The Role of Excitonic Light Collection, Exciton Diffusion to Interfaces, Internal Fields for Charge Separation, and High Charge Carrier Mobilities

Abstract

t g a r t , D 70550 S t u t t g a r t , Germany AbstractOrganic photovoltaic thin film s t r u c t u r e s made by vacuum vapor deposition have been studied. From t h e spectral response as a function of t h e absorption coefficient we conclude t h a t electron -hole generation takes place a t t h e interface between t h e organic thin film and one or b o t h of t h e semitransparent cover electrodes. The observed s h o r t circuit currents, however, are t o o large to be explained on t h e basis of a direct light-induced charge transfer at t h e organic thin film/ electrode interface. Rather, a contribution of t h e bulk-absorbed photons is necessary t o account f o r the observed quantum yield. Transfer of t h e energy t o t h e interface sites can be explained by diffusional migration of excitons. The efficiency of charge separation can be improved by combining donor a n d acceptor type partners in organic double layers. The efficiency of power conversion, however, n o t only depends on a suitable choice of t h e absorption spectra and of t h e ionic energy levels of t h e materials employed, b u t also on t h e internal cell resistance, a f a c t t h a t calls f o r high charge carrier mobilities, and hence f o r using materials with crystal structures t h a t allow s t r o n g n-electron interactions, and f o r deposition in high chemical purity and s t r u c t u r a l perfection.

Exciton diffusion and optical interference in organic donor–acceptor photovoltaic cells

“…However, in our case we do not describe the diffusion of generated charge carriers, but the diffusion of generated neutral excitons in the bulk of the absorbing material. 20,27,48 Further on, our model is extended to take into account the photoinduced electron transfer at the donor-acceptor interface within the thin film structure. Further assumptions in our model are that ͑1͒ the absorbing layer is semi-infinite, so that far away from the illuminated electrode all light is absorbed and optical interference effects cannot occur.…”

Photogeneration and transport of charge carriers in a porphyrin p/n heterojunction

“…[9][10][11][12]21 The photoactive zone in this band model stretches out across the depletion region, in contrast with the interface model where efficient photoinduced charge separation solely takes place at the interface of both organic dyes. Recently, however, the role of the depletion region in organic heterojunctions was reconsidered, 13 since the contribution of the exciton diffusion length in the p-n model is not yet clear.…”

Optical Applications

“…In some cases, this enables the preparation of well-ordered polycrystalline films. This is often an advantage in the operation of the device concerned, for example, for transport of electric charge in field-effect transistors, resulting in relatively high electrical mobilities for FET applications [26], potentially faster switching speeds for electro-optical devices and enhanced chargecarrier separation and lower internal resistance [27] in photo-voltaic applications.…”

“…Furthermore, as emphasized by Karl er 01. [27], the power conversion of organic solar cells is often very low, due to a high internal cell resistance, because the mobility of many organic semiconductors is low. To solve the problem, they advocate the use of more crystalline materials with strong intermolecular .ir-electron interactions and high mobilities, as might yet be achieved with suitably oriented sublimed films of conjugated oligomers.…”