Solar cell technology is changing. New efficiency records are being set. Alta Devices has reached 28.8% efficiency in a thin film single-junction cell at 1-sun, and 30.8% efficiency in a thin-film dual junction cell at 1-sun.
Counter-intuitively, efficient external fluorescence is a necessity for approaching the ultimate limits. A great Solar Cell also needs to be a great Light Emitting Diode. Why would a solar cell, intended to absorb light, benefit from emitting light? Although it is tempting to equate light emission with loss, paradoxically, light emission actually improves the open-circuit voltage, and the efficiency.
The single-crystal thin film technology that achieved these high efficiencies, is created by epitaxial liftoff, and can be produced at cost well below the other less efficient thin film solar technologies. The path is now open to a 30% efficient photovoltaic technology, that can be produced at low cost.

Epitaxial lift off (ELO) of III-V semiconductor materials has often been used to make lightweight, ultrahigh efficiency solar cells. Unfortunately, a major promise - the ability to reuse the expensive substrate material following the removal of the epitaxial active region of the solar cell - has been elusive since the process results in irreversible damage to the parent wafer. Recently, we reported a method to avoid wafer damage caused by ELO using a combination of epitaxial protection layers and non-destructive cleaning procedures of both InP and GaAs substrates [1, 2]. This has resulted, for the first time, in the demonstration of multiple reuse of the parent wafer without incurring changes in either the chemical or morphological status of the original material. This suggests that the wafer itself may be used an unlimited number of times, employing a sequence of epitaxial growth, epitaxial removal using adhesive-free cold weld bonding to a metalized plastic foil, non-destructively cleaning the wafer surface, and then repeat. Indeed, multiple cycles of ELO followed my wafer reuse can be an effective means for achieving very low cost, flexible and extremely lightweight solar cells. In this work, we discuss the progress made in ELO growth of high efficiency p-n junction GaAs solar cells followed by adhesive-free cold-weld bonding to flexible substrates, and then once more reusing the parent wafer. We also discuss other high performance devices (e.g. LEDs, FETs, etc.) that have been grown on original and reused wafers. We will discuss the cost and complexity tradeoffs in this process multiple growths, ELO, and reuse of from a single parent wafer in achieving low cost, high power conversion efficiency III-V solar cells.
[1] K. Lee, K. T. Shiu, J. Zimmerman, and S. R. Forrest, "Multiple growths of epitaxial lift-off solar cells from a single InP substrate," Appl. Phys. Lett., vol. 97, p. 101107, 2010.
[2] K. Lee, J. D. Zimmerman, X. Xiao, K. Sun, and S. R. Forrest, "Reuse of GaAs substrates for epitaxial lift-off by employing protection layers," J. Appl. Phys., vol. 111, p. 033527, 2012.

Monolithically integrated III-V compound semiconductor layers on silicon substrates will enable semiconductor industry to fabricate new interesting device concepts in the near future. The device concepts that have received the most attention include CMOS compatible lasers that could be used in on-chip and chip-to-chip communication, n-channel high mobility transistors and silicon based high-efficiency solar cells. In order to realize these devices, many challenges existing in the polar-on-nonpolar epitaxy (e.g., anti-phase domains, stacking faults, lattice mismatch) needs to be overtaken. Despite the challenges however, it has recently been shown that an atomically smooth low defect density GaP buffer layer can be fabricated on silicon [1]. Therefore, an increasing amount of attention has been addressed on the properties of gallium phosphide based dilute nitride materials (E.g., GaPN, GaAsPN). The benefit of these materials is that the nitrogen incorporation enables strain compensation and the energy band structure engineering.
We have studied the growth of GaP and Ga(As)PN layers on silicon and GaP substrates by metalorganic vapor phase epitaxy. Properties of the grown layers have been examined by Raman scattering, photoreflectance, photoluminescence, IV, atomic force microscopy, Rutherford backscattering and XRD studies[2-5]. We discuss the properties of the fabricated Ga(As)PN layers and focus on the characteristics of the layers that could be utilized to realize a high-efficiency solar cells on silicon substrates. The solar cell device concepts that have drawn our attention include n-GaP heterojunction emitters, GaAsP tandem cells and GaAsPN based intermediate band solar cells (IBSC).
In this work, we present the main experimental results of the fabricated layers. The results show that defects in Ga(As)PN layers on silicon most likely arising from the III-V/Si interface degrade the quality of the layers and that high-quality layers are realized on GaP substrates. Anti-phase domain and stacking fault/threading dislocation type defects have been observed by transverse scan analysis performed by high-resolution XRD setup. Photoreflectance results of Ga(As)PN layers show the conduction band splitting which can then possibly in the future be used to realize IBSC device on silicon substrate. At the moment, we are fabricating our first IBSC devices. In addition, IV measurements performed under 0.54 SUN show that semiconductor devices such as n-GaP/p-Si heterojunction solar cell are realized.
[1]: K. Volz, et al., Journal of Crystal Growth, 315, 37, (2011).
[2]: H. Jussila, et al., J. Appl. Phys. 111, 043518 (2012).
[3]: H. Jussila, et al., Phys. Status Solidi C, 9, 1607 (2012).
[4]: S. Nagarajan, et al., J. Phys. D: Appl. Phys. 46, 165103 (2013).
[5]: H. Jussila, et al., Thin Solid Films, 534, 680-684, (2013).

Multiple batches of thin-film p-n junction GaAs thin-film photovoltaic cells were grown sequentially on a single parent wafer without loss in performance from growth to growth. Through the combination of cold-welding and a modified epitaxial lift-off (ELO) process that eliminates parent wafer damage, the device active region is transferred onto flexible plastic substrates [1]. A combination of alternating arsenide-based and phosphide-based lattice matched epitaxial layers with high etch selectivity grown on both sides of the AlAs sacrificial layer, along with thermal and plasma surface cleaning fully eliminates surface contaminants and damage, creating a pristine surface suitable for subsequent epitaxial layer growth without the use of costly and damaging wafer repolishing methods[2]. Using these methods, three iterations of lightweight and flexible GaAs thin film photovoltaic cells bonded to flexible plastic substrates were fabricated from a single GaAs substrate without any degradation in device performance after regrowth with power conversion efficiencies of ~18%. Wafer recycling without repolishing allows for an indefinite number of growth iterations on a single substrate without damage or loss in the original wafer thickness, providing a path towards cost-effective, high performance, flexible and lightweight thin-film compound semiconductor devices.
[1] K. Lee, K. T. Shiu, J. Zimmerman, and S. R. Forrest, "Multiple growths of epitaxial lift-off solar cells from a single InP substrate," Appl. Phys. Lett., vol. 97, p. 101107, 2010.
[2] K. Lee, J. D. Zimmerman, X. Xiao, K. Sun, and S. R. Forrest, "Reuse of GaAs substrates for epitaxial lift-off by employing protection layers," J. Appl. Phys., vol. 111, p. 033527, 2012

Mono-crystalline germanium (Ge) is the most widely used substrate in concentrated photovoltaics (CPV) high efficiency multijunction solar cells (MJSC). Increasing demand on MJSC would undoubtedly lead to a lack of this rare material, resulting in dramatic increase in the Ge wafer price, not to mention that today, the cost of the Ge substrate already represents a substantial share of the total cell cost. A typical MJSC process uses > 140 µm thick Ge wafers as substrates, whereas a few microns would be sufficient for the bottom cell to match the photogenerated currents at the top and middle subcells. Moreover, using a thin Ge film rather than a thick substrate would reduce electrical and thermal resistances, weight of the cell and increase efficiency of the Ge subcell. In order to separate the active region of the MJSC from its original substrate, a layer transfer process based on simple electrochemical porosification of Ge could be used. Similar processes have already been successfully applied to crystalline silicon thin film solar cells, yielding performances very similar to crystalline cells that are manufactured on standard thick Si wafer.
We have developed a process, using porous germanium, to separate a semiconductor structure from the Ge substrate (APL 102 (1), 011915 (2013)). First, a double porosity layer is formed on top of a p-type Ge substrate in HF based electrolyte; the topmost layer has a low porosity whereas the buried layer is highly porous. After ultra high vaccum (UHV) annealing at high temperature, the porous top layer transforms into quasi-monocristalline germanium (QMG) film and will serve later as seed for epitaxial deposition of III-V top and middle subcells. On the other hand, the buried layer forms a film with large lateral voids that weaken the interface between the QMG film and the substrate, creating a so called “separation layer”. After epitaxial deposition of the III-V MJSC device is done, the cell is bonded to a low cost host substrate, and separated from the original Ge substrate, which is then ready to be re-used in another manufacturing cycle and could potentially yield a large number of MJSC solar cells, as the active region of the Ge subcell is only a few microns thick.
The synthesis and transformation during annealing of mesoporous germanium double layer are key steps in this process. In this presentation, we show the results of electrochemical etching of Ge in HF electrolyte to form the double porosity layers, and its morphology transformation during UHV annealing at high temperature which is due to surface diffusion at constant volume. The monocrystallinity of the layers and their suitability as seed layers for epitaxy of GaAs is confirmed by X-ray diffraction rocking curves.

Upconversion of sub-bandgap photons is a promising approach to exceed the Shockley-Queisser limit in solar technologies. Calculations have indicated that ideal upconverter-enhanced cell efficiencies can exceed 44% for non-concentrated sunlight, but such improvements have yet to be observed experimentally. In this presentation, we develop both theoretical and experimental methods to understand and improve solar upconversion, considering electronic, photonic, and thermodynamic design constraints. First, we develop a thermodynamic model of an upconverter-cell considering a highly realistic narrow-band, non-unity-quantum-yield upconverter. As expected, solar cell efficiencies increase with increasing upconverter bandwidth and quantum yield, with maximum efficiency enhancements found for near-infrared upconverter absorption bands. Our model indicates that existing bimolecular and lanthanide-based upconverters will not improve cell efficiencies more than 1%, consistent with recent experiments. However, our calculations show that these upconverters can significantly increase cell efficiencies from 28% to over 34% with improved quantum yield, despite their narrow bandwidths. Then, we develop the experimental techniques to enhance upconversion efficiencies, tailoring both the optical density of states via plasmonics and the electronic density of states via pressure. Our results highlight the interplay of absorption and quantum-yield in upconversion, and provide a platform for optimizing future solar upconverter designs.

Current photovoltaic technologies harvest only a fraction of incoming solar energy since they are unable to utilize photons with energies below the cell band gap. Placed behind a solar cell, the upconverter converts transmitted low-energy photons to photons with energies higher than the cell band gap. The higher energy photons are absorbed by the solar cell and contribute to the photocurrent. We develop optical models of several state-of-the-art commercial and research thin-film solar cells incorporating the upconversion layer. We present both analytical models based on published EQE data as well as detailed finite difference time domain (FDTD) models that incorporate absorption in all cell layers. We model the improvement in absorption and overall cell performance of amorphous Si, CIGS, GaAs, CdTe, and Cu2O cells with upconverting layers. We incorporate and discuss the effect of interface texture and different cell layers on the absorption of upconverted photons and make suggestions for improving the overall cell design to get the maximum benefit from upconversion. We estimate that the cell efficiency enhancement can range from 0.5% to up to 7% absolute depending on the cell type and upconversion efficiency. This work connects to the fundamental efficiency limit analysis of narrow-bandwidth solar upconversion by our collaborators [1], but presents concrete optical models of current solar cells and discusses the promise of upconversion for particular applications.
[1] J.A. Briggs, A.C. Atre, J.A.Dionne.Narrow-bandwidth solar upconversion: Case studies of existing systems and generalized fundamental limits. Journal of Applied Physics 113, 124509 (2013).

Technological implementation of a high-efficiency quantum dot intermediate-band solar cell (QD-IBSC) must be accompanied with a sufficient photocurrent generation via IB states. The demonstration of QD-IBSCs is presently undergoing two stages. The first is to develop epitaxial growth or lithography technology to fabricate high density QD arrays or superlattice with high quality heterointerface, and the second stage is to realize partially-filled or ideally half-filled IB states in order to maximize the photocurrent generation by two-step absorption of infrared photons in solar spectrum.
For the former requirement, we have developed a strain-compensation or strain-balanced technique in order to grow multi-stacks of self-organized InAs QDs in GaNAs matrix on GaAs substrate by molecular beam epitaxy (MBE). For the latter, either doping of QDs or concentrated illumination of sunlight at around 1000 suns should in principle result in a sufficient population of carriers in IB and hence production of an additive photocurrent in QD-IBSC.
We show that sunlight concentration increases the optical generation rate and hence sufficient carriers are populated in IB, which in turn increases photocurrent production from IB to conduction band. Our InAs/GaNAs QD solar cell shows a conversion efficiency of 20.3% at 100 suns increasing to 21.2% at 1000 suns. The QD-IBSCs which commonly suffer from low absorption by QDs are thus expected to improve fast and perform better under concentrated sunlight illumination.

The advantages of converting solar energy directly into clean electricity have important consequences for realizing a sustainable future. However, the conversion efficiency needs to be improved, and new solar cell concepts are believed to play an important role in order to achieve this target.
For solar cells, infrared absorption and ultra-violet relaxation are considered to be the main factors for conversion efficiency limitation [1]. To reduce these losses, several ideas have been proposed. The intermediate band (IB) solar cell reduces the infrared absorption losses [2]. This concept proposes that by upconversion, the energetic addition of two low energy quanta resulting in one high energy quanta, the infrared light can be efficiently used and the energy conversion efficiency limit can be improved significantly compared to single junction solar cells.
Quantum structures are attractive candidates for design of IB states, and a number of IB solar cells based on InAs/GaAs have been reported. Important InAs quantum structures are quantum dots (QDs) and disklike quantum structures (nanodisks) formed in the early stages of InAs deposition [3]. While the InAs quantum structures have been the focus of IB research for over one decade, the detailed mechanisms of upconversion are still unclear due to the complexity. Upconversion in InAs QDs is generally accepted to occur via the two-step two-photon-absorption mechanism. The operation principle has been verified, but the realized efficiencies are too small for practical applications [4].
In this work we focus on a novel IB solar cell concept using both QDs and nanodisks. The major upconversion mechanisms in the InAs quantum structures were determined by macroscopic measurements of upconverted photoluminescence (PL) and photocurrent (PC) [5,6]. The results showed that the nanodisks with height of a few monolayers and lateral extension up to hundreds of nm show exceptionally high upconversion efficiencies due to the activation of the upconversion via Auger process. Further exploration of the InAs quantum structures with spectroscopic imaging of PL and PC revealed that, depending on the distribution of nanodisks and QDs in different sites, the local PC generation efficiency can change drastically. We discuss the energy transfer between the quantum structures and how this relates to the upconversion efficiency. Based on our finding we propose a novel design of an efficiency enhanced IB solar cell using both QDs and nanodisks.
This work was supported by JST-CREST and by the Strategic Research Infrastructure Project, MEXT, Japan.
[1] W. Shockley and H. J. Queisser, J. Appl. Phys. 32, 510 (1961).
[2] A. Luque and A. Marti Phys. Rev. Lett. 78, 5014 (1997).
[3] R. Heitz et al., Phys. Rev. Lett. 78, 4071 (1997).
[4] A. Marti et al., Phys. Rev. Lett. 97, 247701 (2006).
[5] D. M. Tex and I. Kamiya, Phys. Rev. B 83, 081309(R) (2011).
[6] D. M. Tex, I. Kamiya, and Y. Kanemitsu, Phys. Rev. B 87, 245305 (2013).

Quantum structures are promising candidates for next-generation solar cell materials because of their unique optical properties. Among them, InAs quantum structures attract much attention because InAs/GaAs system forms intermediate bands, which absorb infrared light due to the small band gap energy, and upconversion processes are expected to improve the light-energy conversion efficiency. As previously reported, two-dimensional disklike InAs quantum structures with heights of two and three monolayers (quantum well island: QWI) display efficient upconverted photoluminescence (PL) and photocurrent (PC), which occur through a multiparticle Auger process [1]. This novel application of the QWI may be more suitable than the quantum dots for enhancing the solar cell conversion efficiency. For the further understanding of the upconversion process in QWIs, it is significant to clarify the photocarrier decay and upconversion dynamics. At low temperatures, the photocarrier dynamics can be evaluated by upconverted PL decay dynamics. However, the room-temperature photocarrier dynamics remain unclear because of the very weak PL intensity. Recently, we developed a novel measurement method for femtosecond time-resolved PC dynamics based on femtosecond excitation correlation (FEC) spectroscopy. In this study, we investigated the photocarrier dynamics of QWIs using PC-FEC measurement.
The sample structures were prepared by molecular beam epitaxy. A nominally undoped structure was grown on top of a semi-insulating GaAs (001) substrate and a GaAs buffer. A single InAs layer and a GaAs/AlGaAs quantum well were grown in an AlGaAs matrix. In PC-FEC measurement, two femtosecond laser pulses with a time delay are focused on the sample surface, as is similar to the FEC measurement of PL [2]. We measured the correlation signal of the PC induced by the two pulses as a function of delay time, which approximately corresponds to the photocarrier population in the QWIs. The light source was the optical parametric amplifier based on the KGW:Yb regenerative amplifier (repetition rate: 200 kHz).
We observed the strong FEC signal under excitation at the resonance energy of InAs QWIs with heights of two monolayers. The decay time is around 100 ps, which is weakly dependent on the excitation photon energy. We attribute the FEC decay time to the intrinsic photocarrier lifetime in the InAs QWIs. We will present the principle of the PC-FEC measurement and discuss the temperature dependence of the photocarrier intrinsic lifetime in the InAs QWIs.
Part of this work was supported by The Sumitomo Industries Group CSR foundation, JST-CREST, JSPS KAKENHI (No. 25234567), and the Strategic Research Infrastructure Project of MEXT.
References:
[1] D. Tex, I. Kamiya,and Y. Kanemitsu, Phys. Rev. B 87, 245305 (2013).
[2] D. von der Linde, J. Kuhl, and E. Rosengart, J. Lumin. 24/25, 675 (1981).

Thin-film structures are considered to be essential to lower the cost of solar cells. Thinning the active regions, however, often reduces the optical absorption efficiency. Therefore, the enhancement of optical absorption in thin-films is inevitable to maintain and improve solar cell efficiency. Among the variety of absorption mechanisms in semiconductors, excitonic absorption is one of the most promising candidates to enhance the optical absorption, because it can be added to the normal band-to-band absorption. The excitonic absorption at room temperature occurs more efficiently in the materials with higher exciton binding energies. Unfortunately, semiconductors sensitive to the main part of the solar spectrum such as Si and GaAs exhibit rather lower exciton binding energies than thermal energy at room temperature. However, the excitonic absorption can be considerably enhanced even in these materials by constructing superlattice (SL) and multiple quantum well structures. In this study, we show the absorption characteristics of the solar cells consisting of an AlGaAs / GaAs SL p-i-n structure. Al0.5Ga0.5As (2nm) / GaAs (5nm) SL solar cells are grown on n-type GaAs (001) substrates by a molecular beam epitaxy method. The total active layer thickness is fixed at 2 micrometer or 1 micrometer. In order to compare the effect the excitonic absorption, we fabricate a bulk Al0.14GaAs pin junction. The Al composition of Al0.14GaAs bulk layer corresponds to the averaged one of the SL. For the SL solar cells, the open-circuit voltage is 1.08 V, and the short-circuit current density is 23.6 mA/cm2 under the solar radiation of AM 1.5 spectrum at 100 mW/cm2. The fill factor and the conversion efficiency are 0.83 and 21.0 %, respectively. On the external quantum efficiency of the SL, two distinct maxima appear at approximately 680 and 790 nm with a dip structure in between. The calculated absorption rate with the effect of excitons shows the strong and sharp peaks at the absorption edge around 800 nm. In contrast, the step-like feature is obtained for the electron-hole pair absorption without considering the Coulomb interaction due to the SL density of states. The experimental results clearly indicate that the excitonic absorption is occurred even at room temperature. Thinning the active layer thickness reduce the optical absorption efficiency. Especially, the efficiency is easily decreased in the range near the absorption edge because of the lower absorption coefficient. For the Al0.14GaAs bulk solar cell, the decrease rate of the EQE in the range of 100 nm-wide wavelength from the absorption edge is estimated to be 27 %. On the other hand, the decrease rate for the SL solar cell is estimated to be 15 %. This result suggests the SL solar cells have high absorption coefficient compared with the bulk solar cells because of the excitonic absorptions, and the high absorption efficiency should be maintained when the active layers thickness are reduced.

Quantum dots (QDs) are an exciting group of materials because their bandgap is dependent on their nanocrystal size. The use of QDs allows for precise control over the wavelengths of light which are emitted or absorbed in applications such as light emitting diodes or photovoltaic devices. To produce efficient devices made from QD films, it is necessary to control the rates of charge transport and exciton diffusion among the QDs which make up the film. One way to affect these rates is by changing the ligand present on the surfaces of the QDs, which alters the distance and electronic coupling between neighboring dots. Large, bulky ligands serve to isolate dots from their neighbors while small, compact ligands allow for close packing and enhance electronic coupling. Regardless of whether the goal is to increase or decrease charge transport / exciton diffusion, it is important to be able to quantify properties such as the interparticle spacing and center-to-center distances in QD thin films. In this study, we characterize the morphology of lead sulfide (PbS) thin films made from highly monodisperse PbS QDs (diameter standard deviation < 5%, absorbance peak FWHM ~ 60meV). The native oleic acid ligands are exchanged for alkane monothiols, alkane dithiols, and several other commonly used ligands as well as treated with hydrazine to strip the QDs of all surface ligands. The absorbance spectra of the thin films are measured in order to characterize the shift in the first absorbance peak - which gives an indication of how much the local environment around each QD has changed. To determine the interparticle spacing, the dots are assembled into a close packed monolayer on a transmission electron microscopy (TEM) grid and subsequently have their native ligands exchanged. The TEM images are automatically analyzed to identify the boundaries of each QD as well as its center point. This information is used to calculate the average diameter and average center-to-center distance of all QDs in the image. From this analysis we are able to quantify the change in interparticle spacing as a function of the surface ligand species. These results are compared with those from small angle X-ray scattering (SAXS) on thin films, which also determines the average diameter and average center-to-center distance. We discuss the advantages and disadvantages of the TEM and SAXS measurements as well as the ease of data collection and analysis. Altogether, this study aims to provide a better understanding of how ligand chemistry can be used to control the morphology and electronic properties of a QD film.

Split spectrum photovoltaics, where incident light is divided onto multiple solar cells on the basis of wavelength, are an exciting recent development in the solar energy field. This technology has the potential to exceed record conversion efficiencies by allowing systems to incorporate a large number of active p-n junctions while mitigating the constraints that plague monolithic cells: namely lattice matching and current matching. Each cell in a split spectrum system can be designed to have a different lattice constant (allowing for a larger range or more combinations of materials) and have different operating currents (allowing for more combinations of band spacing).
In this work, we examine a split spectrum system utilizing a single spectrum splitting device, here a dichroic filter, to divide the solar spectrum onto two cells. As opposed to many other split spectrum designs where numerous filters direct light onto numerous single junction cells, in this system each cell is then comprised of multiple active junctions. Each cell is then tailored to absorb its own portion of the solar spectrum at its own operating point. The combination of the two cells allows for the constructing of a photovoltaic system which effectively has four or five active junctions while maintaining lattice and current matching conditions in each cell.
A number of different cutoff frequencies for the dichroic filter are examined. Each cutoff frequency corresponds to its own combination of ideal band placements for both the shorter and longer wavelength cells. Designs which have the potential to have both cells meet the lattice matching and current matching conditions are then simulated using TCAD Sentaurus. Designs which show promise of delivering high system conversion efficiency are then grown and tested. Testing is to be done under a variety of conditions, including that of a solar simulator and environmental conditions. Environmental testing is conducted in part using a custom-built concentration array designed for use with split spectrum solar cells.

Quantum dot (QD) based 3rd generation photovoltaic technology is important to helping the world meet its increasing energy demands. Doping CdSe QDs with impurities such as indium or gallium opens the door to increased conductivity as these elements introduce n-type donors into bulk CdSe. The growth kinetics in the heterogeneous growth-regime and temperature dependent fluorescence provide insight into the doping process. Doped CdSe QD-P3HT hybrid cells show increased photocurrent over hybrid cells fabricated with undoped CdSe QDs at room-temperature and at elevated temperatures indicating possible thermal ionization of n-type donors. Gallium doped CdSe QD based hybrid solar cells show an over-all power conversion efficiency of 2.0% under AM 1.5 solar conditions.

The performance of the light-harvesting active layer of several photovoltaic (PV) technologies (e.g. amorphous silicon and organic photovoltaics) is limited by the overlap of the absorption spectrum with the solar spectrum. Over the last decade the phenomenon of triplet-triplet annihilation-assisted photon upconversion (UC) has been demonstrated for a wide variety of molecular triplet sensitizer/annihilator systems. More recently these efforts have focused on efficient UC of photons from the near-infrared to visible regions of the solar spectrum. These observations suggest that photons below the bandgap of amorphous silicon (a-Si) can be converted to photons with higher energies, which can then be absorbed in the active layer of a-Si devices and converted into carriers, thereby enhancing the photocurrent.
We will outline recent efforts to synthesize novel light-harvesting chromophores that (i) possess large near-infrared extinction coefficients, (ii) manifest ultrafast intersystem crossing to generate the triplet state at unit quantum yield, and (iii) possess triplet lifetimes exceeding several microseconds. We will subsequently discuss factors affecting the optimization of the photon UC efficiency of these sensitizers in combination with various triplet annihilators, the incorporation of the best-performing systems into device architectures employing transparent a-Si devices, and the observed performance of the coupled UC-PV system.

Wide-bandgap metal oxide Nanowires (NW) sensitized with quantum dots (QD) is a promising third generation concept due to their tunable electro-optic properties leading to high efficiency and low cost quantum dot sensitized solar cells (QDSSCs). CdSe/ZnS colloidal QDs have the advantage of band gap tuning, high stability, broad-range photon absorption and multiple carrier generation. Zinc oxide (ZnO) NWs with the advantage of high electron mobility, directionality, and ease of fabrication is a good candidate to use as the conducting electrode. ZnO NWs provides a transparent, high surface area medium for the QD absorbers and a directional path for the carriers. Among challenges faced in practical realization of QDSSs such as slow hole transfer, interface carrier recombination and the poor counter electrode performance, charge transfer dynamics from the absorber QD to the NW is key to better performance. To date, the carrier transfer mechanism has not been thoroughly explored for ZnO NW/QD structures. In this work the QD-to-ZnO NW charge transfer dynamics is analyzed for QDSSs.
In this work, ZnO NWs were synthesized by two methods; bottom-up and top-down. In the first approach, ZnO NWs were grown by hydrothermal method on sputtered zinc oxide (ZnO) coated glass wafer. In the second approach, arrays of ZnO nanowire were fabricated using silica nanosphere colloidal crystal formed by spin coating on a sputtered ZnO film on glass slide and etching in CF4:H2 plasma. The synthesized structures were studied by scanning electron microscope (SEM) and transmission electron microscope (TEM). Upright ordered arrays of ZnO NWs with a diameter range of 50-350nm and length of 1µm with a high crystalline structure were obtained.

The fabricated ZnO NW arrays were sensitized with hydrophobically ligated colloidal CdSe/ZnS QDs. QDs with two different ligands, octadecylamine (ODA) and trioctylphosphine (TOPO), and two various sizes were incorporated in the ZnO NWs using the drop casting and dipcoaing methods. The structural properties of the ZnO NW/QD were studied using high resolution-SEM and TEM. The results show that the QDs are fully filled the gaps between wires and orderly assembled onto the wires.
Charge transfer mechanism in ZnO NW/QD structures was examined by photoluminescence (PL) using steady state and lifetime measurements. The results compared for the structures containing NWs synthesized by two methods. The effects of QD size and ligands on charge transfer dynamics were also studied. A profound quenching of the QD emission peak and lowered average lifetime in ZnO NW/QD structure with respect to the control sample, which is defined as a layer of QD deposited on glass slide, were attributed to the deactivation of the excited QDs via electron transfer to ZnO nanowires. These studies provide insight on charge transfer dynamics at NW/QD interface and better designs for high performance QDSSCs can be engineered accordingly.

Semiconductor bandgap establishes a fundamental upper limit for the conversion efficiency of the photovoltaic (PV) devices. Lattice thermalization and sub-band gap transmission contribute to more than 50% of the total losses in photovoltaic devices. To reduce spectral mismatch losses, two basic solutions might be adopted: designing a novel solar cell to better use the solar spectrum, or modifying the solar spectrum to better match the solar cell. The fabricated solar cells are known as “Third-Gen PV” and considered as leading concepts that are mainly obtained with quantum dots (QDs). However, the toxicology of nanocrystals, health and safety information about nanomaterials and QDs as dominant material for research on advanced PV, are yet relatively unexplored.
In this work a review study on the Third-Gen PV, key factors in QD toxicity, potential pathways by which humans can become exposed to toxic substances, summarized studies on potential exposure pathways were presented. Since the greatest potential for aerosolization of QDs likely arises during the synthesis and manipulation phases of QD manufacturing, workplace exposure to aerosolized QDs requires careful consideration. Experimental results on the detection of various type of QDs and nanoparticles ranging from 6-300nm in the workplace using a particle counter were also presented.
Toxicity of QDs can, at least to some extent, be minimized through selection of an appropriate shell coating, by modulating surface charge or surface coating, selecting a lower overall dosage of QDs, or modulating the overall size of the QD. Airborne inhalation, dermal/ocular absorption, ingestion and injection are a number of potential means by which humans can become exposed to toxic substances. QDs have shown a preference for deposition in organs and tissues and that they do not remain circulating in the bloodstream. QDs are more likely to accumulate in the liver, spleen, kidneys, lymph nodes, and bone marrow. However, the scope of the migration of QDs in vivo is effectively limited by their size. Capping of QDs with ZnS significantly enhances their stability. Exposure of CdSe QDs to air and ultraviolet light leads to the degradation of the QD and the consequent release of free cadmium ions which is been greatly addressed as the major QDs methodological issue.
Nanoparticles follow airstreams very easily and will be collected and retained in standard ventilated enclosures equipped with HEPA filters. Nanomaterials need to be synthesized in enclosed reactors or glove boxes under vacuum, nitrogen and exhaust ventilation, which prevent exposure during the actual synthesis as well as nanodust expulsion. These studies and the obtained experimental results on size selection and counted particles suggest the possibility of handling the QDs for PV applications in such a way to minimize their negative impact on the human health while utilizing them for PV manufactures.

A complete understanding of the reliability for ZnO-based transparent conducting oxides is essential for actual applications in photovoltaic devices or displays requiring long-term stability. The stability and degradation mechanisms under damp-heat environment (humid and hot atmosphere) for sputter-deposited aluminum-doped zinc oxide (ZnO:Al) thin films were systematically studied. The continuous degradations of carrier concentration and mobility with the Fermi-level shift were observed, and these behaviors were resolved by separating the changes in the carrier-transport characteristics of grain boundary and intragrain. By correlating the temperature dependence of electrical characteristics with x-ray photoelectron spectroscopy, the degradation is well explained by the increase of chemisorbed OH^- in the grain boundaries. [1] J. Steinhauser, S. Meyer, M. Schwab, S. Fayuml;, C. Ballif, U. Kroll, and D. Borrello, Thin Solid Films520, 558 (2011). [2] Y. Kim, W. Lee, D.-R. Jung, J. Kim, S. Nam, H. Kim, and B. Park, Appl. Phys. Lett.96, 171902 (2010). Corresponding Author: Byungwoo Park: byungwoo@snu.ac.kr

In this research, we suggest a facile method to synthesize nanoporous NiO thin films for p-type dye-sensitized solar cells (DSSCs). Preparation of porous NiO thin films was successfully achieved by simultaneous sputter deposition of Al and Ni, followed by selective etching of Al and subsequent heat treatment. The initial sputtering power of Al determined the porosity of NiO, and the resulting photovoltaic properties were correlated with the various nanostructures of NiO. Furthermore, we observed that additional NiO coating by nickel acetate (Ni(CH₃COO)₂) had positive effects on surface passivation, showing ~50%-increase in power-conversion efficiency compared to the bare cell. [1] H. Choi, J. Kim, C. Nahm, C. Kim, S. Nam, J. Kang, B. Lee, T. Hwang, S. Kang, D. J. Choi, Y.-H. Kim, and B. Park, Nano Energy (2013). [2] A. Nattestad, A. J. Mozer, M. K. R. Fischer, Y.-B. Cheng, A. Mishra, P. Bäuerle, and U. Bach, Nat. Mater.9, 31 (2010). Corresponding Author: Byungwoo Park: byungwoo@snu.ac.kr

Silicon nanocrystal assembly is an interesting material which has a potential applicability to various functional devices. Its photo-conductive property, however, has not been understood sufficiently, although its knowledge is inevitable in opto-electronic device designing. In this paper, we investigated photo-conductive properties and studied the transport mechanism in Si nanocrystal assembly. We found that Auger excited carriers play important roles in the photo-conduction at room temperature operation.
We deposited Si nanocrystals having a diameter of about 5nm on n-type Si substrate by using pulsed laser ablation method in hydrogen atmospheres [1]. The deposit shows columnar structure with a thickness of 100nm. The photo-conduction properties of the Si nanocrystal assembly between the electrodes deposited on both sides of the sample were measured in the temperature range between 30K and 300K. As for the excitation light source, we used 325nm He-Cd laser line and 488nm Ar laser line.
The current-voltage (I-V) characteristics for the photo-current from 30K to 140K indicated a clear voltage threshold, suggesting Coulomb blockade behavior. We analyzed the experimental date and found that it can be expressed by Middleton and Wingreen&’s scaling law [2]. Thus, the carrier conduction in this temperature region is found to be governed by the percolation transport mechanism. When the operation temperature is increased higher, the I-V characteristics changed to the exponential function behavior with the activation energy of several tens of meV. This indicates that the transport mechanism in this temperature range is variable range hopping. When the temperature is raised further to around room temperature, the photo-current showed steep increase. The activation energy of this temperature range was evaluated to be several hundreds of eV. This phenomenon can be well explained by assuming that the photo-current is caused by the Auger excited carrier transport mechanism.

In conclusion, we elucidated the transport properties of photo-carriers Si nanocrystal assembly in the wide temperature region. It is found that the Si nanocrystal assembly behaves as a network of tunnel junctions between quantum dots when the operation temperature is lower, while the current is governed by variable range hopping in the higher temperature range. When the temperature approaches to room temperature, the steep increase of the photo-current was observed, which can be well explained by the Auger excited carrier transport mechanism. This result suggests that we can expect very high efficiency solar cell operation exceeding Shockley-Queisser limit using Si nanocrystal system.
Reference
[1] MRS Proc.832 F7.24(2005)
[2] A. Middleton and N. Wingreen, Phys. Rev. Lett. 71, 3198 (1993).

Highly mismatched ZnTe1-xOx (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC) because incorporation of small amount of O results in formation of a narrow, O-derived intermediate band IB (E-) well below the upper conduction band CB (E+) edge [1]. Previously, we have reported fabrication of ZnTeO-based IBSC structures using an n-ZnO window layer, and demonstrated a photogenerated current by two-step photon absorption [2]. However, because of the large conduction band offset between ZnTe and ZnO, a small open circuit voltage (Voc) of approximately 0.4 V was observed in this structure, [2]. In order to achieve higher Voc, it is necessary to develop a new n-window layer blocking electrical transport IB(E-). ZnS has a large direct band gap of 3.7 eV with a smaller conduction band offset against ZnTe, and therefore, a suitable n-window material for ZnTeO IBSC. Here, we have grown n-type ZnS thin films by molecular beam epitaxy (MBE), and demonstrated heterojunction ZnS/ZnTe solar cells with an improved Voc.
n-type ZnS thin films were grown by MBE on ZnTe(100) substrates using Al as a dopant. The substrate and the Al cell temperatures were changed between 100 and 300C and between 810 and 950 C, respectively, to optimize the growth conditions of n-type ZnS. Electrical properties of the thin films were characterized by Hall effect measurements. Finally, the solar cells with n-ZnS/i-ZnTe/p-ZnTe structures were fabricated.
At the substrate temperature of 250 C, n-type conductivity was confirmed under a wide range of Al cell temperatures between 810 and 910 C. The highest electron concentration of 4.2 × 10^19 cm^-3 and the lowest resistivity of 5.5 × 10^-3 Omega;cm were obtained at the Al cell temperature of 890 C. Rapid decrease in the electron concentration was observed when the Al cell temperature exceeds 910 C, probably due to self-compensation effects. A current-voltage (J-V) curve of n-ZnS/i-ZnTe/p-ZnTe solar cell under 1 sun illumination showed that the improved Voc of 0.77 V, a large short-circuit current density of 6.7 mA/cm^2, and the fill factor of 0.60, yielding the power conversion efficiency of 3.1 %.
[1] K. M. Yu et al. Phys. Rev. Lett. 91 (2003) 246403.
[2] T. Tanaka et. al. Appl. Phys. Lett. 102 (2013) 052111.

Nanocrystal thin films have been widely applied to photovoltaic devices, optical devices, electronic sensors, etc. As building blocks of nanocrystal thin films, nanoparticles are normally capped by organic ligands preventing aggregating and providing solubility in solutions. However, this insulating organic layer attaching to the surface of nanoparticle influences the performance dramatically such as conductivity, inter-particle charge coupling and most importantly, carrier mobility, etc. In this work, we developed a new theoretical model to study the dependence of ligand chain length V.S. carrier mobility in nanocrystal thin films.
Other than considering surface ligands as an energy barrier for inter-particle carrier transportation, a model of electron mobility in Nanocrystal ligand complex systems is proposed based on theoretically calculated microscopic cross sections of electron and lattice phonon interactions, which are in good agreement with the experiment. Different ligand has the special pattern of connecting with nanocrystal. The ligand chain length determines the effective mass and carrier mobility. This method can be broadened to model electron transport of other nano-crystal and ligand complex materials.

A modified polysulfide redox mediator with incorporation of cyanocobalamin (vitamin B12) was employed in quantum dot-sensitized solar cells (QDSCs), which can improve the electrocatalytic activity and reduce the charge transfer resistance caused by sulfureted contamination on the platinum counter electrode. The enhancement in the electron transfer properties could be regarded as a cause of a new formed vitalizing chelation which could behave as an ohmic characteristic interface between modified electrolyte and platinum counter. As a result, QDSCs with high filling factor of 65% with energy conversion efficiency more than 6% have been achieved.

Converting solar energy to electrical power or chemical potential has long been viewed as one of the greatest challenges of our time. Water-splitting photoelectrochemical cells (PEC) represent an efficient means to harvest solar energy but necessitate the design of photocatalytic constructs with discrete light-harvesting and catalytic components. Towards this end, we have interfaced CdSe quantum dots to lead vanadium oxide (β-Pb_xV₂O₅) nanowires to study their feasibility for photoelectrochemical cells. β-Pb_xV₂O₅ crystallizes with Pb²⁺ ions residing within the channels of a tunnel-like V₂O₅ framework. This material is characterized by mid-gap states that arise between the conduction and valence bands. The mid-gap states are located only slightly positive relative to the water-oxidation potential yet still negative relative to the valence band edges of CdSe quantum dots. Consequently, we propose their use within photocathodes wherein photogenerated holes can easily transfer across the CdSe QD/ β-Pb_xV₂O₅ interface and be propagated to water-oxidizing catalysts.
We have successfully functionalized β-Pb_xV₂O₅ nanowires with CdSe quantum dots using two separate approaches: successive ionic layer adsorption and reaction (SILAR) and linker-assisted attachment via bifunctional linker molecules. Control has been established through the SILAR process as shown by the CdSe layer thickness correlating directly to the number of SILAR cycles. Selective functionalization using linker-assisted attachment allows for direct control of the amount of CdSe on β-Pb_xV₂O₅ nanowires.
The CdSe QD/ β-Pb_xV₂O₅ nanowire heterostructures have been characterized by scanning electron microscopy and transmission electron microscopy to understand surface morphology and layer thickness. X-ray diffraction and Raman spectroscopy have been performed to further characterize the functionalized wires. Experiments examining the photo catalytic degradation of methylene blue have also been performed using our functionalized nanowire photocatalysts. This work was supported by the Research Corporation for Science Advancement through a Scialog Award.

The conversion efficiency of single junction solar cells is limited due to a necessary compromise between current and voltage that can be generated with a single semiconductor material. One way to overcome this limitation is the use of multi-junction solar cells. Today&’s record cell efficiencies of 44% have been achieved with triple junction cells made of III-V compound materials and put under concentration of 942 suns [1]. However, such cells typically are very expensive because of the use of Ge as substrates, and are typically used in concentrating devices. An alternative approach that promises reasonably high efficiencies is the use of a double junction cell made of GaAs on top of Si, which is cheaper and more abundant than Ge. The advantage of this approach is that it uses mature devices and gives a potentially significant efficiency boost compared to a single junction device. However, the disadvantage of this concept is that the lattice constant of silicon and GaAs do not match and a technique has to be found to combine these two cells.
In this project we concentrate on two different approaches:
1. Wafer bonding of the two sub-cells
2. Mechanical stacking of the sub-cells
These techniques can be used to create 2-, 3- or 4-terminal devices.
In this paper we present a theoretical study of GaAs - Si tandem solar cell stacked mechanically or by wafer bonding and investigate the most important parameters that affect the efficiency of this system. These parameters include tunnel junction property, surface recombination, light trapping and photon recycling, etc. This investigation will incorporate both extensive optical and electrical simulation to determine realistic device performance. With this analysis, a comprehensive design rule can be given for designing GaAs on Si cells for different applications in different meteorological regions.
1. Green, M.A., et al., Solar cell efficiency tables (version 41). Progress in Photovoltaics: Research and Applications, 2013. 21(1): p. 1-11.

Surface textures on solar cells can confine light and compensate light absorption losses resulting from a thin absorber layer. Conventional light trapping structures are pyramidal textures on monocrystalline- or isotextures on multicrystalline silicon wafer solar cells. The features of these textures have sizes that are typically in the micrometre range. Conventional textures, however, are not applicable for thin silicon-based solar cells as the absorber layer thickness itself is no larger than a few micrometres [1]. Nanoscale surface textures are hence explored catering for solar cells with a thin absorber layer. This project investigates the optical properties of nanoscale “muffin” structures fabricated from a lithography procedure and simulated using the Rigorous Coupled Wave Analysis (RCWA) method. The “muffin” structures are modelled as a periodic nanostructure in RCWA and study of its optical properties shows a potential for light trapping application, especially in thin silicon solar cells.
The objective of this investigation is to demonstrate a structure with a thickness of a few hundred nanometres that enables light trapping comparable to that of conventional textures with a much larger thickness and is, therefore, applicable, to very thin silicon layers. Preliminary results show that the muffin structure has a high potential for light trapping by concentrating light within a volume close to the texture.
Using RCWA, we investigate theoretically the path length- and absorption enhancement that can be observed for silicon films with a “muffin” surface texture over a broadband wavelength range. The results are compared to those of a planar silicon film and other types of textures. The “muffin” surface textures are further optimized by studying the optical properties for structures with varying shape parameters. By simulation, the nanoscale periodic muffin structures with a small aspect ratio are able to achieve a substantial absorption enhancement.
1. Mavrokefalos, A., et al., Efficient Light Trapping in Inverted Nanopyramid Thin Crystalline Silicon Membranes for Solar Cell Applications. Nano Letters, 2012. 12(6): p. 2792-2796.

Future photovoltaic systems can be greatly benefited by cell and module design that enables simultaneously ultrahigh efficiency (> 50%) and low-cost (< $0.50/Wp) to propel sharp reductions in the levelized cost of electricity. The photonic environment of the cell should be designed to maximize the external radiative efficiency and the open circuit voltage, and key factors that affect open circuit voltage will be discussed. I will also discuss ‘full spectrum&’ photovoltaic modules, which take advantage of advances in low-cost III-V compound cell fabrication and emerging optical and electronic fabrication/assembly methods, features 6-15 independently connected subcells in a spectrum splitting, concentrating photovoltaic receiver. Module architectures utilizing independently connected single junction and multijunction subcells allow flexibility in subcell selection for optimal energy bandgaps and fabrication, and also reduce the constraints posed by current matching requirements. Several different spectrum-splitting optical architectures designed for systems with many (>6) subcells are possible, including designs based on holographic spectrum splitting, specular reflection in dielectric polyhedra and light trapping textured filtered dielectric slabs that perform as nonimaging concentrators.

In order to realize more than 45% efficiency, new approaches for novel materials and structures are necessary. III-V lattice-mismatched material is one of appropriate materials for high efficiency multi-junction solar cells with efficiency of more than 45% due to optimum band gap energy combinations of sub cell materials. However, lattice-mismatching-related dislocation generation in active solar cell layers is one of major problems for high efficiency. InGaAsN is also another appropriate material for 4- or 5-junction solar cell configuration because this material can be lattice-matched to GaAs and Ge substrates. However, present InGaAsN single-junction solar cells have been inefficient because of low minority-carrier lifetime due to N-related recombination centers and low carrier mobility due to alloy scattering and \non-homogeneity of N.
This paper presents our recent results in 1) understanding of lattice-mismatching-related strain relaxation and dislocation behaviour in the InGaAs/GaAs lattice- mismatched system, and 2) understanding of majority and minority carrier traps in GaAsN grown by chemical beam epitaxy (CBE) and their relationships with the electrical properties of the materials and solar cells.
In order to understand dislocation behaviour and reduce dislocation density in lattice-mismatched InGaAs/GaAs, in-situ measurements of reciprocal space mapping of X-ray diffraction during molecular beam epitaxial growth of InGaAs/GaAs have been studied. As a result, mechanism of dislocation motion in lattice-mismatched materials and effects of buffer layer upon strain relaxation and dislocation behaviour have been discussed.
Under the Project of the Europe-Japan Collaborative Research on Concentrator Photovoltaics, high efficiency inverted epitaxially grown InGaP/GaAs/InGaAs 3-junction solar cells with efficiencies of 37.9% under 1-sun and 43.5% under 240-306 -suns have been demonstrated by Sharp. Co. as a result of these studies.
By adapting CBE technique to grow (In)GaAsN thin films, higher mobility and longer minority-carrier lifetime compared to those grown by the other growth methods have been achieved. As a result, 13.7% efficiency (Jsc=20.3mA/cm2, Voc=0.721V, FF=0.815) GaAsN single-junction cells have been obtained. Further improvement in efficiency is necessary for realizing high-efficiency 4-junction