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Register a new userPhotonically-confined solar cells: prospects for exceeding the Shockley-Queisser limit
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The Shockley-Queisser (SQ) limit, introduced by W. Shockley and H. J. Queisser in 1961, is the most well-established fundamental efficiency limit for single-junction photovoltaic solar cells. For widely-studied semiconductors such as Si, GaAs and lead-halide perovskite, the SQ limits under standard solar illumination (1-sun) are 32.7%, 32.5% and 31% for bandgaps of 1.12 eV, 1.43 eV and 1.55 eV, respectively. Here, we propose that the fundamental efficiency limits for single-junction solar cells may be surpassed via photon confinement, substantially raising the theoretical limits to 49%, 45.2% and 42.1% for Si, GaAs and methylammonium lead iodide (MAPbI3) perovskite cells under 1-sun. Such enhancement is possible through the containment of luminescent photons within the solar cell, allowing the suppression of both non-radiative and radiative recombination losses, which were considered inevitable in the classical SQ model. Importantly, restricting photon emission from the solar cells raises the open-circuit voltage (VOC) to values approaching the semiconductor bandgaps, surpassing the theoretical VOC values predicted by the SQ model. The fill factors of the cells are expected to increase substantially, resulting in current-voltage characteristics with very-high squareness for ideal diode operation. Our work introduces a new framework for improving solar cell performance beyond the conventional limits.
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Light management is of great importance to photovoltaic cells, as it determines the fraction of incident light entering the device. An optimal pn-junction combined with an optimal light absorption can lead to a solar cell efficiency above the Shockley-Queisser limit. Here, we show how this is possible by studying photocurrent generation for a single core-shell p-i-n junction GaAs nanowire solar cell grown on a silicon substrate. At one sun illumination a short circuit current of 180 mA/cm^2 is obtained, which is more than one order of magnitude higher than what would be predicted from Lambert-Beer law. The enhanced light absorption is shown to be due to a light concentrating property of the standing nanowire as shown by photocurrent maps of the device. The results imply new limits for the maximum efficiency obtainable with III-V based nanowire solar cells under one sun illumination.
Compared to traditional pn-junction photovoltaics, hot carrier solar cells offer potentially higher efficiency by extracting work from the kinetic energy of photogenerated hot carriers before they cool to the lattice temperature. Hot carrier solar cells have been demonstrated in high-bandgap ferroelectric insulators and GaAs/AlGaAs heterostructures, but so far not in low-bandgap materials, where the potential efficiency gain is highest. Recently, a high open-circuit voltage was demonstrated in an illuminated wurtzite InAs nanowire with a low bandgap of 0.39 eV, and was interpreted in terms of a photothermoelectric effect. Here, we point out that this device is a hot carrier solar cell and discuss its performance in those terms. In the demonstrated devices, InP heterostructures are used as energy filters in order to thermoelectrically harvest the energy of hot electrons photogenerated in InAs absorber segments. The obtained photovoltage depends on the heterostructure design of the energy filter and is therefore tunable. By using a high-resistance, thermionic barrier an open-circuit voltage is obtained that is in excess of the Shockley-Queisser limit. These results provide generalizable insight into how to realize high voltage hot carrier solar cells in low-bandgap materials, and therefore are a step towards the demonstration of higher efficiency hot carrier solar cells.
Charge transfer in polymer devices represents a crucial, though highly inaccessible stage of photocurrent generation. In this article we propose studying the properties and behaviour of organic solar cells through the modification of photocurrent generation when an external magnetic field is applied. By allowing the parameters of our theoretical model not to be constrained to any specific material, we are able to show that not only a modest external magnetic field leads to a significant increase in photocurrent intensity, but also how such magnetic field can be used to study in detail the energy levels and transition rates within the polymer compound. Systematic exploration of key properties in organic composites thus can lead to highly optimised devices in which a magnetic field produces an enhancement in the efficiency of polymer solar cells.
The thermodynamic limit of photovoltaic efficiency for a single-junction solar cell can be readily predicted using the bandgap of the active light absorbing material. Such an approach overlooks the energy loss due to non-radiative electron-hole processes. We propose a practical ab initio procedure to determine the maximum efficiency of a thin-film solar cell that takes into account both radiative and non-radiative recombination. The required input includes the frequency-dependent optical absorption coefficient, as well as the capture cross-sections and equilibrium populations of point defects. For kesterite-structured Cu$_2$ZnSnS$_4$, the radiative limit is reached for a film thickness of around 2.6 micrometer, where the efficiency gain due to light absorption is counterbalanced by losses due to the increase in recombination current.
We explore the degradation behaviour under continuous illumination and direct oxygen exposure of inverted unencapsulated formamidinium(FA)0.83Cs0.17Pb(I0.8Br0.2)3, CH3NH3PbI3, and CH3NH3PbI3-xClx perovskite solar cells. We continuously test the devices in-situ and in-operando with current-voltage sweeps, transient photocurrent, and transient photovoltage measurements, and find that degradation in the CH3NH3PbI3-xClx solar cells due to oxygen exposure occurs over shorter timescales than FA0.83Cs0.17Pb(I0.8Br0.2)3 mixed-cation devices. We attribute these oxygen-induced losses in the power conversion efficiencies to the formation of electron traps within the perovskite photoactive layer. Our results highlight that the formamidinium-caesium mixed-cation perovskites are much less sensitive to oxygen-induced degradation than the methylammonium-based perovskite cells, and that further improvements in perovskite solar cell stability should focus on the mitigation of trap generation during ageing.
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