these materials and their sensitivity to defects have proven challenging. In this work, ZnSiP 2 emerges as a wide band gap absorber that has the potential to meet the requirements needed for a top cell in tandem silicon-based photovoltaics. † Electronic Supplementary Information (ESI) available: