Determining the Internal Quantum Efficiency of PbSe Nanocrystal Solar Cells with the Aid of an Optical Model

Abstract

We determine the internal quantum efficiency (IQE) of the active layer of PbSe nanocrystal (NC) back-contact Schottky solar cells by combining external quantum efficiency (EQE) and total reflectance measurements with an optical model of the device stack. The model is parametrized with the complex index of refraction of each layer in the stack as calculated from ellipsometry data. Good agreement between the experimental and modeled reflectance spectra permits a quantitative estimate of the fraction of incident light absorbed by the NC films at each wavelength, thereby yielding well-constrained QE spectra for photons absorbed only by the NCs. Using a series of devices fabricated from 5.1 ( 0.4 nm diameter PbSe NCs, we show that thin NC cells achieve an EQE and an active layer IQE as high as 60 ( 5% and 80 ( 7%, respectively, while the QE of devices with NC layers thicker than about 150 nm falls, particularly in the blue, because of progressively greater light absorption in the field-free region of the films and enhanced recombination overall. Our results demonstrate that interference effects must be taken into account in order to calculate accurate optical generation profiles and IQE spectra for these thin film solar cells. The mixed modeling/experimental approach described here is a rigorous and powerful way to determine if multiple exciton generation (MEG) photocurrent is collected by devices with EQE < 100%. On the basis of the magnitudes and shapes of the IQE spectra, we conclude that the 1,2-ethanedithiol treated NC devices studied here do not produce appreciable MEG photocurrent.Light absorption in a thin film solar cell can be greatly affected by optical interference if the device is optically thin and has a highly reflective back electrode. When these conditions are met, the buildup of an optical mode structure will change the absorption spectra of the active materials relative to their spectra when free of the optical cavity formed by the device stack. Exponential, Beer's Law type absorption/ generation profiles cannot then be assumed, and accurate values of the internal quantum efficiency (IQE) of the active layers of the device can be obtained only by determining their absorption spectra as they exist within the device structure.The IQE of a solar cell is normally calculated from the ratio of its external quantum efficiency (EQE) and spectral absorptance as IQE(λ) ) EQE(λ)/[1 -F(λ) -τ(λ)], where F(λ) is the spectral reflectance and the device transmittance τ(λ) is usually zero. However, this approach can give an underestimated and misleading IQE spectrum if incident light is absorbed by the electrodes and other nonactive layers of the device in addition to the active semiconductor layer(s).We show here that the quantum efficiency of photons absorbed only within the active layer of a PbSe nanocrystal back-contact Schottky solar cell 1 can be determined by combining reflectance measurements with an optical model capable of discerning the fraction of light absorbed in each of the layers within the ...

Schottky Solar Cells Based on Colloidal Nanocrystal Films

“…A detailed optical and electrical model of the device parametrized by ellipsometry measurements has been developed to help answer this question. 21 However, we have also recently found that the EDT treatment of our PbSe NC films, while enhancing their PV performance, suppresses MEG. 22 Quenching of the MEG quantum yield by the EDT treatment potentially explains why the estimated IQE values reported here do not exceed 100%.…”

Postsynthetic Doping Control of Nanocrystal Thin Films: Balancing Space Charge to Improve Photovoltaic Efficiency

“…101 The photogeneration profile (green line, bottom graph) decays exponentially with the extinction length of 400 nm, typically for 1 μm wavelength light in a PbSe nanocrystal film. 5 The actual current collected under short-circuit (green fill, bottom graph), is the multiplication of the generation profile and collection probability, and drops significantly outside of the space charge region. where ε is the dielectric constant, ε 0 is the permittivity of free space, and q is the fundamental charge.…”

Low-Temperature Approach to High-Yield and Reproducible Syntheses of High-Quality Small-Sized PbSe Colloidal Nanocrystals for Photovoltaic Applications

“…2 Accordingly, colloidal PbSe QDs can be engineered to absorb and emit in a vast spectral range, spanning from ∼800 to 4000 nm with high photoluminescence (PL) emission quantum yields (QYs). [3][4][5][6][7][8] Hence, PbSe QDs are important materials for near-IR (NIR; >700 nm) and mid-IR (>2500 nm) applications, including biological imaging/labeling, 9,10 solar cells, [11][12][13][14][15][16] light-emitting devices, 17 and telecommunications. 18,19 PbSe nanocrystals (NCs) are especially attractive in the PV application, with a multiple exciton generation effect reported.…”