Simon Hänni

Multiscale transparent electrode architecture for efficient light management and carrier collection in solar cells

The challenge for all photovoltaic technologies is to maximize light absorption, to convert photons with minimal losses into electric charges, and to efficiently extract them to the electrical circuit. For thin-film solar cells, all these tasks rely heavily on the transparent front electrode. Here we present a multiscale electrode architecture that allows us to achieve efficiencies as high as 14.1% with a thin-film silicon tandem solar cell employing only 3 μm of silicon. Our approach combines the versatility of nanoimprint lithography, the unusually high carrier mobility of hydrogenated indium oxide (over 100 cm(2)/V/s), and the unequaled light-scattering properties of self-textured zinc oxide. A multiscale texture provides light trapping over a broad wavelength range while ensuring an optimum morphology for the growth of high-quality silicon layers. A conductive bilayer stack guarantees carrier extraction while minimizing parasitic absorption losses. The tunability accessible through such multiscale electrode architecture offers unprecedented possibilities to address the trade-off between cell optical and electrical performance.

Optimization of ZnO Front Electrodes for High-Efficiency Micromorph Thin-Film Si Solar Cells

The quest for increased performances in thin-film silicon micromorph tandem devices nowadays requires an increase of current density. This can be achieved with thin cells by combining both robust cell design and efficient light management schemes. In this paper, we identify three key requirements for the transparent conductive oxide electrodes. First, strong light scattering into large angles is needed on the entire useful wavelength range: A front electrode texture with large enough features is shown to grant a high total current (typically >26 mA/cm2 with a 2.4-μm-thick absorber material), while sharp features are reported to allow for high top cell current (>13 mA/cm2) and reduced reflection at the ZnO/Si interface. Second, sufficiently smooth substrate features are needed to guarantee a high quality of the silicon active material, ensuring good and stable electrical properties (typically Voc around 1.4 V). Third, conduction and transparency of electrodes must be cleverly balanced, requiring high transparent conductive oxide mobility (∼50 cm

Morphology stabilization strategies for small-molecule bulk heterojunction photovoltaics

^2

Light harvesting schemes for high efficiency thin film silicon solar cells

/V/s) to maintain the sheet resistance below 30 Ω/sq while keeping absorption as low as possible. Optimization of these three key requirements using ZnO electrodes allowed us to realize high-efficiency micromorph devices with 13.5% initial and 11.5% stabilized efficiency.

Quantification of Valleys of Randomly Textured Substrates as a Function of Opening Angle: Correlation to the Defect Density in Intrinsic nc-Si:H.

The greater crystallinity of solution-processed small-molecule organic semiconductors, compared to their polymer counterparts, renders the bulk heterojunction (BHJ) more susceptible to phase separation under thermal stress, decreasing device performance. Here we demonstrate and compare strategies to stabilize the donor:acceptor BHJ in DPP(TBFu)2:PC61BM solar cells using molecular additives designed to either afford compatiblization (CP) of the bulk heterojunction, or to in situ link (ISL) the components using a functional azide group. Both additives were found to stop phase segregation of the BHJ under thermal stress. At 5 wt% loading the ISL additive prevents phase segregation, while altering the azide reaction mechanism by using UV-induced linking versus thermal induced linking was found to significantly affect the device performance. Including 5 wt% of the CP additive slowed phase segregation and devices retained 80% of their optimum performance after 3000 min of thermal treatment at 110 °C (compared to 50% with the control). The CP additive at 10 wt% changed drastically the kinetics of phase segregation leading to devices with no decrease in performance over 3000 min thermal treatment. Thin film morphology characterization together with photoluminescence and impedance spectroscopy give further insight into the performance differences between the additives. These results reinforce the conclusion that the compatiblization method is the most promising strategy to engineer highly-efficient thermally-stable organic photovoltaics based on solution-processed small molecules.

Passivated interfaces in fluorinated microcrystalline silicon thin film solar cells

In Thin Film Silicon (TF-Si) solar cells light harvesting schemes must guarantee an efficient light trapping in the thin absorber layers without decreasing the silicon layers quality and consecutively the p-i-n diodes electrical performance. TF-Si solar cells resilience to the substrate roughness is reported to be possibly improved through optimizations of the cell design and of the silicon deposition processes. By further tailoring the superstrate texture, amorphous silicon / microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cells with an initial efficiency up to 13.7 % and a stabilized efficiency up to 11.8 % are demonstrated on single-scale textured superstrates. An alternative approach combining large and smooth features nanoimprinted onto a transparent lacquer with small and sharp textures from as-grown LPCVD ZnO is then shown to have a high potential for further increasing TF-Si devices efficiency. First results demonstrate up to 14.1 % initial efficiency for a TF-Si tandem solar cell.

Post-deposition treatment of microcrystalline silicon solar cells for improved performance on rough superstrates

Optical and electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H) solar cells are strongly influenced by the morphology of underlying substrates. By texturing the substrates, the photogenerated current of nc-Si:H solar cells can increase due to enhanced light scattering. These textured substrates are, however, often incompatible with defect-less nc-Si:H growth resulting in lower Voc and FF. In this study we investigate the correlation between the substrate morphology, the nc-Si:H solar-cell performance, and the defect density in the intrinsic layer of the solar cells (i-nc-Si:H). Statistical surface parameters representing the substrate morphology do not show a strong correlation with the solar-cell parameters. Thus, we first quantify the line density of potentially defective valleys of randomly textured ZnO substrates where the opening angle is smaller than 130° (ρ<130). This ρ<130 is subsequently compared with the solar-cell performance and the defect density of i-nc-Si:H (ρdefect), which is obtained by fitting external photovoltaic parameters from experimental results and simulations. We confirm that when ρ<130 increases the Voc and FF significantly drops. It is also observed that ρdefect increases following a power law dependence of ρ<130. This result is attributed to more frequently formed defective regions for substrates having higher ρ<130.

Single to multi-scale texturing for high efficiency micromorph thin film silicon solar cell

The effect of a passivating buffer layer is studied in single-junction microcrystalline silicon (μc-Si:H:F) solar cells grown by PECVD from a SiF4/H2/Ar precursor mixture. We employ a variation of buffer layer thickness and absorber layer thickness in order to distinguish recombination processes occurring at the interfaces or within the bulk material. By introducing a thin amorphous i-n-layer stack between the microcrystalline intrinsic and n-doped layers an increase in open-circuit voltage (VOC) from 490 mV to 510 mV is observed for a 650 nm thin i-layer with very high Raman crystalline fraction (>85%).

High rate deposition of microcrystalline silicon with silicon oxide doped layers: Highlighting the competing roles of both intrinsic and extrinsinc defects on the cells performances

In this contribution, we investigate the effect of post-deposition treatments on finished non-encapsulated thin-film microcrystalline silicon solar cells and show that annealing in vacuum leads to improved electrical properties of the solar cells, particularly for cells deposited on rough superstrates. Our results suggest that both curing of intrinsic defects in the silicon, which can appear during the deposition of the ZnO back electrode, as well as an improvement of the ZnO back-electrode conductivity itself, occur during an annealing in vacuum, leading to large improvements of the open-circuit voltage and fill factor values. An improvement of the porous zones in the absorber layer, as induced by rough superstrates, is also observed by Fourier-transform photocurrent spectroscopy, implying that these porous zones cannot be considered as being purely bi-dimensional, but have a spatial extension within the absorber layer.

High-efficiency microcrystalline silicon single-junction solar cells

Improving micromorph devices performances nowadays requires a current density increase: good devices typically exhibit an open circuit voltage times fill factor product of one (Voc×FF∼1.4V×0.71=1). Their short-circuit current density (Jsc) value thus dictates their efficiency (expressed in %). Maximizing it with reasonable cells thicknesses necessitates therefore combining robust cell design with adequate light management through transparent electrodes and intermediate reflectors engineering. We will first show how record micromorph devices (13.5% initial and >11.5% stabilized efficiencies) are prepared on optimized single-layer ZnO electrodes. Such electrodes requirements will be discussed: 1) Strong and wide light scattering is needed on the entire useful wavelength range. Large features grant high total currents (>26mA/cm2) while sharp ones allow for high top cell currents (>13mA/cm2). 2) Sufficiently small or smooth substrate features permits high quality cell growth, providing good cell design (typically Voc over 1.4V). 3) Good conduction and transparency for electrodes (requiring ∼50cm2/V/s TCO mobility) should preserve sheet resistance close to 20Ω/□ (for FF>70%) with low absorption. We will then focus on pushing further micromorph devices potential. Either textured intermediate reflectors can fulfill the bottom cell needs, or double-texture substrates can be implemented: light scattering at large wavelengths is here achieved via nanoimprint lithography (a versatile approach to glass-texturing), topped by small and sharp ZnO features guaranteeing high top cell current. By combining excellent TCO with smart under-structures, thin devices delivering high currents with excellent efficiencies are within reach.