The power conversion efficiency of the most efficient organic photovoltaic (OPV) cells has recently increased to over 10%. To enable further increases, the factors limiting the device efficiency in OPV must be identified. In this review, the operational mechanism of OPV cells is explained and the detailed balance limit to photovoltaic energy conversion, as developed by Shockley and Queisser, is outlined. The various approaches that have been developed to estimate the maximum practically achievable efficiency in OPV are then discussed, based on empirical knowledge of organic semiconductor materials. Subsequently, approaches made to adapt the detailed balance theory to incorporate some of the fundamentally different processes in organic solar cells that originate from using a combination of two complementary, donor and acceptor, organic semiconductors using thermodynamic and kinetic approaches are described. The more empirical formulations to the efficiency limits provide estimates of 10-12%, but the more fundamental descriptions suggest limits of 20-24% to be reachable in single junctions, similar to the highest efficiencies obtained for crystalline silicon p-n junction solar cells. Closing this gap sets the stage for future materials research and development of OPV.

Factors Limiting Device Efficiency in Organic Photovoltaics

Point-contact spectroscopy was originally developed for the determination of the electron–phonon spectral function in normal metals. However, in the past 20 years it has become an important tool in the investigation of superconductors. As a matter of fact, point contacts between a normal metal and a superconductor can provide information on the amplitude and symmetry of the energy gap that, in the superconducting state, opens up at the Fermi level. In this paper we review the experimental and theoretical aspects of point-contact spectroscopy in superconductors, and we give an experimental survey of the most recent applications of this technique to anisotropic and multiband superconductors.

https://iopscience.iop.org/article/10.1088/0953-2048/23/4/043001/pdf

Probing multiband superconductivity by point-contact spectroscopy

Thermophotovoltaic energy conversion: Analytical aspects, prototypes and experiences

Major challenges for InGaAs/GaAsP multiple quantum well (MQW) solar cells include both the difficulty in designing suitable structures and, because of the strain-balancing requirement, growing high-quality crystals. The present paper proposes a comprehensive design principle for MQWs that overcomes the trade-off between light absorption and carrier transport that is based, in particular, on a systematical investigation of GaAsP barrier effects on carrier dynamics that occur for various barrier widths and heights. The fundamental strategies related to structure optimization are as follows: (i) acknowledging that InGaAs wells should be thinner and deeper for a given bandgap to achieve both a higher absorption coefficient for 1e-1hh transitions and a lower compressive strain accumulation; (ii) understanding that GaAs interlayers with thicknesses of just a few nanometers effectively extend the absorption edge without additional compressive strain and suppress lattice relaxation during growth; and (iii) understanding that GaAsP barriers should be thinner than 3 nm to facilitate tunneling transport and that their phosphorus content should be minimized while avoiding detrimental lattice relaxation. After structural optimization of 1.23-eV bandgap quantum wells, a cell with 100-period In0.30GaAs(3.5 nm)/GaAs(2.7 nm)/GaAsP0.40(3.0 nm) MQWs exhibited significantly improved performance, showing 16.2% AM 1.5 efficiency without an anti-reflection coating, and a 70% internal quantum efficiency beyond the GaAs band edge. When compared with the GaAs control cell, the optimized cell showed an absolute enhancement in AM 1.5 efficiency, and 1.22 times higher efficiency with 38% current enhancement with an AM 1.5 cut-off using a 665-nm long-pass filter, thus indicating the strong potential of MQW cells in Ge-based 3-J tandem devices. Copyright © 2013 John Wiley & Sons, Ltd.

100‐period, 1.23‐eV bandgap InGaAs/GaAsP quantum wells for high‐efficiency GaAs solar cells: toward current‐matched Ge‐based tandem cells

The analytical drift-diffusion formalism is able to accurately simulate a wide range of solar cell architectures and was recently extended to include those with back surface reflectors. However, as solar cells approach the limits of material quality, photon recycling effects become increasingly important in predicting the behavior of these cells. In particular, the minority carrier diffusion length is significantly affected by the photon recycling, with consequences for the solar cell performance. In this paper, we outline an approach to account for photon recycling in the analytical Hovel model and compare analytical model predictions to GaAs-based experimental devices operating close to the fundamental efficiency limit.

Incorporating photon recycling into the analytical drift-diffusion model of high efficiency solar cells

The band gap of the quantum well (QW) solar cell can be adapted to the incident spectral conditions by tailoring the QW depth. The single-junction strain-balanced quantum well solar cell (SB-QWSC) has achieved an efficiency of 28.3%. The dominant loss mechanism at the high concentrator cell operating bias is due to radiative recombination, so a major route to further efficiency improvement requires a restriction of the optical losses. It has been found that (100) biaxial compressive strain suppresses a mode of radiative recombination in the plane of the QWs. As biaxial strain can only be engineered into a solar cell on the nanoscale, SB-QWSCs are seen to have a fundamental efficiency advantage over equivalent bulk cells. Strain-balanced quantum wells in multi-junction solar cells can current match the sub-cells without the introduction of dislocations. Calculations are shown which predict efficiency limits as a function of QW absorption and band gap for such cells. A dual-junction InGaP/GaAs solar cell with QWs in the bottom sub-cell has been grown and characterized. Laboratory and calculated efficiencies relative to control cells are presented for the reported cell and a modeled device, respectively. Copyright © 2011 John Wiley & Sons, Ltd.

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Recent results for single‐junction and tandem quantum well solar cells

Point contact Andreev reflection (PCAR) spectroscopy is a common technique for determining the spin polarization of a ferromagnetic sample. The polarization is extracted by measuring the bias dependence of the conductance of a metallic/superconducting point contact. Under ideal conditions, the conductance is dominated by Andreev reflection and the Blonder-Tinkham-Klapwijk (BTK) model can be used to extract a value for the polarization. However, PCAR spectra often exhibit unwanted features in the conductance that cannot be appropriately modelled with the BTK theory. In this paper we isolate some of these unwanted features and show that any further extraction of the spin polarization from these non-ideal spectra proves unreliable. Understanding the origin of these features provides an objective criterion for rejection of PCAR spectra unsuitable for fitting with the modified BTK model.

https://iopscience.iop.org/article/10.1088/0953-8984/21/9/095701/pdf

Conductance features in point contact Andreev reflection spectra.

It is possible to tailor the band gap of the strain-balanced quantum well solar cell to match the local solar spectral
conditions by altering the quantum well depth. This has led to a recent single-junction world-record efficiency of 28.3%,
as well as giving advantages for current matching in multi-junction solar cells. Radiative recombination is the dominant
loss mechanism for the strain-balanced quantum well solar cell, so practical improvements focus on techniques for light
management in the cell, such as enhancing the optical path length with epitaxial mirrors. Furthermore, the compressive
strain in the quantum wells suppresses emission into TM-propagating modes, reducing the overall optical loss and
increasing the cell efficiency. As biaxial strain can only be engineered into a cell on the nanoscale, quantum well solar
cells are seen to have a fundamental efficiency advantage over bulk semiconductor cells.

Higher limiting efficiencies for nanostructured solar cells

We present the first experimental data showing a restriction of the radiative recombination in strain-balanced quantum well solar cells. This arises due to the combined effects of quantum confinement and strain splitting the valence band in the quantum wells into heavy and light hole sub-bands. Under increasing compressive strain, the light hole sub-band moves further in energy from the conduction band, suppressing the conduction-band-to-light-hole recombination which couples primarily to photons emitted in the plane of the quantum wells. As a solar cell fundamentally need only emit in the angular range of absorption, the strain-balanced quantum well solar cell operates more closely to this optimal regime than a bulk solar cell.

W. Elder

Papers

Recent results for single‐junction and tandem quantum well solar cells

Conductance features in point contact Andreev reflection spectra.

Higher limiting efficiencies for nanostructured solar cells

Experimental measurement of restricted radiative emission in quantum well solar cells

Efficiency Enhancement in Strain-Balanced Quantum Well Solar Cells via Anisotropic Emission