(Invited) Development of the Hybrid Solar Cells Using Copper Iodide As a Hole Transport Material
Authors: Konno Akinori 
Published: February 2020

Developing highly efficient and low-cost solar cells as an alternative to conventional silicone solar cell has been attracted much interest. In this regard, the dye-sensitized solar cell (DSC) and perovskite solar cell (PSC) has been intensively studied as a promising system due to its unique operating mechanism to result in reasonably high energy conversion efficiency and its much lower fabrication cost.
We have developed a dye-sensitized solid-state cells (DSSCs) using p-CuI or p-CuSCN as a hole-conductor. Although the DSSC is free from the problems associated with the liquid electrolyte, it suffers from its own problems, such as oxidation of CuI under illumination, incomplete pore-filling, loosening of contacts between TiO2 surface and CuI crystallites, and so on. The deposition of CuI inside the voids and on top of the surface of the nanocrystalline TiO2 electrode is achieved by the so called “solution casting” in which the hole conductor (CuI) is dissolved in a suitable solvent and filled into the voids while heating the film to evaporate the solvent. The incomplete pore-filling is a major problem associated with this type of solar cells. Electrodeposition of CuI within the pores of interconnected TiO2 particles has been investigated by the current pulse and continuous current electrodeposition techniques, both in the dark and under illumination. It has been found that the solar cells fabricated using the electrodes prepared by pulse electrodeposition, under illumination, give a collection efficiency of 3.28% with N-719 dye, whereas those prepared by continuous electrodeposition give only 0.75%, both in the absence of triethylamine hydrothiocyanate (THT, a crystal growth inhibiter), under simulated AM 1.5 illumination. CuI films were also grown by the combination of pulse electrodeposition (without THT) followed by solution casting. The cells prepared by these electrodes give even higher collection efficiency of 3.85% compared to 3.38% of the films prepared by only solution casting of CuI in the presence of THT.
The major drawback of CuSCN as p-type semiconductor in this solar cell is its poor hole conductivity (10-2 S m-1) and consequent slower rate of reduction of oxidized dye molecules thus allowing the recombination of electrons injected to conduction band of TiO2 with the oxidized dye molecules and/or with holes in CuSCN. We investigated a versatile procedure adopted for significantly enhancing the p-type conductivity of cuprous thiocyanate to result in significantly improved, controllable and tunable performance of the TiO2/N719 Dye/CuSCN all-solid state solar cell.
Another approach for DSSCs is focused on TiO2/Sb2S3/CuI/Au-FTO solar cell which utilizes Sb2S3 as an extremely thin light absorbing layer and CuI as a hole conducting material. The solar cell shows very poor performance of lower open-circuit voltage of 0.256 V, a short-circuit current density of 9.50 mA cm-2 and a fill factor of 0.39 under 1 sun illumination for a cell with active surface area of 0.25 cm-2. This is due to the recombination of photo-generated electrons with the holes of the highly conducting CuI. This recombination can be overcome to a certain degree by applying a blocking layer on the Sb2S3 layer to cover the voids in the Sb2S3 layer. The application of N719 dye on the Sb2S3 layer as a recombination blocking layer enhances the conversion efficiency of the solar cell by a factor of 2.5 due to the enhancement of all the solar cell parameters to result in an open circuit voltage of 0.37, short-circuit current density of 13.4 mA cm-2, and a fill factor of 0.50. IPCE and dark current measurements reveal that the main role of the dye on the Sb2S3 layer is the suppression of recombination the electrons with the holes in CuI. Interactions of the NSC groups in the N719 dye with CuI are facilitating the transfer of the photo-generated holes in Sb2S3 to the CuI layer for efficient transport towards the counter electrode.
In addition to DSCs, perovskite solar cells (PSCs) have been developed intensively because of rapid increase in its efficiency. PSC has been considered as a kind of derivative of DSCs at first, but recent studies uncovered its unique characteristics different from DSCs. We have also developed PSCs using CuI as a hole transport material. In early studies, application of CuI to PSC gave only poor efficiencies (1-2 %). Recently remarkable improvement has been achieved by introducing carbon nanotube and controlling conditions of CuI film formation. We have successfully fabricated PSCs around 10% of efficiency. The contact between perovskite and CuI seems to play essential role for fabricating efficient PSCs.