Recombination current

A band diagram of a biased n-p-n BIT is shown in figure C2.16.8. Under forward bias, electrons are injected from tlie n type emitter, giving rise to tlie current 7. flowing into tlie p type base. Some of tlie carriers injected into tlie base recombine in tlie base or at tlie surface. This results in a reduction of tlie base current by 7, tlie lost recombination current, and tlie base current becomes 7g = At tlie same time, holes are injected from tlie... 

Lp Pi 50 pm. and the reverse saturation current would be 17 x 10 = 17 pA for a square centimeter of junction area. Typical reverse saturation currents are about one thousand times greater as a result of generation—recombination currents in the depletion region (9). As the reverse voltage bias increases, the field increases in the depletion region until avalanche breakdown occurs, resulting in the characteristic shown in Figure 7. 

The boundary conditions are given by specifying the panicle currents at the boundaries. Holes can be injected into the polymer by thermionic emission and tunneling [32]. Holes in the polymer at the contact interface can also fall bach into the metal, a process usually called interlace recombination. Interface recombination is the time-reversed process of thermionic emission. At thermodynamic equilibrium the rates for these two time-reversed processes are the same by detailed balance. Thus, there are three current components to the hole current at a contact thermionic emission, a backflowing interface recombination current that is the time-reversed process of thermionic emission, and tunneling. Specifically, lake the contact at Jt=0 as the hole injecting contact and consider the hole current density at this contact. 

The hole current in this LED is space charge limited and the electron current is contact limited. There are many more holes than electrons in the device and all of the injected electrons recombine in the device. The measured external quantum efficiency of the device is about 0.5% al a current density of 0.1 A/cm. The recombination current calculated from the device model is in reasonable agreement with the observed quantum efficiency. The quantum efficiency of this device is limited by the asymmetric charge injection. Most of the injected holes traverse the structure without recombining because there are few electrons available to form excilons. 

Recent work with multi-layer polymer LEDs has achieved impressive results and highlights the importance of multi-layer structures [46]. Single-layer, two-layer and three-layer devices were fabricated using a soluble PPV-based polymer as the luminescent layer. The external quantum efficiencies of the single-layer, two-layer, and three-layer devices were 0.08%, 0.55%, and 1%, respectively, with luminous efficiencies of about 0.5 hn/W, 3 lm/W, and 6 lm/W. These results clearly demonstrate improvement in the recombination current because of the increase in quantum efficiency. The corresponding increase in luminous efficiency demonstrates that the improvement in recombination efficiency was achieved without a significant increase in the operating bias. 

The enhancement in luminous efficiency achieved by inserting an ultrathin interlayer between the ITO and NPB is mainly due to the reduction of hole injection from ITO to NPB in OLEDs. For a simple approximation, luminous efficiency (rj) can be related directly to a ratio of the recombination current (/r) to the total current density of OLEDs (/tot). If one denotes the current contributions from holes and electrons in OLEDs as. /h and /e, respectively, then the sum of hole and electron currents, /tot. /h + /e, and tj can be expressed as... 

Generation-recombination current, 22 135 Generation TV International Forum (GIF), 17 557... 

For n-type semiconductor electrodes in which a redox reaction of cathodic hole iiyection reaches its quasi-equilibrium state at the electrode interface, the recombination current of iiqected holes (minority charge carriers) with electrons (minority charge carriers), w, is given by Eqn. 8-70 [Reineke-Memming, 1992] ... 

These observations, and application of Kirchoff s law, suggest that the time dependent electrochemical current density, Je(t), may be written as the product of the fractional intermediate surface coverage, 6 t(t), and the difference between the photo-generation and recombination current densities, JL(t) and Jr(t), respectively. Thus ... 

The recombination current density, Jr, can be treated effectively as a Schottky barrier diode current density. Including both thermionic emission and diffusion charge transport mechanisms (13) Jr can be written as... 

As recombination current in a photocathode was treated as an electron current to the photoanode surface, the photocathode recombination current may be viewed as a hole current. Correspondingly, OH- ions at the photocathode/electrolyte interface lower the energy barrier for hole transport to the photocathode surface. 

The leakage current IR of a reverse-biased junction can be expressed as a sum of several components that have been discussed in detail by Tamg and Pankove (1979). Under the present experimental conditions, the major components are the surface generation-recombination current Iv and the tunneling current /,. 

In conclusion we have shown that the junction current of an organic solar cell measured in the dark cannot be used to interpret data obtained with illuminated solar cells. The current changes from SCLC to diffusion and recombination current when the cell is illuminated. This has allowed us to provide a physical basis for the use of a model, which uses the diode equation for the illuminated solar cells. The model is very successful in interpreting the output characteristics of the illuminated organic solar cells by Brabec and confirmed by us. The true junction dark current (diffusion and recombination current) determined by us remains more than the output current of the illuminated solar cell. Now the two currents do not intersect. This also explains why points of inflexion observed in the current measured in the dark disappear in the output current of the illuminated cell. 

The measured photocurrent response is the sum of the charging and transfer components or alternatively the difference between the photogenerated hole current density qg and the recombination current density /rec... 

Without an applied electric field, the source current is compensated by opposite in direction the surface recombination current js — jr = 0. An assumption has been made that... 

The important message that follows from Eq. (293) is that the light output for organic LEDs operating in the ICEL mode cannot be related directly to the driving current (J) which must not be identified with the recombination current... 

Here, the measured current j is simply the recombination current,, and as long as , Ps and (pFL do not depend on j, remains directly proportional to the driving current. The slope of the linear plot of with j determines the product cPs(Pfl-... 

The triplet exciton concentration increases linearly with the recombination current density, j. 

The treatments of the surface recombination above assume that there is only one trap site. In practice, this will rarely be the case indeed, as the analysis of the a.c. data for p-GaP above illustrated, there may be a near constant distribution of surface states throughout a substantial fraction of the bandgap. For an n-type semiconductor, the total recombination current may then be written, for Gerischer s case (a) [136] above... 

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