Internal and External Quantum Efficiencies of Organic Photodiodes

3 Internal and External Quantum Efficiencies of Organic Photodiodes 

The spectral response, or action spectrum, of a phototodiode is obtained by measuring the electrical response of the device upon monochromatic illumination over a wide range of wavelengths. The external quantum efficiency ij, often also called the incident photon-to-current efficiency (IPCE) of the device, is the ratio of the measured photocurrent (in electrons per unit area and time) to the intensity of incoming monochromatic light in photons per unit area and time. Explicitly, 

In order to study the physics of charge generation and transport in devices, it is helpful to define an internal measure. The internal quantum efficiency (IQE) or quantum yield can be obtained from the external quantum efficiency by considering only the light absorbed in the device. The sum of total absorp-tance A, transmittance T and reflectance R must be unity A + T + R = 1. In the case of thick devices, the transmittance is zero. Therefore the internal quantum efficiency is 

In the case of thin devices, the transmittance may not be zero and the concept of IQE becomes less well defined. Most polymer photovoltaic cells are quite thin and only the photons absorbed in the active regions of the device can contribute to the photocurrent. The useful absorptance in the cells is given by 

It is possible to define a quantum efficiency describing the true efficiency of the material in the photoactive layers, i.e., the efficiency of exciton-to-charge generation (QEC) [19] 

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