Carrier transport and sensitivity issues in heterojunction with intrinsic thin layer solar cells on N-type crystalline silicon: A computer simulation study

Abstract

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.

Amorphous silicon oxide window layers for high-efficiency silicon heterojunction solar cells

“…At 55 C, the cell with an oxide buffer layer exhibits an efficiency of 18.0% (18.6% at 25 C), compared to 17.7% (19.6% at 25 C) obtained from the reference cell. A similar effect has been reported in a simulation study by Rahmouni et al, 51 where thicker buffer layers lead to favorable temperature coefficients due to improving FF. However, in contrast to our findings for a-SiO x :H layers, devices with thicker i a-Si:H passivation layers do not show a better performance at elevated temperatures.…”

Investigation of silicon heterojunction solar cells by photoluminescence under DC-bias

“…Photovoltaic performance is almost insensitive to the c-Si/BSF N id below 5× 10 11 cm −2 [8,9,14] and a value of 10 11 cm −2 has been assumed here in the modelling calculations. However, the calculated FF does not agree with the measured value, because of the high series resistance of the contacts, estimated to be ∼ 3 Ohm.…”

“…The HJ cell V oc is known to be strongly sensitive to the defect density at the emitter/c-Si interface [8,9] and this fact has been used to estimate N id at this junction from modelling and turns out to be 4.5 × 10 11 cm −2 . Photovoltaic performance is almost insensitive to the c-Si/BSF N id below 5× 10 11 cm −2 [8,9,14] and a value of 10 11 cm −2 has been assumed here in the modelling calculations.…”

“…The one-dimensional electrical-optical model ASDMP (Amorphous Semiconductor Device Modelling Program) [6,7], that has recently been extended to equally model HJ solar cells [8], solves the Poisson's equation and the two carrier continuity equations under steady state conditions for the given device structure. The complex refractive indices for each layer of the structure are required as input and have been measured in-house by spectroscopic ellipsometry.…”

Three-dimensional numerical analysis of hybrid heterojunction silicon wafer solar cells with heterojunction rear point contacts

“…24 A subsequent data fitting was performed in order to match the measured lifetime data. As a starting point, the input parameters provided by Rahmouni et al 25 in order to model a standard HIT cell were used, and some slight adjustments in both the bulk and the interface parameters were undertaken in order to fit the measured lifetime data, see Figure 4. Table I summarizes the calibrated simulation input parameters used for the modelling of the c-Si substrate and the intrinsic/doped silicon thinfilms, based on the fitting of the effective lifetime curves.…”