A quantitative model for charge carrier transport, trapping and recombination in nanocrystal-based solar cells.

Deniz Bozyigit,Vanessa Wood,Weyde M M Lin,Olesya Yarema,Nuri Yazdani

doi:10.1038/ncomms7180

Deniz Bozyigit, Vanessa Wood + Show 3 more

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https://doi.org/10.1038/ncomms7180

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Journal: Nature Communications	Publication Date: Jan 27, 2015
Citations: 116	License type: CC BY 4.0

Affiliation: ETH Zurich

Abstract

Improving devices incorporating solution-processed nanocrystal-based semiconductors requires a better understanding of charge transport in these complex, inorganic–organic materials. Here we perform a systematic study on PbS nanocrystal-based diodes using temperature-dependent current–voltage characterization and thermal admittance spectroscopy to develop a model for charge transport that is applicable to different nanocrystal-solids and device architectures. Our analysis confirms that charge transport occurs in states that derive from the quantum-confined electronic levels of the individual nanocrystals and is governed by diffusion-controlled trap-assisted recombination. The current is limited not by the Schottky effect, but by Fermi-level pinning because of trap states that is independent of the electrode–nanocrystal interface. Our model successfully explains the non-trivial trends in charge transport as a function of nanocrystal size and the origins of the trade-offs facing the optimization of nanocrystal-based solar cells. We use the insights from our charge transport model to formulate design guidelines for engineering higher-performance nanocrystal-based devices.

Highlights

Improving devices incorporating solution-processed nanocrystal-based semiconductors requires a better understanding of charge transport in these complex, inorganic–organic materials
The dark current in a diode can provide direct insight into the charge transport, trapping and recombination processes that play an important role in the power conversion efficiency of a solar cell
Understanding the physical processes that determine the dark current in a NC-based solar cell would enable us to assess the origins of performance limitations in these devices and develop guidelines for achieving higher performance

Summary

Introduction

Improving devices incorporating solution-processed nanocrystal-based semiconductors requires a better understanding of charge transport in these complex, inorganic–organic materials. To rationally assess the impact of different fabrication techniques, it is necessary to develop a consistent and predictive model of charge transport in NC-based solar cells Any such model must quantitatively explain the dark current, which is one of the fundamental and, conveniently, most experimentally accessible characteristics of a diode. Understanding the physical processes that determine the dark current in a NC-based solar cell would enable us to assess the origins of performance limitations in these devices and develop guidelines for achieving higher performance. Studies of the dark current of NC-based diodes presented temperature-dependent measurements[15,21], but, by changing only temperature, it is not possible to obtain sufficient information to uniquely identify the physical processes that govern charge transport Data in these studies were explained using variations of the majority carrier emission theory developed for single-crystalline semiconductors that is known as the Schottky diode model. Correlation between the prefactor and activation energy is very common in disordered and defect-rich semiconductors and is known as the Meyer–Neldel rule[25]

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