1. RESONANT TUNNELING MULTIPLE QUNANTUM WELL STRUCTURES IN P-I-N PHOTOVOLTAIC ELEMENT M.P.Telenkov (1,2), Yu.А. Mityagin (1), P.F. Kartzev (3) (1) P.N. Lebedev Physical Institute of RAS, Moscow, Russia (2) National University of Science an Technologies «MISiS»,Moscow, Russia (3) National Research Nuclear University «MEPHI», Moscow, Russia

2. conventional p-i-n junction p-i-n junction with quantum wells absorption is restricted by the band gap absorption is extended towards the longer wavelength Key point - the effective transfer of the photo-carriers from deep quantum wells (with depth higher than kT) into continuous spectrum avoiding recombination processes in the quantum wells

3. The idea is to design the quantum well structure so that in an intrinsic electric field of p-i-n junction the lowest subbands of all the wells would be in resonance The photo-electrons in wells tunnel from deep quantum wells to shallow quantum wells (with depth of order of kT) from which the electrons escape to continuum by thermal excitation as well as direct tunneling

4. GaAs/Ga 1-x In x As; well width – 25 nm; barrier width – 5 nm Well numb er xBarrier height, meV Distance from the 1st subband to the continuum, mV 10.5384.9362.5 20.453354.29331.9 30.409324.58302.7 40.365293.87272.3 50.323263.6242.4 60.282233.17212.3 70.242202.69182.2 80.203172.04152.0 90.165141.48121.8 100.128110.9991.7 110.09280.6461.8 120.05649.6131.5 130.02017.911.25

5. n i – photo-excited electrons concentration in the i-th well; g i – excitation rate of the photo- electrons in the i-thwell; τ i,i+1 - kinetic time of tunneling from i-th well to (i+1)-th well; N – number of the quantum wells in the structure; τ out – tunneling time from the last right well into a continuum; For r=0 If we select the parameters of the wells and barriers so that the inter-well tunneling times would be considerably shorter that the recombination ones we will obtain an additional contribution of the photo-excited electrons in wells to the total photocurrent

6. It is enough to make tunneling from the deepest well in sequence to be much faster than recombination

7. LO-phonon scattering

8. LA-phonon scattering Interface roughness scattering Since the electron and hole concentrations in quantum wells are much lower than that in continuum a Boltzmann distribution was used

9. recombination time achievable in structures of this kind is of 1000 ps in order of magnitude By varying the barrier width we can achieve the tunneling time <~ 10 ps that is about two orders of magnitude lower than the recombination time achievable in structures of this kind By proper choice of the structure parameters it is possible to achieve the necessary tunneling regime and to provide an effective transfer of photo- electrons to the continuum

10. How to avoid the accumulation of the holes in deepest quantum well? At room temperature the intersubband spacings for holes are considerably lower than kT => The photo-excited holes will migrate towards the more deep wells due to resonant tunneling transitions. And finally holes will be accumulated in the deepest well which can be emptied only by hole recombination with the electrons

11. By proper choice of the structure parameters the hole tunneling time also can be made sufficiently low thus providing their effective transfer to the continuum and the contribution to the total photocurrentf

12. Conclusions An effective method of photo-excited charge carrier separation is proposed. The method is based on the phenomena of resonant tunneling in quantum well structure embedded into i-area of the p-i-n photovoltaic element. The parameters of the GaAs/Ga 1-x In x As structure providing resonant tunneling regime in the p-i-n junction electric field were calculated. The kinetic tunneling times were calculated as functions of the well and barrier parameters. A possibility to achieve rather small (<~10 ps) tunneling time values and, correspondingly, to provide an effective transfer of photoelectrons and photoholes from quantum wells by proper selection of barrier widths was demonstrated.

13. Thank you for your attention!