第40卷第1期
2021年2月红外与毫米波学报
J.Infrared Millim.Waves Vol.40，No.1 February，2021
文章编号：1001-9014（2021）01-0007-06DOI：10.11972/j.issn.1001-9014.2021.01.002
Reducing V
oc
loss in InGaAsP/InGaAs dual-junction solar cells
LU Hong-Bo1，2，3，LI Xin-Yi2*，LI Ge2，ZHANG Wei2，HU Shu-Hong1，DAI Ning1*，YANG Gui-Ting2
（1.State Key Laboratory of Infrared Physics，Shanghai Institute of Technology Physics of the Chinese Academy of
Sciences，Shanghai200083，China；
2.State Key Laboratory of Space Power-sources，Shanghai Institute of Space Power-sources，Shanghai200245，
China；
3.University of Chinese Academy of Sciences，Beijing100049，China）
Abstract：Smaller V
oc of1.0eV/0.75eV InGaAsP/InGaAs double-junction solar cell（DJSC）than the V
oc
sum of
individual subcells has been observed，and there is little information of the origin of such V
oc
loss and how to mini⁃
mize it.In this paper，it is disclosed that the dominant mechanism of minority-carrier transport at back-surface-field（BSF）/base interface of the bottom subcell is thermionic emission，instead of defect-induced recombination，which is in contrast to previous reports.It also shows that both InP and InAlAs cannot prevent the zinc diffusion effectively.In addition，intermixing of major III-V element occurs as a result of increasing thermal treatment.To suppress the above negative effects，an initial novel InP/InAlAs superlattice（SL）BSF layer is then proposed and
employed in bottom InGaAs subcell.The V
oc
of fabricated cells reach997.5mV，and a reduction of30mV in Voc loss without lost of Jsc，compared with the results of conventional InP BSF configuration，is achieved.It
would benefit the overall V
oc
for further four-junction solar cells.
Key words：Back-surface field，InGaAsP/InGaAs dual-junction，open-circuit voltage，superlattice.
PACS：88.40.jp，78.67.HC，75.40.Mg
InGaAsP/InGaAs双结太阳电池的开路电压损耗抑制
陆宏波1，2，3，李欣益2*，李戈2，张玮2，胡淑红1，戴宁1*，杨瑰婷2（1.中国科学院上海技术物理研究所，红外物理国家重点实验室，上海200083；
2.上海空间电源研究所，空间电源技术国家重点实验室，上海200245；
3.中国科学院大学，北京100049）
摘要：现有1.0eV/0.75eV InGaAsP/InGaAs双结太阳电池的开路电压小于各子电池的开路电压之和，鲜有研究探索开路电压损耗的来源以及如何抑制。通过研究发现，InGaAs底电池背场/基区界面处的少数载流子输运的主要机制是热离子发射，而不是缺陷诱导复合。SIMS测试表明，采用InP或InAlAs背场均不能有效抑制Zn 掺杂剂的扩散。此外，由于生长过程中持续的高温热处理，III-V族主元素在界面处发生了热扩散。为了抑制上述现象，提出了一种新型InP/InAlAs超晶格背场，并应用到InGaAs底电池中。制备得到的双结太阳电池在维持短路电流密度不变的情况下，开路电压提升到997.5mV，与传统采用InP背场的双结太阳电池相比，开路电压损耗降低了30mV。该研究成果对提升四结太阳电池的整体开路电压有重要意义。
关键词：背场；InGaAsP/InGaAs双结电池；开路电压；超晶格
中图分类号：TM914.4文献标识码：A
Introduction
InGaAsP/InGaAs double-junction solar cells （DJSCs）with approximate bandgap combination of1.0/0.75eV are used in four-junction configuration to har⁃vest900~1700nm sunlight，and are crucially important for device performances［1］.Previous reports Ref.［2-4］
Received date：2020⁃04⁃16，revised date：2020⁃06⁃05收稿日期：2020⁃04⁃16，修回日期：2020⁃06⁃05
Foundation items：Supported by the National Nature Science Foundation of China（61474076and61704106），the Young Elite Scientist Sponsorship Pro⁃gram by China Association for Science and Technology（2017QNRC001）and Shanghai Rising-Star Program（18QB1402500and19QB1403800）. Biography：Lu Hong-Bo，male，Shanghai，master.Research area involves III-V Semiconductor materials and devices.E-mail：lhb2139@163.
*Corresponding author：E-mail：lixy_sisp@163
红外与毫米波学报40卷
show that open -circuit voltage （V oc ）of InGaAsP/InGaAs DJSC is smaller than the sum of individual subcells.To evaluate solar cells with different bandgaps ，bandgap -voltage offset under open -circuit condition （W oc ）is intro⁃duced ［5］.For multijunction solar cells ，Woc can be de⁃scribed as
trimeW oc (J sc )=1q éëê∑i E i g -V oc (J sc )ù
ûú，（1）where E i g is the bandgap of each subcell ，and V oc （Jsc ）
stands for the open -circuit voltage when the device pro⁃duces a given value of short -circuit current （Jsc ）under illumination.And the V oc loss for multijunction solar cells would be defined as the gap between W m oc and the
sum of W i oc of subcells ，as W m oc -∑i
W i
oc ()J sc .
W m
oc
of InGaAsP/InGaAs DJSC at Jsc of conventional
four -junction configuration （about 16.5mA/cm 2）is above 820mV ，higher than the W oc sum of InGaAsP （~330mV ）and InGaAs （~340mV ）individual subcells.Part results from previous report
s are listed in Table 1，and there is no reference reporting the origin of such V oc loss and how to minimize it.
Experience on III -V semiconductor devices reveals that the diffusion and intermixing at heterojunction inter⁃face of InP system always leads to device performance degradation ［6］.Moreover ，from the viewpoint of physics of solar cell device ，the V oc of solar cells majorly depends on the heterojunction interface between base and back -surface field （BSF ）layers.Therefore ，considering the thermal history of DJSC structure ，the bottom InGaAs subcell ，especially the BSF/base interface ，might be the key role to reduce V oc loss.
In this paper ，the evolution of dopant diffusion and recombination at BSF/base interface with increasing ther⁃mal treatment is studied.Based on experimental results ，we propose a novel InP/InAlAs superlattice （SL ）BSF layer for bottom InGaAs subcell.A reduction of 30mV in V oc loss is achieved ，compared with the results of con⁃ventional InP BSF configuration.It shows that such SL BSF would benefit the V oc enhancement for four -junction solar cells.
1Experiments
Growth are done on n -type <100>InP substrates us⁃ing MOVPE technique.The primary group III and
group V precursors used are trimethylgallium （TMGa ），trime⁃thylindium （TMIn ），trimethylgallium （TMAl ），arsine （AsH 3），and phosphine （PH 3）.The dopant precursors used are silane （SiH 4）and diethylzinc （DEZn ）.V/III ra⁃tio of 200~300and growth temperature of 650°C are em⁃
ployed ，as described previously ［7］.
Three periods of isotype p +-barrier （100nm ）/p --In⁃GaAsP （500nm ）/p +-barrier （100nm ）/p ++-In （Al 0.1Ga 0.9）As （100nm ）double heterojunctions （DHs ），separated by InGaAs spacer layers ，are grown in the same stack of MOVPE layers ，as illustrated in Fig.1.Two types of bar⁃riers ，InP and InAlAs respectively ，are employed.After growth ，individual DHs are exposed by a series of selec⁃tive etches.Diluted HCl solution and H 2SO 4：H 2O 2：H 2O solution are used for InP layers and arsenide layers ，re⁃spectively.The overall element profiles in DHs are ob⁃tained through secondary ion mass spectra （SIMS ）mea⁃surement ，while the minority -carrier recombination pro⁃cess in DHs are evaluated using time -resolved photolumi⁃nescence （TRPL ）technique.It should be pointed out that ，the bandgap of p --InGaAsP in DHs is 0.83eV ，for TRPL measurement convenience.
SIMS measurements are performed using Cs +prima⁃ry beam with a fixed 5kV acceleration.The positive ions of the quasi -molecular cluster are collected and detect⁃ed.TRPL measurements with a temporal resolution of ~200ps are performed at room temperature.An H -10330-75PMT is used to collect PL signals.
The schematic cross -section of InGaAsP/InGaAs （1.0/0.75eV ）DJSC structure is shown in Fig.2.The active region of each subcell consists of n -on -p junction （emitter/base ）surrounded by n -type InP window layer and p -type BSF layer.In （Al 0.1Ga 0.9）As tunnel junction is used to connect subcells.The structures are then pro⁃cessed following the standard III -V solar cell device art.
Table 1Previous reported results for InGaAsP/InGaAs DJSC 表1文献报道的InGaAsP/InGaAs 双结电池电性能
Reference Oshima ［2］Wu ［3］
Zhao ［4］Method MBE MBE
MOVPE Bandgap 1.0/0.71
1.05/0.731.07/0.74Illumination AM1.5G AM1.5G AM1.5D
J sc
（mA/cm 2）13.116.110.2
V oc
（mV ）570830977
W oc
（mV ）1140950
833
Fig.1Cross-section of MOVPE stack containing three BSF/In ‐GaAsP/BSF DHs.
图1含3对BSF/InGaAsP/BSF 双异质结结构样品示意图
8
1期
LU Hong -Bo et al ：Reducing V oc loss in InGaAsP/InGaAs dual -junction solar cells
The cells are 1.0×1.0cm 2in size.
In -house photovoltaic current density -voltage （J-V ）measurements are performed under AM0solar simulator ，without GaAs filter.External quantum efficiency （EQE ）measurements are performed to give qualitative insight in⁃to the spectral response.Cells are placed on 25°C cooled stages during measurements.
2Results and discussions
Figure 3shows the element profiles and PL decay curves for topmost DHs （DH1）in InP -barrier and InA⁃lAs -barrier stacks.The zinc concentration around both DH center regions are of the same level about 5-6×1016cm -3，which is similar to the typical doping level of base in solar cells.Sharp zinc diffusion profile near the inter⁃face between InGaAsP and barriers is observed.It should be pointed out that ，the zinc doping level of both InP and InAlAs layers in DHs have been increased to 1-2×1018cm -3，almost one order of magnitude higher than typical doping level used in BSF layer ，to exacerbate such diffusion behavior.Moreover ，the zinc concentra⁃tion in the underneath In （Al 0.1Ga 0.9）As layer reaches 2×1019cm -3
.The nearly identical profiles of zinc atom sug⁃gest that both InP and InAlAs layer are of the similar ef⁃fect as anti -diffusion barriers during the growth.
It evidences in Fig.3that the minority -carrier decay time in InAlAs -barrier DH is much larger than that in InP -barrier DH.In symmetrical DHs ，the effective life⁃time τeff extracted from PL decay is related to both bulk carrier lifetime τbulk and surface recombination velocity S ，as Ref.［8］
1τeff =1τbulk +2S d ，（2）where d is the thickness of confined layer in the DH.For
high -quality materials ，the τbulk approximately equals to
the reciprocal product of spontaneous radiative recombi⁃nation coefficient B and doping concentration N
τbulk =(BN )-1
.（3）
The value of B could be calculated according to Ref.［9］.With the doping concentration acquired from SIMS ，the τbulk in DH1is about 200ns.By mono -expo⁃nential fitting of decay curves ，τeff of 70ns and 110ns are obtained for InP -barrier DH and InAlAs -barrier DH.According to Eq.2，the experimental S for InP/InGaAsP interface and InAlAs/InGaAsP interface are 232cm/s and 103cm/s ，respectively.Such small recombination velocity suggests that the radiative process in the bulk domi
nates the carrier recombination ，and the recombina⁃tion at the interface is nearly neglectable.
The primary mechanism of carrier transport at het⁃erojunction interface includes thermionic emission and defect -induced recombination.When thermionic emis⁃sion dominates ，the recombination current writes ［10］
J 0=q S n 1=q
m 2m 1(2k B T
πm 1
)
1
2
e
-
ΔE k B T
∙n 1
，（4）
where m 1and m 2are effective mass of confined material and barrier ，and ΔE is band offset.It is quite obvious that S would exponentially decreases as the band offset increasing.Notice that electron is the minority -carrier in p -InGaAsP DHs.The values of S for DHs ，in the scenar⁃io of thermionic emission dominating ，are estimated us⁃ing ΔEc and m 1，2from Ref.［
11］，as listed in Table 2.Surface recombination velocities S of 5793cm/s and 0.738cm/s are obtained for InP -barrier DH and InAlAs -barrier DH ，respectively.Both calculated and experi⁃mental values show that ，InAlAs -barrier DH presents smaller surface recombination velocity at heterojunction interface.Consider the complicated carrier transport mechanism at heterojunction interface ，the gap between experimental S will not be so large.For example ，the sharp diffusion profile of zinc would develop built -in field near the interface ，it should reduce the population of mi⁃nority -carrier reaching the interface and therefore ，small⁃er theoretical value of S could be expected ，especially for InP -barrier DH.For InAlAs -barrier DH ，the larger ex⁃perimental S than th
e calculated S implies the minor exis⁃tence of trap -induced nonradiative recombination across the interface.
For InGaAs ，the conduction band offset ΔEc for
InP
Fig.2Cross-section of the InGaAsP/InGaAs double-junction so ‐
lar cell structure.
图2InGaAsP/InGaAs
双结太阳电池结构示意图
Fig.3Element profiles （a ）and PL decay curves （b ）for InP-barrier and InAlAs-barrier DH1s.
图3InP 和InAlAs 背场双异质结的元素深度剖析（a ）和荧光发光衰减曲线（b ）
9
红外与毫米波学报40卷
barrier and InAlAs barrier are0.25eV and0.52eV，re⁃
spectively.The larger offset in conduction band means
smaller thermionic emission velocity.Meanwhile，the va⁃
lence band offsetΔEv for InP barrier and InAlAs barrier
are0.35eV and0.17eV［11］.The smaller offset in va⁃
lence band indicates lower potential barrier for majority-
carrier.Therefore，InAlAs should be more promising
BSF layer in solar cells.
Figure4displays the overall SIMS results for DH
stacks，and Fig.5shows the PL decay curves for individ⁃
ual DHs.Extracted lifetimes are summarized in Table3.
It is obvious that zinc concentration in confined layers ris⁃es with increased thermal history from DH1to DH3.The concentration in DH2is about1.0×1017cm-3，while the concentration in DH3is about2.0×1017cm-3，in spite of the type of barriers.This provides further evidence that both InP and InAlAs present similar ability to block zinc diffusion.Although there is a downward trend from DH1 to DH3，the effective lifetimes in InAlAs-barrier DHs are always longer than those in InP-barrier DHs.It suggests the surface recombination velocity is still dominated by thermionic emission process.With zinc concentration in⁃creasing to1.0×1017cm-3，the bulk carrier lifetimeτbulk in DH2decrease to approximately100ns，according to Eq.2.Therefore，the surface recombination velocity S increase to434cm/s and221cm/s，for InP-barrier DH2 and InAlAs-barrier DH2respectively.Since the drop of band offset is the only cause for thermionic emission re⁃lated increase of S，it is supposed that diffusion of major III-V element across the interface，which would lead to such shrink of band offset，occurs during the thermal treatment.
As shown in Table3，for DHs using the same type of barriers，theτeff in DH3are quite close to theτeff in DH2.With zinc concentration of2.0×1017cm-3，the bulk carrier lifetimeτbulk in DH3is approximately50ns，and S are210.2cm/s and55.6cm/s，for InP-barrier DH3and InAlAs-barrier DH3respectively.The abnormal de⁃crease of S is probably due to the photon recycling effect，which l
eads to the longerτeff and smaller S in DH3than expected.The steady-state PL of DHs confirms the above hy⁃pothesis.As shown in Fig.6，the PL peak for DH1and DH2are of similar intensity，while PL intensities of DH3 are nearly one order of magnitude stronger.Considering the optical configuration of DH stack，the photon recy⁃cling is the most effective in DH3，and is suppressed in DH1and DH2due to extra absorption from underlying narrow bandgap InGaAs spacers.
The results of InGaAsP/InGaAs DJSCs using both InP and InAlAs BSF layers confirm the advantages and effectiveness of InAlAs BSF layer in practical device. Figure7shows light J-V and EQE measurements of the devices.Using InAlAs BSF layer，the cell presents an ef⁃ficiency of9.28%with a V oc of983.2mV，a J sc of15.6 mA/cm2and an FF of0.818.Meanwhile，the device us⁃ing InP BSF present a V oc of967.7mV，a J sc of15.3mA/
Table2Calculated surface recombination velocity at barrier/InGaAsP interface using Eq..（3）
表2采用公式（3）计算得到的背场/InGaAsP界面处的表面复合速率
Barrier InP InAlAs m2（m0）
0.08
0.075
m1（m0）
0.047
0.047
ΔEc（meV）
230
460
S（cm/s）
5793
0.
738 Fig.4Overall SIMS results of（a）InP-barrier DH stack and
（b）InAlAs-barrier DH stack.
图4InP双异质结（a）和InAlAs双异质结（b）的整体SIMS测
试结果Fig.5PL decay curves of（a）InP-barrier DHs and（b）InAlAs-barrier DHs.
图5InP双异质结（a）和InAlAs双异质结（b）的荧光发光衰减曲线
Table3The effective minority-carrier lifetime of the DHs
表3双异质结的有效载流子寿命
τeff（ns）
InP-barrier
InAlAs-barrier
DH1
70.0
110.0
DH2
36.5
53.0
DH3
35.2
45.
Fig.6Steady-state PL of（a）InP-barrier DHs and（b）InAlAs-barrier DHs.Weak peaks marked by asteroids in DH1and DH2 are related to the spacers.
图6InP双异质结（a）和InAlAs双异质结（b）的稳态荧光发光曲线。标*的微弱发光峰与背场相关
10
1期
LU Hong -Bo et al ：Reducing V oc loss in InGaAsP/InGaAs dual -junction solar cells
cm 2and an FF of 0.819.An enhancement of V oc is ob⁃tained ，without any cost of J sc and FF.
It is well established that SL serve as effective barri⁃er for element diffusion or intermixing ，and dislocation threading ，and it has been widely used in semiconductor devices such as high electron mobility transistors ，laser diodes ，electro absorption modulators ［12-16］.Also ，the miniband in SL would not introduce extra potential barri⁃er for carrier transport ［17］.An initial five -period InP （2nm ）/InAlAs （2nm ）SL BSF layer is designed and em⁃ployed in bottom InGaAs subcell of DJSC.A V oc of 997.5mV ，a Jsc of 15.8mA/cm 2and an FF of 0.824are ob⁃tained as in Fig.8.Both V oc and Jsc are boosted ，as ex⁃pected ，in fabricated SL BSF device.The V oc approaches 1.0V ，resulting in a Woc of 752.5mV.A reduction of 30mV in V oc loss for DJSC is achieved ，compared with the conventional InP BSF DJSC.
3Conclusions
In general ，the use of novel SL BSF layer in the bot⁃tom subcell reduces the V oc loss in InGaAsP/InGaAs DJSC.
Experiments show that ，the mechanism of minority -carrier transport at BSF/base interface of the bottom sub⁃cell of InGaAsP/InGaAs DJSCs is dominated by thermion⁃
ic emission ，instead of defect -induced recombination ，which is in contrast to previous reports.It also shows that both InP and InAlAs cannot prevent the zinc diffusion ef⁃fectively.In addition ，intermixing of major III -V element occurs as a result of increasing thermal treatment.
Based on the above results ，an initial 5-period InP/InAlAs SL BSF layer is designed and employed in bottom InGaAs subcell of DJSC.A V oc of 997.5mV ，a J sc of 15.8mA/cm 2and an FF of 0.824are obtained.The V oc approaches 1.0V ，resulting in a W oc of 752.5mV.A reduction of 30mV in V oc loss for DJSC is achieved ，com⁃pared with the results of conventional InP BSF configura⁃tion.It suggests that such SL BSF would benefit the V oc enhancement for four -junction solar cells.References
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Fig.7（a ）Light J-V and （b ）spectra response curves for In ‐
GaAsP/InGaAs solar cells using InP and InAlAs BSF layers
图7采用InP 和InAlAs 背场的InGaAsP/InGaAs 双结电池光照J-V 曲线（a ）和量子效率曲线（b
）
Fig.8Light J-V for InGaAsP/InGaAs DJSC using 5-period InP/
InAlAs SL BSF layer.
图8采用5对InP/InAlAs 超晶格背场的InGaAsP/InGaAs 双结电池光照J-V 曲线
11
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