Recent Developments of Solar Cells from Pbs Colloidal Quantum Dots

applied sciences

Review Recent Developments of Solar Cells from PbS Colloidal Quantum Dots

Tomasz Blachowicz 1 and Andrea Ehrmann 2,*

1 Institute of Physics—Center for Science and Education, Silesian University of Technology, 44-100 Gliwice, Poland; [email protected] 2 Faculty of Engineering Sciences and Mathematics, Bielefeld University of Applied Sciences, 33619 Bielefeld, Germany * Correspondence: [email protected]

Received: 15 February 2020; Accepted: 26 February 2020; Published: 3 March 2020

Abstract: PbS (lead sulfide) colloidal quantum dots consist of crystallites with diameters in the nanometer range with organic molecules on their surfaces, partly with additional metal complexes as ligands. These surface molecules are responsible for solubility and prevent aggregation, but the interface between semiconductor quantum dots and ligands also influences the electronic structure. PbS quantum dots are especially interesting for optoelectronic applications and spectroscopic techniques, including photoluminescence, photodiodes and solar cells. Here we concentrate on the latter, giving an overview of the optical properties of solar cells prepared with PbS colloidal quantum dots, produced by different methods and combined with diverse other materials, to reach high efficiencies and fill factors.

Keywords: colloidal PbS quantum dots; quantum dot solar cells; semiconductor; heterojunction; ligand; open-circuit voltage; short-circuit current; power conversion eﬃciency; ﬁll factor

1. Introduction Usually, material properties are defined by the atomic or molecular composition of a material tested at the macro scale. This rule no longer holds when structures become very small, in the range of nanometers, where scaling effects lead to significantly different optical [1–3], chemical [4–6], magnetic [7–9] or other properties of nanoparticles or quantum dots. An interesting method to combine the bottom-up assembly of devices from nanoparticles with the mechanical flexibility of the device is now accessible, with the use of relatively simple chemical procedures based on colloidal semiconductor quantum dots [10]. A general reason for semiconducting quantum dots research and development is that they are universal technologically tunable systems, with adjustable energy levels for lighting and sensing applications. The core of colloidal semiconductor quantum dots often consists of nanocrystals from some hundred to a thousand atoms of II-VI, III-V or IV-VI semiconductors [11]. On their surface, they bear organic molecules or metal complexes as ligands, as visible in Figure1[12]. These ligands build the surface layer between the semiconductor quantum dot and the outside, in this way both influencing and being influenced by an inner and an outer interface. The inorganic/organic surface between quantum dots and ligands thus impacts the electronic structure of the quantum dot [13], as well as interactions with neighboring quantum dots [14]. A highly interesting material to produce such quantum dots is PbS. This IV-VI semiconductor typically has a sodium chloride crystal structure [15] and a temperature-dependent direct band gap between approx. 0.29 eV and 0.45 eV, making it useful for detecting infrared and visible light [16]. Besides macroscopic crystals and epitaxial thin films, PbS is often prepared in the form of

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Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 16 nanocrystallites or quantum dots by diverse methods, from chemical routes [17] to electrodeposition [18] and[18] microorganism-basedand microorganism-based synthesis synthesis [19]. One[19]. ofOne the of main the areasmain ofareas research of research and development and development in which in PbSwhich nanocrystallites—typically PbS nanocrystallites—typically as colloidal as PbScolloidal quantum PbS dots—arequantum useddots—are is photovoltaic used is photovoltaic applications. Similarapplications. to PbSe, Similar PbS hasto PbSe, high photosensitivityPbS has high photosensitivity in the near-infrared in the spectrumnear-infrared [20], spectrum and allows [20], for and the tuningallows for of the the optical tuning bandof the gap optical to a broadband gap range to a of broad around range 0.7–2.1 of around eV [21, 220.7-2.1], as welleV [21,22], as the electronicas well as structurethe electronic of the structure quantum of dot the films quantum and significantly dot films and shifting significantly the band edgesshifting without the band large edges modifications without oflarge the modifications band gap [23 ,of24 ].the PbS band colloidal gap [23,24]. quantum PbS dotscolloidal also quantum show multiple dots also exciton show generation multiple exciton [25,26]. Allgeneration these properties [25,26]. All make these PbS prop quantumerties make dot solar PbS cellsquantum belong dot to solar the quantumcells belong dot to solar the cellsquantum with dot the highestsolar cells performance with the highest [27–29 ],performance and thus making [27–29], PbS an ad highly thus making interesting PbS materiala highly forinteresting solar cells. material for solarThis cells. review paper gives an overview of most recent studies on solar cells prepared with PbS colloidalThis quantumreview paper dots, gives production an overview methods, of possiblemost recent material studies combinations on solar cells and prepared the resulting with solar PbS cellcolloidal efficiencies. quantum dots, production methods, possible material combinations and the resulting solar cell efficiencies.

Figure 1.1. Pb-terminated truncated octahedral PbS quantumquantum dot, depicting Pb cations, S anions and organic ligands. From [[12],12], reprinted withwith permission.permission. Copyright © WileyWiley 2016.

2. Solar cells Cells with with PbS PbS Quantum Quantum Dots Dots—The – the Principle Principle Function Function With regardsregards to to solar solar cells, cells, the the most most commercially commercially important important technology technology is based is based on silicon. on silicon. There are,There however, are, however, other technologies, other technologies, such as such those as based those on based perovskites on perovskites [30], which [30], typically which typically use an active use layeran active from layer a perovskite from a perovskite material, i.e., material, a material i.e., witha material the crystal with structurethe crystal ABX structure3, with ABX A denoting3, with A a monovalentdenoting a monovalent cation, B a metalcation, cation B a metal and Xcation a halide and anion X a halide [31]. Thisanion active [31]. This layer active is usually layer embedded is usually betweenembedded two between electrodes, two oneelectrodes, of which one has of to which be transparent has to be transparent to allow photons to allow toreach photons the to active reach layer, the aactive hole layer, transport a hole layer transport on the layer one sideon the and one a hole-blockingside and a hole-blocking layer on the layer opposite on the side,opposite in this side, way in definingthis way thedefining orientation the orient of theation current of the flow. current flow. Organic solar cells, also named polymer solar cells, also embed the active parts between two electrodes. Usually, thethe holehole transport transport layer layer is is coated coated on on the the anode anode and and the the electron electron transport transport layer layer on theon the cathode. cathode. The The active active layer layer between between them them contains contains electron electron donors donors and and acceptors acceptors [32 ].[32]. Finally, dye-sensitizeddye-sensitized solar solar cells cells (DSSCs) (DSSCs) are ofare growing of growing interest. interest. While the While efficiencies the efficiencies reachable byreachable them are by stillthem low, are especiallystill low, especially when using when non-toxic using non-toxic and low-cost and low-cost materials materials [33–35], [33–35], they can they be expectedcan be expected to be highly to be useful highly in large-scale useful in applicationslarge-scale applications where overall where efficiency overall is less efficiency important is thanless theimportant cost-benefit than ratiothe cost-benefit [36]. As the ratio aforementioned [36]. As the aforementioned types of solar cells, types DSSCs of solar contain cells, two DSSCs conductive contain electrodestwo conductive surrounding electrodes the surrounding other layers. the Usually, other layers. a semiconductor Usually, a semiconductor like TiO2 or ZnO like is TiO applied2 or ZnO on theis applied top electrode on the top and electrode dyed with and a natural dyed with dye, a orna atural more dye, effi orcient—usually a more efficient—usually toxic—one. A toxic—one. catalyst is appliedA catalyst on is the applied counter on electrode,the counter supporting electrode, suppo electronsrting from electrons an external from an circuit external reaching circuit the reaching active layersthe active of the layers DSSC. of the Between DSSC. aBetween dyed semiconductor a dyed semico andnductor catalyst, and catalyst, an electrolyte an electrolyte fills the gap,fills the usually gap, basedusually on based iodine on/triiodide iodine/triiodide or similar or redoxsimilar couples redox [couples37]. [37]. Generally, quantum dot solar cells enable multi-exciton generation, making such solar cells in principle more efficient than common thin-film or crystal-based ones [38–40]. PbS and other quantum dots can be applied in diverse forms of solar cells, as depicted in Figure 2 [41]. In Schottky junction Appl. Sci. 2020, 10, 1743 3 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 16

Generally,solar cells, a quantum depletion dotregion solar is cellsformed enable due to multi-exciton the band bending generation, between making a metalsuch and a solar p-type cells in principlesemiconductor, more eﬃcient the latter than possibly common being thin-ﬁlm formed or by crystal-based colloidal semiconducting ones [38–40]. quantum PbS and dots. other In quantumthis dots canSchottky be applied barrier, inelectron diverse extraction forms of from solar the cells, device as depictedis favored, in while Figure holes2[ 41 are]. Inwithdrawn Schottky [42]. junction solarTypically, cells, a depletion such Schottky region solar is cells formed and other due solar to the cell band types bending harvesting between light by aPbS metal quantum and dots a p-type semiconductor,absorb especially the latter near-infrared possibly wavelengths being formed [43]. by colloidal semiconducting quantum dots. In this SchottkyLarger barrier, voltages electron than extraction with Schottky from type the solar device cells can is favored, be reached while with depleted holes are heterojunction withdrawn [42]. solar cells [41]. With PbS quantum dots, relatively large efficiencies can be reached [44] thanks to Typically, such Schottky solar cells and other solar cell types harvesting light by PbS quantum dots electrons flowing towards the TiO2 layer, while the hole transfer in the opposite direction is absorbprohibited, especially thus near-infrared obtaining a high wavelengths efficiency of [43 carrier]. separation [45].

FigureFigure 2. Di 2.ﬀ Differenterent quantum quantum dot-based dot-based photovoltaicphotovoltaic cell cells.s. From From [41], [41 reprinted], reprinted with with permission. permission. CopyrightCopyright© Elsevier © Elsevier 2011. 2011.

LargerHeterojunctions voltages than between with Schottky inorganic type and solar organic cells pa canrts can be reachedalso be built with with depleted nanocrystals heterojunction as solarsensitizers, cells [41]. in With combination PbS quantum with a dots,hole transporting relatively large polymer efficiencies [46]. The can high be expectations reached [44 due] thanks to to theoretically high efficiencies could not always be fulfilled, possibly due to reduced contact between electrons flowing towards the TiO2 layer, while the hole transfer in the opposite direction is prohibited, both materials [47,48], inefficient charge separation [49] or charge injection at the often used PbS/TiO2 thus obtaining a high efficiency of carrier separation [45]. interface [50]. HeterojunctionsAs a special between heterojunction inorganic cell, and bulk organic heterojunction parts can alsopolymer be built solar with cells nanocrystals typically combine as sensitizers, in combinationelectron-donating with a conjugated hole transporting polymers polymer with elec [46tron-accepting]. The high expectations fullerenes. Here, due toPbS theoretically belongs to high efficienciesdiverse could possible not alwayssemico benductors fulfilled, which possibly can due be toadded reduced as contactquantum between dots bothin the materials original [47 ,48], inefficientpolymer/fullerene charge separation composite [49] orfor chargelight harvesting injection at[51], the and often sometimes used PbS /alsoTiO 2forinterface electron [ 50transport]. Asinstead a special of the fu heterojunctionllerenes [52]. cell, bulk heterojunction polymer solar cells typically combine electron-donatingFinally, quantum conjugated dots polymers can be applied with as electron-accepting sensitizers in solar fullerenes. cells, similar Here, to the PbS dye belongs molecules to diversein possibleDSSCs. semiconductors Here, the aforementioned which can be added advantage as quantum is used dots so that in the the original band polymergap can /fullerenebe tailored composite by for lightgeometrical harvesting or chemical [51], and modifications sometimes also of the for ligands, electron without transport changing instead the of material the fullerenes of the quantum [52]. dot itself. Like in DSSCs, the excited electron of the quantum dot is inserted into the TiO2 layer, Finally, quantum dots can be applied as sensitizers in solar cells, similar to the dye molecules in leading to the oxidation of the quantum dot, and regeneration into the ground state is obtained by DSSCs.the Here,use of the an aforementionedelectron from the advantageredox mediator is used [53]. so Like that in the some band other gap promising can be tailored approaches, by geometrical the or chemicalreachable modifications efficiencies are of often the ligands, lower than without expected, changing e.g., due the to materialsurface states of the of quantuma different dot nature itself. or Like in DSSCs,back electron the excited transfer electron [54]. of the quantum dot is inserted into the TiO2 layer, leading to the oxidation of the quantumThis is dot,why andwe give regeneration here an overview into the on ground the most state recent is obtained results of by quantum the use dot-based of an electron solar from the redoxcells, mediatorconcentrating [53]. on Like colloidal in some PbS other quantum promising dots, which approaches, can be tailored the reachable to start elightfficiencies absorption are often lowerbetween than expected, 900 nm and e.g., 2000 due nm, to surfaceand are statesthus ideally of a di suitedfferent for nature light harvesting or back electron in the near-infrared transfer [54]. This is why we give here an overview on the most recent results of quantum dot-based solar cells, concentrating on colloidal PbS quantum dots, which can be tailored to start light absorption between 900 nm and 2000 nm, and are thus ideally suited for light harvesting in the near-infrared [55,56]. Appl. Sci. 2020, 10, 1743 4 of 16 Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 16

It[55,56]. should It be should mentioned be mentioned that sometimes that sometimes the boundaries the boundaries between di betweenﬀerent types different of solar types cell of are solar blurred, cell inare which blurred, cases in thewhich studies cases are the sorted studies into are the sorted sections into wherethe sections they ﬁt where best. they fit best.

3. PbS Quantum Dot Schottky Solar Cells One recentrecent idea idea to increaseto increase Schottky Schottky quantum quantu dotm solar dot cells’solar (QDSCs) cells’ (QDSCs) power conversion power conversion efficiency isefficiency to combine is to a pcombine-type PbS a p quantum-type PbS dot quantum layer with dot an layern-type with metal an n oxide-type layermetal for oxide the frontlayer junctionfor the front [57], orjunction to use [57], inverted or to Schottkyuse inverted QDSCs Schottky with QDSCs a low-work-function, with a low-work-function, abstaining from abstaining metal oxidesfrom metal [58]. Maioxides et [58]. al. investigatedMai et al. investigated different interfacial different interfacial electrolytes electrolytes to fit the to electrolyte fit the electrolyte chemistry chemistry to the cell to performancethe cell performance [59]. They [59]. They capped capped PbS PbS quantum quantum dots dots (QDs) (QDs) with with oleic oleic acid acid [58 [58],], and and passivatedpassivated themthem afterwardsafterwards withwith ClCl--. The transparent conductive oxide (TCO) front electrode’selectrode’s workwork functionfunction waswas lowered by spin-coating spin-coating the the TCO TCO electrode electrode with with a afew few nanometers nanometers of ofpolyethylenimine polyethylenimine (PEI) (PEI) or orpoly poly [(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7 [(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)]-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN), respectively. AfterAfter addingadding layerslayers ofof PbSPbS quantumquantum dots,dots, andand MoOMoOxx and AuAu/Ag/Ag as a back electrode, the cell waswas finished. finished. For For the the diff differenterent polymer polymer electrolytes electrolytes used toused reduce to reduce the work the function, work function, the electrical the characteristicselectrical characteristics of the inverted of the Schottky inverted QDSC Schottky changed QDSC clearly, changed as visible clearly, in as Figure visible3. Inin thisFigure way, 3. itIn was this possibleway, it was to reach possible an etoffi ciencyreach an of efficiency 4.5% in an of oxide-free 4.5% in an QDSC oxide-free [59]. QDSC [59].

Figure 3.3. CurrentCurrent density-voltage density-voltage (J-V) (J-V) curves curves of of in invertedverted Schottky QDSCs with two didifferentﬀerent electrolytes. From [[59],59], reprinted withwith permission. Copyright © ElsevierElsevier 2019.

Using ligand-exchanged PbS quantum dots, Fukuda et al.al. reached a more thanthan twice-as-hightwice-as-high photoconversionphotoconversion eefficiencyfficiency [[60].60]. The ligand exchangeexchange means that carrier mobility in a quantumquantum dot thinthin filmfilm isis increasedincreased byby aa chemicalchemical reductionreduction ofof thethe ligandligand lengthlength [[61].61]. In thisthis case,case, soakingsoaking thethe PbSPbS quantum dotdot thin filmfilm in dissolved ethanedithiol (EDT), a typical metal ion ligand, resulted in slightly increasedincreased shortshort circuitcircuit currentscurrents andand openopen circuitcircuit voltages,voltages, asas wellwell asas clearlyclearly enhancedenhanced fillfill factorsfactors [[60].60]. Another way was suggested by Li et al.al. [[62],62], whowho usedused thethe inorganic-organicinorganic-organic hybridhybrid materialmaterial CH NH PbI (MAPI) for PbS nanoparticle passivation and investigated the resulting electrical CH3NH33PbI33 (MAPI) for PbS nanoparticle passivation and investigated the resulting electrical propertiesproperties ofof Schottky Schottky type type solar solar cells cells built withbuilt thesewith core-shellthese core-shell nanoparticles. nanoparticles. It should beIt mentionedshould be thatmentioned passivation, that passivation, in general, isin a general, technology is a steptechnology which reducesstep which surface-related reduces surface-related effects and emphasizes effects and theemphasizes intrinsic bulk-electronicthe intrinsic bulk-electronic properties. Comparing properties. these Comparing core-shell these nanoparticles core-shell with nanoparticles the original with PbS nanoparticlesthe original PbS capped nanoparticles with oleic capped acid/oleylamine, with oleic as acid/oleylamine, prepared before theas prepared ligand exchange before the (Figure ligand4), showedexchange clearly (Figure improved 4), showed electric clearly properties improved of electr theic corresponding properties of the Schottky corresponding type solar Schottky cell with type an approximatelysolar cell with five-timesan approximately higher effi five-timesciency than higher without efficiency a ligand than exchange. without The a increasedligand exchange. open current The voltageincreased was open explained current by thevoltage larger was barrier explained height ofby the the core-shell larger barrier nanoparticles, height inof addition the core-shell to trap statenanoparticles, passivation in inaddition the quantum to trap dotstate surface passivation by the in MAPI the quantum shell, while dot the surface improved by the fill MAPI factor shell, and photocurrentwhile the improved were attributed fill factor to and an enhancedphotocurrent charge were transport, attributed based to an on enhanced the lower distancecharge transport, between neighboringbased on the quantumlower distance dots after between the ligand neighboring exchange quantum [62]. dots after the ligand exchange [62]. Similarly, Yang et al. suggested passivating PbS colloidal quantum dots by hybrid ligands prepared from combinations of oleic amine, octyl-phosphine acid and CdCl2, and found increased efficiencies in Schottky and p-n junction solar cells, which was attributed to oxidation prevention Appl. Sci. 2020,, 10,, xx FORFOR PEERPEER REVIEWREVIEW 5 of 16

Appl.during Sci. device2020, 10, fabrication 1743 and shorter chain lengths in the hybrid ligands corresponding to higher5 of 16 hole mobility [63], as also mentioned in [61].

Figure 4.4. FormationFormation of of the the PbS-MAPI PbS-MAPI core-shell core-shell nanoparticles nanoparticles by ligand by exchange.ligand exchange. From [62 ],From reprinted [62], withreprinted permission. with permission. Copyright Copyright© Elsevier © 2017. Elsevier 2017.

Similarly,Speirs et al. Yang concentrated et al. suggested on a chemical passivating way to PbS p-dope colloidal films of quantum thiol-capped dots byPbS hybrid quantum ligands dot, preparedresulting in from p-ncombinations junctionjunction solarsolar of cellscells oleic withwith amine, anan improvedimproved octyl-phosphine shorshortt circuitcircuit acid and currentcurrent CdCl andand2, and fillfill foundfactor,factor, increased andand thusthus eincreasedincreasedfficiencies efficiencyefficiency in Schottky [64].[64]. and WhileWhile p-n thethe junction tetrabutylammonitetrabutylammoni solar cells, whichum iodide was (TBAI) attributed treatment to oxidation of PbS preventionresulted in duringan n-type device semiconductor, fabrication and EDT-capping shorter chain PbS lengths was used in the to hybrid prepare ligands a p-type corresponding semiconductor, to higher as holealso mobilitydepicted [in63 Figure], as also 5. p mentioned-doping was in performed [61]. by post-treatment withwith aa sodiumsodium hydrosulfidehydrosulfide (NaHS)(NaHS) solution,Speirs and et al.resulted concentrated in an efficiency on a chemical increase way from to p -dope7.1% to films 7.6%. of Investigations thiol-capped PbS of these quantum systems dot, resultingwithout p-doping in p-n junction showed solar decreasing cells with electron an improved and hole short mobilities circuit currentwith a decreasing and fill factor, temperature, and thus increasedcombined e ffiwithciency temperature-independent [64]. While the tetrabutylammonium diffusion lengths iodide in (TBAI) EDT-capped treatment PbS of PbS and resulted decreasing in an ndiffusion-type semiconductor, length with decreasing EDT-capping temperature PbS was used in TBAI-capped to prepare a pPbS-type [65]. semiconductor, as also depicted in FigureThe 5influence. p-doping of the was hole performed mobility by in post-treatment such Schottkyttky with solarsolar a cellscells sodium withwith hydrosulfide PbS-TBAIPbS-TBAI andand (NaHS) PbS-EDTPbS-EDT solution, waswas andsimulated resulted by inHu an et eal.,fficiency with the increase corresponding from 7.1% band to 7.6%. diagramdiagram Investigations beingbeing depicteddepicted of these inin FigureFigure systems 55 [66].[66]. without TheyThey p-dopingfound a strongly showed increasing decreasing fill electron factor and and hole short mobilities circuit current with a decreasingfor increasing temperature, hole mobility combined until withsaturation temperature-independent was reached, which was diffusion explained lengths by a in supp EDT-cappedort of charge PbS carrier and decreasing extraction di toffusion the external length withanode decreasing due to a higher temperature mobility. in TBAI-capped PbS [65].

Figure 5.5. Energy band diagram of a ZnOZnO/PbS-TBAI/PbS-EDT/Au/PbS-TBAI/PbS-EDT/Au solar cellcell underunder illumination,illumination, with PbS-EDT working as an electron blocking layer forming a SchottkySchottky barrier for holes,holes, thusthus avoidingavoiding theirtheir movementmovement toto thethe AuAu anode.anode. From From [[66],[66],66], reprintedreprreprintedinted withwith permission.permission. Copyright Copyright ©© ElsevierElsevier 2016.2016. Appl. Sci. 2020, 10, 1743 6 of 16

The influence of the hole mobility in such Schottky solar cells with PbS-TBAI and PbS-EDT was simulated by Hu et al., with the corresponding band diagram being depicted in Figure5[ 66]. They found a strongly increasing fill factor and short circuit current for increasing hole mobility until saturation was reached, which was explained by a support of charge carrier extraction to the external anode due to a higher mobility. Jang et al. concentrated instead on varying the band gap by varying the quantum dot size, Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 16 in this way enabling building Schottky type solar cells with graded bandgaps. They found higher short circuitJang current et al. concentrated densities combined instead on withvarying lower the band open gap circuit by varying voltages the in quantum comparison dot size, with in cells withthis PbS way quantum enabling dots building with Schottky a uniform type bandgap. solar cells Underwith graded examination, bandgaps. anThey additional found higher thin short electron energy-boostingcircuit current layer densities with combined quantum with dots lower of the open highest circuit bandgap voltages could in comparison be used towith increase cells with both PbS values, in thisquantum way improving dots with the a uniform overall ebandgap.fficiency Under [67]. examination, an additional thin electron energy- boosting layer with quantum dots of the highest bandgap could be used to increase both values, in In summary, PbS quantum dots show a high potential for increasing the efficiency of common or this way improving the overall efficiency [67]. inverse Schottky solar cells. The main research focus is placed on the ligands, since ligand exchange In summary, PbS quantum dots show a high potential for increasing the efficiency of common or otheror inverse forms Schottky of ligand solar modification, cells. The main as research well as focu combinationss is placed on of the PbS ligands, quantum since ligand dotswith exchange different ligands,or other are promising forms of ligand approaches modification, for further as well optimization as combinations of the of PbS Schottky quantum solar dots cell with effi ciency.different ligands, are promising approaches for further optimization of the Schottky solar cell efficiency. 4. PbS Quantum Dot Sensitized Solar Cells 4. PbS Quantum Dot Sensitized Solar Cells Light harvesting properties of metal oxides like TiO2 or ZnO, which are typically applied as semiconductingLight harvesting layers in DSSCs,properties can of be metal enhanced oxides by like decorating TiO2 or ZnO, them which with loware typically bandgap applied metal sulfides. as One ofsemiconducting the techniques layers used in for DSSCs, this purpose can be enhanc is the so-calleded by decorating pseudo-successive them with low ionic bandgap layer absorptionmetal and reactionsulfides. (p-SILAR).One of the techniques Ali et al. us describeded for this in purpose their most is the recent so-called study pseudo-successive how PbS and ionic CdS layer quantum absorption and reaction (p-SILAR). Ali et al. described in their most recent study how PbS and CdS dots were deposited on TiO2 nanoparticles, performing a H2S treatment of the TiO2 nanoparticles in a quantum dots were deposited on TiO2 nanoparticles, performing a H2S treatment of the TiO2 rotary reactor. Using rhodamine B degradation as a measure, they showed the positive influence of the nanoparticles in a rotary reactor. Using rhodamine B degradation as a measure, they showed the H S treatment for the p-SILAR process [68]. 2 positive influence of the H2S treatment for the p-SILAR process [68]. Ul HassanUl Hassan et al. et improvedal. improved the the p-SILAR p-SILAR techniquetechnique by by a a wet wet chemical chemical modification, modification, as depicted as depicted in Figurein Figure6, applying 6, applying a fluorination a fluorination treatment treatment of of PbS PbS andand CdS nanoparticles nanoparticles during during centrifugation, centrifugation, resultingresulting in an in increasedan increased deposition deposition on on ZnOZnO nanoparticlesnanoparticles and and thus thus a significant a significant increase increase of the of the photocurrentphotocurrent density density of correspondingof corresponding solar solar cells cells [[69].69].

FigureFigure 6. Comparison 6. Comparison of of the the normal normal (upper(upper row) and and the the modified modiﬁed p-SILAR p-SILAR process process (lower (lower row). row). FromFrom [69], [69], reprinted reprinted with with permission. permission. Copyright Copyright ©© ElsevierElsevier 2016. 2016.

WithWith common common SILAR, SILAR, Mao Mao et et al. al. realized realized aa synchronoussynchronous PbS PbS and and CdS CdS quantum quantum dot dotdeposition deposition on TiO2, in this way avoiding the excessive growth of PbS quantum dots in TiO2 mesopores and on TiO2, in this way avoiding the excessive growth of PbS quantum dots in TiO2 mesopores and increasingincreasing high-density high-density defect defect passivation passivation on on the the PbS PbS surface, surface, both both leading leading to to a a strong strong increase increase ofof solar cell eﬃsolarciency cell [efficiency70]. Similarly, [70]. Similarly, Kraus et Kraus al. reached et al. reached epitaxial epitaxial nucleation nucleation of PbS of quantum PbS quantum dots dots on a on planar a planar TiO2(100) surface in rutile modification by SILAR [71]. TiO2(100) surface in rutile modiﬁcation by SILAR [71]. Li et al. applied SnO2 instead of TiO2 for quantum dot-sensitized solar cells. They used the aforementioned ligand exchange to reach an improved charge transfer to the sensitized substrate by using short-chain multidentate hydrophilic ligands instead of the common hydrophobic oleic acid ligands, in order to increase electronic coupling to the substrate and at the same time avoid degradation of the PbS-sensitized SnO2 single crystals in the electrolyte [72]. Appl. Sci. 2020, 10, 1743 7 of 16

Li et al. applied SnO2 instead of TiO2 for quantum dot-sensitized solar cells. They used the aforementioned ligand exchange to reach an improved charge transfer to the sensitized substrate by using short-chain multidentate hydrophilic ligands instead of the common hydrophobic oleic acid ligands, in order to increase electronic coupling to the substrate and at the same time avoid degradation of the PbS-sensitized SnO single crystals in the electrolyte [72]. Appl. Sci. 2020, 10, x FOR PEER2 REVIEW 7 of 16 On the base of ZnO/ZnSe core/shell nanorod arrays decorated with PbS quantum dots by SILAR, KamruzzamanOn the prepared base of ZnO/ZnSe solar cells core/shell with a maximumnanorod arra effiysciency decorated of 0.88%, with PbS concentrating quantum dots on by the SILAR, core/ shell structureKamruzzaman and the nanorod prepared array solar density cells with rather a maximu than optimizingm efficiency the of PbS 0.88%, decoration concentrating [73]. on the Anothercore/shell techniquestructure and was the used nanorod by Purcell-Milton array density rather et al., than who optimizing deposited the colloidal PbS decoration quantum [73].dots by Another technique was used by Purcell-Milton et al., who deposited colloidal quantum dots by electrophoretic deposition on TiO2, increasing the quantum dot loading by optimizing the solvent, as visibleelectrophoretic in Figure 7deposition. The clear on di TiOfference2, increasing in absorption the quantum resulted dot loadin in alsog by suggestingoptimizing the dichloromethane solvent, as visible in Figure 7. The clear difference in absorption resulted in also suggesting dichloromethane (DCM) as a possible solvent for PbS and other quantum dots [74]. Electrophoretic deposition was (DCM) as a possible solvent for PbS and other quantum dots [74]. Electrophoretic deposition was also also usedused by Bhardwaj Bhardwaj et et al. al. to todevelop develop nanocomposit nanocomposite-dopede-doped photo-anodes photo-anodes with PbS with quantum PbS quantum dots as a dots as a sensitizer,sensitizer, showing that that the the incorporation incorporation of ofTIO TIO2/Au2/Au nanocomposites, nanocomposites, the optimization the optimization of the of the quantumquantum dot depositiondot deposition temperature temperature and and the the postdepositionpostdeposition annealing annealing of ofthe the sensitized sensitized photo-anode photo-anode improvedimproved the powerthe power conversion conversion effi efficiencyciency [ 75[75].].

FigureFigure 7. UV 7./ VisUV/Vis absorption absorption spectra spectra after after soaking soaking identical identical TiO2 TiO layers2 layers in solutionsin solutions of oleicof oleic acid acid capped CdSecapped colloidal CdSe quantum colloidal dots. quantum From dots. [74 ],From originally [74], originally published published under under a CC-BY a CC-BY license. license.

HuangHuang et al. et appliedal. applied PbS PbS colloidal colloidal quantum quantum dots as as sensitizers sensitizers on on mesoporous mesoporous TiO TiO2/CsPbBr2/CsPbBr3 3 perovskiteperovskite interfaces interfaces in solarin solar cells cells by by spin-coating, spin-coating, in in this this way way approximately approximately doubling doubling the power the power conversionconversion efficiency efficiency in comparisonin comparison with with the thepristine pristine device [76]. [76]. PbS quantumPbS quantum dots dots were were synthesized synthesized inside inside ferritin ferrit proteinin protein shells shells by Hansen by Hansen et al., et which al., which protected protected them from photocorrosion, resulting in quantum dot sensitized solar cells with them from photocorrosion, resulting in quantum dot sensitized solar cells with mesoporous TiO2, mesoporous TiO2, giving efficiencies of around 0.29% when a drop casting method was used [77]. giving effiZhangciencies et al. of concentrated around 0.29% on the when colloidal a drop PbS casting quantum method dots themselves. was used [They77]. synthesized these Zhangquantum et dots al. concentrated by air-stable thiourea, on the colloidal stabilized PbS by Cd quantum post-treatment dots themselves. via cation exchange, They synthesized and found these quantuman increased dots by performance air-stable thiourea, of solar cells stabilized sensitized by Cdwith post-treatment these quantum dots, via cation as compared exchange, to devices and found an increasedproduced performance with quantum of dots solar synthesized cells sensitized from common with these bis(trimethylsilyl) quantum dots, sulfide as compared[78]. to devices producedCore/shell with quantum PbS/CdS dots quantum synthesized dots were from applied common as bis(trimethylsilyl)photosensitizers on sulfideTiO2 to [avoid78]. charge Corerecombination/shell PbS inside/CdS the quantum quantum dots dots wereand at appliedthe quantum as photosensitizers dot/TiO2/electrolyte on interfaces, TiO2 to avoidresulting charge in a significant increase of the power conversion efficiency in comparison with pristine PbS-based recombination inside the quantum dots and at the quantum dot/TiO2/electrolyte interfaces, resulting in a significantquantum dot increase sensitized of thesolar power cells conversion[79]. Park et e ffial.,ciency who synthesized in comparison PbS withquantum pristine dots PbS-based with embedded CuS, reported even higher efficiencies, which were explained by reduced charge carrier quantum dot sensitized solar cells [79]. Park et al., who synthesized PbS quantum dots with embedded recombination and improved absorptivity [80]. CuS, reportedGoing even beyond higher the pure efficiencies, function whichof PbS werequantum explained dots as bysensitizers, reduced Wang charge et al. carrier also used recombination them and improvedas carrier transporters. absorptivity They [80]. prepared a heterostructure from PbS quantum dots as absorbers and hole carriers with ZnO nanowires as electron carriers, and showed efficient charge collection along this hybrid structure, which was attributed to band bending in the ZnO dealing as collector, resulting in efficient charge separation and thus large hole transportation distances [81]. Appl. Sci. 2020, 10, 1743 8 of 16

Going beyond the pure function of PbS quantum dots as sensitizers, Wang et al. also used them as carrier transporters. They prepared a heterostructure from PbS quantum dots as absorbers and hole carriers with ZnO nanowires as electron carriers, and showed efficient charge collection along this hybridAppl. Sci. structure,2020, 10, x FOR which PEER was REVIEW attributed to band bending in the ZnO dealing as collector, resulting8 of in16 efficient charge separation and thus large hole transportation distances [81]. Generally, PbSPbS quantumquantum dotsdots showshow highly interesting sensitizing properties due to the didifferentfferent possibilitiespossibilities toto modify their absorption spectra. In ad addition,dition, they can even be used as charge carriers, allowing for combining twotwo purposespurposes inin oneone materialmaterial layer.layer. After this overview on PbS quantum dots as sensitizerssensitizers in quantum dot sensitized solar cells, thethe nextnext sectionsection willwill reportreport onon furtherfurther ideasideas beyondbeyond SchottkySchottky typetype andand DSSCDSSC typetype solarsolar cells,cells, oftenoften combining didifferentfferent physicalphysical andand chemicalchemical propertiesproperties ofof PbSPbS quantumquantum dots.dots.

5. PbS Quantum Dot Heterojunction Solar Cells Plasmonic metalmetal nanoparticles nanoparticles can can be usedbe used to concentrate to concentrate incident incident sunlight bysunlight plasmon by resonance, plasmon inresonance, this way in increasing this way theincreasing overall the absorption overall absorption of the incident of the light incident in a solar light cell in a without solar cell increasing without theincreasing quantum the dotquantum film thickness dot film thickness over the over carrier the di carrierffusion diffusion length. length. Hong etHong al. et used al. used Au andAu and Ag nanoparticlesAg nanoparticles to introduce to introduce localized localized surface surface plasmon plasmon resonance, resonance, as well as as well the scattering as the scattering of the incident of the light.incident By light. placing By the placing small the Au small nanoparticles Au nanoparticles (diameter (diameter ~ 10 nm) at~ 10 the nm) ZnO at coatedthe ZnO front coated layer, front followed layer, byfollowed the PbS by active the PbS layer active and thelayer larger andAg the nanoparticles larger Ag nanoparticles (diameter ~ (diameter 50 nm), and ~ 50 then nm), the and gold then contact the backgold contact electrode, back the electrode, latter serves the latter mostly serves as amostly far-field as a scatterer, far-field scatterer, while the while Au nanoparticlesthe Au nanoparticles enable near-fieldenable near-field coupling, coupling, in this wayin this increasing way increasing the efficiency the efficiency by approx. by approx. 25% [82 25%]. [82]. Dastjerdi et al. concentrates concentrates on on the the problem problem of of UV UV stability stability of of the the usual usual short short organic organic ligands ligands of ofPbS PbS quantum quantum dots dots used used for for solar solar cells. cells. They They us useded a aCdSe/ZnS CdSe/ZnS quantum quantum dot dot layer to down-shiftdown-shift luminescenceluminescence onon thethe back back of of the the bulk bulk heterojunction heterojunction with with ZnO ZnO nanowires, nanowires, thus th notus not only only increasing increasing the time-stabilitythe time-stability of the of cells,the cells, but alsobut theiralso their power power conversion conversion efficiency, efficiency, since thesince UV the photons—which UV photons— werewhich usually were usually thermalized thermalized or absorbed or absorbed in other in ot layers—wereher layers—were now downshiftednow downshifted to visible to visible light, light, and thusand thus also contributedalso contributed to power to power conversion conversion [83]. [83]. Xie etet al.al. optimized optimized planar planar hybrid hybrid heterojunction heterojuncti PbS-polymeron PbS-polymer solar solar cells cells by tuning by tuning the band the gap band of thegap PbS of the quantum PbS quantum dots between dots between 0.86 eV and0.86 2.1 eV eV and by 2.1 varying eV by the varying synthetization the synthetization temperature temperature (Figure8), ligand(Figure exchange 8), ligand andexchange an optimized and an optimized production production process with process a thinner with a depositedthinner deposited PbS layer. PbS In layer. this way,In this the way, performance the performance could be doubledcould be as doubled compared as tocompared similar hybrid to similar heterojunction hybrid heterojunction PbS/organic solarPbS/organic cells [84 solar]. Using cells a [84]. variable Using band-gap a variable structure band-gap in quantum structure dot in quantum heterojunction dot heterojunction solar cells was solar also suggestedcells was also by Ding suggested et al., whoby Ding used et di ffal.,erent who quantum used different dot sizes quantum to align thedot energysizes to levels align of the the energy active layer,levels resultingof the active in a layer, power resulting conversion in a e powerfficiency conversion increase of efficiency more than increase 20% [85 of]. more than 20% [85].

Figure 8.8. Absorption spectra of PbS quantum dots synthesizedsynthesized at temperaturestemperatures between 45 ◦°CC and 185185 ◦°C.C. From [[84],84], reprintedreprinted withwith permission.permission. Copyright © Elsevier 2019.

The problem of interfacial deficiency between semiconductor and colloidal PbS quantum dots in ordered bulk heterojunction solar cells was investigated by Shi et al. They produced exactly interpenetrating structures of a ZnO nanowire array and PbS colloidal quantum dots by optimizing nanowire growth orientation and using convective assembly to deposit the quantum dots. The resulting dense packing led to a high power conversion efficiency of nearly 10% [86]. Nano-heterojunctions between CdS nanowires surrounded by PbS nanoparticles were developed by Majumder et al. When combining the chemical bath deposition of CdS nanowires with Appl. Sci. 2020, 10, 1743 9 of 16

The problem of interfacial deficiency between semiconductor and colloidal PbS quantum dots in ordered bulk heterojunction solar cells was investigated by Shi et al. They produced exactly interpenetrating structures of a ZnO nanowire array and PbS colloidal quantum dots by optimizing nanowire growth orientation and using convective assembly to deposit the quantum dots. The resulting dense packing led to a high power conversion efficiency of nearly 10% [86]. Appl. Sci.Nano-heterojunctions 2020, 10, x FOR PEER REVIEW between CdS nanowires surrounded by PbS nanoparticles were developed9 of 16 by Majumder et al. When combining the chemical bath deposition of CdS nanowires with the theaforementioned aforementioned SILAR SILAR process process for PbS for nanoparticle PbS nanopa deposition,rticle deposition, an increase an ofincrease the power of conversionthe power conversionefficiency by efficiency more than by one more order than of magnitudeone order of could magnitude be reached could in correspondingbe reached in corresponding solar cells (Figure solar9), whichcells (Figure was attributed 9), which to thewas flat attributed band potential, to the optimizedflat band electronpotential, lifetime optimized and recombination electron lifetime rate [and87]. recombination rate [87].

Figure 9. ProductionProduction process process of of CdS CdS nanowires on FT FTOO (ﬂuorine-doped(fluorine-doped tin oxide) glass, SILAR deposition of PbS nanoparticles and final ﬁnal solar cell assembly. From From [87], [87], reprinted with permission. Copyright © ElsevierElsevier 2019. 2019.

Wang et al.al. optimized optimized the the ZnO/PbS ZnO/PbS heterojuncti heterojunctionon for for a a colloidal quantum dot dot solar cell by passivating defects in the sol-gel ZnO at low temperatures by adding polyethylenimine (PEI) to the precursor solution.solution. InIn thisthis way, way, a a higher higher crystallization crystallization rate rate was was achieved, achieved, the the ZnO ZnO work work function function was wasreduced reduced and theand heterojunction the heterojunction interface interface voltage voltage was increased, was increased, which was which attributed was attributed to improved to improvedcharge carrier charge separation carrier separation in the depletion in the depletio region.n In region. addition, In addition, higher electronhigher electron mobility mobility was found, was whilefound, the while undesired the undesired carrier recombination carrier recombination was suppressed. was suppressed. In this way, In this the powerway, the conversion power conversion efficiency efficiencycould be increased could be byincreased 25%, as by compared 25%, as compared to pure ZnO to/ purePbS heterojunction ZnO/PbS heterojunction solar cells [solar88]. cells [88]. Zhao etet al.al. usedused perovskiteperovskite/PbS/PbS quantum quantum dot dot heterojunction heterojunction films film tos promoteto promote hole hole extraction extraction and anddecrease decrease carrier carrier recombination recombination in a perovskitein a perovskite solar solar cell. cell. In comparison In comparison with with cells cells without without PbS asPbS a ashole a hole transporting transporting material, material, the e ffitheciency efficiency could could nearly nearly be doubled, be doubled, and the and solar the cellssolar showed cells showed much muchhigher higher stability stability [89]. [89]. The interface between TiO2 and PbS in planar heterojunction solar cells was the focus of a study by Zeng et al. [90]. [90]. To To suppress surface defects in the electron transport layer between the active PbS layer andand the the FTO FTO substrate substrate through through a TiO a2 TiOlayer,2 layer, thermo-stable thermo-stable hydroxyl hydroxyl radicals wereradicals pre-implanted were pre- implantedon the TiO2 onnanoparticle the TiO2 surface,nanoparticle in this waysurface, creating in this an electronway creating transport an layer, electron eliminating transport interband layer, eliminatingtraps by passivating interband the traps intrinsic by passivating oxygen-deficient the intrinsic defects andoxygen-deficient matching electron defects affi andnity andmatching work electronfunction. affinity Thus, undesiredand work interfacefunction. recombination Thus, undesired was interface suppressed, recombination and power was conversion suppressed, efficiency and waspower strongly conversion increased efficiency [90]. was strongly increased [90]. Xu et al. improved improved the the interface interface between between the the ac activetive layer layer and the anode of a heterojunction quantum dot solar cell by introducing a graphene oxide layer between the PbS active layer and Au anode. By By post-annealing, post-annealing, the the graphene graphene oxide oxide laye layerr was was partly partly reduced to graphene, worked as a hole-transport layer and significantly reduced the defects at the PbS/Au interface, finally resulting in an increase in efficiency of 12.9% [91]. Imran et al. prepared bulk heterojunction organic solar cells based on the electron-donating organic semiconductor P3HT-poly(3-hexylthiophene) and the electron accepting fullerene PCBM [6,6]-phenyl-C61-butyric acid methyl ester with various ratios of the inorganic semiconductor PbS. Here, the PbS nanoparticles decreased the series resistance, and thus improved the power conversion efficiency of these solar cells [92]. Since traditional PbS quantum dot solar cell structures require high-temperature fabrication of an inorganic buffer layer, an inverted structure was proposed by some groups [93,94]. Wang et al. Appl. Sci. 2020, 10, 1743 10 of 16 hole-transport layer and significantly reduced the defects at the PbS/Au interface, finally resulting in an increase in efficiency of 12.9% [91]. Imran et al. prepared bulk heterojunction organic solar cells based on the electron-donating organic semiconductor P3HT-poly(3-hexylthiophene) and the electron accepting fullerene PCBM [6,6]-phenyl-C61-butyric acid methyl ester with various ratios of the inorganic semiconductor PbS. Here, the PbS nanoparticles decreased the series resistance, and thus improved the power conversionAppl. Sci. 2020, e 10ffi, ciencyx FOR PEER of these REVIEW solar cells [92]. 10 of 16 Since traditional PbS quantum dot solar cell structures require high-temperature fabrication of an inorganicbuilt an effective buffer layer,p-n heterojunction an inverted structure in an inverted was proposed colloidal by quantum some groups dot solar [93, 94cell]. by Wang using et al.NiO built for anthe epff-typeective layer p-n heterojunctionand PbS colloidal in anquantum inverted dots colloidal with iodide quantum ligands dot for solar the cell n-type by using layer. NiO Due for to the plarge-type depletion layer and PbSregion, colloidal a large quantum photocurrent dots with was iodide achieved. ligands At for the the samen-type time, layer. interface Due to the carrier large depletionrecombination region, was a large prohibited photocurrent by an additional was achieved. slightly At the doped same p time,-type interfacequantum carrier dot layer recombination with EDT wasas ligands, prohibited which by animproved additional the slightly device doped voltage,p-type leading quantum to strongly dot layer enhanced with EDT power as ligands, conversion which improvedefficiency [95]. the device voltage, leading to strongly enhanced power conversion efficiency [95]. Spin-coated colloidalcolloidal PbS PbS quantum quantum dot dot films films were were ligand-exchanged ligand-exchanged to methylammonium to methylammonium iodide (MAI)iodide and(MAI) used and in used depleted in depleted heterojunction heterojunction solar cells solar with cells TiO with2 as a TiO semiconductor,2 as a semiconductor, resulting inresulting a more thanin a more doubled than power doubled conversion power conversion efficiency, as efficien comparedcy, as to compared common oleic-acid to common passivated oleic-acid PbS passivated quantum dotPbS filmsquantum [96]. Changdot films et al. [96]. used Chang ligand et exchange al. used to ligand 3-mercaptopropionic exchange to 3-mercaptopropionic acid, thioglycolic acid acid, and 4-mercaptobenozoicthioglycolic acid and acid4-mercaptobenozoic to examine the significant acid to examine impact the of significant ligand structures impact onof ligand the photovoltaic structures performanceon the photovoltaic of quantum performance dot heterojunction of quantum solar dot cells he [97terojunction]. Wang et al.solar and cells Zhang [97]. et Wang al. used et di al.fferent and ligandsZhang et and al. quantumused different dot dimensions ligands and to varyquantum the quantum dot dimensions dot energy to vary levels the [98 quantum,99]. dot energy levelsTo [98,99]. avoid undesired charge recombination between ZnO nanowires and PbS quantum dots in a depletedTo avoid bulk undesired heterojunction charge recombination solar cell, Zang between et al. insertedZnO nanowires an ultrathin and PbS Al2 quantumO3 layer dots between in a thesedepleted materials, bulk heterojunction which clearly solar increased cell, Zang open-circuit et al. inserted voltage, an ultrathin as well as Al the2O3 short-circuitlayer between current these andmaterials, thus thewhich power clearly conversion increased effi open-circuitciency [100 ].volt Pradhanage, as well et al. as reduced the short-circuit interfacial current recombination and thus betweenthe power these conversion materials efficiency by a nanocrystal [100]. Pradhan buffer et layer,al. reduced mixing interfacial ZnO nanocrystals recombination with between PbS quantum these dotsmaterials (Figure by a10 nanocrystal), in this way buffer improving layer, mixing the charge ZnO nanocrystals collection effi wiciencyth PbS and quantum thus the dots overall (Figure power 10), conversionin this way e ffiimprovingciency [101 the]. Zhaocharge et collection al. introduced efficiency a CdIr and treated thus CdSethe overall quantum power dot buconversionffer layer betweenefficiency ZnO [101]. nanoparticles Zhao et al. in andtroduced PbS quantum a CdIr treated dots in CdSe colloidal quantum PbS heterojunction dot buffer layer solar between cells [ZnO102], whilenanoparticles Zang et and al. used PbS anquantum ultrathin dots Mg(OH) in colloidal2 layer PbS for thisheterojunction purpose [103 solar]. cells [102], while Zang et al. used an ultrathin Mg(OH)2 layer for this purpose [103].

Figure 10. SchemeScheme of energy energy band alignment of the solar cell cell with with inserted inserted mixed mixed nanocrystal nanocrystal (MNC) (MNC) layer between ZnO, PbS treated withwith 1-ethyl-3-methylimidazolium1-ethyl-3-methylimidazolium iodide (EMII) as an active layer and PbS-EDTPbS-EDT electronelectron blocking blocking layer, layer, in short-circuitin short-circuit condition. condition. Reprinted Reprinted with with permission permission from [from101]. [101].Copyright Copyright (2017) (2017) American Amer Chemicalican Chemical Society. Society.

Hu et al. examined the influence of CdS thin films used for electron transport in TiO2/PbS Hu et al. examined the influence of CdS thin films used for electron transport in TiO2/PbS heterojunction colloidal quantum dot solar cells. They used a single-source precursor to prepare heterojunction colloidal quantum dot solar cells. They used a single-source precursor to prepare solution-processed CdS thin films by spin-coating and annealing, and found increased power conversion solution-processed CdS thin films by spin-coating and annealing, and found increased power conversion efficiencies compared to cells with CdS layers prepared with chemical bath deposition or RF magnetron sputtering [104]. Another semiconductor, SnO2, was suggested by Khan et al. to produce colloidal quantum dot heterojunction solar cells. Based on its larger band-gap and higher electron mobility, as compared to the often applied ZnO, as well as adequate band alignment with the PbS quantum dots, light absorption and photocarrier extraction were supported, resulting in high power conversion efficiency [105]. Similarly to [74], Wu et al. found a significant influence of the solvent used to prepare PbS/TiO2 heterojunction solar cells from different PbS solutions. They reached the highest power conversion efficiency of 7.64% in an n-octane/isooctane hybrid solvent [106]. Appl. Sci. 2020, 10, 1743 11 of 16 efficiencies compared to cells with CdS layers prepared with chemical bath deposition or RF magnetron sputtering [104]. Another semiconductor, SnO2, was suggested by Khan et al. to produce colloidal quantum dot heterojunction solar cells. Based on its larger band-gap and higher electron mobility, as compared to the often applied ZnO, as well as adequate band alignment with the PbS quantum dots, light absorption and photocarrier extraction were supported, resulting in high power conversion efficiency [105]. Similarly to [74], Wu et al. found a significant influence of the solvent used to prepare PbS/TiO2 heterojunction solar cells from different PbS solutions. They reached the highest power conversion efficiency of 7.64% in an n-octane/isooctane hybrid solvent [106]. To summarize, for heterojunction solar cells similar points as forthe aforementioned types of solar cells are important, tuning the quantum dot band gap to desired values and increasing the time-stability of the common short ligands on PbS quantum dots. Special difficulties also occur according to the band alignment of semiconductor, the PbS quantum dot layer and possible additional charge transport layers. Nevertheless, PbS quantum dots offer a wide range of possible utilizations in heterojunction solar cells, underlined by the large number of studies published in this research area.

6. Conclusions As this excerpt of some of the most recent, highly interesting and promising research on PbS quantum dots for solar cells shows, this material offers a broad bandwidth of applications, from light harvesting to electron transport, and correspondingly a large amount of possible solar cell types in which it can be utilized—in particular, the possibility to tailor the semiconductor band gap by changing quantum dot dimensions or ligands makes PbS an interesting material for solar cells with diverse material combinations. On the other hand, controlling quantum dot and ligand properties is necessary to gain sufficient power conversion efficiencies if this material is used in the solar cells of different types. We hope that our brief review stimulates more research groups working in similar areas to expand their studies towards investigations of this relatively uncomplicated and inexpensive material class, to further increase the possible efficiencies and transfer recent research results into developing industrially suitable processes for the mass market.

Author Contributions: Conceptualization, T.B. and A.E.; visualization, T.B. and A.E.; writing—original draft preparation, A.E. and T.B. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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