Riding on the waves of “carbon dioxide emissions peak” and ‘‘carbon neutrality’’,

the world is moving away from carbon-emitting fossil fuels and is keen to develop cleaner renewable energy sources. Solar energy, as an inexhaustible renewable energy source, has been undoubtedly attracting extensive attention. Photovoltaic (PV) technology, which could directly convert solar energy into electricity, is displaying unprecedented rapid development. Recently, organometallic halide perovskites have been considered as promising optoelectronic materials owing to their excellent optoelectronic advantages and low-cost fabrication processes, and employed in the fields of photovoltaics, light-emitting diodes, detectors, and so on.

Currently, single-junction perovskite solar cells (PSCs) have reached a certified power conversion efficiency (PCE) of 26.0%, approaching the Shockley-Queisser (S-Q) efficiency limit. The win-win cooperation of wide-bandgap (WBG, ~1.7-1.9 eV) perovskite top subcells with low-bandgap (LBG, ~1.1-1.3 eV) perovskite bottom subcells to develop all-perovskite tandem solar cells is an effective strategy to break the S-Q efficiency limit of single-junction PSCs. In addition, the simple solution processing, low manufacturing cost and low energy consumption make all-perovskite tandem solar cells very attractive for next-generation high-performance and low-cost thin-film photovoltaic technology, which have great potential applications for next-generation GW-scale PV technology with a low CO₂ footprint and vehicle and building integrated PV.

In 2016, we reported a novel mixture strategy of combining individual MAPbI₃ and FASnI₃ precursors to prepare high-quality tin-lead (Sn-Pb), i.e., (FASnI₃)_0.6(MAPbI₃)_0.4, perovskite film with a bandgap of 1.25 eV (J. Am. Chem. Soc. 138, 12360-12363 (2016)), offering the great potential to fabricate efficient all-perovskite tandem solar cells via integrating with WBG subcell. In 2017, we increased the thickness of the (FASnI₃)_0.6(MAPbI₃)_0.4 perovskite film from 400 nm to 620 nm by regulating the perovskite growth process. As a result, our LBG PSCs achieved a champion PCE of 17.6% and a certified efficiency of 17.01%, and we reported a 4-terminal (4-T) all-perovskite tandem solar cell with a steady-state efficiency of 21.0% after mechanically stacking with a ∼1.58 eV perovskite top subcell (Nat. Energy 2, 17018 (2017)). Subsequently, we further enlarged the grain size, reduced electric disorder, and prolonged the carrier lifetime of (FASnI₃)_0.6(MAPbI₃)_0.4 film via doping chlorine, reporting 21.0% efficiency Sn-Pb PSCs. We then fabricated 2-T all-perovskite tandem solar cells with chlorine-doped LBG perovskite as bottom subcell absorber, a 1.75 eV WBG perovskite as top subcell absorber, and ultrathin and compact Ag/MoO_x/ITO as an interconnection layer, and yielded the PCE of 21% (Nat. Energy 3, 1093-1100 (2018)).

In 2022, we reported a universal close-space annealing (CSA) strategy that increases grain size, enhances crystallinity and prolongs carrier lifetimes in LBG and WBG perovskite films. Moreover, this CSA method is compatible with perovskite films with various compositions and bandgaps. Benefiting from the higher-quality perovskite absorber layers, our LBG and WBG PSCs achieved champion PCEs of 21.51% and 18.58%, respectively, allowing the fabrication of 25.15% efficiency 4-T and 25.05% efficiency 2-T all-perovskite tandem solar cells (Nat. Energy 7, 744-753 (2022)). After that, we developed a novel self-assembled monolayer (SAM) (4-(7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (4PADCB) as a hole-selective layer in WBG PSCs, which facilitates subsequent growth of high-quality WBG perovskite film over a large area with suppressed interfacial non-radiative recombination, enabling efficient hole extraction. Integrating 4PADCB in devices, we demonstrated a high open-circuit voltage (V_OC) of 1.31 V in 1.77 eV PSCs, corresponding to a V_OC-deficit of 0.46 V (with respect to the bandgap). With these WBG perovskite subcells, we reported 27.0% (26.4% certified stabilized) efficiency all-perovskite tandem solar cells with an aperture area of 1.044 cm² (Nature 618, 80-86 (2023)).

Charge-selective materials play multiple roles in the performance of PSCs, and the hole-selective materials should usually be tailored for perovskites with different bandgaps to target the facile fabrication process and high performance. PTAA (poly (bis (4-phenyl) (2,4,6-trimethylphenyl) amine)) and PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrenesulfonate) are the most commonly employed hole-selective layers in WBG and LBG subcells of all-perovskite tandem solar cells, respectively. However, their use introduces unavoidable interface issues, hindering further performance improvement of both single-junction and tandem solar cells. Therefore, it is highly desired and of significance to search for novel hole-selective layers that could overcome the drawbacks of state-of-the-art ones, and ideally, the new hole-selective layers are simultaneously suitable for both WBG and LBG subcells and high-performance all-perovskite tandems in terms of reduction in fabrication complexity and cost.

Recently, we developed a donor-acceptor-type molecule 4-(7-(4-(bis(4-methoxyphenyl) amino)-2,5-difluorophenyl)benzo[c][1,2,5]thiadiazol-4-yl) benzoic     acid (MPA2FPh-BT-BA, denoted as 2F) via step-by-step molecular design strategy, which could replace PTAA and PEDOT:PSS as a universal hole-selective contact suitable to both 1.77 eV FA_0.8Cs_0.2Pb(I_0.6Br_0.4)₃ WBG and 1.25 eV FA_0.6MA_0.3Cs_0.1Pb_0.5Sn_0.5I₃ LBG subcells for efficient all-perovskite tandem solar cells. It is worth mentioning that the 2F molecule consists of 4-methoxy-N-(4-methoxyphenyl)N-phenylaniline (MPA) as the donor unit, which displays an efficient hole extraction capacity and high oxidizing ability, and an acceptor moiety benzo[c][1,2,5]thiadiazole (BT) that improves the intermolecular stacking and the sulfur (S) and fluorine (F) atoms that tend to interact with uncoordinated ions for passivating surface defects of perovskites. Intriguingly, the orientation of 2F on the ITO is different from that of a typical SAM. SAM can form an approximate monolayer film on the surface of ITO by a self-assembling behavior, however, for 2F, a multilayer film is prone to grow. After being spin coated on ITO, most of the 2F molecules can preferentially anchor to the ITO surface via carboxyl group, while unanchored 2F molecules may pile up to form a multilayer film, and therefore, part of the exposed carboxyl group can interact with the perovskites. Thus, 2F could be employed as a universal hole-selective contact that can be applied to a broad range of PSCs.

In WBG devices, compared with PTAA, 2F not only accelerates hole extraction via energy level regulation but also minimizes interfacial non-radiative recombination by passivating interfacial defects. In LBG devices with PEDOT:PSS replaced by 2F, in addition to suppressing interfacial non-radiative recombination, 2F can also regulate the crystal growth and enhance the Sn-Pb perovskite film quality by slowing down the rapid crystallization and suppressing the Sn²⁺ oxidation for a higher-quality Sn-Pb perovskite absorber. As a result, our best-performing 1.77 eV WBG and 1.25 eV LBG devices achieved the champion PCEs of 19.33% (certified 19.09%) and 23.24%, respectively. In addition, we also employed 2F as hole-selective contact in 1.68 eV FA_0.8Cs_0.2Pb(I_0.8Br_0.2)₃ WBG PSCs and 1.57 eV FA_0.8Cs_0.2Pb(I_0.95Br_0.05)₃ normal-bandgap PSCs. The improved performance of the 2F-tailored devices demonstrates the suitability of 2F as a universal hole-selective contact in PSCs with different bandgaps and compositions.

We have also constructed 2-T all-perovskite tandem solar cells by using 2F as hole-selective contacts for both 1.77 eV WBG top subcells and 1.25 eV LBG bottom subcells, and achieved a champion PCE of 27.22% and a certified PCE of 26.3%. Moreover, the 2F-tailored tandem solar cell displayed the enhanced operational stability, it maintained 80% of its initial efficiency after 301 h of continuous operation under 1-sun illumination at the MPP, while the control tandem device had a shorter lifetime of 84 h.

We have demonstrated a new path and strategy of designing effective charge-selective contact materials: introducing a multifunctional anchoring group and other functionalized atoms or groups can make the hole-selective layers possess multi-functionality including interfacial energy level tunability, surface defect passivation and perovskite growth regulation, applicable to PSCs with different bandgaps. More importantly, our work provides a promising approach to achieve efficient all-perovskite tandem solar cells via employing a universal hole-selective layer suitable to both WBG and LBG subcells. This strategy can effectively simplify the requirements for preparation technology and experimental equipment, which is helpful and significant for reducing the preparation cost and accelerating the commercial production process of tandem devices.

This work titled “A donor-acceptor-type hole-selective contact reducing non-radiative recombination losses in both subcells towards efficient all-perovskite tandems” was published in the latest volume of Nature Energy journal (DOI: 10.1038/s41560-023-01274-z).