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Passivating contacts are becoming a mainstream option in current photovoltaic industry due to their ability to provide an outstanding surface passivation along with a good conductivity for carrier collection. However, their integration usually requires long annealing steps which are not desirable in industry. In this work we study PECVD as a way to carry out all deposition steps: silicon oxide (SiOx), doped polycrystalline silicon (poly-Si) and silicon nitride (SiNx:H), followed by a single firing step. Blistering of the poly-Si layer has been avoided by depositing (p+) microcrystalline silicon (μc-Si:H). We report on the impact of this deposition step on the SiOx layer deposited by PECVD, and on the passivation properties by comparing PECVD and wet-chemical oxide in this hole-selective passivating contact stack. We have reached iVoc > 690 mV on p-type FZ wafers for wet-chemical SiOx\(p+) μc-Si\SiNx:H with no annealing step.

In this work, the fabrication of MoOx-free semitransparent perovskite solar cells (PSC) with Power Conversion Efficiencies (PCE) up to 15.7% is reported. Firstly, opaque PSCs up to 19.7% were fabricated. Then, the rear metal contact was replaced by a highly transparent and conductive indium tin oxide (ITO) film, directly sputtered onto the hole selective layer, without any protective layer between Spiro-OMeTAD and rear ITO. To the best of our knowledge, this corresponds to the most efficient buffer layer-free semitransparent PSC ever reported. Using time-resolved photoluminescence (TRPL) technique on both sides of the semitransparent PSC, Spiro-OMeTAD/perovskite and perovskite/TiO2 interfaces were compared, confirming the great quality of Spiro-OMeTAD/perovskite interface, even after damage-less ITO sputtering, where degradation phenomena result less important than for perovskite/TiO2 one. Finally, a 4-terminal tandem was built combining semitransparent PSC with a commercially-available Aluminium Back Surface Field (Al-BSF) silicon wafer. That silicon wafer presents PCE = 19.52% (18.53% after being reduced to cell size), and 5.75% once filtered, to generate an overall 4 T tandem efficiency of 21.18% in combination with our champion large semitransparent PSC of 15.43%. It means an absolute increase of 1.66% over the original silicon wafer efficiency and a 2.65% over the cut Si cell.

Being able to predict the output power of solar cells by analysis of only the first semiconductor layer – commonly called the absorber layer – i.e. before a number of further deposition steps are carried out – would be a clear advantage both at academic research level and for process monitoring in industry. Since the absorber layer is normally the most difficult to fabricate, the hypothesis is that the power conversion efficiency of the solid-state device is dominated by its optoelectronic properties. One option that allows extracting information on the absorber optoelectronic properties is by formation and interrogation of virtually reversible electrolyte (Schottky) junctions 1. We have recently shown that the short circuit current density and thus efficiency of Cu(In,Ga)Se2 solid-state devices can be predicted by measuring the electrochemical photocurrent density of the respective absorber layers in the presence of Eu3+ under illumination 2. In such a system the Eu3+acts as a scavenger for the electrons generated on the p-type semiconductor upon irradiation with photons with energy greater than the semiconductor band-gap. However, this correlation was found complicated by surface non-ideality, leading in some cases to pronounced charge-carrier recombination. Therefore, here we explore an alternative relationship. Similarly to p-n junctions, the electrical behavior vs bias of p-semiconductor/electrolyte junctions is a direct consequence of the energy-barriers for hole transfer 3. As such, the current-voltage dependence can be expressed by the diode equation 4, including the contribution of parasitic shunt paths (Eq. 1). Eq. 1 JD,L = JPh + J0(exp(-eΔV/kT)-1) + ΔV/RSh where JD,L are the dark and illuminated electrical current-density, JPh is the photocurrent density ( = 0 in the dark and < 0 under illumination), J0 is the reverse saturation current, e is the elementary charge, ΔV is the applied voltage, k is the Boltzmann constant, T is the absolute temperature and RSh is the shunt resistance of the solar cell. The dark-current in reverse bias is related to the parasitic terms RSh and J0, with RSh normally dominating. For good solid-state devices RSh should be as large as possible and J0 as small as possible. From inspection of literature solid-state device data the J0 term is more important in determining the overall efficiency of a device than RSh. However, J0 cannot be distinguished from RShin reverse bias, while in forward bias it is possible. Eu2+ ions are required to interrogate the junction in forward bias. Eu2+ acts as an electron donor in the system, revealing the ability of the semiconductor film to accept electrons when kept under forward bias. Due to the exponential term in Eq. 1, the higher this ability to accept electrons, the higher the expected reverse saturation current from such a semiconductor in a full device whilst the effect of RShis negligible. In this work we demonstrate that the reverse saturation current of CuInSe2 solar cell devices can be predicted by measuring their forward bias characteristics in the presence of Eu2+/3+, allowing the prediction of final solid-state device efficiency. To this end a series of CuInSe2 absorber layers were prepared by intentional deviation from a physical vapour deposition routine and they were split into two. Half were completed into solar cell devices and the other half were tested electrochemically. The chosen absorber layers gave solid-state device power conversion efficiencies between 6 and 12.5% 5. The electrochemical experiments consisted of voltammetric analyses in the dark with Eu2+/3+species. The extraction of a term electrochemically-equivalent to the reverse saturation current density is obtained by linear extrapolation of the forward dark current density to voltages close to the open circuit voltage of the semiconductor-electrolyte junction in the dark. Such electrochemically extracted saturation current is compared with the solid-state device saturation current. These findings open the way to a more reliable electrochemically-based estimation of thin film semiconductor properties with a clear link with solid-state device physics. A comparison between this proposed method and the method involving the measurement of the electrochemical photocurrent density will be made. References 1. Peter, L. M., Semiconductor Electrochemistry. Encyclopedia of Life Support Systems: Oxford, 2010. 2. Colombara, D. et al. Electrochemistry Communications 2014, 48, 99-102. 3. Tan, M. X. et al. Progress in Inorganic Chemistry, John Wiley & Sons, Inc.: 2007; pp 21-144. 4. Shockley, W. Bell System Technical Journal 1949, 28, 435-489. 5. Depredurand, V. et al. PVSC Proceedings, 2011 37th IEEE, 19-24 June 2011; 2011; pp 000337-000342. Acknowledgements LPV, LEM, Nexcis team members and Prof. Laurence M. Peter are acknowledged for help and discussion. Funding was provided by the European Commission through the "Scalenano" program (grant agreement nº 284486)/

In crystalline silicon solar cells, the front surface is textured in order to lower the reflection of the incident light and increase the efficiency of the cell. This texturing whose dimensions are a few micrometers wide and high, often makes it difficult to determine the doping profile measurement. We have measured by secondary ion mass spectrometry (SIMS) and electrochemical capacitance voltage profiling the doping profile of implanted phosphorus in alkaline textured and in polished monocrystalline silicon wafers. The paper shows that SIMS gives accurate results provided the primary ion impact angle is small enough. Moreover, the comparison between these two techniques gives an estimation of the concentration of electrically inactive phosphorus atoms.

Solution processing of inorganic thin films has become an important thrust in material research community because it offers low-cost and high-throughput deposition of various functional coatings and devices. Especially inorganic thin film solar cells – macroelectronic devices that rely on consecutive deposition of layers on large-area rigid and flexible substrates – could benefit from solution approaches in order to realize their low-cost nature. This article critically reviews existing deposition approaches of functional layers for chalcogenide solar cells with an extension to other thin film technologies. Only true solutions of readily available metal salts in appropriate solvents are considered without the need of pre-fabricated nanoparticles. By combining three promising approaches, an air-stable Cu(In,Ga)Se2 thin film solar cell with efficiency of 13.8% is demonstrated where all constituent layers (except the metal back contact) are processed from solutions. Notably, water is employed as the solvent in all steps, highlighting the potential for safe manufacturing with high utilization rates.

The quality control of individual semiconductor thin films during fabrication of multiple layers is important for industry and academia. The ultimate aim of this research is to predict the efficiency of p–n junction solar cells by photoelectrochemical analysis of the bare p-type semiconductor. A linear correlation between the photocurrent measured electrochemically on Cu(In,Ga)Se2 absorber layers through a Eu3+ electrolyte junction and short circuit current and efficiency of the corresponding solid state devices is found. However, the correlation is complicated by pronounced recombination at the semiconductor/electrolyte interface, while the solid state interface behaves more ideally. Keywords: Photocurrent spectroscopy, Cu(In,Ga)Se2, Opto-electronic quality, Photoluminescence, Short-circuit-current

The increasing importance of the Cu(In,Ga)Se2 based thin films photovoltaic industry claims for the development of new assessment and monitoring tools to answer the needs existing in the improvement of the control of the processes involved in the production of solar cells modules. In this frame, a strong interest has been given to the development methodologies for the assessment of the CIGS absorber, nevertheless advanced optical tools for the characterization of the other layers in the solar cells are still missing. In this work, we report a non-destructive optical methodology based on resonant Raman concepts that has been developed for the characterization of Al doped ZnO layers (AZO) that are used as window layer in Cu(In,Ga)Se2 solar cells. Doping the ZnO layer with Al leads to the presence of a characteristic defect induced band at 510 cm−1 spectral region. The correlation of the relative intensity of this band with the resistivity of the layers provides a fast and reliable tool for their electrical monitoring. Analysis of solar cells fabricated with layers of different conductivities has allowed demonstration at cell level of the proposed methodology for the determination of efficiency losses related to degradation of the resistivity of the AZO layers.

The development of low cost industrial processes is one of the key issues to make Cu(In,Ga)Se 2 based solar cells reach grid-parity. Such a process is found by using a two-step technology based on the sequential electro-deposition of a metallic precursor followed by a rapid annealing. Three types of metallic precursors (two-compound systems as copper–indium, copper–gallium and three-compound system as copper–indium–gallium) have been electrodeposited on a molybdenum sputtered soda lime glass and alloyed through a low annealing temperature. Then a selenium film has been evaporated and the stack has been annealed at high temperature in a rapid thermal processing furnace. A one-step heating profile has been used from room temperature to 550 °C in less than 1 min. Samples for which the heating was stopped after different annealing times have been characterized using several techniques: X-ray fluorescence spectrometry for elemental composition, X-ray diffraction and Raman spectroscopy for phase composition, scanning electron microscopy for structural analysis and glow discharge optical emission spectroscopy for diffusion study. Preferential formation reactions of the two-compound based metallic precursors have been studied and compared with the copper–indium–gallium metallic precursor used in a two step process. A gallium free system reacts faster than a gallium-based system and presents well-formed ternary compound after a standard selenization. However, the incorporation of gallium can be improved through a longer annealing time or a higher annealing temperature.

Nanotechnology for Next Generation High Efficiency Photovoltaics Spring International School and Workshop (NEXTGEN NANOPV), SPAIN, APR 20-24, 2015; International audience; Non-destructive characterization of both single layers and completed devices are important issues for the development of efficient and low cost Cu(In,Ga)(S,Se)(2) (CIGS) modules at high yields. This implies for the need of methodologies suitable for the assessment of optical, electrical, and physico-chemical parameters that are relevant for the final device efficiency and that can be used for quality control and process monitoring at different process steps. In these applications, detection of in-homogeneities in the different layers from large area modules is especially relevant, being the presence of these inhomogeneities responsible for the existing gap between the efficiencies achieved in these technologies at cell and module levels. In this context, this work reviews the different optical methodologies that have been developed in the framework of the SCALENANO European project for the advanced assessment of the different layers in high efficiency electrodeposited - based CIGS devices. This has includes different strategies as those based on Raman scattering, Photoluminescence/Electroluminescence (PL/EL) based techniques and new photoelectrochemical based tools and firstly Raman spectroscopy is very sensitive to both composition and crystal quality parameters that are determining for device efficiency. Use of resonant Raman excitation strategies allows achieving a high sensitivity of the Raman spectra to the analysed features in the different regions of the device. This involves selection of the suitable excitation wavelength (in the broad spectral region from UV to IR) for the resonant Raman excitation of the required layer in the device. The strong increase in the intensity of the Raman peaks related to the use of resonant excitation conditions allows also decreasing the measuring time to times compatible with the implementation of these techniques at online process monitoring level. Analysed parameters include the electrical conductivity of the Al-doped ZnO window layer, the thickness of the CdS buffer layer and the chemical composition (SAS+Se) relative content) and presence of relevant secondary phases as Cu-poor ordered vacancy compounds in the surface region of the absorbers. In addition PL/EL imaging are powerful techniques that provide direct access to the optoelectronic properties of the materials and devices. Whereas EL is performed using complete devices by injecting current in analogy to the operation of a light emitting diode, PL allows the characterization of bare absorber materials without the need for any functional or contacting layers. Moreover, semiconductor photo-electrochemistry (PEC) is a versatile technique that enables many opto-electronic properties of semiconductors to be determined. Essentially, a semiconductor on a conducting substrate placed in a solution containing redox species forms a Schottky barrier junction. The formation of such a diode enables basic semiconductor properties to be measured such as doping type, doping density, band gap and the flat band position versus the vacuum energy scale. In all these cases, quality control indicators suitable for the advanced assessment of these processes have been identified and validated for the electrodeposition-based processes developed at Nexcis Company. (C) 2016 Elsevier B.V. All rights reserved.

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