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High power conversion efficiency of intermediate band photovoltaic solar cell based on Cr-doped ZnTe
Solar Energy Materials and Solar Cells 170 (2017) 27–32
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High power conversion efficiency of intermediate band photovoltaic solar cell based on Cr-doped ZnTe Kyoung Su Lee, Gyujin Oh, Dongil Chu, Sang Woo Pak, Eun Kyu Kim
MARK
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Quantum-Function Research Laboratory and Department of Physics, Hanyang University, Seoul 04763, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Intermediate band solar cell ZnTe:Cr Pulsed laser deposition
We report on a high-performance intermediate band solar cell (IBSC) based on Cr-doped ZnTe (ZnTe:Cr) fabricated using a pulsed laser deposition (PLD) method. Chromium (Cr) was uniformly distributed in the ZnTe:Cr thin film with an atomic concentration of about 3.5%, and the ZnTe:Cr thin film showed p-type electrical conductivity. The ZnTe:Cr thin film had higher absorption coefficients than those of undoped ZnTe films in the photon energy range below band gap of ZnTe (2.2 eV). The enhanced absorption coefficients of the ZnTe:Cr thin film were attributed to the photoionization energy between Cr2+ and Cr+ (Cr2+ ⇌ Cr+), which acted as the IB to absorb photons below the bandgap of ZnTe (2.2 eV). Illumination with an AM 1.5G solar spectrum on the ZnTe:Cr IBSC generated a large short circuit current of 21.18 mA/cm2, an open circuit voltage of 0.48 V, and a fill factor of 0.58, yielding a power conversion efficiency (PCE) of 5.9%, the highest reported PCE in an IBSC based on impurity-doped ZnTe.
1. Introduction Intermediate band solar cells (IBSC) have recently attracted renewed interest as a potential approach to achieve high power conversion efficiency (PCE) [1–4]. An IBSC is composed of an IB material sandwiched between two ordinary n and p type semiconductors, which act as selective contacts to the conduction band (CB) and valence band (VB), respectively. In an IB material, sub-bandgap energy photons are absorbed through transitions from the VB to the IB and from the IB to the CB, which together produce the same amount of current as in conventional photons absorbed through the VB–CB transition [5,6]. The benefit of an IBSC is that no additional junction is needed to implement three band gaps in one device, as it is for the multiple junction devices. The theoretical PCE limit for IBSCs is predicted to be 63.2% with blackbody illumination and 65.1% with air mass (AM) 1.5 spectrums, comparable to the theoretical efficiency for optimized triplejunction solar cells with efficiencies of 63.8% and 67.0% under blackbody and AM1.5 illuminations, respectively [7,8]. Feasible approaches to fabricate IBSCs are classified as impurity doping, quantum dots, and highly mismatched alloys, and recent theoretical and experimental works have been attempted in order to dope impurities into semiconductor [9–12]. In our previous studies for oxygen and nitrogen-doped ZnTe (ZnTe:NO) thin films grown by pulsed laser deposition (PLD), we confirmed that the ZnTe:NO layer formed with nitrogen and oxygen
⁎
Corresponding author. E-mail address: [email protected] (E.K. Kim).
http://dx.doi.org/10.1016/j.solmat.2017.05.020 Received 21 January 2017; Received in revised form 30 March 2017; Accepted 10 May 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
plasma exhibits an enhancement of the absorption coefficient in the visible spectral region due to the formation of amorphous TeOx [13–15]. The ZnTe:NO solar cell was subsequently fabricated on the p+-GaAs substrate; however, the PCE value of the ZnTe:NO solar cell was lower than that of an undoped ZnTe solar cell because of the high resistivity of the ZnTe:NO thin film [16]. As an alternative, to reduce the high resistivity of the ZnTe:NO thin film, a Cr-doped ZnTe (ZnTe:Cr) thin film was grown by PLD. It is well known that, when the Zn is replaced by a transition metal, an impurity band, mainly made up of dlevels, can be created in the band gap, depending on the impurity characteristic and the host semiconductor [17]. Systematic studies, based on density functional methods, to find materials with an IB using II–VI host semiconductors have been presented [18,19]. However, the experimental realization of the IBSCs based on transition metal-doped II–VI remains to be achieved. In this work, we present an increased optical absorption coefficient of a ZnTe:Cr thin film in the photon energy below bang gap of ZnTe (2.2 eV), which suggests that the increased absorption coefficient can be attributed to the charge transfer between Cr2+ and Cr+ which acted as the IB in the ZnTe:Cr thin film. To realize an IBSC based on ZnTe:Cr, we fabricated ZnO:Al/ZnTe:Cr/Si IBSC (ZnTe:Cr IBSC) with a PCE of 5.9% using pulsed laser deposition (PLD), and the PCE value of the solar cell was 19 times higher than that of an ZnO:Al/undoped ZnTe/Si solar cell (undoped ZnTe SC).
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mittance spectra of the ZnTe:Cr and undoped ZnTe thin films were created by spectroscopy ellipsometry (SE). The PCE of ZnTe:Cr and undoped ZnTe solar cells were measured with a simulated solar light (100 mW cm−2, AM 1.5) provided by Newport Oriel ® Sol1A Class ABB Solar simulators under an argon atmosphere without encapsulation. Electrical data were recorded using a Keithley 2636A source-measure unit, and the intensity of the simulated solar light was calibrated using a standard Si photodiode detector, which was calibrated at the National Renewable Energy Laboratory. The aperture area of the ZnTe:Cr IBSC and undoped ZnTe SC was 75 mm2.
2. Experiment For deposition of the ZnTe:Cr thin film, a commercial ZnTe target with a Cr content of 4 wt% was used. The purity of the commercial target was 99.99%. P-type Si (100) (1–30 Ω cm) substrates were used for fabricating of ZnTe:Cr and undoped ZnTe based solar cells, and cplane sapphire (Al2O3) substrate was used for the measurement of transmittance for the thin films. Before the deposition of ZnTe:Cr and undoped ZnTe thin films on the p-Si substrate, the p-Si substrates were cleaned using chemical solutions of acetone, methanol, and distilled water for 5 min each in an ultrasonic cleaner. Next, the cleaned Si substrates were dipped in a buffered oxide etch for 1 min to remove native oxide on the p-Si substrate, followed by rinsing in deionized water for 1 min. Then, the Si substrates were transferred to the PLD growth chambers. The vacuum system was evacuated to 1×10−6 Torr, and the substrate temperature was maintained near 250 °C during deposition. A pulsed (10 Hz) Nd:YAG laser operating at a wavelength of 266 nm was used for ablating the ZnTe:Cr, undoped ZnTe, and ZnO:Al targets. The plasma plume was created by illuminating the focused laser pulse onto the target at an angle of 55° with respect to the direction normal to the target surface. We focused the laser spot on the surface of the target using an optical lens, and the focused spot size was approximately 1 mm2. The calculated laser power density illuminating the target was about 3 J/cm2. The structural properties of ZnTe:Cr and undoped ZnTe thin films grown on p-Si substrates were analyzed using θ–2θ scan X-ray diffraction (XRD) measurements with a Cu-Ka (1.5406 Å) X-ray source. To measure the electrical property of ZnTe:Cr, undoped ZnTe, and ZnO:Al thin films grown on a c-plane sapphire substrate, Hall measurements were conducted (see Table S1, Supporting information). The cross-sectional image of ZnTe:Cr IBSC was measured by a tunneling electron microscope (TEM). The compositional analysis of the solar cells was performed using scanning electron tunneling microscopeenergy dispersive X-ray spectroscopy (STEM-EDS). The optical trans-
3. Results and discussion 3.1. Structural and chemical analyses of Cr-doped ZnTe IBSC Fig. 1(a) shows a schematic diagram of our ZnTe:Cr IBSC fabricated using PLD. The thickness of the ZnO:Al and ZnTe:Cr was about 100 and 90 nm, respectively. For the ohmic contacts of ZnTe:Cr IBSC, Ti/Au (30/100 nm) finger grid patterns with a width of 100 µm and pitch of 400 µm were formed on the front surface of the ZnO:Al layer, and then the Ni/Au (30/100 nm) ohmic back contact was formed on the bottom of the p-Si side (see Fig. S1 and Table S2, Supporting information). The cross-sectional TEM image of ZnTe:Cr IBSC is shown in Fig. 1(b), and the thickness of the ZnTe:Cr and ZnO:Al layers was estimated to be 90 and 100 nm, respectively. The real image of ZnTe:Cr IBSC is shown in the inset of Fig. 1(b). To assess the crystal quality and structure of the ZnTe:Cr and undoped ZnTe thin films grown on the c-plane sapphire substrate, the XRD spectra of the thin films were measured in the θ–2θ scan range between 20° and 80°, as shown in Fig. 1(c). All of the vertical lines on the top x-axis are the JCPDS data for the bulk cubic ZnTe structure (JCPDS 01-0582), and the line lengths are proportional to intensity. The thin films show (111), (220), and (311) peaks, which indicated that the crystallinity of the thin films was polycrystalline. The position of the
Fig. 1. (a) Schematic diagram for ZnTe:Cr IBSC. (b) The cross-sectional TEM image and real device image of ZnTe:Cr IBSC. (c) XRD patterns of the ZnTe:Cr and undoped ZnTe thin films grown on c-plane sapphire substrate by PLD. (d) EDS elemental maps Zn, Te, and Cr in ZnTe:Cr layer obtained from the STEM-EDS image.
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ZnTe layers were calculated from ε through the following equation:
(111) main peak in ZnTe:Cr and undoped ZnTe thin films was 25.19° and 25.24°, respectively. The calculated lattice constants of ZnTe:Cr and undoped ZnTe were 6.12 Å and 6.11 Å, respectively. The full width at half maximum (FWHM) of the ZnTe:Cr and undoped ZnTe thin films was 0.44° and 0.43°, respectively. Expecially, the peak positions of (111), (220), and (311) in the ZnTe:Cr film are observed to fit very well with the JCPDS data. Therefore, it was concluded that there was no degradation of the crystal quality of the ZnTe:Cr thin film by doping of Cr into ZnTe, and the structure of thin films showed cubic structure. To analyze the elemental distribution in the ZnTe:Cr layer, we performed an element analysis of ZnTe:Cr IBSC by STEM-EDS. The representative square regions of ZnTe:Cr layer was determined from the STEM image of ZnTe:Cr IBSC, as shown in Fig. 1(d). The STEM–EDS mapping corresponding to elemental Cr in the ZnTe:Cr layer indicated that the Cr was uniformly distributed in ZnTe:Cr layer, and the Cr content in the ZnTe:Cr layer was 3.5 at%. It is well known that Cr atoms can be substituted for the Zn atoms in ZnTe, and the formed Cr2+ ions act as an electron trap and producing acceptor, so that ZnTe:Cr shows ptype conductivity [20]. We also confirmed that the ZnTe:Cr thin film showed p-type conductivity with doping concentration of 1.2 × 1014 cm−3 from Hall measurement.
⎛ (ε 2 + ε 2 )1/2 − ε1 ⎞1/2 2 α = (4π/λ) × ⎜ 1 ⎟ 2 ⎠ ⎝
(1)
where ε1 and ε2 are the real and imaginary parts of the dielectric constant of the ZnTe:Cr and undoped ZnTe layer, respectively, and λ is wavelength. It is known that the lowest direct absorption edges of ZnTe at 300 K are 2.28 eV (Eo) and 3.28 eV (Eo + Δo), and the E1 and E1 + Δ1 transitions occur at 3.78 and 4.34, respectively, where Eo and Eo + Δo are the lowest-direct-band-gap energies, and E1 and E1 + Δ1 are the higher inter-band transition energies [21]. The absorption coefficient values of the ZnTe:Cr and undoped ZnTe layer in the figure start to steeply increase above the Eo, Eo + Δo, E1, and E1 + Δ1. We see from the figure that, below the Eo, the absorption coefficient of the ZnTe:Cr layer was about 6 times higher than that of the undoped ZnTe layer. Using a Tauc plot, the optical band gaps of the ZnTe:Cr and undoped ZnTe layer were calculated to be about 2.14 and 2.17 eV, respectively (see Fig. S2, Supporting information). To determine the optimal growth temperature condition for the n-type emitter of ZnTe:Cr IBSC, the effect of substrate temperature on transmittance and absorption coefficient of ZnO:Al thin films was studied, and it was found that the optimal growth temperature of ZnO:Al thin film was 250 °C (see Fig. S3, Supporting information). In the ZnTe:Cr layer, Cr enters cation sites, and two of its electrons are donated to the bonds, thus forming a deep acceptor level Cr2+(d4). Under an optical excitation above 2.2 eV, two types of charge-transfer processes are known to occur, described by the following two process:
3.2. Optical and electrical characterization of ZnTe:Cr-based IBSC To confirm the enhanced absorption coefficient of the ZnTe:Cr layer by the effect of the IB in the film, the absorption coefficients of the undoped ZnTe and ZnTe:Cr layer were determined using SE. Fig. 2(a) shows the complex dielectric function (ε = ε1 + iε2) of the ZnTe:Cr and undoped ZnTe layer. From the figure, we can clearly see that Cr-doping led to an increase in ε2 for photon energies up to 3.34 eV. The absorption coefficients for the ZnTe:Cr and the undoped ZnTe layer for the photon energy range from 1.2 to 5.2 eV are shown in Fig. 2(b). The absorption coefficients (α) of the ZnTe:Cr and undoped
Cr 2+ + hv → Cr1+ + hVB
(2)
Cr1+ + hv → Cr 2+ + e CB
(3)
where hVB and eCB denote a hole in the valence band and an electron in the conduction band, respectively. The first and second processes are
Fig. 2. (a) The real (ε1) and the imaginary (ε2) parts of the dielectric function for ZnTe:Cr and undoped ZnTe layers determined with SE. (b) Absorption coefficients for ZnTe:Cr and undoped ZnTe layer as a function of photon energy (c) Schematic energy level diagram of the ZnTe:Cr IBSC. (d) Reflectance spectra of the ZnTe:Cr IBSC and undoped ZnTe SC as a function of photon energy. The AM1.5G solar spectrum is drawn as the background.
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undoped ZnTe SC, which means that the ZnTe:Cr IBSC absorbed more wide range photons from visible to near infrared region. However, in the high photon energy ranges from 2.2 to 3.5 eV the reflectance of the undoped ZnTe SC was higher than that of the ZnTe:Cr IBSC. This opposite behavior of optical reflectance in the region of high photon energy may be two layer effect of ZnO:Al and ZnTe:Cr layers on the optical reflectance of the IBSC sample. This result is well consistent with the simulated optical reflectance in Fig. 4(b). Since there was no transmission through the thick Si substrates or photons, which were not reflected and were all absorbed, the light trapping ratio (η) of the solar cells can be calculated using the following Eq. (4): 1100nm
η=
∫300nm (1 − R(λ))(IAM1.5G (λ))/(hc/λ) d λ 1100nm
∫300nm (IAM1.5G (λ))/(hc/λ) d λ
(4)
where R(λ) is the optical reflectance, λ is the wavelength, AM1.5G(λ) is the AM 1.5G solar spectrum, h is the Planck constant, and c is the speed of light. The calculated light trapping ratio of the ZnTe:Cr IBSC and undoped ZnTe SC was about 88% and 82%, respectively. Based on the light absorption profile (1-R(λ)) of the solar cells and assuming one electron–hole pair per every absorbed photon and no recombination (100% internal quantum efficiency), we calculated an upper limit of the short circuit current density (Jup.sc) using the following Eq. (5): 1100nm
Jup.sc = q
∫300nm
(1 − R(λ))(IAM1.5G (λ))/(hc/λ) d λ
(5)
The upper limit of Jup.sc of ZnTe:Cr IBSC was about 38.40 mA/cm2, and the value of Jup.sc was 1.8 times higher than that of the measured Jsc, which indicated that there was still much room for improvement of the PCE of ZnTe:Cr IBSC. 3.3. Performance and electrical characterization of ZnTe:Cr-based IBSC Fig. 3(a) shows the current density–voltage (J–V) characteristics of the ZnTe:Cr IBSC and undoped ZnTe SC under AM 1.5 illumination. The photovoltaic parameters of ZnTe:Cr IBSC and unodoped ZnTe SC are shown in Table 1. The ZnTe:Cr IBSC had an PCE of 5.9%, which is 19 times higher than the PCE of the undoped ZnTe SC. The PCE of ZnTe:Cr IBSC is currently the best value in the IBSC based on II–VI, and the performance of the solar cell is comparable to those of the highperformance CdTe and Cu2ZnSn(S, Se)4 thin film solar cells [24,25]. Because, as shown in the optical reflectance spectra of Fig. 2(d), the ZnTe:Cr IBSC absorbed more light in the region of VNIR, the Jsc of the ZnTe:Cr IBSC was 5.8 times higher than that of the undoped ZnTe SC. To analyze the J–V characteristics of the ZnTe:Cr IBSC and undoped ZnTe SC under AM1.5 illumination, a simple equivalent circuit model including a diode, a series resistance (Rs), and a shunt resistance (Rsh) was used according to the following analytical Eq. (6):
Fig. 3. (a) J-V curve for the ZnTe:Cr IBSC and undoped ZnTe SC under AM 1.5 illumination, (b) EQE, LHE, and APCE spectra of ZnTe:Cr IBSC and undoped ZnTe SC as a function of photon energy.
the photo-generation and photoionization transition, respectively. The charge transport is known to be easily induced in Cr-doped II–VI compound semiconductors [22]. Suto et al. reported that the positions of Cr2+ and Cr1+ states in the ZnTe:Cr layer are 0.85 and 1.3 eV beneath the edge of the CB, respectively [23]. Therefore, the increased optical absorption coefficients of the ZnTe:Cr layer in the photon energy range below ZnTe band gap (2.2 eV) resulted from the charge transfer between Cr2+ and Cr+, indicating that the sub-band gap absorption of the ZnTe:Cr layer increased more than that of the undoped ZnTe. To describe a charge transfer process that increases the absorption coefficient of ZnTe:Cr, the schematic band diagram of ZnTe:Cr IBSC is presented in Fig. 2(c). The optical reflectance spectra of ZnTe:Cr IBSC and undoped ZnTe SC are shown in Fig. 2(d). The optical reflectance values of the ZnTe:Cr IBSC in the ranges from 1.1 to 2.2 eV was lower than that of the
⎧ ⎡ q ⎤ ⎫ V − JAR s J = JL − J0 ⎨exp ⎢ (V + JAR s) ⎥ −1⎬ − ⎣ nkT ⎦ ⎭ ⎩ AR sh
(6)
where k is the Boltzmann constant, T is the absolute temperature, Jo is the reverse saturation current density, n is the diode ideality factor, A is the effective area of the solar cell, and JL is photogenerated current density. The ideality factor of p-n junction diode is determined under forward bias and is normally found to be between 1 and 2 depending on the current flow mechanism of recombination and diffusion [26]. That is, when diffusion current is dominant, the value of ideality factor
Table 1 Photovoltaic parameters of ZnTe:Cr IBSC and undoped ZnTe SC under AM 1.5G illumination. Solar cell architecture
Jsc (mA cm−2)
Voc (v)
FF
PCE (%)
nlight
Jo,light (mA cm−2)
Rs (Ω)
Rsh (Ω)
Undoped ZnTe SC ZnTe:Cr IBSC
3.68 21.18
0.38 0.48
0.32 0.58
0.30 5.90
4.30 1.75
0.08 6.17 × 10−4
77.36 3.52
159.84 814.32
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Fig. 4. (a) The average reflectance of ZnTe:Cr IBSC with MgF2 ARC as a function of MgF2 thickness (b) The reflectance spectra of ZnTe:Cr IBSC with and without 90-nm-thick MgF2 ARC. (c) Comparison of the calculated EQE of ZnTe:Cr IBSC with 90-nm-thick MgF2 ARC and the measured EQE of the ZnTe:Cr IBSC without 90-nm-thick MgF2 ARC. (d) The integrated Jsc of ZnTe:Cr IBSC with and without 90-nm-thick MgF2 ARC.
Fig. S4 verified that the enhanced photovoltaic response of ZnTe:Cr IBSC result from the absorption of sub-bandgap photons through the IB. Therefore, we suggest that the charge transfer acts as the IB to absorb photons below the bandgap of ZnTe, so that the PCE of ZnTe:Cr IBSC was 19 times higher than that of the undoped ZnTe:Cr SC. To improve the performance of the ZnTe:Cr IBSC, an MgF2 antireflection coating (ARC) was deposited on the top of the SC. Before depositing the MgF2 ARC on the solar cell, the transfer matrix method was used to optimize the thickness of the MgF2 ARC. Fig. 4(a) shows the average reflectance value of the ZnTe:Cr IBSC with the MgF2 ARC as a function of MgF2 thickness. The minimum reflectance value of MgF2 for ZnTe:Cr SC was 3.3% at 90 nm. Fig. 4(b) shows the calculated reflectance of the ZnTe:Cr IBSC with and without the 90-nm-thick MgF2. In the high photon energy ranges from 2.2 to 3.5 eV, the calculated reflectance of the ZnTe:Cr IBSC with the 90-nm-thick MgF2 ARC showed a lower reflectance value than the measured reflectance of the ZnTe:Cr IBSC without the MgF2 ARC. In Fig. 4(c), the calculated EQE of the ZnTe:Cr IBSC with the 90-nm-thick MgF2 ARC was more enhanced as a whole than the measured EQE of the ZnTe:Cr IBSC without the 90-nm-thick MgF2 ARC. Using the EQE of the ZnTe:Cr IBSC with and without the 90-nm-thick MgF2 ARC, the values of integrated Jsc were obtained by the following equation:
becomes closer to 1, while it becomes closer to 2 when recombination current is dominant. As shown in Table 1, the value of ideality factor of ZnTe:Cr IBSC is 1.75, which shows that recombination current is more dominant than diffusion current. However, the ideality factor of undoped ZnTe SC is 4.3, which is not between 1 and 2. Because the high ideality factor of undoped ZnTe SC cannot be explained by these mechanisms, further study should be needed to interpret the high ideality factor of undoped ZnTe SC. Fig. 3(b) shows the EQE spectra of the ZnTe:Cr IBSC and undoped ZnTe SC. The EQE values of ZnTe:Cr IBSC above 3.0 eV are relatively lower than those of the energies below 2.2 eV. The low EQE values of ZnTe:Cr IBSC above 3.0 eV can be considered as a result of the front surface recombination or the interface recombination between the ZnO:Al and ZnTe:Cr layer [27], considering shorter penetration depth of high energy photon. However, the EQE values of ZnTe:Cr IBSC below the band gap of ZnTe (2.2 eV) are more enhanced than those of undoped ZnTe SC, and detected for energies down to 1.0 eV. Especially, the EQE values of ZnTe:Cr IBSC and undoped ZnTe at 1.5 eV are 67% and 2%, respectively. By using of EQE data for the samples, the light harvesting efficiency (LHE) and the absorbed photon-to-current conversion efficiency (APCE) were estimated based on LHE = (1−R) × (1−10−A) and APCE = EQE × LHE, where A and R represent absorbance and reflectance, respectively [28]. In the photon energy region from 1.1 to 2.2 eV, the values of LHE and APCE for the ZnTe:Cr IBSC are higher than those of undoped ZnTe SC, which was attributed to the charge transfer between Cr2+ to Cr+ generated in the ZnTe:Cr layer as well as the band to band (2.2 eV) absorption. In addition, the two-photon experiments for the samples in
1032nm
Jint.sc = q
∫324nm
(IAM1.5G (λ) × EQE(λ))/(hc/λ) d λ
(7)
In Fig. 4(d), the Jint.sc of ZnTe:Cr IBSC with and without 90-nm-thick MgF2 was calculated to be 24.4 mA/cm2 and 22.7 mA/cm2, respectively. Therefore, we confirmed, using the 90-nm-thick MgF2 ARC, that 31
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the Jsc of ZnTe:Cr IBSC with 90-nm-thick MgF2 can be increased up to 24.4 mA/cm2. Here, the low contribution of MgF2 ARC to Jsc can be interpreted as the dual role of the ZnO:Al thin film played by n-type emitter and ARC for ZnTe:Cr IBSC, because the reflective index of ZnO:Al thin film is in the middle range between of air and the ZnTe:Cr thin film. 4. Conclusion In conclusion, using the PLD method, we fabricated a ZnTe:Cr IBSC with PCE of 5.9%. The structure of ZnTe:Cr grown on p-Si (100) was a cubic structure, Cr was uniformly distributed in the ZnTe:Cr layer, and the atomic concentration of Cr was 3.5 at%. The ZnTe:Cr layer shows ptype electrical conductivity, and the carrier concentration of the layer was 1.2ⅹ1014 cm−3. In the analysis of SE, the ZnTe:Cr layer had a higher absorption coefficient than the undoped ZnTe layer below 2.2 eV. The experimental results led us to suggest that the enhanced absorption coefficients of the ZnTe:Cr layer resulted from the charge transfer between Cr2+ and Cr+, which plays the role of IB. The analysis of the APCE for the solar cells showed that, in the region of VNIR, the ZnTe:Cr IBSC could absorb more photon energy than the undoped ZnTe SC. Therefore, these results suggest the possibility of achieving high PCE of an IBSC based on the ZnTe:Cr IBSC. Acknowledgments This work was supported in part by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT, & Future Planning (MSIP) (NRF-2014R1A2A1A11053936), and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Trade, Industry, & Energy (MOTIE) of the Republic of Korea (No. 20163030013380). Kyoung Su Lee would like to express thanks to Dr. Juwon Lee for discussion of PLD growth. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2017.05.020. References [1] A. Luque, A. Martí, C. Stanley, Understanding intermediate-band solar cells, Nat. Photonics 6 (2012) 146–152. [2] M.J. Keevers, M.A. Green, Efficiency improvements of silicon solar cells by the impurity photovoltaic effect, J. Appl. Phys. 75 (1994) 4022–4031. [3] A. Luque, A. Martí, E. Antolín, C. Tablero, Intermediate bands versus levels in non-
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
High power conversion efficiency of intermediate band photovoltaic solar cell based on Cr-doped ZnTe Kyoung Su Lee, Gyujin Oh, Dongil Chu, Sang Woo Pak, Eun Kyu Kim
MARK
⁎
Quantum-Function Research Laboratory and Department of Physics, Hanyang University, Seoul 04763, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Intermediate band solar cell ZnTe:Cr Pulsed laser deposition
We report on a high-performance intermediate band solar cell (IBSC) based on Cr-doped ZnTe (ZnTe:Cr) fabricated using a pulsed laser deposition (PLD) method. Chromium (Cr) was uniformly distributed in the ZnTe:Cr thin film with an atomic concentration of about 3.5%, and the ZnTe:Cr thin film showed p-type electrical conductivity. The ZnTe:Cr thin film had higher absorption coefficients than those of undoped ZnTe films in the photon energy range below band gap of ZnTe (2.2 eV). The enhanced absorption coefficients of the ZnTe:Cr thin film were attributed to the photoionization energy between Cr2+ and Cr+ (Cr2+ ⇌ Cr+), which acted as the IB to absorb photons below the bandgap of ZnTe (2.2 eV). Illumination with an AM 1.5G solar spectrum on the ZnTe:Cr IBSC generated a large short circuit current of 21.18 mA/cm2, an open circuit voltage of 0.48 V, and a fill factor of 0.58, yielding a power conversion efficiency (PCE) of 5.9%, the highest reported PCE in an IBSC based on impurity-doped ZnTe.
1. Introduction Intermediate band solar cells (IBSC) have recently attracted renewed interest as a potential approach to achieve high power conversion efficiency (PCE) [1–4]. An IBSC is composed of an IB material sandwiched between two ordinary n and p type semiconductors, which act as selective contacts to the conduction band (CB) and valence band (VB), respectively. In an IB material, sub-bandgap energy photons are absorbed through transitions from the VB to the IB and from the IB to the CB, which together produce the same amount of current as in conventional photons absorbed through the VB–CB transition [5,6]. The benefit of an IBSC is that no additional junction is needed to implement three band gaps in one device, as it is for the multiple junction devices. The theoretical PCE limit for IBSCs is predicted to be 63.2% with blackbody illumination and 65.1% with air mass (AM) 1.5 spectrums, comparable to the theoretical efficiency for optimized triplejunction solar cells with efficiencies of 63.8% and 67.0% under blackbody and AM1.5 illuminations, respectively [7,8]. Feasible approaches to fabricate IBSCs are classified as impurity doping, quantum dots, and highly mismatched alloys, and recent theoretical and experimental works have been attempted in order to dope impurities into semiconductor [9–12]. In our previous studies for oxygen and nitrogen-doped ZnTe (ZnTe:NO) thin films grown by pulsed laser deposition (PLD), we confirmed that the ZnTe:NO layer formed with nitrogen and oxygen
⁎
Corresponding author. E-mail address: [email protected] (E.K. Kim).
http://dx.doi.org/10.1016/j.solmat.2017.05.020 Received 21 January 2017; Received in revised form 30 March 2017; Accepted 10 May 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
plasma exhibits an enhancement of the absorption coefficient in the visible spectral region due to the formation of amorphous TeOx [13–15]. The ZnTe:NO solar cell was subsequently fabricated on the p+-GaAs substrate; however, the PCE value of the ZnTe:NO solar cell was lower than that of an undoped ZnTe solar cell because of the high resistivity of the ZnTe:NO thin film [16]. As an alternative, to reduce the high resistivity of the ZnTe:NO thin film, a Cr-doped ZnTe (ZnTe:Cr) thin film was grown by PLD. It is well known that, when the Zn is replaced by a transition metal, an impurity band, mainly made up of dlevels, can be created in the band gap, depending on the impurity characteristic and the host semiconductor [17]. Systematic studies, based on density functional methods, to find materials with an IB using II–VI host semiconductors have been presented [18,19]. However, the experimental realization of the IBSCs based on transition metal-doped II–VI remains to be achieved. In this work, we present an increased optical absorption coefficient of a ZnTe:Cr thin film in the photon energy below bang gap of ZnTe (2.2 eV), which suggests that the increased absorption coefficient can be attributed to the charge transfer between Cr2+ and Cr+ which acted as the IB in the ZnTe:Cr thin film. To realize an IBSC based on ZnTe:Cr, we fabricated ZnO:Al/ZnTe:Cr/Si IBSC (ZnTe:Cr IBSC) with a PCE of 5.9% using pulsed laser deposition (PLD), and the PCE value of the solar cell was 19 times higher than that of an ZnO:Al/undoped ZnTe/Si solar cell (undoped ZnTe SC).
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mittance spectra of the ZnTe:Cr and undoped ZnTe thin films were created by spectroscopy ellipsometry (SE). The PCE of ZnTe:Cr and undoped ZnTe solar cells were measured with a simulated solar light (100 mW cm−2, AM 1.5) provided by Newport Oriel ® Sol1A Class ABB Solar simulators under an argon atmosphere without encapsulation. Electrical data were recorded using a Keithley 2636A source-measure unit, and the intensity of the simulated solar light was calibrated using a standard Si photodiode detector, which was calibrated at the National Renewable Energy Laboratory. The aperture area of the ZnTe:Cr IBSC and undoped ZnTe SC was 75 mm2.
2. Experiment For deposition of the ZnTe:Cr thin film, a commercial ZnTe target with a Cr content of 4 wt% was used. The purity of the commercial target was 99.99%. P-type Si (100) (1–30 Ω cm) substrates were used for fabricating of ZnTe:Cr and undoped ZnTe based solar cells, and cplane sapphire (Al2O3) substrate was used for the measurement of transmittance for the thin films. Before the deposition of ZnTe:Cr and undoped ZnTe thin films on the p-Si substrate, the p-Si substrates were cleaned using chemical solutions of acetone, methanol, and distilled water for 5 min each in an ultrasonic cleaner. Next, the cleaned Si substrates were dipped in a buffered oxide etch for 1 min to remove native oxide on the p-Si substrate, followed by rinsing in deionized water for 1 min. Then, the Si substrates were transferred to the PLD growth chambers. The vacuum system was evacuated to 1×10−6 Torr, and the substrate temperature was maintained near 250 °C during deposition. A pulsed (10 Hz) Nd:YAG laser operating at a wavelength of 266 nm was used for ablating the ZnTe:Cr, undoped ZnTe, and ZnO:Al targets. The plasma plume was created by illuminating the focused laser pulse onto the target at an angle of 55° with respect to the direction normal to the target surface. We focused the laser spot on the surface of the target using an optical lens, and the focused spot size was approximately 1 mm2. The calculated laser power density illuminating the target was about 3 J/cm2. The structural properties of ZnTe:Cr and undoped ZnTe thin films grown on p-Si substrates were analyzed using θ–2θ scan X-ray diffraction (XRD) measurements with a Cu-Ka (1.5406 Å) X-ray source. To measure the electrical property of ZnTe:Cr, undoped ZnTe, and ZnO:Al thin films grown on a c-plane sapphire substrate, Hall measurements were conducted (see Table S1, Supporting information). The cross-sectional image of ZnTe:Cr IBSC was measured by a tunneling electron microscope (TEM). The compositional analysis of the solar cells was performed using scanning electron tunneling microscopeenergy dispersive X-ray spectroscopy (STEM-EDS). The optical trans-
3. Results and discussion 3.1. Structural and chemical analyses of Cr-doped ZnTe IBSC Fig. 1(a) shows a schematic diagram of our ZnTe:Cr IBSC fabricated using PLD. The thickness of the ZnO:Al and ZnTe:Cr was about 100 and 90 nm, respectively. For the ohmic contacts of ZnTe:Cr IBSC, Ti/Au (30/100 nm) finger grid patterns with a width of 100 µm and pitch of 400 µm were formed on the front surface of the ZnO:Al layer, and then the Ni/Au (30/100 nm) ohmic back contact was formed on the bottom of the p-Si side (see Fig. S1 and Table S2, Supporting information). The cross-sectional TEM image of ZnTe:Cr IBSC is shown in Fig. 1(b), and the thickness of the ZnTe:Cr and ZnO:Al layers was estimated to be 90 and 100 nm, respectively. The real image of ZnTe:Cr IBSC is shown in the inset of Fig. 1(b). To assess the crystal quality and structure of the ZnTe:Cr and undoped ZnTe thin films grown on the c-plane sapphire substrate, the XRD spectra of the thin films were measured in the θ–2θ scan range between 20° and 80°, as shown in Fig. 1(c). All of the vertical lines on the top x-axis are the JCPDS data for the bulk cubic ZnTe structure (JCPDS 01-0582), and the line lengths are proportional to intensity. The thin films show (111), (220), and (311) peaks, which indicated that the crystallinity of the thin films was polycrystalline. The position of the
Fig. 1. (a) Schematic diagram for ZnTe:Cr IBSC. (b) The cross-sectional TEM image and real device image of ZnTe:Cr IBSC. (c) XRD patterns of the ZnTe:Cr and undoped ZnTe thin films grown on c-plane sapphire substrate by PLD. (d) EDS elemental maps Zn, Te, and Cr in ZnTe:Cr layer obtained from the STEM-EDS image.
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ZnTe layers were calculated from ε through the following equation:
(111) main peak in ZnTe:Cr and undoped ZnTe thin films was 25.19° and 25.24°, respectively. The calculated lattice constants of ZnTe:Cr and undoped ZnTe were 6.12 Å and 6.11 Å, respectively. The full width at half maximum (FWHM) of the ZnTe:Cr and undoped ZnTe thin films was 0.44° and 0.43°, respectively. Expecially, the peak positions of (111), (220), and (311) in the ZnTe:Cr film are observed to fit very well with the JCPDS data. Therefore, it was concluded that there was no degradation of the crystal quality of the ZnTe:Cr thin film by doping of Cr into ZnTe, and the structure of thin films showed cubic structure. To analyze the elemental distribution in the ZnTe:Cr layer, we performed an element analysis of ZnTe:Cr IBSC by STEM-EDS. The representative square regions of ZnTe:Cr layer was determined from the STEM image of ZnTe:Cr IBSC, as shown in Fig. 1(d). The STEM–EDS mapping corresponding to elemental Cr in the ZnTe:Cr layer indicated that the Cr was uniformly distributed in ZnTe:Cr layer, and the Cr content in the ZnTe:Cr layer was 3.5 at%. It is well known that Cr atoms can be substituted for the Zn atoms in ZnTe, and the formed Cr2+ ions act as an electron trap and producing acceptor, so that ZnTe:Cr shows ptype conductivity [20]. We also confirmed that the ZnTe:Cr thin film showed p-type conductivity with doping concentration of 1.2 × 1014 cm−3 from Hall measurement.
⎛ (ε 2 + ε 2 )1/2 − ε1 ⎞1/2 2 α = (4π/λ) × ⎜ 1 ⎟ 2 ⎠ ⎝
(1)
where ε1 and ε2 are the real and imaginary parts of the dielectric constant of the ZnTe:Cr and undoped ZnTe layer, respectively, and λ is wavelength. It is known that the lowest direct absorption edges of ZnTe at 300 K are 2.28 eV (Eo) and 3.28 eV (Eo + Δo), and the E1 and E1 + Δ1 transitions occur at 3.78 and 4.34, respectively, where Eo and Eo + Δo are the lowest-direct-band-gap energies, and E1 and E1 + Δ1 are the higher inter-band transition energies [21]. The absorption coefficient values of the ZnTe:Cr and undoped ZnTe layer in the figure start to steeply increase above the Eo, Eo + Δo, E1, and E1 + Δ1. We see from the figure that, below the Eo, the absorption coefficient of the ZnTe:Cr layer was about 6 times higher than that of the undoped ZnTe layer. Using a Tauc plot, the optical band gaps of the ZnTe:Cr and undoped ZnTe layer were calculated to be about 2.14 and 2.17 eV, respectively (see Fig. S2, Supporting information). To determine the optimal growth temperature condition for the n-type emitter of ZnTe:Cr IBSC, the effect of substrate temperature on transmittance and absorption coefficient of ZnO:Al thin films was studied, and it was found that the optimal growth temperature of ZnO:Al thin film was 250 °C (see Fig. S3, Supporting information). In the ZnTe:Cr layer, Cr enters cation sites, and two of its electrons are donated to the bonds, thus forming a deep acceptor level Cr2+(d4). Under an optical excitation above 2.2 eV, two types of charge-transfer processes are known to occur, described by the following two process:
3.2. Optical and electrical characterization of ZnTe:Cr-based IBSC To confirm the enhanced absorption coefficient of the ZnTe:Cr layer by the effect of the IB in the film, the absorption coefficients of the undoped ZnTe and ZnTe:Cr layer were determined using SE. Fig. 2(a) shows the complex dielectric function (ε = ε1 + iε2) of the ZnTe:Cr and undoped ZnTe layer. From the figure, we can clearly see that Cr-doping led to an increase in ε2 for photon energies up to 3.34 eV. The absorption coefficients for the ZnTe:Cr and the undoped ZnTe layer for the photon energy range from 1.2 to 5.2 eV are shown in Fig. 2(b). The absorption coefficients (α) of the ZnTe:Cr and undoped
Cr 2+ + hv → Cr1+ + hVB
(2)
Cr1+ + hv → Cr 2+ + e CB
(3)
where hVB and eCB denote a hole in the valence band and an electron in the conduction band, respectively. The first and second processes are
Fig. 2. (a) The real (ε1) and the imaginary (ε2) parts of the dielectric function for ZnTe:Cr and undoped ZnTe layers determined with SE. (b) Absorption coefficients for ZnTe:Cr and undoped ZnTe layer as a function of photon energy (c) Schematic energy level diagram of the ZnTe:Cr IBSC. (d) Reflectance spectra of the ZnTe:Cr IBSC and undoped ZnTe SC as a function of photon energy. The AM1.5G solar spectrum is drawn as the background.
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undoped ZnTe SC, which means that the ZnTe:Cr IBSC absorbed more wide range photons from visible to near infrared region. However, in the high photon energy ranges from 2.2 to 3.5 eV the reflectance of the undoped ZnTe SC was higher than that of the ZnTe:Cr IBSC. This opposite behavior of optical reflectance in the region of high photon energy may be two layer effect of ZnO:Al and ZnTe:Cr layers on the optical reflectance of the IBSC sample. This result is well consistent with the simulated optical reflectance in Fig. 4(b). Since there was no transmission through the thick Si substrates or photons, which were not reflected and were all absorbed, the light trapping ratio (η) of the solar cells can be calculated using the following Eq. (4): 1100nm
η=
∫300nm (1 − R(λ))(IAM1.5G (λ))/(hc/λ) d λ 1100nm
∫300nm (IAM1.5G (λ))/(hc/λ) d λ
(4)
where R(λ) is the optical reflectance, λ is the wavelength, AM1.5G(λ) is the AM 1.5G solar spectrum, h is the Planck constant, and c is the speed of light. The calculated light trapping ratio of the ZnTe:Cr IBSC and undoped ZnTe SC was about 88% and 82%, respectively. Based on the light absorption profile (1-R(λ)) of the solar cells and assuming one electron–hole pair per every absorbed photon and no recombination (100% internal quantum efficiency), we calculated an upper limit of the short circuit current density (Jup.sc) using the following Eq. (5): 1100nm
Jup.sc = q
∫300nm
(1 − R(λ))(IAM1.5G (λ))/(hc/λ) d λ
(5)
The upper limit of Jup.sc of ZnTe:Cr IBSC was about 38.40 mA/cm2, and the value of Jup.sc was 1.8 times higher than that of the measured Jsc, which indicated that there was still much room for improvement of the PCE of ZnTe:Cr IBSC. 3.3. Performance and electrical characterization of ZnTe:Cr-based IBSC Fig. 3(a) shows the current density–voltage (J–V) characteristics of the ZnTe:Cr IBSC and undoped ZnTe SC under AM 1.5 illumination. The photovoltaic parameters of ZnTe:Cr IBSC and unodoped ZnTe SC are shown in Table 1. The ZnTe:Cr IBSC had an PCE of 5.9%, which is 19 times higher than the PCE of the undoped ZnTe SC. The PCE of ZnTe:Cr IBSC is currently the best value in the IBSC based on II–VI, and the performance of the solar cell is comparable to those of the highperformance CdTe and Cu2ZnSn(S, Se)4 thin film solar cells [24,25]. Because, as shown in the optical reflectance spectra of Fig. 2(d), the ZnTe:Cr IBSC absorbed more light in the region of VNIR, the Jsc of the ZnTe:Cr IBSC was 5.8 times higher than that of the undoped ZnTe SC. To analyze the J–V characteristics of the ZnTe:Cr IBSC and undoped ZnTe SC under AM1.5 illumination, a simple equivalent circuit model including a diode, a series resistance (Rs), and a shunt resistance (Rsh) was used according to the following analytical Eq. (6):
Fig. 3. (a) J-V curve for the ZnTe:Cr IBSC and undoped ZnTe SC under AM 1.5 illumination, (b) EQE, LHE, and APCE spectra of ZnTe:Cr IBSC and undoped ZnTe SC as a function of photon energy.
the photo-generation and photoionization transition, respectively. The charge transport is known to be easily induced in Cr-doped II–VI compound semiconductors [22]. Suto et al. reported that the positions of Cr2+ and Cr1+ states in the ZnTe:Cr layer are 0.85 and 1.3 eV beneath the edge of the CB, respectively [23]. Therefore, the increased optical absorption coefficients of the ZnTe:Cr layer in the photon energy range below ZnTe band gap (2.2 eV) resulted from the charge transfer between Cr2+ and Cr+, indicating that the sub-band gap absorption of the ZnTe:Cr layer increased more than that of the undoped ZnTe. To describe a charge transfer process that increases the absorption coefficient of ZnTe:Cr, the schematic band diagram of ZnTe:Cr IBSC is presented in Fig. 2(c). The optical reflectance spectra of ZnTe:Cr IBSC and undoped ZnTe SC are shown in Fig. 2(d). The optical reflectance values of the ZnTe:Cr IBSC in the ranges from 1.1 to 2.2 eV was lower than that of the
⎧ ⎡ q ⎤ ⎫ V − JAR s J = JL − J0 ⎨exp ⎢ (V + JAR s) ⎥ −1⎬ − ⎣ nkT ⎦ ⎭ ⎩ AR sh
(6)
where k is the Boltzmann constant, T is the absolute temperature, Jo is the reverse saturation current density, n is the diode ideality factor, A is the effective area of the solar cell, and JL is photogenerated current density. The ideality factor of p-n junction diode is determined under forward bias and is normally found to be between 1 and 2 depending on the current flow mechanism of recombination and diffusion [26]. That is, when diffusion current is dominant, the value of ideality factor
Table 1 Photovoltaic parameters of ZnTe:Cr IBSC and undoped ZnTe SC under AM 1.5G illumination. Solar cell architecture
Jsc (mA cm−2)
Voc (v)
FF
PCE (%)
nlight
Jo,light (mA cm−2)
Rs (Ω)
Rsh (Ω)
Undoped ZnTe SC ZnTe:Cr IBSC
3.68 21.18
0.38 0.48
0.32 0.58
0.30 5.90
4.30 1.75
0.08 6.17 × 10−4
77.36 3.52
159.84 814.32
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Fig. 4. (a) The average reflectance of ZnTe:Cr IBSC with MgF2 ARC as a function of MgF2 thickness (b) The reflectance spectra of ZnTe:Cr IBSC with and without 90-nm-thick MgF2 ARC. (c) Comparison of the calculated EQE of ZnTe:Cr IBSC with 90-nm-thick MgF2 ARC and the measured EQE of the ZnTe:Cr IBSC without 90-nm-thick MgF2 ARC. (d) The integrated Jsc of ZnTe:Cr IBSC with and without 90-nm-thick MgF2 ARC.
Fig. S4 verified that the enhanced photovoltaic response of ZnTe:Cr IBSC result from the absorption of sub-bandgap photons through the IB. Therefore, we suggest that the charge transfer acts as the IB to absorb photons below the bandgap of ZnTe, so that the PCE of ZnTe:Cr IBSC was 19 times higher than that of the undoped ZnTe:Cr SC. To improve the performance of the ZnTe:Cr IBSC, an MgF2 antireflection coating (ARC) was deposited on the top of the SC. Before depositing the MgF2 ARC on the solar cell, the transfer matrix method was used to optimize the thickness of the MgF2 ARC. Fig. 4(a) shows the average reflectance value of the ZnTe:Cr IBSC with the MgF2 ARC as a function of MgF2 thickness. The minimum reflectance value of MgF2 for ZnTe:Cr SC was 3.3% at 90 nm. Fig. 4(b) shows the calculated reflectance of the ZnTe:Cr IBSC with and without the 90-nm-thick MgF2. In the high photon energy ranges from 2.2 to 3.5 eV, the calculated reflectance of the ZnTe:Cr IBSC with the 90-nm-thick MgF2 ARC showed a lower reflectance value than the measured reflectance of the ZnTe:Cr IBSC without the MgF2 ARC. In Fig. 4(c), the calculated EQE of the ZnTe:Cr IBSC with the 90-nm-thick MgF2 ARC was more enhanced as a whole than the measured EQE of the ZnTe:Cr IBSC without the 90-nm-thick MgF2 ARC. Using the EQE of the ZnTe:Cr IBSC with and without the 90-nm-thick MgF2 ARC, the values of integrated Jsc were obtained by the following equation:
becomes closer to 1, while it becomes closer to 2 when recombination current is dominant. As shown in Table 1, the value of ideality factor of ZnTe:Cr IBSC is 1.75, which shows that recombination current is more dominant than diffusion current. However, the ideality factor of undoped ZnTe SC is 4.3, which is not between 1 and 2. Because the high ideality factor of undoped ZnTe SC cannot be explained by these mechanisms, further study should be needed to interpret the high ideality factor of undoped ZnTe SC. Fig. 3(b) shows the EQE spectra of the ZnTe:Cr IBSC and undoped ZnTe SC. The EQE values of ZnTe:Cr IBSC above 3.0 eV are relatively lower than those of the energies below 2.2 eV. The low EQE values of ZnTe:Cr IBSC above 3.0 eV can be considered as a result of the front surface recombination or the interface recombination between the ZnO:Al and ZnTe:Cr layer [27], considering shorter penetration depth of high energy photon. However, the EQE values of ZnTe:Cr IBSC below the band gap of ZnTe (2.2 eV) are more enhanced than those of undoped ZnTe SC, and detected for energies down to 1.0 eV. Especially, the EQE values of ZnTe:Cr IBSC and undoped ZnTe at 1.5 eV are 67% and 2%, respectively. By using of EQE data for the samples, the light harvesting efficiency (LHE) and the absorbed photon-to-current conversion efficiency (APCE) were estimated based on LHE = (1−R) × (1−10−A) and APCE = EQE × LHE, where A and R represent absorbance and reflectance, respectively [28]. In the photon energy region from 1.1 to 2.2 eV, the values of LHE and APCE for the ZnTe:Cr IBSC are higher than those of undoped ZnTe SC, which was attributed to the charge transfer between Cr2+ to Cr+ generated in the ZnTe:Cr layer as well as the band to band (2.2 eV) absorption. In addition, the two-photon experiments for the samples in
1032nm
Jint.sc = q
∫324nm
(IAM1.5G (λ) × EQE(λ))/(hc/λ) d λ
(7)
In Fig. 4(d), the Jint.sc of ZnTe:Cr IBSC with and without 90-nm-thick MgF2 was calculated to be 24.4 mA/cm2 and 22.7 mA/cm2, respectively. Therefore, we confirmed, using the 90-nm-thick MgF2 ARC, that 31
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the Jsc of ZnTe:Cr IBSC with 90-nm-thick MgF2 can be increased up to 24.4 mA/cm2. Here, the low contribution of MgF2 ARC to Jsc can be interpreted as the dual role of the ZnO:Al thin film played by n-type emitter and ARC for ZnTe:Cr IBSC, because the reflective index of ZnO:Al thin film is in the middle range between of air and the ZnTe:Cr thin film. 4. Conclusion In conclusion, using the PLD method, we fabricated a ZnTe:Cr IBSC with PCE of 5.9%. The structure of ZnTe:Cr grown on p-Si (100) was a cubic structure, Cr was uniformly distributed in the ZnTe:Cr layer, and the atomic concentration of Cr was 3.5 at%. The ZnTe:Cr layer shows ptype electrical conductivity, and the carrier concentration of the layer was 1.2ⅹ1014 cm−3. In the analysis of SE, the ZnTe:Cr layer had a higher absorption coefficient than the undoped ZnTe layer below 2.2 eV. The experimental results led us to suggest that the enhanced absorption coefficients of the ZnTe:Cr layer resulted from the charge transfer between Cr2+ and Cr+, which plays the role of IB. The analysis of the APCE for the solar cells showed that, in the region of VNIR, the ZnTe:Cr IBSC could absorb more photon energy than the undoped ZnTe SC. Therefore, these results suggest the possibility of achieving high PCE of an IBSC based on the ZnTe:Cr IBSC. Acknowledgments This work was supported in part by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT, & Future Planning (MSIP) (NRF-2014R1A2A1A11053936), and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Trade, Industry, & Energy (MOTIE) of the Republic of Korea (No. 20163030013380). Kyoung Su Lee would like to express thanks to Dr. Juwon Lee for discussion of PLD growth. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2017.05.020. References [1] A. Luque, A. Martí, C. Stanley, Understanding intermediate-band solar cells, Nat. Photonics 6 (2012) 146–152. [2] M.J. Keevers, M.A. Green, Efficiency improvements of silicon solar cells by the impurity photovoltaic effect, J. Appl. Phys. 75 (1994) 4022–4031. [3] A. Luque, A. Martí, E. Antolín, C. Tablero, Intermediate bands versus levels in non-
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