Marko Topič

Effect of surface roughness of ZnO:Al films on light scattering in hydrogenated amorphous silicon solar cells

Abstract Experimental investigation combined with computer modeling is used for analysis of light scattering process in hydrogenated amorphous silicon (a-Si:H) solar cells deposited on textured glass/ZnO:Al substrates. Descriptive scattering parameters—haze and angular distribution functions (ADFs)—for the textured ZnO:Al films with different surface roughness are determined. The haze parameters of all internal interfaces in the a-Si:H solar cells are calculated using equations of scalar scattering theory calibrated on the measurements of the substrates. The ADFs determined for the substrates are modified and applied to the internal interfaces. The scattering parameters are incorporated in our optical model and used to simulate the effect of the ZnO:Al surface roughness on the quantum efficiency (QE) of the solar cells. The simulations reproduce the measured QE of all solar cells with different roughness of the substrate very well.

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.

Optical modeling of a-Si:H solar cells deposited on textured glass/SnO2 substrates

In this article we determine descriptive scattering parameters—haze and angular distribution functions—of scattered light for textured glass/SnO2 Asahi U-type substrates. These scattering parameters are input parameters of our optical model that enables us to analyze multilayer optical systems with rough interfaces. The scalar scattering theory is used to calculate the haze parameters of all internal rough interfaces in the a-Si:H solar cells deposited on the glass/SnO2 substrates. In the equations of the scalar scattering theory the correction functions are introduced in order to match the calculations with the measurements of the haze parameters of the substrates. The angular distribution functions of the substrates are applied to the rough internal interfaces. Using these scattering parameters we investigate the optical behavior of a-Si:H solar cells with different intrinsic layer thicknesses deposited on the textured glass/SnO2 substrates with different roughnesses.

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...

Band‐gap engineering in CdS/Cu(In,Ga)Se2 solar cells

Using a numerical device simulation program, the band‐gap engineering in CdS/Cu(In,Ga)Se2 solar cells is examined. The device physics of different design concepts is analysed. Normal band‐gap grading improves performance, especially due to the additional quasi‐electric field, and the analysis showed that the best results are achieved if the grading extends from the highest band‐gap value at the back up to the space charge region. The double grading concept does not yield further improvement, because the front grading—even if located in the space charge region—repels the minority carriers (electrons) away from the CdS interface, and consequently, the fill factor drops significantly. Notch structures in the base also exhibit lower performance than the uniform band‐gap base due to the lower open‐circuit voltage and poorer fill factor. Therefore, the best results are achieved by a normal grading in a Cu(In,Ga)Se2 base from the edge of the space charge region to the back contact.

Analysis and optimisation of microcrystalline silicon solar cells with periodic sinusoidal textured interfaces by two-dimensional optical simulations

Two-dimensional optical model for simulation of thin-film solar cells with periodical textured interfaces is presented. The model is based on finite element method and uses triangular discrete elements for the structure description. The advantages of the model in comparison to other existing models are highlighted. After validation and verification of the developed simulator, simulations of a microcrystalline silicon solar cell with a sinusoidal grating texture applied to the interfaces are carried out. The analysis and optimization of the two grating parameters—period and height of the grooves—are performed with respect to the maximal short-circuit current density of the cell. Up to 45% increase in the current density is identified for the optimized structure, compared to that of the cell with flat interfaces. Optical losses in the periodically textured silver back reflector are determined.

Analysis of TCO/p(a-Si:C:H) heterojunction and its influence on p-i-n a-Si:H solar cell performance

The ASPIN computer simulator, which enables analysis of transparent conducting oxide (TCO)/a-Si:C:H/a-Si:H/TCO heterostructures, was used to examine the influence of different front TCO/p(a-Si:C:H) heterojunctions on TCO/p-i-n/TCO/metal a-Si:H solar cell performance. Separate analysis of TCO/p(a-Si:C:H) structure for both SnO2 and ZnO indicates that the mismatch between the high contact potential and the measured potential barrier at the p-layer surface can be resolved by a large density of interface defect states, causing a steep potential decrease in the interface. Analysis of the detrimental effects of a-Si:C:H chemical oxidation in SnO2/p(a-Si:C:H), which were simulated by the increased surface state density in the a-Si:C:H, showed that the potential barrier in a p-layer with oxidized surface is increased. The impact of both TCO/p(a-Si:C:H) interface states and a-Si:C:H surface states on the photoelectric properties of p-i-n a-Si:H solar cells is discussed, and a possible improvement of Voc is envisaged.

Examination of blocking current-voltage behaviour through defect chalcopyrite layer in ZnO/CdS/Cu(In,Ga)Se2/Mo solar cell

Abstract Blocking current-voltage behaviour of ZnO/CdS/Cu(In,Ga)Se2/Mo solar cells, which is either temperature- or light-conditioned, is examined using a comprehensive numerical device simulator. Effects of defect states in the defect-chalcopyrite layer and at the CdS/defect-chalcopyrite interface are investigated. Acceptor-like defect states either in a defect-chalcopyrite layer or at the CdS/defect-chalcopyrite interface cause different trapping under red light or white light. This results in different potential profiles throughout the structure, which determine the changeable I−V behaviour under forward bias. Simulation results show that these acceptor-like defect states can also control the temperature-conditioned blocking I−V behaviour.