One of the parameters that limit the efficiency of a thin film solar cell, especially the a-Si and the nc-Si solar cell is the cell thickness. Although thicker film can absorb most of the sun light, the optical generated carriers will recombination through the numerous gap states in the film that obtained lower short circuit current and fill factor. In the controversy, thinner film could not absorb enough sun light that also limit the short circuit current. In this works, we utilize nanowire structure to solve the conflict between the light absorption and the carrier transport. The designed structure has ZnO:Al nanowire array on the substrate. The p-i-n a-Si solar cell structure is grown along the surface of each ZnO: Al nanowire sequentially. Under sunlight illumination, the light is absorbed in the axis direction of the nanowire. However, the carrier transport is along the radial direction of the solar cell. Therefore, the long nanowire could absorb most of the solar light. In the mean time, the thickness of the solar cell still is thin enough for photo-generated carrier transport. The dependence of short circuit current, open circuit voltage and fill factor to the length, diameter and density of ZnO:Al nanowires were simulated.

This paper discusses several topics related to light management that improve our understanding of the performance and potential of the intermediate band solar cell (IBSC). These topics are photon recycling, photon selectivity and light confinement. It is found that neglecting photon recycling leads to underestimate the limiting efficiency of the IBSC in 7 points (56.1 % vs 63.2 %). Light trapping allows to effectively absorbing photons whose energy is associated to the weakest of the optical transitions in the IBSC, allowing also for higher efficiencies with lower device thickness. The impact of photon selectivity on the cell performance is also discussed.

We report on nano-scale optical effects of amorphous silicon layer conformally deposited on randomly textured zinc oxide layers on glass substrates investigated by near-field scanning microscopy. Such textured layers are used in thin-film photovoltaic devices to enhance light trapping. Experimental results are compared to theoretical data, obtained from large scale finite-difference time-domain simulations. Light localization on the surface of the textured interface and a focusing of light by the structure further away are observed. The measurements are compared with simulations, which provide additional insight into the light intensity distribution inside the solar cell on a nm-scale. It will be shown how this information can be used to optimize light trapping in thin-film solar cells using an amorphous silicon solar cell as an example.

Efficient thin-film polycrystalline-silicon (pc-Si) solar cells on inexpensive substrates could substantially lower the price of photovoltaic electricity. We recently showed that good solar cells can be made from pc-Si obtained by epitaxial thickening using thermal CVD of a seed layer made by aluminium-induced crystallization (AIC) of amorphous silicon. We already reported cells in substrate configuration with energy conversion efficiencies up to 8.0% for layers on ceramic alumina substrates. However, much higher efficiencies (η > 10%) are needed for this type of pc-Si solar cells to become cost-effective. To achieve these higher efficiencies, cells will probably have to be made in a superstrate configuration on transparent substrates and advanced light trapping will need to be applied. In this paper we report on our recent progress with pc-Si solar cells made on transparent glass-ceramic substrates.

So far, our best pc-Si solar cells in substrate configuration on glass-ceramics showed an efficiency of 6.4%. By using plasma texturing to lower the front side reflection, we increased the current density of our cells by roughly 1 mA cm-2. The Jsc is much lower for cells on glass-ceramic than for cells on alumina. This is the result of the better diffuse back reflectance of alumina compared to glass. The Voc and fill factor are comparable for cells on both substrates.

To make pc-Si solar cells on glass in superstrate configuration, we will use a-Si/c-Si rear junction emitters. As a first test of the feasibility of this approach, we measured the illuminated IV parameters of pc-Si cells made for the substrate configuration in superstrate configuration. In superstrate configuration, the current density of the cells is much lower than in substrate configuration due to the non-optimized cell design for superstrate illumination. The Voc is slightly smaller in superstrate configuration due to the lower current density.

These results indicate that the glass-ceramic substrates are fully compatible to our poly-Si solar cell process. Furthermore, rear-junction poly-Si cells in superstrate configuration should lead to good cell results once the absorber layer thickness is optimized to the diffusion length of the material and light trapping features adapted to the superstrate configuration are applied.

Spatially and spectrally resolved electroluminescence (EL) measurements are powerful methods to characterize solar cells, if the EL is properly interpreted. The task of interpreting the results and modeling the spectral and absolute EL emission is strongly simplified by considering the link between EL and quantum efficiency. Using this connection, we show how to quantify the influence of light trapping on the solar cell absorptance using EL.

Conductive zinc oxide (ZnO) films are used extensively as transparent electrodes in thin-film photovoltaic solar cells. Compared with the widely used indium tin oxide (ITO) and tin oxide (SnO2), ZnO has a smaller optical bandgap. ZnO is commonly used as a front contact for copper indium gallium diselenide (CIGS) solar cells, but it forms a small, unfavorable conduction-band offset with the CdS layer. The optical bandgap of ZnO could easily be engineering by alloying with MgO or CdO. In this work, we try to optimize the ZnO for CIGS solar cells. The optical and electrical properties of Zn1-xMgxO:Al films fabricated by co-sputtering were studied. Two targets: ZnO:Al and MgO, were used. The ratio of ZnO/MgO was varied continuously on the 6”x6” glass substrate, and the effects of composition on the properties of the Zn1-xMgxO:Al films were investigated. The carrier concentration and mobility of the Zn1-xMgxO:Al films decreased quickly with increasing Mg content. However, the optical properties of the Zn1-xMgxO:Al films do not vary linearly with Mg content, as reported by most papers. The observed optical bandgap of Zn1-xMgxO:Al films is actually first narrowed, then increased with the Mg content. The shift in optical bandgap from narrow to wide occurs at around a composition of x = 0.07. After the point of x = 0.07, the bandgap width star increase but film sheet resistance already too low. Our result therefore suggests that the alloyed Zn1-xMgxO:Al does not benefit the CIGS solar cell.

We use our recently developed microscopic approach to a quantum theory of photovoltaic processes in nanostructures [1]to investigate geometry effects in quantum well photovoltaics, focussing on the role of asymmetry and inter-well coupling in double quantum well (DQW) systems. For that purpose, the IV and power characteristics for DQW systems with different asymmetry and degree of coupling are calculated numerically in the vicinity of the maximum power point. In order to assess the transport properties of a specific QW structure, we isolate these from the effects of absorption by normalizing to the absorptivity. The results obtained from this procedure reveal the escape regime dominating at room temperature and confirm previous experimental observations.

In the nip or substrate configuration thin film silicon solar cells, the choice of front TCO contact is critical because there is a trade off between its transparency which influences the current in the solar cell and its conductivity which influences the series resistance. Here, we investigate the optical behavior of two different TCO front contacts, either a 70 nm thick, nominally flat ITO or a 2 μm thick rough LPCVD ZnO. The back contact consists of LP-CVD ZnO with random texture. First we investigate the influence of the rough and flat front TCOs in μc-Si:H and a-Si:H solar cells. With the back contact geometries used in this work, the antireflection properties of ITO are effective at providing as much light trapping as the rough LP-CVD ZnO. In the second part, we demonstrate that total of 25 to 26 mA/cm2is achievable in nip micromorph tandem cells and show short circuit current up to 11.7 mA/cm2 using an SIO based intermediate reflector.

Advanced light management in thin-film solar cells is important in order to improve the photo-current and, thus, to raise up the conversion efficiencies of the solar cells. In this article two types of periodic structures ¡V one-dimensional diffraction gratings and photonic crystals,are analyzed in the direction of showing their potential for improved light trapping in thin-film silicon solar cells. The anti-reflective effects and enhanced scattering at the gratings with the triangular and rectangular features are studied by means of two-dimensional optical simulations. Simulations of the complete microcrystalline solar cell incorporating the gratings at all interfaces are presented. Critical optical issues to be overcome for achieving the performances of the cells with the optimized randomly textured interfaces are pointed out. Reflectance measurements for the designed 12 layer photonic crystal stack consisting of amorphous silicon nitride and amorphous silicon layers are presented and compared with the simulations. High reflectance (up to 99 %) of the stack is measured for a broad wavelength spectrum. By means of optical simulations the potential for using a simple photonic crystal structure as a back reflector in an amorphous silicon solar cell is demonstrated.

Because conventional photovoltaic (PV) cells are threshold systems in terms of optical absorption, “photon management“is an obvious way to improve their performance.

Calculations to optimize photon utilization in a single-junction PV cell show ˜1.4 eV to be the optimal bandgap for terrestrial solar to electrical power conversion. For Si, with a slightly sub-optimal gap, continuous efforts have yielded single-junction laboratory cells, quite close to the theoretical limit.

One of the repeatedly proposed directions to improve photon management is that of up- and down-conversion of photon energy. In up-conversion two photons with energy hv < EG (the band gap) create one photon with hv > EG, while in down-conversion one photon with energy hv > 2EG, yields two photons with energy hv > EG.

Multi-exciton generation (MEG), although not a "photon management" process, can achieve effects like down-conversion, which, though, is more limited than MEG. In MEG one photon with energy hv > nEG yields n electron-hole pairs with energy EG. Because MEG has clear advantages over down-conversion, in the following we will, instead of considering both, consider MEG.

We find that a straightforward analysis of this approach to “photon management” for a single junction cell under the detailed balance limit shows clearly that, even if we assume (highly unrealistic) 100% efficient up-conversion and MEG, a new theoretical PV conversion limit of 49 %, instead of 31% is arrived at, a maximum possible gain of ≈60%. The main attractive feature of the combination of up-conversion and MEG is a significant broadening of the optimal band-gap range. Rough estimates for the very highest possibly feasible efficiencies for up-conversion and MEG (25% and 70% respectively), yield at most slightly less than 40% PV conversion efficiency, i.e., only a ˜25% gain over conventional single band gap semiconductor.

These results show that up-conversion or MEG are fascinating scientific areas of research, whose implementation can indeed improve PV cell performance. However, truly formidable challenges need to be met to have UC + MEG lead to the type of radical decrease in the (cost)/ (efficiency × lifetime × yield) ratio that we need to allow large-scale economic introduction of PV cells. Parallel pursuit of alternative approaches to improved photon management, such as, for example, lowering the costs of arrangements with multiple solar absorbers and/or multi-junction systems, appears, therefore, critical for the future of PV.

An up-converter (UC) absorbs two or more low-energy photons and emits a single high-energy photon. A down-converter (DC) absorbs a single high-energy photon and emits two or more low energy photons. The current work extends previous limiting efficiency analysis to a combination of UC and DC; an up converter with two levels; and to compare analyses using real air mass data with that modelling the sun as a blackbody. Analysis has been carried out both with the band gap of the cell as an optimized parameter and at a fixed value of 1.1eV. All of UC, DC and two level UC are shown to improve efficiencies for both spectra. Combined UC/DC improves the efficiency further for the 1.1eV band gap but gives a lower efficiency for the optimised band gap. The explanation for this unexpected result is presented based on the small coupling losses that result from absorption/re-emission in the DC. The limiting efficiencies of such an approach are very similar to several other third generation concepts such as impurity PV, Intermediate Band solar cells or three level tandems. However in practice the UC (or DC) approach has the advantage that the optical properties of the UC are decoupled from the electrical properties of the PV cell, and hence each can be optimised independently. This means that it may be the simplest third generation approach to implement using existing PV cells, if a reasonable UC efficiency can be obtained. Nonetheless experimental work on realising UC is at an early stage. Some of the work on rare earth doped UC is reviewed together with the potential to improve the spectral sensitivity to below band gap radiation.

Newly developed SnO2:F films having “double–texture (W-texture)” were evaluated in terms of optical and electrical properties and compared with Asahi type-U substrate. “W-textured” transparent conductive oxide (TCO) film was composed of a combination of 300-500nm large hills and small pyramidal texture covering them. “W-textured” TCO could show a large haze value exceeding 80% even at 800nm. Microcrystalline Si (μc-Si:H) thin film solar cells were fabricated on these TCOs and characterized. The result showed that a quantum efficiency of μc-Si:H solar cells was improved with “W-textured” TCOs significantly in longer wavelength region.

Directional selective optical filters increase the photon confinement of solar cells with a Lambertian light-trapping scheme. These filters restrict the transmission of incoming sunlight to a cone of limited acceptance angle. This paper models the efficiency gain or loss caused by an ideal directional and energy selective filter on top of a solar cell, and compares it to a cell with Lambertian surface and a planar absorber. The enhancement of light trapping by the directional filter is illustrated by the enhancement of the quantum efficiency of normally incident light. Simulations of the annual yield demonstrate that the improved light trapping results in an overall energy gain of more than 10 % for a tracked system.To achieve this gain at the equator,a filter with small acceptance angle of 5 ° that is active below the threshold energy ˜1.5 eV has to used.

We present the results of systematic studies of optical enhancements by textured Ag/ZnO back reflectors in a-SiGe:H and nc-Si:H solar cells. First, the back reflector materials were characterized by AFM, XRD, and TEM. The results showed that the ZnO layers deposited by sputtering exhibit an increased surface texture with the deposition temperature and film thickness. The material structure showed a strong (002) preferential orientation. The large columnar crystal structure determines the surface texture. Second, the solar cell performance was correlated to the back reflector structure. We found that with a thin ZnO layer, a textured Ag layer results in more light scattering than a flat Ag layer. However, when a thick ZnO layer is used, a flat Ag layer can produce similar or more light scattering than a textured Ag layer. Third, we developed a method to estimate the optical enhancement for a-SiGe:H and nc-Si:H solar cells on various structures of Ag/ZnO back reflectors. Comparing the quantum efficiency data from solar cells made using the same recipe but one on a flat stainless steel substrate and another on a textured Ag/ZnO BR substrate revealed that the optical enhancement for the long wavelength light can be as high as 20 to 30. Compared to the theoretical value of 4n2, there is still scope for further improvement

Back surface reflectors (BSRs) with grating structures have been developed to enhance light trapping in thin-film microcrystalline Si (μc-Si:H) solar cells. As a grating structure, a periodic honeycomb-like dimple pattern with periods of 100 ˜ 450 nm has been fabricated on Al substrates by a self-ordering process using anodic oxidation of Al. A clear diffraction effect by the grating structure has been confirmed on the patterned Al using angle-dependent reflection measurement. From quantum efficiency measurements, we found that the periodically patterned BSR can confine the incident light effectively, especially at longer wavelengths. Nevertheless, short circuit current densities obtained from the patterned BSRs are rather comparable to that obtained from the random textured substrate. For further improvement of the light-trapping effect by the grating structure, it is necessary to realize higher diffraction efficiencies while maintaining high total reflectivity on the BSR structure.

Thin film silicon solar cell technology frequently makes use of rough or textured surfaces in order to enhance light absorption within the thin absorber layers by scattering and total internal reflection (“light trapping”). The rough morphology of the optically functional internal surfaces both in superstrate and substrate cells however, not only has a beneficial effect on light scattering properties, but on the other hand may also have deleterious effects on the microscopic structure of the deposited layers, in particular if these layers are nanocrystalline. The narrow valleys in the surface morphology may lead to structural defects, such as cavities and pinholes. By adjusting the morphology, these defects can be avoided.

However, even when structural defects in layers directly deposited on rough interfaces are avoided, the obtained optically defined maximum current density is still much lower than expected. For instance, in n-i-p structures the rough interface (the textured back reflector consisting of nanostructured Ag coated with ZnO) is located at the back of the cell, where only long wavelength light is present. The natively textured Ag film is sputtered at elevated temperature and optimized for diffusely reflecting this long wavelength light. From experiments we infer that the nanostructured metallic surface also gives rise to plasmon absorption in the red and near IR, and that this leads to a parasitic absorption, i.e. at least part of the absorbed energy is not re-emitted to the active layers.

Enhanced light absorption and improved photon harvesting is a major avenue to improving solar cell performance. We simulate and design photonic crystal based loss-less back reflectors. The photonic crystal is a 2-dimensional photonic crystal combined with a distributed Bragg reflector. We have designed and simulated a thin film a-Si:H solar cell with the photonic crystal reflector and an antireflection coating. The photonic crystal has square lattice symmetry and generates strong diffraction of near band edge photons in the absorber layer. The absorption of red and near-IR photons is increased by more than an order of magnitude by the photonic crystal. The photonic crystals are composed of ITO and can easily serve as a conducting back contact. This scheme can be easily extended to other solar absorber layers. We have optimized the geometry of the photonic crystal to maximize absorption using rigorous scattering matrix simulations. The optical path length with the photonic crystal can improve over the limit for a random roughened scattering surface.

We prepared size-controlled silicon quantum dots superlattices (Si-QDSLs) by thermal annealing of stoichiometric hydrogenated amorphous silicon carbide (a-SiC:H)/silicon rich hydrogenated amorphous silicon carbide (a-Si1+xC:H) multilayers for thin-film solar cell applications. Transmission electron microscope (TEM) observation revealed that the size of silicon quantum dots can be controlled by the thickness of the a-Si1+xC:H layers. It was found that hydrogen plasma treatment (HPT) significantly enhanced the photoluminescence (PL) of the Si-QDSLs. From the results of the PL measurement, the bandgap of the Si-QDSLs can be controlled from 1.1 eV to 1.6 eV by varying the diameter of silicon quantum dots. ESR measurement indicated that HPT reduced the defect density in a Si-QDSL from 1.83 ×1019 to 1.67 sup1018 cm-3.

New approach for the determination of the angular distribution of the scattered light at nano-rough surfaces/interfaces from AFM (Atomic Force Microscopy) data is presented. Calculation comes from modeling the electromagnetic field in the tight vicinity of the nano-rough surface by complex solution of Maxwell's equations and subsequent near field to far field transform. This method is demonstrated for four types of transparent conductive oxides (with rough free surfaces) deposited on glass substrates. As a result we have the amount and angular distribution of the scattered light „observed” in both transmission and reflection. Moreover calculation can be done for real sample dimensions (to compare the results with the measurement of the angular distribution function using LED laser) or for a semi-infinite sample which suppresses the interference effects and thus such distribution functions can be used as an input parameter for our 3-dimensional optical model CELL for thin film silicon solar cell modeling.

In the second part of this contribution we describe our experiment of thin film silicon solar cell characterization by Light Beam Induced Current (LBIC). This measurement done for laboratory solar cell structures reveals the light scattering and light trapping properties of the multilayer stack on a glass substrate. We suggest the test structure for the direct back reflector quality comparison and thus also for its optimization.

Monte-Carlo simulations calculate the photon collection of fluorescent collectors in photovoltaic systems. We focus on two collector geometries: solar cells mounted at the collector sides, and solar cells covering the back of the collector. A mirror covers the bare back sides of both systems. On top lies optionally a photonic structure, which acts as an energy selective filter. Ideal systems in their radiative limits are compared to systems where loss mechanisms in the dye, at the mirror, or the filter are included. The examination of loss mechanisms in photovoltaic systems with fluorescent collectors enables us to estimate quality limitations of the used materials and components.