Advances in Solution‐Processed Multijunction Organic Solar Cells

Abstract

Single-junction solar cells are principally limited in performance by two factors (Figure 1a). Electrons excited by photons with energy higher than the bandgap relax to the band edges, releasing surplus energy as heat (thermalization loss). Photons with energy lower than the bandgap are not absorbed (transmission loss). These losses can be alleviated with two or more absorber layers. The first layer should feature a wide bandgap material to reduce the thermalization loss for high-energy photons. The second layer should have a lower bandgap to absorb the low-energy photons that pass the first layer. In such configuration a tandem cell provides less thermalization and less transmission losses than each of the corresponding single-junction cells. In the detailed-balance limit, a double-junction (tandem) cell can reach an efficiency of 42% and a triple-junction cell 49%. [6] To construct a tandem cell, the two complementary absorber layers must be stacked optically and electrically ( Figure 1b). The interconnecting layer (ICL) between the two subcells must pass light and sustain the photocurrent by providing an optically transparent electrical contact for recombination of electrons and holes from the adjacent photoactive layers. The Fermi level of the hole-transporting layer (HTL) and the electron-transporting layer (ETL) that jointly form the ICL must match the relevant highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels in the adjacent photoactive layers (Figure 1b). The ICL should not cause voltage losses and have low resistance. The open-circuit voltage (V OC ) of the tandem solar cell is ideally the sum of the V OC s of the subcells and the photocurrent is limited by the subcell generating less current. To overcome the intrinsic performance limits of single-junction cells, the subcells should absorb complementary regions of the solar spectrum and generate equal photocurrent.In the first organic tandem solar cells, materials were thermally evaporated. Initially only metal clusters were used to interconnect the subcells, [7][8][9] later complemented by p-and n-doped organic transport layers. [10,11] In 2007, the first fully solution-processed tandem polymer solar cells were reported by Gilot et al. [12] and Kim et al. [13] In both examples the ICL featured a layer of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as HTL, stacked on top of either a zinc oxide or a titanium oxide layer as ETL. Kim et al. achieved a PCE of 6.5%. Since then PCEs have steadily increased. Major improvements involved the use of more efficient photoactive blends that afford a high V OC relative to their optical bandgap The efficiency of organic solar cells can benefit from multijunction device architectures, in which energy losses are substantially reduced. Herein, recent developments in the field of solution-processed multijunction organic solar cells are described. Recently, various strategies have been investigated and implemented to improve the performance of these device...

Solution‐Processed Tin Oxide‐PEDOT:PSS Interconnecting Layers for Efficient Inverted and Conventional Tandem Polymer Solar Cells

“…Such solutions can be conveniently adopted in high‐throughput production systems like roll‐to‐roll or slot‐die coating . Aiming at further improving the efficiency, solution‐processed multi‐junction solar cells have been reported, featuring high efficiencies . An outstanding 17.3% efficiency has been recently reported for a tandem polymer solar cell entirely processed from solution (apart from the top electrode), thanks to the development of high performing combinations of photoactive materials .…”

A Cross‐Linked Interconnecting Layer Enabling Reliable and Reproducible Solution‐Processing of Organic Tandem Solar Cells

“…Generally, the ICL in solution‐processed tandem OSCs consists of a hole‐transporting layer (HTL) and an electron‐transporting layer (ETL). A qualified ICL should simultaneously possess the following requirements: (1) high transparency to minimize the parasitic optical losses; (2) efficient and balanced charge extraction from sub‐cells followed by efficient charge recombination at the inner interface of the ICL; (3) satisfactory robustness to protect the underlying active layer against the penetration of solvents during solution‐processing upper layers 16–18…”

Incorporation of Hydrogen Molybdenum Bronze in Solution‐Processed Interconnecting Layer for Efficient Nonfullerene Tandem Organic Solar Cells

“…Building tandem structure is an effective strategy to further improve the PCE. Thermalization loss can be reduced in tandem structure where a wide‐bandgap junction produces high open‐circuit voltage ( V OC ) and low‐bandgap junction extends the photoresponsive spectral region for improved light harvest . Tandem OSCs with PCE of 17.3% have been reported by Meng et al that stimulates the inspiring future for tandem OSCs with higher efficiency.…”

“…In inverted organic tandem solar cells, PEDOT:PSS/ZnO is a widely used ICL in fullerene tandem solar cells . PEDOT:PSS is poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate).…”