Phase analysis and surface morphology

Phase structures of the CuFeS₂ NCs were identified from XRD patterns as shown in Fig. 1. Initially, we started growing these NCs using a procedure identical to that used to prepare FeS₂ NCs.^[Reference Bhandari, Roland, Kinner, Cao, Choi, Jeong and Ellingson^14] The S precursor was injected into the reaction flask containing Fe and Cu ions at 220 °C, and the NCs were allowed to grow at the same temperature for about 2 h. The XRD pattern of these NCs revealed additional peaks, marked by * and ** in Fig. 1, attributed to other phase materials in addition to the peaks of CuFeS₂. The peak marked by * at 27.7° is assigned to covellite (CuS) particles produced during the synthesis of CuFeS₂ NCs. The CuS peak is assigned based on the chemical composition and its diffraction pattern being close to the standard pattern of CuFeS₂. Similarly, the peak marked by ** is assigned to mackinawite (FeS) crystallites. The covellite particle has hexagonal crystal structure while mackinawite has a tetragonal crystal structure.^[Reference Hosseinpour, Alemi, Khandar, Zhao and Xie^27, Reference Xing, Zhang, Cao, Zhao, Wang, Qin, Liu, Jiang and Meng^28] The additional XRD peaks such as at 27.7° were observed when the S injection and growth temperatures were ≤240 °C. These additional peaks observed in the XRD pattern prevent the synthesis of phase pure CuFeS₂ NCs using elemental S powder. Ultimately, the phase pure CuFeS₂ NCs were synthesized when the S precursor was injected at 240 °C, and the NCs were grown at an elevated temperature of 270 °C for about 2 h. As the S precursor was injected at 240 °C, the temperature of reaction solution dropped to ~205 °C, and the temperature was immediately set to growth temperature 270 °C. This S injection and growth temperature produced phase pure NCs based on the XRD pattern which showed no discernible other phases or compositions. This statement regarding the phase purity of the synthesized NCs is supported by the intense and sharp peaks of the XRD pattern of CuFeS₂ NCs presented in Fig. 1. These phase pure CuFeS₂ NCs belong to tetragonal chalcopyrite phase with the XRD pattern matching exactly with the standard expectation for tetragonal CuFeS₂ chalcopyrite material (#PDF 98-000-0156) (space group, ${\rm I}{\bar 4}$2d) obtained from MDI JADE (Jade^™ computer software from Materials Data Inc.). Based on our observation, when the injection and growth temperatures ≤240 °C, it is favorable to the formation of CuFeS₂ NCs in addition to some traces of impurities. During the synthesis, the mole fraction for the elemental S powder was 6 times larger than the metal (Cu and Fe) sources which are essential to obtain the phase pure CuFeS₂ NCs. This result agrees excellently with the XRD pattern of CuFeS₂ material synthesized by solution-based and mechanical alloying method reported previously.^[Reference Pradhan, Ghosh and Samanta^10–Reference Kumar, Uma and Nagarajan^12] In the tetragonal crystal system, the lattice parameters are a = b = 0.5289 nm, c = 0.1042 nm with all the three angles equal to 90°.^[Reference Aldakov, Lefrancois and Reiss^7]

Figure 1.

x-ray diffraction (XRD) patterns of as-synthesized CuFeS₂ nanocrystals (NCs) at various injection (T _i) and growth (T _g) temperatures. The XRD pattern matches well with the standard pattern of chalcopyrite (CuFeS₂) obtained from MDI JADE software (#PDF 98-000-0156), confirming this material is tetragonal CuFeS₂ chalcopyrite NCs.

The structure of CuFeS₂ NCs synthesized by S precursor injection at 240 °C and grown at 270 °C for 5, 10, 15, 30, and 60 min are characterized by XRD patterns shown in Fig. S1. Here, we note that all the synthesis temperatures other than 270 °C were for 2 h. Based on these XRD patterns, NC batches grown longer than 10 min are phase pure tetragonal CuFeS₂ NCs. For NCs with a growth time of 5 min, the products include a measurable quantity of covellite (CuS) particles as indicated by the small peak observed at ~27.7°; note that this XRD feature is similar to that observed for NCs in which the S precursor was injected at ≤240 °C. When the S precursor was injected, the reaction mixture temperature dropped to ~205 °C, and this temperature produced traces of covellite CuS in addition to CuFeS₂. Based on the literature, 205 °C supports the formation of CuS particles^[Reference Hosseinpour, Alemi, Khandar, Zhao and Xie^27, Reference Saldanha, Brescia, Prato, Li, Povia, Manna and Lesnyak^29] and the reaction mixture took ~5 min to reach 270° C. However, the evolution of the XRD pattern with growth time shows that the diffraction peak at ~27.7° disappeared, within the limit of XRD measurement, indicating that the impure phase particles such as CuS reacted with Fe/S complexes in the excess S environment to form the phase pure ternary CuFeS₂ NCs. The crystallite size of the phase pure CuFeS₂ material can be estimated using Debye-Scherrer analysis, based on the equation

$$D_{\rm p} = \displaystyle{{K\lambda } \over {\beta {\rm cos}\theta }},$$

where D _p is the crystallite size, K is the shape factor usually taken close to unity, β (in radian) is the broadening at half maximum or full width at half maximum (FWHM), and θ is the Bragg angle.^[Reference Langford and Wilson^30] Here, the value of K is taken 0.94, and wavelength (λ) is 1.54059 Å for Cu K _α lines of the X-ray. The full width at half maxima (β) was calculated using OriginPro software (OriginLab, Northampton, MA) for the (112) Miller index peaks. An XRD broadening of 0.08° was subtracted from FWHM to calculate the crystallite size of the CuFeS₂ chalcopyrite NCs. This XRD instrumental broadening for corresponding “2θ” was obtained from JADE software from a calibration profile of standard NIST 640e silicon sample. The crystallite sizes of CuFeS₂ NCs obtained from Scherrer analyses is presented in Table I. The calculated value is rounded to the closest whole number in nanometer. These results indicate that the crystallite size of the CuFeS₂ NCs depends upon the growth durations after injecting the S precursor during the synthesis.

Table I.

Crystallite sizes (Scherrer-analysis) of CuFeS₂ NCs synthesized with different growth durations.

Figure 2 shows SEM results of as-synthesized CuFeS₂ chalcopyrite NCs with growth times of 15 min, 30 min, 1 h, and 2 h. The as-synthesized NCs have irregular shapes, unlike most preparations of pyrite FeS₂ NCs, which when synthesized by a similar procedure shows a consistent cubic shape.^[Reference Bhandari, Roland, Kinner, Cao, Choi, Jeong and Ellingson^14] Here, the particle sizes observed by SEM are larger than the crystallite size estimated by Debye-Scherrer equation indicating the crystallites agglomerate to form larger particles. Based on the measured diffractograms, these NC samples all possess tetragonal crystal structure in the chalcopyrite phase. The longest growth time correlates with larger CuFeS₂ crystallite size^[Reference Wang, Wang, Yue, Wang, Yan and Peng^8] with visual evidence of tetragonal shapes with larger facets and longer clean edges as seen in Fig. 2. More SEM results with different resolutions are shown in Fig. S2 (in SI). As shown in Fig. 2, 15 min growth time yields smaller and less fully crystalline NCs than do the longer growth times. However, for 240 °C injection and 270 °C growth temperatures, the phase and compositional purity of the product NCs are not affected over this range of growth time from 15 min to 2 h (Fig. S1). The CuFeS₂ NCs synthesized here have relatively larger particle sizes compared to the previous reports. For example, Wang et al. have reported the spherical and pyramidal shaped tetragonal crystal structure CuFeS₂ chalcopyrite NCs with average sizes of 12 and 30 nm, respectively.^[Reference Wang, Bao and Gupta^11] Liang et al. have synthesized smaller CuFeS₂ NCs with 6.6 nm average size.^[Reference Liang, Ma, Jiao, Pang and Feng^3]

Figure 2.

Scanning electron micrographs of as-synthesized CuFeS₂ chalcopyrite NCs with different growth durations (a) 2 h, (b)1 h, (c) 30 min, and (d) 15 min.

Further, to compare the surface morphology and size of the particles observed in SEM images, STEM images of CuFeS₂ NCs prepared for two growth time (30 min and 1 h) were also obtained as shown in Fig. S3. The size of the CuFeS₂ NCs prepared for a growth time of 30 min is ~20 nm, slightly smaller than crystallite sizes obtained from Scherrer analysis. Similarly, the average particle size prepared for 1 h growth time is ~50 nm. EDS and elemental mapping were used to determine the chemical composition as a function of growth time. EDS confirmed the presence of Cu, Fe, and S atoms in the NCs. The atomic percentages of different elements as a function of growth time are shown in Table SI. Elemental mapping for the 1 h grown CuFeS₂ NC sample is shown in Fig. S4 and corresponding EDS data as a function of growth duration are in Fig. S5. From these results, CuFeS₂ NCs are found to have slightly higher Cu concentration compared with Fe. The atomic ratio of Cu:Fe:S is close to 1:1:2 indicating tetragonal CuFeS₂ chalcopyrite NCs (Table SI), and it agrees with previous results (Cu:Fe:S = 1:0.83:1.96) reported by Wu et al.^[Reference Wu, Zhou, Yang, Liao, Zhang and Li^5] In case of 30 min growth sample (T _i = 240 °C, T _g = 270 °C), the number of Cu and Fe atoms are relatively close (Table SI) to each other although all the NCs were prepared using an identical procedure except the growth time.

Unpolarized transmittance and Raman spectroscopy

Unpolarized transmittance spectra of CuFeS₂ NCs were measured using a Lambda 1050 spectrophotometer (Perkin Elmer) in chloroform solution, referenced to the neat solvent, in a 2 mm path length cuvette. Figure 3 shows the absorbance spectra determined by using Beer's Law from the unpolarized transmittance spectra collected from 250 to 1650 nm. The spectra were recorded for NC samples having growth times of 1, 5, 10, 15, 30, and 60 min, respectively. A strong absorption peak ~465 nm was observed for the NCs with growth time of 1 min. The UV-Vis spectra show that the absorption peak wavelength slightly increases with increasing growth time. The NCs grown for 30 min have the absorption peak at 490 nm, but 60 min grown NCs do not show this absorption peak. The absorption feature observed at ~490 nm matches closely with previous reports.^[Reference Wu, Zhou, Yang, Liao, Zhang and Li^5, Reference Ghosh, Avellini, Petrelli, Kriegel, Gaspari, Almeida, Bertoni, Cavalli, Scotognella, Pellegrino and Manna^6, Reference Gabka, Bujak, Żukrowski, Zabost, Kotwica, Malinowska, Ostrowski, Wielgus, Lisowski and Sobczak^13] A typical absorption spectrum in the range of 400–2500 nm for 1 h grown CuFeS₂ NCs solution is shown in Fig. S6 (SI).

Figure 3.

Optical properties of CuFeS₂ nanocrystals (a) absorbance spectra of as-synthesized CuFeS₂ NCs dispersed in chloroform in 2 mm optical path length cuvette as a function of growth time (b) stability test in 30 days (c) real and imaginary dielectric constants (ε) with photon energy for 15 min grown NCs at 270 °C, and (d) absorption coefficient (α) evaluated from spectra in k using spectroscopic ellipsometry.

Based on the literature, other materials showing absorption peaks in the 290 to 600 nm regions include CuO with an absorption peak at ~290 nm and CuS with an absorption peak at ~ 430 nm.^[Reference El-Trass, ElShamy, El-Mehasseb and El-Kemary^31] Further, Cu_2−xS nanoparticles (NPs) show a broad IR absorption feature ascribed to a localized surface plasmon resonance.^[Reference Saldanha, Brescia, Prato, Li, Povia, Manna and Lesnyak^29] FeO NPs reported by Liu et al. do not have any absorption peaks in the region <600 nm.^[Reference Liu, Cai and Zeng^32] Just prior to initiating the CuFeS₂ synthesis, reactants were evacuated at 120 °C to remove oxygen and water, and the synthesis was carried out in a high purity nitrogen environment to avoid any reaction with atmospheric oxygen. Further, the XRD patterns of CuFeS₂ NCs grown for longer than 10 min at temperature 270 °C is phase-pure tetragonal CuFeS₂ NCs with no other observable impurities (Fig. S1). However, the NCs grown for 5 min at the same temperature may include traces of CuS as evident from the diffraction pattern (Fig. S1) at ~27.7° (vide supra). Wiltrout et al. have reported an absorption peak at ~600 nm arising from bornite (Cu₅FeS₄) NPs, but our XRD studies do not show the presence of Cu₅FeS₄ or any other cubic phases.^[Reference Wiltrout, Freymeyer, Machani, Rossi and Plass^33] Thus, we assign the absorption peaks in the 465–490 nm range to pure tetragonal CuFeS₂ NCs.

Previously, many authors have reported blue shifting of the band gap of the CuFeS₂ NCs compared with its bulk counterpart (~0.6 eV), which they attributed to quantum confinement.^[Reference Liang, Ma, Jiao, Pang and Feng^3, Reference Wu, Zhou, Yang, Liao, Zhang and Li^5, Reference Wang, Bao and Gupta^11] Some authors have correlated an absorption feature observed in chalcopyrite NCs at ~498 nm with a possible surface plasmon resonance.^[Reference Gabka, Bujak, Żukrowski, Zabost, Kotwica, Malinowska, Ostrowski, Wielgus, Lisowski and Sobczak^13] Our spectra show a very narrow red shift (~25 nm) when increasing the NC growth time from 1 min to 30 min at 270 °C. The small red shift observed from 465 to 490 nm may be attributed to the weak quantum confinement or the surface effects of CuFeS₂ NCs. The change in band gap of these CuFeS₂ NCs with growth time is quite different from other quantum confined NCs such as PbS and CdSe. For example, in the case of PbS quantum dots, the absorption peaks shift rapidly from 970 to 1740 nm (1.28 to 0.71 eV) within the growth time of 1–20 min.^[Reference Moreels, Lambert, Smeets, De Muynck, Nollet, Martins, Vanhaecke, Vantomme, Delerue, Allan and Hens^34] The band gap of CuFeS₂ NCs cannot be tuned by quantum confinement as widely as that of PbS, since the Bohr exciton radius of CuFeS₂ NCs should be similar to that of iron pyrite (1.3 nm)^[Reference Li, Doblinger, Vaneski, Rogach, Jackel and Feldmann^35] and CuInS₂ (4.1 nm)^[Reference Castro, Bailey, Raffaelle, Banger and Hepp^36], but much smaller than PbS (18 nm)^[Reference Moreels, Lambert, Smeets, De Muynck, Nollet, Martins, Vanhaecke, Vantomme, Delerue, Allan and Hens^34]. Since the size of the NCs prepared here are larger than the Bohr exciton radius, strong quantum confinement is neither present nor expected.

The electronic structure for bulk chalcopyrite CuFeS₂ has been discussed in available literature.^[Reference Kambara^1, Reference Oguchi, Sato and Teranishi^23, Reference Hamajima, Kambara, Gondaira and Oguchi^24, Reference Xu and Schoonen^37] The theoretical calculation shows an indirect band gap of 0.63 eV in between VB and IB-I, and the width of IB band is ~1.10 eV.^[Reference Ghosh, Avellini, Petrelli, Kriegel, Gaspari, Almeida, Bertoni, Cavalli, Scotognella, Pellegrino and Manna^6] The small band gap of this bulk chalcopyrite arises due to the electronic transition from VB to IB-I predominantly formed by empty Fe 3d orbitals.^[Reference Hamajima, Kambara, Gondaira and Oguchi^24] The suppression of the absorption feature at ~490 nm for larger NCs fabricated at longer growth time may be due to several reasons including scattering by larger particles, defect states or unpassivated dangling bonds on the surface. Further study will be required to understand the influence of CuFeS₂ NC size on defect densities and optical properties. Thus, based on this observation, we conclude that the absorption peaks observed in the spectral range of 465–490 nm (2.66 to 2.53 eV) is not due to the quantum confinement effect; instead it arises from an electronic transition from the VB to the IB-II located within the fundamental band gap (3.1 eV)^[Reference Hamajima, Kambara, Gondaira and Oguchi^24] of CuFeS₂ NCs which is the characteristics of CuFeS₂ materials. Further, the complex optical properties of these CuFeS₂ NCs were studied using spectroscopic ellipsometry technique in the range of 0.735 to 5.887 eV which is discussed in the section “Optical Properties Using Spectroscopic Ellipsometry”.

The stability of these colloidal CuFeS₂ NCs is tracked via absorbance spectra of NCs solution dispersed in chloroform, kept in a vial (solution and in air) and stored in ambient environment. The absorbance spectra for 15 min grown CuFeS₂ NCs measured at different intervals of time (day 1, day 3 etc) after preparation is presented in Fig. 3(b). From day 1 to day 30, no significant changes in the absorption feature of these NCs are observed indicating that these NCs are quite stable in ambient conditions. The NCs concentration in solution may be slightly different in each measurement giving rise to decreased peak intensity, and some agglomeration of NCs is expected over the time. Further, the XRD pattern of CuFeS₂ NC films stored in an ambient environment and measured after 30 days of preparation matches with the standard XRD pattern of chalcopyrite CuFeS₂ (#PDF 98-000-0156) with no other phases materials (Fig. S7). The XRD patterns along with consistently shaped absorbance spectra indicate that these NCs are stable in air.

The active vibrational modes present in the NCs were determined from Raman spectroscopy measurement using a HeNe laser of 632.8 nm wavelength (Fig. S8). The measured Raman spectrum matches quite closely with the spectra reported previously for the bulk CuFeS₂ excited with a 514.5 nm laser.^[Reference Chunrui, Shaolin, Junqing and Kaibin^38] For as-synthesized NCs, active peaks at positions 290, 351, and 470 cm⁻¹ were observed. Based on the literature of similar observations^[Reference Kumar, Uma and Nagarajan^12, Reference Chunrui, Shaolin, Junqing and Kaibin^38], the active peaks at 290, 351, and 470 cm⁻¹ are assigned to be A₁, B₁, and E phonon modes. The active peak at 290 cm⁻¹ is the strongest as evident from the spectrum for the A₁ modes of vibration. Thus, based on XRD pattern, elemental composition, and Raman active modes of vibration, chalcopyrite CuFeS₂ NCs synthesized here are phase pure with tetragonal crystal structure.

Optical properties using spectroscopic ellipsometry

Complex optical properties of the CuFeS₂ NC thin film can be described by the complex dielectric function, ε(E) = ε ₁(E) + i ε ₂ (E), or the complex refractive index, N(E) = n(E) + ik (E) = ε ^1/2 (E), spectra as obtained from spectroscopic ellipsometry measurement and analysis (see SI) where ε ₁ = real part of dielectric constant, ε ₂=imaginary part of dielectric constant, n = refractive index, k = extinction coefficient, and E = photon energy, respectively. These complex quantities, ε(E) and N(E), are also called optical constant which determines reflection and absorption of media when light travel. The spectra in ε and the structural parameters, the surface roughness, and NC film thickness are extracted by fitting the experimental ellipsometric spectra to a model consisting of semi-infinite soda lime glass substrate/NC film/surface layer/air ambient. Spectra in ε for the NC film are parameterized by using multiple critical point parabolic band (CPPB) oscillators^[Reference Subedi, Bhandari, Ellingson and Podraza^39] each of which is described by

$$\varepsilon (E) = \varepsilon _\infty + \sum\limits_n {A_n{\rm e}^{i\varphi _n}{\left( {\displaystyle{{\Gamma _n} \over {2E_n - 2E - i{\it \Gamma} _n}}} \right)}^{\mu _n}} $$

This CPPB oscillator model describes the optical constant of the material at the interband critical point (CP). For each CPPB oscillator, A _n is the amplitude, Γ _n is the broadening, E _n is the critical point energy, μ _n is the exponent, and ϕ _n is the phase projection factor. Excitonic transitions (μ _n = 1) are assumed similar to those used to describe FeS₂ NCs.^[Reference Subedi, Bhandari, Ellingson and Podraza^39] To account the dispersion outside our measured spectral range a constant additive term to ε ₁ denoted by ε _∞ is also included in parameterization. Spectra in ε describing the surface layer of film are modeled using the Bruggeman effective medium approximation^[Reference Bruggeman^40] consisting of 0.834 ± 0.004 of the volume fraction to be identical to CuFeS₂ NCs layer and the remainder void. In addition to the parameters describing spectra in ε, the NCs film thickness (d _b) and surface layer thicknesses (d _s) are also free parameters in the least squares regression fit. The thicknesses as obtained from the parametric model are d _b = 962 ± 4 nm and d _s = 251 ± 2 nm, respectively. The surface layer can be interpreted as a combination of roughness and lower packing density of NCs near the top of the deposited film. The high value of d _s corresponds to the sample non-uniformity of the film near the surface and/or grading in NCs packing density. The fitted values with error bars are reported in Table SII (SI) with mean square error (σ) = 3.5 × 10⁻³. There are three interband transitions represented by the resonance energies of CPPB oscillators at 0.652 ± 0.003, 1.54 ± 0.03, and 2.29 ± 0.01 eV, respectively, along with an additional oscillator needed to account for higher energy transitions outside the measured spectral range. Spectra in ε and N are shown in Fig. 3(c) and Fig. S9 (in SI), respectively. From spectra in N, the absorption coefficient, α = 4 π k (λ)/λ is evaluated which is shown in Fig. 3(d). The CP near 0.65 eV corresponds to the difference in energy between the VB and IB-I which is consistent with the absorption edge at 0.6 eV reported by Teranishi et al.^[Reference Teranishi and Sato^41] Origin of CPs near 1.54 and 2.29 eV are may be due to transition from VB to Fe 3d orbitals. The transition energies for single crystal CuFeS₂ according to Oguchi et al.^[Reference Oguchi, Sato and Teranishi^23] are 1.0 and 2.1 eV, respectively. In comparison, our CPs are blue-shifted relative to a single crystal. The nature of both spectra in ε and α are similar to that of single crystal CuFeS₂^[Reference Oguchi, Sato and Teranishi^23] but with features lower in magnitude. The critical point energies of these NCs are also red-shifted relative to iron pyrite NCs.^[Reference Subedi, Bhandari, Ellingson and Podraza^39]

Electronic and Hole Transport Properties

The electronic properties of a material determine the possible application of the material in the opto-electronic devices. The average sheet-resistance of the solution-processed CuFeS₂ NC thin films is 7.3 (±3.7) × 10⁴ Ω □⁻¹ as provided by 4-point probe measurement with an average resistivity of 0.16 (±0.06) Ω cm. Also, these NC films have p-type conductivity as determined by thermal-probe measurement. The CuFeS₂ NPs synthesized previously by Liang et al. also shows p-type conductivity with a resistivity of 1.21 Ω cm, and carrier concentration of the order 10¹⁸ cm⁻³.^[Reference Liang, Ma, Jiao, Pang and Feng^3] In our case, the lower resistivity may be due to the larger particle size of CuFeS₂ NCs compared with Liang et al. (6.4 nm). Previously, solution-processed FeS₂ NCs, synthesized by hot-injection route, with resistivity ~10–20 Ω cm and carrier concentration ~10¹⁸ cm⁻³ have been applied as low barrier interface layer in CdTe and PKSCs.^[Reference Bhandari, Koirala, Paudel, Khanal, Phillips, Yan, Collins, Heben and Ellingson^17, Reference Huckaba, Sanghyun, Grancini, Bastola, Taek, Younghui, Bhandari, Ballif, Ellingson and Nazeeruddin^18] Here, these CuFeS₂ NC films have lower resistivity compared with FeS₂ NC films and comparable carrier concentration, and hence we were interested to study the hole transport properties of these materials in CdTe solar cells.

Figure 4 displays the device characteristics of the CdS/CdTe solar cells with two back contacts: Cu/Au and Cu/CuFeS₂-NC/Au. The CuFeS₂ layer of thickness ~1 µm was prepared using a NCs with growth time of 2 h at 270 °C. A schematic device structure is shown in Fig. S10 (SI). J-V characteristics measured under simulated AM1.5G irradiance for best devices are shown in Fig. 4(a) for both Cu/Au and Cu/CuFeS₂-NC/Au back contacts. A summary of device parameters for 20 CdTe solar cells with Cu/Au and Cu/CuFeS₂-NC/Au back contacts is presented in Table SIII. By introducing CuFeS₂ as an interface layer, a slight improvement on open-circuit voltage and fill factor was observed in addition to an increased series resistance of the device compared with the standard Cu/Au. The introduction of the CuFeS₂ back contact interface layer resulted in an overall power conversion efficiency that was slightly improved than observed for the Cu/Au back contact design. As a recommended standard practice, the current density (J) as determined by J-V measurement was verified by EQE measurement [Fig. 4(b)].

Figure 4.

Performance characteristics of the best CdTe devices attained with and without the CuFeS₂ NC hole transport layer (a) current-density versus voltage (J-V) curves and (b) external quantum efficiency (EQE) spectra, and (c) schematic diagram for an electronic structure of bulk chalcopyrite (CuFeS₂) materials.

Figure 4(c) displays the schematic diagram for an electronic structure of bulk chalcopyrite CuFeS₂ materials which have IBs formed mainly due to the Fe 3d orbitals in between the VB and CB. The electronic transitions, A and B in Fig. 4(c), <3.1 eV are due to transitions from VB to the IBs.^[Reference Oguchi, Sato and Teranishi^23, Reference Hamajima, Kambara, Gondaira and Oguchi^24] The previous report has assigned the Fermi level at 5.15 eV below the vacuum level for chalcopyrite (CuFeS₂) materials.^[Reference Xu and Schoonen^37] It may require further studies to understand the band offset mismatch in the case of CuFeS₂ NC-film and CdTe interface. Using the CuFeS₂-NC film as an interface layer, we found a slight improvement in the V _OC and FF with respect to Cu/Au back contact, but a slight decrease in J _SC. The decrease in J _SC is compensated by V _OC and FF to yield similar device performance in both cases. The additional effect due to the CuFeS₂ interface layer we observed is the increase in the series resistance of the solar cells, and it may require further treatment to increase the conductivity of the NC interface layer. Two possible reasons for the non-ideal interface layer performance of the CuFeS₂ NC HTL are (i) the large distribution of particle sizes, and (ii) weak adhesion of the HTL film to CdTe. Currently, we are working to control the NC size distribution, to improve the interface between CdTe and the CuFeS₂ NC-film, and to study the hole transport properties of these CuFeS₂ NCs in other PV devices. Based on our preliminary results of using CuFeS₂ NCs as HTLs in CdTe devices, these NCs may find similar applications in other material systems and device structures.