Photovoltaic device energy level

Deep-level states play an important role in solid-state devices through their behavior as recombination centers. For example, deep-level states are tmdesirable when they facilitate electronic transitions that reduce the efficiency of photovoltaic cells. In other cases, the added reaction pathways for electrons result in desired effects. Electroluminescent panels, for example, rely on electronic transitions that result in emission of photons. The energy level of the states caused by introduction of dopants determines the color of the emitted light. Interfacial states are believed to play a key role in electroluminescence, and commercieil development of this technology will hinge on understanding the relationship between fabrication techniques and tile formation of deep-level states. Deep-level states also influence the performance of solid-state varistors. 

In addition to the reqnirements on the energy levels of donor and acceptor for interfacial charge separation, it is necessary that the work function of the electroncollecting electrode shonld be well matched to the LUMO of the acceptor material, and that of the hole-collecting material be well matched to the HOMO of the donor. In bnlk heterojunctions, photovoltaic action can only be achieved if electronic contact is made to the photoactive material nsing snitable asymmetric electrodes. For high efficiency the electrodes shonld also be condnctive and well matched in energy to the electron or hole transport levels. The constraints on electrode materials will be treated in more detail in Sections 7.5 and 7.6.5. The development and optimisation of bilayer and BHJ device types are reviewed in Sections 7.3 and 7.4 respectively. 

Fig. 11.16 a) Schematic of the monolithic combination of a photoelectro-chemical/photovoltaic (PEC/PV) device, b) Idealized energy level diagram for the monolithic PEC/PV photoelectrolysis device. (After ref. [83])... 

It becomes apparent from Equation 19.12a and Equation 19.12b that accurately measuring the internal fields and built-in potential of diodes would not only assist in the understanding of the physics of the device but also permit the gauging of the energy level lineup of the diodes electrodes. EA spectroscopy is a powerful tool as it enables the investigator to do this, not on model structures, but on real finished devices so that results are directly comparable to the current and luminance or photovoltaic characteristics of the device under study. 

These problems may be overcome by the use of organic photovoltaic solar cells [117-120], which consist of a donor and an acceptor material, each possessing its individual HOMO and LUMO energy levels. For an efficient device configuration, the HOMO and LUMO energy levels of the donor material must be higher com-... 

Attempts have been made to deposit TIPS-pentacene from solution as the functional layer in a pentacene/C60 bilayer photovoltaic device. Careful optimization of deposition conditions, optimal concentration of mobile ion dopants, thermal postfabrication annealing, and the addition of an exciton-blocking layer yielded a device with a moderate white-light PCE of 0.52% [41]. Since TIPS-pentacene derivatives rapidly undergo a Diels-Alder reaction with fiillerene, the assembly of potentially more efficient bulk-heterojunction photovoltaic devices from TIPS-pentacene and fiillerene derivatives were not possible [42]. The energy levels of the TIPS-pentacene-PCBM adduct (PCBM is [6,6]-phenyl C61-butyric acid methyl ester) ineffectively supports the photoinduced charge transfer. 

Fig. 1 General band diagram for an OPV device with the donor (red) and acceptor (blue) HOMO/ IP and LUMO/EA levels. All of the energies are relative to the vacuum level (VL). The energy difference AL between the LUMO of the donor and acceptor provides the driving force for charge formation from an exciton on the donor, and gap is the maximum possible open-circuit voltage from an organic photovoltaic device...

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