Abstract
We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS2 (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor. By using time-resolved spectroscopic techniques, we are able to observe photoinduced absorptions that we attribute to the mobile hole carriers in the NC film. In combination with transient photocurrent and photovoltage measurements, we monitor charge dynamics on time scales from 300 fs to 1 ms. Carrier dynamics are investigated for devices with CIS layers composed of either colloidally synthesized 1,3-benzenedithiol-capped nanocrystals or in situ sol-gel synthesized thin films as the active layer. We find that deep trapping of holes in the colloidal NC cells is responsible for decreases in the open-circuit voltage and fill factor as compared to those of the sol-gel synthesized CIS/CdS cell.
Highlights
We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS2 (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor
We find the spectral shape of the photoinduced absorption (PIA) observed in CIS/CdS films to be in good agreement with the steadystate hole plasmon absorption described by Kriegel et al Since both the PIA in CIS/CdS films and the plasmonic bleach in CIS-colloidal nanocrystals (CNCs) films appear within the instrument response, and similar excitation densities are used, it is unlikely that the relative red shift and broadening of the plasmonic bleach is solely caused by an enhancement in heating and charge carrier scattering
While in situ devices showed no significant decay of the hole plasmon within the first microsecond, hole-plasmon induced PIA signals in CNC devices were found to decay by 75% in the same time period, much faster than anticipated from transient photocurrent (TPC) measurements
Summary
We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS2 (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor. Grown semiconductor nanocrystals could enable the use of low-temperature, scaleable atmospheric-pressure deposition methods such as inkjet printing or spray coating.5À27 The main barrier to adoption of (nonsintered) cupric chalcogenide colloidal nanocrystal solar cells is their relatively low power conversion efficiencies (PCEs): generally less than 2% for most candidate materials, with the best devices achieving around 3%.8. These are low with respect to both their thin-film analogues and lead-based nanocrystalline devices.5,28À31 In the latter case, nearly a decade of work on the physics and conduction mechanisms present in these films has informed a series of incremental improvements in efficiency. By linear extrapolation of the EQE curves, we obtain an estimate of the bandgap for CNC and in situ CIS of 1.35 and 1.47 eV, respectively (inset)
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