As the focus of single-junction halide perovskite solar cells now extends to commercialization, new interest in improving efficiency through tandem design is gaining momentum. While perovskite/Si tandem solar cells have delivered efficiencies close to 30%, other configurations are emerging. In this Energy Spotlight, we highlight two recent articles, the first on thin-film photovoltaics composed of two-terminal perovskite/CuInSe2 tandem solar cells, and the second focused on tracking changes in structural and electronic properties of the perovskite-based photovoltaic devices under the stimuli of an electric field and light illumination.
Monolithic Two-Terminal Perovskite/CIS Tandem Solar Cells with Efficiency Approaching 25%
d’Ile de France
, 91120 Palaiseau
Marco A. Ruiz-Preciado*, Fabrizio Gota, Paul Fassl, Ihteaz M. Hossain, Roja Singh, Felix Laufer, Fabian Schackmar, Thomas Feeney, Ahmed Farag, Isabel Allegro, Hang Hu, Saba Gharibzadeh, Bahram Abdollahi Nejand, Veronique S. Gevaerts, Marcel Simor, Pieter J. Bolt, and Ulrich W. Paetzold*
ACS Energy Lett.2022, 7 (7), 2273–2281 (Letter)
The search for reliable and efficient ways to generate electric power is at the core of our pursuit of addressing climate change and transforming our energy economy toward a sustainable future. Solar photovoltaic (PV) technologies, i.e., the direct conversion of sunlight into electricity, have come a long way in the past decade and today exhibit the steepest acceleration in growth rates of cumulative installed capacity, which is quickly approaching a value of 1 TW by the end of the year 2022.
A significant factor for this trend to continue is a steady improvement of PV module efficiencies and the entry of emerging technologies into the commercial space to expand nascent fields of application such as building-integrated PV, vehicle-integrated PV, or agrophotovoltaics. This scenario thus calls for further development of next-generation thin-film PV devices that bear the potential for implementation in flexible, large-area modules along with tailored average visible transmittance.
In their recent work, Ruiz-Preciado, Paetzold, and co-workers are further pushing the limits for all-thin-film devices by combining chalcogenide CuInSe2 (CIS) and halide perovskite solar cells (PSCs) in a monolithic two-terminal tandem device. By adapting the bottom CIS subcell and optimizing the PSC subcell on top, the team achieved a record power conversion efficiency (PCE) of 24.9% measured in the lab and produced a demonstrator with a certified PCE of 23.5% over an active area of 0.5 cm2.
They adopted a multipronged approach of parallel optimization processes for this pivotal step that defines the new state-of-the-art for monolithic CIS/perovskite tandems: First, the use of a co-evaporated CuInGaSe2 layer with drastically reduced Ga content leads to a band gap as low as 1.03 eV. This puts a broad range of perovskite absorber compositions into reach, which in turn facilitates current matching between the monolithically stacked cells. With an average roughness of only 50 nm, the CIS layer enables conformal growth of the PSC on top. In addition, Ruiz-Preciado et al. tailored the surface coating and the various interfaces in the full layer stack, particularly for carrier extraction at the recombination junction between the two subcells and current matching through a MgF2 anti-reflective coating. In summary, these steps pave the avenue for all-thin-film multijunction PV, with the research community aiming for PCEs of over 30%.
Stressing Halide Perovskites with Light and Electric Fields
Department of Chemistry and
Institute for Functional Intelligent Materials
, National University of Singapore
, Singapore 117543
Sarah Wieghold*, Emily M. Cope, Gregory Moller, Nozomi Shirato, Burak Guzelturk, Volker Rose, and Lea Nienhaus*
ACS Energy Lett.2022, 7 (7), 2211–2218 (Letter)
The past few years have seen a surge of worldwide research interest and rapid growth in the field of lead halide perovskites─one of the most promising candidates for next-generation photovoltaic (PV) and optoelectronic devices. The fundamental interest and technological relevance of these materials are closely linked to their superior optoelectronic properties, bolstered by their scalable production at a low cost. Further understanding a wealth of elusive phenomena discovered in these materials and their devices hinges on the ability to obtain nanoscale and even atomic-scale insights into their physicochemical and optoelectronic properties under “real-world” operating conditions.
To this end, Sarah Wieghold and co-workers employed advanced scanning tunneling microscopy (STM) techniques to perform a systematic characterization of the structural and electronic changes of a mixed A-site cation/mixed halide perovskite under external stimuli of an electric field and light illumination, mimicking the operation environment of the perovskite-based PV devices at room temperature.
In particular, the authors first utilized the single-molecule absorption STM (SMA-STM) technique that relies on electronic detection of the optical absorption processes using the STM tip as a local probe, to probe the surface inhomogeneity of this perovskite thin film. The back-illumination geometry mitigates the local heating issue, ensuring a stable STM measurement. This study revealed that the presence of an electric field without light illumination leads to a slight reduction of the electronic bandgap. Surprisingly, such an effect leading to reduction of the electronic bandgap gets more pronounced upon continuous light illumination, suggesting the presence of lattice distortion under the applied external stimuli.
To better understand the structural change, the synchrotron X-ray STM (SX-STM) technique that relies on detecting the tunneling current produced by the X-ray adsorption process associated with the iodine M4,5 edge is utilized to provide nanoscale spatial insight into variations of the absorption spectral signatures triggered by optical illumination. Taking advantage of SX-STM, the authors verified that the photogeneration of charge carriers leads to PbI6 octahedral distortion (elongation or compression), manifested by changes of the iodine–iodine bond length.
This work not only offers new insights into structural and electronic changes in the model hybrid perovskite film triggered by the additional stimuli of the electric field and light illumination but also underpins the great potential of SMA-STM and SX-STM characterization techniques to map out the optoelectronic and structural responses of the perovskite-based devices to external stimuli with nanoscale spatial resolution.
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