Origin of Photocarrier Losses in Iron Pyrite (FeS₂) Nanocubes

Abstract

Iron pyrite has received significant attention due to its high optical absorption. However, the loss of open circuit voltage (Voc) prevents its further application in photovoltaics. Herein, we have studied the photophysics of pyrite by ultrafast laser spectroscopy to understand fundamental limitation of low Voc by quantifying photocarrier losses in high quality, stoichiometric, and phase pure {100} faceted pyrite nanocubes. We found that fast carrier localization of photoexcited carriers to indirect band edge and shallow trap states is responsible for major carrier loss. Slow relaxation component reflects high density of defects within the band gap which is consistent with the observed Mott-variable range hopping (VRH) conduction from transport measurements. Magnetic measurements strikingly show the magnetic ordering associated with phase inhomogeneity, such as FeS2-δ (0 ≤ δ ≤ 1). This implies that improvement of iron pyrite solar cell performance lies in mitigating the intrinsic defects (such as sulfur vacancies) by blocking the fast carrier localization process. Photocarrier generation and relaxation model is presented by comprehensive analysis. Our results provide insight into possible defects that induce midgap states and facilitate rapid carrier relaxation before collection.

Scientific and Technological Assessment of Iron Pyrite for Use in Solar Devices

“…Thea ctive iron pyrite layer and the top and bottom electronand hole-collecting layers are distinctly visible.D espite the low photovoltage output, this approach provest hat solar-cell [8] Reprinted withpermission from Ref. [8].C opyright 2016 AmericanChemicalS ociety.…”

“…Recently,w ee valuated iron pyrite to replace Pt and C counter electrodes. [38] Its transition-metal chemistry,F e-S co- [8] Reprintedwithpermission from Ref. [8].C opyright 2016 AmericanC hemical Society.…”

“…Representation of the spray pyrolysis, hot-injection, and sulfurization processes. [8,38] Reprintedw ithpermission.…”

“…2018,6,[8][9][10][11][12][13][14][15][16][17][18][19][20] 2018 Wiley-VCH Verlag GmbH &Co. KGaA,W einheim devices could be made,i np rinciple,u sing iron pyrite nanostructures.T oi mprove the photovoltage output, it would be necessary to develop methods for surface treatmenta nd intrinsic defect reduction in iron pyrite.A lso,s hunting pathways could be blocked by making better junctions, which could result in good current rectification to improve device characteristics.…”

“…[7][8][9][10][11] Despitep ossessing such desirable properties,t he success in preparing aw orking solar cell from iron pyrite has been hampered by alack of control on the chemistry of the material and by crystald efects that are believed to be responsible for the low photovoltage output from pyrite solar cells.T he crystal defects manifest as specific material conditions that include Fe-S phase impurities (troilite, greigite,p yrrhotite,e tc. ), stoichiometric deviations, [12][13][14] intrinsic bulk and surface defects, [15][16][17][18] reduced surface energy band gaps, [19] Fermi-level pinning, [20] surface-state-induced band bendinga nd ionized deep donors, [9] and surface inversion layers.…”