Selective emitter formation with a single screen-printed p-doped paste deposition using out-diffusion in an RTP-step

We present a new way to realise a selective emitter structure using a single screen-printed phosphorous paste deposition as dopant source to obtain a doping differential, with rapid thermal diffusion. The heavily doped part of the emitter is situated underneath the deposited phosphorous lines, and the lightly doped emitter in between is obtained via the gas phase, because phosphorous out-diffuses from the paste during the high-temperature step. SIMS profiles show a difference of a factor 4 in magnitude for the surface concentration between the regions underneath and beside the deposited paste. Photovoltaic results show an improvement in efficiency of 0.7% in comparison with the reference cell (homogeneous emitter), due to a 2 mA/cm² short-circuit current increase.     

Local aluminum-silicon contacts by layer selective laser ablation

We demonstrate damage free selective laser ablation of silicon nitride from a silicon nitride/amorphous silicon double layer. This approach allows local contact formation to passivated silicon. Thereby the remaining amorphous silicon dissolves in evaporated aluminum by annealing. This technique is especially useful for contacting thin emitters since it avoids any damage to the silicon substrate. We demonstrate a local contact resistivity of 0.8±0.3 mΩ cm² on a phosphorous diffused emitter with a peak doping density of 2×10²⁰ cm⁻³. Laser treated as well as non-treated areas show the same carrier lifetime of 2000 μs on 100 Ω cm mono-crystalline silicon, proving the selective ablation.     

Self-aligned locally diffused emitter (SALDE) silicon solar cell

This paper presents, for the first time, a low-cost, high-throughput manufacturing approach for fabricating n-base dendritic web silicon solar cells with selectively doped emitters and self-aligned aluminum contacts using rapid thermal processing (RTP) and screen printing. The self-aligned locally diffused emitter (SALDE) structure is p⁺ nn⁺⁺ where aluminum is screen-printed on a boron-doped emitter and fired in a belt furnace to form a deep self-doped p⁺-layer and a self-aligned positive contact to the emitter according to the well-known aluminum-silicon (Al---Si) alloying process. The SALDE structure preserves the shallow emitter (20.2 μm) everywhere except directly beneath the emitter contact. There the junction depth is greater than 5 μm, as desired, in order to shield carriers in the bulk silicon from that part of the silicon surface covered by metal where the recombination rate is high. This structure is realized by using n-base (rather than p-base) substrates and by utilizing screen-printed aluminum (rather than silver) emitter contacts. Prototype dendritic web silicon (web) cells (25 cm² area) with efficiencies up to 13.2% have been produced.     

The electrical properties and hydrogen passivation effect in mono crystalline silicon solar cell with various pre-deposition times in doping process

The doping process in diffusion furnace consisting of pre-deposition and drive-in steps is essential to create p–n junction in crystalline silicon solar cell fabrication, and its optimization is necessary to obtain the high conversion efficiency. In this work, pre-deposition time was varied to study the electrical properties of solar cells and its effect on the hydrogen passivation with various phosphorous doping profiles. As a result, solar cell conversion efficiency of 17.8% with 7 min pre-deposition was achieved. Dopant (phosphorous) concentration in the emitter measured by SIMS indicated that the surface with shorter pre-deposition time had lower dopant concentration. High concentration of phosphorous on the surface appears to be the source for the electron consumed by the stored hydrogen in making the neutral H₂ gas during firing. The formation of neutral hydrogen gas is thermodynamically and stochastically more favorable than the reaction between Si with dangling bond and H. This means that the passivation by the stored H during firing is strongly controlled by the dopant on the surface. This result obtained herein lays the foundations to understand the relationship between the doping profile of diverse dopant species and its passivation effect.     

The effect of emitter recombination on the effective lifetime of silicon wafers

The application of photoconductance measurements of the effective lifetime of silicon wafers to determine the saturation current density of diffused emitter regions is reviewed. To illustrate the technique, a sequence of experiments is presented with phosphorus diffusions of various types: oxide passivated and unpassivated, lightly doped and heavily doped. Different material qualities (FZ, CZ and multicrystalline silicon) are considered, as well as different substrate resistivities. The dependence of the effective minority carrier lifetime on injection level is discussed. The limitations imposed by emitter recombination on the measurable minority carrier lifetimes are clarified and demonstrated experimentally. This bound varies with the dopant density and thickness of the silicon wafer.     

A new model of very high efficiency buried emitter silicon solar cell

In this work, we report a new model of symmetrical silicon solar structure where the emitter is buried, thus, creating many depletion regions in series inside the cell. The photocurrent in this model is computed for AM0 solar spectrum and is compared to the classical p-n junction. The first results of the calculation show that the buried emitter solar cell (BESC) has 10–35% more short-circuit current than the classical cell depending on the surface recombination velocity S. The biggest difference is obtained for S = 10⁵cm/s. The ratio of the photocurrent in the BESC to the classical photocurrent as a function of the absorption coefficient a goes from 1.9 for small α to 6.5 times for α = 10⁷cm⁻¹ with a minimum of 1.1 for α around 1800 cm⁻¹ for S = 10⁵cm/s.     

The influence of porous silicon coating on silicon solar cells with different emitter thicknesses

The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n⁺p silicon junction whose emitter depth was varied between 0.5 and 2.2 μm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n⁺ emitter. The electrical properties of the samples with PS were improved with decrease of the n⁺p junction depth. Our results demonstrate short-circuit current values of about 35–37 mA/cm² using n⁺ region with 0.5 μm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modeling. The spectral response measurements were performed at a wavelength range of 0.4–1.1 μm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400–500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19–20% efficiency by the proposed simple technology.     

Recovery of minority carrier lifetime in low-cost multicrystalline silicon

Lifetime of minority carriers has been widely identified to be the key material parameter determining the conversion efficiency of pn-junction silicon solar cells. Impurities and defects in the silicon crystal lattice reduce the charge carrier lifetime and thus limit the performance of the solar cells. Removal of impurities by silicon material purification is often contradictory with low cost production of photovoltaic devices. In this paper, we present experimental results of an efficient gettering technique which can be applied to low cost processing of multicrystalline silicon solar cells without any additional process steps or compromises with optimal device design parameters. This technique is based on well-known phosphorous gettering. We have discovered that if the silicon wafers are kept in the furnace after the emitter diffusion at the 700°C, significant improvement in the lifetime will take place. At this temperature the properties of the pn-junction remain unaffected meanwhile many lifetime killers are still mobile. The time needed for this temperature program can be easily modified in order to respond to the material quality variations in substrates originating from different parts of multicrystalline ingot. Better control of lifetime can lead to higher degree of starting material utilization.     

Sheet resistance uniformity in drive-in step for different multi-crystalline silicon wafer dispositions

In this work, we present a study of emitters realized using different configurations of the silicon wafers in the quartz boat. The phosphorous liquid source is sprayed onto p-type multi-crystalline silicon substrates and the drive-in is made at high temperature in a muffle furnace. Three different configurations of the wafers in the boat are tested: separated, back to back and compact block of wafers. A fourth configuration is also used in source-receptor mode. The emitter phosphorous concentration profile is obtained by SIMS analysis. The resulting emitters are characterized by sheet resistance measurements and a comparison is made between the wafers within the same batch and from one batch to another. The uniformity and the standard deviation of the sheet resistance are calculated in each case. The emitter sheet resistance mapping of the wafer set in the middle of the boat for a given process gives a mean R_sq 14.66 Ω/sq with a standard deviation of 1.76% and uniformity of 18.7%. Standard deviations of 2.116% and 1.559% are obtained for wafers in the batch when using the spaced and compact configurations, respectively. The standard deviation is reduced to 0.68% when the wafers are used in source/receptor mode. A comparison is also made between wafers with different dilution of phosphorous source in ethanol.From these results we can conclude that the compact configuration offers better uniformity and lower standard deviation. Furthermore, when combined with the source-receptor configuration these parameters are significantly improved.This study allows the experimenter to identify the technological parameters of the solar cell emitter manufacturing and target precisely the desired values of the sheet resistance while limiting the number of rejected wafers.     

Aluminium BSF in silicon solar cells

The purpose of this work is to develop a back surface field (BSF) for industrial crystalline silicon solar cells and thin-film solar cells applications. Screen-printed and sputtered BSFs have been realised on structures which already have a n⁺p back junction due to the diffusion of the phosphorus in both faces of the wafer during solar cell emitter elaboration. Rapid thermal annealing temperatures from 700°C to 1000°C have been used. Thickness of the BSF has been measured by SIMS and confronted to the theoretical expected value and simulations. Electrical and optical measurements have been done in order to characterise the BSF. For 250 μm thick industrial solar cells, 6% relative increase in photocurrent has been reached.     

Internal quantum efficiency improvement of polysilicon solar cells with porous silicon emitter

The present study aimed to develop a simple analytical model to investigate the potential use and implications of porous silicon (PS) as an antireflective coating in thin polysilicon solar cells. It analytically solved the complete set of equations necessary to determine the contribution that this material has on the internal quantum efficiency (IQE) of the cell when acting as an antireflective coating agent. The increase in the IQE, the contribution of the different regions of the cell, and the effects of the physical parameters of each region were derived and investigated in comparison with conventional solar cells.The findings revealed that the internal quantum efficiency of the solar cell with PS emitter is higher than that of the conventional one particularly for short-wavelengths (λ < 0.6 μm). Furthermore, for photons with higher energy, the emitter contribution in the IQE is more significant than the base and depletion regions. For photons with smaller energy, on the other hand, the absorption coefficients are also smaller, which leads to a higher generation rate in the base region and, hence, to a more pronounced contribution from this region to IQE. Last but not least, the improvement of IQE is observed to increase with decreased PS thickness and with heavily doped PS emitter (N_d⁺⁺ = 10²⁰ cm⁻³).     

Multicrystalline silicon solar cells with porous silicon emitter

A review of the application of porous silicon (PS) in multicrystalline silicon solar cell processes is given. The different PS formation processes, structural and optical properties of PS are discussed from the viewpoint of photovoltaics. Special attention is given to the use of PS as an antireflection coating in simplified processing schemes and for simple selective emitter processes as well as to its light trapping and surface passivating capabilities. The optimization of a PS selective emitter formation results in a 14.1% efficiency mc-Si cell processed without texturization, surface passivation or additional ARC deposition. The implementation of a PS selective emitter into an industrially compatible screenprinted solar cell process is made by both the chemical and electrochemical method of PS formation. Different kinds of multicrystalline silicon materials and solar cell processes are used. An efficiency of 13.2% is achieved on a 25 cm² mc-Si solar cell using the electrochemical technique while the efficiencies in between 12% and 13% are reached for very large (100–164 cm²) commercial mc-Si cells with a PS emitter formed by chemical method.     

Progress in emitter wrap-through solar cell fabrication on boron doped Czochralski-grown silicon

We report on RISE-EWT (Rear Interdigitated Single Evaporation-Emitter Wrap-Through) solar cells on full area (12.5×12.5 cm²) pseudo square boron doped Czochralski-grown silicon wafers. We investigate the main efficiency optimisation factors of these cells by investigating the dependence of RISE-EWT cell parameters on the base dopant concentration N_A. We furthermore detail the effects of large feature sizes in base and emitter regions at the rear of the solar cell and investigate these effects with particular attention to the edge regions. EWT solar cells typically exhibit rather low fill factors. However, our results show that the improved fill factors can be achieved by increasing N_A, which in return leads to optimised efficiency values. For our RISE-EWT solar cells made from boron doped Cz-Si wafers, this benefit is maintained even after light-induced degradation. Our investigation of edge area related effects shows the importance of proper cell design in these areas, leading to a further 2.8% absolute improvement in the fill factor. Combining increased base dopant concentration with optimised edge design, we achieve 19.0% efficiency on (12.5×12.5 cm²) boron doped Cz silicon wafers before light-induced degradation, resulting in 18.1% efficiency in the light-degraded state.     

Comparison of different emitter diffusion methods for MINP solar cells: Thermal diffusion and RTP

A solar cell technology with an extremely small thermal budget was developed for MINP cells. MINP solar cells with efficiencies of up to 15.3% have been achieved by rapid thermal processing (RTP). An emitter diffusion process was simulated and developed that yields a doping profile with a tunnelling oxide in a single furnace step. The P concentration profile was investigated by SIMS measurements and compared to the calculated profile. The SIMS results of the 850°C processing temperature differ from the calculated profile, but the 800°C values showed an excellent conformity. The surface passivation can be improved by an increase of the deposition temperature of the antireflection film. The maximum temperature was appointed with 400°C for MINP and 300°C for pn cells. In comparison with pn cells the temperature stability of MINP cells is significantly higher.     

Selective emitter using porous silicon for crystalline silicon solar cells

This study is devoted to the formation of high–low-level-doped selective emitter for crystalline silicon solar cells for photovoltaic application. We report here the formation of porous silicon under chemical reaction condition. The chemical mixture containing hydrofluoric and nitric acid, with de-ionized water, was used to make porous on the half of the silicon surface of size 125×125 cm. Porous and non-porous areas each share half of the whole silicon surface. H₃PO₄:methanol gives the best deposited layer with acceptable adherence and uniformity on the non-porous and porous areas of the silicon surface to get high- and low-level-doped regions. The volume concentration of H₃PO₄ does not exceed 10% of the total volume emulsion. Phosphoric acid was used as an n-type doping source to make emitter for silicon solar cells. The measured emitter sheet resistances at the high- and low-level-doped regions were 30–35 and 97–474 Ω/□ respectively. A simple process for low- and high-level doping has been achieved by forming porous and porous-free silicon surface, in this study, which could be applied for solar cells selective emitter doping.     

Lifetime extension of in‐core self‐powered neutron detector using new emitter materials

In this paper, new in‐core self‐powered neutron detector emitter candidate materials, ie, vanadium, cobalt, and silver, have been examined in their lifetimes compared with the commonly used rhodium emitter. Using a new quantitative lifetime evaluation model, the lifetimes of vanadium and cobalt were determined to be longer than that of rhodium, but these materials were also shown to have the disadvantage of low signal intensities. Under normal operating conditions, we showed that rhodium emitter can be used for 2 cycles of pressurized water reactors (PWRs) with lifetime of 4.35 years, whereas silver can be used for 5 cycles of PWRs with lifetime of 8.04 years. Three sensitivity tests were performed for rhodium and silver about (1) the emitter size, (2) the fuel assembly burnup, and (3) the emitter temperature variations. From the test results, we observed that the lifetimes of rhodium and silver emitters remained 2 and 5 cycles long, respectively. We concluded that silver can significantly extend the in‐core detector's lifetime in PWR operation.     

Development of an emitter for industrial silicon solar cells using the doped oxide solid source diffusion technique

The aim of this paper is to demonstrate for the first time the feasibility of fabricating large-area screen-printed monocrystalline silicon solar cells using the Doped Oxide Solid Source (DOSS) diffusion technique. This process was applied to form the n⁺p emitter junction from highly doped sources prepared in a POCl₃ ambient. The diffusions were performed under a pure nitrogen flow in the temperature range 900–1050°C. In this investigation attention was devoted to the influence of the source doping level on the fill factor. The solar cells were fabricated on industrial quality 4-inch Cz wafers using a simple processing sequence incorporating screen-printed contacts and a TiO₂ antireflection coating deposited by spin-on. Fill factors as high as 79% were obtained. The potential benefit of retaining for passivation purposes the thin residual oxide grown during phosphorus diffusion was evaluated. These first experiments led to a cell efficiency close to 10%.     

Formation of highly aluminum-doped p-type silicon regions by in-line high-rate evaporation

Highly aluminum-doped p-type silicon regions are formed by in-line high-rate evaporation of aluminum. We deposit aluminum layers of 28 μm thickness at dynamic deposition rates of 20 μm×m/min on p-type silicon substrates. Due to the high substrate temperature of up to 770 °C during deposition an Al-doped p⁺ region is formed. Using the camera-based dynamic infrared lifetime mapping technique we measure emitter saturation current densities of 695±65 fA/cm² for the fully metalized Al-p⁺ regions, which corresponds to an implied solar cell open-circuit voltage of 635±2 mV.     

Reduction of the phosphorous cross-contamination in n-i-p solar cells prepared in a single-chamber PECVD reactor

A new approach to reduce phosphorous contamination in the intrinsic layer during the deposition of amorphous silicon (a-Si:H) n-i-p solar cells prepared in single-chamber reactors is presented. This novel process consists of a hydrogen etching plasma performed after the n-layer deposition, which prevents a recycling of phosphorous from the reactor walls when exposed to a hydrogen-rich plasma during the subsequent i-layer deposition. The implemented process reduces the phosphorous cross-contamination in the i-layer, as corroborated by secondary ion mass spectroscopy measurements. Furthermore, the end of the etching process can be easily monitored by measuring the DC bias voltage at the powered electrode. By applying this process, we were able to improve the fill factor from 70% up to 75%, without degradation in the other parameters of the cell, neither in the initial nor in the stabilized state. Finally, by implementing this process in a-Si:H/a-Si:H tandem solar cells we obtained an initial efficiency of 10.3% (V_oc=1.76 V, FF=74.5%, J_sc=7.8 mA cm⁻²); light soaking test resulted in a stabilized efficiency of 8.5%.     

Analytical modelling and minority current measurements for the determination of the emitter surface recombination velocity in silicon solar cells

A new analytical model is used to describe the emitter of silicon solar cells in order to gain information on the surface recombination velocity S. The procedure takes advantage of the combined use of experimental measurements, done to determine the emitter saturation current J_oe, and analytical modelling to relate J_oe to S. Several experiments have been carried out on silicon solar cells having different emitter profiles subjected to various surface treatments. The influence of the surface on significant cell parameters has been analysed.