Better Cu(In,Ga)Se2 solar cells based on surface treated stoichiometric absorbers

in Physica Status Solidi A. Applications and Materials Science (2017), 214, No. 1

Doping, Defects And Solar Cell Performance Of Cu-rich Grown CuInSe2

Doctoral thesis (2016)

Cu-rich grown CuInSe2 thin-film solar cells can be as efficient as Cu-poor ones. However record lab cells and commercial modules are grown exclusively under Cu-poor conditions. While the Cu-rich ... [more ▼]

Cu-rich grown CuInSe2 thin-film solar cells can be as efficient as Cu-poor ones. However record lab cells and commercial modules are grown exclusively under Cu-poor conditions. While the Cu-rich material’s bulk properties show advantages, e.g. higher minority carrier mobilities and quasi-Fermi level splitting - both indicating a superior performance - it also features some inherent problems that led to its widespread dismissal for solar cell use. Two major challenges can be identified that negatively impact the Curich’s performance: a too high doping density and recombination close to the interface. In this work electrical characterisation techniques were employed to investigate the mechanisms that cause the low performance. Capacitance measurements are especially well suited to probe the electrically active defects within the space-charge region. Under a variation of applied DC bias they give insights into the shallow doping density, while frequency and temperature dependent measurements are powerful in revealing deep levels within the bandgap. CuInSe2 samples were produced via a thermal co-evaporation process and subsequently characterized utilizing the aforementioned techniques. The results have been grouped into two partial studies. First the influence of the Se overpressure during growth on the shallow doping and deep defects is investigated and how this impacts solar cell performance. The second study revolves around samples that feature a surface treatment to produce a bilayer structure - a Cu-rich bulk and a Cu-poor interface. It is shown that via a reduction of the Se flux during absorber preparation the doping density can be reduced and while this certainly benefits solar cell efficiency, a high deficit in open-circuit voltage still results in lower performance compared to the Cu-poor devices. Supplementary measurements trace this back to recombination close to the interface. Furthermore a defect signature is identified, that is not present in Cu-poor material. These two results are tied together via the investigation of the surface treated samples, which do not show interface recombination and reach the same high voltage as the Cu-poor samples. The defect signature, normally native to the Cu-rich material, however is not found in the surface treated samples. It is concluded that this deep trap acts as a recombination centre close to the interface. Shifting it towards the bulk via the treatment is then related to the observed increase in voltage. Within this thesis a conclusive picture is derived to unite all measurement results and show the mechanisms that work together and made it possible to produce a high efficient Cu-rich thin-film solar cell. [less ▲]

Vapour phase alkali species for Cu(In,Ga)Se2 solar cells

Scientific Conference (2015, September)

Alkalis are essential in Cu(In,Ga)Se2 absorber layers for efficient solar cells. Current doping methods rely on solid state diffusion of an alkali through to the absorber layer, e.g. a thin NaF layer on ... [more ▼]

Alkalis are essential in Cu(In,Ga)Se2 absorber layers for efficient solar cells. Current doping methods rely on solid state diffusion of an alkali through to the absorber layer, e.g. a thin NaF layer on Mo or NaCl dissolved in a metal precursor ink[1]. The apparent concentration of alkali in the final absorber is determined by the initial alkali dosing and the use of an interfacial barrier to stop alkali diffusion from the substrate. Until now the vapor–absorber interface as a source or sink of alkali doping has been largely ignored. We show that device efficiency improves from 2 to 8% by gas phase Na adsorption alone. Conversely initial results show that Na can also be desorbed to the gas phase. Although these efficiencies are lower than those obtained by including Na directly in the precursor (device efficiency 13.3% [1]), the findings are relevant to all chalcogenide growers as they show that exact doping, and thus control of device efficiency, is only possible when gas phase adsorption/desorption processes are controlled. [less ▲]

Highly conductive ZnO films with high near infrared transparency

in Progress in Photovoltaics (2015)

We present an approach for deposition of highly conductive nominally undoped ZnO films that are suitable for the n-type window of low band gap solar cells. We demonstrate that low-voltage radio frequency ... [more ▼]

We present an approach for deposition of highly conductive nominally undoped ZnO films that are suitable for the n-type window of low band gap solar cells. We demonstrate that low-voltage radio frequency (RF) biasing of growing ZnO films during their deposition by non-reactive sputtering makes them as conductive as when doped by aluminium (ρ≤1·10−3Ω cm). The films prepared with additional RF biasing possess lower free-carrier concentration and higher free-carrier mobility than Al-doped ZnO (AZO) films of the same resistivity, which results in a substantially higher transparency in the near infrared region (NIR). Furthermore, these films exhibit good ambient stability and lower high-temperature stability than the AZO films of the same thickness. We also present the characteristics of Cu(InGa)Se2, CuInSe2 and Cu2ZnSnSe4-based solar cells prepared with the transparent window bilayer formed of the isolating and conductive ZnO films and compare them to their counterparts with a standard ZnO/AZO bilayer. We show that the solar cells with nominally undoped ZnO as their transparent conductive oxide layer exhibit an improved quantum efficiency for λ > 900 nm, which leads to a higher short circuit current density JSC. This aspect is specifically beneficial in preparation of the Cu2ZnSnSe4 solar cells with band gap down to 0.85 eV; our champion device reached a JSC of nearly 39 mAcm−2, an open circuit voltage of 378 mV, and a power conversion efficiency of 8.4 %. [less ▲]

Alternative etchning for improved Cu-rich CuInSe2 solar Cells

in Materials Research Society Symposia Proceedings (2015), 1771

The influence of Se pressure on the electronic properties of CuInSe2 grown under Cu-excess

in Applied Physics Letters (2014), 105

Influence of the Se environment on Cu-rich CIS devices

in Physica B. Condensed Matter (2013), B 439