The progress in research and development in the bilateral collaboration between Delft University of Technology and HyET Solar on lightweight and flexible thin-film photovoltaic (PV) foils, will be presented. The PV technologies under development are based on thin-film silicon, perovskites or a hybrid combination of both materials. Advances in terms of performance, reliability and processing of thin film PV foils and materials will be presented. In this contribution we will focus on i) the development of textured substrates to enhance the optical performance of thin-film PV devices, ii) development of bi-layered indium-free transparent conductive oxide (TCOs) to decouple the optical and electrical functionality of the transparent front contact-window layers in thin-film PV devices, iii) volume-deficiency and defect engineering of germanium alloys to realize a low band gap p-i-n junction for multi-junction PV architectures; iv) upscaling the deposition rates of perovskites absorber layers using co-evaporation processing, and v) development of in-house ElectroLuminescence and IR analyses on PV foils as a helpful feedback loop to quickly start up a module production line and optimize its processing steps.

This study investigates the transparent conductive oxides (TCOs) as front contact for thin-film solar cell applications by developing a bilayer design that decouples the optical and electrical functionalities. The bilayer front contact structure combines hydrogenated indium oxide (IOH) and non-intentionally doped zinc oxide (ZnO) materials. This design achieves enhanced optoelectrical properties with a mobility of 120 cm2/Vs and a carrier density of 1.97 & sdot;1019 cm-3. Notably, the bilayer outperforms the expected average of its constituent layers in both transparency and conductivity, reflecting the benefits of optimized layer architecture. When integrated as the front electrode in a hydrogenated nanocrystalline silicon (nc-Si:H) solar cell, the IOH/ZnO bilayer yields a fill factor of 64.56 % and a power conversion efficiency of 7.85 %. When using an ITO front contact, the nc-Si:H solar cell reveals a fill factor of 56.27 % and an efficiency of 6.80 %. By successfully decoupling optical and electrical properties, the optimized IOH/ZnO bilayer offers a significant advancement over single-layer TCO configurations, presenting an innovative pathway for enhanced performance in thin-film solar cell technology.

Germanium based films with a low bandgap can be integrated in a multijunction solar cell as the bottom cell absorber. To develop such films, the effect of deposition temperature, pressure and RF power in a PECVD setup has been studied. Higher temperatures achieve more dense, stable and intrinsic films. The interplay of power and pressure is more complex, allowing for further manipulation of the bandgap and electrical properties. The best films produced during this study achieve bandgaps of 0.8-0.9 eV, with an activation energy with values of about half of the bandgap energy, suggesting and intrinsic behavior. Moreover, more detailed measurements show that the Ge:H materials deposited are dense amorphous films with predominantly small voids within the structure. The photoresponse was about 100 times lower than that of a-Si:H, which agrees with previous literature. Finally, no oxidation was observed after 2 weeks of air exposure. Based on these results, Ge:H films are determined to be a suitable candidate for further implementation in a solar cell, allowing a better utilization of the infrared light in the solar spectrum.

This work discusses the application of injection-dependent electroluminescence (EL) for screening the defects responsible for hotspot formation in laser scribed serially interconnected thin amorphous silicon (a-Si) solar modules. Fraction of modules having divergent electroluminescence signals are selected from a large batch of modules and is utilized in this study. The defects are classified as mode A, mode B and mode C based on the luminescence contrast seen in the EL images captured with a current injection of ISC and 20% ISC. Mode A refers to the defects that exhibit less luminescence across the entire cell area in both the EL images. In contrast, mode B and mode C defects show less luminescence only in a localized region in EL images with a current injection of ISC. However; in 20% of ISC EL images, mode B defects show reduction in luminescence on the entire cell area and luminescence contrast pattern remains the same as in ISC injected EL image for mode C defects. Furthermore, these defects are tested for hotspot endurance by exposing the module in a light soaking system under short circuit conditions by selectively shading the cells with these defects. The results show that only mode B defects resulted in localized hotspot formation. Both mode A and mode C defects did not result in localized heating as the former has uniform distribution of shunts and the latter do not draw significant enough current that can result in hotspots. These results substantiate that the injection dependent EL imaging is an effective approach to filter out the defects that can cause hotspot formation in thin a-Si solar modules.

-Photovoltaic (PV) panel installations in buildings and transportation hubs pose additional safety challenges as the glare from the panels can impose adverse impacts like flash blindness in human eyes. This study substantiates that polymer encapsulated thin film modules offer significantly low glare levels that are essential for building integrated and transport hub installations. In this work, the glare hazard potential associated with matt ethylene tetrafluoroethylene (ETFE)-based polymer sheet used as the frontsheet for the production of flexible thin amorphous silicon (a-Si) PV modules is studied and compared with standard PV glass used in crystalline silicon (c-Si) PV panels. The specular reflectance extracted from the measured total and diffuse reflectance for an angle of incidence (AOI) of 8(degrees) and the angular intensity distribution (AID) of specular reflectance measured for AOI ranging from 10(degrees) to 80(degrees) are utilized for glare assessment of the frontsheets. The mean value of specular reflectance extracted from the measured total and diffused reflectance is as low as < 0.5% for the polymer frontsheet and is > 4% for glass. The AID measurements suggest that the reflection from the polymer frontsheet is highly diffusive in nature in contrast to glass and the measured specular reflectance is always close to a magnitude lower than that from glass for all AOI. With the increase in AOI, the specular AID reflectance increases exponentially for glass to become as high as 40%, which is almost 20 times less than that from the polymer frontsheet for an AOI of 80 degrees. degrees . Further, the c-Si test structure with glass and thin a-Si PV module with matt ETFE-based polymer as frontsheet showed similar specular reflectance trends as that of glass and the polymer frontsheet, respectively.

Fabricating high-performing solar cells requires techniques to facilitate high optical yield. In thin film solar cells, light scattering at textured interfaces is used to enhance the absorption path length in the photovoltaic active layer. This work presents two different texturing methods to integrate texture on glass superstrates. These features can sustain the crack-free growth of device-quality nanocrystalline thin-film silicon materials and multi-junction photovoltaic devices. This work is a detailed study of different texturing options, broadly classified as random and periodic. Random texturing is achieved with the help of sacrificial texturing using intrinsic zinc oxide and indium tin oxide on glass. A correlation of roughness is established with light interaction for random textures. This is different for two different sacrificial texturing methods. It was demonstrated that high optical scattering yields could be achieved for both ITO sacrificial texture and i-ZnO sacrificial texture but at different roughness value ranges. These two texturing techniques are combined to make a superimposed texture with their combined morphology. Periodic Hexagonal textures on glass are created with the help of photolithography. These hexagons show curved bases compared to flatter bases, as reported with crystalline silicon substrates. Also, the necessity of two-dimensional axes characterization of hexagons is established.

Dedicated optical models are crucial for advancing the modelling of next-generation solar cells. Incorporating various textures of different shapes and sizes into solar cells significantly improves light management. This study optimizes the optical design and predicts the performance of a novel thin-film tandem solar cell device. The top cell features a hydrogenated amorphous silicon (a-Si:H) absorber layer, while the bottom cell incorporates a low-bandgap tin-lead-based perovskite (Sn-Pb PVK) absorber layer, all supported on a flexible aluminum substrate Optical simulations exhibit 24 mA/cm² as total implied photo-current density when the absorber layer thicknesses are current-matched. The maximum absorptance reaches 80% at 500 nm for a-Si:H and only 62% at 800 nm for Sn-Pb PVK sub-cells. Experimental results show open-circuit voltages of 0.9 V for a-Si:H and 0.85 V for Sn-Pb PVK solar cells. Based on the highest achieved fill factor of 0.77, the researchers estimated a power conversion efficiency exceeding 16%.

Our study focuses on the optimization of front contact design by exploring a novel bilayer configuration that employs transparent conductive oxides (TCOs) to enhance the efficiency of thin-film silicon solar cells. The TCOs investigated include sputtered hydrogenated indium oxide (IOH), cerium-doped indium oxide (ICO), cerium and hydrogen co-doped indium oxide (ICOH), and intrinsic zinc oxide (i-ZnO). We highlight the suitability of these TCOs in a bilayer design, first analyzing their opto-electrical properties as monolayers and subsequently in bilayer configurations. The IOH/i-ZnO bilayer architecture, in particular, demonstrates promising opto-electrical properties on both flat glass and micro-textured glass substrates. IOH/i-ZnO on flat glass substrate demonstrates remarkable mobility (143.44 cm (2)/Vs) and a carrier concentration in the order of 1019 cm(-3). The mean of reflectance (R) trends consistently exceeds 80%, while the mean of transmittance (T) trends falls below 20% beyond 500 nm. The interference effects within the bilayers are minimized for designs on micro-textured glass, preserving values within a desirable range. These findings represent an innovative approach to front contact design for thin-film silicon solar cells, emphasizing the potential of bilayer configurations to advance solar cell technology.

Glare from the photovoltaic (PV) panels is a serious concern, especially for PV installations in areas such as transportation hubs and buildings as it can induce problems like flash blindness to human eyes. Hence, it is extremely important to assess the glare from the PV panels or the top encapsulants used in PV panels. This work utilizes the angular intensity distribution (AID) reflectance measurements to compare the glare hazard associated with a polymer sheet (PS) used as the top layer for encapsulating thin flexible amorphous silicon panels and glass encapsulant used in crystalline silicon wafer-based PV modules. The total reflectance measured from both glass and PS is nearly similar for the wavelength range of 400 nm to 700 nm. The variation in AID reflectance for PS and glass follows broader and narrow normal distribution, respectively with the maximum peak at an angle twice as that angle of incidence (AOI). This suggests that the AID reflectance from PS is highly diffusive as compared to glass. Further, the measured specular AID reflectance from glass is always nearly an order of magnitude higher for AOI ranging from 10° to 80°. The study also reveals that the specular AID from the glass increases drastically for AOI beyond 50°. Based on the specular reflectance estimated from AID, modules with PS can be the preferred choice for PV installations where glare can be a potential concern.

Excellent surface passivation induced by ( i )a‐Si:H is critical to achieve high‐efficiency silicon heterojunction (SHJ) solar cells. This is key for conventional single‐junction cell applications but also for bottom cell application in tandem devices. In this study, we investigated the effects of ( i )a‐Si:H deposition temperature on passivation quality and SHJ solar cell performance. At the lower end of temperatures ranging from 140°C to 200°C, it was observed with Fourier‐transform infrared spectroscopy (FTIR) that ( i )a‐Si:H films are less dense, thus hindering their surface passivation capabilities. However, with additional hydrogen plasma treatments (HPTs), those ( i )a‐Si:H layers deposited at lower temperatures exhibited significant improvements and better passivation qualities than their counterparts deposited at higher temperatures. On the other hand, even though we observed the highest V OC s for cells with ( i )a‐Si:H deposited at the lowest temperature (140°C), the related FFs are poorer as compared to their higher temperature counterparts. The optimum trade‐off between V OC and FF for the SHJ cells was found with temperatures ranging from 160°C to 180°C, which delivered independently certified efficiencies of 23.71%. With a further improved p ‐layer that enables a FF of 83.3%, an efficiency of 24.18% was achieved. Thus, our study reveals two critical requirements for optimizing the ( i )a‐Si:H layers in high‐efficiency SHJ solar cells: (i) excellent surface passivation quality to reduce losses induced by interface recombination and simultaneously (ii) less‐defective ( i )a‐Si:H bulk to not disrupt the charge carrier collections.

Transparent conductive oxides (TCOs) are used as front electrode of thin film silicon (TF-Si) solar cells to increase power conversion efficiency. Metal oxides doped with different materials can be deployed as TCO. The preferred TCO is usually selected using a trade-off between transparency and conductivity. This work proposes a bi-layer front contact to address the limitation of this trade-off. IOH and i-ZnO are chosen as the best candidates for such architecture due to their good opto-electrical properties. A thin layer of IOH ensures good lateral conductivity and high transparency in the visible part of the solar spectrum. An additional i-ZnO layer provides minimized parasitic absorption losses along with low transverse resistivity. The best opto-electrical properties are achieved when deposition temperature and power density are set at 25°C and 1.5 W/cm 2 , 200°C and 2 W/cm 2 for IOH and i-ZnO respectively.

In this study, undoped hydrogenated amorphous silicon (a-Si:H) thin films deposited under moderate dilution ratios of silane by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) have been investigated using steady-state photoconductivity and improved dual beam photoconductivity (DBP) methods to identify changes in multiple gap states in annealed and light-soaked states. Four different gap states were identified in annealed state named as A, B, C, and X states. The peak energy positions of these Gaussian distributions are consistent with those recently identified by Fourier transform photocurrent spectroscopy (FTPS). After in situ light soaking, their density increases with different rates as peak energy positions and half-widths remain unaffected. The electron-occupied A and B states located below the dark Fermi level and their density and ratios in the annealed and light-soaked states correlate well with those defects detected by time-domain pulsed electron paramagnetic resonance (EPR) experiments. The A, B, and X states located closer to the middle of the bandgap anneal out at room temperature in dark and define the “fast” states. However, the C states show no sign of room temperature annealing such that they must define the “slow” states in undoped a-Si:H. The results found in this study indicate that the anisotropic disordered network is a more appropriate model than previously proposed defect models based on the continuous random network to define the nanostructure of undoped a-Si:H, where multiple defects, D0 and non-D0 defects, can be identified by using the improved DBP method.

Silicon heterojunction (SHJ) solar cells have achieved a record efficiency of 26.81% in a front/back-contacted (FBC) configuration. Moreover, thanks to their advantageous high VOC and good infrared response, SHJ solar cells can be further combined with wide bandgap perovskite cells forming tandem devices to enable efficiencies well above 33%. In this study, we present strategies to realize high-efficiency SHJ solar cells through combined theoretical and experimental studies, starting from the optimization of Si-based thin-film layers to the implementation of electrodes with reduced indium and silver usage. Advanced opto-electrical simulations, which enable comprehensive theoretical understandings of the main physical mechanisms governing carriers’ collection and light management, provide clear pathways for device designs and experimental optimizations. We present the fabricated FBC-SHJ solar cells in both monofacial and bifacial configurations with the best efficiencies of 24.18% and 23.25%, respectively. We point out that to achieve optimum device performance, the compositional materials should be holistically optimized and evaluated as part of the contact stacks with adjacent layers. As an outlook beyond the classical FBC-SHJ solar cell architecture, we propose various novel SHJ-based solar cell architectures. Their potential performance was assessed and compared via rigorous opto-electrical simulations and a maximal efficiency of 27.60% was simulated for FBC-SHJ solar cells featuring localized contacts.

Accelerating the deployment of Photovoltaic (PV) systems is a key contributing factor in achieving climate neutrality. Even though solar power is one of the cheapest energy sources and its deployment is growing rapidly around the world, an even faster growth is required to achieve existing climate goals. Besides the role that finance and permitting can play as enablers or barriers to this, the key elements to enable fast PV deployment are the use of education, and science and data-driven tools to empower citizens, installers, and investors to make their decisions based on robust scientific evidence. This perspective article aims to summarize the key concepts presented and discussed during the side event at COP27 on PV resources towards climate neutrality. The article will accomplish this by highlighting two key aspects: (1) the advantages of using solar-related education and data-driven tools, and (2) showcasing the significance of education, improved data and tools, community involvement, and PV mapping in expediting the deployment of PV systems.

Two terminal multi-junction (MJ) photovoltaic (PV) devices are well established concepts to increase the solar to-electrical power conversion in reference to single PV junctions. In multi-junction PV devices two consecutive sub-cells are interconnected using a tunnel recombination junction (TRJ) in which the light excited holes of one sub-cell recombine with the light excited electrons of the other sub cell. An ideal TRJ is an ohmic contact with non-rectifying behaviour. TRJ's based on p-and n-doped silicon-oxides have been successfully applied in a variety of hybrid multi-junction PV devices in which tunnelling and trap-assisted tunnelling over width of 5-20 nm rules the TRJ's recombination kinetics. In this contribution the qualitative fundamental working principles of tunnel recombination junctions based on p-and n-doped silicon and silicon-oxide alloys are revealed using both electrical modelling and experiments based on a unique set of tandem lab cells (four types based on four different PV materials) combined with structural variations in TRJ architectures. The study results in design rules for the integration of silicon-oxide based TRJ's and provides fundamental insights into the sensitivity of the electrical performance of the TRJ's to doping concentrations, to alignment of the conduction and valence bands of consecutive sub-cells, to the nature of interface defects, to the growth of amorphous and crystalline phases and its dependence on substrate or seed layers and to the nanoscale thicknesses of the TRJ layers.

The oxidation and carbisation kinetics of porous amorphous and nano-crystalline hydrogenated germanium (a-Ge:H and nc-Ge:H) films exposed to ambient air and deionized water have been studied using vibration modes observed by infrared spectroscopy. Based on infrared analysis, a two-step process of first oxidation by water and secondly carbisation by carbon dioxide (CO2) is proposed that partly mimics the (photo-)catalytic processes in artificial (photo)synthesis. It is shown that water acts like the precursor for oxidation of porous a-Ge:H and nc-Ge:H in the first step. The incorporation of oxygen in a-Ge:H and nc-Ge:H alloys occurs preferentially at Ge-dangling bonds and not at the Ge-Ge back bonds like in hydrogenated silicon alloys (next of kin IV-valence element). The formation of germanium oxide (GeO) tissue at void surfaces locally creates Ge alloys with significantly lower energy levels for the valence band that can align with the half reaction for water reduction. The heterogeneous nature of a-Ge:H and nc-Ge:H oxidation will result in local catalytic generation of electrons and protons. It is proposed that these charge carriers and ions act as precursors for the second-step reaction based on carbisation that includes both the adsorption of CO2 and formation of CO and formaldehyde.

Surface textures that result in high optical yields are crucial for high efficiency photovoltaic (PV) devices. In this work three different texturing approaches are presented that result in smooth concave structures devoid of sharp features. Such features can sustain the crack-free growth of device quality nano- to poly-crystalline materials such as nano-crystalline silicon, perovskites or C(I)GS, facilitating routes towards hybrid multijunction PV devices. A sacrificial implanted poly-c-Si layer is used to develop a random surface texture for the first texturing approach (Tsac). The influence of the processing conditions, such as layer thickness, implantation energy, dose and ion type, annealing time and temperature, of the sacrificial layer on the developed surface features is investigated. Additionally, a photolithographically developed honeycomb texture (Thoney) is presented. The influence of mask design on the honeycomb features is discussed and a relation is established between the honeycomb period and crack formation in nano-crystalline silicon layers. The reflective properties (spectral reflection, haze in reflection and angular intensity distribution) of these approaches are characterized and compared to a third texturing approach, Tsp, the result of chemically smoothened pyramidal <111> features. It was demonstrated that high optical scattering yields can be achieved for both Thoney and Tsp. Additionally, the performance of a-Si/nc-Si tandem devices processed onto the different textures is compared using both optical device simulations and real device measurements. Simulations demonstrate strong improvements in Jsc-sum (≈45%), in reference to a flat surface, and high Voc*FF of over 1 V are demonstrated for Tsp.

Excellent surface passivation induced by (i)a-Si:H is critical to achieve high-efficiency silicon heterojunction (SHJ) solar cells. In this study, we investigated the effects of (i)a-Si:H deposition temperature on passivation quality and solar cell performance. Among the deposition temperatures we investigated ${(140\ -200^{\mathrm{o}}\mathrm{C})}$ , lower temperatures seem to result in less dense (i)a-Si:H films, which hinder their surface passivation capabilities. However, with additional hydrogen plasma treatments (HPTs), those (i)a-Si:H layers exhibited significant improvements and better passivation qualities than their higher temperature counterparts. On the other hand, even though we observed the highest VOCs for cells with (i)a-Si:H deposited at the lowest temperature (140 ${{}^{\circ}\mathrm{C})}$ , the related FFs are poorer as compared to their higher temperature counterparts. The optimum trade-off between VOC and FF was found with temperatures ranging from ${160^{\circ}\mathrm{C}}$ to ${180^{\circ}\mathrm{C}}$ , which delivered independently certified efficiencies of 23.71%. Thus our study reveals two critical requirements for optimizing the (i)a-Si:H layers in high-efficiency SHJ solar cells: (i) excellent surface passivation quality to reduce losses induced by interface recombinations and (ii) less-defective (i)a-Si:H bulk to improve the charge carrier collections.

A logical next step for achieving a cost price reduction per Watt peak of photovoltaics (PV) is multijunction PV devices. In two-terminal multijunction PV devices, the photo-current generated in each subcell should be matched. Intermediate reflective layers (IRLs) are widely employed in multijunction devices to increase reflection at the interface between subcells to enhance current generation in the subcell(s) positioned before the IRL, in reference to the incident light. In this work, the results of over 65 multijunction devices are presented, in order to explore the effect of different current matching approaches. The influence of variations in absorber thickness as well as thickness variations of different IRLs based on silicon-oxide, various transparent conductive oxides (TCO), and metallic layers on all-silicon multijunction PV devices is studied. Specifically, hybrid, 2-terminal, monolithically integrated silicon heterojunction (SHJ) and thin film nanocrystalline silicon (nc-Si:H) and amorphous silicon (a-Si:H) tandem and triple junction devices are processed. Based on these experiments, certain design rules for optimal current matching operation in multijunction devices are formulated. Finally, taking these design rules into account, record all-silicon multijunction devices are processed. Conversion efficiencies close 15% and Voc approximate to 2$$ {V}_{\mathrm{oc}}\approx 2 $$ V are demonstrated for triple junction SHJ/nc-Si:H/a-Si:H devices. Such conversion efficiencies for a wireless, high-voltage wafer-based all-silicon 2-terminal multijunction PV device opens the way for efficient autonomous solar-to-fuel synthesis systems as well as other wireless innovative approaches in which the multijunction solar cell is used not only as a photovoltaic current-voltage generator, but also as an ion-exchange membrane, electrochemical catalysts, and/or optical transmittance filter.