Donor/Acceptor Heterojunction Organic Solar Cells

electronics

Article Donor/Acceptor Heterojunction Organic Solar Cells

Pasquale Cusumano 1,* , Claudio Arnone 1, Marco Angelo Giambra 2 and Antonino Parisi 1

1 Dipartimento di Ingegneria (DING), University of Palermo, Viale delle Scienze Ediﬁcio 9, I-90127 Palermo, Italy; [email protected] (C.A.); [email protected] (A.P.) 2 CNIT-Consorzio Nazionale Interuniversitario per le Telecomunicazioni, via Moruzzi 1, 56124 Pisa, Italy; [email protected] * Correspondence: [email protected]

Received: 8 December 2019; Accepted: 20 December 2019; Published: 1 January 2020

Abstract: The operation and the design of organic solar cells with donor/acceptor heterojunction structure and exciton blocking layer is outlined and results of their initial development and assessment are reported. Under halogen lamp illumination with 100 mW/cm2 incident optical power density, the devices exhibits an open circuit voltage VOC = 0.45 V, a short circuit current density JSC between 2 and 2.5 mA/cm2 with a fill factor FF 50%, an external quantum efficiency (electrons/s over ≈ incident photons/s) EQE 5% and a power conversion efficiency of about 0.5%. Measurements of the ≈ photoelectrical characteristics with time are also reported, confirming that non encapsulated organic solar cells have limited stability in ambient atmosphere.

Keywords: organic solar cells; organic photovoltaics; donor/acceptor heterojuntion; exciton blocking layer; lifetime and degradation

1. Introduction Organic amorphous semiconductors in the form of polymers or oligomers (also called small molecules) have by now made their way in Electronics and Optoelectronics [1]. This is due to a simple and cheap chemical synthesis, not critical film deposition techniques even on flexible plastic substrates and, last but not the least, their unique and useful optoelectronic properties [2]. Organic thin films can be produced by several methods, such as high vacuum thermal evaporation for oligomers and spin-coating, drop-casting, screen printing, spray-coating and inkjet-printing usually for polymers. Applications include color displays based on organic light emitting diodes (OLEDs), paper (flexible) Electronics and, more recently, organic solar cells (SCs). Organic photovoltaics is indeed a very active research field, promising high efficiency, large area and low cost SCs [3,4]. Organic SCs with improved structures and materials, including bulk heterojunction [5] and perovskites [6], have reached power conversion efficiencies above 10% [7] but stability and lifetime is still an issue even with encapsulated devices [8]. In this contribution we report the design, initial development and assessment of donor (D)/acceptor (A) heterojunction SCs. The D/A heterojunction is the archetypal structure of several organic SC structures, as first reported by Tang in 1986 [9]. Considering small molecule organic materials, photons absorption creates excitons, i.e., electron-hole pairs localized on the molecules and made stable by their Coulomb attractive force. These are also termed Frenkel excitons and can move by diffusion-driven hopping between nearest molecules under a concentration gradient. Their binding energy is quite high, even up to 1 eV, due to the low dielectric constant of the organic materials (εr ~ 3). For this reason, organic SCs are also called excitonic SCs. Efficient organic SCs require the separation of the electron hole-pair (exciton dissociation) followed by charge transport and collection at the electrical contacts. Due to the high binding energy of the

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ElectronicsFrenkel excitons,2020, 9, x FOR direct PEER dissociation REVIEW by the internal electric field, although possible, has a very2 lowof 8 probability, <10%. However, efficient excitons dissociation can be achieved at a heterojunction interface interface with proper energy levels offset for charge separation [9]. Frenkel excitons that reach the with proper energy levels offset for charge separation [9]. Frenkel excitons that reach the interface lose interface lose some energy and become charge transfer (CT) excitons, where electron and hole are now some energy and become charge transfer (CT) excitons, where electron and hole are now localized to localized to neighboring and chemically different molecules on the two sides of the interface [9,10]. neighboring and chemically different molecules on the two sides of the interface [9,10]. CT excitons CT excitons are then easily split by the internal electric field allowing free electrons and holes to be are then easily split by the internal electric field allowing free electrons and holes to be collected at collected at the electrodes. the electrodes. Figure 1a shows the structure of an SC heterojunction made on a glass substrate, with a donor Figure1a shows the structure of an SC heterojunction made on a glass substrate, with a donor organic material (D) able to transport holes and an acceptor material (A) for electron transport. Figure organic material (D) able to transport holes and an acceptor material (A) for electron transport. Figure1b 1b shows the open circuit band diagram of the D-A heterojunction SC, that is in flat band condition, shows the open circuit band diagram of the D-A heterojunction SC, that is in flat band condition, where where the energy levels are referenced to the vacuum level, taken as 0 eV. The energy level of the the energy levels are referenced to the vacuum level, taken as 0 eV. The energy level of the highest highest occupied molecular orbital (HOMO) is equivalent to the valence band edge in inorganic occupied molecular orbital (HOMO) is equivalent to the valence band edge in inorganic crystalline crystalline semiconductors. The energy level of the lowest unoccupied molecular orbital (LUMO) is semiconductors. The energy level of the lowest unoccupied molecular orbital (LUMO) is equivalent to equivalent to the conduction band edge. Consequently, the level offset at the D-A interface must be as the conduction band edge. Consequently, the level offset at the D-A interface must be as indicated in indicated in Figure 1b, with the D material having electron affinity (EA) smaller than that of the A Figure1b, with the D material having electron a ffinity (EA) smaller than that of the A material, in order material, in order to easily release electrons (hence the term donor). As the A material accepts electrons to easily release electrons (hence the term donor). As the A material accepts electrons (acceptor), this (acceptor), this must have a ionization potential (IP) higher than that of the D material, for releasing must have a ionization potential (IP) higher than that of the D material, for releasing holes to the D holes to the D layer. The work functions of anode and cathode materials are also shown. The energy of layer. The work functions of anode and cathode materials are also shown. The energy of the Frenkel the Frenkel exciton, generated by optical absorption, is lower than the HOMO-LUMO bangap by a exciton, generated by optical absorption, is lower than the HOMO-LUMO bangap by a value exactly value exactly equal to the exciton binding energy. The built-in voltage Vbi is given by the work equal to the exciton binding energy. The built-in voltage V is given by the work function difference function difference between cathode and anode materialsbi and is ideally equal to the open circuit between cathode and anode materials and is ideally equal to the open circuit voltage VOC of the SC. voltage VOC of the SC.

FigureFigure 1. 1. (a)( aD)-A D-A SC SCstructure; structure; (b) Open (b) Open circuit circuit band diagram band diagram (flat band) (ﬂat with band) Frenkel with excitons Frenkel becoming excitons CTbecoming excitons CT upon excitons charge upon transfer charge at transfer the D-A at interface.the D-A interface. On the A On side the there A side is there the cathode is the cathode metal electrodemetal electrode (usually (usually Aluminum) Aluminum) and on and the on D the side D side the thetransparent transparent anode anode electrode electrode (usually (usually Indium Indium TinTin Oxide, ITO) to allow the transmission of the incident photonsphotons inside the SC.

AA short short circuited circuited SC SC and its band diagram are shown in Figure2 2a,ba,b respectively. respectively. The The band band diagramdiagram with with constant constant Fermi Fermi energy energy level level helps helps to to understand understand the the photovoltaic photovoltaic effect effect in in organic organic SCs. SCs. AA photogenerated Frenkel Frenkel exciton exciton that that reaches reaches th thee D D-A-A interface interface becomes becomes a a CT CT exciton exciton upon upon charge charge transfertransfer and and this this is is then then dissociated dissociated by by the the internal internal electric electric field. field. The The free free electrons electrons are are transferred transferred to to thethe A A side side and and the the free holes to the D side.side. Therefore, an external short circuit current density ( (JJSC)) flowsflows through through the the external external connection connection from from anode anode to to cathode. When When a a load load is is connected connected to to the the SC SC thethe external external current current density density is is smaller than J SC andand the the D D-A-A heterojuntion heterojuntion is is forward forward self self-biased-biased to to a a voltagevoltage less less than than V Vbibi. .The The SC SC operates operates as as an an electri electricc power power generator. generator. As a final remark, the difference in energy between the LUMO of A and the HOMO of D must be smaller than the Frenkel exciton energy to ensure an ideally 100% probability of CT exciton formation at the D-A interface with subsequent charge separation by the internal electric field. If this requirement is not met, CT excitons cannot be formed and most Frenkel excitons reaching the interface will recombine and are lost, resulting in SC poor performance.

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Figure 2. 2. (a(a) )Short Short circuited circuited SC; SC; (b () bRelevant) Relevant band band diagram diagram showing showing the thephotovoltaic photovoltaic effect e ffofect the of D the-A solar cell. The exciton diffusion lengths LD and LA are indicated. Ideally the difference in work D-A solar cell. The exciton diffusion lengths LD and LA are indicated. Ideally the difference in work function energy levels between anode and cathode is VOC = Vbi of the solar cell. function energy levels between anode and cathode is VOC = Vbi of the solar cell.

OrganicAs a final materials remark, thehave di highfference optical in energy absorption between coefficients the LUMO in the of Avisible and the range HOMO (α ~ of10 D5 cm must−1 [1]) be withsmaller corresponding than the Frenkel absorption exciton energylength toL = ensure 1/α of an the ideally order 100% of hundreds probability of ofnm, CT which exciton is formation indeed the at typicalthe D-A thickness interface of with organic subsequent SCs. Being charge this separation shorter than by the the wavel internalength electric of the field. photons, If this light requirement reflection is fromnot met, the CT metal excitons cathode cannot creates be formed a standing and most wave Frenkel pattern excitons in the device reaching thickness the interface with will an associatedrecombine distributionand are lost, of resulting photogenerated in SC poor excitons. performance. This pattern is actually made of a single antinode due to the 5 1 smallOrganic thickness materials of the absorbing have high layers, optical creating absorption a concentration coefficients in gradient. the visible As rangea result, (α ~in 10 generalcm− the[1]) excitonswith corresponding in the A layer absorption diffuse length toward L = the1 /α Dof-A the interface order of and hundreds the metal of nm, cathode. which Only is indeed excitons the photogeneratedtypical thickness within of organic an SCs.exciton Being diffusion this shorter length than from the thewavelength D-A interface of the photons, can reach light this reflection and be dissociated.from the metal The cathodephotoactive creates region a standing of the organic wave patternSC is close in the to the device D-A thickness interface withand is an equal associated to the sumdistribution of exciton of diffusion photogenerated lengths, excitons. LD in D and This L patternA in A, from is actually the inter madeface, of as a indicated single antinode in Figure due 2 tob. Typically,the small thicknessorganic materials of the absorbing have short layers, exciton creating diffusion a concentration lengths, of gradient.the order Asof several a result, nm in [1]. general But goodthe excitons D and in A the materials A layer suitable diffuse toward for organic the D-A SCs interface have exciton and the diffusion metal cathode. length Only one order excitons of magnitudephotogenerated higher. within This an is exciton a relevant diffusion point, length because from the the D-A photoactive interface region can reach is thiswider and and be condissociated.sequently The a higher photoactive power regionconversion of the efficiency organic SCcan is be close expected. to the D-A interface and is equal to the sumActually, of exciton the di ffsimpleusion lengths,two-layer L DDin-A DSC and shown LA in in A, Figure from 2 the has interface, a drawback: as indicated the excitons in Figure in the2 b.A layerTypically, can alsoorganic diffuse materials away havefrom shortthe D exciton-A interface diffusion toward lengths, the Al of thecathode. order The of several deposition nm [ 1of]. Butthe megoodtal Dfilm and over A materials the organic suitable layers for is organic made SCsby vacuum have exciton thermal diff usionevaporation, length one as the order last of step magnitude of the fabricationhigher. This process. is a relevant For this point, reason, because in the the early photoactive stage of film region formation, is wider the and first consequently particles that a higher reach thepower A layer conversion produce effi someciency damage can be in expected. the soft organic material. This creates a high density of defects and trapsActually, with the energy simple levels two-layer below D-Athe LUMO SC shown level in of Figure the A2 haslayer a drawback:close to the the metal excitons cathode in the [11]. A Theselayer can defects also di actffuse as away exciton from traps, the D-A enhancing interface the toward non the-radiative Al cathode. recombination The deposition of the of the excitons metal reachingfilm over the the met organical surface layers (exciton is made quenching). by vacuum thermalAs a result, evaporation, most excitons as the are last lost step in oftheir the path fabrication to the cathode.process. For this reason, in the early stage of film formation, the first particles that reach the A layer produceTo avoid some this damage phenomenon in the soft an organic exciton material. blocking This layer creates (EBL) a highcan be density inserted of defects between and the traps A layer with andenergy the levelsmetal belowcathode the [11,12]. LUMO The level HOMO of the-LUMO A layer bandgap close to the of metalthe EBL cathode must be [11 larger]. These than defects that of act the as Aexciton layer, traps,for being enhancing optically the transparent, non-radiative and recombinationhave proper thickness. of the excitons However reaching the blocking the metal layer surface must allow(exciton electrons quenching). transport As a through result, most it. This excitons is believed are lost to in occur, their patheven toin thethis cathode. three-layer case, through energyTo states avoid below this phenomenon the LUMO energy an exciton level blocking created in layer the (EBL)EBL by can damage be inserted during between the metal the cathode A layer depositionand the metal [11,12]. cathode This [ 11is ,schematically12]. The HOMO-LUMO shown in the bandgap open circuit of the EBLband must diagram be larger in Figure than that3a. Hence of the thisA layer, three for-layer being SC optically can be expected transparent, to exhibit and have a better proper photovoltaic thickness. Howeverperformanc thee blockingas compared layer to must the simpleallow electrons two-layer transport D-A SC. through it. This is believed to occur, even in this three-layer case, through

Electronics 2020, 9, 70 4 of 8 energy states below the LUMO energy level created in the EBL by damage during the metal cathode deposition [11,12]. This is schematically shown in the open circuit band diagram in Figure3a. Hence this three-layer SC can be expected to exhibit a better photovoltaic performance as compared to the simple two-layer D-A SC. Electronics 2020, 9, x FOR PEER REVIEW 4 of 8

FigureFigure 3. 3.( a(a)) Open Open circuitcircuit bandband diagramdiagram ofof aa three-layer three-layer D-A D-A solar solar cell cell with with exciton exciton blocking blocking layer layer (EBL).(EBL). The The arrow arrow indicates indicates electrons electrons transport transport through through defect defect states in states the EBL; in the (b) Molecular EBL; (b) Molecular structure ofstructure the organic of the materials organic used materials for the used fabrication for the of fabrication the SCs (b1) of CuPc,the SCs (b2) (b C60,1) CuPc, and (b3)(b2) BCP.C60, and (b3) BCP. 2. Materials and Methods 2. MaterialsThis section and describesMethods the D-A three-layer SCs fabricated by the authors. The organic materials are CopperThis section phtalocyanine describes (CuPc, the D- Sigma-AldrichA three-layer SCs product fabricated nr. 546674), by theused authors. as the The D organic material,materials fullereneare Copper (C60, phtalocyanine ABCR product (CuPc, nr. Sigma AB109139)-Aldrich as product the A organic nr. 546674), material, used and as the bathocuproine D organic material, (BCP, Sigma-Aldrichfullerene (C60,product ABCR product nr. 140910) nr. AB109139) as the material as the for A theorganic EBL. material, The molecular and bathocuproine structures of these(BCP, materialsSigma-Aldrich are shown product in Figure nr. 140910)3b. The as relevant the material HOMO for andthe LUMOEBL. The energy molecular levels structures are shown of in these the bandmaterial diagrams are ofshown the three-layer in Figure 3 D-Ab. The organic relevant SC inHOMO Figure 3anda. LUMO energy levels are shown in the bandMeasured diagram of exciton the three diff-usionlayer D lengths-A organic of many SC in organic Figure 3 materialsa. are reported in [10], including someMeasured spreading exciton range for diffusion their values. lengths The of C60 many has organic an exciton materials diffusion are length reported LA =in40 [10], nm including whereas the CuPc has a much shorter exciton diffusion length L = 5 nm [13]. Due to the very low charge some spreading range for their values. The C60 has an excitonD diffusion length LA = 40 nm whereas carrier mobility in amorphous organic materials, it is convenient to choose the thicknesses of the D the CuPc has a much shorter exciton diffusion length LD = 5 nm [13]. Due to the very low charge andcarrier A layers mobility close in to amorphous their respective organic exciton materials, diffusion it is lengthconvenient [10]. Asto choose a matter the of thicknesses fact excitons of can the be D eandfficiently A layers dissociated close to onlytheir withinrespective an exciton exciton di diffusionffusion length length from [10]. the As D-A a matter interface. of fact Moreover, excitons duecan tobe theefficiently low mobility, dissociated most chargeonly within carriers an exciton can be collecteddiffusion atlength the electrodes from the D only-A interface. if the layers Moreover, are not toodue thick.to the Based low mobility, on those considerations,most charge carriers we chose can 40be nmcollected for the at thickness the electrodes of the A only layer if andthe layers 20 nm forare thenot Dtoo layer. thick. To Based prevent on those the damage-induced considerations, we states chose from 40 nm reaching for the thethickness A layer, of thethe thicknessA layer and of 20 the nm EBL for (BCP)the D mustlayer. be To not prevent too thin the but damage also not-induced too thick states for allowingfrom reaching electrons the toA flowlayer through, the thickness it and achievingof the EBL a(BCP) low series must resistance be not too of the thin SC. but Based also on not the too study thick reported for allowing in [14 ], electrons we chose to a thicknessflow through of 12 it nm. and All devices are fabricated in a single run by thermal evaporation in high vacuum (5 10 7 mbar) achieving a low series resistance of the SC. Based on the study reported in [14], we chose× a thickness− onof commercial12 nm. ITO-coated glass substrates (Merck Display Technologies, ITO film 100 nm thick with sheetAll resistance devices

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ITO anode solar ITO anode solar cell cell

organic organic Al cathode layers Al cathode layers

((aa)) ((bb)) Figure 4. (a) Top view of the layers arrangement for the sample with 4 organic solar cells of 9 mm2 FigureFigure 4. 4.(a ()a Top) Top view view of of the the layers layers arrangement arrangement for thefor samplethe sample with with 4 organic 4 organic solar solar cells ofcells 9 mm of 29area mm2 area each. Black area is ITO film, grey stripes are Al film; (b) Spectrum of the halogen lamp as each.area Black each. area Black is ITO area ﬁlm, is ITO grey film, stripes grey are stripes Al ﬁlm; are (b ) Al Spectrum film; (b of) Spectrum the halogen of lamp the halogen as compared lamp to as compared to the AM1.0 solar spectrum. thecompared AM1.0 solar to the spectrum. AM1.0 solar spectrum.

3.3.3. PhotoelectricalPhotoelectrical Photoelectrical CharacterizationCharacterization Characterization TheTheThe SCs SCs SCs are are are tested tested tested in in in ambient ambient ambient atmosphere atmosphere atmosphere and and and without without without encapsulation encapsulation encapsulation immediately immediately immediately after after after film film film deposition.deposition.deposition. For For aa quickquick testtest test ofof of thethe the devices,devices, devices, aa commercial acommercial commercial 50 50 50WW TungstenWTungsten Tungsten halogenhalogen halogen lamplamp lamp isis usedused is used asas lightlight as lightsourcesource source andand the andthe desireddesired the desired incidentincident incident opticaloptical optical powerpower power densitydensity density isis setset byby is simply setsimply by simplyvaryingvarying varying thethe verticalvertical the verticaldistancedistance distancebetweenbetween between the the lamplamp the and and lamp thethe and SC SC surface. thesurface. SC surface. The The halogenhalogen The halogen lamplamp opticaloptical lamp optical spectrum,spectrum, spectrum, measured measured measured by by an an Oceanby Ocean an OceanOpticsOptics Optics USB4000 USB4000 USB4000 UV UV--VISVIS UV-VIS fiber fiber fiber optic optic optic spectrometer spectrometer spectrometer limited limited limited to to to the the the 2020 200–85000––850850 nmnm nm spectral spectral spectral range, range, is is is acceptablyacceptablyacceptably close closeclose to to theto the the visible visible visible part part ofpart the of of AM1.0 the the AM1.0 AM1.0 solar spectrum, solar solar spectrum, spectrum, as shown as as in shown shownFigure 4 inb. in The Figure Figure decreasing 44b.b. The The lampdecreasingdecreasing emission lamp lamp above emission emission 700 nm above above is not 700 700 relevant nm nm is is not not here relevant relevant because here here the because SCbecause absorption the the SC SC isabsorptionabsorption very small, is is very duevery tosmall, small, the largeduedue to HOMO-LUMO to the the large large HOMO HOMO bandgaps.--LUMOLUMO The bandga bandga opticalps.ps. spectrum The The optical optical is used spectrum spectrum to calibrate is is used used the to light to calibrate calibrate intensity the the of light light the halogenintensityintensity lamp of of the the and halogen halogen then to lamp lamp set the and and distance then then to to between set set the the distance thedistance lamp betweenbetween and the SC thethe surfacelamp lamp andand for the athe standard SC SC surface surface value for for of a a standard value2 of 100 mW/cm2 for the incident optical power density. The current density J vs. 100standard mW/cm valuefor the of 100 incident mW/cm optical2 for power the incident density. optical The current power density density. J vs. The forward current voltage density V J isvs. measuredforwforwardard voltage voltage by a Keithley V V is is measured measured 6487 source-meter. by by a a Keithley Keithley 6487 6487 source source--meter.meter. AAA typical typical typical J-V J J--VV plot plot plot for forfor the the the organic organicorganic SC SCis SC reported is is reported reported in Figure in in FigureFigure5a for 5a a5a range for for a a of range range incident of of incident incident optical power optical optical power density, from 45 2mW/cm2 to 150 2mW/cm2. We notice a good photovoltaic effect and a current density,power fromdensity, 45 mWfrom/cm 45 mW/cmto 150 mW2 to /150cm mW/cm. We notice2. We a goodnotice photovoltaic a good photovoltaic effect and effect a current and a density current fairlydensitdensit monotonicyy fairly fairly monotonic with monotonic the increasing with with the the incident increasing increasing optical incident power incident density. optical optical This power power indicates density. density. minimum ThisThis saturation indicates indicates 2 eminimumffminimumects at the saturationsaturation used values effectseffects of incident atat thethe usedused optical valuesvalues power ofof density. incidentincident The opticaloptical dark powerpower current density.density. results The inThe a fewdarkdarkµ currentAcurrent/cm . 2 2 2 2 2 Atresults 100 mW in a/ cmfew ,μ theA/cm SC2. exhibits At 100 mW/cm V = 0.452, the V SC and exhibits J = 2.35 VOC mA= 0.45/cm V andwith JSC a fill= 2.35 factor mA/cm FF 2 50%.with a fill results in a few μA/cm . At 100 mW/cmOC , the SC exhibitsSC VOC = 0.45 V and JSC = 2.35 mA/cm≈ with a fill factorfactor FF FF ≈ ≈ 50%. 50%. 1 1 1 1

0.5 ) Voltage (V) 2 0.5 ) Voltage (V)

0 ) 2 2 Voltage V (V)

0 ) 2 Voltage V (V) 0 0.1 0.2 0.3 0.4 0.5 0 0 0.1 0.2 0.3 0.4 0.5 0 0 0.1 0.2 0.3 0.4 0.5 -1 0 0.1 0.2 0.3 0.4 0.5 -1 -0.5 -0.5

-2 -1 -2 -1 15 min 45 mW/cm2 -1.5 15 min -1.5 5 h

45 mW/cm2 (mA/cm J density Current

Currnt density J (mA/cm J density Currnt -3 70 mW/cm2 5 h Current density J (mA/cm J density Current Currnt density J (mA/cm J density Currnt -3 10070 mW/cm2 -2 24 h -2 24 h 150100 mW/cm2 mW/cm2 150 mW/cm2 48 h -4 -2.5 48 h -4 -2.5

((aa)) ((bb)) FigureFigure 5. 5. a(a) J-V characteristics of the best SC under illumination with increasing incident optical Figure 5.( )(a J-V) J- characteristicsV characteristics of theof best the best SC under SC under illumination illumination with increasing with increasing incident incident optical poweroptical density;power density (b) J-V; characteristics (b) J-V characteristics of the best of SCthe withbest timeSC with under time continuous under continuous illumination illumination at 100 mW at/cm 1002 power density; (b) J-V characteristics of the best SC with time under continuous illumination at 100 incidentmW/cm2 power incident density. power density. mW/cm2 incident power density.

TheThe external external quantum quantum efficiency efficiency (EQE) (EQE) of of a a solar solar cell cell is is the the ratio ratio electrons/s electrons/s to to incident incident photons/s. The electrons/s are calculated from JSC and the photons/s are calculated from the incident photons/s. The electrons/s are calculated from JSC and the photons/s are calculated from the incident opticaloptical power power and and its its op opticaltical spectrum. spectrum. This This gives gives EQE EQE ≈ ≈ 5%. 5%. The The power power conversion conversion efficiency efficiency (PCE) (PCE)

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The external quantum efficiency (EQE) of a solar cell is the ratio electrons/s to incident photons/s. The electrons/s are calculated from JSC and the photons/s are calculated from the incident optical power and its optical spectrum. This gives EQE 5%. The power conversion efficiency (PCE) is the ratio ≈ of the maximum electric power the SC can deliver to an optimum load to the incident optical power. The PCE is calculated to be about 0.5%. Both EQE and PCE values are lower than previously reported for SC with similar structure [10] and this can be ascribed to the low value of JSC being about one order of magnitude smaller. In fact to get a high value of EQE and PCE one must have a high Jsc and VOC with FF close to 1. A low JSC is mainly due to excitons loss at non radiative recombination sites for both Frenkel excitons in the D and A organic layers and CT excitons at the D/A interface. These non radiative recombination sites can have different origins. (i) foreign impurities incorporated in the organic layers because the organic materials used for thin film deposition are not purified, (ii) trapping defects created in the organic layers by the exposure of the SC to oxygen and water (humidity) [15,16] because our SCs are non encapsulated and operated in ambient atmosphere. Moreover some structural disorder and traps could also have been generated and propagated through the EBL in the A layer during metal cathode deposition and this will add a further non radiative recombination path for Frenkel excitons in the A layer. However we are not able to give proof for the latter hypothesis.

4. Lifetime Tests Organic materials are very sensitive to oxygen and water (humidity) present in ambient atmosphere and this can lead to degradation of the performances and eventually to device breakdown [15,16]. For evaluating the aging rate, we measured the J-V characteristics of the SC with time under continuous illumination, with 100 mW/cm2 incident power density. The results are shown in Figure5b. A slight decrease in VOC and a substantial decrease in JSC is evident in a time interval of 48 h. The J-V curves get less steep with time and this means that the SC attains a higher series resistance RS and a decreasing ﬁll factor FF. The values of VOC,JSC,RS and FF with time are shown in Figure6. The series resistance of the SC is calculated from the slope of the J-V curve above VOC (forward bias). Moreover the ﬁll factor is calculated as: P FF = max (1) VOCIsc where Pmax is the maximum electric power the SC can deliver to an optimum load, VOC is the open circuit voltage, ISC is the short circuit current and VOC ISC is the absolute maximum power the SC could ideally deliver. Pmax is easily derived from the J-V curves. From Figure6a after 5 h the short circuit current density decreases of about 25% as compared to the value at 15 min and then the decrease rate of the short circuit current density slows down with time reaching about 40% after 48 h. Figure6b shows the RS and FF vs. time. It can be noticed that, after 5 h, RS almost quadruplicates and then continues to increase at a smaller rate. The FF decreases from 50% to about 40% after 5 h time and so by a relatively small amount and then again the decrease slows down. Electronics 2020, 9, 70 7 of 8 Electronics 2020, 9, x FOR PEER REVIEW 7 of 8

2.5 0.436 30 50

0.434 25

0.432 )

2 2 0.43 20 40 FF FF (%)

Jsc )

0.428 (V) W

( 15 OC

Voc 0.426 S

(mA/cm V

R Rs SC

J 1.5 0.424 10 30 FF 0.422 5 0.42

1 0.418 0 20 0 10 20 30 40 50 0 5 10 15 20 25 30 35 40 45 50 Time (h) Time (h) (a) (b)

FigureFigure 6.6. (a) Short circuit circuit current current density density JSC JSC andand open open circuit circuit voltage voltage VOC V;OC (b;() Sberies) Series resistance resistance RS and RS andfill factor ﬁll factor FF as FF a function as a function of time of for time the for best the SC best under SC undercontinuous continuous illumination illumination at 100 mW/cm at 100 mW2 incident/cm2 incidentpower density. power density.

5. ConclusionsIn summary, an initial relatively fast degradation takes place within five hours from fabrication. Then the degradation progresses at a lower rate. A proper encapsulation in inert atmosphere [17,18] of In this paper we report our experience in the fabrication of organic SCs having the D-A the devices is found to be effective in preventing oxygen and water molecules (extrinsic factors) from heterojunction structure with EBL. The best devices exhibit an open circuit voltage VOC = 0.45 V, a short diffusing into the organic layers. In fact chemical reactions with oxygen and water molecules are the circuit current density JSC = 2.35 mA/cm2 with a fill factor FF ≈ 50%, an external quantum efficiency dominant degradation mechanism in air. On the contrary, encapsulated organic SCs have durability (electrons/s over incident photons/s) EQE ≈ 5% and a power conversion efficiency of about 0.5%. and lifetime dominated by photoelectric and thermal stability (intrinsic factors) of the organic materials. Non-encapsulated devices are very sensitive to oxygen and humidity induced degradation. This 5.degradation Conclusions process was monitored in a time interval of 48 h. Our results confirm that non-encapsulated organic solar cells have limited stability in ambient atmosphere. In this paper we report our experience in the fabrication of organic SCs having the D-A Author Contributions: Conceptualization, P.C.; methodology, P.C.; investigation, P.C.; resources, P.C. and heterojunction structure with EBL. The best devices exhibit an open circuit voltage VOC = 0.45 C.A.; data curation, P.C.; writing—original draft preparation, P.C. and C.A.; writing—review and editing, C.A., V, a short circuit current density J = 2.35 mA/cm2 with a fill factor FF 50%, an external quantum M.A.G. and A.P.; funding acquisition,SC P.C. ≈ efficiency (electrons/s over incident photons/s) EQE 5% and a power conversion efficiency of about ≈ 0.5%.Funding: Non-encapsulated This research was devices funded are by very University sensitive of to Palermo, oxygen under and humidity grant number induced ORPA073ZR8 degradation. “Organic This degradationsolar cells development”. process was monitored in a time interval of 48 h. Our results confirm that non-encapsulated organicAcknowledgments: solar cells haveIn this limited section stabilityyou can acknowledge in ambient atmosphere. any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind Author(e.g., materials Contributions: used forConceptualization, experiments). P.C.; methodology, P.C.; investigation, P.C.; resources, P.C. and C.A.; data curation, P.C.; writing—original draft preparation, P.C. and C.A.; writing—review and editing, C.A., M.A.G. andConflicts A.P.; funding of Interest: acquisition, The authors P.C. Alldeclare authors no conflict have read of interest and agreed. to the published version of the manuscript. Funding: This research was funded by University of Palermo, under grant number ORPA073ZR8 “Organic solar cellsReferences development”.

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