Background: Elongated nanostructures, such as nanowires, have attracted significant attention for application in silicon-based solar cells. The high aspect ratio and characteristic radial junction configuration can lead to higher device performance, by increasing light absorption and, at the same time, improving the collection efficiency of photo-generated charge carriers. This work investigates the performance of ultra-thin solar cells characterised by nanowire arrays on a crystalline silicon bulk. Results: Proof-of-concept devices on a p-type mono-crystalline silicon wafer were manufactured and compared to flat references, showing improved absorption of light, while the final 11.8% (best-device) efficiency was hindered by sub-optimal passivation of the nanowire array. A modelling analysis of the optical performance of the proposed solar cell architecture was also carried out. Results showed that nanowires act as resonators, amplifying interference resonances and exciting additional wave-guided modes. The optimisation of the array geometrical dimensions highlighted a strong dependence of absorption on the nanowire cross section, a weaker effect of the nanowire height and good resilience for angles of incidence of light up to 60°. Conclusion: The presence of a nanowire array increases the optical performance of ultra-thin crystalline silicon solar cells in a wide range of illumination conditions, by exciting resonances inside the absorber layer. However, passivation of nanowires is critical to further improve the efficiency of such devices.

@en

Interdigitated back contact (IBC) crystalline silicon (c-Si) solar cells are attracting a lot of attention because of their capability to reach world record conversion efficiency. Because of the relatively complex contact pattern, their design and optimization typically require advanced numerical simulation tools. In this work, a TCAD-based simulation platform has been developed to account accurately and in detail the optical and passivation mechanisms of front texturization. Its validation has been carried out with respect to a novel homo-junction IBC c-Si solar cell based on ion implantation and epitaxial growth, comparing measured and simulated reflectance, transmittance, internal quantum efficiency, external quantum efficiency spectra, and current density–voltage characteristics. As a result of the calibration process, the opto-electrical losses of the investigated device have been identified quantitatively and qualitatively. Then, an optimization study about the optimal front surface field (FSF) doping, front-side texturing morphology, and rear side geometry has been performed. The proposed simulation platform can be potentially deployed to model other solar cell architectures than homo-junction IBC devices (e.g., passivated emitter rear cell, passivated emitter rear locally diffused cell, hetero-IBC cell). Simulation results show that a not-smoothed pyramid-textured front interface and an optimal FSF doping are mandatory to minimize both the optical and the recombination losses in the considered IBC cell and, consequently, to maximize the conversion efficiency. Similarly, it has been showed that recombination losses are affected more by the doping profile rather than the surface smoothing. Moreover, the performed investigation reveals that the optimal FSF doping is almost independent from the front texturing morphology and FSF passivation quality. According to this result, it has been demonstrated that an IBC cell featuring an optimal FSF doping does not exhibit a significant efficiency improvement when the FSF passivation quality strongly improves, proving that IBC cell designs based on low-doped FSF require a very outstanding passivation quality to be competitive. Deploying an optimization algorithm, the adoption of an optimized rear side geometry can potentially lead to an efficiency improvement of about 1%abs as compared with the reference IBC solar cell. Further, by improving both emitter and c-Si bulk quality, a 22.84% efficient solar cell for 280-μm thick c-Si bulk was simulated.@en

Black silicon (b-Si) nanotextures can significantly enhance the light absorption of crystalline silicon solar cells. Nevertheless, for a successful application of b-Si textures in industrially relevant solar cell architectures, it is imperative that charge-carrier recombination at particularly highly n-type doped black Si surfaces is further suppressed. In this work, this issue is addressed through systematically studying lowly and highly doped b-Si surfaces, which are passivated by atomic-layer-deposited Al2O3 films or SiO2/Al2O3 stacks. In lowly doped b-Si textures, a very low surface recombination prefactor of 16 fA/cm2 was found after surface passivation by Al2O3. The excellent passivation was achieved after a dedicated wet-chemical treatment prior to surface passivation, which removed structural defects which resided below the b-Si surface. On highly n-type doped b-Si, the SiO2/Al2O3 stacks result in a considerable improvement in surface passivation compared to the Al2O3 single layers. The atomic-layer-deposited SiO2/Al2O3 stacks therefore provide a low-temperature, industrially viable passivation method, enabling the application of highly n- type doped b-Si nanotextures in industrial silicon solar cells.@en

In this work, the application of carrier-selective passivating contacts based on tunneling silicon-dioxide and ion-implanted poly-Si in front and rear contacted Si solar cells is presented. This paper addresses the need to minimize the contact recombination while still keeping high short circuit current. We aim to solve such trade-off with a novel solar cell architecture called Passivated Rear and Front ConTacts (PeRFeCT). Such design employs a selective passivating contact combined with standard homojunction on the front side in order to minimize contact recombination, while achieving high optical transparency and a full area passivating contact on the rear side. The opto-electrical modeling of this front/rear contacted architecture indicates a potential efficiency above 26%. As technology demonstration, we also report on the optimization of front surface field and processing of 2.8 × 2.8 cm2 wide solar cells leading to a 20.1% conversion efficiency.
@en

The optical analysis of optically-textured and electrically-flat ultra-thin crystalline silicon (c-Si) slabs is presented. These slabs were endowed with decoupled front titanium-dioxide (TiO2) / back silicon-dioxide (SiO2) dielectric textures and were studied as function of two types of back reflectors: standard silver (Ag) and dielectric modulated distributed Bragg reflector (MDBR). The optical performance of such systems was compared to that of state-of-the-art flat c-Si slabs endowed with so-called front Mie resonators and to those of similar optical systems still endowed with the same back reflectors and decoupled front/back texturing but based on textured c-Si and dielectric coatings (front TiO2 and back SiO2). Our optimized front dielectric textured design on 2-µm thick flat c-Si slab with MDBR resulted in more photo-generated current density in c-Si with respect to the same optical system but featuring state-of-the-art Mie resonators ( + 6.4%), mainly due to an improved light in-coupling between 400 and 700 nm and light scattering between 700 and 1050 nm. On the other hand, the adoption of textured dielectric layers resulted in less photo-generated current density in c-Si up to −20.6% with respect to textured c-Si, depending on the type of back reflector taken into account.@en

Ion-implanted poly-crystalline silicon (poly-Si), in combination with a tunnel oxide layer, is investigated as a carrier-selective passivating contact in c-Si solar cells based on an interdigitated back contact (IBC) architecture. The optimized poly-Si passivating contacts enable low interface recombination, resulting in implied VOC (iVOC) of about 720 mV and 704 mV for n-type and p-type, respectively, before any hydrogenation step. It is found that high-quality passivation can be obtained when confining the dopants within the poly-Si layers and realizing a shallow diffusion of dopants into the c-Si bulk, meaning a sharp decrease in doping concentration in the c-Si at the poly-Si/c-Si interface. The doping profile at the poly-Si/c-Si interface can be influenced by poly-Si layer thickness, poly-Si ion-implantation parameters, and post-implantation annealing conditions. The detailed discussion on the passivation properties of the poly-Si passivating contacts and their preparation conditions are presented in this paper. In addition, IBC solar cells with/without front surface field (FSF) are fabricated, with the optimized poly-Si passivating contacts as back surface field, BSF (n-type poly-Si), and emitter (p-type poly-Si). The best cell shows an efficiency of 21.2% (VOC=692 mV, JSC=39.2 mA/cm2, FF=78.3%, and pFF=83.5%).@en

Front and rear contacted wafer-based c-Si solar cells characterized by P-diffused emitter and Al-based back surface field currently constitute the dominant solar cell architecture in the photovoltaic market. The key success of this technology is based on the simple and cost effective fabrication process. However, its conversion efficiency is limited. High-efficiency c-Si solar cells architectures have been demonstrated at laboratory and industrial scale with the aim of decreasing the levelized cost of electricity (LCOE) by increasing efficiency. For this reason, high-efficiency solar cells are expected to increase their market share in next decade. In particular, interdigitated back contacted (IBC) c-Si solar cell architecture, which the current world record efficiency is based on, is expected to gain shortly relevance at industrial level. In this work, activities at TUDelft on the fabrication of IBC c-Si solar cells are reported. In particular, a novel method for realizing high-efficiency IBC c-Si solar cells based on single-side and (relatively) low-temperature doping techniques is demonstrated. In particular, epitaxial growth of B-doped Si is used to form the emitter, while P-ion implantation is deployed to form both front and back surface fields. To pattern the rear junction, a self-aligned process based on one lithographic step has been developed. In addition, metal lift-off is used to define the metal contacts of both polarities. By using this process, efficiency higher than 20% has been demonstrated.@en

Ion-implanted passivating contacts based on poly-crystalline silicon (polySi) are enabled by tunneling oxide, optimized, and used to fabricate interdigitated back contact (IBC) solar cells. Both n-type (phosphorous doped) and p-type (boron doped) passivating contacts are fabricated by ion-implantation of intrinsic polySi layers deposited via low-pressure chemical vapor deposition and subsequently annealed. The impact of doping profile on the passivation quality of the polySi doped contacts is studied for both polarities. It was found that an excellent surface passivation could be obtained by confining as much as possible the implanted-and-activated dopants within the polySi layers. The doping profile in the polySi was controlled by modifying the polySi thickness, the energy and dose of ion-implantation, and the temperature and time of annealing. An implied open-circuit voltage of 721 mV for n-type and 692 mV for p-type passivating contacts was achieved. Besides the high passivating quality, the developed passivating contacts exhibit reasonable high conductivity (Rsh n-type = 95 Ω/□ and Rsh p-type = 120 Ω/□). An efficiency of 19.2% (Voc = 673 mV, Jsc = 38.0 mA/cm2, FF = 75.2%, and pseudo-FF = 83.2%) was achieved on a front-textured IBC solar cell with polySi passivating contacts as both back surface field and emitter. By improving the front-side passivation, a VOC of 696 mV was also measured.@en

Climate changes due to increase of CO2 emission are becoming a serious issue for this planet. The so called climate crisis has been the main topic of the last United Nations Climate Change Conference (COP 21) . Direct conversion of sunlight into electricity is one of the most promising technology for achieving the COP 21 agreement. Wafer-based crystalline silicon (c-Si) solar cells account for more than 90% of the total PV market because silicon is a non-toxic and abundant material and Si-based PV modules have demonstrated long term stability and high durability. To maintain this technology dominant also in the coming years, continuous improvement in conversion efficiency without increasing processing costs are required. In this thesis novel solutions based on opto-electrical surface engineering are presented as potential solutions to increase conversion efficiency and/or decrease the costs of wafer-based c-Si solar cells. In particular, advanced light management techniques were developed to enhance light absorption in thin c-Si absorber and to fabricate customized PV products for building integrated photovoltaic (BIPV) applications. This thesis begins with introducing, theoretical limits (Chapter 1), working principles and current status (Chapter 2) of waferbased c-Si solar cells. In Chapter 3 the losses analysis of industrial multi-crystalline silicon (mc-Si) solar cell was performed by using the ASA simulation tool. Such analysis pointed out the main opto-electrical losses for a mc-Si solar cell which were tackled in the next Chapters. In particular, Chapter 4 deals with design and fabrication of advanced light trapping scheme for minimizing optical losses of state-of-the-art c-Si solar cells. To this aim a combination of surface textures with different geometrical scales were used in order to trigger several optical effects. In particular, nano-texturing fabricated via reactive ion etching (RIE) on the front side and micro texturing based on alkaline etching on the rear side were used providing broadband light-in coupling and light scattering. Almost ideal back reflectors such as Ag or Distributed Bragg reflectors (DBR) were applied on the rear side. By using such light trapping scheme, the so-called 4n2 absorption enhancement limit, which has been elusive for more than 30 years was experimentally demonstrated on a broad wavelength range. The interdigitated back contact (IBC) c-Si solar cell was indicated as the most promising solar cell architecture to apply such light trapping scheme. This technology was not available within the PVMD group. Therefore, in Chapter 5 a simplified self-aligned process for fabrication high efficiency IBC c-Si solar cells was demonstrated. The process involved the combination of ion implantation and epitaxial growth of in-situ doped Si. The process flow was optimized to minimize the thermal budget and the number of lithographic steps. By using only two lithographic steps, a conversion efficiency equal to 20.2% on 9 cm2 device was demonstrated. For such solar cell architecture it was shown that a lightly doped front surface field improves carrier collection. After developing a process flow for fabricating IBC c-Si solar cells, the application of the advanced light management technique to IBC was presented in Chapter 6. To this aim two major issues were tackled. The first was related to the removal of surface defects induced by the RIE process to decrease surface recombination. To achieve this goal a cost effective process was developed. The second dealt with adapting the light trapping scheme to the IBC process integration. To this aim, the decoupled front (nano-textured) and rear side (micotexturing) light trapping scheme of Chapter 4 was modified by superposing both texture scales on the front side of the wafer. This approach is called modulated surface texture (MST). The combination of the advanced light trapping and surface passivation schemes was employed in IBC c-Si solar cells. Top efficiency of 19.8% for MST-IBC solar cell was demonstrated. Advanced light management techniques were also applied to bifacial c-Si solar cells. The objective of this study was twofold: (i) enhancing cell efficiency by increasing the internal rear internal reflectance and (ii) providing novel solutions for BIPV applications. In particular, DBR and TiO2 particles in the form of white paint were used as back reflectors of bifacial c-Si solar cells. The DBR enabled the possibility of fabricating rear side coloured bifacial modules, which can be attractive for BIPV applications.@en

The photovoltaic (PV) industry has recently become more oriented toward n-type c-Si solar cells. Among the different n-type solar cell architectures, bifacial cells are quickly emerging. The open-rear configuration of a bifacial device results in high transmittance (T) losses at long wavelengths (>1000 nm). This limitation is usually overcome at the module level either by using a bifacial encapsulation or by placing a reflective foil on the rear side. In this paper, we have investigated the application of a distributed Bragg reflector (DBR) and TiO2-based white paint (WP) as alternative metal-free back-reflector options applied to the textured open-rear of bifacial n-Pasha cells. Because of the high T losses at long wavelengths of the DBR applied on textured surface, its design and fabrication is studied in detail. The dielectric (DBR and WP) and optimized Ag back-reflectors, which are used as a reference, are applied to bifacial n-Pasha cells, and their performance is evaluated. In particular, we demonstrate T below 20% at 1200 nm by optimizing the DBR thickness for textured surfaces. In addition, the optimized DBR and WP show performance comparable with a state-of-the-art Ag back-reflector. The highest increase of the conversion efficiency is measured for the WP back-reflector: +0.34% absolute compared with n-Pasha measured with no-additional back-reflector.@en