Diego Colombara

Thermodynamic aspects of the synthesis of thin-film materials for solar cells.

A simple and useful thermodynamic approach to the prediction of reactions taking place during thermal treatment of layers of multinary semiconductor compounds on different substrates has been developed. The method, which uses the extensive information for the possible binary compounds to assess the stability of multinary phases, is illustrated with the examples of Cu(In,Ga)Se(2) and Cu(2)ZnSnSe(4) as well as other less-studied ternary and quaternary semiconductors that have the potential for use as absorbers in photovoltaic devices.

Detecting ZnSe secondary phase in Cu2ZnSnSe4 by room temperature photoluminescence

Secondary phases, such as ZnSe, occur in Cu2ZnSnSe4 and can be detrimental to the resulting solar cell performance. Therefore, it is important to have simple tools to detect them. We introduce subband gap defect excitation room temperature photoluminescence of ZnSe as a practical and non-destructive method to discern the ZnSe secondary phase in the solar cell absorber. The PL is excited by the green emission of an Ar ion laser and is detected in the energy range of 1.2–1.3 eV. A clear spatial correlation with the ZnSe Raman signal confirms this attribution.

Deliberate and Accidental Gas-Phase Alkali Doping of Chalcogenide Semiconductors: Cu(In,Ga)Se2

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of gas-phase alkali transport in the kesterite sulfide (Cu2ZnSnS4) system (re)open the way to a novel gas-phase doping strategy. However, the current understanding of gas-phase alkali transport is very limited. This work (i) shows that CIGSe device efficiency can be improved from 2% to 8% by gas-phase sodium incorporation alone, (ii) identifies the most likely routes for gas-phase alkali transport based on mass spectrometric studies, (iii) provides thermochemical computations to rationalize the observations and (iv) critically discusses the subject literature with the aim to better understand the chemical basis of the phenomenon. These results suggest that accidental alkali metal doping occurs all the time, that a controlled vapor pressure of alkali metal could be applied during growth to dope the semiconductor, and that it may have to be accounted for during the currently used solid state doping routes. It is concluded that alkali gas-phase transport occurs through a plurality of routes and cannot be attributed to one single source.

Doping mechanism in pure CuInSe2

We investigate the dopant concentration and majority carrier mobility in epitaxial CuInSe2thin films for different copper-to-indium ratios and selenium excess during growth. We find that all copper-poor samples are n-type, and that hopping conduction in a shallow donor state plays a significant role for carrier transport. Annealing in sodium ambient enhances gallium in-diffusion from the substrate wafer and changes the net doping of the previously n-type samples to p-type. We suggest that sodium incorporation from the glass might be responsible for the observed p-type doping in polycrystalline Cu-poor CuInSe2 solar cell absorbers.

Thin-film Photovoltaics Based on Earth-abundant Materials

This book chapter presents an overview of thin-film silicon (both amorphous and microcrystalline) technology and its application to the fabrication of tandem and multiple-junction solar cells. It reviews the material relevant properties for this application and discuss the critical points for achieving high device efficiency. The state-of-the-art, current limitations and prospective concepts are discussed in details.

CHAPTER 5:Thin-film Photovoltaics Based on Earth-abundant Materials

At some stage in the near future, the rapid expansion of photovoltaic solar energy conversion based on thin films of semiconductors such as cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS) could become subject to constraints arising from materials availability and security. For this reason, the development of alternative PV technologies based on earth-abundant elements has become a research priority. This chapter deals with a range of compound semiconductors that could be used to replace CdTe or CIGS as the light-absorbing layer in thin film solar cells. Because this is a rapidly expanding field, the authors have chosen to place the main emphasis on important fundamental aspects and emerging issues rather than just on device performance. The highly promising kesterite copper zinc tin sulfide/selenide (CZTS(Se)) is discussed in detail in order to illustrate the importance of phase equilibria and thermodynamics when considering the quaternary systems that are alternatives to CIGS. The chapter continues with an in depth discussion of the current state of understanding of the electronic properties of CZTS(Se) before reviewing the different synthetic methods that are being used to prepare kesterite layers for devices. The approach taken by the authors clearly demonstrates the central importance of thermodynamics and kinetics in understanding the formation and thermal stability of CZTS(Se) layers. After a short section on the most important opto-electronic properties of absorber layers, the chapter concludes with a comprehensive survey of a range of other potential absorber materials such as pyrite (FeS2), tin sulfide (SnS), copper tin sulfide (Cu2SnS3) and the copper bismuth/antimony sulfide family.

Understanding quaternary compound Cu2ZnSnSe4 synthesis by microscopic scale analyses at an identical location

The synthesis of multinary compound films from layered precursors is only partially understood. Identical location microscopy resolves the multi-step synthesis of Cu2ZnSnSe4 from metallic stacks on the micron-scale. Large scale metal alloying and the seemingly illogical observation that ZnSe segregates preferentially on locations previously poor of zinc are revealed.

Crystallographic and optoelectronic properties of the novel thin film absorber Cu2GeS3

Thin films of Cu2GeS3 are grown by annealing copper layers in GeS and S gaseous atmosphere above 460°C. Below 500°C the cubic polymorph is formed, having inferior optoelectronic properties compared to the monoclinic phase, formed at higher temperature. The bandgap of the cubic phase lies below that of the monoclinic phase: they are determined from absorption measurements to be 1.23 and 1.55 eV respectively. Photoluminescence measurements are performed and only the monoclinic Cu2GeS3 shows a photoluminescence signal with a peak maximum at 1.57 eV. We attribute this difference between cubic and monoclinic to the higher quasi fermi level splitting of the monoclinic phase. Wavelength dependent photoelectrochemical measurements demonstrate the Cu2GeS3 to be p-type with an apparent quantum efficiency of less than 3 % above the band gap.

Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers

Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells.Sodium doping is necessary to achieve high performance in polycrystalline chalcopyrite solar cells, but retards gallium interdiffusion, and thus efficiency optimisation. Here, Colombara et al. show that in contrast to the polycrystalline case, sodium accelerates atomic interdiffusion in monocrystalline samples.