Silicon solar cells that employ passivating contacts featuring a heavily doped polysilicon layer on a thin silicon oxide (TOPCon) have been demonstrated to facilitate remarkably high cell efficiencies, amongst the highest achieved to date using a single junction on a silicon substrate. Importantly, it has been shown that the polysilicon‐based passivating contacts have a high degree of compatibility with existing mass production processes and toolsets, making them an attractive choice for photovoltaic (PV) cell manufacturers to increase the efficiency of their products. With several large PV manufacturers recently announcing plans to push the TOPCon technology into mass production, we review the significant industrial research and development activities that have been undertaken to push the boundaries of the technology and optimise its integration into the existing mass production pipeline. From an industrial perspective, TOPCon fabrication methodology options as well as necessary technological advances in front‐side fabrication, cell metallisation and module integration are discussed. The TOPCon technology development is contextualised in terms of larger trends in PV manufacturing, and we look towards the direction of future industrial development.

Improving the passivation of contacts in silicon solar cells is crucial for reaching high‐efficiency devices. Herein, the impact of the contact work function on the obtained passivation is examined and quantified using a novel method—quasi‐steady‐state photoluminescence—which provides access to the surface saturation current density after metallization ( J 0s,m ). The obtained J 0s,m indicates an improvement of the surface passivation when contacts with high work function are applied onto Si wafers passivated with aluminum oxide, regardless of the wafer doping type. This improvement is mainly due to the amplification of the imbalance between the electron and hole concentrations near the Si interface. The passivation quality is reduced when using contacts with low work function in which the recombination rate increases via the charge‐assisted carrier population control. Herein, the vital importance of selecting suitable metals to minimize contact recombination in high‐efficiency solar cells is pointed.

Identifying solar cell efficiency shortfalls in production lines is crucial to troubleshoot and optimize manufacturing processes. With the adoption of luminescence imaging as a key end-of-line characterization tool, a wealth of information is available to evaluate cell performance and classify defects, suitable for user input-free deep-learning analysis. We propose an automated reconstruction method, based on state-of-the-art generative adversarial networks, to remove defective regions in luminescence images. The reconstructed and original images are compared to estimate the efficiency loss. The method is validated on intentionally damaged cells by reconstructing defect-free images and then predicting the efficiency loss. The method can differentiate between different types of defects and pinpoint the defects that lead to the highest efficiency shortfall, enabling manufacturers to troubleshoot production lines in a fast and cost-effective manner- The proposed approach unlocks the potential of luminescence imaging as an effective end-of-line characterization tool.

The performance of solar cells influences the reliability, lifetime, cost, and safety of photovoltaic power plants. The electrical performance of these cells, as well as, their degradation rates over time, can vary between individual cells within the same photovoltaic module. Current-voltage measurements can provide detailed data on cell performance, however, it cannot be performed on individual cells in encapsulated modules. In this study, we propose a contactless and nondestructive method that is based on Suns-photoluminescence measurements to extract the current-voltage characteristics of individual solar cells under operating conditions in the field. Applications of the method to identify the extent of various degradation mechanisms, such as light-induced and potential-induced degradation, are demonstrated and discussed. The main advantages of the proposed method include its low cost, and its fast and easy applicability.

Understanding the impact of metal contacts on the recombination within a passivated silicon wafer is crucial for the optimization of various photovoltaic devices such as passivating-contact-based solar cells. To investigate the effect of the metal work function, a selection of metals is applied to aluminum-oxide-passivated n-type crystalline silicon wafers. The saturation current density of the metalized contact (J(0m)) is determined using the quasi-steady-state photoluminescence method and used as a figure of merit to quantify the effect. We find that J(0m) increases with the metal work function and that this effect is modulated with the passivation layer thickness. It is more pronounced for thinner passivation layers, which can be attributed to a significant change in the populations of electrons and holes near the silicon surface induced by the metal. Meanwhile thicker layers prevent the charge transfer between the silicon and metal more efficiently leading to insignificant changes in J(0m). Based on these findings, we suggest a suitable metal work function range to optimize contact recombination in silicon-based solar cells.

The surface recombination at metallized surfaces of a solar cell is of significant interest for cell process development, considering the limiting effect such recombination has on cell efficiency. This recombination is difficult to measure accurately, because the enhanced recombination at the metal contacts causes an inhomogeneous minority carrier profile, which limits the viability of conventional photoconductance-based recombination measurements. In this study, a photoluminescence-based system is used to measure the injection-dependent effective lifetime of full area metallized samples. Several techniques to extract surface recombination parameters from these data are compared. A simulation-based approach is shown to be superior to the simplified analyses that are more commonly applied to data of this type, in the case where the depth profile of the minority carrier density is nonuniform.

The surface recombination at metallized surfaces of a solar cell is of significant interest to process developers, considering the limiting effect such recombination has on cell efficiency. This recombination is difficult to measure accurately because the higher recombination at metallized surfaces compared to other passivated surfaces causes an inhomogeneous minority carrier profile. In this study, a novel photoluminescence-based system is used to measure the injection dependent effective lifetime of metallized samples, and several techniques to extract surface recombination parameters are compared. A simulation-based approach is shown to be superior to the simplified analyses that are more commonly applied to data of this type, in the case where the minority carrier profile is non-uniform.

Degradation of photovoltaic modules is one of the key aspects determining the power output and thus the financial viability of solar power plants. Furthermore, it is known that individual cells in PV modules degrade at different rates. The specific cell's degradation rate depends on many factors and generally exhibits an injection dependent nature. Thus, a current-voltage measurement of a specific cell can provide valuable information to determine the extent of degradation and identify the prevalent degradation mechanism. Although various inspection methods can be used to identify degraded cells, so far none of them provide the ability to extract the current-voltage characteristics of an underperforming cell by a contactless and non-destructive method. In this study, we present a photoluminescence based, contactless, fast, simple, and highly sensitive method to obtain the implied current-voltage curve of individual cells already embedded in solar modules. The method is validated by comparisons with Suns-V oc measurements.

Illumination intensity dependent open-circuit voltage measurements, commonly known as Suns- $V_{{\rm{oc}}}$, are often used to measure the current–voltage characteristic of a solar cell without the impact of series resistance. Deviations at high illumination levels between Suns-$V_{{\rm{oc}}}$ measurements and contactless measurements, such as injection-dependent photoluminescence, have previously been reported. These deviations are analyzed in detail in this paper and shown to cause significant errors when converting Suns-$V_{{\rm{oc}}}$ data to injection-dependent minority carrier lifetimes. Numerical modeling is used to identify shading and contact recombination as main causes of these deviations. Experimental data are used to demonstrate the magnitude of this effect for a range of different cell types.

As silicon solar cells approach limiting efficiencies, recombination at the metal-silicon interface is becoming an important limiting factor. Current methods to characterize this recombination are limited in that either: (1) they can only be applied to test structures that are not representative of the final solar cell; or (2) they require electrical contact to both sides of the p-n junction. In this study, we present a novel measurement technique to study recombination at the metal-silicon interface. The method is based on the quasi-steady-state photoluminescence technique, which is ideally suited because it is contactless and can be applied to completely processed cells with full rear metal coverage, precursor structures or other test structures. The new method is first validated against commonly used open circuit voltage measurements, and then applied to a range of metallized device structures to extract the contact recombination saturation current density.

Illumination intensity dependent open circuit voltage measurements (commonly known as Suns-V-oc) are often used to measure the current-voltage characteristic of a solar cell without the impact of series resistance. Deviations have previously been reported between Suns-V-oc measurements and contactless measurements, such as injection-dependent photoluminescence (Suns-PL) at high illumination levels. These deviations are analyzed in detail in this paper and shown to cause significant errors when converting Suns-V-oc data to injection-dependent minority carrier lifetimes. Experimental data are used to demonstrate the magnitude of this effect for a range of different solar cell types.

Current-voltage (I-V) measurements of a solar cell or of a photovoltaic (PV) module are critical to determine the overall power output of a PV system whose degradation rate has a critical impact on the levelized cost of electricity and on the bankability of PV projects [1]. Currently existing characterization techniques, such as infra-red (IR) thermography [2], ultra-violet (UV) fluorescence [3], electroluminescence (EL) [4]–[6] and photoluminescence (PL) [7] imaging have proven to be helpful in identifying performance-limiting module faults. However, all these characterization techniques fail to provide quantitative information regarding the nature of each fault. In order to investigate the cause of these faults, additional characterization techniques must be developed. I-V measurements can provide quantitative information regarding various performance limiting faults in solar cells. In many cases they can be used to identify the fault type and its typical causes. Yet, in the case of a PV module, only the terminal characteristics are obtained using conventional methods. Obtaining the I-V parameters of an individual cell within a module can be very useful, however, such information is not easily accessible in modules installed in the field since (1) the solar cells are embedded in a laminated module rendering their contacts inaccessible, and (2) the modules are connected to a solar array that is operating during daylight conditions.

Understanding the impact of metal contacts on the recombination within a passivated crystalline silicon (c-Si) wafer is crucial for the optimization of various photovoltaic devices such as passivating-contactbased solar cells. In this type of device, the metal contacts are offset from the c-Si wafer surface by additional layers [1]. The latter (1) minimize recombination losses—either by chemically reducing the density of defects at the wafer surface or by increasing the imbalance between the majority and minority carrier density near the surface—and (2) are selective for one type of carrier [1, 2]. An asymmetric population of electrons and holes near the c-Si surface can be obtained by applying a contact layer with a different work function than that of c-Si [2]. Therefore, we expect that the presence of a metal contact forms an extremely thin accumulation or inversion layer close to the wafer surface. This imbalance in concentration of the two carrier types will change the recombination statistics at the passivated surface. Usually, the well-established quasi-steady-state (QSS) photoconductance (PC) technique is used to measure the injection-dependent minority-carrier lifetime (τeff) and to extract the surface recombination current density (J0s). However, the QSSPC technique is not easily applicable to metallized structures due to the dominating conductivity of metals in comparison with the semiconductor conductivity. Several other techniques exist that allow determining the saturation current density at the metallized surface (J0m) [3-5]. Yet, each of them has its own limitations, as discussed in Ref. [6]. Recently, Dumbrell et al. presented a robust and contactless method based on QSS photoluminescence (PL) from which τeff of any metallized structure can be obtained, giving access to J0m [7]. In this study, we use J0m as figure of merit to investigate the impact of the metal work function on the recombination in passivating contacts. N-type Czochralski-grown silicon wafers are passivated by aluminum oxide (AlOx) films with a thickness of 5 or 20 nm deposited by atomic layer deposition. Five different metals were thermally evaporated at the rear side of Structures A and B, as shown in Fig. 1(a). We measured τeff of the samples using the QSSPL method, which enabled us to extract their total surface saturation current density (J0s,total). J0m is then calculated from J0s,total of the metallized samples and J0s of Structures C and D [Fig. 1(b)]. We find that applying metals with a work function smaller than that of n-type c-Si (<4.2 eV) on the 5-nm-thick AlOx passivation layer, J0m values are reduced. This indicates an improvement of surface passivation. This can be explained by an increase in electron density and a decrease in hole density near the c-Si surface related to the presence of the metal. As the density of electrons is much larger than the density of holes, the recombination rate at the interface—which is determined by their product— is reduced. We attribute the change in J0m to the change in the hole concentration near the interface. It is also important to note that negative fixed charges are present in the AlOx layers. These induce a local depletion of the electron concentration at the AlOx/c-Si interface and may thereby mitigate the positive effect of the metal work function. When applying the metals with a work function larger than that of n-type c-Si (>4.2 eV) on the 5-nm-thick AlOx passivation layer, a decrease in the electron density and an increase in the hole density near the c-Si surface is expected. The presence of negative fixed charges in the AlOx layers amplifies this asymmetric population of electrons and holes. Consequently, the recombination rate at the AlOx/cSi interface is reduced. Therefore, we expected that J0m will be reduced in comparison with J0s of the non-metallized reference sample. The high J0m of the sample with the metal work function of 5.22 eV might be attributed to the abnormally poor surface passivation of this sample. The impact of the metal work function on J0m is strongly reduced in the case of thicker passivation layer (Structure B). We attribute this to the reduced charge transfer probability between the c-Si and the metal through the AlOx layer once it becomes too thick. Hence, the asymmetric carrier population induced by the difference in work function between them is not achieved. In summary, we find that J0m increases with the metal work function and that this effect is modulated with the passivation layer thickness. It is more pronounced for thinner passivation layers, which can be attributed to a significant change in the populations of electrons and holes near the silicon surface induced by the metal. Meanwhile thicker layers prevent the charge transfer between the silicon wafer and the metal leading to insignificant changes in J0m. Based on these findings, we suggest that suitable metals should exhibit work function values below that of n-type c-Si—or above for the case of p-type cSi—to benefit from the asymmetric carrier population induced by the metal in passivating-contact-based solar cells. 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 0 5 10 15 20 Without metal Structure A Structure B