Lead sulphide colloidal quantum dots (CQD) are a prime candidate for opto-electronic applications in the near infrared spectral region that can be precisely targeted due to the quantum size effect. Cast from solution, CQD solar cells achieved impressive power conversion efficiencies of around 10% in single junction devices and have thus almost risen on a par with organic photovoltaics.
A limiting factor for their performance is the low open circuit voltage (VOC) relative to their absorption onset. This reduction is often ascribed to trap mediated recombination. Indeed, their large surface to volume ratio makes CQDs prone to surface defects either during synthesis or successive ligand exchange. Whereas successful strategies to counter this trap formation were carried out e.g. via successive treatment with different ligands or the employment of core-shell type CQDs, the precise nature of these states has hitherto not been examined comprehensively.
In this work, we thus chose to investigate the population of PbS CQD trap states under the variation of external parameters, including particle size, ligand type, washing steps and ageing. We introduce a versatile FTIR-based pump-probe set-up, which is capable of probing energies as low as 0.07 eV and observe a broad region of photoinduced absorption (0.1 to 0.5 eV) for all samples investigated. This absorption is found to consist of two overlapping bands located around 0.22 and 0.32 eV – independent of external factors. Merely the strength of the signals is affected by experimental conditions. We link the emergence of these bands to surface traps, predominantly evoked by atom rearrangement, which leads to changes in the Pb/S atom partial charges. The particle-ligand interaction can furthermore be examined by the emergence of infrared active vibrations that we attribute to the electric field of trapped surface charges acting on the molecules resonance frequencies.
Our results offer a deeper insight into the formation and origin of trap states in PbS CQDs, whilst simultaneously delivering a sensitive tool to assess the degree of surface passivation offered by different ligands. Ultimately, this will help the community to further improve the manufacturing protocols for CQD solar cells leading to a better performance.