Effective harvesting, detection, and conversion of IR radiation due to quantum dots with built-in charge
© Sablon et al; licensee Springer. 2011
Received: 16 August 2011
Accepted: 7 November 2011
Published: 7 November 2011
We analyze the effect of doping on photoelectron kinetics in quantum dot [QD] structures and find two strong effects of the built-in-dot charge. First, the built-in-dot charge enhances the infrared [IR] transitions in QD structures. This effect significantly increases electron coupling to IR radiation and improves harvesting of the IR power in QD solar cells. Second, the built-in charge creates potential barriers around dots, and these barriers strongly suppress capture processes for photocarriers of the same sign as the built-in-dot charge. The second effect exponentially increases the photoelectron lifetime in unipolar devices, such as IR photodetectors. In bipolar devices, such as solar cells, the solar radiation creates the built-in-dot charge that equates the electron and hole capture rates. By providing additional charge to QDs, the appropriate doping can significantly suppress the capture and recombination processes via QDs. These improvements of IR absorption and photocarrier kinetics radically increase the responsivity of IR photodetectors and photovoltaic efficiency of QD solar cells.
One of the main goals for the next generation of infrared [IR] imaging systems and solar cell photovoltaic devices is to increase the photoresponse to IR radiation . To enhance the IR photoresponse, it is necessary to (1) improve electron coupling to IR radiation and (2) increase the photocarrier lifetime, i.e., to suppress recombination losses. However, it is not easy to increase IR absorption without enhancement of recombination losses because by introducing electron levels, which provide strong IR transitions, we inevitably create additional channels for inverse processes that increase recombination losses.
This trade-off between IR absorption and recombination processes are well understood for a number of technologies and corresponding materials. For example, in the early 1960s, semiconductors with impurities which provide electron levels inside a semiconductor bandgap and induce IR transitions from localized impurity states to conducting states received significant attention. However, midgap impurities drastically enhance the recombination processes, i.e., the Shockley-Read-Hall recombination, and deteriorate the photovoltaic conversion efficiency [2, 3].
To accommodate the solar spectrum and utilize its IR portion, modern photovoltaic technology mainly employs multi-junction cells with different bandgaps . In these devices, each p-n junction cell is designed to effectively harvest solar energy within a certain spectral window close to the bandgap. According to theoretical modeling, in a multi-junction solar cell with five or more junctions, the ultimate photovoltaic efficiency may exceed 70%. However, current technology enables only triple-junction cells (Ge-substrate junction-InGaAs-AlInGaP) with the maximum conversion efficiency of approximately 42% for concentrator cells. Strong technological limitations are caused by the need for lattice match, thermal expansion match, and current match in the cascade of heterojunctions [5, 6].
Quantum-well structures are intensively investigated for applications in IR imaging and solar energy conversion. Some enhancement in conversion efficiency was observed in solar cells, based on planar quantum wells, due to increased resonance absorption. Quantum-well IR sensing is currently a well-established technology, which is widely used for detection and imaging at liquid nitrogen temperatures and below. However, at higher temperatures, the photoresponse tremendously decreases due to a strong reduction of photocarrier lifetime.
Recently, quantum-dot [QD] structures have attracted much attention due to their ability to enhance absorption of IR radiation via multiple energy levels introduced by QDs [7–9]. In QDs, the carriers are confined in all three dimensions. Electron states in separate dots can be connected via manageable tunneling coupling between QDs. Therefore, QD media provide numerous possibilities for nanoscale engineering of electron spectra by varying the dot size and shape as well as the concentration of QDs and geometry of a QD structure. Besides tunable IR absorption, QD structures offer wide flexibility for nano-engineering of electron processes via the built-in-dot charge, correlation of dot positions, and selective doping. The built-in charge induced by selective doping creates potential barriers around dots and prevents capture of carriers of the same sign as the built-in-dot charge.
In very recent works, we have reported a radical improvement on the responsivity of QD infrared photodetectors [QDIP]  and QD solar cell efficiency  due to strong inter-dot doping, which creates substantial built-in-dot charge. While up to now the incorporation of QDs improves the solar cell's performance just by a few percent , we demonstrated that QDs with the built-in charge of approximately six electrons per dot provide a 50% increase in photovoltaic efficiency . We also observed approximately 25 times increase of the photoresponse of QDIP when the built-in-dot charge increases from one electron to six electrons per dot . Research on the capabilities of QD media with built-in-dot charge is still far from completion.
In this work, we investigate the physical processes behind these radical improvements. We study the potential relief created by the built-in-dot charge and calculate potential barriers, which separate the conducting states in the media from the localized QD states. Taking into account the effects of the built-in-dot charge on the IR absorption and photoelectron kinetics, we propose a simple model, which adequately describes effects of doping on the operation of unipolar optoelectronic QD devices, such as QDIPs. We also analyze our data related to the operation of a QD solar cell and present basic contours of the model for the description of doping-induced effects in the kinetics of bipolar photocarriers in QD structures. We conclude that in both cases, the built-in-dot charge strongly enhances electron coupling to electromagnetic radiation and suppresses the most effective capture processes. These two factors allow us to improve the performance of QDIPs and QD solar cells.
Unipolar kinetics in QD structures: IR photodetectors
Donor concentration (1011 cm-2)
Number of electrons in dot, n
Built-in-dot charge, n q
To calculate the built-in-dot charge and investigate the potential profiles around dots, we used the nextnano3 software, which allows for simulation of multilayer structures combined with different materials of realistic geometries in one, two, and three spatial dimensions . This simulation tool self-consistently solves Schrödinger, Poisson, and current equations for electrons and holes. The conduction and valence bands of the structures are defined within a single-band or multi-band k·p model, which includes a strain.
As seen in Figure 2, the potential barriers around QDs are strongly asymmetric. The barriers in the QD planes, i.e., in the direction perpendicular to the current, are substantially smaller than the barriers in the direction of the current. This asymmetry has strong consequences for the kinetics of photocarriers.
where V(Q) is the height of the local potential barrier, which is a function of the built-in-dot charge Q = en q.
As shown in Figure 5, the height of the potential barrier in the direction parallel to the QD plane is substantially smaller than that in the perpendicular direction. Therefore, we expect that the capture processes in QD planes will dominate in the relaxation processes. Based on Figure 5, the corresponding barrier height is V || = bn q, where b = 2.5 meV. In the case of the intra-dot doping, the dot charge n q is equal to the dot population n reduced by the number of dopants p in the dot, i.e., n q = n - p. In the case of the inter-dot doping, the built-in-dot charge q is obviously equal to n.
Here, A is some constant which does not depend on doping. The pre-exponential factor in Equation 2 describes the increase of the absorption with increasing number of electrons in the dot n. The exponential factor describes the effect of potential barriers around dots on the photoelectron lifetime. It is proportional to the dot charge n q determined by the number of electrons and number of dopants in the dot.
Thus, the proposed relatively simple model provides a very good description of doping effects on the photoresponse of QD structures. We believe that such good agreement with the experiment evidences that the model adequately takes into account the main effects of doping on photoelectron kinetics.
Bipolar kinetics: solar cell with built-in-dot charge
The heterostructure solar cells are presently dominating the market of high-efficiency solar cells. They have a conversion efficiency of up to 42%, have high degradation robustness (enables applications in outer space), and allow for high concentration of solar energy. Despite the impressive achievements in heterostructure technologies, the pace of improvement of solar cell efficiency is very slow. It is limited by the following factors: thermalization losses, losses related to junction and contact voltages, and recombination losses. Multi-junction solar cells with different bandgaps have been developed to minimize thermalization losses in heterostructure solar cells. In these devices, each p-n junction cell is designed to effectively harvest solar energy within a certain spectral window close to the bandgap. To date, the triple-junction cells reach a maximum conversion efficiency of approximately 42%, in the case of concentrator cells. Technological limitations are determined by the need to match crystalline lattices, thermal expansion coefficients, and the most difficult, to match all the photoinduced currents in the cascade of heterojunctions.
QD structures are considered very promising photovoltaic nanomaterials due to their ability to extend the conversion of solar energy into the IR range [7–9]. Up to now, most of the emphasis has been placed on the QD solar cell with an intermediate band, which is formed from discrete QD levels due to tunneling coupling between QDs. Theoretical calculations predict that the intermediate-band solar cell can provide an efficiency of approximately 63%. However, intensive experimental efforts to improve the performance of intermediate-band solar cells show limited success. In comparison with a reference cell, the short-circuit photocurrent of the QD intermediate-band cells increases only by a few percent . It is well understood that the addition of QDs significantly increases the absorption of IR radiation, but simultaneously, QDs drastically increase recombination processes. For this reason, the corresponding recombination losses are hardly compensated by the conversion of IR radiation. To solve this problem, one should further suppress the photocarrier capture into QDs.
As we have discussed in the previous section in relation to QDIP, potential barriers around dots provide an effective and reliable way to control the photoelectron processes at room temperatures. However, the bipolar kinetics of electrons and holes in QD structures is much more complex. The built-in-dot charge suppresses solely the capture processes of the carriers of the same sign as the dot charge. Again, this suppression is strong and has an exponential dependence on the dot charge (Equation 1). Under radiation, in stationary conditions of the dynamic equilibrium, the built-in-dot charge equates the capture rates of electrons and holes. Thus, to minimize recombination losses, the built-in-dot charge should be used for the suppression of the most effective capture processes. Here, we investigate this concept and study the effects of built-in-dot charge on IR harvesting, recombination, and efficiency of QD solar cells.
The effect of the built-in-dot charge on the capture processes has been studied by employing photoluminescence [PL] measurements. PL in QD solar cells was measured under short-circuit conditions. To stimulate PL, we used the 514-nm line from an Argon-ion laser. PL signals from the samples were measured by an InGaAs detector array.
The IR harvesting and conversion have been investigated by measurements of the photoresponse under 1 Sun radiation with the GaAs filter which eliminates high-energy photons with a wavelength less than 880 nm. I-V characteristics obtained with the GaAs filter are corrected for reflectivity losses and presented in Figure 9. As seen, the IR photoresponse significantly increases due to the built-in-dot charge. In the device doped to provide two electrons per dot, we observe an increase in the photocurrent of approximately 7.0 mA/cm2 compared with the reference cell. The photocurrent from the sample with six electrons per dot increases by approximately 9 mA/cm2. As expected, the GaAs reference cell does not show any IR photoresponse. These measurements directly demonstrate strong harvesting and effective conversion of IR radiation by solar cells with the built-in-dot charge.
Our new approach, based on engineering 3-D potential barriers introduced by QDs with built-in-dot charge, provides real opportunities to radically improve the performance of IR photodetectors and solar cells. These improvements are due to effective harvesting of IR radiation via intra-band QD transitions and transitions from QDs to the conducting states in the matrix. Potential barriers also prevent photoelectron capture and increase the photoelectron lifetime. Modern technologies allow for fabrication of specific structures with various positions/distributions of QDs. This provides numerous possibilities for engineering very specific 3-D barriers. We believe that in future electronic and sensing technologies, the 3-D barriers will be employed in every device as nowadays, technology employs semiconductor heterostructures with one-dimensional barriers.
The work is supported by the Air Force Office of Scientific Research.
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