Caleb K. Miskin

Device comparison of champion nanocrystal-ink based CZTSSe and CIGSSe solar cells: Capacitance spectroscopy

Capacitance spectroscopy has been used to compare charge carrier and defect properties of champion nanocrystal-ink based CZTSSe and CIGSSe solar cells, with efficiencies reported here at 9.2% and 14.2%, respectively. Differences in energy level, frequency/temperature response, and contributions to bulk conductivity have been identified for the different materials. Due to these differences, contributions to the free carrier density have been associated with a single defect for CIGSSe, while associated with two defects in CZTSSe. Additionally, carrier freeze-out out at low temperatures has been identified for both devices, contributing to increasing series resistance at low temperatures as determined from the bulk conductivity. In addition to differences in defect formation, CZTSSe has been characterized with a reduced Vbi when compared to CIGSSe.

Analysis of temperature-dependent current-voltage characteristics for CIGSSe and CZTSSe thin film solar cells from nanocrystal inks

Thin film solar cells with CIGSSe and CZTSSe absorber layers fabricated from nanocrystal inks represent economically scalable technologies for alternative sources of energy. Although these two materials share similar properties important to functioning as a photovoltaic absorber, lab scale CIGSSe devices have achieved power conversion efficiencies 1.5 to 2 times higher than their CZTSSe counterparts. In the current work, CIGSSe and CZTSSe devices similarly processed from nanocrystal inks and exhibiting efficiencies of 14.2% and 9.2%, respectively, are characterized by temperature-dependent current-voltage (IVT) analysis to reveal limitations to CZTSSe device performance compared to CIGSSe.

Directing solar photons to sustainably meet food, energy, and water needs

As we approach a “Full Earth” of over ten billion people within the next century, unprecedented demands will be placed on food, energy and water (FEW) supplies. The grand challenge before us is to sustainably meet humanity’s FEW needs using scarcer resources. To overcome this challenge, we propose the utilization of the entire solar spectrum by redirecting solar photons to maximize FEW production from a given land area. We present novel solar spectrum unbundling FEW systems (SUFEWS), which can meet FEW needs locally while reducing the overall environmental impact of meeting these needs. The ability to meet FEW needs locally is critical, as significant population growth is expected in less-developed areas of the world. The proposed system presents a solution to harness the same amount of solar products (crops, electricity, and purified water) that could otherwise require ~60% more land if SUFEWS were not used—a major step for Full Earth preparedness.

A direct solution deposition approach to CdTe thin films

A direct solution deposition approach to CdTe thin films is presented. The difficulty of co-dissolving Te and desirable Cd salts is overcome through a diamine–thiol solvent mixture. Thin films of densely-packed, micron-sized grains are achieved after annealing without the need for chalcogen or CdCl2 vapor treatments.

High efficiency Cu 2 ZnSnS 4 nanocrystal ink solar cells through improved nanoparticle synthesis and selenization

In this work we present the improved efficiency of nanocrystal ink based Cu2ZnSn(S, Se)4 (CZTSSe) solar cells to 9.15%. CZTSSe devices prepared from nanocrystal inks are known for the presence of an unintended amorphous/fine-grained layer (unsintered layer) near the back contact. We demonstrate the ability to reduce the proportion of the unsintered layer in the final film through improved nanocrystal synthesis techniques and tailored thermal annealing in selenium atmosphere (selenization). Interestingly, our selenization process has not led to similar device performance in films prepared from particles using other recipes from the literature.

Speciation of CuCl and CuCl2 Thiol-Amine Solutions and Characterization of Resulting Films: Implications for Semiconductor Device Fabrication

Thiol-amine mixtures are an attractive medium for the solution processing of semiconducting thin films because of their remarkable ability to dissolve a variety of metals, metal chalcogenides, metal salts, and chalcogens. However, very little is known about their dissolution chemistry. Electrospray ionization high-resolution tandem mass spectrometry and X-ray absorption spectroscopy were employed to identify the species formed upon dissolution of CuCl and CuCl2 in 1-propanethiol and n-butylamine. Copper was found to be present exclusively in the 1+ oxidation state for both solutions. The copper complexes detected include copper chlorides, copper thiolates, and copper thiolate chlorides. No complexes of copper with amines were observed. Additionally, alkylammonium ions and alkylammonium chloride adducts were observed. These findings suggest that the dissolution is initiated by proton transfer from the thiol to the amine, followed by coordination of the thiolate anions with copper cations. Interestingly, the mass and X-ray absorption spectra of the solutions of CuCl and CuCl2 in thiol-amine were essentially identical. However, dialkyl disulfides were identified by Raman spectroscopy as an oxidation product only for the copper(II) solution, wherein copper(II) had been reduced to copper(I). Analysis of several thiol-amine pairs suggested that the dissolution mechanism is quite general. Finally, analysis of thin films prepared from these solutions revealed persistent chlorine impurities, in agreement with previous studies. These impurities are explained by the mass spectrometric finding that chloride ligands are not completely displaced by thiolates upon dissolution. These results suggest that precursors other than chlorides will likely be preferred for the generation of high-efficiency copper chalcogenide films, despite the reasonable efficiencies that have been obtained for films generated from chloride precursors in the past.

Minority carrier electron traps in CZTSSe solar cells characterized by DLTS and DLOS

We report observations of minority carrier interactions with deep levels in 6-8% efficient Cu2ZnSn(S, Se)4 (CZTSSe) devices using conventional and minority deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS). Directly observing defect interactions with minority carriers is critical to understanding the recombination impact of deep levels. In devices with Cu2ZnSn(S, Se)4 nanoparticle ink absorber layers we identify a mid-gap state capturing and emitting minority electrons. It is 590±50 meV from the conduction band mobility edge, has a concentration near 1015/cm3, and has an apparent electron capture cross section ~10-14 cm2. We conclude that, while energetically positioned nearly-ideally to be a recombination center, these defects instead act as electron traps because of a smaller hole cross-section. In CZTSe devices produced using coevaporation, we used minority carrier DLTS on traditional samples as well as ones with transparent Ohmic back contacts. These experiments demonstrate methods for unambiguously probing minority carrier/defect interactions in solar cells in order to establish direct links between defect energy level observations and minority carrier lifetimes. Furthermore, we demonstrate the use of steady-state device simulation to aid in the interpretation of DLTS results e.g. to put bounds on the complimentary carrier cross section even in the absence its direct measurement. This combined experimental and theoretical approach establishes rigorous bounds on the impact on carrier lifetime and Voc of defects observed with DLTS as opposed to, for example, assuming that all deep states act as strong recombination centers.

Comparison and interpretation of admittance spectroscopy and deep level transient spectroscopy from co-evaporated and solution-deposited Cu 2 ZnSn(S x , Se 1−x )4 solar cells

Cu2ZnSn(S, Se)4 (CZTSe) is an earth-abundant semiconductor with potential for economical thin-film photovoltaic devices. Short minority carrier lifetimes contribute to low open circuit voltage and efficiency. Deep level defects that may contribute to lower minority carrier lifetimes in kesterites have been theoretically predicted, however very little experimental characterization of these deep defects exists. In this work we use admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS) to characterize devices built using CZTSSe absorber layers deposited via both coevaporation and solution processing. AS reveals a band of widely-distributed activation energies for traps or energy barriers for transport, especially in the solution deposited case. DLTS reveals signatures of deep majority and minority traps within both types of samples.