Perovskite tandem solar cells are highly desirable to realize power conversion efficiencies (PCEs) beyond the single junction Shockley-Queisser (SQ) limit. Previously we employed an integrated approach to successfully realize highly efficient, bandgap-matched perovskite-perovskite tandem solar cell with 18.5% PCE and high photovoltages (~2 V) reaching 80% of theoretical limit.1 It is imperative to note that this performance of perovskite-perovskite tandem is limited by the photovoltage loss in individual subcells. Photovoltages (Voc) of the best performing small bandgap (~1.2 eV) and large bandgap (~1.8 eV) perovskite solar cells (PVSCs) are currently limited to 88% and 80% of the SQ limit. Efforts to improve photovoltages of small and large bandgap PVSCs to higher values are critical for further advancement of perovskite tandems.
Herein, we improve the optoelectronic quality of small bandgap perovskites via defect passivation through incorporation of fluoroalkyl-substituted fullerene via a graded heterojunction structure. Remarkably high Voc ~ 0.89 V was realized for corresponding small bandgap (~1.2 eV) PVSCs, which is ~92% of the SQ limit, comparable to the state-of-the-art inorganic technologies and is the best among PVSCs reported till date.2 On the other hand, for large bandgap perovskites, through engineering material characteristics via composition of organic cation, we simultaneously improve optoelectronic quality and alleviate instabilities due to light-induced phase segregation. The resultant large bandgap (~1.8 eV) PVSCs had much improved Voc around 1.3-1.35 eV, which corresponds to 85-87% of the SQ limit and is a significant enhancement with respect to the current state-of-the-art. The above realized improvements in photovoltages of small and large bandgap devices will be pivotal for unleashing the complete potential of perovskite-perovskite tandems and get past the PCE of single junction PVSCs.
 Rajagopal, A. et al. Highly Efficient Perovskite-Perovskite Tandem Solar Cells Reaching 80% of the Theoretical Limit in Photovoltage. Adv. Mater. 29, 1702140 (2017).
 Rajagopal, A. et al. Defect Passivation via a Graded Fullerene Heterojunction in Low-Bandgap Pb–Sn Binary Perovskite Photovoltaics. ACS Energy Lett. 2531–2539 (2017).