Kunal Ghosh

Explanation of the device operation principle of amorphous silicon/ crystalline silicon heterojunction solar cell and role of the inversion of crystalline silicon surface

The device operation principle of amorphous silicon/crystalline silicon heterojunction solar cell is discussed. The band diagram obtained by the computer model developed in the commercial simulator Sentaurus shows that the c-Si surface is inverted at the interface between a-Si and c-Si (heterointerface). A strong inversion gives a strong electric field at the c-Si surface, which in turn facilitates the transport of minority carriers across the heterointerface. A high performance device requires a strongly inverted c-Si surface. Calculations are performed to show that the doping of the doped a-Si layer, the thickness of the intrinsic layer, and the defect state density at the heterointerface all affect the inversion of the crystalline silicon surface. Unlike homojunction devices, the defects in heterojunction devices have a greater role in transport mechanism than in recombination mechanism. The results show that in devices with a large number of defects at the interface, the fill factor degrades with little change in open circuit voltage. This explains why it is relatively easy to obtain VOCs approaching 700 mV with heterojunctions but often with low fill factors.

2D modeling of Silicon Heterojunction Interdigitated Back Contact solar cells

Silicon Heterojunction Interdigitated Back Contact (SHJ-IBC) solar cells were studied by two dimensional modeling using Sentaurus TCAD tools. It was shown that low fill factor caused by the S-shape behavior of experimental J-V curves of standard interdigitated back contact cells can be recovered by making small openings in the intrinsic buffer layer. The small openings in the buffer layer also substantially reduce the influence of the relative dimensions of the silicon strips as when compared to cells with a continuous buffer layer.

Results from coupled optical and electrical sentaurus TCAD models of a gallium phosphide on silicon electron carrier selective contact solar cell

We report results from coupled optical and electrical Sentaurus TCAD models of a gallium phosphide (GaP) on silicon electron carrier selective contact (CSC) solar cell which show that Auger-limited open-circuit voltages up to 787 mV (on a 10 μm monocrystalline silicon substrate) and efficiencies up to 26.7% (on a 150 μm monocrystalline silicon substrate) may be possible for front-contacted devices which exhibit low interface recombination velocity (IRV) at the GaP/Si interface and which employ random pyramidal texturing, a detached silver reflector, rear locally diffused point contacts and a SiO2/Al2O3 rear oxide passivation stack.

Growth and characterization of GaAs1−xSbx barrier layers for advanced concept solar cells

The InAs∕GaAsSb material system is a promising medium for the implementation of a quantum dot solar cell due to a favorable valence band alignment. The quantum dot solar cell requires a highly dense, highly ordered array of quantum dots for overlap of wave functions to form a band in the band gap of the host material. Since the GaAsSb barriers are a III-V-V ternary the alloy composition is particularly sensitive to variations in temperature. We have studied the variation in Sb content for thin layers of GaAsSb in GaAs for various fluxes of Sb. The purpose was to be able to predict the required Sb flux at a particular temperature to obtain a desired Sb composition. The target Sb composition for this study was 12% with the composition obtained confirmed by x-ray diffraction. By also studying the reciprocal space maps of the 12% samples, it is inferred that the composition can be maintained for a large temperature range. The implications of these results for the growth of InAs quantum dots on GaAsSb barriers...

Hot hole transport in a-Si/c-Si heterojunction solar cells

The transport behavior of photogenerated minority carriers in an a-Si/c-Si heterojunction solar cell is dependent on the energy distribution function (EDF) of the carriers impinging on the hetero-interface. The high field region at the interface results in a strongly non-Maxwellian distribution of holes incident on the surface, which has implications for current collection and a significant impact on the overall efficiency of the device. This work studies the effect of the high field transport on photogenerated carriers at the hetero-interface through a combination of Monte Carlo simulations and analysis of defect assisted transport. A three band warped non-parabolic band model is implemented to describe the valence band in order to accurately represent high energy photocarriers. Also, percolation path theory is applied to study defect assisted transport in the intrinsic amorphous region by considering mechanisms such as defect capture through tunneling, emission through Poole - Frenkel effect, and emission through tunneling.

Structural and optical investigations of GaN-Si interface for a heterojunction solar cell

In recent years the development of heterojunction silicon based solar cells has gained much attention, lead largely by the efforts of Panasonics HIT cell. The success of the HIT cell prompts the scientific exploration of other thin film layers, besides the industrially accepted amorphous silicon. The band gap, mobilities, and electron affinity of GaN make it an interesting candidate to solve problems of parasitic absorption while selectively extracting electrons. Using a novel MBE based growth technique, thin films of GaN have been deposited at temperature significantly lower than industry standards. Crystalline measurements and absorption data of GaN are presented. Additionally, effects of deposition on the silicon wafer lifetimes are presented.

Non PN junction solar cells using carrier selective contacts

A novel device concept utilizing the approach of selectively extracting carriers at the respective contacts is outlined in the work. The dominant silicon solar cell technology is based on a diffused, top-contacted p-n junction on a relatively thick silicon wafer for both commercial and laboratory solar cells. The VOC and hence the efficiency of a diffused p-n junction solar cell is limited by the emitter recombination current and a value of 720 mV is considered to be the upper limit. The value is more than 100 mV smaller than the thermodynamic limit of VOC as applicable for silicon based solar cells. Also, in diffused junction the use of thin wafers (< 50 um) are problematic because of the requirement of high temperature processing steps. But a number of roadmaps have identified solar cells manufactured on thinner silicon wafers to achieve lower cost and higher efficiency. The carrier selective contact device provides a novel alternative to diffused p-n junction solar cells by eliminating the need for complementary doping to form the emitter and hence it allows the solar cells to achieve a VOC of greater than 720 mV. Also, the complete device structure can be fabricated with low temperature thin film deposition or organic coating on silicon substrates and thus epitaxially grown silicon or kerfless silicon, in addition to standard silicon wafers can be utilized.

Experimental and theoretical verification of the presence of inversion region in a-Si/c-Si heterojunction solar cells with an intrinsic layer

Photovoltaic devices based on amorphous silicon (a-Si)/ crystalline silicon (c-Si) heterostructure exhibits excellent surface passivation with the highest open circuit voltage being reported on these devices. A plausible explanation for these devices to show low recombination is that the junction is induced in c-Si and an inversion region is present at the heterointerface. In this work, the presence of the inversion region at the heterointerface between intrinsic a-Si and c-Si is theoretically shown by a computer model developed in the commercial simulator Sentaurus and experimentally corroborated by lateral conductance measurement technique.

Growth of InAs quantum dots on GaAsSb for the realization of a quantum dot solar cell

The InAs/GaAsSb quantum dot/barrier material system has been identified as a candidate for implementing the quantum dot (QD) solar cell for an Sb content of ∼ 12%. We present results from the growth of this system on GaAs substrates by Molecular Beam Epitaxy (MBE). The results show that the growth of GaAsSb requires special care in order to ensure the highest quality interface and also to maintain the Sb composition. When InAs QDs are grown on the GaAsSb, the role of strain in determining the properties of the QDs is seen to be profound. Results from PL studies show that the sizes of the QDs are controlled by the GaAsSb layer thickness and hence the residual strain at the GaAsSb surface. In addition a change from type I to type II transitions can be affected by this method. The implications of these results plus the influence of the substrate choice, and so the strain, on the system properties will be discussed in terms of a QD solar cell design with a strain balanced design enabling a much larger active region and hence higher absorption.

Material selection for three level transition using Quantum well structure

Nanostructured devices (Quantum dot and Quantum well) have been proposed as a way to overcome the Shockley-Quiesser limit of a single junction solar cell as they have the potential to show three quasi Fermi levels. In this paper the material combinations that can be used in a Quantum Well solar cell to realize a multiple quasi-Fermi level device will be discussed. The calculations were done on different possible combinations of direct band gap III–V semiconductors, with the effect of strain being taken into account by applying 6 band K.p model. A detailed balance calculation was done on the materials selected to determine their maximum efficiency under 1 sun AM0. The material combination of Al0.63In0.37 As as the barrier and InAs0.16P0.84 as the well with InP as the substrate is found to be the best material combination giving a theoretical efficiency of 43% under 1 sun AM0, compared to the maximum three-level efficiency of 47% under the same conditions.