Solution-processing of Cu(In,Ga)(S,Se)2 solar cells from metal chalcogenides: aspects of absorber crystallisation and interface formation

Photovoltaic (PV) solar cells provide a simple and smart way for direct conversion of sunlight into electricity, which is a clean alternative to conventional fossil fuel-based energy conversion methods. As a result of ongoing research, photovoltaic technologies are becoming both more efficient and cheaper. Thin film solar cells, particularly chalcogenides, are a promising area of research as they offer substantial cost savings as a consequence of reduced material usage and cheaper atmospheric fabrication processes.Cu(In,Ga)(S,Se)2(CIGS)solar cells are currently the best performing commercially deployed thin film PV technology. However the best performing CIGS devices are fabricated using sophisticated vacuum-based co-evaporation or co-sputtering systems requiring high capital costs and high energy budgets. Lower costs are achievable using alternative atmospheric deposition methods. These typically involve two step processes consisting of electrodeposition, nanoparticle or molecular solution coatings followed by high temperature annealing in a chalcogen containing atmosphere. [Continues.

Quantifying minority carrier lifetime and collection efficiency of solar cells by combined optical and electrical characterisation techniques

This work presents the use of a combined measurement system for spectrally-resolved photoluminescence (PL), time-resolved photoluminescence (TRPL) and transient photocurrent decay (TPCD) to characterise the physical properties of solar cells and their materials. A physical model is proposed to quantify the localised carrier collection efficiency of solar cells from the measured localised minority carrier lifetime from TRPL measurements and the localised minority carrier diffusion time from TPCD measurements. A single excitation laser source is used to measure TRPL and TPCD at the same spot on the solar cell. Combined PL, TRPL and TPCD measurements are conducted on a CdS/CdTe and a CIGS sample. The resulting PL spectra for both samples show that the emission spectra can yield information on the material bandgap. TRPL and TPCD yield localised carrier lifetime and diffusion times of τTRPL=3.91ns and τTPCD=40.5ns respectively for the CdS/CdTe sample, and τTRPL=2.45ns and τTPCD=196.8ns respectively for the CIGS sample. The ratio between the τTRPL and τTPCD values is shown to be proportional to the localised carrier collection efficiency, yielding collection efficiencies of 21.97% and 7.93% for the CdS/CdTe and CIGS sample, respectively. The initial results show that the localised carrier collection efficiency may be affected by the sample’s metal contact configuration. In short, this combined measurement approach can offer a novel and useful method of characterising the material quality of solar cells and the localised carrier collection efficiency of finished PV devices

Development of ZnTe as a back contact material for thin film cadmium telluride solar cells

Cadmium telluride (CdTe) is high-efficiency commercialised thin film photovoltaic technology. However, developing a stable low-resistivity back contact to the CdTe solar cells is still an issue. High work function and low level of doping of this material don't allow to create an ohmic contact with metals directly. Copper is commonly used to lower the back contact barrier in CdTe solar cells, but an excessive amount of copper diffusing through the cell is harmful for the device performance and stability. In this work a copper-doped ZnTe (ZnTe:Cu) buffer layer was incorporated in between CdTe and gold metal contact by high-rate pulsed DC magnetron sputtering. The back contact was then activated by rapid thermal processing (RTP) resulting in spectacular improvement in key device performance indicators, open circuit voltage (VOC) and fill factor (FF)

Hydrazine-free metal chalcogenide precursor solutions for sprayed CuIn(S,Se)2 thin film solar cells

CuIn(S,Se)2 (CIS) absorbers have been prepared by dissolving copper and indium sulphides in a hydrazine-free mixture of solvents, followed by spray deposition of the solution on molybdenum coated glass substrates. It was found that the choice of the solvent for dilution of the precursor solution can have an influence on the absorber quality and consequently, the device performance. The sprayed absorbers were implemented in solar cell devices, resulting in power conversion efficiencies (PCE) exceeding 7%

An innovative approach for fabrication of Cu2ZnSnSe4 absorber layers using solutions of elemental metal powders

An innovative approach has been demonstrated for the deposition of Cu2ZnSnSe4 (CZTSe) absorber layers. Using a unique, safe solvent combination, moving away from hydrazine, elemental Cu, Zn, Sn and S/Se can be fully dissolved at ambient conditions, with the composition easily controlled. The preparation and the spray deposition of these solutions are performed in air, allowing a quick, easy and inexpensive process. Upon selenisation, large crystals are observed which are identified as the CZTSe kesterite phase using X-ray diffraction and Raman, the latter showing no distinctive signs of any binary or ternary secondary phases. Using this method, CZTSe absorber layers have been prepared for use in thin-film solar cells reaching efficiency of 3.2%. With further device optimisation, improved device performance will be achieved

A comparison of different selenisation approaches for solution-processed Cu(In,Ga)(S,Se)2 solar cells

Hydrazine-free Cu(In,Ga)Se2 (CIGS) absorbers were fabricated using a low-cost atmospheric deposition method. The structural and electrical properties of thin film absorbers and the resulting solar cells processed using two different selenisation approaches were compared. A double selenisation process showed improved crystal structure compared to a single selenisation step, resulting in improved absorption throughout the spectrum and conversion efficiencies reaching 9.3%

Variation of Cu content of sprayed Cu(In,Ga)(S,Se)2 solar cells based on a Thiol-Amine solvent mixture

Cu(In,Ga)(S,Se)2 (CIGS) thin films were formed by a low cost solution-based approach using metal sulfide precursors. The stoichiometry of the absorber layer is tailored in order to improve film morphology and electrical properties. Cuy ln0.7Ga0.3Se2 films were prepared with a varied Cu content (0.8>y>1.1) and were completed in solar cell devices. The compositional, structural and electrical properties of the devices were investigated. Increased Cu content improves lateral crystallization, but results in the formation of Cu-rich secondary phases in-between CIGS grain boundaries. Characterization of the completed devices shows that Cu content has an important effect on the device electrical properties and the dominant recombination mechanisms

Scalable deposition of high-efficiency perovskite solar cells by spray-coating

Spray-deposition is a low-cost, roll-to-roll compatible technique that could potentially replace spin-coating for the deposition of highly efficient perovskite solar cells. Here, perovskite active layers were fabricated in air using an ultrasonic spray system and compared with equivalent spin-coated films. A chlorine-containing perovskite ink with a wide processing window coupled with an antisolvent extraction resulted in perovskite films with relatively rougher surfaces than those spin-coated. A power conversion efficiency (PCE) of 17.3% was achieved with an average of 16.3% from 24 devices. Despite observing differences in film roughness and structure, the performance of sprayed perovskite solar cells was comparable to that of the spin-coated cells processed in an inert atmosphere, showing the versatility of perovskite processing

Exploring metastable defect behavior in solution-processed antimony doped CIGS thin film solar cells

This study investigates the metastable defect behavior from temperature dependent current density-voltage (JVT) and capacitance spectroscopy measurements in solutionprocessed antimony (Sb) doped CIGS thin film solar cells. From the Voc(T) analysis, the main recombination mechanism is found to be Schottky-Read-Hall recombination in the bulk. A detailed study of the carrier concentration, defect density and energy level defects was performed using capacitance spectroscopy. Admittance spectroscopy measurements revealed an admittance step at low temperatures with an activation energy of 42 meV

Solution processing of CuIn(S,Se)2 and Cu(In,Ga)(S,Se)2 thin film solar cells using metal chalcogenide precursors

In order to realize the true low cost potential of Cu(In,Ga)(S,Se)2 (CIGS) thin film solar cells, high performance needs to be combined with simple and easily controllable atmospheric-based deposition processes. A molecular solution-based approach for CIGS deposition is proposed, using metal chalcogenide precursors dissolved in an amine-thiol solvent combination. CIGS thin films were sprayed with varying Ga content and the sprayed films were incorporated into solar cells. The effect of the Ga content on the material and device properties is investigated. A champion power conversion efficiency of 9.8% (active area) was achieved, which highlights the potential of this methodology