Influence of defects and band offsets on carrier transport mechanisms in amorphous silicon/crystalline silicon heterojunction solar cells

We have investigated the carrier transport mechanisms in undoped a-Si:H/p-type c-Si heterojunctions with and without a μc-Si buffer layer, as well as their effects on the photovoltaic properties of the junction. The conduction behavior of the junction is strongly affected by the defect state distribution and band offset at the hetero-interface. The recombination process involving the interface states on the thin film silicon (a-Si:H/μc-Si) side dominates at low forward bias (V<0.3 V), whereas multistep tunneling capture emission (MTCE) dominates in the higher bias region (0.3<V<0.55 V) until the conduction becomes space charge limited (V>0.55 V). The MTCE process seems to be more closely related to the bulk defects in the thin film silicon than the interface states. In addition, the position of a trapping level, where the tunneling process occurs, seems to be determined by the hole energy at the edge of the c-Si and the trap distribution in the thin film silicon. Despite the domination of MTCE in the indicated voltage range, the reduced band offset at the interface increases current levels by the enhanced diffusion and/or emission processes. The insertion of a 200 Å thick μc-Si buffer layer between the a-Si:H (700 Å)/c-Si increases the solar cell efficiency to 10%, without an antireflective coating, by improving both the carrier transport and the red response of the cell.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells

We have investigated the photovoltaic (PV) characteristics of both glow discharge deposited hydrogenated amorphous silicon (a-Si:H) on crystalline silicon (c-Si) in a n⁺ a-Si:H/undoped a-Si:H/p c-Si type structure, and DC magnetron sputtered a-Si:H in a n-type a-Si:H/p c-Si type solar cell structure. It was found that the PV properties of the solar cells were influenced very strongly by the a-Si/c-Si interface. Properties of strongly interface limited devices were found to be independent of a-Si thickness and c-Si resistivity. A hydrofluoric acid passivation prior to RF glow discharge deposition of a-Si:H increases the short circuit current density from 2.57 to 25.00 mA/cm² under 1 sun conditions.DC magnetron sputtering of a-Si:H in a Ar/H₂ ambient was found to be a controlled way of depositing n type a-Si:H layers on c-Si for solar cells and also a tool to study the PV response with a-Si/c-Si interface variations. 300 Å a-Si sputtered onto 1–10 ω cm p-type c-Si resulted in 10.6% efficient solar cells, without an A/R coating, with an open circuit voltage of 0.55 V and a short circuit current density of 30 mA/cm² over a 0.3 cm² area. High frequency capacitance-voltage measurements indicate good junction characteristics with zero bias depletion width in c-Si of 0.65 μm. The properties of the devices have been investigated over a wide range of variables like substrate resistivity, a-Si thickness, and sputtering power. The processing has focused on identifying and studying the conditions that result in an improved a-Si/c-Si interface that leads to better PV properties.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer

Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (μc-Si) buffer layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are effectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a fill factor comparable to the conventional HJ silicon solar cell. The optimized configuration consisted of an undoped a-Si:H emitter layer (700 Å), providing an excellent light absorption and defect passivation, and a thin μc-Si buffer layer (200 Å), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion efficiency of 10% without an anti-reflective coating.     

Interface effects on the carrier transport and photovoltaic properties of hydrogenated amorphous silicon/crystalline silicon solar cells

Current-voltage-temperature (I-V-T) characteristics evaluated near 150K and 300K were used to study the photovoltaic property variations in hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) solar cells. The possible carrier transport mechanisms in such devices were examined from the I-V-T data which indicated a significant influence of the amorphous /crystalline interface on the short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the solar cells. Carrier transport near 300K for forward biases was by a multi-tunneling mechanism and became space charge limited with increasing bias. For devices having low J_sc and V_oc an additional region was seen in both forward and reverse biases, at low temperatures, where the current simply varied linearly with the applied bias. This characteristic manifested in both high and low temperatures region for devices with still lower photovoltaic properties, which has been reasoned to be due to a higher interface density. Passivating the c-Si surface with HF just prior to the amorphous layer deposition resulted in a large improvement in the properties. The most significant effect was on the J_sc which improved by an order of magnitude. The treatment also affected the lower temperature I-V-T data in that the current fell to very low levels. The spectral response of the treated solar cells showed enhanced blue/violet response compared with the unpassivated devices. The interface passivation plus reducing a-Si thickness has improved the solar cell efficiency from 0.39% to 9.5%.     

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.     

Present status and future of crystalline silicon solar cells in Japan

Crystalline silicon solar cells show promise for further improvement of cell efficiency and cost reduction by developing process technologies for large-area, thin and high-efficiency cells and manufacturing technologies for cells and modules with high yield and high productivity.In this paper, Japanese activities on crystalline Si wafers and solar cells are presented. Based on our research results from crystalline Si materials and solar cells, key issues for further development of crystalline Si materials and solar cells will be discussed together with recent progress in the field. According to the Japanese PV2030 road map, by the year 2030 we will have to realize efficiencies of 22% for module and 25% for cell technologies into industrial mass production, to reduce the wafer thickness to 50–100 μm, and to reduce electricity cost from 50 Japanese Yen/kWh to 7 Yen/kWh in order to increase the market size by another 100–1000 times.     

A review of thin-film crystalline silicon for solar cell applications. Part 1: Native substrates

Approximately half the cost of a finished crystalline silicon solar module is due to the silicon itself. Combining this fact with a high-efficiency potential makes thin-film crystalline silicon solar cells a growing research area. This paper, written in two parts, aims to outline world-wide research on this topic. The subject has been divided into techniques which use native substrates and techniques which use foreign substrates. Light trapping, vapour- and liquid-phase deposition techniques, cell fabrication and some general considerations are also discussed with reference to thin-film cells.     

A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates

One of the most promising ways to reduce the cost of photovoltaics is thin-film crystalline silicon solar cells. This paper, together with part 1, reviews the current state of research in thin-film crystalline silicon solar cells. Deposition on silicon, novel techniques which use a high-quality, reusable silicon substrate and light trapping have been described in part 1 of this paper. This paper describes deposition on glass and ceramics and discusses cell designs for thin-film crystalline silicon solar cells.     

非晶硅太阳能路灯系统的设计

介绍太阳能路灯系统的组成,各部分的工作原理、工作特点,并以沈阳汉锋工厂内安装的太阳能路灯为例,来说明非晶硅太阳能路灯系统的设计方法。具有弱光性、低成本的非晶硅太阳能电池与高亮度、长寿命的LED光源相结合将是未来照明工程的发展趋势。     

Mechanism for the anomalous degradation of Si solar cells induced by high-energy proton irradiation

High-energy and high-fluence proton irradiation of Si space solar cells has provoked an anomalous increase in short-circuit current, followed by its abrupt decrease and cell failure. A model is proposed which explains the phenomena by expressing a reduction in the carrier concentration of the base region, in addition to a decrease of minority-carrier diffusion length. The reduction in carrier concentration due to majority-carrier trapping by radiation-induced defects has the effect of (1) broadening the depletion region width and (2) increasing the resistivity of the base layer. The anomalous change in the quantum efficiency of the cells under high-fluence ( 10¹⁴cm⁻²) irradiation is also explained by considering the generation of a donor-type defect level with the irradiation.     

Laser fired contacts applied to the rear surface of heterojunction silicon solar cells

In this work, we fabricate heterojunction silicon solar cells on p-type substrates whose rear surface configuration is based on dielectric passivation and laser fired contacts (LFC cells). This is an alternative to boron-doped amorphous silicon film, with which we also fabricate solar cells for direct comparison (HJ cells). As substrates, 3.5 and 0.8 Ω cm p-type double-side polished FZ c-Si wafers are used. Regarding surface passivation for highly doped substrates, LFC configuration has some advantage due to the higher difficulty in creating an efficient amorphous back surface field. Additionally, those substrates are also more advantageous in terms of carrier injection when the rear surface is locally contacted. Thus LFC cells made on 0.8 Ω cm substrates reach V_oc values up to 680 mV, in the same range as that of their HJ cell counterpart, with better FF demonstrating that LFC configuration is a feasible alternative for highly doped substrates. Focusing on the impact of the distance between rear contacts on cell performance, we found a trade-off between open circuit voltage V_oc and fill factor FF. Finally electroluminescence characterization and the dependence of V_oc on pitch, modeled by Fischer's equation, indicate that the depassivated area due to the laser processing of the contacts is bigger than the contacted area.     

CF4/O2 dry etching of textured crystalline silicon surface in a-Si:H/c-Si heterojunction for photovoltaic applications

We investigated a dry cleaning procedure of the crystalline substrate, both mono- and multi crystalline silicon, to leave an uncontaminated surface using an etching process involving CF₄/O₂ mixture. A detailed investigation was performed to find compatibility and optimisation of amorphous layer depositions both on flat and textured silicon by changing the plasma process parameters. We found evidence that plasma etching acts by removing the native oxide and the damages of textured silicon and by leaving an active layer on silicon surface suitable for the emitter deposition. SEM analysis confirmed that it is possible to find plasma process conditions where no appreciable damages and change in surface morphology are induced. By using this process we achieved on amorphous crystalline heterostructure a photovoltaic conversion efficiency of 13% on 51 cm² and 14.5% on 1.26 cm² active area. We also investigated compatibility of the process with industrial production of large area devices.     

Optimization of fabrication process of high-efficiency and low-cost crystalline silicon solar cell for industrial applications

The process conditions for a high-efficiency and low cost crystalline silicon solar cell were optimized. Novel approaches such as wafer cleaning and saw -damage removal using 0.5 wt% of 2,4,6-trichloro-1,3,5-triazine, silicon surface texturing with optimized pyramid heights (∼5 μm), and a third step of drive-in after phosphosilicate glass (PSG) removal followed by oxide removal were investigated. A simple method of chemical etching adopted for edge isolation was optimized with edge etching of 5-10 μm, without any penetration of chemicals between the stacked wafers. The conversion efficiency, open-circuit voltage, short-circuit current, and fill factor of the cell fabricated with the optimized process were a maximum of 17.12%, 618.4 mV, 5.32 A, and 77% under AM1.5 conditions, respectively.     

Characterization of crystalline silicon grown by plasma-enhanced CVD for thin-film solar cells

High growth-rate Si epitaxy by plasma-enhanced chemical vapor deposition (PECVD) has been investigated for a thin-film solar cell application. A high growth rate of 50 μm/h was obtained at 1050°C with plasma which is 50% larger than that by the conventional CVD without plasma. The electrical properties are almost the same for epitaxial layers with and without plasma. For undoped n-type layers, the Hall mobility and carrier density were about 600 cm²/V s and low 10¹⁵ cm⁻³, respectively. The electron diffusion length in doped p-type layers was about 20 μm. These electrical properties for the layer with plasma, in spite of higher growth rate, are comparable or better than those without plasma.     

Activation of dopant in the p-layer of amorphous silicon solar cells under illumination

We have investigated the effect of light-soaking on the p-doped layer of amorphous silicon (a-Si:H) solar cells by low temperature (50–300 K) AC conductance measurements. The experimental results are interpreted on the basis of an equilibration model of the doped material. The model takes into account the finite dimension of the layer and its presence inside a complex structure. It is shown that the Fermi level shifts after light soaking, which can result in activation of the doping impurities.     

Stability of thin film solar cells having less-hydrogenated amorphous silicon i-layers

In this study, highly stabilized hydrogenated amorphous silicon films and their solar cells were developed. The films were fabricated using the triode deposition system, where a mesh was installed between the cathode and the anode (substrate) in a plasma-enhanced chemical vapor deposition system. At a substrate temperature of 250 °C, the hydrogen concentration of the resulting film (Si–H=4.0 at%, Si–H₂<1×10²⁰ cm⁻³) was significantly less than that of conventionally prepared films. The films were used to develop the i-layers of solar cells that exhibited a significantly low degradation ratio of 7.96%.     

Analysing consequence of solar irradiance on amorphous silicon solar cell in variable underwater environments

Harvesting underwater Solar energy using photovoltaic (PV) technology leads to an innovative approach to utilize it in monitoring various underwater sensors, devices, or other autonomous systems using modern-day power electronics. Another huge advantage of placing PV cells underwater comes from the fact that the water itself can provide cooling and cleaning for the cells. Such advantages come with many challenges and constraints due to the underwater spectral change and decrease in Solar radiation with an increase in water depth. In this work, an experimental set-up has been realized to create an underwater environment and further characterized in the indoor environment using the Solar simulator. Moreover, the transfer of Solar radiation through water and the performance of amorphous silicon Solar cell underwater up to 0.2 m has been analysed in changing underwater environments. This investigation shows a better understanding of solar radiation underwater and the amorphous silicon solar cell underwater at shallow depths with considering the water depth up to 0.2 m, salinity 3.5%, total dissolved salts, and other impurities affecting the solar radiation and the performance of amorphous silicon Solar cell in underwater conditions. In addition to that, the maximum power output P_max of amorphous silicon Solar cell is 0.0367 W at 0.2 m in the case of DI water. In contrast, in real seawater and artificial seawater with 3.5% salinity, it shows 0.0337 W and 0.0327 W, respectively.     

Investigation of Ag-bulk/glassy-phase/Si heterostructures of printed Ag contacts on crystalline Si solar cells

The interface structure of screen-printed silver contacts on a crystalline silicon solar cell has been studied by transmission electron microscopy (TEM). TEM results confirmed that the glassy-phase plays an important role in contact properties. There are at least three different microstructures present in optimal fired contacts. The location where silver-bulk directly contacts silicon is observed through SEM, and this is actually a very thin glass layer in between. In addition, high-density silver embryos on silicon were found for samples fired optimally. The results presented in this study suggest that Ag-bulk/thin-glass-layer/Si contact is the most decisive path for current transportation.     

Degradation and annealing of amorphous silicon solar cells by current injection: experiment and modeling

In this paper we report in detail on the effect of current injection in amorphous silicon solar cells. A set of devices has been degraded and then annealed at different current intensities. Device performances during the whole experiment have been monitored by current–voltage characteristics and quantum efficiency curves. It has been found that annealing rate increases with current intensity, while stabilized photovoltaic parameters decrease. Time evolution of efficiency and short-circuit current during degradation has been reproduced by a numerical device modeling, resulting in a pronounced increase of defects near the p–i interface. The model also demonstrated that annealing results are not well reproduced if current-induced annealing is not energy selective.