The use of silicon nitride in buried contact solar cells

Silicon nitride offers many potential benefits to the family of buried contact fabrication sequences including improved design flexibility and efficiency. The main device structures of the buried contact family comprise the standard buried contact, the simplified buried contact and the double-sided buried contact cells. The physical properties of silicon nitride allow it to be used for surface passivation, as an anti-reflection coating, as a diffusion source material and as a masking dielectric. The use of silicon nitride in each buried contact fabrication sequence is described in this work.     

Fabrication of buried contact silicon solar cells using porous silicon

We report the fabrication of buried contact solar cells using porous silicon as sacrificial layer to create well-defined channels (for buried contacts) in silicon. In this paper, the salient features of the technology have been presented. No detrimental effect was found in the performance of buried contact solar cell with partially filled contact area compared to the solar cells having conventional planar contacts. However, a marked difference in the short circuit current density was seen when channel was fully filled with metal by screen printing, without degradation in the open-circuit voltage. It is expected that improved processing in combination with optimized buried metallic contact parameters may yield higher efficiencies that may result in substantial decrease in solar cell cost.     

Study on the edge isolation of industrial silicon solar cells with waterjet-guided laser

In this paper, we review the device physics to explain the necessity of edge isolation as well as the various common methods for performing this important step. In order to assess these processes, different aspects have to be taken into consideration and will be discussed. Finally, experimental results of edge isolation on commercial multicrystalline solar cells with waterjet-guided laser (wavelength 1064 nm) and conventional laser (wavelength 355 nm) are presented and discussed. From the experiments, we conclude that the waterjet-guided laser shows the potential to improve the laser scribing process in terms of pn junction damage and mechanical stability.     

Advanced buried contact solar cell structure

This paper discusses the feasibility of overcoming the recombination losses associated with the rear metal contact of buried contact solar cells. The buried contact locally diffused rear (BCLD) structure has been proposed as one of the structures that will limit the rear surface recombination and improve on cell efficiency. The one-dimensional (1D) modeling, using PC-1D has given an energy conversion efficiency of close to 21% with all the possible losses taken into account. This means that by proper optimization of the processing steps energy conversion efficiency in excess of 19% would be achieved on large-area cells.     

Impact and options for boron diffusions in buried contact solar cells

Boron-diffused rear surfaces are used in place of an Al/Si alloy to form a back surface field (BSF) for single-sided buried contact (SSBC) cells. The findings are that a heavy boron diffusion over a large area can retain high bulk lifetime and have a low saturation current. SSBC solar cells that were made had the entire rear heavily boron diffused. These cells had open-circuit voltages of 650, 645, and 640 mV on 3 and 10 Ω cm p-type, and 100 Ω cm (rear junction) n-type, respectively.     

Characterization of high open-circuit voltage double sided buried contact (DSBC) silicon solar cells

The double sided buried contact (DSBC) silicon solar cells have consistently shown high open-circuit voltages (V_oc) than its single sided buried contact counterpart because of better rear surface passivation. The rear surface passivation which is provided by the rear floating junction is effective only when there is no leakage in the rear floating junction. However, the partial shunting of the rear floating junction can cause a drop in the fill factor of the cell which has been the only parameter limiting the realization of the structure's potentials. In this paper, LBIC (light beam induce current), spectral response, dark I-V and J_sc-V_oc measurements for DSBC cells have been carried out to help explain some of the experimentally observed attributes of this structure. The partly shunted rear floating junction has been identified by LBIC measurement as low current regions near the rear metal contacts.     

Effect of edge junction isolation on the performance of laser doped selective emitter solar cells

The effect of laser and chemical edge junction isolation on electrical performance of industrially manufactured laser doped selective emitter solar cells with light induced plated n-type contacts is investigated in this work. Directly after the formation of the aluminium back surface field, photoluminescence images indicates that laser edge junction isolation causes substantial damage around the perimeter of the cell, extending several millimeters from the laser edge isolation groove. On finished devices, regions of high series resistance are evident around the perimeter, caused by parasitic plating nucleating in the damaged laser grooved region which induce shunting and inhibits further plating taking place in the surrounding regions. The use of chemical edge junction isolation eliminates both of these issues and can result in efficiency gains of more than 2% absolute compared to that fabricated using laser edge isolation, suggesting a far superior method of edge junction isolation for the industrial manufacture of laser doped selective emitter solar cells with light induced plated contacts.     

Nondestructive local analysis of current-voltage characteristics of solar cells by lock-in thermography

By evaluating dark lock-in thermography images taken at one reverse and three forward biases, images of all two-diode-parameters J₀₁, J₀₂, n (ideality factor of J₀₂), and G_p (the parallel Ohmic conductivity) of the dark current-voltage characteristic are obtained. A local series resistance is explicitly considered and may be provided as a series resistance image, e.g. resulting from luminescence imaging. The results enable a separate investigation of factors influencing the depletion region recombination current and the diffusion current, which is governed by the bulk lifetime. Local I-V characteristics of special sites may be simulated.     

Metallization improvement on fabrication of interdigitated backside and double sided buried contact solar cells

Metallization based on electroless metal plating of nickel and copper is a simple, cost-effective process used in the fabrication of Buried Contact silicon solar cells. Whereas the electroless Ni–Cu metallization scheme works well for metal deposition on early Buried Contact solar cells, in which deposition was required only on phosphorus diffused contact regions, more care is required for advanced Buried Contact solar cell designs that require simultaneous deposition on to both phosphorus and boron diffused contact regions. In this paper, we examine two key issues related to the metallization in these solar cells. Firstly we demonstrate an improved buffered hydrofluoric acid etch process for simultaneous removal of borosilicate and borophosphosilicate glasses from the contact regions prior to electroless deposition of nickel with good etch selectivity against silicon dioxide masking films. Secondly, we demonstrate an improved process for nucleation of the nickel layer on both phosphorus and boron diffused contact areas based on immersion palladium chloride activation of the plating surfaces. N-type double-sided buried contact solar cells metallized by processing introduced in this study show improvement on absolute efficiency of more than 3%.     

Surface passivation in high efficiency silicon solar cells

Surface passivation for crystalline silicon solar cells is particularly important for devices with open-circuit voltages in excess of 650 mV. Thick passivating thermal oxides, originally developed for use with buried contact solar cells, are shown to produce the most effective and stable surface passivation particularly in conjunction with lightly phosphorus diffused surfaces. However, for improved optical performance, antireflection coatings are only effective with surface oxide thicknesses reduced to 100–200 Å. Thinner passivating oxides cause significant voltage loss, most of which can be recovered through hydrogen passivation. Throughout this study, variation in surface passivation approaches has produced open-circuit voltages ranging from 620 mV to record voltages of 720 mV.     

Airbrush spray-coating of polymer bulk-heterojunction solar cells

We report on the fabrication of efficient polymer solar cells via airbrush coating as a promising method for low cost and large area production. We used a dual action airbrush for deposition of the active layer from a poly(3-hexylthiophene):[6,6]-Phenyl C61 butyric acid methyl-ester (P3HT:PCBM) blend dissolved in a co-solvent mixture. The resulting devices were measured under AM1.5G conditions and compared with spin-coated ones in air and nitrogen atmosphere. High power conversion efficiencies (η=4.1%) were obtained by optimizing the parameters of the spray system (i.e. film thickness, time of spray, distance between sample and airbrush, substrate temperature, etc.). The measurements also showed good repeatability and uniformity despite a relatively rougher surface.     

Novel carbon‐based material for perovskite solar cells back‐contact

Perovskite solar cells are one of the most promising photovoltaic technology, presenting the fastest power conversion efficiency (PCE) growth from 3.8 % to 24.2 % in just 10 years. However, there are still challenges hindering its commercialization such as the expensive back‐contact made of gold. Carbon‐based materials, mainly carbon pastes made of carbon black and graphite, have already proven to be good candidates as back‐contacts because of their features such as low cost, high conductivity, and high stability. In this work, the replacement of gold back‐contact by a carbon paper with a microporous layer coated with a PEDOT:PSS dispersion is reported. To the best of the author's knowledge, this material has never been reported for perovskite solar cells. A PCE of 9.22 % was obtained, representing 62 % of the PCE obtained for the same cell but with a gold back‐contact.     

Evolution of space solar cells

The solar cells used in space for over 40 years are reviewed by discussing the semiconductor materials which have provided the best cells. Most emphasis was on high efficiency, combined with good tolerance to charged particle bombardment, and the steady increase in efficiency is discussed. The most important requirement is that the cells must be highly reliable, consistent in performance, and stable while operating in space. The need for highest reliability makes the costs less important. The progress to date has provided a good foundation for future applications for space cells.     

Fabrication of double sided buried contact (DSBC) silicon solar cell by simultaneous pre-deposition and diffusion of boron and phosphorus

Double Sided Buried Contact (DSBC) silicon solar cells have recently been developed at the University of New South Wales. The processing sequence requires three high temperature steps. These three high temperature steps have been reduced to two by simultaneous pre-deposition and diffusion of boron and phosphorus using the spin-on sources. The preliminary results of DSBC cells fabricated by fewer (two) high temperature steps have demonstrated energy conversion efficiency close to 15% for planar substrates. This efficiency should translate into efficiency in excess of 17% with the inclusion of surface texturing.     

A study of stainless steel-based dye-sensitized solar cells and modules

Stainless steel (StSt) has been applied as substrate material for efficient, flexible, nanoporous TiO₂ dye-sensitized solar cells (DSSCs) with the aim of improving the photochemical properties of current plastic-based flexible DSSCs. DSSCs with a StSt substrate show almost equivalent properties in efficiency and convenience to cells with a F-doped tin oxide (FTO) glass substrate. Specifically, the metal substrate allows application of high-temperature sintering processes and shows high conductance even after sintering. Cells fabricated with the StSt substrates have been investigated as individual cells and as modules. A comparison between conventional DSSCs with a FTO glass substrate and flexible DSSCs with a StSt substrate is presented. In addition, Pt-coated electrodes, which can serve as window electrodes for StSt-based DSSCs, are fabricated via two different methods, i.e., chemical reduction and annealing, and compared.     

A simple passivation technique for the edge area of silicon solar cells improves the efficiency

The efficiency of silicon solar cells (SC) can strongly be degraded by localized defects especially at the edge of SC (e.g. scratches) which are introduced during the production of the SC and may cause local shunts. A new optimized chemical etching procedure has been developed which allows a very effective passivation of shunts at the SC edges without reducing the surface area, i.e. without a reduction of the I_sc current. In contrast to other techniques like plasma etching (“coin staking”) or cutting off the edges, this procedure could be implemented cheaply in a large-scale production.The newly developed passivation method always leads to an improvement in the efficiency η of slightly or severely degraded SCs which is typically around 10–30%, but can be as large as 100%, while good SCs are totally unaffected with respect to η but still showing an improvement of the leakage current.     

Inkjet printed nickel silicide applied as front contact seed-layer for electrolessly copper plated silicon solar cells

Abstract

The use of high purity nickel silicon nano-inks represents a new approach to the design of conductive electrode structures, with potential to reduce electrode cost by up to 75% by reducing silver usage. In addition to reducing manufacturing costs, utilising nickel silicon inks would greatly improve product throughput. However, careful design and incorporation of a nickel barrier layer are required to prevent copper poisoning of the solar cell. Once proven, this technology could be readily incorporated into existing production facilities (the majority of solar cell process tooling remaining as standard), thereby providing a ready route to a mass market. The status of the Innovate UK funded Propress R&D project is reported and the specialised equipment, nano-inks and processes developed are described. An evaluation of the cell efficiencies obtained using this alternative cell processing technology is presented, alongside stability and performance data.     

Comparison of 1D and 3D analysis of the front contact influence on GaAs concentrator solar cell performance

An extensive analysis of the front contact influence on concentrator GaAs solar cell performance has been carried out. The fill factor, open circuit voltage and efficiency have been calculated by varying the front contact specific resistance and the metal sheet resistance for 500X, 1000X and 2000X. An optimum front grid design has also been developed. The simulations have been carried out using a 3D model based on distributed circuit units, and by a classic lumped model, showing the need to use distributed models to achieve an accurate concentrator solar cell modeling as well as a precise front grid design.     

Assessment of a low-cost gold-free metallization for III–V high concentrator solar cells

A gold-free metallization is proposed to be used as the grid contact in III–V concentrator solar cells. This metallization is based on the Cu/Ge system which has been reported to attain very low specific contact resistances on n-GaAs. In this letter, we show that metal layers with low resistivity (13 μΩ cm) can be obtained if the copper content in the alloy is around 28% in weight for a wide range of annealing temperatures (400–450 °C). Finally, this metallization has been used to manufacture single-junction GaAs high concentrator solar cells. Efficiencies of 26.2% at 1000 suns have been reached.     

Photoluminescence analysis of InGaP top cells for high-efficiency multi-junction solar cells

A way of evaluate the minority-carrier lifetime by using photoluminescence (PL) measurement is proposed which includes self-absorption. The room-temperature PL intensity is analyzed theoretically for bulk crystals and a device with n⁺-p junction configuration, based on a one-dimensional model. Photoluminescence analysis of In_0.5Ga_0.5P solar cells grown on GaAs and Si substrates by MOCVD (metal organic vapor deposition) have been carried out and compared with the properties of the In_0.5Ga_0.5P solar cells. By improving minority-carrier lifetime, high-efficiency In_0.5Ga_0.5P cells on GaAs substrates with an efficiency of 18.5% have been made.