• Energy Research
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AbstractOne way of improving the efficiency of solar cells is to subdivide the broad solar spectrum into smaller energy ranges and to convert each range with a cell of appropriately matched bandgap. The most common approach to implementing this idea has been to use a monolithic or mechanical stack of cells arranged in order of increasing bandgap, with the highest bandgap cell uppermost. This provides automatic filtering of incident sunlight so that each cell absorbs and converts the optimal spectral range. The potential of an earlier experimental approach based on steering light in different wavelength bands to non‐stacked cells recently has been re‐explored with good results. The present work extends this previous work by putting measurements on a more rigorous basis and by improving the ‘composite’ experimental efficiency of selected cells to beyond 43%, the highest reported to date for any combination of photovoltaic devices. Copyright © 2009 John Wiley & Sons, Ltd.

Abstract A 2-terminal, dual-junction, epitaxially integrated, GaAsP/Si tandem solar cell with an 3rd party certified efficiency of 23.4 % was fabricated via MOCVD growth on an ex-situ produced Si sub cell. The drastic efficiency improvement over the authors previous peer-reviewed demonstration of such a device architecture is examined. Critical advancements in top cell design to maximize short wavelength response were critical in enabling improved top cell response. An in-depth analysis of this champion tandem cell has identified key loss mechanisms which elucidate the pathway for further efficiency gains. First, voltage dependent collection efficiency in the GaAsP top cell is the primary cause of fill factor losses currently limiting efficiency. Analysis of spectrally resolved I–V measurements and analytical device modeling and indicate poor diffusion length due to elevated dislocation densities as the likely cause for the voltage dependent collection efficiency. Second, modeling for the GaAs0.75P0.25 top cell, using experimental data at multiple dislocation densities, provides quantitative understanding of the current and voltage losses associated with threading dislocations providing a clear efficiency pathway with reduction in dislocation density. Lastly Si subcell modeling identifies the pathway for further Si subcell advances over the present, simplistic design, which has yet to employ the known benefits of rear surface texture or dielectric passivation.

We report on efforts to design high-efficiency silicon homojunction subcells for use in multijunction stack devices. Both simulation and experimental works have been performed looking at a silicon solar cell under a truncated spectrum below 1.5 eV filtered by the upper layers in the multijunction stack. Good agreement is seen between the modeling and experimental results, identifying different emitter design requirements when the solar cell operates under a full or truncated spectrum. A well-passivated front surface, i.e., with low-interface surface recombination velocity, required a lightly doped emitter profile to maximize open-circuit voltage ( $V_{{\rm OC}}$ ), while a high-interface recombination surface requires a heavily doped for higher $V_{{\rm OC}}$ values. The impact on short-circuit current density ( $J_{{\rm SC}}$ ) is found to be minimal, even with large variations in the interface recombination and emitter profiles. In a tandem stack, an interface with low- and high-interface recombination velocities would require lightly doped and intermediate-doped emitters, respectively, for maximum conversion efficiency (η).

Severe silicon lifetime degradation was found after its high-temperature treatment in III–V material growth chambers for the fabrication of III–V/Si multijunction solar cells. Further improvement of the cell efficiency requires insights into the root cause of such lifetime degradation and how to protect the silicon lifetime accordingly. While the exact origins of such degradation remain largely unknown, most published work concluded that extrinsic impurities that diffuse into the silicon bulk during the thermal treatment are the sole reason. In this article, we show that while not necessarily present in every float-zone silicon wafer, grown-in defects that can be thermally activated is also a key mechanism behind the observed silicon lifetime degradation during anneal in our molecular beam epitaxy chamber. As such, annealing of the silicon wafer in the furnace at 1000 °C to annihilate the grown-in defects, together with the deposition of a SiNX diffusion barrier to prevent the extrinsic impurities from diffusing into the silicon bulk, are both required to preserve the silicon lifetime throughout the III–V material growth steps.

AbstractConsolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since June 2016 are reviewed. Copyright © 2016 John Wiley & Sons, Ltd.

Abstract Metals are an integral part of solar cell structures—as a metal contact and/or a back reflector. However it is a less known fact that there are plasmonic losses associated with these structures and it is important to minimise such losses especially when placed in proximity to scattering media like textures on the front of the cell that can introduce an angular dependence to the light incident on the rear. This study investigates the losses in a metal reflector when placed adjacent to a dielectric layer in a typical solar cell rear geometry. The experimental measurement was realised using a novel custom built optical setup for characterising intensity of the reflected light at various internal incident angles at the Si–dielectric interface. Our results show that the thickness of the dielectric layer, the refractive index of the dielectric layer and the type of the rear metal can all affect the degree of such losses and the distribution of the angular reflection. Both measured and simulation results indicate that a 250–320 nm SiO2 along with an Ag back reflector gives the best internal rear angular reflection in Si but is dependant on the wavelength of interest. It also shows that textured or scattering front interfaces with internal incident angles greater than 28° have the potential to provide light trapping through total internal reflection and at the same time minimise plasmonic losses at the metal reflector interface.

Abstract In this article, a novel rear structure using Ag nanoparticles to create surface plasmons to enhance light trapping is applied on the rear of planar high efficiency PERT (Passivated Emitter and Rear Totally Diffused) silicon wafer cells, targeting the spectrum range from 1000 nm to 1200 nm. Variations of this rear structure that combine Ag nanoparticles, dielectric layers and back metal reflectors were studied and analysed. Thickness of the rear surface passivation SiO 2 spacer layer was optimised to achieve maximum optical enhancement using surface plasmons but with minimum electronic losses due to recombination effects. The effect of the precursor evaporated Ag film thickness was also studied as a means to vary the size/shape of the nanoparticles. The measured external quantum efficiency ( EQE ) of the best performing rear reflector shows a maximum enhancement of more than 4-fold at 1160 nm. This corresponds to a 16% photocurrent increase (calculated from 900 nm to 1200 nm) compared to the cell with conventional Al rear reflector. Moreover, from the measured spectral response and optical absorption data, we successfully separated and analysed the electrical and optical properties of the novel rear light trapping designs. Light trapping features were quantified using optical parameters characterised by an effective optical path length factor Z , while electrical parameters such as surface recombination velocity S (cm/s) and effective minority charge carrier lifetime τ bulk (μs) were also extracted. Relative errors for these parameters were also calculated. For the cell with the best performing rear structure, we report a maximum Z factor enhancement of around 6-fold using Ag nanoparticles in conjunction with a detached Ag reflector, in comparison to the reference at 1200 nm.