The current losses due to parasitic absorption in the indium tin oxide (ITO) and amorphous silicon (a-Si:H) layers at the front of silicon heterojunction solar cells are isolated and quantified. Quantum efficiency spectra of cells in which select layers are omitted reveal that the collection efficiency of carriers generated in the ITO and doped a-Si:H layers is zero, and only 30% of light absorbed in the intrinsic a-Si:H layer contributes to the short-circuit current. Using the optical constants of each layer acquired from ellipsometry as inputs in a model, the quantum efficiency and short-wavelength current loss of a heterojunction cell with arbitrary a-Si:H layer thicknesses and arbitrary ITO doping can be correctly predicted. A 4 cm2 solar cell in which these parameters have been optimized exhibits a short-circuit current density of 38.1 mA/cm2 and an efficiency of 20.8%.

/pdf/current-losses-at-the-front-of-silicon-heterojunction-solar-9rf44rhs66.pdf

Current Losses at the Front of Silicon Heterojunction Solar Cells

We have measured C‐V characteristics and temperature dependence of J‐V characteristics of undoped hydrogenated amorphous silicon (a‐Si:H) heterojunctions formed on p‐type crystalline silicon ( p c‐Si) substrates with different resistivities. It has been found that an abrupt heterojunction model is valid for a‐Si:H/p c‐Si heterojunctions, and the electron affinity of a‐Si:H has been estimated as 3.93±0.07 eV from C‐V characteristics. The forward current of all the junctions studied shows voltage and temperature dependence expressed as exp(−ΔEa f/kT) exp(AV), where ΔEa f and A are constants independent of voltage and temperature, being successfully explained by a multitunneling capture‐emission model. The reverse current is proportional to exp(−ΔEar/kT)(VD−V)1/2, where VD is the diffusion voltage and ΔEar is a constant. This current is probably limited by generation‐recombination process.

Electrical properties of n-amorphous/p-crystalline silicon heterojunctions

We have achieved a very high conversion efficiency of 21·5% in HIT cells with a size of 100·3 cm2. One of the most striking features of the HIT cell is its high open-circuit voltage Voc, in excess of 710 mV. This is due to the excellent surface passivation at the a-Si/c-Si heterointerface realized by Sanyo's successful technologies for fabricating high-quality a-Si films and solar cells with low plasma damage processes. We have studied ways to treat the surface to produce a good interface throughout our fabrication processes. We have also investigated the deposition conditions of a-Si layers for optimizing the barrier height for the minority carriers in the heterojunction. Our approach for obtaining HIT cells with a high Voc is reviewed here. Copyright © 2005 John Wiley & Sons, Ltd.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pip.646

Obtaining a higher Voc in HIT cells

Intrinsic hydrogenated amorphous silicon films can yield outstanding electronic surface passivation of crystalline silicon wafers. In this letter the authors confirm that this is strongly determined by the abruptness of the interface. For completely amorphous films the passivation quality improves by annealing at temperatures up to 260°C, most likely by film relaxation. This is different when an epitaxial layer has been grown at the interface during film deposition. Annealing is in such a case detrimental for the passivation. Consequently, the authors argue that annealing followed by carrier lifetime measurements allows determining whether the interface is abrupt.

Abruptness of a-Si :H/c-Si interface revealed by carrier lifetime measurements

Silicon wafer solar cells continue to be the leading photovoltaic technology, and in many places are now providing a substantial portion of electricity generation. Further adoption of this technology will require processing that minimises losses in device performance. A fundamental mechanism for efficiency loss is the recombination of photo-generated charge carriers at the unavoidable cell surfaces. Dielectric coatings have been shown to largely prevent these losses through a combination of different passivation mechanisms. This review aims to provide an overview of the dielectric passivation coatings developed in the past two decades using a standardised methodology to characterise the metrics of surface recombination across all techniques and materials. The efficacy of a large set of materials and methods has been evaluated using such metrics and a discussion on the current state and prospects for further surface passivation improvements is provided.

/pdf/dielectric-surface-passivation-for-silicon-solar-cells-a-zv6y6fla1t.pdf

Dielectric surface passivation for silicon solar cells: A review

We have investigated the current-voltage characteristics of metal/undoped a-Si: H/n+ (or p+) c-Si structures using Mg, Au, Pt and Al. It has been indicated that the energy band of undoped a-Si: H shows a downward bending against Mg, being analogous to n+a-Si: H/undoped a-Si: H contact, while the band shows an upward bending against Au, Pt and Al. Mg/undoped a-Si: H contact is found to allow injection of electrons into the conduction band of a-Si: H, providing us a good ohmic contact property. This contact does not exhibit the thermal-degradation at least up to 100°C.

Ohmic Contact Properties of Magnesium Evaporated onto Undoped and P-doped a-Si: H

Investigated are the current‐voltage characteristics of metal (Au, Mg)/B‐doped hydrogenated amorphous silicon (a‐Si:H)/crystalline silicon (n+, p+) diodes for various doping levels of B in a‐Si:H. From junction studies we determine the conduction type of B‐doped a‐Si:H on the basis of ‘‘dominant’’ carrier concentration, and find that the p‐n transition occurs at B2H6/SiH4∼10−6 although the conductivity minimum appears at B2H6/SiH4∼10−4.

Metal-semiconductor junctions and amorphous-crystalline heterojunctions using B-doped hydrogenated amorphous silicon

We have measured C-V characteristics and temperature dependence of J-V characteristics of undoped a-Si:H heterojunctions formed on p-type c-Si substrates with different resistivities. It has been found out that an abrupt heterojunction model is valid for a-Si:H/p c-Si heterojunctions, and the electron affinity of a-Si:H has been estimated as 3.93 t 0.07 eV from C-V characteristics. The forward cument of all the junctions studied.shows voltage and temperature dependence expressed as exp(-M ^/kT) x exp(Av), where ot\". and A are constants independent of vg]{age and templratu?5. in. reverse current is -p*roportional to exp(AE^-/kT) x (V^_V)'r', where M is constant independent of voltage and temperature and Vo-is the diffusion voltage. ar

Electrical Properties of n--p Amorphous-Crystalline Silicon Heterojunctions

PURPOSE:To obtain a semiconductor device with the electrode being in ohmic- contact without deteriorating the characteristics of a semicondutor layer by directly coating the electrode with magnesium. CONSTITUTION:A gate insulating film 22 consisting of Si3N4, SiO2 or the like is laminated on N crystalline silicon 21 functioning as a gate, and a high-resistance amorphous silicon layer 23 as source-drain is laminated. Metallic magnesium is prepared directly on the surface of the amorphous silicon layer 23 through evaporation, and functions as a source electrode 24 and a drain electrode 25. Accordingly, the semiconductor device in which the electrodes are prepared directly to the high-resistance amorphous silicon layer 23 can be obtained without laminating an amorphous doping film of high impurity concentration at all.

Tetsuhiro Okuno

Papers

Electrical properties of n-amorphous/p-crystalline silicon heterojunctions

Ohmic Contact Properties of Magnesium Evaporated onto Undoped and P-doped a-Si: H

Metal-semiconductor junctions and amorphous-crystalline heterojunctions using B-doped hydrogenated amorphous silicon

Electrical Properties of n--p Amorphous-Crystalline Silicon Heterojunctions

Electrode being in ohmic-contact with semiconductor