Letter pubs.acs.org/JPCL
Efficient Monolithic Perovskite/Silicon Tandem Solar Cell with Cell Area >1 cm2 Jérémie Werner,*,† Ching-Hsun Weng,† Arnaud Walter,† Luc Fesquet,† Johannes Peter Seif,† Stefaan De Wolf,† Bjoern Niesen,*,†,‡ and Christophe Ballif†,‡ †
Ecole Polytechnique Fédérale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin-Film Electronics Laboratory, Rue de la Maladière 71b, 2002 Neuchâtel, Switzerland ‡ CSEM, PV-Center, Jaquet-Droz 1, 2002 Neuchâtel, Switzerland S Supporting Information *
ABSTRACT: Monolithic perovskite/crystalline silicon tandem solar cells hold great promise for further performance improvement of well-established silicon photovoltaics; however, monolithic tandem integration is challenging, evidenced by the modest performances and small-area devices reported so far. Here we present first a low-temperature process for semitransparent perovskite solar cells, yielding efficiencies of up to 14.5%. Then, we implement this process to fabricate monolithic perovskite/silicon heterojunction tandem solar cells yielding efficiencies of up to 21.2 and 19.2% for cell areas of 0.17 and 1.22 cm2, respectively. Both efficiencies are well above those of the involved subcells. These single-junction perovskite and tandem solar cells are hysteresis-free and demonstrate steady performance under maximum power point tracking for several minutes. Finally, we present the effects of varying the intermediate recombination layer and hole transport layer thicknesses on tandem cell photocurrent generation, experimentally and by transfer matrix simulations.
I
ically stacked on top of each other. Perovskite-based fourterminal tandems have recently been demonstrated both with chalcogenide and crystalline silicon bottom cells, showing efficiencies of up to 20.5 and 22.8%, respectively, both with perovskite cell areas 6 min maximum power point tracking, as shown in Figure 2a. This is the semitransparent perovskite solar cell with the highest performance reported so far. Figure 2 shows that the semitransparent cell measured in substrate configuration, illuminated through the MoOx/IO:H/ITO electrode (similar to the situation in a monolithic tandem) suffers from a current loss of ∼1.5 mA/ cm2, seen in both current−voltage (J−V) and external quantum efficiency (EQE) measurements. This can be attributed to parasitic absorption in the spiro-OMeTAD layer, in agreement with previously reported results.13 Interestingly, in this illumination direction, the fill factor is slightly higher than in the standard superstrate configuration. With this efficient and reliable semitransparent perovskite solar cell and the IZO intermediate recombination layer, we fabricated monolithic tandem solar cells with an efficiency as high as 19.5% in forward direction scan, with an open-circuit voltage (Voc) of 1703 mV, fill factor (FF) of 70.9%, and shortcircuit current density (Jsc) of 16.1 mA/cm2. This cell was measured through a laser-cut mask with 1.22 cm2 aperture area and showed a steady efficiency of 19.2%, when measured with a maximum power point tracking system. The complete set of results is detailed in Figure 3 and Table 1, and demonstrates that the tandem performance is better than those of both subcells. The small difference in Jsc between J−V (16.1 mA/ cm2) and EQE (16.8 mA/cm2) measurements is due to the 162
DOI: 10.1021/acs.jpclett.5b02686 J. Phys. Chem. Lett. 2016, 7, 161−166
Letter
The Journal of Physical Chemistry Letters
Figure 2. (a) J−V and (b) EQE measurements of a low-temperature processed semitransparent perovskite solar cell with the same layer stack as used for the top cell in the monolithic tandem device but deposited on ITO-coated glass substrates, measured in both substrate and superstrate configurations, indicated, respectively, by “From spiro-side” and “From PCBM-side”. The cell was measured at a scan rate of 100 mV/s, with an antireflective foil. The inset shows a maximum power point tracking measurement of the cell. Further optical data, including transmittance, reflectance, and absorptance measurements, can be found in the Supporting Information, Figure S1.
Figure 3. (a) EQE spectra of a perovskite/SHJ monolithic tandem with (solid lines) and without (dashed lines) anti-reflective foil (ARF) as well as the corresponding reflectance (green curves). The integrated Jsc for both top and bottom cells are given in the legend (without ARF/with ARF). (b) J−V measurements of the best perovskite/SHJ monolithic tandem with 1.22 cm2 aperture area and of the single junction perovskite and DSP-SHJ cells. Reverse (solid lines) and forward (dashed lines) scans are shown for perovskite single-junction and tandem cells. The dotted red curve shows the J−V curve of the SHJ cell when illuminated at an intensity of 0.53 suns. (c) J−V curves of the best perovskite/SHJ monolithic tandem with 0.17 cm2 aperture area. The insets to panels b and c show the maximum power point tracking curves of the tandem cells.
shadowing induced by the metal contact fingers, which cover 5% of the cell surface. Efficiencies of up to 21.2% were reached on smaller tandem cells with an aperture area of 0.17 cm2, as shown in Figure 3c. The cell also shows negligible hysteresis, and Jsc is confirmed by EQE measurements as shown in Figure S3. Compared with the cell with a larger area, we observe a gain in FF due to reduced current and series resistance. The planar and nonscattering configuration of the tandem devices results in strong optical interferences. This can clearly be identified in the EQE spectra, as shown in Figure 3a. Therefore, several strategies were investigated to reach the high performances presented in this paper. To increase the current of our devices, we applied microtextured anti-reflective foils on the front side of the cells during characterization.26,27 As shown in Figure 3a, this strategy helps to drastically reduce reflection losses and increases the current in the 1.22 cm2-sized tandem cell by ∼10% in the top cell and by ∼16% in the bottom cell. Consequently, it passes from a bottom-limited to a top-limited situation. Previous studies on multijunction organic solar cells have shown that optical interferences can be tuned to maximize the
light intensity in the absorber layers by changing the effective optical path length. This is usually achieved by the insertion of an optical spacer.28,29 Motivated by these findings, we experimentally tested the effect of thickness variations in our devices. First, we varied the intermediate recombination layer thickness between ∼25 and 70 nm. The resulting EQE curves are shown in Figure 4a. We can observe a decrease in bottom cell current with increasing IZO thickness. The lower thicknesses lead to a situation where the perovskite top cell is current-limiting, whereas the highest thickness shifts the limitation to the bottom cell. An optimum situation with closely matching currents is therefore achieved with a 40 to 50 nm thick IZO layer. Similar results were obtained when using an ITO recombination junction, as shown in Figure S4. Then, the spiro-OMeTAD hole transport layer thickness was varied. We choose to test this layer in particular because of its known high parasitic absorption, especially for wavelengths
