Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material: The Clear Role of Chalcogen Ratio on Light Absorption and Charge Transport for Cu2–xZn1+xSn(S1

Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material: The Clear Role of Chalcogen Ratio on Light Absorption and Charge Transport for Cu2–xZn1+xSn(S1–ySey)4. Braun, M. B., Korala, L., Kephart, J. M., & Prieto, A. L. 1(3):1053–1059.

Paper doi abstract bibtex

Photovoltaic (PV) devices based on bulk polycrystalline Cu2ZnSn(S1–xSex)4 (CZTSSe) as the absorber material have historically shown the best efficiency with high Se compositions. The selenization process, which is employed in the formation of absorber layer, has been shown to result in maximum device efficiency at a lower than predicted optimal band gap (Eg= ∼1.1 eV as compared to the 1.34 eV predicted by the Shockley–Queisser detailed balance model). It is still not clear if this deviation is due to changes in the chalcogen composition, grain growth in the film, or increased order in the lattice. In contrast, CZTSSe nanocrystals (NCs) offer a unique opportunity to evaluate the effect of chalcogen ratio on light absorption, charge transport, and photovoltaic performance excluding the impact of the uncertain effects of the conventional selenization step and, importantly, offer a potential path to a dramatic reduction in PV manufacturing cost. Despite an abundance of literature reports on this compound, there is to date no systematic study of the effects of controlled composition of the chalcogen on photocarrier generation and extraction at an optimal and constant cation ratio in a single system. This is required to determine the interplay between light absorbance and transport without compositional convolution and, in turn, to identify the best chalcogen ratio for the unannealed NC PV devices. Here we show that the entire family of Cu2–zZn1+zSn(S1–ySey)4 NCs can be made by a simple one-pot synthetic method with exquisite control over cation content and particle size across the entire range of chalcogen compositions. These NCs are then used to make solution-processed and electrically conductive CZTSSe NC films in the full range of S/(S + Se) ratios via ligand exchange without postdeposition annealing. The transport properties assessed by Hall-effect measurements revealed an intrinsic increase in film conductivity with selenium incorporation. These measurements are then correlated with the PV performance at the full range of band gaps (Eg = 1.0–1.5 eV), leading to an observed maximum in power conversion efficiency centered around Eg = 1.30 eV, which is much closer to the predicted Shockley–Queisser optimal band gap, an outcome predominantly dictated by the compromise between electrical conductivity and band gap.

@article{braun_synthetic_2018,
	title = {Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material: The Clear Role of Chalcogen Ratio on Light Absorption and Charge Transport for Cu2–{xZn}1+{xSn}(S1–{ySey})4},
	volume = {1},
	url = {https://doi.org/10.1021/acsaem.7b00198},
	doi = {10.1021/acsaem.7b00198},
	shorttitle = {Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material},
	abstract = {Photovoltaic ({PV}) devices based on bulk polycrystalline Cu2ZnSn(S1–{xSex})4 ({CZTSSe}) as the absorber material have historically shown the best efficiency with high Se compositions. The selenization process, which is employed in the formation of absorber layer, has been shown to result in maximum device efficiency at a lower than predicted optimal band gap (Eg= ∼1.1 {eV} as compared to the 1.34 {eV} predicted by the Shockley–Queisser detailed balance model). It is still not clear if this deviation is due to changes in the chalcogen composition, grain growth in the film, or increased order in the lattice. In contrast, {CZTSSe} nanocrystals ({NCs}) offer a unique opportunity to evaluate the effect of chalcogen ratio on light absorption, charge transport, and photovoltaic performance excluding the impact of the uncertain effects of the conventional selenization step and, importantly, offer a potential path to a dramatic reduction in {PV} manufacturing cost. Despite an abundance of literature reports on this compound, there is to date no systematic study of the effects of controlled composition of the chalcogen on photocarrier generation and extraction at an optimal and constant cation ratio in a single system. This is required to determine the interplay between light absorbance and transport without compositional convolution and, in turn, to identify the best chalcogen ratio for the unannealed {NC} {PV} devices. Here we show that the entire family of Cu2–{zZn}1+{zSn}(S1–{ySey})4 {NCs} can be made by a simple one-pot synthetic method with exquisite control over cation content and particle size across the entire range of chalcogen compositions. These {NCs} are then used to make solution-processed and electrically conductive {CZTSSe} {NC} films in the full range of S/(S + Se) ratios via ligand exchange without postdeposition annealing. The transport properties assessed by Hall-effect measurements revealed an intrinsic increase in film conductivity with selenium incorporation. These measurements are then correlated with the {PV} performance at the full range of band gaps (Eg = 1.0–1.5 {eV}), leading to an observed maximum in power conversion efficiency centered around Eg = 1.30 {eV}, which is much closer to the predicted Shockley–Queisser optimal band gap, an outcome predominantly dictated by the compromise between electrical conductivity and band gap.},
	pages = {1053--1059},
	number = {3},
	journaltitle = {{ACS} Applied Energy Materials},
	shortjournal = {{ACS} Appl. Energy Mater.},
	author = {Braun, Max B. and Korala, Lasantha and Kephart, Jason M. and Prieto, Amy L.},
	urldate = {2023-03-31},
	date = {2018-03-26},
	file = {ACS Full Text Snapshot:C\:\\Users\\abbyo\\Zotero\\storage\\DR3MMVTU\\acsaem.html:text/html;Braun et al_2018_Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material.pdf:C\:\\Users\\abbyo\\OneDrive\\Documents\\Zotero\\Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material_2018\\Braun et al_2018_Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material.pdf:application/pdf},
}

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