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Abstract
The spatial distribution of electric field in photovoltaic multiple quantum wells (MQWs) is extremely important to dictate the mutual competition of photoelectric conversion and optical transition. Here, electric-field-driven photoluminescence (PL) in both steady-state and transient-state has been utilized to directly investigate the internal photoelectric conversion processes in InGaN-based MQW photovoltaic cell. As applying the reversed external electric field, the compensation of the quantum confined stark effect (QCSE) in InGaN QW is beneficial to help the photoabsorbed minor carriers drift out from the localized states, whereas extremely weakening the PL radiative recombination. A directly driven force by the reversed external electric field decreases the transit time of photocarriers drifting in InGaN QW. And hence, the overall dynamic PL decay including both the slow and fast processes gradually speeds up from 19.2 ns at the open-circuit condition to 3.9 ns at a negative bias of −3 V. In particular, the slow PL decay lifetime declines more quickly than that of the fast one. It is the delocalization of photocarriers by electric-field drift that helps to further enhance the high-efficiency photoelectric conversion except for the tunneling transport in InGaN-based MQW photovoltaics. Therefore, it can be concluded that the electric-field PL probe may provide a direct method for evaluating the photoelectric conversion in multilayer quantum structures and related multijunction photovoltaic cells.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The use of green, renewable and sustainable alternative energy sources is seen as a key component to combat the irreversible phenomenon like climate change. Photovoltaic energy conversion, which directly converts sunlight directly into electricity, holds great promise to generate enough renewable, carbon-free and clean electric power to accommodate the ever-increasing energy demands of mankind [1]. As important optoelectronic semiconductors, III-Nitrides and related quantum structures have been already proved to achieve the commercialization such as high-brightness light emitting diodes or laser diodes with emission wavelengths varying from blue/green to ultraviolet [2,3]. Meanwhile, InGaN-based alloys have a direct and adjustable band gap from 0.7 to 3.4 eV covering the nearly whole solar spectrum, high absorption coefficient (~105 cm−1) and superior radiation resistance, which is favorable for highly efficient and harsh solar photovoltaics in both terrestrial and space-based applications [4–7]. It has been proposed that the use of InGaN alloys with In composition of approximately 40% or multijunctions of III-Nitrides can achieve high-efficiency solar cells with the conversion efficiency of >50% [8,9]. However, realization of a thick high-quality InGaN with high In composition is still challenging until date due to the severe In segregation, existence of piezoelectric field and high defect density [10,11]. InGaN/GaN multiple quantum well (MQW), as an alternative functional structure for optical-electrical conversion, can simultaneously possess light-detecting function under light irradiation and light-emitting one under current injection, enabling photovoltaic detection, solar cells, adjustable optical display, pressure imaging sensing, optical communication, optoelectronic modulators and so on [12–15]. It is an ideal candidate photovoltaic material by using the bandgap engineering, quantum polarization engineering and micro/nano structure design to enhance optical-to-electric conversion efficiency.
In terms of photovoltaic operation, the photoelectric conversion efficiency is a key parameter of photovoltaic cell, which is not only dependent on the conversion of absorbed photons into electrons and holes but also the electrical transport of these non-equilibrium carriers inside the photovoltaic device. There are strong polarization charges at the InGaN/GaN interfaces which results in high internal electric field in the InGaN well layer, which is opposite to the p-n depletion field in [0001]-polarity InGaN/GaN MQWs [16–18]. The spatial distribution of electrical field in photovoltaic MQWs is extremely important to dictate the mutual competition of radiative recombination and photocurrent generation. Therefore, the fundamental physics of carrier generation, transport, separation, collection and recombination in polar MQWs must be taken into consideration for a design of ideal photovoltaic devices with high power conversion efficiency.
In this work, we have investigated the photoelectric conversion of InGaN-based photovoltaic cell by electric-field driven PL and time-resolved PL (TRPL). The InGaN/GaN MQW i-region between n-type and p-type layers serves as both photoabsorption and photoelectric conversion. Its electric-field distribution will affect carrier dynamics including carrier transport and carrier recombination as well as the photovoltaic performance. By the modulation of electric field, the effect of the external electric field and internal polarization field on carrier escape and separation will be fully investigated in our work. The distinct regimes of carrier escape and recombination are identified in electric-field driven PL and TRPL. The electric-field dependence of the fast and slow PL decay lifetimes in InGaN/GaN MQWs has been observed. The competitive mechanism of carrier escape and recombination in MQW structure has been well discussed. Accordingly, it can be proved that the electric-field driven PL probe in steady-state and transient-state, is a very promising, versatile, and fast experimental technique that can provide insight into the photoelectric conversion of photovoltaic semiconductors and devices.
2. Experimental section
The photovoltaic multilayer structure of In0.15Ga0.85N/GaN MQWs was grown on c-plane sapphire substrate by low-pressure metal organic chemical vapor deposition system. From bottom to top, it consisted of 2 µm undoped GaN buffer layer, 1 µm n-type GaN layer, ten periods of In0.15Ga0.85N/GaN (3 nm/8 nm) quantum well active layers and 100 nm p-type Mg-doped GaN capper layer. The gallium, indium and nitrogen sources were trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3), respectively. Biscyclopentadienyl magnesium (CP2Mg) and silane (SiH4) were used as the p-type and n-type doping sources, respectively. A typical planar p-i MQW-n photovoltaic cell with a chip size of 1 mm × 1 mm was fabricated as follows. First, active regions were defined by Cl2-based inductively coupled plasma dry etching down to n-type GaN layer. Then, Ti/Al/Ni/Au for n-type Ohmic contact and the indium tin oxide (ITO) for p-type semitransparent window were deposited by using electron beam evaporation, respectively. The semitransparent ITO enabled a uniform electric field across the active region and kept a low optical loss for the incident light [19]. On the top of ITO layer, the Ti/Au electrode pad was used for wire bonding. Figure 1(a) and 1(b) show the schematic diagram and actual map of as-fabricated InGaN-based MQW photovoltaic cell, respectively. A highly uniformed blue-color PL imaging of the whole photovoltaic cell in Fig. 1(c) demonstrates its good optical quality.
Fig. 1 (a) Schematic description of the InGaN/GaN photovoltaic cell structure. (b) The plane-view optical microscopy image and (c) the spatial PL image of a fabricated photovoltaic cell with 1 × 1 mm2 mesa.
The current-voltage (I-V) characteristic was carried out under illumination by using a Keithley 2400 source meter. The electric-field driven PL measurement system was used to evaluate the steady-state PL recombination of InGaN-based photovoltaic device by applying external bias voltage. The electric-field dependent carrier dynamics of InGaN-based MQW photovoltaics were analyzed using time-resolved PL setup (Edinburgh Instruments FLS980). Both the steady-state PL and transient-state TRPL measurements are excited by a 375 nm laser diode with ~1.5 mm beam diameter, which has a wide range of optical power from 0 to 6.03 mW/cm2. The incident photons of 375 nm can penetrate through GaN thin film, and are only absorbed by In0.15Ga0.85N QW active layers. It is favorable for studying the photoelectric conversion in QW structure, including the carrier recombination and escape processes. In addition, the PL imaging of InGaN-based MQW photovoltaics is measured by inverted fluorescence microscope (Olympus CKX53). A 100 W mercury lamp as the excitation source is filtered in the range of 340 to 390 nm by an exciter filter (BP340-390), while combined with a dichroic beamsplitter centered at 410 nm (DM410) for PL imaging. And it can fully excite the whole chip of 1 × 1 mm2 to further examine the PL uniformity. All the electrical and optical measurements were performed at room temperature of 23°C.
3. Results and discussion
To evaluating the performance of photoelectric conversion, the I-V characteristic of InGaN-based photovoltaic cell was measured under different optical powers excited by a 375 nm laser source in Fig. 2(a). And the incident photons can penetrate through GaN thin film and are only absorbed by In0.15Ga0.85N QW active layers. It is advantageous to directly investigate the optical-electric conversion in MQW i-region. As the optical power of 375 nm laser increases, the total number of the absorbed photons correspondingly increases and hence generates more and more photoinduced carriers in MQW i-region. Due to the quantum confinement of QW, both carrier escape and carrier capture may be influenced by both piezoelectric polarization effect and the band offset of GaN barrier and In0.15Ga0.85N well. The photogenerated electrons and holes have to overcome the QW confinement and separate with each other, thus contributing to effective photocurrent for energy conversion. As shown in Fig. 2(a), the absolute value of the photocurrent density has increased rapidly as the increase of optical power from 0 to 6.03 mW/cm2. In addition, it has a great dependence on the applied bias voltage following the Eq. (1) [20].
Fig. 2 (a) The current-voltage curves of the InGaN/GaN photovoltaic cell under different optical powers illuminated by the 375nm laser diode. (b) Its key parameters of Voc and Isc as a function of the incident optical power.
Where q is the electron charge, K is the Boltzmann constant, T is the temperature, n is the ideality factor, P is the optical power of 375 nm laser. The net photocurrent (Inet) represents the electrical current only induced by the photogenerated carriers in Eq. (2). The overall photocurrent (Io) is the sum of the dark current (Id) and net photocurrent. Meanwhile, other key parameters including the open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF) and photoelectric conversion efficiency (η) are also described in Eqs. (3)~(6), respectively. Therein, the FF is defined as the ratio of maximum obtainable power to the product of the open-circuit voltage and short-circuit current. The η is the ratio of maximum obtainable power to the optical power of incident light.
As shown in Fig. 2(b), the Voc and Isc of InGaN-based MQW photovoltaic cell have been evaluated as a function of the incident optical power P. As increasing the P, the Isc exhibits an almost linearly scaling up while the Voc has a logarithmic increasing as presented in Fig. 2(b). The Isc and Voc are consistent with the ideal relationships with the excitation power in Eqs. (3) and (4), respectively. It manifests that the MQW structure can be used in photovoltaic applications such as solar cells or photodetectors. Under the 375 nm optical excitation of 6.03 mW/cm2, the InGaN MQW photovoltaic cell shows an open-circuit voltage of 2.36 V, a short-circuit current density of 0.743 mA/cm2 and a fill factor of 76.6%, hence achieving a maximum photoelectric conversion efficiency of 22.27%.
In addition, the carrier capture and escape processes in MQW can be directly evaluated by the electric-field driven PL spectra. With regard to the photovoltaic applications, the applied bias voltage is kept below the open-circuit voltage, which ranges from Voc to −3 V in our experimental measurement. Lower than the Voc, the electroluminescence of InGaN-based MQW is rather weak below the measurement limit. So it can be negligible in our spectrum setup, demonstrating not any influence on the PL measurement. Figure 3(a) presents the steady-state PL spectra of InGaN-based MQW photovoltaic cell under different external bias voltages. At the constant excitation power of 6.03 mW/cm2, it is observed that its PL characteristic has a great dependence on the external bias voltage. The integrated PL intensity reduces a lot from 1.195*106 at open-circuit Voc to 0.303*106 at short-circuit 0 V, and even to 0.0934*106 at negatively biased −3 V in Fig. 3(b). The ratio of the integrated PL intensity at 0 V and −3 V to that at Voc is 25.4% and 7.8%, respectively. The reduction of the integrated PL intensity indicates that more and more photogenerated carriers don’t participate in the radiative recombination in InGaN QW as decreasing the external electric field from Voc to −3 V. Especially, the negative bias voltage could further help the approximately 92% of photoexcited carriers to separate apart driven by electric-field drift. It means that a plenty of these separated carriers could escape from the InGaN QW to generate photocurrent. This newly generated photocurrent contributes to the net photocurrent. As shown in Fig. 3(b), the net photocurrent increases gradually from 0.612 mA/cm2 at Voc to 0.743 mA/cm2 at 0 V, and then to 0.807 mA/cm2 at −3 V. Even at open-circuit condition, it can be seen that the free photocarriers in InGaN QW cannot be restricted by GaN barrier and directly drifted to p or n sides to generate photocurrent. As reducing the external voltage, the net photocurrent has a corresponding relationship with the integrated PL intensity. The more the integrated PL intensity decreases, the more the net photocurrent generates. The consistent trend between both of them demonstrates the electric field-induced charge separation plays an important role on the effective photoelectric conversion in the MQW-structure photovoltaics.
Fig. 3 (a) The steady-state PL spectra of the InGaN-based MQW photovoltaic cell under different external bias voltages at the constant excitation power of 6.03 mW/cm2. The dependence of (b) its integrated PL intensity and net photocurrent, (c) its PL peak wavelength and FWHM on the external bias voltage, respectively.
As well known, the photogenerated electron-hole pairs by the excitation of 375 nm laser are spatially localized at the bottom of conduction band and the top of valence band. Due to the spontaneous and piezoelectric polarization in [0001]-polarity MQW region, a strong internal polarization field induces to form the oblique triangular QW, which is opposite to the built-in field of p-n junction. It makes the photoabsorbed minor carriers highly localized in the InGaN QWs. As shown in Fig. 3(c), the PL peak energy has a blue shift from 455.5 nm to 448.9 nm with decreasing the external voltage from Voc to −3 V. It manifests the compensation of the QCSE in InGaN QWs by the reversed external electric field [21,22]. Correspondingly, the photocarriers can jump out of the localized states driven by external electric field, thus broadening the energy distribution of radiative recombination [23]. As applying voltage from Voc to −3 V, it is observed that the FWHMs of PL spectra increase from 18.02 nm to 23.97 nm in Fig. 3(c).
The localization, recombination, transport and escaping of photocarriers lead to complex dynamics of PL mechanism in InGaN-based MQW under different electric fields, which plays a great role on the photoelectric conversion processes. To get insight into its PL recombination dynamics, the TRPL characteristic of InGaN-based photovoltaic cell was carried out under the modulation of external electric field. Figure 4(a) presents its TRPL decay curves as the function of external electric field from Voc to −3 V. It is concerned that the TRPL decay is evaluated at peak PL wavelength of 460 nm, which corresponds to the dynamic recombination between the electron and hole ground states in In0.15Ga0.85N QW. And its PL photon counts as time are normalized at different electric fields for comparison. As a general trend, the transient decay of PL intensity becomes much quicker as applying external voltage from Voc to −3 V in Fig. 4(a). The dynamic recombination deviates from single-exponential decay, indicating the existence of multiple PL transitions. So the TRPL decay curves need to be fitted by bi-exponential numerical fitting procedure to obtain the carrier lifetimes of two recombination processes [24].
Fig. 4 (a) The normalized TRPL decay curves of InGaN-based photovoltaic cell as the function of external bias voltage at room temperature. (b) Carrier lifetimes τ1 and τ2 of the slow and fast PL decays, (c) their respective weight ratio A1/(A1 + A2), and (d) the effective carrier lifetime τeff of the overall TRPL decay under different bias voltages.
Where L0 is the PL intensity at t = 0, τ1 and τ2 are the carrier lifetimes of two PL decays with their respective total intensities A1 and A2. And τeff is defined as an effective carrier lifetime to evaluate the kinetic process of multiple decays by averaging two components of time constants τ1 and τ2 with their weights A1 and A2. Based on the Eq. (7), the overall PL decay curves are split into one slow and one fast decays at room temperature, corresponding to the carrier lifetimes of τ1 and τ2. The slow process can be assigned to the localized states from In compositional fluctuation or piezoelectric polarization, while the fast decay may be related to the free photocarriers. There exists a strong dependence of decay lifetimes τ1 and τ2 on the external electric field. As reducing the external voltage from Voc to −3 V, it can be seen that the slow decay lifetime τ1 decreases more abruptly from 29 to 8.5 ns than the fast one τ2 from 5.1 to 2.7 ns in Fig. 4(b). The trend of these decay lifetimes is in good agreement with the previous results [25,26]. The applied external electric field may provide a directly driven force to influence the transit time of photocarriers, which reflects the time that the carriers pass through InGaN QW under electric-field drift. When positively biased at Voc, the photoabsorbed carriers are prone to inducing radiative recombination before jumping out of the localized states due to a long transit time. As increasing the reversed bias voltage, the transit time is gradually shortened. Between Voc to 1 V, it is observed that the delocalization of photocarriers by electric-field drift mainly influences the slow decay of radiative recombination at the localized states. The decay lifetime τ1 declines faster than that of τ2. Further applying bias voltage from 1 V to −3 V, a stronger electric-field drift with a shorter transit time has a great impact on both fast and slow kinetic decay processes in Fig. 4(b). It can make the photogenerated electrons and holes to separate quickly and even move out of InGaN QW. Meanwhile, Fig. 4(c) compares the respective weights A1 and A2 of these two PL decays. The PL weight ratio of slow decay to fast decay defined as A1/(A1 + A2) reduces from 20.9% at 3 V to 7.4% at −3 V, which decreases abruptly between 1 V and −1 V. It manifests that the PL emission in InGaN QW is gradually dominated by fast recombination process as increasing the reversed bias voltage.
As a whole, we can further extract the effective carrier lifetime τeff by using the numerical Eq. (8), which contains information on both slow and fast components of the overall decay profile. Figure 4(d) shows that τeff has decreased obviously from 19.2 ns at Voc to 3.9 ns at −3 V as changing the external electric field. It can be concluded that the strong reversed electric field may shorten the transit time, which inhibits the slow PL decay recombination in InGaN QW. On the assumption that the nonradiative component in the TRPL decay has no change with the external electric field, the photocarriers escaping out of InGaN QW can fully convert into photocurrent instead of PL recombination, thus extremely enhancing the photoelectric conversion for photovoltaic applications.
In combination with the obtained steady-state PL and TRPL characteristics, the physical mechanism of electric-field modulation on the photoelectric conversion has been proposed in Fig. 5. The deactivation paths of photoexcited minority carriers mainly consist of PL radiative recombination, electrical drifting and tunneling transport except for the nonradiative recombination. Besides, there exists a mutual competition among each of them by the electric-field modulation. For simplicity, one single QW is taken as an example to illustrate the photoelectric conversion in InGaN-based photovoltaic cell. The total electric-field vector in optical absorption layer of i-InGaN QW consists of three parts: the built-in electric field formed by p-n junction (Ebi), polarization-induced electric field (Epi) and external electric field (Eex). It is noted that the Epi is a constant at the fixed In-composition InGaN layer, which has a direction opposite to the Ebi, resulting in a down-shifted or even a reversed electric field in the InGaN QW region. When Eex is zero bias in Fig. 5(b), the p-i-n photovoltaic cell is under the short-circuit condition. The total electric field in InGaN QW is only dominated by the Ebi and Epi electric fields. Further by applying the Eex, its photoelectric conversion capability is well compared with that worked at both positive bias and negative bias in Figs. 5(a) and 5(c), respectively. It can be seen that the oblique triangular QW becomes more flat as changing the Eex from positive bias to zero and then to negative bias. When positively biased in Fig. 5(a), the distribution of photogenerated electrons and holes is highly centralized at the bottom of the triangular QW. They may occupy at the localized states near the band edge due to the weaker electric-field driven drift, leading to a stronger PL recombination. Now the net photocurrent is mainly contributed by the tunneling transport of photogenerated carriers [27,28]. On the contrary, when negatively biased in Fig. 5(c), a plenty of photocarriers are prone to be delocalized from the localized states and escape from the InGaN QW by a larger drift force. So the proportion of the slow radiative recombination in the total PL has been reduced apparently. And hence the high-efficiency photoelectric conversion has been achieved by photocarrier escaping from the QW confinement. Therefore, the electric-field driven PL probe shows a great advantage to a deep understanding of MQW-structure photovoltaics for guiding the improvement of the photoelectric conversion efficiency.
Fig. 5 The physical mechanism of the photoelectric conversion in InGaN-based QW photovoltaic cell modulated by (a) positive bias, (b) zero bias and (c) negative bias, respectively.
4. Conclusions
In summary, the photoelectric conversion of InGaN-based MQW photovoltaic cell has been fully investigated. The electric-field modulation on steady-state PL and TRPL is used to directly evaluate the carrier separation and recombination processes in InGaN-MQW light absorption region. There exists a mutual competition of the electric-field drift and PL recombination evaluated by the electric-field PL probe. The ratio of the integrated PL intensity at 0 V and −3 V to that at Voc is 25.4% and 7.8%, respectively. The net photocurrent is mainly contributed by the tunneling transport of photogenerated carriers at positively biased Voc while further enhanced by the fast electric-field drift at negative bias. The more the integrated PL intensity decreases, the more the net photocurrent generates. By the reversed external electric field, the compensation of the QCSE in InGaN QWs is beneficial to help the photoabsorbed minor carriers drift out from the localized states. As a result, by reducing the quantum confinement or carrier localization, InGaN-based MQW photovoltaics with polarization-free and In-homogeneity can give a suitable way for high-efficiency photoelectric conversion for photovoltaic applications.
Funding
National Natural Science Foundation of China (Grant No. 51402064, 61274134); Beijing Natural Science Foundation (Grant No. 4173077); Fundamental Research Funds for the Central Universities (Grant No. FRF-BR-16-018A, FRF-TP-17-022A1, FRF-UM-15-032, 06400071); USTB Start-up Program (Grant No. 06105033); Beijing Innovation and ResearchBase (Grant No. Z161100005016095); Youth Innovation Promotion Association of Chinese Academy of Sciences (2015387).
Acknowledgments
The authors would like to thank Prof. Chi Zhang, Dr. Yaokun Pang and Dr. Huabo Zhao from Beijing Institute of Nanoenergy and Nanosystems for the great help of experimental measurement and discussion. And we also thank Dr. Chengyue Yang from Institute of Microelectronics of Chinese Academy of Sciences for chip bonding.
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