On the energy conversion efficiency of the bulk photovoltaic effect

The bulk photovoltaic effect (BPVE) leads to directed photo-currents and photo-voltages in bulk materials. Unlike photo-voltages in p-n junction solar cells that are limited by carrier recombination to values below the bandgap energy of the absorbing material, the BPVE photo-voltages have been shown to greatly exceed the bandgap energy. Therefore the BPVE is not subject to the Shockley-Queisser limit for sunlight to electricity conversion in single junction solar cells and experimental claims of efficiencies beyond this limit have been made. Here, we show that BPVE energy conversion efficiencies are, in practice, orders of magnitude below the Shockley-Queisser limit of single junction solar cells and are subject to different, more stringent limits. The name BPVE stands for two different fundamental effects, the shift current and the injection current. In both of these, the voltage bias necessary to produce electrical energy, accelerates both, intrinsic and photo-generated, carriers. We discuss how energy conservation alone fundamentally limits the BPVE to a bandgap-dependent value that exceeds the Shockley Queisser limit only for very small bandgaps. Yet, small bandgap materials have a large number of intrinsic carriers, leading to high conductivity which suppresses the photo-voltage. We discuss further how slightly more stringent fundamental limits for injection (ballistic) currents may be derived from the trade-off between high resistivity, needed for a high voltage, and long ballistic transport length, needed for a high current. We also explain how erroneous experimental and theoretical claims of high efficiency have arisen. Finally, we calculate the energy conversion efficiency for an example 2D material that has been suggested as candidate material for high efficiency BPVE based solar cells and show that the efficiency is very similar to the efficiency of known 3D materials.Comment: 23 pages, 6 figure

Performance Analysis and Fault Diagnosis Method for Concentrator Photovoltaic Modules

Concentrator Photovoltaic (CPV) systems use high efficiency multi-junction solar cells with efficiencies >40%, but the module efficiency is often much lower. The increased complexity of a CPV module, with optics, receiver and the tracker gives an increased probability that faults will arise during the operational lifetime. In addition, a location like India has varied atmospheric conditions that further complicates the diagnosis of faults. It is therefore important to decouple effects due to the external environment (such as the atmosphere) from effects due to the degradation of the module. By applying a computer model to outdoor CPV test data in Bangalore, India we have established a method to assess the performance of the CPV module and finally we present a method to diagnose faults in the module.Comment: 7 pages, 12 figure

Multi-dimensional modeling of solar cells with electromagnetic and carrier transport calculations

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Electronic and optical properties of SixGe1-x-ySny alloys lattice-matched to Ge

We present a combined experimental and theoretical analysis of the evolution of the near-band-gap electronic and optical properties of SixGe1-x-ySny alloys lattice-matched to Ge and GaAs substrates. We perform photoreflectance (PR) and photoluminescence (PL) measurements on SixGe1-x-ySny epitaxial layers grown via chemical vapor deposition for Si (Sn) compositions up to x=9.6% (y=2.5%). Our measurements indicate the presence of an indirect fundamental band gap, with PL observed Ë 200-250 meV lower in energy than the direct E0 transition identified by PR measurements. The measured PL is Ge-like, suggesting that the alloy conduction band (CB) edge is primarily derived from the Ge L-point CB minimum. Interpretation of the PR and PL measurements is supported by atomistic electronic structure calculations. Effective alloy band structures calculated via density functional theory confirm the presence of an indirect fundamental band gap, and reveal the origin of the observed inhomogeneous broadening of the measured optical spectra as being alloy-induced band hybridization occurring close in energy to the CB edge. To analyze the evolution of the band gap, semiempirical tight-binding (TB) calculations are employed to enable calculations for large supercell sizes. TB calculations reveal that the alloy CB edge is hybridized in nature, consisting at low Si and Sn compositions of an admixture of Ge L-, G-, and X-point CB edge states, and confirm that the alloy CB edge retains primarily Ge L-point CB edge character. Our experimental measurements and theoretical calculations confirm a direct transition energy close to 1 eV in magnitude for Si and Sn compositions x=6.8%-9.6% and y=1.6%-2.2%

The Potential for Concentrator Photovoltaics: A Feasibility Study in India

India has aggressive plans for scaling up photovoltaic installations in the coming decades. Currently fixed tilt, flat plate crystalline silicon (c‐Si) technology sets the standard for cost and performance and is both robust and relatively easy to deploy. Concentrator photovoltaics (CPV) systems have a different cost structure; using solar cells with the highest efficiencies, system efficiencies greater than 30% are possible, but the system is also more sensitive to meteorological conditions. India has a complex and varied atmosphere that prevents a straightforward comparison of technologies, and hence, in this paper, we use a computer model to simulate the power output from CPV systems located in locations in India where the Aerosol Robotic Network (AERONET) stations are based and additionally, in Bangalore where we have a CPV test station. We quantify the increased intermittency suffered by CPV systems that arises from the larger dynamic range in direct beam irradiance over global irradiance. Nevertheless, by calculating the target system costs required to attain a competitive levelized cost of electricity (LCOE), we find that CPV systems in some, but not all locations have the opportunity to compete against dual‐axis tracked and inclined c‐Si based PV in India

Overview and loss analysis of III–V single-junction and multi-junction solar cells

The development of high-performance solar cells offers a promising pathway toward achieving high power per unit cost for many applications. Because state-of-the-art efficiencies of single-junction solar cells are approaching the Shockley-Queisser limit, the multi-junction (MJ) solar cells are very attractive for high-efficiency solar cells. This paper reviews progress in III–V compound single-junction and MJ solar cells. In addition, analytical results for efficiency potential and non-radiative recombination and resistance losses in III–V compound single-junction and MJ solar cells are presented for further understanding and decreasing major losses in III–V compound materials and MJ solar cells. GaAs single-junction, III–V 2-junction and III–V 3-junction solar cells are shown to have potential efficiencies of 30%, 37% and 47%, respectively. Although in initial stage of developments, GaAs single-junction and III–V MJ solar cells have shown low ERE values, ERE values have been improved as a result of several technology development such as device structure and material quality developments. In the case of III–V MJ solar cells, improvements in ERE of sub-cells are shown to be necessary for further improvements in efficiencies of MJ solar cells