No abstract

Directional and energy selective optical surface structures attached to solar cells may result in both, enhancement or deterioration of photovoltaic performance of a solar cell. On the one hand, restricting the cell acceptance to the small incidence angle of direct and circumsolar irradiation enhances the maximum path length of the light in a solar cell with Lambertian surfaces even above the Yablonovitch limit (ultra-light-trapping). On the other hand, restrictions to small acceptance angles imply losses of diffuse sunlight, even for perfectly tracked cells. Using temporally resolved solar irradiation spectra, we simulate the enhancement and loss in the annual energy yield of silicon solar cells of various thicknesses. We assume an idealized angular and energy selective filter on top of the Lambertian surface of the absorber and compare the results to a Lambertian surface only. We find a maximum annual gain in the energy density of 32.5% for 1 mu m, and of similar to 10% for 10 mu m and 100 mu m thick perfectly tracked crystalline silicon solar cells. The simulation implies various tracking modes and two different locations. Finally, we introduce two possible realizations of such a filter; a Ru-gate stack and inverted opals. In experimental measurements, we could verify the absorptance enhancement by such a structure applied on top of a thin silicon wafer

The influence of a retro-reflective texture cover on light in-coupling and light-trapping in thin film silicon solar cells is investigated. The texture cover is applied to the front glass of the cell and leads to a reflectance as low as r % 3% by reducing the reflection at the air/glass interface and indirectly also reducing the reflections from the internal interfaces. For weakly absorbed light in the long wavelength range, the texture also enhances the light-trapping in the solar cell. We demonstrate an increase of the short circuit current density of exemplary investigated thin film silicon tandem solar cells by up to 0.95 mA cm À2 and of the conversion efficiency by up to 0.74% (absolute). For a planar microcrystalline solar cell, the enhancement of light-trapping was determined from the reduced reflection in the long wavelength range to be up to 17%, leading to an increase of the external quantum efficiency of up to 12%.

Perovskite solar cells (PSCs) have shown great potential for next-generation photovoltaics. One of the main barriers to their commercial use is their poor long-term stability under ambient conditions and, in particular, their sensitivity to moisture and oxygen. Therefore, several encapsulation strategies are being developed in an attempt to improve the stability of PSCs in a humid environment. The lack of common testing procedures makes the comparison of encapsulation strategies challenging. In this paper, we optimized and investigated two common encapsulation strategies: lamination-based glass–glass encapsulation for outdoor operation and commercial use (COM) and a simple glue-based encapsulation mostly utilized for laboratory research purposes (LAB). We compare both approaches and evaluate their effectiveness to impede humidity ingress under three different testing conditions: on-shelf storage at 21 °C and 30% relative humidity (RH) (ISOS-D1), damp heat exposure at 85 °C and 85% RH (ISOS-D3), and outdoor operational stability continuously monitoring device performance for 10 months under maximum power point tracking on a roof-top test site in Berlin, Germany (ISOS-O3). LAB encapsulation of perovskite devices consists of glue and a cover glass and can be performed at ambient temperature, in an inert environment without the need for complex equipment. This glue-based encapsulation procedure allowed PSCs to retain more than 93% of their conversion efficiency after 1566 h of storage in ambient atmosphere and, therefore, is sufficient and suitable as an interim encapsulation for cell transport or short-term experiments outside an inert atmosphere. However, this simple encapsulation does not pass the IEC 61215 damp heat test and hence results in a high probability of fast degradation of the cells under outdoor conditions. The COM encapsulation procedure requires the use of a vacuum laminator and the cells to be able to withstand a short period of air exposure and at least 20 min at elevated temperatures (in our case, 150 °C). This encapsulation method enabled the cells to pass the IEC 61215 damp heat test and even to retain over 95% of their initial efficiency after 1566 h in a damp heat chamber. Above all, passing the damp heat test for COM-encapsulated devices translates to devices fully retaining their initial efficiency for the full duration of the outdoor test (>10 months). To the best of the authors’ knowledge, this is one of the longest outdoor stability demonstrations for PSCs published to date. We stress that both encapsulation approaches described in this work are useful for the scientific community as they fulfill different purposes: the COM for the realization of prototypes for long-term real-condition validation and, ultimately, commercialization of perovskite solar cells and the LAB procedure to enable testing and carrying out experiments on perovskite solar cells under noninert conditions.

We suggest a design for a coating that could be applied on top of any solar cell having at least one diffusing surface. This coating acts as an angle and wavelength selective filter, which increases the average path length and absorptance at long wavelengths without altering the solar cell performance at short wavelengths. The filter design is based on a continuous variation of the refractive index in order to minimize undesired reflection losses. Numerical procedures are used to optimize the filter for a 10 microm thick monocrystalline silicon solar cell, which lifts the efficiency above the Auger limit for unconcentrated illumination. The feasibility to fabricate such filters is also discussed, considering a finite available refractive index range.

We present a meaningful characterization method for tandem solar cells. The experimental method allows for optimizing the output power instead of the current. Furthermore, it enables the extraction of the approximate AM1.5g efficiency when working with noncalibrated spectra. Current matching of tandem solar cells under short-circuit condition maximizes the output current but is disadvantageous for the overall fill factor and as a consequence does not imply an optimization of the output power of the device. We apply the matching condition to the maximum power output; that is, a stack of solar cells is power matched if the power output of each subcell is maximal at equal subcell currents. The new measurement procedure uses additional light-emitting diodes as bias light in theJVcharacterization of tandem solar cells. Using a characterized reference tandem solar cell, such as a hydrogenated amorphous/microcrystalline silicon tandem, it is possible to extract the AM1.5g efficiency from tandems of the same technology also under noncalibrated spectra.

The next technological step in the exploration of metal‐halide perovskite solar cells is the demonstration of larger‐area device prototypes under outdoor operating conditions. The authors here demonstrate that when slot‐die coating the halide perovskite layers on large areas, ribbing effects may occur but can be prevented by adjusting the precursor ink's rheological properties. For formamidinium lead triiodide (FAPbI3) precursor inks based on 2‐methoxyethanol, the ink viscosity is adjusted by adding acetonitrile (ACN) as a co‐solvent leading to smooth FAPbI3 thin‐films with high quality and layer homogeneity. For an optimized content of 46 vol% of the ACN co‐solvent, a certified steady‐state performance of 22.3% is achieved in p‐i‐n FAPbI3‐perovskite solar cells. Scaling devices to larger areas by making laser series‐interconnected mini‐modules of 12.7 cm2, a power conversion efficiency of 17.1% is demonstrated. A full year of outdoor stability testing with continuous maximum power point tracking on encapsulated devices is performed and it is demonstrated that these devices maintain close to 100% of their initial performance during winter and spring followed by a significant performance decline during warmer summer months. This work highlights the importance of the real‐condition evaluation of larger area device prototypes to validate the technological potential of halide perovskite photovoltaics.

A directionally selective multilayer filter is applied to a hydrogenated amorphous silicon solar cell to improve the light trapping. The filter prevents non-absorbed long-wavelength photons from leaving the cell under oblique angles leading to an enhancement of the total optical path length for weakly absorbed light within the device by a factor of kappa(r) = 3.5. Parasitic absorption in the contact layers limits the effective path length improvement for the photovoltaic quantum efficiency to a factor of kappa(EQE) = 1.5. The total short-circuit current density increases by DeltaJ(sc) = 0.2 mAcm(-2) due to the directional selectivity of the Bragg-like filter.