B.C. Richards 1, Young Lin 1, Pravin Patel 1, Daniel Chumney 1, Paul R. Sharps 1 Chris Kerestes 1,2, David Forbes 2, Kristina Driscoll 2, Adam Podell 2, Seth Hubbard 2 1 EMCORE Corporation, Albuquerque, New Mexico, USA 2 NanoPower Research Labs, Rochester Institute of Technology, Rochester, NY, USA ABSTRACT Lattice matched, triple-junction solar cells with straincompensated quantum dots (QDs) in the GaAs middle cell were grown by Metal-Organic Chemical Vapor Deposition (MOCVD). Devices with different numbers of QD layers are compared to baseline devices with no QDs. Quantum efficiency and light I-V measurements show an increase in short circuit current density and degradation of the open circuit voltage for QD solar cells. The QDs do not improve the overall efficiency of the devices, and the performance degrades as more QD layers are added. The QD solar cells show improved relative radiation resistance compared to baseline devices, but the improvement is insufficient to make up for the initial loss of performance. Index Terms photovoltaic cells, quantum dots, III-V semiconductor materials. I. INTRODUCTION The triple-junction (3J) GaAsP 2 /(In)GaAs/Ge solar cell with 29.5% average conversion efficiency for 1 sun, AM0 conditions represents the state-of-the-art for space power systems, and has been widely deployed on commercial and government satellites. Several avenues for surpassing the 30% efficiency level have been proposed and investigated, including inverted metamorphic multi-junction (IMM) [1], dilute nitride [2], and quantum dot (QD) or quantum well (QW) [3, 4] solar cells. All three of these approaches to increasing solar cell efficiency rely on optimizing the available bandgaps of multi-junction solar cell materials to more efficiently utilize the solar spectrum of available energy. In principle, the middle cell of the 3J solar cell is the current limiting sub-cell of the device, although in actual manufacturing the device is intentionally designed to be top cell limited for the purposes of radiation tolerance. On the other hand, the germanium (Ge) bottom cell typically produces significantly more current than the top and middle cells. The incorporation of QDs into the middle cell can extend the absorption of the middle cell further into the infrared region of the solar spectrum, thereby capturing some of the radiation that would otherwise be absorbed by the Ge bottom cell. This would in turn increase the current of the middle cell and potentially enable a higher-efficiency device. In addition to being the current-limiting junction, the middle cell of the 3J device is also the most susceptible to radiation damage. QDs in the middle cell, if they could improve the radiation resistance of the middle cell, could also provide a boost to the end-of-life performance of the 3J solar cell. This study presents the results of incorporating QDs into the (In)GaAs middle cell of a lattice-matched 3J solar cell. Performance as a function of number of QD layers is presented as well as relative resistance to high energy particle radiation. II. STRAIN-COMPENSATED QD GROWTH BY MOCVD State-of-the-art solar cells in high volume manufacturing for space are grown by metal-organic chemical vapor deposition (MOCVD). Indium arsenide (InAs) QDs are grown in the Stranski-Krastonov growth mode by growing fractional, strained monolayers that, at some critical thickness, selfassemble into the most energetically favorable state to form 3-dimensional QD islands. Varying the MOCVD growth parameters of the dots, such as InAs flow rates, temperature, and V/III compound ratio affects the size, density, and uniformity of the QDs. These properties in turn determine how suitable the QDs will be for absorbing light in the spectral region of interest for QD solar cells. The structural properties of the QDs also affect the quality of the subsequently grown crystal lattice. The 3D nature of the QDs can introduce threading dislocation defects into the crystal lattice that reduce the quality of the crystal and hence lower the open-circuit voltage of the device. This degradation would be in addition to the loss that is sustained simply by incorporating a lower bandgap material into the device. Thus, the quality of the QDs, both optically and structurally, is important for achieving high quality solar cells. Furthermore, because the InAs layers are in compressive strain with respect to the host crystal lattice, the growth of more than a few layers of QDs results in rapidly deteriorating device quality unless additional layers are integrated to compensate for the strain of the QD layers. In this work, GaP strain compensating layers were grown in addition to the www.emcore.com 1
InAs QD layers in order to realize strain-neutral devices. A schematic of an n-layer QD structure used in this work is shown in figure 1. This is the structure that was inserted into the middle cell of the 3J solar cell. The structural and optical quality of the QDs was evaluated by atomic force microscopy (AFM) and photoluminescence (PL) measurements, respectively. AFM measurements require an additional layer of surface QDs. Good QD quality was achieved on test structures for stacks of up to 20x QD layers. A typical strain-compensated 10x QD structure with surface dots showed average QD height of 2.1 nm, average diameter of 23 nm and density of 2.9 10 10 /cm 2. PL measurements revealed that the buried QD ensemble PL is typically centered around 1 μm with a FWHM of around 100 nm. X-ray diffraction measurements show that the structure is well strain-balanced. Figure 1. Cross section schematic of the structure of an n- layer QD stack. III. QD-ATJ SOLAR CELLS The optimized QD recipe was incorporated into the middle cell of the EMCORE ATJ 3J solar cell. The ATJ solar cell has been in high volume manufacturing for space applications since 2001 and has a lot average efficiency of 27.5% under 1 sun, AM0 illumination. The ATJ solar cell is grown on 100 mm germanium wafers in the n-on-p configuration. Layers of QDs in integral multiples of 5 were added to the middle cell, and the wafers were processed into 2 cm x 2 cm solar cells. This form factor was chosen for these experiments in order to provide better statistical data versus the standard two-per-wafer form factor of the ATJ solar cell. The processed devices were then tested in quantum efficiency (QE) and light current-voltage (LIV) measurements. The QE measurements were performed on the top and middle junctions of the device, since the bottom junction overproduces current and is not expected to be affected by the subsequent epitaxial growth of the QDs. The LIV measurements were performed on a two-zone steady state solar simulator calibrated to 1 sun, AM0 conditions matched to the top two junctions. Because the middle cell will suffer the worst radiation degradation of the 3J stack, the standard ATJ solar cell is designed to be top cell limited at beginning-of-life so that the top and middle junctions will be current matched at end-oflife. In order to be able to detect the effect of QDs, the QD devices in this study were grown middle cell limited. Because of the current boost due to the QDs, the 15 and 20 layer devices were grown with a thicker top cell than the 5 and 10 layer devices. Thickening the top cell provides more current to the top cell so that the middle cell remains the current limiting junction. The QE measurements revealed a boost in the response at the long wavelength tail of the middle cell QE curve, indicating that the QDs are in fact collecting additional light that is not collected by the middle cell of a standard triple junction device. Significantly, the QE results also did not reveal any degradation of the top cell caused by the addition of the QDs. This indicates that the material quality of the top cell is at least reasonably high and has not been drastically impacted by the addition of the QDs. Integration of the QE curves at the long-wavelength tail of the middle cell where the QD response causes a boost in QE shows an approximately 20 μa per QD layer improvement in the overall current in the spectral region where the QDs make a contribution to the absorption. Figure 2 shows internal QE results for 5, 10, 15 and 20 layer QD devices as well as a baseline ATJ device with no QDs. Internal QE removes the effects of changes in reflectance due to the different layer thicknesses of the devices that are seen in external QE. The QE results show a clear boost in the middle cell response from the QDs in the spectral range of approximately 900 nm to 1000 nm, with a response that increases as more QD layers are added. The QE responses of the top cells of the different devices are largely indistinguishable. www.emcore.com 2
for a full wafer (12 cells) of 2 cm x 2 cm devices for the best of the QD wafers in each lot, and the baseline data is taken from a large number of 2 cm x 2 cm baseline devices. The average efficiency of the 15 and 20 layer devices falls approximately 0.5% and 1.0% in absolute terms compared to the baseline devices, whereas the 5 and 10 layer devices are approximately 0.2% improved and unchanged, respectively, compared to the baseline devices. The median efficiency of the baseline devices is higher than any of the QD devices. Figure 2. Internal quantum efficiency measurements of the top (left) and middle (right) junctions of QD ATJ solar cells with 5, 10, 15 and 20 layers of QDs in the middle cells compared to a baseline solar cell with no QDs. The contribution of the QDs is seen in the long wavelength tail of the middle cells between 900 and 1000 nm. Light I-V measurements show degradation in the solar cell performance as more QD layers are added. The overall current improvement saturates at 15 layers of QDs, whereas the loss in open circuit voltage accelerates above 10 layers of QDs. The fill factor also decreases slightly as more layers are added. The improvement in current of the QD devices is not enough to make up for the degradation in open circuit voltage, and so the QD devices show lower performance than the baseline devices with no QDs, and the performance decreases with increasing number of QD layers. The acceleration of the degradation in open circuit voltage above 10 layers of QDs indicates that the top cell material is starting to degrade due to defects introduced by the QD layers in the lower epitaxial structure. As long as the diffusion length of charge carriers in the top cell is longer than the thickness of the top cell, a decreasing diffusion length due to material defects will not be observable in QE. However, LIV is much more sensitive to such degradation. Furthermore, the InGaP used in the top cell is particularly sensitive to underlying material quality compared to the other compounds in the structure. Figure 3 shows a boxplot of the light I-V results for short circuit current density (J sc ), open circuit voltage (V oc ), fill factor (FF) and efficiency. The measurements were obtained The results of the LIV measurements indicate that QDs are not a good approach for surpassing the 30% efficiency level for multi-junction solar cells. Even with very well strainbalanced material, the loss in V oc due to the lower bandgap material in the middle cell as well as slightly degraded material in the subsequent epitaxy are not offset by the improvement in J sc that the QD layers provide. The 20 layer devices showed a median device efficiency for the best performing wafer of 26.5%. We believe that this is the best efficiency shown to date for any QD solar cell with this number of QD layers. However, the performance is not as high as the baseline devices without QDs. Figure 3. Light current-voltage measurements for QD solar cells with different numbers of QD layers compared to baseline devices. IV. RADIATION RESISTANCE As mentioned in the introduction, an important characteristic of solar cells designed for space environments is their ability to withstand the radiation exposures which they will undergo. The ATJ solar cell showed a remaining power factor of approximately 89% after exposure to 1 MeV electrons at www.emcore.com 3
fluences of 5 10 14 /cm 2 during S111 qualification. The radiation hardness of QD solar cells has been the subject of discussion, and so a number of devices from this study were sent to the National Institute of Standards and Technology (NIST) Radiation Interactions and Dosimetry Laboratory for 1 MeV electron irradiation at 5 10 14 /cm 2 and 1 10 15 /cm 2 fluences. The devices that were sent for radiation included 5, 10, 15 and 20 layer QD ATJ devices as well as baseline devices. For each device design, six cells were irradiated at each fluence. LIV measurements on the post-radiation cells were performed to look at the effect of the number of QD layers on radiation hardness. The results of these measurements show that QDs do improve the relative radiation hardness of the solar cells, and more QD layers lead to devices that degrade less severely following radiation. Figure 4 shows efficiency versus fluence for the QD ATJ series of devices. Figure 5 shows the degradation relative to the pre-radiation measurements. The figures show the slower degradation rate for QD solar cells. The 20 layer QD devices showed an approximately 4% absolute improvement in radiation degradation as fraction of beginning of life efficiency compared to the baseline devices. However, it should be noted that the beginning of life efficiency of the QD solar cells is lower so that the end of life efficiency of the baseline and the 20 layer devices is approximately the same. This indicates that the improved radiation performance of the devices would not result in a net gain over the life of a satellite mission. Figure 5. Radiation resistance as fraction of beginning-oflife (BOL) efficiency remaining for two dosage levels of electron radiation versus number of QD layers. V. CONCLUSION Strain balanced QD layers have been successfully incorporated into the middle cell of a triple junction GaAsP2/(In)GaAs/Ge solar cell. The QDs show good structural and optical quality, and devices with 5, 10, 15, and 20 layers of QDs were fabricated and characterized. QE measurements show that the QDs extend the absorption of the middle cell into the infrared region of the solar spectrum, effectively absorbing light that would otherwise be absorbed by the over-producing Ge bottom cell. LIV measurements show an improvement in Jsc, but a degradation of Voc of the solar cell. The Jsc increases linearly through 15 layers of QDs, but the Voc degradation accelerates above 10 layers of QDs. The average efficiency of the 5 layer devices is unchanged with respect to the baseline devices, but the efficiency degrades with additional QD layers, with the 20 layer devices showing a 1.0% absolute performance degradation compared to the baseline devices. The QD devices show improved relative radiation resistance compared to the baseline devices, but due to the poorer beginning-of-life performance do not result in a net performance improvement over the life of a typical satellite mission. Figure 4. Efficiency versus fluence for QD ATJ and baseline ATJ solar cells. www.emcore.com 4
REFERENCES [1] A. B. Cornfeld, et al., Development of a large area inverted metamorphic multi-junction (IMM) highly efficient AM0 solar cell, Proc. 33rd IEEE Photovoltaic Spec.Conf., pp.1-5 (2008). [2] A. Erol, Dilute III-V Nitride Semiconductors and Materials Systems: Physics and Technology. Springer- Verlag, Berlin, Heidelberg, New York, 2010. [3] C. G. Bailey, et al., Open Circuit Voltage Improvement of InAs/GaAs Quantum Dot Solar Cells Using Reduced InAs Coverage, IEEE Journal of Photovoltaics, vol.2, no. 3, pp. 269-275 (2012). [4] N. J. Ekins-Daukes, et al., Strain-balanced GaAsP/InGaAs quantum well solar cells Applied Physics Letters, vol. 75, iss. 26, pp. 4195-4197 (1999). [5] C. D. Cress, Effects of ionizing radiation on nanomaterials and III-V semiconductor devices, Ph.D. dissertation, Microsystems Engineering, Rochester Institute of Technology (2008). www.emcore.com 5