SiOx(C)/SiNx dual-layer anti-reflectance film coating for improved cell efficiency

Light-induced plating (LIP), in which the current driving the metal reduction process is derived from illuminated solar cells, is an attractive technique for solar cell metallization because of its potential simplicity. However, applying the LIP techniques on standard acidic-textured multicrystalline silicon wafers with a silicon nitride-coated surface presents a challenge. The use of a spray-on carbon-doped non-stoichiometric silicon oxide [SiO_x(C)] dielectric film before nickel and silver plating can greatly reduce background plating while helping decrease the reflectance on the front of silicon solar cells. The sprayed dielectric films have low refractive indices of 1.3–1.4, depending on the annealing temperature. Simulation studies show that the SiO_x(C)/SiN_x dual-layer anti-reflective coating has a lower weighted reflectance against an AM 1.5 G spectrum compared with the SiN_x single coating. Finally, the performance of the laser-doped solar cells with a standard SiN_x as an anti-reflectance coating were compared with those with the SiO_x(C)/SiN_x double-layer stack. An efficiency of 16.74% on a large, commercial-grade, p-type, multicrystalline silicon substrate was achieved.     

Spray-coated organic solar cells with large-area of 12.25 cm

This study evaluated the possibility of utilizing a spray-coating process for large-area organic solar cells (OSCs) combined with a metal electrode geometry. The effects of the cell area in spray-coated OSCs were investigated systematically by introducing a metal sub-electrode and grid-electrode to realize large-area cells of up to 12.25 cm². The series resistance could be reduced significantly by inserting a metal grid-electrode into the indium tin oxide (ITO) anode, yielding a power conversion efficiency of 2.11% at a cell area of 12.25 cm² and 2.49% at an effective photocurrent generated area of 11.23 cm² under AM.1.5 simulated illumination. This is comparable to the 3.13% obtained in the cell produced by spray-coating at a cell area of 0.38 cm².     

Effect of series resistance on metal-wrap-through multi-crystalline silicon solar cells

Metal-wrap-through (MWT) is a promising technique to improve the solar cell performance cost effectively because it can be easily integrated into the current production line with only two additional processing steps. Metal filling through the via-holes is a key to obtain low series resistances and good FFs. In this study, several screen printing process conditions were examined to find out an optimal filling state of the metal contacts. Various shapes of the filled metals in the via-holes were formed with different printing conditions, and the shape of the filled metal results in different series resistance values. Optimization of the printing conditions dramatically reduced the series resistance of the MWT cells. The maximum and minimum series resistance values of the cells obtained are 8.56 and 0.114 Ω cm², respectively. As a result, we achieved an efficiency of 16.3% using the optimal printing condition on 156 mm×156 mm solar-grade multi-crystalline silicon wafer, which was 0.8% absolute higher than the baseline cell efficiency.     

Bulk and contact resistance in P3HT:PCBM heterojunction solar cells

In this paper, the series resistance of poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic solar cells (OSC) has been studied. The series resistance of thermal annealed and un-annealed devices with different active layer thicknesses was measured. The series resistance of the organic solar cells consists of the bulk resistance of the active layer itself and the specific contact resistance between the active layer and the electrode. The bulk resistance and contact resistance were extracted from the measured series resistance using the vertical transmission line model (TLM) method. By fabricating solar cell devices with different active layer thicknesses, a relationship of the series resistance with thickness was established from which bulk and contact resistances were derived. We have also found that thermal annealing helps reduce both contact resistance and bulk resistance significantly; the contact resistance dropped by a factor of 2, while the bulk resistance decreased by a factor of 8. Results have shown that for an annealed P3HT:PCBM device that has an active layer thickness of 85 nm (optimum thickness for high efficiency), 17% of the total series resistance was due to the contact resistance, and bulk resistance contributed the rest 83%. The bulk resistance value for thermal annealed organic solar cell device with an active area of 0.1 cm² was found to be 150 Ω, and the measured specific contact resistance was 3.1 Ω cm². The measured bulk and contact resistance values are much higher as compared to the high efficiency silicon solar cells. Bulk resistance and contact resistance need to be further decreased in order to achieve higher organic solar cell efficiency.     

Novel process of grain boundary metallisation on mc Si solar cells

The electronic properties of multicrystalline silicon are heavily influenced by impurities concentrated along the grain boundaries that increase the recombination activity near the crystallite borders. Dopants can also diffuse preferentially down the grain boundaries, which leads to a low resistance path down the grain. These and other effects decrease the efficiency of multicrystalline silicon solar cells. Additionally, the efficiency is lowered by the shading of areas of silicon by metallisation lines due to the reduction of the active conversion area of the cell. We present a new way to combine the grain boundaries and the front contact grid with the aim to improve the efficiency of multicrystalline silicon solar cells. A first approach has been developed to produce multicrystalline silicon solar cells with a front contact metallisation following the grain boundaries: The different grain boundaries of a multicrystalline silicon wafer are detected by optical scanning of the wafer surface. Together with the emitter sheet resistivity this image serves as an input to calculate a net of finger lines that follow the grain boundaries wherever possible. Onto these detected grain boundaries the metallisation is performed by evaporative deposition of copper and photolithography. We report on the successful implementation of such a grid on 100×100 mm² wafers.     

Improved LDSE processing for the avoidance of overplating yielding 19.2% efficiency on commercial grade crystalline Si solar cell

A record in laser doped selective emitter (LDSE) solar cells with an efficiency η=19.2% is reported. In this study, we investigate the effect of SiN_x films for laser doped selective emitter solar cells with plated front contacts. It is observed that the condition of processes such as silicon nitride and laser doping (LD) is of critical importance prior to light induced plating. If these processes are not performed optimally, localized shunts may form during the light induced plating (LIP) process that then inhibit plating in the surrounding areas. In the previous work an efficiency of 18.3% has been achieved, even though the fill factor was only 74.2% and the cell suffered from additional shunting and shading losses due to overplating. However, in this work, we demonstrate that with the optimization of the PECVD SiN_x and metallization processes, cells have reached efficiencies of more than 19% on commercial grade p-type CZ Si substrates.     

Localized irradiation effects on tunnel diode transitions in multi-junction concentrator solar cells

Multi-junction concentrator solar cells incorporate tunnel diodes that undergo a transition from high-conductance tunneling to low-conductance thermal diffusion behavior, typically at threshold current densities of the order of 10²-10³ mA/mm². We present experimental evidence of a prominent heretofore unrecognized dependence of threshold current density on the degree of localized irradiation, in different solar cell architectures. We also show that solar cells with non-uniform metallization can exhibit an additional spatial dependence to the tunnel diode threshold current density. These previously undiscovered phenomena - which should be observable in all non-uniformly irradiated photovoltaic tunnel diodes - are interpreted as being derived from the lateral spreading of excess majority carriers (analogous to current spreading in light-emitting diodes (LEDs)). The consequences for concentrator photovoltaics are addressed.     

Assessment of a low-cost gold-free metallization for III–V high concentrator solar cells

A gold-free metallization is proposed to be used as the grid contact in III–V concentrator solar cells. This metallization is based on the Cu/Ge system which has been reported to attain very low specific contact resistances on n-GaAs. In this letter, we show that metal layers with low resistivity (13 μΩ cm) can be obtained if the copper content in the alloy is around 28% in weight for a wide range of annealing temperatures (400–450 °C). Finally, this metallization has been used to manufacture single-junction GaAs high concentrator solar cells. Efficiencies of 26.2% at 1000 suns have been reached.     

Metallization improvement on fabrication of interdigitated backside and double sided buried contact solar cells

Metallization based on electroless metal plating of nickel and copper is a simple, cost-effective process used in the fabrication of Buried Contact silicon solar cells. Whereas the electroless Ni–Cu metallization scheme works well for metal deposition on early Buried Contact solar cells, in which deposition was required only on phosphorus diffused contact regions, more care is required for advanced Buried Contact solar cell designs that require simultaneous deposition on to both phosphorus and boron diffused contact regions. In this paper, we examine two key issues related to the metallization in these solar cells. Firstly we demonstrate an improved buffered hydrofluoric acid etch process for simultaneous removal of borosilicate and borophosphosilicate glasses from the contact regions prior to electroless deposition of nickel with good etch selectivity against silicon dioxide masking films. Secondly, we demonstrate an improved process for nucleation of the nickel layer on both phosphorus and boron diffused contact areas based on immersion palladium chloride activation of the plating surfaces. N-type double-sided buried contact solar cells metallized by processing introduced in this study show improvement on absolute efficiency of more than 3%.     

Nano-structured Cu2O solar cells fabricated on sparse ZnO nanorods

Nano-structured Cu₂O/ZnO nanorod (NR) heterojunction solar cells fabricated on indium tin oxide (ITO)-coated glass are studied. Substrate film and NR density have a strong influence on the preferred growth of the Cu₂O film. The X-ray diffractometer (XRD) analysis results show that highly (2 0 0)-preferred Cu₂O film was formed when plating on plain ITO substrate. However, a highly (1 1 1)-preferred Cu₂O film was obtained when plating on sparse ZnO NRs. SEM, TEM and XRD studies on sparse NR samples indicate that the Cu₂O nano-crystallites mostly initiate its nucleation on the peripheral surfaces of the ZnO NRs, and are also highly (1 1 1)-oriented. Solar cells with ZnO NRs yielded much higher efficiency than those without. In addition, ZnO NRs plated on a ZnO-coated ITO glass significantly improve the shunt resistance and open-circuit voltage (V_oc) of the devices, with consistently much higher efficiency obtained than when ZnO NRs are directly plated on ITO film. However, longer NRs do not improve the efficiency due to low short-circuit current (J_sc) and slightly higher series resistance. The best conversion efficiency of 0.56% was obtained from a Cu₂O/ZnO NRs heterojunction solar cell fabricated on a 80 nm ZnO-coated ITO glass with V_oc=0.514 V, J_sc=2.64 mA/cm² and 41.5% fill factor.     

Enhancement of photovoltaic properties of multicrystalline silicon solar cells by combination of buried metallic contacts and thin porous silicon

Photovoltaic properties of buried metallic contacts (BMCs) with and without application of a front porous silicon (PS) layer on multicrystalline silicon (mc-Si) solar cells were investigated. A Chemical Vapor Etching (CVE) method was used to perform front PS layer and BMCs of mc-Si solar cells. Good electrical performance for the mc-Si solar cells was observed after combination of BMCs and thin PS films. As a result the current-voltage (I-V) characteristics and the internal quantum efficiency (IQE) were improved, and the effective minority carrier diffusion length (Ln) increases from 75 to 110 μm after BMCs achievement. The reflectivity was reduced to 8% in the 450-950 nm wavelength range. This simple and low cost technology induces a 12% conversion efficiency (surface area = 3.2 cm²). The obtained results indicate that the BMCs improve charge carrier collection while the PS layer passivates the front surface.     

High Efficiency Large Area Back Contact Concentrator Solar Cells with a Multilevel Interconnection

Very high efficiencies have been demonstrated under concentration with silicon solar cells having interdigitated contacts on the backside. However, only laboratory cells of small dimension have reached very high efficiencies. The need for developing a multilevel metallization technology for back contact concentrator solar cells of large area is demonstrated. The particular features required for such a multilevel interconnection are studied and a process using anodic oxidation of aluminum is presented. Back contact silicon solar cells of 0.64 cm² have been processed in this technology resulting in 26.2% efficiencies at 10W/cm² (100 suns AM1.5, 25.5 ^°C). the highest efficiency reported to date for a solar cell of this area. The one-sun efficiency of this cell is 21.7% (AMI.5, 25.2^°C). We propose also a new design for the metallization of back contact cells which allows an increase in the size of the cell without increasing the series resistance.     

Physical understanding of the behavior of silver thick-film contacts on n-type silicon under annealing conditions

High-efficiency silicon solar cells with evaporated front contacts and an oxide-passivated rear require post metallization annealing (PMA). In an industrial environment the evaporated front contacts are replaced by screen printed contacts for fast processing and cost reasons. The PMA conditions necessary for optimum rear side passivation can be inferior to such a front side metallization. In order to design a PMA supporting contact in future, this paper investigates what mechanism deteriorates the contact resistance of screen printed front side metallization during nitrogen PMA. Scanning electron microscopy (SEM) on samples with increased contact resistance reveals an altered microstructure at the silver-silicon contact interface that is proposed to impede current flow and hence increases the contact resistance. We present a model that describes the mechanism of contact deterioration during nitrogen PMA.     

ITO-free flexible inverted organic solar cell modules with high fill factor prepared by slot die coating

We report on the fabrication of inverted ITO-free P3HT:PCBM solar cell modules on glass and PET foil as substrate where the organic functional layers are deposited with slot die coating, a reel to reel compatible coating technique. The active layers have been processed in ambient atmosphere, which will be of advantage in a future production and is especially remarkable as the metallic cathode is already deposited on the substrate at this stage of fabrication. The modules comprise two busses of 11 cell elements connected in series each. The series connection leads to an open circuit voltage of up to 6.88 V on glass substrate, which translates to 625 mV per cell element, a very competitive value for P3HT:PCBM based solar cells on glass. Although the designated area is as large as 41 cm² and the active area 26.4 cm², we obtain fill factors of up to 65% for these modules, which again is a typical value for small area laboratory cells. Remarkably the values for PET foil as substrate with an open circuit voltage of 6.5 V and a fill factor of 64% are very close to the results on glass and to our knowledge the highest fill factors for flexible organic solar cells, even if compared to small area devices. The short circuit current densities and therefore efficiencies are also comparable to small area devices, if only the photoactive area is accounted for. Therefore we have demonstrated that the scale up of organic solar cells can be achieved with a suited circuitry scheme.     

Nano-sized Ag-inserted amorphous ZnSnO3 multilayer electrodes for cost-efficient inverted organic solar cells

A sheet resistance- and optical transmittance-tunable amorphous ZnSnO₃ (ZTO) multilayer electrode created through the insertion of a nano-scale Ag layer is demonstrated as an indium-free transparent conducting electrode for cost-efficient inverted organic solar cells (IOSCs). Due to the antireflection effect, the ZTO/Ag/ZTO/glass exhibited a high transmittance of 86.29% in the absorption wavelength region of the organic active layer and a low resistivity of 3.24×10⁻⁵ Ω cm, even though the ZTO/Ag/ZTO electrode was prepared at room temperature. The metallic conductivity of the electrode indicates that its electrical conductivity is dominated by the nano-scale Ag metal layer. In addition, optimization and control of the thickness of the nano-scale Ag layer are important in obtaining highly transparent ZTO/Ag/ZTO electrodes, because antireflection is strongly influenced by Ag thickness. Moreover, IOSCs fabricated on optimized ZTO/Ag/ZTO electrodes with Ag thicknesses of 12 nm showed power conversion efficiencies (2.55%) comparable to that of an IOSC prepared on a crystalline ITO electrode (2.45%), due to the low sheet resistance and high optical transmittance in the range of 400-600 nm. The performances of ZTO/Ag/ZTO multilayer electrodes indicate that ZTO/Ag/ZTO multilayers are promising as indium-free, transparent electrode substitutes for conventional ITO electrodes in cost-efficient IOSCs.     

A study of shading and resistive loss from the fingers of encapsulated solar cells

Optimised solar cell design is dependent on the assumed shading and resistance losses associated with front contacts. In this study, a spectrophotometer with integrating sphere attachment was used to measure the reflection from the front surface of encapsulated silver electroplated front contact solar cells. The results obtained are in good agreement with a previous study by one of the authors using a different method. The measured effective shading loss is about one third of the coverage fraction of the cell grid because of trapping of light reflected from the grid. The grid loss in 4×5 cm silver electroplated front contact solar cells was found to be similar to the predicted loss from buried grid and rear point contact solar cells operating at 30 suns concentration.     

Techno-economic evaluation of concentrating solar power generation in India

The Jawaharlal Nehru National Solar Mission (JNNSM) of the recently announced National Action Plan on Climate Change (NAPCC) by the Government of India aims to promote the development and use of solar energy for power generation and other uses with the ultimate objective of making solar competitive with fossil-based energy options. The plan includes specific goals to (a) create an enabling policy framework for the deployment of 20,000 MW of solar power by 2022; (b) create favourable conditions for solar manufacturing capability, particularly solar thermal for indigenous production and market leadership; (c) promote programmes for off grid applications, reaching 1000 MW by 2017 and 2000 MW by 2022, (d) achieve 15 million m² solar thermal collector area by 2017 and 20 million by 2022, and (e) deploy 20 million solar lighting systems for rural areas by 2022. The installed capacity of grid interactive solar power projects were 6 MW until October 2009 that is far below from their respective potential.     

The determination of the optimum transparency factor for solar cell metallization grids

A method for calculating the optimum transparency factor of a solar cell metallization grid as a function of the concentration factor is presented. The method is used to discuss the relative influence of the parameters determining the series resistance on the variation of the solar cell efficiency with the concentration factor. The method can be extended to solar cells other than the conventional cells for which it was primarily developed.     

Process development of amorphous silicon/crystalline silicon solar cells

We have already investigated some crucial limiting process steps of the amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell technology and some specific characterization tools of the ultrathin amorphous material used in devices. In this work, we focus our attention particularlyon the technology of the ITO front contact fabrication, that also is used as an antireflective coating. It is pointed out that this layer acts as a barrier layer against the diffusion of metal during the annealing treatments of the front contact grid. The criteria of the selection of the metal to be used to obtain good performance of the grid and the deposition methods best suited to the purpose are shown. We were able to fabricate low temperature heterojunction solar cells based p-type Czochralski silicon, and a conversion efficiency of 14.7% on 3.8 cm² area was obtained without back surface field and texturization.     

Highly stable transparent and conducting gallium-doped zinc oxide thin films for photovoltaic applications

Transparent and highly conducting gallium zinc oxide (GZO) films were successfully deposited by RF sputtering at room temperature. A lowest resistivity of ∼2.8×10⁻⁴ Ω cm was achieved for a film thickness of 1100 nm (sheet resistance ∼2.5 Ω/□), with a Hall mobility of 18 cm²/V s and a carrier concentration of 1.3×10²¹ cm⁻³. The films are polycrystalline with a hexagonal structure having a strong crystallographic c-axis orientation. A linear dependence between the mobility and the crystallite size was obtained. The films are highly transparent (between 80% and 90% including the glass substrate) in the visible spectra with a refractive index of about 2, very similar to the value reported for the bulk material. These films were applied to single glass/TCO/pin hydrogenated amorphous silicon solar cells as front layer contact, leading to solar cells with efficiencies of about 9.52%. With the optimized deposition conditions, GZO films were also deposited on polymer (PEN) substrates and the obtained results are discussed.