Mark J. Keevers

Efficiency improvements of silicon solar cells by the impurity photovoltaic effect

A theoretical investigation of the impurity photovoltaic (IPV) effect for improving silicon solar‐cell efficiency is presented. The approach is better than previous analyses because of the improved treatment of generation and recombination via impurities, and because it includes the effects of optical competition and light trapping. The approach is applied to the nonmidgap, deep‐level impurity indium as the IPV effect impurity incorporated into an idealized silicon solar cell. The analysis is based on experimentally determined parameters for indium. Improvements of cell current, subgap spectral response, and energy conversion efficiency are quantified. The analysis reveals the importance of light trapping and proper selection of indium and dopant concentrations. The impurity photovoltaic effect is predicted to improve solar‐cell efficiency.

Absorption edge of silicon from solar cell spectral response measurements

The optical absorption coefficient of crystalline silicon near the band edge is determined to values as low as 10−7 cm−1 by sensitive photocurrent measurements on high efficiency silicon solar cells. Structure due to three‐ and four‐phonon assisted absorption processes is observed. Discrepancies between absorption coefficient values around 10−2 cm−1 reported in the literature are resolved. The role of disorder theory in understanding the absorption edge of crystalline semiconductors such as silicon is discussed.

Lifetime limiting recombination pathway in thin-film polycrystalline silicon on glass solar cells

The minority carrier lifetimes of a variety of polycrystalline silicon solar cells are estimated from temperature-dependent quantum efficiency data. In most cases the lifetimes have Arrhenius temperature dependences with activation energies of 0.17–0.21 eV near room temperature. There is also a rough inverse relationship between lifetime and the base dopant concentration. Judging by this inverse law, the activation energies of the lifetimes, and the absence of plateau behavior in the lifetimes of the higher doped cells at low temperatures, it is inferred that the dominant recombination pathway involves the electronic transition between shallow states which are 0.05–0.07 eV below the conduction band and 0.06–0.09 eV above the valence band, respectively, consistent with the shallow bands in silicon dislocations. The modeled recombination behavior implies that deep levels do not significantly affect the lifetimes for most of the cells at and below room temperature.

Four-Terminal Tandem Solar Cells Using CH3NH3PbBr3 by Spectrum Splitting.

In this work, the use of a high bandgap perovskite solar cell in a spectrum splitting system is demonstrated. A remarkable energy conversion efficiency of 23.4% is achieved when a CH3NH3PbBr3 solar cell is coupled with a 22.7% efficient silicon passivated emitter rear locally diffused solar cell. Relative enhancements of >10% are demonstrated by CH3NH3PbBr3/CH3NH3PbI3 and CH3NH3PbBr3/multicrystalline-screen-printed-Si spectral splitting systems with tandem efficiencies of 13.4% and 18.8%, respectively. The former is the first demonstration of an all perovskite split spectrum system. The CH3NH3PbBr3 cell on a mesoporous structure was fabricated by the vapor-assisted method while the planar CH3NH3PbI3 cell was fabricated by the gas-assisted method. This work demonstrates the advantage of the higher voltage output from the high bandgap CH3NH3PbBr3 cell and its suitability in a tandem system.

Extended infrared response of silicon solar cells and the impurity photovoltaic effect

Abstract Sub-bandgap spectral response measurements on silicon solar cells are used to characterise the infrared response of present devices, and to investigate the impurity photovoltaic (IPV) effect for improving their infrared response. The former has, aside from establishing a baseline case, led to an improved determination of the subgap absorption coefficient of crystalline silicon. Absorption coefficient values as low as 10 −7 cm −1 have been determined, revealing structure due to 3- and 4-phonon assisted absorption. These values are compared with a more recent determination of the absorption edge based on photoluminescence measurements. The influences of free carrier absorption, bandgap narrowing, and the Franz-Keldysh effect on cell infrared response are considered. Investigation of the IPV effect of indium in high efficiency bulk and thin film cells reveals that indium improves their infrared response. The cross section for electron photoemission from the indium level, a crucial parameter for modelling indiums IPV effect, is determined.

ON THE PERTURBATION SERIES FOR RADIATIVE EFFECTS

We present a generalization of the recently developed first-order perturbation approximation for the computation of radiative effects such as layer heating rates and surface fluxes. The basis for this generalization is Dysons equation for Greens function or the inverse transport operation.

Anomalous temperature dependence of diode saturation currents in polycrystalline silicon thin-film solar cells on glass

Temperature dependent Suns-Voc measurements are performed on four types of polycrystalline silicon thin-film solar cells on glass substrates, all of which are made by solid phase crystallization∕epitaxy of amorphous silicon from plasma enhanced chemical vapor deposition or e-beam evaporation. Under the two-diode model, the diode saturation currents corresponding to n=1 recombination processes for these polycrystalline silicon p‐n junction cells follow an Arrhenius law with activation energies about 0.15–0.18eV lower than that of single-crystal silicon p‐n diodes of 1.206eV, regardless of whether the cells have an n- or p-type base. This discrepancy manifests itself unambiguously in a reduced temperature sensitivity of the open-circuit voltage in thin-film polycrystalline silicon solar cells compared to single-crystal silicon cells with similar voltages. The physical origin of the lowered activation energy is attributed to subgap levels acting either as minority carrier traps or shallow recombination centers.

Fabrication and characterisation of parallel multijunction thin film silicon solar cells

The parallel multijunction solar cell design offers the exciting possibility of high efficiency at low cost. To date, there has been no detailed report on the experimental characteristics of these devices. This paper reports on the beginnings of a detailed experimental investigation of the parallel multijunction solar cell. Progress is reported on the fabrication of parallel multijunction thin-film silicon solar cells (on inert single-crystal silicon substrates), specifically designed and fabricated to serve as experimental test-beds for the detailed study of cell performance limiting mechanisms. Of particular interest is the importance of junction space-charge-region recombination in heavily defected parallel multijunction cells.

Light trapping and reflection control for silicon thin films deposited on glass substrates textured by embossing

A glass substrate texture of pyramids formed by embossing, suitable for polycrystalline silicon solar cells 5-20 /spl mu/m thick, is described. A monocrystalline silicon wafer textured with inverted pyramids was used as a die. We evaluate antireflection and light trapping properties for an undoped a-Si:H silicon film (volumetric thickness 5.6 /spl mu/m; no reflector) simultaneously deposited on this texture and sandblasted glass, using total reflectance and transmittance measurements. J/sub sc/ enhancement potential from light trapping in equally thick polycrystalline silicon films on the same substrates is estimated to be 6.7 (sandblasted), 8.7 (embossed) mA/cm/sup 2/. Light trapping characteristics obtained by spectral photoconductance measurements of the specimens are compared.

Development of a fabrication process for parallel multijunction thin film silicon solar cells on wafer substrates

The parallel multijunction (PMJ) cell design theoretically enables high efficiency thin film polysilicon solar cells at lower cost. Since its initial proposal in 1994 the PMJ cell has been the subject of a number of theoretical studies, however, no detailed experimental investigation has yet been reported. Any systematic study of the PMJ solar cell will require suitably designed and fabricated devices to serve as experimental test-beds. This paper reports the successful development of a fabrication sequence for PMJ cells in CVD-epilayers on inert single-crystal silicon substrates, producing cells with efficiencies up to 13%. The processing sequence is based on photolithography, anisotropic wet etching, high temperature furnace steps and evaporated metallisation. Full details of the processing sequence are provided, with explanations of particular process choices, including the method of parallel electrical connection of like-polarity layers, the use of a thick photoresist (Shipley SJR5740), avoiding pitfalls, and procedures to minimise cell shunt behaviour. The establishment of this baseline fabrication sequence for PMJ cells opens up a wealth of opportunities for systematic studies of cell performance limiting mechanisms, such as junction recombination, and the implications of various cell design and processing options, particularly those likely to be of more commercial relevance, such as laser scribing, laser doping, rapid thermal processing and electroless metal plating. Copyright