Compensated silicon has long been a topic of interest in semiconductor physics due to the complex interactions between donors and acceptors.Today, the growing emphasis on circular manufacturing and end-of-life module recycling drives the attention toward n-type silicon, the current market standard. The possibility of reusing p-type feedstock from decommissioned modules compensated by donor addition could enable its integration into modern n-type architectures such as TOPCon, SHJ, and IBC. However, a key question remains open: does n-type compensated silicon (particularly when derived from recycled sources) exhibit the required optoelectronic quality for these advanced device structures?This work presents first answers to this question through a comparative study of two compensated silicon ingots: (i) an intentionally co-doped n-type ingot grown from conventional polysilicon; and (ii) a compensated ingot grown from recycled p-type material with phosphorus addition. The study aims to elucidate how dopant compensation affects key material parameters such as carrier lifetime and mobility, resistivity, and defect activity, thereby assessing its suitability for advanced n-type solar cell technologies. The results of this work confirm that recycled compensated silicon is a viable route toward high-efficiency n-type solar cells.

This study compares two methods for recovering metals from Al-BSF solar cell fragments: alkaline etching and acid leaching. Etching experiments were carried out in an installation processing six fragments simultaneously. A 35% KOH + 5% ethanol aqueous solution completely removed the metal contacts, yielding high-quality silicon substrates for reuse, as confirmed by carrier lifetime tests; the etching solution could be reused up to three times before silica precipitation occurred. Large differences in Ag and Al particle sizes enabled efficient sieving and high recovery rates (75% pure Ag recovery). The methodology was further extended to bifacial PERC cells. In this case, a 25% KOH + 5% ethanol aqueous solution also removed the contacts, but similar Ag and Al particle sizes made separation by sieving difficult. In both Al-BSF and PERC, acid leaching achieved comparable Ag recoveries (i.e., Ag mg per gram of cell) of 4 mg/g and 3.7 mg/g, respectively, but required precipitation steps to increase the recovery of pure silver. These findings highlight the efficiency of alkaline etching, particularly for Al-BSF cells, and demonstrate that particle size distribution strongly influences the separation efficiency in metallurgical recovery from solar cells.

Light-induced degradation (LID) of boron-doped (B-doped) Czochralski silicon (Cz-Si) due to the formation of boron-oxygen (B-O) defects is a very well-known phenomenon that limits the stability of solar cells. To overcome the detrimental effects of B-O defects, the silicon industry has adopted gallium-doped wafers. However, it has been reported that Ga-doped Cz-Si-based solar cells are affected by light and elevated temperature induced degradation (LeTID), and the origin of this phenomenon at wafer level remains unclear. In this work we investigate light-induced changes in Ga-doped p-type Cz-Si wafers of different resistivities and compare them to the well-known degradation response of B-doped substrates. Through controlled illumination–temperature experiments and photoconductance-based carrier lifetime measurements, we analyse the evolution of minority carrier lifetime and the metastable nature of the lifetime-limiting defects. It was found that upon light-soaking at room temperature for 24 hours, Ga-doped wafers exhibit an average increment in the carrier lifetime, but a drop is observed under dark annealing at 200 ºC for 10 min. This behavior, which has an opposite dynamic to the traditional B-O activation/deactivation process in B-doped wafers, might suggest the existence of a metastable defect in Ga-doped samples. By applying a higher temperature during the light-soaking step, the average carrier lifetime values of Ga-doped samples decrease around 30%, and no recovery is observed under dark annealing conditions at 70 or 200 ºC. Moreover, our results of cyclability suggest that the defects dynamic in Ga-doped Cz-Si is not reversible. The results provide new insights into the mechanisms governing stability in Ga-doped silicon at bare wafer level and their implications for reliable, high-efficiency solar cell manufacturing.

Environmental stability of perovskite solar cells (PSCs) can be improved by a thin layer of low-dimensional (LD) perovskite sandwiched between the perovskite absorber and the hole transport layer (HTL). This layer, called ‘capping layer,’ has mostly been optimized by trial and error. In this study, we present a machine-learning framework to rationally design and optimize perovskite capping layers. We ‘featurize’ 21 organic halide salts, apply them as capping layers onto methylammonium lead iodide (MAPbI3) thin films, age them under accelerated conditions combining illumination and increased humidity and temperature, and determine features governing stability using random forest regression and SHAP (SHapley Additive exPlanations). We find that a low number of hydrogen-bonding donors and a small topological polar surface area of the organic molecules correlate with increased MAPbI3 film stability. The top performing organic halide salt, phenyltriethylammonium iodide (PTEAI), successfully extends the MAPbI3 stability lifetime by 4±2 times over bare MAPbI3 and 1.3±0.3 times over state-of-the-art octylammonium bromide (OABr). Through morphological and synchrotron-based structural characterization, we found that this capping layer consists of a Ruddlesden-Popper perovskite structure and stabilizes the photoactive layer by “sealing off” the grain boundaries and changing the lead surface chemistry, through the suppression of lead (II) iodide (PbI2) formation and methylammonium loss.

Carrier lifetime measurements are performed using the photoconductance decay (PCD) technique on samples exhibiting varying degrees of inhomogeneity in carrier lifetime. The PCD measurement is regarded as the prevailing method for evaluating the electrical performance of silicon wafers or surface passivation layer structures for photovoltaic applications by reporting the effective carrier lifetime versus the injection level tau(Delta n). However, we found that in case of substantial carrier lifetime inhomogeneity within the detected area, the reliability of the results is highly questionable. We used a simple model to compute the expected shape of the recorded tau(Delta n) curves. While in the case of a uniform carrier lifetime distribution the results are independent of the measuring parameters, they are strongly influenced if measuring samples with inhomogeneous carrier lifetime. It was observed that changing the excitation light power or the duration of the light pulse resulted in a substantial and consistent shift in tau(Delta n) curves. Furthermore, the tau(Delta n) results of the decay measurement exhibited a significant deviation from the areal average of steadystate carrier lifetime depending on the degree of carrier lifetime inhomogeneity. This phenomenon was investigated experimentally as well, using a laser-based PCD setup on various sample types. These samples featured substantial areal carrier lifetime fluctuations, either in bulk lifetime or surface passivation quality. The experimental results obtained confirmed the expectations derived from the modeling. Recording the PCD curves with multiple laser excitation parameters and using the steady state carrier lifetime results, more precise and reliable carrier lifetime results can be obtained from inhomogeneous samples.

Upgraded metallurgical-grade silicon (UMG-Si) has the potential to reduce the cost of photovoltaic (PV) technology and improve its environmental profile. In this contribution, we summarize the extensive work made in the research and development of UMG technology for PV, which has led to the demonstration of UMG-Si as a competitive alternative to polysilicon for the production of high-efficiency multicrystalline solar cells and modules. The tailoring of the processing steps along the complete Ferrosolar’s UMG-Si manufacturing value chain is addressed, commencing with the purification stage that results in a moderately compensated material due to the presence of phosphorous and boron. Gallium is added as a dopant at the crystallization stage to obtain a uniform resistivity profile of ∼1 Ω cm along the ingot height. Defect engineering techniques based on phosphorus diffusion gettering are optimized to improve the bulk electronic quality of UMG-Si wafers. Black silicon texturing, compatible with subsequent gettering and surface passivation, is successfully implemented. Industrial-type aluminum back surface field (Al-BSF) and passivated emitter and rear cell (PERC) solar cells are fabricated, achieving cell efficiencies in the range of those obtained with conventional polysilicon substrates. TOPCon solar cell processing key steps are also tested to further evaluate the potential of the material in advanced device architectures beyond the PERC. Degradation mechanisms related to light exposure and operation temperature are shown to be insignificant in UMG PERC solar cells when a regeneration step is implemented, and PV modules with several years of outdoor operation demonstrated similar performance to reference ones based on poly-Si. Life cycle analysis (LCA) is carried out to evaluate the environmental impact of UMG-based PV technology when compared to poly-Si-based technology, considering different scenarios for both the manufacturing sites and the PV installations.

As the installed capacity of photovoltaic (PV) systems continues to grow, also does the necessity to address the accumulating waste from decommissioned PV modules. Millions of modules will need to be retired in the next few decades, and this poses a challenge that requires the development of cost-effective and efficient recycling strategies, with a focus on essential components such as solar cells, which are known for their significant environmental impact and energy budget. This study explores strategies to recover metal from front and rear contacts, as well as the potentiality of silicon substrate recrystallization. Alkaline-organic solutions allow for complete metal detachment with small-to-minimal silicon loss, and recrystallization of recovered substrates provides promising results in obtaining wafers suitable for the newly established industry requirements. Al-BSF solar cells fabricated from recycled materials exhibit improved performance, proving the feasibility of reclaiming precious metals and silicon substrates from PV modules. These encouraging first results represent an important step towards cycling PV systems within a circular economy framework, thereby minimizing waste and maximizing resource utilization.

Due to the exponential increase in installed photovoltaic (PV) capacity, a large amount of waste is expected, once the PV modules reach the end of their useful life, typically in the range of 25-30 years. This reality urgently calls for the design and adoption of affordable, effective, and massive strategies of recycling and reutilisation of all PV components, and particularly of those most energy -demanding in their manufacturing and thus responsible for the largest quote of environmental impact: the solar cells. This work explores two key processes for the recycling of silicon solar cells, namely the recovery of the metal contacts and the recrystallisation of the silicon substrate. On the one hand, the demetallisation of first -generation solar cells in KOH-ethanol-water solutions is explored. A 2 3 factorial design of experiments was developed to optimize the process, indicating that the temperature of the bath is the most influential variable. With 10 % KOH, 5 % ethanol and 60 degrees C for 105 min, or alternatively, 15 % KOH, 5 % ethanol and 65 degrees C for 60 min, it was possible to completely detach the metal front (mainly Ag) and back contacts (mainly Ag and Al) with an overall silicon weight loss of only 13 and 15 %, respectively, with resulting carrier lifetimes in the remaining silicon matrix above 100 mu s, high enough for new solar cell fabrication targeting efficiencies exceeding 20 %. Additionally, Czochralski recrystallisation of old multicrystalline wafers has been tested to evaluate the possibility of reinjecting recovered silicon at an earlier stage of the PV value chain. After successful growth and slicing in wafers, a thorough optoelectronic characterisation program has been implemented, demonstrating the fulfilment of the basic requirements needed to make new solar cells, in terms of wafer resistivity (-1 Omega cm), oxygen content (-10 18 cm -3 ), mobility (-1000 cm 2 /V & sdot; s) and carrier lifetimes (above 100 mu s threshold in all cases and reaching maximum values of 350 mu s). Demetallised samples and recrystallised wafers are subsequently used to manufacture P/Al solar cells, and results show that the solar cell performance is not limited by the material quality. The recovery and reutilisation of the two most valuable elements of Si -based PV -modules, namely the precious metals and the ultrapurified silicon, key for the establishment of PV as an example of circular economy, are thus demonstrated.

High-quality multicrystalline Upgraded Metallurgical Grade Silicon (UMG-Si) offers significant advantages over conventional polysilicon-based PV technology, associated to lower cost, lower energy budget and lower carbon footprint. The aim of this study is twofold: on the one hand, to ascertain the efficiency potential of solar cells based on this material in terms of carrier lifetime; and on the other hand, to explore, as a result of that, the adoption of high-efficiency cell architectures by establishing an effective rear-side passivation scheme for the implementation of passivated emitter rear contact (PERC) devices. The carrier lifetime and the surface passivation efficacy are investigated for different passivating layer configurations after single and double P-diffusion gettering processes. Layer stacks consisting of Al2O3, SiOxNy and a-SiNx:H capping overlayers have been optimized, on industrial size, saw-damage-etched UMG wafers and results compared to those obtained using reference iodine-ethanol (IE) passivation. Diagnosis based on minority carrier lifetime and implied Voc (iVoc) measurements helped monitor the impact of parameter optimization on wafer quality, particularly after firing processes. Carrier lifetimes over 600 us at 10^15 cm-3 injection level as well as up to 790 us locally have been measured in UMG-Si wafers passivated with IE after a Phosphorus Diffusion Gettering (PDG), demonstrating the suitability of the material for high-efficiency cell architectures. Values higher than 300 us have been obtained with Al2O3-based passivation layers for gettered UMG wafers, with implied Voc values up to 710 mV. These record-breaking lifetimes and iVoc figures obtained with p-type multicrystalline UMG-Si material demonstrate a significant upgrading of its electronic quality by means of industry-scalable technical processes.

Bandgap engineering and quantum confinement in semiconductor heterostructures provide the means to fine-tune material response to electromagnetic fields and light in a wide range of the spectrum. Nonetheless, forming semiconductor heterostructures on lattice-mismatched substrates has been a challenge for several decades, leading to restrictions for device integration and the lack of efficient devices in important wavelength bands. Here, we show that the van der Waals epitaxy of two-dimensional (2D) GaSe and InSe heterostructures occur on substrates with substantially different lattice parameters, namely silicon and sapphire. The GaSe/InSe heterostructures were applied in the growth of quantum wells and superlattices presenting photoluminescence and absorption related to interband transitions. Moreover, we demonstrate a self-powered photodetector based on this heterostructure on Si that works in the visible-NIR wavelength range. Fabricated at wafer-scale, these results pave the way for an easy integration of optoelectronics based on these layered 2D materials in current Si technology.

Current recycling strategies applied to photovoltaic (PV) modules have not yet solved the issue of effectively reusing and reconditioning their active part, namely the solar cells. This paper presents first results in the research carried out within the RESILIENS project to develop a cost-effective and environmentally meaningful technological recycling process for silicon solar cells that allows the recovery and reutilization of silicon and precious metals. On the one hand, demetallization of two different silicon solar cell technologies has been successfully realized via an alkaline route, optimizing process conditions. On the other hand, fragments of old silicon wafers have been successfully recrystallized, and the resulting wafers show resistivities and charge carrier lifetimes after a phosphorus diffusion gettering compatible with further high efficiency solar cell processing.

Upgraded metallurgical grade (UMG) silicon (Si) has raised interest as an alternative material for solar cells due to its low cost, low environmental impact and low CAPEX. Maximum cell efficiencies at the level of those obtained from high purity poly-Si have been reported. However, a higher defect density and the compensated doping character result in UMG-based cell efficiencies varying over wider ranges in frequency distribution charts. In this report we characterize mc-Si UMG samples with different defect densities, comparing them with mono crystalline silicon (mono-Si) UMG and commercial high-performance multicrystalline silicon (mc-Si) samples, analysing the impact of carrier trapping by means of photoconductance (PC) decay measurements, and its evolution after applying a phosphorous diffusion gettering (PDG) process. When analyzing the decay time constant of the PC measurements, slow (66.8 +/- 14.3 ms) and fast (16.1 +/- 3.5 ms) traps are found in mc-Si samples, while no evidence of trapping is found in mono-UMG samples. Slow traps are effectively removed after the PDG process, while fast traps do remain. The influence of dislocations clusters and the possible role of oxygen, as revealed by Fourier-transform infrared spectroscopy (FTIR) is discussed. Finally, the improvement in minority carrier lifetime due to the PDG treatment is reported for each sample type, reaching values up to 140 its in mc-Si samples with neither slow traps nor interstitial oxygen FTIR-peaks.

Upgraded Metallurgical-Grade (UMG) silicon (Si) has raised interest as an alternative material for silicon solar cells due to its low cost, low environmental impact and low CAPEX. Maximum cell efficiencies similar to those obtained with high purity poly-Si have been reported. However, a higher defect density and the compensated doping character result in UMG-based efficiencies varying over wider ranges in frequency distribution charts. Developing methods to precisely identify impurities has been an active field of work for many years. Among others, Injection Dependent Lifetime Spectroscopy (IDLS) is considered a very sensitive technique suitable for an accurate determination of the defect's fingerprint. In this work we present a comprehensive analysis of UMG-Si by IDLS on wafers with different starting carrier lifetimes. After P-diffusion gettering optimization and related lifetime improvements up to two orders of magnitude, we hypothesize Cu as the main impurity that governs the minority carver lifetime and its associated defect parameters, energy levels and symmetry factors, based on IDLS results. Furthermore, we point out the importance of relevant factors affecting IDLS analysis, that could lead to defect misinterpretation, such as the mobility model employed in the lifetime analysis (as UMG-Si is a compensated material) and the impact of the surface recombination velocity. We estimate the Cu concentration to be in the range7-13.10(12) cm(-3), based on the agreement between simulations and lifetime measurements.

By means of Terahertz Spectroscopy we demonstrate a boost in charge carrier mobility in upgraded-metallurgical grade silicon wafers treated by phosphorous diffusion gettering. Mobilities parallel those of high quality poly-Si.

Herein, it is demonstrated how the carrier mobility and carrier lifetime of upgraded‐metallurgical grade silicon (UMG‐Si), a feedstock alternative to electronic‐grade, high‐purity polysilicon dominating photovoltaic technology, can be largely improved upon the gettering action of industrially compatible phosphorous diffusion gettering (PDG) process at the wafer level. The results, based on ultrafast THz spectroscopy and inductively coupled photoconductive decay measurements, show outstanding increments in the carrier lifetime of PDG‐treated multi‐crystalline UMG‐Si wafers from 50 ps to 25 μs and a boost in intra‐grain charge carrier mobility from 283 ± 37 up to 726 ± 23 cm2 Vs−1. Most remarkably, the latter figure parallels the carrier mobility observed in monocrystalline wafers manufactured from polysilicon, thereby demonstrating the effectiveness of a simple pre‐conditioning step in upgrading the electronic properties of UMG‐Si up to the device level.

Upgraded metallurgical-grade (UMG) Si is obtained via a purification route alternative to the one used for conventional polysilicon and with significantly reduced environmental impact. Additionally, despite a lower purity level in the feedstock than polysilicon, UMG-Si has demonstrated potential for the fabrication of highly efficient and low-cost solar cells. Low initial bulk carrier lifetimes recorded in UMG-Si bare wafers can be improved by means of an adequate Phosphorus Diffusion Gettering (PDG) process to the level of mc-Si. In this letter, optimized PDG processes for UMG-Si are reported, resulting in increased values between 20 and 250 times the original carrier lifetimes and record figures above 645 us.

In this work, we demonstrate the beneficial effect of post-growth rapid thermal annealing (RTA) on the performance of -1 eV GaAsSbN-based solar cells. Different configurations of the dilute nitride material are studied: a bulk quaternary GaAsSbN layer and type-II GaAsSb/GaAsN superlattices (SL) with different period thickness. The RTA treatment leads in both types of structures to an enhanced external quantum efficiency (EQE) and reduced values of the dark saturation current and series resistance. The origin of these improvements is attributed to reduced densities of N and Sb-rich clusters and point defects. Remarkably, all solar cell parameters are substantially improved, this being particularly significant for the short circuit current density (JSC) and power conversion efficiency (PCE), the latter with a relative enhancement as high as 500%. The large increment of JSC is an important step towards current-matching in multi-junction solar cells containing GaAsSbN structures.

III-V compound semiconductors and SiGe alloys can be combined to develop multijunction solar cells on Silicon substrates with optimum bandgap combinations. Current implementations of such devices have reached efficiencies over 20%, using thick -and thus costly- buffer layers which induce the appearance of cracks in large area samples. As a strategy to mitigate these two issues (thick buffers and cracking), a GaAsP/SiGe tandem solar cell has been developed employing group IV reverse graded buffer layers grown on Ge/Si virtual substrates with a subsurface Silicon porous layer. Reverse buffer layers facilitate a reduction in the threading dislocation density with limited thicknesses but can also induce cracks. To minimise this, a porous silicon layer has been incorporated close to the Ge/Si interface so that the ductility of this layer suppresses crack propagation. In terms of solar cell performance, this porous layer reduces the problem of cracks, not totally supressing them though. Accordingly, the low shunt resistance observed in previous designs has been increased thus improving solar cell efficiency, which is still notably behind designs using thicker forward graded buffer layers. The first results of this new architecture are presented here.

Experimental evidence indicating the beneficial impact of a phosphorous diffusion gettering (PDG) in the reduction of trapping centers is shown, as observed by means of inductively coupled photoconductance (PC) decay and lifetime measurements carried out on upgraded metallurgical‐grade silicon (UMG‐Si) wafers. The presence of trapping species dominating the long time range of the PC decay of UMG material (slow traps), which is effectively removed after a PDG conducted at 780 °C, is detected. Notwithstanding, a second trapping mechanism, characterized by a shorter time constant, still governs the response at very low injection levels after the gettering. Furthermore, the beneficial effect of the PDG is studied as a function of processing time, showing minority carrier bulk lifetime improvements up to 18‐fold, up to the range of 70 μs. Thereby, the way for developing gettering strategies capable of successfully removing trap centers and improving the bulk lifetime of unconventional Si material is paved.

We demonstrate type-II GaAsSb/GaAsN superlattices (SL) as a suitable structure to form the lattice-matched 1.0-1.15 eV subcell that would allow the implementation of the optimum monolithic multi-junction solar cell design. The separation of Sb and N atoms during growth leads to an improved composition homogeneity and a lower defect density than in the bulk GaAsSbN counterparts. The type-II band alignment SLs provide long radiative lifetimes that facilitate carrier collection as compared to equivalent type-I SLs. Moreover, the radiative lifetime can be controllably tuned through the period thickness, which is not possible in type-I SLs. A reduced period thickness results in enhanced absorption due to increased wavefunction overlap, as well as in a change in the transport regime from diffusive to quasiballistic, providing improved carrier extraction efficiency. As a result, the short period SL single junction solar cells show an enhanced power conversion efficiency of 134% over the equivalent bulk devices.