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PUBLISHED: 16 MAY 2016 | ARTICLE NUMBER: 16067 | DOI: 10.1038/NENERGY.2016.67
Monocrystalline CdTe solar cells wi...
0 250 500 750 1,000 1,250 1,500 1,750
Seff = 1.2 ± 0.7 cm s−1
Seff = 1...
Electrode ITO
(ND = 5 × 1017 cm−3)
(ND = 5 × 1...
ITO a-Si:H
Hole-contact layer Open-circuit voltage, Voc (V)
0.0 0.2 0.4 0.6
Voltage (V)
0.8 1.0 1.2
used a step size of 0.001◦
with a time step of 0.5 s. Detailed compute...
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ASU Zhang Nature Energy paper on CdTe DH_5_16_2016


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ASU Zhang Nature Energy paper on CdTe DH_5_16_2016

  1. 1. ARTICLES PUBLISHED: 16 MAY 2016 | ARTICLE NUMBER: 16067 | DOI: 10.1038/NENERGY.2016.67 Monocrystalline CdTe solar cells with open-circuit voltage over 1 V and efficiency of 17% Yuan Zhao1,2 , Mathieu Boccard2 , Shi Liu1,2 , Jacob Becker1,2 , Xin-Hao Zhao1,3 , Calli M. Campbell1,3 , Ernesto Suarez1,2 , Maxwell B. Lassise1,2 , Zachary Holman2 and Yong-Hang Zhang1,2 * The open-circuit voltages of mature single-junction photovoltaic devices are lower than the bandgap energy of the absorber, typically by a gap of 400 mV. For CdTe, which has a bandgap of 1.5 eV, the gap is larger; for polycrystalline samples, the open-circuit voltage of solar cells with the record efficiency is below 900 mV, whereas for monocrystalline samples it has only recently achieved values barely above 1 V. Here, we report a monocrystalline CdTe/MgCdTe double-heterostructure solar cell with open-circuit voltages of up to 1.096 V. The latticed-matched MgCdTe barrier layers provide excellent passivation to the CdTe absorber, resulting in a carrier lifetime of 3.6 µs. The solar cells are made of 1- to 1.5-µm-thick n-type CdTe absorbers, and passivated hole-selective p-type a-SiCy:H contacts. This design allows CdTe solar cells to be made thinner and more efficient. The best power conversion efficiency achieved in a device with this structure is 17.0%. S ilicon and GaAs solar cells have recently been demonstrated with efficiencies that are 87% of their respective detailed- balance limits1 . Like Si and GaAs, CdTe has a near optimum bandgap and a high absorption coefficient near the band edge, and is thus an excellent material for photovoltaic technology2 . However, the efficiency of the best CdTe cell is only 67% that of its detailed- balance limit owing to excessive non-radiative recombination1 and the difficulty in forming hole contacts by p-type doping3 . Indeed, the record Si and GaAs cells have monocrystalline absorbers with wide-bandgap barrier/passivating layers at the absorber interfaces4 , whereas the record CdTe cell has a polycrystalline absorber. Furthermore, existing CdTe cell structures do not have a wide- bandgap material that can both provide carrier confinement and also offer a low interface recombination velocity (IRV)1 . The cells thus have a low open-circuit voltage (Voc) of 0.876 V compared to a detailed-balance Voc of 1.23 V; this is largely responsible for the relatively low efficiency of CdTe cells5 . High quasi-Fermi-level splitting is a prerequisite for high Voc, and requires long bulk carrier lifetime and low IRV. However, typical lifetimes in polycrystalline CdTe thin films are of the order of only several nanoseconds6 , which, together with low achievable doping levels in the p-type regions, limit the quasi-Fermi-level splitting, and thus the Voc to 0.936 V. Assuming an acceptor density of 1015 cm−3 and a carrier lifetime of 66 ns, as was demonstrated in bulk CdTe6 , a Voc as high as 1.026 V should have been possible as early as 1987, yet a Voc of only 0.910 V was measured for a monocrystalline CdTe wafer, a record that stood for decades7 . This impasse seems recently to have come to an end as interest in the material system has resurfaced and voltages over 1 V have been demonstrated in a monocrystalline CdTe cell8 . For the standard polycrystalline CdTe cell configuration with a CdS layer at the front and a metallic layer at the back, an IRV of approximately 105 cm s−1 was measured6 , thereby limiting the effective lifetime to a few nanoseconds and the maximum possible Voc to roughly 0.9 V, depending on the CdTe thickness6 . Provided that excellent bulk carrier lifetime and low IRV are achieved, the high chemical potential (quasi-Fermi-level splitting) must be extracted at the contacts as an electrical potential to achieve high Voc. For conventional polycrystalline thin-film CdTe solar cells, the n-type CdS layer at the front has a typical donor density of approximately 1018 cm−3 and acts as an effective electron contact9 , while a lightly p-type doped CdTe absorber layer is used in conjunction with an additional hole contact. This results in a built-in voltage (Vbi) inside the cell that is smaller than the achievable quasi- Fermi-level splitting in the absorber material5,10 , so that the chemical potential cannot be fully extracted as an electrical potential. This paper addresses the three challenges to achieving high Voc and high efficiency in CdTe solar cells: long bulk carrier lifetimes, low IRV, and a heavily doped p-type contact. Using epitaxial CdTe as a demonstration platform, and employing new passivation and p-type contact layers in a double-heterostructure cell design, we demonstrate a Voc beyond the 1 V barrier and a substantial increase in efficiency for monocrystalline CdTe solar cells. Absorber quality and interface optimization To achieve long carrier lifetimes, we leverage high-quality CdTe epitaxially grown on InSb (001) substrates using molecular beam epitaxy (MBE)11 and CdTe/Mgx Cd1−x Te double-heterostructure (DH) designs11–13 . The complete desorption of the oxide layer on InSb substrates under a Sb flux and the near-perfect lattice match between InSb and both CdTe (0.03% mismatch) and MgTe (0.9% mismatch) enable extremely low defect density, and thus very good structural and optical properties. The DH designs offer optimal confinement for minority carriers and excellent passivation of the surfaces of the CdTe absorber layer. To reduce the IRV, we employ a DH in which a CdTe absorber layer is sandwiched between two Mgx Cd1−x Te barrier layers. These wide-bandgap barriers effectively confine the minority carriers to the narrower-bandgap CdTe absorber14,15 . Furthermore, the CdTe/Mgx Cd1−x Te interfaces themselves are close to perfect, eliminating recombination-active defects at the absorber interfaces. Figure 1a shows time-resolved photoluminescence (TRPL) data for a set of four CdTe/Mgx Cd1−x Te DH samples, each consisting 1 Center for Photonics Innovation, Arizona State University, Tempe, Arizona 85287, USA. 2 School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, Arizona 85287, USA. 3 School for Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287, USA. *e-mail: NATURE ENERGY | 1 © 2016 Macmillan Publishers Limited. All rights reserved
  2. 2. ARTICLES NATURE ENERGY DOI: 10.1038/NENERGY.2016.67 0 250 500 750 1,000 1,250 1,500 1,750 Seff = 1.2 ± 0.7 cm s−1 Seff = 1.4 ± 0.6 cm s−1 PLintensity(a.u.) Time (ns) 2.8 dCdTe (nm) 348 541 220 3.6 272 2.2 2.2 3 4 5 6 7 8 9 10 2/d (μm−1) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 1/non(μs−1)τ eff (μs)τ a b Figure 1 | CdTe double-heterostructure photoluminescence decay and interface recombination velocity. a, Normalized room-temperature time-resolved photoluminescence decay for a set of four DH samples, each consisting of two 30 nm Mg0.46Cd0.54Te barriers and a CdTe layer with a thickness between 220 nm and 541 nm. The curves have been shifted along the y-axis for clarity. The fitted lifetimes are shown in the inset table. b, Inverse non-radiative recombination lifetime 1/τnr versus inverse CdTe layer thickness 2/d. The effective interface recombination velocities were extracted by fitting these data. The error bars of 1/τnon were determined by considering the uncertainty of the estimated radiative lifetimes due to the estimation of doping densities. of two 30-nm-thick intrinsic Mg0.46Cd0.54Te barriers and a CdTe middle layer with n-type background doping of the order of 1014 cm−3 and a thickness between 220 nm and 541 nm, which was determined by detailed analysis of high-resolution X-ray diffraction measurements. All samples exhibit effective carrier lifetimes— determined by fitting single exponentials to the TRPL decay tails— exceeding 2 µs, which attests to the high quality of the CdTe layers and the CdTe/Mgx Cd1−x Te heterointerfaces. The longest lifetime of 3.6 µs is substantially longer than the previous records for crystalline bulk CdTe (ref. 6) and CdTe/Mgx Cd1−x Te DHs (refs 12,13). The IRV can be parsed by varying the CdTe bulk layer thickness. The expression for effective (measured) lifetime τeff is shown in equation (1), where τrad and τnon are the radiative and non-radiative lifetimes, respectively. The radiative lifetime is related to the photon recycling factor γ (ref. 16), the material radiative recombination coefficient B, and the doping concentration ND. Because the photon recycling effect is stronger for thicker samples, the radiative lifetime becomes longer for DH samples with thicker CdTe absorber layers. The non-radiative lifetime is related to the bulk Shockley–Read– Hall (SRH) lifetime τSRH and the interface recombination. In equation (1), Seff is the effective IRV and d is the thickness of the CdTe layer. 1 τeff = 1 τrad + 1 τnon =(1−γ)BND + 1 τSRH + 2Seff d (1) Because the radiative lifetime is dependent on the sample thickness, only the non-radiative lifetimes were used to extrapolate the effective IRV; the non-radiative lifetimes were calculated from the measured effective lifetimes and an estimated radiative lifetime. The radiative lifetime was calculated assuming B = 4.3 × 10−9 cm3 s−1 , ND = 1.5 × 1014 cm−3 , and an error bar of ±25% for the estimation of the doping concentration17 . Figure 1b plots the inverse non- radiative lifetime (1/τnr) versus the inverse CdTe layer thickness (2/d) for the four samples shown in Fig. 1a, which have 30-nm- thick Mg0.46Cd0.54Te barriers, and another set of four samples with identical layer structure and alloy composition but with 22-nm- thick barriers. Weighted fittings of the data using the error bars yield effective IRVs of 1.2 ± 0.7 cm s−1 and 1.4 ± 0.6 cm s−1 , which are comparable to or better than the best values reported for GaAs/Al0.5Ga0.5As (18 cm s−1 ) and GaAs/Ga0.5In0.5P (1.5 cm s−1 )18,19 . CdTe solar cell design The studied device structure shown in Fig. 2a affords new opportunities with respect to addressing the challenge of p-type doping in CdTe: with interface passivation provided by the Mgx Cd1−x Te barrier layers, the contact layers can be defective. Such a desirable property enables a much broader choice of contact- layer materials, which may be either crystalline or amorphous. This structure maintains the voltage of the solar cell by preventing the contact layers from compromising the absorber quality, as the minority carriers in the CdTe absorber will be confined by the barriers. That is, heterostructure barriers offer an alternative way to construct a junction in CdTe solar cells that circumvents the major challenge of p-type doping and opens the door to many novel device structure designs—a similar approach is used in HIT solar cells20 . One caveat is that the front contact layer should be as transparent as possible to minimize parasitic absorption, which reduces the photogenerated current of the solar cell. We used a 5- to 15-nm-thick heavily doped p-type amorphous silicon (a-Si:H, estimated doping level of 1018 cm−3 ) or amorphous silicon carbide (a-SiCy :H, y ∼ 6%) layer as the p-type contact. These layers were deposited by plasma-enhanced chemical vapour deposition on the front Mgx Cd1−x Te barrier, followed by an indium tin oxide (ITO) electrode deposited by sputtering (Fig. 2a). The schematic band diagrams are shown in equilibrium in Fig. 2b and at open circuit in Fig. 2c. The intent of the design is that the barrier/contact stacks block the transport of minority carriers to the contacts while permitting majority carriers to flow unimpeded— minority carriers referring to the minority carrier type of each respective contact layer, not the absorber. The Mgx Cd1−x Te barrier at the front (hole-contact side) should be properly chosen, without compromising the effectiveness of its passivation of the CdTe absorber, to enable transport of holes across the barrier while simultaneously blocking electrons by the large conduction-band offset. Note that the simulated open-circuit band diagram in Fig. 2c indicates a small Voc loss at the p-type contact because of the negative valence-band offset between a-Si:H and CdTe. The motivation for 2 © 2016 Macmillan Publishers Limited. All rights reserved NATURE ENERGY |
  3. 3. NATURE ENERGY DOI: 10.1038/NENERGY.2016.67 ARTICLES Electrode ITO n-Mg0.24Cd0.76Te (ND = 5 × 1017 cm−3) n-CdTe (ND = 5 × 1017 cm−3) n-InSb (ND = 5 × 1017 cm−3) n-InSb substrate n-CdTe n-CdTe n-CdTe n-CdTe n-MgCdTe n-MgCdTe MgxCd1−xTe a-SiCy:H MgCdTe a-SiCy:H a-Si:H i-MgCdTe Contact layer Absorber Back side barrier Contact layer Buffer Conduction band Voc Quasi-Fermi levels Valence band 0 −2.0 −1.5 −1.0 −0.5 0.0 Energy(eV) 0.5 1.0 1.5 2.0 100 200 300 Distance from surface (nm) 1,400 1,650 Front side barrier a b c d Figure 2 | Device design and band diagram. a, Layer structure of the CdTe/MgxCd1−xTe DH solar cell with an a-SiCy:H (y = 0–6%) hole-contact layer. b–d, Schematic band diagrams at equilibrium (b) and open circuit (c) and an equilibrium band diagram drawn to scale for the hero cell (d). The band diagrams shown in b and c represent several different structure designs and are thus not drawn to scale with respect to energy and length. The parameters used for the calculation are given in Table 1. Table 1 | Parameters used for the quantified band diagram calculation. ITO a-Si:H i-MgCdTe n-CdTe n-MgCdTe Bandgap 4 eV 1.8 eV 2.088 eV 1.5 eV 1.97 eV Electron affinity 4.9 eV 3.9 eV 3.871 eV 4.28 eV 3.951 eV Doping n-type p-type Intrinsic n-type n-type Doping density Degenerate 1 × 1018 cm−3 NA 1 × 1016 cm−3 5 × 1017 cm−3 Intrinsic carrier concentration Metal-like 8 × 104 cm−3 6 × 103 cm−3 5 × 105 cm−3 6 × 103 cm−3 Nc/Nv Metal-like 1 0.144 0.144 0.144 Thickness 70 nm 8 nm 10 nm 1.4 µm 50 nm adding carbon to form a-SiCy :H is to achieve a smaller valence-band offset, and thus a lower voltage drop. As the conduction-band offset is large, the 50-nm-thick Mg0.24Cd0.76Te barrier at the back (electron- contact side) was heavily doped n-type to facilitate transport of electrons and impede holes. Although more than eight wafers of different designs were used for the study reported here, we focus on the following two designs in this paper: Design A consists of a hole-contact layer (8 nm a-SiCy :H + 4 nm a-Si:H, y ∼ 6%), a 10-nm-thick undoped Mg0.30Cd0.70Te front barrier, and a 1-µm-thick absorber with n-type In doping of 3 × 1016 cm−3 ; Design B consists of a hole-contact layer (8 nm a-Si:H), a 10-nm-thick undoped Mg0.30Cd0.70Te front barrier, and a 1.4-µm-thick absorber with n-type In doping of 1 × 1016 cm−3 for the top 1 µm and 5 × 1017 cm−3 for the bottom 0.4 µm. Figure 2d shows an equilibrium band diagram drawn to scale for the hero cell design (Design B) with the highest efficiency. Solar cell characterization After the growth of the underlying DH, the wafers were processed into devices. Figure 3a shows the average and maximum Voc of a series of solar cells with 8- to 12-nm-thick a-Si:H and a-SiCy :H hole-contact layers. For each contact material, the front Mgx Cd1−x Te barrier width and height (Mg composition, x) were also explored. A (low) Voc was measured even in the absence of an intentional hole- contact layer, because ITO itself is slightly hole selective with its relatively high work function of 4.8 eV. Inserting a heavily doped p-type a-Si:H contact layer yields a greatly enhanced Voc because of the increase in Vbi, which we determined to be 1.1 V using capacitance–voltage (C–V) measurements. As anticipated from TRPL studies of DHs, the Voc rises as the front barrier height or width increases because electrons are further confined to the CdTe absorber layer as thermionic emission and tunnelling are suppressed. The Voc further increases—to a maximum measured value of 1.096 V—when p-type a-SiCy :H is used, which has a wider bandgap and lower (negative) valence-band offset than that of a- Si:H. The solar cells with the highest Voc values, however, do not tend to have the highest efficiencies owing to smaller fill factors (FF). Figure 3b shows the FF against the Voc for all solar cells measured so far; notice that the cells with a-SiCy :H all have lower FF than their a-Si:H counterparts. We attribute the large FF loss to a lower doping level than that in the a-Si:H layer, which inhibits transport across the heterojunction interfaces between the a-SiCy :H hole-contact layer and the ITO layer and Mgx Cd1−x Te front barrier layer; this effect effectively increases the lumped series resistance of the cell. A 0.21-cm2 solar cell of Design A and an evaporated silver front grid was tested by the National Renewable Energy Laboratory (NREL). The certified current–voltage and external quantum efficiency (EQE) characteristics are shown in Fig. 4a and indicate an efficiency of 14.66% ± 1.4%. Although the Voc of this particular device is slightly under 1 V at 0.9954 V ±0.3%, another device of Design B had a certified Voc of 1.0542 V ±0.5%, which is approximately 150 mV greater than the long-standing record, and nearly 40 mV greater than the recently demonstrated 1 V devices7,8 . Measurements of further devices with set-ups calibrated using the device measured by NREL in the authors’ laboratories reveal that many devices of Design A have demonstrated Voc consistently over 1 V without greatly sacrificing the output power under operating conditions, and the best tested device (Design B) had a Voc of 1.036 V, a Jsc of 22.3 mA cm−2 , a FF of 73.6%, and a power conversion efficiency of 17.0%, as shown in Fig. 5a. The maximum Voc measured from all the tested devices was 1.096 V, which is quickly approaching the theoretical limit of 1.17 V for CdTe solar cells with NATURE ENERGY | © 2016 Macmillan Publishers Limited. All rights reserved 3
  4. 4. ARTICLES NATURE ENERGY DOI: 10.1038/NENERGY.2016.67 0.4 ITO a-Si:H Hole-contact layer Open-circuit voltage, Voc (V) Barrier thickness (nm) a-SiC:H 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 0.5 0.6 0.7 0.8 0.9 Open-circuitvoltage,Voc(V) Fillfactor,FF(%) 1.0 1.1 30% Mg 40% 1.2 0 10 20 30 40 50 60 70 80 90 a-Si:H a-SiCy:H ITO only 10 a b 5 10 5 10 Figure 3 | Effects of different passivation and contact layers on device performance. a, Box plot indicating the average and maximum Voc for several solar cell designs with different hole-contact layers and barrier thicknesses and heights. The upper and lower bounds of the boxes indicate the 25th and 75th quartiles. b, FF versus Voc for all individual devices measured and analysed for a. 0 0.0 0.2 0.4 0.6 Voltage (V) 300 400 500 600 700 800 900 Wavelength (nm) 0.8 1.0 1.2 5 10 15 Currentdensity(mAcm−2) 0 20 40 JPhoto = 23.93 mA cm−2 Device parameters: Jsc = 21.663 mA cm−2 ± 1.4% Voc = 0.9954 V ± 0.3% FF = 67.98% ± 0.4% = 14.66% ± 1.4%η 60 80 100 Externalquantumefficiency(%) 20 25a b Figure 4 | NREL certified device results. J–V curve (a) and EQE (b) for a sample with a 10-nm-thick Mg0.30Cd0.70Te barrier and an 8-nm-thick a-Si:H hole-contact layer. The device under test is square with an area of 0.21 cm2 . an absorbing substrate. All these device characteristics are greater than the previous records of Voc (1.017 V) and efficiency (15.2%) for monocrystalline CdTe (ref. 8). The significant increase in both Voc and conversion efficiency is attributed to the much improved bulk carrier lifetime and reduced IRV through the use of Mgx Cd1−x Te passivation/barrier layers, and the heavily doped a-Si:H or a-SiCy :H hole-contact layer. The Jsc values of both Design A and B are also higher than the previous record monocrystalline cell7 , primarily owing to the higher quantum efficiencies at shorter wavelengths (below 600 nm), as seen in Fig. 5b. An AM1.5G-weighted integration of the EQE, shown in blue, provides a Jsc of 22.3 mA cm−2 . However, there is still considerable current loss, as indicated by the large gap between the EQE and 1 − R curves. The loss in this region is attributed to parasitic optical absorption in the ITO, highly defective a-SiCy :H, and Mgx Cd1−x Te layers, as well as transmission loss. The breakdown of these different losses by their mechanisms and the simulated absorptance of this structure are shown in Fig. 5c. The ITO, a-Si:H and Mgx Cd1−x Te layers all absorb incident sunlight before it reaches the CdTe absorber, and are responsible for Jsc losses of 1.2 mA cm−2 , 1.4 mA cm−2 and 0.6 mA cm−2 , respectively. Using a thinner hole- contact layer or a wider-bandgap material can drastically reduce the parasitic optical absorption at these energies, resulting in additional current generation of over 3 mA cm−2 . Of course, external reflection also plays an important role in current loss within the device. By the very nature of the index matching between the ITO, a-Si:H and CdTe, the structure (with no initial concern given to reflectance) already exhbits relatively good anti-reflective properties—especially at 500 nm, where near-complete absorption is observed. However, there is still considerable room for improvement, as the photons lost owing to reflection amount to 2.1 mA cm−2 of the potential photo- current. The use of multilayer anti-reflection coatings can help regain some of this loss, with SiO2 and MgF2 proving to be excellent candidates. As these are wafer-based devices, inevitably, a small 4 © 2016 Macmillan Publishers Limited. All rights reserved NATURE ENERGY |
  5. 5. NATURE ENERGY DOI: 10.1038/NENERGY.2016.67 ARTICLES 0.0 0.2 0.4 0.6 Voltage (V) 0.8 1.0 1.2 0 5 10 15 Currentdensity(mAcm−2) EQEand1−R(%) Power(mW) 20 25 0.0 0.2 0.4 0.6 0.8 0 10 20 30 40 50 60 70 80 90 100 Reflectance,transmittance andabsorptance(%) 0 10 20 30 40 50 60 70 80 90 1001.0a b c 300 400 500 600 700 800 900 Wavelength (nm) 300 400 500 600 700 800 900 Wavelength (nm) Device parameters: Jsc = 22.3 mA cm−2 Losses: (mA cm−2 ) Transmission = 0.5 Reflectance = 2.1 ITO = 1.2 a-Si:H = 1.4 MgCdTe = 0.6 Voc = 1.04 V VM = 830 mV Parasitic loss Reflectance loss 1 − R EQE JM = 20.5 mA cm−2 PM = 534 mW FF = 73.6% = 17.0%η Figure 5 | Optimum device performance. a, Measured J–V curve and associated device parameters. b, Measured EQE and 1 −reflectance (1 − R) with a calculated photo-current of 22.3 mA cm2 . c, Simulated absorptance spectrum for the highest-performing CdTe solar cell device with a calculated photo-current of 23 mA cm2 . The device under test (Design B) has a 10-nm-thick Mg0.30Cd0.70Te barrier layer, an 8-nm-thick a-Si:H hole-contact layer and an area of 0.03 cm2 . portion of light is lost to transmission into the substrate. Simulated transmission loss for this structure amounts to 0.5 mA cm−2 , but can be improved through the use of a thicker absorber. Altogether, nearly 20% of the potential Jsc is lost to rectifiable design issues—leaving room for considerable improvement in the current, and ultimately the efficiency, of future devices. Conclusions We have provided clear evidence that CdTe is an excellent material for solar cells and other optoelectronics applications through the demonstration of key material quality records that place this material system well beyond previous limits. The record minority carrier lifetime (3.6 µs), limited partially by radiative recombination, and IRV (as low as 1.2 cm s−1 ) achieved in the CdTe/Mgx Cd1−x Te double-heterostructures are comparable to or even better than the best values reported for GaAs/Al0.5Ga0.5As (18 cm s−1 ) and GaAs/Ga0.5In0.5P (1.5 cm s−1 ) double-heterostructures, and thus indicative of the potential for high Voc solar cell devices. The innovative approach for hole contact using a heavily doped a-Si:H or a-SiCy :H hole-contact layer in conjunction with the double-heterostructure design, namely a Mgx Cd1−x Te front passivation/barrier layer, allows the large implied Voc values resulting from the long carrier lifetime and low IRV to be realized in functioning devices. Mgx Cd1−x Te/CdTe/Mg0.24Cd0.76Te double- heterostructure solar cells with the novel hole-contact layers (Design A) have demonstrated Voc consistently over 1 V without greatly sacrificing the output power under operating conditions, and an NREL certified maximum measured efficiency of 14.66% ± 1.4% with a Voc of 0.9954 V ±0.3%. The maximum certified Voc of a device with similar layer structure design (Design B) is 1.0542 V ±0.5%, and additional measurements of further devices with calibrated set-ups in the authors’ laboratories reveal that the best tested device (Design B) has a Voc of 1.036 V, a Jsc of 22.3 mA cm−2 , a fill factor of 73.6%, and a power conversion efficiency of 17.0%. The maximum Voc measured from these devices is 1.096 V, which is quickly approaching the theoretical limit of 1.17 V for CdTe solar cells with an absorbing substrate. It is worth noting that the use of the double-heterostructure design enables a much broader choice of contact-layer materials with various degrees of perfection (crystalline or amorphous) for both types, p-type in particular for the CdTe case, and maintains the high performance of the solar cell without being compromised, as the minority carriers in the CdTe absorber will be confined by the barriers. Therefore, the combination of the double-heterostructure design and the amorphous hole-contact layer offers an alternative way to circumvent the major challenge of p-type doping, and opens the door to many novel device structure designs, such as the use of ZnTe (ref. 21), MoOx (ref. 22) and CuZnS for the hole- contact layers. These results on monocrystalline CdTe/MgCdTe double-heterostructures establish possibly achievable metrics for polycrystalline CdTe thin-film solar cells, should the presented approach be transferred to such technologies. Methods MBE material growth. All samples discussed in this article were grown on InSb (001) substrates using a dual-chamber VG V80H MBE system. InSb substrates are first prepared with an oxide removal process within the III–V growth chamber. The substrates are heated to 500 ◦ C (measured using a thermocouple) at a rate of 25 ◦ C min−1 , with the Sb cell shutter opened at 350 ◦ C to suppress any Sb desorption. The substrate temperature is then measured by a finely-tuned pyrometer, and further increased at a rate of 5 ◦ C min−1 , with 3-min holding periods between each ramp until the pyrometer reads a substrate temperature of 475 ◦ C. Slow, deliberate temperature control is necessary to ensure that the substrate does not surpass its melting point, which is very close to the oxide removal temperature. This temperature is held until streaky pseudo-(1 × 3) reflection high-energy electron diffraction (RHEED) reconstruction patterns are observed, indicating the removal of the surface oxide. After the surface oxide has been removed, the substrate temperature is brought down to a pyrometer reading of 390 ◦ C for the n-type InSb:Te buffer growth—the cells are controlled so as to give a Sb/In flux ratio of 1.5 and a growth rate of 10.8 nm min−1 . The tellurium cell temperature is used to dope the InSb buffer layer to 5 × 1017 electrons. The samples are then transferred through the ultrahigh vacuum preparation chamber to the II–VI growth chamber, avoiding surface oxidation. During the substrate temperature ramp before the II–VI material growth, the samples are exposed to a Cd flux for several minutes to prevent the formation of a group III–VI alloy on the surface. An n-type CdTe:In buffer layer is then grown on the substrate at 280 ◦ C (pyrometer reading) with an initial Cd/Te flux ratio of 3.0 to further prevent the formation of In3Te2 at the InSb/CdTe interface. The indium dopant cell temperature is set to dope the CdTe buffer layer to 5 × 1017 cm−3 . After two minutes of growth, the Cd/Te flux ratio is adjusted to an optimum 1.5. The surface quality is monitored through RHEED imaging. Streaky RHEED patterns appear after approximately 10 min, and after a 500 nm buffer, the surface is ready for active layer growth. It is important to note that the substrate temperature reading will decrease to approximately 265 ◦ C during the buffer growth as the emissivity of the wafer surface changes. All additional II–VI layers were grown at the same substrate temperature of 265 ◦ C and the same 1.5 Cd:Te flux ratio. Magnesium incorporation and indium doping concentration are controlled by varying the cell temperatures. Magnesium alloying has a negligible effect on growth rate, and thus all nominal thicknesses are calculated from a 1.6 Å s−1 growth rate. The Mg0.24Cd0.76Te back-side barrier is grown with a Mg:Te flux ratio of 0.39. The intrinsic Mgx Cd1−x Te layer has a magnesium incorporation range of 0.30–0.46 throughout the experiments grown using a Mg:Te flux ratio of 0.5–0.84. XRD measurements. High-resolution X-ray diffraction (XRD) measurements were carried out using a PANalytical X’Pert PRO MRD diffractometer. The incident beam is first focused through a hybrid monochromator module and the diffracted beam is collected through a triple-axis detector. The measurements NATURE ENERGY | © 2016 Macmillan Publishers Limited. All rights reserved 5
  6. 6. ARTICLES NATURE ENERGY DOI: 10.1038/NENERGY.2016.67 used a step size of 0.001◦ with a time step of 0.5 s. Detailed computer simulations of the XRD patterns were used to accurately determine the layer thickness of all those barrier layers in the PL samples. Steady state photoluminescence (PL) measurements. General material quality was characterized using the photoluminescence (PL) collection system, which consists of a spectrometer with a 0.85 m focal length, a photomultiplier tube (PMT), and a germanium detector—for CdTe samples, a PMT is used. A 532 nm diode-pumped solid state (DPSS) 40 mW laser is used as the excitation source and the incident power is adjusted to 0.92 mW using a neutral density filter; the beam radius on the sample is measured to be 0.54 mm. This corresponds to a power density of 100 mW cm−2 , similar to one sun power density. A chopper is used to modulate the laser beam and send a reference signal to a lock-in amplifier, which improves the signal-to-noise ratio. Time-resolved photoluminescence (TRPL) measurements. Carrier lifetimes and interface recombination velocities were determined using TRPL measurements with a time-correlated single-photon-counting (TCSPC) system. A Becker-Hickl SPC-830 single-photon-counting card is used for data acquisition. The excitation sources are an ultrafast titanium-sapphire laser and a Fianium fibre laser, which emit wavelengths in the range of 700 nm–950 nm and 450 nm–750 nm, respectively. The repetition rate of the Ti:sapphire laser (0.4 MHz–80 MHz) and the Fianium laser (0.1 MHz–20 MHz) can be adjusted accordingly. A spectrometer is used to collect the PL from the sample at a specific wavelength and a high-speed PMT detector is used to detect the photons in the wavelength range from 300 nm to 900 nm. The detection wavelength is set to 820 nm, which is the PL peak position of CdTe at room temperature. Device processing and characterization. The p-doped amorphous silicon layer was deposited after air exposure, without prior surface treatment, by plasma- enhanced chemical vapour deposition (PECVD) in a P-5000 tool using silane, hydrogen and tri-methyl boron, at a pressure of 2.5 torr, a nominal susceptor temperature of 250 ◦ C and a radiofrequency (RF) power of 36 W. Deposition time was adjusted to obtain a 12-nm-thick layer. A 73-nm-thick layer of tin-doped indium oxide (ITO, 95%/5%) was then sputtered in an MRC sputtering tool with direct current (d.c.) sputtering, at room temperature, a pressure of 2.5 mtorr and a power of 1 kW, yielding a film with <100 sq.−1 sheet resistance. More details on these processes can be found on A laser-cut shadow mask was used during ITO sputtering to define circular pads of 2-mm diameter to 3-mm diameter. To ensure a good electric contact from the back of the device to the measurement chuck during electrical characterization, a 100-nm-thick layer of silver was sputtered on the back of the devices with d.c. sputtering at 1 kW without prior treatment of the surface. Light I–V measurements. Solar cell parameters such as the open-circuit voltage, fill factor and power conversion efficiency were extracted from light I–V measurements taken using an Oriel Class A Solar Simulator. The Newport Class A solar simulator generates a 4-inch-diameter collimated beam using a xenon arc lamp and a series of filters designed to provide 0.1 W cm−2 at the surface of the testing stage. Electrical contact is made using a two-point probe controlled by a Keithley 2400 multimeter. The incident beam intensity is set using a calibrated Oriel silicon detector. No spectral mismatch factor was used, and the efficiency measurements of the same cell measured at ASU and NREL were 14.57% and 14.66%, respectively. However, to more accurately represent the output current of the device, the integrated response of the EQE weighted against the standard reference spectrum was used to determine the short-circuit current density. The reported J–V curves have been corrected to fit the Jsc as measured by the EQE. The scans were completed in the forward direction with a 10 mV step and a dwell time of approximately 20 ms at each step. A mask/aperture was used during all light I–V measurements. The aperture was necessary as the cell areas were not perfectly defined using mesas. The Jsc was seen to vary with device size, as specified by the aperture. Hysteresis was not checked for at ASU; however, NREL did perform a hysteresis check and reported a 4% variation in FF and Pmax. External quantum efficiency (EQE) measurements. Quantum efficiency is a wavelength-dependent collection efficiency that helps analyse how different areas of the device affect current generation. The EQE is measured under short-circuit conditions using an Oriel QEPVSI quantum efficiency measurement system. This system is composed of a xenon arc lamp, a chopper set to generate 100 Hz square waves, a monochromator, and a series of focusing optics to create a 2 mm ×2 mm square beam incident on the surface of the device under test. The output current of the device is fed into a transimpedance amplifier whose output voltage is sent to a lock-in amplifier. The signal is then referenced to a calibrated silicon detector head which is under the same light bias via a beam splitter. PC1D simulation. The band-edge diagrams shown in Fig. 2 were calculated using PC1D, a one-dimensional semiconductor device simulator. Optics simulation. The absorptance of each layer is calculated using wave optics, taking into account the optical constants (n & k) and the thickness of each layer. The substrate is assumed to be infinitely thick. C–V measurements. Capacitance–voltage (C–V) measurements are conducted after the deposition of p-type a-Si:H on the CdTe/MgCdTe DHs, using a mercury probe with a contact area of 4.56 × 10−5 m2 , and a Hewlett Packard 4284A Precision LCR meter. The built-in voltage is determined by plotting 1/C2 versus V, and extrapolating the curve to the x-axis. The intersection on the x-axis gives the extrapolated built-in voltage. Received 29 December 2015; accepted 19 April 2016; published 16 May 2016 References 1. Green, M. A., Emery, K., Hishikawa, Y., Warta, W. & Dunlop, E. D. Solar cell efficiency tables (version 47). Prog. Photovolt. Res. Appl. 24, 3–11 (2016). 2. Adachi, S. Optical Constants of Crystalline and Amorphous Semiconductors: Numerical Data and Graphical Information (Springer, 1999); 3. Yang, J.-H. et al. Enhanced p-type dopability of P and As in CdTe using non-equilibrium thermal processing. J. Appl. Phys. 118, 025102 (2015). 4. Kayes, B. M. et al. 27.6% conversion efficiency, a new record for single-junction solar cells under 1 sun illumination. Conf. Rec. 37th IEEE Photovolt. Spec. Conf. 000004–000008 (2011). 5. Gessert, T. A. et al. Research strategies toward improving thin-film CdTe photovoltaic devices beyond 20% conversion efficiency. Sol. Energy Mater. Sol. Cells 119, 149–155 (2013). 6. Kuciauskas, D. et al. Minority carrier lifetime analysis in the bulk of thin-film absorbers using subbandgap (two-photon) excitation. IEEE J. Photovolt. 3, 1319–1324 (2013). 7. Nakazawa, T., Takamizawa, K. & Ito, K. High efficiency indium oxide/cadmium telluride solar cells. Appl. Phys. Lett. 50, 279–280 (1987). 8. Burst, J. M. et al. CdTe solar cells with open-circuit voltage greater than 1 V. Nature Energy 1, 16015 (2016). 9. Dhere, R. et al. Influence of Cds/CdTe interface properties on the device properties. Conf. Rec. 26th IEEE Photovolt. Spec. Conf. 435–438 (1997). 10. Gloeckler, M., Sankin, I. & Zhao, Z. CdTe solar cells at the threshold to 20% efficiency. IEEE J. Photovoltaics 3, 1389–1393 (2013). 11. DiNezza, M. J., Zhao, X.-H., Liu, S., Kirk, A. P. & Zhang, Y.-H. Growth, steady-state, and time-resolved photoluminescence study of CdTe/MgCdTe double heterostructures on InSb substrates using molecular beam epitaxy. Appl. Phys. Lett. 103, 193901 (2013). 12. Zhao, X., Dinezza, M. J., Liu, S., Campbell, C. M. & Zhao, Y. Determination of CdTe bulk carrier lifetime and interface recombination velocity of CdTe/MgCdTe double heterostructures grown by molecular beam epitaxy. Appl. Phys. Lett. 105, 252101 (2014). 13. Liu, S. et al. Carrier lifetimes and interface recombination velocities in CdTe/MgxCd1-xTe double heterostructures with different Mg compositions grown by molecular beam epitaxy. Appl. Phys. Lett. 107, 041120 (2015). 14. Hartmann, J. M. et al. CdTe/MgTe heterostructures: growth by atomic layer epitaxy and determination of MgTe parameters. J. Appl. Phys. 80, 6257–6265 (1996). 15. Kuhn-Heinrich, B. et al. Optical investigation of confinement and strain effects in CdTe/(CdMg)Te quantum wells. Appl. Phys. Lett. 63, 2932–2934 (1993). 16. Steiner, M. A. et al. Optical enhancement of the open-circuit voltage in high quality GaAs solar cells. J. Appl. Phys. 113, 123109 (2013). 17. Zhao, X.-H. et al. Time-resolved and excitation-dependent photoluminescence study of CdTe/MgCdTe double heterostructures grown by molecular beam epitaxy. J. Vac. Sci. Technol. B 32, 040601 (2014). 18. Olson, J. M., Ahrenkiel, R. K., Dunlavy, D. J., Keyes, B. & Kibbler, A. E. Ultralow recombination velocity at Ga0.5In0.5P/GaAs heterointerfaces. Appl. Phys. Lett. 55, 1208–1210 (1989). 19. Molenkamp, L. W. & van’t Blik, H. F. J. Very low interface recombination velocity in (Al,Ga)As heterostructures grown by organometallic vapor-phase epitaxy. J. Appl. Phys. 64, 4253–4256 (1988). 20. Masuko, K. et al. 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  7. 7. NATURE ENERGY DOI: 10.1038/NENERGY.2016.67 ARTICLES Acknowledgements We would like to thank all those among the ASU MBE group members who, although not directly associated with this work, contributed to its success through experimental preparation and discussion, principally Z. He for his efforts in materials and device characterization experimental design. We would also like to thank T. Moriarty, a Senior Scientist at the National Renewable Energy Laboratory, for certification measurements carried out in the PV Cell Performance Laboratory. This work is partially supported by the Department of Energy BAPVC Program under Award Number DE-EE0004946, NSF/DOE QESST ERC under Award Number DE-EE0006335, and the AFOSR Grant FA9550-15-1-0196. Author contributions Y.-H.Z. proposed the ideas to use InSb substrate and DH structure; Y.Z. modelled the device and first proposed the use of a-Si:H as a hole-contact layer on the front MgCdTe barrier; M.B. and Y.Z. then extended the idea to the a-SiCy :H hole-contact layer; S.L. designed and grew DH PL samples; C.M.C., M.L. and E.S. grew the device wafers and participated in editing of the manuscript; X.-H.Z. did XRD measurements and analysis, and together with S.L. analysed the TRPL results and built the theoretical model, M.B. deposited the ITO and hole-contact layers, and processed all the devices; Y.Z., J.B. and M.B. characterized and modelled the device and analysed the results; the manuscript was mainly written by Y.Z., J.B., M.B., X.-H.Z., Z.H., Y.-H.Z., with Y.-H.Z. leading the entire project. Additional information Reprints and permissions information is available online at Correspondence and requests for materials should be addressed to Y.-H.Z. Competing interests The authors declare no competing financial interests. NATURE ENERGY | © 2016 Macmillan Publishers Limited. All rights reserved 7