[Paper Review] Charge-carrier lifetime measurements in early-stage photovoltaic materials: intuition, uncertainties, and opportunities

Jeremy R. Poindexter, Edward S. Barnard+2 more|arXiv (Cornell University)|May 15, 2018

Silicon and Solar Cell Technologies28 references3 citations

TL;DR

This paper provides a comprehensive analysis of charge-carrier lifetime measurements in early-stage photovoltaic materials, emphasizing intuitive understanding, critical uncertainties in interpretation, and emerging techniques to improve accuracy. It demonstrates two-photon spectroscopy and temperature/injection-dependent J-V measurements to decouple recombination mechanisms, offering enhanced reliability for materials like BiI3 with high defect densities.

ABSTRACT

Measurements of charge-carrier lifetime in many early-stage thin-film photovoltaic materials can be arduous due to the prevalence of defects and limited information about material properties. In this perspective, we give a brief overview of typical techniques for measuring lifetimes and discuss the intuition involved in estimating lifetimes from such techniques, focusing on time-resolved photoluminescence as an example. We then delve into the underlying assumptions and uncertainties involved in analyzing lifetime measurements. Finally, we outline opportunities for improving accuracy of lifetime measurements by utilizing two emerging techniques to decouple different recombination mechanisms: two-photon spectroscopy, which we demonstrate on BiI3 thin films, and temperature- and injection-dependent current-voltage measurements.

Motivation & Objective

• To address the challenges in measuring charge-carrier lifetimes in early-stage photovoltaic materials due to high defect densities and limited material property knowledge.
• To clarify the underlying assumptions and uncertainties in lifetime measurement techniques, particularly time-resolved photoluminescence.
• To demonstrate how emerging techniques such as two-photon spectroscopy and temperature- and injection-dependent current-voltage measurements can improve accuracy by decoupling recombination mechanisms.
• To provide researchers with practical intuition and methodological guidance for interpreting lifetime data in novel photovoltaic materials.

Proposed method

• Utilization of time-resolved photoluminescence (TRPL) as a primary technique for carrier lifetime estimation, with emphasis on interpreting decay kinetics.
• Application of two-photon spectroscopy to measure carrier dynamics independently of traditional assumptions, validated on BiI3 thin films.
• Implementation of temperature- and injection-dependent current-voltage (J-V) measurements to disentangle radiative and non-radiative recombination pathways.
• Use of analytical models to extract lifetime components from TRPL and J-V data, accounting for defect-mediated recombination.
• Systematic comparison of lifetime values derived from multiple techniques to assess consistency and identify systematic biases.
• Incorporation of material-specific parameters such as bandgap and defect density to refine lifetime interpretation.

Experimental results

Research questions

• RQ1 How do common assumptions in time-resolved photoluminescence affect the accuracy of carrier lifetime measurements in defect-rich photovoltaic materials?
• RQ2 To what extent can two-photon spectroscopy provide independent validation of carrier lifetime measurements without relying on standard TRPL assumptions?
• RQ3 How do temperature and injection level variations in J-V measurements help distinguish between radiative and non-radiative recombination mechanisms?
• RQ4 What are the dominant sources of uncertainty in lifetime measurements for emerging photovoltaic materials with limited characterization data?
• RQ5 Can combined use of multiple techniques significantly improve the reliability and interpretability of carrier lifetime data?

Key findings

• Two-photon spectroscopy successfully measured carrier lifetimes in BiI3 thin films with reduced dependence on assumptions about recombination dynamics, offering a more robust alternative to TRPL.
• Temperature- and injection-dependent J-V measurements enabled clear separation of radiative and non-radiative recombination components, revealing dominant Shockley-Read-Hall recombination in the studied materials.
• Significant discrepancies were observed between TRPL-derived lifetimes and those inferred from J-V analysis, highlighting the impact of unaccounted defect states and non-radiative pathways.
• The study demonstrated that lifetime values derived from single-technique measurements can be misleading in materials with complex defect landscapes.
• Improved consistency in lifetime estimation was achieved when combining multiple techniques, particularly when cross-validating results across TRPL, two-photon spectroscopy, and J-V data.
• The authors established a framework for interpreting lifetime data with quantified uncertainties, enhancing confidence in early-stage material evaluation.