Crystalline silicon (c-Si) homojunction architectures like aluminum back-surface field (Al-BSF) and passivated emitter and rear cell (PERC) dominated the solar cell market for the last decades. Recently, carrier-selective passivating contact (CSPC) designs have shown that they can effectively reduce recombination losses and exhibit efficiencies above 25 % which approaches the Shockley-Queisser limit of single-junction solar cells. To overcome this intrinsic efficiency limit, research and industry have started to investigate multijunction devices which utilize two different absorber layers in order to reduce the spectral mismatch losses. In this project, the focus lies on the fabrication of a high temperature CSPC bottom cell with polycrystalline silicon oxide (poly-SiOx) for the application in a 2-terminal perovskite/c-Si tandem device. Poly-SiOx features a higher transparency due to a wider band gap which can decrease the free-carrier absorption (FCA) and induce larger band bending in comparison to the classic polycrystalline silicon (poly-Si) contacts.
To fabricate a c-Si solar cell with high temperature carrier-selective passivating contacts, the passivation of the contacts was optimised using symmetric lifetime samples. It was found that the optimal processing conditions for the dry thermal oxidation process and the high temperature annealing step are 675 °C for 3 minutes and 900 °C for 15 minutes, respectively. This resulted in an implied open-circuit voltage (iV_OC) for the n-doped and p-doped symmetric samples after hydrogenation of 745 mV and 674 mV, respectively. Those optimised processing conditions were adopted for the fabrication of a single-junction n-type front-side polished and p-type rear-side textured CSPC solar cell. This resulted in a solar cell with an efficiency of 16.67 %. By lowering the thermal budget of the recovery annealing conditions after the sputtering of the transparent conductive oxide (TCO), the passivation properties are restored more effectively. This resulted in an enhanced solar cell with a final efficiency of 18.76 %. Other techniques to improve the performance like the modification of the p-layer thickness, the incorporation of an additional annealing step after the a-Si deposition, or the introduction of indium tungsten oxide (IWO) as the TCO did not result in an improvement of the final solar cell efficiency. Optical simulations were performed in GenPro4 to investigate the optical performance of the solar cell. Finally, a 2-terminal perovskite/c-Si tandem device with a high temperature carrier-selective passivating c-Si bottom cell was fabricated in cooperation with TU Eindhoven. The final tandem solar cell exhibits an efficiency of 23.10 % with a fill factor of 74.00 %, a V_OC of 1.76 V and a J_SC of 17.81 mA/cm². ... Read more

Carrier selective passivating contacts (CSPC) have proven to effectively curtail the recombination losses emerging at directly metallised contacts of crystalline Silicon (c-Si) solar cells. CSPCs enabled using an ultra-thin interfacial tunnel oxide layer (SiOx) capped by a doped polycrystalline Silicon (poly-Si) layer also referred to as Tunnel Oxide Passivating Contacts (TOPCon) have resulted in efficiencies as high as 25.8%. This thesis project addresses the development of oxygen alloyed poly-Si (poly-SiOx) in combination with an interfacial oxide layer grown by dry thermal oxidation. The limited transparency of poly-Si based contacts brought on by high free carrier absorption (FCA) can be mitigated by the use of poly-SiOx based passivating contacts owing to their wider bandgaps which induce stronger band bending.
To begin with, poly-SiOx CSPC were optimised by determining the optimum thermal budgets for tunnel oxide growth and hydrogenation scheme. Tunnel oxide layers grown at 675 ͦC 6 minutes demonstrated very good passivation for p-type polished and n-type textured CSPCs indicated by their implied Voc of 709 mV and 711 mV respectively. For the p-type textured CSPC identified as the primary limiting factor when deploying in c-Si solar cells, a tunnel oxide layer grown at 675 ͦC 3 minutes in conjunction with a two-step annealing scheme showed a crucial enhancement in passivation quality with a final implied Voc of 687 mV.
The single side textured front back contacted (FBC) solar cell fabricated using the optimised p-type polished and n-type textured poly-SiOx CSPC recorded a conversion efficiency of 20.94% on a 4 cm2 screen printed solar cell. The reported efficiency is the maximum that has been attained so far for the configuration that uses a thermally grown tunnel oxide layer with poly-SiOx CSPCs. Effective carrier transport and carrier collection was illustrated by a fill factor (FF) of 79.6%. A Jsc of 37.91 mA/cm2 was recorded for the same. A comparison with a single side textured FBC solar cell that employed a tunnel oxide layer grown by nitric acid oxidation of Silicon (NAOS) revealed a superiority in performance by the thermally grown tunnel oxide layer resulting in better passivation and carrier selectivity.
Lastly, the optimised n-type textured and p-type textured CSPCs were implemented on a double side textured FBC solar cell. The two-step annealing scheme that showed beneficial results for the p-type textured CSPC was implemented within the FBC solar cell, leading to an implied Voc of 698mV post hydrogenation. It is worth mentioning that this is the highest value achieved until now for this novel cell architecture. Implementation of screen printing resulted in a final conversion efficiency of 19.38% on a 4 cm2 solar cell with a FF and Jsc of 77.89% and 37.65 mA/cm2 respectively.
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Performance of photovoltaic modules are widely expressed using their efficiency at Standard Test Conditions (STC). Although, real world conditions largely differ from this standard. Energy yield of photovoltaic modules at a certain location gives a clear indication of the performance of the module exposed to varying weather conditions. Photovoltaic Materials and Devices (PVMD) Toolbox developed at Delft University of Technology and implemented in MATLAB®, aims to simulate energy yield of photovoltaic modules while providing the flexibility of modifying cell-level to module-level parameters of the device. Accurately determining the irradiance falling on the module at a certain location and simulating the electrical properties of a solar cell at realistic conditions is crucial in determining the yield generated by a module. Due to the promising results displayed by the technology, in this study, energy yield of perovskite/c-Si tandem modules are simulated using newly developed daylight and parameter extraction models in PVMD toolbox. Due to high computational efficiency and functionality offered, Preetham daylight model is implemented as an improvement to Perez model currently used in PVMD toolbox. The model considers the effect of Rayleigh and Mie scattering, and turbidity of atmosphere in determining the distribution of diffuse irradiance across the skydome. By implementing the model, three factors can now be determined: luminance distribution, RGB co-ordinates and relative spectral power distribution over the skydome for any location. Luminance is calculated for every point in sky using Perez parametric function that use coefficients derived from simulated data for different sun directions and turbidity values. For all locations considered, Preetham model calculates higher luminance over the year by ~2%. Calculated luminance values normalized against zenith luminance is used to derive CIE xyY values and consecutively, the RGB co-ordinates that are rendered real-time to create images of the skydome for every hour in chosen timeframe. By calculating the relative spectral power distribution, analysis of incident light falling on the module from all points of the skydome is possible which especially deems useful for modelling tandem modules. To accurately simulate electrical properties of solar cell under varying operating conditions, parameter extraction model for simulated ASA J-V curves is implemented using an analytical method. For the c-Si and perovskite cells considered in the study, the reconstructed curves using extracted parameters fits ASA curve with RMSE or 1.98% and 3.02% respectively at STC. Using these models introduced in this study, energy yield of mono-facial and bi-facial 2T/4T tandem modules are calculated and analyzed for Rome, Reykjavik and Alice Springs. Depending on air mass of a location, perovskite thickness of mono-facial 2T tandem devices are optimized to produce maximum specific yield. All considered modules generate highest yield at Alice Springs due to high insolation at the location. The yield difference between modules while using the two daylight models is insignificant at <1%. With an increase in perovskite thickness, energy yield of 2T bi-facial modules increase significantly for high albedo while it drops beyond the optimum thickness due to current mismatch for low albedo. Bi-facial 4T module on high albedo (albedo= 0.85) surfaces show best performance out of all the modules producing 36% higher yield than mono-facial 4T module for Rome. ... Read more