Interdigitated back contact silicon heterojunction solar cells Towards an industrially applicable structuring method

We report on the investigation and comparison of two different processing approaches for interdigitated back contacted silicon heterojunction solar cells our photolithography based reference procedure and our newly developed shadow mask process. To this end, we analyse fill factor losses in different stages of the fabrication process. We find that although comparably high minority carrier lifetimes of about 4 ms can be observed for both concepts, the shadow masked solar cells suffer yet from poorly passivated emitter regions and significantly higher series resistance. Approaches for addressing the observed issues are outlined and first solar cell results with efficiencies of about 17 and 23 for shadow masked and photolithographically structured solar cells, respectively, are presente

Material properties of high mobility TCOs and application to solar cells

The benefit of achieving high electron mobilities in transparent conducting oxides TCOs is twofold they first exhibit superior optical properties, especially in the NIR spectral range, and secondly their low resistivity enables the usage of thinner films. Remarkably high mobilities can be obtained in Al doped zinc oxide by post deposition annealing under a protective layer. The procedure has not only shown to increase mobility, but also strongly reduces sub bandgap absorption. Extensive optical, electrical and structural characterization is carried out in the films in order to clarify the microscopic origins of the changes in material properties. While the annealing of defect states, most likely deep acceptors, seems clear, earlier results also suggest some influence of grain boundaries. Tailing, on the contrary, seems to be linked to extended defects. In application to a Si H c Si H thin film solar cells the films have already shown to increase spectral response. When reducing the film thickness, the main challenge is to provide a suitable light trapping scheme. Normally this is achieved by a wet chemical etching step in diluted HCl, which provides a surface structure with suitable light scattering properties. Therefore a TCO independent light scattering approach using textures glass was applied in conjunction with the high mobility zinc oxide. The substrate enables the use of very thin TCO layers with a strongly reduced parasitic absorptio

Zinc oxide films grown by galvanic deposition from 99% metals basis zinc nitrate electrolyte

The use of relatively low purity zinc nitrate for electrochemical deposition of compact ZnO films is attractive for large scale production because of the cost saving potential. ZnO films were grown on SnO2:F and magnetron sputtered ZnO:Al templates using a three electrode potentiostatic system in galvanic mode. The electrolyte consisted of a 0.1 M zinc nitrate solution (either 99.998% or 99% purity) and 1 mM aluminium nitrate for extrinsic doping, when required. Moderate deposition rates of up to 0.9 nm s−1 were achieved on ZnO:Al templates with lower rates of up to 0.5 nm s−1 on SnO2:F templates. Observation of SEM images of the films revealed a wall-like morphology whose lateral thickness (parallel to the substrate) reduced as aluminium was added to the system either in the electrolyte or from the substrate. However, pre- deposition activation of the template by applying a negative voltage (approximately −2 V) allowed the growth of compact films even for the low purity electrolyte. The optical band gap energy of intrinsically doped films was lower than that of the Al doped films. The composite electrical conductivity of all the films studied, as inferred from sheet resistance and Hall effect measurements of the ZnO/template stacks was much less than that of the uncoated templates. A strong E2 (high) mode at around 437 cm−1 was visible in the Raman spectra for most films confirming the formation of ZnO. However, both the Raman modes and XRD reflections associated with wurtzite ZnO diminished for the Al doped films indicating a high level of mainly oxygen related defects. Based on these data, further studies are underway to improve the doping efficiency of aluminium, the crystalline structure and thus the conductivity of such films

Nanocrystalline n Type Silicon Front Surface Field Layers From Research to Industry Applications in Silicon Heterojunction Solar Cells

Nanocrystalline silicon and silicon oxide nc Si Ox H layers grown by plasma enhanced chemical vapor deposition PECVD have shown low parasitic absorption and excellent contact properties when implemented as n type front surface field FSF contact in rear junction silicon heterojunction SHJ solar cells [1 3]. In this contribution we present results from the successful process transfer from the lab at PVcomB at the Helmholtz Zentrum Berlin HZB , to the industrial pilot line at Meyer Burger Germany GmbH MBG . Conversion efficiencies gt; 22.5 were demonstrated on SHJ cell 4 cm2 [2, 3]. The excellent cell performance in the lab and the potential to reduce parasitic absorption in the front stack by using nc SiOx H motivated the process transfer from HZB to MBG. Initial cross processing experiments on 244 cm2 wafers showed the benefit of using nc Si H as FSF layer. We here also emphasize the role of the Si texture on a fast nc Si H nucleation. After cross processing experiments a successful transfer of the nc Si H process and fine tuning resulted in a median cell efficiency of 23.4 . This is in the same range as the MGB reference on 244 cm2 cells, noteworthy, at the same throughput. Currently work is ongoing to further improve the optical performance of the cells by adding oxygen CO

Nanocrystalline silicon oxide interlayer in monolithic perovskite silicon heterojunction tandem solar cells with total current density gt;39 mA cm2

Silicon heterojunction solar cells are implemented as bottom cells in monolithic perovskite silicon tandem solar cells. Commonly they are processed with a smooth front side to facilitate wet processing of the lead halide perovskite cell on top. The inherent drawback of this design, namely, enhanced reflection of the cell, can be significantly reduced by replacing the amorphous or nanocrystalline silicon front side n layer of the silicon cell by a nanocrystalline silicon oxide n layer. It is deposited with the same commonly used plasma enhanced chemical vapor deposition and can be tuned to feature opto electrical properties for enhanced light coupling into the Si bottom cell, namely, low parasitic absorption and an intermediate refractive index of 2.6. We demonstrate that a 80 100 nm thick layer results in 0.9 mA cm 2 current gain in the bottom cell yielding tandem cells with a top cell bottom cell total current above 39 mA cm 2 . These first nc SiO x H coupled tandem cells reach an efficiency gt;23.

Optoelectrical analysis of TCO Silicon oxide double layers at the front and rear side of silicon heterojunction solar cells

Silicon Heterojunction has become a promising technology to substitute passivated emitter and rear contact PERC solar cells in pursuance of lower levelized cost of electricity through high efficiency devices. While high open circuit voltages and fill factors are reached, current loss related to the front and rear contacts, such as the transparent conductive oxide TCO layers is still a limiting factor to come closer to the efficiency limit of silicon based solar cells. Furthermore, reducing indium consumption for the TCO has become mandatory to push silicon heterojunction technology towards a terawatt scale production due to material scarcity and costs. To address these issues dielectric layers, such as silicon dioxide or nitride cappings are implemented to reduce TCO thicknesses both diminishing parasitic absorption and material consumption. However, reducing the TCO thickness comes in cost of resistive losses. Furthermore, the TCO properties do vary with thickness and neighboring layer configuration altering the optimization frame of the device. In this paper we present a detailed analysis to quantify the optoelectrical losses trade off associated to the TCO thickness reduction in such layer stacks. Through the analysis we show and explain why experimental bifacial cells with 20 nm front and rear TCO perform at a similar level to reference cells with 75 nm under front and rear illumination reaching efficiency close to 24 at 92 bifaciality. We present as well a simple interconnection method via screen printing metallization to implement a thin TCO silicon dioxide silver reflector enhancing current density from 39.6 to 40.4 mA cm2 without compromising resistive losses resulting in a 0.2 absolute solar cell efficiency increase from a bifacial design 23.5 23.7 . Finally, following this approach we present a certified champion cell with an efficiency of 24.