Within the last years, Q CELLS has developed a silicon solar cell structure yielding an energy conversion efficiency exceeding 24%. The cell structure features a so-called passivating contact consisting of an interfacial oxide and an n-type polysilicon layer on the rear side, double-sided screen-printed metal contacts, a module-optimized anti-reflective coating, a homogeneous front-side emitter, and an n-type silicon substrate. For the fabrication of this type of solar cell, a lean processing sequence has been applied by using exclusively mass-production processes in Q CELLS’ pilot R&D line in Thalheim, Germany. For module integration, state-of-the-art technology, such as half-cells, multi-wire interconnection, standard encapsulants, and zero-gap technology, can be applied. Hence, this solar cell structure has the potential to fully close the small remaining gap to the highest efficiency cell technologies, such as heterojunction and rear-contact solar cells at very competitive manufacturing cost. In analogy to Q CELLS' PERC-like Q.ANTUM technology, and in contrast to conventional passivated emitter and rear cells (PERC), this novel solar cell is shown to reliably suppress light-induced degradation.

In this paper, we propose a hybrid quantum dot (QD)/solar cell configuration to improve performance of interdigitated back contact (IBC) silicon solar cells, resulting in 39.5% relative boost in the short-circuit current (JSC) through efficient utilisation of resonant energy transfer (RET) and luminescent downshifting (LDS). A uniform layer of CdSe1-xSx/ZnS quantum dots is deposited onto the AlOx surface passivation layer of the IBC solar cell. QD hybridization is found to cause a broadband improvement in the solar cell external quantum efficiency. Enhancement over the QD absorption wavelength range is shown to result from LDS. This is confirmed by significant boosts in the solar cell internal quantum efficiency (IQE) due to the presence of QDs. Enhancement over the red and near-infrared spectral range is shown to result from the anti-reflection properties of the QD layer coating. A study on the effect of QD layer thickness on solar cell performance was performed and an optimised QD layer thickness was determined. Time-resolved photoluminescence (TRPL) spectroscopy was used to investigate the photoluminescence dynamics of the QD layer as a function of AlOx spacer layer thickness. RET can be evoked between the QD and Si layers for very thin AlOx spacer layers, with RET efficiencies of up to 15%. In the conventional LDS architecture, down-converters are deposited on the surface of an optimised anti-reflection layer, providing relatively narrowband enhancement, whereas the QDs in our hybrid architecture provide optical enhancement over the broadband wavelength range, by simultaneously utilising LDS, RET-mediated carrier injection, and antireflection effects, resulting in up to 40% improvement in the power conversion efficiency (PCE). Low-cost synthesis of QDs and simple device integration provide a cost-effective solution for boosting solar cell performance.

Contact resistivity measurements for electroless nickel plated (ENP) contacts formed on heavily boron diffused p + silicon are presented. In this work, the contact resistivity values for an alkaline based ENP solution which does not require prior surface sensitisation or activation is obtained from circular transmission line method measurements and compared to identical structures formed with evaporated aluminium contacts. It is found that in comparison to evaporated aluminium contacts, ENP contacts have a higher contact resistivity for comparative samples with identical p + diffusion profiles in the underlying silicon. Nevertheless, contact resistivity values as low as 0.24 m$\Omega .cm ^{2}$ are measured for a boron surface concentration of $(1.4 \times 10) ^{19} cm ^{-3}$.

In this work, we develop a fabrication process for an interdigitated back contact solar cell using BBr3 diffusion to form the p+ region and POCl3 diffusion to form the n+ regions. We use the industry standard technology computer‐aided design modelling package, Synopsys Sentaurus, to optimize the geometry of the device using doping profiles derived from electrochemical capacitance voltage measurements. Cells are fabricated using n‐type float‐zone silicon substrates with an emitter fraction of 60%, with localized back surface field and contact holes. Key factors affecting cell performance are identified including the impact of e‐beam evaporation, dry etch damage, and bulk defects in the float zone silicon substrate. It is shown that a preoxidation treatment of the wafer can lead to a 2 ms improvement in bulk minority carrier lifetime at the cell level, resulting in a 4% absolute efficiency boost.

Silicon nitride (SiNx) synthesised by low-temperature plasma enhanced chemical vapour deposition (PECVD) is the most extensively used antireflection coating for crystalline silicon solar cells because of its tunable refractive index in combination with excellent levels of surface and bulk passivation. This has attracted a significant amount of research on developing SiNx films towards an optimal electrical and optical performance. Typically, recipes are first optimised in lab-scale reactors and subsequently, the best settings are transferred to high-throughput reactors. In this paper, we show that for one particular, but widely used, PECVD reactor configuration this upscaling is severely hampered by an important experimental artefact. Specifically, we report on the unintentional deposition of a dual layer structure in a dual mode AK 400 plasma reactor from Roth & Rau which has a significant impact on its surface passivation performance. It is found that the radio frequency (RF) substrate bias ignites an unintentional depositing plasma before the ignition of the main microwave (MW) plasma. This RF plasma deposits a Si-rich intervening SiNx layer (refractive index = 2.4) while using a recipe for stoichiometric SiNx. This layer was found to be 18 nm thick in our case and had an extraordinary impact on the Si surface passivation, witnessed by a reduction in effective surface recombination velocity from 22.5 to 6.2 cm/s. This experimental result may explain some “out of the ordinary” excellent surface passivation results reported recently for nearly stoichiometric SiNx films and has significant consequences when transferring these results to high-throughput deposition systems.

Most c-Si solar cells adopt monofacial thin film structures and consequently much effort, particularly at the equipment design level, is devoted to ensure monofacial thin film processing. This often leads to more complex production equipment with lower throughput. However, in some cases, bifacial processing can be tolerated, and it is shown that a bifacial deposition of aluminium oxide (AlOx) using atomic layer deposition (ALD) can even result in improved performance of the solar cell. The merits of ALD AlOx at the rear of p-type PERC solar cells are well established and this work shows that a thin AlOx layer on the front of a PERC solar cell can also significantly reduce the contact resistance of screen printed Ag without affecting the optical properties of the solar cell. In this work, a equidistant linear transmission line method (TLM) pattern is used to characterise the contact resistance and specific contact resistivity. This technique has the advantage of being able to measure the fingers of completed cells, and this also explores the error introduced by measuring the resistance across interjacent fingers. A 3 or 5 nm AlOx film reduces the contact resistivity to 0.06 mΩ.cm2 and 0.11 mΩ.cm2, respectively, significantly lower than a value of 0.25 mΩcm2 achieved with an uncapped reference. Also, the peak temperatures required to achieve a good contact was lower, and the optimum firing temperature window was found to be wider, in comparison to the reference. A 10 nm ALD AlOx film is found to have a detrimental effect on the contact resistance which cannot be mitigated by a higher firing temperature.

The effect of positive fixed charge on the recombination rate at SiNx-passivated p+ surfaces is studied in this work. It is shown that a high positive fixed charge on a low defect density, passivated doped surface can result in a near injection level independent lifetime in a certain injection level range. This behaviour is modelled with advanced computer simulations using Sentaurus TCAD, which replicates the measurements conditions during a photoconductance based effective minority carrier lifetime measurement. The resulting simulations show that the shape of the injection level dependent lifetime is a result of the surface recombination rate, which is non-linear due to the surfaces moving into inversion with increasing injection level. As a result, the surface recombination rate switches from being limited by electrons to holes. Equations describing the surface saturation current density, J0s, during this regime are also derived in this work.

Single-sided atomic layer deposition (ALD) is challenging as often some degree of wrap-around deposition onto the nontarget side occurs. This is particularly challenging for aluminum oxide (AlO x ) deposition on silicon solar cells, as is required on the rear of passivated emitter rear cell (PERC) solar cells. The effects of unintended ALD-deposited AlO x on the front side of PERC solar cells are explored in this work, with a specific focus on the impact on contact resistance. The contact resistance is determined from transmission line measurement structures on samples with up to 10 nm of AlO x on top of silicon nitride (SiN x ). These values are used as input parameters for computer simulations to simulate the effect of varying degrees of wrap-around on p-PERC cells. Finally, p-type aluminum back surface field solar cells with 3, 5, 7, and 10 nm AlO x layers on top of the SiN x are fabricated using various firing recipes and screen printing pastes to assess the effect on fabricated devices. It was found that a 3 nm AlO x layer can improve the cell performance relative to the uncapped case, whereas 5, 7, and 10 nm resulted in a lower solar cell efficiency due to higher series resistance.

A recently developed, novel method for the extraction of fixed interface charge, Q(f), and the surface recombination parameters, S-n0 and S-p0, from the injection-level dependent effective minority carrier lifetime is applied to HfOx passivated highly doped c-Si samples. This contactless technique can, unlike conventional capacitance-voltage measurements, be applied to highly doped surfaces provided the surface becomes heavily depleted or inverted as the injection level increases. By fitting the measured injection level dependent effective lifetime curve before and after annealing, Q(f) and the surface recombination velocity parameters, S-n0 and S-p0 are independently resolved and extracted. It was shown that the significantly higher effective lifetime for 15 nm and 30 nm thick HfOx films was a result of differing passivation mechanisms; the 15 nm film improves field effect passivation after annealing, whereas the surface passivation mechanism of the 30 nm film after anneal was predominantly a chemical passivation effect. This work presents interface parameter measurements of doped surfaces not accessible by the more common capacitance-voltage techniques.

A new design for high efficiency silicon solar cells with the use of limited p-n junction area and point Al rear contact is proposed. In this work, planar test devices with different emitter area and spacing are fabricated to demonstrate the limited junction area (LIA) design can improve VOC for a well-designed cell structure. Interim lifetime and PL measurements during fabrication confirm lower dark saturation current (I0) and higher implied VOC (iVOC) in the LIA samples with a robust surface passivation scheme. Higher VOC and efficiency are also simulated with Quokka on some LIA devices compared to the full area emitter control sample.

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.

A limited area p-n junction silicon solar cell is designed and the effect of limited area emitter on the output open circuit voltage (VOC) and efficiency is studied by a 3-D simulator Quokka. The recombination property of emitter, surface and bulk are all extracted from the test structures and this paper reports potential voltage that can be achieved on the limited area junction solar cells by the modelling. The result indicates that this approach will help improve VOC when the emitter area is reduced and the voltage will be more than 700mV. The result also shows that there is an optimum emitter width for the highest efficiency on the limited area junction solar cell.

A method of patterning atomic layer deposition (ALD) deposited aluminium oxide dielectrics (Al2O3) using an inkjet printer is outlined. This method has applications in creating PERC cell type rear contacts. It is simpler, and uses fewer chemicals than immersive etching techniques such as those involved in photolithography, and the patterning does not cause damage to the silicon evident in laser ablation techniques. Holes with varying diameter between 61–159 µm were etched onto a 50 nm thick ALD Al2O3 passivating dielectric at numerous print settings. The relationship between the printing parameters and the feature size of the holes is discussed.

A study of limited area p-n junction silicon solar cells using photoluminescence (PL) is used to demonstrate the pathways to an increase in open circuit voltage (VOC). A high voltage baseline structure is used to ensure that the junction dominates the recombination. Both quasi-steady-state photo-conductance (QSSPC) and PL measurements indicate higher implied voltage on the limited p-n junction area solar cells compared to full area junction cells. This work aims at achieving more than 740 mV VOC on the limited area junction structure, through optimising the passivation layers and comprehensive analysis of voltage losses throughout the structure. Various choices of passivation layers and a new structure design for separating and analysing J0 from each component on the limited area junction solar cell will be presented in this paper.

Local rear metal contacting through passivating dielectric layers has the ability to increase silicon solar cell efficiencies to over 20%. To-date most contact schemes have involved the formation of localised aluminium-alloyed regions through patterned AlOx or SiNx passivating layers. Recently electrochemically-formed anodic aluminium oxide (AAO) layers have been shown to enhance minority carrier lifetimes of phosphorus–diffused p-type CZ wafers when formed over an intervening layer of SiO2 or SiNx, suggesting that these layers may find applications as passivation layers for cells. We report here on the inkjet patterning of AAO layers formed over a thermally-grown thin oxide layer on p-type silicon surfaces. The process, which involves the inkjet printing of 50% (w/w) phosphoric acid, was used to form well-resolved arrays of holes with a diameter as small as 20-40μm in the dielectric stack. Alloying of aluminium, which was evaporated over the patterned dielectric stack, resulted in the formation of localised back surface field (BSF) regions having a thickness up to 8μm. Future work will focus on adapting this process for use in local rear metal contacting of silicon solar cells.