Studies of implanted boron emitters for solar cell applications

B implanted emitters are investigated in the back junction cell configuration and their material properties are tested in double side implanted Si wafers. B has been implanted at 5 keV at various dose conditions varying from 1 × 10¹⁴ up to 3 × 10¹⁵ at./cm² and activated at 1000°C for 10 min. N‐type 8 × 8 cm² mono‐crystalline cells are fabricated and measured. Both fill factor and efficiency increase for high‐B doses. However, at 10¹⁵ at./cm² B dose the V_oc drops, which is in agreement with lifetime degradation in the wafer. Defect evolution simulations of B_nI_m clusters formation is correlated with lifetime degradation. Copyright © 2011 John Wiley & Sons, Ltd.     

Understanding of the annealing temperature impact on ion implanted bifacial n‐type solar cells to reach 20.3% efficiency

Ion implantation has the advantage of being a unidirectional doping technique. Unlike gaseous diffusion, this characteristic highlights strong possibilities to simplify solar cell process flows. The use of ion implantation doping for n‐type PERT bifacial solar cells is a promising process, but mainly if it goes with a unique co‐annealing step to activate both dopants and to grow a SiO₂ passivation layer. To develop this process and our SONIA cells, we studied the impact of the annealing temperature and that of the passivation layers on the electrical quality of the implanted B‐emitter and P‐BSF. A high annealing temperature (above 1000 °C) was necessary to fully activate the boron atoms and to anneal the implantation damages. Low J_0BSF (BSF contribution to the saturation current density) of 180 fA/cm² was reached at this high temperature with the best SiO₂ passivation layer. An average efficiency of 19.7% was reached using this simplified process flow (“co‐anneal process”) on large area (239 cm²) Cz solar cells. The efficiency was limited by a low FF, probably due to contaminations by metallization pastes. Improved performances were achieved in the case of a “separated anneals” process where the P‐BSF is activated at a lower temperature range. An average efficiency of 20.2% was obtained in this case, with a 20.3% certified cell. Copyright © 2014 John Wiley & Sons, Ltd.     

Influence of boron clustering on the emitter quality of implanted silicon solar cells: an atom probe tomography study

The use of ion implantation doping instead of the standard gaseous diffusion is a promising way to simplify the fabrication process of silicon solar cells. However, difficulties to form high‐quality boron (B) implanted emitters are encountered when implantation doses suitable for the emitter formation are used. This is due to a more or less complete activation of Boron after thermal annealing. To have a better insight into the actual state of the B distributions, we analyze three different B emitters prepared on textured Si wafers: (1) a BCl₃ diffused emitter and two B implanted emitters (fixed dose) annealed at (2) 950°C and at (3) 1050°C (less than an hour). Our investigations are in particular based on atom probe tomography, a technique able to explore 3D atomic distribution inside a material at nanometer scale. Atom probe tomography is employed here to characterize B atomic distribution inside textured Si solar cell emitters and to quantify clustering of B atoms. Here, we show that implanted emitters annealed at 950 °C present maximum clusters due to poor solubility at lower temperature and also highest emitter saturation current density (J_0e = 1000 fA/cm²). Increasing the annealing temperature results in greatly improved J_0e (131 fA/cm²) due to higher solubility and a consequently lower number of clusters. BCl₃ diffused emitters do not contain any B clusters and presented the best emitter quality. From our results, we conclude that clustering of B atoms is the main reason behind higher J_0e in the implanted boron emitters and hence degraded emitter quality. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical and experimental investigation of aluminum‐boron codoping of silicon

We present a detailed study on aluminum‐boron codoping of silicon by alloying from screen‐printed aluminum pastes containing boron additives (Al–B pastes). We derive an analytical model for the formation of the Al–B acceptor profiles by quantitatively describing (i) the composition of the Al–B–Si melt and (ii) the incorporation of Al and B acceptor atoms into the recrystallizing Si lattice. We show that measured Al–B dopant profiles can be excellently described by this model, which therefore offers a straightforward method for the comprehensive investigation of alloying from Al–B pastes. The formation of a characteristic kink in the Al–B dopant profile curve can thus be ascribed to the exhaustion of the B additive dissolution during alloying. By intentionally adding elemental B powder to an Al paste, we demonstrate that only a low percentage of the B powder actually dissolves into the melt. We show that this incomplete dissolution of the B additive strongly affects the recombination characteristics of Al–B–p⁺ regions and, thus, is an important element of alloying from Al–B pastes. This study therefore provides improved understanding of aluminum‐boron codoping of silicon. Copyright © 2015 John Wiley & Sons, Ltd.     

The effect of boron,oxygen, and fluorine in ion implanted GaAs

Effects of boron, fluorine, and oxygen in GaAs have been investigated by electrical characterization using current-voltage, capacitance-voltage and deep level transient spectroscopy techniques. Ion implantation at 100 keV energy was conducted with doses of 10¹¹ and 10¹²/cm². Carrier compensation was observed in each implanted sample. The compensation effect strongly depended on ion implantation condition and ion species. More free carriers were compensated for higher dose and heavier species; however, severe surface damage would also be induced that degrade electrical performance. Rapid thermal annealing treatment showed the heavier ion implanted samples to be more thermally stable. Defect levels for each implanted species were compared and identified. A native shallow defect E4 was easily removed by ion implantation. In higher dose and heavier ion implantation, both electron and hole traps were induced. However, in some cases, heavier ion implantation also removed native defects. Acceptor-type surface states were created by implantation that degrade material electrical characteristics and also reduce the effect of compensation. The damage induced traps were mostly point-defects or point-defect/impurity complexes as evidenced by sensitivity to heat treatment.     

Process development and comparison of various boron emitter technologies for high‐efficiency (~21%) n‐type silicon solar cells

This paper shows for the first time a comparison of commercial‐ready n‐type passivated emitter , rear totally diffused solar cells with boron (B) emitters formed by spin‐on coating, screen printing, ion implantation, and atmospheric pressure chemical vapor deposition. All the B emitter technologies show nearly same efficiency of ~20%. The optimum front grid design (5 busbars and 100 gridlines), calculated by an analytical modeling, raised the baseline cell efficiency up to 20.5% because of reduced series resistance. Along with the five busbars, rear point contacts formed by laser ablation of dielectric and physical vapor deposition Al metallization resulted in another 0.4% improvement in efficiency. As a result, 20.9% efficient n‐type passivated emitter, rear totally diffused cell was achieved in this paper. Copyright © 2016 John Wiley & Sons, Ltd.     

N‐type solar‐grade silicon purified via the metallurgical route: characterisation and fabrication of solar cells

This study focuses on the characterisation and the fabrication of solar cells using n‐type multicrystalline silicon purified via the metallurgical route. Electrical and chemical analyses were performed on wafers taken from several positions along the crystallised ingot. The impact of the fabrication processing steps was investigated via effective carrier lifetime measurements. Solar cells were processed, and their efficiencies were found to be dependent on the position of the wafer along the ingot height, that is, the wafer's resistivity. A maximum conversion efficiency of 15.0% was obtained on cells from the bottom part of the ingot. In this study, the minimum resistivity value of 0.4 Ω cm resistivity is given in order to reach adequate cell efficiency. Light‐soaking experiments were then performed on the fabricated cells. No significant variations of the cell performances were observed even after 110 h at 60 °C, meaning that the fabricated cells are stable under illumination. Copyright © 2012 John Wiley & Sons, Ltd.     

Excellent boron emitter passivation for high‐efficiency Si wafer solar cells using AlOx/SiNx dielectric stacks deposited in an industrial inline plasma reactor

Excellent passivation of boron emitters is realised using AlO_x/SiN_x dielectric stacks deposited in an industrial inline plasma‐enhanced chemical vapour deposition reactor. Very low emitter saturation current density (J_0e) values of 10 and 45 fA/cm² are obtained for 180 and 30 Ω/sq planar p⁺ emitters, respectively. For textured p⁺ emitters, the J_0e was found to be 1.5–2 times higher compared with planar emitters. The required thermal activation of the AlO_x films is performed in a standard industrial fast‐firing furnace, making the developed passivation stack industrially viable. We also show that an AlO_x thickness of 5 nm in the AlO_x/SiN_x stack is sufficient for obtaining a J_0e of 18 fA/cm² for planar 80 Ω/sq p⁺ emitters, which corresponds to a 1‐sun open‐circuit voltage limit of the solar cell of 736 mV at 25 °C. Copyright © 2012 John Wiley & Sons, Ltd.     

A 3‐in‐1 doping process for interdigitated back contact solar cells exploiting the understanding of co‐diffused dopant profiles by use of PECVD borosilicate glass in a phosphorus diffusion

Boron and phosphorus doping of crystalline silicon using a borosilicate glass (BSG) layer from plasma‐enhanced chemical vapor deposition (PECVD) and phosphorus oxychloride diffusion, respectively, is investigated. More specifically, the simultaneous and interacting diffusion of both elements through the BSG layer into the silicon substrate is characterized in depth. We show that an overlying BSG layer does not prevent the formation of a phosphorus emitter in silicon substrates during phosphorus diffusion. In fact, a BSG layer can even enhance the uptake of phosphorus into a silicon substrate compared with a bare substrate. From the understanding of the joint diffusion of boron and phosphorus through a BSG layer into a silicon substrate, a model is developed to illustrate the correlation of the concentration‐dependent diffusivities and the emerging diffusion profiles of boron and phosphorus. Here, the in‐diffusion of the dopants during diverse doping processes is reproduced by the use of known concentration dependences of the diffusivities in an integrated model. The simulated processes include a BSG drive‐in step in an inert and in a phosphorus‐containing atmosphere. Based on these findings, a PECVD BSG/capping layer structure is developed, which forms three different n⁺⁺−, n⁺− and p⁺−doped regions during one single high temperature process. Such engineered structure can be used to produce back contact solar cells. Copyright © 2016 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.     

Gapless point back surface field for the counter doping of large‐area interdigitated back contact solar cells using a blanket shadow mask implantation process

Gapless interdigitated back contact (IBC) solar cells were fabricated with phosphorous back surface field on a boron emitter, using an ion implantation process. Boron emitter (boron ion implantation) is counter doped by the phosphorus back surface field (BSF) (phosphorus ion implantation) without gap. The gapless process step between the emitter and BSF was compared to existing IBC solar cell with gaps between emitters and BSFs obtained using diffusion processes. We optimized the doping process in the phosphorous BSF and boron emitter region, and the implied V_oc and contact resistance relationship of the phosphorous and boron implantation dose in the counter doped region was analyzed. We confirmed the shunt resistance of the gapless IBC solar cells and the possibility of shunt behavior in gapless IBC solar cells. The highly doped counter doped BSF led to a controlled junction breakdown at high reverse bias voltages of around 7.5 V. After the doping region was optimized with the counter doped BSF and emitter, a large‐area (5 inch pseudo square) gapless IBC solar cell with a power conversion efficiency of 22.9% was made.     

Impact of excess phosphorus doping and Si crystalline defects on Ag crystallite nucleation and growth in silver screen‐printed Si solar cells

Good quality contacts between metal and silicon emitter are crucial for high crystalline solar cell efficiencies. We investigate the impact of defects originating from electrically inactive phosphorus on contact formation within silver thick film metallized silicon solar cells. For this purpose, emitters with varying sheet resistance, depth, and dead layer were metallized with silver pastes from different generations. Macroscopic contact resistivity measurements were compared with the microscopic contact configurations studied by scanning electron microscopy. The density of direct contacts between Ag crystallites grown into Si and the Ag finger bulk is essential for low contact resistivity. The presence of glass‐free regions needed for such direct contacts depends on the paste composition and on the surface texture, and does not vary with the Si emitter properties. Indeed, the decrease in contact resistivity correlates with increasing density of Ag crystallites embedded in the Si surface. Furthermore, the density of Si surface‐embedded Ag crystallites scales proportional to the electrically inactive P and is independent of the sheet resistance. Using the newest silver paste, the Ag crystallite density is independent of the emitter doping, but the Ag crystallite size increases as a function of the thickness of the dead layer. Transmission electron microscopy characterization of the excess P‐doped Si crystal lattice shows that significant strain and Si bond weakening may play a major role for both Ag crystallite nucleation and growth. Finally, we studied Si crystal defects by metallizing nanocracks, dislocations, and grain boundaries and found that Ag crystallite nucleation is defect‐property dependent. Copyright © 2013 John Wiley & Sons, Ltd.     

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Atomic‐layer‐deposited aluminium oxide (Al₂O₃) is applied as rear‐surface‐passivating dielectric layer to passivated emitter and rear cell (PERC)‐type crystalline silicon (c‐Si) solar cells. The excellent passivation of low‐resistivity p‐type silicon by the negative‐charge‐dielectric Al₂O₃ is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al₂O₃ film covered by a 200 nm plasma‐enhanced‐chemical‐vapour‐deposited silicon oxide (SiO_x) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single‐layer of Al₂O₃, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.     

Ar^＋注入多孔硅与Ar^＋注入硅多孔结构的光致发光研究

研究了中等能量(30 key)Ar+注入多孔硅和中等能量(30 key)Ar+先注入单晶硅后再进行电化学腐蚀成的多孔结构的光致发光特性.研究结果表明:中等能量(30 key)Ar+注入多孔硅后,多孔硅原有的发光峰消失,主要是Ar+对多孔硅表面氧的剥离作用.使得与氧相关发光的结构消失,多孔硅不再发光;中等能量(30 kev)Ar+先注入单晶硅后冉电化学腐蚀成的多孔结构中,通常多孔硅原有的580 nIn附近发光峰强度随注入Ar+剂量的增加而增强,并有红移;同时在谱峰处于470 nm附近的微弱发光峰不因注入Ar+而明显变化.     

Polycrystalline silicon thin‐film solar cells on glass by aluminium‐induced crystallisation and subsequent ion‐assisted deposition (ALICIA)

A research project is under way at The University of New South Wales aiming at the realisation of a novel type of polycrystalline silicon thin‐film solar cell on glass. The idea is to first create a thin large‐grained polycrystalline seed layer on glass by aluminium‐induced crystallisation of amorphous silicon and then to epitaxially thicken the seed layer with ion‐assisted deposition. By mid‐2003 this ALICIA project had achieved laboratory cells with voltages of up to 163 mV, as reported elsewhere. In the present paper we give an overview of recent progress (improved Si epitaxy process, improved control of base doping profile due to the use of phosphorus dopants instead of gallium, hydrogen passivation) that has improved the voltages of ALICIA solar cells to 270 mV. Furthermore, the strategy for further voltage improvements is presented. At the present point in time only the voltages of ALICIA cells are known, but obviously solar cells also require current for good efficiency. Hence much improvement in both voltage and current is still needed. Copyright © 2004 John Wiley & Sons, Ltd.     

Structural and Electronic Properties of Semiconductor‐Sensitized Solar‐Cell Interfaces

A recent study has reported a power‐conversion efficiency of 5.1% for solar cells employing mesoporous TiO₂ films sensitized with quantum dots of stibnite (Sb₂S₃). Here, a first‐principles atomic‐scale investigation of the interface between TiO₂ and Sb₂S₃ is presented. The proposed atomistic interface model is free of defects, and the calculated energy‐level alignment at the interface indicates that the ideal open‐circuit voltage is as high as 1.6 V. Films sensitized with the isostructural compounds bismuthinite (Bi₂S₃) and antimonselite (Sb₂Se₃), which exhibit band gaps closer to the ideal Shockley–Queisser value are also examined. In the case of Bi₂S₃ the calculations indicate that the lowest unoccupied molecular orbital is too low in energy to inject electrons into TiO₂, in agreement with experimental data. For antimonselite (Sb₂Se₃) the calculations predict a type‐II heterojunction with TiO₂, and suggest that Sb₂Se₃ sensitization may lead to higher power conversion efficiencies than found in the TiO₂/Sb₂S₃ system.     

Analysis of the EWT‐DGB solar cell at low and medium concentration and comparison with a PESC architecture

Silicon represents an interesting material to fabricate low‐cost and relatively simple and high‐efficient solar cells in the low and medium concentration range. In this paper, we discuss a novel cell scheme conceived for concentrating photovoltaic, named emitter wrap through with deep grooved base (EWT‐DGB), and compare it with the simpler passivated emitter solar cell. Both cells have been fabricated by means of a complementary metal–oxide–semiconductor‐compatible process in our laboratory. The experimental characterization of both cells is reported in the range 1–200 suns in terms of conversion efficiency, open circuit voltage, short circuit current density and fill factor. In particular, for the EWT‐DGB solar cells, we obtain an encouraging 21.4% maximum conversion efficiency at 44 suns. By using a calibrated finite‐element numerical electro‐optical simulation tool, validated by a comparison with experimental data, we study the potentials of the two architectures for concentrated light conditions considering possible realistic improvements with respect to the fabricated devices. We compare the solar cell figures of merit with those of the state‐of‐the‐art silicon back‐contact back‐junction solar cell holding the conversion efficiency record for concentrator photovoltaic silicon. Simulation results predict a 24.8% efficiency at 50 suns for the EWT‐DGB cell and up to 23.9% at 100 suns for the passivated emitter solar cell, thus confirming the good potential of the proposed architectures for low to medium light concentration. Finally, simulations are exploited to provide additional analysis of the EWT‐DGB scheme under concentrated light. Copyright © 2017 John Wiley & Sons, Ltd.     

High‐quality surface passivation of silicon solar cells in an industrial‐type inline plasma silicon nitride deposition system

We have studied the surface passivation of silicon by deposition of silicon nitride (SiN) in an industrial‐type inline plasma‐enhanced chemical vapor deposition (PECVD) reactor designed for the continuous coating of silicon solar cells with high throughput. An optimization study for the passivation of low‐resistivity p‐type silicon has been performed exploring the dependence of the film quality on key deposition parameters of the system. With the optimized films, excellent passivation properties have been obtained, both on undiffused p‐type silicon and on phosphorus‐diffused n⁺ emitters. Using a simple design, solar cells with conversion efficiencies above 20% have been fabricated to prove the efficacy of the inline PECVD SiN. The passivation properties of the films are on a par with those of high‐quality films prepared in small‐area laboratory PECVD reactors. Copyright © 2004 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.     

High‐Resolution Spectroscopic Mapping of the Chemical Contrast from Nanometer Domains in P3HT:PCBM Organic Blend Films for Solar‐Cell Applications

A high‐resolution near‐field spectroscopic mapping technique is successfully applied to investigate the influence of thermal annealing on the morphology of a poly(3‐hexylthiophene) and [6,6]‐penyl‐C₆₁ butyric acid methyl ester (P3HT:PCBM) blend film. Based on the simultaneously recorded morphological and spectroscopic information, the interplay among the blend film morphology, the local P3HT:PCBM molecular distribution, and the P3HT photoluminescence (PL) quenching efficiency are systematically discussed. The PL and Raman signals of the electron donor (P3HT) and acceptor (PCBM) are probed at an optical resolution of approximately 10 nm, which allows the chemical nature of the different domains to be identified directly. In addition, the local PL quenching efficiency, which is related to the electron transfer from P3HT to PCBM, is quantitatively revealed. From these experimental results, it is proposed that high‐resolution near‐field spectroscopic imaging is capable of mapping the local chemical composition and photophysics of the P3HT:PCBM blends on a scale of a few nanometers.