arXiv subjects

Eric S. Toberer

Publications and source records attributed to Eric S. Toberer.

23 records · Page 2Linked to original sources

arXiv2015

Effects of Low Temperature Annealing on the Transport Properties of Zinc Tin Nitride

ZnSnN2 has recently garnered increasing interest as a potential solar absorber material due to its direct bandgap that is predicted to be tunable from 1.0-2.1 eV based on cation disorder. One important challenge to the further development of this material for photovoltaics (PV) is to reliably synthesize films with carrier density less than or equal to 10^17 electrons/cm^3. In this work, we perform a systematic annealing study on compositionally-graded Zn-Sn-N thin films to determine the effects on carrier density and transport of such post-growth treatment. We find that annealing up to 6 hr under an activated nitrogen atmosphere results in a reduction in carrier density by ~80% for zinc-rich films, and by ~50% for stoichiometric films. However, we also find that annealing reduces mobility as a function of increasing annealing time. This result suggests that initial film disorder hampers the benefits to film quality that should have been gained through annealing. This finding highlights that carefully managed initial growth conditions will be necessary to obtain PV-quality ZnSnN2 absorber films.

cond-mat.mtrl-sci↗

arXiv2015

Combinatorial Insights into Doping Control and Transport Properties of Zinc Tin Nitride

ZnSnN2 is an Earth-abundant analog to the III-Nitrides with potential as a solar absorber due to its direct bandgap, steep absorption onset, and disorder-driven bandgap tunability. Despite these desirable properties, discrepancies in the fundamental bandgap and degenerate \emph{n}-type carrier density have been prevalent issues in the limited amount of literature available on this material. Using a combinatorial RF co-sputtering approach, we have been able to explore a growth-temperature-composition space for Zn(1+x)Sn(1-x)N(2) over the ranges 35-340 degrees C and 0.30-0.75 Zn/(Zn+Sn). In this way, we were able to identify an optimal set of deposition parameters for obtaining as-deposited films with wurtzite crystal structure and carrier density as low as 1.8 x 10^(18) cm^(-3). Films grown at 230 degrees C with Zn/(Zn+Sn) = 0.60 were found to have the largest grain size overall (70 nm diameter on average) while also exhibiting low carrier density (3 x 10^(18) cm^(-3)) and high mobility (8.3 cm^(2) V^(-1) s^(-1)). Furthermore, we report evidence of a Burstein-Moss shift widening the apparent bandgap as cation composition becomes increasingly Sn-rich, and tunable carrier density as a function of cation composition (lower carrier density for higher Zn content), which suggests the formation of defect complexes. Collectively, these findings provide important insight into the fundamental properties of the Zn-Sn-N material system, and also highlight the potential to utilize ZnSnN2 for photovoltaics.

cond-mat.mtrl-sci↗

arXiv2015

Effects of disorder on carrier transport in Cu$_2$SnS$_3$

In recent years, further improvements in the efficiency of Cu$_2$ZnSn(S,Se)$_4$ photovoltaic devices have been hampered due to several materials issues, including cation disorder. Cu$_2$SnS$_3$ is a promising new absorber material that has attracted significant interest in recent years. However, similar to CZTS, Cu$_2$SnS$_3$ displays cation disorder. In this work, we develop synthetic techniques to control the disorder in Cu$_2$SnS$_3$ thin films. By manipulating the disorder in this material, we observe crystal structure changes and detect improvements in the majority carrier (hole) transport. However, when the minority carrier (electron) transport was investigated using optical pump terahertz probe spectroscopy, minimal differences were observed between the ordered and disordered Cu$_2$SnS$_3$. By combining these results with first-principles and Monte Carlo theoretical calculations, we are able to conclude that even ostensibly "ordered" Cu$_2$SnS$_3$ displays minority carrier transport properties corresponding to the disordered structure. The presence of extended planar defects in all samples, observed in TEM imaging, suggests that disorder is present even when it is not detectable using traditional structural characterization methods. The results of this study highlight some of the challenges to the further improvement of Cu$_2$SnS$_3$-based photovoltaics, and have implications for other disordered multinary semiconductors such as CZTS.

cond-mat.mtrl-sci↗

arXiv2012

Improved Thermoelectric Cooling Based on the Thomson Effect

Traditional thermoelectric Peltier coolers exhibit a cooling limit which is primarily determined by the figure of merit, zT. Rather than a fundamental thermodynamic limit, this bound can be traced to the difficulty of maintaining thermoelectric compatibility. Self-compatibility locally maximizes the cooler's coefficient of performance for a given zT and can be achieved by adjusting the relative ratio of the thermoelectric transport properties that make up zT. In this study, we investigate the theoretical performance of thermoelectric coolers that maintain self-compatibility across the device. We find such a device behaves very differently from a Peltier cooler, and term self-compatible coolers "Thomson coolers" when the Fourier heat divergence is dominated by the Thomson, as opposed to the Joule, term. A Thomson cooler requires an exponentially rising Seebeck coefficient with increasing temperature, while traditional Peltier coolers, such as those used commercially, have comparatively minimal change in Seebeck coefficient with temperature. When reasonable material property bounds are placed on the thermoelectric leg, the Thomson cooler is predicted to achieve approximately twice the maximum temperature drop of a traditional Peltier cooler with equivalent figure of merit (zT). We anticipate the development of Thomson coolers will ultimately lead to solid state cooling to cryogenic temperatures.

cond-mat.mtrl-sci↗

arXiv2010

Entropic Stabilization and Retrograde Solubility in Zn4Sb3

Zn4Sb3 is shown to be entropically stabilized versus decomposition to Zn and ZnSb though the effects of configurational disorder and phonon free energy. Single phase stability is predicted for a range of compositions and temperatures. Retrograde solubility of Zn is predicted on the two-phase boundary region between Zn4Sb3 and Zn. The complex temperature dependent solubility can be used to explain the variety of nanoparticle formation observed in the system: formation of ZnSb on the Sb rich side, Zn on the far Zn rich side and nano-void formation due to Zn precipitates being reabsorbed at lower temperatures.

cond-mat.mtrl-sci↗