Patent Publication Number: US-2011067752-A1

Title: Solar cell with epitaxially grown quantum dot material

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/038,230, filed Jan. 21, 2005. U.S. patent application Ser. No. 11/038,230 claims the benefit of U.S. Provisional Application No. 60/537,259, filed Jan. 20, 2004. U.S. patent application Ser. No. 11/038,230 and U.S. Provisional Application No. 60/537,259, are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to photovoltaic solar cells. More particularly, the invention relates to III-V multijunction semiconductor solar cells having an epitaxially grown quantum dot material. 
     BACKGROUND OF THE INVENTION 
     The sun emits a wide optical spectrum that peaks in the visible and has 60% of its photon flux in the wavelength range spanning from ˜350 nm to ˜1350 nm. This wavelength range corresponds to 80% of the sun&#39;s total power flux of ˜1.3 kW/m 2  at the earth. 
     It has been known for decades that the best approach to convert the sun&#39;s optical power into electrical power is through solar cells that make use of absorption transitions in semiconductors. Photon energy is harnessed in this way by exciting electrons from the semiconductor&#39;s valence band across the bandgap into the conduction band. The photocarriers thus generated, i.e. the electrons and holes, are then swept across a p-n or p-i-n junction fabricated by doping different regions of the semiconductor structure, and are used to produce electricity. Semiconductors or semiconductor alloys with bandgaps E g  absorb impinging photons having energies greater than or equal to E g  as opposed to photons having energies less than E g . Equivalently, it can be said that photons having wavelengths corresponding to energies greater than E g  are absorbed while photons having longer wavelengths are not. 
     Since the energy of a photon in excess of E g  is effectively lost through thermal processes, it is well established that a combination of materials having different bandgaps must be used to adjust the voltage and current of the solar cell in order to optimize the conversion efficiency of solar light into electricity. To that effect, multijunction solar cells, also known as tandem solar cells, have been developed for applications requiring higher conversion efficiencies. 
     From a fabrication and crystal perspective, the choice of semiconductors or semiconductor alloys is practically restricted to materials that can be grown on common substrates, such as GaAs, Ge, Si, or InP substrates, with a minimum of defects. To date, the best optical to electrical conversion efficiencies, the conversion efficiency being defined as the electrical power that the device can produce divided by the optical power received from a light source such as, for example, the sun, are around 30% and have been achieved by growing a monolithic multijunction cell having a GaInP top subcell (E g ˜1.8 eV), a GaAs middle subcell (E g ˜1.4 eV), and a Ge bottom subcell (E g ˜0.7 eV) on a Ge substrate. Since the subcells are typically connected in series through tunnel junctions, it is recognized that to improve further the conversion efficiency, the bandgaps of the materials have to be changed or, a fourth subcell added. 
     The total voltage of the multijunction cell is essentially the sum of the voltages generated by the individual subcells, where the voltage of each subcell is proportional to the subcell&#39;s bandgap. To optimize the conversion efficiency, the subcells should be current-matched, otherwise the subcell generating the weakest current limits the overall current. In the case above, GaInP has a bandgap that can absorb 25% of the total solar photon flux (sometimes referred to as the AM0 spectrum), whereas only 14% of the total solar photon flux transmitted through the GaInP subcell can be absorbed by the GaAs subcell, and 38% of the total solar photon flux transmitted through the GaAs subcell can be absorbed by the Ge subcell. 
     This clearly leads to a current imbalance in the multijunction cell. Relatively speaking, the GaAs subcell does not absorb enough solar photons while the Ge subcell captures too many. To equilibrate the current balance between the subcells, the middle subcell, i.e. the subcell disposed between the GaInP and the Ge subcells, would have a smaller bandgap. For example, a middle subcell having a bandgap of ˜1.16 eV (corresponding to an optical wavelength of approximately 1100 nm) would imply that all the three subcells would each absorb approximately 25% of the total solar photon flux. The remaining 25% of the solar photon flux would not be absorbed since the three subcells are transparent to the longer wavelength photons (i.e. photons with wavelengths greater than 1.8 μm are not absorbed). 
     As mentioned above a four-subcell arrangement can improve the current balance. If a material with Eg˜1.0 eV is introduced between the GaAs and the Ge subcells, it yields the following distribution in the absorption of the solar photon flux: ˜25% of the photons absorbed by the first subcell (GaInP), ˜14% by second subcell (GaAs), ˜19% by the third subcell (Eg˜1 eV), and ˜19% by the fourth subcell (Ge). However, this four-subcell arrangement is still current-limited by the GaAs subcell. To make the four-subcell arrangement better balanced in terms of current, the thickness of the first subcell can be adjusted (reduced) to let some of the shorter wavelength photons reach the second subcell. In this scenario, the second subcell absorbs more photons having energies greater than that of its bandgap thus leading to more thermally wasted energy. This was described by Olson et al. in U.S. Pat. No. 5,223,043 incorporated herein by reference. 
     Research and development to find new materials and novel multijunction arrangements to improve the efficiency of solar cells has been very active. For example, Olson, in U.S. Pat. No. 4,667,059, disclosed dual GaInP/GaAs cells on a GaAs substrate; Ho et al., in U.S. Pat. No. 5,405,453, disclosed dual GaInP/GaAs cells on a Ge substrate; Wanlass, in U.S. Pat. No. 5,019,177, disclosed dual InP/GaInAsP cells on InP; Freundlich et al., in U.S. Pat. No. 5,407,491, disclosed dual InP/InGaAs cells on an InP substrate; Chang et al., in U.S. Pat. No. 5,330,585, disclosed the dual AlGaAs/GaAs cells on a GaAs substrate; these patents being incorporated herein by reference. 
     These examples of dual cells and the triple cell made of GaInP/GaAs/Ge on a Ge substrate can have conversion efficiencies close to 30% as long as compromises in the design or in the quality of the materials are made. The compromise in the case of the dual cells having GaAs as the smallest bandgap is that the longer wavelength photons are not absorbed, they are transmitted though all the layers. In the case of dual cells with the smaller InGaAs or InGaAsP bandgaps, the compromise is that the shorter wavelength photons are losing their excess energies in heat. It is also worth nothing that GaAs or Ge substrates have the advantage of a lower cost compared to InP substrates. 
     To extend the photo-absorption of GaInP/GaAs cells to longer wavelengths, Freundlich, in U.S. Pat. No. 6,372,980, incorporated herein by reference, disclosed solar cells with InGaAs quantum wells, the solar cells having modeled efficiencies in excess of 30%. Other schemes have also been disclosed to try to improve the efficiency of solar cells. For example, Freundlich et al., U.S. Pat. No. 5,851,310, incorporated herein by reference, disclosed the use of strained quantum wells grown on an InP substrate. Also, Suzuki in U.S. Pat. No. 6,566,595 (later referred to as &#39;595), incorporated herein by reference, disclosed the use of quantum well layers having a plurality of projections with different sizes, the goal being of better matching the sun&#39;s spectrum by using materials having different bandgaps. 
     Similar is the disclosure by Sabnis et al. in U.S. Pat. No. 4,206,002, incorporated herein by reference, for bulk graded bandgap multijunction solar cells. In the case of the &#39;595 patent however, the overall efficiency is unlikely to be improved since the tailoring of the absorption spectrum involves distributing quantum well or quantum dot materials of different sizes in the plane of the layers. This compromises the spatial density of the material that can be used to absorb light and is likely to require thicker layers to absorb the same number of photons as would be absorbed in uniform layers or, larger surfaces which would reduce the conversion efficiency. Chaffin et al., in U.S. Pat. No. 4,688,068, incorporated herein by reference, also disclosed the use of quantum wells in multijunction cells. 
     As disclosed by Kurtz et al. in U.S. Pat. No. 6,252,287, incorporated herein by reference, InGaAsN lattice-matched to GaAs is also a promising material for tailoring the bandgap of layers lattice-matched to GaAs for optimizing the conversion efficiency. 
     Other aspects of the fabrication of monolithic multijunction solar cells such as antireflection windows, tunnel junctions, and surface metallization have matured with the extensive developments of photovoltaic solar cells as disclosed in numerous patents and publication in that field as seen in several of the patents identified above (for example, U.S. Pat. Nos. 4,694,115; 5,009,719; 4,419,530; 4,575,577). 
     In the field of semiconductor nanostructures, it is well known that high-quality, defect-free, self-assembled quantum dots can be obtained during the early stage of growth of highly strained semiconductors (see for example: S. Fafard, et al., “ Manipulating the Energy Levels of Semiconductor Quantum Dots”, Phys. Rev. B  59, 15368 (1999) and S. Fafard, et al., “ Lasing in Quantum Dot Ensembles with Sharp Adjustable Electronic Shells”, Appl. Phys. Lett.  75, 986 (1999)). Such quantum dot material can be grown in multiple layers to achieve thick active regions for devices such as Quantum Dot Infrared Photodetectors, as disclosed by Fafard et al. in U.S. Pat. No. 6,239,449, incorporated herein by reference. There, the interband absorption properties of the quantum dot material can be tailored to cover various wavelength ranges in the near infrared and visible portions of the optical spectrum. The composition, size and shape of the quantum dot material are adapted to change the quantization energies and the effective bandgap of the quantum dot material, where the effective bandgap of the material is defined as essentially being the lowest energy transitions at which photons can be absorbed and is determined by the quantized energy levels of the heterostructure. 
     Self-assembled quantum dots come in a wide range of high quality materials that can be pseudomorphically grown on GaAs or InP. For example, InAlAs/AlGaAs on GaAs substrates absorbs in the red or the near-infrared, InAs/InAlAs on InP substrates absorbs in the 1.5 μm wavelength range, and InAs/InGaAs on InP substrates absorbs in the 1.9 μm wavelength range. More importantly however, In(Ga)As/GaAs self-assembled quantum dot material grown on GaAs substrates, is particularly well-suited for absorption below the GaAs bandgap in the spectral region spanning from 885 nm to 1150 nm, or up to ˜1350 nm depending on the growth parameters. In(Ga)As/GaAs self-assembled quantum dot layers can be grown uniformly and with high densities. Furthermore, multiple layers can be grown with the same uniformity or, when desirable, with different sizes and/or compositions by simply controlling the growth parameters. Additionally, the In(Ga)As/GaAs self-assembled quantum dot material has been shown to produce devices which are orders of magnitude more radiation robust than conventional material (see for example: P. G. Piva et al., “ Enhanced Degradation Resistance of Quantum Dot Lasers to Radiation Damage”, Appl. Phys. Lett.  77, 624 (2000)). The radiation and defect hardnesses are particularly great advantages for space applications where the solar cells are getting exposed to radiations. 
     As can be appreciated from the prior art discussed above, there is a real need for high quality materials having desired absorption spectra, that can be easily incorporated in multijunction solar cells to improve further the conversion efficiency. A reliable material that can balance the absorption between the bandgaps of GaAs and Ge is of particular interest. 
     SUMMARY OF THE INVENTION 
     In a first aspect, the present invention provides a multijunction semiconductor photovoltaic solar cell. The solar cell comprises a plurality of subcells disposed in series, each subcell has formed therein a p-n junction or a p-i-n junction. The plurality of subcells includes a quantum dot subcell, the quantum dot subcell is a subcell with strained epitaxially-grown semiconductor layers that include self-assembled quantum dots. The quantum dot subcell is pseudomorphically grown on another subcell. 
     The solar cell can comprise tunnel junctions formed between the subcells, the tunnel junctions for electrically connecting the subcells. 
     The solar cell can comprise an electrically conductive substrate upon which the plurality of subcells is formed. Each of the subcells has an effective bandgap energy, the subcells the subcells can be disposed in order of increasing effective bandgap energy, a subcell with a lowest effective bandgap energy being closest to the electrically conductive substrate. 
     The solar cell can be such that each subcell is for absorbing a substantially same fraction of solar photons. 
     The plurality of subcells can consist of three subcells, a first subcell having the lowest effective bandgap energy, a third subcell having a highest effective bandgap energy, and a second subcell disposed between the first subcell and the third subcell, the second subcell being the quantum dot subcell. The electrically conductive substrate can be a Ge or a GaAs substrate. The first subcell can include Ge. The strained epitaxially-grown semiconductor layers can include strained InGaAs quantum dot layers intercalated with GaAs, AlGaAs, or GaPAs layers. The third subcell can include GaInP, AlGaAs or AlGaInP. 
     The solar cell of can be such that the first subcell is epitaxially grown on the electrically conductive substrate. Alternatively, the electrically conductive substrate can be a Ge substrate and the first subcell can an interdiffused portion of the Ge substrate. 
     The electrically conductive substrate can be n-doped, and an n-side of each of the p-n junction or p-i-n junction formed in each subcell is closer to the substrate than a respective p-side of the p-n junction or the p-i-n junction. 
     The electrically conductive substrate can be p-doped, and a p-side of the p-n junction of p-i-n junction formed in each subcell is closer to the substrate than a respective n-side of the p-n junction or p-i-n junction. 
     The solar cell can be such that one of the subcells includes a Bragg reflector or a distributed Bragg reflector to modify an absorption characteristic of the solar cell. 
     In a second aspect, the present invention provides a multijunction semiconductor photovoltaic solar cell. The solar cell comprises a plurality of subcells disposed in series, each subcell has formed therein a p-n junction or a p-i-n junction. The plurality of subcells includes a quantum dot subcell, the quantum dot subcell is a subcell with strained epitaxially-grown semiconductor layers that include self-assembled quantum dots. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of examples only, with reference to the attached Figures, wherein: 
         FIG. 1  shows the percentage of the AM0 solar photon and power fluxes integrated from 200 nm. 
         FIG. 2  depicts a monolithic three-subcell photovoltaic solar cell of the present invention. 
         FIG. 3  shows photovoltaic spectra of self-assembled quantum dot materials. 
         FIG. 4  depicts a self-assembled quantum dot material of the present invention. 
         FIG. 5  depicts a monolithic four-subcell photovoltaic solar cell of the present invention. 
         FIG. 6  depicts a monolithic two-subcell photovoltaic solar cell of the present invention. 
         FIG. 7  depicts a conduction band of a quantum dot. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides monolithic semiconductor photovoltaic solar cells comprising at least one subcell having a self-assembled quantum dot material. Also provided is a method for making such solar cells, the method using epitaxial growth of self-assembled quantum dot material in at least one subcell of the solar cell. 
     The first embodiment is a high efficiency monolithic three-junction photovoltaic solar cell. Three-junction photovoltaic solar cells can be fabricated by stacking p-n or n-p junctions made of different semiconductor materials. As stated above, conversion efficiencies of approximately 30% have been obtained using a Ge bottom subcell grown on a Ge substrate, with a GaAs middle subcell and a GaInP or AlGaAs top subcell. The efficiency of such a multijunction photovoltaic solar cell is improved in the present invention by using self-assembled quantum dot material in the middle subcell instead of bulk GaAs. 
     The principle of the invention is illustrated in  FIG. 1  where plot  10  shows the percentage of the AM0 solar photon flux, integrated from the 200 nm wavelength, as a function of wavelength, and plot  12  shows the percentage of the AM0 solar power flux integrated from 200 nm wavelength as a function of wavelength. Reference numerals on plot  10  indicate semiconductor materials and their absorption edges. These semiconductor materials can be used in converting photons (solar or otherwise) to electrical carriers. The plotted points of  FIG. 1  are referred to in the description as cells, subcells, particular semiconductor material or absorption edges, depending on their particular context. 
     To optimize the conversion efficiency of a solar cell, the current of each subcell must be substantially equal since the subcells are connected in series. Neglecting the reflection at the surface of the device, which is a valid approximation for solar cells equipped with an antireflection coating, the light intensity transmitted at a depth z from the surface of the semiconductor is given by I(z)=I o exp(−αz). I o  is the input intensity and α is the semiconductor absorption coefficient, which is a function of the wavelength dependent density of states of the material (i.e. α is wavelength dependent). For sufficiently thick semiconductor material, only light having a wavelength longer than the semiconductor bandgap equivalent wavelength (or with an energy less than the bandgap energy) will transmit through the semiconductor layer since the density of states drops, as does α, for photon energies less than the bandgap energy. For direct bandgap semiconductors, at wavelengths shorter than the bandgap wavelength, α is in the  10   4  cm −1  to  10   5  cm −1  range and each impinging photon can create a pair of photocarriers, i.e. an electron and a hole. 
     The current in a subcell is proportional to the fraction of photon flux absorbed by the subcell. As can be inferred from plot  10 , a GaInP/GaAs/Ge solar cell absorbs about 25% of the photon flux in its top GaInP subcell  14 , approximately 14% in its middle GaAs subcell  16 , and approximately 38% in the bottom Ge subcell  18 . Replacing the top GaInP subcell  14  with an AIGaAs subcell  20  would yield similar results. 
     The imbalance in the absorption of the solar photon flux by subcells  14 ,  16  and  18  leads to a current imbalance. That is, the Ge bottom subcell  18  is generating the most current and the middle GaAs subcell  16  is limiting the overall current and conversion efficiency. The overall conversion efficiency can be improved by substituting GaAs material  16  with a material having an effective bandgap wavelength of about 1070 nm (1.16 eV). As will be seen in detail below, such a material can be a self-assembled quantum dot material  22 . By using quantum dot material  22  in the middle subcell, the solar photon flux absorption of each of the three subcells is about 25% of the total solar photon flux and the current generated by each subcell will be equal. The theoretical efficiency can be calculated to give the thermodynamic limit of photovoltaic energy conversion. The theoretical efficiency takes into account the bandgap of the subcell, the impinging photon flux and its spectral distribution to estimate the corresponding open-circuit voltage (V oc ) and the short-circuit current (J sc ) as described by, for example, Baur et al in paper 3P-B5-07 of the WCPEC-3 Proceedings Osaka, 2003. For an optimized configuration, the conversion efficiency could theoretically exceed 40%. 
     The details of such a monolithic three-junction photovoltaic solar cell is illustrated is  FIG. 2  (not to scale), which sketches an embodiment for the solar cell of the present invention. The multijunction solar cell  24  comprises a substrate  26 , first subcell  28 , second subcell  30  pseudomorphically grown by epitaxy and comprising self-assembled In(Ga)As/GaAs quantum dot material, and third subcell  32 . The In(Ga)As/GaAs quantum dot material of second subcell  30  can be tailored to obtain an effective bandgap of about 1.16 eV. Other techniques can be used to produce similar nanostructures using other epitaxial techniques such as selective area epitaxy, templated epitaxy, epitaxy with stained-induced bandgap modified heterostructures, Volmer-Weber growth modes, modified Stranski-Krastanow growth modes, Frank-Van der Merwe growth modes combined or not with high-resolution microfabrication, or, non-epitaxial techniques involving, for example, colloidal quantum dots; however, the optical and/or structural properties of such nanostructures are typically not suited for improving the efficiency of multijunction solar cell devices. 
     According to this embodiment of the invention, substrate  26  can be conductive GaAs or preferably conductive Ge, both of them having a similar lattice constant. The doping of substrate  26  can be n-type or p-type. Whether substrate  26  is n-type with an n-p or an n-i-p junction grown on top or, p-type with a p-n or a p-i-n junction grown on top, is not fundamental to the present invention. For illustration purposes, this embodiment will use an n-type substrate with n-p or n-i-p junction. Other possible combinations, which could include an undoped substrate and buried back contacts, are equally possible. Substrate  26  may be metallized to form ohmic contact  34  on the backside, as illustrated in  FIG. 2 , and a buffer and/or back field layers  36  can be grown between substrate  26  and first subcell  28  to optimize various structural, electrical, or optical properties. First subcell  28  is preferably made of Ge and includes an n-p junction to create a depletion region. A tunnel junction  38  is used to connect first subcell  28  with second subcell  30 . 
     As will be readily understood by a person skilled in the art, tunnel junction  38  is preferably made of a high quality material that can be epitaxially grown on first subcell  28 . Tunnel junction  38  is preferably highly doped to provide good electrical conduction and to support high current densities and, is preferably transparent to photons traversing it. For this embodiment tunnel junction  38  can be made of a highly doped GaAs n-p junction but many other combinations supporting the requirements herein are equally valid. 
     Second subcell  30  comprises a self-assembled In(Ga)As/GaAs quantum dot material, adapted to, or tailored to, obtain an effective bandgap of about 1.16 eV. More details about second subcell  30  are given in  FIG. 4  and its corresponding description where it is disclosed to contain an n-p or an n-i-p junction comprising a plurality of layers with high-quality self-assembled In(Ga)As/GaAs quantum dots of a specific shape, composition, and density, grown pseudomorphically by epitaxy. 
     Second subcell  30  is electrically connected to third subcell  32  via tunnel junction  40 . The requirements of tunnel junction  40  are similar to those of tunnel junction  38  discussed above. In this embodiment tunnel junction  40  can be made of a highly doped InGaP or AlGaAs n-p junction but many other combinations are equally valid. Third subcell  32  is essentially an n-p junction preferably made of doped GaInP or doped AlGaAs, or a similar alloy latticed-matched to GaAs, with a bandgap around 1.8 eV. Third subcell  32  may include window  42 , antireflection layer  44 , and contact  46  as is customary in multijunction solar cells. 
     The top part of  FIG. 2  shows graph  48  of spectral intensity as a function of wavelength for solar spectrum  50 .  FIG. 2  also illustrates absorption ranges  52 ,  54  and  56  of solar spectrum  50  for third subcell  32 , second subcell  30  and first subcell  28  respectively. In view of the discussion relating to  FIG. 1 , it will be clear for someone skilled in the art that such a three-junction photovoltaic solar cell has good current matching between the subcells together with high conversion efficiencies. 
       FIG. 3  shows photovoltaic spectrum  58  of high quality self-assembled quantum dot materials, i.e. materials that may be comprised within second subcell  30 , grown within a p-i-n junction. Plot  58  shows the spectrum of a first material measured at 20° C. The features of plot  58  are quantum dot material ground states  60 , quantum dot material excited states  62  and wetting layer states  64 , a wetting layer being a thin continuous layer that usually forms during the epitaxy of the self-assembled quantum dots. In this self-assembling epitaxial process, the first monolayer, or first few monolayers, are deposited in uniform two-dimensional layers called the wetting layers. Quantum dots then self-assemble from the additional material deposited and/or in part from the previous wetting layer material. Plot  58  was measured using a white light source transmitted through GaAs layers. Signal decrease  68  is observed for energies greater than the GaAs bandgap. Quantum dot ground states  60  can be referred to as the effective bandgap of a self-assembled quantum dot material. For bulk semiconductors not comprising semiconductor heterostructures, the effective bandgap is simply the bandgap of the semiconductor material. 
     It is known that quantum dots energy levels can be adjusted by controlling their shape, composition, and density during growth [for example see: S. Fafard, et al., “Manipulating the Energy Levels of Semiconductor Quantum Dots”, Phys. Rev. B 59, 15368 (1999) or S. Fafard, et al., “Lasing in Quantum Dot Ensembles with Sharp Adjustable Electronic Shells”, Appl. Phys. Lett. 75, 986 (1999)]. For plot  58 , the self-assembled In(Ga)As/GaAs quantum dot material was tailored to have an effective bandgap of about 1.16 eV. The material measured in plot  58  contains 14 layers of In(Ga)As quantum dots separated with 10 nm barriers made of GaAs 
     An embodiment of second cell  30  is shown in  FIG. 4 . There, a plurality of III-V semiconductor alloy layers is grown epitaxially on tunnel junction  38 , which comprises heavily p-doped layer  78  and heavily n-doped layer  80 . An emitter  82  is formed by the combination of highly n-doped layer  84  and n-doped layer  86 . Emitter  82  is preferably made of GaAs or from another alloy lattice-matched to GaAs and has a bandgap close to the bandgap of GaAs. Similarly, a collector  88  is later grown using p-type doping for layer  90  and highly p-doped layer  92 . Silicon is preferably used for n-type doping while beryllium is preferably used for p-type doping. Obviously, other dopants may be used such as, for example, Zinc (Zn), Tellurium (Te), or others. 
     Emitter  82  and collector  88  form an n-i-p junction together with intrinsically undoped self-assembled quantum dot material  94  disposed between emitter  82  and collector  88 . The doping profiles of emitter  82  and collector  88  are such that they provide a depletion region extending substantially across self-assembled quantum dot material  94 . Similar configurations can be designed using an n-p junction instead of an n-i-p one, or by reversing the order of the p and n doping. Additionally, since self-assembled quantum dot material  94  is pseudomorphically grown on GaAs, Bragg reflectors or distributed Bragg reflector (DBR) cavities comprising alternate layers of high and low index of refraction semiconductors may be grown within the emitter and/or the collector, to enhance the reflectivity and change the absorption properties of the subcell and consequently of solar cell  24 . Similar self-assembled quantum dot material  94 , together with emitter  82  and collector  88 , as depicted in  FIG. 4 , will be used in the description of all embodiments of the present invention. 
     Self-assembled quantum dot material  94  comprises a first quantum dot layer  96 , first barrier  98 , second quantum dot layer  100 , second barrier  102  and so on up to N th  self-assembled quantum dot material  104  and N th  barrier  106 . It will be clear for one skilled in the art that the N quantum dot layers need not be identical in thickness or composition. Furthermore, layers can be inserted in self-assembled quantum dot material  94  to optimize optical, structural or electrical properties of solar subcells such as second subcell  30 . For example layers with other bandgaps or with a another lattice constant can be grown above and/or below quantum dot layers  96 ,  100 ,  104  to modify the optical and/or the structural properties of quantum dot material  94 . Also, layers with other bandgaps or with another lattice constant can be grown within barriers  98 ,  102 ,  106 . The growth of such intermediate layers within the barriers is particularly important to control the total strain built in the structure. For example, the thickness of semiconductor layers having a lattice constant smaller than the epitaxial layer and smaller than the lattice constant of the substrate can be chosen to yield layers that would compensate the strain introduce when using a semiconductor with a larger lattice constant for the quantum dot layers. For example, GaPAs or GaInP can be grown within the barriers to compensate the strain of the InAs or InGaAs quantum dots. Furthermore, the plurality of layers could contain a number of sub-groups of layers having similar properties, such that N layers would be composed of m sub-groups each containing a number m i  of quantum dot and barriers layers having a similar size, composition, and effective bandgap. 
     In the present embodiment, growth materials and parameters can be chosen to obtain self-assembled quantum dot material  94  with desired absorption characteristics such as, for example, an absorption edge at 1.16 eV. Pseudomorphic growth of the self-assembled quantum dot material  94  is obtained by epitaxy using, for example, a molecular beam epitaxy (MBE) system. The MBE system is used for growing, for example, GaAs or AlGaAs layers on a semiconductor material lattice-matched to GaAs. Epitaxy systems other than MBE systems can be used. They may include, for example, chemical beam epitaxy (CBE), metal organic chemical vapor deposition (MOCVD) or other similar hybrid systems or combination thereof. To obtain the desired optical, electrical and structural properties, the growth temperature is maintained in a range that optimizes the desired properties while avoiding high temperatures that could cause intermixing of layers or diffusion of dopants present, for example, in emitter  82  or collector  88  layers. 
     As an example, when intermixing or diffusion of the dopants in emitter  82  layers is not a concern, the growth of the GaAs layers is preferably done in a temperature range comprised between 400° C. and 800° C., preferably between 520° C. and 630° C. and most preferably between 600° C. and 630° C. In the case where intermixing and/or diffusion of dopants is a concern during the epitaxial growth of the quantum dot layers, the growth temperature is preferably comprised between of 450° C. and 550° C. and most preferably between of 490° C. and 530° C. The growth temperature of the quantum dot layers is used to adjust the shape and composition of the quantum dots. The temperature during the overgrowth of the barrier of each quantum dot layer may be varied at different stages of the overgrowth to further control the size and composition of the quantum dots and therefore the absorption characteristics of self-assembled quantum dot material  94 . 
     The combination of growth temperature, the group V over-pressure or the III/V ratio, the quantum dot material, the amount of material used to obtain the self-assembled growth transition between a uniform quasi two-dimensional film to three-dimensional islands, the growth rate or the pauses used during the growth, and the overgrowth conditions such as growth temperature and growth rate, are chosen to obtain quantum dot layers having a high in-plane density of highly uniform quantum dots having desired energy levels. This allows high conversion efficiencies of impinging solar photons into electricity. 
     As will be apparent to one skilled in the art, there are many combinations of parameters that can accomplish the desired absorption characteristics. However, for illustration purposes of the present embodiment, the desired absorption of the self-assembled quantum dot material  94  can be obtained by growing InAs on GaAs, the thickness of InAs being comprised between 0.6 nm and 0.8 nm and preferably comprised between 0.68 nm and 0.72 nm. The preferred growth rate of InAs is comprised between 0.001 and 3 nm/s and most preferably between 0.01 and 0.03 nm/s, with a growth pause following the InAs deposition, the growth pause preferably ranging from 0 to 300 seconds. The growth of the InAs quantum dot layer is followed with the over-growth of a barrier layer having a thickness ranging from 6 nm to 50 nm, the barrier layer preferably being GaAs or A x Ga 1-x As, x being comprised between 0 and 1 but preferably comprised between 0 and 0.35. The growth sequence of quantum dot layer and barrier layers is repeated a number of times as stated above. 
     As mentioned above, a specific temperature cycling of the substrate may be used to adjust the shape, composition, and uniformity of the quantum dots during the overgrowth of the quantum dot layers and barrier layers. In this case, the temperature of substrate  26  is increased preferably above the InAs disorption temperature which is roughly 530° C. for MBE growth and can depend, amongst different factors, on the growth method and on the use of a surfactant. Once the temperature has been increased above the InAs disorption temperature, it is decreased back to a nominal value preferably comprised between 450° C. and 550° C. and most preferably between 490° C. and 530° C. This is followed by the growth of a subsequent quantum dot layer. In the case where the desired absorption edge is 1.16 eV, the temperature cycling performed during the over-growth may occur when the thickness of the barrier is comprised between 1 nm and 50 nm, preferably between 2.0 nm and 10.0 nm, and most preferably between 4.5 nm and 6.5 nm. 
     In the preferred embodiment, the number of quantum dot layers is comprised between 1 and 100 preferably between 30 and 80. It is possible to grow more layers if necessary. The larger the number of layers of self-assembled quantum dot layer material  94 , the larger the absorption coefficient of second cell  30  will be, which is desirable to increase the current of second cell  30 . 
     The distance between the quantum dot layers, i.e. the barrier thickness, is adjusted to (A) change the desired characteristics of the absorption spectrum; (B) control the vertical stacking of the self-assembled quantum dots; and (C) maintain the overall strain level below that related to the critical thickness that leads to the onset of lattice relaxation. For thicknesses above the critical thickness, the quantum dot material could start developing material dislocations and defects. The critical thickness can be measured and/or estimated by using, for example, Matthew&#39;s law. For the average InGaAs composition of interest with low indium content the critical thickness is expected to be between 1 and 2 microns. The critical thickness is smaller for higher average indium content. The distance between the quantum dot layers can therefore be used to adjust the average indium composition of the quantum dot material and avoid dislocations and defects caused by strain and lattice relaxation. As discussed above, the quantum dot layers typically have a larger lattice constant than the rest of the structure and therefore the embodiment can also incorporate thin layers of semiconductors such as GaPAs or InGaP with the opposite strain (i.e. smaller lattice constant) for the purpose of reducing the average strain in the quantum dot material if necessary. For example, as mentioned above, layers with a different lattice constant can be grown above and/or below the quantum dot layers  96 ,  100 ,  104  to modify the structural properties of the quantum dot material  94 , or similarly layers with a different lattice constant can be grown within the barriers  98 ,  102 ,  106 . 
     A second embodiment of the invention provides high efficiency monolithic four-junction photovoltaic solar cells. Self-assembled quantum dot material  94  may be adapted to absorb photons with energies greater than about 1.0 eV, indicated as material  21  in  FIG. 1 . Such a material can be used to fabricate a high-efficiency monolithic four-junction photovoltaic solar cell depicted in  FIG. 5  where the subcells will be current-matched if each of the subcells absorbs about 19% of the solar flux. 
     The four-junction solar cell comprises substrate  108  upon which first subcell  110  is fabricated. First subcell  110  preferably includes germanium with an appropriate doping profile, the germanium being grown on substrate  108  by epitaxy or by other crystal growth methods. Alternatively, first subcell  110  can be fabricated by intermixing or implanting dopants in bulk germanium material such as, for example, a Ge substrate to create the appropriate doping profile. For example, when III-V semiconductor materials are grown on p-type Ge, the intermixing of the group V within the Ge of the substrate will form an n-type Ge region and therefore a p-n junction. Similarly for an n-type Ge substrate, the intermixing of the group III within the Ge of the substrate would form a p-type region and therefore an n-p junction. Second subcell  112  is pseudomorphically grown on first subcell  110  by epitaxy and comprises a self-assembled In(Ga)As/GaAs quantum dot material adapted to obtain an effective bandgap of about 1.0 eV. Third subcell  114  is pseudomorphically grown on second subcell  112  by epitaxy and is followed by fourth subcell  116 , which is pseudomorphically grown on third subcell  114  by epitaxy. 
     In this embodiment, substrate  108  can be conductive GaAs or preferably Ge, each of them having a similar lattice constant. The doping of substrate  108  can be n-type or p-type. Whether substrate  108  is n-type with an n-p or an n-i-p junction grown on top or, p-type with a p-n or a p-i-n junction grown on top, is not fundamental to the present invention. For illustration purposes, this embodiment will use an n-type substrate with n-p or n-i-p junctions. Other possible combinations, which could include an undoped substrate and buried back contacts, are equally possible. In the final steps of the process, substrate  108  may be metallized to form ohmic contact  118 , as illustrated in  FIG. 5 . 
     A buffer and/or back field layer  120  can be fabricated on substrate  108  prior to the growth of first subcell  110  to optimize various structural, electrical, or optical properties. First subcell  110  is preferably made of Ge and includes an n-p junction to provide a depletion region. Tunnel junction  122  is used to connect first subcell  110  with second subcell  112 . As will be readily understood by a person skilled in the art, tunnel junction  122  is preferably made of a high quality material which can be epitaxially grown on first subcell  110  and is highly doped to provide good electrical conduction and to support high current densities. Tunnel junction  122 , as all tunnel junctions described herein, is preferably substantially transparent to photons traversing it. 
     For this embodiment the tunnel junction  122  can be made of a highly doped GaAs n-p junction but other types of tunnel junctions are possible such as AlGaAs, or alloys of AlGaInAsP with a lattice constant close to that of GaAs. As previously stated, second subcell  112  comprises self-assembled In(Ga)As/GaAs quantum dot material tailored to obtain an effective bandgap of about 1.0 eV. The details of second subcell  122  are similar to the ones disclosed in  FIG. 4  and its corresponding description, but with modifications as far as the growth parameters of the self-assembled quantum dot material. Further details regarding the growth of second subcell  112  appear below. For now, suffice to say that second subcell  112  comprises a self-assembled quantum dot material and an n-p or n-i-p junction. The self-assembled quantum dot material includes a plurality of layers with high quality self-assembled In(Ga)As/GaAs quantum dots of specified shape, composition, and density grown pseudomorphically by epitaxy. 
     Second subcell  112  is connected to third subcell  114  via tunnel junction  124 . Tunnel junction  124  is preferably made of a high quality material, which can be epitaxially grown on third cell  112  and is highly doped to provide good electrical conduction and to support high current densities. Tunnel junction  124  is substantially transparent to photons traversing it. In this embodiment, tunnel junction  124  can be made of a highly doped GaAs, InGaP, AlGaAs, or AlGaInAsP n-p junction, the alloy used having a lattice constant close to that of GaAs and a bandgap equal or greater than that of GaAs. 
     Third subcell  114  is essentially an n-p junction preferably made of doped GaAs or of an AlGaInAsP or GaInNAs alloy latticed-matched to GaAs and having a bandgap around 1.4 eV. For some configurations, to help balance the current of the subcells, it might be desirable to adjust the thickness and absorption characteristics of third subcell  114  such that third subcell  114  lets part of the light impinging on it reach second subcell  112 . Third subcell  114  is connected to fourth subcell  116  via tunnel junction  126  which can be made of a highly doped InGaP or AlGaAs n-p junction but other alloys supporting the requirements mentioned above are equally valid, for example AlInGaP or ZnSe alloys. 
     Fourth subcell  116  is essentially an n-p junction preferably made of doped GaInP or AlGaAs, or a similar AlGaInAsP alloy latticed-matched to GaAs, and has a bandgap around 1.8 eV. Preferably, the thickness and absorption characteristics of fourth subcell  116  are such that fourth subcell  116  lets part of the light impinging on it reach third subcell  114 . Furthermore, fourth subcell  116 , third subcell  114 , second subcell  112  and first subcell  110  are such that the respective currents generated by photons absorbed by the respective cells are balanced. Fourth subcell  116  may include window  128 , antireflection coating  130 , and electrical contact  132 , as is customary in the fabrication of multijunction solar cells. 
     The top part of  FIG. 5  shows graph  134  of the spectral intensity as a function of wavelength for solar spectrum  50 .  FIG. 5  also illustrates absorption ranges  136 ,  138 ,  140  and  142  of solar spectrum  50  for fourth subcell,  116 , third subcell  114 , second subcell  112  and first subcell  110  respectively. It will be clear for someone skilled in the art, with the help of the description of  FIG. 1 , that such a four-junction photovoltage solar cell will have good current matching between the subcells with about 19% of the solar photon flux absorbed in each subcell, and consequently, high conversion efficiencies. 
     A measured photovoltaic spectrum of a high quality self-assembled quantum dot material grown within a p-i-n junction and having an absorption band edge of about 1.0 eV is shown as plot  70  in  FIG. 3 . There, plot  70 , measured at 20° C., shows spectral features associated with the quantum dot ground states  71 , the quantum dot excited states  72  and the wetting layer states  76 . This particular sample contains one single layer of In(Ga)As quantum dots embedded in GaAs barriers. 
     The growth conditions of second subcell  112  can be adjusted so that the size and the composition of the quantum dots, together with the composition of the material adjacent to the quantum dots, yield a self-assembled quantum dot material with an absorption edge at an energy lower than 1.16 eV at about 1.0 eV. As discussed previously, there are many combinations of growth parameters that can accomplish the desired goal. However, for illustration purposes of the embodiment just described, the desired absorption characteristics of self-assembled quantum dot material  94  can be obtained by growing InAs on GaAs, the thickness of InAs being comprised preferably between 0.4 nm and 0.8 nm, and most preferably between 0.50 nm and 0.58 nm. The preferred growth rate of InAs is comprised between 0.001 and 3 nm/s and more preferably between 0.01 and 0.03 nm/s, with a growth pause following the InAs growth, the growth pause preferably ranging from 0 second to 300 seconds. The growth of the InAs quantum dot layer is followed by the over-growth of a barrier layer having a thickness ranging from 6 nm to 50 nm, the barrier layer preferably being a GaAs or an AlGaAs alloy with Al composition smaller than about 10% having a slightly higher bandgap. The growth sequence of quantum dot layer and barrier layers is repeated a number of times as previously stated. The preferred number of quantum dot layers is between 50 and 150 layers, or as required to balance the absorption in the subcells. 
     As mentioned above, a specific temperature cycling of substrate  108  may be used to adjust the shape, composition, and uniformity of the quantum dots during the overgrowth of the quantum dot layer. In this case, the temperature of substrate  108  is increased preferably above the InAs disorption temperature and decreased back to its nominal value before the growth of the subsequent quantum dot layer. In this case where the desired absorption edge is about 1.0 eV, the temperature cycling performed during the over-growth may occur when the thickness of the barrier is comprised between 1 nm and 50 nm, and preferably between 2.0 nm and 10.0 nm, and most preferably between 7.5 nm and 10.0 nm. Alternatively, alloys of slightly lower bandgap material than GaAs such InGaAs with low concentration of indium, or graded bandgap materials, can also be used adjacent to the quantum dot layers to extend absorption to longer wavelengths. 
     A method for incorporating self-assembled quantum dot material within a subcell of a multijunction monolithic photovoltaic solar cell may comprise the steps of: providing a substrate having a top surface with a lattice constant; providing a subcell, lattice-matched to said lattice constant, upon the previous layer; providing a tunnel junction, lattice-matched to said lattice constant, upon the previous subcell and repeating the last two steps until the subcell comprising self-assembled quantum dot material has to be incorporated. The last two steps are omitted if the self-assembled quantum dot material has to be incorporated in the bottom subcell. Further steps include: epitaxially depositing buffer layers, upon the previous layer, of semiconductor materials that are lattice-matched to the said lattice constant and having a buffer dopant concentration; epitaxially depositing back field layers, upon said buffer layers, of a semiconductor materials that are lattice-matched to the said lattice constant and having a back field dopant concentration; epitaxially depositing a first barrier layer, upon said back field, of a semiconductor material that is lattice-matched to the said lattice constant, and having a barrier dopant concentration and a barrier thickness grown at a barrier temperature. Additional steps are: epitaxially depositing a quantum dot layer, comprising a high density of uniform self-assembled quantum dots with a low density of defects and having a shape and a size, upon previous barrier, using a semiconductor with a nominal composition for the quantum dots, that is highly strained to the said lattice constant, and having a quantum dot dopant concentration, a quantum dot thickness, a quantum dot growth temperature, a quantum dot growth rate, a quantum dot group V overpressure or III-V ratio; epitaxially depositing a barrier layer, upon the previous quantum dot layer, after pausing the growth for a growth interruption time, of a semiconductor material that is lattice-matched to the said lattice constant, having a barrier dopant concentration, a barrier thickness, a barrier growth rate, and a barrier temperature profile for the temperature of the substrate during the overgrowth of the quantum dots; and repeating the last 2 steps for a number of periods, wherein said composition, said size, and said shape of the quantum dots are controlled and can be changed throughout the stacking profile via the growth parameters. More steps include: epitaxially depositing top field layers, upon the previous barrier layer, of semiconductor materials that are lattice-matched to the said lattice constant and having a top field dopant concentration; epitaxially depositing a tunnel junction, upon said top field layers, of highly doped semiconductor materials that are lattice-matched to the said lattice constant and having a dopant concentration that is of the same type as said top field dopant for the initial part of the tunnel junction and abruptly changed to the opposite type for the final part of the tunnel junction; providing a subcell lattice-matched to said lattice constant upon the previous tunnel junction providing a tunnel junction lattice-matched to said lattice constant upon the previous subcell and repeating the last 2 steps to complete the number of subcells comprised in the said multijunction solar cell. Finally, the method includes: providing a window upon the top subcell, providing an antireflection coating upon said window and providing a contact layer connected to said top subcell. In the method just described, the dopant concentration of the said buffer layers is between 1×10 16  cm −3  and 1×10 19  cm −3 , said dopant concentration of the said back field layers is between 1×10 16  cm −3  and 1×10 19  cm −3 , said barrier dopant concentration is between 1×10 13  cm −3  and 1×10 17  cm −3 , said quantum dot thickness is between 0.4 nm to 5.0 nm, said quantum dot growth temperature is between 450° C. and 540° C., said quantum dot growth rate is between 0.0001 nm/s and 0.2 nm/s, said growth interruption time is between 0 s and 600 sec, said barrier temperature profile is either constant or varying between 450° C. and 650° C., said barrier growth rate is between 0.01 nm/s and 1 nm/s, said barrier thickness is between 3 nm and 60 nm, said dopant concentration of the said top field layers is between 1×10 16  cm −3  and 1×10 19  cm −3 . The lattice constant may be the lattice constant of GaAs, the alloy compositions of said buffer layers, said back field layers, said barrier material, and said top field layers, are between that of Al 0.3 Ga 0.7 As and GaAs, and said nominal composition of the quantum dots is between that of In 0.3 Ga 0.7 As and InAs; the lattice constant may be the lattice constant of GaAs, the alloy compositions of said buffer layers, said back field layers, said barrier material, and said top field layers, are between that of Al 0.9 Ga 0.1  and Al 0.1 Ga 0.9 As, and said nominal composition of the quantum dots is between that of In 0.3 Al 0.7 As and InAs; or, the lattice constant may be the lattice constant of GaAs, the alloy compositions of said buffer layers, said back field layers, said barrier material, and said top field layers, are that of GaAlInP alloys latticed-matched to GaAs, and said nominal composition of the quantum dots is InP. 
     A third embodiment of the invention provides high efficiency monolithic dual-junction photovoltaic solar cells. In another embodiment, a self-assembled quantum dot material is adapted to be used in a dual-junction solar cell. In order to have a high efficiency photovoltaic dual-junction solar cell, a first subcell having a self-assembled quantum dot material similar to self-assembled quantum dot material  94 , but with an absorption edge at 0.92 eV, is required together with a second subcell having a material absorbing photons with energies greater than about 1.6 eV. The second subcell may also include a self-assembled quantum material similar to self-assembled quantum dot material  94 . Such a dual-junction solar cell would have balanced currents generated in each subcell. Furthermore, each subcell would absorb about 31% of the total solar photon flux, as shown on plot  10  in  FIG. 1  where bandgap  15  (1.6 eV) and bandgap  23  (0.92 eV) are depicted. 
     A dual-junction photovoltaic solar cell of the present invention is depicted in  FIG. 6 . The dual-junction solar cell comprises a substrate  144  upon which a first subcell  146  is pseudomorphically grown by epitaxy and comprises a first self-assembled In(Ga)As/GaAs quantum dot material, tailored to obtain an effective bandgap of about 0.92 eV. The details of first subcell  146  are similar to the ones of second subcell  30  of  FIG. 2 , disclosed in  FIG. 4  and its corresponding description. A second subcell  148  is pseudomorphically grown by epitaxy on first subcell  146  and may comprise a second self-assembled quantum dot material, preferably having AlInAs/AlGaAs quantum dots, and tailored to obtain an effective bandgap of about 1.6 eV. In this embodiment, substrate  144  is preferably a conductive GaAs substrate or a conductive Ge substrate, each of them having a similar lattice constant. 
     As discussed previously for other multijunction embodiments, the doping of substrate  144  can be n-type or p-type. Whether substrate  144  is n-type with an n-p or an n-i-p unction grown on top or, p-type with a p-n or a p-i-n junction grown on top, is not fundamental to the present invention. For illustration purposes, this embodiment will use an n-type substrate with n-p or n-i-p junctions. Other possible combinations, which could include an undoped substrate and buried back contacts, are equally possible. In the final steps of the process, substrate  144  may be metallized to form ohmic contact  150 , as illustrated in  FIG. 6 . 
     A buffer and/or back field layer  152  can be grown on substrate  144  prior to growth of first subcell  146  in order to optimize various structural, electrical, or optical properties. First subcell  146  is electrically connected to second subcell  148  via tunnel junction  154 . In this embodiment tunnel junction  154  may comprise a highly doped InGaP, or AlGaAs n-p junction having a bandgap greater than about 1.7 eV. Alternatively, other similar AlInGaAsP alloys can be used. Second subcell  148  is essentially an n-p or an n-i-p junction pseudomorphically grown by epitaxy. As stated above, second subcell  148  may comprise a self-assembled AlInAs/AlGaAs quantum dot material, tailored to obtain an effective bandgap of about 1.6 eV. Alternatively, second cell  148  may be made of doped bulk GaInP or AlGaAs or other similar InAlGaAsP alloys having an alloy composition giving a bandgap of about 1.6 eV. Second subcell  148  may have window  156 , antireflection coating  158 , and electrical contact  160 , as is customary in the fabrication of multijunction solar cells. 
     The top part of  FIG. 6  shows a graph  162  of the spectral intensity as a function of wavelength for solar spectrum  50 . Graph  162  also illustrates absorption ranges  164  and  166  of solar spectrum  50  for first cell  146  and second cell  148  respectively. It will be clear for someone skilled in the art, with the help of the description of  FIG. 1 , that such a dual-junction photovoltage solar cell will have good current matching between the subcells together with high conversion efficiencies. 
     For this embodiment, the structure of subcells having a self-assembled quantum dot material layer structure would be similar to that disclosed in  FIG. 4  and its associated description. However, the growth conditions are changed to obtain the desired optical, electrical and structural properties. In particular, the size and the composition of the quantum dot or the material adjacent to the quantum dots are modified to extend the absorption of the self-assembled quantum dot material to longer wavelengths for first subcell  146 , and to shorter wavelengths for second subcell  148 . As discussed previously, there are many combinations of parameters that can accomplish the desired goal. 
     For illustrations purposes of the present embodiment, the desired absorption characteristics of the self-assembled quantum dot material of first cell  146  can be obtained by growing InAs on GaAs, the thickness of InAs being comprised between 0.5 and 0.8 nm, preferably between 0.50 and 0.58 nm. The preferred growth rate of InAs is comprised between 0.001 and 3 nm/s, more preferably between 0.01 and 0.03 nm/s, with a growth pause following the InAs deposition (growth), the growth pause preferably ranging from 0 to 300 seconds. The growth of the InAs quantum dot layer is followed by the over-growth of a barrier layer having a thickness ranging from 6 nm to 50 nm, the barrier layer preferably being a GaAs layer or an alloy having a similar bandgap. The growth sequence of quantum dot layer and barrier layers is repeated a number of times as stated previously. 
     As mentioned above, a specific temperature cycling of substrate  144  may be used to adjust the shape, composition, and uniformity of the quantum dots during the overgrowth of the quantum dot layer. In this case, the temperature of substrate  144  is increased preferably above the InAs disorption temperature and decreased back to its nominal value before the growth of the subsequent quantum dot layer. In the case where the desired absorption edge is about 0.92 eV, the temperature cycling performed during the over-growth may occur when the thickness of the barrier is comprised between 1 and 50 nm, preferably between 2 and 11 nm, more preferably between 7.5 and 11 nm. Additionally, alloys of slightly lower bandgap material than GaAs such as InGaAs with low concentration of indium, or graded bandgap material, can also be grown adjacent to the quantum dot layer in order to extend the absorption to longer wavelengths. 
     A similar method is used to obtain the self-assembled quantum dot material having the desired properties for second subcell  148 ; however, AlInAs quantum dots are used instead of InAs quantum dots and the GaAs barrier material is replaced by an AlGaAs barrier material. The nominal percentage of Al in the barrier can be between 0% and 100%, but is preferably between 0% and 35%, in order to maintain a direct bandgap material. More preferably, the Al percentage is comprised between 25% and 35%. The nominal percentage of In in the quantum dots can be between 35% and 100%, but is preferably between 50% and 75%, and more preferably between 55% and 70%. The thickness of AlInAs used to form the self-assembled quantum dot layer is preferably comprised between 0.7 nm and 1.2 nm, and more preferably between 0.8 nm and 0.9 nm. 
     Alternatively, for second subcell  148 , an equivalent structure can be fabricated by using InP quantum dots instead of AlInAs quantum dots and GaInP barriers instead of AlGaAs barriers. Other alloys latticed-matched to GaAs, such as GaAlInP or GaInPAs, can be used in the barriers. 
     The present invention can be applied to other embodiments and materials, for example, a dual-junction monolithic solar cell grown on an InP substrate. This dual-junction cell has a first subcell, which is essentially an n-p or n-i-p junction, preferably comprising InAs quantum dot material within InGaAs barriers lattice-matched to InP. The dual-junction cell also has a second subcell, which is essentially an n-p junction preferably made of doped bulk AlInAs or of a similar alloy latticed-matched to InP such as AlInGaAs or GaInPAs. The first subcell of this embodiment has an extended absorption range compared to Ge since it absorbs photons of energies as small as about 0.65 eV. The disposition of the first and second subcells is the same as depicted in  FIG. 6 . The second subcell may be optional in some further embodiments. In an embodiment where the second subcell is optional, a lower conversion efficiency might be obtained, but to the benefit of a simpler manufacturing and lower cost and/or higher radiation or defect hardness. 
     Another embodiment of interest uses a germanium substrate to form a high efficiency dual-junction monolithic photovoltaic solar cell. Here again, the disposition of the first and second subcells is as depicted in  FIG. 6 . A first subcell, which is essentially an n-p or n-i-p junction, is made of Ge pseudomorphically grown on the Ge substrate by epitaxy or other similar deposition, implantation, or interdiffusion techniques as discussed for subcell  110  of  FIG. 5 . The second subcell, which is essentially an n-p or n-i-p junction, preferably comprises InGaAs quantum dot material within AlGaAs barriers (or similar alloys such as, for example, AlGaP alloys) pseudomorphically grown by epitaxy with a lattice constant close to that of Ge. Such a dual-junction cell using a Ge substrate might not be as efficient as other possible embodiments discussed herein but, will benefit from simpler manufacturing, higher flexibility from the wide combination of choices for the barrier layers and of the quantum dot layers, and from higher radiation and defect robustness leading to higher end-of-life efficiencies. 
     Another embodiment particularly interesting uses a silicon substrate to form a high efficiency dual-junction monolithic photovoltaic solar cell. Here again, the disposition of the first and second cells is as depicted in  FIG. 6 . A first subcell, which is essentially an n-p or n-i-p junction, preferably comprises Ge or SiGe quantum dot material and Si barriers pseudomorphically grown on the Si substrate by epitaxy or other similar deposition techniques. Thin film methods or combinations of methods for growing semiconductor crystals can also be used. The second subcell, which is essentially an n-p or n-i-p junction, preferably comprises InP quantum dot material within GaP barriers (or similar alloys such as such as AlGaP) pseudomorphically grown on Si by epitaxy. The dual-junction cell using a Si substrate can also be designed to use self-assembled quantum dot material in the second sub-cell grown on a conventional crystalline or poly-crystalline Si first cell in order to obtain improved efficiencies. The second subcell may be optional in some further embodiments. In an embodiment where the second subcell is optional, a lower conversion efficiency might be obtained, but to the benefit of a simpler manufacturing and lower cost. 
     As mentioned above, self-assembled quantum dot materials have been shown to produce devices that are orders of magnitude more radiation and defect robust. The higher radiation and defect robustness is a consequence of the combination of: (A) spatial confinement isolating regions with defects, (B) favourable diffusion length damage coefficient due to the nanostructures, (C) elimination of current limiting restrictions in the subcells most affected by radiation, and (D) solar cell design without a current-limiting cell. This makes devices having self-assembled quantum material particularly attractive for space applications. The solar cells of the embodiments describe above would benefit from radiation hardness in applications where the devices are exposed to radiations. For example, an optimized solar cell will have a beginning-of-life efficiency of about 40%. Assuming that this optimized solar cell has a radiation hardness improved by two orders of magnitude with respect to conventional solar cells, the end-of-life efficiency would be higher than 38% for an end-of-life defined as the equivalent to a total dose of 1×10 15  cm −2  of 1 MeV electron radiation. 
     It can be desirable to have embodiments of the present invention that exploit and/or to optimize the defect hardness, even while partially sacrificing some of the conversion efficiency. For example, a dual-junction solar cell may include a Si substrate upon which is metamorphically grown a plurality of GaAs/AlGaAs layers to make a transition buffer layer. On this transition buffer layer is epitaxially grown a first subcell comprising self-assembled In(Ga)As/GaAs quantum dot material, tailored to obtain an effective bandgap of about 0.92 eV. A second subcell, which could be optional in some embodiments, is grown by epitaxy on the first subcell and may comprise self-assembled quantum dot material preferably made of AlInAs/AlGaAs quantum dots, and adapted to obtain an effective bandgap of about 1.6 eV. In this embodiment, a significant concentration of defects will be presence due to the large lattice mismatch between Si and GaAs. However, the conversion efficiency could be acceptable for some applications due to the defect hardness of the self-assembled quantum dot material. 
     In addition to defect hardness, other advantages of the invention may be realized. For example, a self-assembled quantum dot material can enhance the conversion efficiency by recovering some of the photon energy that is in excess of the semiconductor bandgap, which would otherwise be lost when photons having higher energies than the effective bandgap impinge on the multi-junction solar cell. As is well known, the energy in excess of the effective bandgap can generate phonons. Some of these phonons will be re-absorbed within the quantum dots and be used in thermionic emission processes by raising photocarriers from confined quantum dot states to higher, unconfined, states, before being swept across the depletion region. Hence the higher efficiency. 
     Other, similar schemes to optimize the conversion efficiency can be incorporated in embodiments of the invention. For example, self-assembled quantum dot materials can be doped with a controlled residual doping to act as detectors in the longer infrared wavelengths. Detection processes would use intraband absorption simultaneously with the interband transitions, and therefore increase the subcell currents by using a larger fraction of the total solar photon flux. This is shown in  FIG. 7  where a biased conduction band edge  200  is shown to form a potential well  202  at a quantum dot. Electronic quantized energy levels  204 ,  206  and  208  are shown in potential well  202  together with the quasi-Fermi level  212 . Electrons  300  are shown to populate levels  204  and  206 . Long wavelength infrared intraband absorption transitions (shown as  216  and  218 ), which are not normally present in solar cells, create additional photocarriers that drift in the device as shown by arrows  310 . This type of absorption could be significant since approximately 20% of the solar photon flux is in the energy range lying below the Ge bandgap. 
     The proposed approach can also be extended to other embodiments and material systems. For example semiconductor self-assembled quantum dots containing diluted nitrides such as InGaAsN or similar semiconductor alloys containing a small fraction of nitrogen, or In(Ga)N quantum dots within Ga(Al)N barriers, or using antimony-based material system such as InSb self-assembled quantum dots in Ga(Al)Sb barriers or similar alloys. Obviously the invention could also benefit applications other than solar energy conversion, but also requiring the efficient conversion of a broadband source of photons into electrical signals. 
     A series of different embodiments of the invention have been presented. All the embodiments related to monolithic semiconductor photovoltaic solar cells comprising at least one subcell having a self-assembled quantum dot material. Details on how to tailor the effective bandgap of a subcell having a self-assembled quantum material were given. The tailoring of the bandgap allows for solar cells having higher conversion efficiencies than prior art solar cells. Embodiments including two, three and four subcells were discussed. 
     The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto.