Investigation of the Trap-Induced Power Conversion Limit for CdS/CdSe Cascade Quantum Dot Sensitized Solar Cells Fabricated by Using the Successive Ionic Layer Adsorption and Reaction Process

Abstract

CdS/CdSe quantum dot sensitized solar cells (QDSSC) fabricated by successive ionic layer adsorption and reaction (SILAR) processes was investigated, and a rate-equation model for the trapinduced power conversion efficiency (PCE) limit was developed and used to explain the experimental results. The cascade structure with a CdS:CdSe (7:7) cycle ratio showed the highest PCE of 2.55%. However, excess cycles of CdSe beyond the optimum condition decrease the device performance. The current loss when exceeding the maximum PCE condition is attributed to the trap-induced charge field that impedes the carrier extraction from the absorber layer to the titanium dioxide (TiO2) and increases recombination due to dislocation generation when the critical thickness for pseudomorphic growth is exceeded. The simulation results show that the increase in the number of dislocations beyond the critical thickness increases the recombination rate and impedes charge transfer at the interface between TiO2 and the QDs.