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.

In order to simulate the performance of the present day state-of-the-art multijunction solar cells in its entirety, an integrated electrical-optical model has been developed. The one-dimensional ab initio electrical model for the analysis of the transport properties of such devices can handle a very general semiconductor device structure where the material properties vary with position and the gap state properties with position and energy. The original semi-empirical optical model used takes into account both specular interference effects, and diffused reflectances and transmittances due to interface roughness. The latter are derived from angular-resolved photometric measurements and used as input parameters to the numerical programme. Comparison of the illuminated current density-voltage (J-V) characteristics, calculated on the basis of (a) a simple exponential absorption law and (b) the optical model, reveals an increase of ˜1 mA cm−2 in the short-circuit current and ˜8% in the cell conversion efficiency for case (b). Also the long wavelength quantum efficiency (QE) shows a marked improvement, while the blue QE decreases since proper account is taken of the absorption in the transparent conducting oxide and reflection from the device. The combined model is being applied to simulate the characteristics of wideband-gap-emitter-layer solar cells deposited in a three chamber conventional glow discharge reactor onto (i) highly textured SnO2 and (ii) weakly textured indium tin oxide substrates. The cells have been characterised experimentally by J-V and QE measurements. Preliminary results indicate that the integrated model matches the experimental J-V and QE data with a more realistic set of material parameters as compared to case (a).

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A first principles computer model for simulating the performance of amorphous semiconductor solar cells has been developed. With a suitable choice of parameters, the calculated results for the illuminated J-V characteristics and solar cell quantum efficiencies are shown to agree well with experiments. The model has been used in this paper to study the sensitivity of the light J-V characteristics to various device and material parameters in p-i-n homojunction solar cells. The single most important factor controlling the open circuit voltage, short circuit current, fill factor, and cell efficiency is the transparent conducting oxide/p-a-Si:H contact barrier height φbo, when φbo is less than a certain critical value. Also shown is that practically no improvement in cell performance can be achieved by decreasing the dangling-bond midgap state density, described by Gaussian distribution functions, to lower than 1016 cm−3, unless the valence-band tail states are also reduced. Moreover, results indicate that light-induced defect states have neutral capture cross sections of 10−15 cm2, which is at least one order of magnitude higher than the corresponding quantity for the gap states in annealed materials. Finally, low band microscopic carrier mobilities are found to have a strong detrimental influence on solar cell performance.

Hydrogenated polymorphous silicon (pm-Si:H) is a nanostructured silicon thin film produced by plasma enhanced chemical vapor deposition under conditions close to powder formation. It has a lower initial and stabilized density of states, and a hole mobility considerably higher than state-of-the-art a-Si:H, which makes this material an interesting candidate for solar cell applications. In this article, we present experimental studies in conjunction with computer modeling to analyze and explain the relative performances of solar cells in which either a-Si:H or pm-Si:H is used as the intrinsic layer. Our results reveal large differences in the transport and metastability behavior of the two types of solar cells. Moreover, we observe a more damaged p/i interface for the pm-Si:H cells, although the p and n layers have been deposited under identical conditions. As a consequence, the cells fabricated initially with pm-Si:H did not perform better than standard a-Si:H based cells, despite the fact that the model confirmed the better transport properties of pm-Si:H films with respect to a-Si:H. Using insight gained from modeling, the deposition parameters were optimized to ultimately yield pm-Si:H based solar cells with conversion efficiencies higher than a-Si:H based cells in both the annealed and the light stabilized states.