Recombination Dynamics in PbS Nanocrystal Quantum Dot Solar Cells Studied through Drift–Diffusion Simulations

Abstract

The significant performance increase in nanocrystal (NC)-based solar cells over the last decade is very encouraging. However, many of these gains have been achieved by trial-and-error optimization, and a systematic understanding of what limits the device performance is lacking. In parallel, experimental and computational techniques provide increasing insights into the electronic properties of individual NCs and their assemblies in thin films. Here, we utilize these insights to parameterize drift–diffusion simulations of PbS NC solar cells, which enable us to track the distribution of charge carriers in the device and quantify recombination dynamics, which limit the device performance. We simulate both Schottky- and heterojunction-type devices and, through temperature-dependent measurements in the light and dark, experimentally validate the appropriateness of the parameterization. The results reveal that Schottky-type devices are limited by surface recombination between the PbS and aluminum contact, while heterojunction devices are currently limited by NC dopants and electronic defects in the PbS layer. The simulations highlight a number of opportunities for further performance enhancement, including the reduction of dopants in the nanocrystal active layer, the control over doping and electronic structure in electron- and hole-blocking layers (e.g., ZnO), and the optimization of the interfaces to improve the band alignment and reduce surface recombination. For example, reduction in the percentage of p-type NCs from the current 1–0.01% in the heterojunction device can lead to a 25% percent increase in the power conversion efficiency.

Sub-nanosecond Intrinsic Response Time of PbS Nanocrystal IR-Photodetectors

“…Another common mechanism of trap-assisted recombination in PbS-NC films with high trap density is the Shockley–Read–Hall (SRH) recombination, also observed to be invariant with temperature. ,, However, in view of the monoexponential decay in the 2PC curves (Figure c), only one mechanism seems to be dominant. Whereas the SRH recombination rate depends on the trap density, TAA recombination depends on both the trap density and the excess carrier density .…”

Design and optimization of the performance of PbS quantum dot based vertical photodetector

“…Table 1 [15,22] displays various material parameters fed to the software for the required calculations. Table 2 [14] represents the interface defect parameters between different layers.…”

“…In the past two decades, research on PbS quantum dot photovoltaic devices has continued, both experimentally [10][11][12] and theoretically [13], but among them the theoretical calculations have mainly focused on PbS quantum dot solar cells [14][15][16][17][18], and research on PbS quantum dot photodetectors still requires further development.…”

Fourth-generation solar cells: a review

“…In addition, these simulation methods are capable of providing information on the dopant and trap state densities of the nanocrystal layers, such that the appropriate selection and optimization of blocking contacts has been carried out to enhance the device performance further. 112 These novel high photovoltaic energy conversion efficiency quantum dot sensitized solar cells (QDSSCs) are based on II–VI semiconductor materials such as CdS. CdS is an n-type semiconductor that has a direct band gap energy of 2.42 eV.…”