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We investigate the influence of the deposition parameters for intrinsic amorphous silicon absorber layers on light-induced degradation (LID) of thin-film silicon solar cells. The focus is on absorber layers with different bandgaps: on one side, solar cells with a wide-bandgap absorber layer that provides open-circuit voltages up to 1.04V; on the other, cells with short-circuit current densities of 18.2mA/cm(2) with a 300-nm-thick narrow-bandgap absorber layer, and 20mA/cm(2) at reverse bias for a cell with a 1000-nm-thick absorber layer. Between these extremes, we varied the hydrogen-to-silane ratio and the deposition pressure during the absorber layer deposition. The light-induced degradation of these materialscovering the deposition regimes of low-pressure, protocrystalline, polymorphous, and high-pressure amorphous siliconincorporated in single-junction amorphous silicon solar cells is detailed here. For each pressure, we found an optimum hydrogen dilution with least LID close to the amorphous-to-microcrystalline transition. The relative LID is similar for all pressures at optimized hydrogen dilutions. Further, we present the influence of absorber layer thickness, p-layer thickness, and deposition rate on the kinetics of light-induced degradation to facilitate the choice of a material for its application in several types of multi-junction thin-film silicon solar cells. We show that the degradation kinetics depends, in semi-logarithmic scale, only weakly on time but more on deposition conditions. Copyright (c) 2014 John Wiley & Sons, Ltd.

Silicon heterojunction solar cells use crystalline silicon (c-Si) wafers as optical absorbers and employ bilayers of doped/intrinsic hydrogenated amorphous silicon (a-Si:H) to form passivating contacts. Recently, we demonstrated that such solar cells increase their operating voltages and thus their conversion efficiencies during light exposure. We found that this performance increase is due to improved passivation of the a-Si:H/c-Si interface and is induced by injected charge carriers (either by light soaking or forward-voltage biasing of the device). Here, we discuss this counterintuitive behavior and establish that: (i) the performance increase is observed in solar cells as well as modules; (ii) this phenomenon requires the presence of doped a-Si:H films, but is independent from whether light is incident from the a-Si:H(p) or the a-Si:H(n) side; (iii) UV and blue photons do not play a role in this effect; (iv) the performance increase can be observed under illumination intensities as low as 20 W m(-1) (0.02-sun) and appears to be almost identical in strength when under 1-sun (1000 W m(-1)); (v) the underlying physical mechanism likely differs from annealing-induced surface passivation.

At the front contact of thin film silicon solar cells, the junction between the p-doped window layer and the n-type transparent electrode results in a barrier that must be surmounted by the charge carriers. The barrier height is governed by the work function difference of these two materials. For different compositions of the well known In2O3-SnO2 (ITO) system, we find that higher oxygen partial pressure during sputter deposition increases the work function of the deposited films. Over the same range of oxygen partial pressures, ITO electrodes with low tin content applied to n-i-p type thin film silicon solar yield a gain in Voc by up to 40 mV. *

In this contribution, the replication of nano-textures enhancing the short circuit current of thin film silicon solar cells onto low cost substrates (glass or PolyEthylene Naphtalate (PEN)) is studied. Optical and morphological analysis is performed to asses the quality of these replicas. Single and tandem a-Si:H solar cells are deposited on top of the master and replica structures to verify their suitability to be used as substrates for solar cells in substrate (n-i-p) configuration. We find similar stabilized efficiencies (> 8 %) for tandem cells on a master and a PEN replica of a different texture. The use of a soft stamp to make these replicas is smoothing the original features of the masters. Replications with higher fidelity of fine features are obtained with solid stamps. Such replicas do not exhibit discernible differences when compared to their master textures by atomic force microscopy measurements. Moreover a comparison of single junction amorphous cells made on a replica and on its corresponding master shows no difference in term of light trapping capability of the texture. Keywords: Thin film Solar Cells, Replication Process, Back Reflector

We introduce passivating hetero-interfaces in single-junction microcrystalline silicon (mc-Si:H) solar cells. We investigate the effect of different i–n layer stacks in thin mc-Si:H devices, in which recombination is significant at the interfaces as well as in the bulk material. By applying amorphous silicon passivating layers at the mc-Si:H i–n interface, we show a device with a high open-circuit voltage (Voc) of 608 mV, for a standard Raman crystalline fraction of the i-layer (>50%). This Voc is the highest reported value for a state-of-the-art mc-Si:H device made by plasma-enhanced chemical vapor deposition. We also report an efficiency of 9.45% for a solar cell with an absorber layer as thin as 650 nm on an area greater than 1 cm2, and show with a simple crystalline silicon model that for such thin mc-Si:H devices or mc-Si:H devices with a very high bulk-material quality, well-mastered interfaces and doped layers are of paramount importance for high efficiency.

An approach that models the fabrication and enables the characterization of randomly self-textured silicon thin film solar cells is developed and are compared to experimental results. The optical and electrical properties of the solar cells depend on the morphology of the randomly self-textured substrate and the formation of the individual layers of the solar cell. The influence of the interface morphology on the optical and electrical properties is investigated by 3D morphological algorithms. The calculated interface morphologies are compared to measured surfaces, and used as input parameters to simulate the optical wave propagation and to determine the formation of regions with reduced order (cracks) in the solar cells. The calculations are compared to experimentally realized 1.1 mu m thick microcrystalline silicon solar cells prepared on randomly self-textured substrates with high energy conversion efficiency of up to 9.4%. Guidelines for the optical and electronic optimization are provided.

Thin-film Si solar cells employ a back reflector (BR) for a more efficient use of the long wavelength light. Here, we have carried out a cross evaluation of metal (Ag-based) and dielectric (white paint-based) BR designs. Conclusive results have been reached regarding the most suitable BR type depending on the front electrode morphology, both with crater-like and pyramidal texture. The ZnO/Ag BR is found to be optically more efficient because of improved light trapping, although the gain tends to vanish for rougher front electrodes. Thanks to non-conventional Raman intensity measurements, this dependence on the front texture has been linked to the different weight of front and back interfaces in the light trapping process for the different morphologies. With rougher substrates, because the minor optical gain is accompanied by sputter-induced electronic deterioration of the solar cell during the ZnO buffer layer deposition, the white paint-based BR design is preferred. Copyright (c) 2016 John Wiley & Sons, Ltd.

Passivating contacts are key enablers for high efficiency c-Si solar cells. Here, we present our latest advancement in terms of implementation and understanding in fired passivating contacts (FPC) used as rear p + passivating contact in p-type solar cells. We study three different layer configurations and show that the microstructural layer properties play a major role on the surface passivation and charge carrier transport. Upon optimization we demonstrate implied open circuit voltage of 722 mV corresponding to saturation current density of -7 fA/cm(2) and contact resistances below 10 m Omega cm(2), when metallized with ITO/Ag. P-type c-Si solar cells employing a screen-printed P-diffused emitter co-fired with the FPC on the rear side with conversion efficiency up to 22.5% are demonstrated. Temperature-dependent measurements on test structures and solar cells reveal that tunneling is the main transport mechanism for FPC layers that crystallize during the firing process, whereas for more amorphous FPC layers, an additional component due to thermionic emission is also present. Finally, we present the efficiency potential of solar cells employing the developed FPC layer as hole selective rear side passivating contact.

We investigate the impact of guided modes on the absorption enhancement in solar cells. By considering a dielectric thin film, we show that the absorption can be enhanced beyond the previous predictions even after averaging over wide wavelength ranges.

Hydrogenated amorphous silicon (alpha-Si:H) has been used for decades doped and as intrinsic absorber layers in thin-film silicon solar cells. Whereas their effiency was improved for a long time by the deposition of higher quality absorber layers, recent improvements can be attributed to a better understanding of the interfaces, allowing for their specific engineering. In this review, we briefly resume the state-of-the-art of a-Si:H solar cell technology from growth and characterization of single layers to full solar cells and multijunction devices. Focusing on the absorber layer quality first, we highlight thereafter aspects of interface problematics and discuss the growth and role of doped microcrystalline silicon-oxide layers and approaches of 3D-solar-cell designs in more detail. Although the findings summarized in this review were obtained from thin-film solar cells, we show that alpha-Si:H is a very versatile material with properties that are of high interest for application in other devices such as heterojunction solar cells, detectors, or optoelectronic devices.

We present electron- and hole-selective passivating contacts based on wet-chemically grown interfacial SiOx and overlying in-situ doped silicon carbide (SiCx) deposited by plasma-enhanced chemical vapor deposition. After annealing at 850 degrees C, excellent surface passivation on the p-type planar crystalline silicon wafer is obtained for both electron- and hole-selective contacts. Their potential is demonstrated at the device level by employing a simple process flow, in which the junction formation of the two polarities is achieved with a single coannealing step. Both-side-contacted patterning-free planar p-type cells with an area of 4 cm(2) and screen-printed metallization reach a fill factor of 83.4% and a open-circuit voltage of 726 mV. Zirconium-doped indium oxide with excellent optoelectrical properties is used as a front electrode. The decrease in the parasitic absorption in the front electrode results in higher photogenerated current. By realizing front-side-textured and rear-side-planar p-type cells, an efficiency of up to 22.6% is achieved.

Silver nanoparticles embedded in a dielectric material have strong scattering properties under light illumination, due to localized surface plasmons. This effect is a potential way to achieve light trapping in thin-film solar cells. In this paper we study light scattering properties of nanoparticles on glass and ZnO, and on silver coated with ZnO, which represent the back reflector of a solar cell. We find that large nanoparticles embedded in the dielectric at the back contact of amorphous silicon solar cells lead to a remarkable increase in short circuit current of 20% compared to co-deposited cells without nanoparticles. This increase is strongly correlated with the enhanced cell absorption in the long wavelengths and is attributed to localized surface plasmons. We also discuss the electrical properties of the cells. Copyright # 2010 John Wiley & Sons, Ltd.

We present a p-type passivating rear contact that complies with integration into standard solar cell manufacturing with phosphorus-diffused front side. Our contact structure consists of a thin SiOx tunneling layer grown by wet chemistry and a stack of layers deposited in one single run by plasma-enhanced chemical vapor deposition. The layers of the stack were tailored to protect the interfacial oxide layer, to act as a source for boron diffusion into the wafer and to connect to the external metallisation with low contact resistivity. We found that this stack tolerated annealing at 900 degrees C over a wide range of dwell times: for 15 min anneals we obtained dark saturation current densities (J(o)) as low as 10 fA center dot cm(-2) (after hydrogenation) and after 12-fold increase of the annealing time to 180 min, J(0) was only increased to 12 fA center dot cm(-2). These values corresponded to implied open circuit voltages (iV(oc)) of 718 and 715 mV, respectively. To test passivating rear contacts under realistic operation conditions, we combined them with an n-type heterojunction into hybrid solar cells. With conversion efficiencies abovementioned 22% and V-oc > 705 mV, these devices demonstrated high level of rear surface passivation. Finally, we demonstrated the integration of the hole selective rear contact with a POCl3 diffusion process. To this end, we added a phosphorus diffusion barrier to our layer stack by depositing one additional layer of amorphous SiOx on top of the stack. For symmetric samples with this layer structure on both sides, we observed iV(oc) values of 714 and 712 mV on n- and p-type silicon wafers after hydrogenation, respectively. Co-diffused cells with POCl3 front diffused emitter and rear passivating contact resulted so far in efficiencies of 20.4% and 20.1% for n- and p-type wafers, respectively.

To further increase the efficiency of multijunction thin-film silicon (TF-Si) solar cells, it is crucial for the front electrode to have a good transparency and conduction, to provide efficient light trapping for each subcell, and to ensure a suitable morphology for the growth of high-quality silicon layers. Here, we present the implementation of highly transparent modulated surface textured (MST) front electrodes as light-trapping structures in multijunction TF-Si solar cells. The MST substrates comprise a micro-textured glass, a thin layer of hydrogenated indium oxide (IOH), and a sub-micron nano-textured ZnO layer grown by low-pressure chemical vapor deposition (LPCVD ZnO). The bilayer IOH/LPCVD ZnO stack guarantees efficient light in-coupling and light trapping for the top amorphous silicon (a-Si:H) solar cell while minimizing the parasitic absorption losses. The crater-shaped micro-textured glass provides both efficient light trapping in the red and infrared wavelength range and a suitable morphology for the growth of high-quality nanocrystalline silicon (nc-Si:H) layers. Thanks to the efficient light trapping for the individual subcells and suitable morphology for the growth of high-quality silicon layers, multijunction solar cells deposited on MST substrates have a higher efficiency than those on single-textured state-of-the-art LPCVD ZnO substrates. Efficiencies of 14.8% (initial) and 12.5% (stable) have been achieved for a-Si:H/nc-Si:H tandem solar cells with the MST front electrode, surpassing efficiencies obtained on state-of-the-art LPCVD ZnO, thereby highlighting the high potential of MST front electrodes for high-efficiency multijunction solar cells. Copyright (c) 2015 John Wiley & Sons, Ltd.