The performance of an OPV is characterized by the current-voltage dependence (I-V curve) depicted in Fig. 6.2.

The directly-measurable parameters that allow a quantitative analysis of the performance of a PV cell are:

(i) The open-circuit voltage Voc corresponding to zero-current in the cell, which is dependent on the offset of the energy levels (highest occupied molecular orbital of the donor: HOMOD and lowest unoccupied molecular orbital of the acceptor: LUMOA) of the DA interface of the cell (Fig. 6.3). The short-circuit current Isc which is related to the photo-generated charges. The fill factor (FF) which is related to the ratio of the red and grey areas in Fig. 6.2, which reflects the quality of a photovoltaic (PV) cell, or more precisely, it is determined by the mobilities of charge carriers [4, 9].

(ii) The incident-photon-to-current efficiency (IPCE) is defined as the ratio of the number of incident photons and the number of photo-induced charge carriers which can be generated. Unlike the internal quantum efficiency, IPCE accounts for losses by reflection, scattering, and recombination.

(iii) The power conversion efficiency (PCE), mentioned above, depends on the FF, Voc, Isc, and the incident light intensity. PCE is maximal for larger values of the first three parameters. A standard AM1.5 spectrum is used for PV characterizations.

(iv) The PCE can be decomposed in terms of different “sub-efficiencies” related to each step towards the generation of the photocurrent. The overall per­formance of an OPV depends on the sequence of steps mentioned above and illustrated in Fig.

p

Fig. 6.2 The I-V curves characteristics of PV cells with (short dash) and without (long dash) solar illumination. A diode-like behaviour is observed in the absence of illumination. The short — circuit current Isc (V = 0) and the open-circuit voltage Voc (I = 0) are shown. MPP and Pmax denote the maximum power point and maximum power output of the PV cell, respectively

ПР = Па X nd X П x Пс

Fig. 6.3 Schematic representation of the main fundamental steps describing the photo-current conversion by an organic solar cell along with the related efficiencies in terms of which the power conversion efficiency (PCE) parameter, np, is shown

The incident light, with energy falling inside the HOMO-LUMO gap, is absorbed, leading to the creation of a strongly-bound Frenkel exciton by promoting an electron from its ground state to a higher energy level. The exciton is of a strongly “bound” nature because the electron-hole pair is subject to a strong mutual attractive interaction [6]. In OPVs additional steps are then required for electron- hole extraction, which is in contrast to inorganic PVs where the free carriers are released immediately. Therefore the transport processes are much more complex in OPVs due to the polaronic nature of the carriers. In the absence of an applied electric field, the exciton diffuses inside the organic semiconductor and becomes spatially dissociated into bound, positive and negative carriers (a polaron-holon pair). The effective dissociation occurs at the DA interface followed by transport of the polarons and holon to the relevant electrodes where they are extracted gener­ating the final photocurrent (Fig. 6.4). During its diffusion the exciton may recombine before it reaches the DA interface, with a consequent loss of the absorbed energy by dissipation. Indeed, the diffusion length of excitons to the DA interface is much shorter than the optical absorption length. This is of prime importance in understanding the different loss mechanisms that should be avoided to improve the dissociation and collection of charges before recombination.

Figure 6.5 shows different loss mechanisms, either by exciton recombination or by charge-carrier trapping, which may occur during the different steps towards photocurrent production explained above. Loss by recombination can even occur after a successful exciton dissociation recombination, and trapping during the charge transport or collection at the DA interface and electrodes, is a further loss. After a recombination, the loss leading to dissipation of the absorbed solar-energy can be either radiative or non-radiative, the former being detected via fluorescence and/or phosphorescence, whilst the later via a phonon creation. When compared with inorganic devices, the organic analogous have two important limitations: first, their narrow absorption-window, and second, the tendency for thermal motion to perturb

the rather soft charge-transport system. Therefore, efficient energy-conversion for practical applications requires significant improvements of charge transport and transfer processes. Energy loss during the photogeneration process should be min­imized by maximizing the number of excitons that dissociate into free charge — carriers, rather than simply recombining without contributing to the photocurrent. A rich research field has been stimulated aimed at extending the spectral sensitivity of OPVs to cover a broader wavelength region and therefore the band-gap tuning.

In this context, it has been proposed that stacking different OPVs would help to achieve this goal. The problem of inefficient exciton-dissociation could be solved thanks to the introduction of tandem[3] device architectures and BHJ [7-11]. The interpenetrating networks of n-and p-semiconductors (Fig. 6.1) improve the charge separation and transport to their respective electrodes for collection.