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 .
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.
 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