Interdigitated Back Contact Solar Cells Research Articles

All-screen-printed Dopant Paste Interdigitated Back Contact Solar Cell

We report results on the development of a cost-effective all-screen-printed interdigitated back contact (IBC) solar cell process. The rear side interdigitated doping pattern is achieved using screen printed n-type and p-type dopant pastes, and thermal drive-in. Our process provides excellent spatial definition of the doping pattern necessary for fabrication of an IBC solar cell. The doping approach used in this work overcomes limitations associated with conventional pastes, notably poor lateral doping control and reduced bulk lifetime. We demonstrate that wafer lifetime remains high, well above 1.5 msec, at the end of the solar cell process, enabling high cell efficiencies. Contacts are also achieved using screen printed fire-through metal pastes. We present pilot line data for n-type CZ silicon, 156mm pseudo-square cells achieving just over 21% efficiency.

Properties of Liquid Phase Crystallized Interdigitated Back-contact Solar Cells on Glass

We fabricated interdigitated back-contact silicon hetero-junction solar cells based on thin-film absorbers on glass. The Si absorbers were directly deposited on the glass and crystallized using liquid phase crystallization. To compare whether our contact system is applicable to a wide range of initial absorber conditions two different types of precursors were prepared with absorber thicknesses between 2.5-9μm. Furthermore, glass superstrates, doping densities and interlayer stacks were varied. With a KOH random pyramid light trapping texture at the back more than 30mA/cm2 were achieved on an electron beam evaporated precursor material and 655mV on a PECVD precursor material.

Potential-induced Degradation for Encapsulated n-type IBC Solar Cells with Front Floating Emitter

Studying encapsulated n-type interdigitated back contact (IBC) solar cells with front floating emitter (FFE) we observe a power degradation mechanism which behaves similar to the PID-p degradation for n-typeIBC solar cells with front surface field described in or for bifacial n-typesolar cells in . This power drop is not shunt-related and arises as loss of short circuit current and open circuit voltage in analogy to the findings in . The power degradation occurs when biasing FFE-IBC minimodules with negative potential against the ground and can be fully regenerated by reversing the polarity of the bias potential. The susceptibility to PID for our IBC minimodules depends on the encapsulation material to higher or lesser extent; until now no material was identified which can totally prevent PID. Bifacial IV measurements and EL recordings let us conclude that the PID we observe is non-local and causes a homogenous degradation of the front surface of the solar cell. Silvaco Atlas simulations show that a total loss of surface passivation is not sufficient to explain the effect.

Forming aluminum electrodes by screen printing and electron-beam evaporation for high performance interdigitated back contact solar cells

Interdigitated back contact (IBC) solar cell is one of the most efficient silicon solar cells commercially available to date. This paper studies the application of aluminum electrodes, deposited by electron-beam evaporation, instead of commonly used silver paste. Furthermore, screen printing instead of optical lithography method was used to generate etching masks. Together, we develop a low-cost technology for production. The specific contact resistivity of Al was found to be even lower than that of traditionally used Ag paste. The specific contact resistivity (ρc) and the recombination parameter (Jo) were found to be dependent on the doping level. Therefore, there is a design trade-off between a low Jo obtained with a low surface doping concentration, and a low ρc obtained with a high surface doping concentration. This process produced an IBC solar cell with a size of 4cm2, an open circuit voltage of 680mV, a conversion efficiency (η) of 22.72%, a fill factor of 0.80, and a short circuit current density of 41.69mA/cm2. The optimal design for the Al contact IBC cell was found by comparing the 3D Quokka© simulations of the Al contact and Ag paste contact IBC cells. The simulation indicated that further decrease in the Al contact size and spacing using screen printing and optimize the doping level could result in η up to 23.4% in production, comparable to the performance achieved in small cells using optical lithography.

Analysis of n-type IBC solar cells with diffused boron emitter locally blocked by implanted phosphorus

Interdigitated back-contact (IBC) solar cells were fabricated with a process sequence combining local ion implantation of phosphorus and full area BBr3 furnace diffusion resulting in conversion efficiencies of up to 22.4%. The highly doped emitter and BSF are in direct contact to each other (p+n+ junction) leading to a controlled junction breakdown at low reverse-bias voltages of around 5V. The breakdown was located at the p+n+ junction and found to be homogeneously distributed over the whole cell area. This is not critical for module integration as the absolute temperature rise of a reverse-biased cell was determined to be less than 35K. After reverse breakdown, the conversion efficiency degraded by 1–2% absolute due to additional recombination at the p+n+ junction. The cell performance could be fully recovered by a short annealing at 300°C indicating that the Al2O3 passivation was altered by the reverse breakdown. This might be a fundamental issue for Al2O3 passivated IBC solar cells without gap between emitter and BSF, independent from the doping method.

Local Series Resistance Imaging of Silicon Solar Cells With Complex Current Paths

Next-generation highest efficiency silicon solar cells are often designed with elaborate doping and contacting structures that are sensitive to series-resistance-induced losses. Such structures exhibit complex asymmetric current paths, which present conceptual and technical challenges for spatially resolved characterization, which have thus far not been addressed and discussed. We show how series resistance imaging by luminescence can be adapted and interpreted for such complex devices using interdigitated back-contact solar cells as a prime example. Experiments that are supported by simulations for several cell geometries and base materials show that global series resistance and, thus, fill factor (FF) is governed by both electron and hole transport as a function of the resistances along their different current paths. A conductive boundary-based simulation environment for current–voltage (I-V) curves and luminescence images is used to further explain current transport physics and parameter correlations. Agreement within ±30% between series resistance derived from arithmetically averaged local values and conventional global measurements is achieved. The developed 2-D/3-D luminescence-experiment simulation environment opens the path for future accurate comparisons between other local and global characterization methods. Thus, we enable quantitative analysis of local series resistance phenomena induced by design or by production defects for complex silicon solar cells.

Laser-Doped Back-Contact Solar Cells

We present laser-doped interdigitated back-contacted solar cells with a record efficiency $\bm{\eta}$ = 22.0%. The high versatility and spatial resolution of our laser doping process enable local n-type and p-type doping with a precision below 30 $\mu \hbox{m}$ and avoid any masking for doping. Nevertheless, the presented solar cells use photolithography (masking) to define the contact area and metallization layout. Quasi-steady-state photoconductance measurements prove the low-saturation current density of the laser doping. We process solar cells with varied pitch and emitter fraction and compare their measured current density–voltage characteristics with a 3-D solar cell simulation. The good agreement of the simulation and experimental data allows a reliable efficiency forecast when optimizations are applied. Furthermore, the influence of the base-busbar region on solar cell performance is discussed.

A simulation study on the electrical structure of interdigitated back-contact silicon solar cells

In this paper, a simulation for interdigitated back-contact (IBC) silicon solar cells was performed by using Silvaco TCAD ATLAS to investigate the cell’s electrical properties. The impacts of various parameters, including the depth of the front surface field(FSF), the FSF peak doping concentration, the depths of the emitter and the back surface field(BSF), the peak doping concentrations of the emitter and BSF, the base doping, and the bulk lifetime on the output characteristics like the light current-voltage curves and the internal quantum efficiency of the IBC solar cell, were investigated. The light absorption was determined by adjusting the antireflection coating and the Al thickness. The FSF must be thin and have a low doping concentration for high-efficiency IBC cells. If the conversion efficiency is to be improved, a thick emitter and a high doping concentration are needed. Because of the low resistivity of the Si substrate, the series resistance was reduced, but recombination was increased. With a high-resistivity Si substrate, the opposite trends were observed. By counter-balancing the series resistance and the recombination, we determined by simulation that the optimized resistivity for the IBC cells was 1 Ω·cm. Because all metal electrodes in the IBC cells are located on the back side, a higher minority carrier lifetime showed a higher efficiency. After the various parameters had been optimized, texturing and surface recombination were added into the simulation. The simulated IBC cells showed a short-circuit current density of 42.89 mA/cm2, an open-circuit voltage of 714.8 mV, a fill factor of 84.04%, and a conversion efficiency of 25.77%.

Ultrathin interdigitated back-contacted silicon solar cell with light-trapping structures of Si nanowire arrays

With the very high efficiency that has been achieved in silicon-based solar cells, the international technology roadmap for photovoltaics foresees a steady decrease in the thicknesses of such cells over the next decade. In this paper, we present an ultrathin interdigitated back-contacted silicon solar cell fabricated using 30-μm-thick Si substrates. In consideration of the special light-trapping and passivation requirements for ultrathin wafers, Si nanowire arrays coated with Al2O3 were used to significantly reduce the reflectance in the visible region of the solar spectrum. The 15-nm-thick conformal Al2O3 coating improved the effective minority carrier lifetime of the silicon nanowires and exhibited competitive passivation performance. Furthermore, the photovoltaic properties of the fabricated ultrathin solar cell were investigated and a relatively high conversion efficiency of 16.61% was determined for a thickness of 30μm. The findings of this study confirm the feasibility of producing ultrathin silicon-based photovoltaic devices.

Passivated contacts to laser doped p+ and n+ regions

In this work, tunnel SiO2/a-Si:H stacks are trialed as passivated contacts to laser doped p+ and n+ regions. The passivation performance and contact resistivity are investigated as a function of the tunnel SiO2 thickness and annealing condition. We find that the SiO2/a-Si:H stack provides excellent passivation to laser doped n+ regions, with corresponding low recombination current density (Jo) values. A lower level of surface passivation is achieved by the SiO2/a-Si:H stack on laser doped p+ regions. A post-deposition forming gas anneal (FGA) at 400°C is found to improve the passivation performance to laser doped p+ regions and deteriorate the passivation to laser doped n+ regions. Acceptable contact resistivity (ρc) values have been obtained for both laser doped n+ and p+ regions after aluminum metallization and a post FGA to activate the alloying process between the a-Si:H and aluminum layer. In the final part of this work implementation of the passivated contacts to laser doped regions into a simplified interdigitated back-contact (IBC) solar cell fabrication process is proposed. Simulation result suggests that IBC device with an efficiency of up to 23% can be achieved using the obtained experimental results.

Analysis of the impact of geometrical and technological parameters on recombination losses in interdigitated back-contact solar cells

This paper presents a detailed analysis of the recombination losses in an interdigitated back-contact (IBC) solar cell by means of three-dimensional numerical simulation. In particular, we discuss about the influence of geometrical and technological parameters such as the bulk thickness, the emitter contact fraction and the passivation effectiveness of the gap region on the saturation current density and on the carrier collection efficiency at region and mechanism-wise level. Moreover, the simulation results in terms of main figures of merit of the solar cell are reported and discussed. The paper shows that, except for the parasitic resistive losses, the optimum contact fraction at the emitter and base strongly depends on the presence of physical competing mechanisms, such as the internal optical bottom reflectivity and the recombination losses at the passivated emitter and base. In addition, the study underlines the critical role played by the passivation properties of the gap region, which may potentially be detrimental in terms of Fill-Factor and conversion efficiency.

Understanding the Influence of Busbars in Large-Area IBC Solar Cells by Distributed SPICE Simulations

In this paper, we model a large-area high-efficiency interdigitated back-contact (IBC) solar cell by means of a distributed electrical network. The simulation tool allows accounting for the distributed resistive effects in diffusions and metallization. The model also considers the electrical shading effect and resistive losses due to both back-surface field (BSF) and emitter busbars. A calibrated model is used to investigate the case of a large-area (15.6 × 15.6 cm2) IBC cell, in which we investigate the influence of key busbar parameters: number of busbars, busbar width, soldering pitch (for module connection), and metal sheet resistance. The predictive simulations allow finding out the optimum number of busbars, arising from a tradeoff between the electrical shading effect due to the BSF busbars and resistive losses due to the emitter busbars and the fingers. Moreover, we show how the distance between soldering points on the metal busbars influences the choice of the busbar width. We found out that if an adequate number (>7) of soldering points is adopted, the busbar width should be kept lower than 0.5 mm. On the other hand, the adoption of a thick Cu-plating (15 μm) leads to an increase of efficiency of $0.2\%_{{\rm abs}}$ with respect to the case of sputtered Al metal (3 μm thick).

Simulation of interdigitated back contact solar cell with trench structure

We performed two-dimensional technology computer-aided design simulations for interdigitated back contact (IBC) solar cells with rear trench structures (TS), denoted here as TS-IBC solar cells. First, we calculated a reference simulation model for conventional IBC solar cells. We then assumed a trench structure at the rear surface of the IBC solar cell. For this structure, we analyzed solar cell performance as a function of various trench depths and type. It was found that emitter trench formation affects minority carrier collection, such that the short-circuit current density increases with increasing trench depth. However, the back-surface field (BSF) trench exhibited poor minority carrier collection, which reduced the conversion efficiency of the TS-IBC solar cells. It was also found that for the same trench depth (30 μm), the difference in conversion efficiencies of an IBC solar cell with an emitter trench and that with a BSF trench was 0.6%. We are thus convinced that the emitter trench structure is more important than the BSF trench structure.

Ion-Implanted Laser-Annealed p+ and n+ Regions: A Potential Solution for Industrially Feasible High-Efficiency N-Type Interdigitated Back-Contact Solar Cells

The main challenge for interdigitated back-contact (IBC) solar cells is to reduce the fabrication complexity, which consists of multiple high-temperature processing and patterning steps. Patterned ion implantation has been proposed to simplify the manufacture of IBC solar cells, and the annealing of boron and phosphorus implanted areas is still a problem for the application. In this study, a new method consisting of laser annealing and a subsequent low-temperature oxidation (LA&OX) has been developed to co-anneal boron implanted p + and phosphorus implanted n + regions by a single step. We found that an additional laser annealing before oxidation could improve the electrical properties of boron-implanted p + regions effectively; however, it has almost no effect on the phosphorus-implanted n + regions. An industrially feasible IBC solar cell fabrication technology has been proposed based on the patterned ion implantation and LA&OX processing. The main fabrication steps of the IBC solar cell could be reduced to ten steps, and only one high-temperature oxidation step is required. As-designed IBC cell shows a potential efficiency higher than 23% according to simulations with the experimental parameters.

CMOS-Enabled Interdigitated Back-Contact Solar Cells for Biomedical Applications

We demonstrate a 980-nm laser-driven CMOS-enabled interdigitated back-contact (IBC) solar cell for biomedical applications. The design of this device leverages from the CMOS process to allow 2-D junction formation and uniform series resistance, but suffers from poor minority carrier properties in bulk substrate. This issue is partially solved in this paper by thinning down the substrate to 60 μm, leading to an ultimate efficiency of 15%. After inserting an emulated tissue loss, an 1.67-mm 2 IBC solar cell is capable of generating an electrical power of 159 μW under a concentrated illumination intensity of 10 mW, which is within the conservative limit to human skin.

Design, fabrication and characterisation of a 24.4% efficient interdigitated back contact solar cell

AbstractThe interdigitated back contact (IBC) solar cells developed at the Australian National University have resulted in an independently confirmed (Fraunhofer Institut für Solare Energiesysteme (ISE) CalLab) designated‐area efficiency of 24.4 ± 0.7%, featuring short‐circuit current density of 41.95 mA/cm2, open‐circuit voltage of 703 mV and 82.7% fill factor. The cell, 2 × 2 cm2 in area, was fabricated on a 230 µm thick 1.5 Ω cm n‐type Czochralski wafer, utilising plasma‐enhanced chemical vapour deposition (CVD) SiNx front‐surface passivation without front‐surface diffusion, rear‐side thermal oxide/low‐pressure CVD Si3N4 passivation stack and evaporated aluminium contacts with a finger‐to‐finger pitch of 500 µm. This paper describes the design and fabrication of lab‐scale high‐efficiency IBC cells. Characterisation of optical and electronic properties of the best produced cell is made, with subsequent incorporation into 3D device modelling used to accurately quantify all losses. Loss analysis demonstrates that bulk and emitter recombination, bulk resistive and optical losses are dominant and suggests a clear route to efficiency values in excess of 25%. Additionally, laser processing is explored as a means to simplify the manufacture of IBC cells, with a confirmed efficiency value of 23.5% recorded for cells fabricated using damage‐free deep UV laser ablation for contact formation. Meanwhile all‐laser‐doped cells, where every doping and patterning step is performed by lasers, are demonstrated with a preliminary result of 19.1% conversion efficiency recorded. Copyright © 2014 John Wiley & Sons, Ltd.

Ozone Base Cleaning: Impact on High Efficiency Interdigitated Back Contact Solar Cells

Interdigitated back contacted (IBC) silicon solar cells have long been shown as high conversion efficiency solar cells, with values as high as 25.0% reported by Sunpower [1]. Such high efficiencies are enabled by the cell structure, with the absence of front side metal shading, which can then be optimised from optical and passivation perspectives. High bulk lifetime silicon and a low series resistance metallisation scheme can also enhance the efficiencies. Other advantages include the easier module incorporation given both metal contacts are located on the rear of the cell.

3-D Simulation of Interdigitated-Back-Contact Silicon Solar Cells With Quokka Including Perimeter Losses

An interdigitated-back-contact (IBC) version of Quokka, a recently developed free and fast solar cell simulation program, is presented. It is capable of simulating IBC unit cells with a variety of interdigitated contact and diffusion patterns in both 2-D and 3-D. The program is evaluated by comparing simulated and experimental current–voltage ( I–V ) curves of high-efficiency IBC solar cells. The simulations include the perimeter effects of edges and busbars by simulating the inner unit cell in 3-D, and accounting for the edges and busbars by 2-D unit cell approximations. The simulation agrees well with the experiment under 1-sun conditions with different aperture areas. Furthermore, simulations of the inner unit cell are successfully validated against Sentaurus Device, for both the I–V curve and detailed free energy losses at maximum power point. The results evidence the validity of the quasi-neutral and conductive-boundary approximations employed by Quokka for fast simulation of IBC solar cells.

Heterojunction Interdigitated Back-Contact Solar Cells Fabricated on Wafer Bonded to Glass

Future wafer-based silicon solar cells will be fabricated on thin (<;140 μm) wafers. However, technologies to handle thin wafers during cell processing are not yet available for industry. In this paper, a flow to handle thin wafers during rear side cell processing is developed and demonstrated on 4-in 200 μm-thick wafers. The flow involves bonding the wafers to glass after front-side processing followed by a low-temperature p-n heterojunction formation on the rear side. 2.5 × 2.5 cm2 amorphous/crystalline silicon heterojunction interdigitated back-contact solar cells are fabricated by use of lithography while bonded to glass, and they show an efficiency of up to 17.7%. Shunts, infrared light absorption, and rear side interface passivation are identified as the main efficiency losses. Dedicated experiments suggest that the passivation losses are related to the degradation of the adhesive during wafer cleaning. Hence, methods to improve the compatibility of the adhesive with the cleaning process are discussed.

Potential-induced Degradation at Interdigitated Back Contact Solar Cells

Potential-induced degradation (PID) is characterized by the power loss of solar modules under high voltage stress across the module layer stack between framing/glass surface and solar cells. Standard silicon solar cells with a front side emitter may suffer from PID through massive shunting (PID-s) under high voltage stress conditions. PID was also reported in the past for cell concepts with a local emitter at the back side. For this case the underlying physical mechanism is not fully understood.In this contribution the PID effect is investigated for interdigitated back contact solar cells (IBC cells). Parts of the front side of the cells are exposed to high-voltage stress using a recently developed cell test setup at variable temperature, voltage and polarity. Cells are investigated by means of electroluminescence (EL), IR-thermography, illuminated and dark I-V measurements before as well as after PID tests. Cell fragments are investigated after PID stressing using the electron beam induced current (EBIC) method in combination with scanning electron microscopy (SEM).PID tests with a positive voltage respect to the grounded cell cause a locally degraded EL signal in the region where the PID stress was applied. In contrast, PID tests with the opposite polarity do not affect the EL behavior at all. I-V curves and thermography images indicate that PID stress does not significantly increase Rshunt. The local decrease of the EL intensity indicates increased non-radiative recombination. SEM/EBIC reveals neither local shunts nor distinct local degradation of the EBIC signal that could be attributed to PID tests with positive voltage.The results indicate a degradation process related to a degradation of the front side passivation layer (PID-p), in contrast to the well-known PID-s effect. Based on the results a model concept for PID-p of IBC solar cells is proposed. Accordingly, the potential impact on the module power output under the influence of high voltage stress is assessed.