1616 IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 11, NOVEMBER 2012

Design of Organic Vertical-Cavity Surface-EmittingLaser for Electrical Pumping

Vahid Qaradaghi, Vahid Ahmadi, Senior Member, IEEE, and Gholamreza Abaeiani, Member, IEEE

Abstract—In this letter, we propose a new structure of anorganic vertical-cavity surface-emitting laser to be excited elec-trically. The static and dynamic characteristics of the device areanalyzed numerically. We show that the structure can reach thelasing threshold under electrical pump at a current density of30 A/cm2. With electrical pulse injection, the photon density peakrises, and we can apply a current density as high as 100 A/cm2.Also, the results show that a high interval causes a high peak.

Index Terms—Current density, electrical pump, organic losses,organic vertical-cavity surface-emitting laser (OVCSEL), photondensity.

I. INTRODUCTION

O RGANIC laser diodes (OLDs) are expected to provideunique features such as the tunability over the whole

visible range, low cost, and large-area fabrication. These prop-erties would make OLDs suitable for a huge number of ap-plications in bioanalytics, digital printing, and fluorescencespectroscopy [1].

Organic semiconductor lasers excited by optical pump havebeen demonstrated in [2]. Organic lasers with electrical pump-ing have not been practical yet. For electrical excitation, highcurrent density is necessary. This causes high dissipation whichresults in heating and eventual destruction of the device. Lowmobility and other factors such as contact losses and potentialbarrier between the organic layer and the contact or betweenorganic layers are the important factors that prevent lasingthreshold. Decreasing metal contact losses and potential bar-rier effects and also using high-quality-factor resonators willdecrease the mobility effect, so we can have a chance toget the lasing threshold under electrical excitation. Chakarounet al. reported a structure of an organic vertical-cavity surface-emitting laser (OVCSEL) [3]. They have used high-reflectionand low-absorption DBR mirror layers for reaching high qualityfactor. However, the designed structure has no capability ofreaching the lasing threshold under electrical injection. Here,

Manuscript received July 24, 2012; accepted July 29, 2012. Date of publica-tion September 13, 2012; date of current version October 19, 2012. The reviewof this letter was arranged by Editor O. Manasreh.

V. Qaradaghi is with the Department of Nanotechnology, Tarbiat ModaresUniversity, Tehran 14115-143, Iran.

V. Ahmadi is with the Faculty of Electrical and Computer Engi-neering, Tarbiat Modares University, Tehran 14115-194, Iran (e-mail:v_ahmadi@modares.ac.ir).

G. Abaeiani is with the Semiconductor Department, Laser and OpticsResearch School, Tehran 19835-63113, Iran.

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2012.2211330

Fig. 1. (a) Schematic structure of the proposed OVCSEL, (b) schematic bandgap of the active region, and (c) valence and conduction band energy under abias of 10 V.

we propose a new structure which is capable of reaching thethreshold condition under electrical pumping. The optimizedstructure includes one thin layer of LiF between the cathodeand the electron injection layer (EIL) to make electrons moveeasily to the active region. Also, we use four hole transmissionlayers (HTLs) instead of one layer to decrease the high potentialbarrier effect.

II. STRUCTURE OF THE PROPOSED OVCSEL

The schematic structure of the proposed OVCSEL is shownin Fig. 1(a). The active region consists of eight organic layerswith two contacts for carrier injection and DBR mirror layersat the top and bottom as reflectors. The device has thin anodeand cathode layers to support both electrical injection andtransparency of the resonance action. For this purpose, the

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QARADAGHI et al.: DESIGN OF OVCSEL FOR ELECTRICAL PUMPING 1617

anode is chosen as a three-layer structure of ITO (12 nm)/Ag(6 nm)/ITO (12 nm) to decrease the sheet resistance and supportbetter the hole injection, and the cathode is chosen as a two-layer structure of Al (3 nm)/Ag (5 nm) to support betterthe electron injection. Ta2O5/SiO2 as DBR layers has highreflection and low absorption. The active region consists ofseveral organic layers, including Alq3 [4] as EIL, BCP [5] asthe electron transmission layer (ETL) and the hole blockinglayer, Alq3:DCM [1] as the emission layer (EML), and PEDOT[6] as the hole injection layer (HIL). A thin LiF [7] layerbetween Alq3 and Al decreases the potential barrier to zero[8]. For electrical lasing, the potential barrier between twoadjacent layers should be decreased to minimum values. Thiscauses carriers to transmit in the system easily. Fig. 1(b) showsthe schematic band gap of the organic layers in the proposedOVCSEL. As shown in the figure, the HTLs include four layers:MPMP, TPD-OMe-19, TPFI-OMe-22, and TPFI-H21 [9], [10].We choose these four layers from one material family whichare the same in material characteristics but different in HOMOenergy bands. Using these four layers, we can break the highbarrier potential between HIL and HTL (0.5 eV) to four steps of0.1 eV, which causes the hole carriers to reach the EML easily.Therefore, the holes are not piled up between layers. Note thatthe accumulation of holes results in a local electric field whichdissociates singlet excitons.

Fig. 1(c) shows the conduction and valence band energy ofthe active region under a bias of 10 V. As can be observed,there is no effective potential barrier between layers in the newstructure; therefore, carriers move easily from layers, and theyrecombine in Alq3:DCM where the mobility of electrons andholes is low relative to the other layers.

III. RESULTS AND DISCUSSION

To analyze the photon and exciton dynamics, first, we solvethree coupled equations, which are the Poisson, continuity, anddrift–diffusion equations [11]. Solving these three equationsgives the electric field and the carrier concentration inside thedevice which are used for solving three coupled singlet exciton,triplet exciton, and photon equations [1]. The mobility of car-riers is electric field dependent, and the recombination rate isassumed to be Langevin type. We consider all the losses in thesystem, such as bimolecular annihilation process, field-inducedexciton dissociation, induced absorption, and contact losses.We also take the organic–organic potential barrier effects intoaccount which are important factors in reaching the lasingcondition.

Low mobility of carriers in organic layers is quite chal-lenging for the lasing condition which requires high carrierconcentration. It causes carriers to accumulate between het-erostructure layers with different mobilities and energy lev-els. The accumulation of carriers causes a high local electricfield that dissociates singlet and triplet excitons. Therefore, weshould have a device with high-mobility layers. In this regard,we use PEDOT with relatively higher mobility as HIL. Underelectrical excitation with current density above 10 A/cm2,various processes become important which reduce the sin-glet exciton density [1], so it is necessary to have a device

Fig. 2. Photon density versus time for different injected current densities(without optical pumping).

Fig. 3. Photon densities versus current densities at steady state for differentoptical pump energy densities.

with high quality factor (Q = λ0/Δλ) to decrease the lasingthreshold.

Fig. 2 shows the photon density of the proposed device versustime for different current densities when an electric pump isapplied to the device. Because of the low potential barrierbetween HTLs, holes easily move from these layers to EML.As there is no potential barrier between EIL and ETL, electronsmove easily from the cathode to EML. When an electricalcurrent is injected to the device, there is no carrier or spacecharge (SC); therefore, polarons move easily to EML, and thephoton density peaks significantly. As can be seen, the first peakin photon density is very high relative to steady state.

Fig. 3 shows the photon density versus current density atsteady state with and without optical pump. The photon densityincreases with current density to a level that losses in the deviceare not large enough to affect photon density. This level is theupper limit for current density. Larger current density resultsin higher losses that decrease photon density. By using opticalpump, singlet excitons are generated in EML, leading to higherphoton density and lower threshold current density. Note thatthe maximum photon density remains at a current density of35 A/cm2 for different values of optical pump energy. Whenthe optical pump energy increases to above 60 J/cm2, noelectrical current injection is required for lasing.

1618 IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 11, NOVEMBER 2012

Fig. 4. Output photon density versus optical energy density for different inputcurrent densities.

Fig. 5. (a) Output photon density for electrical pulse and CW excitations.(b) Output photon density for different time intervals of input electrical pulses.

Fig. 4 shows the photon density versus optical pump energyfor electrooptical excitation. The figure shows that, for electri-cal current above 30 A/cm2, the device starts lasing withoutoptical pump. However, high current densities increase the lossinside the device; hence, we need optical pump injection tocompensate the lost singlet excitons.

High current injection provides high carrier concentrationand greater local electric field which reduces photon density.With pulse current injection, charge carriers are not accumu-lated in layers to produce losses for larger current densities.Therefore, we can obtain much larger photon density.

Fig. 5(a) shows a comparison of photon densities for CW andpulse current injection. With CW injection, at the initial time,

there are no carriers in layers, so the carriers can move easilythrough the layers and reach EML. After a while, carriers arepiled up in the layers, and the loss (including triplet quenching,polaron absorption, and electric field dissociation) gets larger,reducing the photon density. With pulse current injection, welet the carriers leave the layers easily to overcome the limits ofthe SC limit (SCL) effect. The time interval between pulses isvery important to control the photon density.

Fig. 5(b) shows the photon density versus time for differentintervals of pulse injection. When the time interval betweentwo pulses increases, the SCL effect decreases more. There-fore, longer time intervals between pulses cause larger photondensity peak as shown in the figure.

IV. CONCLUSION

We have proposed a new structure of OVCSEL and studiedits static and dynamic characteristics under electrooptical ex-citation. The main losses, including bimolecular annihilationprocess, induced absorption process, and field-induced excitondissociation losses, are included in the simulation. The resultsshow that the proposed device structure starts lasing at a cur-rent density of about 35 A/cm2 without optical pump. Withpulse injection, we could apply a current density as high as100 A/cm2 to obtain large photon density. Also, we havestudied the effects of time interval between current pulses onthe output photon density. The results show that increasing thetime interval results in higher photon density peak.

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