Semiconductor quantum dots (QDs) have drawn considerable interest for more than 20 years because of the optoelectronic advantages based on a zero-dimensional system. The photovoltaic applications using self-assembled quantum dots (SAQDs) and colloidal quantum dots (CQDs) have the potential to enhance the photogeneration of carriers through the QD energy level or band.12.3.4.–5 An increase in the maximum attainable thermodynamic conversion efficiency is theoretically predicted by expanding the available spectrum for the photoexcitation. However, the fundamental physics of carrier transport and carrier collection must be considered for the real photovoltaic operation. Understanding of their basic photovoltaic mechanisms in SAQDs and CQDs is essential to a design of ideal solar cell structures having high power conversion efficiency (PCE). Additionally, technical progress in the growth of SAQDs and the synthesis of CQDs is most important to construct the realistic solar cell structures, which include appropriate materials with high crystal quality, well-controlled heterointerface, and surface passivation. The recent progress in SAQD solar cells and CQDSCs is remarkable. Consequently, for their future prospects, it is significant to discuss their improved properties and current problems.
Recently, several papers on QDSCs, which were specialized in either SAQDs or CQDs, have been published. For III-V compounds, such as InAs/GaAs SAQDSCs, Okada et al.6 have reviewed the latest progress on intermediate band solar cells (IBSCs) focused on the thermodynamics of solar energy conversion, the device physics, and the two-step intersub-band absorption/recombination. Wu et al.7 have summarized recent developments in QD optoelectronic devices, including -wavelength QD lasers, QD infrared photodetectors, and QD-IBSCs. Zheng et al.8 reviewed the recent progress in QDSC, especially on the enhanced optical absorption. On the other hand, for CQD-based solar cells, Kramer and Sargent9 have thoroughly reviewed the architecture of CQDSCs with special focus on the material and device. Duan et al.10have reviewed the recent advance in materials for QD-sensitized solar cell application. Wang11 reviewed the device physics in CQDSC. In this paper, we united highlights of more recent advances in both SAQDSCs and CQDSCs. The main subject of this review is recent experimental results and understanding of carrier dynamics in both solar cell operations.
In SAQD-SCs (Chapter 2), we focus on IBSCs with SAQDs. The IBSC was proposed as a way of breaking the detailed balance efficiency limits of conventional single gap solar cells.1 The fundamental operation of the IBSC is based on two-step photoexcitation of carriers from a valence band to a conduction band via a miniband of coupled QDs, which is located within a band gap of the host material. Although the two-step photoexcitation enhances photocurrent, an open-circuit voltage () often decreases. According to theoretical calculations using a detailed balance model, a large QD number of more than and high concentrated sunlight are needed to achieve high PCE.1213.–14 To prepare the ultrahigh-density QDs with an electronic coupling, stacking growth1522.214.171.124.20.–21 and in-plane high-density growth22,23 techniques have been developed. Recently, many researchers are attempting to explain the experimental results of QD-IBSCs by a more realistic model, including various processes: photoexcitation, carrier separation, carrier transport, and recombination. Therefore, we review such carrier dynamics in the QD-IBSC operation in this chapter.
The state-of-the-art photovoltaic devices using CQDs are presented in Chapter 3. The most common approach to the synthesis of CQDs is the controlled nucleation and growth of nanoparticles in a chemical solution of precursors containing the metal and anion sources. Such a convenient method has many advantages for utilizing various materials and reducing the process cost.24,25 In this chapter, recent progress in CQD-sensitized solar cells and CQD heterojunction solar cells (HSCs) is reviewed briefly. In particular, the PCE of CQDSCs improved remarkably in the last several years. The carrier dynamics including photogeneration, spatial separation, transfer, and recombination are key issues for increasing the efficiency. Recently, development of well-controlled heterointerface and effective surface passivation is pushed forward to suppress the undesired carrier recombination and enhance the carrier transport.
InAs/GaAs Quantum Dot Solar Cell: Effect of Carrier Dynamics on
The issue that has received the most attention in recent InAs/GaAs QDSC research is the reduction of the when compared to the GaAs control cell.26126.96.36.199.–31 Experimentally confirmed short circuit currents for both GaAs- and InAs QD-based solar cells are almost identical (note: Some InAs /GaAs QDSC showed slightly higher than GaAs SC, but the increase amount is fairly small and usually ).15,3233.34.–35 Under such a situation, the lower observed in QDSC was attributed mainly to the larger dark recombination current when compared to GaAs control cell. To understand the dark recombination current in QDSC, modeling of the carrier dynamics is indispensable.6,3637.–38 A simplified two-voltage region model is proposed by us, as shown in in Figs. 1(a)–1(c).39,40
For a p-i-n type QDSC under low forward bias voltage [see Fig. 1(b)], the voltage is mainly applied across the space charge region (SCR). For the recombination behavior within this region, the dark current for GaAs and InAs/GaAs QDSC is described using a two-diode model as follows:
where the is the Shockley–Read–Hall (SRH) recombination dark saturation current, and its magnitude is usually proportional to the density of defects.28 On the other hand, is the radiative or diffusion recombination dark saturation current, and its magnitude is basically related to the material properties such as bandgap (the lower the bandgap, the higher the ) and diffusion length (the shorter diffusion length, the higher the ).37,40,41 The generation of defects is usually more enhanced for the InAs QDs-embedded GaAs because of the accumulated strain compared with the i-GaAs layer without QDs, which inevitably cause the increase of and the decrease of .
When the p-i-n solar cell is under high forward bias [see Fig. 1(c)], the SCR region becomes negligible and the voltage drop is applied equally across the whole device; the dark currents for GaAs SC and InAs/GaAs QDSC are described as follows:
It can be seen from here that the total dark current in the InAs/GaAs QDSC might be higher than the GaAs SC due to the increased caused by the reduced effective in the QD region, thus causing the reduction of .
The reduction of in InAs/GaAs QDSC has recently become a central issue for further efficiency improvement. In this review, we have limited our focus on the works that have mainly dealt with interplay between carrier dynamics and device performance. Meanwhile, to ensure timeliness and currency, the works reviewed here were sorted and scrutinized from the literatures published in the past 2 to 3 years. The model described in this section will act as a supportive guide to better interpret the results summarized from the reviewed papers.
Carrier Escape Nature and Electric Field Effect
The photoelectrons generated by InAs QD light absorption usually, at first, have to escape from the potential formed between InAs QD and GaAs buffer. Understanding the carrier escape mechanism is crucial to further improving the device performance.42,43 Sellers et al.42 have evaluated the carrier escape mechanism in InAs/GaAs QDSC by using photocurrent measurements under sub-bandgap illumination. The 1.17-eV photon source and 0.8-eV photo source were chosen, and their fluencies were varied to reveal the fundamental carrier escaping principle. The inset Fig. 2(a) depicts the three competing mechanisms for carrier extraction from intermediate states in QD: tunneling (pathway 1), thermal (pathway 2), and optically driven (pathway 3).37,44 When the sample was solely under illumination of a 1.17-eV photon source, photocurrent was found to increase linearly with laser fluence over the range of fluences studied, as shown in Fig. 2(b). The linear increase suggests that thermal and tunneling escape mechanisms dominate any carrier escape driven optically by 1.17-eV photons. The effect of adding a 0.8-eV photon source was presented in Fig. 2(c). Here, was used to describe the increased photocurrent due to the addition of 0.8-eV light. Again, they found that the value increased linearly with increasing 0.80-eV fluence under a fixed fluence value of 1.17-eV photon source. If the photocurrent was limited solely by the generation of carriers within the QDs, should become saturated for high fluences of 0.8-eV photons.42 The absence of saturation under these conditions suggests that there is a continuous supply of carriers that can be optically excited by 0.8-eV photons. The authors have proposed a carrier “retrapping model” to explain the experimental results.42 Based on this model, depends linearly on the fluence of 0.8-eV photons because the addition of an optical escape pathway reduces the mean time carriers spent in traps in either the QD or wetting layer and, therefore, increases the conductivity (therefore, the photocurrent) even for a fixed number of carriers.30 It is worthwhile to mention here that in a recent work, Asahi et al.45 have reported a saturation of two-step photocurrents. However, unlike the work reported here involving the two-step photoexcitation from valence band to intermediate states and from intermediate states to conduction band, Asahi et al.45 observed the saturation by tuning “two-step” photoexcitation from valance band to conduction band and from intermediate states to conduction band.
Related to the dynamic nature of carrier escape and separation mechanism, it has been shown that the excitonic dynamics of electrons and holes could be responsible for the nonadditive behavior of the photocurrent contributed by the QDs in a QDSC.37 This nonadditive characteristic was observed when the total photocurrent of the cell was much lower than the sum of the photocurrents contributed by the barrier and the QDs separately, as reported for undoped InAs/GaAs solar cells.46 Cedola et al.46 have investigated the excitonic and nonexcitonic dynamic nature of the carrier escape of the InAs/GaAs QDSC by numerical simulation. The energy band model is illustrated in Fig. 3(a). By assuming identical or separate time constants for the intersubband carrier transfer processes in the ground and excited states, a device-level model developed in combining drift-diffusion equations for the barrier and rate equations for the QD kinetics was successfully applied.47,48 Figure 3(b) shows their simulated results. The nonexcitonic cell clearly shows a nearly linear behavior in the short-circuit condition (). On the other hand, the excitonic cell exhibits a strong nonlinear operation ).49 The linear behavior of the nonexcitonic QDSC indicates that practically all the extra carriers contributed by the QDs are being collected at the cell contacts.40,42 On the contrary, the nonadditive characteristic shown by the QD current in the excitonic QDSC suggest that all the carriers photogenerated at the QDs are recombining. Further analysis has been performed by visualizing the processes of carrier interchange between states at each QD layer. The results revealed that the increment of the recombination is mainly due to the increased hole population in the QDs while playing a central role in the response of the excitonic device.
The effect of the internal electric field on carrier escape and separation was investigated quantitatively by Kasamatsu et al.50 They have experimentally fabricated InAs/GaAs QDSC with different built-in electric fields by controlling the thickness of intrinsic buffer layers surrounding the InAs/GaAs QDs. The internal electric fields applied to the QDs were estimated as 46 and when the total intrinsic layer thicknesses were 299.3 and 69.8 nm, respectively. The authors calibrated the electric field effect by monitoring the carrier dynamic behavior using time-resolved photoluminescence (PL) measurements.50 Figure 4 displays the detection wavelength dependence of the PL decay profile measured at 3K. The sample with shows a double exponential decay. The rapid initial decay is slightly shorter than 2 ns, and the following slow decay becomes longer than 2 ns. For an extremely large internal electric field as , the decay is very fast and can be described by double exponential components of s1 and s2 ().51 A strong electric field of causes tunneling-assisted electron escape that occurs easily. The electron escape rapidly reduces the PL intensity with a time constant s1 of 0.46 ns. This short lifetime in the high electric field will prevent photoexcitation of electrons in the intermediate band (IB) in QDs.5152.–53 Meanwhile, as mentioned in Sec. 2.1, the dark saturation current is treated as radiative recombination based on the detailed balance principle. For the sample with a high internal electric field, the existence of additional tunneling recombination path will increase the dark saturation current , and the of this SC eventually reduces according to Eqs. (2) and (4).37,50
In addition to the internal electric field, the external electric field has also been reported to exert influence on the carrier dynamics. Shiokawa et al.54 have reported an inplane ultrahigh-density InAs (2.2 mL) QDs with density of grown on the GaAsSb/GaAs(001) by molecular beam epitaxy. By applying voltage ranging from to , the PL decay time showed strong bias dependence, as shown in Fig. 5(a). In particular, the long decay time of 10 ns was obtained at .54 By applying forward bias, it favors the formation of in-plane coupling among the QDs. This is again confirmed in Fig. 5(b) by the PL peak energy shift under different bias voltages. For the forward bias condition, the PL peak shifted toward the low energy side because the enhanced coupling among QDs caused the electron to relax at much lower ground energy states in the QDs with large size.22,23,55 Meanwhile, the reverse bias was also found to shift the PL peak energy to long wavelength side. This was partially attributed to the strong Stark effect and partially to the fast electron escape rate, which greatly suppressed the high energy interband transition in the QDs.
Doping Effect on InAs/GaAs Quantum Dot Solar Cell Performance
Typically, QDs grown in the intrinsic region are subjected to high electric field, and carriers are quickly swept away from the nanostructures as soon as they are excited into the bulk continuum.5657.58.59.–60 Doping QD in the intrinsic region will affect the band profile and, therefore, the electric field in the intrinsic region. Accordingly, the change in electric field will also affect carrier dynamics as well as the solar cell device performance. Polly et al.61 have reported on both the theoretical and experimental results of the effect of delta-doping in the InAs/GaAs QD on the solar cell performance. Figure 6(a) shows the simulated results where the doping of () has dramatically flattened the i-region and has shifted the intrinsic region to the emitter. A consequence of this shift in the electric field can be seen in Fig. 6(b), which shows the reduced SRH recombination rate at 1.0 V applied bias. They have also found that doping the hole () pushed the intrinsic region toward base layer while reducing the SRH recombination similarly to the electron doping.47 As shown in Fig. 6(c), the experimental results were consistent with the simulated results. The doping resulted in the highest of 0.932 V, which can be considered a direct effect from the decreased SRH recombination due to the shift of intrinsic region.29,40 However, the doping-induced band flattening has weakened the carrier collection due to the decreased electric field in the QD region. This negative effect was directly linked to the reduction of for all doped samples. It is interesting to note that the experimental results from Polly et al. are not consistent with the theoretical simulation results by Yoshida et al.62 The discrepancy is probably because in the simulation an ideal IBSC operation modeling was applied including the electron occupancy in IB, which is usually more difficult to realize by the current doping technique. In other words, the doping in the IB is not able to form the Fermi-level, as shown in the IB region of Fig. 4(b), in the work of Yoshida et al.62 Therefore, besides the region near the top emitter and the bottom base layer, the occupancy rate in other regions of IB is also not close to the optimal value of used in the work of Yoshida et al.
With respect to the doping effect on QDSC, Li et al.63 have recently reported interesting results regarding electron-doping in QD devices, especially its potential impact on the electron capture potential, which affects (1) carrier collection efficiency and (2) below bandgap photon absorption via transitions to quantum confined states. Figure 7(a) shows the current–voltage characteristics of the fabricated solar cells with different electron doping concentrations. The , to devices exhibit a of 10.1, 11.8, and , respectively. The authors have interpreted the results as a consequence of so-called “charging” effect. As depicted in Fig. 7(b), when above-bandgap photogenerated electrons move through the QDs layer, the repulsive force exerted by the negatively charged QDs can alter the electron trajectory in such a way as to reduce the probability of electron trapping.64 The Coulomb potential exerted by the negatively charged QDs is a competing process that acts on the mobile electrons and competes with the QDs trapping potential. When electrons are captured, the trapping effects are progressively deactivated.65 For the effect (2), the authors found that the device had a much higher external quantum efficiency (EQE) value measured at the QD transition energy (1.1 eV) as compared to the doped QD devices. This was considered a result of the available number of unoccupied confined electron states being reduced with further doping. According to the Fermi’s Golden rule, the total transition rate and the absorption in QDs decrease correspondingly, as schematically illustrated in Fig. 7(c).37,38 These conclusions were further verified by the PL results shown in Fig. 7(d) in which the intensity of QDs emission reduces as the doping increases.
Effect of Quantum Dot Location on Carrier Transportation and Recombination
QD position in the intrinsic region also showed great influence on carrier dynamics and the device performance.41,6667.–68 Driscoll et al.69 performed simulations on three positions for InAs QD located in the intrinsic region: (1) near the n-doped base, (2) exact center of i-region, and (3) near the p-doped emitter.69From the simulation, they found that was nearly identical for the three positions as all were located in a high electric field region and carrier collection remained efficient for all three conditions. However, the model predicts an additional loss of 20 mV in the for the QD located in the exact center of the intrinsic region versus those located near the doped edges.69 The reduction of the was mainly attributed to SRH recombination, which showed the maximum value when the electron and hole density were similar at the place such as the center of the intrinsic region.66 They have also fabricated three samples experimentally with the three QD locations mentioned above to verify the simulated results.69 Figure 8(a) summarized the EQE results for the three samples along with the baseline GaAs cell. It is clear that the bulk spectral response from the QD devices are similar to that of the baseline cell, indicating that neither the introduction of the QDs nor their position has had any adverse effects on the carrier collection efficiency.69 However, dramatic variation in the was observed, as shown in Fig. 8(b). The emitter-shifted cell exhibited a substantial decrease in down to 0.863 V, as compared to the base-shifted and centered cells with promising voltages of 0.957 and 0.945 V, respectively. The reduction in the emitter-shifted cell was contradictory to the previous theoretical simulation results.49 A new simulation model was developed by taking the unintentional background n-type doping into consideration, and the reduction of in the emitter-shifted sample was well reproduced and became consistent with the experimental results.
Kechiantz70 have reported a novel QDSC structure in which a stack of strain-compensated GaSb/GaAs type-II QDs were embedded in the p-doped emitter region, thus spatially far separated from the depletion region. The original motivation for this theoretical work was to find an ultimate solution to the suppression of the reduction of in QD while leaving the advantages of QD intact so that it can still act effectively as additional light absorption source to increase , as well as the building block for the realizing IBSC with the conservation of .71,72 The calculation predicts that the concentration from 1-sun to 500-sun increases the efficiency from 30% to 50%, respectively, without the degradation of the .
Other Featured Carrier Dynamics in InAs/GaAs Quantum Dot Solar Cell
Interdot transportation through Urbach tail
Li et al.73 have proposed an “extended Urbach tail” model, shown in Fig. 9(a), to explain the interdot carrier transport and below-bandgap photon absorption. They have modeled the Urbach tail with two parameters and , where is the characteristic width of the absorption edge and is a scaling factor.74 As shown in Fig. 9(b), a fit to the linear dependence up to the edge of the QD transition energy estimated the value for and for bulk and QDSC and found 55 meV for QD compared with 15 meV for the bulk GaAs layer.74 This indicates that an exponentially increasing continuum density of states occurred in the surrounding GaAs matrix, which facilitates the energy relaxation of excited electrons within the QD conduction band potential. The measured for the QDSC is 0.77 V while the bulk GaAs control cell is 0.94 V. The deduced conclusion agrees well with the dark curve fitting analysis results, where they have found that the observed change of lower is due to the change of the SRH recombination coefficient.37
Quantum dot scattering effect on transportation
Semichaevsky and Johnson75 have used a multiscale model for carrier transport to simulate a p-i-n solar cell that includes InAs/GaAs QDs. Their results suggest that, while contributing to the photocurrent due to absorption of photons with energies less than the bulk GaAs bandgap, stacked layers of InAs QD arrays with high in-plane densities used in a solar cell can inhibit the transport of photocarriers originating from the absorption of photons with energies above bulk GaAs bandgap.75 Quantum scattering of carriers by the confinement potential, resulting in longer paths travelled by the carriers and thus an increased nonradiative (NR) recombination in the intrinsic region of the cell.76 The reflected carriers also form an additional space charge that reduces the built-in field in the heterostructure region. As suggested in Eq. (2), an increased NR recombination will cause the reduction of .37,42,48
Upper limit of Voc for nearly defect-free InAs/GaAs quantum dot solar cell
One question has recently been raised regarding the realistic upper limit of for InAs/GaAs QDSC. 30,32,34,37,47 Under the ideal case, the device is assumed to be defect-free; therefore, the SRH or NR recombination will be completely eliminated. As discussed in Sec. 2.1, for a InAs/GaAs QDSC to be able to reach the same as GaAs SC, the dark saturation current must be minimized to the value of .38 Jolley et al.77 have addressed this issue by performing both experimental and theoretical studies. As shown in Fig. 10(a), a comparison between the InAs/GaAs QDSC and GaAs SC showed that the QDSC has a smaller activation energy of than GaAs SC, which is regarded as the leading cause for the reduction of .78 Figure 10(b) shows the proposed principle in which the QD layer next to the n-layer will have a raised potential, which results in a reduction of the energy required to transport a hole from the p-layer to the QD layers and, therefore, a reduction in dark current activation energy.79,80 The crucial point to compensate for this 0.1 V reduction in activation energy is that the optical excitation time must be sufficiently short to reduce the carrier occupation in QDs. Given the fast thermal processes, it is expected that intersubband optical excitation time would have to be on the order of 1 ns or below to have a large impact on the dark current processes; this is, however, hardly achievable.81 Therefore, 0.1 V difference in between InAs/GaAs QDSC and GaAs SC can be regarded as the realistic upper limit for the current InAs/GaAs QDSC and is expected to hold even in the complete absence of crystal defects in the device.
It is noteworthy to mention that in very recent research, Varghese et al.82 have reported inspiring results by demonstrating a complete voltage recovery in the InAs/GaAs QDSCs by completely suppressing the fast capture of photoelectrons from the GaAs conduction band to the localized states in QDs. The mechanism of this approach reflects exactly the mechanism described in Fig. 10(b), which we proposed for the upper limit of 0.1 V difference.
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