Tabulated values of the Shockley–Queisser limit for single junction solar cells

Nanostructured materials for solar energy conversion

Note: EPFL Reference LESO-PB-CONF-2003-017 Record created on 2005-01-12, modified on 2016-08-08

The use of quantum dots can turn the old concept of a luminescent solar collector into a practical concentrator. The quantum efficiency, tunability of absorption threshold, and size of the redshift make quantum dots an ideal replacement for the organic dyes whose performance limited this inexpensive technology. Progress in photovoltaic cells, in particular, the ability of quantum-well cells to tune the band gap, also suggests high efficiency is possible in solar and thermophotovoltaic applications. A thermodynamic model is used to show quantitatively how the separation of absorption and luminescent peaks under global illumination is related to the spread of quantum-dot sizes. Hence, the redshift can be determined during the growth process. The model can be used to optimize concentrator performance and to study the effect of reabsorption, which is important for high concentration even if the quantum efficiency is unity. This model provides a quantitative explanation for the contribution of the spread of sizes to the redshift, which should help in the extraction of the much smaller, single-dot effects.

Quantum-dot concentrator and thermodynamic model for the global redshift

A strain-balance multiquantum well (MQW) approach to enhance the GaAs solar cell efficiency is reported. Using a p-i-n diode structure, the strain-balanced GaAsP/InGaAs MQW is grown on a GaAs substrate and equals a good GaAs cell in terms of power conversion efficiency. The cell design is presented together with measurements of the forward bias dark current density, quantum efficiency, and 3000 K light-IV response. Cell efficiencies under standard air mass (AM) 1.5 and AM 0 illumination are projected from experimental data and the suitability of this cell for enhancing GaInP/GaAs tandem cell efficiencies is discussed.

Strain-balanced GaAsP/InGaAs quantum well solar cells

Quantum well solar cells

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.

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

It is known that quantum well solar cells (QWSCs) can enhance short circuit current and power conversion efficiency in comparison with similar, conventional solar cells made from the quantum well (QW) barrier material alone. In this article we report measurements of the dark‐current and open‐circuit voltage (Voc) of a number of quantum well cells in three different lattice‐matched material systems, namely, Al0.35Ga0.65As/GaAs, GaInP/GaAs, and InP/InGaAs. We also present the results obtained from comparable control cells without wells formed either from the material of the barriers or the well material alone. Our results clearly demonstrate in all three cases that, at fixed voltage, QWSC dark currents are systematically lower than would be expected from control cells with the same effective absorption edge. Measurements of Voc in a white‐light source show that the open‐circuit voltages of the QWSCs are higher than those of control cells formed from the well material. Furthermore, this enhancement is more t...

Voltage enhancement in quantum well solar cells

The radiative behavior of quantum‐well (QW) devices depends upon the quasi‐Fermi‐level separation ΔEf induced in the quantum well. We present a method of obtaining ΔEf in absolute units from the emission spectra of optically or electrically biased QWs. Emission spectra are calibrated by comparison with measurements of the limiting photocurrent. A theoretical model is then used to separate out the effects of carrier generation rate and field‐dependent QW absorption. We apply the method to the low‐temperature photoluminescence spectra of a set of single QW p‐i‐n photodiodes at different electric fields. We show that modeled emission spectra agree closely with measured spectra in flatband conditions. We also observe a field‐dependent loss in emission intensity—leading to a reduction in ΔEf of several meV—which we attribute mainly to carrier escape from the QW. The derived values for field‐dependent nonradiative efficiency are consistent with independent measurements of low‐temperature dc photocurrent, and wi...

Determination of the quasi‐Fermi‐level separation in single‐quantum‐well p‐i‐n diodes

We have studied the variation with applied bias and temperature of steady state photoluminescence (DCPL) and photoconductivity (DCPC) from a series of GaAs/AlGaAs single quantum well, p-i-n structures with different well widths. We present the DCPC and DCPL results, which when combined, allow us to assess how significant nonradiative recombination is in the samples and hence the quality of the material. We discuss the qualitative features in the light of a new theoretical approach presented here for the first time. This includes contributions from escape (of both electrons and holes) and makes it possible to extract from the experimental data two parameters, each reflecting the competition between escape and one of the recombination processes (radiative or nonradiative) in the absence of the other. We further comment qualitatively on the bias and temperature dependence of these different processes.

E. S. M. Tsui

Papers

Quantum well solar cells

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

Voltage enhancement in quantum well solar cells

Determination of the quasi‐Fermi‐level separation in single‐quantum‐well p‐i‐n diodes

Steady state photocurrent and photoluminescence from single quantum wells as a function of temperature and bias