Solar cells based on colloidal quantum dot solids: Seeking enhanced photocurrent

Abstract

We describe an all-inorganic metal / PbSe quantum dot (QD) film / metal sandwich photovoltaic (PV) cell that produces a large short-circuit photocurrent (≫ 21 mA cm−2) by way of a Schottky junction at the negative (rear) contact. The PV cell consists of a PbSe QD film, deposited via layer-by-layer (LbL) dip coating that yields an EQE of 55– 65% in the visible and up to 25% in the infrared region of the solar spectrum, with a spectrally corrected AM1.5G power conversion efficiency of 2.1%. This QD device produces one of the largest short-circuit currents of any nanostructured solar cell, without the need for sintering, superlattice order or separate phases for electron and hole transport. To ascertain if the photocurrent is enhanced by multiple exciton generation (MEG), we determined the internal quantum efficiency (IQE) of the QD active layers of our devices by combining external quantum efficiency (EQE) and total reflectance measurements with an optical model of the device stack. Good agreement between the experimental and modeled reflectance spectra permits a quantitative estimate of the fraction of incident light absorbed by the QD films at each wavelength, thereby yielding well-constrained QE spectra for photons absorbed only by the QDs. Using a series of devices fabricated from 5.1 ± 0.4 nm diameter PbSe QDs treated with 1,2– ethaneditiol (EDT), we show that thin QD cells achieve an EQE and an active layer IQE as high as 60 ± 5% and 80 ± 7%, respectively, but that there is no evidence for MEG enhancement of the photocurrent. These results are consistent with femtosecond transient absorption measurements showing that the MEG process is quenched by the EDT treatment used to electronically couple the QDs. However, we find enhanced MEG yields using other film treatments, suggesting that MEG photocurrent may be collected from devices prepared using these alternative film treatment chemistries.