In our contribution we discuss the application of impedance spectroscopy to organic semiconductor devices. Electronic and optoelectronic devices composed by this new class of materials show important differences with respect to the classical inorganic-semiconductors. We focus on two important aspects, typical of organic devices: trap states and negative capacitance effects. We discuss how to interpret and characterize the impedance spectra starting from the general semiconductor theory. The argumentations are then applied to the measured spectra of different devices, from organic solar cells, to homo- and hetero-junction diodes. Trap states are measured in small-molecule solar cells based on a p-i-n structure [1]. Trap states can modify the electric field inside the device, considerably affecting charge generation and transport and consequently the solar cell efficiency [2]. Specific variations in the device structure allow us to identify the traps position and nature. In order to completely characterize these states we have implemented previously reported theoretical works [3] and calculated the energy diagram and the Impedance Spectrum of the solar cell for different traps distributions. The calculated results are compared to measured impedance spectra for different traps distributions in order to confirm the validity of our discussion. The specific trap states distribution is then evaluated by a fitting procedure. Negative capacitance is often observed in the impedance spectra of different organic devices. The physical origin of this effect is debated in literature and no clear explanation as been proposed up to now [4]. We show here that Negative Capacitance effects can be explained starting from the relaxation semiconductor theory when applied to organic semiconductors [5]. The semi-insulating nature of these materials cannot be described by standard semiconductor theory simply assuming low mobilities, since they exhibit electronic transport phenomena which are generically different. We present first a short review of the organic devices in which negative capacitance has been observed and we explain them with a unified approach, based on relaxation semiconductor theory. The implications in device performance coming from our findings are also discussed. Small-molecule devices are fabricated and characterized in order to confirm the proposed arguments. Starting from a simple metal/organic/metal structure we design devices which deliberately present or not negative capacitance effects. The results obtained from these simple structures are then generalized and observed in homo- and hetero-junction diodes as well as in complete p-i-n organic solar cells.
[1] M.Riede, T.Mueller, W.Tress, R.Schueppel, and K.Leo, Nanotechnology, 19, 424001, 2008.
[2] D.Ray, L.Burtone, K.Leo, and M.Riede, Physical Review B, 82, 125204, 2010.
[3] J.Bisquert, Physical Review B, 77, 235203, 2008. D. L. Losee, Journal of Applied Physics, 46, 2204, 1975.
[4] C.Lungenschmied, E.Ehrenfreund, and N.Sariciftci, Organic Electronics, 10, 115-118, 2009 / J.Bisquert, Physical chemistry chemical physics: PCCP, 13, 4679-85, 2011 / J.Bisquert, G.Garciabelmonte, A.Pitarch, and H.Bolink, Chemical Physics Letters, 422, 184-191, 2006.
[5] W.vanRoosbroeck and H.Casey, Physical Review B, 5, 2154-2175, 1972 /W.vanRoosbroeck, Physical Review, 119, 636-652, 1960.
In our contribution we discuss the application of impedance spectroscopy to organic semiconductor devices. Electronic and optoelectronic devices composed by this new class of materials show important differences with respect to the classical inorganic-semiconductors. We focus on two important aspects, typical of organic devices: trap states and negative capacitance effects. We discuss how to interpret and characterize the impedance spectra starting from the general semiconductor theory. The argumentations are then applied to the measured spectra of different devices, from organic solar cells, to homo- and hetero-junction diodes. Trap states are measured in small-molecule solar cells based on a p-i-n structure [1]. Trap states can modify the electric field inside the device, considerably affecting charge generation and transport and consequently the solar cell efficiency [2]. Specific variations in the device structure allow us to identify the traps position and nature. In order to completely characterize these states we have implemented previously reported theoretical works [3] and calculated the energy diagram and the Impedance Spectrum of the solar cell for different traps distributions. The calculated results are compared to measured impedance spectra for different traps distributions in order to confirm the validity of our discussion. The specific trap states distribution is then evaluated by a fitting procedure. Negative capacitance is often observed in the impedance spectra of different organic devices. The physical origin of this effect is debated in literature and no clear explanation as been proposed up to now [4]. We show here that Negative Capacitance effects can be explained starting from the relaxation semiconductor theory when applied to organic semiconductors [5]. The semi-insulating nature of these materials cannot be described by standard semiconductor theory simply assuming low mobilities, since they exhibit electronic transport phenomena which are generically different. We present first a short review of the organic devices in which negative capacitance has been observed and we explain them with a unified approach, based on relaxation semiconductor theory. The implications in device performance coming from our findings are also discussed. Small-molecule devices are fabricated and characterized in order to confirm the proposed arguments. Starting from a simple metal/organic/metal structure we design devices which deliberately present or not negative capacitance effects. The results obtained from these simple structures are then generalized and observed in homo- and hetero-junction diodes as well as in complete p-i-n organic solar cells.
[1] M.Riede, T.Mueller, W.Tress, R.Schueppel, and K.Leo, Nanotechnology, 19, 424001, 2008.
[2] D.Ray, L.Burtone, K.Leo, and M.Riede, Physical Review B, 82, 125204, 2010.
[3] J.Bisquert, Physical Review B, 77, 235203, 2008. D. L. Losee, Journal of Applied Physics, 46, 2204, 1975.
[4] C.Lungenschmied, E.Ehrenfreund, and N.Sariciftci, Organic Electronics, 10, 115-118, 2009 / J.Bisquert, Physical chemistry chemical physics: PCCP, 13, 4679-85, 2011 / J.Bisquert, G.Garciabelmonte, A.Pitarch, and H.Bolink, Chemical Physics Letters, 422, 184-191, 2006.
[5] W.vanRoosbroeck and H.Casey, Physical Review B, 5, 2154-2175, 1972 /W.vanRoosbroeck, Physical Review, 119, 636-652, 1960.
