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Tandem Solar Cells

Thue Trofod Larsen-Olsen

One of the factors that limit the efficiencies of polymer solar cells is the narrow absorption bands of the organic semiconductors constituting the active layers. This issue is well illustrated in figure 1a, where the absorption spectra of two bulk heterojunctions (BHJs) based on typical donor polymers mixed with the acceptor PCBM, is compared with the solar irradiance spectrum. P3HT absorption sets in around 660 nm, which corresponds to a band-gap of around 1.9 eV (using the practical conversion factor of 1240 eV nm); while for MH306 (a specific low band gap polymer developed at DTU) the band-gap is around 1.5 eV, read more about low band gap polymers in the Materials chapter. From figure 1a it is clear that both the high and low band-gap BHJ will only absorb a small fraction of the available sunlight.

This is where the idea of tandem solar cells comes in; by combining several junctions in one solar cell, the effective absorption window of the solar cell will cover a larger part of the solar spectrum. Such a tandem solar cell stack is shown in figure 1b, where the two BHJs are stacked on top of each other and connected through the recombination layer. Apart from broadening the absorption spectra, tandem cells increase the possible achievable efficiency by minimizing the thermal relaxation loss (thermalisation), which is proportional with the energetic difference between a given absorbed photon and the band-gap of the absorbing semiconductor (see How do polymer solar cells work), “converting” the losses into photovoltage instead DOI:10.1002/pip.1024.

Figure 1. (a) The terrestrial solar irradiance spectra (black) compared with the absorption spectra of two different bulk heterojunctions (BHJs), one using the high band-gap polymer P3HT mixed with PCBM (red) and one with the low band-gap polymer MH306 also mixed with PCBM (green). (b) The principle of a polymer tandem solar cell where two BHJs are stacked on top of each other, each absorbing different parts of the incoming sunlight. The junctions are connected in series through the recombination layer.

The individual BHJs in a tandem solar cell (referred to as sub-junctions) can be connected either in series or in parallel. The simplest configuration in terms of processing is series connection as shown in figure 2a, a device structure which is also referred to as a two-terminal configuration. The parallel connection is shown in figure 2b, and as shown this configuration demands a middle electrode which can be connected to the external circuit and is therefore also known as a three-terminal tandem solar cell. The middle electrode is the tricky thing about the parallel tandem cell, as it is difficult to fabricate a layer which is both transparent and sufficiently conductive to allow extraction of charges laterally without substantial loss, but some have succeededDOI:10.1016/j.orgel.2009.06.010DOI:10.1002/adma.200902750DOI:10.1063/1.3095594. Hadipour et al.DOI:10.1063/1.2786024 went a step further and made a four-terminal device, having an optical spacer electrically separating the two sub-junctions, such that the device could be operated in either series or parallel configuration according to the external circuiting.

Figure 2. (a) the series connected tandem cell, implying only two terminals for connection to an external circuit. (b) The parallel connected tandem cell, where a middle transparent electrode acts as third terminal, allowing for parallel connection to an external circuit. The current-voltage relations between tandem and the individual BHJs (1 and 2) are shown to the left.

As seen in figure 2, the voltage and current of tandem device is derived from Kirchoff’s laws and dictated by the current-voltage characteristics of the individual junctionsDOI:10.1016/j.orgel.2008.03.009. For the series tandem this leads to the following prediction of the series tandem open-circuit voltage: $$V_{oc,\tan }^s = {V_{oc,1}} + {V_{oc,2}}$$ This assumes a good ohmic contact between the individual sub-junctions and the recombination layer, so that all holes from one junction are joined with all electrons from the other. For this reason the tandem $V_{oc}$ becomes an important quality parameter for such a device. Because of charge conservation, the current in the series tandem will be limited by the lowest of the sub-junction currents, to an extent which is determined by the shape of the I-V characteristics of the individual sub-junctions, as explained elsewhere DOI:10.1016/j.solmat.2011.08.025. This generally means that the optimal series connected tandem cell will have to have a balanced current production in all sub-junctions, which in practical terms is a question of matching active materials and active layer thicknesses, a challenge referred to as current-matching. In research, this is often resolved by the use of optical simulationsDOI:10.1016/j.solmat.2006.05.009DOI:10.1039/c3ee40388b. In the case of the parallel tandem the currents are added up instead, leading to the following relation for the short-circuit current: $$I_{sc,\tan }^p = {I_{sc,1}} + {I_{sc,2}}$$ And again analogous with the series tandem, the parallel tandem voltage is limited by the lowest sub-junction voltage to a degree determined by the sub-junction I-V shape.

Generally one can say that whether to choose the series of parallel configuration should depend on which of the sub-junction I-V parameters match the best; if the current matches well - go for series, if the voltage matches best - go for parallel. It can be argued that because the photovoltage is generally less sensitive to e.g. BHJ thickness variations and lighting conditions, it could be advantages in many cases. However, the extra complications regarding the fabrication of a third terminal makes the series connection a popular choice.

Towards roll-to-roll processed polymer tandem solar cells

The last years, polymer tandem solar cells (PTSCs) have seen a surge in interest as single junction device efficiencies have come nearer to the projected efficiency limit of 10-12% DOI:10.1063/1.2181635DOI:10.1002/adma.200501717, while it is projected that PTSCs will be able to reach 15-23% depending on the model asumptions.DOI:10.1002/adma.200702337DOI:10.1002/aenm.201400084DOI:10.3390/ma5101933 This has led to some very impressive efficiency records.DOI:10.1038/ncomms2411DOI:10.1021/ja401434xDOI:10.1002/aenm.201400084 However, the very high efficiencies have still only been achieved on tiny area solar cells, using rigid glass substrates and slow and energy intensive fabrication steps. The challenge is then to transfer these small-scale results to large area, roll-to-roll (R2R) fabrication on flexible plastic substrates, printing or coating all layers.

Figure 3. The general stack of an all printed tandem polymer solar cell. Notice that the tandem solar cell is an ‘inverted’ geometry solar cell as the top electrode is the positive contact.

For solution processed solar cells it is preferable to use the so-called inverted geometry, as this allows the use of a printed silver top electrode. The generalized stack is shown in figure 3, showing the minimum of 8 discreet layers that need to be printed or coated on top of each other. As seen, the recombination layer consists of two layers, a hole transporting layer (HTL) and an electron transporting layer (ETL), whereby the tandem stack becomes a repetition of the stack “ETL/Active layer/HTL” sandwiched between two electrodes. The most widely used recombination layer consists of PEDOT:PSS as HTL and ZnO as ETL. Few reports tries to move towards R2R processed PTSCs. Li et al.DOI:10.1002/aenm.201400084 succeeded in fabricating a 5.56% efficient PTSC on a flexible plastic substrate, using R2R compatible coating, however, the device utilized a pre-fabricated transparent front electrode of indium-tin oxide (ITO), and an evaporated back electrode of MoOx and silver. Larsen-Olsen et al. successfully demonstrated PTSCs fabricated by R2R slot-die coating. However, the efficiencies were limited and the devices also used a pre-fabricated ITO electrode.DOI:10.1016/j.solmat.2011.08.025 The first all-printed and coated PTSCs were reported recently by Andersen et al.DOI:10.1016/j.solmat.2013.07.006, presenting a roll-coated PTSC consisting of 12 layers.

Below you can see a video explaining how polymer tandem solar cells can be fabricated on a mini roll coater:

Challenges related to multilayer solution processing

One of the main challenges when fabricating PTSCs from solution comes from the difficulty of having multiple layers which are soluble in common solvents. This is especially critical for the active layers in the stack, as these layers are generally processed from similar organic solvents such as chlorobenzene or chloroform. For this reason the recombination layer separating the two active layers must function as an effective solvent barrier, as the processing of the 2nd active layer will otherwise destroy or harm the 1st active layer as illustrated in figure 4. In some reports this have been achieved with special formulation of the PEDOT:PSS HTL material used DOI:10.1063/1.3387863DOI:10.1002/adma.201100221. While Andersen et al. combined several types of HTL to achieve an effective solvent barrierDOI:10.1016/j.solmat.2013.07.006.

Figure 4. A tandem stack having a recombination layer with insufficient solvent barrier properties, e.g. due to cracks or coating defects. As shown this will compromise the 1st active layer when processing the 2nd active layer.

The concept of orthogonal processing, where the processing solvent of the 2nd active layer will not solubilize the 1st active layer, has been explored by the group of professor Frederik C. Krebs as another solution to the challenge: Hagemann et al. used a so-called thermocleavable material in the 1st active layer which could be made insoluble after processing, thus allowing for coating of the 2nd active layer DOI:10.1016/j.solmat.2008.05.005. In another report Larsen-Olsen et al. achieved orthogonal processing conditions, by using water as processing solvent for the 2nd active layerDOI:10.1016/j.solmat.2011.08.025.

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