Christiana B. Honsberg

Enhancing the surface passivation of TiO2 coated silicon wafers

In this letter, we demonstrate good surface passivation of lightly diffused n-type solar cell emitters using titanium dioxide (TiO2) thin films treated with a furnace oxidation process. Transient-photoconductance decay, x-ray photoelectron spectroscopy, and scanning electron microscopy measurements indicate that the silicon dioxide layer formed at the TiO2:Si interface provides excellent surface passivation. Emitter dark saturation current densities of 4.7×10−14 A/cm2 are achieved by this method, demonstrating that TiO2 films are compatible with high-efficiency solar cell structures.

Detailed balance efficiency limits with quasi-Fermi level variations [QW solar cell]

A central assumption in detailed balance efficiency limit calculations has been that the light generated carriers are collected by drift transport processes and have an infinite mobility, giving rise to constant quasi-Fermi levels (RFLs) across the solar cell. However, recent experimental and theoretical results for quantum well (QW) devices indicate that the QFLs need not be constant across the device. It is shown in this paper that transport mechanisms which cause a variation in the difference between the electron and hole QFLs give an increase in the limiting efficiency compared to previous detailed balance calculations. Further, QW solar cells which employ hot carrier transport across a well will have an efficiency limit in excess of a tandem solar cell while using the same number of semiconductor materials.

Buried contact silicon solar cells

Abstract Despite the growing commercial success of the conventional buried contact solar cell (BCSC), significant improvements in cell performance, simplicity of fabrication, and costs are being made. Five strands to this work with five corresponding variations of the cell structure are responsible for the developments. Hybrid solar cells (standard BCSC front surface with photolithographically defined rear metal contact scheme) have demonstrated open-circuit voltages approaching 700 mV while a simpler cell design requiring no photolithography has demonstrated open-circuit voltages as high as 685 mV. Large area, 20 sun BCSC concentrator cells have been developed with very low metal shading losses (below 3%) due to the redesigning of the groove structure to recess the metal to below the top surface. The resulting record efficiency of 21.5% has been independently confirmed (Sandia). The most recent BCSC structure, where the emphasis is on simplicity and low cost, has the number of high temperature processing steps reduced to one, while efficiencies in the vicinity of those achieved by the conventional BCSC are anticipated. The highest efficiencies demonstrated to date with any of the BCSC structures are well above 21% (Sandia) although all five variations of the BCSC structure appear capable of achieving similar performance levels in the future.

685 mV open-circuit voltage laser grooved silicon solar cell

Abstract The recombination limiting the voltage of the present buried contact solar cell (BCSC) can be reduced by replacing the present high recombination sintered aluminium back with a floating rear junction for passivation, heavy boron diffusion below the rear contact, and by limiting the rear surface contact area. Analysis of these implementations in the double sided laser grooved (DSLG) structure shows that the floating junction passivation is effective in reducing the recombination component at the rear surface and that the boron diffusion in the rear groove comprises up to half of the total saturation current. Limiting the area of the heavily diffused boron grooves allows open-circuit voltages of 685 mV while maintaining the simplicity of the BCSC processing sequence. An open-circuit voltage of 685 mV represents nearly a 50 mV increase over the conventional BCSC.

Potential cost reduction of buried-contact solar cells through the use of titanium dioxide thin films

Abstract This paper explores the potential of applying titanium dioxide (TiO 2 ) thin films to the buried-contact (BC) solar cell. The aim is to develop a lower-cost BC technology that can be applied to multicrystalline silicon (mc-Si) wafers, the predominant substrate of the photovoltaics (PV) industry. The original BC solar cell used a thick, thermally grown, silicon dioxide (SiO 2 ) layer as the front surface dielectric coating. Upon commercialisation of the BC technology, BP Solar replaced this layer with silicon nitride (Si 3 N 4 ), which exhibits improved optical properties. It is anticipated that production costs can be further reduced by using a low temperature deposited front surface dielectric coating, such as TiO 2 , thereby reducing the number of lengthy high temperature processing steps, and developing a process such that it can be applied to mc-Si wafers. TiO 2 is chosen because of its optimal optical properties for glass-encapsulated silicon solar cells and familiarity of PV manufacturers with this material. The results presented resolve the issue of surface passivation with TiO 2 and demonstrate that TiO 2 /SiO 2 stacks, achieved during a brief high-temperature oxidation process after TiO 2 thin film deposition, are compatible with high-efficiency solar cells. However, TiO 2 cannot perform all the necessary functions of the thick SiO 2 or Si 3 N 4 layer, due to its inability to act as a phosphorus diffusion barrier. In light of these results, three alternate BC solar cell fabrication sequences are presented, and an initial conversion efficiency of 11.5% has been achieved from the first batch of solar cells in a non-optimised processes.

Green power: status and perspectives

The main objective of the so-called green power marketing is to provide selective customers of electric energy with choices to purchase electric energy from sustainable, environmentally friendly sources. This paper presents a rationale for marketing of green power provides a brief overview of some of the existing efforts in the United States and abroad, and attempts to identify the conditions and future trends for survival of renewable energy in the energy marketplace. Our goal is primarily to provide a representative sample of the current technology status, rather than a comprehensive coverage of worldwide initiatives in this area.

Quantum dot intermediate band solar cell material systems with negligible valence band offsets

In this paper, material triads (quantum-dot/barrier/substrate) are presented that may implement quantum dot intermediate band solar cells with conversion efficiencies greater than 60%. Triads whose barrier material and substrate material are lattice-matched are presented. In addition, triads are presented with the lattice constant of the substrate in-between the lattice constant of the barrier and the lattice constant of the quantum dot. The latter case provides triads that may remove strain during epitaxial growth.

Impact of stress relaxation in GaAsSb cladding layers on quantum dot creation in InAs/GaAsSb structures grown on GaAs (001)

We describe InAs quantum dot creation in InAs/GaAsSb barrier structures grown on GaAs (001) wafers by molecular beam epitaxy. The structures consist of 20-nm-thick GaAsSb barrier layers with Sb content of 8%, 13%, 15%, 16%, and 37% enclosing 2 monolayers of self-assembled InAs quantum dots. Transmission electron microscopy and X-ray diffraction results indicate the onset of relaxation of the GaAsSb layers at around 15% Sb content with intersected 60° dislocation semi-loops, and edge segments created within the volume of the epitaxial structures. 38% relaxation of initial elastic stress is seen for 37% Sb content, accompanied by the creation of a dense net of dislocations. The degradation of In surface migration by these dislocation trenches is so severe that quantum dot formation is completely suppressed. The results highlight the importance of understanding defect formation during stress relaxation for quantum dot structures particularly those with larger numbers of InAs quantum-dot layers, such as those proposed for realizing an intermediate band material.

Rear surface effects in high efficiency silicon solar cells

Rear surface effects in PERL solar cells can lead not only to degradation in the short circuit current and open circuit voltage, but also fill factor. Three mechanisms capable of changing the effective rear surface recombination velocity with injection level are identified, two associated with oxidised p-type surfaces, and the third with two dimensional effects associated with a rear floating junction. Each of these will degrade the fill factor if the range of junction biases corresponding to the rear surface transition, coincides with the maximum power point. Despite the identified nonidealities, PERL cells with rear floating junctions (PERF cells) have achieved record open circuit voltages for silicon solar cells, while simultaneously achieving fill factor improvements relative to standard PERL solar cells. Without optimisation, a record efficiency of 22% has been demonstrated for a cell with a rear floating junction. The results of both theoretical and experimental studies are provided.

Design trade-offs and rules for multiple energy level solar cells

Abstract Solar cell efficiency limit are receiving renewed examination due to the realisation that many types of solar cell structures can theoretically achieve similar efficiencies to those of multiple p–n homojunctions, without the need for a large number of different semiconductors. This paper shows that the proposed high efficiency device structures fit into one of three general classes and therefore only three ideal efficiency limit calculations are required. However, many of the suggested structures violate assumptions in the ideal efficiency limit calculations. Hence, these calculations should be modified to include additional transport, generation and recombination effects.