The Role of Silicon Heterojunction and TCO Barriers on the Operation of Silicon Heterojunction Solar Cells: Comparison between Theory and Experiment (original) (raw)
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Journal of The …, 2011
The fabrication of amorphous silicon/crystalline silicon ͑a-Si:H/c-Si͒ heterojunction solar cell and an understanding of the fundamental conduction mechanism in the device are presented. In the first part, the effect of intrinsic amorphous silicon ͓a-Si:H͑i͔͒ layer thickness on the performance of a-Si:H/c-Si solar cells has been studied. The thickness of a-Si:H͑i͒ layer formed on n-type c-Si substrate was controlled accurately with spectroscopy ellipsometry ͑SE͒. Based on SE results, we discuss the influence of the a-Si:H͑i͒ thickness on the interface quality and thereby cell performance. Then, in the latter part, we present the temperaturedependent current density-voltage curves, in the dark, in order to elucidate the dominant transport mechanisms in a-Si:H/c-Si heterojunction solar cells with and without incorporation of a-Si:H͑i͒ layers. Finally, using optimum design considerations, we obtained a solar cell efficiency of 17.43%.
Amorphous silicon oxide window layers for high-efficiency silicon heterojunction solar cells
Journal of Applied Physics, 2014
In amorphous/crystalline silicon heterojunction solar cells, optical losses can be mitigated by replacing the amorphous silicon films by wider bandgap amorphous silicon oxide layers. In this article, we use stacks of intrinsic amorphous silicon and amorphous silicon oxide as front intrinsic buffer layers and show that this increases the short-circuit current density by up to 0.43 mA/cm 2 due to less reflection and a higher transparency at short wavelengths. Additionally, high open-circuit voltages can be maintained, thanks to good interface passivation. However, we find that the gain in current is more than offset by losses in fill factor. Aided by device simulations, we link these losses to impeded carrier collection fundamentally caused by the increased valence band offset at the amorphous/crystalline interface. Despite this, carrier extraction can be improved by raising the temperature; we find that cells with amorphous silicon oxide window layers show an even lower temperature coefficient than reference heterojunction solar cells (À0.1%/ C relative drop in efficiency, compared to À0.3%/ C). Hence, even though cells with oxide layers do not outperform cells with the standard design at room temperature, at higher temperatures-which are closer to the real working conditions encountered in the field-they show superior performance in both experiment and simulation. V
Materials Science and Engineering: B, 2009
a b s t r a c t "Heterojunction with Intrinsic Thin layer (HIT)" solar cells on P-type crystalline silicon (c-Si) wafers, have been studied using the detailed electrical-optical numerical simulator "Amorphous Semiconductor Device Modeling Program (ASDMP)", in conjunction with experiments. The aim is to understand what factor has the maximum impact on the conversion efficiency of these cells. For this purpose, we characterize the amorphous-crystalline interfaces by the defect states on the surface of the c-Si wafer (N ss ), the wafer itself by its minority carrier lifetime; and the contacts by the recombination velocities of free carriers there. We find that N ss plays a crucial role in limiting cell performance, so much so that for N ss > 10 12 cm −2 , the cell efficiency falls sharply regardless of the quality of the c-Si absorber layer or the contacts. Defect states on the surface of the P-type wafer facing the emitter N-a-Si:H, primarily influence the open-circuit voltage (which may change by over 200 mV) and the fill factor (FF), while defects on the surface contacting with the amorphous BSF layer in double HIT, mostly influence the short-circuit current density and FF. On the other hand, the variation of the minority carrier lifetime in the P-type wafer between 0.1 and 2.5 ms improves V oc by a maximum of 75 mV while the contacts have practically no influence.
Silicon heterojunction solar cells toward higher fill factor
Progress in Photovoltaics: Research and Applications, 2020
One of the most limiting factors in the record conversion efficiency of amorphous/crystalline silicon heterojunction solar cells is the not impressive fill factor value. In this work, with the aid of a numerical model, the ways to enhance the cell fill factor up to 85% are investigated in detail, considering the properties of conventional amorphous-doped films, wider Energy gap layers, and transparent conductive oxide films. The band alignment among the various materials composing the heterojunction is the key to high efficiency but becomes an issue for the solar cell fill factor, if not well addressed. One of the most interesting outcomes of this work is the evidence of hidden barriers arising between the transparent conductive oxide and both selective contacts, due to the mismatch between their work functions. The measurement of light current-voltage characteristics performed at low temperature is proposed as a way to identify the presence of these barriers in efficient solar cells that do not possess high fill factor values. Experimental J-V characteristics compared with numerical simulations demonstrated that the sometimes neglected cell base contact needs instead a more careful consideration. To this aim, a model to predict the presence of a hidden barrier at the base contact that limits the cell fill factor is proposed.
In order to compensate the insufficient conductance of heterojunction thin films, transparent conductive oxides (TCO) have been used for decades in both-sides contacted crystalline silicon heterojunction (SHJ) solar cells to provide lateral conduction for efficient carrier collection. In this work, we substitute the TCO layers by utilizing the lateral conduction of c-Si absorber, thereby enabling a TCO-free design. A series resistance of 0.32 Ωcm2 and a fill factor of 80.7% were measured for a TCO-free back-junction SHJ solar cell with a conventional finger pitch of 1.8 mm, thereby proving that relying on lateral conduction in the c-Si bulk is compatible with low series resistances. Achieving high efficiencies in SHJ solar cells with TCO-free front contacts requires suppressing deterioration of the passivation quality induced by direct metal-a-Si:H contacts and in-diffusion of metal into the a-Si:H layer. We show that an ozone treatment at the a-Si:H/metal interface suppresses the m...
Solar Energy Materials and Solar Cells, 2014
We briefly review the basic concepts of junction capacitance and the peculiarities related to amorphous semiconductors, paying tribute to Cohen and to his pioneering work. We extend the discussion to very high efficiency silicon heterojunction (SiHET) solar cells where both an amorphous semiconductor, namely hydrogenated amorphous silicon, and heterojunctions are present. By presenting both modeling and experimental results, we demonstrate that the conventional theory of junction capacitance based on the depletion approximation in the space charge region, cannot reproduce the capacitance data obtained on SiHET cells. The experimental temperature dependence is significantly stronger than that of the depletion-layer capacitance, while the bias dependence yields underestimated values of the diffusion potential, leading to strong errors if applied to the determination of band offsets using the procedure proposed precedingly in the literature. We demonstrate that this is not related to the amorphous nature of a-Si:H, but to the existence of a strongly inverted c-Si surface layer that requires minority carriers to be taken into account in the analysis of the junction capacitance.
15th International Conference on Concentrator Photovoltaic Systems (CPV-15), 2019
Silicon heterojunction (SHJ) solar cell technology has the potential to be the next mainstream industrial solar cell design due to its high efficiency and lean production process with only four main process steps. While two-side contacted SHJ cells have very high open circuit voltages (Voc) >740 mV, they tend to be lower in short circuit current density (Jsc) and fill factor (FF). Understanding the series resistance (Rs) components of such cells is crucial as these cells have two extra TCO/a-Si/Si contact resistances due to the optically absorptive passivating electrodes. Reducing the Rs components contribution is essential to improve the FF. In this paper, we report a straightforward and simple analytical model to break down the Rs of our SHJ solar cell having >23% efficiency into its components with the aid from common characterization methods, namely transfer length method (TLM) and Cox and Strack method. We derived the silicon bulk to transparent conductive oxide (TCO) contact resistivity through the amorphous-silicon (a-Si:H) intrinsic/p-doped stacks, a parameter that is not measureable directly, from experimental SHJ solar cell results, using front-junction, rear-junction and front finger number variation setups. We found it to be 0.30 ± 0.07 Ωcm 2. Further reducing this value is one of the keys to improve SHJ solar cell's FF.
IEEE Journal of Photovoltaics, 2018
We have analyzed a-Si:H(p)/a-Si:H(i)/c-Si(n) heterojunction silicon solar cell having the S-shaped current densityvoltage characteristics with a low fill factor and open-circuit voltage, using quantum efficiency (QE) characterization technique under forward/reverse voltage and different light (blue, infrared, and white) bias conditions. The curvature of S-shape is sensitive to excitation light intensities because of modification in junction barrier potential (variation in quasi-Fermi levels splitting). With forward-bias voltage alone near/above S-shaped region, cell's QE is uniformly reduced because of reduction in junction field and dominance of barrier for collection of holes. However, with blue and white light at bias voltages close to S-shaped characteristics, a uniform improvement of QE in broad wavelength region is observed because of defects saturation at the junction interface and photoconductivity in the a-Si layers. With white light and voltage bias, cell's QE is anomalously improved and it has even crossed the QE response at no voltage/light bias conditions in the blue region because of defects saturation in a-Si:H layers, whereas under infrared light and voltage bias conditions defect saturation is not displayed in the QE because of carrier generation in a deeper region of the cell after crossing unabsorbed photons front region.
Journal of Applied Physics, 2010
Heterojunction with intrinsic thin layer or "HIT" solar cells are considered favorable for large-scale manufacturing of solar modules, as they combine the high efficiency of crystalline silicon ͑c-Si͒ solar cells, with the low cost of amorphous silicon technology. In this article, based on experimental data published by Sanyo, we simulate the performance of a series of HIT cells on N-type crystalline silicon substrates with hydrogenated amorphous silicon ͑a-Si:H͒ emitter layers, to gain insight into carrier transport and the general functioning of these devices. Both single and double HIT structures are modeled, beginning with the initial Sanyo cells having low open circuit voltages but high fill factors, right up to double HIT cells exhibiting record values for both parameters. The one-dimensional numerical modeling program "Amorphous Semiconductor Device Modeling Program" has been used for this purpose. We show that the simulations can correctly reproduce the electrical characteristics and temperature dependence for a set of devices with varying I-layer thickness. Under standard AM1.5 illumination, we show that the transport is dominated by the diffusion mechanism, similar to conventional P/N homojunction solar cells, and tunneling is not required to describe the performance of state-of-the art devices. Also modeling has been used to study the sensitivity of N-c-Si HIT solar cell performance to various parameters. We find that the solar cell output is particularly sensitive to the defect states on the surface of the c-Si wafer facing the emitter, to the indium tin oxide/P-a-Si:H front contact barrier height and to the band gap and activation energy of the P-a-Si:H emitter, while the I-a-Si:H layer is necessary to achieve both high V oc and fill factor, as it passivates the defects on the surface of the c-Si wafer. Finally, we describe in detail for most parameters how they affect current transport and cell properties.
Solar Energy Materials and Solar Cells
In this study we make a detailed comparison between indium tin oxide (ITO), aluminum-doped zinc oxide (ZnO:Al) and hydrogenated indium oxide (IO:H) when applied on the illuminated side of rear-junction silicon heterojunction (SHJ) solar cells. ITO being the state of the art material for this application, ZnO:Al being an attractive substitute due to its cost effectiveness and IO:H being a transparent conductive oxide (TCO) with high-mobility and excellent optical properties. Through numerical simulations, the optically optimal thicknesses for a double layer anti-reflective coating system, consisting of the respective TCO and amorphous silicon oxide (a-SiO2) capping layers are defined. Through two-dimensional electrical simulations, we present a comparison between front-junction and rear-junction devices to show the behavior of series resistance (Rs) in dependence of the TCO sheet resistance (Rsh) and the device effective lifetime (τeff). The study indicates that there is a τeff dependent critical TCO Rsh value, above which, the rear-junction device will become advantageous over the front-junction design in terms of Rs. Solar cells with the respective layers are analyzed. We show that a thinner TCO optimized layer will result in a benefit in cell performance when implementing a double layer anti-reflective coating. We conclude that for a highest efficiency solar cell performance, a high mobility TCO, like IO:H, is required as the device simulations show. However, the rear-junction solar cell design permits the implementation of a lower conductive TCO in the example of the cost-effective ZnO:Al with comparable performance to the ITO, opening the possibility for substitution in mass production.