The absorption of sunlight in a molecular organic semiconductor results in the formation of molecular excitons, and in accordance with the dipole process and the discussion in Sections 3.6 and 3.7 the ground-state singlet is excited into an excited singlet molecular exciton. This exciton is localized to a single molecule, which is generally on the nanometer length scale. Unless the exciton can be dissociated and its hole and electron extracted no current can result. In contrast to this, photon absorption in inorganic semiconductors used in solar cells results in separated holes and electrons that are free to flow independently of each other and hence directly contribute to current flow.
A key challenge in the development of organic solar cells is to overcome the localization and pairing in the form of excitons of optically generated holes and electrons. Once dissociated, charges can flow from molecule to molecule by a hopping process. Materials and device architectures designed to facilitate exciton dissociation are the key to successful organic solar cells.
The simplest organic solar cell structure is the single-layer device shown in Figure 6.28. Photons create molecular excitons in the organic semiconductor layer. The device relies on the differing workfunctions between cathode and anode to generate an electric field high enough to collect these charges. The diffusion length of carriers is generally on the order of 10 nm and the thickness of the semiconductor layer is much more than 10 nm to achieve a reasonable degree of photon absorption. The majority of the generated excitons are never dissociated and only a small fraction of generated carriers are collected by the electrodes. In fact the electric field is only high right at the abrupt electrode-semiconductor interfaces, and this is where exciton dissociation can most readily occur. Efficient carrier collection, however, requires the participation of both carrier types, which is not favoured by this approach. The energy level diagram for this structure is shown in Figure 6.29.
Figure 6.27 Phosphorescent iridium-based emitters: Red – Ir(thpy)3, Green-Ir(ppy)3, Blue – Ir(ppz)3. Chemical Structure reproduced from Organic light-emitting materials and devices, ed by Z. Li and EL. Meng 9781574445749 (2007) Taylor and Francis
Figure 6.28 Single-layer organic solar cell consisting of a single organic semiconductor layer, a low workfunction cathode and a transparent anode. Device efficiency is well below 1%
A substantial increase in the collection of charge may be accomplished by the planar heterojunction solar cell, which is shown in Figure 6.30. Its energy band diagram is shown in Figure 6.31. The introduction of an electron transport acceptor layer (ETL) and a hole transport donor layer (HTL) creates a strong electric field at the heterojunction interface that greatly enhances exciton dissociation there. In principle both the donor and acceptor layers can absorb photons and become populated with excitons. These excitons can then diffuse towards the heterojunction interface and dissociate there.
The sharp and narrow high field region at the heterojunction interface in organic solar cells may be contrasted with the inorganic p-n junction. The width of the depletion region in inorganic junctions is determined by the spatial extent over which carriers recombine to establish an equilibrium condition. At the organic interface in organic junctions, charge carriers in the HOMO and LUMO levels transfer from molecule to molecule by hopping and only minimal charge transfer occurs, leaving the equivalent of a depletion region of very small thickness. The potential difference between the HOMO and LUMO levels falls across a very small spatial range of dimension in the nanometer scale giving rise to a high electric field at the junction.
In practice, however, the donor layer is specifically designed to absorb photons and become populated with excitons. Since the mobility of holes is relatively higher than electrons, as we saw in Section 6.10, the donor HTL allows the holes to diffuse towards the heterojunction interface. The electrons will remain bound to the holes since exciton
Figure 6.29 Energy level diagram for single-layer organic solar cell. The absorption of light creates excitons through the promotion of molecular electrons from the HOMO level to the LUMO level. Electrode workfunctions are set to match the HOMO and LUMO levels to facilitate the collection of the electrons and holes as shown
Figure 6.30 Organic planar heterojunction solar cell structure showing donor and acceptor organic layers
dissociation will not readily take place until the excitons reach the interface. This interface now enables the collection of both electrons and holes, which drift across their respective layers: Holes reach the ITO electrode through the hole-conducting layer, and electrons reach the cathode through the electron-conducting layer.
The terminology ‘donor’ and ‘acceptor’ used to describe the two layers forming the heterojunction comes about since electrons that are dissociated from the excitons in the donor layer at the junction are transferred or donated across this junction from donor molecules and accepted by acceptor molecules in the acceptor layer. The holes from the dissociated excitons remain in the donor layer and drift to the anode. The terminology is a molecular analogue of the terms ‘donor’ and ‘acceptor’ applied to dopants used in inorganic semiconductors; however, the organic molecules donate and accept electrons to/from neighbouring molecules rather than to/from energy bands. Since the acceptor layer becomes populated with electrons this layer needs to be an electron conductor and charge
Figure 6.31 Heterojunction solar cell showing donor and acceptor LUMO and HOMO levels. Excitons are generated throughout the donor layer and these excitons are dissociated when they diffuse to the donor-acceptor interface. Finally the separated holes and electrons can drift to their respective electrodes
is carried in its LUMO level. Conversely the donor layer, being populated with holes, needs to be a hole conductor and these holes are carried in its HOMO level.
The thickness of the p-type donor layer is controlled by the diffusion length of the excitons which must reach the interface to be dissociated. A donor layer that is too thick will lower efficiency since a significant fraction of the generated excitons will recombine before they can reach the interface. A layer that is too thin will result in less absorption of light. A solar cell efficiency of only a few percent is achievable with the planar heterojunction design.
Since diffusion lengths in organic materials are approximately 10 nm, the useful absorption depth in the donor layer is only about 10 nm, which means that incomplete absorption of sunlight limits the performance of the heterojunction solar cell. The thickness for virtually complete absorption of sunlight is closer to 100 nm in organic materials; however, a donor layer of this thickness would result in poor efficiency and most generated excitons would recombine without reaching the interface.
A successful approach to improving performance further is to arrange several interfaces within the light path of the incoming sunlight, and to make each donor layer thin enough to allow effective exciton diffusion to the nearest heterojunction interface. A portion of sunlight is absorbed in each thin donor layer and the remaining light can then continue to a subsequent layer. This approach relies on a bulk heterojunction layer that incorporates multiple donor and acceptor regions. The device structure is as shown in Figure 6.32.
The bulk heterojunction layer can be formed using a variety of nanostructures, and the development of techniques and materials for the achievement of these nanostructures has been a focal point in further improving organic solar cells. Bulk heterojunction solar cells have attained the highest efficiency levels available in organic solar cells that use a single organic absorption band to absorb light.
The length scale of the desired nanostructures is in the nanometer range, and it is very desirable to use self-organization of the organic materials to achieve a low-cost method to create these nanostructures. For example, the donor and acceptor organic materials can be mixed together and then deposited onto the solar cell substrate. If the mixed material segregates spontaneously under suitable conditions to form the desired bulk heterojunction then self-organization has been achieved. This dramatically lowers the cost of processing since submicron lithography and patterning techniques are avoided. Although these techniques are well known and highly developed for inorganic semiconductor device processing they are not cost effective for large-area solar cells.
Figure6.32 Bulk heterojunction organic solar cell. A number of small (~10nm) donor regions are organized within the bulk heterojunction layer and optimized to absorb sunlight and allow exciton diffusion to a nearby junction
An important requirement of bulk heterojunctions is to provide for the effective conduction of current away from the donor and acceptor regions and for the collection of this current by the electrodes. Two specific examples of heterostructures are shown in Figure 6.33. Figure 6.33a shows a morphology that limits the effectiveness of current collection because the donor and acceptor layers are not well connected to the electrodes. This is not an accurate representation of the morphology in real systems, but serves for illustration purposes. Figure 6.33b shows a more desirable structure since the donor and acceptor materials are arranged to allow for effective connection to the electrodes. The achievement of organic layers that self-organize into optimal structures at low cost is an area of ongoing research.
Another important area of current development is the use of multilayer organic solar cells in which various layers act to absorb different portions of the solar spectrum in a manner analogous to the inorganic multiple junction solar cells described in Sections 4.11 and 4.13. This is particularly important for organic solar cells since the absorption bandwidth of a given organic material is small. This is because the n and n * bands are much narrower than the conduction and valence bands in inorganic semiconductors, which limits the absorption bandwidth.
The heterojunction structures we have discussed are capable of providing two absorption bands if excitons can be generated and harvested in both the donor and acceptor materials. The energy gaps of these two layers can be different and two absorption bands can be realized. A challenge associated with this is achieving high enough diffusion lengths in both the donor and acceptor layers and collecting carriers effectively. A single organic material having all the attributes needed specifically for an ideal acceptor, including good electron mobility, high optical absorption, and effective electron capture from the donor material, has not yet been found. Nevertheless numerous organic material blends and mixtures are being investigated to obtain multiple absorption bands in organic solar cell structures.
To date, efficiencies as high as 7-8% have been reached in the laboratory for organic solar cells. The attributes of low cost, low weight and flexibility are the key drivers behind this development, although the available materials ultimately control the achievement of high performance. A few popular organic materials will now be reviewed.