Reciprocal Relation Between Intraband Carrier Generation and Interband Recombination at the Heterointerface of Two-Step Photon Up-Conversion Solar Cells

Abstract

Photon up-conversion processes are considered beneficial for energy-conversion devices. The recently proposed two-step photon up-conversion (TPU) solar-cell design employs an intermediate level with a high electron occupation probability. To understand the device physics, a quantitative evaluation of the different current-generation and loss mechanisms is required. In the present work, we use a TPU solar cell containing an InAs/GaAs quantum-dot layer located 10 nm in front of an Al 0.3 Ga 0.7 As/GaAs heterointerface. We study the relation between the photocurrent (PC), radiative recombination, and nonradiative recombination as a function of the bias voltage. The radiative interband recombination is evaluated by integrating the photoluminescence (PL) over the range from 1000 to 1300 nm. The magnitudes of the PC and PL signals generated via interband excitation of the GaAs layer depend on the bias voltage; a higher forward bias reduces the PC and increases the PL intensity. We verify that, under additional infrared light illumination at 1319 nm, which induces intraband transitions, the PC density increases while the PL intensity significantly decreases. This PC enhancement exhibits a maximum at −0.6 V, which reflects the optimum internal electric field strength for maximizing the TPU efficiency.

Diffusion photocurrent influenced by intraband excitation in an intermediate band solar cell with type-I band alignment

“…In a previous study, we presented a reciprocal relation between intraband carrier generation and interband radiative recombination at the QDs in a heterojunction SC. 29) Similar to that work, we qualitatively examined the state filling in the QDs by measuring the PL spectra under short-circuit conditions. Figure 4(a) shows how the PL spectrum at 200 K depends on P IB-CB (other temperatures resulted in similar trends).…”

Intraband Transitions at a CsPbBr3/GaAs Heterointerface in a Two-Step Photon Upconversion Solar Cell

“…These accumulated electrons partially recombine with the holes that reach HI-I by diffusion. We described this scenario in previous publications [10][11][12][13] . A part of the electrons at HI-I can overcome the energy barrier by thermal excitation (evidence for thermionic emission is provided in In uence of the Temperature).…”

“…In particular, in an Al 0.3 Ga 0.7 As/GaAs-based TPU-SC, there is no IR-induced EQE enhancement when the wavelength of the photons for interband excitation is shorter than 680 nm (the Al 0.3 Ga 0.7 As bandgap). This result is attributed to the fact that high-energy photons are strongly absorbed by the WGS, [10][11][12][13] which causes a reduction of the number of photons that reach the NGS layer, leading to a reduction of the electron density at the heterointerface. As a result, the IR-induced photocurrent enhancement gradually disappears as the wavelength of the photons for interband excitation becomes shorter; if there are no photogenerated electrons in the NGS CB, the IR laser beam has no effect, because the photogenerated electrons in the WGS CB do not accumulate at the energy barrier [10][11][12] .…”

“…The enhancement of absorption in IBSCs is caused by a sequential excitation of electrons from the VB to the IB and from the IB to the CB using sub-bandgap photons (in addition to the direct band-to-band transitions). Because the band-to-band transitions are independent of the excitation via the IB, an IBSC is equivalent to a parallel circuit: one branch contains one diode corresponding to band-toband transitions, and the other branch contains two diodes connected in series corresponding to the sequential absorption of sub-bandgap photons via the IB [9][10][11][12][13][14] . As a result, an IBSC is not affected by current and voltage mismatches, which are problematic in the case of multi-junction solar cells 1 .…”

“…Such a trade-off relation between fabrication complexity and e ciency hinders the commercialization of IBSCs. On the other hand, we demonstrated a so-called two-step photon upconversion solar cell (TPU-SC) based on Al 0.3 Ga 0.7 As and GaAs including a single layer of InAs QDs at the heterointerface (the QDs were prepared by using the Stranski-Krastanov growth mode) [10][11][12][13] . The fabrication of such a TPU-SC is relatively simple compared to IBSCs.…”