The realistic energy yield potential of GaAs-onSi tandem solar cells: a theoretical case study Haohui Liu,1,2,* Zekun Ren,3 Zhe Liu,1 Armin G. Aberle,1 Tonio Buonassisi,3,4 and Ian Marius Peters1,4 1
Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, 7 Engineering Drive 1, 117574, Singapore 2 NUS Graduate School for Integrative Sciences & Engineering (NGS), 28 Medical Drive, 117456, Singapore 3 Singapore-MIT Alliance for Research and Technology (SMART), 1 CREATE Way, 138602, Singapore 4 Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA * [email protected]
Abstract: Si based tandem solar cells represent an alternative to traditional compound III-V multijunction cells as a promising way to achieve high efficiencies. A theoretical study on the energy yield of GaAs on Si (GaAs/Si) tandem solar cells is performed to assess their energy yield potential under realistic illumination conditions with varying spectrum. We find that the yield of a 4-terminal contact scheme with thick top cell is more than 15% higher than for a 2-terminal scheme. Furthermore, we quantify the main losses that occur for this type of solar cell under varying spectra. Apart from current mismatch, we find that a significant power loss can be attributed to low irradiance seen by the sub-cells. The study shows that despite non-optimal bandgap combination, GaAs/Si tandem solar cells have the potential to surpass 30% energy conversion efficiency. ©2015 Optical Society of America OCIS codes: (350.6050) Solar energy; (010.5620) Radiative transfer.
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Cells 75(3–4), 707–714 (2003). 32. E. F. Fernández, F. Almonacid, J. A. Ruiz-Arias, and A. Soria-Moya, “Analysis of the spectral variations on the performance of high concentrator photovoltaic modules operating under different real climate conditions,” Sol. Energy Mater. Sol. Cells 127, 179–187 (2014). 33. G. S. Kinsey and K. M. Edmondson, “Spectral response and energy output of concentrator multijunction solar cells,” Prog. Photovolt. Res. Appl. 17(5), 279–288 (2009). 34. D. A. Clugston and P. A. Basore, “PC1D version 5: 32-bit solar cell modeling on personal computers,” in Proc. of 26th IEEE Photovoltaic Specialists Conference (Anaheim, 1997), pp. 207–210. 35. H. Haug, B. R. Olaisen, Ø. Nordseth, and E. S. Marstein, “A Graphical User Interface for Multivariable Analysis of Silicon Solar Cells Using Scripted PC1D Simulations,” Energy Procedia 38, 72–79 (2013). 36. G. J. Bauhuis, P. Mulder, E. J. Haverkamp, J. C. C. M. Huijben, and J. J. 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1. Introduction Recently, there is a revived interest in silicon based multijunction solar cell concepts within the photovoltaic research community [1–10]. This interest is motivated by the intrinsic efficiency limits of around 30% [11] of single-junction solar cells combined with a plateauing of the record efficiencies of GaAs and Si solar cells at levels close to this limit. It is believed #233542 - $15.00 USD (C) 2015 OSA
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that silicon based multijunction solar cells are a promising way to surpass the efficiency milestone of 30% for non-concentrating solar cells, while keeping the cost low by avoiding the use of expensive Ge or GaAs substrates. Achieving this is important as there is a growing demand on high-efficiency non-concentrating terrestrial PV systems. A very promising configuration is the GaAs on Si dual-junction solar cell [4, 12–15]. Using Si as the bottom cell keeps the cost low as Si solar cells have reached a state of development in which they can be produced at a low price [16]. Our calculation based on detailed balance limit method [17] shows that a double-junction solar cell with a silicon bottom cell has an efficiency limit of around 45% with optimum top cell bandgap of 1.7-1.8 eV. GaAs has a bandgap of 1.42 eV, which is non-ideal for a combination with Si. However, the efficiency limit of the GaAs Si tandem solar cell is 39% for a current matched 2-terminal configuration and 42% for a non-current-matched 4-terminal configuration, which are still much higher than the single-junction limit. Moreover, GaAs and Si solar cells are mature technologies, and it can be expected that both sub-cells work at a considerable fraction of their limit efficiency. We will show in this work that current GaAs and Si materials have the potential to achieve more than 30% conversion efficiency under laboratory testing and outdoor conditions using a 4-terminal configuration. Thus, the GaAs on Si approach is relevant and interesting from a technological perspective, especially in the near term. Also, calculations done for GaAs on Si tandem may shed useful insights on other tandems such as perovskites on Si due to their similar bandgap combination. GaAs/Si dual-junction solar cell can be connected in a 2-terminal or a 4-terminal configuration. The 2-terminal configuration consists of monolithically integrated GaAs and Si sub-cells that are series connected via a tunnel junction. Historically, the approach of heteroepitaxial growth of III-V material on Si substrate had produced interesting results [18–21]. However, due to difficulties to further improve material quality, a very high efficiency tandem device has not been demonstrated yet. Recently, layer transfer and wafer bonding emerged as promising approaches to circumvent material growth challenges and produce high-quality multijunction devices [2–4, 10, 22–24]. The 4-terminal configuration involves integrating two independently connected sub-cells. This configuration relaxes the current matching constraint of series connection. It allows the use of best single-junction solar cells as sub-cells without having to overcome the material and process challenges associated with monolithic multijunction cells. Moreover, it offers more freedom in circuit wiring at the module level [25]. 4-terminal integration of sub-cells is challenging, but had produced high tandem efficiencies of over 30% under concentration [12]. With the availability of better quality sub-cells, the 4-terminal approach may still be an interesting and viable option. It is known that multijunction solar cells exhibit more spectral sensitivity than singlejunction solar cells [26, 27]. Therefore the actual energy yield potential of GaAs/Si tandem solar cells deployed in the outdoor environment needs to be carefully assessed, instead of just looking at the efficiency under standard testing conditions. Furthermore, studying the energy yield allows quantifying the potential benefits of a 4-terminal contact scheme. Many extensive and systematic studies were performed to study the spectral effect on the current and power production of triple-junction concentrator photovoltaic (CPV) cells and modules [27–33]. These studies mostly used simulated spectra as input and simulated solar cell characteristics using detailed balance method or 1D analytical models. They investigated the effects of various atmospheric parameters such as air mass, aerosol optical depth, and water vapor. Most findings indicate that current mismatch loss due to spectrum effect can be significant, and should be taken into account when designing multijunction solar cells. However, these studies focused on CPV applications, and did not take into account other variations in outdoor conditions such as rapid changes in spectral composition and intensity due to cloud coverage. Therefore, these results are only partially relevant for the proposed Si based tandem solar cells in non-concentrating systems operating in different climates. In this paper, we used a new method to calculate the energy yield, employing extensive optical simulation with measured real-time spectrum data as input, and device model with realistic cell parameters to simulate device performance. The period of investigation is from
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Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A384
Feb 2013 to Jan 2014, completing a course of one full year. Two locations are chosen for this study, Singapore and Denver, which represent distinctly different climate conditions. The calculated energy yield is used to assess the realistic performance potential of GaAs/Si tandem solar cell in different environments, as well as the effect of various yield loss channels for this type of solar cell. 2. Device model and illumination data The device structures used in this study are shown in Fig. 1. We considered a 1D structure with the following features: antireflection coating (ARC), GaAs (together with InGaP window layer and back surface field) and Si layers, and Al back contact which also serves as back reflector. This generalized structure does not incorporate any 2D effects like contact schemes, elaborate photon management features, losses related to adhesive interconnection layers or tunnel junctions. Two configurations were investigated: a 2-terminal series connected tandem structure with 200 nm GaAs top cell and 200 μm Si bottom cell (referred to hereafter as 2T), and a 4terminal tandem structure with 1.0 μm GaAs top cell independently connected with a 200 μm bottom cell (referred to as 4T). The GaAs cell thickness of 200 nm was chosen for 2T so as to achieve current matching under standard testing condition with AM1.5G spectrum. For 4T, the top cell thickness was optimized with respect to tandem efficiency, resulting in the used value of 1.0 µm.
Fig. 1. Structure of the simplified GaAs on Si tandem solar cell being investigated. 2T configuration makes use of a 200 nm GaAs top cell, and 4T configuration makes use of a 1 μm top cell. In 4T, a hypothetical insulation layer with zero thickness separates the two sub-cells.
The transfer matrix method (TMM) was used to calculate the depth-resolved photogeneration profile within the different layers, for varying input spectra. Normal light incidence was assumed in TMM calculation, but intensity reduction at higher incidence angles are accounted for in the spectrum data. The electrical characteristics of the two subcells were simulated using PC1D [34, 35]. The electrical characteristics of the tandem device were calculated using a circuit model to connect the sub-cells. The device parameters for the two sub-cells used in this simulation are shown in Table 1. These parameters represent typical baseline solar cell devices [36, 37] with non-record efficiencies that can be found in laboratories and also in commercial production. PC1D simulation shows that under standard testing conditions, the combination of these two sub-cells results in a 4-terminal efficiency of over 30% and a moderately high 2-terminal efficiency higher than most single-junction solar cells (see AM1.5G efficiency in Table 2). In principle, better quality GaAs or Si sub-cells can be used to produce even higher efficiency tandem device.
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Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A385
Table 1. Device parameters of GaAs/Si tandem solar cell model used in PC1D simulation. Si Bottom cell Thickness (µm) Background doping (cm−3) Diffusion (cm−3) Bulk lifetime (µs)
200 1.5 × 1016 (p-type) 7.2 × 1020 (n-type front, erfc) 200 Front 6 × 105 Surface recombination velocity (cm/s) Back 1 × 103 Voc (mV)* 605 (2T) / 585 (4T) Jsc (mA/cm2)* 18.6 (2T) / 8.8 (4T) Typical single-junction efficiencies 19% *values are for sub-cells in a tandem configuration, under AM1.5G illumination.
GaAs top cell 0.2 (2T) / 1 (4T) 1 × 1017 (p-type) 6 × 1017 (n-type front, uniform) 0.4 Front = Back = 7 × 104 1109 (2T) / 1081 (4T) 18.9 (2T) / 28.5 (4T) 26%
The time-dependent solar spectrum data used to model realistic energy yield are derived from spectroradiometer measurements performed in the two locations. The Denver data were provided by the National Renewable Energy Laboratory (NREL), Solar Radiation Research Laboratory, Baseline Measurement System [38]. The Singapore data were measured at the Solar Energy Research Institute of Singapore (SERIS). The recording frequency is one spectrum per minute, and the recorded wavelength range was 350 nm to 1060 nm. The period of study is from Feb 2013 to Jan 2014. Within this period, we picked one spectrum every 5 minutes. This time resolution is particularly necessary for Singapore, as rapid movement of low-altitude clouds results in rapid fluctuation in illumination intensity as well as spectrum. One way to characterize spectra is to obtain the average photon energy (APE) value, which is calculated by dividing the integrated irradiance in a certain wavelength range with the total photon number in that same wavelength range: λ2
λ2
λ I (λ )d λ = λ I (λ )d λ APE = λ λ I (λ ) λ Φ(λ )d λ λ hc λ d λ 1
1
2
2
1
(1)
1
where I (λ ) is the wavelength resolved intensity distribution of a spectrum, and Φ (λ ) is the wavelength resolved photon flux density. The APE value characterizes how blue- or red-rich a spectrum is. In general, the spectra in Singapore tend to be significantly more blue-rich than AM1.5G spectrum, and have average APE values of over 1.9 eV. This is because Singapore has an air mass closer to AM1.0 instead of AM1.5. APE of Denver spectra is closer to AM1.5G value of 1.88 eV. However, there is a wide variation in spectral compositions, and average APE value is not sufficient in describing the illumination condition. For a more comprehensive description, we sorted the collected spectra in the whole year into bins of different APE values, with an interval of 0.05 eV. Then we took the average spectrum within each bin and generated the representative spectra for different spectral compositions. These representative spectra are shown in Fig. 2. Note that these spectra are used here for illustrative purposes. Yield calculations are performed using the actual spectra. As can be seen, the Singapore spectra are significantly different from Denver’s, even when they are within the same APE interval. This is due to markedly different atmospheric parameters such as water vapor. The different atmospheric conditions affect radiative transfer differently, resulting in different absorption and scattering of sunlight. In addition, there is much more fluctuations in intensity and spectrum in Singapore than in Denver. Therefore, studying energy yield in these two locations can shed insight on how much performance variation there may be for GaAs/Si tandem solar cells in different climates.
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Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A386
Fig. 2. Spectra with different spectral composition in (a) Singapore and in (b) Denver, characterized by different APE ranges (values in the figure indicate the left bound of an interval). These spectra are obtained from averaging real measured spectra.
To calculate energy yield, the absorption profile in each layer of the device was calculated using TMM, with the recorded spectrum data as input. After translating the absorption profile into cumulative photogeneration profiles and loading them into PC1D, the I-V characteristic was simulated. Repeating this procedure for every spectral data point, the time-resolved power output was obtained. The annual energy yield is simply the integration of the power output over the period of one year. 3. Results and discussion 3.1 Effect of realistic spectral composition and intensity on efficiency GaAs/Si tandem solar cell efficiency is affected by illumination conditions. This is not only due to losses from current mismatch, but also due to the intrinsic effect of changing spectral composition and intensity level on the efficiency limit of a tandem solar cell, even for a 4terminal configuration. Different spectral compositions will imply different current generations for the sub-cells, and thus different 4-terminal tandem efficiencies. This will be referred to as spectral effect in this paper. For the same spectral composition, changes in the intensity levels also affect efficiency by affecting the Voc. This will be termed intensity effect. On top of this, current mismatch introduces additional losses for 2-terminal devices, and will be called current mismatch effect. To see the effect of various illumination conditions on tandem solar cell efficiency, the representative spectra measured in Singapore are grouped according to their APE values as described in the methodology section, and tandem cell efficiencies are simulated under these spectra scaled to different intensity levels. The efficiency as a function of APE values and intensity levels in Singapore is shown in Fig. 3. For a given APE, the efficiency depends logarithmically on the intensity level, as expected from the diode equation. In comparison, the variation of efficiency due to changing APE values can be more significant. For 2T, extreme spectra with high APE result in a larger drop in efficiency than low intensities. 4T also suffers from spectrum variation, but to a much lesser extent due to the absence of the currentmatching requirement. For 4T, a general rule is: the higher the APE, the lower the efficiency. This is because the bottom cell receives a much smaller portion of the available irradiance under a blue rich spectrum. It should also be noted that the more blue rich the spectrum is, the smaller the total available current will be for a given intensity. As a result, Jsc of the bottom cell decreases significantly, Voc and FF decrease as well. This is not compensated by the marginal increase of the top cell efficiency. Therefore, it can be said that one major effect of changing illumination condition is a change in current generation in the two sub-cells,
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Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A387
especially the Si bottom cell. This makes photon management of the bottom cell a potentially important issue when designing GaAs/Si tandems.
Fig. 3. Simulated tandem efficiency for (a) 2T and (b) 4T configurations under different spectral compositions and intensity levels. Efficiency can vary significantly under different illumination conditions. The variation is from 17 to 28% for 2T and 27-33% for 4T. APE value for AM1.5G spectrum is indicated by a dashed line.
3.2 Annual energy yield Efficiency variation under different APE and intensity levels do not translate directly into impact on annual energy yield, as different conditions occur with different frequency throughout the year. Also, efficiencies at different time points are weighted differently in terms of contribution to total power production. Therefore we simulated the annual energy yield of both 2T and 4T configuration in the period of Feb 2013 to Jan 2014 using the methodology described previously. The summary of results is shown in Table 2 and Fig. 4. For this combination of material systems, 4T has a distinctive yield advantage compared to 2T, especially for Singapore, where 4T yield is about 20% higher than that of 2T. This is much more than the AM1.5G efficiency difference of about 15% between the two configurations. An important figure of merit we choose to quantify the tandem solar cell performance under realistic conditions is the annual harvesting efficiency, which is defined as the ratio between the total electric energy generated in a year and the total solar energy received during the same period. The harvesting efficiency is lower than the AM1.5G efficiency under standard testing conditions by an amount of ~1% (absolute) for 4T and 1-2% (absolute) for 2T. Table 2. Summary of annual energy yield calculation for 2T and 4T in Singapore and Denver. Location
Configuration
Singapore (Yearly insolation = 1588 kWh/m2) Denver (Yearly insolation = 1958 kWh/m2)
2T
Harvesting efficiency 25.3%
4T
30.3%
31.1%
481
2T
26.1%
27.0%
511
4T
30.5%
31.1%
597
AM1.5G efficiency 27.0%
Yearly yield (kWh/m2) 402
The decrease in energy conversion efficiency comes from several loss channels identified earlier: spectral effect, intensity effect, and current mismatch effect. To better understand the contribution of each loss channel, we performed a breakdown of these loss mechanisms by turning them on one at a time. The benchmark for comparison is a 4-terminal connected
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Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A388
device with the top cell thicknesses of 2T or 4T configuration, under standard testing condition with an illumination of the standard AM1.5G spectrum at room temperature. The breakdown for simulated devices in 2T and 4T configuration in Singapore is shown in Fig. 5. As learned from Fig. 3, even without current mismatch loss, changing spectrum still may lead to a drop in harvesting efficiency for 4T. This spectral effect is smaller for 2T. This is because with a thinner top cell, the intrinsic variation in 4-terminal connected 2T tandem efficiency (still with 200 nm top cell) due to spectrum change is smaller than that of 4T. Changing intensities under outdoor condition results in a decrease of efficiency from that under standard test condition, as low intensity occurs quite often. This effect is more pronounced in tandem solar cells compared to single-junction solar cells, as the incoming radiation is divided among the sub-cells. On top of this, 2-terminal device suffers from current mismatch as the incoming spectrum deviates from the optimal spectrum it is optimized to. This is particularly so in Singapore, because the average spectrum in Singapore is significantly different from AM1.5G spectrum. Not taking this into account when designing GaAs/Si tandem solar cells can lead to additional current mismatch loss of up to 1% (absolute). Our calculations show that by simply fine tuning the top cell thickness to achieve current matching for the average Singapore spectrum, the current mismatch loss can be reduced significantly, to a level close to that of Denver’s. However, even with minimal current mismatch loss, the additional benefit provided by adding a Si bottom cell is not significant, unless better Si bottom cell is used.
Fig. 4. The calculated average daily energy yield for 12 months in (a) Singapore and (b) Denver. On average, the yield of the 4T configuration is over 15% higher than that of the 2T configuration.
We have also briefly estimated the loss due to elevated operating temperature (thermal effect) by setting it to 45 °C in PC1D models. This is the expected Si module temperature based on measurements made in SERIS. With more efficient tandem solar cells, the temperature is expected to be lower. The inclusion of temperature effect introduced another 1.5% (absolute) drop in the harvesting efficiency. This breakdown of harvesting efficiency drop is dependent on characteristics of solar irradiance in a certain location. For Denver, the relative contribution of different loss channels is different, with much less spectral and current mismatch losses.
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Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A389
Fig. 5. Loss breakdown of annual harvesting efficiency and yield for (a) 2T and (b) 4T configuration in Singapore. Significant loss comes from current mismatch loss due to variation in spectral composition and non-optimized top cell thickness.
4. Conclusion In this study, we theoretically calculated the energy yield of GaAs/Si tandem solar cell through a case study using measured spectrum data from 2013 to 2014. Our results show that 4-terminal GaAs/Si tandem solar cell using existing cell technologies can achieve 30% harvesting efficiency under outdoor conditions. With better sub-cells, the harvesting efficiency can potentially be even higher. This suggests that GaAs on Si, despite having nonideal bandgap combination, is still an interesting technology option to realize high efficiency, and thus should not be dismissed. The yield potential for 2- and 4-terminal configuration is very different for GaAs/Si tandem solar cells. The 4-terminal configuration has a distinctive advantage in yield potential, which is over 20% higher than for the 2-terminal scheme in Singapore and 17% higher in Denver. This difference is larger than what is predicted using the AM1.5G efficiency values. We have also shown in detail how the tandem efficiency changes with varying intensity and spectral composition. This efficiency variation results in an annual harvesting efficiency that is significantly lower than the efficiency under standard test conditions. The drop in harvesting efficiency can be attributed to different loss channels. The contribution of different loss channels depends on the location of interest. In general, low irradiance loss and current mismatch loss play the largest role, especially in Singapore where rapid fluctuations in illumination condition is common due to low-altitude cloud movements. Overall, it can be concluded that outdoor illumination condition at the specific site of deployment should be taken into account when designing GaAs/Si tandem solar cells. Also, the theoretical energy yield benefit of a 4-terminal device emphasizes the need for further technology development in this design space. With careful optimization, a GaAs/Si tandem solar cell employing already available sub-cells is a promising candidate for surpassing 30% energy conversion efficiency in non-concentration applications. Acknowledgment This work is supported by the Solar Energy Research Institute of Singapore (SERIS) and the Singapore-MIT Alliance for Research and Technology (SMART). SERIS is sponsored by the National University of Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). We would also like to thank the NREL Solar Radiation Research Laboratory and André Nobre from SERIS’ Solar Energy Systems Cluster for providing the spectral irradiance data, and Jonathan Mailoa from MIT for providing valuable suggestions and comments.
#233542 - $15.00 USD (C) 2015 OSA
Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A390
Solar Energy Research Institute of Singapore (SERIS), National University of Singapore, 7 Engineering Drive 1, 117574, Singapore 2 NUS Graduate School for Integrative Sciences & Engineering (NGS), 28 Medical Drive, 117456, Singapore 3 Singapore-MIT Alliance for Research and Technology (SMART), 1 CREATE Way, 138602, Singapore 4 Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA * [email protected]
Abstract: Si based tandem solar cells represent an alternative to traditional compound III-V multijunction cells as a promising way to achieve high efficiencies. A theoretical study on the energy yield of GaAs on Si (GaAs/Si) tandem solar cells is performed to assess their energy yield potential under realistic illumination conditions with varying spectrum. We find that the yield of a 4-terminal contact scheme with thick top cell is more than 15% higher than for a 2-terminal scheme. Furthermore, we quantify the main losses that occur for this type of solar cell under varying spectra. Apart from current mismatch, we find that a significant power loss can be attributed to low irradiance seen by the sub-cells. The study shows that despite non-optimal bandgap combination, GaAs/Si tandem solar cells have the potential to surpass 30% energy conversion efficiency. ©2015 Optical Society of America OCIS codes: (350.6050) Solar energy; (010.5620) Radiative transfer.
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Olson, “The influence of spectral solar irradiance variations on the performance of selected single-junction and multijunction solar cells,” Sol. Cells 31(3), 259–278 (1991). 28. S. P. Philipps, G. Peharz, R. Hoheisel, T. Hornung, N. M. Al-Abbadi, F. Dimroth, and A. W. Bett, “Energy harvesting efficiency of III–V triple-junction concentrator solar cells under realistic spectral conditions,” Sol. Energy Mater. Sol. Cells 94(5), 869–877 (2010). 29. N. L. A. Chan, T. B. Young, H. E. Brindley, N. J. Ekins-Daukes, K. Araki, Y. Kemmoku, and M. Yamaguchi, “Validation of energy prediction method for a concentrator photovoltaic module in Toyohashi Japan,” Prog. Photovolt. Res. Appl. 21(8), 1598–1610 (2013). 30. W. Xiaoting and A. Barnett, “The Effect of Spectrum Variation on the Energy Production of Triple-Junction Solar Cells,” IEEE J. Photovolt. 2(4), 417–423 (2012). 31. K. Araki and M. Yamaguchi, “Influences of spectrum change to 3-junction concentrator cells,” Sol. Energy Mater. Sol. Cells 75(3–4), 707–714 (2003). 32. E. F. Fernández, F. Almonacid, J. A. Ruiz-Arias, and A. Soria-Moya, “Analysis of the spectral variations on the performance of high concentrator photovoltaic modules operating under different real climate conditions,” Sol. Energy Mater. Sol. Cells 127, 179–187 (2014). 33. G. S. Kinsey and K. M. Edmondson, “Spectral response and energy output of concentrator multijunction solar cells,” Prog. Photovolt. Res. Appl. 17(5), 279–288 (2009). 34. D. A. Clugston and P. A. Basore, “PC1D version 5: 32-bit solar cell modeling on personal computers,” in Proc. of 26th IEEE Photovoltaic Specialists Conference (Anaheim, 1997), pp. 207–210. 35. H. Haug, B. R. Olaisen, Ø. Nordseth, and E. S. Marstein, “A Graphical User Interface for Multivariable Analysis of Silicon Solar Cells Using Scripted PC1D Simulations,” Energy Procedia 38, 72–79 (2013). 36. G. J. Bauhuis, P. Mulder, E. J. Haverkamp, J. C. C. M. Huijben, and J. J. 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1. Introduction Recently, there is a revived interest in silicon based multijunction solar cell concepts within the photovoltaic research community [1–10]. This interest is motivated by the intrinsic efficiency limits of around 30% [11] of single-junction solar cells combined with a plateauing of the record efficiencies of GaAs and Si solar cells at levels close to this limit. It is believed #233542 - $15.00 USD (C) 2015 OSA
Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A383
that silicon based multijunction solar cells are a promising way to surpass the efficiency milestone of 30% for non-concentrating solar cells, while keeping the cost low by avoiding the use of expensive Ge or GaAs substrates. Achieving this is important as there is a growing demand on high-efficiency non-concentrating terrestrial PV systems. A very promising configuration is the GaAs on Si dual-junction solar cell [4, 12–15]. Using Si as the bottom cell keeps the cost low as Si solar cells have reached a state of development in which they can be produced at a low price [16]. Our calculation based on detailed balance limit method [17] shows that a double-junction solar cell with a silicon bottom cell has an efficiency limit of around 45% with optimum top cell bandgap of 1.7-1.8 eV. GaAs has a bandgap of 1.42 eV, which is non-ideal for a combination with Si. However, the efficiency limit of the GaAs Si tandem solar cell is 39% for a current matched 2-terminal configuration and 42% for a non-current-matched 4-terminal configuration, which are still much higher than the single-junction limit. Moreover, GaAs and Si solar cells are mature technologies, and it can be expected that both sub-cells work at a considerable fraction of their limit efficiency. We will show in this work that current GaAs and Si materials have the potential to achieve more than 30% conversion efficiency under laboratory testing and outdoor conditions using a 4-terminal configuration. Thus, the GaAs on Si approach is relevant and interesting from a technological perspective, especially in the near term. Also, calculations done for GaAs on Si tandem may shed useful insights on other tandems such as perovskites on Si due to their similar bandgap combination. GaAs/Si dual-junction solar cell can be connected in a 2-terminal or a 4-terminal configuration. The 2-terminal configuration consists of monolithically integrated GaAs and Si sub-cells that are series connected via a tunnel junction. Historically, the approach of heteroepitaxial growth of III-V material on Si substrate had produced interesting results [18–21]. However, due to difficulties to further improve material quality, a very high efficiency tandem device has not been demonstrated yet. Recently, layer transfer and wafer bonding emerged as promising approaches to circumvent material growth challenges and produce high-quality multijunction devices [2–4, 10, 22–24]. The 4-terminal configuration involves integrating two independently connected sub-cells. This configuration relaxes the current matching constraint of series connection. It allows the use of best single-junction solar cells as sub-cells without having to overcome the material and process challenges associated with monolithic multijunction cells. Moreover, it offers more freedom in circuit wiring at the module level [25]. 4-terminal integration of sub-cells is challenging, but had produced high tandem efficiencies of over 30% under concentration [12]. With the availability of better quality sub-cells, the 4-terminal approach may still be an interesting and viable option. It is known that multijunction solar cells exhibit more spectral sensitivity than singlejunction solar cells [26, 27]. Therefore the actual energy yield potential of GaAs/Si tandem solar cells deployed in the outdoor environment needs to be carefully assessed, instead of just looking at the efficiency under standard testing conditions. Furthermore, studying the energy yield allows quantifying the potential benefits of a 4-terminal contact scheme. Many extensive and systematic studies were performed to study the spectral effect on the current and power production of triple-junction concentrator photovoltaic (CPV) cells and modules [27–33]. These studies mostly used simulated spectra as input and simulated solar cell characteristics using detailed balance method or 1D analytical models. They investigated the effects of various atmospheric parameters such as air mass, aerosol optical depth, and water vapor. Most findings indicate that current mismatch loss due to spectrum effect can be significant, and should be taken into account when designing multijunction solar cells. However, these studies focused on CPV applications, and did not take into account other variations in outdoor conditions such as rapid changes in spectral composition and intensity due to cloud coverage. Therefore, these results are only partially relevant for the proposed Si based tandem solar cells in non-concentrating systems operating in different climates. In this paper, we used a new method to calculate the energy yield, employing extensive optical simulation with measured real-time spectrum data as input, and device model with realistic cell parameters to simulate device performance. The period of investigation is from
#233542 - $15.00 USD (C) 2015 OSA
Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A384
Feb 2013 to Jan 2014, completing a course of one full year. Two locations are chosen for this study, Singapore and Denver, which represent distinctly different climate conditions. The calculated energy yield is used to assess the realistic performance potential of GaAs/Si tandem solar cell in different environments, as well as the effect of various yield loss channels for this type of solar cell. 2. Device model and illumination data The device structures used in this study are shown in Fig. 1. We considered a 1D structure with the following features: antireflection coating (ARC), GaAs (together with InGaP window layer and back surface field) and Si layers, and Al back contact which also serves as back reflector. This generalized structure does not incorporate any 2D effects like contact schemes, elaborate photon management features, losses related to adhesive interconnection layers or tunnel junctions. Two configurations were investigated: a 2-terminal series connected tandem structure with 200 nm GaAs top cell and 200 μm Si bottom cell (referred to hereafter as 2T), and a 4terminal tandem structure with 1.0 μm GaAs top cell independently connected with a 200 μm bottom cell (referred to as 4T). The GaAs cell thickness of 200 nm was chosen for 2T so as to achieve current matching under standard testing condition with AM1.5G spectrum. For 4T, the top cell thickness was optimized with respect to tandem efficiency, resulting in the used value of 1.0 µm.
Fig. 1. Structure of the simplified GaAs on Si tandem solar cell being investigated. 2T configuration makes use of a 200 nm GaAs top cell, and 4T configuration makes use of a 1 μm top cell. In 4T, a hypothetical insulation layer with zero thickness separates the two sub-cells.
The transfer matrix method (TMM) was used to calculate the depth-resolved photogeneration profile within the different layers, for varying input spectra. Normal light incidence was assumed in TMM calculation, but intensity reduction at higher incidence angles are accounted for in the spectrum data. The electrical characteristics of the two subcells were simulated using PC1D [34, 35]. The electrical characteristics of the tandem device were calculated using a circuit model to connect the sub-cells. The device parameters for the two sub-cells used in this simulation are shown in Table 1. These parameters represent typical baseline solar cell devices [36, 37] with non-record efficiencies that can be found in laboratories and also in commercial production. PC1D simulation shows that under standard testing conditions, the combination of these two sub-cells results in a 4-terminal efficiency of over 30% and a moderately high 2-terminal efficiency higher than most single-junction solar cells (see AM1.5G efficiency in Table 2). In principle, better quality GaAs or Si sub-cells can be used to produce even higher efficiency tandem device.
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Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A385
Table 1. Device parameters of GaAs/Si tandem solar cell model used in PC1D simulation. Si Bottom cell Thickness (µm) Background doping (cm−3) Diffusion (cm−3) Bulk lifetime (µs)
200 1.5 × 1016 (p-type) 7.2 × 1020 (n-type front, erfc) 200 Front 6 × 105 Surface recombination velocity (cm/s) Back 1 × 103 Voc (mV)* 605 (2T) / 585 (4T) Jsc (mA/cm2)* 18.6 (2T) / 8.8 (4T) Typical single-junction efficiencies 19% *values are for sub-cells in a tandem configuration, under AM1.5G illumination.
GaAs top cell 0.2 (2T) / 1 (4T) 1 × 1017 (p-type) 6 × 1017 (n-type front, uniform) 0.4 Front = Back = 7 × 104 1109 (2T) / 1081 (4T) 18.9 (2T) / 28.5 (4T) 26%
The time-dependent solar spectrum data used to model realistic energy yield are derived from spectroradiometer measurements performed in the two locations. The Denver data were provided by the National Renewable Energy Laboratory (NREL), Solar Radiation Research Laboratory, Baseline Measurement System [38]. The Singapore data were measured at the Solar Energy Research Institute of Singapore (SERIS). The recording frequency is one spectrum per minute, and the recorded wavelength range was 350 nm to 1060 nm. The period of study is from Feb 2013 to Jan 2014. Within this period, we picked one spectrum every 5 minutes. This time resolution is particularly necessary for Singapore, as rapid movement of low-altitude clouds results in rapid fluctuation in illumination intensity as well as spectrum. One way to characterize spectra is to obtain the average photon energy (APE) value, which is calculated by dividing the integrated irradiance in a certain wavelength range with the total photon number in that same wavelength range: λ2
λ2
λ I (λ )d λ = λ I (λ )d λ APE = λ λ I (λ ) λ Φ(λ )d λ λ hc λ d λ 1
1
2
2
1
(1)
1
where I (λ ) is the wavelength resolved intensity distribution of a spectrum, and Φ (λ ) is the wavelength resolved photon flux density. The APE value characterizes how blue- or red-rich a spectrum is. In general, the spectra in Singapore tend to be significantly more blue-rich than AM1.5G spectrum, and have average APE values of over 1.9 eV. This is because Singapore has an air mass closer to AM1.0 instead of AM1.5. APE of Denver spectra is closer to AM1.5G value of 1.88 eV. However, there is a wide variation in spectral compositions, and average APE value is not sufficient in describing the illumination condition. For a more comprehensive description, we sorted the collected spectra in the whole year into bins of different APE values, with an interval of 0.05 eV. Then we took the average spectrum within each bin and generated the representative spectra for different spectral compositions. These representative spectra are shown in Fig. 2. Note that these spectra are used here for illustrative purposes. Yield calculations are performed using the actual spectra. As can be seen, the Singapore spectra are significantly different from Denver’s, even when they are within the same APE interval. This is due to markedly different atmospheric parameters such as water vapor. The different atmospheric conditions affect radiative transfer differently, resulting in different absorption and scattering of sunlight. In addition, there is much more fluctuations in intensity and spectrum in Singapore than in Denver. Therefore, studying energy yield in these two locations can shed insight on how much performance variation there may be for GaAs/Si tandem solar cells in different climates.
#233542 - $15.00 USD (C) 2015 OSA
Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A386
Fig. 2. Spectra with different spectral composition in (a) Singapore and in (b) Denver, characterized by different APE ranges (values in the figure indicate the left bound of an interval). These spectra are obtained from averaging real measured spectra.
To calculate energy yield, the absorption profile in each layer of the device was calculated using TMM, with the recorded spectrum data as input. After translating the absorption profile into cumulative photogeneration profiles and loading them into PC1D, the I-V characteristic was simulated. Repeating this procedure for every spectral data point, the time-resolved power output was obtained. The annual energy yield is simply the integration of the power output over the period of one year. 3. Results and discussion 3.1 Effect of realistic spectral composition and intensity on efficiency GaAs/Si tandem solar cell efficiency is affected by illumination conditions. This is not only due to losses from current mismatch, but also due to the intrinsic effect of changing spectral composition and intensity level on the efficiency limit of a tandem solar cell, even for a 4terminal configuration. Different spectral compositions will imply different current generations for the sub-cells, and thus different 4-terminal tandem efficiencies. This will be referred to as spectral effect in this paper. For the same spectral composition, changes in the intensity levels also affect efficiency by affecting the Voc. This will be termed intensity effect. On top of this, current mismatch introduces additional losses for 2-terminal devices, and will be called current mismatch effect. To see the effect of various illumination conditions on tandem solar cell efficiency, the representative spectra measured in Singapore are grouped according to their APE values as described in the methodology section, and tandem cell efficiencies are simulated under these spectra scaled to different intensity levels. The efficiency as a function of APE values and intensity levels in Singapore is shown in Fig. 3. For a given APE, the efficiency depends logarithmically on the intensity level, as expected from the diode equation. In comparison, the variation of efficiency due to changing APE values can be more significant. For 2T, extreme spectra with high APE result in a larger drop in efficiency than low intensities. 4T also suffers from spectrum variation, but to a much lesser extent due to the absence of the currentmatching requirement. For 4T, a general rule is: the higher the APE, the lower the efficiency. This is because the bottom cell receives a much smaller portion of the available irradiance under a blue rich spectrum. It should also be noted that the more blue rich the spectrum is, the smaller the total available current will be for a given intensity. As a result, Jsc of the bottom cell decreases significantly, Voc and FF decrease as well. This is not compensated by the marginal increase of the top cell efficiency. Therefore, it can be said that one major effect of changing illumination condition is a change in current generation in the two sub-cells,
#233542 - $15.00 USD (C) 2015 OSA
Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A387
especially the Si bottom cell. This makes photon management of the bottom cell a potentially important issue when designing GaAs/Si tandems.
Fig. 3. Simulated tandem efficiency for (a) 2T and (b) 4T configurations under different spectral compositions and intensity levels. Efficiency can vary significantly under different illumination conditions. The variation is from 17 to 28% for 2T and 27-33% for 4T. APE value for AM1.5G spectrum is indicated by a dashed line.
3.2 Annual energy yield Efficiency variation under different APE and intensity levels do not translate directly into impact on annual energy yield, as different conditions occur with different frequency throughout the year. Also, efficiencies at different time points are weighted differently in terms of contribution to total power production. Therefore we simulated the annual energy yield of both 2T and 4T configuration in the period of Feb 2013 to Jan 2014 using the methodology described previously. The summary of results is shown in Table 2 and Fig. 4. For this combination of material systems, 4T has a distinctive yield advantage compared to 2T, especially for Singapore, where 4T yield is about 20% higher than that of 2T. This is much more than the AM1.5G efficiency difference of about 15% between the two configurations. An important figure of merit we choose to quantify the tandem solar cell performance under realistic conditions is the annual harvesting efficiency, which is defined as the ratio between the total electric energy generated in a year and the total solar energy received during the same period. The harvesting efficiency is lower than the AM1.5G efficiency under standard testing conditions by an amount of ~1% (absolute) for 4T and 1-2% (absolute) for 2T. Table 2. Summary of annual energy yield calculation for 2T and 4T in Singapore and Denver. Location
Configuration
Singapore (Yearly insolation = 1588 kWh/m2) Denver (Yearly insolation = 1958 kWh/m2)
2T
Harvesting efficiency 25.3%
4T
30.3%
31.1%
481
2T
26.1%
27.0%
511
4T
30.5%
31.1%
597
AM1.5G efficiency 27.0%
Yearly yield (kWh/m2) 402
The decrease in energy conversion efficiency comes from several loss channels identified earlier: spectral effect, intensity effect, and current mismatch effect. To better understand the contribution of each loss channel, we performed a breakdown of these loss mechanisms by turning them on one at a time. The benchmark for comparison is a 4-terminal connected
#233542 - $15.00 USD (C) 2015 OSA
Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A388
device with the top cell thicknesses of 2T or 4T configuration, under standard testing condition with an illumination of the standard AM1.5G spectrum at room temperature. The breakdown for simulated devices in 2T and 4T configuration in Singapore is shown in Fig. 5. As learned from Fig. 3, even without current mismatch loss, changing spectrum still may lead to a drop in harvesting efficiency for 4T. This spectral effect is smaller for 2T. This is because with a thinner top cell, the intrinsic variation in 4-terminal connected 2T tandem efficiency (still with 200 nm top cell) due to spectrum change is smaller than that of 4T. Changing intensities under outdoor condition results in a decrease of efficiency from that under standard test condition, as low intensity occurs quite often. This effect is more pronounced in tandem solar cells compared to single-junction solar cells, as the incoming radiation is divided among the sub-cells. On top of this, 2-terminal device suffers from current mismatch as the incoming spectrum deviates from the optimal spectrum it is optimized to. This is particularly so in Singapore, because the average spectrum in Singapore is significantly different from AM1.5G spectrum. Not taking this into account when designing GaAs/Si tandem solar cells can lead to additional current mismatch loss of up to 1% (absolute). Our calculations show that by simply fine tuning the top cell thickness to achieve current matching for the average Singapore spectrum, the current mismatch loss can be reduced significantly, to a level close to that of Denver’s. However, even with minimal current mismatch loss, the additional benefit provided by adding a Si bottom cell is not significant, unless better Si bottom cell is used.
Fig. 4. The calculated average daily energy yield for 12 months in (a) Singapore and (b) Denver. On average, the yield of the 4T configuration is over 15% higher than that of the 2T configuration.
We have also briefly estimated the loss due to elevated operating temperature (thermal effect) by setting it to 45 °C in PC1D models. This is the expected Si module temperature based on measurements made in SERIS. With more efficient tandem solar cells, the temperature is expected to be lower. The inclusion of temperature effect introduced another 1.5% (absolute) drop in the harvesting efficiency. This breakdown of harvesting efficiency drop is dependent on characteristics of solar irradiance in a certain location. For Denver, the relative contribution of different loss channels is different, with much less spectral and current mismatch losses.
#233542 - $15.00 USD (C) 2015 OSA
Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A389
Fig. 5. Loss breakdown of annual harvesting efficiency and yield for (a) 2T and (b) 4T configuration in Singapore. Significant loss comes from current mismatch loss due to variation in spectral composition and non-optimized top cell thickness.
4. Conclusion In this study, we theoretically calculated the energy yield of GaAs/Si tandem solar cell through a case study using measured spectrum data from 2013 to 2014. Our results show that 4-terminal GaAs/Si tandem solar cell using existing cell technologies can achieve 30% harvesting efficiency under outdoor conditions. With better sub-cells, the harvesting efficiency can potentially be even higher. This suggests that GaAs on Si, despite having nonideal bandgap combination, is still an interesting technology option to realize high efficiency, and thus should not be dismissed. The yield potential for 2- and 4-terminal configuration is very different for GaAs/Si tandem solar cells. The 4-terminal configuration has a distinctive advantage in yield potential, which is over 20% higher than for the 2-terminal scheme in Singapore and 17% higher in Denver. This difference is larger than what is predicted using the AM1.5G efficiency values. We have also shown in detail how the tandem efficiency changes with varying intensity and spectral composition. This efficiency variation results in an annual harvesting efficiency that is significantly lower than the efficiency under standard test conditions. The drop in harvesting efficiency can be attributed to different loss channels. The contribution of different loss channels depends on the location of interest. In general, low irradiance loss and current mismatch loss play the largest role, especially in Singapore where rapid fluctuations in illumination condition is common due to low-altitude cloud movements. Overall, it can be concluded that outdoor illumination condition at the specific site of deployment should be taken into account when designing GaAs/Si tandem solar cells. Also, the theoretical energy yield benefit of a 4-terminal device emphasizes the need for further technology development in this design space. With careful optimization, a GaAs/Si tandem solar cell employing already available sub-cells is a promising candidate for surpassing 30% energy conversion efficiency in non-concentration applications. Acknowledgment This work is supported by the Solar Energy Research Institute of Singapore (SERIS) and the Singapore-MIT Alliance for Research and Technology (SMART). SERIS is sponsored by the National University of Singapore (NUS) and Singapore’s National Research Foundation (NRF) through the Singapore Economic Development Board (EDB). We would also like to thank the NREL Solar Radiation Research Laboratory and André Nobre from SERIS’ Solar Energy Systems Cluster for providing the spectral irradiance data, and Jonathan Mailoa from MIT for providing valuable suggestions and comments.
#233542 - $15.00 USD (C) 2015 OSA
Received 2 Feb 2015; revised 13 Mar 2015; accepted 13 Mar 2015; published 20 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A382 | OPTICS EXPRESS A390
