Low intensity conduction states in FeS₂: implications for absorption, open-circuit voltage and surface recombination

Abstract

Pyrite (FeS2), being a promising material for future solar technologies, has so far exhibited in experiments an open-circuit voltage (OCV) of around 0.2 V, which is much lower than the frequently quoted 'accepted' value for the fundamental bandgap of ∼0.95 eV. Absorption experiments show large subgap absorption, commonly attributed to defects or structural disorder. However, computations using density functional theory with a semi-local functional predict that the bottom of the conduction band consists of a very low intensity sulfur p-band that may be easily overlooked in experiments because of the high intensity onset that appears 0.5 eV higher in energy. The intensity of absorption into the sulfur p-band is found to be of the same magnitude as contributions from defects and disorder. Our findings suggest the need to re-examine the value of the fundamental bandgap of pyrite presently in use in the literature. If the contribution from the p-band has so far been overlooked, the substantially lowered bandgap would partly explain the discrepancy with the OCV. Furthermore, we show that more states appear on the surface within the low energy sulfur p-band, which suggests a mechanism of thermalization into those states that would further prevent extracting electrons at higher energy levels through the surface. Finally, we speculate on whether misidentified states at the conduction band onset may be present in other materials.

Increasing the band gap of FeS2 by alloying with Zn and applying biaxial strain: A first-principles study

“…6b and c. Obviously, along either the x axial or y (or z) axial directions, the absorption edge blue shifts with the increase of compressive strain even the band gap decreases, and red shifts under the increasing tensile strain even the band gap increases. This also happens on the strained FeS 2 [12,21,23,26,36]. More specifically, right move of the sharp increase in electronic-states intensity make the adsorption edge blue shift, while left move of the sharp increase causes the red shift.…”

“…The limiting factor for a high efficiency is related to its low open-circuit voltage (OCV, less than 0.2 eV, which is much lower than the pyrite band gap of 0.95 eV) [4]. But the reason for the low OCV remains unclear and various possible explanations have been proposed, including bulk defects [5][6][7][8], intrinsic surface states [8][9][10][11], the low intensity states at the bottom of the conduction band [12], and the presence of competing phases [9]. Most importantly, according to Shockley-Queisser theory [13], the band gap of FeS 2 is narrow for photovoltaic applications.…”

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

“…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. [21] These issues were extensively investigated in various studies in the literature,y et they are not fully underIron pyrite (FeS 2 )h olds an enormous potential as al ow cost and non-toxic photoelectrochemicala nd energy-harvesting material owing to its interesting optical, electronic, and chemical properties along with elemental abundance.I nthis Review,l ow cost and scalable processing techniques to synthesize phase-pure pyrite thin films and nanocubes are described, and the application of this material in various energy-harvesting devices such as dye-sensitized solar cells, photodiodes,a nd heterojunctions olar cells is discussed.…”

Accurate Prediction of Band Structure of FeS2: A Hard Quest of Advanced First-Principles Approaches

“…However, practical application of FeS 2 -based optoelectronic devices is seriously hampered by its unexpected low efficiency due to a loss of open-circuit voltage V OC ( Cabán-Acevedo et al, 2014 ). A number of factors possibly responsible for the low V OC have been suggested and examined, including the intrinsic and defect surface states ( Bronold et al, 1994 ; Sun et al, 2011 ; Herbert et al, 2013 ; Lazić et al, 2013 ; Cabán-Acevedo et al, 2014 ; Limpinsel et al, 2014 ; Walter et al, 2017 ), bulk sulfur deficiency ( Birkholz et al, 1991 ; Cabán-Acevedo et al, 2014 ; Shukla et al, 2016 ) and presence of the metastable marcasite phase as a small-gap impurity ( Spagnoli et al, 2010 ; Sun et al, 2011 ; Schena et al, 2013 ).…”