Myles A. Steiner

Optical enhancement of the open-circuit voltage in high quality GaAs solar cells

The self-absorption of radiated photons increases the minority carrier concentration in semiconductor optoelectronic devices such as solar cells. This so-called photon recycling leads to an increase in the external luminescent efficiency, the fraction of internally radiated photons that are able to escape through the front surface. An increased external luminescent efficiency in turn correlates with an increased open-circuit voltage and ultimately conversion efficiency. We develop a detailed ray-optical model that calculates Voc for real, non-idealized solar cells, accounting for isotropic luminescence, parasitic losses, multiple photon reflections within the cell and wavelength-dependent indices of refraction for the layers in the cell. We have fabricated high quality GaAs solar cells, systematically varying the optical properties including the back reflectance, and have demonstrated Voc = 1.101 ± 0.002 V and conversion efficiencies of (27.8 ± 0.8)% under the global solar spectrum. The trends shown by th...

Enhanced external radiative efficiency for 20.8% efficient single-junction GaInP solar cells

We demonstrate 1.81 eV GaInP solar cells approaching the Shockley-Queisser limit with 20.8% solar conversion efficiency, 8% external radiative efficiency, and 80–90% internal radiative efficiency at one-sun AM1.5 global conditions. Optically enhanced voltage through photon recycling that improves light extraction was achieved using a back metal reflector. This optical enhancement was realized at one-sun currents when the non-radiative Sah-Noyce-Shockley junction recombination current was reduced by placing the junction at the back of the cell in a higher band gap AlGaInP layer. Electroluminescence and dark current-voltage measurements show the separate effects of optical management and non-radiative dark current reduction.

Non-linear luminescent coupling in series-connected multijunction solar cells

The assumption of superposition or linearity of photocurrent with solar flux is widespread for calculations and measurements of solar cells. The well-known effect of luminescent coupling in multijunction solar cells has also been assumed to be linear with excess current. Here we show significant non-linearities in luminescent coupling in III-V multijunction solar cells and propose a simple model based on competition between radiative and nonradiative processes in the luminescent junction to explain these non-linearities. We demonstrate a technique for accurately measuring the junction photocurrents under a specified reference spectrum, that accounts for and quantifies luminescent coupling effects.

Effects of Internal Luminescence and Internal Optics on

For solar cells dominated by radiative recombination, the performance can be significantly enhanced by improving the internal optics. We demonstrate a detailed model for solar cells that calculates the external luminescent efficiency and discuss the relationship between the external and internal luminescence. The model accounts for wavelength-dependent optical properties in each layer, parasitic optical and electrical losses, multiple reflections within the cell, and assumes isotropic internal emission. For single-junction cells, the calculation leads to Voc, and for multijunction cells, the calculation leads to the Voc of each junction as well as the luminescent coupling constant. In both cases, the effects of the optics are most prominent in cells with high internal radiative efficiency. Exploiting good material quality and high luminescent coupling, we demonstrate a two-junction nonconcentrator cell with a conversion efficiency of (31.1 ± 0.9)% under the global spectrum.

V_{\bf oc}

We present results for quadruple-junction inverted metamorphic (4J-IMM) devices under the concentrated direct spectrum and analyze the present limitations to performance. The devices integrate lattice-matched subcells with rear heterojunctions, as well as lattice-mismatched subcells with low threading dislocation density. To interconnect the subcells, thermally stable lattice-matched tunnel junctions are used, as well as a metamorphic GaAsSb/GaInAs tunnel junction between the lattice-mismatched subcells. A broadband antireflection coating is used, as well as a front metal grid designed for high concentration operation. The best device has a peak efficiency of (43.8 ± 2.2)% at 327-sun concentration, as measured with a spectrally adjustable flash simulator, and maintains an efficiency of (42.9 ± 2.1)% at 869 suns, which is the highest concentration measured. The Voc increases from 3.445 V at 1-sun to 4.10 V at 327-sun concentration, which indicates high material quality in all of the subcells. The subcell voltages are analyzed using optical modeling, and the present device limitations and pathways to improvement are discussed. Although further improvements are possible, the 4J-IMM structure is clearly capable of very high efficiency at concentration, despite the complications arising from utilizing lattice-mismatched subcells.

and

High quality, direct-bandgap solar cells emit significant luminescence at their band-edge when forced to operate in forward bias, thereby creating a possible source of photocurrent in lower bandgap junctions of a multijunction cell. We study the effects of luminescent coupling on the measurement of the subcell photocurrents for a series-connected III–V multijunction solar cell. We describe a technique that uses a set of LEDs and a Xenon-lamp white-light source to accurately determine the subcell photocurrents under a reference spectrum, taking the luminescent coupling current into account. The technique quantifies the luminescent coupling efficiencies and compensates for any spectral overlap between the LEDs and the other junctions. Since quantum efficiency curves are used in the adjustment of the simulator spectrum, we also show how to correct those curves to remove the effects of luminescent coupling.

J_{\bf sc}

Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III–V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% ± 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III–V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed.

of III--V Solar Cells

Luminescent coupling in multijunction solar cells is the phenomenon in which a junction in forward bias radiates photons that are absorbed in and converted to photocurrent by the junction beneath the radiating one. This effect can be significant in modern high-efficiency multijunction cells. We have previously developed a combined measurement and analytical approach to characterize the short-circuit current including the effects of nonlinear coupling in terms of measurable parameters η and φ that describe the coupling strength and linearity, respectively. Here, we develop an analytical model for the full current-voltage characteristic V (J) of a multijunction cell in the presence of luminescent coupling, in terms of the η and φ parameters. We compare the model with the measured V (J) parameters of GaInP/GaAs two-junction cells that exhibit differing degrees of luminescent coupling, and show that the model well describes the measurements. We then use the model to explore the consequences of luminescent coupling on the operating parameters of an idealized two-junction cell as a function of the top-junction thickness, focusing on the open-circuit voltage, fill factor, and efficiency. The results demonstrate that the strong luminescent coupling can significantly alter the dependence of cell efficiency on junction thickness, and that consequently the well-known optical-thinning design rules must be modified.

Quadruple-Junction Inverted Metamorphic Concentrator Devices

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.

Measuring IV Curves and Subcell Photocurrents in the Presence of Luminescent Coupling

We analyze the implications of luminescent coupling on multijunction cell design and performance, using a recently developed formalism that uses the measured luminescent coupling parameters as inputs to an analytical model of the full current-voltage (J-V) characteristic of the cell. This calculation of the full J-V curve allows the determination of the cell open-circuit voltage, short-circuit current, fill factor, and efficiency in the presence of luminescent coupling. We show that luminescent coupling affects critical aspects of the cell design that include the optimal junction thicknesses and bandgaps, and affects the dependence of the cell performance on the spectral content of the light illuminating it.