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Ultra-sensitive External Quantum Efficiency (EQE) Measurement

Our home-built, ultra-sensitive EQE measurement setup is a useful tool to investigate photo-generation and absorption processes of charge carriers via inter- and intramolecular states such as charge transfer states or sub-gap tail and trap states in photovoltaic systems based on organic and inorganic semiconductors. Using a high-performance spectrophotometer with integrated double holographic grating monochromators as a probe light source and a low noise multi-stage amplifier system with variable gain and electrical bandwidths enables via lock-in method dynamic ranges of > 100 dB and allows for the detection of photocurrents smaller than femto ampere.

Our ultra-sensitive EQE measurement setup provides:

  • an extended, ultra-low stray light wavelength regime from 175 nm up to 3300 nm

  • a pre-current amplifier with variable gain (up to 109 VA-1) and integrated low noise voltage source ( V) for EQE measurements under electrical bias

  • electrical bandwidths (integration times) from 106 Hz (1 ms) to  Hz (30 ks)

  • different pump light sources for EQE measurements under light bias

  • low noise sample holder with integrated temperature control stage for temperature dependent EQE measurements


FIgure 1: Experimental setup of our home-built, ultra-sensitive external quantum efficiency (EQE) measurements with spectrophotometer (PerkinElmer, Lambda950, lock-in amplifier (Stanford Research Systems, SR860), multi-blade chopper (Thorlabs, MC2000B), pre-current amplifier (Femto, DLPCA-200) and mounted optics. A detailed description of the measurement setup can be found in [1].


Figure 2: External quantum efficiency (EQE) plotted as a function of wavelength for a 300 nm thick PBTTT:PC70BM device measured under short-circuit (no voltage applied on the device). The EQE measurement was performed at a (probe light) chopping frequency of 273 Hz and an electrical bandwidth of 33.33 mHz. EQE signals above the noise floor at photon energies well below the bandgap reveals sub-gap absorption features which could be assigned to tail or trap states or micro-cavity effect.


 [1] Zeiske, et al., Sensitivity of Sub-Bandgap External Quantum Efficiency Measurements of Solar Cells under Electrical and Light Bias, ACS Photonics, DOI: 10.1021/acsphotonics.9b01531

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