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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 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.

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≈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. [ABSTRACT FROM AUTHOR]

Solar fuels are a promising way to store solar energy seasonally. This paper proposes an earth‐abundant heterostructure to split water using a photovoltaic‐electrochemical device (PV‐EC). The heterostructure is based on a hybrid architecture of a thin‐film (TF) silicon tandem on top of a c‐Si wafer (W) heterojunction solar cell (a‐Si:H (TF)/nc‐Si:H (TF)/c‐Si(W)) The multijunction approach allows to reach enough photovoltage for water splitting, while maximizing the spectrum utilization. However, this unique approach also poses challenges, including the design of effective tunneling recombination junctions (TRJ) and the light management of the cell. Regarding the TRJs, the solar cell performance is improved by increasing the n‐layer doping of the middle cell. The light management can be improved by using hydrogenated indium oxide (IOH) as transparent conductive oxide (TCO). Finally, other light management techniques such as substrate texturing or absorber bandgap engineering were applied to enhance the current density. A correlation was observed between improvements in light management by conventional surface texturing and a reduced nc‐Si:H absorber material quality. The final cell developed in this work is a flat structure, using a top absorber layer consisting of a high bandgap a‐Si:H. This triple junction cell achieved a PV efficiency of 10.57%, with a fill factor of 0.60, an open‐circuit voltage of 2.03 V and a short‐circuit current density of 8.65 mA/cm2. When this cell was connected to an IrOx/Pt electrolyser, a stable solar‐to‐hydrogen (STH) efficiency of 8.3% was achieved and maintained for 10 hours. This paper focuses on designing a hybrid triple junction solar cell consisting of a‐Si:H/nc‐Si:H/c‐Si for solar water splitting, as a promising way to store energy in the long term. The triple junction cell achieved an efficiency of 10.57%, open‐circuit voltage (Voc) of 2.03 V, short‐circuit current density (Jsc) of 8.65 mA/cm2 and fill factor (FF) of 0.60, and an STH efficiency of 8.3% for 10 hours. [ABSTRACT FROM AUTHOR]

Abstract: Time-resolved cavity ringdown (τ-CRD) spectroscopy has been applied to monitor the silyl (SiH3) radicals and nanoparticles in a pulsed very high frequency (VHF) silane-hydrogen plasma under microcrystalline silicon (μc-Si:H) deposition conditions. The measured cavity loss reveals four time intervals (I up to VI) in the first 4s of the plasma pulse. By variation of the laser wavelength, it is demonstrated that the small cavity loss at 220nm reflects the SiH3 absorption in interval I. In intervals II and III, an additional cavity loss appears. This additional cavity loss corresponds to Rayleigh and Mie scattering by growing nanoparticles. Interval IV reflects the loss of nanoparticles between the electrodes during the afterglow of the plasma pulse. The evolution of the nanoparticle generation determined from the τ-CRD measurements are further confirmed by additional scanning electron microscopy analyses on the nanoparticles created in the plasma pulse. [Copyright &y& Elsevier]

A hybrid tandem solar cell consisting of a thin-film, nanocrystalline silicon top junction and a silicon heterojunction bottom junction is proposed as a supporting solar cell for photoelectrochemical applications. Tunneling recombination junction engineering is shown to be an important consideration in designing this type of solar cell. The best hybrid cell produced has a spectral utilization of 30.6 mA cm −2 a J SC of 14 mA cm −2 , a V OC of 1.1 V, a fill factor of 0.67 and thus an efficiency of 10.3%. A high solar-to-hydrogen efficiency of 7.9% can be predicted when using the hybrid cell in conjunction with current a-SiC photocathode technology. [ABSTRACT FROM AUTHOR]

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. • Experimental results of over 100+ different tandem device architectures are presented. • Four types of tandem PV devices are presented based on four different silicon alloys. • p-layer design is structurally varied across different multijunction PV architectures. • Energy band diagrams provide fundamental insight into the working principles of TRJ's. • The study results in design rules for the integration of silicon-oxide based TRJ's. [ABSTRACT FROM AUTHOR]

Amorphous and nano-crystalline germanium is of potential interest for a wide range of electronic, optical, opto-electronic and photovoltaic applications. In this work the influence of deposition temperature on hydrogenated germanium (Ge:H) films was characterized, using over 200 Ge:H and over 70 SiGe:H films. The demonstrated temperature-induced densification of Ge:H films resulted in more stable films with a lower bandgap energy and dark conductivity and higher activation energy. [ABSTRACT FROM AUTHOR]

Bandgap energy profiling is applied in a variety of materials for photovoltaic technologies, such as chalcogenides, III–V materials and perovskites. Bandgap profiling of the absorber layer is used to fight the fundamental loss mechanisms imposed by the bandgap energy of the absorber for the maximum voltage and current that a photovoltaic device can generate. The bandgap profile can be affected by a number of profiling strategies, such as the difference between the maximum and minimum bandgap energy, the position of the minimum bandgap energy, the width over which this minimum bandgap energy occurs and the total absorber width. These parameters have a complex effect on output characteristics of a photovoltaic device. Varying multiple parameters at once further increases the complexity, limiting the effectiveness of rigorous physical opto-electrical modelling. In this work we therefore present an expedient semi-empirical approach for the optimal bandgap profiling of stoichiometric absorbers. Using PECVD processed amorphous silicon germanium as a model, we present a unique set of semi-empirical relations that simulate the V OC and J SC of solar cells as a function of the bandgap energy profile. For this model, the influence of deposition conditions such as the relative germane flow rate, the deposition power and substrate temperature on the opto-electrical properties of a-SiGe:H films is first characterized. Opting for the relative germane flow rate to control the bandgap profiling, the experimental results of a large number of solar cells with profiled a-SiGe:H absorber are presented, varying: 1. the absorber thickness, 2. the peak germane flow rate, so minimum bandgap energy, and 3. the introduction of a plateau at the minimum bandgap energy. Using this experimental data and optical simulations, the expedience and effectiveness of the semi-empirical approach is demonstrated. • The opto-electrical properties of PECVD processed a-SiGe:H are characterized. • The performance of a-SiGe:H solar cells are reported as a function bandgap profiling. • A semi-empirical modelling approach is presented for stoichiometric absorbers. • Optimal profiling for different device architectures is expediently demonstrated. [ABSTRACT FROM AUTHOR]