Origin of Photocarrier Losses in Iron Pyrite - ACS Publications

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Origin of Photocarrier Losses in Iron Pyrite (FeS2) Nanocubes

Sudhanshu Shukla,†,‡ Guichuan Xing,§ Hu Ge,‡ Rajiv Ramanujam Prabhakar,∥ Sinu Mathew,⊥,¶ Zhenghua Su,∥ Venkatram Nalla,# Thirumalai Venkatesan,⊥,¶ Nripan Mathews,‡ Thirumany Sritharan,‡ Tze Chien Sum,§ and Qihua Xiong*,§,+ †

Energy Research Institute, Interdisciplinary Graduate School, and ‡School of Materials Science and Engineering, Nanyang Technological University, Singapore 637371, Singapore § Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ∥ Energy Research Institute, Nanyang Technological University, Singapore 637371, Singapore ⊥ Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore ¶ NUSNNI-NanoCore, National University of Singapore, Singapore 117576, Singapore # Centre for Disruptive Photonic Technologies (CDPT), Nanyang Technological University, Singapore 639798, Singapore + NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 637371, Singapore S Supporting Information *

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. KEYWORDS: iron pyrite, nanocubes, transient absorption, variable range hopping, photovoltaics, magnetization, carrier dynamics

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materials including a-Si, CZTS, CuO, Cu2O, Cu2S, CoO3, Zn3P2, and FeS2.9 Of those, FeS2 has received much attention lately due to its abundance and attractive semiconducting properties. Iron pyrite (FeS2) has a cubic crystal structure with good semiconducting and optical absorption properties that make it attractive as a material for solar energy conversion. It has a remarkably high optical absorption coefficient (α > 105 cm−1 for hν > 1.3 eV), a nearly ideal energy band gap Eg of ∼0.95 eV

ow cost and clean solar photovoltaic electricity generation is one of the key challenges of modern era since the demand for power is continuously increasing.1,2 Solar energy conversion via photovoltaic (PV) systems is certainly a viable option as it does not generate any pollution in the conversion process. However, the major impediment to widespread PV use is the high cost of the modules as well as the requirement for large land area. Consequently, the major motivating factor for PV research has been to achieve high efficiency and low cost with the use of environmentally benign materials.3,4 Earth-abundant semiconducting materials offer great opportunity for large-scale PV deployment by leveraging the cost concerns and environmental issues.5−8 Extensive research effort has been devoted to the development of such © XXXX American Chemical Society

Received: January 5, 2016 Accepted: March 10, 2016

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DOI: 10.1021/acsnano.6b00065 ACS Nano XXXX, XXX, XXX−XXX

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microscopy (HRTEM), and Raman spectroscopy for crystallinity and phase evaluation. Charge carrier relaxation pathways were identified from exponential decays of photoexcited charge carriers in TA dynamics. On the basis of the slow and fast relaxation components of photoexcited carriers, a recombination model is formulated here that could account for the recombination mechanisms of charge carriers in iron pyrite through shallow and deep defect trap states. Temperature dependence of electrical charge transport measurement revealed a disordered semiconductor behavior with MottVRH type transport, implying a high density of defects. Also, temperature dependence of magnetization revealed a weak magnetic order at low temperatures which could be attributed to the defects and their influence. These findings support a carrier relaxation model where the relaxation pathway is through multiple electronic states present within the band gap.

and sufficiently long minority carrier diffusion length (100− 1000 nm). Furthermore, a high carrier mobility of μ ∼ 360 cm2 V−1 s−1 has been reported for n-type bulk single crystals.10 Nonetheless, the efficiency for a pyrite single crystal photoelectrochemical cell has not exceed 2.8% despite an extraordinarily high short circuit current (Jsc = 42 mA/cm2), fill factor of 50% and external quantum efficiency reaching as high as 90%. This low efficiency is attributed to its low open circuit voltage of Voc ∼ 187 mV which has still not been exceeded, even after nearly 30 years since this original report.11 Early extensive research on iron pyrite, primarily led by Tributsch and co-workers, attributed the problem of poor photovoltage to possible defect states within the forbidden energy region, and the challenge of control on phase purity and stoichiometry.10 Recently, independent studies confirmed the existence of a surface inversion layer on pyrite surface and conclude that the surface states and ionization of high density deep donor defect states are responsible for low voltages, consistent with the prediction of Jaegermann et al.12−14 Interestingly, recent reports of synthesis of pure pyrite nanostructures by solution processing caused a resurgence of interest in pyrite as a PV material as this might make it possible to avoid impurities and the surface inversion problem.15−17 Solution processed nanoparticles offer a unique platform to study fundamental as well as device physics due to their better control of quality, composition, size and shape, and the relative ease of making films by simple dip or spin coating.18 However, post treatment is generally required to remove any residual organics, such as attached insulating ligands, before device applications to get appreciable currents in films through enhanced electronic interparticle coupling.19 Nanostructured iron pyrite has been deployed, (i) in a heterojunction and p−i− n configuration photodiode, (ii) as a catalyst in dye sensitized solar cell (DSSC), (iii) as a photoconductor, and (iv) in bulk heterojunction inorganic−organic hybrid solar cells.20−25 However, regardless of their application as a photoresponsive layer in such devices, pyrite nanocrystals have not proven their effectiveness in solar cell configuration which is a setback for the adoption of pyrite nanoparticles in solar cells technology.26 Despite this setback, we believe that the high responsivity to optical excitation in pyrite nanocrystals, coupled with their good crystallinity and stability, gives rise to the potential for their use as the absorber layer in solar cells if adequate photovoltages could be derived through appropriate post treatments and better heterojunctions fabrication. To achieve this, the physics of photoexcitation and charge carrier dynamics in pyrite nanocrystals must be well understood, because loss of photovoltage clearly implies major loss of photocarriers through trapping pathways within the crystal. Understanding the fundamental photophysics of charge carriers in pyrite nanocrystals is key to improve their poor photovoltaic performance. In this work, we extensively investigate and correlate the physical property characterizations with the charge carrier generation and relaxation dynamics in as-prepared (without sulfurized as-coated films) and sulfur treated {100} faceted iron pyrite nanocubes by transient absorption (TA) measurements. Since {100} surface inhibits surface reconstruction, contrary to {111} or {210} face, {100} faceted nanocubes were chosen for the present study.27−29 In addition, spin-coated as-prepared iron pyrite nanocubes were sulfur treated to make heterojunction solar cells. Iron pyrite films were characterized by Xray diffraction (XRD), high resolution transmission electron

RESULTS AND DISCUSSION Panels a and b of Figure 1 show the X-ray diffraction (XRD), Raman spectra of as-prepared and sulfurized films obtained from nanocubes, respectively. All XRD peaks could be indexed to the cubic pyrite phase (JCPDS 42-1340). No peaks from possible impurity phases such as pyrrhotite (Fe1−xS), iron monosulfide (FeS), marcasite (orthorhombic polymorph of FeS2), or iron oxide were detected. It is evident from Figure 1 that sulfurization process retains the shape and phase purity of the as-prepared sample. This allows us to improve the conductivity of the films without affecting the phase purity and the cube morphology by removing the insulating organic ligands and enhancing inter particle coupling (Figure S1, Supporting Information). The size of the nanocubes produced in our case is between 80 and 150 nm, which is desirable for solar cell applications.15,30,31 This size range takes advantage of the long diffusion length of carriers in pyrite and minimizes the scattering of charge carriers. A film made from larger nanocubes also minimizes excess contact surface area where undesirable carriers scattering and recombination impede the charge transport. Lattice constants computed for the as-coated and sulfurized films from the XRD spectra are 5.392 and 5.412 Å, respectively. This is very close to the value of 5.418 Å reported for a high quality, pyrite sample by Birkholz et al.32 Since the sensitivity of the XRD technique in detecting small fractions of impurity phases is limited, we resorted to confocal microRaman spectroscopy with laser excitation at 532 nm with 0.14 mW power. Lower laser illumination intensity used as oxidation occurred at higher intensities (Figure S2, Supporting Information). Three characteristic phonon modes were observed in the spectra at wavenumber 341 (Ag), 377 (Eg), and 425 cm−1 (Tg(3)), respectively. The Ag and Eg modes are from sulfur−sulfur bond vibrations and Tg (3) is vibrational mode.33,34 Out of five active Raman active modes, the Tg (1) and Tg (2) phonon modes are generally not observed due to significant light extinction caused by other modes and inefficient light scattering at room temperature.35 Both asprepared and sulfurized films showed only pyrite peaks confirming the high phase purity of the films. No signals were obtained from possible impurity phases such as FeS, Fe3S4, Fe1−xS including the polymorph marcasite phase. Nanocube morphology of as-prepared and sulfurized films is evident from SEM images of Figure 1c,d with clear facets. Since the cube shape remained intact even after sulfurization, this is a good high temperature heat treatment platform to perform passivation and ligand removal without affecting the morpholB

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Figure 1. Structural characterization. (a) XRD pattern, (b) confocal micro-Raman spectra, and SEM morphology of (c) as-prepared and (d) sulfurized iron pyrite nanocubes.

Figure 2. HRTEM image of sulfurized iron pyrite nanocube. (a) Edge of the nanocube, inset shows full nanocube image; (b) lattice fringes and stacking faults marked with yellow dashed lines and arrows; (c) selected area electron diffraction (SAED) pattern indicating the points corresponding to marked planes (red spots).

Optical absorption of sulfurized pyrite nanocubes thin films was measured by UV−Vis absorption spectrophotometer and is shown in Figure 3a. An optical absorption coefficient α > 1 × 105 cm−1 for hν > 1.3 eV was obtained. Such a high optical absorption is a notable characteristics of iron pyrite.38,39 In Figure 3a, absorption below band gap is observed which corresponds to defect level excitations and transitions within the exponential band tails (region left to the dashed red line).40−42 The shape of the curve below the band gap absorption (referred as Urbach tail) also indicates the degree of disorder in the semiconducting material.43,44 From Tauc analysis, shown in Figure 3b, intercepts of the linear extrapolations of the different slopes on the energy axis could be interpreted as optical transitions corresponding to those specific energy levels.45 The electronic behavior of pyrite is governed by these energy level transitions.46 The band gap estimated is 1.02 eV, which is well within the range (0.80−1.10 eV) commonly reported for pyrite.10,29,47 However, this band gap value appears to be direct as it fits linearly with n = 2 value in the parameter (αhν) n in the Tauc equation (α is calculated

ogy for basic studies retaining the cube shape. To further confirm the crystalline quality and phase purity of the nanocubes, high-resolution transmission electron microscopy (HRTEM) was performed as shown in Figure 2. Single crystallinity of the as-coated (Supporting Information) and sulfurized nanocubes is evident in the HRTEM images. The lattice-resolved, high-resolution image yields a lattice constant of 5.48 Å, in congruence with the (100) plane lattice spacing shown in Figure 2b. Selected area electron diffraction (SAED) pattern shows two pairs of planes which could be indexed to (200) and (20̅ 0), and (020) and (020̅ ) as shown in Figure 2c. The zone axis of the cube is [001] which proves that they are terminated with {100} facets. Planar defects such as stacking faults could be discerned in the lattice as marked with yellow lines and arrows in Figure 2b. Although such defects have been previously reported in nanoparticles, their presence in these cubes may have significance in this study, as they may have an important role in governing the electronic properties of pyrite nanoparticles.17,36,37 C

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Figure 3. (a) UV−Vis absorption spectra and (b) Tauc plot analysis of sulfurized iron pyrite film. Indirect and direct band edge intercept at 0.72 and 1.02 eV, respectively.

Figure 4. Differential transient absorption spectra after photoexcitation of sulfurized iron pyrite nanocubes as a function of time delay in (a) femto−picosecond range, (c) nano−microsecond range. Carrier decay dynamics probed at 950 nm (photobleaching) and 1400 nm (photoinduced absorption) by fitting the transients in (b) picosecond range, fitted with single exponential decay function (solid black line) with decay time constant (charge transfer time, τct) 1.8 ps and (d) microsecond range, fitted with biexponential decay function with time constants 50 ns (τd1) and 990 ns (τd2) associated with the long-lived trap states and the eventual recombination process.

dynamics of these states are critical in order to establish carrier loss mechanism in iron pyrite. Given the presence of high density defect states, the next important step is to understand the carrier dynamics in iron pyrite as defect states within the band gap will have repercussions on the optoelectronic performance. Pump probe or transient absorption (TA) spectroscopy was conducted on as-prepared and sulfurized samples to understand charge carrier relaxation in pyrite. TA spectroscopy allows us to monitor the photogenerated charge carrier dynamics in asprepared and sulfurized pyrite samples. The broad band dynamical TA spectra in the femtosecond and microsecond

by absorption spectra). Such indirect to direct transition is not unusual in semiconductors with high density of defect states and disorder, and has been reported even for iron pyrite single crystals.12,48 Notably, the discrepancies in commonly reported optical band gap of pyrite could be an indication of the presence of fuzzy band edges in pyrite, a quintessential feature in disordered semiconductors.49,50 Potential fluctuations arising from defects perturb the electronic band edges and could result in broadening of electronic states leading to band tails extending below the band edges, which could manifest as sub-band gap optical absorption.51−53 Assessment of the carrier D

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Scheme 1. Representative Schematic of the Photophysical Processes Involved in Iron Pyrite Based on Optical Pump Probe Spectroscopya

a (1) Optical excitation of electron from valence to conduction band, (2) rapid carrier localization of the excited carrier to indirect band edge and low lying shallow defect states, (3) slower electron relaxation to midgap deep defect states/band (long lived trap states),and (4) electron recombination process with the valence band holes.

probe time delay ranges were measured with a HELIOS and EOS from ultrafast systems. Following photoexcitation with above bandgap 650 nm pump pulses, the hot charge carriers rapidly relax to the band edge and trap states. Figure 4a,c (Figure S4a,c) shows the pseudo color plots of the change in absorption (ΔA represented by the color scale) against wavelength (nm) and probe time delay (ps and μs) for the sulfurized sample (as-prepared sample). Similar TA spectra and dynamics for the as-prepared and sulfurized samples suggest that the intrinsic electronic property (crystallization) of pyrite is not changed much by the sulfurization. This result further confirms that the dominant effect of this post treatment is to remove the attached insulating ligands for better interparticle coupling. Figure 4a also shows a prominent negative ΔA band or photobleaching (PB) signature near the bandgap at around 950 nm. This PB band shows a fast decay with a fitted lifetime of 1.8 ps. This lifetime closely matches with the building up time of a broad positive ΔA band or photoinduced absorption (PIA) signature at wavelengths >1000 nm. The band edge PB signal should originate from near bandgap stimulated emission of the photogenerated carriers and state-filling of the band edge states; the below band gap broad PIA band should originate from the localized charge carrier absorption. Therefore, the band edge PB fast decay represents photogenerated charge carrier localization from the direct gap edge to indirect band edge and midgap trap states. Since, the sample is of p-type nature (Hall measurement, Supporting Information Table ST1), fast charge carrier localization most likely originates from the photogenerated electron localization and trapping. The recombination time of the trapped electrons with holes is of the order of few hundreds of nanoseconds (Figure 4d). On the basis of our analysis, we have drawn a representative schematic of the complete photophysical processes involved in iron pyrite nanocubes as shown in Scheme 1. These processes clearly indicate the high carrier losses in iron pyrite due to fast localization and recombination assisted by intermediate gap states. Our model also supports the mechanism of rapid

electron relaxation through intraband states proposed by Ceder et al. through theoretical and STS analysis.49 Next, we examine the impact of such high defect states on charge carrier transport in iron pyrite. To study the physics of carrier transport and the effects of intrinsic defects, we conducted temperature-dependent electrical transport and magnetization measurements. Figure 5a shows the dependence of resistivity (Ln ρ) on 1/T (inset shows the resistance versus temperature data) which resembles a typical semiconducting behavior. When the results are plotted against T−1/4, a good straight line could be fitted from room temperature (300 K) to about 50 K as shown in Figure 5b. This indicates a good agreement with Mott variable range hopping (VRH) mechanism for charge transport given by the equation ρ = ρo exp(To/T)1/d+1 where d in the exponent is the dimensionality of the system, To is the characteristic temperature, and ρo is a weak temperature dependent pre-exponential factor (value of ρ at T → ∞) which depends on the electron−phonon interactions.54 It is to be highlighted that the density of states (DOS) is assumed to be non-zero and constant at the Fermi level within the thermal energy interval between localized states in the equation. The resistance below 50 K was high and therefore difficult to measure; hence, we focused in the range 300−50 K. A high characteristic temperature of To = 4.5 × 106 K was obtained by linear fitting of Ln(ρ) vs T(−1/4). Such conducting behavior is marked by charge transport through the localized states via phonons (phonon assisted transport). At finite temperature, VRH conduction is caused by the localized states that depend on dimensionality (d) of the system.55 Since the charge transport in our films is through nanocubes, the value of d = 3 could be intuitively anticipated due to threedimensional isotropy of nanocubes. Moreover, deviation from d = 3 is not anticipated as the characteristic length scales are too small for iron pyrite to observe such transitions or crossover in pyrite nanostructures. This argument agrees well with the experimental data fitted with three- dimensional VRH equation. Previous transport studies on iron pyrite single crystals and E

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Figure 5. Electrical transport of iron pyrite. (a) Resistivity vs 1/T plot. Inset shows resistance vs temperature plot; (b) Ln ρ vs T−1/4 plot fitted linearly with Mott-VRH type transport. Magnetic measurements on iron pyrite, (c) magnetization vs temperature plot from 300 to 10 K and (d) M−H curve at temperature 10 and 300 K showing superparamagnetic and diamagnetic response, respectively.

H measurements conducted in the two different regimes are shown in Figure 5d. Typical diamagnetic behavior is observed at 300 K, though at 10 K, a very weak magnetic ordering is exhibited. This raises the possibility of the presence of unexpected phases such as FeS2−δ (0 ≤ δ ≤ 1) which could be the consequence of nonstoichiometry at localized regions such as sulfur point defects within the bulk and the surface. Such concerns have been stressed in number of studies.42,61 Since such phases were not detected at room temperature, this could be attributed to the point defects clustering at low temperatures which might create nanoscale regions of nonstoichiometry and consequently local magnetic ordering.62 At low temperature, magnetism may be due to freezing of defect spins which enable magnetic exchange to occur versus thermal fluctuations which tend to kill the alignment and hence magnetism. Moreover, the origin of magnetic signal can be explained in terms of geometrical consideration of the Fe−S coordination in the crystal. LFT and MO theory predicts the formation of midgap defect states/band due to sulfur vacancies. The Fe atom in iron pyrite is in octahedral coordination (Fe−S6) with S. Due to strong ligand field (S atoms), Fe d-states split and form valence and conduction bands which are derived from t2g and eg states, respectively. Electron distribution in these states is in low spin configuration, and hence, pyrite is nonmagnetic. When S vacancies are formed, the local coordination of Fe (such as Fe− S5, FeS4,..., etc.) is reduced which leads to symmetry reduction. This causes further splitting of Fe d-states and introduces additional states in the forbidden energy gap. Electron distribution is no longer in low spin configuration (fully filled orbitals) giving rise to some high spin configurations due to splitting of t2g state, resulting in localized magnetic spins.32,63 At low temperatures when the thermal vibrations are low, the

nanostructures also showed Mott-VRH type conduction phenomena.12,56 The fact that Mott-VRH type conduction behavior is often observed in different pyrite nanostructures, regardless of the synthesis procedures used, reinforces the existence of smaller localization lengths and intrinsic defect mechanisms in iron pyrite. Disorder and intrinsic defects cause localized states (Anderson localization) within the band gap and electron hops from one site to another spatially separated site. The selection of the site is based on the criteria of minimum activation energy (minimum energy difference) configuration that maximizes the hopping probability.57 The presence of high density of defect states reflected by our optical pump probe studies is in agreement with the charge transport behavior observed in the temperature dependence of resistance in the iron pyrite films. It is thus clear that the disorder and intrinsic defects in the pyrite crystals cause the defect states that trap the charge carriers and consequently assist recombination. To eliminate such defects, the nature and characteristics of such defects need to be understood. A very rational and logical argument would be to look for the possible existence of any phase inhomogeneities in iron pyrite. Intrinsic defects are arguably the most plausible explanation for the universal conduction behavior of iron pyrite and its sluggish photoresponse (Figure S5, Supporting Information). We conducted temperaturedependent magnetic measurements on our sulfurized iron pyrite nanocube film samples. In general, FeS2 is a diamagnetic semiconductor having all the electrons filled in t2g state in low spin configuration.58,59 Fe and other Fe−S phases exhibit magnetic order.60 Temperature-dependent magnetization data is shown in Figure 5c. It can be clearly seen that below about 50 K, magnetic ordering is evident. This might occur if the ordering energy dominates the thermal energy. Isothermal M− F

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Figure 6. (a) Current density vs voltage (J−V) curve of iron pyrite heterojunction solar cell, (b) external quantum efficiency of the heterojunction solar cell, (c) schematics of energy band alignment of different layers of the solar cell, and (d) cross-sectional SEM of the measured solar cell.

the reason for the lower photovoltage. Figure 6c shows the schematic of charge transfer process in a pyrite nanocubes/CdS heterojunction device. Energy band position of CdS and FeS2 NCs are favorable for charge carriers to be extracted by the hole and electron selective contacts. Cross-sectional SEM image of the device is shown in Figure 6d. Active layer, top and bottom electron and hole collecting layers can be distinctly seen. The low photovoltage value obtained is consistent with the charge carrier dynamics of pyrite elucidated previously. Our model predicts the recombination pathway through trapping in band tail localized states followed by deep state relaxation and subsequent recombination via hole capture or directly through nonradiative processes.

crystals tend to reduce their total energy by S vacancy diffusion into some low energy configurations where exchange coupling of the unpaired electrons could occur through the Fe ions.64 This could lead to ferromagnetism in localized regions of the crystal where it is detectable by a sensitive magnetometer. Since point defects are a source of localized gap states, the magnetic information is also consistent with the photophysical processes proposed above in iron pyrite. The characteristics of defects depend mostly on the growth conditions. Our results suggest immediate link between localized gap states and existence of such phases intrinsic to pyrite chemistry. Iron pyrite heterojunction solar cells were evaluated using CdS as the heterojunction partner for charge extraction due to its favorable energy level configuration for electron transfer. Figure 6a shows the current density versus voltage (J−V) curve of the solar cell under 1 Sun (100 mW/cm2) illumination. A photovoltage of around 94 mV was obtained with FF of 0.28 and current density of 0.4 mA/cm2 showing that the sulfurized iron pyrite nanoparticles could be used as solar absorber material. Figure 6b shows the external quantum efficiency (EQE) of the FeS2−CdS heterojunction solar cell. Photocurrent spectra consistently followed the optical absorption spectra of pyrite, which reflects the ability of photogenerated carriers to be converted to electrical current. Only electronic contribution is considered for generation of photocurrents as thermal contributions would be negligible at femtosecond excitation followed by relaxation in approximately microsecond time scale (Figure S5, Supporting Information). This further corroborates that current flows in pyrite through trapped charges that are distributed within the band gap which is clearly

CONCLUSION We have studied high purity as-prepared and sulfurized iron pyrite nanocubes prepared by hot-injection method. We have demonstrated that sulfurization treatment of nanocubes do not affect the shape and phase purity of nanocubes while increasing the conductivity, making them suitable for fundamental optical and transport studies and device applications. Nature of optical absorption of iron pyrite corresponds to that of a disordered semiconductor. Optical pump−probe studies conducted on pyrite sample to study photocarrier generation and relaxation process have found strong photocarrier generation occurring at the direct band edge (∼950 nm). Decay of the photoexcited carriers consisted of a fast and a slow component. The fast component was attributed to rapid carrier localization of photoexcited carriers to indirect band edge and shallow defect G

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SEM images were acquired using JEOL JSM-7600F field-emission scanning electron microscope. HRTEM analysis was performed using JEOL TEM 2010 and 2100F. Optical absorption measurement was done using PerkinElmer Lambda 950 spectrophotometer. Four probe Hall measurements were performed using MMR variable temperature Hall measurement system. Transient photocurrent measurements were conducted using mode-locked Ti:sapphire laser seeded Ti:sapphire regenerative amplifier pumped OPA (spectra physics, Mai Tai-Spitfire Ace-TOPAS Prime). Current density versus voltage (J−V) curves of the solar cell were measured using solar simulator (San-EI electric, XEC-301S) equipped with a 450 W xenon lamp, which was coupled to Agilent semiconductor parameter analyzer (4155C). Solar cell performance was measured under illumination of AM 1.5 (100 mW/cm2) calibrated using silicon reference cell (Fraunhofer).

states with τct of 1.8 ps. The slow component was attributed to carrier relaxation through deep midgap states and recombination process via direct and hole capture process. Temperaturedependent transport measurements showed the operation of VRH type conduction mechanism from 50 to 300 K, which is consistent with high density of defect states detected by optical TA measurements. Temperature-dependent magnetic measurements showed magnetic ordering arising in iron pyrite at low temperatures, contrary to nonmagnetic behavior at room temperature. These magnetic impurities directly correlate with phase inhomogeneity in iron pyrite, such as localized formation of FeS2−δ (0 ≤ δ ≤ 1) that induces defect states/ band within the energy gap that traps the charge carriers by fast localization of carriers and promote recombination losses. Our findings suggest that photovoltages can still be obtained from pyrite nanocubes, though heterojunction solar cells prepared using sulfurized iron pyrite nanocubes showed photovoltage of only 94 mV with current density of 0.4 mA/cm2. However, significant increases to efficiency may be obtained by two factors, (i) boosting the current through enhanced interparticle coupling or realizing a single domain films free of boundaries, (ii) increment in photovoltage by mitigating the defect states present within the band gap by not only limiting to post treatment process but also finding adequate growth strategy that results in low bulk defects. We believe that our findings will help in the understanding of the rarely explored photophysics of iron pyrite nanocubes in detail and provide deep insight on intricate relationship between defects (intrinsic and disorder induced) and gap states responsible for photocarrier losses. Our future work will be focused on quantifying these defects, eliminating them varying growth conditions and post treatment processes, and subsequently, studying their relationship to pyrite solar cell performance.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00065. Additional experimental data (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Authors would also like to thank Kalon Gopinadhan for help in transport measurements. V.N. would like to thank the Ministry of Education, Singapore, for financial support (Grant MOE2011-T3-1-005). T.C.S. gratefully acknowledge the financial support from Nanyang Technological University start-up grant M4080514, the Ministry of Education AcRF Tier 1 grant RG101/15 and AcRF Tier 2 grants MOE2013-T21-081 and MOE2014-T2-1-044; and the NRF through the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE Programme. S.M would like to acknowledge SinBeRISE and Mid-IR photonics project bridge funding (MIPS R-398-000-082-646) from ODPRT, NUS. Q.X. gratefully acknowledges Singapore National Research Foundation via an Investigatorship Award (NRF-NRFI2015-03), and Singapore Ministry of Education through two AcRF Tier2 grants (MOE2011-T2-2-051 and MOE2015-T2-1-047).

METHODS Synthesis. The {100} faceted iron pyrite nanocubes were synthesized by hot-injection method as reported previously.65 The starting materials for the synthesis were 98% anhydrous iron(II) chloride (FeCl2), 70% oleylamine (OLA), and sulfur powder from Sigma-Aldrich. A total of 0.5 mmol (63.5 mg) of FeCl2 with 5 mL of OLA were mixed in a trineck flask and degassed for 30 min, and subsequently held at 110 °C for 1 h to form Fe−OLA complex. Thereafter, the temperature was raised to 180 °C and 3 mmol (96 mg) of sulfur mixed in OLA was injected into the flask while maintaining the temperature at 180 °C. The duration of reaction was 24 h. After reaction, the solution was cooled to room temperature naturally; a large amount of methanol was added to precipitate the FeS 2 nanocubes followed by centrifugation and dispersion in hexane. Then, 50 μL of Pyrite NCs suspension with a concentration of (0.1M) was spin-coated on the substrate. Sulfurization process was carried out in a two zone furnace. Sulfur powder was kept at 200 °C and nanocube film was kept at 500 °C in argon atmosphere for 30 min. Tube was evacuated and purged with argon three times before ramping up the temperature for sulfurization process. Device Fabrication. At first pyrite nanocubes were spin coated on molybdenum bottom contact and sulfurized by above-mentioned process. Then, CdS buffer layer of ∼60 nm thickness was deposited on sulfurized pyrite nanocube thin film on Mo by chemical bath deposition (CBD). Subsequently, 50 nm i-ZnO followed by 600 nm ZnO/Al layer were deposited by RF and DC magnetron sputtering. Finally, silver glue was printed on AZO layer to form top contact fingers. Characterization. Thin films X-ray diffraction (XRD) were taken from Shimadzu 6000 using Cu Kα (λ = 1.54178 Å) radiation. T64000 micro-Raman spectrometer with an incident power of 0.14 mW and laser illumination of 532 nm wavelength was used for Raman analysis.

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DOI: 10.1021/acsnano.6b00065 ACS Nano XXXX, XXX, XXX−XXX