Interdigitated Back Contact Silicon Heterojunction Research Articles

Interdigitated‐back‐contacted silicon heterojunction solar cells featuring novel MoOx‐based contact stacks

AbstractThe fabrication process of interdigitated‐back‐contacted silicon heterojunction (IBC‐SHJ) solar cells has been significantly simplified with the development of the so‐called tunnel‐IBC architecture. This architecture utilizes a highly conductive (p)‐type nanocrystalline silicon (nc‐Si:H) layer deposited over the full substrate area comprising pre‐patterned (n)‐type nc‐Si:H fingers. In this context, the (p)‐type nc‐Si:H layer is referred to as blanket layer. As both electrodes are connected to the same blanket layer, the high lateral conductivity of (p)nc‐Si:H layer can potentially lead to relatively low shunt resistance in the device, thus limiting the performance of such solar cells. To overcome such limitation, we introduce a thin (<2 nm) full‐area molybdenum oxide (MoOx) layer as an alternative to the (p)nc‐Si:H blanket layer. We demonstrate that the use of such a thin MoOx minimizes the shunting losses thanks to its low lateral conductivity while preserving the simplified fabrication process. In this process, a novel (n)‐type nc‐Si:H/MoOx electron collection contact stack is implemented within the proposed solar cell architecture. We assess its transport mechanisms via electrical simulations showing that electron transport, unlike in the case of tunnel‐IBC, occurs in the conduction band fully. Moreover, the proposed contact stack is evaluated in terms of contact resistivity and integrated into a proof‐of‐concept front/back‐contacted (FBC) SHJ solar cells. Contact resistivity as low as 100 mΩcm2 is achieved, and fabricated FBC‐SHJ solar cells obtain a fill factor above 81.5% and open‐circuit voltage above 705 mV. Lastly, the IBC‐SHJ solar cells featuring the MoOx blanket layer are fabricated, exhibiting efficiencies up to 21.14% with high shunt resistances above 150 kΩcm2. Further optimizations in terms of layer properties and fabrication process are proposed to improve device performance and realize the efficiency potential of our novel IBC‐SHJ solar cell architecture.

Process-induced losses by plasma leakage in lithography-free shadow masked interdigitated back contact silicon heterojunction architectures

Process-induced losses by plasma leakage in lithography-free shadow masked interdigitated back contact silicon heterojunction architectures

Low-Resistance Hole Contact Stacks for Interdigitated Rear-Contact Silicon Heterojunction Solar Cells

Achieving low contact resistivity for the p-contact in silicon heterojunction (SHJ) solar cells is challenging when classic n-type transparent conductive oxides (TCOs), such as indium tin oxide (ITO), are used in the contact stack. Here, we report on SHJ solar cells with interdigitated back-contact (IBC) and a direct aluminum (Al) metallization applied to the p-contact. We find that carefully annealing an Al/a-Si:H(p) (p-type amorphous silicon) contact at moderate temperatures leads to a specific contact resistivity that is half as low as its silver (Ag)/ITO counterpart. This is explained by Al diffusing into a-Si:H(p) upon temperature treatment, forming a partially crystallized aluminum silicide layer. For a sufficiently high doping level in a-Si:H(p), this enables an efficient tunnel-recombination of holes from a-Si:H(p) to the Al contact. An estimate for this tunneling-dominated specific contact resistivity is calculated as a function of the interface doping density. Best fabricated IBC SHJ solar cells with Al p-contact yield a fill factor of 77.5% and a power conversion efficiency of 22.3%. The main differences to devices with an Ag/ITO/a-Si:H(p) contact stack are a decrease in open-circuit voltage by 14 mV and a slightly higher series resistance ( R s). While the first aspect can be ascribed to increased interface recombination, the second one is unexpected and requires further investigation. Interestingly, omitting an intermediate TCO does not lead to current losses in devices with Al contacts, which is further investigated by optical simulations. Finally, electrical equivalent circuit simulations are conducted to describe the electrical behavior of the investigated devices.

Application of a Genetic Algorithm in Four-Terminal Perovskite/Crystalline-Silicon Tandem Devices

Perovskite/crystalline-silicon (c-Si) tandem solar cells offer a viable roadmap for reaching power conversion efficiencies beyond 30%. In this configuration, however, the silicon cell now mainly receives an infrared rich illumination spectrum where the absorption coefficient of silicon is poor. To boost the light absorption in this wavelength interval, transmitted through the top cell, a solution is to introduce a dedicated nanoscale texture on the front side of the silicon bottom cell and tweaking the optical elements in its design. These optical elements are manifold, with tunable geometrical dimensions, layer thicknesses, and refractive indices. For optically optimizing nanostructured silicon solar cells, electromagnetic wave solving methods, such as the rigorous coupled-wave analysis (RCWA), are normally used but they become computationally infeasible when several design variables are involved in an optimization problem. In this work, a natural selection algorithm, known as the genetic algorithm, is coupled with RCWA for extracting the optimal values of various design parameters in four-terminal perovskite/c-Si tandem devices in parallel. Both two-side-contacted and interdigitated back-contacted silicon heterojunction cell structures, featuring an inverse nanopyramid grating texture on the front, are optimized for five interdependent variables using a genetic algorithm for the bottom cell application and benchmarked against their random pyramid textured analogs. Our study shows that an optimized inverse nanopyramid grating texture can outperform the standard random pyramid texture when considering incidence angle variations. More importantly, the study illustrates how a genetic algorithm can support modeling complex solar cell structures with numerous degrees of freedom.

Three-Terminal Perovskite/Silicon Tandem Solar Cells with Top and Interdigitated Rear Contacts

We present a three-terminal (3T) tandem approach for the interconnection of a perovskite top cell with an interdigitated back-contact (IBC) silicon heterojunction (SHJ) bottom cell. The general via...

Lithography-free and dopant-free back-contact silicon heterojunction solar cells with solution-processed TiO2 as the efficient electron selective layer

Lithography-free interdigitated back-contact silicon heterojunction (IBC-SHJ) solar cells with dopant-free metal oxides (TiO2 and MoOx) as the carriers selective transport layers were investigated. Spin-coating and hot-wire reactive-sublimation deposition together with low cost mask technology were used to fabricate the solar cells. Insertion of a SiOx layer with the thickness of about 2.4 nm between the intrinsic amorphous Si (a-Si:H(i)) passivation layer and the spin-coated TiO2 layer greatly improves the solar cell performance due to the enhanced field-effect passivation of the a-Si:H(i)/SiOx/TiO2 layer stack. Efficiency up to 20.24% was achieved on the lithography-free and dopant-free IBC-SHJ devices with a-Si:H(i)/SiOx/TiO2 layer stack as the electron selective transport layer, a-Si:H(i)/MoOx as the hole selective transport layer, and WOx as the antireflection layer. The novel IBC-SHJ solar cells show significant advantages in simplification of the technology and process compared with the IBC-SHJ devices whose back surface pattering and carrier selective layers relied on photolithography and plasma enhanced chemical vapor deposition (PECVD).

The n-type Si-based materials applied on the front surface of IBC-SHJ solar cells**Project supported by the National Key Research Program of China (Grant Nos. 2018YFB1500500 and 2018YFB1500200), the National Natural Science Foundation of China (Grant Nos. 51602340, 51702355, and 61674167), and JKW Project, China (Grant No. 31512060106).

Interdigitated back contact silicon hetero-junction (IBC-SHJ) solar cells exhibit excellent performance owing to the IBC and SHJ structures. The front surface field (FSF) layer composed of electric field passivation and chemical passivation has been proved to play an important role in IBC-SHJ solar cells. The electric field passivated layer n+-a-Si: H, an n-type Si alloy with carbon or oxygen in amorphous phase, is simulated in this study to investigate its effect on IBC-SHJ. It is indicated that the n+-a-Si: H layer with wider band gap can reduce the light absorption on the front side efficaciously, which hinders the surface recombination of photo-generated carriers and thus contributes to the improvement of the short circuit current density Jsc. The highly doped n+-a-Si: H can result in the remakable energy band bending, which makes it outstanding in the field passivation, while it makes little contribution to the chemical passivation. It is noteworthy that when the electric field intensity exceeds 1.3 × 105 V/cm, the efficiency decrease caused by the inferior chemical passivation is only 0.16%. In this study, the IBC-SHJ solar cell with a front n+-a-Si: H field passivation layer is simulated, which shows the high efficiency of 26% in spite of the inferior chemical passivation on the front surface.

A new interdigitated back contact silicon solar cell with higher efficiency and lower sensitivity to the heterojunction defect states

A new interdigitated back contact silicon heterojunction (IBC-SHJ) solar cell with two additional buffer layers is investigated. The buffer layers are two thin p-type crystalline silicon (c-Si) layers placed at front and back surface of c-Si/a-Si:H hetero-junction interfaces. Improvement of the overall efficiency and decrease in sensitivity of the output parameters to c-Si/a-Si:H interface defect density through the controlling of carrier concentration by introducing the new structure is achieved. The impact of new buffer layers on the cell performance is studied with numerical modeling. The main output parameters of the IBC-SHJ solar cell including buffer layers are compared with the conventional IBC-SHJ solar cell. It is found that the new IBC-SHJ solar cell has a higher fill factor due to reduced series resistance. With a deeper study on the impact of the buffer layers, it is realized that the field effect passivation has a key role in the improvement of the new cell performance. By modifying the energy band structure through the buffer layers, recombination at the c-Si/a-Si:H heterojunction interfaces is reduced due to the field effect passivation of the c-Si surface, and the efficiency of the solar cell is increased. Optimizing doping concentration and thickness of the buffer layers leads to an increase of 5% in fill factor and an increase of 1.2% in efficiency compared with the conventional similar solar cell.

Potential of interdigitated back-contact silicon heterojunction solar cells for liquid phase crystallized silicon on glass with efficiency above 14%

Liquid phase crystallization of silicon (LPC-Si) on glass is a promising method to produce high quality multi-crystalline Si films with macroscopic grains. In this study, we report on recent improvements of our interdigitated back-contact silicon heterojunction contact system (IBC-SHJ), which enabled open circuit voltages as high as 661mV and efficiencies up to 14.2% using a 13µm thin n-type LPC-Si absorbers on glass. The influence of the BSF width on the cell performance is investigated both experimentally and numerically. We combine 1D optical simulations using GenPro4 and 2D electrical simulations using Sentaurus™ TCAD to determine the optical and electrical loss mechanisms in order to estimate the potential of our current LPC-Si absorbers. The simulations reveal an effective minority carrier diffusion length of 26µm and further demonstrate that a doping concentration of 4 × 1016cm−3 and a back surface field width of 60µm are optimum values to further increase cell efficiencies.

Selective Dry Etching of p-Type Si Films for Photolithography Processing of Interdigitated Back Contact Silicon Heterojunction Solar Cells

Highly doped p-type Si thin films are hardly etched by alkaline etchants unlike i- and n-type Si films. In addition, the use of a HNO3/HF/H2O mixture to etch p-type Si films can lead to inhomogeneous etching of p-type Si films and consequently result in excessive surface roughness and inferior surface passivation. For these reasons, we have implemented a dry etching method to selectively pattern p-type Si films for interdigitated back contact (IBC) silicon heterojunction (SHJ) solar cells using a SiO x masking layer. In this way, the underlying passivation layer and the Si surfaces can be undamaged. In this study, the etch rates of all Si films that make up IBC-SHJ solar cells have been investigated in order to obtain high etch selectivity and establish proper patterning steps. Furthermore, we have fabricated IBC-SHJ solar cells on planar wafers by using dry and wet patterning methods developed in this study. As a result, a conversion efficiency of 15.7% is obtained with V oc of 682 mV, J sc of 33.8 mA/cm2, and FF of 0.68.

Technology computer aided design of 29.5% efficient perovskite/interdigitated back contact silicon heterojunction mechanically stacked tandem solar cell for energy-efficient applications

A 29.5% efficient perovskite/SiC passivated interdigitated back contact silicon heterojunction (IBC-SiHJ) mechanically stacked tandem solar cell device has been designed and simulated. This is a substantial improvement of 40% and 15%, respectively, compared to the transparent perovskite solar cell (21.1%) and Si solar cell (25.6%) operated individually. The perovskite solar cell has been used as a top subcell, whereas 250- and 25-μm-thick IBC-SiHJ solar cells have been used as bottom subcells. The realistic technology computer aided design analysis has been performed to understand the physical processes in the device and to make reliable predictions of the behavior. The performance of the top subcell has been obtained for different acceptor densities and hole mobility in Spiro-MeOTAD along with the impact of counter electrode work function. To incorporate the effect of material quality, the influence of carrier lifetimes has also been studied for perovskite top and IBC-SiHJ bottom subcells. The optical and electrical behavior of the devices has been obtained for both standalone as well as tandem configuration. Results reported in this study reveal that the proposed four-terminal tandem device may open a new door for cost-effective and energy-efficient applications.

Simulation and optimization of n-type interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cell structure using Silvaco Tcad Atlas

Simulation models of interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cells, not only help in understanding the cell behavior in line with the experimental results but also help further in predicting the cell performance, adding to the cost effectiveness in the cell processing.IBC-SiHJ solar cells that combine the hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) heterojunction and interdigitated back contact (IBC) concepts are very promising in order to reach the highest one-junction efficiency (η). In this paper, we have studied these solar cells by two dimensional modeling using Silvaco Tcad Atlas software which has recently extended its capability to simulate these devices and given accurate bulk and interface complex defect models and allowed special specification of transport physics for the hetero-interface. The study has been done on the IBC-SiHJ structure based on n-type crystalline silicon (c-Si) by introducing a very thin intrinsic hydrogenated amorphous silicon (i-a-Si:H) layer between the c-Si base and the doped a-Si:H layers and varying the values of the following parameters: c-Si substrate and back-surface field (BSF) doping concentration, thickness of i-a-Si:H layer (Thi-a-Si) and rear side geometry. The impact of these parameters has been tested by generating the current–voltage characteristics under illumination. It is shown that the open circuit voltage (VOC) and η of IBC-SiHJ solar cells increase with decreasing i-a-Si:H thickness. The η improves further with the increase of p-type emitter width (2Wp), the decrease of n-type BSF width (2Wn) and gap width (Wg) which are explained by the simulation. The S-shaped J–V curves with low fill factor (FF) observed previously in experiments are confirmed by simulation. To improve FF, Thi-a-Si and Wg should decrease. Results indicate that to achieve high η, c-Si substrate and BSF doping concentration must be optimized. The Wg (spacing between BSF and the emitter) must be kept as small as possible to avoid recombination of minority carriers in the base. The optimum geometry corresponds to a minimum size BSF region and a maximum size emitter region. With these optimizations, an enhanced η 23.20% is demonstrated by the simulation.

Simulation of interdigitated back-contact silicon heterojunction solar cells with quantum transport model

A simulation of interdigitated back-contact silicon heterojunction (IBC-SHJ) solar cells was performed using a quantum transport model to consider the quantum effect at the crystalline/amorphous (c/a) heterojunction interface. It was found that the impact of the quantum effect on the open-circuit voltage is comparable to that of the interface defect density at the c/a interface, indicating the importance of implementation of the quantum model. The optimal back-contact design was also discussed from the viewpoint of mass production, in which the design rule is relaxed. The degradation of the conversion efficiency by widening the gap between the p- and n-aSi:H layers can be compensated by improving passivation quality at the c/a interface.

Laser‐fired contact for n‐type crystalline Si solar cells

AbstractLaser‐fired contacts to n‐type crystalline silicon were developed by investigating novel metal stacks containing Antimony (Sb). Lasing conditions and the structure of metals stacks were optimized for lowest contact resistance and minimum surface damage. Specific contact resistance for firing different metal stacks through either silicon nitride or p‐type amorphous silicon was determined using two different models and test structures. Specific contact resistance values of 2–7 mΩcm2 have been achieved. Recombination loss due to laser damage was consistent with an extracted local surface recombination velocity of ~20 000 cm/s, which is similar to values for laser‐fired base contact for p‐type crystalline silicon. Interdigitated back contact silicon heterojunction cells were fabricated with laser‐fired base contact and proof‐of‐concept efficiencies of 16.9% were achieved. This localized base contact technique will enable low cost back contact patterning and innovative designs for n‐type crystalline solar cell. Copyright © 2014 John Wiley & Sons, Ltd.

Approach for a Simplified Fabrication Process for IBC-SHJ Solar Cells with High Fill Factors

In this paper, interdigitated back contacted silicon heterojunction (IBC-SHJ) solar cell results as well as two dimensional device simulations are presented. The simulation indicates that for the minority contact (emitter) the coverage of the metallisation should be nearly 100%, while this is less critical for the majority contact (BSF). We present experimental results for an IBC-SHJ solar cell with a metallisation fraction of 100% for the emitter and approximately 65% for the BSF, with a high fill factor of 77.7% and an independently confirmed energy conversion efficiency of 19.4%.

Point Contact Technology for Silicon Heterojunction Solar Cells

This study deals with the introduction of Point Contact technology to enhance the efficiency of Interdigitated Back Contact (IBC) Silicon Heterojunction (Si-HJ) solar cells. For the study of point contacted Emitter and Back Surface Field (BSF), respectively Rear emitter (RE) Si-HJ cells and conventional Si-HJ cells are fabricated with different contact area fractions and different Emitter and BSF layers. It is shown that point contact area fraction should be above 16% to avoid performance losses for the different emitter stack. Emitter stack p-doped amorphous silicon on top of intrinsic amorphous silicon seems to be the most promising layer since it allows very high surface passivation level. Concerning point contacted BSF, further improvement of passivating stack and contact layer is still needed to allow an enhancement of cell efficiency. With an optimized geometry and further improvement of the passivating and contact layers, such structures may be suitable for an application on IBC Si-HJ devices.

Interdigitated Back-Contacted Silicon Heterojunction Solar Cells With Improved Fill Factor and Efficiency

We compare recently reported results of efficient back-contacted amorphous/crystalline silicon heterojunction solar cells with fill factors up to 78.8% with calculated j-V characteristics that are derived from an area-weighted summation of recombination currents taken from lifetime measurements. We compare solar cell structures with and without an intrinsic buffer layer beneath the p-type amorphous silicon emitter. We find that the inclusion of the buffer layer reduces the fill-factor potential by changing the ideality of the recombination current. However, analyzing the series resistance by illumination-dependent j-V-measurements, we find that the major loss mechanism of the fill factor is the limitation of the charge-carrier transport.

Development of Interdigitated Back Contact Silicon Heterojunction (IBC Si-HJ) Solar Cells

Abstract This study deals with the development and optimization of Interdigitated Back Contact (IBC) Silicon Heterojunction (Si-HJ) solar cells based on n-type crystalline silicon (c-Si) substrates. Both one-dimensional (1D) and 2D aspects of IBC Si-HJ cells are explored in this work. Rear Emitter (RE) Si-HJ cells are fabricated to study the influence of the emitter stack on 1D resistive losses. It is shown that the rear emitter stack has to be carefully designed to maintain a high surface passivation level without inducing series resistance (RSeries). On IBC Si-HJ structures, the influence of 2D features such as emitter contact fraction are confirmed both experimentally and by modeling. Using the screen printing technology, 25 cm2 IBC Si-HJ structures have been fabricated with an efficiency of 15.7%.

Effects of the front surface field in n-type interdigitated back contact silicon heterojunctions solar cells

Abstract Previous simulations of interdigitated back contact silicon heterojunction (IBC-SiHJ) solar cells have indicated that front surface passivation is a critical factor in the performance of such cells. This is why we here focus on the effect of a front surface field (FSF) layer by 2D numerical modelling. A FSF layer made of a highly doped thin crystalline silicon top layer makes the cell performance insensitive to the surface recombination velocity up to quite high values (5000 cm/s). It also reduces the lateral resistance losses due to the increased lateral current through the doped layer particularly in IBC-SiHJ solar cells with large pitches. A FSF layer can also be produced by doped hydrogenated amorphous silicon due to the induced accumulation layer at the crystalline silicon surface. The positive effect of such layer strongly depends on the a-Si:H/c-Si interface quality.

2D simulations of interdigitated back contact heterojunction solar cells based on n‐type crystalline silicon

AbstractThe combination of amorphous silicon/crystalline silicon heterojunction (a‐Si:H/c‐Si) concept with interdigitated back contact leads to a very promising structure for high efficiency solar cell. In this work, we study the interdigitated back contact silicon heterojunction (IBC‐SiHJ) structure by two‐dimensional numerical simulations and we focus on IBC‐SiHJ solar cells based on n‐type c‐Si. Assuming low surface recombination velocity (10 cm/s), we study the effect of the following parameters: bulk lifetime and doping concentration of c‐Si, density of defects at the a‐Si:H/c‐Si interface. The influence of these parameters has been tested by generating the current‐voltage (I‐V) curves. Results indicate that with a high crystalline substrate quality and low recombining a‐Si:H/c‐Si interfaces efficiencies approaching 25% can be reached. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)