Ngwe Soe Zin

Miniature silicon solar cells for High Efficiency Tandem Cells

In this paper, a discussion is made of the design of silicon cells to be used in a six-junction tandem solar cell structure as part of the Very High Efficiency Solar Cell (VHESC) program. Minority carrier recombination at surfaces and in the volume, internal quantum efficiency, resistance losses, free carrier parasitic absorption, optical reflection, light trapping, and light absorption must be traded off against each other. Modelling was used to analyse the various parameters and produce estimates of short circuit current, fill factor and open-circuit voltage of the cell. In addition, quasi-steady-state photoconductance measurements to analyse carrier recombination and emitter saturation current (Joe) as well as to predict the open-circuit voltage of solar cell is presented. For metallisation of such small solar cells, alternate methods of making contact such as light-induced plating and electrolyte plating in addition to evaporating metal on the contacts were explored and employed. Numerical resistive loss modelling was made to calculate the optimum metal thickness achieved by light-induced and electroplating to minimise resistive losses. Experiments were conducted to determine the proper plating rate by light-induced and electrolyte plating. Cells were fabricated by standard silicon processing techniques followed by testing of IV curves using current-voltage flash-tester to achieve the target efficiency.

Design, characterization and fabrication of silicon solar cells for ≫50% efficient 6-junction tandem solar cells

A major objective for photovoltaic conversion is to develop high efficiency solar cells. Many approaches are under investigation - Multiple Junction Solar Cell, Multiple Spectrum Solar Cell, Multiple Absorption Path Solar Cell, Multiple Energy Solar Cell, and Multiple Temperature Solar Cells [1]. The Multiple Junction Solar Cell approach based on a six-junction tandem solar cell has been adopted to achieve conversion efficiency of greater than 50% in the VHESC program sponsored by DARPA [2]. In six-junction tandem solar cells, individual solar cells are stacked on one another and each solar cell absorbs the best-matched slice of the solar spectrum. Silicon is one of the cells in the tandem stacks, and absorbs photon energy of 1.42 – 1.1 eV. The role of the silicon cell is to convert 7% of the light incident on the tandem stack into electricity. Other cells in the stack contribute the balance of the electricity. Key design parameters for the silicon cells are that it should have dimensions of 2.5 × 8 mm2 and it needs to transfer light with energy of less than 1.1ev to the underlying solar cells. In this paper, discussion is made of the design of the silicon cell. Minority carrier recombination at surfaces and in the volume, internal quantum efficiency, resistance losses, free carrier parasitic absorption, optical reflection, light trapping, and light absorption must be traded off against each other. PC1D modeling is used to analyze the various parameters and produce estimates of short circuit current, fill factor and open-circuit voltage of the cell [3]. In addition, characterization of solar cell by photoconductance measurement to analyze carrier recombination and emitter saturation current as well as to predict the open-circuit voltage of solar cell [4, 5] is presented. Discussion of cell fabrication process followed by I–V testing is presented. Completed solar cells were tested in ANU using an in-house fabricated current-voltage flash tester [6] under AM1.5D.

Rounded rear pyramidal texture for high efficiency silicon solar cells

Interdigitated back-contact (IBC) solar cells developed in the past two years have efficiencies in the range 24.4%-25.6% As high as these efficiencies are, there are opportunities to increase them further by improving on the light trapping. Silicon solar cells incorporating double-sided pyramidal texture are capable of superior light trapping than cells with texture on just the front. One of the principle losses of double-sided pyramidal texture is the light that escapes after a second pass through the cell when the facet angles are the same on the front and rear. This contribution investigates how this loss might be reduced by changing the facet angle of the rear pyramids. A textured pyramid rounding is introduced to improve the light trapping. The reduction in surface recombination that rounding the facets introduces is also evaluated. With confocal microscopy, spectrophotometry and ray tracing, the rounding etch time required to yield the best light trapping is investigated. With photoconductance lifetime measurements, the surface recombination is found to continue to decrease as the rounding time increases. The spectrophotometry and ray tracing suggests that the double sided textured samples featuring rounded rear pyramids have superior light trapping to the sample with a planar rear surface. The high-efficiency potential of rounded textured pyramids in silicon solar cells is demonstrated by the fabrication of 24% efficient back-contact silicon solar cells.

Contact Resistivity of Evaporated Al Contacts for Silicon Solar Cells

The contact resistivity of evaporated Al on doped silicon is examined for a range of process conditions common to the fabrication of laboratory silicon solar cells. The effects of silicon surface preparation prior to evaporation, sintering temperature, the use of a shutter, and evaporation power are investigated. The presented evaporation conditions yielded the lowest published contact resistivity between Al- and phosphorus-doped Si over a large range of doping concentration. It is also demonstrated that a contact resistivity below 10-6 Ω·cm2 can be achieved without sintering. Three-dimensional simulations are utilized to compare the obtained results for evaporated Al contacts with those for passivated contacts.

Investigation of lifetime degradation of RIE-processed silicon samples for solar cells

Reactive Ion Etching (RIE) is observed to cause substantial effective carrier lifetime degradation in silicon wafers. Degradation of lifetime is permanent for samples where RIE etches into silicon, while the lifetime degradation is temporary for samples where RIE etches only dielectric layers of SiO2 grown on the wafer. The degradation of the effective lifetime of RIE-etched silicon samples can be minimized by exposing only a few percent of the wafer to the etch.

Development of silicon solar cells for six-junction tandem stack cells

This paper presents the development of small (2.5×8.0mm2) silicon solar cells, to be used in a six-junction tandem device. PC1D, numerical modeling and quasi steady state photoconductance (QSSPC) measurement were used to predict the targeted efficiency of silicon solar cells. Early batch of cells had problems of shunting, series resistance and high carrier recombination. Various techniques - junction isolation, pin-hole analysis, diffusion drive-in, light-induced plating, lifetime degradation studies and implied-Voc - were used to improve the performance of the solar cells.

Etch-back simplifies interdigitated back contact solar cells

The process of making Interdigitated Back Contact (IBC) solar cell is implemented by a novel simplified etch-back technique, while aiming for no compromise on high-efficiency potentials. Simplified etch-back creates localized heavy and light phosphorus and boron diffusions simultaneously. This process also leaves localised heavy diffusions to be approximately a micron higher than neighbouring light diffusion regions. In comparison to the IBC solar cells that ANU developed to date [1], key advantages of this technique feature reduction in cell process steps; requires only two diffusions to create p, p+, n and n+ diffusions; no high-temperature oxidation masking steps required as diffusion barriers; independent optimization of contact recombination, lateral carriers transport and surface passivation; and potential higher silicon bulk lifetime and reduced contamination due to low thermal budget. Based on the etch-back technique, the total saturation current density deduced from the test structures for the IBC cell is below 30 fA/cm2.

Boron diffusion induced shunts

Various types of shunt occur in solar cells, including process errors, defects, tunnel shunts between adjacent opposing diffusions and pinholes in insulating dielectric layers used to separate opposite-polarity regions. We have found that boron diffusions into small windows in dielectric layers generate pinholes in the layers following the removal of borosilicate glass (BSG) after the diffusion. These “boron-spots” lie close to the edge of the diffusion windows. If a phosphorus diffused region underlies the dielectric then subsequent metallisation can short circuit the two regions.

Single junction, horizontally-stacked and vertically-stacked MSS cells

In this paper a detailed discussion of different options of silicon solar cell design meant to be used in conjunction with a tandem cell stack is presented. Conventional single junction (SJ), horizontally-stacked (HS) and vertically-stacked (VS) silicon solar cell approaches are discussed. The design considerations of each individual silicon solar cell approach (SJ, HS and VS) are detailed, and benefits and drawbacks of these cells are presented. Expected conversion efficiencies of SJ, HS and VS silicon solar cells under a condition — illuminated with a light spot of 1.9 mm diameter through 2 W/cm2 intensity under the infrared light spectrum — are deduced for the purpose of selecting a suitable design for the tandem stack.

Characterization of dielectric layer, laser damage and edge recombination in miniature silicon solar cells

Miniature silicon solar cells (8 × 2.0 mm2) are being fabricated for use in tandem-cell concentrator systems. Several factors combine to make the achievement of high efficiency problematical. These include surface, bulk and edge recombination. The latter is relatively important because the surface area of the edge of a small cell is a large fraction of the total surface area. Surface recombination in the cells is caused by the loss of passivating hydrogen beneath a conformal LPCVD SiNx coating, induced by high temperature annealing. Bulk carrier lifetime degradation mechanisms that we have encountered include silicon crystal damage induced by laser scribing of the cells, which affects a relatively large proportion of the volume of the cell. The Quasi-steady state photoconductance (QSSPC) measurement technique was used for the carrier lifetime degradation study. Firstly, a detailed study was undertaken of the carrier lifetime degradation due to the loss of hydrogen in conformally deposited LPCVD silicon nitride grown samples and the effect of hydrogenation on these layers, when subjected to various anneal times and temperatures. Additionally, LPCVD nitride was studied to determine whether it can be used as a layer that can prevent or resist possible contamination, induced by prolonged high temperature anneals. Secondly, a comparison was made between reference samples and samples that were exposed to laser scribing and dicing to determine whether laser scribing is suitable for the shaping of miniature silicon solar cells. Finally, cells with different pn junction designs were fabricated and tested to study edge recombination.