Monday, February 10, 2020

Trap-Assisted Injection Gain in Organic Photoconductors

Journal of Applied Physics publishes a paper "Physics of trap assisted photomultiplication in vertical organic photoresistors" by Mehdi Daanoune, Raphaël Clerc, Bruno Flament, and Lionel Hirsch from Université de Lyon and Université Bordeaux, France.

"Several experimental groups have reported recently an intriguing high level of gain (Photomultiplication) in vertical organic photoresistance (as well as in other technologies, such as perovskite for instance). This mechanism is sometimes named as “Trap-Assisted Photomultiplication.” This paper investigates the origin of this mechanism by means of drift diffusion simulations, analytical theory, and experiments, considering the particular case of PCDTBT:PC60BM photoresistors, although some conclusions are likely to apply in other technologies. It turns out that an excess of charges (induced by electron–hole carrier generation) may trigger additional carrier injection, leading to photomultiplication, under specific circumstances. We call this mechanism “gain by injection enhancement.” Electron (respectively, hole) trapping for P only (respectively, N only) devices can play this role efficiently. As these additional carriers came from contacts, significant dark current injection is thus needed to achieve a large value of gain, explaining why this mechanism can occur only in P (or N) only photoresistors (and not photodiodes or intrinsic photoresistors, i.e., with midgap contacts). In such devices, however, the detectivity remains intrinsically limited by the high level of dark injection currents required to get gain, and consequently, this type of device may be interesting, in particular, in technologies where it is not possible to achieve low dark currents using photodiodes. However, penalized by the slow trap dynamics, the cut-off frequency of these devices remains extremely low (less than 100 Hz). Also, this gain takes a high value only at low irradiance, making photoresistor responsivity light dependent. All these results bring new light in analyzing and optimizing photoresistors, opening a large field of investigation to take advantage of gain by injection enhancement."


  1. And the first author ?

  2. Thanks for quoting my paper ! Raphael Clerc.

  3. I think that the effects described in this paper, including large photocurrent gain and its physical mechanism, are very similar to what's happening in impurity photodetectors, and in quantum well infrared photodetectors.

    Search for papers by Boris Fuks for impurity photodetectors, and for papers by Maxim Ershov (me) for QWIPs.

    Responsivity drop at high power, at high frequency, injections effects,
    transient photoresponse with two time constants, etc.

  4. Thanks for the info, never heard about it. I will have a closer look at these works.

  5. The basic mechanism is very simple.
    The light excites the carriers (let's say, electrons) from their "seats" in the volume of the "photoconductor" (a "seat" may be an impurity, a quantum well/dot, or a trap).
    Photoexcited electrons are extracted form the photoconductor volume into "collector" - hence, you need to have applied voltage, to create electric filed, to extract them.
    That's primary photocurrent.
    Then an uncompensated positive charge is left in the photoconductor, that increases the electric field or lowers the barrier at the injection contact.
    More carriers are injected form the "emitter", and flow through the photoconductor into the collector.
    That's secondary, or multiplied photocurrent.
    If probability of their capture to the vacant "seat" is low, many secondary electrons flow through the detector in response to one photoexcited electron.
    That's photocurrent gain.
    It can be small (smaller than one), or large (1-10-100-1000-...) - if capture probability is low.
    Eventually the vacant "seat" gets an electron, and equilibrium is re-established.

    This mechanism produces a wealth of interesting and counter-intuitive (at a first glance) physical phenomena in such photodetectors.

    As en example - impurity photodetectors were notorious for very slow transient, history-dependent photoresponse, nonlinear ("hook") photoresponse, etc.
    It made them very difficult to use and calibrate in space.
    Until Fuks explained this and showed how to deal with this.
    Similarly - with quantum well infrared photodetectors.
    At very low temperatures and low background radiation, they behave in a weird way, which puzzled people for a long time.

    CMOS image sensors operate very differently, so these phenomena and problems are, luckily, not present there.

    One interesting effect that I am sure you will observe in your photodetectors is negative capacitance (but not at any operating conditions).

  6. Photocurrent gain is not new in image sensors. The old vidicon tube used antimony trisulfide in this mode with voltage-variable gain and a gamma of 2/3 from the non-linear effect of induced electric fields from the accumulating charge.

    1. Sure, photocurrent gain is an old thing.

      Understanding, where it comes from, at the device/microscopic level, and device/aplication implications, are not always understood well.

      People usually assume "ohmic" injection contact, that magically supplies the missing carriers.
      Injection contacts are not "ohmic", they rely on non-ideal effects turning on this extra injection, leading to all sorts of unusual device behaviors.


All comments are moderated to avoid spam and personal attacks.