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4 introductionorder in such a complicated morphology and by the possibilityfor the trapped charge carriers to recombine with themobile ones before proceeding to the contacts [13].The generation and collection of carriers contribute tothe short-circuit current (I SC ), that is the maximum currentfrom a solar cell, occurring at zero voltage. This parameter,together with the open-circuit voltage (V OC ) and the fill factor,determines the energy conversion efficiency of a solarcell [11]. I SC depends on the area of the solar cell, the numberof photons, the spectrum of the incident light and theoptical properties of the solar cell. The open-circuit voltageis the maximum voltage available from a solar cell, and thisoccurs at zero current. For ohmic contacts V OC is governedby the energy levels of HOMO and LUMO of donor andacceptor [11], therefore, it can be raised by carefully positioningthese levels [11]. Obviously, when the device worksat either open circuit or short circuit conditions the powerP = VI is zero.Another important quantity is the fill factor (FF), definedas the ratio between the maximum output power (P max )and V OC I SC (see Figure 1.2). Since the efficiency is givenby the ratio between the power output P out and the solarpower input P in , it can be expressed in terms of FF by therelation η = V OC I SC FFP in.Figure 1.2.: Characteristic voltage-current of a solar cell.An important problem limiting the efficiency of the organicsolar cell is related to the collection of the photonsover the whole solar spectrum. To obtain good efficienciesthe absorption spectrum of the photoactive material mustmatch the solar emission spectrum and it must be suffi-