Oct 28, 2014 - Kesterite Cu2ZnSn(S,Se)4 and perovskite CH3NH3PbI3 solar cells were ... chalcogenide-perovskite monolithic architecture and does not focus ...
Perovskite-kesterite monolithic tandem solar cells with high open-circuit voltage Teodor Todorov, Talia Gershon, Oki Gunawan, Charles Sturdevant, and Supratik Guha Citation: Applied Physics Letters 105, 173902 (2014); doi: 10.1063/1.4899275 View online: http://dx.doi.org/10.1063/1.4899275 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/105/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Suns-VOC characteristics of high performance kesterite solar cells J. Appl. Phys. 116, 084504 (2014); 10.1063/1.4893315 Band tailing and efficiency limitation in kesterite solar cells Appl. Phys. Lett. 103, 103506 (2013); 10.1063/1.4820250 Indications of short minority-carrier lifetime in kesterite solar cells J. Appl. Phys. 114, 084507 (2013); 10.1063/1.4819849 Admittance spectroscopy in kesterite solar cells: Defect signal or circuit response Appl. Phys. Lett. 102, 202105 (2013); 10.1063/1.4807585 Copper-phthalocyanine-based organic solar cells with high open-circuit voltage Appl. Phys. Lett. 86, 082106 (2005); 10.1063/1.1871347
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APPLIED PHYSICS LETTERS 105, 173902 (2014)
Perovskite-kesterite monolithic tandem solar cells with high open-circuit voltage Teodor Todorov, Talia Gershon, Oki Gunawan, Charles Sturdevant, and Supratik Guhaa) IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, USA
(Received 5 September 2014; accepted 3 October 2014; published online 28 October 2014) We report a monolithic tandem photovoltaic device with earth-abundant solution processed absorbers. Kesterite Cu2ZnSn(S,Se)4 and perovskite CH3NH3PbI3 solar cells were fabricated monolithically on a single substrate without layer transfer. The resulting devices exhibited a high open circuit voltage (Voc) of 1350 mV, close to the sum of single-absorber reference cells voltages and outperforms any monolithic tandem chalcogenide device (including Cu(In,Ga)Se2) reported to date. Ongoing optimization of several device elements including the severely limiting top contact electrode is expected to yield superior currents and efficiency. Importantly, our device architecture demonstrates the compatibility and synergistic potential of two of the most promising emerging C 2014 photovoltaic materials and provides a path for optimization towards >20% efficiency. V AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4899275]
Chalcogenide solar cells such as Cu(In,Ga)Se2 (CIGS) and Cu2ZnSn(S,Se)4 (CZTSSe) can benefit from a partner material suitable for a tandem photovoltaic device that would allow broader spectrum light harvesting and enhanced efficiency.1 Researchers have attempted different approaches in the past including mechanical stacking of separate devices with different band gap absorbers via external wiring of CIGS-CdTe,2 CIGS-dye sensitized solar cell,3 CIGS-CGS4 and CIGS-CIGS5 cells with different ratios of In:Ga. However, the use of multiple transparent conducting layers with related losses due to transmission and series resistance as well as the additional interconnections needed for external wiring of the two separate solar cells limits the practicality of these concepts. A preferred monolithic design on a single substrate would theoretically be capable of delivering superior performance and lower fabrication cost. However, the processing and materials constraints associated with the preparation of highest-efficiency chalcogenide devices such as elevated absorber crystal growth temperatures (above 400 C), the rapid deterioration of the p-n junction above 220 C and non-transparent Mo bottom contact have been a significant barrier to the fabrication of efficient CIGS or CZTSSe based monolithic tandem devices. One attempt towards a CIGS-organic solar cell tandem via a sophisticated layer transfer reached an efficiency of 3.8%.6 With earthabundant CZTSSe solar cells, neither monolithic nor mechanically stacked tandem devices have been reported to date. Recently, perovskite organo-lead halide solar cells have made rapid progress with the demonstration of solar cells with over 16% efficiency.7 The low processing temperatures of metal-organic (MO) perovskite devices (15% efficiency is projected to reach tandems with over 20% efficiency. The device (SEM cross-section shown in Fig. 1) was built on a molybdenum coated soda-lime glass substrate and
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Appl. Phys. Lett. 105, 173902 (2014) TABLE I. Device parameters of various devices at simulated 1 sun illumination. RS is the series resistance and n is the ideality factor extracted using Lambert W function fitting method.16
Device CZTSSe Perovskite CZTSSe-shadoweda Tandem-projectedb Tandem
g %
FF %
Voc V
Jsc mA/cm2
RS X.cm2
n
11.6 12.3 4.5 16.0 4.6
68.2 76.6 64.4 73.6 60.4
0.477 0.953 0.452 1.401 1.353
35.7 16.8 15.5 15.5 5.6
0.63 2.53 0.66 3.20 15.70
1.66 2.76 1.85 4.41 6.40
a
FIG. 1. Cross-sectional SEM image of a monolythic tandem CZTSSe/ perovskite solar cell structure identifying individual layers.
comprises a bottom CZTSSe-based solar cell, prepared by a solution process described in detail previously.11,12 The CZTSSe absorber used in the study was prepared via a pure solution deposition approach, where the precursors dissolved in hydrazine are spin-coated onto a Mo-coated soda lime glass substrate and subsequently annealed to form a largegrained kesterite layer with a band gap of approximately 1.1 eV (details in Ref. 13). CdS was then deposited by chemical bath deposition, followed by a sputtered ITO top electrode. On top of this completed device, we prepared a top cell based on a perovskite absorber in a manner similar to that described previously.14 A PEDOT:PSS layer was spun directly on the top ITO contact of the completed CZTSSe cell, and was then annealed in air at 140 C to remove excess water. The samples were then moved to a nitrogenfilled glove box for subsequent processing. The NH3CH3PbI3 perovskite absorber was prepared by adapting the vapor-assisted solution process described by Chen15 consisting of spin-coating a layer of PbI2 and then annealing it on a hot plate in a vapor of NH3CH3I until conversion to the perovskite phase was complete. A PCBM layer was then prepared by spin-coating from solution in chlorobenzene on top of the perovskite layer. A thin Al contact was then deposited by thermal evaporation on top of the PCBM layer. The Al thickness (20–25 nm) was tuned to allow 30%–40% optical transmission enabling device characterization (Fig. 2). As we will discuss below, the poorly transparent and highly resistive (sheet resistance > 200 X/sq) Al layer is the main
FIG. 2. (a) Relationship between the average transmission (over the range of 400–1200 nm) and sheet resistance for a given aluminum film thickness. The Al layer used in our device is expected to be similar to the middle data point. (b) Tauc plot generated from the transmission curve of the stand-alone perovskite layer assuming the relationship I=I0 ¼ eax and a perovskite thickness of 200 nm.
“CZTSSe-shadowed” is a CZTSSe cell shadowed by a perovskite absorber. The “tandem-projected” cell is a theoretical estimate based on the CZTSSeshadowed cell and the perovskite cell. b
performance limiting factor, requiring further optimization of the transparent top contact. Device area of 0.45 cm2 was defined by mechanical scribing. We also fabricated separate stand-alone CZTSSe (glass\Mo\CZTSSe\CdS\ZnO\ITO) with e-beam evaporated Ni-Al grid contact and superstrate perovskite devices (Glass/ITO/PEDOT:PSS/NH3CH3PbI3/ PCBM/Al) by identical procedures, with the exception of a thick (>50 nm) Al contact, for comparison as shown in Table I. We did not observe any evidence of optical or electrical deterioration of the ITO layer in the superstrate cell as is most likely the case with the ITO layer in the tandem device. A current density-voltage (J-V) curve of a typical standalone CZTSSe device (single-cell) is shown in Figure 3 (blue), and the parameters are tabulated in Table I. The Voc values of the reference CZTSSe devices prepared identically as the ones used for the tandem structures had range between 460–490 mV. Overlaid with the CZTSSe J-V curve is a similar measurement performed on a stand-alone perovskite reference device fabricated on ITO-coated glass substrate (red) with parameters tabulated in Table I. Due to the larger band gap of this material, the Jsc value is smaller and the Voc is larger (typically 850–960 mV for our devices) than the values corresponding to the CZTSSe device. From the Tauc plot shown in Fig. 2(b), we estimate the band gap of the perovskite layer as 1.58 eV. The band gap of our bottom
FIG. 3. J-V measurements of stand-alone devices (CZTSSe with Ni-Al grid contacts and a superstrate perovskite on ITO-coated glass), and a tandem CZTSSe/perovskite device.
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CZTSSe layer is estimated to be 1.1 eV from quantum efficiency measurement. The J-V characteristics of the tandem CZTSSe/perovskite device at 1 sun (red) is shown in Fig. 2 and tabulated in Table I. We note that the Voc value of 1353 mV is nearly equal to the sum of the individual Voc values of the CZTSSe and perovskite devices, thus confirming that we have effectively formed a series-connected, monolithic, two-terminal tandem solar cell, and also confirming the integrated processing compatibility of these two materials systems (chalcogenide and the perovskite) for tandem photovoltaic device fabrication. The efficiency of our tandem device is still limited by low current and fill factor with two main contributing elements. First is the significant optical transmission and reflection losses in the top Al layer17 (Fig. 2(a)) resulting in extremely low Jsc. There is a tradeoff between optical transparency and sheet resistance in thin metallic films such as Al, where films with 30%–40% optical transmission still have sheet resistances that are too high for optimal performance (>200 X/sq, Fig. 2); this is in contrast to >90% transmission for indium tin oxide (ITO) with sheet resistance of
