† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 61565008 and 61205205) and the General Program of Yunnan Applied Basic Research Project, China (Grant No. 2016FB009).
Inhibiting the radiative radiation is an efficient approach to enhance quantum yields in a solar sell. This work carries out the inhibition of radiative recombination rate (RRR) in a quantum photocell with two coupled donors. We perform explicit calculations of the transition rates, energy gaps and the absorbed solar wavelength-dependent RRR, and find that two different regimes play the crucial roles in inhibiting RRR. One is the quantum coherence generated from two different transition channels, the other includes the absorbed photon wavelength and gaps between the donor and acceptor in this proposed photocell model. The results imply that there may be some efficient ways to enhance the photoelectron conversion compared to the classic solar cell.
The photoelectron conversion efficiency[1–4] is a continuous concern about solar cells or photovoltaic devices. When research of the photoelectric conversion efficiency encounters the bottleneck,[5–14] scientists turn their interesting to quantum technology,[15–25] and they hoped to break through the efficiency limitation, and further to improve the photoelectric conversion efficiency by means of quantum mechanics.
When it comes to efficiency losses, there are two categories efficiency loss processes in the photovoltaic devices. One is the extrinsic losses such as series resistance,[26–28] parasitic recombination and contact shadowing,[29] they are theoretically avoidable and consequently are not considered in fundamental limiting efficiency. The other is the intrinsic losses such as non-absorption of photons with energy below the bandgap,[30–32] thermalisation loss[33–35,37] due to strong interaction between excited carriers and lattice phonons and emission loss according to Kirchoff’s law,[37,38] they are unavoidable in device design and will still be present in an idealized solar cell.[6] What is more, the radiative upward transition and its reversal, the radiative downward transition coexist simultaneously,[39] which has been considered as the fundamental limit[5] on the conversion efficiency. Scully[15] proposed a scheme to reduce radiative recombination via the quantum coherence with or without the drive field. Svidzinsky et al.[17] manifested that quantum coherence could increase the flow of electrons through the load and enhance cell power via the reduction of radiative recombination. Creatore et al. showed that quantum interference induced by the dipole-dipole interaction between molecular excited states guarantees an enhanced light-to-current conversion and power.[40]
In this paper, we explicitly consider the regulation mechanism of radiative recombination rate (RRR) in a quantum photocell with two coupled donors, and this intention requires a more detailed density matrix analysis via the master equations for this quantum open system. Furthermore, we focus on the inhibited RRR by quantum coherence generated from two different transition channels and by other controlled parameters of this photocell system. As compared to Ref. [15], some more explicit principal issues rather than practical implications are discussed deeply.
Before explicitly visiting the performance of quantum photocell (seen in Fig.
![](cpb_29_6_064207/cpb_29_6_064207_ieqn1.gif)
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Meanwhile, the acceptor’s states |c〉 with any excess energy will radiate phonons into the ambient environment. The recombination between the acceptor and the donor is also considered with a decay rate of Χ Γ, where Χ is the RRR, a dimensionless fraction. This loss channel brings the system back into the ground state without producing a work current, which could be a significant source of inefficiency. Finally, the state |v〉 decays back to the ground state |b〉, closing the cycle. In spite of another crucial loss procedure, the phonon-mediated energy relaxation still exits in this model, which are included in our kinetic model via the relaxation rates γ1, γ2, and
![](cpb_29_6_064207/cpb_29_6_064207_ieqn9.gif)
![](cpb_29_6_064207/cpb_29_6_064207_ieqn10.gif)
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Considering the steady solution to Eq. (
It is know that the losses due to radiative recombination in principle represent one of major loss mechanisms.[15] Therefore, based on Fig.
The current–voltage characteristic (red curves) and power (blue curves) delivered to the external load circuit versus the RRR Χ are plotted in Fig.
Therefore, it is of great significance to inhibit the radiative recombination in this quantum photocell model. Scully[15] proposed that the quantum yields in a different quantum photocell could be enhanced by the quantum coherence with or without driving fields compared to the classic one. In this quantum photocell, the Fano quantum coherence can be generated from two transition channels, i.e., |ai〉 ↔ |b〉 channel, and |ai〉 ↔ |c〉 channel. In the following, the RRR Χ will be discussed as functions of the transition rates γ1 and γ2 in Fig.
In this proposed photocell model, it should be mentioned that the quantum coherence is expressed by
The quantum coherence has been proved to be a very effective way to inhibit radiative recombination in this proposed quantum photocell. Are there any other ways to inhibit radiative recombination except for the quantum coherence? In the following, Fig.
The physical regime reflected in Fig.
Before concluding this work, we would like to point out some items. Although we try to use some experimental parameters to carry out theoretical discussion, we have not carried out the corresponding experimental verification. Our results may enlighten the current field of experimental research, and can even propose some interesting experimental research within this theoretical framework. We also remark that the theoretical significance will be considered in our forthcoming investigations.
In this work, we have proposed a inhibited RRR scenario in a quantum photocell with two coupled donors. After gazing at the influence of the RRR on the quantum yields, we explore the relationships among the quantum coherence generated by two different transition channels, gaps between the donor and acceptor, and the absorbed photon wavelength dependent RRR Χ. The results reveal that the quantum coherence generated by the monotone increments of transition rates γ1 and γ2 cannot furthest inhibit the RRR, whereas the quantum coherence generated by two equal γ1 and γ2 has a better inhibition on the RRR, which is different from the proposal of radiative recombination reduced by the mentioned quantum coherence scheme.[15] When the energy gap between donor and acceptor is small, the transport of photo-generated electrons is more smoothly carried out, and the inhibition effect on the RRR is more robust. When the short-wavelength photons are absorbed by this photocell, more photo-generated electrons with more kinetic energy could be transported to the external load more quickly, which results in the smaller RRR. These theoretical results may enlighten the current field of experimental research, and can even be helpful for proposal of some interesting experimental research within this theoretical framework.
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