With a global market share of about 90%, crystalline silicon is by far the most important photovoltaic technology today. This article reviews the dynamic field of crystalline silicon photovoltaics from a device-engineering perspective. First, it discusses key factors responsible for the success of the classic dopant-diffused silicon homojunction solar cell. Next it analyzes two archetypal high-efficiency device architectures – the interdigitated back-contact silicon cell and the silicon heterojunction cell – both of which have demonstrated power conversion efficiencies greater than 25%. Last, it gives an up-to-date summary of promising recent pathways for further efficiency improvements and cost reduction employing novel carrier-selective passivating contact schemes, as well as tandem multi-junction architectures, in particular those that combine silicon absorbers with organic–inorganic perovskite materials.

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High-efficiency crystalline silicon solar cells: status and perspectives

The optimization of diamond films as valuable engineering materials for a wide variety of applications has required the development of robust methods for their characterization. Of the many methods used, Raman microscopy is perhaps the most valuable because it provides readily distinguishable signatures of each of the different forms of carbon (e.g. diamond, graphite, buckyballs). In addition it is non-destructive, requires little or no specimen preparation, is performed in air and can produce spatially resolved maps of the different forms of carbon within a specimen. This article begins by reviewing the strengths (and some of the pitfalls) of the Raman technique for the analysis of diamond and diamond films and surveys some of the latest developments (for example, surface-enhanced Raman and ultraviolet Raman spectroscopy) which hold the promise of providing a more profound understanding of the outstanding properties of these materials. The remainder of the article is devoted to the uses of Raman spectroscopy in diamond science and technology. Topics covered include using Raman spectroscopy to assess stress, crystalline perfection, phase purity, crystallite size, point defects and doping in diamond and diamond films.

Raman spectroscopy of diamond and doped diamond.

The isotope effect on the lattice thermal conductivity for group IV and group III-V semiconductors is calculated using the Debye-Callaway model modified to include both transverse and longitudinal phonon modes explicitly. The frequency and temperature dependences of the normal and umklapp phonon-scattering rates are kept the same for all compounds. The model requires as adjustable parameters only the longitudinal and transverse phonon Gr\"uneisen constants and the effective sample diameter. The model can quantitatively account for the observed isotope effect in diamond and germanium but not in silicon. The magnitude of the isotope effect is predicted for silicon carbide, boron nitride, and gallium nitride. In the case of boron nitride the predicted increase in the room-temperature thermal conductivity with isotopic enrichment is in excess of 100%. Finally, a more general method of estimating normal phonon-scattering rate coefficients for other types of solids is presented.

Estimation of the isotope effect on the lattice thermal conductivity of group IV and group III-V semiconductors

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%.

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Current Losses at the Front of Silicon Heterojunction Solar Cells

We explore substoichiometric molybdenum trioxide (MoOx, x < 3) as a dopant-free, hole-selective contact for silicon solar cells. Using an intrinsic hydrogenated amorphous silicon passivation layer between the oxide and the silicon absorber, we demonstrate a high open-circuit voltage of 711 mV and power conversion efficiency of 18.8%. Due to the wide band gap of MoOx, we observe a substantial gain in photocurrent of 1.9 mA/cm2 in the ultraviolet and visible part of the solar spectrum, when compared to a p-type amorphous silicon emitter of a traditional silicon heterojunction cell. Our results emphasize the strong potential for oxides as carrier selective heterojunction partners to inorganic semiconductors.

Silicon heterojunction solar cell with passivated hole selective MoOx contact

In this letter, we report on our investigations of hydrogenated amorphous silicon suboxides (a-SiOx:H) used as a high quality passivation scheme for heterojunction solar cells. The a-SiOx:H films were deposited using high frequency (70MHz) plasma enhanced chemical vapor deposition by decomposition of carbon dioxide, hydrogen, and silane at a substrate temperature of around 155°C. High effective lifetimes of outstanding 4ms on 1Ωcm n-type float-zone material and a surface recombination velocity of ⩽2.6cm∕s have been repeatedly obtained. Optical analysis revealed a distinct decrease of blue light absorption in the a-SiOx:H films compared to commonly used intrinsic amorphous silicon passivation used in heterojunction cells.

High quality passivation for heterojunction solar cells by hydrogenated amorphous silicon suboxide films

Thermochemical polishing of CVD diamond films

Glucose determination using a re-usable enzyme-modified ion track membrane sensor.

A passivation scheme, featuring nanocomposite amorphous silicon suboxides (a-SiOx:H) is investigated and analyzed in this work. The a-SiOx:H films are deposited by high-frequency plasma-enhanced chemical-vapor deposition via decomposition of silane (SiH4), carbon dioxide (CO2), and hydrogen (H2) as source gases. The plasma deposition parameters of a-SiOx:H films are optimized in terms of effective lifetime, while the oxygen content and the resulting optical band gap EG of the a-SiOx:H films are controlled by varying the CO2 partial pressure χO=[CO2]/([CO2]+[SiH4]). Postannealing at low temperatures of those films shows a beneficial effect in form of a drastic increase of the effective lifetime. This improvement of the passivation quality by low temperature annealing for the a-SiOx:H likely originates from defect reduction of the film close to the interface. Raman spectra reveal the existence of Si–(OH)x and Si–O–Si bonds after thermal annealing of the layers, leading to a higher effective lifetime, as it ...

Crystalline silicon surface passivation by high-frequency plasma-enhanced chemical-vapor-deposited nanocomposite silicon suboxides for solar cell applications

Disclosed is a novel method for creating local contacts in solar cells. In the method, a surface passivation that has been applied to a semiconductor substrate is locally etched away using a plasma process with the help of a thin stretched, elastic foil. If necessary, deep doping gradients are then locally created at the same points by means of a hydrogen plasma treatment with the help of thermal donors so as to increase the diffusion length of the charge carriers in the direction of the contacts. Finally, local heterostructure contacts are applied through the same mask openings. The contacts are characterized by a much lower saturation current than common diffused contacts and are therefore particularly suitable for high-performance solar cells.

Wolfgang R. Fahrner

Papers

High quality passivation for heterojunction solar cells by hydrogenated amorphous silicon suboxide films

Thermochemical polishing of CVD diamond films

Glucose determination using a re-usable enzyme-modified ion track membrane sensor.

Crystalline silicon surface passivation by high-frequency plasma-enhanced chemical-vapor-deposited nanocomposite silicon suboxides for solar cell applications

Local heterostructure contacts