Addition of benzyne to C 70 results in four isomeric monoadducts (compounds 1a-d) in a 42:35: 13:10 ratio as determined by 1 H NMR. These compounds were separated by repeated passes through HPLC columns. The major isomer (1a) resulted from addition to the highly pyramidalized C1-C2 bond as shown by 13 C NMR and UV/vis spectroscopy. The structure of the C1-C2 adduct (1a) was confirmed by X-ray crystallography. The second isomer (1b) was assigned as an adduct to the C5-C6 bond on the basis of 13 C NMR and UV/vis spectroscopy. Compound 1c exhibited a 13 C spectrum consistent with an adduct to the C7 and C8 positions. The presence of sp 3 13 C NMR resonances proved that the C7-C8 bond is still intact, making this the first identification of direct addition to a 5-6 ring fusion in a fullerene, and the first example of an adduct to a 5-6 ring fusion where the ring fusion bond remains intact. The fourth isomer (1d) displayed spectral data consistent with a compound with C 1 symmetry and is assigned as an adduct to the C7-C21 bond.S0002-7863(97)03957-7 CCC: $15.00

Recent literature reports have shown the ability to manipulate Cu-Zn cation ordering for Cu2ZnSnSe4 (CZTSe) via low temperature treatments. Theoretical arguments suggest that one of the major roadblocks to higher VOC—significant band tailing—could be improved with increased cation order; however, few direct measurements have been reported and significant device improvements have not yet been realized. This report investigates electrical properties, defects, and devices from quenched and slow-cooled single crystals of CZTSe. The extent of disorder was characterized by Raman spectroscopy as well as x-ray diffraction, where the change in Cu-Zn order can be detected by a changing c/a ratio. Quenched samples show higher acceptor concentrations, lower hole mobilities, and a lower-energy photoluminescence (PL) peak than crystals cooled at slower rates, consistent with a reduction in the bandgap. In addition, samples quenched at the highest temperatures showed lower PL yield consistent with higher quantities of deep defects. Devices fabricated using slow-cooled CZTSe single crystals showed improved efficiencies, most notably with increased VOC; however, low temperature intensity-dependent photoluminescence measurements continue to indicate the existence of potential fluctuations. We discuss the possibility that potential fluctuations in slow-cooled samples may be related to the inability to achieve a long range order of the Cu-Zn sub-lattice resulting in local regions of high and low levels of cation order, and consequent local variations in the bandgap. The presence of significant potential fluctuations, even after the slow-cooling step, suggests the difficulty in eliminating band-tailing in this system, and thus, additional approaches may be needed for significant reduction of the VOC deficit.

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination mechanisms to structural properties and processing of photovoltaic (PV) absorber materials is critical to the design of efficient solar cells. Carrier dynamics in PV absorbers have conventionally been characterized by time-resolved photoluminescence (TRPL), but TRPL may not be suitable or straightforward for all absorbers. Alternative non-contact methods can enable measurement of ultrafast carrier dynamics for a wider range of materials. Here we demonstrate the complementary use of time-resolved terahertz spectroscopy (TRTS) and near-infrared transient reflectance (NIR-TR) spectroscopy along with TRPL to elucidate photoexcited carrier dynamics in a high-quality copper-poor, zinc-rich kesterite Cu2ZnSnSe4 (CZTSe) single crystal. The single-crystalline nature of the sample eliminates complications arising from grain boundaries, secondary phases, and interfaces associated with thin film growth. A single-crystal-based PV device exhibited an efficiency of 6.2% and Voc of 400 mV, consistent with the quasi-Fermi level splitting determined using absolute photoluminescence. NIR-TR showed picosecond-scale cooling and relaxation of carriers into a distribution of band tail states while TRTS revealed a characteristic time scale of 200-260 ps for recombination. Hall effect and TRTS measurements revealed electron and hole mobilities in the range of 50-100 cm 2 /Vs. These dynamics result in a characteristic minority carrier diffusion length of less than 200 nm, leading to incomplete carrier collection, as confirmed by a strongly decreasing external quantum efficiency at long wavelengths. Our approach combining ultrafast spectroscopy and device measurements can lead to more detailed

The limited efficiency of Cu 2 ZnSn(SSe) 4 (CZTSSe) solar cells has been widely attributed to band tailing due to high densities of Cu Zn and Zn Cu antisite defects. It has been proposed that the partial replacement of Cu by Ag should reduce the antisite defect density, leading to reduced band tailing and increased cell efficiencies. This article examines antisite defects in Ag 2 ZnSnSe 4 (AZTSe) crystals grown at high temperatures from a stoichiometric mixture of elements by scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and photoluminescence (PL). The elemental distribution was examined directly by atomic resolution STEM energy-dispersive X-ray (EDX) mapping and simultaneous annular dark field (ADF) imaging. EDX mapping suggested the complete intermixing of Ag and Zn on the Wyckoff 2c and 2d sites in the AZTSe kesterite unit cell and around 14% substitution of Zn for Ag on the 2a site, while ADF images showed evidence for local nanometer-sized regions within the disordered matrix with partial ordering of Zn and Ag on the 2c and 2d sites. These observations show that AZTSe had a high density of Ag Zn and Zn Ag antisite defects, in contrast with room-temperature photoluminescence showing relatively narrow emission lines close to the band edge and thus minimal band tailing. The interpretation of these results and their wider significance for understanding the role of antisite defects in CZTSSe and Ag-substituted CZTSSe cells is discussed. KEYWORDS: thin film solar cells, Ag 2 ZnSnSe 4 , Cu 2 ZnSnSe 4 , antisite defects, scanning transmission electron microscopy, atomic-resolution energy-dispersive X-ray analysis

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