Miha Filipič

Complex Refractive Index Spectra of CH3NH3PbI3 Perovskite Thin Films Determined by Spectroscopic Ellipsometry and Spectrophotometry.

The complex refractive index (dielectric function) of planar CH3NH3PbI3 thin films at room temperature is investigated by variable angle spectroscopic ellipsometry and spectrophotometry. Knowledge of the complex refractive index is essential for designing photonic devices based on CH3NH3PbI3 thin films such as solar cells, light-emitting diodes, or lasers. Because the directly measured quantities (reflectance, transmittance, and ellipsometric spectra) are inherently affected by multiple reflections, the complex refractive index has to be determined indirectly by fitting a model dielectric function to the experimental spectra. We model the dielectric function according to the Forouhi-Bloomer formulation with oscillators positioned at 1.597, 2.418, and 3.392 eV and achieve excellent agreement with the experimental spectra. Our results agree well with previously reported data of the absorption coefficient and are consistent with Kramers-Kronig transformations. The real part of the refractive index assumes a value of 2.611 at 633 nm, implying that CH3NH3PbI3-based solar cells are ideally suited for the top cell in monolithic silicon-based tandem solar cells.

Infrared light management in high-efficiency silicon heterojunction and rear-passivated solar cells

Silicon heterojunction solar cells have record-high open-circuit voltages but suffer from reduced short-circuit currents due in large part to parasitic absorption in the amorphous silicon, transparent conductive oxide (TCO), and metal layers. We previously identified and quantified visible and ultraviolet parasitic absorption in heterojunctions; here, we extend the analysis to infrared light in heterojunction solar cells with efficiencies exceeding 20%. An extensive experimental investigation of the TCO layers indicates that the rear layer serves as a crucial electrical contact between amorphous silicon and the metal reflector. If very transparent and at least 150 nm thick, the rear TCO layer also maximizes infrared response. An optical model that combines a ray-tracing algorithm and a thin-film simulator reveals why: parallel-polarized light arriving at the rear surface at oblique incidence excites surface plasmons in the metal reflector, and this parasitic absorption in the metal can exceed the absorption in the TCO layer itself. Thick TCO layers—or dielectric layers, in rear-passivated diffused-junction silicon solar cells—reduce the penetration of the evanescent waves to the metal, thereby increasing internal reflectance at the rear surface. With an optimized rear TCO layer, the front TCO dominates the infrared losses in heterojunction solar cells. As its thickness and carrier density are constrained by anti-reflection and lateral conduction requirements, the front TCO can be improved only by increasing its electron mobility. Cell results attest to the power of TCO optimization: With a high-mobility front TCO and a 150-nm-thick, highly transparent rear ITO layer, we recently reported a 4-cm2 silicon heterojunction solar cell with an active-area short-circuit current density of nearly 39 mA/cm2 and a certified efficiency of over 22%.

CH 3 NH 3 PbI 3 perovskite / silicon tandem solar cells: characterization based optical simulations

In this study we analyze and discuss the optical properties of various tandem architectures: mechanically stacked (four-terminal) and monolithically integrated (two-terminal) tandem devices, consisting of a methyl ammonium lead triiodide (CH(3)NH(3)PbI(3)) perovskite top solar cell and a crystalline silicon bottom solar cell. We provide layer thickness optimization guidelines and give estimates of the maximum tandem efficiencies based on state-of-the-art sub cells. We use experimental complex refractive index spectra for all involved materials as input data for an in-house developed optical simulator CROWM. Our characterization based simulations forecast that with optimized layer thicknesses the four-terminal configuration enables efficiencies over 30%, well above the current single-junction crystalline silicon cell record of 25.6%. Efficiencies over 30% can also be achieved with a two-terminal monolithic integration of the sub-cells, combined with proper selection of layer thicknesses.

Amorphous silicon oxide window layers for high-efficiency silicon heterojunction solar cells

In amorphous/crystalline silicon heterojunction solar cells, optical losses can be mitigated by replacing the amorphous silicon films by wider bandgap amorphous silicon oxide layers. In this article, we use stacks of intrinsic amorphous silicon and amorphous silicon oxide as front intrinsic buffer layers and show that this increases the short-circuit current density by up to 0.43 mA/cm2 due to less reflection and a higher transparency at short wavelengths. Additionally, high open-circuit voltages can be maintained, thanks to good interface passivation. However, we find that the gain in current is more than offset by losses in fill factor. Aided by device simulations, we link these losses to impeded carrier collection fundamentally caused by the increased valence band offset at the amorphous/crystalline interface. Despite this, carrier extraction can be improved by raising the temperature; we find that cells with amorphous silicon oxide window layers show an even lower temperature coefficient than referenc...

Analysis of lateral transport through the inversion layer in amorphous silicon/crystalline silicon heterojunction solar cells

In amorphous/crystalline silicon heterojunction solar cells, an inversion layer is present at the front interface. By combining numerical simulations and experiments, we examine the contribution of the inversion layer to lateral transport and assess whether this layer can be exploited to replace the front transparent conductive oxide (TCO) in devices. For this, heterojunction solar cells of different areas (2 × 2, 4 × 4, and 6 × 6 mm2) with and without TCO layers on the front side were prepared. Laser-beam-induced current measurements are compared with simulation results from the ASPIN2 semiconductor simulator. Current collection is constant across millimeter distances for cells with TCO; however, carriers traveling more than a few hundred microns in cells without TCO recombine before they can be collected. Simulations show that increasing the valence band offset increases the concentration of holes under the surface of n-type crystalline silicon, which increases the conductivity of the inversion layer. U...

Analysis of Local Minority Carrier Diffusion Lengths in Liquid-Phase Crystallized Silicon Thin-Film Solar Cells

We developed a method to quantify the local minority carrier diffusion lengths in interdigitated back-contact solar cells having a 10 μm thick liquid phase crystallized (LPC) Si absorber by light-beam induced current (LBIC) measurements. The method is verified by 2-D simulations of the LBIC signals using ASPIN3. The effective minority carrier diffusion lengths determined this way range between 33–44 μm inside a grain, which proves that advanced cell concepts like an IBC system are well suited for the LPC absorbers. Furthermore, the method has the potential to help improving the optimization of contact system geometries and it may be used to understand the influences of different grain orientations and improve the LPC-Si absorber fabrication process.

Revisiting light trapping in silicon solar cells with random pyramids

Random pyramids are the most widely used texture in monocrystalline silicon solar cells for reducing front- surface reflection and trapping weakly absorbed light. In prior efforts to evaluate the light-trapping performance of random pyramids through optical simulations, the base angle of the pyramids was assumed to be 54.7°, as is expected from the orientation of the crystallographic planes. In this contribution, we benchmark the light-trapping capability of real random pyramids—which have a distribution of base angles—against both ideal, 54.7° random pyramids, and a Lambertian scatterer. We do so by calculating the path length enhancement and fraction of rays remaining trapped as a function of passes through the wafer, and this information is used to calculate short- circuit current density as a function of wafer thickness. Interestingly, the excellent performance of real random pyramids—they are close to Lambertian—arises precisely because they are imperfect and have a distribution of angles.

Design challenges for light harvesting in photovoltaic devices

Device modelling and characterization are indispensable tools in the design of photovoltaic devices. In the contribution we present two challenging issues related to accurate modelling and efficient characterization of light scattering at nanotextured interfaces or other nanophotonic structures used in solar cell technologies. The model based on finite element method, which is upgraded with the Huygens’ expansion theorem is presented. It enables to calculate the angular distribution function of scattered light in the near and far field. It accounts also for the antireflection effects originating from nanoroughnesses. To characterize scattered light efficiently a camera based angular resolved spectroscopy system is presented. It captures the spatial angular distribution function in broad angular range at one shot.

Optimization of interdigitated back contact geometry in silicon heterojunction solar cell

In-house developed 2D semiconductor simulator ASPIN3 is used to simulate amorphous silicon / crystalline silicon heterojunction cells with interdigitated contacts on the back side. Our focus is on finding the optimal widths of emitter and back surface field stripes as well as the width of the gap between them. Analysis of the three dimensional parameter space reveals that high efficiencies can be achieved for relatively large widths, over 100 μm, allowing the use of simple patterning techniques to create the cells.

Amorphous silicon/crystalline silicon heterojunction solar cells — Analysis of lateral conduction through the inversion layer

We examine the contribution of the inversion layer, present at the amorphous silicon/crystalline silicon interface, to the lateral conduction of photogenerated charge carriers. We employ numerical simulation and experiments to determine if this layer can be exploited to replace the transparent conductive oxide layer (TCO). We found that current collection is constant when the TCO is present, but carriers can only travel a few hundred μm when the TCO is omitted. Simulations predict that increasing the valence band offset increases the conductivity of the inversion layer.