While organic semiconductors have been shown to form efficient devices, including commercialized light-emitting diodes, and thin film transistors and solar cells nearing the market, a fundamental understanding of the physical processes that contribute to efficient mobile carrier generation and transport remains under debate. Popular perceptions of the physical processes, which contribute
to efficient photovoltaic current generation in BHJ solar cells, are as follows.
Photons incident upon a “plastic” solar cell initiate a series of optical and electrical processes ultimately resulting in the generation of charge in the external circuit. A photon absorbed by a polymer chain produces an excitation along the conjugated backbone.4 Figure 9.1 outlines the time evolution of the photoexcitation and resulting charge carriers induced in a BHJ thin film in response to a single ultrafast pulse of light. Processes that have been observed by experiment are identified along with their rough time scale, and the quasiparticle/particle evolution highlights the commonly used physical picture of current generation in BHJ solar cells. This includes
(1) absorption of a photon resulting in a primary photoexcitation,
(2) relaxation of the primary excitation to a singlet exciton, (3) exciton diffusion to a donor-acceptor interface, (4) charge transfer to create mobile electrons and holes via the charge transfer state, (5) charge transport to the electrodes and charge collection, (6) residual presence of deep-level trapped carriers. Studies of loss processes for each of these steps, charge generation, charge transport, and charge collection, produce conflicting evidence on the role of excitons and charge transfer (CT) states, in particular, in efficient photovoltaic conversion.
Debate about the nature and evolution of the primary photoexcitation in conjugated polymers is ongoing.48-12 There are two main schools of thought. On the one hand, conjugated polymers have been treated as one-dimensional semiconductors, where mobile electrons and holes are directly photogenerated in energy bands and eventually localize into a bound exciton state.4813 On the other hand, evidence exists for a more molecular approach, where the primary photoexcitation is already a bound exciton.91415 The limit between the two cases depends to some extent on both disorder and on the time scale at which the photoexcitation is observed. Thus, disorder in the conjugated polymer chain determines the delocalization of the excited state and hence its more semiconductor-like or more molecular-like behavior. In highly ordered polydiacetylene quantum wires, quantum coherence reaching tens of micrometers has been observed (macroscopic delocalization of the excited state).16 In most cases, disorder in the dissolved or solid-state materials prevents the delocalization of the excited state over the entire chain and breaks it into much smaller, more molecular-like chromophores of variable size. Those can, however, couple together at short time delays (<200 fs), giving rise to collective delocalized states.1718 The primary photoexcitation observed directly after light absorption is therefore much more delocalized than the excited state species (exciton) formed following a series of complex relaxation processes.3417-33
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Elapsed time from excitation (s)
Figure 9.1 Species density temporal evolution in a BHJ thin film in response
to a single ultrafast pulse of light with line width ~10 fs.
In the BHJ, CS at the donor-acceptor interface splits the excitation into a domain-segregated electron and hole. Many time – resolved studies have shown that the CS to the fullerene materials is nearly quantitative and ultrafast (<100 fs for many optimized systems).534-43 Based on relaxation time scales found during time – resolved fluorescence studies on pristine polymers, the ultrafast CS can compete with primary excitation relaxation processes such as excited-state localization and exciton diffusion.21920
Moreover, CS on the order of 100 fs (or less) limits exciton diffusion to the 0.1-0.2 nm length scale, according to published exciton diffusion coefficients.64445 However, nanoscale phase separation746-51 is known to be necessary for efficient BHJ photovoltaics,3’4’52 so that there is a discrepancy between the distance that an exciton can diffuse and the distance that it needs to diffuse in order to ensure ultrafast CS at the fullerene interface. We will discuss alternative mechanisms in which the primary photoexcitation, which is distinctively different from the bound exciton since it is more delocalized, reaches an interface and undergoes CS.
A body of research puts forward that all generation of free charge carriers passes through intermediate charge transfer (CT) states, in which electrons and holes are still coulombically bound. They are responsible for most geminate recombination events—a note: geminate recombination refers to recombination of charge-separated carriers that originally stem from the same photoexcitation. CT states, distinct electronic states existing at the interface between donor and acceptor materials and formed by the dissociation of the photoexcitation, are known in literature as a loss mechanism for polymer : polymer and polymer : small-molecule blends. CT states can be optically or electrically populated, and emission from these states can be measured via photoluminescence or electroluminescence, respectively. However, CT state emission has been measured, in contactless photo – and contacted electroluminescence experiments, to be orders of magnitude lower for blends, which produce efficient solar cells than for lossy BHJ blends. In other words, CT states seem unlikely to intervene in charge dissociation for efficient BHJ solar cells. Electroluminescence experiments on their own do not rule out the possibility that some or all charges pass through these states. However, the observation of free charge carriers on the hundreds of femtosecond time scale in efficient BHJ blends makes the probability that a majority of carriers pass through a bottleneck electronic state much less likely. It has recently been put forward that dissociation of the excited state at the D/A interface leads directly to free charges via delocalized states, so that trapping in the relaxed CT state is only a minor loss process.53 The exact scope and magnitude of the role CT states play in the operation of an efficient BHJ solar cell remains a topic of intense debate and research.
A large fraction of the incident light absorbed in the BHJ in optimized devices is converted to photoinduced charge carriers that are collected as current in an external circuit—a process shown to be greater than 90% efficient in at least one of the donor-acceptor systems. Photogenerated current is reduced by internal recombination loss prior to charge collection at the electrodes. Competition between carrier sweep-out by the internal field and the loss of photogenerated carriers by recombination is one of the key issues to overcome for high-efficiency devices. Due to the relatively low charge-carrier mobility in common BHJ materials, the PCE does not scale linearly with the thickness of the active layer in polymer BHJ solar cells. As the active layer thickness increases, the path length of photogenerated electrons and holes to the electrodes increases. Thin devices can be made with almost 100% conversion efficiency of absorbed photons to electrons collected in the external circuit.454 However, increasing the thickness of the active layer in these devices typically does not increase overall efficiency.4’8-12’55’56 The solar cell active layer thickness, which maximizes PCE, is usually smaller than necessary for optimal light harvesting. The short-circuit current increases moderately (nonlinearly) with increasing active layer thicknesses,57 but often, the open-circuit voltage is reduced and the fill factor drops dramatically. While a larger fraction of the incident photons are absorbed in thick film devices, the resulting photogenerated charge carriers are not efficiently extracted into the external circuit.
Recombination of free charge carriers, which unlike CS processes is dependent on the total thickness of the film, becomes the dominant loss mechanism as the active layer thickness increases, limiting short – circuit current, open-circuit voltage, and fill factor. Unfortunately, as researchers seek to increase the efficiency of polymer photovoltaics and companies begin the process of up-scaling this technology, increasing the thickness of the active layer of the BHJ becomes necessary due to the minimum film thicknesses that can be reproducibly fabricated from scalable printing processes (roll-to – roll printing, etc.).55,58
Recombination mechanisms are best classified by rate, as this is what principally distinguishes them experimentally. Geminate recombination refers to recombination of species that originate from the same absorbed photon and is thus a monomolecular process. This encompasses the recombination of the primary photo-excitation, of the exciton before the quasi-particle charge separates and of bound electron-hole pairs in the CT state. Geminate recombination rate is predicted to be proportional to the population of bound electron-hole pairs (in the exciton or CT state), X, by decay rate, kf:
Ro=kf X (9.1)
Trap-based recombination describes recombination through a trap state located within the energy gap of a material. In inorganic materials, where charge transport properties originate from the ordering of atoms in a lattice, trap states originate from interruptions in the ordered lattice. In organic materials, charge transport properties arise from inter- and intramolecular ordering. Trap states can include electrically active impurities, intramolecular energetically localized states, disordered energetic states at the interface between two molecules, or at the interface between two materials. In the case of BHJ, where majority charge-carrier populations are spatially separated by BHJ phase separation, recombination likely occurs in traps at the interface between the two phase-separated regions. The rate at which this trap is filled is assumed to be much faster than the recombination rate, such that an immobile population of trapped charge exists with which mobile carriers of the opposite sign may recombine. Equation (9.2) describes the trap-based recombination rate through a density of single-occupancy electron traps, nt, where ne t is the density of occupied traps, (nt – net) is the density of unoccupied states available for electron occupation, and kte(h) is the rate of electron and hole capture:
Й1 = kt, e«eOt – n, t} + kt, hnhe, t (9.2)
Bimolecular recombination describes interband recombination of an electron and hole (separated or in an excited state) which do not originate from the same photon. In the case of the BHJ, where majority charge-carrier populations are spatially separated by BHJ phase separation, recombination likely occurs at the interface between the two phase-separated regions. Because this recombination process depends on the spatial localization of a mobile electron and hole, the recombination rate is proportional to the mobile electron-hole product minus contribution from the intrinsic carrier density of both charge species (usually negligible).
The decay rate for bimolecular recombination in organic BHJ solar cells has been found to be related to the Langevin recombination rate, у= едят / £0^eff,
R> = 7(«e«h – n2) (9.3)
In an Auger recombination event, an excited state decaying to the ground state transfers its kinetic energy to a neighboring excited state, visualized in Fig. 9.2d as an electron-electron interaction with some decay rate, kAe(h). The kinetically excited state usually relaxes back to its initial excited state via phonon interactions.
R3= kA, ene(nenh – n2) + kA, hn(nenh – n2) (9.4)
Because of Auger recombination’s third-order dependence on charge density, high charge densities are needed to observe Auger recombination, and it has not yet been observed in organic BHJ solar cells.
In the simplest cases of these recombination mechanisms, where the recombination rate is relatively low and ne ~ nh, ne >> n;, and nt >> ne, t, recombination rate equations can be simplified as summarized in Fig. 9.2.
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b. c. d.
Figure 9.2 Recombination mechanisms by rate: (a) Geminate recombination rate proportional to the bound electron-hole density; (b) Trap – based recombination rate in the simplest case proportional to the density of mobile holes; (c) Bimolecular (interband) recombination rate proportional to the mobile electron and hole densities; (d) Auger recombination (transition energy transferred to neighboring particle) rate proportional to mobile holes, and square of mobile electrons. |
Charge recombination has been studied via a variety of experimental methods, including time-of-flight (TOF),59 steady – state current-voltage,60-63 impedance spectroscopy,64-66
photoinduced charge extraction by linearly increasing voltage
(photo-CELIV),67 double injection currents,68 and transient absorption and transient photovoltage measurements.6970 At applied voltages above the maximum power point and at the open-circuit condition, bimolecular recombination reduces the current density and limits the fill factor, thereby decreasing PCE.60,70 Determining primary recombination mechanisms and their physical origins is therefore essential to the future of organic BHJ solar cells.
In addition to processes internal to the BHJ materials, extraction of charge into an external circuit depends on one’s ability to contact organic materials and draw charge out of the device. Metals including aluminum, silver, and gold are commonly used to contact the devices, as well as transparent conducting oxides, conducting polymers, carbon nanotubes, and graphene. Contact interlayers have been shown to be necessary as a buffer layer between the anode and cathode and the active layer of the device, the BHJ. These interlayers are thought to be necessary for a number of reasons. First, interlayers are necessary to “break the symmetry” of the device, or to create selectivity for primarily electron collection at the electron contact and primarily hole collection at the hole contact. Interlayers play a similar role to prevent injection of the opposite carrier. Common interlayer materials, to date, include semiconducting polymers like PEDOT:PSS, thin layers of metals with high electron affinity such as calcium and lithium fluoride, polymer polyelectrolytes, self-assembled monolayers, and transparent metal oxides including zinc oxide, molybdenum oxide, nickel oxide, and titanium oxide. Many of the metal oxides are additionally promising because of their stabilizing role in extending device lifetimes from seconds or minutes to months and years. Researchers are in the process of developing a rubric for ideal contacts, though this problem is much more difficult than it may seem (for further information, see Chapter 4).
Improvements in PCE in the field of organic photovoltaics are usually the result of entirely new materials, each with new requirements for contact layers and interlayers. Nevertheless, we will discuss here the materials parameters, which current research has shown to be critical to charge extraction: work function and energy band alignment, charge selectivity, interface defect density, transparency, and interface chemical interactions, and explore case studies in which new BHJ materials have necessitated specific contacts.
