Theoretical efficiencies are derived in a detailed balance calculation for thermophotovoltaic solar energy conversion, where solar radiation is absorbed by an intermediate absorber, which emits radiation inside an evacuated housing towards a solar cell. For ideal components with no optical losses and only radiative recombination in the solar cell, maximal efficiencies are found of 85% for full concentration of the incident sunlight on a black absorber, and of 54% for no concentration and a selective absorber absorbing only for hω > 0.92 eV. This is considerably larger than the efficiency for directly illuminated solar cells with also only radiative recombination, the Shockley-Queisser limit, which is 41% for full concentration and 30% for no concentration.In order to approach efficiency limits for real TPV systems, several non-idealities have been introduced: (a) realistic assumptions about the geometry of the intermediate absorber, (b) optical losses of 5% for photons with energy below the band gap of the solar cell and (c) non-radiative recombination in the solar cell of the same amount as radiative recombination. This reduces the efficiency for non-concentrated sunlight to only 32.8%, but for very high concentrations of 10 000 and above suitable absorber geometries still seem to allow efficiencies close to 60%.
Boron diffusion is commonly associated with the formation of an undesirable boron-rich layer (BRL), which is often made responsible for degradation of the carrier lifetime in the bulk. We investigate the phenomenology of the BRL formation, which results from BBr 3 boron diffusion processes, and its impact on sheet resistance and bulk lifetime. Our measurements show that boron silicate glass (BSG) and BRL thicknesses vary between 50 and 600 nm and 0 and 80 nm respectively within the two-dimensional wafer surface of one sample for one diffusion process. Both thicknesses strongly depend on the gas composition during composition and deposition time. Further results show that BRL formation is favored by high concentrations of BBr 3 vapor and of oxygen during B 2 O 3 deposition. Also, high drive-in temperatures promote the growth of the BRL. We find that a BRL of more than 10 nm thickness causes a degradation of the carrier lifetime in the bulk of the silicon wafer. In particular, we show that this bulk lifetime degradation occurs during the cool-down ramp after the diffusion process. We show that carrier lifetime degradation can be avoided either by limiting the process temperature to 850 • C and thus preventing BRL formation or through reconverting the BRL by a drive-in step in oxidizing atmosphere at 920 • C.
Thermophotonics (TPX) is a recently proposed concept, which generalizes thermophotovoltaics (TPV) by allowing the radiation from the heat source to be increased by an internal electrochemical potential difference. This paper examines the basic working principle of TPX by means of detailed balance calculations. In these calculations TPX is not only modelled by the highly idealized standard Shockley-Queisser analysis but also in a more realistic manner, in which two typical parasitic loss mechanisms are included: (a) non-radiative recombination and (b) non-zero absorptivity/emissivity in the sub-bandgap energy spectrum. We show that TPX, in principle, allows a much higher rate of electrical power extraction from a heat source than TPV. When applying TPX to the conversion of solar energy we furthermore show that for a realistic absorber geometry TPX has a substantially higher theoretical conversion efficiency than TPV, particularly in the presence of parasitic sub-bandgap absorption/emission. Additionally, the range of suitable bandgap energies for TPX is greatly enhanced towards larger values over that of TPV. An essential requirement, however, is very high external electro-luminescent quantum efficiency.
We make up the free energy balance for thermalized electrons and holes in a solar cell. Equations for the loss rates of free energy due to recombination and transport of carriers are derived. The well known expression for Joule heat dissipation also holds for the free energy loss by diffusive transport. All loss rates have units of mW/cm2. Thus transport losses become directly comparable in magnitude to recombination losses. The latter are usually quantified in mA/cm2 rather than mW/cm2. The impact of various loss mechanisms on the power output of the cell, also in mW/cm2, becomes directly apparent.
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