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Super high efficiency Cu(In,Ga)Se2 thin-film solar cells approaching 25%

Periodic Reporting for period 3 - Sharc25 (Super high efficiency Cu(In,Ga)Se2 thin-film solar cells approaching 25%)

Reporting period: 2018-01-01 to 2018-10-31

Prime objective of the Sharc25 project is to develop super-high efficiency Cu(In,Ga)Se2 (CIGS) solar cells for next generation of cost-efficient solar module technology with the world leading expertise establishing the new benchmarks of global excellence. Both CIGS developers such as Empa (low temperature CIGS) and ZSW (high temperature CIGS) have developed new processing concepts which open new prospects for further breakthroughs leading to paradigm shift for increased performance of solar cells approaching to the practically achievable theoretical limits. In this way the costs for industrial solar module production < 0.35€/Wp and installed systems < 0.60€/Wp can be achieved, along with a reduced Capex < 0.75€/Wp for factories of > 100 MW production capacity, with further scopes for cost reductions through production ramp-up.

In this project the performance of single junction CIGS solar cells will be pushed from ~21% towards 25% by a consortium with multidisciplinary expertise. The key limiting factors in state-of-the-art CIGS solar cells are the non-radiative recombination and light absorption losses. Novel concepts are studied to overcome major recombination losses: combinations of increased carrier life time in CIGS with emitter point contacts, engineered grain boundaries for active carrier collection, shift of absorber energy bandgap, and bandgap grading for increased tolerance of potential fluctuations. Innovative approaches are applied for light management to increase the optical path length in the CIGS absorber and combine novel emitter, front contact, and anti-reflection concepts for higher photon injection into the absorber

Both CIGS developers (Empa and ZSW) could increase their solar cell efficiencies significantly, mainly by improved processes for CIGS absorber fabrication and advanced interface design. The best result of a small-area CIGS thin-film solar cells was 22.6%. In addition, parasitic absorption of the CdS buffer layer could be significantly reduced due to implementation of novel materials as subsequent high resistive layer. Further, a reflector was successfully introduced at the molybdenum back contact to increase the optical path length in the CIGS absorber. The Sharc25 project has provided deep insights into highly efficient CIGS thin-film solar cells using advanced characterization methods, analytical tools, device simulation, and density functional modeling.
High-efficiency Cu(In,Ga)Se2 (CIGS) thin-film solar cells (with efficiency equal to or above 20%) were sent out during the whole project by Empa and ZSW to the consortium for in-depth material and device characterization. Theoretical results from device simulations and density functional theory (DFT) modeling were provided to support the scientists in the lab.

With the introduction of RbF (and CsF) post-deposition treatment (PDT) of CIGS absorbers a step forward to even higher efficiencies could be made. Finally, using the RbF-PDT procedure combined with an advanced buffer system with very thin solution-grown CdS and sputtered (Zn,Mg)O, a certified cell efficiency of 22.6% could be reached. The RbF-PDT helped to improve the efficiency of low bandgap (Eg = 1.0 eV) CuInSe2 solar cell with a strong Ga grading in vicinity of the back contact to a certified efficiency value of 18%. In addition to (Zn,Mg)O as substitution for the commonly used i-ZnO as high resistive layer, TiOx or ZnTiOx films grown by atomic layer deposition (ALD) enabled a further reduction of the CdS buffer layer thickness to reduce parasitic absorption and enhance cell efficiency.

The improved alkali metal PDT processes were successfully transferred to the industry partners NICE Solar Energy and Flisom.

Various methods for deposition of front and back side passivation layers for CIGS absorber layers were tested and Al2O3 and HfOx grown by ALD are the most promising candidates. After determination of optimal size and pitch for adequate openings by device simulations, several procedures were adapted from Si technology to polycrystalline CIGS with its relatively rough surface. The main focus was on hole-mask colloidal lithography for patterning of passivation layers at the CIGS front side and nano-imprint lithography for the back side. With theoretical device modeling the origin of optical losses and possibilities to increase efficiency further – e.g. with light trapping and various types of back reflectors – have been determined. An aluminium back reflector in combination with InZnO spacer layer and very thin molybdenum resulted experimentally in an increase of efficiency of 0.4% absolute.

Two international workshops were organized in December 2017 at ZSW in Stuttgart (Germany) and in October 2018 at Empa in Dübendorf (Switzerland). Both Sharc25 workshops, open to the public, were successfully held with around 70 international attendees for each event.
The most obvious progress beyond state of the art is achieved in the device performance: With the demonstration of a solar cell with an efficiency of 22.6% a major step towards the main project objective of 24% cell efficiency was reached. Very important for future improvements of the performance as well as expected impact on the costs is the progress achieved on understanding. The Sharc25 project enabled the following technical progress beyond state of the art:

• Complete data set of optical properties of all functional layers in device quality
• Multidimensional numerical model for record-efficiency solar cells
• Simulation-based guidelines for improving the CIGS/buffer interface
• Simulation-based guidelines for optimized point contact size in passivation layers
• Information on chemical distribution of elements in the bulk and interfaces of high quality absorber layers
• Method for measuring potential and band gap fluctuation in the bulk of high quality absorber layers
• Information on carrier recombination in the bulk of high quality absorber layers
• Formation energies of alkali-based doping defects in CIGS with different composition
• Determination of refractive indices for different CIGS compositions and development of a simulation tool
• Detailed know-how on optical losses in the different layers of the device
• Simulation-based guidelines for optimized light trapping and back reflector layers

The consortium developed optimized fabrication processes based on the findings described above with the objective to reach efficiencies as high as possible. Novel device concepts and processes were developed to overcome major recombination losses, e.g.: increasing carrier life time, active carrier collection, and adapting energy bandgap grading. Novel approaches for light management were applied and assessed to increase the optical path length in the CIGS absorber and combine novel emitter, front contact, and anti-reflection concepts for higher photon injection into the CIGS absorber layer.

The novel device architecture and processing concepts developed in Sharc25 can provide the decisive advantage to the European PV industry to realize maximized CIGS module performance with optimized thin-film production processes and PV technology. Without this significant technological progress the sustainable development of European PV industry would be at high risk. Not only will the module manufacturers benefit from this technology head start but also many branches of industry along the value chain of CIGS solar modules and PV systems.
ZSW researcher with CIGS solar cells. Photo: Copyright ZSW
Empa researchers in the lab. Photo: Copyright Empa
Road towards 25% CIGS solar cell efficiency. Image: Copyright Sharc25