Vanessa L. Pool

Chemical Bath Deposition of p-Type Transparent, Highly Conducting (CuS)x:(ZnS)1-x Nanocomposite Thin Films and Fabrication of Si Heterojunction Solar Cells.

P-type transparent conducting films of nanocrystalline (CuS)x:(ZnS)1-x were synthesized by facile and low-cost chemical bath deposition. Wide angle X-ray scattering (WAXS) and high resolution transmission electron microscopy (HRTEM) were used to evaluate the nanocomposite structure, which consists of sub-5 nm crystallites of sphalerite ZnS and covellite CuS. Film transparency can be controlled by tuning the size of the nanocrystallites, which is achieved by adjusting the concentration of the complexing agent during growth; optimal films have optical transmission above 70% in the visible range of the spectrum. The hole conductivity increases with the fraction of the covellite phase and can be as high as 1000 S cm(-1), which is higher than most reported p-type transparent materials and approaches that of n-type transparent materials such as indium tin oxide (ITO) and aluminum doped zinc oxide (AZO) synthesized at a similar temperature. Heterojunction p-(CuS)x:(ZnS)1-x/n-Si solar cells were fabricated with the nanocomposite film serving as a hole-selective contact. Under 1 sun illumination, an open circuit voltage of 535 mV was observed. This value compares favorably to other emerging heterojunction Si solar cells which use a low temperature process to fabricate the contact, such as single-walled carbon nanotube/Si (370-530 mV) and graphene/Si (360-552 mV).

Thermal engineering of FAPbI3 perovskite material via radiative thermal annealing and in situ XRD

Lead halide perovskites have emerged as successful optoelectronic materials with high photovoltaic power conversion efficiencies and low material cost. However, substantial challenges remain in the scalability, stability and fundamental understanding of the materials. Here we present the application of radiative thermal annealing, an easily scalable processing method for synthesizing formamidinium lead iodide (FAPbI3) perovskite solar absorbers. Devices fabricated from films formed via radiative thermal annealing have equivalent efficiencies to those annealed using a conventional hotplate. By coupling results from in situ X-ray diffraction using a radiative thermal annealing system with device performances, we mapped the processing phase space of FAPbI3 and corresponding device efficiencies. Our map of processing-structure-performance space suggests the commonly used FAPbI3 annealing time, 10 min at 170 °C, can be significantly reduced to 40 s at 170 °C without affecting the photovoltaic performance. The Johnson-Mehl-Avrami model was used to determine the activation energy for decomposition of FAPbI3 into PbI2.

Rapid thermal processing chamber for in-situ x-ray diffraction

Rapid thermal processing (RTP) is widely used for processing a variety of materials, including electronics and photovoltaics. Presently, optimization of RTP is done primarily based on ex-situ studies. As a consequence, the precise reaction pathways and phase progression during the RTP remain unclear. More awareness of the reaction pathways would better enable process optimization and foster increased adoption of RTP, which offers numerous advantages for synthesis of a broad range of materials systems. To achieve this, we have designed and developed a RTP instrument that enables real-time collection of X-ray diffraction data with intervals as short as 100 ms, while heating with ramp rates up to 100 °Cs(-1), and with a maximum operating temperature of 1200 °C. The system is portable and can be installed on a synchrotron beamline. The unique capabilities of this instrument are demonstrated with in-situ characterization of a Bi2O3-SiO2 glass frit obtained during heating with ramp rates 5 °C s(-1) and 100 °C s(-1), revealing numerous phase changes.

Formation of nanoscale composites of compound semiconductors driven by charge transfer

Mixing of different materials is routinely used to create alloys or composites with new functionalities and properties tailored for specific applications. We have synthesized a uniform stoichiometric composite of CdO and SnTe, two compound semiconductors with distinctly different electrical properties and electronic band structure. The carrier concentration in the composite varies from about 1020 cm-3 electrons in CdO to about 1021 cm-3 holes in SnTe with a semi-insulating material in the intermediate composition range. The optical absorption edge shows a non-monotonic dependence on the composition. These unusual properties are explained by a nanocomposite whose formation is driven by charge transfer between the constituent compounds.

Transformation from crystalline precursor to perovskite in PbCl 2 -derived MAPbI 3

Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI3) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl2 + 3MAI) (MA = CH3NH3). The as-spun film consists of crystalline MA2PbI3Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices.The existence of a crystalline precursor is key to perovskite film formation, but the precise chemistry of the precursor and its transformation into perovskite are poorly understood. Here, the authors identify the crystal structure and conversion chemistry of the precursor for PbCl2-derived methylammonium lead iodide perovskites.

Front contact metallization of Si solar cells: Insights from in-situ X-ray diffraction

The front contact metallization of Si solar cells begins with printing a mixture of an Ag powder, glass frit (mixture of metal oxides such as PbO, SiO2, B2O3, and Bi2O3) and an organic binder over the antireflection coating that is subsequently fired up to about 825 °C. It is known that the frit allows the paste to react with and burn through the anti-reflective coating such that the metal can react with the underlying c-Si during firing. However, the precise phase transformations between Ag, Si, SiNx, and the frit constituents, which happen within a few seconds during rapid thermal processing (RTP), giving rise to an Ag-Si contact, are not well understood in the absence of in-situ characterization under the actual processing conditions. We have carried out in-situ x-ray diffraction studies on sample mixtures of different component powders (Ag, sinks, PbO-fruit and Si) under realistic processing conditions using an in-situ rapid thermal processing setup. We track the phase progression and reaction pathways at a time resolution of 100 milliseconds. We show the direct evidence of SiNx oxidation by PbO between 600-750 °C. On subsequent heating to higher temperature, up to 825 °C, Ag dissolves into the frit etches the c-Si surface and is deposited on etch pits forming intimate electrical contacts.