Observation of suppressed radiative recombination in single quantum well p-i-n photodiodes

Abstract

We have measured electroluminescence (EL) spectra of GaAs/InGaAs and AlGaAs/GaAs single quantum well (QW) p-i-n photodiodes at temperatures between 200 and 300 K and forward biases close to the open circuit voltage. Integrated EL spectra vary like eqV/nkT with an ideality factor n=1.05±0.05 over five decades, indicating purely radiative processes. The spectra are calibrated into absolute units enabling comparison to be made with the predictions of a theoretical model. For each temperature and bias we calculate the EL spectrum and radiative current expected in the detailed balance limit, integrating the theoretical emission spectrum over the surface of the device, in order to establish the quasi-Fermi potential separation, Δφf, in the QW and, where possible, in the host material. For the GaAs/InGaAs cell we are able to model emission from the QW and the host material simultaneously. We find that, in all cases, the QW emission is overestimated by theory if it is assumed that Δφf=V. QW emission corresponds instead to a value of Δφf which a few tens of mV less than V. In contrast, emission from the host material, where visible, is well fitted by the model with Δφf=V at all biases and temperatures. We attribute the variation in Δφf to irreversible thermally assisted escape from the QWs.

High-voltage quantum well waveguide solar cells

“…A number of different physical mechanisms can lead to a suppression in radiative current relative to that predicted by Equation (1), including the finite volume of the InGaAs well, photon recycling, and perturbations in the diode quasi-Fermi level [16][17]. While work is on-going to clarify the role of each of these mechanisms in our devices [9], photon recycling is an intriguing phenomenon that can potentially be leveraged to further reduce the dark current and hence increase the operation voltage of InGaAs quantum well solar cells operating in the radiative limit.…”

Progress in quantum well solar cells

“…In fact a thorough experimental analysis of the radiative components of the dark recombination current in strained single quantum well (SQW) devices and in a lattice matched double quantum well (DQW) devices [14,15] showed evidence of reduced radiative recombination in quantum wells with respect to level predicted by the generalised detailed balance model.…”

“…However, we assume that the non-radiative contribution in the i-region is described by the SHR n¨2 model discussed above. The radiative contribution to the QW recombination can be estimated by a detailed balance argument [10]. This relates the photons absorbed to the photons radiated and depends on the absorption coefficient a(E, F) as a function of energy E and field F and is calculated from first principles [10] in the programme used to fit the spectral response of the QWs assuming unity quantum efficiency for escape from the wells.…”

“…The radiative contribution to the QW recombination can be estimated by a detailed balance argument [10]. This relates the photons absorbed to the photons radiated and depends on the absorption coefficient a(E, F) as a function of energy E and field F and is calculated from first principles [10] in the programme used to fit the spectral response of the QWs assuming unity quantum efficiency for escape from the wells. It should be noted that the important parameters for both the ideal Shockley (minority carrier diffusion lengths) and the QW radiative current levels (absorption coefficient a(E, F)) are therefore determined by the spectral response fits in the bulk and QW regions, respectively, as in Fig.…”

Experimental measurement of restricted radiative emission in quantum well solar cells

“…A calculation of the carrier distribution across the depletion region allows the rate of non-radiative Shockley-ReadHall recombination to be estimated assuming a representative value for the non-radiative lifetime. The radiative recombination current is calculated from the absorption profile using the generalized Planck formalism [18,19]. This assumes such features as an isotropic emission profile, high carrier mobility and ideal carrier transport.…”