Device Physics and Device Structure

The HSCs work in the same way as OSCs, where the conjugated polymers serve as light absorber and electron donor (D), the inorganic nanocrystals serve as electron acceptor (A). Figure 9.1 gives the schematic illustration of the energy level alignment and the photocurrent generation mechanism in HSCs. Upon illumina­tion, the photocurrent is generated via the following processes [36, 38]:

(1) Photo absorption and exciton generation. The conjugated polymers mainly account for the light absorption in HSCs. In some cases, the inorganic nano­crystals could also absorb light, but the majority of the light absorption are attributed to the conjugated polymers. The light harvesting efficiency (gLHE) is determined by the bandgap (Eg) and the absorption coefficient of the polymers. After absorbing the incident photons, electrons will be excited from the highest occupied molecular orbit (HOMO) to the lowest unoccupied molecular orbit (LUMO). Due to the low dielectric constant of the polymers, the elec­trons in LUMO and the holes in HOMO are not free charges but excitons with strong Coulomb interaction. The exciton binding energy (Eb) is typically

Fig. 9.1 Schematic illustration of the energy levels alignment and photocurrent generation mechanism in hybrid solar cells. Reproduced with permission from Ref. [36]

0.2-0.5 eV. Therefore, the exciton generation efficiency (gg), which is regarded as the possibility of generating one exciton by one photon, is critical to the device performance. In addition, the excitons have a good chance to recombine [38].

(2) Exciton diffusion. Before separating into free charges, the excitons have to diffuse to the D/A interfaces. The exciton diffusion efficiency (gdiff) depends on how much the excitons could successfully diffuse to the D/A interface before recombination takes place. Noted that the exciton diffusion length is 4-20 nm for most conjugated polymers [36-38], the D/A domains in HSCs should also be in this range for high exciton diffusion efficiency.

(3) Exciton dissociation. Once reaching the D/A interface, excitons could be dissociated into free electrons and holes if the energy offset between the LUMO of the polymer and the conduction band (CB) of the inorganic nano­crystals overcomes the binding energy of the excitons; therefore, the relative positions of the LUMO of the Donors and the CB level of the inorganic acceptors determine the exciton dissociation efficiency (gd).

(4) Charge transfer and collection. After the exciton dissociation, the free elec­trons and holes need to transfer through the inorganic nanocrystals and con­jugated polymers until they are collected at the electrodes. The charge transfer efficiency (gtr) is related to the intrinsic properties of the materials, e. g., the carrier mobility, the crystallinity and the purity, and so on. Besides, continuous pathways are also needed for efficient charge transfer. While the charge collection efficiency (gcc) mainly depends on the energy level alignments of the active layer and the electrodes as well as the contact between them.

Therefore, the external quantum efficiency (EQE) of a HSC could be calculated

through the following equation [38]:

Fig. 9.2 Spectral photon flux density of the AM 1.5 G solar irradiation

Then the short circuit current (Jsc) could be obtained as [39]:

kmax

Jsc = q J U(k)EQE(k)dk (9.2)

kmin

The Ф(к) is spectral photon flux (see Fig. 9.2).

The theoretical maximum open circuit voltage (Voc, max) is determined by the energy level difference between the CB of the inorganic acceptor and the HOMO of the polymer donor, i. e., [38]

eVoc, max = |Ehomo, d I — |Ecb, a| (9 з)

=Eg + |£lumo, d| — |Ecb, a|

For a given inorganic material with fixed conduction band, the photovoltaic performance of HSC is mainly determined by the Eg and LUMO of the conjugated polymer and this could be understood from the views of Jsc and Voc. On one hand, the polymer bandgap (Eg) should be as narrow as possible to absorb as much light as it can to generate more photocurrent and the LUMO of the polymer should lies at least Eb higher than the CB of the inorganic acceptor for efficient exciton dissociation. On the other hand, the energy difference between the CB of the inorganic crystal and the HOMO of the polymer should be as large as possible for high Voc as described in Eq. 9.3. Due to the two contradictory requirements for high Jsc and Voc, the Eg and the LUMO level of the conjugated polymer should be balanced in a real HSC for optimized device performance.

Usually, the HSCs show a similar structure with OSCs, i. e., an active layer consisting of conjugated polymers and inorganic nanocrystals sandwiched by two electrodes with different work functions. According to the different morphologies of the active layer, the architectures of HSCs can be classified as the following three types: (1) bilayer heterojunction, (2) bulk heterojunction, and (3) ordered heterojunction, as shown in Fig. 9.3.

Fig. 9.3 Three types of device configuration of hybrid solar cells: a bilayer heterojunction, b bulk heterojunction, c ordered heterojunction. Reproduced with permission from Ref. [38]

The bilayer heterojunction HSCs can be easily fabricated by consecutive depositing the acceptor and donor layers on the substrate; however, the efficiencies of bilayer heterojunction HSCs are typically much lower than that of BHJ HSCs [40, 41], due to the reduced D/A interface area and the dilemma of efficient light harvesting and exciton diffusion [42].

Similar problems have also been encountered by the organic bilayer hetero­junction solar cells and well solved by the design of bulk heterojunction, which is a three dimensional interpenetrating network of the donor and acceptor materials. Learning from that, the concept of BHJ could also be applied into HSCs for improving their performance. The BHJ HSCs can be fabricated through: (1) infil­trating of polymers into inorganic network, (2) simultaneous deposition of the blend of polymer and inorganic nanocrystals, and (3) in situ growth of one material into another.

If the BHJ is formed in ordered inorganic nanostructures, e. g., ordered nanorod, nanowire, and nanotube arrays, it is also called ordered heterojunction. It is believed to be the ideal morphology for HSCs, for the direct charge passway and tunable D/A domain size, both of which are crucial for efficient charge dissociation and transport [37].