(253c) The Potential of Solution Processed Silver Indium Diselenide for Photovoltaic Devices

Inorganic thin film materials are extensively investigated as solar cell absorber layers for their advantageous material properties. Specifically, the chalcopyrite material Cu(In,Ga)Se₂ (CIGSe) has achieved high device efficiencies in excess of 23% at the lab scale and has been deployed in commercial applications by multiple private organizations¹. Current CIGSe research focuses on improving device performance and exploring novel synthesis strategies and approaches, one of which is the alloying of the CIGSe system with silver to create (Ag,Cu)(In,Ga)Se₂ (ACIGSe). The addition of silver has been shown to confer benefits to the material, including the passivation of certain defects, the reduction of the melting point of the alloy which facilitates superior material growth at lower temperatures, and an improvement in the structural order of the resulting material².

However, incorporation of silver in to the ACIGSe material system can lead to phase segregation of the material³ and maintaining homogeneity over the large areas required for module fabrication is expected to become more difficult with this more complex stoichiometry. Although challenges are expected to complicate the use of ACIGSe materials, results thus far clearly demonstrate a unique opportunity presented by the incorporation of silver in to the chalcopyrite system.

Another way of taking advantage of the benefits of silver addition is to fully substitute silver for copper and remove gallium from the system, simplifying the stoichiometry and eliminating phase segregation concerns while still harnessing the benefits of silver incorporation. Silver indium diselenide, AgInSe₂ (AISe), is a material that has previously been shown to have a low defect concentration, a high carrier mobility, and a band gap well matched with the solar spectrum⁴. Despite the promising material properties of AISe, research efforts into this material, specifically for photovoltaic applications, is sparse.

To investigate this material more thoroughly, we have developed a solution processing route for the formation of silver indium diselenide using amine-thiol chemistry previously studied by our group⁵. Silver sulfide and indium metal were separately dissolved in a mixture of butylamine and 1,2 ethanedithiol before combining these solutions in a 1:1 Ag:In molar ratio and coating this ink on to molybdenum coated Eagle XG (an alkali-free glass) substrates using a doctor blading method. These as-cast films were annealed on a hotplate set to 300C to form thin films of orthorhombic silver indium disulfide. The silver indium disulfide films were then annealed in a tubular furnace in an atmosphere of argon and elemental selenium at 475-500C for 20-25 minutes. These selenized films formed in the chalcopyrite crystal structure and were not found to contain secondary phases such as Ag₂Se or AgIn₅Se₈ by XRD analysis.

To investigate the electrical properties of AISe, Hall Effect measurements were conducted on the selenized films. Cr/Au contacts were deposited in the van der Pauw configuration by thermal evaporation to create samples for Hall Effect. Measurements were conducted in magnetic fields varying from -9T to 9T. Samples were found to have n-type conductivity with a carrier concentration of ~10¹³ cm^-3 and a Hall mobility of ~12 cm²/Vs, an encouraging value for solution-processed chalcogenide films. It is known that Hall measurements commonly underestimate the true carrier mobility of samples because the in-plane direction of carrier transport in Hall measurements causes carriers to pass through more grain boundaries than would be encountered during solar cell operation, in which carriers would move perpendicular to the plane of the film. Thus, the true carrier mobility of these films is expected to be higher. The n-type conductivity suggests that AISe cannot be made in to a device using the standard CIGSe device architecture and a new device architecture must be proposed.

Kelvin Probe Force Microscopy was conducted to analyze the electrical characteristics of the grain boundaries in AISe films. The electrical properties of grain boundaries are known to play a significant role in determining the device performance of CIGS thin films⁶ and likewise are expected to play a significant role in the development of high performing AISe films. Preliminary results suggest that the band bending present at the grain boundaries will repel minority charge carriers, helping to prevent minority charge carrier recombination at grain boundaries that can negatively impact device performance. Typically, passivation treatments are required in CIGSe films to ensure favorable band bending at grain boundaries but it seems that favorable band bending is present in AISe without requiring a targeted chemical treatment to passivate grain boundaries.

Steady-state photoluminescence measurements were conducted at room temperature on selenized films and a strong, narrow photoluminescence peak was observed, suggesting low non-radiative recombination losses for a material in the early stages of investigation. The narrow (~60meV full width at half maximum) peak observed in room-temperature measurements suggest there is low structural disorder within the film, in agreement with the observations of previous investigations.

In this work we have presented the synthesis of solution-processed silver indium diselenide and demonstrated phase-pure films which exhibit promising electrical properties. Based on these results we suggest that silver indium diselenide deserves a closer investigation as an absorber layer material and should be further explored by the research community.

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