The global energy demand necessitates sustainable solutions, and solar energy is a key contender. Perovskite solar cells (PSCs) have emerged as a promising alternative to silicon cells due to their ease of fabrication and high power conversion efficiency (PCE), rising from ~3.8% to ~25% within a decade. However, the Shockley-Queisser (SQ) limit for single-junction solar cells restricts their maximum efficiency to around 33%. A significant loss mechanism is the conversion of excess kinetic energy from hot carriers (generated by high-energy photons) into heat. Carrier multiplication (CM), also known as multiple exciton generation (MEG), offers a potential solution. CM generates multiple electron-hole pairs from a single high-energy photon, theoretically allowing for efficiencies exceeding the SQ limit (up to 44.4%). Halide perovskites are attractive candidates for CM due to their slow hot carrier cooling. While studies have demonstrated CM in halide perovskite nanocrystals (NCs) and films using transient absorption (TA) spectroscopy, the demonstration and evaluation of CM enhancement in actual PSC devices remain crucial. A key indicator of CM in a PSC is an external quantum efficiency (EQE) or internal quantum efficiency (IQE) exceeding 100% without bias. However, typical Pb-based perovskites have bandgaps around 1.7 eV, placing the CM threshold energy (>3.4 eV, <365 nm) in a region of weak solar spectrum intensity. Mixed Pb-Sn perovskites, with narrower bandgaps (~1.17–1.30 eV), offer a more suitable platform as their CM threshold aligns better with the solar spectrum's high-intensity region.

Extensive research has explored the phenomenon of carrier multiplication (CM) in various materials, particularly in semiconductor nanocrystals. Several studies have demonstrated compelling spectroscopic evidence of strong CM effects in halide perovskites, showing promise for exceeding the Shockley-Queisser efficiency limit in solar cells. These studies have primarily utilized transient absorption spectroscopy on bare perovskite films, offering insights into the fundamental mechanisms of CM. However, translating these findings into tangible improvements in the performance of actual perovskite solar cells (PSCs) has been a significant challenge. The lack of clear evidence for CM enhancement in real-world PSC devices highlights the need for further research to investigate the interplay of various factors affecting CM efficiency within the complex device architecture. Previous work has shown efficient multiple exciton generation (MEG) in perovskite nanocrystals (NCs), leading to photodetector enhancements in the UV/deep UV region. Nevertheless, the demonstration of MEG enhancements in actual PSC devices remains lacking, underlining the need for studies focusing on real-world device performance rather than isolated material properties.

The research utilized a mixed Pb-Sn perovskite system, Cs0.05FA0.5MA0.45Pb0.5Sn0.5I3, with a bandgap of ~1.24 eV. Initially, transient absorption (TA) spectroscopy was performed on thin films to assess CM behavior. Careful procedures were implemented to mitigate artifacts in the TA measurements. The initial photobleaching (PB) signal amplitude was used to determine the absorbed photon flux required for the same initial PB. A reduction in this flux at high pump photon energies above the 2Eg threshold indicated CM. The CM quantum yield (QYCM) was calculated by comparing gradients obtained above and below the threshold. The CM efficiency was determined through fitting methods. Perovskite solar cells (PSCs) were fabricated using both Cs0.05FA0.5MA0.45Pb0.5Sn0.5I3 and a MAPbI3 reference. The short-circuit current density (Jsc) under monochromatic illumination at various photon energies was measured. Internal quantum efficiency (IQE) was calculated using an equation that accounts for light absorption by the perovskite layer, eliminating the influence of excitation energies from different lasers. Broadband illumination studies were conducted on PSCs with optimized device structures. To minimize parasitic losses and recombination, the researchers used quartz substrates and a self-assembled monolayer (SAM) of 2PACz as the hole transport layer (HTL). The J-V curves under one sun (AM 1.5 G) illumination were measured for devices with varying perovskite layer thicknesses. EQE and IQE were calculated to assess CM in the devices under broadband illumination. Optical modeling was performed to validate the IQE exceeding 100%. Various characterization techniques were employed, including X-ray diffraction (XRD), UV-Vis-NIR spectroscopy, current-voltage (J-V) measurements, external quantum efficiency (EQE) measurements, atomic force microscopy (AFM), and ultraviolet photoelectron spectroscopy (UPS).

Transient absorption spectroscopy on Cs0.05FA0.5MA0.45Pb0.5Sn0.5I3 thin films confirmed efficient CM with a low threshold of 2Eg and a high efficiency of 99.4 ± 0.4%. Perovskite solar cells (PSCs) based on this material showed a peak IQE of ~161% (average 157 ± 4%) under unbiased conditions, indicating robust CM. Monochromatic illumination studies revealed an increase in Jsc and IQE above the CM threshold (473 nm excitation) in the mixed Pb-Sn PSCs, absent in the MAPbI3 reference devices. Broadband illumination studies on optimized quartz-based PSCs (with 2PACz as HTL) further validated CM, with IQE values exceeding 100% at a CM threshold of ~2Eg (500 nm). The maximum IQE reached ~160% at 3.33Eg in the thin perovskite layer devices. Optical modeling supported the experimental findings of IQE > 100%. A comparison with literature values for PbSe and PbS devices showed that the mixed Pb-Sn PSCs possess a lower CM threshold and comparable or superior peak IQE values.

The observed IQE exceeding 100% provides strong evidence for CM in the mixed Pb-Sn PSCs. However, the study also highlights factors limiting the impact of CM on overall PSC performance. These factors include competing requirements for optimal perovskite thickness (thin for CM from high-energy photons, thick for efficient low-energy photon harvesting), carrier losses due to defects, parasitic absorption losses from substrates and intermediate layers, and energy level alignment issues at the perovskite/HTL interface. The use of a quartz substrate and 2PACz HTL mitigated some of these losses, revealing the significant contribution of CM. The findings suggest that CM effects are present in mixed Pb-Sn PSCs but may be masked by other factors. Future strategies to fully leverage CM include using alternative transparent conducting layers with high UV transmittance, engineering perovskites with enhanced optical thickness at longer wavelengths, interface engineering to reduce recombination losses, and developing improved ETL materials. Perovskite tandem configurations could also utilize thin mixed Pb-Sn PSCs to exploit CM at short wavelengths in combination with other cells optimized for longer wavelengths.

This research presents compelling evidence for carrier multiplication (CM) in mixed Pb-Sn perovskite solar cells, achieving an internal quantum efficiency exceeding 110% without external bias. Transient spectroscopy, monochromatic, and broadband illumination studies confirm the high CM efficiency and low threshold. However, the typical PSC architecture limits the impact of CM due to various parasitic and recombination losses. By optimizing the device structure, an IQE greater than 160% was demonstrated. Future research should focus on device architecture redesign to fully unlock the potential of CM in next-generation perovskite solar cells and tandem configurations to exceed the Shockley-Queisser limit.

While the study successfully demonstrates CM in mixed Pb-Sn PSCs and identifies limiting factors, several limitations should be considered. The optimization of the device architecture focused primarily on minimizing parasitic losses and improving charge extraction. A more comprehensive investigation of defect engineering and interfacial engineering could further enhance the device performance. Additionally, long-term stability studies are necessary to assess the practical viability of these high-efficiency devices. The study predominantly focused on a specific mixed Pb-Sn perovskite composition, and further research is needed to explore the CM effect in a broader range of compositions and perovskite structures.