Photoconversion Mechanism at the Pn-Homojunction Interface in Single Organic Semiconductor

materials

Article Photoconversion Mechanism at the pn-Homojunction Interface in Single Organic Semiconductor

Ji-Hyun Lee 1,2, Armand Perrot 1,3, Masahiro Hiramoto 1,2 and Seiichiro Izawa 1,2,*

1 Institute for Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan; [email protected] (J.-H.L.); [email protected] (A.P.); [email protected] (M.H.) 2 SOKENDAI, The Graduate University for Advanced Studies, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan 3 École Nationale Supérieure de Chimie de Paris, 11 Rue Pierre et Marie Curie, 75005 Paris, France * Correspondence: [email protected]

Received: 18 March 2020; Accepted: 3 April 2020; Published: 7 April 2020

Abstract: Clarifying critical differences in free charge generation and recombination processes between inorganic and organic semiconductors is important for developing efficient organic photoconversion devices such as solar cells (SCs) and photodetector. In this study, we analyzed the dependence of doping concentration on the photoconversion process at the organic pn-homojunction interface in a single organic semiconductor using the temperature dependence of J–V characteristics and energy structure measurements. Even though the organic pn-homojunction SC devices were fabricated using a single host material and the doping technique resembling an inorganic pn-homojunction, the charge generation and recombination mechanisms are similar to that of conventional donor/acceptor (D/A) type organic SCs; that is, the charge separation happens from localized exciton and charge transfer (CT) state being separated by the energy offset between adjacent molecules, and the recombination happens from localized charge carrier at two adjacent molecules. The determining factor for photoconversion processes is the localized nature of charges in organic semiconductors. The results demonstrated that controlling the delocalization of the charges is important to realize efficient organic photoconversion devices.

Keywords: organic solar cell; pn junction; interface; doping; single organic semiconductor; photoconversion

1. Introduction Free charge generation processes by light absorption in inorganic and organic semiconductors are completely different. Inorganic semiconductors have a large dielectric constant; thus, free charges are directly formed after the light absorption in a single semiconductor material [1]. In contrast, a strongly bounded Frenkel type exciton forms after light absorption in organic semiconductors, because they generally have a smaller dielectric constant when compared with that of inorganic semiconductors [2]. The exciton can be separated using the energy offset between the two organic semiconductor materials called donor and acceptor in organic solar cells (OSCs) [2,3]. The question is whether free charge formation is possible in single organic semiconductor material by light absorption, similar to that in inorganic semiconductors. Recently, we have reported that pn-homojunction interfaces in a single organic semiconductor films formed by doping can achieve efficient charge separation [4]. The organic pn-homojunction solar cell SC device showed a high internal quantum efficiency of 30%. Further investigation for clarifying the mechanism of the photoconversion at the pn homojunction interface can help to answer the basic question of how free charges form beyond the strong coulomb binding and recombine in a single

Materials 2020, 13, 1727; doi:10.3390/ma13071727 www.mdpi.com/journal/materials Materials 2020, 13, x FOR PEER REVIEW 2 of 8 Materials 2020, 13, 1727 2 of 8 single organic semiconductor film, and what is the critical difference in these processes between organicorganic semiconductor and inorganic ﬁlm,semiconductors. and what is the critical diﬀerence in these processes between organic and inorganicIn this semiconductors. study, we analyzed the doping concentration dependence on the photoconversion processIn thisin organic study, pn we-homojunction analyzed the SC doping devices concentration mainly by a temperature dependence dependence on the photoconversion measurement. processThe measurement in organic pn gives-homojunction information SC devicesabout therma mainlyl byactivation a temperature for the dependence charge generation measurement. and Therecombination measurement pathways gives information [5,6]. Upon about combining thermal activation an energy for the structure charge generation measurement and recombination near the pn- pathwayshomojunction, [5,6]. Uponwe revealed combining the an energydetailed structure mechanis measurementm of photoconversion near the pn-homojunction, at the single we revealedorganic thesemiconductor detailed mechanism interface. of photoconversion at the single organic semiconductor interface.

2.2. ResultsResults andand DiscussionDiscussion TheThepn pn-homojunction-homojunction SC SC devicesdevices werewere fabricatedfabricated inin thethe samesame mannermanner asas previouslypreviously reportedreported [[4]4].. WeWe usedused diindenoperylene (DIP), (DIP), which which is is an an ambipolar ambipolar organic organic semiconductor semiconductor molecule, molecule, as the as host, the host,and MoO and3 MoO and3 Csand2CO Cs3 as2CO p-3 andas p -n-dopants, and n-dopants, respectively. respectively. The device The was device fabricated was fabricated by thermal by thermalevaporation evaporation under high under vacuum, high and vacuum, the structure and the was structure indium was tin oxide indium (ITO)/MoO tin oxide3: 10 (ITO) nm/MoO/MoO33:- 10doped nm/MoO DIP:3 -doped50 nm/Cs DIP:2CO 503-doped nm/Cs 2DIP:CO3 -doped50 nm/bathocuproine DIP: 50 nm/bathocuproine (BCP): 10 nm/Al: (BCP): 60 10 nm nm (Figure/Al: 60 nmS1). (FigureDopants S1). were Dopants introduced were introduced into the intosemiconducto the semiconductorr layer via layer co-deposition via co-deposition techniques techniques and andthe theconcentration concentration of ofthe the dopants dopants relative relative to to the the semiconductor semiconductor volume volume was was controlled controlled by varyingvarying thethe ratioratio between between the the deposition deposition rates rates of the of two the species two [4species]. Figure [4]. S2 showsFigure the S2 typical showsJ–V thecharacteristics typical J–V ofcharacteristicspn-homojunction of pn devices-homojunction and the performances devices and arethe summarizedperformances in are Table summarized S1. As previously in Table reported, S1. As JpreviouslySC drastically reported, increased JSC and drasticallyVOC decreased increased with and anincrease VOC decreased in doping with concentration. an increase The in degreesdoping ofconcentration.JSC increase wereThe degrees eight times of JSC from increase undoped were toeight 5% times doping from in theundoped DIP devices. to 5% doping In contrast, in theV DIPOC decreaseddevices. In from contrast, 1.13 V toVOC 0.83 decreased V. The ﬁll from factor 1.13 (FF) V values to 0.83 increased V. The marginally fill factor in(FF) the devicevalues withincreased high dopingmarginally concentration. in the device The with increase high doping of external concentr quantumation. e ﬃTheciency increase (EQE) of external spectra without quantum the efficiency change in(EQE) spectral spectra shape without (Figure the S3) change indicated in spectral that charge shape separation (Figure S3) from indicated the absorption that charge of the separation ground statefrom ofthe DIP absorption was accelerated of the ground by doping. state of DIP was accelerated by doping. ToTo investigateinvestigate thethe chargecharge generationgeneration andand recombinationrecombination processes,processes, wewe measuredmeasured thethe J–VJ–V characteristicscharacteristics ofof the the devices devices at at temperatures temperatures from from 30 30 to to −6060 °C,C, asas shownshown inin FigureFigure1 .1. − ◦

0.2 Temp. / K

) ―302

-2 0.15 ―292 0.1 ―283 0.05 ―272 0 ―262 ―252 -0.05 ―242 -0.1 ―232 -0.15 222

Current density (mA cm (mA density Current ― -0.2 ―212 -0.500.511.52 Voltage (V)

Figure 1. J–V curves for the 5% doped pn-homojunction device at various temperatures under AM Figure 1. J–V curves for the 5% doped2 pn-homojunction device at various temperatures under AM 1.5 irradiation 1.5 irradiation (100 mW cm− ). (100 mW cm−2). Typically, JSC decreases while VOC increases with a decrease in temperature in the OSCs [6]. The pnTypically,-homojunction JSC decreases devices while followed VOC theincreases same tendency. with a decrease Firstly, in the temperature temperature in dependence the OSCs [6]. of TheJSC ispn expressed-homojunction by the devices following followed Arrhenius the same Equation tenden (1):cy. Firstly, the temperature dependence of JSC is expressed by the following Arrhenius Equation (1): JSC = J0(Plight)exp( Ea/kT), (1) JSC = J0(Plight)exp(−Ea/kT), (1)

Materials 2020, 13, 1727 3 of 8 Materials 2020, 13, x FOR PEER REVIEW 3 of 8 wherewhere J0J(0P(Plightlight) is) the is the pre-exponential pre-exponential factor, factor, Ea isE atheis activation the activation energy, energy, k is Boltzmannk is Boltzmann constant, constant, and Tand is temperatureT is temperature [7]. The [7 origin]. The of origin Ea was of attributedEa was attributed to the activation to the process activation during process charge during separation charge [5,8].separation The calculated [5,8]. The E calculateda of the devicesEa of with the devices 5% and with 1% 5%doped and DIP 1% dopedfrom the DIP Arrhenius from the Arrheniusplots in Figure plots 2ain are Figure 74.42 aand are 110 74.4 meV, and 110respectively. meV, respectively.

(a) 0 ■ 5% : Ea=74.4 meV -0.5 ♦ 1% : Ea=110 meV

)) -1 -2 -1.5

-2

-2.5

) (ln(mA cm (ln(mA ) -3 sc J -3.5 ln( -4

-4.5 0.003 0.0035 0.004 0.0045 0.005 -1 (b) 1/T (K ) 2

1.8

1.6

1.4 (V)

OC 1.2 V

0.8 eff eff ▲ 1% : Eg =1.95 eV, Eg -eVOC=0.95 ■ 5% : E eff=1.75 eV, E eff-eV =0.92 0.6 g g OC 0 50 100 150 200 250 300 T (K)

Figure 2. Temperature dependence of (a) JSC and (b) VOC for the 1% doped (red) and 5% doped (blue) Figure 2. Temperature dependence of (a) JSC and (b) VOC for the 1% doped (red) and 5% doped (blue) pn-homojunction devices. pn-homojunction devices.

The 5% doped device showed a lesser Ea than 1% doped, indicating that the charge separation The 5% doped device showed a lesser Ea than 1% doped, indicating that the charge separation was accelerated in the device with high doping concentration. was accelerated in the device with high doping concentration. Next, the temperature dependence of VOC is expressed by the following equation: Next, the temperature dependence of VOC is expressed by the following equation: eff qVqVOCOC = Egeff ++ nkTln(JnkTln(phJph/J00/J),00 ),(2) (2) where q is elementary charge, Egeffeff is effective bandgap energy at the interface, n is ideality factor, Jph where q is elementary charge, Eg is effective bandgap energy at the interface, n is ideality factor, Jph is the photogenerated current density, and J00 is the pre-exponential factor of the reverse saturation is the photogenerated current density, and J00 is the pre-exponential factor of the reverse saturation current density [9]. Equation (2) represents the VOC, which is determined by mainly by two parts, that current density [9]. Equation (2) represents the VOC, which is determined by mainly by two parts, is,that the is, former the former energetic energetic term term and andlatter latter recomb recombinationination loss loss term, term, which which are are dependent dependent on on the the temperature [6]. The VOC plots as a function of temperature are shown in Figure 2b. Egeffeff of the devices temperature [6]. The VOC plots as a function of temperature are shown in Figure2b. Eg of the devices withwith 5% andand 1%1% doped doped DIP DIP calculated calculated from from the interceptthe intercept of the plotsof the are plots 1.75 andare 1.951.75 eV,and respectively. 1.95 eV, respectively. On the other hand, the VOC loss values induced by the recombinationeff Egeff/q − VOC of both On the other hand, the VOC loss values induced by the recombination Eg /q VOC of both devices devices showed similar values of 0.95 and 0.92 V, respectively. The results indicate− that the decrease showed similar values of 0.95 and 0.92 V, respectively. The results indicate that the decrease in VOC in VOC owing to an increase in doping concentration is the result of the reduction in the effective energy gap at the interface, not because of the difference in the recombination processes.

Materials 2020, 13, 1727 4 of 8

owingMaterials to 2020 an, increase13, x FOR PEER in doping REVIEW concentration is the result of the reduction in the effective energy gap4 of at 8 the interface, not because of the difference in the recombination processes. TheThe energeticenergetic structurestructure of of the the doped doped DIP DIP films film wass was estimated estimated by by energy-level energy-level mapping mapping using using the Kelvinthe Kelvin probe probe (KP) method(KP) method to measure to measure the work the function work function (WF) [10 –(WF)12]. Figure[10–12].3 shows Figure the 3 schematicshows the imageschematic of the image KP measurement of the KP measurement and the result and of the the result 5% doped of the DIP5% doped film. DIP film.

(a) MoO p 3 Cs2CO3 n doped DIP doped DIP

7 (eV) function Work 6 5

Vbi 4 3 Wp 2 0 1020304050

Thickness of MoO3 5% doped (nm)

(b) MoO p 3 Cs2CO3 n doped DIP doped DIP 7 6 Wn 5 V 4 bi 3 2 Workfunction (eV) 50 60 70 80 90 100

Thickness of Cs2CO3 5% doped (nm)

Figure 3. Work function (WF) of (a) 5% MoO -doped and (b) 5% Cs CO -doped diindenoperylene Figure 3. Work function (WF) of (a) 5% MoO3 3-doped and (b) 5% Cs2 2CO33-doped diindenoperylene (DIP) films on the Cs CO -doped film and MoO -doped film, respectively, as a function of the thickness. (DIP) films on the 2Cs2CO3 3-doped film and 3MoO3-doped film, respectively, as a function of the thickness. The results of the 1% doped DIP film are shown in Figure S4. The WF of the p-doped layer n n- was measuredThe results on of an the Al 1%/BCP doped/ -doped DIP layerfilm ar substratee shown andin Figure that of S4. dopedThe WF layer of the was p-doped measured layer on was an p pn ITOmeasured/MoO3 /on-doped an Al/BCP/ layer substrate.n-doped layer The steep substrate change and in thethat WF of nearn-doped the layer-interface was measured was observed on an in all of the films. The ionization potentials (IP) of the undoped and doped DIP films were evaluated by ITO/MoO3/p-doped layer substrate. The steep change in the WF near the pn-interface was observed photoelectronin all of the films. yield The spectroscopy ionization potentials (PYS) in Figure (IP) of S5. the The undoped IPs of the and undoped doped DIP and films doped were films evaluated did not changeby photoelectron significantly, yield indicating spectroscopy that doping(PYS) in does Figu notre S5. aff Theect theIPs IPof ofthe the undoped films. Small and doped changes films in thedid absorptionnot change spectra significantly, (ABS) shownindicating in Figure that doping S3b also does indicate not affect that the the bandgap IP of the and films. electron Small a ffichangesnity (EA) in ofthe the absorption film were spectra not aff (ABS)ected shown by doping. in Figure Therefore, S3b also the indicate energetic that structure the bandgap of doped and electron DIP films affinity was determined(EA) of the film only were by the not WF affected difference by doping. [10]. Therefore, the energetic structure of doped DIP films pn was Thedetermined energy diagramsonly by the of WF the difference active layer [10]. in the -homojunction devices were estimated based on theThe result energy of the diagrams KP measurement of the active [13 layer]. The in drawback the pn-homojunction of the KP measurement devices were is estimated that it can based only measureon the result the WF of the at the KP surface measurem of theent films. [13]. WeThe cannot drawback observe of the the KP WF measurement change of the is underlying that it can layeronly aftermeasure the depositionthe WF at the of thesurface over of layer. the films. To estimate We ca thennot energetic observe structurethe WF change of whole of the films, underlying we calculated layer after the deposition of the over layer. To estimate the energetic structure of whole films, we calculated the electric potential distribution derived from Poisson’s equation [13]. The total depletion layer width (W) of pn-junction for uniformly doped semiconductor is given by the following equation:

( ) 𝑊= , (3)

Materials 2020, 13, 1727 5 of 8 the electric potential distribution derived from Poisson’s equation [13]. The total depletion layer width (W) of pn-junction for uniformly doped semiconductor is given by the following equation: v tu + 2εrε0Vbi Np− + Nn W = + , (3) Materials 2020, 13, x FOR PEER REVIEW qNp−Nn 5 of 8

- + qNn xp = qNn xn, (4) qNn-xp = qNn+xn, (4) , + where εr, ε0, Vbi, Nn− Np , xp, and xn denote the relative dielectric constant of the semiconductor, where εr, ε0, Vbi, Nn−, Np+, xp, and xn denote the relative dielectric constant of the semiconductor, the the dielectric permittivity in a vacuum, the built-in-potential, ionized dopant concentration of the p dielectric permittivity in a vacuum, the built-in-potential, ionized dopant concentration of+ the p and and n layer, and the depletion layer width in the p- and n-type regions, respectively. The Np and Nn− n layer, and the depletion layer width in the p- and n-type regions, respectively. The Np+ and Nn− values can be calculated by the measured xp, xn, and Vbi values in Figure3 and Figure S4. The electric values can be calculated by the measured xp, xn, and Vbi values in Figures 3 and S4. The electric potential distribution V(x) at the pn-junction can be found by integrating Poisson’s equation as follows: potential distribution V(x) at the pn-junction can be found by integrating Poisson’s equation as follows: + qNn 2 V(x) = (x + xn) + Vbi ( xn 5 x 5 0), (5) −2εrε0 − 𝑉(𝑥) =− (𝑥+𝑥) +𝑉 (−𝑥 ≦𝓍≦0), (5) qN p− 2 𝑉((𝑥) = 𝑥 −𝑥 (0 ≦ 𝓍 ≦ 𝓍 ), V x xp x 0 5 x 5 xp,(6) (6) 2ε rε0 − Figure4 4 shows shows thethe calculatedcalculated energyenergy diagramdiagram ofof thethe activeactive layerlayer inin thethe pnpn-homojunction-homojunction devices. devices.

6 5 4 Vacu u m l evel 3 5% doped DIP WDIP5% = 20 nm 2 1% doped DIP VDIP5% = 1.40 eV 1 LUMO Fermi level

Energy (eV) Energy 0 VDIP1% = 0.99 eV -1 -2 W = 60 nm HOMO -3 DIP1% -50 -30 -10 10 30 50 Thickness (nm)

Figure 4. Vacuum level, HOMO, and LUMO energy levels relative to the Fermi level in the 1% doped Figure 4. Vacuum level, HOMO, and LUMO energy levels relative to the Fermi level in the 1% doped DIP (red) and 5% doped DIP (blue) devices as a function of ﬁlm thickness. These results are based on DIP (red) and 5% doped DIP (blue) devices as a function of film thickness. These results are based on the KP measurement shown in Figure3 and Figure S4 and the calculation by Equations (5) and (6). the KP measurement shown in Figure 3 and Figure S4 and the calculation by Equations (5) and (6).

The W and Vbi values of the 5% and 1% doped DIP devices are 20 nm and 1.40 eV and 60 nm and 0.99The eV,W and respectively. Vbi values This of the shows 5% and that 1% the doped higher DIP doping devices concentration are 20 nm and decreased 1.40 eV the andW 60value nm and 0.99 eV, respectively. This shows that the higher doping concentration decreased the W value and increased the Vbi value. increasedThe charge the Vbi generation value. and recombination processes in the pn-homojunction devices are discussed basedThe on thecharge results generation of the temperature and recombination dependence processes of J–V characteristics in the pn-homojunction and the energy devices structure are discussed based on the results of the temperature dependence of J–V characteristics and the energy measurements. Firstly, JSC increased eight times owing to doping and Ea for charge separation decreasedstructure measurements. from 110 meV inFirstly, the 1% JSC doped increasedpn-homojunction eight times devicesowing to to doping 74.4 meV and in theEa for 5% dopedcharge pnseparation-homojunction decreased devices. from 110 In thismeV study, in the we1% useddoped the pn same-homojunction host material, devices and to only74.4 meV the interfacial in the 5% doped pn-homojunction devices. In this study, we used the same host material, and only the energetics were modiﬁed by doping. The light intensity dependence of JSC in the 1% and 5% doped devicesinterfacial in Figureenergetics S6 showed were modified that the slopesby doping. of both The the light devices intensity were dependence close to unity, of indicating JSC in the that1% and the bimolecular5% doped devices recombination in Figure is negligibleS6 showed under that the a short slopes circuit of both condition the devices [14]. Thus, were the close origin to ofunity, the indicating that the bimolecular recombination is negligible under a short circuit condition [14]. Thus, the origin of the difference in the Ea induced by doping is attributed to the charge separation process and not to the exciton diffusion, nor the bimolecular recombination process. The charge separation processes are mainly separated into the two processes: exciton to CT state, which is defined as a bounded charge pair by the coulomb attraction at the interface, and the CT state to the charge separated state [2]. The energy structure measurement revealed that the energy offset between adjacent molecules at the pn-interface was 0.07 and 0.26 eV for 1% and 5% doped devices, respectively. The offset at one molecular layer apart from the pn-interface was 0.06 and 0.22 eV in the 1% and 5% doped devices, respectively (Figure 5).

Materials 2020, 13, 1727 6 of 8

difference in the Ea induced by doping is attributed to the charge separation process and not to the exciton diffusion, nor the bimolecular recombination process. The charge separation processes are mainly separated into the two processes: exciton to CT state, which is defined as a bounded charge pair by the coulomb attraction at the interface, and the CT state to the charge separated state [2]. The energy structure measurement revealed that the energy offset between adjacent molecules at the pn-interface was 0.07 and 0.26 eV for 1% and 5% doped devices, respectively. The offset at one molecular layer apart from the pn-interface was 0.06 and 0.22 eV in the 1% and 5% doped devices, respectively (Figure5). Materials 2020, 13, x FOR PEER REVIEW 6 of 8

2 5% off 1% : 0.07 eV - E 5% : 0.26 eV

- 1% - 1% : 0.06 eV 1 - 5% : 0.23 eV

Fermi level eff 1% : 1.95 eV 0 Eg 5% : 1.75 eV 5% + Energy (eV) Energy 1% -1 +

-2 5% MoO3 doped 5% Cs2CO3 doped

FigureFigure 5.5.Schematic Schematic energy energy diagram diagram and and charge charge dissociation dissociation and and recombination recombination mechanisms mechanisms in organic in pn-organichomojunction pn-homojunction SCs. The SCs. inset The shows inset theshows energy the ener gapgy between gap between adjacent adjacent molecules molecules at the atpn- theinterface pn- withinterface 1% (red) with and1% (red) 5% (blue) and 5% doping. (blue) doping.

TheThe conventional conventional donor donor/acceptor/acceptor (D (D/A)-type/A)-type OSCs OSCs have have an an energy energy offset offset only only at the at D the/A interface,D/A andinterface, an off setand larger an offset than larger 0.3 eV than not 0.3 only eV acceleratesnot only accelerates the exciton the dissociation, exciton dissociation, but also but suppresses also suppresses geminate recombination from the CT state [15]. By the analogy with the D/A-type OSCs, geminate recombination from the CT state [15]. By the analogy with the D/A-type OSCs, the energy offset the energy offset between adjacent molecules at the pn-interface is crucial for charge separation in the between adjacent molecules at the pn-interface is crucial for charge separation in the pn-homojunction pn-homojunction devices. The reason for the smaller Ea in the 5% doped device, compared with that devices. The reason for the smaller E in the 5% doped device, compared with that in the 1% device, in the 1% device, was that larger energya offset close to 0.3 eV in the 5% doped device not only was that larger energy offset close to 0.3 eV in the 5% doped device not only accelerated exciton accelerated exciton dissociation, but also suppressed geminate recombination from the CT state. dissociation, but also suppressed geminate recombination from the CT state. In contrast to JSC, VOC decreased from 0.98 V in the 1% doped device to 0.83 V in the 5% doped In contrast to J , V decreased from 0.98 V in the 1% doped device to 0.83 V in the 5% doped device. TemperatureSC dependenceOC of VOC revealed that the VOC difference in the doping concentration V V device.was attributed Temperature to the dependenceEgeff difference, of notOC torevealed the recombination that the OC lossdi difference.fference in E thegeff reflects doping the concentration energy eff eff wasof charge attributed recombination to the Eg centerdifference, [16]. In not the to case the recombinationof inorganic SCs, loss Eg dieff ffcorrespondserence. Eg toreflects the energy the energy of eff ofthe charge bandgap recombination of the semiconductor center [16]. In material the case ofbecause inorganic the SCs,chargeEg recombinationcorresponds tohappens the energy from of the bandgapdelocalized of thecharges semiconductor on the conduction material and because valence the bands charge [16]. recombination In contrast, E happensgeff in the fromconventional delocalized eff chargesD/A-type on OSCs the conduction corresponds and to the valence CT state bands energy [16 ].because In contrast, localizedEg electronsin the conventionalon the LUMO Dof/ A-typethe OSCsacceptor corresponds and holes to on the the CT HOMO state energy of the becausedonor recombine localized electrons[17]. In the on case the LUMOof the organic of the acceptor pn- andhomojunction holes on the SCs HOMO in this study, of the E donorgeff decreased recombine with [ 17an]. increase In the caseof Vbi of. The the E organicgeff differencepn-homojunction between eff eff SCs1% inand this 5% study,is 0.20 EeV,g whichdecreased is almost with same an increasevalue with of theVbi energy. The E offsetg di differencefference between between 1%1% and 5% is5% 0.20 at eV,the whichadjacent is almostmolecules same at valuethe pn with-interface the energy(Figure o ff5).set The di ffresulterence indicates between that 1% the and charge 5% at the adjacentrecombination molecules in the at organic the pn-interface pn-homojunction (Figure SCs5). The happens result from indicates localized that holes the chargeand electrons recombination at the intwo the adjacent organic pnmolecules.-homojunction The mechanism SCs happens is similar from localizedto that of holes the CT and state electrons recombination at the two in adjacent the molecules.conventional The D/A-type mechanism OSCs, is similar even though to that ofthe the device CT states in this recombination study were infabricated the conventional using a single D/A-type OSCs,host material even though and a thedoping devices technique in this resembling study were the fabricated inorganic using pn-homojunction a single host SCs. material and a doping technique resembling the inorganic pn-homojunction SCs. 3. Conclusions In summary, the temperature dependence of J–V characteristics and energy structure measurement revealed that the increase in JSC and decrease in VOC with an increase in doping concentration in the organic pn-homojunction SC devices was the result of the acceleration of charge separation and the change in energy of the recombination center. The charge separation mechanism in the device is that the localized exciton and CT state are separated by the energy offset between adjacent molecules, and the recombination happens from the localized charge carrier at two adjacent molecules. These mechanisms are similar to those of conventional D/A-type OSCs, not to those of the

Materials 2020, 13, 1727 7 of 8

3. Conclusions In summary, the temperature dependence of J–V characteristics and energy structure measurement revealed that the increase in JSC and decrease in VOC with an increase in doping concentration in the organic pn-homojunction SC devices was the result of the acceleration of charge separation and the change in energy of the recombination center. The charge separation mechanism in the device is that the localized exciton and CT state are separated by the energy offset between adjacent molecules, and the recombination happens from the localized charge carrier at two adjacent molecules. These mechanisms are similar to those of conventional D/A-type OSCs, not to those of the inorganic SCs. In the case of inorganic pn-homojunction, larger Vbi formed by higher doping concentration leads to both higher JSC and VOC [1]. The same tendency is favorable to obtain high efficiency in organic pn-homojunction SCs. The primordial difference in the photoconversion process between inorganic and organic pn-homojunction SCs comes from the delocalized and localized nature of charges in inorganic and organic semiconductors, respectively. Recently, some organic semiconductor materials showed a band-like nature of charges [18,19]. Utilizing these kinds of materials as hosts in organic pn-homojunction SCs could lead to the realization of direct free charge formation and band-to-band recombination.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/13/7/1727/s1, Figure S1: chemical structure, schematic of the device, energy levels; Figure S2: J–V curves; Figure S3: EQE and absorption spectra; Figure S4: WF as a function of the thickness; Figure S5: PYS spectra; Figure S6: light intensity dependence of JSC Table S1: summary of the device performances. Author Contributions: Conceptualization, S.I. and J.-H.L.; methodology, S.I. and J.-H.L.; investigation, J.-H.L. and A.P.; writing—original draft preparation, J.-H.L.; writing—review and editing, S.I.; supervision, M.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded in part by JSPS KAKENHI (Grant-in-Aid for Young Scientists, No. 18K14115), the Foundation of Chubu Science and Technology Center, and the Mazda foundation. Acknowledgments: The authors are grateful to Keisuke Tajima and Kyohei Nakano at RIKEN for assistance with the PYS measurements. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Battaglia, C.; Cuevas, A. High-efficiency crystalline silicon solar cells: Status and perspectives. Energy Environ. Sci. 2016, 9, 1552–1576. [CrossRef] 2. Clarke, T.M.; Durrant, J.R. Charge photogeneration in organic solar cells. Chem. Rev. 2010, 110, 6736–6767. [CrossRef][PubMed] 3. Hiramoto, M.; Kubo, M.; Shinmura, Y.; Ishiyama, N.; Kaji, T.; Sakai, K.; Ohno, T.; Izaki, M. bandgap science for organic solar cells. Electronics 2014, 3, 351–380. [CrossRef] 4. Izawa, S.; Perrot, A.; Lee, J.H.; Hiramoto, M. Organic pn homojunction solar cell. Org. Electron. 2019, 71, 45–49. [CrossRef] 5. Izawa, S.; Nakano, K.; Suzuki, K.; Hashimoto, K.; Tajima, K. Dominant effects of first monolayer energetics at donor/acceptor interfaces on organic photovoltaics. Adv. Mater. 2015, 27, 3025–3031. [CrossRef][PubMed] 6. Shintaku, N.; Hiramoto, M.; Izawa, S. Effect of trap-assisted recombination on open-circuit voltage loss in phthalocyanine/fullerene solar cells. Org. Electron. 2018, 55, 69–74. [CrossRef] 7. Riedel, I.; Dyakonov, V. Influence of electronic transport properties of polymer-fullerene blends on the performance of bulk heterojunction photovoltaic devices. Phys. Status Solidi Appl. Res. 2004, 201, 1332–1341. [CrossRef] 8. Gao, F.; Tress, W.; Wang, J.; Inganäs, O. Temperature dependence of charge carrier generation in organic photovoltaics. Phys. Rev. Lett. 2015, 114, 1–5. [CrossRef][PubMed] 9. Yamamoto, S.; Orimo, A.; Ohkita, H.; Benten, H.; Ito, S. Molecular understanding of the open-circuit voltage of polymer:Fullerene solar cells. Adv. Energy Mater. 2012, 2, 229–237. [CrossRef] Materials 2020, 13, 1727 8 of 8

10. Izawa, S.; Shintaku, N.; Hiramoto, M. Effect of band bending and energy level alignment at the donor/acceptor interface on open-circuit voltage in organic solar cells. J. Phys. Chem. Lett. 2018, 9, 2914–2918. [CrossRef] [PubMed] 11. Ishii, H.; Hayashi, N.; Ito, E.; Washizu, Y.; Sugi, K.; Kimura, Y.; Niwano, M.; Ouchi, Y.; Seki, K. Kelvin probe study of band bending at organic semiconductor/metal interfaces: Examination of fermi level alignment. Phys. Status Solidi Appl. Res. 2004, 201, 1075–1094. [CrossRef] 12. Shintaku, N.; Hiramoto, M.; Izawa, S. Doping for controlling open-circuit voltage in organic solar cells. J. Phys. Chem. C 2018, 122, 5248–5253. [CrossRef] 13. Yamashina, Y.; Shinmura, Y.; Ishiyama, N.; Kaji, T.; Hiramoto, M. Mapping of band-bending in organic pn –homojunctions. J. Appl. Phys. 2015, 117, 125501. [CrossRef] 14. Koster, L.J.A.; Kemerink, M.; Wienk, M.M.; Maturová, K.; Janssen, R.A.J. Quantifying bimolecular recombination losses in organic bulk heterojunction solar cells. Adv. Mater. 2011, 23, 1670–1674. [CrossRef] [PubMed] 15. Nakano, K.; Chen, Y.; Xiao, B.; Han, W.; Huang, J.; Yoshida, H.; Zhou, E.; Tajima, K. Anatomy of the energetic driving force for charge generation in organic solar cells. Nat. Commun. 2019, 10, 2520. [CrossRef][PubMed] 16. Hegedus, S.S.; Shafarman, W.N. Thin-film solar cells: Device measurements and analysis. Prog. Photovolt. Res. Appl. 2004, 12, 155–176. [CrossRef] 17. Burke, T.M.; Sweetnam, S.; Vandewal, K.; McGehee, M.D. Beyond Langevin recombination: How equilibrium between free carriers and charge transfer states determines the open-circuit voltage of organic solar Cells. Adv. Energy Mater. 2015, 5, 1500123. [CrossRef] 18. Bussolotti, F.; Yang, J.; Yamaguchi, T.; Yonezawa, K.; Sato, K.; Matsunami, M.; Tanaka, K.; Nakayama, Y.; Ishii, H.; Ueno, N.; et al. Hole-phonon coupling effect on the band dispersion of organic molecular semiconductors. Nat. Commun. 2017, 8, 173. [CrossRef][PubMed] 19. Liu, C.; Minari, T.; Lu, X.; Kumatani, A.; Takimiya, K.; Tsukagoshi, K. Solution-processable organic single crystals with bandlike transport in field-effect transistors. Adv. Mater. 2011, 23, 523–526. [CrossRef][PubMed]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).