High efficiency carrier multiplication in PbSe nanocrystals (NCs) has been demonstrated, with impact ionization (II) occurring with up to 100% efficiency. This process, which is the inverse of Auger recombination (AR), allows a single photon to generate multiple excitons, significantly increasing the power conversion efficiency of NC-based solar cells. The study shows that II in PbSe NCs is highly efficient, fast, and occurs in a wavelength range that can enhance solar cell performance. Solar cells based on PbSe NCs can achieve higher power conversion efficiency by utilizing II. The maximum theoretical efficiency for solar cells under concentrated illumination is 43.9%, but this can be exceeded with efficient II. The study uses transient absorption (TA) to monitor carrier dynamics in PbSe NCs, revealing that II leads to rapid biexciton formation, which then undergoes AR. The efficiency of II is measured by comparing the amplitude of the fast decay component with the long-lived excitonic background. The efficiency of II increases with decreasing band gap energy (Eg) and is highest when the photon energy is more than three times the band gap. For photon energies of 3.8Eg, efficiencies up to 118% are observed, indicating the formation of triexcitons. The fast relaxation times observed in NCs are inversely proportional to their volume, confirming the role of AR in biexciton decay. The study also shows that reducing the threshold for II can further increase power conversion efficiency. For example, minimizing the II threshold to 2Eg can increase relative power conversion efficiency by 37%. PbSe NCs offer advantages such as size-tunable band gaps and strong absorption across a wide spectral range, making them suitable for high-efficiency solar cells and other optoelectronic applications. The results suggest that II can significantly enhance solar cell performance by enabling more efficient use of the solar spectrum.High efficiency carrier multiplication in PbSe nanocrystals (NCs) has been demonstrated, with impact ionization (II) occurring with up to 100% efficiency. This process, which is the inverse of Auger recombination (AR), allows a single photon to generate multiple excitons, significantly increasing the power conversion efficiency of NC-based solar cells. The study shows that II in PbSe NCs is highly efficient, fast, and occurs in a wavelength range that can enhance solar cell performance. Solar cells based on PbSe NCs can achieve higher power conversion efficiency by utilizing II. The maximum theoretical efficiency for solar cells under concentrated illumination is 43.9%, but this can be exceeded with efficient II. The study uses transient absorption (TA) to monitor carrier dynamics in PbSe NCs, revealing that II leads to rapid biexciton formation, which then undergoes AR. The efficiency of II is measured by comparing the amplitude of the fast decay component with the long-lived excitonic background. The efficiency of II increases with decreasing band gap energy (Eg) and is highest when the photon energy is more than three times the band gap. For photon energies of 3.8Eg, efficiencies up to 118% are observed, indicating the formation of triexcitons. The fast relaxation times observed in NCs are inversely proportional to their volume, confirming the role of AR in biexciton decay. The study also shows that reducing the threshold for II can further increase power conversion efficiency. For example, minimizing the II threshold to 2Eg can increase relative power conversion efficiency by 37%. PbSe NCs offer advantages such as size-tunable band gaps and strong absorption across a wide spectral range, making them suitable for high-efficiency solar cells and other optoelectronic applications. The results suggest that II can significantly enhance solar cell performance by enabling more efficient use of the solar spectrum.