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Thin-film solar cells, due to their low material usage and flexible substrates compatibility, have the potential to fill market niches for photovoltaic (PV) technologies. Among them, the Cu(In,Ga)Se2 (CIGS) solar cell is a widely deployed mature technology. However, the usage of scarce material like Ga and In is considered a major hindrance for its further scaling up. The commercial opportunity of its promising counterpart Cu2ZnSnS4 (CZTS) with a similar structure but low-cost potential, has not yet been fully recognized. In this paper, the bottom-up approach is used to build models of cost analysis for CZTS on the different substrates with their probability distribution simulated by the Monte Carlo method. The resulting production costs are $41–52 per m2, making them economically attractive. Prospective strategies for further cost reduction are also suggested. The fundamental technical features of CZTS are reviewed, identifying the large efficiency potential of this PV technology. Analysis of different market opportunities is performed to fit the market demands better. Moreover, possible constraints and promising pathways towards the commercialization of the emerging CZTS technology are proposed.

The hydrogen plasma passivation of thin film polycrystalline silicon (pc-Si) was investigated in conjunction with plasma texturing process to make efficient heterojunction solar cells. The pc-Si layers were first treated using direct and remote hydrogen plasma technologies. The heterojunction solar cells were then fabricated by subsequent deposition of i/n+ a-Si:H. Hydrogenation at high temperature (610 °C) results in enhanced dissolution and diffusion of hydrogen in pc-Si by a factor of about 3 and 4, respectively, in comparison with those at low temperature (420 °C). The hydrogen atoms in the pc-Si layer mainly bond to the silicon dangling bonds and form complexes with dopant atoms. In addition, platelets defects are generated by the hydrogen plasma in the sub-surface region of pc-Si hydrogenated at 420 °C and cause higher saturation current in the space charge region whilst they form in the region deeper than 1 μm at 610 °C. Removal of the platelets using SF6/N2O plasma post-texturing after low-temperature hydrogenation not only enhances the short circuit current but also improves the open circuit voltage and the fill factor simultaneously. Combining plasma pre-texturing with high-temperature hydrogenation, the best 2 µm-thick pc-Si heterojunction solar cell reaches an efficiency of 8.54%.

Solid-phase crystallisation (SPC) of Si films on glass prepared by three deposition methods, plasma enhanced chemical vapour deposition (PECVD), electron-beam evaporation, and a combination of both, are compared for different annealing temperatures. Three independent techniques, optical transmission microscopy, UV reflectance spectroscopy, and X-ray diffraction, are used to characterise the crystallisation kinetics and film properties. The activation energy for the incubation is estimated to be 2.7–3.0 eV. The scanning electron microscopy images of polycrystalline Si films after Secco etching show a gradually decreasing average grain size in each film type for higher SPC temperatures. The crystal quality of all film types degrades at higher crystallisation temperatures. Solar cells fabricated from these polycrystalline Si films were characterised by Suns- V oc and spectral response measurements. According to both the resulting open-circuit voltage and the short-circuit current, the electronic quality of all polycrystalline Si film types and the corresponding cell performance degrade for higher crystallisation temperatures but to a different extent depending on the film deposition method.

Solid-phase crystallisation of Si thin films on glass fabricated by plasma enhanced chemical vapour deposition is compared at different annealing temperatures. Four independent techniques, optical transmission microscopy, Raman and UV reflectance spectroscopy, and X-ray diffraction, are used to characterise the crystallisation kinetics and film properties. The 1.5 μm thick films with the n+/p−/p+ solar cell structure have incubation times of about 300, 53, and 14 min and full crystallisation times of about 855, 128, and 30 min at 600 °C, 640 °C, and 680 °C respectively. Estimated activation energies for incubation and crystal growth are 2.7 and 3.2 eV respectively. The average grain size in the resulting polycrystalline Si films measured from scanning electron microscopy images gradually decreases with a higher annealing temperature and the crystal quality becomes poorer according to the Raman, UV reflection, and X-ray diffraction results. The dopant activation and majority carrier mobilities in heavily doped n+ and p+ layers are similar for all crystallisation temperatures. Both the open-circuit voltage and the spectral response are lower for the cells crystallised at higher temperatures and the minority carrier diffusion lengths are shorter accordingly although they are still longer than the cell thickness for all annealing temperatures. The results indicate that shortening the crystallisation time by merely increasing the crystallisation temperature offers little or no merits for PECVD polycrystalline Si thin-film solar cells on glass.

The effects of electrostatic fluctuations due to charged extended defects and strain-induced bandgap fluctuations are examined in polycrystalline silicon on glass solar cells. The analysis is based on models previously applied to Cu(In,Ga)Se2 solar cells, but with a new interpretation of the local ideality factor associated with electrostatic fluctuations. It is shown that electrostatic fluctuations become influential to the cell voltage properties as the absorber dopant concentration falls below a certain threshold (a few 10 15 cm � 3 ), and the degradations to the open circuit voltage and fill factor are expected to increase with further lowering of dopant density. It is equally plausible that the electrostatic fluctuations originate from charged dislocations or grain boundaries. Bandgap fluctuations on the other hand can be detrimental to the open circuit voltage of cells of any absorber dopant density. However, this voltage degrading effect is seen only in the cells deposited by electronbeam evaporation, and not amongst those made by plasma enhanced chemical vapour deposition. Copyright # 2011 John Wiley & Sons, Ltd.

Crystalline silicon on glass (CSG) solar cell technology was developed to address the difficulty that silicon wafer-based technology has in reaching the very low costs required for large-scale photovoltaic applications as well as the perceived fundamental difficulties with other thin-film technologies. The aim was to combine the advantages of standard silicon wafer-based technology, namely ruggedness, durability, good electronic properties and environmental soundness with the advantages of thin-films, specifically low material use, large monolithic construction and a desirable glass superstrate configuration. The challenge has been to match the different preferred processing temperatures of silicon and glass and to obtain strong solar absorption in notoriously weakly-absorbing silicon of only 1.4 μm thickness, the thinnest active layer of the key thin-film contenders. A rugged, durable silicon thin-film technology has been developed arguably with the lowest likely manufacturing cost of these contenders and confirmed efficiency for small pilot line modules already in the 8–9% energy conversion efficiency range, on the path to 12–13%.

Strong contact adhesion is an important requirement for durable, manufacturable solar cells. Advanced contacting technologies require new methods to measure adhesion. We describe a scratch test for measuring contact adhesion that involves scanning a weighted stylus across the cell while measuring the horizontal force F D required to dislodge the contacts. F D is characteristic of the adhesive bond but independent of the contact height, stylus weight and scan speed. We observe that contact peeling depends also on the tensile strength of the metal finger. The tests provide a valuable way to assess and optimize the adhesion of metal contacts.

Line-focus diode laser is applied to advance crystalline silicon thin-film solar cell technology. Three new processes have been developed: 1) defect annealing/dopant activation; 2) dopant diffusion; 3) liquid phase crystallisation of thin films. The former two processes are applied to either create a solar cell device from pre-crystallised films or improve its performance while reducing the maximum temperature experienced by substrate. The later process is applied to amorphous silicon films to obtain high crystal and electronic quality material for thin-film solar cells with higher efficiency potential. Defect annealing/dopant activation and dopant diffusion in a few micron thick poly-Si films are achieved by scanning with line-focus 808 nm diode laser beam at 15-24 kW/cm2 laser power and 2~6 ms exposure. Temperature profile in the film during the treatment is independent from laser power and exposure but determined by beam shape. Solar cell open-circuit voltages of about 500 mV after such laser treatments is similar or even higher than voltages after standard rapid-thermal treatments while the highest temperature experienced by glass is 300C lower. Amorphous silicon films can be melted and subsequently liquid-phase crystallised by a single scan of line laser beam at about 20 kW/cm2 power and 10-15 ms exposure. Solar cells made of laser-crystallised material achieve 557 mV opencircuit voltage and 8.4% efficiency. Electronic quality of such cells is consistent with efficiencies exceeding 13% and it is currently limited by research-level simplified cell metallisation.

Photoreflectance measurements of multilayered Al x Ga 1−x As GaAs semiconductor structure in the regime of high modulating laser powers are reported. The main photoreflectance feature observed at low powers at about 1.4 eV is related to Franz-Keldysh oscillations caused by electric field at the Al x Ga 1−x As GaAs interface. The value of electric field deduced from these oscillations is in agreement with that determined from calculation using the accepted value of the Al x Ga 1−x As GaAs band offset and the doping level in the GaAs layer. On increasing the power of the modulating laser, the spectrum changes its character and after decomposition reveals a feature at the energy corresponding to GaAs bandgap. The intensity of this line increases with the modulating laser power, moreover the line is in phase with the Franz-Keldysh oscillations. These observations are consistent with the assignment of the line to the photoreflectance at the underlying GaAs buffer/GaAs substrate interface. This finding underlines the altered character of the photoreflectance spectra at high light intensities, compared to the standard low intensity regime.

Electron-beam (e-beam) evaporation provides both exciting opportunities and challenges for the preparation of poly-crystalline silicon (poly-Si) thin film solar cells. A conversion efficiency of 6.7% was recently achieved for solid phase crystallized poly-Si mini-modules on planar SiN-coated glass deposited at a deposition rate of 600 nm/min, demonstrating the excellent electronic quality of e-beam evaporated silicon. Even at significantly increased background pressures of 5×10−6 mbar, the photovoltaic performance of the mini-modules was considerably high, showing a decline in open circuit voltage of 17 mV per cell. The implementation of light trapping structures into the device led to an efficiency increase of 1.1%, yielding module efficiencies of 7.8%. By systematically studying the implementation of ZnO:Al as a front contact layer into the poly-Si solar cell device structure, we unraveled novel features that prove the supreme suitability of ZnO:Al for poly-Si thin film solar cells. Not only can etched ZnO:Al be utilized as a front side texture, but its electrical properties can also improve during the crystallization process of the Si layer, showing a record charge carrier mobility of 67 cm2/Vs after thermal annealing. In addition, ZnO:Al drastically modifies the crystallization kinetics of the Si on ZnO:Al, enabling us to control the crystallization process by adjusting the deposition temperature. The nucleation process of Si on ZnO:Al was found to be influenced by a variation of the deposition temperature of the amorphous Si in a critical temperature regime of 200 °C to 300 °C. The nucleation rate decreased significantly with decreasing deposition temperature, while the activation energy for nucleation increased from 2.9 eV at a deposition temperature of 300 °C to 5.1 eV at 200 °C, resulting in poly-Si which comprised grains with features sizes of several µm.

Solid-phase crystallization (SPC) of Si thin films on glass deposited by electron-beam evaporation is compared at different annealing temperatures. Four independent film characterization methods, optical transmission microscopy, Raman and UV reflectance spectroscopy (UV-R), and X-ray diffraction (XRD), reveal that 1.4 micron thick films with n+/p-/p+ solar cell structures have the incubation times of about 285, 32, and 10 min and the full crystallization times of about 825, 94, and 24 min at 600°C, 640°C, and 680°C respectively. The estimated activation energies for the incubation and crystal growth are 3.0 and 3.2 eV respectively. The scanning electron microscopy (SEM) images of the resulting polycrystalline Si (poly-Si) films after Secco etching show that the average grain size gradually decreases with a higher SPC temperature. Similarly, according to the Raman, UV-R, and XRD spectra the crystal quality of the poly-Si films is poorer for the higher SPC temperatures. The sheet resistivities, which are dominated by the properties of the two thin but heavily doped n+ and p+ layers, are similar for all SPC temperatures. The open-circuit voltages (Voc) of about 450 mV measured by Suns-Voc method are also similar for all SPC temperatures and comparable to previously reported values for the best evaporated poly-Si cells. However, the minority carrier diffusion length of about 0.5 micron in the absorber layers for all SPC temperatures extracted from the external quantum efficiency (EQE) data is much shorter than the cell thickness and than previously reported, which can be due to relatively high boron concentration in the absorber found by means of the capacitance-voltage (C-V) analysis.

A few microns thick silicon films on glass coated with a dielectric intermediate layer can be crystallised by a single pass of a line-focused diode laser beam. Under favorable process conditions relatively large linear grains with low defect density are formed. Most grain boundaries are defect-free low-energy twin-boundaries. Boron-doped laser crystallised films are processed into solar cells by diffusing an emitter from a phosphorous spin-on-dopant source, measuring up to 539 mV open-circuit voltage prior to metallisation. After applying a point-contact metallisation the best cell achieves 7.8% energy conversion efficiency, open-circuit voltage of 526 mV and short-circuit current of 26 mA/cm2 . The efficiency is significantly limited by a low fill-factor of 56% due to the simplified metallisation approach. The internal quantum efficiency of laser crystallised cells is consistent with low front surface recombination. By improving cell metallisation and enhancing light-trapping the efficiencies of above 13% can be achieved.

Summary form only given. We have demonstrated that gratings of different strengths in unhydrogenated fiber display different stabilities at elevated temperatures, however, gratings written in hydrogenated fiber did not show this trend. These observations should provide further insight into the mechanism of photo-induced index change during both writing and annealing.