Recent Progress Towards Quantum Dot Solar Cells with Enhanced Optical Absorption
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Quantum dot solar cells, as a promising candidate for the next generation solar cell technology, have received tremendous attention in the last 10 years. Some recent developments in epitaxy growth and device structures have opened up new avenues for practical quantum dot solar cells. Unfortunately, the performance of quantum dot solar cells is often plagued by marginal photon absorption. In this review, we focus on the recent progress made in enhancing optical absorption in quantum dot solar cells, including optimization of quantum dot growth, improving the solar cells structure, and engineering light trapping techniques.
KeywordsSolar Cell Intermediate Band Droplet Epitaxy Single Junction Solar Cell Generation Solar Cell
The world energy and environmental crisis urgently calls for development of renewable energies. Among various renewable energy sources, solar energy is abundant and clean. Although solar energy has been an ideal renewable energy, the harvesting of the free and abundant sunshine can be quite costly, which limits the wide deployment of solar power. The next generation of solar cells with high efficiency over 50 % is in urgent need to achieve affordable rates below 0.10 €/kWh (0.14 $/kWh) . In the last 10 years, a lot of efforts have been devoted to low-dimensional structures as building blocks for next generation solar cells [2, 3, 4, 5, 6, 7]. Among these nanostructures, the zero-dimensional nature of quantum dots (QDs) with discrete energy levels makes an ideal candidate for intermediate band-based solar cells with a theoretical efficiency of 63 % . Since Luque and Martí proposed the concept of intermediate band solar cell (IBSC), QD solar cells (QDSCs) have attracted great attention and substantial progress has been made in this field [9, 10, 11, 12, 13, 14].
Compared with conventional single junction solar cells, an IBSC allows two sub-bandgap photons to create an electron-hole pair via a mid-gap intermediate band. The intermediate energy band introduces additional photon absorption, which in turn contributes to higher photocurrent . The improved utilization of the solar spectrum via intermediate band-assisted transitions to absorb otherwise wasted low-energy photons can largely improve photocurrent and potentially exceed the Shockley–Queisser limit [15, 16, 17]. Although the early work has provided solid understanding of the operational principles of IBSCs [18, 19, 20, 21, 22, 23, 24], the experimental studies of QD-IBSCs have not achieved any notable improvement in their overall conversion efficiency. QDSCs have often shown improved short-circuit currents compared with the bulk single junction solar cell without QDs, but the overall contribution to efficiency enhancement from the QDs is marginal. Therefore, research efforts in the last 10 years have been mainly focused on improving the photocurrent generation.
In this paper, we review the recent progress made in QDSCs with main focus on the recent effects involving photocurrent enhancement, which has been the major limited to realize high-efficiency QDSCs. A variety of methods used to enhance the optical absorption and photocarrier collection have been reviewed. Finally, this review summarizes the progress of QDSCs with enhanced photocurrent. More comprehensive discussion can also be found in Ref. [14, 25].
Principles of Quantum Dot Solar Cells
QDSCs share same device structures with the quantum well solar cells (QWSCs), which incorporate low-dimensional nanomaterials made from narrow bandgap semiconductors and hence boost the device efficiency by capturing low-energy photons below the primary bandgap. Compared with QWSCs, QDs, instead of QWs, are used at a solar cell junction. The atom-like density of states in QDs not only enables additional photocurrent generation via the discrete energy levels but also preserves the open-circuit voltage . The carrier confinement in all three-dimensions in QDs can enable isolated quasi-Fermi levels which are required to realize of IBSCs [4, 8]. As a result, much higher conversion efficiency is expected from QDSCs compared with QWSCs. Therefore, the unique properties of QDs and the attractive concept of IBSCs have led to intensive research efforts on QD-IBSCs. The research of QD-IBSCs is also largely benefited from the well-established fabrication methods of high-quality QDs in the last couple of decades. Most of the QDSCs adopt a device structure with self-assembled QDs imbedded between the emitter and base of a bulk single junction solar cell, as shown in Fig. 1b. In(Ga)As/GaAs QD system is most used because of its mature fabrication techniques and well-understood optical properties. On the other hand, the transition energies in In(Ga)As/GaAs QDs are quite different from the optimal values for the ideal IBSC, and high-efficiency QDSCs have not been realized yet, although a high theoretical efficiency of 52.8 % is still predicted . Nonetheless, In(Ga)As/GaAs QDSCs have successfully demonstrated the basic operating principles of the IBSCs , including splitting of quasi-Fermi levels  and QD-mediated two-photon absorption [11, 27]. Therefore, in the last few years, many of the research efforts of QDSCs have been focused on realizing practical QD-IBSCs with high efficiency. In order to achieve this goal, the major challenges associated with QDSCs are yet to be addressed, including recombination in the QDs (radiative and non-radiative), marginal photocurrent collected from the QDs, and degradation of open-circuit voltage . The radiative recombination via the QD intermediate band can be largely suppressed under concentrated light when CB-VB recombination dominates. However, additional non-radiative recombination paths are presented in the QDSCs due to accumulated strain in S-K QDs . To tackle this issue, improvement in QD fabrication and development of new growth techniques have been explored [29, 30, 31, 32]. In addition to the strain-induced defects that largely limit the QD absorption volume, the sub-bandgap absorption in QDSCs is rather low and only contributes to ~1 % of the overall efficiency . Moreover, the slightly improved photocurrent has been largely undermined by the voltage loss as a result of thermal coupling of the QD states and the continuum states [10, 30, 33]. Therefore, the major research activities have been focused on addressing these challenges facing QD-IBSCs. The following sections will review the recent efforts to achieve practical high-efficiency QDSCs through improving photocurrent.
Recent Efforts to Improve Photocurrent of QDSCs
Although the addition of QDs in a single junction solar cell normally shows additional photocurrent, improvement in short-circuit current is well below the expectation for high-efficiency solar cells. The marginal improvement in the device efficiency with QDs is largely attributed to the non-radiative recombination, low QD absorption volume, and low optical transition rate . In order to obtain high photocurrent, both the QD material quality and device structure have to be optimized. Moreover, photonic structures can also be used to boost the light absorption in the QDSCs. Here, these efforts are summarized.
Optimization of QDs
The difficulty to increase the absorption volume QDs, e.g., the number of QD layers, is that the accumulated strain generates various types of defects and largely undermines the improvement of photon absorption [22, 37]. To minimize the number of strain-induced defects that are deleterious to both optical and electronic properties, strain-compensation layers are deposited for multiple stacked QDSCs . By using GaP strain compensation layers, InAs QDs with good structural and optical properties up to 50 layers have been reported . The improved material quality has also led to increase in short-circuit current and reduced dark current . Additionally, the reduced strain-induced defects also decrease non-radiative recombination, and then, high open-circuit voltage can be obtained . Bailey et al. reported 0.5 % enhancement in absolute efficiency from a 40-layer QDSC with reduced InAs coverage and GaP strain compensation layers compared with the GaAs reference cell .
A number of different materials have also been explored to improve QD quality. Highly stacked QDs up to 100 layers are also achieved by using dilute nitride GaAsN strain compensation layers [41, 42]. The effectively compensated strain results in a distinct improvement in short-circuit current as high as 2.47 mA/cm2 . Strain-balanced In0.47Ga0.53As/GaAs1 − xPx QDs have also been reported with improved quality as well as uniformity on GaAs (311) substrates . Furthermore, strain-compensated InAs/GaNAs QDs with additional strain-mediating GaInNAs layers can not only shift the absorption to long wavelength but also increase the surface density of QDs . Strain reducing layers is also beneficial for realizing high-performance QDSCs. It has also been reported that an addition of Ga0.90In0.10As strain-reducing layers in an InAs/GaAs QDSC results in a 1.19 % improvement of the conversion efficiency of a GaInP/Ga(In)As/Ge triple junction solar cell due to reduced Shockley–Read–Hall recombination centers .
Apart from the strained S-K QDs and SML QDs, quantum structures grown by different modes can be used as promising alternatives for improving photocurrent. Quantum well dots (QWD), two-dimensional layers with lateral modulation of thickness and composition, have unity surface coverage, which facilitates higher absorption as compared with S-K QDs and demonstrates significantly improve sub-bandgap photocurrent . Strain-free quantum structures fabricated by droplet epitaxy have also show promise in boosting photon absorption [52, 53, 54, 55]. Based on these strain-free nanostructures grown by droplet epitaxy, additional photocurrent was clearly demonstrated [56, 57, 58, 59]. Although further efforts to improve material quality are needed, the two-photon absorption observed in strain-free QDSCs opens new opportunities for QD-based high-efficiency intermediate band solar cells [59, 60].
Optimization of Device Structures
Engineering the QDs locally to change the carrier dynamics can also lead to a higher short-circuit current. A simple but effective way to achieve this goal is doping in the QD region, which has been reported to reduce non-radiative recombination via defect passivation  and to improve the photocarrier collection by build-in field . The doping in the QD region forms charged QDs that also reduce the probability of electron capture. Although state filling can also decrease interband quantum dot absorption , the charged QDs enhance the collection of photocarriers generated above bandgap and lead to overall improvement in photocurrent [67, 68]. It has also been shown that the positioning of the QD layers can also largely affect the performance of QDSCs , which also reflects the effects of doping .
Substantial efforts have also been made to type II QDs to improve short-circuit current [20, 71, 72, 73, 74, 75]. QDSCs can benefit from largely enhanced absorption coefficient, particularly for transitions from extended states to bound states, by using type II QDs rather than type I QDs , as depicted in Fig. 4b. Yet, it is still needed to find new material system with even high absorption coefficient to compete with the higher bound-to-bound state absorption coefficient in type I QDs. Another attractive feature of the type II QDSCs is the extremely long radiative lifetime over 200 ns . Such a long carrier radiative lifetime facilitates the photocarrier collection as long as non-radiative recombination centers are suppressed with the presence of additional strain . Moreover, the reduced Auger recombination rate in type II structure can also benefit the QDSC performance .
A very interesting and promising method to improve photocurrent of QDSCs is light trapping. To full fill the promise of QDSCs, the QD density needs to be significantly improved (>1000). Such a requirement poses a significant challenge for material growth. If the optical path can be improved, high density of QDs is not necessarily required . For example, given a QD density achievable by existing growth techniques, an optical absorption enhancement over 50 can potentially realize high-efficiency QDSCs beyond the Shockley–Queisser limit .
In the present paper, we have briefly reviewed the efforts to improve the photocurrent in QDSCs. A number of different methods have so far been examined to improve the optical absorption as well as photocarrier collection in QDSCs. Although each of these methods shows promise in boosting the cell performance in terms of photocurrent, there is still a lot of room to improve. Till now, the absorption from the QDs is still much inferior to the bulk absorption. Undoubtedly, novel designs and further improved growth of QDSCs need to be in place to achieve efficiency exceeding that of single junction solar cells. Nonetheless, the progress made so far discussed here, including growth of high-density QDSCs, modification of carrier dynamics, and light trapping, provides helpful guidelines for further development of high-efficiency QDSCs.
This work was supported by the National Natural Science Foundation of China under Project no. 51302030 and 61474015 and National Basic Research Program (973) of China under Project no. 2013CB933801.
ZZ collected the documents and wrote the manuscript. PY drew and prepared all figures in the manuscript. HJ and ZMW provided the indispensable guidance. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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