Article pubs.acs.org/JPCC
Evaluating Charge Carrier Mobility Balance in Organic Bulk Heterojunctions using Lateral Device Structures Zi-En Ooi,*,† Eric Danielson,‡ Kelly Liang,‡ Christopher Lombardo,‡ and Ananth Dodabalapur*,‡ †
Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602 ‡ Microelectronics Research Center, The University of Texas at Austin, Austin, Texas 78758, United States ABSTRACT: Lateral geometry devices are useful structures to probe the charge transport properties of organic bulk heterojunction materials. In this work, we describe the lateral device as effectively having three distinct zones: two photogenerative space-charge regions next to each electrode and a middle zone that has high recombination rate. We analyze how photocurrent depends on electron and hole mobilities and recombination strength and, based on this analysis, demonstrate a steady-state method for extracting both carrier mobilities and the recombination coefficient from potentiometry and photocurrent data.
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INTRODUCTION Generally, an organic solar cell (OSC) is composed of a photoactive organic film sandwiched between two electrode layers, with each electrode having a different work-function. The device is usually fabricated on a flat transparent substrate, such as glass, with the various layers forming a vertical stack. The organic film is often a binary blend of polymeric and molecular semiconductor materials, with one material responsible for electron transport and the other for hole transport. Charge carriers are generated upon absorption of light in the organic film and need to be transported to opposite electrodes. The photoactive organic semiconductor blend film, also known as the bulk heterojunction (BHJ) film, thus serves multiple functions and is crucial to the overall efficiency of the OSC. Every BHJ blend has three main characteristic electronic properties that largely govern the performance of OSCs: electron mobility, hole mobility, and recombination coefficient. A well-known technique to estimate them is photoinduced charge extraction by a linearly increasing voltage (photoCELIV).1 Photo-CELIV has the advantage that the testing is performed on a vertical structure (identical to an actual solar cell), but will typically find just one of the two carrier mobilities without being able to distinguish which one. Another method using space-charge limited single-carrier injection currents can be used to separately estimate hole and electron mobilities2 and often complements photo-CELIV. This article discusses a steady-state method to estimate the electron and hole mobilities, as well as the recombination coefficient, of bulk heterojunction films. This method involves the use of lateral device structures that, unlike the usual vertical OSC structure, have two electrodes that lie either on top of, or underneath the BHJ film, with the photoactive region © 2014 American Chemical Society
determined by the channel between the two electrodes, as schematically shown in Figure 1. Such lateral devices resemble
Figure 1. Schematic of a lateral bulk heterojunction device. The substrate does not participate in device operation, and should be insulating. However, having an Si/SiO2 substrate allows for routine FET measurements, if required.
field-effect transistors, but with the gate disconnected and left floating. The mobilities estimated from this method are therefore based on lateral transport and serve to complement the existing vertical device techniques to provide a more complete picture of charge transport in a possibly anisotropic medium.3,4 Calculations and experiments by Kirchartz et al.5,6 showed that charge carrier distributions, which occur in thin vertical solar cells, can affect experimental observations of recombinaReceived: February 21, 2014 Revised: July 24, 2014 Published: July 24, 2014 18299
dx.doi.org/10.1021/jp501823s | J. Phys. Chem. C 2014, 118, 18299−18306
The Journal of Physical Chemistry C
Article
injection, the process described above leads to a buildup of positive space-charge near the anode. A similar process can also occur near the cathode (positive electrode), which leads to accumulation of negative space-charge there. Under steady-state operation, three distinct zones therefore exist: (i) A central recombination zone (RZ), in which net generation is close to zero (i.e., the quasi-equilibrium region). (ii) A space-charge region (SCR) adjacent to the anode, where holes accumulate. (iii) Another similar SCR adjacent to the cathode, where electrons accumulate. When the applied electric field is increased, a larger amount of space-charge can be supported, implying an expansion in size of the SCRs, a corresponding contraction of the RZ, and a resulting rise in internal quantum efficiency (IQE). Eventually, with a sufficiently large applied electric field, the RZ vanishes and the IQE begins to saturate. Evidence of the above description was shown by Lombardo et al. using scanning confocal photocurrent microscopy (SCPM).7,12 In SCPM, photocurrent was measured from a 20 μm lateral PSBTBT:PC61BM (poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] and [6,6]-phenyl-C61-butyric acid methyl ester) device as a small (
