We have reduced the processing temperature of the bulk absorber layer in CH3NH3PbI3−xClx perovskite solar cells from 500 to <150 °C and achieved power conversion efficiencies up to 12.3%. Remarkably, we find that devices with planar thin-film architecture, where the ambipolar perovskite transports both holes and electrons, convert the absorbed photons into collected charge with close to 100% efficiency.

Low-temperature processed meso-superstructured to thin-film perovskite solar cells

Organic solar cells are promising for technological applications, as they are lightweight and mechanically robust. This study presents flexible organic solar cells that are less than 2 μm thick, have very low specific weight and maintain their photovoltaic performance under repeated mechanical deformation.

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Ultrathin and lightweight organic solar cells with high flexibility

We report on the use and effect of the alkali elements rubidium and caesium in the place of sodium and potassium in the alkali post deposition treatment (PDT) as applied to Cu(In,Ga)Se2 (CIGS) solar cell absorbers. In order to study the effects of the different alkali elements, we have produced a large number of CIGS solar cells with high efficiencies resulting in a good experimental resolution to detect even small differences in performance. We examine the electrical device parameters of these fully functional devices and observe a positive trend in the I –V parameters when moving from devices without PDT to KF-, RbF-, and eventually to CsF-PDT. A diode analysis reveals an improved diode quality for cells treat-ed with heavier alkalis. Furthermore, secondary ion mass spectrometry (SIMS) measurements reveal a competitive mechanism induced within the class of alkali elements in the CIGS absorber induced by the alkali post deposition treatment. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

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Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%

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.

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

We report on a new thin-film Cu(In,Ga)Se2 (CIGS) solar cell record efficiency of 217% In order to better understand this newest development, we describe the specific solar cell data as obtained from I–V measurement and diode analysis, quantum efficiency and secondary neutral mass spectrometry measurements We hope that such a description will help to exploit the potential of this promising thin-film solar technology even further (© 2015 WILEY-VCH Verlag GmbH &Co KGaA, Weinheim)

Properties of Cu(In,Ga)Se2 solar cells with new record efficiencies up to 21.7%

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.

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

Development of a Cu(In,Ga)Se 2 thin film solar cell on a polyimide film with a conversion efficiency of 17.1 %, measured under standard test conditions at the European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) of the European Commission, Ispra, is reported. The drastic improvement from the previous record of 14.1% efficiency is attributed to a more optimized compositional grading, better structural and electronic properties of the absorber layer as well as reduced reflection losses. Basic film and device properties, which led to the improvement in the efficiency record of flexible solar cells are presented for the new process and compared to the old process.

Cu(In,Ga)Se2 solar cell grown on flexible polymer substrate with efficiency exceeding 17%

For the development of high efficiency Cu(In,Ga)Se 2 (CIGS) flexible solar cells on polymer films with efficient and robust deposition process the so-called three-stage evaporation processes were modified. CIGS layers were grown by co-evaporation of constituent elements at low substrate temperature compatible with the stability of the polyimide film. The CIGS layers grown with our standard process are compositionally graded, especially the Ga has a strong grading profile across the layer thickness. Different variations of the standard evaporation process were investigated, especially in view of changing the energy band gap profiles in the absorber layer for achieving further improvements in high efficiency solar cells. Further on, the incorporation of sodium during the CIGS deposition was optimized. Structural and chemical composition properties of CIGS layers were characterized with SEM, XRF, SIMS and the photovoltaic properties were characterized with I–V and quantum efficiency measurements. A flexible CIGS solar cell on polyimide film with a record efficiency of 17.6% has been developed.

P. Bloesch

Papers

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

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

Cu(In,Ga)Se2 solar cell grown on flexible polymer substrate with efficiency exceeding 17%

Optimization of composition grading in Cu(In,Ga)Se2 for flexible solar cells and modules