Enrico Avancini

Low-temperature-processed efficient semi-transparent planar perovskite solar cells for bifacial and tandem applications

Semi-transparent perovskite solar cells are highly attractive for a wide range of applications, such as bifacial and tandem solar cells; however, the power conversion efficiency of semi-transparent devices still lags behind due to missing suitable transparent rear electrode or deposition process. Here we report a low-temperature process for efficient semi-transparent planar perovskite solar cells. A hybrid thermal evaporation–spin coating technique is developed to allow the introduction of PCBM in regular device configuration, which facilitates the growth of high-quality absorber, resulting in hysteresis-free devices. We employ high-mobility hydrogenated indium oxide as transparent rear electrode by room-temperature radio-frequency magnetron sputtering, yielding a semi-transparent solar cell with steady-state efficiency of 14.2% along with 72% average transmittance in the near-infrared region. With such semi-transparent devices, we show a substantial power enhancement when operating as bifacial solar cell, and in combination with low-bandgap copper indium gallium diselenide we further demonstrate 20.5% efficiency in four-terminal tandem configuration.

High-Efficiency Polycrystalline Thin Film Tandem Solar Cells

A promising way to enhance the efficiency of CIGS solar cells is by combining them with perovskite solar cells in tandem devices. However, so far, such tandem devices had limited efficiency due to challenges in developing NIR-transparent perovskite top cells, which allow photons with energy below the perovskite band gap to be transmitted to the bottom cell. Here, a process for the fabrication of NIR-transparent perovskite solar cells is presented, which enables power conversion efficiencies up to 12.1% combined with an average sub-band gap transmission of 71% for photons with wavelength between 800 and 1000 nm. The combination of a NIR-transparent perovskite top cell with a CIGS bottom cell enabled a tandem device with 19.5% efficiency, which is the highest reported efficiency for a polycrystalline thin film tandem solar cell. Future developments of perovskite/CIGS tandem devices are discussed and prospects for devices with efficiency toward and above 27% are given.

Surface Passivation for Reliable Measurement of Bulk Electronic Properties of Heterojunction Devices.

Quantum efficiency measurements of state of the art Cu(In,Ga)Se2 (CIGS) thin film solar cells reveal current losses in the near infrared spectral region. These losses can be ascribed to inadequate optical absorption or poor collection of photogenerated charge carriers. Insight on the limiting mechanism is crucial for the development of more efficient devices. The electron beam induced current measurement technique applied on device cross-sections promises an experimental access to depth resolved information about the charge carrier collection probability. Here, this technique is used to show that charge carrier collection in CIGS deposited by multistage co-evaporation at low temperature is efficient over the optically active region and collection losses are minor as compared to the optical ones. Implications on the favorable absorber design are discussed. Furthermore, it is observed that the measurement is strongly affected by cross-section surface recombination and an accurate determination of the collection efficiency is not possible. Therefore it is proposed and shown that the use of an Al2 O3 layer deposited onto the cleaved cross-section significantly improves the accuracy of the measurement by reducing the surface recombination. A model for the passivation mechanism is presented and the passivation concept is extended to other solar cell technologies such as CdTe and Cu2 (Zn,Sn)(S,Se)4 .

Single-graded CIGS with narrow bandgap for tandem solar cells

Abstract Multi-junction solar cells show the highest photovoltaic energy conversion efficiencies, but the current technologies based on wafers and epitaxial growth of multiple layers are very costly. Therefore, there is a high interest in realizing multi-junction tandem devices based on cost-effective thin film technologies. While the efficiency of such devices has been limited so far because of the rather low efficiency of semitransparent wide bandgap top cells, the recent rise of wide bandgap perovskite solar cells has inspired the development of new thin film tandem solar devices. In order to realize monolithic, and therefore current-matched thin film tandem solar cells, a bottom cell with narrow bandgap (~1 eV) and high efficiency is necessary. In this work, we present Cu(In,Ga)Se2 with a bandgap of 1.00 eV and a maximum power conversion efficiency of 16.1%. This is achieved by implementing a gallium grading towards the back contact into a CuInSe2 base material. We show that this modification significantly improves the open circuit voltage but does not reduce the spectral response range of these devices. Therefore, efficient cells with narrow bandgap absorbers are obtained, yielding the high current density necessary for thin film multi-junction solar cells.

Evolution of carbon impurities in solution-grown and sputtered Al:ZnO thin films exposed to UV light and damp heat degradation

The electrical properties of polycrystalline ZnO thin films with grain boundary limited charge transport depend significantly on the degree of chemisorbed adsorbate (CxOyHz) coverage at grain boundaries and on the surface. Here we report the evolution of carbon adsorbates in chemical bath deposited (CBD) aluminum doped ZnO (AZO) thin films during post-deposition UV light exposure and degradation in humid atmosphere. As CBD AZO intrinsically contains carbon impurities stemming from the addition of citrate during the deposition, sputtered AZO is examined in parallel as a nominally carbon-free reference. UV illumination decomposes citrate impurities in CBD AZO, but the residual carbon compounds are not effectively removed from the bulk part of the layer. At the same time, charge-trapping oxygen species at grain boundaries are desorbed, increasing the carrier density and mobility. Exposure to humid atmosphere decreases the electrical conductivity of CBD AZO, which is caused by the chemisorption of environmental oxygen species and the formation of zinc carbonate.

Refractive indices of layers and optical simulations of Cu(In,Ga)Se2 solar cells

Abstract Cu(In,Ga)Se2 -based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se2 solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se2 does not depend on the Cu content in the explored composition range, while the absorption coefficient value is primarily determined by the Cu content. An expression for the absorption spectrum is proposed, with Ga and Cu compositions as parameters. This set of parameters allows accurate device simulations to understand remaining absorption and carrier collection losses and develop strategies to improve performances.

Analysis of Ga grading in CIGS absorbers with different Cu content

This work investigates the effect of Cu content and Ga grading on the performance of CIGS cells, by means of numerical simulations and comparison with corresponding experiments. Different Ga profiles and Cu average concentrations are considered. We show that the optical effect of Cu content must be properly taken into account to model NIR absorption. As far as the GGI profile is concerned, we show that the main improvement can be obtained by increasing the GGI ratio toward the back-side; an optimized notch bandgap profile can be designed with the help of these indications.

A numerical study of the use of C-V characteristics to extract the doping density of CIGS absorbers

In this work we use numerical simulations to study how the doping profile extracted by the commonly used C-V technique is influenced by the solar cell main features, as the doping and thickness of different layers, or the conduction band offsets at hero-interfaces. The doping profiles dependence on temperature is also investigated. The effect of both acceptor and donor deep defects of different energy and density on the simulated doping profile has been analyzed and correlated with experimental results, in order to give indications for the correct interpretation of measured doping profile.