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

A low band gap liquid-processed Cu2ZnSn(Se1−xSx)4 (CZTSSe) kesterite solar cell with x ≈ 0.03 is prepared from earth abundant metals, yielding 10.1% power conversion efficiency. This champion cell shows a band gap of 1.04 eV, higher minority-carrier lifetime, lower series resistance and lower Voc deficit compared to our previously reported higher band gap (Eg = 1.15 eV; x ≈ 0.4) cell with similar record efficiency. The ability to vary the CZTSSe band gap using sulfur content (i.e., varying x) facilitates the examination of factors limiting performance in the current generation of CZTSSe devices, as part of the thrust to achieve operational parity with CdTe and Cu(In,Ga)(S,Se)2 (CIGSSe) analogs.

Low band gap liquid-processed CZTSe solar cell with 10.1% efficiency

Photovoltaic thin film solar cells based on kesterite Cu2ZnSn(Sx,Se1–x)4 compounds (CZTSSe) have reached >12% sunlight-to-electricity conversion efficiency. This is still far from the >20% record devices known in Cu(In1–y,Gay)Se2 and CdTe parent technologies. A selection of >9% CZTSSe devices reported in the literature is examined to review the progress achieved over the past few years. These devices suffer from a low open-circuit voltage (Voc) never better than 60% of the Voc max, which is expected from the Shockley-Queisser radiative limit (S-Q limit). The possible role of anionic (S/Se) distribution and of cationic (Cu/Zn) disorder on the Voc deficit and on the ultimate photovoltaic performance of kesterite devices, are clarified here. While the S/Se anionic distribution is expected to be homogeneous for any ratio x, some grain-to-grain and other non-uniformity over larger area can be found, as quantified on our CZTSSe films. Nevertheless, these anionic distributions can be considered to have a negligible impact on the Voc deficit. On the Cu/Zn order side, even though significant bandgap changes (>10%) can be observed, a similar conclusion is brought from experimental devices and from calculations, still within the radiative S-Q limit. The implications and future ways for improvement are discussed.

Is the Cu/Zn disorder the main culprit for the voltage deficit in kesterite solar cells?

This review summarizes the current status of Cu(In,Ga)(S,Se)2 (CIGS) thin film solar cell technology with a focus on recent advancements and emerging concepts intended for higher efficiency and novel applications. The recent developments and trends of research in laboratories and industrial achievements communicated within the last years are reviewed, and the major developments linked to alkali post deposition treatment and composition grading in CIGS, surface passivation, buffer, and transparent contact layers are emphasized. Encouraging results have been achieved for CIGS-based tandem solar cells and for improvement in low light device performance. Challenges of technology transfer of lab's record high efficiency cells to average industrial production are obvious from the reported efficiency values. One section is dedicated to development and opportunities offered by flexible and lightweight CIGS modules. Copyright © 2016 John Wiley & Sons, Ltd.

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Progress in thin film CIGS photovoltaics – Research and development, manufacturing, and applications

Our effort towards the attainment of high performance devices has yielded several devices with total-area conversion efficiencies above 16%, the highest measuring 16.8% under standard reporting conditions (ASTM E892-87, Global 1000 W/m/sup 2/). The first attempts to translate this development to larger areas resulted in an efficiency of 12.5% for a 16.8-cm/sup 2/ monolithically interconnected submodule test structure, and 15.3% for a 4.85-cm/sup 2/ single cell. Achievement of a 17.2% device efficiency fabricated for operation under concentration (22-sun) is also reported. All high efficiency devices reported here are made from graded bandgap absorbers. Bandgap grading is achieved by compositional Ga/(In+Ga) profiling as a function of depth. The fabrication schemes to achieve the graded absorbers, the window materials and contacting are described.

High efficiency Cu(In,Ga)Se/sub 2/-based solar cells: processing of novel absorber structures

An important development in polycrystalline Cu(In,Ga)Se2 (CIGS) thin‐film photovoltaic solar cells is the attainment of a high voltage device simultaneous with state‐of‐the‐art conversion efficiency. This letter describes a CIGS‐based solar cell that demonstrates an open‐circuit voltage (Voc) approaching 700 mV and a total‐area conversion efficiency of 12.2%. The high value of Voc was achieved by grading In/Ga through the absorber by a computer‐controlled physical vapor deposition (PVD) process that utilizes variable metal fluxes.

Graded band-gap Cu(In,Ga)Se2 thin-film solar cell absorber with enhanced open-circuit voltage

The conversion of renewable bioethanol into high-energy-density higher alcohols has become essential for meeting the increasing global demand to achieve carbon neutrality. In this study, Zn- and nitrogen-codoped Ni-based lignin-derived carbon catalysts (NiZn@NC) were prepared by solvent volatile self-assembly and in situ reductive carbonization using pulp and paper waste stream alkali lignin as the carbon source. Lignin amphipathic derivatives with −COOH and −NH2 groups would coordinate with metal ions to form a stable lignin–metal framework; thus, the lignin-derived carbon layer disperses the NiZn bimetallic catalyst and prevents from corroding. At an amination reagent/lignin mass ratio of 1:2, an ethanol conversion of 75.2% and a high alcohol yield of 41.7% were achieved over the Ni20Zn1@NC catalyst. Experimental results and density functional theory calculations showed that Zn doping improved the electronic environment and defect structures of metallic Ni and carbon carrier, which effectively inhibited C–C cleavage and suppressed the byproduct formation, such as methane. Thereby, the synergetic effect between Ni and Zn facilitated the efficient conversion of aqueous ethanol into higher alcohols by the Guerbet reaction. This work provides a strategy of in situ pyrolytic doping and stabilizing of renewable biomass macromolecules as the frameworks for the construction of highly active and cost-efficient catalysts for ethanol upgrading. 

Efficient Catalytic Upgrading of Ethanol to Higher Alcohols via Inhibiting C–C Cleavage and Promoting C–C Coupling over Biomass-Derived NiZn@NC Catalysts

Exploring highly active and stable electrocatalysts for oxygen evolution reaction (OER) is crucial for developing water splitting and rechargeable metal-air batteries. In this study, a hybrid electrocatalyst of CoFe alloy and CoxN heterojunction encapsulated and anchored in N-doped carbon support (CoFe-CoxN@NC) was in situ coupled via the pyrolysis of a novel coordination polymer derived from lignin biomacromolecule. CoFe-CoxN@NC exhibited outstanding OER activity with a low overpotential of 270 mV at 10 mA cm−2 and stability with an increment of 20 mV, comparable to commercial Ir/C. DFT calculations showed that CoxN and N-doped graphitic encapsulation can reduce the binding strength between *O and CoFe alloy, limit metal leaching and agglomeration, and improve electron transfer efficiency, considerably enhancing the OER activity and stability. In situ coupling approach for preparing alloy and nitride heterojunctions on N-doped lignin-derived carbon offers a promising and universal catalyst design for developing renewable energy conversion technologies. 

In situ
 coupling of lignin‐derived carbon‐encapsulated CoFe‐Co
 
 x
 
 N heterojunction for oxygen evolution reaction

As the flexible producer of substrates, it is of increasing importance to create the infinite coordination-polymer particles (CPPs) for high-efficiency solar energy conversion. However, the inadequate advanced organic ligands and modification largely hinder their application. In this work, competitive evolution-morphological and structural change from Zn-based crystallites to amorphous particles is achieved. The controllable contribution of organic linkers selectively derived six Zn-CPPs with multivariate characters. Based on the diversity of these substructures, hollow zinc oxide particles were initially formed by self-pyrolysis of CPPs and effectively modified by ultrathin doped-nanosheets. Especially, the obtained double-sided heterojunctions show advantages of fully-covered active sites, bringing together the efficient light-excited charge transfer nanochannels, which exhibit an excellent solar H2 releasing activity (e.g., 4512.5 μmol h-1 g-1 ) and stable cyclability.

Hierarchical Hollow Zinc Oxide Nanocomposites derived from Morphology-tunable Coordination Polymers for Enhanced Solar Hydrogen Production.

Infinite coordination-polymer particles (CPPs) are promising materials for solar energy conversion with high efficiency. However, the range of organic ligands that may be used to create CPPs is limited, as are strategies for modification, thereby hindering the applications of such material. In this paper, competitive evolution–morphological and structural change from Zn-based crystallites to amorphous particles is described. Controlled contribution of organic linkers selectively derived six Zn-CPPs with multivariate characters. Based on the diversity of these substructures, hollow zinc oxide particles were initially formed by self-pyrolysis of CPPs and effectively modified by ultrathin doped nanosheets. The obtained double-sided heterojunctions offer fully-covered active sites, bringing together efficient light-excited charge-transfer nanochannels, which exhibit an excellent solar H2-releasing activity (e.g., 4512.5 μmol h−1 g−1) and stable cyclability. 

Yi Qi

Papers

Graded band-gap Cu(In,Ga)Se2 thin-film solar cell absorber with enhanced open-circuit voltage

Efficient Catalytic Upgrading of Ethanol to Higher Alcohols via Inhibiting C–C Cleavage and Promoting C–C Coupling over Biomass-Derived NiZn@NC Catalysts

In situ coupling of lignin‐derived carbon‐encapsulated CoFe‐Co x N heterojunction for oxygen evolution reaction

Hierarchical Hollow Zinc Oxide Nanocomposites derived from Morphology-tunable Coordination Polymers for Enhanced Solar Hydrogen Production.

Hierarchical Hollow Zinc Oxide Nanocomposites derived from Morphology‐tunable Coordination Polymers for Enhanced Solar Hydrogen Production