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Reports: DNI1050629-DNI10: Development of Single-Crystalline Organic Solar Cells for the Fundamental Studies of Photo-Voltaic Effect in Highly Ordered Organic Semiconductors

Vitaly Podzorov, PhD, Rutgers, the State University of New Jersey (New Brunswick)

During the year 09/2013 – 08/2014 we have finalized our work on the non-linear effects in photoconductivity of rubrene that we have discovered during the previous year(s). We showed that photoconductivity in pristine molecular crystals of high purity exhibits several distinct regimes, in which photocurrent s as a function of cw (continuous wave) excitation intensity G is described by a power law, s µ Gα, with the exponent α sequentially taking values 1, 1/3 and ¼.  We have showed that this photocurrent is generated almost exclusively at the surface of pristine rubrene crystals, while the bulk photocurrent is dramatically smaller and follows a different set of exponents, 1 and ½.  We have developed an analytical model based on exciton fission, fusion and triplet-charge quenching to describe these non-trivial effects in photoconductivity of highly ordered organic semiconductors. Phenomenological and fundamental understanding of strong nonlinearities in photoconductivity provided by this work is important for gaining deep insights into the physics of excitons and charge carriers in organic semiconductors and utilization of these materials in such emerging photonics applications, as organic photovoltaic cells.      Figure 1 shows the typical result of photoconductivity measurements in pristine rubrene performed in a wide range of excitation intensities covering more than six decades in light power.  The most striking observation is that photoconductivity closely follows a power law, sPC µ Ga, with the exponent a sequentially taking values 1, 1/3 and ¼, as the excitation power increases. 

Fig. 1.  Dependence of the steady-state photocurrent on the excitation power in pristine rubrene.  Photocurrent follows a power law, sPC(G) µ Ga, with the three distinct regimes described by the power exponent a = 1, 1/3 and ¼ (note the double-log scale).  Before continuing with the investigation of this non-trivial photoconductivity, we demonstrated that most of the photocurrent in pristine rubrene crystals flows at the very surface of the crystal, perhaps within a ~ nm below the (a,b) surface, while bulk photocurrent is contributing less than ~ 1% to the total photoconductivity.  This step of the experiment is described in detail in our recently submitted manuscript.  It suggests that photoconductivity in pristine rubrene is predominantly a surface effect.  This realization alone lends a strong support to the model, in which photon absorption leads to a generation of long-lived excitonic species (for rubrene, triplets generated via singlet fission) capable of reaching the surface of the crystal from the bulk due to long-range diffusion, where they dissociate and generate a surface photocurrent.  After the relative contributions of the surface and bulk photoconductivities have been established, we considered the two limiting cases, the surface-dominated and bulk-dominated photoconductivity, measured in the same crystal (Fig. 2).  In addition, photoluminescence emitted from the (a,b)-facet of the crystal has been measured simultaneously with photoconductivity (Fig. 2a).  The bulk photoconductivity is smaller than the surface one by ~ 1-2 orders of magnitude, and, more interestingly, has a qualitatively different excitation intensity dependence (a power-law exponent a = 1/3 (surface-dominated) vs. a = ½ (bulk-dominated)).    

Fig. 2. Photoluminescence (a) and photoconductivity (b) simultaneously measured in rubrene crystals as a function of excitation density.  Photoconductivity is shown for pristine crystal (black), and for the same crystal after it has been exposed to a high-vacuum gauge (red).  Thin solid lines are power-law fits sPC µ Ga with exponent α indicated next to each line (note the double-log scale).    Photoconductivity of pristine crystal (black open symbols in Fig. 2b) exhibits the three well-defined regimes characterized by the power-law exponents a = 1, 1/3 and ¼.  The bulk photoconductivity of the same crystal has a different behavior with the transitions from a = 1 to ½ and 0.4 (red curve in Fig. 2b).  On the lowest intensity end (below 1014 - 1015 cm-3 s-1), the simple linear dependence is always observed in our crystals.  At high excitation powers, a significant increase of PL yield is observed (a “bump” in PL in Fig. 2a), with the inflection point at around 1020 cm-3 s-1, signifying a transition from one linear regime to another one with ~ 10 times greater slope.  This crossover in PL approximately coincides with the transition from a = 1/3 to ¼ in the surface photoconductivity of pristine crystals.  Such a drastic increase of photoluminescence quantum yield thus correlates well with a reduction in the photocarrier generation efficiency, implying that the same new channel responsible for formation of additional emissive singlets is also responsible for an extra photocarrier loss.  We have formulated a microscopic model that rationalizes our experimental observations.  Our model is based on the rate equations describing the dynamics of charge carrier density (n), and the density of singlet (S) and triplet (T) excitons, in the presence of singlet-to-triplet and triplet-to-singlet conversions occurring via fission and fusion, as well as triplet-carrier quenching.  The detailed differential equations, their steady-state solutions, data analysis and discussion are included in a manuscript that we have recently submitted:  P. Irkhin, H. Najafov and V. Podzorov, "Steady-state photoconductivity and multi-particle interactions in rubrene single crystals", submitted to Phys. Rev. Lett. (submission date 09/20/2014).