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