Alexander R. Uhl

Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells

Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20%. This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance. Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters. Ion exchange processes, well known in other research areas, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction.

Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films

Solar cells based on polycrystalline Cu(In,Ga)Se(2) absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry.

Doping of polycrystalline CdTe for high-efficiency solar cells on flexible metal foil

Roll-to-roll manufacturing of CdTe solar cells on flexible metal foil substrates is one of the most attractive options for low-cost photovoltaic module production. However, various efforts to grow CdTe solar cells on metal foil have resulted in low efficiencies. This is caused by the fact that the conventional device structure must be inverted, which imposes severe restrictions on device processing and consequently limits the electronic quality of the CdTe layer. Here we introduce an innovative concept for the controlled doping of the CdTe layer in the inverted device structure by means of evaporation of sub-monolayer amounts of Cu and subsequent annealing, which enables breakthrough efficiencies up to 13.6%. For the first time, CdTe solar cells on metal foil exceed the 10% efficiency threshold for industrialization. The controlled doping of CdTe with Cu leads to increased hole density, enhanced carrier lifetime and improved carrier collection in the solar cell. Our results offer new research directions for solving persistent challenges of CdTe photovoltaics.

Highly transparent and conductive ZnO: Al thin films from a low temperature aqueous solution approach.

A solution deposition approach for high-performance aluminum-doped zinc oxide (AZO) thin films (visible transparency > 90% and sheet resistance down to 25 Ω/sq) with process temperatures not exceeding 85 °C is presented. This allows the non-vacuum deposition of AZO on temperature sensitive substrates such as polymer films for flexible and transparent electronics, or inorganic and organic thin film photovoltaics.

Progress towards 14% efficient CdTe solar cells in substrate configuration

CdTe solar cells are conventionally grown in superstrate configuration. However, the growth in substrate configuration offers more control of junction properties as recrystallization of CdTe and junction formation with CdS can be decoupled. In this paper the influence of various annealing treatment conditions of the CdS layer on its morphology and phase and on the device properties is presented. The presence of CdCl2 during this annealing treatment is important for the phase change of the CdS layer to hexagonal wurtzite and for high efficiencies. A CdCl2 treatment of the CdS at 360 °C improves the efficiency of the device without the adverse effect of pinhole formation in the CdS. CdTe solar cells in substrate configuration with more than 13% efficiency are achieved as a progress towards 14% efficiency.

Ultrasonically sprayed Zinc sulfide buffer layers for Cu(In,Ga)(S,Se) 2 solar cells

Zinc sulfide (ZnS) buffer layer deposited by an ultrasonic spray pyrolysis (USP) method is a feasible alternative to the chemical bath deposited cadmium sulfide (CdS) buffer layer. In the present work we report the results of a low-cost, non-vacuum and in-line compatible method to grow ZnS thin films. We investigated the properties of USP-ZnS films grown at different substrate temperatures and spray solution precursors. Rutherford backscattering spectrometry measurements were done for quantitative chemical composition information, revealing chlorine impurities depending on the deposition temperature as well as on the chemical precursors. By optimizing the spray parameters of USP-ZnS buffer layers on Cu(In,Ga)(S,Se)2 absorbers, a maximum solar cell efficiency of 10.8% after air-annealing was achieved, whilst the CdS reference fabricated on a similar absorber reached 11.4%. A significant increase of the short circuit current is observed as compared to the CdS reference due to a gain in the blue wavelength region.

Na distribution in CIGS solar cells grown with modified three-stage processes

Highest efficiency CIGS solar cells are generally grown with a three-stage co-evaporation process where the absorber layer is in a copper-rich regime for a period of time at the end of the second stage. We investigated the influence of changing the maximum [Cu]/[In+Ga] ratio at the end of stage 2 on the distribution of sodium throughout the absorber layer when sodium is supplied by diffusion from the soda-lime glass substrate. Secondary ion mass spectrometry (SIMS) was used for depth profiling of the Na content, the surface concentration of Na was determined by wavelength dispersive X-ray analysis (WDX) from top view scanning electron micrographs. Raman investigation of the phase composition of the surface and SIMS compositional depth profiles of the investigated absorber layers suggested the possibility of the formation of a Na-rich compound on the absorber layer surface for CIGS grown with low Cu excess while absorbers grown with high excess showed a more evenly distributed Na depth profile. New WDX results further support these claims as a surface [Na]/[Cu+Na] ratio of up to 0.2 for layers grown with low Cu excess was measured while the Na surface values of absorbers grown with high Cu excess are below the detection limit.