Recombination layers

Next to efficient active layer materials, the other critical component of a tandem solar cell is the intermediate eontact layer or recombination layer. This layer is responsible for the eleetrieal eonnection of the subcells and the effective operation of the deviee. There is no current generation in this area and 

The next step was the use of solution processed charge transport layers in the recombination contact of polymer solar cells. In 2007, Gilot et al showed that ZnO nanoparticles processed from acetone can be covered with a layer of pH-neutral PEDOTrPSS spincoated from a dispersion in water to fabricate a ZnO/PEDOT PSS recombination layer that is transparent to light and makes an almost loss-less recombination layer for tandem and triple junction cells. This approach bears similarity to the p- and n-doped wide bandgap transport layers developed for evaporated small molecule tandem cells. The ZnO/PEDOT PSS recombination contact was initially employed in 

Several excellent reviews exist on polymer tandem solar cells and we refer to these publications for a comprehensive overview. In this chapter we focus on the materials requirements for creating efficient polymer solar cells and intermediate contact layers. In the following paragraphs, we will outline the operational principles in more detail. We will then review the most important photoactive materials used in polymer multi-junction cells and outline the material combinations that can used for the recombination layer. We conclude with an overview of recent achievements that have pushed the efficiency to well over 10% and address the progress in processing of large area tandem polymer solar cells. 

2 Optimization and Characterization of Multi-Junction Polymer Solar Cells 

Next to optical interference there is also electrical interference. In a series-connected tandem solar cell the conservation of charge dictates that the photocurrent through the device must be constant and that the voltages of the subcells add up to give the voltage of the tandem device this is represented for current density and voltage by the relations  

The starting point is to determine the J-V characteristics of the two single junction cells as a function of layer thickness and the optical constants [n, k) of the photoactive layers. Using optical modeling of the layer stack of the single junctions, it is then possible to calculate the IQE as the ratio 

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A thin recombination layer is employed between a substrate and a photodetective layer in the imager of JP-A-61067958. The lattice constant of the recombination layer is different from the lattice constant of the substrate and the photodetective layer. To prevent misfit dislocation, the thickness of the recombination layer is chosen to be thinner than a critical value. 

In the inverted region the recombination layer is first extended and then shifted from the contact as AGr increases (Fig. 3.5). The bell-shaped Wr t) can be approximated by the rectangular model of the layer (3.55), whose inner border is r, = / A and external border is r, R I A (Fig. 3.6). In highly polar solvents... 

The remarkable feature of this phenomenon is that the quantum yield does not depend on the initial separation, as long as the latter is less than the inner radius of the remote recombination layer r,-. This layer screens the ions that start from its interior. The smaller is the separation quantum yield, the faster is the recombination inside the layer. However, the survival probability of ions sharply increases when the starting point is shifted outside (Fig. 3.27). 

Figure 3.27. The separation quantum yield (survival probability at t = oo) as a function of the initial distance between the ions for D = 1CT5 cm2/s and three recombination rates (from top to bottom), Wo = 10,100,1000 ns-1 (a = 5 A, = 10A,L = 5 A). At the top (a) the start from inside the recombination layer related to the left, horizontal branches of the curves (b) the outside start related to the right branches approaching the maximum tp = 1 at ro —> oo. (From Refs. 32 and 158.)...

When diffusion becomes slower, the residence time in the recombination layer increases and the layer becomes nontransparent for particles that start from inside that is, for inner starts Z universally decreases with diffusion [Fig. 3.26(h)] ... 

Here z = WqLR is identical to the EM parameter provided Wo = k-el. Z(D) behavior for inner starts is qualitatively similar for contact starts from the exponential reaction zone. The noncontact starts from the exponential reaction zone are also similar to the starts from inside the rectangular recombination zone. Only for starts outside this zone, Z(D) is a monotonically increasing function of the diffusion coefficient [Fig. 3.26(h)]. For thin layers (qL << 1) this dependence coincides with that predicted by the CA Eq. (3.209). This is a consequence of the artificially sharp borders of the rectangular recombination layer. Ions that are born outside do not react in principle unless diffusion delivers them into the reaction zone. 

This is the reason for a qualitatively different diffusional dependence of Z for ions that are born in and out of the recombination layer. However, the initial separation of ions also depends on the diffusion when it controls the ionization. Therefore ro cannot be the same for all viscosities, as was implied above. Moreover, the initial distribution of ions is far from being similar to that in Eq. (3.202) and has to be specified separately. All this will be done in the next section. 

In general such distributions are neither contact nor infinitely thin, as in Eq. (3.202), and their subsequent evolution in the course of geminate recombination can be essentially different. In particular, when the backward transfer occurs in the remote recombination layer (see Fig. 3.27), the starting positions of ions can be either inside or outside it and their separation crucially depends on this initial distance. The initial distribution (3.299) allows one to specify the fractions of ions that are in and out and estimate their true contribution to the total charge separation yield. 

Figure 3.39. The viscosity dependence of the recombination efficiency Z at fixed starts from the interior of the remote rectangular recombination layer (thin solid line), from inside of this layer (long dashed and dashed lines) and outside of it (dotted line). The true Z(D) dependence, calculated from cp by means of IET, is indicated by the thick solid line. The horizontal line represent Z = 0.0585 A2/ns. The parameters, defining the rectangular recombination layer are the following / = 1.1 a, re — 1.4a, Wo — 0.156ns-1, a — 10A. (From Ref. 151.)...

Since the recombination rate was found to be more extended than the ionization one (lR > If), there is no surprise that the curve Z( >) goes through the maximum as the upper curve in Figure 3.41. This effect was attributed in Section VII.B to extension of the recombination layer due to the high exergonicity of recombination. Although this factor is absent when the model of exponential rates is used, lR can be stretched instead to reach the same effect ... 

Rothe C, A1 Attar HA, Monkman AP (2005) Absolute measurements of the triplet-triplet annihilation rate and the charge-carrier recombination layer thickness in... 

A small microcavity effect, as seen in Sec. 2, is necessary for maintaining a good emission of the device. For that purpose, internal reflections Ranode and Rcathode must not be reduced to zero, and the organic layers inside the OLED act as cavity layers, so that the position of the emitting layer (the thin recombination layer) must be at a resonance peak of the electric field. 

Both particles, electron and hole—coming from the different electrodes—move from opposite directions towards the recombination layer. There they can combine and form excitons. This may happen near to the layer interface, on matrix molecules within the layer, and/or at doped emitter molecules. In suitable cases, as required for OLEDs, this leads to a population of excited states of the emitter material which subsequently emits light. Obviously, this process should occur with high efficiency. Details of the mechanism of exciton formation and population processes of excited emitter states are discussed in the next section. 

Application of direct current voltage induces injection of electrons and holes at cathode and anode producing radical anions and radical cations, respectively, and their recombination excites the EL compound. The injected charges are transported by hopping between the carrier compounds at a high voltage of 10 V cm . The carrier mobility is not high, around 10 10 cm V s , but the time needed to reach the recombination layer is very short, about 10 s, because of the thin layer of around 100 nm, so that the response of the device is very quick [113]. 

A typical organic tandem cell consists of two subcells stacked on top of each other, each of which has different absorption range (Figure 12.8). The two subcells are connected in series via a thin recombination layer. Hence, better coverage of the solar spectrum and reduction of the thermalization loss can be realized due to the use of materials with different bandgaps. The total thickness of the active films in... 

Figure 11.2 Schematic representations of the regular and inverted device layouts for series-connected organic tandem cells. Under operation the indium tin oxide (ITO) electrode is biased positive with respect to the metal electrode in the regular configuration and negative in the inverted configuration. The difference is caused hy the positioning of the hole and electron collecting layers (HCL and ECL) with respect to photoactive layers. The intermediate contact formed hy HCL and ECL is referred to as the recombination layer. Note that in practice the device layout may vary from these schematics.

As can be seen in Table 11.1 there is a large variety of recombination layers. Typical materials used as n-type layers are solution-processed metal oxides such as ZnO nanoparticles, Li doped ZnO, ° and sol-gel TiO. . These metal oxides are sometimes functionalized with Cgo self-assembled monolayers (Cgo-SAMs) ° or PFN (Figure 11.10) to improve hole blocking. 

Figure 11.12 Device structure of the polymer tandem solar cells with PEDOT PSS (PHIOOO) modified by PEIE as the recombination layer. Reproduced from ref. 67.

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