With the global warming and the exhaustion of fossil fuels, the development and utilization of clean energy is urgent. Solar energy as the easiest to collect clean energy, compared with wind energy and other clean energy has great advantages and practical research significance. The space electric field region formed at the interface between two kinds of semiconductors formed by different doping methods of P-type semiconductor and N-type semiconductor is called PN junction, which is also the basic structure of solar cells. In this paper, the basic principles of diffusion and drift of PN junction are explained by formula and graph inference. The spatial electric field region formed at their interface by diffusion between two semiconductors formed by different doping methods of p-type semiconductors and n-type semiconductors is called the PN junction, which is also the basic structure of solar cells. This article uses formulas and pictures to reason and illustrate the basic principles of diffusion and drift of pn junctions. The solar cell characteristics were analyzed using simulation software. Among them, the open circuit voltage and short circuit current of solar cells are mainly analyzed by combining the voltage and current curves, the characteristics of the filling coefficient of solar cells are analyzed by using formulas, and finally the theoretical efficiency of solar cells is analyzed by using simulation results. At the same time, based on the actual situation, the efficiency changes of three common solar cells, namely Silicon Solar Cell, Thin-film Solar Cell and III-V Solar Cell, were analyzed and compared through the change of the year. The following paper gives the current technical limitations of solar cells and the prospect of future solar cells.

Busbar pull tests are the generally accepted method of measuring metal-silicon adhesion for silicon solar cells. However, this method cannot be used to measure metal finger adhesion and consequently cannot be used for cells being interconnected with new technologies that require no busbars to be formed on the cell or for when finger and busbar adhesion can differ. To address this need, a stylus-based adhesion tester has been developed that enables direct finger adhesion measurements. However, the operation of this tester results in different finger impact failure modes depending on the relative cohesive and adhesive properties of the metal fingers, and these different finger impact modes need to be differentiated before the measured forces can be used for comparative purposes. In this paper, we report on the implementation of an automatic classifier of failure modes for stylus based adhesion testing. The classifier analyses video frames corresponding to when the stylus laterally impacts fingers on the solar cell surface and classifies them using a support vector machine classifier trained on features extracted from a set of example images. A classification accuracy of 94.4% +/- 0.4%(abs) was achieved on a test set of images comprising finger impacts on both screen-printed and plated silicon solar cells when a "Histogram of Oriented Gradients" (HOG) algorithm was used to generate the image features. Using a Matlab implementation, it took 3.2 s to classify 500 test images using the HOG features. This suggests that only 7-8 s would be required to analyse the finger impacts for a 156 mm industrial silicon solar cell, with even further reduced processing times made possible by software optimisation and reducing the number of scans across the solar cell.

This work demonstrates the first application of LA-ICP-MS for detection of substrate contamination from plated metals in silicon solar cells.

This paper reports on the observed diffusion of plated copper through silver capping layers during thermal stability testing of copper-plated silicon solar cells. Distortion of Suns-V OC curves was observed on plated solar cells after extended thermal exposure at 200 °C for 500 hrs, reducing the accuracy of Arrhenius analyses of long-term stability. Cross-sectional imaging revealed the source of the distortion to be high contact resistance due to void formation between the bulk copper and the silver capping layer in the contact stack, the voids occurring as a result of copper diffusing through the capping metal. The extent of void formation was shown to be dependent on the microstructure of the capping layer, highlighting the limitations of using plated metals as diffusion barriers.

Application of 266-nm picosecond (ps) laser ablation and copper (Cu)-plated metallization to p-type selective emitter (SE) passivated emitter and rear cells (PERC) is reported in this paper. Use of a 266-nm ps laser resulted in similar laser-induced periodic surface structures as observed for 355-nm ps laser ablation of a silicon (Si) nitride antireflection coating (ARC) on random-textured Si solar cell surfaces. In addition, it is shown that 266-nm ps laser ablation results in the formation of amorphous Si with an underlying distorted crystalline Si layer at the laser-ablated surfaces. The successful alignment of laser-ablated openings to the heavily doped SE regions resulted in a comparable cell efficiency of Cu-plated SE PERC cells to screen-printed controls, with a maximum cell efficiency of 20.6% being achieved for the Cu-plated cells. The plated cell performance was limited by the recombination losses, and in particular nonideal recombination caused by the use of a shallow emitter, which had been optimized for screen-printed metallization. Engineering of an SE with a junction depth of 0.52 mu m in the heavily doped regions resulted in a 0.3% absolute increase in pseudo fill factor and demonstrated the importance of displacing the p-n junction from the laser-ablated Si surface. Although 355-nm ps laser ablation has been demonstrated to result in strong busbar adhesion in previous reports of Cu-plated cells, significant variability in the busbar adhesion of the fully plated SE PERC cells resulted by 266-nm ps laser ablation. The predicted increased sensitivity of 266-nm laser ablation to the ARC thickness and the possibility that surface oxides were not uniformly removed across wafers before plating may have affected the uniformity of silicide formation and hence the adhesion of the plated busbars.

In this study, silicon solar cells with copper-plated front side metallisation were exposed to long-term reliability thermal stress conditions and the material integrity of the plated contacts after stress testing was investigated using imaging and electrical measurements. Significant voltage ‘bend-back’ was observed in Suns-VOC measurements at high illumination intensities (> 1Sun) following thermal stress testing at 200°C for 500h of laser-ablated cells with a nickel/copper/silver plated front metal grid. Using a combination of focussed ion beam milling, high resolution imaging and energy dispersive X-ray spectroscopy, it was shown that large voids can form between the silver capping layer and the main copper stack during thermal annealing. However, even more revealing was the detection of a new metal layer comprising largely of diffused copper overlying the silver capping. The cause of the Schottky ‘bend-back’ behaviour was theorised to be due to increased contact resistance arising from the voids which are presumed to form as a result of grain boundary diffusion of copper through the silver capping layer. Errors of 5–10% in the determination of pFF from Suns-VOC occur as a result, with the scale of the error dependent on the capping method and sintering conditions. Collapsing the voids was subsequently shown to remove the Schottky behaviour and improve reliability of the fitted diode parameters extracted from Suns-VOC measurements.

Historically, busbar pull tests have been used as a measure of metal-silicon adhesion for silicon solar cells; however, such measurements cannot be easily applied to evaluate finger adhesion and the propensity of metal fingers to peel. Finger adhesion will be increasingly important as the width of fingers decrease and busbars are effectively removed from the cell metallization. In this paper, we correlate metal-plated finger dislodgement measurements, which have been obtained using a stylus-based metallization testing tool, and busbar pull test forces with nanoindentation measurements of the Young's modulus in order to determine key determinants of strong finger adhesion. It is proposed that metal fingers with a higher Young's modulus dislodge at lower stylus impact forces because the energy associated with the impact is less easily dissipated along the fingers and consequently remains more focused on the impact location, causing not only finger dislodgement but more extensive finger peeling as well. It is shown how plating rate, chemistry, grid geometry, and postplating annealing can all contribute to plated metal finger adhesion, therefore necessitating an understanding of these factors for reliable plated metallization.

Light-induced plating presents a potentially lower-cost alternative to screen-printed Ag for silicon solar cell metallization. This paper presents results of experiments that investigated the effects of bias current density and post-plating rapid thermal processing (RTP) on plated Cu finger microstructure and texture. It is shown that the Cu, if not annealed after plating, self-anneals with time resulting in grain growth and increased (200) to (111) grain texture which is associated with increased tensile stress. The rate of self-annealing is much faster with high plating current densities. Post-plating RTP annealing enables fast annealing and stable Cu fingers with a low ratio of (200) to (111) grain texture. These findings have important implications for plated finger adhesion and highlight the importance of annealing after plating for reliable Cu plated metallization.

Light-induced plating (LIP) presents a potentially lower-cost alternative to screen-printed silver (Ag) for front surface silicon solar cell metallization. This paper presents the results of experiments that investigated the effects of bias current density, post-plating rapid thermal processing (RTP) and electrolyte chemistry on plated copper (Cu) finger microstructure, crystal texture and resistance. It is shown that the Cu fingers, if not rapidly annealed immediately after plating, self-anneal resulting in grain growth and an increased (200) to (111) grain texture ratio which is indicative of increased tensile stress in the Cu fingers. The resistance in the fingers also decreases due to reduced scattering arising from the fewer and larger grains formed during self-annealing. The rate of self-annealing is faster with higher plating current densities and can vary for different plating electrolytes, presumably due to differing levels and types of incorporated impurities. The changes in plated Cu microstructure due to self-annealing may impact electrical reliability and adhesion of metal contacts plated using LIP for silicon solar cells where the grid geometry requirements may cause differences in the plating rate and hence self-annealing rate across a cell. Use of an RTP after Cu plating can reduce this variability with a lower and stable ratio of (200) to (111) grain texture resulting after thermal annealing, however voids can form near grain boundaries in thermally-annealed Cu fingers as they cool. (C) 2016 The Electrochemical Society. All rights reserved.

We report fabrication of nanostructured, laser-doped selective emitter (LDSE) silicon solar cells with power conversion efficiency of 18.1% and a fill factor (FF) of 80.1%. The nanostructured solar cells were realized through a single step, mask-less, scalable reactive ion etch (RIE) texturing of the surface. The selective emitter was formed by means of laser doping using a continuous wave (CW) laser and subsequent contact formation using light-induced plating of Ni and Cu. The combination of RIE-texturing and a LDSE cell design has to our knowledge not been demonstrated previously. The resulting efficiency indicates a promising potential, especially considering that the cell reported in this work is the first proof-of-concept and that the fabricated cell is not fully optimized in terms of plating, emitter sheet resistance and surface passivation. Due to the scalable nature and simplicity of RIE-texturing as well as the LDSE process, we consider this specific combination a promising candidate for a cost-efficient process for future Si solar cells.

This paper compares the use of quasi-steady state photoluminescence (QSSPL) and photoconductance (QSSPC) measurements for injection-dependent carrier lifetime analysis of the state of boron-oxygen (B-O) complexes in boron-doped Czochralski wafers passivated with different dielectrics. Use of QSSPL measurements enabled effective carrier lifetime measurements over a larger range of injection levels than possible with QSSPC, potentially increasing the accuracy of estimates of the capture cross-section ratio of the deep-level B-O Shockley-Read Hall (SRH) defect. Although the capture cross-section ratios estimated using QSSPL (9.7 ± 1.7) and QSSPC (9.7 ± 1.9) for wafers symmetrically-passivated with silicon nitride and subsequently rapidly fired were very similar and close to the value of 9.3 reported by Rein et al. for the B-O defect, significant differences in the ratio and larger variances in the measurements within a group resulted for wafers with silicon nitride that was not annealed and wafers which were passivated with the thermal oxide. It is suggested the low-injection QSSPL measurements may introduce inaccuracies arising from the conduction of excited carriers via the diffused emitter away from the measurement area. This may suggest that there is little advantage in using QSSPL over the more commonly-used QSSPC measurements for these SRH analyses. Additionally, the study confirmed that stable regeneration of the effective carrier lifetime was only achieved when wafers were passivated with silicon nitride and underwent a rapid thermal anneal after deposition. If wafers were not rapidly thermally-annealed after silicon nitride deposition, then the recovery of effective lifetime observed with regeneration was not stable during a second light soak. Furthermore, the effective carrier lifetime of wafers passivated with a thermal oxide continued to decrease with light soaking, regeneration and a second light soaking with the appearance of “rings” in the PL images of oxide-passivated wafers as they underwent light soaking and regeneration highlighting the possible role of oxide precipitates in reactions occurring with light soaking and regeneration.

Copper plating can reduce the consumption of silver for silicon photovoltaic manufacturing, whilst also offering the potential to increase cell efficiency by way of reduced shading due to very narrow fingers and contacting silicon surfaces with low phosphorus concentrations. However, it can be challenging to plate busbars and fingers that have sufficient adhesion to the silicon. To date adhesion has typically been assessed by busbar pull tests however we propose that this measurement does not consider the properties of copper-plated fingers and may not be a good indicator of whether fingers may dislodge or peel with subsequent processing or during module fabrication. In this paper we investigate the factors of silicon interface roughness and plated copper properties using a combination of busbar pull tests and stylus-based adhesion measurements. We show that average 180° pull test forces of 2.1 N/mm can be achieved when a UV ps laser is used to ablate the silicon nitride, however ensuring strong finger adhesion is a far more complex problem with no accepted standard to determine what is “sufficient” adhesion. Although use of a ps laser to ablate the silicon nitride can result in plated metal adhering so strongly to the silicon that fragments of silicon are broken off with the finger when it is dislodged by the stylus, use of fast plating rates can result in reduced finger dislodgement forces and excessive finger peeling whereas busbar pull test forces are largely unchanged because the increased plating current is directly mostly through the fingers of the plated metal grid during light-induced plating. The plating of busbars and fingers on a cell presents challenges for uniform silicon-metal adhesion and this paper highlights the importance of finger adhesion measurement for process quality control for nickel/copper plating of p-type silicon cells in a manufacturing environment.

Light-induced plating (LIP)of Ni/Cu presents a potentially lower-cost alternative to screen-printed Ag for silicon solar cell metallization. This paper presents results of self-annealing and post-plating rapid thermal processing (RTP) of plated Cu finger microstructure, texture and resistance. It is shown that the plated Cu conductors, if not thermally-annealed immediately after plating, self-anneal with time resulting in grain growth and increased (200) to (111) grain texture which occurs with increased tensile stress. Post-plating RTP annealing enables fast annealing and stable Cu fingers with a low ratio of (200) to (111) grain texture. These findings have important implications for plated finger adhesion and highlight the importance of annealing after plating for reliable Cu plated metallization.

A light-induced anodization (LIA) method, which uses the light-induced current of a silicon solar cell to anodize aluminum and form an anodic aluminum oxide (AAO) layer that can reduce recombination at a p-type surface of the cell, is reported. This method can result in anodic oxides with uniform properties over the surfaces of industrially sized silicon wafers since the current flows through the wafer rather than in the surface aluminum layer, as it does when the aluminum is directly electrically contacted. Uniform AAO layers can be formed within 4 min on p-type silicon surfaces by anodizing 300-nm-thick aluminum layers, and when formed over a thin interfacial silicon dioxide layer, the interface state density at the p-type silicon wafer interface can be reduced to values as low as 1 × 10 10 cm -2 ·eV -1 and a fixed charge density of 2 × 10 11 cm -2 . Photoconductance carrier lifetime measurements indicate that these AAO dielectric stack layers are capable of passivating silicon surfaces to a level similar to that provided by silicon nitride layers, which are deposited using plasma-enhanced chemical vapor deposition, although at potentially a much-reduced cost. The high-quality surface passivation, rapid room temperature processing, and large-area uniformity of LIA make it a promising method for producing passivation materials that can further improve efficiency and potentially reduce the cost of photovoltaic solar cells and other p-n junction devices.

A stylus-based adhesion tester was used to characterise the adhesion of metal fingers on silicon solar cells. The measured lateral force together with the failure modes of finger cut-off and dislodgment, were identified for screen-printed silver contacts and nickel/copper plated laser-doped selective emitter cells. In-situ images were recorded to visualise the dynamics of the adhesion testing process. The contours of the measured lateral force were mapped to demonstrate the variability of finger adhesion characteristics across two representative solar cells.

Light-induced plating of Si solar cells was analyzed using in-situ linear sweep voltammetry using a potentiostat configured with a reference electrode. The current-voltage relationships recorded depend on the characteristics of the solar cell, the electrochemical reactions at the interface and the electrolyte resistance. Linear sweep voltammetry can be used to ensure that the solar cell operates under forward bias during plating, thereby eliminating the potential of non-uniform plating which can occur if the solar cell operates under reverse bias.

This paper reports on the use of AAO layers as a source of p-type dopants for laser doping processes that forms localised p+ regions on Si surfaces. Sheet resistances as low as 2Ω/sq were demonstrated when a laser was used to scribe through a region of AAO using a speed of 500mm/s and power of 9W. Unlike laser-doping through spin-coated polyboron sources, it was shown that laser doping through AAO layers can be performed without introducing any voids into the Si and form a local BSF ~5µm into Si which is advantageous for PERL cell structure. This co-doping process was used to fabricate rear-passivated cells with efficiencies of up to 19.9%. However, although the heavily-doped local p+ regions could reduce Rs to values as low as 0.54Ωcm2, there was a penalty in terms of a high ideality factor in the Vmp –Voc voltage range which limited FFs to ~76%.

Tunnel oxides for carrier-selective contacts must be sufficiently thin that carriers can tunnel through the oxide whilst minimising recombination at the silicon interface if high open circuit voltages are to be achieved. We report the formation of ultra-thin silicon oxide layers by a field-induced anodisation process in which the anodisation current is directed through the wafer in such a way that very uniform oxides can be grown. Spectroscopic analyses of these thin silicon oxide layers demonstrate the ability to adjust their atomic density and valence band offset by varying the anodisation conditions, suggesting that anodisation may provide a viable method to form tuneable silicon oxide layers for carrier selectivity. (C) 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

This paper reports a patterning and metallization method for silicon solar cells fabrications. Patterning was achieved by the inkjet printing of a dye-based ink as a mask to protect the photoresist from UV-light initiated crosslinking. The patterned photoresist was used to facilitate the etching of a pattern in the underlying dielectric layer and also to act as a metal plating mask. This method resulted in fine point openings in the photoresist layer with a diameter of 15 μm and line openings with a width of 30 μm. Nickel/copper plated homogeneous emitter silicon solar cells with an efficiency of 18.2% on small size Cz wafers were fabricated using this method, may find applications in the metallization of future heterojunction, rear contact and PERC cells.