Mitigating Plasmonic Absorption Losses at Rear Electrodes in High‐Efficiency Silicon Solar Cells Using Dopant‐Free Contact Stacks

Although charge‐carrier selectivity in conventional crystalline silicon (c‐Si) solar cells is usually realized by doping Si, the presence of dopants imposes inherent performance limitations due to parasitic absorption and carrier recombination. The development of alternative carrier‐selective contacts, using non‐Si electron and hole transport layers, has the potential to overcome such drawbacks and simultaneously reduce the cost and/or simplify the fabrication process of c‐Si solar cells. Nevertheless, devices relying on such non‐Si contacts with power conversion efficiencies (PCEs) that rival their classical counterparts are yet to be demonstrated. In this study, one key element is brought forward toward this demonstration by incorporating low‐pressure chemical vapor deposited ZnO as the electron transport layer in c‐Si solar cells. Placed at the rear of the device, it is found that rather thick (75 nm) ZnO film capped with LiF_x/Al simultaneously enables efficient electron selectivity and suppression of parasitic infrared absorption. Next, these electron‐selective contacts are integrated in c‐Si solar cells with MoO_x‐based hole‐collecting contacts at the device front to realize full‐area dopant‐free‐contact solar cells. In the proof‐of‐concept device, a PCE as high as 21.4% is demonstrated, which is a record for this novel device class and is at the level of conventional industrial solar cells.     

A Polymer/Carbon‐Nanotube Ink as a Boron‐Dopant/Inorganic‐Passivation Free Carrier Selective Contact for Silicon Solar Cells with over 21% Efficiency

Traditional silicon solar cells extract holes and achieve interface passivation with the use of a boron dopant and dielectric thin films such as silicon oxide or hydrogenated amorphous silicon. Without these two key components, few technologies have realized power conversion efficiencies above 20%. Here, a carbon nanotube ink is spin coated directly onto a silicon wafer to serve simultaneously as a hole extraction layer, but also to passivate interfacial defects. This enables a low‐cost fabrication process that is absent of vacuum equipment and high‐temperatures. Power conversion efficiencies of 21.4% on an device area of 4.8 cm² and 20% on an industrial size (245.71 cm²) wafer are obtained. Additionally, the high quality of this passivated carrier selective contact affords a fill factor of 82%, which is a record for silicon solar cells with dopant‐free contacts. The combination of low‐dimensional materials with an organic passivation is a new strategy to high performance photovoltaics.     

20.1%‐efficient crystalline silicon solar cell with amorphous silicon rear‐surface passivation

We have developed a crystalline silicon solar cell with amorphous silicon (a‐Si:H) rear‐surface passivation based on a simple process. The a‐Si:H layer is deposited at 225°C by plasma‐enhanced chemical vapor deposition. An aluminum grid is evaporated onto the a‐Si:H‐passivated rear. The base contacts are formed by COSIMA (contact formation to a‐Si:H passivated wafers by means of annealing) when subsequently depositing the front silicon nitride layer at 325°C. The a‐Si:H underneath the aluminum fingers dissolves completely within the aluminum and an ohmic contact to the base is formed. This contacting scheme results in a very low contact resistance of 3.5 ±0.2 mΩ cm² on low‐resistivity (0.5 Ω cm) p‐type silicon, which is below that obtained for conventional Al/Si contacts. We achieve an independently confirmed energy conversion efficiency of 20.1% under one‐sun standard testing conditions for a 4 cm² large cell. Measurements of the internal quantum efficiency show an improved rear surface passivation compared with reference cells with a silicon nitride rear passivation. Copyright © 2005 John Wiley & Sons, Ltd.     

CdS/CdTe thin‐film solar cells with Cu‐free transition metal oxide/Au back contacts

Superstrate CdS/CdTe thin‐film solar cells with Cu‐free transition metal oxide (TMO)/Au and Au‐only back contacts have been fabricated. The TMOs include MoO_3‐x, V₂O_5‐x, and WO_3‐x. The incorporation of the TMO buffer layers at the back contacts resulted in significant improvement on open‐circuit voltage (V_OC) as compared with the cells with Cu‐free Au‐only back contacts. Among the cells using TMO buffer layers, the ones with MoO_3‐x buffer layers exhibited the best performance, yielding an efficiency of 14.1% under AM1.5 illumination with V_OC of 815 mV and a fill factor of 67.9%. Though the performance is slightly behind the best reference cell with a Cu/Au back contact fabricated in our lab with V_OC of 844 mV, fill factor of 76.3%, and efficiency of 15.7%, the use of Cu‐free back contacts may lead to improved long‐term cell stability. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental and simulated analysis of front versus all‐back‐contact silicon heterojunction solar cells: effect of interface and doped a‐Si:H layer defects

Front silicon heterojunction and interdigitated all‐back‐contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high efficiency and low cost because of their good surface passivation, heterojunction contacts, and low temperature fabrication processes. The performance of both heterojunction device structures depends on the interface between the crystalline silicon (c‐Si) and intrinsic amorphous silicon [(i)a‐Si:H] layer, and the defects in doped a‐Si:H emitter or base contact layers. In this paper, effective minority carrier lifetimes of c‐Si using symmetric passivation structures were measured and analyzed using an extended Shockley–Read–Hall formalism to determine the input interface parameters needed for a successful 2D simulation of fabricated baseline solar cells. Subsequently, the performance of front silicon heterojunction and IBC‐SHJ devices was simulated to determine the influence of defects at the (i)a‐Si:H/c‐Si interface and in the doped a‐Si:H layers. For the baseline device parameters, the difference between the two device configurations is caused by the emitter/base contact gap recombination and the back surface geometry of IBC‐SHJ solar cell. This work provides a guide to the optimization of both types of SHJ device performance, predicting an IBC‐SHJ solar cell efficiency of 25% for realistic material parameters. Copyright © 2013 John Wiley & Sons, Ltd.     

Laser processing of Al2O3/a‐SiCx:H stacks: a feasible solution for the rear surface of high‐efficiency p‐type c‐Si solar cells

We explore the potential of laser processing aluminium oxide (Al₂O₃)/amorphous silicon carbide (a‐SiC_x:H) stacks to be used at the rear surface of p‐type crystalline silicon (c‐Si) solar cells. For this stack, excellent quality surface passivation is measured with effective surface recombination velocities as low as 2 cm/s. By means of an infrared laser, the dielectric film is locally opened. Simultaneously, part of the aluminium in the Al₂O₃ film is introduced into the c‐Si, creating p+ regions that allow ohmic contacts with low‐surface recombination velocities. At optimum pitch, high‐efficiency solar cells are achievable for substrates of 0.5–2.5 Ω cm. Copyright © 2012 John Wiley & Sons, Ltd.     

Comparison of TiO2 and other dielectric coatings for buried‐contact solar cells: a review

This paper compares the optical, electronic, physical and chemical properties of dielectric thin films that are commonly used to enhance the performance of bulk silicon photovoltaic devices. The standard buried‐contact (BC) solar cell presents a particularly challenging set of criteria, requiring the dielectric film to act as: (i) an anti‐reflection (AR) coating; (ii) a film compatible with surface passivation; (iii) a mask for an electroless metal plating step; (iv) a diffusion barrier for achieving a selective emitter; (v) a film with excellent chemical resistance; (vi) a stable layer during high‐temperature processing. The dielectric coatings reviewed here include thermally grown silicon dioxide (SiO₂), silicon nitride deposited by plasma‐enhanced chemical vapour deposition (a‐ SiN_x :H) and low‐pressure chemical vapour deposition (Si₃N₄), silicon oxynitride (SiON), cerium dioxide (CeO₂), zinc sulphide (ZnS), and titanium dioxide (TiO₂). While TiO₂ dielectric coatings exhibit the best optical performance and a simple post‐deposition surface passivation sequence has been developed, they require an additional sacrificial diffusion barrier to survive the heavy groove diffusion step. A‐ SiN_x :H affords passivation through its high fixed positive charge density and large hydrogen concentration; however, it is difficult to retain these electronic benefits during lengthy high‐temperature processing. Therefore, for the BC solar cell, Si₃N₄ films would appear to be the best choice of dielectric films common in industrial use. Copyright © 2004 John Wiley & Sons, Ltd.     

Solar‐Energy Conversion in TiO2/CuInS2 Nanocomposites

The search for low‐cost thin‐film solar cells, to replace silicon multi‐crystalline cells in due course, calls for new combinations of materials and new cell configurations. Here we report on a new approach, based on semiconductor nanocomposites, towards what we refer to as the three‐dimensional (3D) solar‐cell concept. Atomic layer chemical vapor deposition is employed for infiltration of CuInS₂ inside the pores of nanostructured TiO₂. In this way it is possible to obtain a nanometer‐scale interpenetrating network between n‐type TiO₂ and p‐type CuInS₂. X‐ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, scanning electron microscopy, transmission electron microscopy, and current–voltage measurements are used to characterize the nanostructured devices. The 3D solar cells obtained show photovoltaic activity with a maximum monochromatic incident photon‐to‐current conversion efficiency of 80 % and have an energy‐conversion efficiency of 4 %.     

Low‐temperature flexible Ti/TiO2 photoanode for dye‐sensitized solar cells with binder‐free TiO2 paste

An energy‐economical dye‐sensitized solar cell (DSSC) with highly flexible Ti/TiO₂ photoanode was developed through a low‐temperature process, using a binder‐free TiO₂ paste. Ti foils, coated with the binder‐free TiO₂ films were annealed at various temperature. Scanning electron microscopic (SEM) images of the films show uniform, mesoporous and crack‐free surface morphologies as well as interpenetrated TiO₂ network. DSSCs with binder‐free TiO₂ films annealed at 450, 350, 250 and 120°C show solar‐to‐electricity conversion efficiencies (η) of 4.33, 4.34, 3.72 and 3.40%, respectively, which are comparable to the efficiency of 4.56% obtained by using a paste with binder and annealing it at 450°C; this observation demonstrates the benefits of a binder‐free TiO₂ paste for the fabrication of energy‐fugal DSSCs. On the other hand, when organic binder was used in the TiO₂ paste for film preparation, a drastic deterioration in the cell performance with decreasing annealing temperature is noticed. Laser‐induced photo‐voltage transient technique is used to estimate the electron lifetime in various Ti/TiO₂ films. Electrochemical impedance spectroscopic (EIS) analysis shows that the lower the annealing temperature of the TiO₂ coated Ti foil, the larger the charge transfer resistance at the TiO₂/dye/electrolyte interface (R_ct2). Copyright © 2011 John Wiley & Sons, Ltd.     

Light‐enhanced surface passivation of TiO2‐coated silicon

Titanium dioxide is shown to afford good passivation to non‐diffused silicon surfaces and boron‐diffused surfaces after a low‐temperature anneal. The passivation most likely owes to the significant levels of negative charge instilled in the films, and passivation is enhanced by illumination—advantageous for solar cells—indicating that a titanium dioxide photoreaction is at least partly responsible for the low surface recombination. We demonstrate a surface recombination velocity of less than 30 cm/s, on a 5‐Ω cm n‐type silicon, and an emitter saturation current density of 90 fA/cm² on a 200‐Ω/sq boron diffusion. If these titanium dioxide passivated boron‐diffused surfaces were employed in a crystalline silicon solar cell, an open‐circuit voltage as high as 685 mV could be achieved. Given that TiO₂ has a high refractive index and was deposited with atmospheric pressure chemical vapour deposition, an inexpensive technique, it has the potential as a passivating antireflection coating for industrial boron‐diffused silicon solar cells. Copyright © 2011 John Wiley & Sons, Ltd.     

>16% thin‐film epitaxial silicon solar cells on 70‐cm2 area with 30‐µm active layer,porous silicon back reflector,and Cu‐based top‐contact metallization

We demonstrate the use of a copper‐based metallization scheme for the specific application of thin‐film epitaxial silicon wafer equivalent (EpiWE) solar cells with rear chemical vapor deposition emitter and conventional POCl₃ emitter. Thin‐film epitaxial silicon wafer equivalent cells are consisting of high‐quality epitaxial active layer of only 30 µm, beneath which a highly reflective porous silicon multilayer stack is embedded. By combining Cu‐plating metallization and narrow finger lines with an epitaxial cell architecture including the porous silicon reflector, a J_sc exceeding 32 mA/cm² was achieved. We report on reproducible cell efficiencies of >16% on >70‐cm² cells with rear epitaxial chemical vapor deposition emitters and Cu contacts. Copyright © 2011 John Wiley & Sons, Ltd.     

Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero‐interface band structures while maintaining surface passivation

Interdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (V_OC) due to the surface passivation and heterojunction contacts, and high short circuit current density (J_SC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence V_OC and J_SC, but degrade fill factor (FF) from an “S” shape J–V curve. Two‐dimensional (2D) simulation using “Sentaurus device” demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero‐interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC‐SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since J_SC and V_OC also decrease due to poor surface passivation. A narrow band gap a‐Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero‐interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC‐SHJ solar cell structures and optimize cell performance. Copyright © 2010 John Wiley & Sons, Ltd.     

Front and rear silicon‐nitride‐passivated multicrystalline silicon solar cells with an efficiency of 18.1%

A solar cell process designed to utilise low‐temperature plasma‐enhanced chemical vapour deposited (PECVD) silicon nitride (SiN_x) films as front and rear surface passivation was applied to fabricate multicrystalline silicon (mc‐Si) solar cells. Despite the simple photolithography‐free processing sequence, an independently confirmed efficiency of 18.1% (cell area 2 × 2 cm²) was achieved. This excellent efficiency can be predominantly attributed to the superior quality of the rear surface passivation scheme consisting of an SiN_x film in combination with a local aluminium back‐surface field (LBSF). Thus, it is demonstrated that low‐temperature PECVD SiN_x films are well suited to achieve excellent rear surface passivation on mc‐Si. Copyright © 2002 John Wiley & Sons, Ltd.     

Preparation and measurement of highly efficient a‐Si:H single junction solar cells and the advantages of μc‐SiOx:H n‐layers

Reducing the optical losses and increasing the reflection while maintaining the function of doped layers at the back contact in solar cells are important issues for many photovoltaic applications. One approach is to use doped microcrystalline silicon oxide (μc‐SiO_x:H) with lower optical absorption in the spectral range of interest (300 nm to 1100 nm). To investigate the advantages, we applied the μc‐SiO_x:H n‐layers to a‐Si:H single junction solar cells. We report on the comparison between amorphous silicon (a‐Si:H) single junction solar cells with either μc‐SiO_x:H n‐layers or non‐alloyed silicon n‐layers. The origin of the improved performance of a‐Si:H single junction solar cells with the μc‐SiO_x:H n‐layer is identified by distinguishing the contributions because of the increased transparency and the reduced refractive index of the μc‐SiO_x:H material. The solar cell parameters of a‐Si:H solar cells with both types of n‐layers were compared in the initial state and after 1000 h of light soaking in a series of solar cells with various absorber layer thicknesses. The measurement procedure for the determination of the solar cell performance is described in detail, and the measurement accuracy is evaluated and discussed. For an a‐Si:H single junction solar cell with a μc‐SiO_x:H n‐layer, a stabilized efficiency of 10.3% after 1000 h light soaking is demonstrated. Copyright © 2015 John Wiley & Sons, Ltd.     

Enhanced Performance of I2‐Free Solid‐State Dye‐Sensitized Solar Cells with Conductive Polymer up to 6.8%

An iodine‐free solid‐state dye‐sensitized solar cell (ssDSSC) is reported here, with 6.8% energy conversion efficiency—one of the highest yet reported for N719 dye—as a result of enhanced light harvesting from the increased transmittance of an organized mesoporous TiO₂ interfacial layer and the good hole conductivity of the solid‐state‐polymerized material. The organized mesoporous TiO₂ (OM‐TiO₂) interfacial layer is prepared on large‐area substrates by a sol‐gel process, and is confirmed by scanning electron microscopy (SEM) and grazing incidence small‐angle X‐ray scattering (GISAXS). A 550‐nm‐thick OM‐TiO₂ film coated on fluorine‐doped tin oxide (FTO) glass is highly transparent, resulting in transmittance increases of 8 and 4% compared to those of the bare FTO and conventional compact TiO₂ film on FTO, respectively. The high cell performance is achieved through careful control of the electrode/hole transport material (HTM) and nanocrystalline TiO₂/conductive glass interfaces, which affect the interfacial resistance of the cell. Furthermore, the transparent OM‐TiO₂ film, with its high porosity and good connectivity, exhibits improved cell performance due to increased transmittance in the visible light region, decreased interfacial resistance ( Ω ), and enhanced electron lifetime ( τ ). The cell performance also depends on the conductivity of HTMs, which indicates that both highly conductive HTM and the transparent OM‐TiO₂ film interface are crucial for obtaining high‐energy conversion efficiencies in I₂‐free ssDSSCs.     

Development of back‐junction back‐contact silicon solar cells based on industrial processes

We have presented simplified industrial processes to fabricate high performance back‐junction back‐contact (BJBC) silicon solar cells. Good optical surface structures (solar averaged reflectance 2.5%) and high implied open‐circuit voltage (0.695 V) have been realized in the BJBC cell precursors through wet chemical processing, co‐diffusion, P ion implantation and annealing oxidation, as well as laser patterning and plasma enhanced chemical vapour deposition passivation processes. We have achieved a certified high efficiency of close to 22% on BJBC silicon solar cells with the size of 4.04 cm² by using screen printing and co‐firing technologies. The manufacturing process flow further successfully yields efficiency of around 21% BJBC silicon solar cells with enlarged sizes of 6 × 6 cm². The present work has demonstrated that the commercialization of low‐cost and high‐efficiency BJBC solar cells is possible because we have used processes compatible with existing production lines. Copyright © 2017 John Wiley & Sons, Ltd.     

The 2,2,6,6‐Tetramethyl‐1‐piperidinyloxy Radical: An Efficient,Iodine‐ Free Redox Mediator for Dye‐Sensitized Solar Cells

A promising redox system, offering an alternative to the widely used iodide/triiodide couple, based on the stable organic radical 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) has been employed in dye‐sensitized solar cells. The photovoltaic performance of this new redox couple has been evaluated by employing nanocrystalline TiO₂ films with different thickness. Judicious selections of a 5.0 μm photoanode made from TiO₂ mesoscopic particles and an organic sensitizer with a high molar extinction coefficient yield an overall solar‐to‐electric power conversion efficiency of 5.4 % under AM 1.5 illumination at 100 mW cm^–2, which is unprecedented for an iodine‐free mediator system.     

Silicon solar cells based on all‐laser‐transferred contacts

Crystalline silicon solar cells based on all‐laser‐transferred contacts (ALTC) have been fabricated with both front and rear metallization achieved through laser induced forward transferring. Both the front and rear contacts were laser‐transferred from a glass slide coated with a metal layer to the silicon substrate already processed with emitter formation, surface passivation, and antireflection coating. Ohmic contacts were achieved after this laser transferring. The ALTC solar cells were fabricated on chemically textured p‐type Cz silicon wafers. An initial conversion efficiency of over 15% was achieved on a simple cell structure with full‐area emitter. Further improvements are expected with optimized laser transferring conditions, front grid pattern design, and surface passivation. The ALTC process demonstrates the advantage of laser processing in simplifying the solar cell fabrication by a one‐step metal transferring and firing process. Copyright © 2013 John Wiley & Sons, Ltd.     

High efficiency large area n‐type front junction silicon solar cells with boron emitter formed by screen printing technology

In this paper, we report on commercially viable screen printing (SP) technology to form boron emitters. A screen‐printed boron emitter and ion‐implanted phosphorus back surface field were formed simultaneously by a co‐annealing process. Front and back surfaces were passivated by chemically grown oxide capped with plasma‐enhanced chemical vapor deposition silicon nitride stack. Front and back contacts were formed by traditional SP and firing processes with silver/aluminum grid on front and local silver back contacts on the rear. This resulted in 19.6% efficient large area (239 cm²) n‐type solar cells with an open‐circuit voltage V_oc of 645 mV, short‐circuit current density J_sc of 38.6 mA/cm², and fill factor of 78.6%. This demonstrates the potential of this novel technology for production of low‐cost high‐efficiency n‐type silicon solar cells. Copyright © 2014 John Wiley & Sons, Ltd.     

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

In this work, we report on ion‐implanted, high‐efficiency n‐type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser‐patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiN_x, followed by screen‐printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO₂/SiN_x passivation stack on the back to form the pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO₂/SiN_x passivation layers, without inducing appreciable damage or defects on the surface of the n⁺ back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n‐type passivated emitter, rear totally diffused cell. Copyright © 2014 John Wiley & Sons, Ltd.