Item Type Article
Authors Babics, Maxime;de Bastiani, Michele;Ugur, Esma;Xu, Lujia;Bristow, Helen Laura;Toniolo, Francesco;Raja,
Waseem;Subbiah, Anand Selvin;Liu, Jiang;Torres Merino, Luis Victor;Aydin, Erkan;Sarwade, Shruti;Allen, Thomas;Razzaq, Arsalan;Wehbe, Nimer;Salvador, Michael;De Wolf, Stefaan Citation Babics, M., De Bastiani, M., Ugur, E., Xu, L., Bristow, H., Toniolo,
F., Raja, W., Subbiah, A. S., Liu, J., Torres Merino, L. V., Aydin, E., Sarwade, S., Allen, T. G., Razzaq, A., Wehbe, N., Salvador, M. F.,
& De Wolf, S. (2023). One-year outdoor operation of monolithic perovskite/silicon tandem solar cells. Cell Reports Physical Science, 101280. https://doi.org/10.1016/j.xcrp.2023.101280 Eprint version Publisher's Version/PDF
DOI 10.1016/j.xcrp.2023.101280
Publisher Elsevier BV
Journal Cell Reports Physical Science
Rights Archived with thanks to Cell Reports Physical Science under a Creative Commons license, details at: http://
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Link to Item http://hdl.handle.net/10754/687580
perovskite/silicon tandem solar cells
In this work, Babics et al. report the outdoor performance of a perovskite/silicon tandem solar cell during a complete calendar year. The device retains 80% of its initial efficiency. Local environmental factors such as temperature, solar spectrum, and soiling strongly affect tandem solar cells’ performance.
Maxime Babics, Michele De Bastiani, Esma Ugur, ..., Nimer Wehbe, Michael F. Salvador, Stefaan De Wolf
[email protected] (M.D.B.) [email protected] (S.D.W.)
Highlights
Outdoor operation of a perovskite/silicon tandem solar cell for a full year
The tandem solar cell retains 80%
of its initial efficiency
Temperature and solar spectrum are critical for tandem
performance
Babics et al., Cell Reports Physical Science4, 101280
February 15, 2023ª2023 The Authors.
https://doi.org/10.1016/j.xcrp.2023.101280
Report
One-year outdoor operation of monolithic perovskite/silicon tandem solar cells
Maxime Babics,
1,4Michele De Bastiani,
1,2,4,* Esma Ugur,
1Lujia Xu,
1Helen Bristow,
1Francesco Toniolo,
1,2Waseem Raja,
1Anand S. Subbiah,
1Jiang Liu,
1Luis V. Torres Merino,
1Erkan Aydin,
1Shruti Sarwade,
1Thomas G. Allen,
1Arsalan Razzaq,
1Nimer Wehbe,
3Michael F. Salvador,
1and Stefaan De Wolf
1,5,*
SUMMARY
Perovskite/silicon tandem solar cells have gained significant atten- tion as a viable commercial solution for ultra-high-efficiency photo- voltaics. Ongoing research efforts focus on improving device performance, stability, and upscaling. Yet, paradoxically, their out- door behavior remains largely unexplored. Here, we describe their performance over a complete calendar year outdoors in the area of the Red Sea coast of Saudi Arabia, which represents a hot and hu- mid environment. After 1 year, our test device retains 80% of its initial power conversion efficiency. Further, we find three critical factors affecting current matching: the module temperature; devia- tions of the local, actual solar spectrum from the AM1.5G standard, which dictates optical design requirements of the subcells; and module soiling due to a spectrally non-uniform transmission of light through the accumulated dust. Overall, our results underline the promise of perovskite/silicon tandem solar cells as a future high-per- formance technology, yet device tailoring toward targeted deploy- ment may be desired to achieve maximum energy yields.
INTRODUCTION
In the course of the last decade, photovoltaics (PV) have become the cheapest en- ergy in many locations in the world.1The power conversion efficiency (PCE) of solar cells is one important factor contributing to the decrease of the levelized cost of electricity (LCOE) of PV.2Conventional large-area crystalline silicon (c-Si) solar cells can now reach PCEs beyond 26% in laboratories. Yet, this value is approaching the practical PCE limit of around 29% for this type of technology, which will eventually end the efficiency race of single-junction devices.3,4 Multi-junction solar cells, in particular perovskite/silicon tandems, have the potential to reach a PCE above 40%.5 Experimentally, the PCE of lab-scale tandems has already reached values over 31%.6Currently, most of the research efforts are dedicated to further enhancing performance, improving stability in the controlled lab environment, and technology upscaling.7,8However, the outdoor characteristics of this new technology, at least in the scientific literature, are not yet well documented. This contrasts with conven- tional c-Si panels, whose outdoor behavior is well known and benefit from the expe- rience of the cumulative terawatt of this technology now globally deployed.9The behavior of perovskite/silicon tandems is less straightforward due to the ionic nature of the perovskite material and its monolithic incorporation in a tandem stack.10,11As a result, there is a pressing need to identify and mitigate possible failure modes in the field to avoid major delays in commercialization.
1KAUST Solar Center (KSC), Physical and Engineering Division (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia
2Now at Department of Chemistry & INSTM Universita` di Pavia, Via T. Taramelli 14, 27100 Pavia, Italy
3Imaging and Characterization Corelabs, Surface Analysis Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia
4These authors contributed equally
5Lead contact
*Correspondence:
[email protected](M.D.B.), [email protected](S.D.W.) https://doi.org/10.1016/j.xcrp.2023.101280
Cell Reports Physical Science4, 101280, February 15, 2023ª2023 The Authors.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1
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The first outdoor study of perovskite/silicon tandems originated in the year 2020 when Aydin and Allen et al. collected outdoor data for 7 days.12The impact of the device temperature and the solar spectrum on the current density of tandems was thoroughly investigated. Later, Liu et al. correlated the degradation of the perov- skite subcell to the outdoor degradation of tandems using 43 days of data.13De Bas- tiani et al. extended the outdoor work to bifacial tandems and looked at an extended period of 6 months, the longest period reported in the literature so far.14,15 Commonly, all these studies employed glass-glass encapsulation without an inner encapsulant material, which, however, will eventually lead to a degradation of the electrodes due to the volatile compounds released by the perovskite over time.15,16From these reports, one of the main lessons we learned is the necessity of robust encapsulation to minimize encapsulation-related failures.
RESULTS AND DISCUSSION Device fabrication and field operation
In this study, we encapsulated single-junction c-Si and perovskite/silicon tandem solar cells with an industry-compatible and robust method, which is discussed in detail in Notes S1and S2 andFigure S1. In brief, two metal wires are soldered on both sides of the cell to extend the electrode contacts, then the device is encapsulated between two layers of thermoplastic polyurethane (TPU) and glass, followed by module lamination. We fabricated a perovskite/silicon tandem with the structure given in Figure S2A.12For the top cell absorber, we use a triple-cation perovskite Cs0.05MA0.14FA0.81Pb(I0.72Br0.28)3with a band gap of 1.68 eV, and passivated sputtered nickel oxide (NiOx) serves as a hole transport layer (HTL).17The corresponding JV curves measured under standard test conditions (STC; 1 sun, AM1.5G spectrum, 25C) after encapsulation are shown inFigure S2B, and the performance before and after encapsu- lation is provided inTable S1. After encapsulation, the tandem device has an open-cir- cuit voltage (Voc) of 1.74 V, a short-circuit current (Jsc) of 18.2 mA cm2, a fill factor (FF) of 67.3%, and a PCE of 21.4%, measured under STC. The device was then installed in the KAUST outdoor testing facility (2218017.0"N 3906030.100E) on a fixed rack facing south with a tilt angle of 25(Figure S3). Its performance was tracked during a full calendar year from April 2021 to April 2022.
Our test environment for the field investigations represents one of the harshest con- ditions for PV operation: the Arabian Peninsula is characterized by a high level of sun irradiance above 2,000 kWh m2per year (Figure S4). Specifically, the global hori- zontal irradiance (GHI) for the KAUST outdoor testing site is2,240 kWh m2per year. The plane-of-array irradiance (irradiance received on the tandem perpendic- ular to its surface) is provided inFigure 1A. The peak irradiation reaches values be- tween 850 and 1,000 W m2per day during most of the year. We evaluate a total level of irradiation of around 2,600 kWh m2 for the testing period (assuming 6 kWh m2per missing day), significantly above most other locations in the world.
The Arabian Peninsula is also characterized by an elevated outdoor temperature (Figure 1B). During the daytime, the temperature is around 30C from April to November and stayed above 20C during winter. Abnormal surges in temperature (for instance due to heat waves) also occur sporadically throughout the year, where the maximum outdoor temperature recorded during the testing period was 47.7C.
The Red Sea coast of Saudi Arabia is also characterized by high relative humidity (RH) between 60% and 90% during the year (Figure S5). Therefore, the combination of high irradiance, high temperature, and high RH makes this given location a chal- lenging environment for perovskite-based solar cells but a most relevant location to study the outdoor resilience of PV technologies.
Evaluating the PV parameters recorded throughout the year, depicted inFigure 1, we observe a power stabilization in the first week after installation (detailed data from the first days are provided inFigure S6). During this time, the Vocincreased from1.71 to 1.77 V. After this period, the Voc remained stable over the next 8 months until mid-December, when lower outdoor temperatures brought the de- vice Voc up to 1.80 V. After 1 year of operation, the Voc of the device was measured to be still above 1.75 V. From this, we infer that no significant density of deep-level states was generated in the bulk of the perovskite.13,15 The FF also shows a stabilization after field installation and increases from 74% to 78%
over the first week. The power stabilization is mostly linked to the stabilization of the passivation of the device, as also evidenced by its Voc, which is related to an improvement of the NiOx interface over time. Contrary to other HTLs (seeFig- ure S7), the passivated NiOx requires time to achieve optimal hole extraction per- formance. Likely, this is due to two mechanisms: on one side, halide redistribution was proven to favor the neutralization of NiOx surface defects18; on the other side,
Figure 1. Outdoor performance of the perovskite/silicon tandem between April 2021 and April 2022 located in the KAUST testing site, Saudi Arabia (A) Plane-of-array irradiance.
(B) Outdoor temperature.
(C) Voc. (D) FF.
(E)Jsc.
(F) Power generation density (PGD).
(G) Normalized PGD (PGD/irradiance/PGDt=0).
The missing days are due to maintenance-related power shutdowns on site. During this period, the device was kept onsite under open circuit.
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the employed dye-passivation mechanism is fully functional only after several hours of operation.17The FF stayed above 78% for the first 4 months and then gradually decreased from August until December to values around 72%. After 1 year of operation, the FF was still above 70%. The outliers of the FF throughout the year can be attributed to clouds passing by during the JV sweep, resulting in anomalous JV characteristics (Figure S8). Knowing that the FF is the parameter that is most prone to degrade outdoors,15the results are promising toward achieving stable tandem technology. The Jscmeasured in the field decreased from 15.5 to 13.5 mA cm2after 1 year; we note that the relatively modest starting value largely relates to the not-yet-optimized optical design of encapsulated tandems when installed in the field. Stable the first months until mid-July (normalized current to irradiance providedFigure S9), theJscthen decreased before stabilizing. Looking at the normalized power output (power generation density [PGD])/irradiance/
PGDt=0), it followed the behavior of the current density and the FF: stable for the first 3 months, decreasing the next 4 months, and retaining around 80% of its initial efficiency after 1 year.
Analysis of the tandem after 1 year
To better understand the tandem performance after 1 year, we performed a detailed analysis of the encapsulated device back in the laboratory. The JV curves before and after the outdoor test are provided inFigure 2A. Measured under the AM1.5G spec- trum, theJschas decreased from 18.2 to 16.7 mA cm2after 1 year, representing a 12% loss. The external quantum efficiencies (EQEs) depicted inFigure 2B reveal that Figure 2. Analysis of the encapsulated device after 1 year outdoors
(A and B)JVcurves (A) and EQE (B) of the tandem before and after the field test.
(C) Reference silicon heterojunction device.
(D) Picture of the device after 1 year with various locations selected for analyzing hyperspectral images. The red square indicates the aperture in the mask used to illuminate only the tandem stack. The area outside the red square device was shaded.
(E) PL peak mapping on various locations on the device after 1 year.
the band gap of the perovskite redshifted by15 nm. A high bromide content and halide segregation due to the presence of light, electrical bias, and temperature could cause this irreversible change.19,20The observed decrease in EQEs of the c-Si implies a possible degradation of the electrical contacts and an overall decrease in charge collection efficiency. We exclude c-Si subcell degradation since the refer- ence silicon heterojunction (SHJ) single-junction device with the same characteristics as the tandem bottom cell did not change in a year outdoors; instead, it performs even slightly better, which may be linked to the known light-induced performance enhancement for this technology (Figure 2C).21,22In addition, the c-Si bottom cell is protected from the perovskite by the ITO recombination junction and the NiOx HTL, preventing potential degradation due to ion migration.23The change of the perovskite band gap is confirmed by photoluminescence (PL) imaging measure- ments where we probed different locations of the device, indicated inFigure 2D, including an area outside the aperture mask. We found that the active area of the de- vice displays non-uniform compositions with PL emissions peaks around 1.65 eV and as low as 1.61 eV. This contrasts with the area that was not exposed to light, which showed more homogeneous emissions peaks around 1.68 eV. Here, we note that the perovskite absorber does not contain any additives or passivation layer to prevent halide segregation.13Reducing the bromide content and using a lower-band-gap perovskite by switching to a bifacial tandem configuration is also a strategy to over- come this type of degradation.24,25Finally, we applied secondary-ion mass spec- troscopy (SIMS) on the encapsulant in contact with the top side of the tandem to trace the possible diffusion of perovskite-related ions with time.23The SIMS profiles from a fresh reference encapsulated tandem and the 1-year device are given inFig- ure S10, and experimental details are provided inNote S3. After 1 year, the TPU en- capsulant contained more positive and negative ions than the reference sample, in particular, cesium. However, this increase in concentration is not significant compared with our previous finding where, in the case of harmful contamination due to potential induced degradation (PID), we found that ion concentrations in the encapsulant increased by orders of magnitude.26
Detailed impact of the outdoor environment
Compared with indoor measurements at STC (i.e., at 25C), the temperature of a PV module outdoors can reach a temperature of 60C–70C.12Outdoor data are ideal for accurately documenting the effect of temperature on device performance. In particular, the heatwaves happening multiple times per year in the region are an op- portunity to monitorin situthe effect of temperature on the device performance, assuming that no degradation is happening within this short period. We selected the 11thand 13thof June 2021, when a heat wave suddenly changed the outdoor temperature. The 11thof June was representative of a summer day: the air was hu- mid (RH up to 80%) with a weak wind from the Red Sea (North-West origin), the air temperature was around 32C, and the device reached 44C when the solar irra- diance was maximum around 12.30 p.m. On the 13thof June, a strong hot and dry wind from the desert (South-East origin) between 10 a.m. and 2 p.m. was recorded.
As a result, the outdoor temperature reached 46C, and the tandem temperature was 56C. The device parameters against the irradiance and temperature for these two days are overlapped and depicted inFigures 3C–3F. As anticipated, the Vocwas strongly affected by the device temperature.27,28For instance, with an irradiance of 861 W m2, the Vocdecreased from 1.766 to 1.711 V when the device temperature increased from 44.6C to 55.1C (0.34% K1). On the contrary, theJscincreased with temperature (+0.44% K1). The FF decreased from 77.5% to 74.2% with the temperature rise. TheJSCand FF behaviors can be attributed to the change in sub- cell currents induced by the temperature. Since the overall current of the tandem is
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silicon limited, an increase in temperature would reduce the current mismatch, therefore increasing theJscand decreasing the FF. The relation between current mismatch, FF, and power generation has been discussed previously.12,29,30Looking at the c-Si solar cell results on the same days, the dependence of the FF on temper- ature was not as pronounced (Figure S11). From this experiment, we can conclude that if current mismatch via silicon limiting current is a strategy to achieve high FF and high efficiency, this strategy could be jeopardized by an increased temperature of the device and the resulting reduced current mismatch.
The local spectral irradiance is the environmental parameter regaining attention in the multi-junction community.31,32In the last 20 years, research institutes and PV manufacturers adopted the AM1.5 Global spectrum (ASTM G173-03) as a standard to report and compare PV performances. This spectrum is a yearly average of mul- tiple locations in the United States and does not systematically extend to other loca- tions in the world.33,34In fact, the spectral irradiance depends on the atmospheric composition and the air mass, the latitude, and the tilt angle of the panels and has daily and seasonal variations.35If the local changes in spectral irradiance have negli- gible consequences on the device performance for single-junction c-Si PV technol- ogies, the spectral irradiance plays a critical role for multi-junction solar cells regarding the number of photons available per subcell, thus affecting current gen- eration, current matching, and power output.36Moreover, the larger the number Figure 3. Tracking of environmental conditions and their effect on performance during outdoor testing (A) Wind rose diagram measured on the 11thand 13thof June 2021.
(B) Relative humidity, wind speed, ambient temperature, and device temperature.
(C–F) Device characteristics: (C) Voc, (D)Jsc, (E) FF, and (F) PGD depending on the irradiance and the device’s temperature.
of junctions, the more important this effect will become. In addition, the long-term stability of perovskite/silicon tandem solar cells under the current mismatch is yet to be studied and discussed. InFigure 4A, we show the AM1.5G spectrum as well as a representative spectrum measured on our testing site. While both spectra display a comparable integrated power density up to 1,200 nm (836 W m2), their main dif- ference is the photon flux after 650 nm, meaning that the silicon subcell current will be the most different relative to the laboratory data. To evaluate the impact of both spectra on the tandem current, we show a hypothetical/constructed, but yet realistic, EQE of a tandem cell with a perfect current matching of 20 mA cm2under AM1.5G.
The same device under our local spectrum will generate 20.4 and 18.8 mA cm2for the perovskite and the silicon subcells, respectively, representing a considerable mismatch of 1.6 mA cm2(6% relative current loss). This rationale can be extended to triple-junction devices (Figure S12;Table S2), where a device with aJscof 13 mA cm2under AM1.5G will see its silicon current generation reduced to 12.1 mA cm2, thus representing an even higher relative current loss (6.9%). While efforts are currently being undertaken in laboratories to achieve the best current density and efficiency under the AM1.5G spectrum, we anticipate that tandem PV manufacturers may have to adapt to the local spectrum at the installation site and develop strate- gies to control and modify subcells’ current accordingly to achieve optimal power output.
An additional outdoor challenge faced by PV is dust accumulation (soiling) on the module glass. This reduces the photon flux reaching the solar cells, consequently affecting the energy yield of a PV plant.37,38Soiling is especially severe in the Middle East, where local and distant soils accumulate and solidify on the panels (Fig- ure S13A).39To quantify the effect of soiling on tandems, we measured EQEs of a perovskite/silicon tandem cell installed outdoors for several days, allowing natural soiling on them (Figure 4B). As anticipated, the overall EQE of the cell is lower after the accumulation of the dust layer. Interestingly, theDEQE (EQEcleanEQEdusty) is different for both subcells: soiling has a more severe impact on the perovskite sub- cell than the silicon subcell. To further understand this, we measured the absorption of the soil collected on the device (Figure S13B), and we observed a wavelength- dependent light absorption: the absorption is higher in the blue region of the solar spectrum, in agreement with previous reports.39 To explain the non-uniform Figure 4. Effect of the local solar spectrum and soiling on tandem device performance
(A) Constructed EQE of a perovskite/silicon tandem giving 20 mA cm2for both subcells under AM1.5G. The local spectrum measured on site and the AM1.5G spectrum are also superimposed.
(B) Effect of soiling on the encapsulated device EQE.
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transmission, two origins have been proposed in the literature: the composition of the dust with minerals that absorb more in the blue region of the solar spectrum and the wavelength dependency of the scattering of the submicrometers dust par- ticles.40,41If soiling already represents a challenge for any PV power plant, it will further affect tandem PV modules due to a soiling-induced current mismatch.
Development of perovskite/tandem technology should be inclusive and address all the challenges including outdoor operation. Doing this early on will allow the design of materials and devices with reduced failure modes and accelerate the commercialization process. Transitioning from the lab to field operation, we note several findings that need to be addressed: light and elevated temperature- induced degradation (LETID) is the main challenge for perovskite/silicon tandem solar cells and requires more attention when aiming for multiple decades of oper- ation. Another challenge will be to achieve subcell current matching during oper- ation to maximize the energy yield. Due to the constant change of temperature, solar spectrum, and dust accumulation, the tandem solar cells will often operate in a current, limiting subcell configuration. Despite these challenges, we demon- strate a perovskite/silicon tandem that retains 80% of its original power output af- ter 1 year of exposure under the harshest conditions. This is an encouraging result, and we foresee that long-term, high-efficiency perovskite silicon devices are within reach.
EXPERIMENTAL PROCEDURES Resource availability
Lead contact
Further information and requests for resources should be directed to and will be ful- filled by the lead contact, Stefaan De Wolf ([email protected]).
Materials availability
This study did not generate new unique reagents.
Data and code availability
All data supporting the findings of this study are available from thelead contact upon reasonable request.
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.xcrp.
2023.101280.
ACKNOWLEDGMENTS
This work was supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award nos. OSR-2021-4833, OSR-CARF/CCF-3079, IED OSR-2020-4611, IED OSR-2019-4580, OSR-CRG2020- 4350, OSR-2020-CPF-4519, OSR-CRG2019-4093, and IED OSR-2019-4208. We acknowledge the use of the KAUST Solar Center and support from its staff.
AUTHOR CONTRIBUTIONS
Conceptualization, M.B. and M.D.B.; investigation, M.B., M.D.B., E.U., F.T., H.B., N.W., and L.X.; validation, J.L., L.V.T.M., and W.R.; resources, A.S.S., E.A., S.S., A.R., and T.G.A.; writing – original draft, M.B. and M.D.B.; writing – review & editing, E.A., M.F.S., W.R., S.D.W., and T.G.A.; supervision, S.D.W.
DECLARATION OF INTERESTS The authors declare no competing interests.
Received: September 29, 2022 Revised: November 21, 2022 Accepted: January 16, 2023 Published: February 6, 2023
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