Incorporation of gel electrolyte in dye-sensitized solar cells could widen applications

Researchers have developed a variety of different photoelectrochemical cells that convert light to electricity. The most prominent is the dye sensitized solar cell, or DSSC. This consists of two electrodes made of a transparent, electrically conducting substrate: dye-absorbed nanocrystalline titanium dioxide is deposited on one electrode. Since the original model cell reported by O'Regan and Grätzel in 1991,¹ 15 years of research have led to the development of efficient and low cost DSSCs in the laboratory.² However, problems related to reliability, durability and engineering in the construction of integrated products have prevented wide commercialization.

Most efforts have focused on manufacturing high efficiency DSSCs.^3,4 To our knowledge, these have only been obtained through high optical density values at the TiO₂ interface, which require thick layers of metal oxide and a high surface concentration of dye. This results in solar cells with little or no transparency, which limits potential applications in architectural elements such as windows, facades, and semi-transparent roofs. Furthermore, high efficiency DSSCs usually employ liquid electrolytes. But liquid environments often leak or degrade photochemically, causing poor long-term stability.⁵ We consider transparency and stability to be essential to future applications. As such, we have directed our efforts toward the creation of semi-transparent solar cells employing a quasi-solid-state construction that will increase their long-term stability (see Figure 1).

Others have introduced the use of gel electrolytes as a possible route toward the production of stable DSSCs.⁶ Our approach, inspired by the gel electrolytes in electrochromic devices,⁷ uses a simple gel electrolyte based on a poly-methyl-meta-acrylate (PMMA) polymer matrix. In it is embedded a liquid electrolyte, such as acetonitrile (CH₃CN), and a gelling solvent such as propylene carbonate (PC). The gel electrolyte recipe was first optimized with 5cm² DSSCs. We then produced and tested a large area solar module composed of a 25 × 25cm² master plate containing 23 serially connected cells on the same fluorine doped tin oxide (FTO) plate.

This simple configuration with alternating deposition of counter electrode and photoanode results in a low-current/high voltage scheme that consists of balanced cells that perform similarly when illuminated from either side. This serial connection strategy allows a low-loss DSSC power extraction, solving the problem of high current densities. It also avoids the introduction of complicated, interdigitated silver networks on the FTO surface.

For details on the preparation of the cells and modules, we refer to the literature.^8,9

Optimization of gel mixtures was carried out on 2.5cm² active area DSSCs (see Figure 2). A low molar ratio of LiI/I₂ led to a peculiar lifetime profile. Figure 3 shows a fast decrease in cell performance under standard illumination followed by an unexpected recovery after the cell was placed in the dark for several hours.

The decrease in cell performance is primarily due to a decrease in shunt resistance R_sh. This indicates a higher recombination rate at the photoanode electrode, which results in a lower fill factor.^8,9 This behavior could result from depletion of the electrolyte I^-, which prevents dye regeneration and therefore decreases cell performance. This depletion could be caused by either a low diffusion rate of I^- through the polymer matrix or a high rate of I₂ evaporation, confirmed by the fast bleaching of the typical I₂ yellow color under illumination. Thus, the concentration of the redox couple must be optimized to avoid I^- depletion.

The 625cm² dye sensitized solar modules (DSSMs) consisted of 23 cells, each with an active area of 12.5cm². Serial connection on the same FTO master plate was achieved through scribing 11 lines, thus removing the conductive oxide (Figure 4). Using a gel electrolyte in the 625cm² modules eliminated the need for Surlyn® sealing of each cell. These modules were produced in a five-stage process requiring only one heating step. This was crucial to achieving good cell performance since the use of multiple heating steps causes FTO sheet resistance to increase, leading to high ohmic loss of photogenerated current.^8,9

The use of gel electrolyte improved the shelf life of the DSSMs, as seen in Figure 5 and Table 1. Cells one month old have performed similarly or even better than their initial measurements.

a. I_sc is calculated over 12.5cm², while the efficiency is calculated over the entire active area of 287.5cm².

b. DSSM_5_23_G3 with one month of shelf life

In conclusion, the combination of a very thin TiO₂ layer and a low concentration of dye at the metal oxide surface allow a gel electrolyte to be used in DSSCs. This makes a quasi-solid state DSSC possible. Despite the low light harvesting and the module transmittance of more than 50% in the visible region, we achieved an efficiency close to 1% for cells with an active area of 2.5cm² and 0.3% for modules with an active area of 287.5cm².

This work could not have been performed without the contributions of Frederik C. Krebs and Keld West. This work was supported by the Danish Technical Research Council (FTP 274-05-0053) and Public Service Obligation (PSO 103032 FU 3301).