Full-area Rear Research Articles

Crystalline Silicon (c-Si)-Based Tunnel Oxide Passivated Contact (TOPCon) Solar Cells: A Review

Contact selectivity is a key parameter for enhancing and improving the power conversion efficiency (PCE) of crystalline silicon (c-Si)-based solar cells. Carrier selective contacts (CSC) are the key technology which has the potential to achieve a higher PCE for c-Si-based solar cells closer to their theoretical efficiency limit. A recent and state-of-the-art approach in this domain is the tunnel oxide passivated contact (TOPCon) approach, which is completely different from the existing classical heterojunction solar cells. The main and core element of this contact is the tunnel oxide, and its main role is to cut back the minority carrier recombination at the interface. A state-of-the-art n-type c-Si-based TOPCon solar cell featuring a passivated rear contact was experimentally analyzed, and the highest PCE record of ~25.7% was achieved. It has a high fill factor (FF) of ~83.3%. These reported results prove that the highest efficiency potential is that of the passivated full area rear contact structures and it is more efficient than that of the partial rear contact (PRC) structures. In this paper, a review is presented which considers the key characteristics of TOPCon solar cells, i.e., minority carrier recombination, contact resistance, and surface passivation. Additionally, practical challenges and key issues related to TOPCon solar cells are also highlighted. Finally, the focus turns to the characteristics of TOPCon solar cells, which offer an improved and better understanding of doping layers and tunnel oxide along with their mutual and combined effect on the overall performance of TOPCon solar cells.

Electron‐Selective Contacts ATO/LiF/Al‐Based High‐Performance N‐Type Silicon Solar Cells (Adv. Mater. Interfaces 31/2022)

Silicon Solar Cells In article number 2201512, Zengguang Huang and colleagues prepared Al-doped TiO2 (ATO) thin films by atomic-layers-deposited (ALD) technique. The films were observed to possess the highest minority carrier lifetime of 253.0 μs and a significantly low contact resistivity ρc of 19.7 mΩ cm2 on solar-grade silicon. A highest efficiency of 19.9% was obtained for an n-type c-Si solar cell with a full-area rear ATO/LiF/Al electron-selective contact.

Aluminum–silicon interdiffusion in silicon heterojunction solar cells with a-Si:H(i)/a-Si:H(n/p)/Al rear contacts

We characterize a-Si:H(i)/a-Si:H(n)/Al and a-Si:H(i)/a-Si:H(p)/Al contacts implemented on the rear side of silicon heterojunction solar cells. Electrical test structures and full-area solar cells employing these contacts demonstrate promising performance. For example, a-Si:H(i)/a-Si:H(p)/Al test structures with a 40 nm thick a-Si:H(p) layer that were annealed at 180 °C had contact resistivities of 48 mΩ · cm2 and implied open-circuit voltage losses after metallization of only 9 mV. Similarly, solar cells with full-area rear a-Si:H(i)/a-Si:H(n)/Al contacts that were annealed at 150 °C had open-circuit voltages of 717 mV and contact resistivities of 9.4 mΩ · cm2. For thinner doped a-Si:H layers and higher annealing temperatures, the contacts become less stable and performance degrades. Complementary transmission electron microscopy and energy-dispersive x-ray spectroscopy analysis show the Al–Si interactions at these interfaces that explain the range of exhibited performance. This analysis leads to a better understanding of the materials properties limiting the contact stability.

Titanium Nitride Electron-Conductive Contact for Silicon Solar Cells By Radio Frequency Sputtering from a TiN Target.

Efficient and stable electron selective materials compatible with commercial production are essential to the fabrication of dopant-free silicon solar cells. In this work, we report an air-stable TiN (titanium nitride) polycrystalline film, deposited using radio frequency sputtering process, as an electron selective contact in silicon solar cells. TiN films deposited at 300 W and 1.5 mTorr exhibit a low contact resistivity of 2.0 mΩ·cm2. Furthermore, the main factors and mechanisms affecting the carrier selectivity properties are also explored. TiN layers as full area rear electron contacts in n-type silicon solar cells have been successfully implemented, even though TiN film contains some oxygen. This process yields a 17% increment in relative efficiency in comparison with reference devices (n-Si/Al contact). Hence, considering the low thermal budget, scalable technique, and low contact resistivity, the TiN layers can pave the way to fabricate high-efficiency selective contact silicon solar cells with a higher degree of reproducibility.

21.3%-efficient n-type silicon solar cell with a full area rear TiOx/LiF/Al electron-selective contact

In this work, we investigate an efficient electron-selective passivating contact with TiOx/LiF/Al contact structure, which offers both low surface recombination and specific contact resistance. Optimized TiOx layer thickness of 4 nm provides high quality surface passivation, achieving minority carrier lifetime of 3.03 ms on 5 Ω cm n-type wafers, with a saturated current density J0 of 23 fA/cm2. In addition, inserting a 1 nm LiF between the 4 nm TiOx and Al reduces the contact resistivity to 18 mΩ cm2. The low contact resistivity of TiOx/LiF/Al contact is attributed to barrier reduction from the low work function of LiF/Al stack. A champion solar cell efficiency of 21.3% has been achieved for an n-type crystalline silicon device with a full-area rear TiOx/LiF/Al contact, demonstrating the excellent potential of this passivating contact for fabricating high-efficiency silicon solar cells.

Hole-selective molybdenum oxide as a full-area rear contact to crystalline p-type Si solar cells

We examine thermally evaporated MoOx films as a full-area rear contact to crystalline p-type Si solar cells for efficient hole-selective contacts. Prior to front- and rear-metallization, the implied open-circuit voltage (iVoc) is evaluated to be 646 mV with implied fill factor (iFF) of 82.5% for the tunnel SiOx/MoOx rear contacted cell structure with the passivated emitter on the textured surface, showing it is possible to achieve an implied 1-sun efficiency of 20.8%. Numerical simulation reveals that the electron affinity (χ) of the MoOx material strongly influences the performance of the MoOx contacted p-Si cell. Simulated band diagrams show that the values in χ of the MoOx layer must be sufficiently high in order to lower junction recombination, indicating that the highest efficiency of 21.1% is achievable for a high χ of 5.6 eV of MoOx films and back surface recombination velocity of <100 cm/s at p-Si/MoOx.

Investigation of different contact geometries for partial rear metal contact of high-efficiency silicon solar cells

In this paper, we have investigated the dependence of the parameters of partial rear metal contact (PRC) solar cells on two commonly used geometrical configurations, viz. grid contact and square contact. It is demonstrated that while the geometry of rear metallic contact having the same fractional coverage changes the base spreading resistance significantly, rear surface passivation and back reflectance depend only on the fractional coverage of the real metal contact and are independent of the geometry of the contact. The performed analysis indicates that the base spreading resistance is much higher for square contact cells compared to that of grid contact cells having the same fractional coverage. While open-circuit voltage and short-circuit current are found to be essentially independent of the contact geometry, the fill factor is significantly affected by the geometry, indicating that the design of high-efficiency cells with partial rear grid contact is less critical in comparison to that of square contact cells for optimized performance. Results indicate that for a 180 μm cell, an efficiency enhancement of 14% is possible for <10% fractional rear metallization over the baseline efficiency of about 19% for solar cells having full-area rear metallization. For 50 μm thin cells, the corresponding improvement in efficiency is 17%, leading to 22% efficiency solar cells.

Numerical Modeling of c-Si PV Modules by Coupling the Semiconductor with the Thermal Conduction, Convection and Radiation Equations

Commonly, the thermal behavior of solar cell modules is calculated with analytical approaches using non wavelength-dependent optical data. Here, we employ ray tracing of entire solar modules at wavelengths of 300-2500nm to calculate heat sources. Subsequently, finite element method (FEM) simulations are used to solve the semiconductor equations coupled with the thermal conduction, thermal convection, and thermal radiation equations. The implemented model is validated with measurements from an outdoor test over the period of an entire year. Our ray tracing analysis of different solar modules under the AM.15G spectrum shows that, for a standard module about 18.9% of the sun's intensity becomes parasitically absorbed. A loss analysis shows that the biggest parasitic heat source is the cell's full-area rear side metallization. We hence propose the use of a SiNx layer as rear side mirror to reduce the parasitic absorption to 11.7%. This change can lead to a 3.2°C lower module operating temperature, which results in an about 5W higher electrical power output when considering a typical 260W module.

Simple silicon solar cells featuring an a-Si:H enhanced rear MIS contact

This paper presents the experimental demonstration of silicon solar cells that incorporate an enhanced MIS passivated contact scheme on a phosphorus diffused surface. By depositing intrinsic a-Si:H on an ultrathin SiOx layer and alloying with an overlying aluminium layer, the interface passivation has been vastly improved over that of conventional MIS contacts, whilst maintaining a low contact resistance. This paper focuses on the optimisation of the Al/a-Si:H alloying process and the influence of the tunnelling SiOx layer thickness. A conversion efficiency of 21.0% has been achieved for n-type cells fabricated with a front boron diffusion and a full area rear MIS passivated phosphorus diffusion. The cells exhibit a moderate Voc=666mV and FF=0.805, whereas Jsc 39.3mA/cm2 is relatively low due to a non-optimal antireflection coating and back surface reflector, and hence will be subject to further improvement.

Development of a Conductive Distributed Bragg Reflector for Heterojunction Solar Cells Using &lt;italic&gt;N &lt;/italic&gt;-Doped Microcrystalline Silicon and Aluminum-Doped Zinc Oxide Films

We report on the feasibility of integrating two conductive thin-film materials-n-doped hydrogenated microcrystalline silicon μc-Si:H(n) and Al-doped zinc oxide ZnO:Al-to form a conductive distributed Bragg reflector (DBR) at the rear of a silicon heterojunction solar cell, which simultaneously possesses high electrical conductance and high optical reflectance in the 900 ± 200 nm wavelength range. An optimization of the DBR is undertaken, considering the parasitic absorption in the thin films. Although an increased absorption loss is observed using the thicker films proposed by the DBR optimization, a significant increase in internal rear reflectance will compensate for this effect. If a full-area rear metal contact is used, in combination with a single DBR unit block, the rear's sheet resistance decreases from 100 to 40 Ω/□, and the weighted average reflection from 700 to 1100 nm increases from 79.5% to 88.2% as compared with the nonoptimized thicknesses for the conductive films.

Analysis of recombination losses in screen-printed aluminum-alloyed back surface fields of silicon solar cells by numerical device simulation

The Al-alloyed back surface field (Al-BSF) is a current topic in research and development of crystalline Si solar cells for mass production. We simulate the solar cells in full-size with numerical device modeling, based on incomplete ionization and updated parameters of the Al–O complex in the Al-BSF region, combined with SPICE circuit models. The simulations show that for full-area rear contact, the full-area Al-BSFs should optimally be about 6 μm deep; in shallower Al-BSFs, recombination at the metal contact dominates; and in deeper Al-BSFs, recombination via Al–O defects dominates. The recombination in the Al-BSF dominates the cell's total recombination in well optimized cells. The optimally attainable cell efficiency then is about 18.8% after degradation of the B–O complex in the boron-doped wafer, or 19.1% after deactivating the B–O complexes as completely as currently possible. In PERC cells, having screen-printed Al-BSF fingers and a rear Al2O3 passivation layer, cell efficiency reaches about 19.6% or 20.3% after degradation or deacivation, respectively, with an improved homogeneous emitter that is feasible for mass production, and with screen-printing front contacts. Ideally, the design strategies for PERC structure with various wafer resistivities and rear finger widths are shown. The reduction in cell efficiency due to three different types of voids (cavities) in local Al-BSFs is also quantified in detail.

Modeling, geometric optimization and isolation of the edge region in silicon solar cells

Commercial front-contact solar cells can often be represented as an equivalent circuit with two diodes. A first diode models low-level injection Shockley-Read-Hall recombination in the bulk of the solar cell and associated with an ideality factor of one, while the second diode models several loss mechanisms as depletion region recombination or edge recombination. The ideality factor of the second diode is most often greater than one, and in the case of edge recombination a value of two is observed in many cases. Even values above two can be explained by e.g. coupled defect level recombination [10]. As the associated impact of the second diode on the IV curve of a solar cell is generally strong, an optimization of the edge region is sought. This can be done through minimizing the dark saturation current of the edge as well as by geometrical optimization of the grid and edge isolation groove. In this paper, an equivalent circuit model for the edge region of a solar cell with full-area rear contact and H-grid front contacts is presented. Using approximations for the short-circuit current in the edge region, which are derived from numerical simulations, the model allows for the extraction of dark-saturation currents as well as for geometrical grid optimization. After validation of the model on experimental data, projections are made on a variety of solar cells, pointing out the influence of the edge design on the overall cell performance.

Optimizing annealing steps for crystalline silicon solar cells with screen printed front side metallization and an oxide‐passivated rear surface with local contacts

AbstractSilicon solar cells that feature screen printed front contacts and a passivated rear surface with local contacts allow higher efficiencies compared to present industrial solar cells that exhibit a full area rear side metallization. If thermal oxidation is used for the rear surface passivation, the final annealing step in the processing sequence is crucial. On the one hand, this post‐metallization annealing (PMA) step is required for decreasing the surface recombination velocity (SRV) at the aluminum‐coated oxide‐passivated rear surface. On the other hand, PMA can negatively affect the screen printed front side metallization leading to a lower fill factor. This work separately analyzes the impact of PMA on both, the screen printed front metallization and the oxide‐passivated rear surface. Measuring dark and illuminated IV‐curves of standard industrial aluminum back surface field (Al‐BSF) silicon solar cells reveals the impact of PMA on the front metallization, while measuring the effective minority carrier lifetime of symmetric lifetime samples provides information about the rear side SRV. One‐dimensional simulations are used for predicting the cell performance according to the contributions from both, the front metallization and the rear oxide‐passivation for different PMA temperatures and durations. The simulation also includes recombination at the local rear contacts. An optimized PMA process is presented according to the simulations and is experimentally verified. The optimized process is applied to silicon solar cells with a screen printed front side metallization and an oxide‐passivated rear surface. Efficiencies up to 18.1% are achieved on 148.8 cm2 Czochralski (Cz) silicon wafers. Copyright © 2009 John Wiley &amp; Sons, Ltd.