Solution-processed organic tandem solar cells with embedded optical spacers

Abstract

We demonstrate a solution-processed polymer tandem solar cell in which the two photoactive single cells are separated by an optical spacer. The use of an optical spacer allows for an independent optimization of both the electronic and optical properties of the tandem cell. The optical transmission window of the bottom cell is optimized to match the optical absorption of the top cell by varying the layer thickness of the optical spacer. The two bulk heterojunction subcells have complementary absorption maxima at λmax∼850nm for the top cell and λmax∼550nm for the bottom cell. The subcells are electronically coupled in series or in parallel using four electrical contacts. The series configuration leads to an open-circuit voltage of >1V, which is equal to the sum of both subcells. The parallel configuration leads to a high short-circuit current of 92A∕m2, which is equal to the sum of both subcells. The parallel configuration results in a much higher efficiency compared to the series configuration.

Device operation of organic tandem solar cells

“…As stated above the tandem cell we consider is based on a 250 nm P3HT:PCBM blend for the bottom cell and a 80 nm MDMO-PPV:PCBM blend for the top cell, separated by an optical spacer with a thickness of 190 nm. For this thickness the optical spacer maximizes the transmitted light for the wavelengths that correspond to the absorption spectrum of the MDMO-PPV [9]. The experimental J-V characteristics of the individual bottom and top cell of this structure were already shown in Fig.…”

“…However, the narrow absorption properties combined with low charge carrier mobilities limit the performance of the organic solar cells [1,2]. One way to improve the absorption of organic solar cells is by using tandem (or multi-junction) structures [3][4][5][6][7][8][9][10]. Because of the different band gap of the active layer each sub cell then absorbs light in a different part of the solar spectrum.…”

“…In order to compare the calculated results with experiment, a 4-electrode tandem cell is used in which the bottom and top cells are separated by an optical spacer [9] (inset Fig. 5).…”

Polymer‐Fullerene Bulk‐Heterojunction Solar Cells

“…[111] Monolithic four-terminal devices [112] were reported using a transparent and insulating polymer (polytrifluoroethylene) to separate the two stacked cells. [113] The most innovative device architecture, which is also accounted for under tandem cells, is probably the so-called folded reflective tandem device, [114] as depicted in Figure 12.…”

Incremental optimization in donor polymers for bulk heterojunction organic solar cells exhibiting high performance

“…27,28,45 In this context, tandem solar cell devices, which consists of two or more solar cells connected in series or parallel, could be a promising solution. 28,[96][97][98] The connection between the two devices is achieved using a recombination layer, an optically transparent layer that facilitate the alignment of quasi-Fermi levels of the acceptor of the first cell with the donor of the second cell while offering minimal resistance (Fig. 11).…”