P. Chatterjee

Photovoltaic performance of a‐Si:H homojunction p‐i‐n solar cells: A computer simulation study

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 lig...

The open circuit voltage in amorphous silicon p-i-n solar cells and its relationship to material, device and dark diode parameters

We review the sensitivity of the open-circuit voltage (Voc) to material, device and dark diode parameters, and try to assess the possibilities of improving this quantity in hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells, having a wide band-gap emitter layer. We find that parameters that affect Voc can broadly be classified into two categories: those that alter the built-in potential (Vbi) appreciably, and those that produce small or no change in Vbi, but can still alter Voc by changing the dark recombination current in the intrinsic absorber or the interfaces. The study is carried out using an electrical-optical model based on the solution of the Poisson’s and the continuity equations. In agreement with existing work, we find that Voc is very sensitive to all parameters that appreciably alter Vbi, viz., the transparent conducting oxide∕P contact barrier height, the P layer thickness and its activation energy, although Voc is found to be more sensitive to these parameters than Vbi itself. Howeve...

A computer analysis of the effect of a wide‐band‐gap emitter layer on the performance of a‐Si:H‐based heterojunction solar cells

A first principles computer model for simulating the performance of amorphous‐silicon‐based solar cells has been applied to study the effect on solar cell performance of using an emitter layer with band gap larger than that of the intrinsic absorber. We surprisingly find that if the transparent conducting oxide/p‐layer contact barrier height φb0 is held constant, the effect of increasing the band gap of the emitter layer Eμ(p) is to depress the open‐circuit voltage Voc, fill factor (FF), and cell efficiency η, although the short‐circuit current Jsc increases as expected. The main reason for the decrease of Voc, FF, and η at constant φb0 is that as Eμ(p) expands, the field corresponding to the gradient in the electron affinity at the p/i interface increases also, leading more and more to a collapse of the field over the intrinsic absorber layer. Considering the effect of φb0, we find that Jsc in these structures is practically independent of this parameter. However, φb0 exerts considerable influence on Voc...

Analysis and optimization of the performance of polymorphous silicon solar cells: Experimental characterization and computer modeling

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 co...

Computer analysis of a-Si:H p-i-n solar cells with a hydrogenated microcrystalline silicon p layer

An integrated electrical–optical model has been used to analyze the performance of hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells having a p-type hydrogenated microcrystalline silicon (p-μc-Si:H) window layer. Our attempts to simulate various experimentally measured solar cell characteristics of such cells available in the literature indicate that for the same thickness, μc-Si:H p layers may have different mobility band gaps (Eμ), which may be linked to different crystalline volume fractions (Fc). Modeling reveals that there is both an optimum value for Eμ (therefore Fc), as well as an optimum thickness of the μc-Si:H p layer for obtaining the best solar cell performance. A thin (8–10 nm) film having a rather low crystalline volume fraction (high Eμ) was found from our computer analysis to give the best results. This is because as the band gap of μc-Si:H p layer decreases (i.e., its crystallinity increases), the valence band offset at the p/i interface increases, leading to a collapse of the bu...

Metastability study and optimization of polymorphous silicon solar cells: the state-of-the-art

Optimization of polymorphous silicon solar cells is carried out through a combination of experiments and modelling of the device characteristics. The improved transport properties of polymorphous silicon films show up in the higher stability of the cells, while the more reactive plasma conditions used in pm-Si:H deposition as compared to a-Si:H result in a deterioration of the TCO/p/i layers (interfaces). This is clearly detected from the modelling results and supported by experiments in which the TCO substrate is protected against the hydrogen rich plasma used in pm-Si:H deposition. Thus the efficiency limiting factors of pm-Si:H cells are clearly presented, their higher stability demonstrated and guidelines to further improve their efficiency established.

No benefit from microcrystalline silicon N layers in single junction amorphous silicon p-i-n solar cells

The use of phosphorous-doped microcrystalline silicon (μc-Si:H) as the n-type electrode in single junction hydrogenated amorphous silicon solar cells has been studied both experimentally and through computer modeling. The aim is to understand why, in spite of a considerable decrease in the activation energy of the n layer—from 0.2 eV in n-a-Si:H to 0.03 eV in n-μc-Si:H—the open-circuit voltage of solar cells fabricated using these two types of n layer remains almost unchanged. Experimental determination of the work function of n-μc-Si:H and n-a-Si:H by the “flatband heterojunction” technique, has revealed that n-μc-Si:H has a higher electron affinity. Thus, in spite of the fact that the difference in activation energy is 0.17 eV, the difference in built-in potential between the two types of cells reduces to about half. Moreover, modeling of the output characteristics of solar cells, having these two types of N layer, indicates a sharp localization of the field at the N/I interface for the cell with a μc-S...

Experimental study and modeling of reverse-bias dark currents in PIN structures using amorphous and polymorphous silicon

Polymorphous silicon (pm-Si:H) is a nanostructured silicon thin film, with a lower defect density of states and better electronic properties than standard amorphous silicon. We have studied the reverse-bias dark current in PIN structures using this material as the intrinsic layer and compared the results to amorphous silicon PIN devices. All the structures were grown using a standard plasma enhanced chemical vapor deposition process. For thick pm-Si:H devices, we have achieved reverse-bias dark current densities about ten times lower than those obtained using amorphous silicon as the intrinsic layer. This is consistent with the lower defect density of states in polymorphous silicon, which is about 7×1014 cm−3 against 5×1015 cm−3 for amorphous silicon. For a 2.5-μm-thick pm-Si:H diode, the current density obtained is as low as 3 pA cm−2 at −3 V. However, for thinner structures (∼0.5 μm), polymorphous and amorphous silicon show nearly the same reverse-bias leakage current. The experimental dark as well as i...

Origin of the modulated photocurrent in the presence of bias light in quantum efficiency measurements

It is demonstrated mathematically that in quantum efficiency measurement in the presence of a constant bias illumination, one of three possible phenomena occurs: (1) monochromatic‐light‐produced field redistribution has negligible impact and almost all photocarriers in the quantum efficiency measurement are from the probe monochromatic light; (2) the monochromatic light causes appreciable improvement in the electric field in some part of the device, causing a channeling of bias‐light‐produced photocarriers into the modulated current response. This creates the possibility of apparent quantum efficiencies greater than unity; and (3) the monochromatic light has a significant detrimental influence on the electric field in some part of the device. This may result in negative values of quantum efficiency, although the lock‐in measurement must be suitably modified to detect the effect. Using our detailed computer‐analysis model, two conditions are identified in which quantum efficiencies greater than unity can b...

Hydrogenated amorphous silicon films with low defect density prepared by argon dilution: application to solar cells

Abstract Structural study of several amorphous silicon (a-Si:H) films deposited by rf-PECVD from a silane–argon mixture have revealed that the 90% argon diluted sample has the lowest microstructure and defect density. This sample has been further compared with a-Si:H layer deposited from undiluted silane by using each of them as the intrinsic (i) layer in p–i–n solar cells. The output characteristics of these devices have been studied experimentally and by theoretical modeling. These studies demonstrate that the improvement in the stabilized output characteristics of the cell with argon diluted i-layer results both from the structural improvement of the a-Si:H layer, as well as a reduction of the p/i interface defects.